Genetic or pharmacological disruption of the MSH3 Y245/K246 IDL binding pocket slows CAG repeat expansion.

Recent genetic studies have shown somatic expansion of the CAG repeat is the key process driving Huntington's disease (HD) pathogenesis. Recognition of insertion-deletion loops (IDLs), lesions prone to form within the CAG repeat, by Mutsβ (MSH3/MSH2) is thought to be the primary event in the expansion process. This starts a cascade that leads to error-prone repair and incorporation of additional CAG units into the repeat. In vitro data shows MSH3 binds IDLs through a DNA-binding pocket formed by MSH3 residues Y245/K246. In this study, we investigated the significance of this DNA-binding motif in CAG repeat expansion using cell lines harbouring long, unstable HTT CAG repeats. Genetic disruption of the MSH3 Y245/K246 motif significantly reduced DNA interaction, exhibited MMR deficiency in a frameshift mutator assay, and abrogated repeat expansion in a U2OS cell line expressing mutant HTT exon 1. Pharmacological blockade of this site using a small molecule targeting the DNA-binding pocket similarly reduced DNA binding and repeat expansion in a U2OS cell line. Crucially, this molecule also slowed CAG repeat expansion in medium spiny neurones derived from HD patient-iPSCs. Targeting of the MSH3 IDL binding pocket may represent a possible therapeutic strategy.