A functional polymorphism in the reduced folate carrier gene and DNA hypomethylation in mothers of children with autism

Abstract

The biologic basis of autism is complex and is thought to involve multiple and variable gene-environment interactions. While the logical focus has been on the affected child, the impact of maternal genetics on intrauterine microenvironment during pivotal developmental windows could be substantial. Folate-dependent one carbon metabolism is a highly polymorphic pathway that regulates the distribution of one-carbon derivatives between DNA synthesis (proliferation) and DNA methylation (cell-specific gene expression and differentiation). These pathways are essential to support the programmed shifts between proliferation and differentiation during embryogenesis and organogenesis. Maternal genetic variants that compromise intrauterine availability of folate derivatives could alter fetal cell trajectories and disrupt normal neurodevelopment. In this investigation, the frequency of common functional polymorphisms in the folate pathway was investigated in a large population-based sample of autism case-parent triads. In case-control analysis, a significant increase in the reduced folate carrier (RFC1) G allele frequency was found among case mothers, but not among fathers or affected children. Subsequent log linear analysis of the RFC1 A80G genotype within family trios revealed that the maternal G allele was associated with a significant increase in risk of autism whereas the inherited genotype of the child was not. Further, maternal DNA from the autism mothers was found to be significantly hypomethylated relative to reference control DNA. Metabolic profiling indicated that plasma homocysteine, adenosine, and S-adenosylhomocyteine were significantly elevated among autism mothers consistent with reduced methylation capacity and DNA hypomethylation. Together, these results suggest that the maternal genetics/epigenetics may influence fetal predisposition to autism.

Figure 1

Diagram of folate-dependent transsulfuration pathway indicating interaction of MTHFR C677T, MTHFR A1298C, MTRR A66G and membrane transport function of RFC1 A80G. The up-down arrows indicate the metabolite alterations observed among the autism mothers. The inhibition of DNA methyltransferase by SAH is also diagrammed. Abbreviations: THF: tetrahydrofolate; 5-CH₃ THF: 5-methyltetrahyrdofolate; MTHFR: methylene tetrahydrofole reductase; MS; methionine reductase; MTRR: methionine synthase reductase; RFC: reduced folate carrier; SAM: S-adenosylmethionine; SAH: S-adenosylhomocysteine; SAHH: SAH hydrolase; MTase:methyltransferase

Figure 2

Scatterplots showing the correlations between metabolites among the autism mothers. 2A: Inverse correlation between plasma homocysteine and folate concentrations; 2B: Positive correlation between SAH and homocsyteine; 2C: Positive correlation between SAH and adenosine; 2D: Negative correlation between SAH and SAM/SAH ratio.

Figure 3

Scatterplots showing the relationship between the SAM/SAH ratio and DNA methylation by RFC1 genotype. 3A: RFC1 wildtype AA; 3B: RFC1 heterozygous AG; 3C: RFC1 homozygous variant GG.