Megaloblastic anemia caused by vitamin B-12 deficiency due to impaired absorption. The impaired absorption of vitamin B-12 is secondary to atrophic gastritis and loss of gastric parietal cells.

Congenital pernicious anemia (PA), or intrinsic factor deficiency, 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).

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.'