Women who take folic acid periconceptionally reduce their risk of having a child with a neural tube defect (NTD) by >50%. A variant form of methylenetetrahydrofolate reductase (MTHFR) (677C-->T) is a known risk factor for NTDs, but the prevalence of the risk genotype explains only a small portion of the protective effect of folic acid. This has prompted the search for additional NTD-associated variants in folate-metabolism enzymes. We have analyzed five potential single-nucleotide polymorphisms (SNPs) in the cytoplasmic, nicotinamide adenine dinucleotide phosphate-dependent, trifunctional enzyme methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase/formyltetrahydrofolate synthetase (MTHFD1) for an association with NTDs in the Irish population. One SNP, R653Q, in this gene appears to be associated with NTD risk. We observed an excess of the MTHFD1 "Q" allele in the mothers of children with NTD, compared with control individuals. This excess was driven by the overrepresentation of QQ homozygotes in the mothers of children with NTD compared with control individuals (odds ratio 1.52 [95% confidence interval 1.16-1.99], P=.003). We conclude that genetic variation in the MTHFD1 gene is associated with an increase in the genetically determined risk that a woman will bear a child with NTD and that the gene may be associated with decreased embryo survival.

Alterations in homocysteine, methionine, folate, and/or B 12 homeostasis have been associated with neural tube defects, cardiovascular disease, and cancer. Methionine synthase, one of only two mammalian enzymes known to require vitamin B 12 as a cofactor, lies at the intersection of these metabolic pathways. This enzyme catalyzes the transfer of a methyl group from 5-methyl-tetrahydrofolate to homocysteine, generating tetrahydrofolate and methionine. Human patients with methionine synthase deficiency exhibit homocysteinemia, homocysteinuria, and hypomethioninemia. They suffer from megaloblastic anemia with or without some degree of neural dysfunction and mental retardation. To better study the pathophysiology of methionine synthase deficiency, we utilized gene-targeting technology to inactivate the methionine synthase gene in mice. On average, heterozygous knockout mice from an outbred background have slightly elevated plasma homocysteine and methionine compared to wild-type mice but seem to be otherwise indistinguishable. Homozygous knockout embryos survive through implantation but die soon thereafter. Nutritional supplementation during pregnancy was unable to rescue embryos that were completely deficient in methionine synthase. Whether any human patients with methionine synthase deficiency have a complete absence of enzyme activity is unclear. These results demonstrate the importance of this enzyme for early development in mice and suggest either that methionine synthasedeficient patients have residual methionine synthase activity or that humans have a compensatory mechanism that is absent in mice.Methionine synthase (MS; EC 2.1.1.13), one of two B 12 -dependent mammalian enzymes, catalyzes the remethylation of homocysteine to methionine and the concurrent demethylation of 5-methyltetrahydrofolate (5-Me-THF) to tetrahydrofolate (THF). Methylcobalamin, a derivative of vitamin B 12 , is the cofactor for this reaction. MSs are highly conserved, large (about 140-kDa) monomeric Zn metalloproteins and consist of three domains: a catalytic domain that contains the binding sites for 5-Me-THF and homocysteine; a B 12 domain, where the methylcobalamin cofactor is tightly bound; and an activation domain. The cob(I)alamin form of the cofactor is methylated by 5-Me-THF, generating enzyme-bound methylcob (III)alamin and THF. Then methylcob(III)alamin transfers its methyl group to homocysteine to produce methionine and returns to the cob(I)alamin form. Occasionally, the highly reactive cob(I)alamin cofactor is oxidized to the nonfunctional cob(II)alamin form during catalysis. The enzyme is reactivated by an S-adenosylmethionine (AdoMet) and NADPH-dependent reductive methylation of enzyme-bound cob(II)alamin to methylcob(III)alamin. The enzyme(s) involved in this reductive methylation has not been well characterized, and two pathways have been proposed. The first pathway involves a single protein, MS reductase, a soluable P 450 -reductase-like protein that contains binding sites for NADPH, flavin adenine dinucleotide, and fla...

The thermolabile variant (677TT) of methylenetetrahydrofolate reductase (MTHFR) is a known risk factor for neural tube defects (NTDs). The relationship between a second MTHFR polymorphism (1298A fi C) and NTD risk has been inconsistent between studies. We genotyped 276 complete NTD triads (mother, father and child affected with an NTD) and 256 controls for MTHFR 1298A fi C. Our findings do not support a role for the 1298A fi C polymorphism in NTDs (OR 0.85 (95% CI 0.49-1.47), p= 0.55), nor do we observe a combined effect with the 677C fi T polymorphism.

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