FUS post-transcriptional splicing is autoregulated via RNA condensation with therapeutic potential for ALS-FUS

Mutations in the FUS gene cause aggressive and often juvenile forms of amyotrophic lateral sclerosis (ALS-FUS). In addition to mRNA, the FUS gene gives rise to a partially processed RNA with retained introns 6 and 7. We demonstrate that these FUSint6&7-RNAs form nuclear condensates scaffolded by the highly structured intron 7 and associated with nuclear speckles. Using hybridization-proximity labelling proteomics, we show that the FUSint6&7-RNA condensates are enriched in splicing factors and the m6A reader YTHDC1. These ribonucleoprotein structures facilitate post-transcriptional FUS splicing and depend on m6A/YTHDC1 for their maintenance. FUSint6&7-RNAs become hypermethylated in cells expressing mutant FUS, leading to their enhanced condensation and consequently, splicing. We further demonstrate that FUS protein is repelled by m6A. Thus, ALS-FUS mutations may cause an abnormal activation of FUS post-transcriptional splicing via altered RNA methylation. Strikingly, ectopic expression of FUS intron 6&7 sequences dissolves the endogenous FUSint6&7-RNA condensates, downregulating FUS mRNA and protein. Overall, we describe an RNA condensation-dependent mechanism regulating FUS splicing that can be harnessed for developing new therapies.