A005 Inducible CAG repeat expansion in a human iPSC model of Huntington’s disease

<h3>Background</h3> Somatic expansion of the pathogenic CAG repeat region in the huntingtin (HTT) gene has been linked to hastened onset of Huntington’s disease (HD). DNA mismatch repair genes identified through human genetic studies affect CAG repeat instability in iPSC and mouse models of HD. Data from our lab have shown that homozygous knockout of PMS1 prevented CAG repeat expansion in a patient-derived iPSC model of HD while heterozygous knockout had no effect. <h3>Aim</h3> Genetically engineer an inducible model of somatic expansion by complementing PMS1-/- HD-iPSCs with inducible PMS1 <h3>Methods/Techniques</h3> We co-transfected PMS1-/- HD-iPSCs with a plasmid containing PMS1 under the control of a doxycyline-inducible Tet-On promotor, and two transcription activator-like effector nucleases (TALENs) for insertion into a safe harbour site. We screened the transfected cells and created clonal lines with homozygous and heterozygous insertion. We assessed protein expression and somatic expansion in these complemented lines. <h3>Results/Conclusions</h3> Complemented PMS1-/- HD-iPSCs show inducible expression of PMS1 protein after culturing in the presence of doxycycline. PMS1 expression levels in the complemented lines are ~20-25% of endogenous expression levels in PMS1+/+ HD-iPSCs. Assaying somatic expansion in complemented PMS1-/- iPSCs over 8 weeks showed that somatic expansion is PMS1-dependent and that expansion rates correlate with the PMS1 protein level. These data show that PMS1 is rate-limiting for somatic expansion and could be an attractive therapeutic target in HD and other repeat-expansion disorders. Further, our inducible system can be used to probe the molecular mechanisms of somatic expansion.