C015 The function of PMS1 as a genetic modifier of Huntington’s disease

<h3>Background</h3> Genome-wide association studies have identified loci proximal to DNA repair genes which moderate Huntington’s disease age-at-onset, many of which are involved in mismatch repair (e.g., FAN1, MSH3 and PMS1; GeM-HD, 2015, 2019). The mechanism of PMS1 remains unknown although polymorphisms proximal to PMS1 tag HD AAO-hastening and -delaying 2AM1 and 2AM2 haplotypes, respectively (GeM-HD, 2019; Lee et al., 2022). <h3>Aims</h3> The aims of this project include determining whether PMS1 expression in HD-relevant cells modifies the rate of somatic HTT CAG repeat expansion. Additionally, we aim to identify functional domains of PMS1 which moderate the CAG repeat somatic expansion phenotype. <h3>Methods</h3> PMS1 -/- U2OS and HD patient-derived stem cells (iPSCs) will be generated via CRISPR-Cas9 gene editing, with U2OS being reconstituted with PMS1 structural variants. Using fragment analysis, somatic expansion of an exogenous 118 CAG HTT exon 1 construct will be monitored in U2OS cells, while the endogenous locus will be measured in iPSCs and iPSC-derived medium spiny neurons. <h3>Results</h3> We have successfully generated PMS1 -/- lines and show that PMS1 expression promotes somatic expansion. Moreover, PMS1 loss-of-function affects expression of other mismatch repair proteins. Experiments to establish the effect of specific domains of PMS1 (e.g., MLH1-binding region) on somatic expansion are currently in progress with preliminary data available. <h3>Conclusions</h3> PMS1 expression is a significant driver of somatic expansion in HD cells, highlighting this gene as a therapeutic target. Human genetic data show it is relatively tolerant of loss-of-function and exploration of specific domains involved will be useful in tailoring therapeutic strategies. <h3>References</h3> Genetic Modifiers of Huntington’s Disease (GeM-HD) Consortium. Identification of genetic factors that modify clinical onset of Huntington’s disease. <i>Cell</i> 2015;<b>162</b>(3):516–526. Genetic Modifiers of Huntington’s Disease (GeM-HD) Consortium. CAG repeat not polyglutamine length determines timing of Huntington’s disease onset. <i>Cell</i> 2019;<b>178</b>(4):887-900. Lee JM, Huang Y, Orth M, Gillis T, Siciliano J, Hong E, Mysore JS, Lucente D, <i>et al.</i> Genetic modifiers of Huntington disease differentially influence motor and cognitive domains. <i>Am J Hum Genet</i> 2020;<b>109</b>(5):885-899.