Defective lysosomal release of vitamin B12 (cb1F): a hereditary cobalamin metabolic disorder associated with sudden death.

Here we report on a girl who presented with failure to thrive, developmental delay, minor facial anomalies, stomatitis, skin rashes, macrocytosis, mild homocystinemia(uria), and methylmalonic acidemia(uria). Fibroblast studies showed abnormal intracellular cobalamin (vitamin B12) metabolism. Reduced incorporation of 14C from [14C] propionate and [14C] methyltetrahydrofolate into TCA-precipitable macromolecules reflected decreased synthesis of adenosylcobalamin and methylcobalamin respectively. The diagnosis of cb1F mutation was established by demonstrating the accumulation of unmetabolized free cyanocobalamin in fibroblasts and by lack of genetic complementation with fibroblasts from the only other known cb1F patient. The defect is in the lysosomal release of endocytosed cobalamin. Administration of hydroxocobalamin resulted in clinical and biochemical improvement but sudden death occurred at age 5 months. The absence of brain pathological changes suggests that early treatment may prevent the neurological complications in cobalamin cofactor deficiency.