A011 Blocking Huntingtin’s transcription protects against somatic instability

<h3>Background</h3> Somatic instability (SI) of the Htt CAG tract is a primary driver of HD age of onset. Reducing instability has become a key area of clinical interest alongside lowering mHTT, but despite its importance, the underlying mechanism behind instability remains unclear. It has been theorized that transcription is involved in the process, but this has not yet been demonstrated in animal models. It is still unknown whether HTT lowering modalities that reduce transcription also reduce SI. <h3>Aims</h3> Examine the impact of a variety of different Htt-lowering mechanisms on CAG instability. Identify the cellular mechanisms of CAG instability. <h3>Methods/Techniques</h3> We utilized multiple Htt-lowering systems with different mechanisms of action, including ASOs, GalNAc-conjugated siRNAs, ZFPs, and a Lac repression mouse model, measuring knockdown by qPCR, bDNA, western blot, and MSD assays. Somatic instability was measured using a CAG-spanning PCR and fragment analysis, and transcription was measured using an intron-exon spanning (pre-mRNA targeting) qPCR assay. <h3>Results/Outcome</h3> We find that the rate of somatic Htt CAG instability can be modified by specific modalities of Htt-lowering. Htt-lowering agents that block transcription reduce instability, while Htt-lowering methods that do not affect transcription do not significantly block instability. <h3>Conclusions</h3> Htt transcription is a key contributor in the progression of somatic instability.