COBALAMIN METABOLISM IN METHIONINE-DEPENDENT HUMAN TUMOR AND LEUKEMIA-CELL LINES

Objective: To identify the defect in cobalamin metabolism in the human melanoma cell line MeWoLCl, and to determine how frequent this defect is in other methionine-dependent tumour cell lines. Design: Biochemic al and somatic cell genetics study. Interventions: Aspects of cobalami n metabolism were measured in a panel of 14 human tumour cell lines th at were unable to proliferate normally in medium in which methionine h ad been replaced by its metabolic precursor homocysteine (methionine-d ependent cell lines). Results: The human melanoma cell line MeWoLCl wa s unique among these cell lines, in that it was characterized by decre ased uptake of cobalamin, decreased synthesis of coenzyme derivatives, and decreased functional activity of the cobalamin-dependent enzymes methionine synthase and methylmalonylCoA mutase, This phenotype was id entical to that observed in fibroblasts from patients with the cblC an d cblD inborn errors of cobalamin metabolism. The defect in cobalamin metabolism in MeWoLCl was complemented in somatic cell complementation analysis by cblA, cblB, cblD, cblE and cblG fibroblasts, but not by c blC fibroblasts, strongly suggesting that the defect in this cell line affects the cblC locus. Similar changes in cellular cobalamin metabol ism were not seen in any other methionine-dependent cell line in the p anel, suggesting that there may be multiple causes of methionine depen dence, and that inactivation of the cblC locus may not be a common cau se of this phenotype in transformed cells. Conclusions: The defect und erlying methionine dependence in MeWoLCl appears to involve the locus that is affected in patients with the cblC inborn error of metabolism. This defect does not seem to be common among other methionine-depende nt cell lines.