C009 Characterising the role MSH3 and MutS components play in CAG repeat expansion

Background DNA damage response (DDR) genes have emerged as major regulators of Huntington’s Disease (HD) onset and progression. DDR genes MSH3, FAN1, and MLH1, which are modifiers of HD, also regulate repeat instability in cell and animal models and influence the rate of somatic expansion in patient blood. These findings suggest somatic expansion of the HTT CAG repeat tract beyond a toxic threshold drives disease pathogenesis and therefore modulation of DDR components has significant therapeutic potential. MSH3, unlike many other DNA repair genes implicated in somatic expansion, is known to tolerate loss-of-function variation making it an ideal therapeutic target. Methods This study uses cell lines carrying unstable expanded repeats to investigate the functions of MSH3 that drive CAG repeat expansions. CRISPR/Cas9 has been used to knock-out MSH3 or introduce mutations disrupting the PCNA interacting peptide (PIP) motif (Q21A), the MLH1 interacting peptide (MIP) motif (F27A/F28A) or MSH3 ATPase activity (E976A). Results MSH3 knock out ablates HTT CAG expansions in HD induced pluripotent stem cells (iPSCs) and iPSC-derived neurons. Mutation of the ATPase domain halted somatic expansion in iPSCs and iPSC-derived neurons, suggesting that the ATPase domain is critical for expansions. Interestingly, the MIP box mutation (F27A/F28A) reduced the MSH3-MLH1 interaction but had little effect on CAG repeat expansions. The effect of MSH3 variants on protein-protein interactions and mismatch repair is also explored. Conclusions This study provides mechanistic insights by which MSH3 promotes repeat expansions.