Huntington's disease (HD) is a fatal neurodegenerative disorder caused by the expansion of a polymorphic CAG repeat tract beyond a threshold of ~ 36 units. The expanded CAG repeat is translated into a polyglutamine stretch at the amino-terminus of the huntingtin protein, triggering cell death in a subset of neurons in the striatum and cortex. The expanded CAG repeat also exhibits dramatic instability in the germline and in somatic tissues. The long-term goals of this research are to elucidate the molecular pathways that underlie the instability of the HD CAG repeat and the specific neurodegeneration triggered by mutant huntingtin. In HD knock-in mice, which accurately recapitulate the human HD mutation, the Msh2 gene is a modifier of repeat instability and an early striatal phenotype. In this study, we will test the hypothesis that Msh2 acts in the mismatch repair pathway to modify CAG repeat instability and early phenotypes in HD knock-in mice. We will perform genetic crosses with mice deficient in specific mismatch repair genes and mouse line carrying a mutation in Msh2's ATPase domain. These experiments will provide mechanistic insight into Msh2's role in CAG repeat instability and phenotypic expression of the mutant HD allele. To determine whether candidate DNA repair genes are modifiers of the age of onset of HD in humans genetic association studies will be performed using 'extreme'individuals with onset ages deviating from values predicted by CAG repeat size. To gain further insight into the mechanism by which Msh2 modifies repeat instability and early striatal disease in the mouse we will generate a conditional knockout of the Msh2 gene using the Cre-loxP system. We will specifically inactivate Msh2 in forebrain neurons using a Camklla-driven Cre transgene to test the hypothesis that Msh2 acts in striatal neurons to modify instability and early phenotypes. Together, these studies will provide insight into mechanisms of repeat instability and pathogenesis, leading to rational therapeutic strategies aimed at slowing or halting this devastating disease.