One-carbon metabolism is under the influence of folate, vitamin B12 and genetic polymorphisms of methylenetetrahydrofolate reductase (MTHFR 677 C→T and 1298 A→C), of methionine synthase (MTR 2756 C→G), methionine synthase reductase (MTRR 66 A→G) and transcobalamin (TCN 776 C→G). The pathogenesis of neural tube defect (NTD) may be related to this metabolism. The influence of the MTHFR 677 C→T polymorphism reported in The Netherlands and Ireland can be questioned in southern Italy, France and Great Britain. MTRR, combined with a low level of vitamin B12, increases the risk of NTD and of having a child with NTD in Canada, while TCN 776 GG and MTRR 66 GG mutated genotypes associated with the MTHFR 677 CC wild-type are predictors of NTD cases in Sicily. Down syndrome (DS) is due to a failure of normal chromosomal segregation during meiosis, possibly related to one-carbon metabolism. MTHFR 677 C→T and MTRR 66 A→G polymorphisms are associated with a greater risk of having a child with DS in North America, Ireland and The Netherlands. In contrast, MTHFR 677 C→T has no influence on DS risk in France and Sicily, while homocysteine and MTR 2756 AG/GG genotypes are predictors of DS risk in Sicily. In conclusion, NTD and DS are influenced by the same genetic determinants of onecarbon metabolism. The distinct data produced in different geographical areas may be explained by differences in the nutritional environment and genetic characteristics of the populations.
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