Metabolic endophenotype and related genotypes are associated with oxidative stress in children with autism

Abstract

Autism is a behaviorally defined neurodevelopmental disorder usually diagnosed in early childhood that is characterized by impairment in reciprocal communication and speech, repetitive behaviors, and social withdrawal. Although both genetic and environmental factors are thought to be involved, none have been reproducibly identified. The metabolic phenotype of an individual reflects the influence of endogenous and exogenous factors on genotype. As such, it provides a window through which the interactive impact of genes and environment may be viewed and relevant susceptibility factors identified. Although abnormal methionine metabolism has been associated with other neurologic disorders, these pathways and related polymorphisms have not been evaluated in autistic children. Plasma levels of metabolites in methionine transmethylation and transsulfuration pathways were measured in 80 autistic and 73 control children. In addition, common polymorphic variants known to modulate these metabolic pathways were evaluated in 360 autistic children and 205 controls. The metabolic results indicated that plasma methionine and the ratio of S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH), an indicator of methylation capacity, were significantly decreased in the autistic children relative to age-matched controls. In addition, plasma levels of cysteine, glutathione, and the ratio of reduced to oxidized glutathione, an indication of antioxidant capacity and redox homeostasis, were significantly decreased. Differences in allele frequency and/or significant gene-gene interactions were found for relevant genes encoding the reduced folate carrier (RFC 80G > A), transcobalamin II (TCN2 776G > C), catechol-O-methyltransferase (COMT 472G > A), methylenetetrahydrofolate reductase (MTHFR 677C > T and 1298A > C), and glutathione-S-transferase (GST M1). We propose that an increased vulnerability to oxidative stress (endogenous or environmental) may contribute to the development and clinical manifestations of autism.

Figure 1

An overview of the pathways involved in folate-dependent methionine transmethylation and transsulfuration. Methylenetetrahydrofolate (MTHFR) catalyzes the synthesis of 5-methyltetrahydrofolate (5-CH₃-THF) from 5,10,methylenetetrahydrofolfate (5,10-CH₂THF). The methyl group from 5-CH₃THF is transferred to homocysteine to regenerate methionine via the folate/B12-dependent methionine synthase (MS) reaction. Methionine synthase reductase (MSR) maintains the B12 cofactor in a reduced state for optimal MS activity. Methionine is then activated to S-adenosylmethionine (SAM), the major methyl donor for multiple cellular methyltransferase (MTase) reactions. After methyl group transfer, SAM is converted to SAH which is further metabolized to homocysteine and adenosine by a reversible reaction catalyzed by SAH hydrolase (SAHH). Homocysteine may be permanently removed from the methionine cycle by irreversible conversion to cystathionine by B6-dependent cystathionine beta synthase (CBS) which initiates the transsulfuration pathway. Cystathionine is subsequently converted to cysteine, the rate limiting amino acid for the synthesis of the tripeptide, glutathione (Glu-Cys-Gly). Reduced active glutathione (GSH) is in dynamic equilibrium with the oxidized disulfide GSSG form of glutathione. Reduced folates are transported from the plasma into the cell by the reduced folate carrier (RFC). Transport of folate into the intestinal mucosa is mediated by glutamate carboxypepsidase II (GCPII). Vitamin B12 is transported into the cell bound to the B12 transport protein transcobalmin II (TCII)