PRMT5 regulates splicing of detained introns from DNA damage repair genes to protect against replication stress.

Protein arginine methyltransferase 5 (PRMT5) catalyzes the formation of symmetric dimethyl arginine on a broad range of cellular targets, and PRMT5 inhibitors (PRMT5i) show drastic anti-proliferative effects in preclinical studies. Several models have been proposed to explain and predict sensitivity to PRMT5 inhibition including through p53 activation and DNA damage response regulation. Here, we interrogate the underlying mechanisms of PRMT5i sensitivity in Merkel cell carcinoma, a high-grade neuroendocrine carcinoma of the skin that is that is highly sensitive to p53 activation. We observed that although PRMT5 inhibition activated p53, p53 loss did not impact PRMT5i sensitivity. To identify critical pathways altered by PRMT5i, we conducted a genome-wide CRISPR/Cas9 screen and found that PRMT5i-treated cells have an increased dependency on splicing factors. Furthermore, proteomic analysis found that levels of mRNA splicing factors increased after PRMT5i treatment. Characterization of the transcriptome revealed a disruption in mRNA splicing of detained introns in both PRMT5i sensitive and resistant lines. We observed that basal levels of detained introns correlated with PRMT5 inhibitor sensitivity, and that PRMT5 was required to maintain genome integrity through ATR/CHK1 signaling. Our findings support a threshold model of genome stability, in which cells can tolerate an increase in detained introns until a point where DNA damage pathways are triggered, and cells undergo apoptosis.