PRMT5-mediated SRSF1 methylation promotes virus-positive Merkel cell carcinoma tumorigenesis by increasing Tip60 exon inclusion [PacBio Sequel IIe]

Protein Arginine Methyltransferase 5 (PRMT5) is a key regulator of gene expression and RNA splicing, with its therapeutic potential demonstrated through synthetic lethality in MTAP-deleted cancers. Emerging evidence also suggests that MYC-driven or p53 wild-type tumors may be more sensitive to PRMT5 inhibition, though the underlying mechanisms remain unclear. Virus-positive Merkel cell carcinoma (MCC) serves as an ideal model to explore this question, as it is driven by the MYC paralog MYCL, and most virus-positive MCCs retain wild-type p53. In this study, we investigated PRMT5’s impact on the Tip60-EP400 complex, given MCC’s reliance on the interaction between MYCL and this complex. We used second-generation sequencing (Illumina) to characterize PRMT5-mediated bulk gene expression and long-read RNA sequencing (PacBio Iso-Seq) to analyze PRMT5-mediated alternative splicing. The mechanism underlying PRMT5 deficiency's selective impact on specific splice sites while leaving others unaffected remains unclear. Our findings suggest that PRMT5-mediated methylation of SRSF1 enhances its recruitment to m6A-modified RNA, ensuring proper splicing of Tip60 and maintaining Tip60-EP400 activity. In contrast, PRMT5 inhibition disrupts this recruitment, leading to widespread splicing defects, including both exon skipping and intron retention. These results provide new insights into PRMT5’s role in splicing regulation in cancer cells and may have broader implications for targeting splicing dysregulation in MYC-driven cancers.