Molecular Aspects of Thimerosal-induced Autism

The developmental disorder autism has both genetic and environmental origins, and its forty-fold increase during the past two decades reflects an increased role for environmental factors. It has been proposed that increased use of vaccines containing the ethylmercury derivative thimerasol is the major contributing factor. Published research from my laboratory has revealed that thimerosal is an exceptionally potent inhibitor of biochemical pathways that transfer single carbon atoms between molecules. These “methylation” pathways are critically involved in several important functions including the regulation of gene expression and the molecular mechanism of attention. Recent studies from my lab indicate that thimerosal exerts its toxic effect on methylation by interfering with formation of the active form of vitamin B12, also known as cobalamin. Dietary B12 must be converted to methylB12 (methylcobalamin) in order to assist in the transfer of single-carbon methyl groups from the folic acid pathway by the enzyme known as methionine synthase. By reducing methylB12 formation, thimerosal inhibits this enzyme and thereby interferes with methylation events. Autistic children have abnormal plasma levels of methylation-related metabolites and exhibit higher frequencies of genetic mutations that affect this pathway. These genetic risk factors make them less able to detoxify thimerosal and also increase their sensitivity to its mechanism of toxicity. In many cases, autism can be effectively treated by the administration of methylB12 along with other agents that augment methylation capacity. Taken together, these facts indicate that increased exposure to thimerosal has combined with genetic risk factors in a sensitive subpopulation to cause the recent rise in autism.