Environmental drivers of trinucleotide repeat instability and Huntington's disease onset

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PROJECT SUMMARY Trinucleotide repeat disorders is a class of neurological diseases defined by repetitive changes in DNA. Many trinucleotide repeat disorders including Huntington’s disease (HD), a rare and fatal neurodegenerative disease, are inherited. HD is caused by expanded repeats of the CAG trinucleotide sequence in the Huntingtin (HTT) gene. Expansions greater than 36 CAG repeats leads to a pathogenic transcript and proteins causing a late onset, but severe and terminal form of neurodegeneration. The length of the CAG repeat sequences throughout the genome is unstable with a high potential to expand across generations. While HD is mostly inherited, a small proportion of cases arise through sporadic expansion of CAG repeats. Approximately 60% of the variance in the onset in HD symptoms is attributable to the number of CAG repeats a person carries; more CAG expansions are associated with earlier onset. Over half of the remaining variability in the duration to symptom onset has been attributed to environmental factors that remain undiscovered. Environmental chemical exposure could con- tribute to repeat instability and subsequently, sporadic forms of trinucleotide repeat disorders such as HD. In- deed, chemical-induced oxidative stress causes CAG repeat expansion mutations. We previously demonstrated that mitochondria inhibiting pesticides cause oxidative stress in mouse neurons and elicit gene expression sig- natures of HD. One member of this pesticide class, pyraclostrobin, is applied at very high levels on food to inhibit fungal pathogens. Predicted human exposure levels suggest that they are sufficient to inhibit human mitochon- dria and therefore, could contribute to HD disease risk and severity. Our preliminary results demonstrate that pyraclostrobin causes CAG repeat expansion in cultured cells. We will test the hypothesis that pyraclostrobin accelerates the course of HD, but is also capable of producing new pathogenic repeat expansions in non-carri- ers. We will apply a diverse set of molecular, histopathological, and behavioral tools to characterize the HD phenotypes in a widely accepted mouse model of HD upon pyraclostrobin exposure across the life course. Our results will provide the foundation necessary to establish prevention strategies for those at familial risk for trinu- cleotide repeat disorders. Moreover, our work re-defines the role of environmental chemicals as mutagens and expands their role as contributors to canonical genetic diseases.