Molecular basis for methionine synthase reductase deficiency in patients belonging to the cbIE complementation group of disorders in folate/cobalaminmetabolism

Methionine synthase reductase (MSR) deficiency is an autosomal recessive di sorder of folate/cobalamin metabolism leading to hyperhomocysteinemia, hypo methioninemia and megaloblastic anemia. Deficiency in MSR activity occurs a s the result of a defect in the MSR enzyme, which is required for the reduc tive activation of methionine synthase (MS), MS itself is responsible for t he folate/cobalamin-dependent conversion of homocysteine to methionine, We have recently cloned the cDNA corresponding to the MSR protein, a novel mem ber of the ferredoxin-NADP(+) reductase (FNR) family of electron transferas es, We have used RT-PCR, heteroduplex, single-strand conformation polymorph ism (SSCP) and DNA sequence analyses to reveal 11 mutations in eight patien ts from seven families belonging to the cblE complementation group of patie nts of cobalamin metabolism that is defective in the MSR protein. The mutat ions include splicing defects leading to large insertions or deletions, as well as a number of smaller deletions and point mutations. Apart from an in tronic substitution found in two unrelated patients, the mutations appear s ingular among individuals. Of the eleven, three are nonsense mutations, all owing for the identification of two patients for whom little if any MSR pro tein should be produced. The remaining eight involve point mutations or in- frame disruptions of the coding sequence and are distributed throughout the coding region, including proposed FMN, FAD and NADPH binding sites, These data demonstrate a unique requirement for MSR in the reductive activation o f MS.