PRMT5 inhibition triggers functional ATM deficiency and sensitizes pancreatic cancer to CHK1 blockade.

Introduction: PRMT5 inhibitors are under clinical investigation for pancreatic ductal adenocarcinoma (PDAC), but strategies to maximize their therapeutic efficacy remain undefined. Here, we sought to determine whether pharmacologic inhibition of PRMT5 creates therapeutically exploitable vulnerabilities in PDAC. Methods: We employed immunofluorescence, western blotting, and comet assays for molecular analyses, Incucyte live-cell imaging and clonogenic assays for in vitro growth assessment, RNA-seq for transcriptomic profiling followed by RT-qPCR array validation of selected targets, and xenograft models with immunohistochemistry to evaluate in vivo effects. Results and discussion: We report that pharmacologic inhibition of PRMT5 markedly reduces ATM levels across multiple PDAC cell lines using different PRMT5 inhibitors, resulting in a functional ATM-deficient state. This reduction in ATM rewires DNA damage response signaling, increasing PDAC cell reliance 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. Congruently, RNA-seq demonstrated downregulation of cell-cycle and DNA repair genes along with 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 adversely affect body weight. Treated tumors showed reduced ATM and Ki67 with elevated γH2A.X. Together, these findings identify PRMT5 inhibition as a trigger of reduced ATM function that exposes a therapeutically actionable vulnerability to CHK1 inhibition, offering a rational, mechanistically-based combination strategy for this dismal disease.