C008 The role of MutL complexes in CAG repeat expansion in a human iPSC model of Huntington’s disease

<h3>Introduction</h3> Huntington’s disease (HD) is caused by an inherited expanded CAG repeat that further expands in post-mitotic neurons in HD individuals. This somatic CAG repeat expansion has been implicated as a key driver of disease and is a potential disease modifying therapeutic target. The mechanism facilitating somatic CAG repeat expansion has not yet been fully elucidated but results from human genetics studies and experiments using mouse and cell models of HD have implicated mismatch repair. Our study has focused on the role of MutL complexes in somatic CAG repeat expansion. The MutL complexes are key components of mismatch repair and consist of 3 heterodimers: MutLα (MLH1-PMS2), MutLβ (MLH1-PMS1) and MutLγ (MLH1-MLH3). <h3>Methods</h3> We have used CRISPR-Cas9 to knock out the 4 MutL proteins (MLH1, PMS2, PMS1 and MLH3) in a patient derived induced pluripotent stem cell (iPSC) model of HD with an unstable CAG repeat that expands in culture. <h3>Results/Conclusions</h3> Homozygous knockout of MLH1, MLH3 or PMS1 prevented CAG repeat expansion, whilst heterozygous knockout of these genes did not alter the rate of CAG repeat expansion. Homozygous knockout of PMS2 reduced, but did not prevent, expansion, whilst heterozygous knockout of PMS2 increased the rate of expansion. This evidence suggests that both MutLβ and MutLγ are critical drivers of expansion but the role of MutLα is more complex. To extend this work we have generated HD-iPSC derived inducible glutamatergic cortical neurons that show expansion in culture.