Aberrant splicing exonizes C9orf72 repeat expansion in ALS/FTD

A nucleotide repeat expansion (NRE) (GGGGCC)n within the first annotated intron of the C9orf72 (C9) gene is a common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). While previous studies have shown that C9 NRE produces several toxic dipeptide repeat (DPR) proteins, the mechanism by which an intronic RNA segment can access the cytoplasmic translation machinery remains unclear. By selectively capturing and sequencing NRE-containing RNAs (NRE-capture-seq) from patient-derived fibroblasts and neurons, we found that, in contrast to previous models, C9 NRE is retained as part of an extended exon 1 due to the usage of various downstream alternative 5′ splice sites. These aberrant splice isoforms accumulate in C9-ALS/FTD brains, and their production is promoted by serine/arginine-rich splicing factor 1 (SRSF1). Antisense oligonucleotides targeting either SRSF1 or the aberrant C9 splice isoforms reduced the levels of DPR. Together, our findings revealed a crucial role of aberrant splicing in the biogenesis of NRE-containing RNAs and demonstrated potential therapeutic strategies to target these pathogenic transcripts. By selectively isolating and sequencing the rare RNA transcripts containing C9orf72 repeat expansion from ALS–FTD neurons, the authors uncover an alternative splicing mechanism that explains the retention of this intron segment in a translated mRNA.