Gene polymorphism and folate metabolism: a maternal risk factor for Down syndrome

The high birth frequency of Down syndrome (DS), trisomy 21 (T21), has been a subject of interest to the clinicians and researchers due to its complexity in phenotypic expression. In addition to the maternal age, identification of the mechanistic basis for T21 requires an understanding of the cellular-molecular events and other biochemical pathways that could promote maternal meiotic nondisjunction. Recent studies have linked the increased frequency of polymorphism of methylenetetrahydrofolate reductase (MTHFR, C677T) and methionine synthase gene (MTRR, A66G) in mothers with DS child. Based on evidence that abnormal folate and methyl metabolism can lead to DNA hypomethylation and abnormal chromosomal segregation, researchers have observed that mothers with mutation in MTHFR (C677T) and MTRR (A66G) gene have elevated levels of plasma homocysteine. This was found to be associated with a 2.6 to 2.9 fold increased risk of having child with DS compared to mothers without the mutation. Subsequent studies evaluating Italian, Irish, French, and Indian-Gujarati women could not demonstrate an association of MTHFR gene polymorphism in mothers with DS child. However, the Irish study did find an increased risk of DS associated with the MTRR polymorphism and an interactive effect of MTRR and MTHFR polymorphisms with increased risk. Interestingly, an increase in plasma homocysteine was found to be a risk factor for DS in several of the studies. Despite the differences, the published studies suggest a common theme of abnormal folate metabolism associated with increased risk of having a child with DS. These observations suggest that there seems to be a geographic variation in gene polymorphism and it could not be attributable to meiotic nondysjunction in all mothers with DS child but increased homocysteine in all different study group does suggest that there may be a gene-nutritional or gene-gene or gene-nutritional-environmental factors involved in increased frequency of meiotic nondisjunction which needs transnational and multinational study design.