The Putative Role of Environmental Mercury in the Pathogenesis and Pathophysiology of Autism Spectrum Disorders and Subtypes

Abstract

Exposure to organic forms of mercury has the theoretical capacity to generate a range of immune abnormalities coupled with chronic nitro-oxidative stress seen in children with autism spectrum disorder (ASD). The paper discusses possible mechanisms explaining the neurotoxic effects of mercury and possible associations between mercury exposure and ASD subtypes. Environmental mercury is neurotoxic at doses well below the current reference levels considered to be safe, with evidence of neurotoxicity in children exposed to environmental sources including fish consumption and ethylmercury-containing vaccines. Possible neurotoxic mechanisms of mercury include direct effects on sulfhydryl groups, pericytes and cerebral endothelial cells, accumulation within astrocytes, microglial activation, induction of chronic oxidative stress, activation of immune-inflammatory pathways and impairment of mitochondrial functioning. (Epi-)genetic factors which may increase susceptibility to the toxic effects of mercury in ASD include the following: a greater propensity of males to the long-term neurotoxic effects of postnatal exposure and genetic polymorphisms in glutathione transferases and other glutathione-related genes and in selenoproteins. Furthermore, immune and inflammatory responses to immunisations with mercury-containing adjuvants are strongly influenced by polymorphisms in the human leukocyte antigen (HLA) region and by genes encoding effector proteins such as cytokines and pattern recognition receptors. Some epidemiological studies investigating a possible relationship between high environmental exposure to methylmercury and impaired neurodevelopment have reported a positive dose-dependent effect. Retrospective studies, on the other hand, reported no relationship between a range of ethylmercury-containing vaccines and chronic neuropathology or ASD. On the basis of these results, we would argue that more clinically relevant research is required to examine whether environmental mercury is associated with ASD or subtypes. Specific recommendations for future research are discussed.

Keywords: Autism spectrum disorders; Cytokines; Immune; Inflammation; Mercury; Oxidative stress.