Huntington’s disease, one of more than 50 inherited repeat expansion disorders1, is a dominantly inherited neurodegenerative disease caused by a CAG expansion in HTT2. Inherited CAG repeat length is the primary determinant of age of onset, with human genetic studies underscoring that the disease is driven by the CAG length-dependent propensity of the repeat to further expand in the brain3–9. Routes to slowing somatic CAG expansion, therefore, hold promise for disease-modifying therapies. Several DNA repair genes, notably in the mismatch repair pathway, modify somatic expansion in Huntington’s disease mouse models10. To identify novel modifiers of somatic expansion, we used CRISPR–Cas9 editing in Huntington’s disease knock-in mice to enable in vivo screening of expansion-modifier candidates at scale. This included testing of Huntington’s disease onset modifier genes emerging from human genome-wide association studies as well as interactions between modifier genes, providing insight into pathways underlying CAG expansion and potential therapeutic targets.
