Abstract B088: Epigenetic reprograming in ATM-deficient pancreatic cancer: targeting EZH2- mediated oncogenic traits

Abstract Despite extensive research, the complex biology of pancreatic ductal adenocarcinoma (PDAC) continues to impede progress in improving its prognosis. Its resistance to conventional therapies, along with diverse mutations, necessitate improved molecular stratification to develop tailored approaches and to improve survival of PDAC patients. Our previous studies on ATM- deficient models revealed that serine/threonine kinase ATM loss induces homologous recombination deficiency (HRD), promoting invasiveness and desmoplastic reactions, while also sensitizing tumors to platinum-based therapies and PARP1 inhibition. Interestingly, our data also identified the histone-methyltransferase EZH2 as a direct ATM substrate to operate as a crucial regulator of ATM-specific oncogenic traits. In detail, murine and human ATM-deficient tumors accumulated EZH2, suggesting that ATM status contributes to rewiring the epigenetic landscape of pancreatic tumors, subsequently supporting oncogenic features. To investigate how EZH2 impacts ATM-deficient PDAC pathogenesis, we generated conditional double ATM and EZH2-knockout in the Kras G12D/+ ; Ptf1a Cre/+ mice (AKC, Atm fl/fl ; LSL-Kras G12D/+ ; Ptf1a Cre/+ , and EAKC, Ezh2 fl/fl ; Atm fl/fl ; LSL-Kras G12D/+ ; Ptf1a Cre/+ ). EZH2 depletion in AKC mice prolonged overall survival and reinstated a differentiated phenotype. Furthermore, PDAC patients with negative ATM and EZH2 expression from the TCGA-PAAD cohort (PanCancer Atlas) also showed significantly extended progression-free survival compared to ATM neg EZH2 pos patients. In line, CRISPR/Cas9-mediated EZH2 depletion in ATM-deficient pancreatic cancer cells affected their epigenetic landscape and significantly upregulated transcriptomic programs associated with cell differentiation, thereby restoring differentiated tumor features ex vivo. We showed that ATM status significantly impacts EZH2 activity through a post-translational pathway, promoting tumor supportive EZH2 activity in pancreatic cancer. ATM kinase facilitates EZH2 ubiquitination and degradation by recruiting the Cullin1 E3-ubiquitin ligase complex. Additionally, combining an EZH2 inhibitor with PARP inhibition enhances cytotoxicity in ATM- deficient HRD PDAC. Overall, our findings demonstrate that ATM deficiency drives a pro- oncogenic mesenchymal differentiation program via the histone-methyltransferase EZH2. We emphasize the importance of stratifying patients based on ATM status before considering pharmacological interference with EZH2, offering a promising approach for targeting ATM- deficient HRD tumors. Citation Format: Elodie Roger, Anna Härle, Lukas Perkhofer, Elisabeth Hessmann, Alexander Kleger, Johann Gout. Epigenetic reprograming in ATM-deficient pancreatic cancer: targeting EZH2- mediated oncogenic traits [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research; 2024 Sep 15-18; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl_2):Abstract nr B088.