Smith-Lemli-Opitz syndrome (SLOS) is a congenital multiple-anomaly / cognitive impairment syndrome caused by an abnormality in cholesterol metabolism resulting from deficiency of the enzyme 7-dehydrocholesterol (7-DHC) reductase. It is characterized by prenatal and postnatal growth restriction, microcephaly, moderate-to-severe intellectual disability, and multiple major and minor malformations. The malformations include distinctive facial features, cleft palate, cardiac defects, underdeveloped external genitalia in males, postaxial polydactyly, and 2-3 syndactyly of the toes. The clinical spectrum is wide; individuals with normal development and only minor malformations have been described.

INSR-related severe syndromic insulin resistance comprises a phenotypic spectrum that is a continuum from the severe phenotype Donohue syndrome (DS) (also known as leprechaunism) to the milder phenotype Rabson-Mendenhall syndrome (RMS). DS at the severe end of the spectrum is characterized by severe insulin resistance (hyperinsulinemia with associated fasting hypoglycemia and postprandial hyperglycemia), severe prenatal growth restriction and postnatal growth failure, hypotonia and developmental delay, characteristic facies, and organomegaly involving heart, kidneys, liver, spleen, and ovaries. Death usually occurs before age one year. RMS at the milder end of the spectrum is characterized by severe insulin resistance that, although not as severe as that of DS, is nonetheless accompanied by fluctuations in blood glucose levels, diabetic ketoacidosis, and – in the second decade – microvascular complications. Findings can range from severe growth delay and intellectual disability to normal growth and development. Facial features can be milder than those of DS. Complications of longstanding hyperglycemia are the most common cause of death. While death usually occurs in the second decade, some affected individuals live longer.

Pallister-Killian syndrome (PKS) is a dysmorphic condition involving most organ systems, but is also characterized by a tissue-limited mosaicism; most fibroblasts have 47 chromosomes with an extra small metacentric chromosome, whereas the karyotype of lymphocytes is normal. The extra metacentric chromosome is an isochromosome for part of the short arm of chromosome 12: i(12)(p10) (Peltomaki et al., 1987; Warburton et al., 1987).

GLB1-related disorders comprise two phenotypically distinct lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB (MPS IVB). The phenotype of GM1 gangliosidosis constitutes a spectrum ranging from severe (infantile) to intermediate (late-infantile and juvenile) to mild (chronic/adult). Type I (infantile) GM1 gangliosidosis begins before age 12 months. Prenatal manifestations may include nonimmune hydrops fetalis, intrauterine growth restriction, and placental vacuolization; congenital dermal melanocytosis (Mongolian spots) may be observed. Macular cherry-red spot is detected on eye exam. Progressive central nervous system dysfunction leads to spasticity and rapid regression; blindness, deafness, decerebrate rigidity, seizures, feeding difficulties, and oral secretions are observed. Life expectancy is two to three years. Type II can be subdivided into the late-infantile (onset age 1-3 years) and juvenile (onset age 3-10 years) phenotypes. Central nervous system dysfunction manifests as progressive cognitive, motor, and speech decline as measured by psychometric testing. There may be mild corneal clouding, hepatosplenomegaly, and/or cardiomyopathy; the typical course is characterized by progressive neurologic decline, progressive skeletal disease in some individuals (including kyphosis and avascular necrosis of the femoral heads), and progressive feeding difficulties leading to aspiration risk. Type III begins in late childhood to the third decade with generalized dystonia leading to unsteady gait and speech disturbance followed by extrapyramidal signs including akinetic-rigid parkinsonism. Cardiomyopathy develops in some and skeletal involvement occurs in most. Intellectual impairment is common late in the disease with prognosis directly related to the degree of neurologic impairment. MPS IVB is characterized by skeletal dysplasia with specific findings of axial and appendicular dysostosis multiplex, short stature (below 15th centile in adults), kyphoscoliosis, coxa/genu valga, joint laxity, platyspondyly, and odontoid hypoplasia. First signs and symptoms may be apparent at birth. Bony involvement is progressive, with more than 84% of adults requiring ambulation aids; life span does not appear to be limited. Corneal clouding is detected in some individuals and cardiac valvular disease may develop.

Untreated tyrosinemia type I usually presents either in young infants with severe liver involvement or later in the first year with liver dysfunction and renal tubular dysfunction associated with growth failure and rickets. Untreated children may have repeated, often unrecognized, neurologic crises lasting one to seven days that can include change in mental status, abdominal pain, peripheral neuropathy, and/or respiratory failure requiring mechanical ventilation. Death in the untreated child usually occurs before age ten years, typically from liver failure, neurologic crisis, or hepatocellular carcinoma. Combined treatment with nitisinone and a low-tyrosine diet has resulted in a greater than 90% survival rate, normal growth, improved liver function, prevention of cirrhosis, correction of renal tubular acidosis, and improvement in secondary rickets.

Systemic primary carnitine deficiency (CDSP) is a disorder of the carnitine cycle that results in defective fatty acid oxidation. It encompasses a broad clinical spectrum including the following: Metabolic decompensation in infancy typically presenting between age three months and two years with episodes of hypoketotic hypoglycemia, poor feeding, irritability, lethargy, hepatomegaly, elevated liver transaminases, and hyperammonemia triggered by fasting or common illnesses such as upper respiratory tract infection or gastroenteritis. Childhood myopathy involving heart and skeletal muscle with onset between age two and four years. Pregnancy-related decreased stamina or exacerbation of cardiac arrhythmia. Fatigability in adulthood. Absence of symptoms. The latter two categories often include mothers diagnosed with CDSP after newborn screening has identified low carnitine levels in their infants.

Barth syndrome is characterized in affected males by cardiomyopathy, neutropenia, skeletal myopathy, prepubertal growth delay, and distinctive facial gestalt (most evident in infancy); not all features may be present in a given affected male. Cardiomyopathy, which is almost always present before age five years, is typically dilated cardiomyopathy with or without endocardial fibroelastosis or left ventricular noncompaction; hypertrophic cardiomyopathy can also occur. Heart failure is a significant cause of morbidity and mortality; risk of arrhythmia and sudden death is increased. Neutropenia is most often associated with mouth ulcers, pneumonia, and sepsis. The nonprogressive myopathy predominantly affects the proximal muscles, and results in early motor delays. Prepubertal growth delay is followed by a postpubertal growth spurt with remarkable "catch-up" growth. Heterozygous females who have a normal karyotype are asymptomatic and have normal biochemical studies.

While the majority of individuals with Costello syndrome share characteristic findings affecting multiple organ systems, the phenotypic spectrum is wide, ranging from a milder or attenuated phenotype to a severe phenotype with early lethal complications. Costello syndrome is typically characterized by failure to thrive in infancy as a result of severe postnatal feeding difficulties; short stature; developmental delay or intellectual disability; coarse facial features (full lips, large mouth, full nasal tip); curly or sparse, fine hair; loose, soft skin with deep palmar and plantar creases; papillomata of the face and perianal region; diffuse hypotonia and joint laxity with ulnar deviation of the wrists and fingers; tight Achilles tendons; and cardiac involvement including: cardiac hypertrophy (usually typical hypertrophic cardiomyopathy), congenital heart defect (usually valvar pulmonic stenosis), and arrhythmia (usually supraventricular tachycardia, especially chaotic atrial rhythm/multifocal atrial tachycardia or ectopic atrial tachycardia). Relative or absolute macrocephaly is typical, and postnatal cerebellar overgrowth can result in the development of a Chiari I malformation with associated anomalies including hydrocephalus or syringomyelia. Individuals with Costello syndrome have an approximately 15% lifetime risk for malignant tumors including rhabdomyosarcoma and neuroblastoma in young children and transitional cell carcinoma of the bladder in adolescents and young adults.

Danon disease is a multisystem condition with predominant involvement of the heart, skeletal muscles, and retina, with overlying cognitive dysfunction. Males are typically more severely affected than females. Males usually present with childhood onset concentric hypertrophic cardiomyopathy that is progressive and often requires heart transplantation. Rarely, hypertrophic cardiomyopathy can evolve to resemble dilated cardiomyopathy. Most affected males also have cardiac conduction abnormalities. Skeletal muscle weakness may lead to delayed acquisition of motor milestones. Learning disability and intellectual disability, most often in the mild range, are common. Additionally, affected males can develop retinopathy with subsequent visual impairment. The clinical features in females are broader and more variable. Females are more likely to have dilated cardiomyopathy, with a smaller proportion requiring heart transplantation compared to affected males. Cardiac conduction abnormalities, skeletal muscle weakness, mild cognitive impairment, and pigmentary retinopathy are variably seen in affected females.

Cardiofaciocutaneous (CFC) syndrome is characterized by cardiac abnormalities (pulmonic stenosis and other valve dysplasias, septal defects, hypertrophic cardiomyopathy, rhythm disturbances), distinctive craniofacial appearance, and cutaneous abnormalities (including xerosis, hyperkeratosis, ichthyosis, keratosis pilaris, ulerythema ophryogenes, eczema, pigmented moles, hemangiomas, and palmoplantar hyperkeratosis). The hair is typically sparse, curly, fine or thick, woolly or brittle; eyelashes and eyebrows may be absent or sparse. Nails may be dystrophic or fast growing. Some form of neurologic and/or cognitive delay (ranging from mild to severe) is seen in all affected individuals. Neoplasia, mostly acute lymphoblastic leukemia, has been reported in some individuals.

3-hydroxyacyl-CoA dehydrogenase deficiency is an inherited condition that prevents the body from converting certain fats to energy, particularly during prolonged periods without food (fasting).\n\nInitial signs and symptoms of this disorder typically occur during infancy or early childhood and can include poor appetite, vomiting, diarrhea, and lack of energy (lethargy). Affected individuals can also have muscle weakness (hypotonia), liver problems, low blood glucose (hypoglycemia), and abnormally high levels of insulin (hyperinsulinism). Insulin controls the amount of glucose that moves from the blood into cells for conversion to energy. Individuals with 3-hydroxyacyl-CoA dehydrogenase deficiency are also at risk for complications such as seizures, life-threatening heart and breathing problems, coma, and sudden death. This condition may explain some cases of sudden infant death syndrome (SIDS), which is defined as unexplained death in babies younger than 1 year.\n\nProblems related to 3-hydroxyacyl-CoA dehydrogenase deficiency can be triggered by periods of fasting or by illnesses such as viral infections. This disorder is sometimes mistaken for Reye syndrome, a severe disorder that may develop in children while they appear to be recovering from viral infections such as chicken pox or flu. Most cases of Reye syndrome are associated with the use of aspirin during these viral infections.

Berardinelli-Seip congenital lipodystrophy (BSCL) is usually diagnosed at birth or soon thereafter. Because of the absence of functional adipocytes, lipid is stored in other tissues, including muscle and liver. Affected individuals develop insulin resistance and approximately 25%-35% develop diabetes mellitus between ages 15 and 20 years. Hepatomegaly secondary to hepatic steatosis and skeletal muscle hypertrophy occur in all affected individuals. Hypertrophic cardiomyopathy is reported in 20%-25% of affected individuals and is a significant cause of morbidity from cardiac failure and early mortality.

Cornelia de Lange syndrome (CdLS) encompasses a spectrum of findings from mild to severe. Severe (classic) CdLS is characterized by distinctive facial features, growth restriction (prenatal onset; <5th centile throughout life), hypertrichosis, and upper-limb reduction defects that range from subtle phalangeal abnormalities to oligodactyly (missing digits). Craniofacial features include synophrys, highly arched and/or thick eyebrows, long eyelashes, short nasal bridge with anteverted nares, small widely spaced teeth, and microcephaly. Individuals with a milder phenotype have less severe growth, cognitive, and limb involvement, but often have facial features consistent with CdLS. Across the CdLS spectrum IQ ranges from below 30 to 102 (mean: 53). Many individuals demonstrate autistic and self-destructive tendencies. Other frequent findings include cardiac septal defects, gastrointestinal dysfunction, hearing loss, myopia, and cryptorchidism or hypoplastic genitalia.

Myofibrillar myopathy (MFM) is a noncommittal term that refers to a group of morphologically homogeneous, but genetically heterogeneous chronic neuromuscular disorders. The morphologic changes in skeletal muscle in MFM result from disintegration of the sarcomeric Z disc and the myofibrils, followed by abnormal ectopic accumulation of multiple proteins involved in the structure of the Z disc, including desmin, alpha-B-crystallin (CRYAB; 123590), dystrophin (300377), and myotilin (TTID; 604103). Genetic Heterogeneity of Myofibrillar Myopathy Other forms of MFM include MFM2 (608810), caused by mutation in the CRYAB gene (123590); MFM3 (609200), caused by mutation in the MYOT gene (604103); MFM4 (609452), caused by mutation in the ZASP gene (LDB3; 605906); MFM5 (609524), caused by mutation in the FLNC gene (102565); MFM6 (612954), caused by mutation in the BAG3 gene (603883); MFM7 (617114), caused by mutation in the KY gene (605739); MFM8 (617258), caused by mutation in the PYROXD1 gene (617220); MFM9 (603689), caused by mutation in the TTN gene (188840); MFM10 (619040), caused by mutation in the SVIL UNC45B gene (611220); MFM11 (619178), caused by mutation in the UNC45B gene (611220); and MFM12 (619424), caused by mutation in the MYL2 gene (160781). 'Desmin-related myopathy' is another term referring to MFM in which there are intrasarcoplasmic aggregates of desmin, usually in addition to other sarcomeric proteins. Rigid spine syndrome (602771), caused by mutation in the SEPN1 gene (606210), is another desmin-related myopathy. Goebel (1995) provided a review of desmin-related myopathy.

Mutations in the PRKAG2 gene (602743) give rise to a moderate, essentially heart-specific, nonlysosomal glycogenosis with clinical onset typically in late adolescence or in the third decade of life, ventricular pre-excitation predisposing to supraventricular arrhythmias, mild to severe cardiac hypertrophy, enhanced risk of sudden cardiac death in midlife, and autosomal dominant inheritance with full penetrance (summary by Burwinkel et al., 2005).

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the MYL2 gene.

Mitochondrial phosphate carrier deficiency (MPCD) is an autosomal recessive disorder characterized by onset of cardiorespiratory insufficiency soon after birth. Patients usually require intervention in the neonatal period. The disorder may result in death in infancy, although those that survive have stabilization or amelioration of symptoms with age. Most affected individuals have hypotonia, delayed motor development, and exercise intolerance, but cognitive development is normal. Laboratory studies typically show increased serum lactate, although this may not be present. Muscle biopsy shows abnormal mitochondria and lipid accumulation. There is phenotypic variability likely depending on the location of the mutation (summary by Bhoj et al., 2015).

Alpha-B crystallin-related myofibrillar myopathy is an autosomal dominant muscular disorder characterized by adult onset of progressive muscle weakness affecting both the proximal and distal muscles and associated with respiratory insufficiency, cardiomyopathy, and cataracts. There is phenotypic variability both within and between families (Fardeau et al., 1978; Selcen and Engel, 2003). A homozygous founder mutation in the CRYAB gene has been identified in Canadian aboriginal infants of Cree origin who have a severe fatal infantile hypertonic form of myofibrillar myopathy; see 613869. For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the MYL3 gene.

Oculoectodermal syndrome (OES) is characterized by the association of epibulbar dermoids and aplasia cutis congenita. Affected individuals exhibit congenital scalp lesions which are atrophic, nonscarring, hairless regions that are often multiple and asymmetric in distribution, and may have associated hamartomas. Ectodermal changes include linear hyperpigmentation that may follow the lines of Blaschko and, rarely, epidermal nevus-like lesions. Epibulbar dermoids may be uni- or bilateral. Additional ocular anomalies such as skin tags of the upper eyelid and rarely optic nerve or retinal changes or microphthalmia can be present. Phenotypic expression is highly variable, and various other abnormalities have occasionally been reported, including growth failure, lymphedema, and cardiovascular defects, as well as neurodevelopmental symptoms such as developmental delay, epilepsy, learning difficulties, and behavioral abnormalities. Benign tumor-like lesions such as nonossifying fibromas of the long bones and giant cell granulomas of the jaws have repeatedly been observed and appear to be age-dependent, becoming a common manifestation in individuals aged 5 years or older (summary by Boppudi et al., 2016).

Isolated complex I deficiency is a rare inborn error of metabolism due to mutations in nuclear or mitochondrial genes encoding subunits or assembly factors of the human mitochondrial complex I (NADH: ubiquinone oxidoreductase) and is characterized by a wide range of manifestations including marked and often fatal lactic acidosis, cardiomyopathy, leukoencephalopathy, pure myopathy and hepatopathy with tubulopathy. Among the numerous clinical phenotypes observed are Leigh syndrome, Leber hereditary optic neuropathy and MELAS syndrome (see these terms).

Autosomal dominant myosin storage congenital myopathy-7A (CMYO7A) is a skeletal muscle disorder with wide phenotypic variability. The age at symptom onset can range from early childhood to late adulthood. Affected individuals have proximal muscle weakness affecting the upper and lower limbs and distal muscle weakness of the lower limbs, resulting in gait difficulties and scapular winging (scapuloperoneal myopathy). Additional features may include thin habitus, high-arched palate, foot drop, pes cavus, calf pseudohypertrophy, and decreased reflexes. The severity is also variable: some patients develop respiratory insufficiency, joint contractures, and scoliosis in the first decades, whereas others are clinically unaffected, but show subtle signs of the disorder on examination. Serum creatine kinase may be normal or elevated. The disease is usually slowly progressive and most patients remain ambulatory. Skeletal muscle biopsy can show different abnormalities, including hyaline bodies, type 1 fiber predominance, congenital fiber-type disproportion (CFTD), and nonspecific myopathic changes with myofibrillar disarray. Intrafamilial variability is common (Dye et al., 2006; Pegoraro et al., 2007; review by Tajsharghi and Oldfors, 2013). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Uruguay faciocardiomusculoskeletal syndrome (FCMSU) is an X-linked disorder in which affected males have a distinctive facial appearance, muscular hypertrophy, and cardiac ventricular hypertrophy leading to premature death. Additional features include large, broad, and deformed hands and feet, congenital hip dislocation, and scoliosis (summary by Xue et al., 2016).

A rare glycogen storage disease with fetal or neonatal onset of severe cardiomyopathy with non-lysosomal glycogen accumulation and fatal outcome in infancy. Patients present with massive cardiomegaly, severe cardiac and respiratory complications and failure to thrive. Non-specific facial dysmorphism, bilateral cataracts, macroglossia, hydrocephalus, enlarged kidneys and skeletal muscle involvement have been reported in some cases.

Autosomal recessive myosin storage congenital myopathy-7B (CMYO7B) is a skeletal muscle disorder characterized by the onset of scapuloperoneal muscle weakness in early childhood or young adulthood. Affected individuals have difficulty walking, steppage gait, and scapular winging due to shoulder girdle involvement. The severity and progression of the disorder is highly variable, even within families. Most patients develop respiratory insufficiency, nocturnal hypoventilation, and restrictive lung disease; some develop hypertrophic cardiomyopathy. Additional features include myopathic facies, high-arched palate, scoliosis, and muscle wasting with thin body habitus. Serum creatine kinase may be normal or elevated. Skeletal muscle biopsy shows variable findings, including myosin storage disease, type 1 fiber predominance, centralized nuclei, and multiminicore disease (Onengut et al., 2004; Tajsharghi et al., 2007; Beecroft et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Friedreich ataxia (FRDA) is characterized by slowly progressive ataxia with onset usually before age 25 years (mean age at onset: 10-15 yrs). FRDA is typically associated with dysarthria, muscle weakness, spasticity particularly in the lower limbs, scoliosis, bladder dysfunction, absent lower-limb reflexes, and loss of position and vibration sense. Approximately two thirds of individuals with FRDA have cardiomyopathy, up to 30% have diabetes mellitus, and approximately 25% have an "atypical" presentation with later onset or retained tendon reflexes.

Mitochondrial complex IV deficiency nuclear type 5 (MC4DN5) is an autosomal recessive severe metabolic multisystemic disorder with onset in infancy. Features include delayed psychomotor development, impaired intellectual development with speech delay, mild dysmorphic facial features, hypotonia, ataxia, and seizures. There is increased serum lactate and episodic hypoglycemia. Some patients may have cardiomyopathy, abnormal breathing, or liver abnormalities, reflecting systemic involvement. Brain imaging shows lesions in the brainstem and basal ganglia, consistent with a diagnosis of Leigh syndrome (see 256000). Affected individuals tend to have episodic metabolic and/or neurologic crises in early childhood, which often lead to early death (summary by Debray et al., 2011). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Sengers syndrome is an autosomal recessive mitochondrial disorder characterized by congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy, exercise intolerance, and lactic acidosis. Mental development is normal, but affected individuals may die early from cardiomyopathy (summary by Mayr et al., 2012). Skeletal muscle biopsies of 2 affected individuals showed severe mtDNA depletion (Calvo et al., 2012).

Hypertrophic cardiomyopathy is a heart condition characterized by thickening (hypertrophy) of the heart (cardiac) muscle. When multiple members of a family have the condition, it is known as familial hypertrophic cardiomyopathy. Hypertrophic cardiomyopathy also occurs in people with no family history; these cases are considered nonfamilial hypertrophic cardiomyopathy. \n\nIn familial hypertrophic cardiomyopathy, cardiac thickening usually occurs in the interventricular septum, which is the muscular wall that separates the lower left chamber of the heart (the left ventricle) from the lower right chamber (the right ventricle). In some people, thickening of the interventricular septum impedes the flow of oxygen-rich blood from the heart, which may lead to an abnormal heart sound during a heartbeat (heart murmur) and other signs and symptoms of the condition. Other affected individuals do not have physical obstruction of blood flow, but the pumping of blood is less efficient, which can also lead to symptoms of the condition. Familial hypertrophic cardiomyopathy often begins in adolescence or young adulthood, although it can develop at any time throughout life.\n\nThe symptoms of familial hypertrophic cardiomyopathy are variable, even within the same family. Many affected individuals have no symptoms. Other people with familial hypertrophic cardiomyopathy may experience chest pain; shortness of breath, especially with physical exertion; a sensation of fluttering or pounding in the chest (palpitations); lightheadedness; dizziness; and fainting.\n\nWhile most people with familial hypertrophic cardiomyopathy are symptom-free or have only mild symptoms, this condition can have serious consequences. It can cause abnormal heart rhythms (arrhythmias) that may be life threatening. People with familial hypertrophic cardiomyopathy have an increased risk of sudden death, even if they have no other symptoms of the condition. A small number of affected individuals develop potentially fatal heart failure, which may require heart transplantation.\n\nNonfamilial hypertrophic cardiomyopathy tends to be milder. This form typically begins later in life than familial hypertrophic cardiomyopathy, and affected individuals have a lower risk of serious cardiac events and sudden death than people with the familial form.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the TTN gene.

Nonfamilial hypertrophic cardiomyopathy tends to be milder. This form typically begins later in life than familial hypertrophic cardiomyopathy, and affected individuals have a lower risk of serious cardiac events and sudden death than people with the familial form.\n\nWhile most people with familial hypertrophic cardiomyopathy are symptom-free or have only mild symptoms, this condition can have serious consequences. It can cause abnormal heart rhythms (arrhythmias) that may be life threatening. People with familial hypertrophic cardiomyopathy have an increased risk of sudden death, even if they have no other symptoms of the condition. A small number of affected individuals develop potentially fatal heart failure, which may require heart transplantation.\n\nThe symptoms of familial hypertrophic cardiomyopathy are variable, even within the same family. Many affected individuals have no symptoms. Other people with familial hypertrophic cardiomyopathy may experience chest pain; shortness of breath, especially with physical exertion; a sensation of fluttering or pounding in the chest (palpitations); lightheadedness; dizziness; and fainting.\n\nIn familial hypertrophic cardiomyopathy, cardiac thickening usually occurs in the interventricular septum, which is the muscular wall that separates the lower left chamber of the heart (the left ventricle) from the lower right chamber (the right ventricle). In some people, thickening of the interventricular septum impedes the flow of oxygen-rich blood from the heart, which may lead to an abnormal heart sound during a heartbeat (heart murmur) and other signs and symptoms of the condition. Other affected individuals do not have physical obstruction of blood flow, but the pumping of blood is less efficient, which can also lead to symptoms of the condition. Familial hypertrophic cardiomyopathy often begins in adolescence or young adulthood, although it can develop at any time throughout life.\n\nHypertrophic cardiomyopathy is a heart condition characterized by thickening (hypertrophy) of the heart (cardiac) muscle. When multiple members of a family have the condition, it is known as familial hypertrophic cardiomyopathy. Hypertrophic cardiomyopathy also occurs in people with no family history; these cases are considered nonfamilial hypertrophic cardiomyopathy. 

Familial hypertrophic cardiomyopathy-3 (CMH3) is an autosomal dominant disorder characterized by increased myocardial mass with myocyte and myofibrillar disarray (Thierfelder et al., 1994). For a general phenotypic description and a discussion of genetic heterogeneity of hypertrophic cardiomyopathy, see CMH1 (192600).

Hypertrophic cardiomyopathy is a heart condition characterized by thickening (hypertrophy) of the heart (cardiac) muscle. When multiple members of a family have the condition, it is known as familial hypertrophic cardiomyopathy. Hypertrophic cardiomyopathy also occurs in people with no family history; these cases are considered nonfamilial hypertrophic cardiomyopathy. \n\nIn familial hypertrophic cardiomyopathy, cardiac thickening usually occurs in the interventricular septum, which is the muscular wall that separates the lower left chamber of the heart (the left ventricle) from the lower right chamber (the right ventricle). In some people, thickening of the interventricular septum impedes the flow of oxygen-rich blood from the heart, which may lead to an abnormal heart sound during a heartbeat (heart murmur) and other signs and symptoms of the condition. Other affected individuals do not have physical obstruction of blood flow, but the pumping of blood is less efficient, which can also lead to symptoms of the condition. Familial hypertrophic cardiomyopathy often begins in adolescence or young adulthood, although it can develop at any time throughout life.\n\nThe symptoms of familial hypertrophic cardiomyopathy are variable, even within the same family. Many affected individuals have no symptoms. Other people with familial hypertrophic cardiomyopathy may experience chest pain; shortness of breath, especially with physical exertion; a sensation of fluttering or pounding in the chest (palpitations); lightheadedness; dizziness; and fainting.\n\nWhile most people with familial hypertrophic cardiomyopathy are symptom-free or have only mild symptoms, this condition can have serious consequences. It can cause abnormal heart rhythms (arrhythmias) that may be life threatening. People with familial hypertrophic cardiomyopathy have an increased risk of sudden death, even if they have no other symptoms of the condition. A small number of affected individuals develop potentially fatal heart failure, which may require heart transplantation.\n\nNonfamilial hypertrophic cardiomyopathy tends to be milder. This form typically begins later in life than familial hypertrophic cardiomyopathy, and affected individuals have a lower risk of serious cardiac events and sudden death than people with the familial form.

Noonan syndrome with multiple lentigines (NSML) is a condition in which the cardinal features consist of lentigines, hypertrophic cardiomyopathy, short stature, pectus deformity, and dysmorphic facial features including widely spaced eyes and ptosis. Multiple lentigines present as dispersed flat, black-brown macules, mostly on the face, neck, and upper part of the trunk with sparing of the mucosa. In general, lentigines do not appear until age four to five years but then increase to the thousands by puberty. Some individuals with NSML do not exhibit lentigines. Approximately 85% of affected individuals have heart defects, including hypertrophic cardiomyopathy (typically appearing during infancy and sometimes progressive) and pulmonary valve stenosis. Postnatal growth restriction resulting in short stature occurs in fewer than 50% of affected persons, although most affected individuals have a height that is less than the 25th centile for age. Sensorineural hearing deficits, present in approximately 20% of affected individuals, are poorly characterized. Intellectual disability, typically mild, is observed in approximately 30% of persons with NSML.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

GNPTAB-related disorders comprise the phenotypes mucolipidosis II (ML II) and mucolipidosis IIIa/ß (ML IIIa/ß), and phenotypes intermediate between ML II and ML IIIa/ß. ML II is evident at birth and slowly progressive; death most often occurs in early childhood. Orthopedic abnormalities present at birth may include thoracic deformity, kyphosis, clubfeet, deformed long bones, and/or dislocation of the hip(s). Growth often ceases in the second year of life; contractures develop in all large joints. The skin is thickened, facial features are coarse, and gingiva are hypertrophic. All children have cardiac involvement, most commonly thickening and insufficiency of the mitral valve and, less frequently, the aortic valve. Progressive mucosal thickening narrows the airways, and gradual stiffening of the thoracic cage contributes to respiratory insufficiency, the most common cause of death. ML IIIa/ß becomes evident at about age three years with slow growth rate and short stature; joint stiffness and pain initially in the shoulders, hips, and fingers; gradual mild coarsening of facial features; and normal to mildly impaired cognitive development. Pain from osteoporosis becomes more severe during adolescence. Cardiorespiratory complications (restrictive lung disease, thickening and insufficiency of the mitral and aortic valves, left and/or right ventricular hypertrophy) are common causes of death, typically in early to middle adulthood. Phenotypes intermediate between ML II and ML IIIa/ß are characterized by physical growth in infancy that resembles that of ML II and neuromotor and speech development that resemble that of ML IIIa/ß.

This syndrome is characterized by severe hypotonia, lactic acidemia and congenital hyperammonemia. It has been described in three newborns born to consanguineous parents. Ultrasound examination during the 36th week of pregnancy revealed generalized edema. Hypertrophic cardiomyopathy and tubulopathy developed within the first week of life and the infants died within the first month. The activities of enzymes in the mitochondrial respiratory chain were reduced in the muscles of the patients. Mutations were identified in the MRPS22 gene on chromosome 3q23, encoding a mitochondrial ribosomal protein

Any familial isolated dilated cardiomyopathy in which the cause of the disease is a mutation in the ACTN2 gene.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the CSRP3 gene.

An autosomal dominant subtype of familial hypertrophic cardiomyopathy caused by mutation(s) in the ACTC1 gene, encoding actin, alpha cardiac muscle 1.

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by the clinical triad of: joint contractures that begin in early childhood; slowly progressive muscle weakness and wasting initially in a humero-peroneal distribution that later extends to the scapular and pelvic girdle muscles; and cardiac involvement that may manifest as palpitations, presyncope and syncope, poor exercise tolerance, and congestive heart failure along with variable cardiac rhythm disturbances. Age of onset, severity, and progression of muscle and cardiac involvement demonstrate both inter- and intrafamilial variability. Clinical variability ranges from early onset with severe presentation in childhood to late onset with slow progression in adulthood. In general, joint contractures appear during the first two decades, followed by muscle weakness and wasting. Cardiac involvement usually occurs after the second decade and respiratory function may be impaired in some individuals.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the VCL gene.

An autosomal dominant subtype of familial hypertrophic cardiomyopathy caused by mutation(s) in the MYH6 gene, encoding myosin-6.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the TNNC1 gene.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

A rare, syndromic, hereditary optic neuropathy disorder characterized by early-onset, severe, progressive visual impairment, optic disc pallor and central scotoma, variably associated with dyschromatopsia, auditory neuropathy (e.g. mild progressive sensorineural hearing loss), sensorimotor axonal neuropathy and, occasionally, moderate hypertrophic cardiomyopathy.

Myofibrillar myopathy-6 is an autosomal dominant severe neuromuscular disorder characterized by onset in the first decade of rapidly progressive generalized and proximal muscle weakness, respiratory insufficiency, cardiomyopathy, and skeletal deformities related to muscle weakness. Muscle biopsy shows fiber-type grouping, disruption of the Z lines, and filamentous inclusions, and sural nerve biopsy shows a neuropathy, often with giant axonal neurons. Most patients are severely affected by the second decade and need cardiac transplant, ventilation, and/or a wheelchair (summary by Jaffer et al., 2012). For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy (MFM), see MFM1 (601419).

Growth retardation, developmental delay, and facial dysmorphism (GDFD) is an autosomal recessive multiple congenital anomaly syndrome characterized by severe psychomotor retardation, poor overall growth, and dysmorphic facial features. Additional features may include cardiac malformations and deafness (summary by Daoud et al., 2016).

Infantile-onset autophagic vacuolar myopathy is characterized by increased cardiac and skeletal muscle glycogen with normal acid maltase (GAA; 606800). Skeletal muscle biopsy shows characteristic intracytoplasmic vacuoles that stain for sarcolemmal proteins and complement proteins. Similar pathologic findings are seen in Danon disease (300257), caused by mutation in the LAMP2 gene (309060) on chromosome Xq24, and X-linked myopathy with excessive autophagy (XMEA; 310440), which has been mapped to Xq28.

Myopathy, lactic acidosis, and sideroblastic anemia-2 (MLASA2) is an autosomal recessive disorder of the mitochondrial respiratory chain. The disorder shows marked phenotypic variability: some patients have a severe multisystem disorder from infancy, including cardiomyopathy and respiratory insufficiency resulting in early death, whereas others present in the second or third decade of life with sideroblastic anemia and mild muscle weakness (summary by Riley et al., 2013). For a discussion of genetic heterogeneity of MLASA, see MLASA1 (600462).

Congenital dyserythropoietic anemia type IVa (CDAN4A) is an autosomal dominant red blood cell disorder characterized by ineffective erythropoiesis and hemolysis resulting in anemia. Circulating erythroblasts and erythroblasts in the bone marrow show various morphologic abnormalities. Affected individuals with CDAN4A also have increased levels of fetal hemoglobin (summary by Arnaud et al., 2010). For a discussion of genetic heterogeneity of congenital dyserythropoietic anemia, see CDAN1 (224120).

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the MYOZ2 gene.

An autosomal dominant subtype of familial hypertrophic cardiomyopathy caused by mutation(s) in the JPH2 gene, encoding junctophilin-2.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the PLN gene.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the NEXN gene.

HSD10 mitochondrial disease (HSD10MD) most commonly presents as an X-linked neurodegenerative disorder with highly variable severity and age at onset ranging from the neonatal period to early childhood. The features are usually multisystemic, consistent with mitochondrial dysfunction. Some affected males have a severe infantile form associated with cardiomyopathy that may result in death in early childhood, whereas other rare patients may have juvenile onset or even atypical presentations with normal neurologic development. More severely affected males show developmental regression in infancy or early childhood, often associated with early-onset intractable seizures, progressive choreoathetosis and spastic tetraplegia, optic atrophy or retinal degeneration resulting in visual loss, and mental retardation. Heterozygous females may show non-progressive developmental delay and intellectual disability, but may also be clinically normal. Although the diagnosis can be aided by the observation of increased urinary levels of metabolites of isoleucine breakdown (2-methyl-3 hydroxybutyrate and tiglylglycine), there is not a correlation between these laboratory features and the phenotype. In addition, patients do not develop severe metabolic crises in the neonatal period as observed in other organic acidurias, but may show persistent lactic acidosis, most likely reflecting mitochondrial dysfunction (summary by Rauschenberger et al., 2010; Zschocke, 2012). In a review of this disorder, Zschocke (2012) noted that although it was originally thought to be an inborn error of branched-chain fatty acid and isoleucine metabolism resulting from decreased HSD17B10 dehydrogenase activity (HSD17B10 'deficiency'), subsequent studies have shown that the HSD17B10 gene product has additional functions and also acts as a component of the mitochondrial RNase P holoenzyme, which is involved in mitochondrial tRNA processing and maturation and ultimately mitochondrial protein synthesis. The multisystemic features of HSD10MD most likely result from the adverse effect of HSD17B10 mutations on mitochondrial function, rather than from the effects on the dehydrogenase activity (see PATHOGENESIS).

Mitochondrial encephalo-cardio-myopathy due to <i>TMEM70</i> mutation is characterized by early neonatal onset of hypotonia, hypetrophic cardiomyopathy and apneic spells within hours after birth accompanied by lactic acidosis, hyperammonemia and 3-methylglutaconic aciduria.

Mitochondrial complex V (ATP synthase) deficiency, nuclear type 3 (MC5DN3) is an autosomal recessive disorder with variable manifestations. Affected individuals present soon after birth or in early infancy with hypotonia, respiratory distress, and poor sucking. They have global developmental delay with mildly impaired intellectual disability. Additional features may include dystonia, ataxia, peripheral neuropathy, and seizures. Congenital cataracts, hearing impairment, and mild left cardiac ventricular hypertrophy have been reported in 1 patient each. Laboratory studies show increased lactate; some patients have hyperammonemia, 3-methylglutaconic aciduria, and hyperCKemia (Mayr et al., 2010; Zech et al., 2022). For a general phenotypic description of the nuclear type of mitochondrial complex V deficiency and a discussion of genetic heterogeneity of mitochondrial complex V deficiency, see 604273.

Multiple mitochondrial dysfunctions syndrome-2 (MMDS2) with hyperglycinemia is a severe autosomal recessive disorder characterized by developmental regression in infancy. Affected children have an encephalopathic disease course with seizures, spasticity, loss of head control, and abnormal movement. Additional more variable features include optic atrophy, cardiomyopathy, and leukodystrophy. Laboratory studies show increased serum glycine and lactate. Most patients die in childhood. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including MMDS2, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014). For a general description and a discussion of genetic heterogeneity of multiple mitochondrial dysfunctions syndrome, see MMDS1 (605711).

Hyperglycinemia, lactic acidosis, and seizures (HGCLAS) is a severe autosomal recessive disorder characterized by onset of hypotonia and seizures associated with increased serum glycine and lactate in the first days of life. Affected individuals develop an encephalopathy or severely delayed psychomotor development, which may result in death in childhood. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including HGCLAS, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014).

Autosomal recessive mitochondrial complex III deficiency is a severe multisystem disorder with onset at birth of lactic acidosis, hypotonia, hypoglycemia, failure to thrive, encephalopathy, and delayed psychomotor development. Visceral involvement, including hepatopathy and renal tubulopathy, may also occur. Many patients die in early childhood, but some may show longer survival (de Lonlay et al., 2001; De Meirleir et al., 2003). Genetic Heterogeneity of Mitochondrial Complex III Deficiency Mitochondrial complex III deficiency can be caused by mutation in several different nuclear-encoded genes. See MC3DN2 (615157), caused by mutation in the TTC19 gene (613814) on chromosome 17p12; MC3DN3 (615158), caused by mutation in the UQCRB gene (191330) on chromosome 8q; MC3DN4 (615159), caused by mutation in the UQCRQ gene (612080) on chromosome 5q31; MC3DN5 (615160), caused by mutation in the UQCRC2 gene (191329) on chromosome 16p12; MC3DN6 (615453), caused by mutation in the CYC1 gene (123980) on chromosome 8q24; MC3DN7 (615824), caused by mutation in the UQCC2 gene (614461) on chromosome 6p21; MC3DN8 (615838), caused by mutation in the LYRM7 gene (615831) on chromosome 5q23; MC3DN9 (616111), caused by mutation in the UQCC3 gene (616097) on chromosome 11q12; and MC3DN10 (618775), caused by mutation in the UQCRFS1 gene (191327) on chromosome 19q12. See also MTYCB (516020) for a discussion of a milder phenotype associated with isolated mitochondrial complex III deficiency and mutations in a mitochondrial-encoded gene.

Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen.

Hypertrophic cardiomyopathy (CMH) is characterized by unexplained cardiac hypertrophy: thickening of the myocardial wall in the absence of any other identifiable cause for left ventricular hypertrophy such as systemic hypertension or valvular heart disease. Myocyte hypertrophy, disarray, and fibrosis are the histopathologic hallmarks of this disorder. Clinical features are diverse and include arrhythmias, sudden cardiac death, and heart failure. With an estimated prevalence of 1 in 500, CMH is the most common cardiovascular genetic disease and the most common cause of sudden death in competitive athletes in the United States (summary by Song et al., 2006). For a discussion of genetic heterogeneity of familial hypertrophic cardiomyopathy, see CMH1 (192600).

Mitochondrial complex IV deficiency nuclear type 6 (MC4DN6) is an autosomal recessive multisystem metabolic disorder with a highly variable phenotype. Some patients present in the neonatal period with encephalomyopathic features, whereas others present later in the first year of life with developmental regression. Manifestations include hypotonia, feeding difficulties, and global developmental delay. Many, but not all, patients develop hypertrophic cardiomyopathy, which may result in early death. Additional more variable features may include poor overall growth, microcephaly, seizures, neurodegeneration, spasticity, visual defects, retinopathy, and hepatic steatosis. Brain imaging in some patients shows features consistent with Leigh syndrome (see 256000). Laboratory studies show increased serum lactate and decreased levels and activity of mitochondrial respiratory complex IV (summary by Kennaway et al., 1990 and Oquendo et al., 2004). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Any dilated cardiomyopathy in which the cause of the disease is a mutation in the MYPN gene.

Cardiofaciocutaneous (CFC) syndrome is characterized by cardiac abnormalities (pulmonic stenosis and other valve dysplasias, septal defects, hypertrophic cardiomyopathy, rhythm disturbances), distinctive craniofacial appearance, and cutaneous abnormalities (including xerosis, hyperkeratosis, ichthyosis, keratosis pilaris, ulerythema ophryogenes, eczema, pigmented moles, hemangiomas, and palmoplantar hyperkeratosis). The hair is typically sparse, curly, fine or thick, woolly or brittle; eyelashes and eyebrows may be absent or sparse. Nails may be dystrophic or fast growing. Some form of neurologic and/or cognitive delay (ranging from mild to severe) is seen in all affected individuals. Neoplasia, mostly acute lymphoblastic leukemia, has been reported in some individuals.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

The nephronophthisis (NPH) phenotype is characterized by reduced renal concentrating ability, chronic tubulointerstitial nephritis, cystic renal disease, and progression to end-stage renal disease (ESRD) before age 30 years. Three age-based clinical subtypes are recognized: infantile, juvenile, and adolescent/adult. Infantile NPH can present in utero with oligohydramnios sequence (limb contractures, pulmonary hypoplasia, and facial dysmorphisms) or postnatally with renal manifestations that progress to ESRD before age 3 years. Juvenile NPH, the most prevalent subtype, typically presents with polydipsia and polyuria, growth retardation, chronic iron-resistant anemia, or other findings related to chronic kidney disease (CKD). Hypertension is typically absent due to salt wasting. ESRD develops at a median age of 13 years. Ultrasound findings are increased echogenicity, reduced corticomedullary differentiation, and renal cysts (in 50% of affected individuals). Histologic findings include tubulointerstitial fibrosis, thickened and disrupted tubular basement membrane, sporadic corticomedullary cysts, and normal or reduced kidney size. Adolescent/adult NPH is clinically similar to juvenile NPH, but ESRD develops at a median age of 19 years. Within a subtype, inter- and intrafamilial variability in rate of progression to ESRD is considerable. Approximately 80%-90% of individuals with the NPH phenotype have no extrarenal features (i.e., they have isolated NPH); ~10%-20% have extrarenal manifestations that constitute a recognizable syndrome (e.g., Joubert syndrome, Bardet-Biedl syndrome, Jeune syndrome and related skeletal disorders, Meckel-Gruber syndrome, Senior-Løken syndrome, Leber congenital amaurosis, COACH syndrome, and oculomotor apraxia, Cogan type).

Combined oxidative phosphorylation deficiency-16 (COXPD16) is an autosomal recessive multisystem disorder with hypertrophic cardiomyopathy as a major feature.

RHPD2 is an autosomal recessive multisystemic disorder with severe abnormalities apparent in utero and often resulting in fetal death or death in infancy. The main organs affected include the kidney, liver, and pancreas, although other abnormalities, including cardiac, skeletal, and lung defects, may also be present. Affected individuals often have situs inversus. The disorder results from a defect in ciliogenesis and ciliary function, as well as in cell proliferation and epithelial morphogenesis; thus, the clinical manifestations are highly variable (summary by Grampa et al., 2016). For a discussion of genetic heterogeneity of renal-hepatic-pancreatic dysplasia, see RHPD1 (208540).

Mitochondrial DNA depletion syndrome-12B is an autosomal recessive mitochondrial disorder characterized by childhood onset of slowly progressive hypertrophic cardiomyopathy and generalized skeletal myopathy resulting in exercise intolerance, and, in some patients, muscle weakness and atrophy. Skeletal muscle biopsy shows ragged-red fibers, mtDNA depletion, and accumulation of abnormal mitochondria (summary by Echaniz-Laguna et al., 2012). For a discussion of genetic heterogeneity of mtDNA depletion syndromes, see MTDPS1 (603041).

Combined oxidative phosphorylation deficiency-17 (COXPD17) is an autosomal recessive disorder of mitochondrial dysfunction characterized by onset of severe hypertrophic cardiomyopathy in the first year of life. Other features include hypotonia, poor growth, lactic acidosis, and failure to thrive. The disorder may be fatal in early childhood (summary by Haack et al., 2013).

FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay. Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes. All affected individuals alive at the time they were reported (median age: 3.5 years) demonstrated significant developmental delay. Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia). Survival varies; the median age of reported deaths was two years (range 2 days – 75 months), although surviving individuals as old as 36 years have been reported. To date FBXL4-related mtDNA depletion syndrome has been reported in 50 individuals.

Deficiency of very long-chain acyl-coenzyme A dehydrogenase (VLCAD), which catalyzes the initial step of mitochondrial beta-oxidation of long-chain fatty acids with a chain length of 14 to 20 carbons, is associated with three phenotypes. The severe early-onset cardiac and multiorgan failure form typically presents in the first months of life with hypertrophic or dilated cardiomyopathy, pericardial effusion, and arrhythmias, as well as hypotonia, hepatomegaly, and intermittent hypoglycemia. The hepatic or hypoketotic hypoglycemic form typically presents during early childhood with hypoketotic hypoglycemia and hepatomegaly, but without cardiomyopathy. The later-onset episodic myopathic form presents with intermittent rhabdomyolysis provoked by exercise, muscle cramps and/or pain, and/or exercise intolerance. Hypoglycemia typically is not present at the time of symptoms.

Most characteristically, Aicardi-Goutières syndrome (AGS) manifests as an early-onset encephalopathy that usually, but not always, results in severe intellectual and physical disability. A subgroup of infants with AGS present at birth with abnormal neurologic findings, hepatosplenomegaly, elevated liver enzymes, and thrombocytopenia, a picture highly suggestive of congenital infection. Otherwise, most affected infants present at variable times after the first few weeks of life, frequently after a period of apparently normal development. Typically, they demonstrate the subacute onset of a severe encephalopathy characterized by extreme irritability, intermittent sterile pyrexias, loss of skills, and slowing of head growth. Over time, as many as 40% develop chilblain skin lesions on the fingers, toes, and ears. It is becoming apparent that atypical, sometimes milder, cases of AGS exist, and thus the true extent of the phenotype associated with pathogenic variants in the AGS-related genes is not yet known.

Combined oxidative phosphorylation defect type 20 is a rare mitochondrial oxidative phosphorylation disorder characterized by variable combination of psychomotor delay, hypotonia, muscle weakness, seizures, microcephaly, cardiomyopathy and mild dysmorphic facial features. Variable types of structural brain anomalies have also been reported. Biochemical studies typically show decreased activity of mitochondrial complexes (mainly complex I).

Mitochondrial complex IV deficiency nuclear type 9 (MC4DN9) is an autosomal recessive multisystem metabolic disorder characterized by neonatal hypertrophic cardiomyopathy resulting in death in early infancy. Patient tissues show decreased levels and activity of mitochondrial respiratory complex IV (summary by Huigsloot et al., 2011). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Complex lethal osteochondrodysplasia of the Symoens-Barnes-Gistelinck type is characterized by severe skeletal osteopenia, microcephaly, multiple fractures, and congenital anomalies including ascites, pleural effusion, and intracranial ventriculomegaly (Symoens et al., 2015).

Any mitochondrial DNA depletion syndrome in which the cause of the disease is a mutation in the OPA1 gene.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Mitochondrial complex IV deficiency nuclear type 13 (MC4DN13) is an autosomal recessive metabolic disorder characterized by the onset of hypertrophic cardiomyopathy soon after birth. Affected individuals have hypotonia, weakness, and failure to thrive, resulting in death in infancy. Laboratory studies show increased serum lactate and decreased levels and activity of mitochondrial respiratory complex IV (summary by Baertling et al., 2015). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Mitochondrial short-chain enoyl-CoA hydratase 1 deficiency (ECHS1D) represents a clinical spectrum in which several phenotypes have been described: The most common phenotype presents in the neonatal period with severe encephalopathy and lactic acidosis and later manifests Leigh-like signs and symptoms. Those with presentation in the neonatal period typically have severe hypotonia, encephalopathy, or neonatal seizures within the first few days of life. Signs and symptoms typically progress quickly and the affected individual ultimately succumbs to central apnea or arrhythmia. A second group of affected individuals present in infancy with developmental regression resulting in severe developmental delay. A third group of affected individuals have normal development with isolated paroxysmal dystonia that may be exacerbated by illness or exertion. Across all three groups, T2 hyperintensity in the basal ganglia is very common, and may affect any part of the basal ganglia.

Any hypertrophic cardiomyopathy in which the cause of the disease is a mutation in the TCAP gene.

MPSPS is an autosomal recessive inborn error of metabolism resulting in a multisystem disorder with features of the mucopolysaccharidosis lysosomal storage diseases (see, e.g., 607016). Patients present in infancy or early childhood with respiratory difficulties, cardiac problems, anemia, dysostosis multiplex, renal involvement, coarse facies, and delayed psychomotor development. Most patients die of cardiorespiratory failure in the first years of life (summary by Kondo et al., 2017).

Combined oxidative phosphorylation deficiency-31 is an autosomal recessive multisystem disorder characterized by left ventricular noncompaction (LVNC), global developmental delay, and severe hypotonia. More variable features include seizures, cataract, and abnormal movements. The disorder becomes apparent soon after birth or in early infancy, and patients may die in early childhood. Biochemical studies are consistent with a defect in mitochondrial function (summary by Eldomery et al., 2016). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

MTDPS12A is characterized by severe hypotonia due to mitochondrial dysfunction apparent at birth. Affected infants have respiratory insufficiency requiring mechanical ventilation and have poor or no motor development. Many die in infancy, and those that survive have profound hypotonia with significant muscle weakness and inability to walk independently. Some patients develop hypertrophic cardiomyopathy. Muscle samples show mtDNA depletion and severe combined mitochondrial respiratory chain deficiencies (summary by Thompson et al., 2016). For a discussion of genetic heterogeneity of mtDNA depletion syndromes, see MTDPS1 (603041).

Harel-Yoon syndrome is a syndromic neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, truncal hypotonia, spasticity, and peripheral neuropathy. Other more variable features such as optic atrophy may also occur. Laboratory studies in some patients show evidence of mitochondrial dysfunction (summary by Harel et al., 2016).

Familial cardiomyopathy caused by mutation in the FLNC gene has been described as hypertrophic, restrictive, dilated, or arrhythmogenic right ventricular cardiomyopathy. Affected individuals, especially those with dilated cardiomyopathy, are at risk for arrhythmias and sudden death. Arrhythmias without cardiomyopathy, and left ventricular noncompaction, have also been reported (Ortiz-Genga et al., 2016; Verdonschot et al., 2020).

Noonan syndrome-like disorder with loose anagen hair is characterized by facial features similar to those observed in Noonan syndrome (163950), including hypertelorism, ptosis, downslanting palpebral fissures, low-set posteriorly angulated ears, and overfolded pinnae. In addition, patients display short stature, frequently with growth hormone (GH; see 139250) deficiency; cognitive deficits; relative macrocephaly; small posterior fossa resulting in Chiari I malformation; hypernasal voice; cardiac defects, especially dysplasia of the mitral valve and septal defects; and ectodermal abnormalities, in which the most characteristic feature is the hair anomaly, including easily pluckable, sparse, thin, slow-growing hair (summary by Bertola et al., 2017). Reviews Komatsuzaki et al. (2010) reviewed the clinical manifestations of patients with Noonan syndrome, Costello syndrome (218040), and cardiofaciocutaneous syndrome (CFC; see 115150) compared to patients with mutations in the SHOC2 gene. They noted that although there is phenotypic overlap among the disorders, loose anagen/easily pluckable hair had not been reported in mutation-positive patients with Noonan, CFC, or Costello syndrome, and appeared to be a distinctive feature of SHOC2 mutation-positive patients. Genetic Heterogeneity of Noonan Syndrome-Like Disorder with Loose Anagen Hair NSLH2 (617506) is caused by mutation in the PPP1CB gene (600590) on chromosome 2p23.

Autosomal recessive cutis laxa type IID (ARCL2D) is characterized by generalized skin wrinkling with sparse subcutaneous fat and dysmorphic progeroid facial features. Most patients also exhibit severe hypotonia as well as cardiovascular and neurologic involvement (summary by Van Damme et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive cutis laxa, see ARCL1A (219100).

An inherited condition caused by autosomal dominant mutation(s) in the PPP1CB gene, encoding serine/threonine-protein phosphatase PP1-beta catalytic subunit. The condition is characterized by facial features similar to those seen in Noonan syndrome but may also include short stature, cognitive deficits, relative macrocephaly, small posterior fossa resulting in Chiari I malformation, hypernasal voice, cardiac defects, and ectodermal abnormalities, which typically presents as slow-growing, sparse, and/or unruly hair.

Combined oxidative phosphorylation deficiency-8 (COXPD8) is an autosomal recessive disorder caused by dysfunction of the mitochondrial respiratory chain. The main clinical manifestation is a lethal infantile hypertrophic cardiomyopathy, but there may also be subtle skeletal muscle and brain involvement. Biochemical studies show combined respiratory chain complex deficiencies in complexes I, III, and IV in cardiac muscle, skeletal muscle, and brain. The liver is not affected (summary by Gotz et al., 2011). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Joubert syndrome-32 (JBTS32) is an autosomal recessive developmental disorder characterized by delayed psychomotor development, intellectual disability, dysmorphic facial features, and postaxial polydactyly. Brain imaging shows cerebellar abnormalities consistent with the molar tooth sign (MTS) (summary by De Mori et al., 2017). For discussion of genetic heterogeneity of Joubert syndrome, see JBTS1 (213300).

Noonan syndrome with multiple lentigines (NSML) is a condition in which the cardinal features consist of lentigines, hypertrophic cardiomyopathy, short stature, pectus deformity, and dysmorphic facial features including widely spaced eyes and ptosis. Multiple lentigines present as dispersed flat, black-brown macules, mostly on the face, neck, and upper part of the trunk with sparing of the mucosa. In general, lentigines do not appear until age four to five years but then increase to the thousands by puberty. Some individuals with NSML do not exhibit lentigines. Approximately 85% of affected individuals have heart defects, including hypertrophic cardiomyopathy (typically appearing during infancy and sometimes progressive) and pulmonary valve stenosis. Postnatal growth restriction resulting in short stature occurs in fewer than 50% of affected persons, although most affected individuals have a height that is less than the 25th centile for age. Sensorineural hearing deficits, present in approximately 20% of affected individuals, are poorly characterized. Intellectual disability, typically mild, is observed in approximately 30% of persons with NSML.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

A rare mitochondrial disease due to a defect in mitochondrial protein synthesis characterized by initially normal growth and development followed by the infantile-onset of failure to thrive, psychomotor delay, poor feeding, dyspnea, severe hypertrophic cardiomyopathy and hepatomegaly. Laboratory studies report increased plasma lactate and alanine, abnormal liver enzymes and decreased activity of mitochondrial respiratory chain complexes I, III, IV, and V. Caused by compound heterozygous mutation in the MRPL3 gene on chromosome 3q22.

MC1DN20 is an autosomal recessive multisystem disorder characterized by infantile onset of acute metabolic acidosis, hypertrophic cardiomyopathy, and muscle weakness associated with a deficiency of mitochondrial complex I activity in muscle, liver, and fibroblasts (summary by Haack et al., 2010). For a discussion of genetic heterogeneity of mitochondrial complex I deficiency, see 252010.

CMH27 is a severe, early-onset cardiomyopathy with morphologic features of both dilated and hypertrophic disease, characterized by biventricular involvement and atypical distribution of hypertrophy. Heterozygotes are at increased risk of developing cardiomyopathy (Almomani et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of hypertrophic cardiomyopathy, see CMH1 (192600). An oligogenic form of hypertrophic cardiomyopathy, involving heterozygous mutations in the ALPK3, TTN (188840), and MYL3 (160790) genes has also been reported in 1 family.

Mitochondrial complex I deficiency nuclear type 29 (MC1DN29) is an autosomal recessive metabolic disorder that usually presents in childhood, adolescence, or adulthood with exercise intolerance and easy fatigue with myalgias and muscle weakness. However, a severe multisystem presentation with chronic renal failure and cardiomyopathy in infancy has been reported (Sanchez-Caballero et al., 2016; Alston et al., 2016). For a discussion of genetic heterogeneity of mitochondrial complex I deficiency, see 252010.

Combined oxidative phosphorylation deficiency-10 (COXPD10) is an autosomal recessive disorder resulting in variable defects of mitochondrial oxidative respiration. Affected individuals present in infancy with hypertrophic cardiomyopathy and lactic acidosis. The severity is variable, but can be fatal in the most severe cases (summary by Ghezzi et al., 2012). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Combined oxidative phosphorylation deficiency-37 is an autosomal recessive multisystem disorder apparent at birth or in the first months of life. Affected individuals have hypotonia, failure to thrive, and neurodegeneration with loss of developmental milestones, as well as liver dysfunction. Some patients may have hypertrophic cardiomyopathy, loss of vision and hearing, and/or seizures. Mitochondrial respiratory dysfunction is apparent in liver and skeletal muscle tissue. Most patients die in childhood (summary by Zeharia et al., 2016). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Oculopharyngodistal myopathy-1 (OPDM1) is an autosomal dominant disorder characterized by adult-onset ptosis, external ophthalmoplegia, facial muscle weakness, distal limb muscle weakness and atrophy, and pharyngeal involvement, resulting in dysphagia and dysarthria. Skeletal muscle biopsy shows myopathic changes with rimmed vacuoles. There are variable manifestations of the disorder regarding muscle involvement and severity (summary by Ishiura et al., 2019). Genetic Heterogeneity of Oculopharyngodistal Myopathy See also OPDM2 (618940), caused by trinucleotide repeat expansion in the GIPC1 gene (605072) on chromosome 19p13; OPDM3 (619473), caused by trinucleotide repeat expansion in the NOTCH2NLC gene (618025) on chromosome 1q21; and OPDM4 (619790), caused by trinucleotide repeat expansion in the RILPL1 gene (614092) on chromosome 12q24. Oculopharyngeal muscular dystrophy (OPMD; 164300) is a similar disorder with overlapping features. It is caused by a similar heterozygous trinucleotide repeat expansion in the PABPN1 gene (602279) (summary by Durmus et al., 2011).

Mitochondrial complex V (ATP synthase) deficiency nuclear type 6 (MC5DN6) is an autosomal recessive progressive and degenerative disorder characterized by episodic regression of gross motor skills beginning in early childhood. The episodes are associated with metabolic stress, including fever, illness, and general anesthesia. Patients develop gait difficulties or loss of ambulation, as well as other variable abnormalities, including abnormal movements, hemiplegia, and persistent lethargy. Brain imaging shows degenerative features in the basal ganglia and brainstem consistent with a diagnosis of Leigh syndrome (see 256000) (summary by Barca et al., 2018). For a discussion of genetic heterogeneity of mitochondrial complex V deficiency, see MC5DN1 (604273).

Neonatal lethal pontocerebellar hypoplasia, hypotonia, and respiratory insufficiency syndrome (PHRINL) is an autosomal recessive multisystem disorder with onset in utero and death in the neonatal period. Rare patients may survive a few months. Affected infants show respiratory insufficiency and almost no spontaneous movement at birth, usually requiring mechanical ventilation and admission to the neonatal intensive care unit. Additional features include corneal clouding, seizures, dysmorphic facies, contractures, and progressive pontocerebellar hypoplasia with simplified gyral pattern and white matter abnormalities. Some patients may have cardiac anomalies or cardiac hypertrophy. Laboratory studies show evidence consistent with mitochondrial defects and/or abnormal cholesterol or lipid metabolism. Depending on the type of mutation or deletion, some patients may have a less severe disorder (see GENOTYPE/PHENOTYPE CORRELATIONS) (summary by Desai et al., 2017).

Autosomal dominant chromosome 1p36.33 duplication syndrome is a severe multisystemic disorder characterized by neonatal respiratory insufficiency, hypotonia, and cardiomyopathy, resulting in death in the first weeks of life. Affected infants may also have seizures, contractures, and corneal opacities. Brain imaging shows variable anomalies, such as white matter changes, and laboratory studies suggest that the phenotype results from metabolic defects in mitochondrial and cholesterol homeostasis (summary by Gunning et al., 2020).

Combined oxidative phosphorylation deficiency-40 (COXPD40) is an autosomal recessive mitochondrial disorder with onset in utero or soon after birth. Affected individuals have severe hypertrophic cardiomyopathy, poor growth, and sensorineural hearing loss. Laboratory studies show evidence of mitochondrial dysfunction, such as lactic acidosis. Patient-derived tissues and cells show variably decreased activities of mitochondrial respiratory complexes I, III, IV, and V. The disorder is lethal, with no reported patients surviving past infancy (summary by Friederich et al., 2018). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Combined oxidative phosphorylation deficiency-44 (COXPD44) is an autosomal recessive mitochondrial disorder with multisystemic manifestations. Most affected individuals present in infancy or early childhood with global developmental delay, hypotonia, and abnormal movements. Most patients develop seizures, often associated with status epilepticus, and some patients may have optic atrophy. One patient with hypertrophic cardiomyopathy has been reported. Serum lactate may be increased, although that finding is inconsistent. Detailed biochemical analysis shows variable combined deficiencies of mitochondrial oxidative complexes that appear to be tissue-specific (summary by Wei et al., 2020). For discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Mitochondrial complex IV deficiency nuclear type 2 (MC4DN2) is an autosomal recessive multisystem metabolic disorder characterized by the onset of symptoms at birth or in the first weeks or months of life. Affected individuals have severe hypotonia, often associated with feeding difficulties and respiratory insufficiency necessitating tube feeding and mechanical ventilation. The vast majority of patients develop hypertrophic cardiomyopathy in the first days or weeks of life, which usually leads to death in infancy or early childhood. Patients also show neurologic abnormalities, including developmental delay, nystagmus, fasciculations, dystonia, EEG changes, and brain imaging abnormalities compatible with a diagnosis of Leigh syndrome (see 256000). There may also be evidence of systemic involvement with hepatomegaly and myopathy, although neurogenic muscle atrophy is more common and may resemble spinal muscular atrophy type I (SMA1; 253300). Serum lactate is increased, and laboratory studies show decreased mitochondrial complex IV protein and activity levels in various tissues, including heart and skeletal muscle. Most patients die in infancy of cardiorespiratory failure (summary by Papadopoulou et al., 1999). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Mitochondrial complex IV deficiency nuclear type 1 (MC4DN1) is an autosomal recessive metabolic disorder characterized by rapidly progressive neurodegeneration and encephalopathy with loss of motor and cognitive skills between about 5 and 18 months of age after normal early development. Affected individuals show hypotonia, failure to thrive, loss of the ability to sit or walk, poor communication, and poor eye contact. Other features may include oculomotor abnormalities, including slow saccades, strabismus, ophthalmoplegia, and nystagmus, as well as deafness, apneic episodes, ataxia, tremor, and brisk tendon reflexes. Brain imaging shows bilateral symmetric lesions in the basal ganglia, consistent with a clinical diagnosis of Leigh syndrome (see 256000). Some patients may also have abnormalities in the brainstem and cerebellum. Laboratory studies usually show increased serum and CSF lactate and decreased levels and activity of mitochondrial respiratory complex IV in patient tissues. There is phenotypic variability, but death in childhood, often due to central respiratory failure, is common (summary by Tiranti et al., 1998; Tiranti et al., 1999; Teraoka et al., 1999; Poyau et al., 2000) Genetic Heterogeneity of Mitochondrial Complex IV Deficiency Most isolated COX deficiencies are inherited as autosomal recessive disorders caused by mutations in nuclear-encoded genes; mutations in the mtDNA-encoded COX subunit genes are relatively rare (Shoubridge, 2001; Sacconi et al., 2003). Mitochondrial complex IV deficiency caused by mutation in nuclear-encoded genes, in addition to MC4DN1, include MC4DN2 (604377), caused by mutation in the SCO2 gene (604272); MC4DN3 (619046), caused by mutation in the COX10 gene (602125); MC4DN4 (619048), caused by mutation in the SCO1 gene (603664); MC4DN5 (220111), caused by mutation in the LRPPRC gene (607544); MC4DN6 (615119), caused by mutation in the COX15 gene (603646); MC4DN7 (619051), caused by mutation in the COX6B1 gene (124089); MC4DN8 (619052), caused by mutation in the TACO1 gene (612958); MC4DN9 (616500), caused by mutation in the COA5 gene (613920); MC4DN10 (619053), caused by mutation in the COX14 gene (614478); MC4DN11 (619054), caused by mutation in the COX20 gene (614698); MC4DN12 (619055), caused by mutation in the PET100 gene (614770); MC4DN13 (616501), caused by mutation in the COA6 gene (614772); MC4DN14 (619058), caused by mutation in the COA3 gene (614775); MC4DN15 (619059), caused by mutation in the COX8A gene (123870); MC4DN16 (619060), caused by mutation in the COX4I1 gene (123864); MC4DN17 (619061), caused by mutation in the APOPT1 gene (616003); MC4DN18 (619062), caused by mutation in the COX6A2 gene (602009); MC4DN19 (619063), caused by mutation in the PET117 gene (614771); MC4DN20 (619064), caused by mutation in the COX5A gene (603773); MC4DN21 (619065), caused by mutation in the COXFA4 gene (603883); MC4DN22 (619355), caused by mutation in the COX16 gene (618064); and MC4DN23 (620275), caused by mutation in the COX11 gene (603648). Mitochondrial complex IV deficiency has been associated with mutations in several mitochondrial genes, including MTCO1 (516030), MTCO2 (516040), MTCO3 (516050), MTTS1 (590080), MTTL1 (590050), and MTTN (590010).

Mitochondrial complex IV deficiency nuclear type 7 (MC4DN7) is an autosomal recessive metabolic encephalomyopathic disorder with highly variable manifestations. Only a few patients have been reported. Some patients have normal early development then show rapid neurodegeneration with progressive muscle weakness, gait disturbances, and cognitive decline in mid to late childhood. Other features may include seizures and visual impairment. Brain imaging shows progressive leukodystrophy with cystic lesions. In contrast, at least 1 patient has been reported who presented in the neonatal period with metabolic acidosis, hydrocephalus, hypotonia, and cortical blindness. This patient developed hypertrophic cardiomyopathy resulting in early death. All patients had increased serum lactate and decreased levels and activity of mitochondrial respiratory complex IV (summary by Massa et al., 2008 and Abdulhag et al., 2015). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Mitochondrial complex IV deficiency nuclear type 10 (MC4DN10) is an autosomal recessive multisystem metabolic disorder characterized by the onset of severe symptoms soon after birth. Affected infants have respiratory and neurologic distress, metabolic lactic acidosis, and dysmorphic features, including microphthalmia. Death occurs in early infancy. Postmortem examination has demonstrated systemic involvement with hepatomegaly, hypertrophic cardiomyopathy, renal hypoplasia, and adrenal hyperplasia. There is also abnormal brain myelination and cavitating brain lesions. Patient tissues show decreased levels and activity of mitochondrial respiratory complex IV (summary by Weraarpachai et al., 2012). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Neurodevelopmental disorder with cardiomyopathy, spasticity, and brain abnormalities (NEDCASB) is an autosomal recessive multisystemic disorder characterized by global neurodevelopmental delay, severely impaired intellectual development, poor overall growth, and spasticity of the lower limbs resulting in gait difficulties. Most affected individuals also develop progressive hypertrophic cardiomyopathy in childhood or have cardiac developmental anomalies. Additional more variable features include dysmorphic facies and axonal sensory peripheral neuropathy. Brain imaging tends to show thin corpus callosum and polymicrogyria (summary by Garcia-Cazorla et al., 2020).

Neurodevelopmental disorder with hypotonia, facial dysmorphism, and brain abnormalities (NEDHFBA) is an autosomal recessive neurologic syndrome characterized by global developmental delay with severely impaired intellectual development, hypotonia and muscle weakness, often resulting in the inability to walk or sit, and characteristic coarse facial features. Additional features include feeding difficulties, respiratory distress, scoliosis, poor visual function, and rotary nystagmus. Brain imaging shows variable abnormalities, including enlarged ventricles, decreased white matter volume, white matter changes, thin corpus callosum, and cerebellar hypoplasia (summary by Loddo et al., 2020).

Combined oxidative phosphorylation deficiency-52 (COXPD52) is an autosomal recessive infantile mitochondrial complex II/III deficiency characterized by lactic acidemia, multiorgan system failure, and abnormal mitochondria. Intrafamilial variability has been reported (Farhan et al., 2014; Hershkovitz et al., 2021). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Infantile-onset myofibrillar myopathy-12 with cardiomyopathy (MFM12) is a severe autosomal recessive disorder affecting both skeletal and cardiac muscle tissue that is apparent in the first weeks of life. Affected infants show tremor or clonus at birth, followed by onset of rapidly progressive generalized muscle weakness and dilated cardiomyopathy and cardiac failure, usually resulting in death by 6 months of age. Skeletal and cardiac muscle tissues show hypotrophy of type I muscle fibers and evidence of myofibrillar disorganization (summary by Weterman et al., 2013). For a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Individuals with TANGO2-related metabolic encephalopathy and arrhythmias can present in acute metabolic crisis (hypoglycemia, elevated lactate, mild hyperammonemia) or with developmental delay, regression, and/or seizures. The acute presentation varies from profound muscle weakness, ataxia, and/or disorientation to a comatose state. Individuals can present with intermittent acute episodes of rhabdomyolysis. The first episode of myoglobinuria has been known to occur as early as age five months. Acute renal tubular damage due to myoglobinuria can result in acute kidney injury and renal failure. During acute illness, transient electrocardiogram changes can be seen; the most common is QT prolongation. Life-threatening recurrent ventricular tachycardia or torsade de pointes occurs primarily during times of acute illness. Individuals who do not present in metabolic crises may present with gait incoordination, progressively unsteady gait, difficulty with speech, or clumsiness. Intellectual disability of variable severity is observed in almost all individuals. Seizures are observed outside the periods of crises in more than 75% of individuals. Hypothyroidism has been reported in more than one third of individuals.

Peripheral neuropathy with variable spasticity, exercise intolerance, and developmental delay (PNSED) is an autosomal recessive multisystemic disorder with highly variable manifestations, even within the same family. Some patients present in infancy with hypotonia and global developmental delay with poor or absent motor skill acquisition and poor growth, whereas others present as young adults with exercise intolerance and muscle weakness. All patients have signs of a peripheral neuropathy, usually demyelinating, with distal muscle weakness and atrophy and distal sensory impairment; many become wheelchair-bound. Additional features include spasticity, extensor plantar responses, contractures, cerebellar signs, seizures, short stature, and rare involvement of other organ systems, including the heart, pancreas, and kidney. Biochemical analysis may show deficiencies in mitochondrial respiratory complex enzyme activities in patient tissue, although this is not always apparent. Lactate is frequently increased, suggesting mitochondrial dysfunction (Powell et al., 2015; Argente-Escrig et al., 2022). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Combined oxidative phosphorylation deficiency-23 (COXPD23) is an autosomal recessive disorder characterized by early childhood onset of hypertrophic cardiomyopathy and/or neurologic symptoms, including hypotonia and delayed psychomotor development. Laboratory investigations are consistent with a defect in mitochondrial function resulting in lactic acidosis, impaired activities of respiratory complexes I and IV, and defective translation of mitochondrial proteins. Brain imaging shows abnormal lesions in the basal ganglia, thalamus, and brainstem. The severity of the disorder is variable, ranging from death in early infancy to survival into the second decade (summary by Kopajtich et al., 2014). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen.

The phenotypes of dihydrolipoamide dehydrogenase (DLD) deficiency are an overlapping continuum that ranges from early-onset neurologic manifestations to adult-onset liver involvement and, rarely, a myopathic presentation. Early-onset DLD deficiency typically manifests in infancy as hypotonia with lactic acidosis. Affected infants frequently do not survive their initial metabolic decompensation, or die within the first few years of life during a recurrent metabolic decompensation. Children who live beyond the first two to three years frequently exhibit growth deficiencies and residual neurologic deficits (intellectual disability, spasticity, ataxia, and seizures). In contrast, isolated liver involvement can present as early as the neonatal period and as late as the third decade. Evidence of liver injury/failure is preceded by nausea and emesis and frequently associated with encephalopathy and/or coagulopathy. Acute metabolic episodes are frequently associated with lactate elevations, hyperammonemia, and hepatomegaly. With resolution of the acute episodes affected individuals frequently return to baseline with no residual neurologic deficit or intellectual disability. Liver failure can result in death, even in those with late-onset disease. Individuals with the myopathic presentation may experience muscle cramps, weakness, and an elevated creatine kinase.

Immunodeficiency-93 and hypertrophic cardiomyopathy (IMD93) is an autosomal recessive disorder characterized by onset of recurrent viral and bacterial infections, particularly with encapsulated bacteria, and hypertrophic cardiomyopathy in the first months or years of life. Immunologic workup typically shows decreased circulating B cells and hypo- or agammaglobulinemia, sometimes with neutropenia or T-cell lymphocytosis, although laboratory findings may be variable among patients. Ig replacement therapy is beneficial. Cardiac involvement can also include atrial septal defect, valvular insufficiency, and pre-excitation syndrome. Rare myopathic or neurologic involvement has been reported, but these features are not consistently part of the disorder and may be related to other genetic defects (summary by Niehues et al., 2020 and Saettini et al., 2021).

Noonan syndrome-14 (NS14) is a recessive developmental disorder within the RASopathy clinical spectrum. Patients exhibit developmental delay, impaired intellectual development, and short stature, as well as distinctive dysmorphic features including bitemporal narrowing, hypertelorism, low-set posteriorly rotated ears, prominent nasal bridge, low posterior hairline with a short webbed neck, and pectus excavatum (Motta et al., 2021). For a general phenotypic description and discussion of genetic heterogeneity of Noonan syndrome, see NS1 (163950).

Hepatorenocardiac degenerative fibrosis (HRCDF) is a primarily fibrotic disease affecting the liver, kidney, and heart, with considerable variability in disease onset and expression. Affected individuals develop degenerative hepatic fibrosis in childhood or early adulthood, with variable later onset of fibrocystic kidney disease and hypertrophic cardiomyopathy (Devane et al., 2022).

Mitochondrial complex II deficiency is an autosomal recessive multisystemic metabolic disorder with a highly variable phenotype. Some patients have multisystem involvement of the brain, heart, and muscle with onset in infancy, whereas others have only isolated cardiac or muscle involvement. Measurement of complex II activity in muscle is the most reliable means of diagnosis; however, there is no clear correlation between residual complex II activity and severity or clinical outcome. In some cases, treatment with riboflavin may have clinical benefit (summary by Jain-Ghai et al., 2013). Complex II, also known as succinate dehydrogenase, is part of the mitochondrial respiratory chain. Genetic Heterogeneity of Mitochondrial Complex II Deficiency See MC2DN2 (619166), caused by mutation in the SDHAF1 gene (612848) on chromosome 19q13; MC2DN3 (619167), caused by mutation in the SDHD gene (602690) on chromosome 11q23; and MC2DN4 (619224), caused by mutation in the SDHB gene (185470) on chromosome 1p36. Fullerton et al. (2020) reviewed the genetic basis of isolated mitochondrial complex II deficiency.

Mitochondrial complex I deficiency nuclear type 39 (MC1DN39) is an autosomal recessive nuclear disorder of mitochondrial respiratory chain complex I characterized by intrauterine growth retardation and anemia and postpartum hypertrophic cardiomyopathy, lactic acidosis, encephalopathy, and a severe complex I defect with a fatal outcome (Correia et al., 2021)

Combined oxidative phosphorylation deficiency-57 (COXPD57) is an autosomal recessive multisystem mitochondrial disease with varying degrees of severity from premature death in infancy to permanent disability in young adulthood (Lee et al., 2022). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Neurodevelopmental disorder with absent speech and movement and behavioral abnormalities (NEDSMB) is an autosomal recessive disorder characterized by global developmental delay and severely impaired intellectual development with aggressive behavior. Mild dysmorphic features and hypodontia are also present (Faqeih et al., 2023).

Congenital myopathy-21 with early respiratory failure (CMYO21) is an autosomal recessive muscle disorder associated with diaphragmatic weakness and spinal rigidity. The age at symptom onset is highly variable, ranging from infancy to adulthood; the severity of the respiratory impairment, which can lead to death in the most severe cases, is also variable. Additional features, including developmental delay and hypertrophic cardiomyopathy, have been observed in one patient each (Weihl et al., 2023; Al-Kasbi et al., 2022). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Combined oxidative phosphorylation deficiency-59 (COXPD59) may present as a lethal infantile form of Leigh syndrome (see 256000) or as a milder disorder with hypertrophic cardiomyopathy, lactic acidosis, attention deficit-hyperactivity disorder (ADHD) and survival into adulthood (summary by Amarasekera et al., 2023). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).