Treatment of Huntington’s disease with a pan-HTT-targeting CRISPR nuclease

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by an expansion of a CAG trinucleotide repeat in the huntingtin (HTT) gene, which leads to a mutant protein that destroys neurons in the brain. Despite intense effort, there remains no approved disease-modifying therapy for HD. Here, we develop a pan-HTT-targeting CRISPR-Cas9 system that, when delivered to the striatum of R6/2 and YAC128 mice by adeno-associated virus serotype 5 (AAV5), lowered mutant HTT mRNA and protein by 55%-80% via its induction of frameshift-inducing insertion or deletion (indel) mutations in HTT exon 1. Cas9 targeting improved motor coordination and locomotor activity, decreased anxiety-like deficits, reduced clasping and weight loss, limited striatal atrophy, and decreased the formation of intranuclear inclusions immunoreactive for the mutant HTT protein. In Hu21/21 mice, which carry the wild-type human HTT gene in lieu of the mouse ortholog, Cas9 lowered the HTT protein by 44% but induced no measurable behavioral deficits and had no adverse effect on neuronal viability, though its targeting was associated with neuroinflammation. Altogether, our results demonstrate the ability of a newly developed pan-HTT-targeting Cas9 system to affect HD-related phenotypes across models and provide insights into its tolerability.