Abstract
Mutations in the FUS gene cause aggressive amyotrophic lateral sclerosis (ALS-FUS). Beyond mRNA, FUS generates partially processed transcripts retaining introns 6 and 7. We demonstrate that these FUSint6&7-RNA molecules form nuclear condensates, scaffolded by the highly structured intron 7 and associated with nuclear speckles. Using hybridization-proximity labeling proteomics, we show that the FUSint6&7-RNA condensates are enriched for splicing factors and the N6-methyladenosine (m6A) reader YTHDC1. These ribonucleoprotein structures facilitate posttranscriptional FUS splicing and depend on m6A/YTHDC1 for integrity. In cells expressing mutant FUS, FUSint6&7-RNAs become hypermethylated, which in turn stimulates their condensation and splicing. We further show that FUS protein is repelled by m6A. Thus, ALS-FUS mutations may cause abnormal activation of FUS posttranscriptional splicing through altered RNA methylation. Notably, ectopic expression of FUS intron 7 sequences dissolves endogenous FUSint6&7-RNA condensates, down-regulating FUS mRNA and protein. Our findings reveal a condensation-dependent mechanism regulating FUS splicing, with possible therapeutic implications for ALS.