Homocystinuria and Megaloblastic Anemia Responsive to Vitamin B12Therapy

Abstract

We describe an inborn error of vitamin B₁₂ metabolism in an infant who had severe developmental delay, megaloblastic anemia, and homocystinuria. There was no evidence of methylmalonic aciduria or deficiency of folate or vitamin B₁₂. Treatment with hydroxocobalamin, but not with cyanocobalamin and folic acid, resulted in rapid clinical and biochemical improvement. Cultured fibroblasts showed an absolute growth requirement for methionine, defective incorporation of radioactivity from [¹⁴C]5-methyltetrahydrofolate into protein, and normal incorporation pf radioactivity from [¹⁴C]propionate, thus assigning the intracellular defect to methionine synthesis. The proportion of intracellular methylcobalamin in the fibroblasts was decreased, but that of 5′-deoxyadenosylcobalamin was normal. Methionine synthetase activity in cell extracts was normal, as was cobalamin incorporation into cultured cells. This defect differs from those described previously in being limited to methylcobalamin accumulation and defective use of 5-methyltetrahydrofolate by intact cells with normal activity of methylmalonyl CoA mutase. (N Engl J Med 1984; 310:686–90.)