Selective targeting of mutant huntingtin intron-1 improves rescue provided by antisense oligonucleotides

Huntington's disease (HD) arises from the toxic gain of function caused by a CAG expansion in the coding region of the HTT gene. HD is increasingly appreciated to emerge from multiple pathogenic processes, including somatic instability in mutant HTT's ( mHTT ) CAG repeat tract, which leads to diverse deleterious consequences. These include the alternative processing of HTT pre-mRNA to generate the HTT1a transcript that encodes the very toxic, mHTT isoform referred to as HTT1a. We set out to compare the efficacy and safety of allele-selective lowering of mHTT compared to non-allele-selective lowering using antisense oligonucleotides (ASOs) in heterozygous Htt Q111 (Q111) mice. We developed a mutant specific ASO (MutASO) targeting Htt intron-1 that selectively reduced mutant full-length HTT, as well as HTT1a, in the brains of Q111 mice. Compared to the rescue provided by a pan-allele-targeting ASO (PanASO) that lowers wild-type HTT and full-length mHTT (sparing HTT1a), the MutASO essentially eliminated aggregate formation, and provided marked protection from transcriptional dysregulation in HD knock-in mice. Thus, by targeting the ASO to the region upstream of the cryptic polyadenylation sites required to generate the HTT1a transcript, our allele-selective MutASO potently reduced HTT1a protein levels. Here, our findings advocate that HTT1a may have a disproportionate impact on aggregate formation and transcriptional dysregulation and that lowering the levels of HTT1a could provide benefit when designing HTT-lowering based therapeutic strategies for HD.