PRMT5 Inhibition Triggers Functional ATM Deficiency and Sensitizes Pancreatic Cancer to CHK1 Blockade
PRMT5 inhibitors are under clinical investigation for pancreatic ductal adenocarcinoma (PDAC), but strategies to maximize their therapeutic efficacy remain undefined. Here, we report that pharmacologic inhibition of PRMT5 markedly reduces levels of the DNA damage response kinase ATM across multiple PDAC cell lines and different PRMT5 inhibitors, leading to a state of functional ATM haploinsufficiency. We find that this functional deficiency rewires checkpoint signaling, rendering PDAC cells reliant on the ATR-CHK1 pathway for survival. Consequently, we tested combined PRMT5 and CHK1 inhibition and found synergistic suppression of PDAC growth, accompanied by enhanced Caspase 3/7 activation, Annexin V staining, and DNA damage accumulation. Congruent with these results, RNA-seq demonstrated downregulation of cell-cycle and DNA repair genes along with the upregulation of cell death pathways, providing mechanistic insight into the cooperative effect. Moreover, in subcutaneous xenografts, the combination substantially reduced tumor volume, prolonged median survival, and did not affect overall health or body weight. Treated tumors showed reduced ATM and Ki67 with elevated ?H2A.X. Together, these findings identify PRMT5 inhibition as a trigger of functional ATM haploinsufficiency that exposes a therapeutically actionable vulnerability to CHK1 inhibition, offering a rational, mechanistically-based combination strategy for this dismal disease. Overall design: Pancreatic mRNA profiles of L3.6pl cells that were treated with 200 nM EPZ015938 and 1.95-2.34 nM prexasertib, alone or in combination, for 5 days. Three biological replicates per condition.  DMSO controls after 5 and 7 days of treatment were treated as a single control.