C01 The effect of mismatch repair proteins in a huntington’s disease cellular model

Background Huntington’s disease (HD) is caused by an expanded CAG repeat in exon 1 of the Huntingtin gene (HTT). Repeats greater than 35 CAGs in length cause disease and the age at disease onset is inversely correlated with CAG repeat length. Although about 60% of the variation in age at onset of disease can be attributed to the CAG repeat length, other genetic factors have been found to modify age at disease onset. Recent genome wide association studies (GWAS) have identified several single nucleotide variants (SNVs) in genes encoding DNA damage repair factors that modify age at disease onset. There is a particular enrichment in mismatch repair genes with MLH1 and MSH3 implicated. Evidence from mouse models of HD suggests that MLH1 and MSH3 are involved in driving expansion of the CAG repeat in somatic cells. This somatic expansion is hypothesised to accelerate pathogenesis and decrease the age of disease onset. Methods To investigate the role of the mismatch repair genes MLH1 and MSH3 in HD we have used an induced pluripotent stem cell (iPSC) model of HD with 109 CAG repeats which shows expansion when cultured. We have used CRISPR Cas9 gene editing technology to knockout (KO) MLH1 and MSH3 in this cell line. Results and Conclusions MLH1 KO ablates somatic expansion of the expanded HTT CAG repeat in iPSCs when cultured for 59 days and preliminary data suggests that MSH3 KO reduces somatic expansion of the expanded HTT CAG repeat in iPSCs.