21-24 – Targeting the C9orf72 Intronic G4C2 Hexanucleotide Repeat Expansion in C9ALS/FTD Using Antisense Oligonucleotides (ASOs)

ASOs have emerged as a promising therapeutic approach for Amyotrophic Lateral Sclerosis (ALS). Tofersen, an ASO, targets ALS caused by a mutation in the SOD1 gene, while Jacifusen, another ASO, is designed for ALS linked to a FUS gene mutation. The leading cause of ALS and Frontotemporal Dementia (FTD), collectively known as C9ALS/FTD, is a GGGGCC (G4C2) hexanucleotide repeat expansion (HRE) within the first intron of the C9orf72 gene, which is central to the pathogenesis of C9ALS/FTD. However, despite targeting this mechanism, ASOs developed for C9ALS/FTD have not yet demonstrated clinical benefit. The three primary pathogenic mechanisms in C9ALS/FTD are repeat-associated non-ATG (RAN) translation, nuclear RNA inclusions, and C9orf72 loss of function. Bromodomain inhibitors, under specific experimental conditions, can enhance C9orf72 transcription, reduce toxic DPR inclusions, whilst increasing the accumulation of RNA foci which is potentially neuroprotective. However, due to the off-target effects of bromodomain inhibitors, we are developing ASOs that selectively silence specific bromodomain and extra-terminal domain (BET) proteins. These ASOs offer greater precision compared to traditional bromodomain inhibitors like JQ1 and PFI-1.
Our goal is to use ASOs to knock down BRD2, BRD3, and BRD4 mRNA, along with their specific isoforms. Initially, we will conduct this in iPSCs and iPSC-derived neurons to confirm the expression of the different BET isoforms and demonstrate their potential for silencing. We will assess the changes in C9orf72 expression and the resulting impact on DPR formation and RNA foci levels. Following this, we will apply the approach to patient-derived iPSC lines containing known pathogenic C9orf72 HREs.