Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is a progressive renal disorder characterized by excessive urinary Ca(2+) and Mg(2+) excretion. There is progressive loss of kidney function, and in about 50% of cases, the need for renal replacement therapy arises as early as the second decade of life (summary by Muller et al., 2006). Amelogenesis imperfecta may also be present in some patients (Bardet et al., 2016). A similar disorder with renal magnesium wasting, renal failure, and nephrocalcinosis (HOMG5; 248190) is caused by mutations in another tight-junction gene, CLDN19 (610036), and is distinguished by the association of severe ocular involvement. For a discussion of phenotypic and genetic heterogeneity of familial hypomagnesemia, see HOMG1 (602014).

Gitelman syndrome is an autosomal recessive renal tubular salt-wasting disorder characterized by hypokalemic metabolic alkalosis with hypomagnesemia and hypocalciuria. It is the most common renal tubular disorder among Caucasians (prevalence of 1 in 40,000). Most patients have onset of symptoms as adults, but some can present in childhood. Clinical features include transient periods of muscle weakness and tetany, abdominal pains, and chondrocalcinosis (summary by Glaudemans et al., 2012). Gitelman syndrome is sometimes referred to as a mild variant of classic Bartter syndrome (607364). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364.

Cronkhite-Canada syndrome is characterized by gastrointestinal hamartomatous polyposis, alopecia, onychodystrophy, skin hyperpigmentation, and diarrhea. It is associated with high morbidity (summary by Sweetser et al., 2012).

Autosomal dominant hypocalcemia-1 is associated with low or normal serum parathyroid hormone concentrations (PTH). Approximately 50% of patients have mild or asymptomatic hypocalcemia; about 50% have paresthesias, carpopedal spasm, and seizures; about 10% have hypercalciuria with nephrocalcinosis or kidney stones; and more than 35% have ectopic and basal ganglia calcifications (summary by Nesbit et al., 2013). Thakker (2001) noted that patients with gain-of-function mutations in the CASR gene, resulting in generally asymptomatic hypocalcemia with hypercalciuria, have low-normal serum PTH concentrations and have often been diagnosed with hypoparathyroidism because of the insensitivity of earlier PTH assays. Because treatment with vitamin D to correct the hypocalcemia in these patients causes hypercalciuria, nephrocalcinosis, and renal impairment, these patients need to be distinguished from those with other forms of hypoparathyroidism (see 146200). Thakker (2001) suggested the designation 'autosomal dominant hypocalcemic hypercalciuria' for this CASR-related disorder. Genetic Heterogeneity of Autosomal Dominant Hypocalcemia Autosomal dominant hypocalcemia-2 (HYPOC2; 615361) is caused by mutation in the GNA11 gene (139313) on chromosome 19p13.

High urine magnesium in the presence of hypomagnesemia.

A rare, primary bone dysplasia characterized by prenatal and postnatal growth retardation, short stature, cortical thickening and medullary stenosis of the long bones, absent diploic space in the skull bones, hypocalcemia due to the hypoparathyroidism, small hands and feet, delayed mental and motor development, intellectual disability, dental anomalies, and dysmorphic features, including prominent forehead, small deep-set eyes, beaked nose, and micrognathia.

Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal forms of Bartter syndrome typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome (see BARTS3, 607364) present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364.

Familial hypomagnesemia with secondary hypocalcemia is a rare autosomal recessive disorder characterized by very low serum magnesium levels. Hypocalcemia is a secondary consequence of parathyroid failure and parathyroid hormone resistance as a result of severe magnesium deficiency. The disease typically manifests during the first months of life with generalized convulsions or signs of increased neuromuscular excitability, such as muscle spasms or tetany. Untreated, the disease may be fatal or lead to severe neurologic damage. Treatment includes immediate administration of magnesium, usually intravenously, followed by life-long high-dose oral magnesium (review by Knoers, 2009). Genetic Heterogeneity of Hypomagnesemia A form of hypomagnesemia due to kidney defects and high urinary magnesium excretion associated with hypocalciuria (HOMG2; 154020) is caused by mutation in the FXYD2 gene (601814). Renal hypomagnesemia-3 (HOMG3; 248250), associated with hypercalciuria and nephrocalcinosis, is caused by mutation in the CLDN16 gene (603959). Renal hypomagnesemia-4 (HOMG4; 611718), which is normocalciuric, is caused by mutation in the EGF gene (131530). Renal hypomagnesemia-5 (HOMG5; 248190), associated with hypercalciuria, nephrocalcinosis, and severe ocular involvement, is caused by mutation in the CLDN19 gene (610036). Renal hypomagnesemia-6 (HOMG6; 613882) is caused by mutation in the CNNM2 gene (607803). Renal hypomagnesemia-7 with or without dilated cardiomyopathy (HOMG7; 620152) is caused by mutation in the RRAGD gene (608268). Patients with Gitelman syndrome (263800) and Bartter syndrome (see 241200) also show hypomagnesemia, and steatorrhea and severe chronic diarrhea states, such as Crohn disease (see 226600) and Whipple disease, that can result in severe hypomagnesemia.

Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal forms of Bartter syndrome typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome (see BARTS3, 607364) present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). For a discussion of genetic heterogeneity of Bartter syndrome, see 607364.

Primary hypomagnesemia comprises a rare heterogeneous group of disorders characterized by renal or intestinal magnesium wasting that results in symptoms of magnesium depletion such as tetany and seizures. Renal hypomagnesemia-4 (HOMG4) is characterized by low serum magnesium levels, decreased urinary tubular magnesium reabsorption, seizures with onset in early infancy, and moderately impaired intellectual development (summary by Geven et al., 1987; Groenestege et al., 2007). For a discussion of genetic heterogeneity of hypomagnesemia, see 602014.

Syndrome with characteristics of seizures, sensorineural deafness, ataxia, intellectual deficit, and electrolyte imbalance. It has been described in five patients from four families. The disease is caused by homozygous or compound heterozygous mutations in the KCNJ10 gene, encoding a potassium channel expressed in the brain, spinal cord, inner ear and kidneys. Transmission is autosomal recessive.

Cystinosis comprises three allelic phenotypes: Nephropathic cystinosis in untreated children is characterized by renal Fanconi syndrome, poor growth, hypophosphatemic/calcipenic rickets, impaired glomerular function resulting in complete glomerular failure, and accumulation of cystine in almost all cells, leading to cellular dysfunction with tissue and organ impairment. The typical untreated child has short stature, rickets, and photophobia. Failure to thrive is generally noticed after approximately age six months; signs of renal tubular Fanconi syndrome (polyuria, polydipsia, dehydration, and acidosis) appear as early as age six months; corneal crystals can be present before age one year and are always present after age 16 months. Prior to the use of renal transplantation and cystine-depleting therapy, the life span in nephropathic cystinosis was no longer than ten years. With these interventions, affected individuals can survive at least into the mid-forties or fifties with satisfactory quality of life. Intermediate cystinosis is characterized by all the typical manifestations of nephropathic cystinosis, but onset is at a later age. Renal glomerular failure occurs in all untreated affected individuals, usually between ages 15 and 25 years. The non-nephropathic (ocular) form of cystinosis is characterized clinically only by photophobia resulting from corneal cystine crystal accumulation.

HUPRA syndrome is a severe autosomal recessive multisystem disorder characterized by onset in infancy of progressive renal failure leading to electrolyte imbalances, metabolic alkalosis, pulmonary hypertension, hypotonia, and delayed development. Affected individuals are born prematurely (summary by Belostotsky et al., 2011).

HOMGSMR1 is characterized by onset of seizures associated with low serum magnesium in the first year of life. Affected individuals show variable degrees of delayed psychomotor development (summary by Arjona et al., 2014). Genetic Heterogeneity of Hypomagnesemia, Seizures, and Mental Retardation HOMGSMR2 (618314) is caused by mutation in the ATP1A1 gene (182310) on chromosome 1p13.

HOMG5 is an autosomal recessive disorder characterized by severe renal magnesium wasting, progressive renal failure, nephrocalcinosis, and severe visual impairment (Konrad et al., 2006). Amelogenesis imperfecta may also be present in some patients (Yamaguti et al., 2017). For a discussion of genetic heterogeneity of renal hypomagnesemia, see 602014.

Dopamine beta-hydroxylase (DBH) deficiency is characterized by lack of sympathetic noradrenergic function but normal parasympathetic and sympathetic cholinergic function. Affected individuals exhibit profound deficits in autonomic regulation of cardiovascular function that predispose to orthostatic hypotension. Although DBH deficiency appears to be present from birth, the diagnosis is not generally recognized until late childhood. The combination of ptosis of the eyelids in infants and children, together with hypotension, is suggestive of the disease. In the perinatal period, DBH deficiency has been complicated by vomiting, dehydration, hypotension, hypothermia, and hypoglycemia requiring repeated hospitalization; children have reduced exercise capacity. By early adulthood, individuals have profound orthostatic hypotension, greatly reduced exercise tolerance, ptosis of the eyelids, and nasal stuffiness. Presyncopal symptoms include dizziness, blurred vision, dyspnea, nuchal discomfort, and chest pain; symptoms may worsen in hot environments or after heavy meals or alcohol ingestion. Life expectancy is unknown, but some affected individuals have lived beyond age 60 years.

Diarrhea-10 (DIAR10) is a protein-losing enteropathy characterized by intractable secretory diarrhea and massive protein loss due to leaky fenestrated capillaries. Features include early-onset anasarca, severe hypoalbuminemia, hypogammaglobulinemia, and hypertriglyceridemia, as well as electrolyte abnormalities. Some patients exhibit facial dysmorphism and cardiac and renal anomalies. Intrafamilial variability has been observed, and the disease can be severe, with death occurring in infancy in some patients (Broekaert et al., 2018; Kurolap et al., 2018). For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700).

HOMGSMR2 is characterized by generalized seizures in infancy, severe hypomagnesemia, and renal magnesium wasting. Seizures persist despite magnesium supplementation, and are associated with significantly impaired intellectual development (Schlingmann et al., 2018). For a discussion of genetic heterogeneity of HOMGSMR, see 616418.

Neurodevelopmental disorder with hypotonia and dysmorphic facies (NEDHYDF) is characterized by global developmental delay and hypotonia apparent from birth. Affected individuals have variably impaired intellectual development, often with speech delay and delayed walking. Seizures are generally not observed, although some patients may have single seizures or late-onset epilepsy. Most patients have prominent dysmorphic facial features. Additional features may include congenital cardiac defects (without arrhythmia), nonspecific renal anomalies, joint contractures or joint hyperextensibility, dry skin, and cryptorchidism. There is significant phenotypic variability in both the neurologic and extraneurologic manifestations (summary by Tan et al., 2022).

Combined oxidative phosphorylation deficiency-55 (COXPD55) is characterized by global developmental delay, hypotonia, short stature, and impaired intellectual development with speech disabilities in childhood. Indolent progressive external ophthalmoplegia phenotype has been described in 1 patient (summary by Olahova et al., 2021).

Neurodevelopmental disorder with language delay and seizures (NEDLDS) is an autosomal recessive disorder characterized by global developmental delay with mild to severely impaired intellectual development and speech delay with poor or absent language. Affected individuals develop early-onset seizures that are usually well-controlled with medication. Additional features may include axial hypotonia, peripheral hypertonia, hypothyroidism, and nonspecific dysmorphic features or brain imaging abnormalities (Lu et al., 2022).