Synergistic Effects of MTHFR, MTRR, and MTR Gene Variants on Serum Folate Levels and Cognitive Function in Chinese Preschoolers: A Cross-Sectional Study

<b>Background/Objectives</b>: Subnormal folate levels have a detrimental impact on the growth and development of preschoolers. We aimed to investigate the association between independent/synergistic effects of the gene polymorphisms (methyltetrahydrofolate reductase (MTHFR) <i>C677T</i> and <i>A1298C</i> polymorphisms, alongside methionine synthase reductase (MTRR) <i>A66G</i> polymorphism and the methionine synthase (MTR) <i>A2756G</i> polymorphism) and serum folate levels as well as cognitive levels in Chinese preschoolers aged 5-7 years. <b>Methods</b>: Data were sourced from 614 children, acquired through the "Long-term Health Effects Assessment Project of Infants and Toddlers Nutritional Pack (LHEAITNP)" program were used. Folate serum concentrations were measured using a microbiologically modified technique. The genotypes of MTHFR <i>A1298C</i> and <i>C677T</i>, together with MTRR <i>A66G</i>, were identified by Kramer's Allele-Specific PCR (KASP) technique. The cognitive scores of children were assessed by questionnaire. <b>Results</b>: MTHFR <i>677TT</i> and MTR <i>2756AG + GG</i> correlated negatively with serum folate levels (<i>TT</i> vs. <i>CC + CT</i>, <i>p</i> = 0.0009 and <i>AG + GG</i> vs. <i>AA</i>, <i>p</i> = 0.0057, respectively). MTHFR <i>C677T</i> and <i>A1298C</i> were independently linked to an elevated risk of suboptimal cognitive development (<i>TT</i> vs. <i>CC + CT</i>, <i>p</i> = 0.0009 and <i>AA</i> vs. <i>CA + CC</i>, <i>p</i> < 0.0001, respectively). The joint impact of these risk genotypes showed significantly increased risk of folate deficiency and inferior cognitive function compared to non-risk genotypes, particularly in those with more than two risk genotypes. The findings were corroborated by a cumulative effects model (<i>p</i> < 0.05). <b>Conclusions</b>: Our results indicate the substantial association between folate-homocysteine metabolism gene variants and serum folate status/cognitive performance in Chinese preschoolers. Potential gene-nutrient interactions worthy of longitudinal investigation.