Cas9 nickase-mediated contraction of CAG/CTG repeats is associated with phenotypic improvements in a Huntington's disease mouse model.

Expanded CAG/CTG repeats cause more than 15 different diseases that all remain without a disease-modifying treatment. Because repeat length accounts for most of the variation in disease severity, contracting CAG/CTG repeats presents an attractive therapeutic avenue. Here, we show that the CRISPR-Cas9 nickase targeted to CAG/CTG repeats leads to efficient repeat contractions in Huntington's disease (HD) patient-derived neurons and astrocytes and in myotonic dystrophy type 1 patient-derived neurons. The approach was allele selective and free of detectable off-target mutations. Injection of the Cas9 nickase into the striatum of the R6/1 mouse model of HD using adeno-associated viral vectors led to repeat contractions in more than half of striatal neurons. Upon injection, we observed a reduction in the number of inclusion bodies, an improved gene expression profile, and amelioration of locomotion deficits in the open field test. The effects were greater than expected from the contractions induced, suggesting that non-cell autonomous mechanisms may be involved. Our results provide the proof of concept that correction of CAG/CTG repeats can improve HD phenotypes in a mouse disease model.