Abstract 2116 The effects of spliceosomal U1 snRNA mutations in multiple cancers on the biogenesis of U1 snRNP and the snRNP repertoire

In eukaryotes, newly synthesized pre-messenger RNAs undergo post-transcriptional processing to produce protein-coding mRNAs. Small nuclear ribonucleoproteins (snRNPs) are non-coding RNA-protein complexes and major components of spliceosomes in splicing. U1 snRNP, the most abundant RNP particle among the spliceosomal snRNPs, plays a key role in splicing as well as preventing premature termination of nascent transcripts, known as telescripting. Recent studies showed that frequent mutations in U1 snRNA were identified in multiple cancers, suggesting potential disruptions in splicing through the cryptic splice sites usage. However, it is unknown whether the U1 snRNA mutants form mature U1 snRNPs. The stability and abundance of snRNPs are determined by the formation of the Sm core, a seven-membered protein ring surrounding a uridine-rich sequence. The assembly of Sm core on snRNAs is facilitated by a multi-subunit survival of motor neurons (SMN) complex. In this study, we employed a quantitative in vitro Sm core assembly assay to measure the formation snRNPs in response to mutations in U1 snRNA. Our results showed that mutations at the 5'-end of U1 snRNA led to a reduction of up to 75% in the formation of the Sm core. In addition, we observed that mutations at various loops in U1 snRNA showed moderate effects on Sm core assembly. Furthermore, the competitions with wild-type snRNAs indicated that the 5'-end of U1 snRNA mutant inhibited the formation of all snRNPs formation. These findings suggest that U1 snRNA mutations in cancer prevent the formation of functional U1 snRNPs and disrupt the SMN complex's Sm core assembly activity for all snRNAs. This connection potentially underscores the impact on snRNP repertoire and snRNP-dependent protein-coding gene expression. This work is supported by the National Institute of General Medical Sciences of the National Institutes of Health under award number R15GM152936 and by the University of Texas Arlington start-up funds.