Development of a cellular reporter assay to measure activity of MutSβ, a therapeutic target for Huntington’s disease

Genetic modifiers of age of onset in Huntington’s disease (HD) provide compelling evidence that somatic expansion of the CAG repeats is a critical driver of pathogenesis and demonstrate that repeat instability is modulated by DNA mismatch repair (MMR). A component of this pathway, MutSβ, a heterodimer comprised of MSH2 and MSH3, has emerged as a potential target for small-molecule therapeutic intervention. However, a robust cellular assay to interrogate genetic and pharmacological modifiers of MutSβ has not been reported. We have repurposed and optimized a tetranucleotide reporter assay to measure MutSβ activity in MMR-competent cells. We show that repeat instability is modulated by MSH3 protein levels and by its ATPase activity. In addition, we show that an inhibitor of HDAC3 modulates repeat instability, demonstrating the utility of the assay for pharmacological studies.