Dose-dependent reduction of somatic expansions but not Htt aggregates by di-valent siRNA-mediated silencing of MSH3 in HdhQ111 mice

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by CAG trinucleotide repeat expansions in exon 1 of the HTT gene. In addition to germline CAG expansions, somatic repeat expansions in neurons also contribute to HD pathogenesis. The DNA mismatch repair gene, MSH3, identified as a genetic modifier of HD onset and progression, promotes somatic CAG expansions, and thus presents a potential therapeutic target. However, what extent of MSH3 protein reduction is needed to attenuate somatic CAG expansions and elicit therapeutic benefits in HD disease models is less clear. In our study, we employed potent di-siRNAs to silence mouse Msh3 mRNA expression in a dose-dependent manner in Hdh Q111/+ mice and correlated somatic Htt CAG instability with MSH3 protein levels from simultaneously isolated DNA and protein after siRNA treatment. Our results reveal a linear correlation with a proportionality constant of ~ 1 between the prevention of somatic Htt CAG expansions and MSH3 protein expression in vivo, supporting MSH3 as a rate-limiting step in somatic expansions. Intriguingly, despite a 75% reduction in MSH3 protein levels, striatal nuclear HTT aggregates remained unchanged. We also note that evidence for nuclear Msh3 mRNA that is inaccessible to RNA interference was found, and that MSH6 protein in the striatum was upregulated following MSH3 knockdown in Hdh Q111/+ mice. These results provide important clues to address critical questions for the development of therapeutic molecules targeting MSH3 as a potential therapeutic target for HD.