PRMT5 inhibition disrupts detained intron splicing and impairs ATR signaling with increased DNA damage.

PRMT5 catalyzes symmetric dimethyl arginine on numerous proteins, with PRMT5 inhibitors (PRMT5i) inducing anti-proliferative effects in preclinical studies. Several models have been proposed to explain sensitivity to PRMT5i including through p53 activation and DNA damage response (DDR) regulation. Here, we interrogate the mechanisms of PRMT5i sensitivity in Merkel cell carcinoma, a neuroendocrine skin cancer sensitive to p53 activation. To identify critical pathways altered by PRMT5i, we performed CRISPR/Cas9 screening, proteomic, and transcriptomic analyses. Our results indicate that PRMT5i sensitivity is independent of p53 activation and is characterized by an increase in detained introns (DIs) and dependency on mRNA processing factors. Sensitivity correlated with elevated basal DI levels and was associated with replication-associated DNA damage accompanied by impaired ATR/CHK1 signaling. These findings are consistent with a threshold model of sensitivity in which excessive DI accumulation is associated with replication-associated DNA damage and apoptosis following PRMT5 inhibition.