Cobalamin deficiency and the pathogenesis of nervous system disease

Neuropathy commonly complicates cobalamin (Cb1) deficiency in humans, monkeys, fruit bats, and pigs. The neuropathy is characterized by demyelination of the posterolateral columns of the spinal cord (subacute combined degeneration). The lesion was thought to arise primarily from impairment of the adenosylcobalamin-dependent methylmalonyl CoA mutase reaction, leading to the formation of abnormal odd-chain and branched-chain fatty acids and their incorporation into myelin with resultant demyelination. Data from recently developed animal models of the Cb1 neuropathy induced by exposure to nitrous oxide do not substantiate this hypothesis, but rather identify impairment of the methylcobalamin-dependent methionine synthetase reaction as the more important basic defect. The key evidence for this hypothesis is the ability of methionine to delay the onset of Cb1 neuropathy in experimental Cb1 deficiency. In the Cb1-deficient pig, adenosylhomocysteine accumulates in neural tissue, presumably owing to the inability to recycle homocysteine via the defective methionine synthetase reaction. Accumulation of adenosylhomocysteine results in a fall in the adenosylmethionine:adenosylhomocysteine methylation ratio, and this change is believed to cause defective methylation and demyelination in the nervous system. However, in the Cb1 neuropathy in the fruit bat, adenosylhomocysteine does not accumulate in the nervous system, the methylation ratio does not change, and no defect can be demonstrated in the methylation of myelin lipid or basic protein. Although a central role for methionine in the pathogenesis of the Cb1 neuropathy has been established, defective methylation attendant upon impairment of the methionine synthetase reaction may not be the universal defect underlying the Cb1 neuropathy. This would suggest that the methionine effect could be mediated via its role in formate metabolism or polyamine synthesis, or by some as yet unidentified pathway.