Intrinsic factor deficiency (IFD), or congenital pernicious anemia, is a rare disorder characterized by the lack of gastric intrinsic factor in the presence of normal acid secretion and mucosal cytology and the absence of GIF antibodies that are found in the acquired form of pernicious anemia (170900). See also pernicious anemia due to defect in the receptor for vitamin B12/intrinsic factor (261100).

Celiac disease is a systemic autoimmune disease that can be associated with gastrointestinal findings (diarrhea, malabsorption, abdominal pain and distension, bloating, vomiting, and weight loss) and/or highly variable non-gastrointestinal findings (dermatitis herpetiformis, chronic fatigue, joint pain/inflammation, iron deficiency anemia, migraines, depression, attention-deficit disorder, epilepsy, osteoporosis/osteopenia, infertility and/or recurrent fetal loss, vitamin deficiencies, short stature, failure to thrive, delayed puberty, dental enamel defects, and autoimmune disorders). Classic celiac disease, characterized by mild to severe gastrointestinal symptoms, is less common than non-classic celiac disease, characterized by absence of gastrointestinal symptoms.

Folate-responsive megaloblastic anemia (MEGAF) is an autosomal recessive metabolic disorder characterized by megaloblastic anemia resulting from decreased folate transport into erythrocytes. Although serum levels of folate are normal, there is folate deficiency in tissues, including erythrocytes and possibly nerve cells. Serum homocysteine levels are increased and vitamin B12 levels may be decreased. Treatment with oral folate corrects the anemia and normalizes homocysteine (summary by Svaton et al., 2020)

Diarrhea-6 is a relatively mild, early-onset chronic diarrhea that may be associated with increased susceptibility to inflammatory bowel disease, small bowel obstruction, and esophagitis (Fiskerstrand et al., 2012). For a discussion of genetic heterogeneity of diarrhea, see DIAR1 (214700).

3-Methylglutaconic aciduria type I (MGCA1) is a rare autosomal recessive disorder of leucine catabolism. The metabolic landmark is urinary excretion of 3-methylglutaconic acid (3-MGA) and its derivatives 3-methylglutaric acid (3-MG) and 3-hydroxyisovaleric acid (3-HIVA). Two main presentations have been described: one with onset in childhood associated with the nonspecific finding of psychomotor retardation, and the other with onset in adulthood of a progressive neurodegenerative disorder characterized by ataxia, spasticity, and sometimes dementia; these patients develop white matter lesions in the brain. However, some asymptomatic pediatric patients have been identified by newborn screening and show no developmental abnormalities when reexamined later in childhood (summary by Wortmann et al., 2010). Genetic Heterogeneity and Classification of Methylglutaconic Aciduria Methylglutaconic aciduria is a clinically and genetically heterogeneous disorder. Type II MGCA (MGCA2), also known as Barth syndrome (BTHS; 302060), is caused by mutation in the tafazzin gene (TAZ; 300394) on chromosome Xq28. It is characterized by mitochondrial cardiomyopathy, short stature, skeletal myopathy, and recurrent infections; cognitive development is normal. Type III MGCA (MGCA3; 258501), caused by mutation in the OPA3 gene (606580) on chromosome 19q13, involves optic atrophy, movement disorder, and spastic paraplegia. In types II and III, the elevations of 3-methylglutaconate and 3-methylglutarate in urine are modest. Type IV MGCA (MGCA4; 250951) represents an unclassified group of patients who have severe psychomotor retardation and cerebellar dysgenesis. Type V MGCA (MGCA5; 610198), caused by mutation in the DNAJC19 gene (608977) on chromosome 3q26, is characterized by early-onset dilated cardiomyopathy with conduction defects, nonprogressive cerebellar ataxia, testicular dysgenesis, and growth failure in addition to 3-methylglutaconic aciduria (Chitayat et al., 1992; Davey et al., 2006). Type VI MGCA (MGCA6; 614739), caused by mutation in the SERAC1 gene (614725) on chromosome 6q25, includes deafness, encephalopathy, and a Leigh-like syndrome. Type VII MGCA (MGCA7B, 616271 and MGCA7A, 619835), caused by mutation in the CLPB gene (616254) on chromosome 11q13, includes cataracts, neurologic involvement, and neutropenia. Type VIII MGCA (MGCA8; 617248) is caused by mutation in the HTRA2 gene (606441) on chromosome 2p13. Type IX MGCA (MGCA9; 617698) is caused by mutation in the TIMM50 gene (607381) on chromosome 19q13. Eriguchi et al. (2006) noted that type I MGCA is very rare, with only 13 patients reported in the literature as of 2003. Wortmann et al. (2013) proposed a pathomechanism-based classification for 'inborn errors of metabolism with 3-methylglutaconic aciduria as discriminative feature.'

Imerslund-Grasbeck syndrome-2 (IGS2) is an autosomal recessive disorder characterized by onset of megaloblastic anemia associated with decreased serum vitamin B12 (cobalamin, Cbl) in infancy or early childhood. Low molecular weight (LMW) proteinuria is frequently present, but usually occurs later and is usually mild or subclinical. Patients often present with vague symptoms, including failure to thrive, loss of appetite, fatigue, lethargy, and/or recurrent infections. Treatment with vitamin B12 results in sustained clinical improvement of the anemia. The proteinuria is nonprogressive, and affected individuals do not have deterioration of kidney function; correct diagnosis is important to prevent unnecessary treatment. The disorder results from a combination of vitamin B12 deficiency due to selective malabsorption of the vitamin, and impaired reabsorption of LMW proteins in the proximal renal tubule. These defects are caused by disruption of the AMN/CUBN (602997) complex that forms the 'cubam' receptor responsible for intestinal uptake of B12/GIF (CBLIF; 609342). In the kidney, AMN/CUBN interacts with the endocytic receptor megalin (LRP2; 600073), which is important for the reabsorption of plasma proteins (summary by Grasbeck, 2006, De Filippo et al., 2013, and Storm et al., 2013). For a discussion of genetic heterogeneity of Imerslund-Grasbeck syndrome, see 261100.

Neurodevelopmental disorder with seizures, hypotonia, and brain imaging abnormalities (NEDSHBA) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay, severe to profound intellectual impairment, early-onset refractory seizures, hypotonia, failure to thrive, and progressive microcephaly. Brain imaging shows cerebral atrophy, thin corpus callosum, and myelination defects. Death in childhood may occur (summary by Marafi et al., 2020).