Revisiting the outcome of adult wild type Htt inactivation in the context of gene editing

Huntingtin (HTT)-lowering strategies are central to therapeutic approaches for Huntington's disease. Recent studies reported the induction of age- and cell type-specific phenotypes by conditional HTT knockout, but these experimental conditions did not precisely mimic HTT-lowering or gene-editing conditions in terms of the cells targeted and brain distribution, and no transcriptional profiles were provided. Here, we used the AAV delivery system commonly used in CNS gene therapy programs and the self-inactivating KamiCas9 gene-editing system to investigate the long-term consequences of wild-type mouse huntingtin (Htt) inactivation in adult neurons and, thus, the feasibility and safety of Htt inactivation in these cells. Behavioral and neuropathological analyses and single-nuclei RNA sequencing (snRNA-seq) indicated that Htt editing in 77% of striatal neurons and 16% of cortical projecting neurons in adult mice induced no behavioral deficits or cellular toxicity. Single-nuclei RNA-seq in 11.5-month-old animals showed that Htt inactivation did not alter striatal-cell profiles or proportions. Few differentially expressed genes were identified and Augur analysis confirmed an extremely limited response to Htt inactivation in all cell types. Our results therefore indicate that wild-type Htt inactivation in adult striatal and projection neurons is well tolerated in the long term.