Evidence of Abnormal Folate Metabolism and DNA Hypomethylation in Mothers of Children with Autism

Background:   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). As such, normal functioning of this pathway is essential to support the rapid shifts between proliferation, differentiation and cell death that are essential for normal fetal programming and organogenesis.  Maternal polymorphic variants, nutritional deficiencies and/or environmental exposures that negatively affect availability of folate one-carbon precursors have been associated with increased risk of structural birth defects, chromosomal anomalies, schizophrenia, and prematurity.  We recently reported preliminary evidence suggesting that folate-dependent transmethylation and transsufuration metabolism was abnormal in some autism mothers (James et al, JADD 2009).

Objectives:  To measure plasma transmethylation metabolites and DNA methylation status in a larger independent local cohort of autism and control mothers and to determine the frequency of folate-relevant polymorphisms in DNA from 530 case-parent triads and 560 controls obtained from the NIMH repository.

Methods:  Fasting plasma samples were obtained from 58 local autism mothers and 80 matched control mothers and analyzed for concentrations of folate, methionine, S-adenosylmethionine (SAM), S-adenosylmethionine (SAH), adenosine, and homocysteine using HPLC with electrochemical detection.  Global DNA methylation (% 5-methylcystosine/total cytosine) was measured by LC/mass spectrometry.  NIMH repository DNA samples were analyzed using TaqMan primer-probe sets (ABI PRISM 7300) for MTHFR C677T, MTHFR A1298C, MTRR A66G, TCII C776G, and RFC1 A80G as potential contributors to abnormal maternal one-carbon metabolism.

Results:   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-adenosylhomocysteine were significantly elevated among autism mothers which is biochemically consistent with reduced methylation capacity and global DNA hypomethylation.  In the case-control analysis of over 2100 NIMH repository DNA samples, 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.  Plasma folate levels were significantly reduced among the local autism mothers.

Conclusions:    Taken together, these results support a broader paradigm of autism gene-environment interaction that encompasses the mother as a genetic/epigenetic case as well as a potential fetal environmental factor.  Inclusion of maternal genetic/epigenetics in the autism gene-environment paradigm could provide new insights into the etiology of this complex disorder.