Diarrhea-2 with microvillus atrophy, with or without cholestasis (DIAR2) is characterized by onset of intractable life-threatening watery diarrhea during infancy. Two forms are recognized: early-onset microvillus inclusion disease (MVID) with diarrhea beginning in the neonatal period, and late-onset, with first symptoms appearing after 3 or 4 months of life. Definite diagnosis is made by transmission electron microscopy demonstrating shortening or absence of apical microvilli with pathognomonic microvillus inclusions in mature enterocytes and peripheral accumulation of periodic acid-Schiff (PAS)-positive granules or vesicles in immature enterocytes (Muller et al., 2008). The natural course of MVID is often fatal, but partial or total weaning from parenteral nutrition has been described. For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700).

IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) syndrome is characterized by systemic autoimmunity, typically beginning in the first year of life. Presentation is most commonly the clinical triad of watery diarrhea, endocrinopathy (most commonly insulin-dependent diabetes mellitus), and eczematous dermatitis. Most children have other autoimmune phenomena including cytopenias, autoimmune hepatitis, or nephropathy; lymphadenopathy, splenomegaly, alopecia, arthritis, and lung disease related to immune dysregulation have all been observed. Fetal presentation of IPEX includes hydrops, echogenic bowel, skin desquamation, IUGR, and fetal akinesia. Without aggressive immunosuppression or bone marrow transplantation, the majority of affected males die within the first one to two years of life from metabolic derangements, severe malabsorption, or sepsis; a few with a milder phenotype have survived into the second or third decade of life.

Mitochondrial DNA (mtDNA) deletion syndromes predominantly comprise three overlapping phenotypes that are usually simplex (i.e., a single occurrence in a family), but rarely may be observed in different members of the same family or may evolve from one clinical syndrome to another in a given individual over time. The three classic phenotypes caused by mtDNA deletions are Kearns-Sayre syndrome (KSS), Pearson syndrome, and progressive external ophthalmoplegia (PEO). KSS is a progressive multisystem disorder defined by onset before age 20 years, pigmentary retinopathy, and PEO; additional features include cerebellar ataxia, impaired intellect (intellectual disability, dementia, or both), sensorineural hearing loss, ptosis, oropharyngeal and esophageal dysfunction, exercise intolerance, muscle weakness, cardiac conduction block, and endocrinopathy. Pearson syndrome is characterized by sideroblastic anemia and exocrine pancreas dysfunction and may be fatal in infancy without appropriate hematologic management. PEO is characterized by ptosis, impaired eye movements due to paralysis of the extraocular muscles (ophthalmoplegia), oropharyngeal weakness, and variably severe proximal limb weakness with exercise intolerance. Rarely, a mtDNA deletion can manifest as Leigh syndrome.

PMM2-CDG, the most common of a group of disorders of abnormal glycosylation of N-linked oligosaccharides, is divided into three clinical stages: infantile multisystem, late-infantile and childhood ataxia–intellectual disability, and adult stable disability. The clinical manifestations and course are highly variable, ranging from infants who die in the first year of life to mildly affected adults. Clinical findings tend to be similar in sibs. In the infantile multisystem presentation, infants show axial hypotonia, hyporeflexia, esotropia, and developmental delay. Feeding problems, vomiting, faltering growth, and developmental delay are frequently seen. Subcutaneous fat may be excessive over the buttocks and suprapubic region. Two distinct clinical courses are observed: (1) a nonfatal neurologic course with faltering growth, strabismus, developmental delay, cerebellar hypoplasia, and hepatopathy in infancy followed by neuropathy and retinitis pigmentosa in the first or second decade; and (2) a more severe neurologic-multivisceral course with approximately 20% mortality in the first year of life. The late-infantile and childhood ataxia–intellectual disability stage, which begins between ages three and ten years, is characterized by hypotonia, ataxia, severely delayed language and motor development, inability to walk, and IQ of 40 to 70; other findings include seizures, stroke-like episodes or transient unilateral loss of function, coagulopathy, retinitis pigmentosa, joint contractures, and skeletal deformities. In the adult stable disability stage, intellectual ability is stable; peripheral neuropathy is variable, progressive retinitis pigmentosa and myopia are seen, thoracic and spinal deformities with osteoporosis worsen, and premature aging is observed; females may lack secondary sexual development and males may exhibit decreased testicular volume. Hypogonadotropic hypogonadism and coagulopathy may occur. The risk for deep venous thrombosis is increased.

Congenital disorders of glycosylation (CDGs) are a genetically heterogeneous group of autosomal recessive disorders caused by enzymatic defects in the synthesis and processing of asparagine (N)-linked glycans or oligosaccharides on glycoproteins. Type I CDGs comprise defects in the assembly of the dolichol lipid-linked oligosaccharide (LLO) chain and its transfer to the nascent protein. These disorders can be identified by a characteristic abnormal isoelectric focusing profile of plasma transferrin (Leroy, 2006). CDG1D is a type I CDG that generally presents with severe neurologic involvement associated with dysmorphism and visual impairment. Liver involvement is sometimes present (summary by Marques-da-Silva et al., 2017). For a discussion of the classification of CDGs, see CDG1A (212065).

Proprotein convertase-1/3 deficiency is an autosomal recessive disorder characterized by neonatal severe generalized malabsorptive diarrhea and failure to thrive. As the disease progresses, additional endocrine abnormalities develop, including diabetes insipidus, growth hormone deficiency, primary hypogonadism, adrenal insufficiency, and hypothyroidism (summary by Wilschanski et al., 2014).

Immunodeficiency-41 is an autosomal recessive complex disorder of immune dysregulation. Affected individuals present in infancy with recurrent viral, fungal, and bacterial infections, lymphadenopathy, and variable autoimmune features, such as autoimmune enteropathy and eczematous skin lesions. Immunologic studies show a defect in T-cell regulation (summary by Goudy et al., 2013).

Congenital disorders of glycosylation (CDGs) are a genetically heterogeneous group of autosomal recessive disorders caused by enzymatic defects in the synthesis and processing of asparagine (N)-linked glycans or oligosaccharides on glycoproteins. Type I CDGs comprise defects in the assembly of the dolichol lipid-linked oligosaccharide (LLO) chain and its transfer to the nascent protein. These disorders can be identified by a characteristic abnormal isoelectric focusing profile of plasma transferrin (Leroy, 2006). For a discussion of the classification of CDGs, see CDG1A (212065). CDG Ib is clinically distinct from most other CDGs by the lack of significant central nervous system involvement. The predominant symptoms are chronic diarrhea with failure to thrive and protein-losing enteropathy with coagulopathy. Some patients develop hepatic fibrosis. CDG Ib is also different from other CDGs in that it can be treated effectively with oral mannose supplementation, but can be fatal if untreated (Marquardt and Denecke, 2003). Thus, CDG Ib should be considered in the differential diagnosis of patients with unexplained hypoglycemia, chronic diarrhea, liver disease, or coagulopathy in order to allow early diagnosis and effective therapy (Vuillaumier-Barrot et al., 2002) Freeze and Aebi (1999) reviewed CDG Ib and CDG Ic (603147). Marques-da-Silva et al. (2017) systematically reviewed the literature concerning liver involvement in CDG.

Congenital tufting enteropathy (CTE) is a rare inherited intractable diarrhea of infancy characterized by villous atrophy and absence of inflammation, with intestinal epithelial cell dysplasia manifesting as focal epithelial tufts in the duodenum and jejunum. CTE presents in the first few months of life with chronic watery diarrhea and failure to thrive, and most affected individuals require parenteral nutrition for normal growth and development (summary by Sivagnanam et al., 2008). Semiquantitative assessment of the epithelial surface in CTE patients revealed that 80 to 90% contained tufts, compared to only 16% in patients with celiac disease and less than 10% in normal jejunum (Reifen et al., 1994). For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700).

Congenital disorders of glycosylation (CDGs) that represent defects of dolichol-linked oligosaccharide assembly are classified as CDG type I. For a general description and a discussion of the classification of CDGs, see CDG1A (212065).

A rare autosomal recessive primary immunodeficiency characterized by absence of HLA class II molecules on the surface of immune cells, leading to severely impaired cellular and humoral immune response to foreign antigens, severe CD4+ T-cell lymphopenia, and hypogammaglobulinemia. The disease clinically manifests with early onset of severe and recurrent infections mainly of the respiratory and gastrointestinal tract, protracted diarrhea with failure to thrive, and autoimmune disease, and is frequently fatal in childhood.

IMD31C is a disorder of immunologic dysregulation with highly variable manifestations resulting from autosomal dominant gain-of-function mutations in STAT1 (600555). Most patients present in infancy or early childhood with chronic mucocutaneous candidiasis (CMC). Other highly variable features include recurrent bacterial, viral, fungal, and mycoplasmal infections, disseminated dimorphic fungal infections, enteropathy with villous atrophy, and autoimmune disorders, such as hypothyroidism or diabetes mellitus. A subset of patients show apparently nonimmunologic features, including osteopenia, delayed puberty, and intracranial aneurysms. Laboratory studies show increased activation of gamma-interferon (IFNG; 147570)-mediated inflammation (summary by Uzel et al., 2013 and Sampaio et al., 2013).

Any neonatal inflammatory skin and bowel disease in which the cause of the disease is a mutation in the ADAM17 gene.

Trichohepatoenteric syndrome (THES), generally considered to be a neonatal enteropathy, is characterized by intractable diarrhea (seen in almost all affected children), woolly hair (seen in all), intrauterine growth restriction, facial dysmorphism, and short stature. Additional findings include poorly characterized immunodeficiency, recurrent infections, skin abnormalities, and liver disease. Mild intellectual disability (ID) is seen in about 50% of affected individuals. Less common findings include congenital heart defects and platelet anomalies. To date 52 affected individuals have been reported.

Common variable immunodeficiency-8 with autoimmunity is an autosomal recessive disorder of immune dysregulation. Affected individuals have early childhood onset of recurrent infections, particularly respiratory infections, and also develop variable autoimmune disorders, including idiopathic thrombocytopenic purpura, autoimmune hemolytic anemia, and inflammatory bowel disease. The presentation and phenotype are highly variable, even within families (summary by Lopez-Herrera et al., 2012 and Alangari et al., 2012). Immunologic findings are also variable and may include decreased B cells, hypogammaglobulinemia, and deficiency of CD4+ T regulatory (Treg) cells (Charbonnier et al., 2015). For a general description and a discussion of genetic heterogeneity of common variable immunodeficiency, see CVID1 (607594).

Diarrhea-7 (DIAR7) is a protein-losing enteropathy characterized by early-onset nonbloody watery diarrhea and unresponsiveness to soy-based or elemental formulas. Patients experience failure to thrive, hypogammaglobulinemia with recurrent infections, and require albumin infusions and parenteral nutrition. Hypertriglyceridemia and digital clubbing have been observed (Stephen et al., 2016). The malabsorption can result in severe deficiency of vitamin D and other nutrients (Gupta et al., 2020). For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700).

Autoinflammation with infantile enterocolitis is an autosomal dominant disorder characterized by onset of recurrent flares of autoinflammation in early infancy. Affected individuals tend to have poor overall growth and gastrointestinal symptoms in infancy associated with laboratory evidence of activated inflammation. This initial presentation is followed by recurrent febrile episodes with splenomegaly and sometimes hematologic disturbances, arthralgias, or myalgias. The disorder results from overactivation of an arm of the immune response system (Romberg et al., 2014; Canna et al., 2014).

Trichohepatoenteric syndrome (THES), generally considered to be a neonatal enteropathy, is characterized by intractable diarrhea (seen in almost all affected children), woolly hair (seen in all), intrauterine growth restriction, facial dysmorphism, and short stature. Additional findings include poorly characterized immunodeficiency, recurrent infections, skin abnormalities, and liver disease. Mild intellectual disability (ID) is seen in about 50% of affected individuals. Less common findings include congenital heart defects and platelet anomalies. To date 52 affected individuals have been reported.

Diarrhea-9 (DIAR9) is a form of neonatal-onset chronic diarrhea characterized by an osmotic diarrhea that is not substrate specific, abnormal crypt and villus architecture, and significant fat malabsorption (O'Connell et al., 2018). For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700).

Diarrhea-11 (DIAR11) is characterized by onset of intractable malabsorptive diarrhea within the first few weeks of life (Oz-Levi et al., 2019). For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700).

Osteootohepatoenteric syndrome (OOHE) is characterized by a variable combination of bone fragility, hearing loss, cholestasis, and congenital diarrhea. Some patients also display mild developmental delay and intellectual disability (Esteve et al., 2018).

Immunodeficiency-82 with systemic inflammation (IMD82) is a complex autosomal dominant immunologic disorder characterized by recurrent infections with various organisms, as well as noninfectious inflammation manifest as lymphocytic organ infiltration with gastritis, colitis, and lung, liver, CNS, or skin disease. One of the more common features is inflammation of the stomach and bowel. Most patients develop symptoms in infancy or early childhood; the severity is variable. There may be accompanying fever, elevated white blood cell count, decreased B cells, hypogammaglobulinemia, increased C-reactive protein (CRP; 123260), and a generalized hyperinflammatory state. Immunologic workup shows variable B- and T-cell abnormalities such as skewed subgroups. Patients have a propensity for the development of lymphoma, usually in adulthood. At the molecular level, the disorder results from a gain-of-function mutation that leads to constitutive and enhanced activation of the intracellular inflammatory signaling pathway. Treatment with SYK inhibitors rescued human cell abnormalities and resulted in clinical improvement in mice (Wang et al., 2021).

Microvillus inclusion disease (DIAR12) is a congenital enteropathy characterized by neonatal-onset intractable secretory diarrhea, resulting in severe dehydration and metabolic acidosis. Patients may tolerate limited enteral feeding, but are dependent on total parenteral nutrition (TPN) and require eventual small bowel and/or liver transplantation. Pathologic hallmarks include variable loss of brush-border microvilli, microvillus inclusions, and accumulation of subapical vesicles in villus enterocytes (summary by Wiegerinck et al., 2014). Another form of microvillus inclusion disease, MVID1 (DIAR2; 251850), is caused by mutation in the MYO5B gene (606540). For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700). Mutations in the STX3 gene that affect only isoform A (STX3A) cause DIAR12, whereas mutations in STX3 affecting both STX3A and isoform B (STX3B), which predominates in retinal tissue, cause a syndrome involving severe early-onset retinal dystrophy and MVID (RDMVID; 619446).

Immunodeficiency-85 and autoimmunity (IMD85) is an autosomal dominant immunologic disorder characterized by onset of atopic eczema and recurrent respiratory infections in the first decade of life. Affected individuals also develop autoimmune enteropathy with vomiting, diarrhea, and poor overall growth. More variable features may include autoimmune oligoarthritis, interstitial pneumonitis, and EBV viremia. Laboratory studies show hypogammaglobulinemia and abnormal T-cell function, consistent with a combined immunodeficiency (Keskitalo et al., 2019).

Immunodeficiency-87 and autoimmunity (IMD87) is an autosomal recessive immunologic disorder with wide phenotypic variation and severity. Affected individuals usually present in infancy or early childhood with increased susceptibility to infections, often Epstein-Barr virus (EBV), as well as with lymphadenopathy or autoimmune manifestations, predominantly hemolytic anemia. Laboratory studies may show low or normal lymphocyte numbers, often with skewed T-cell subset ratios. The disorder results primarily from defects in T-cell function, which causes both immunodeficiency and overall immune dysregulation (summary by Serwas et al., 2019 and Fournier et al., 2021).

Netherton syndrome (NETH) is a rare and severe autosomal recessive skin disorder characterized by congenital erythroderma, a specific hair-shaft abnormality, and atopic manifestations with high IgE levels. Generalized scaly erythroderma is apparent at or soon after birth and usually persists. Scalp hair is sparse and brittle with a characteristic 'bamboo' shape under light microscopic examination due to invagination of the distal part of the hair shaft to its proximal part. Atopic manifestations include eczema-like rashes, atopic dermatitis, pruritus, hay fever, angioedema, urticaria, high levels of IgE in the serum, and hypereosinophilia. Life-threatening complications are frequent during the neonatal period, including hypernatremic dehydration, hypothermia, extreme weight loss, bronchopneumonia, and sepsis. During childhood, failure to thrive is common as a result of malnutrition, metabolic disorders, chronic erythroderma, persistent cutaneous infections, or enteropathy (summary by Bitoun et al., 2002).