Autism and Folate-dependent One-carbon Metabolism: Serendipity and Critical Branch-point Decisions in Science.

Folate-dependent one-carbon metabolism is present in every cell of the body. It represents a central systems biology hub that reverberates into countless other pathways with more specialized roles in specialized cell types throughout the body. I have spent 25 years of research on this core biochemical pathway with several unanticipated iterations that led me from Down syndrome to congenital heart defects to leukemia and finally to autism about 12 years ago. Figure 1 provides an overview of the three interdependent pathways involved in folate-dependent methionine "transmethylation" and "transsulfuration." Methionine is necessary for the synthesis of S-adenosylmethionine (SAM), the major methyl donor for all cellular methylation reactions. It is also the major precursor for cysteine, the rate-limiting amino acid for glutathione synthesis linking transmethylation and transsulfuration pathways. Methionine levels can be negatively affected by genetic and environmental factors that reduce folate availability and/or oxidative inhibition of the methionine synthase enzyme. Because these three metabolic pathways are mutually interdependent, genetic or environmental perturbation of folate or methionine metabolism will indirectly impact glutathione synthesis, and conversely, alterations in glutathione synthesis will alter flux through pathways of folate and methionine metabolism. This interdependency translates into broader impact on essential cellular functions.