Polymorphisms of methylenetetrahydrofolate reductase (MTHFR) and methionine synthase (MTR) are related to cognitive dysfunction and mental disability. These genes, along with folate and vitamin B12 levels, are regulators of one-carbon metabolism, which synthesizes S-adenosylmethionine (SAM) as a methyl donor for arsenic methylation. The aim of this study was to explore whether polymorphisms of MTHFR and MTR influence arsenic methylation capacity and plasma folate and vitamin B12 levels and if these influences cause developmental delay in preschool children. A total of 178 children with developmental delay and 88 without developmental delay were recruited from August 2010 to March 2014. A high-performance liquid chromatography–hydride generator and atomic absorption spectrometer were used to determine urinary arsenic species. Plasma folate and vitamin B12 concentrations were measured by SimulTRAC-SNB radioassay. Polymorphisms of MTHFR C677T, MTHFR A1298C, and MTR A2756G were examined by polymerase chain reaction and restriction fragment length variation. The results show that MTHFR C677T C/T and T/T genotypes had a lower risk of developmental delay than the C/C genotype (odds ratio [OR] = 0.47; 95% confidence interval, 0.26–0.85). Subjects with the MTHFR C677T C/C genotype had significantly lower plasma folate and vitamin B12 levels than those with the MTHFR C677T C/T and T/T genotype. The MTHFR C677T C/C genotype combined with high total urinary arsenic and poor arsenic methylation capacity indices significantly increased the OR of developmental delay in a dose–response manner. This is the first study to show the combined effect of MTHFR C677T genotype and poor arsenic methylation capacity on developmental delay.
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