Abstract B075: Sirtuin 6 histone deacetylase is required for the integrated stress response and resistance to inhibition of transcriptional cyclin dependent kinases in Pancreatic Ductal Adenocarcinoma

Abstract Pancreatic Ductal Adenocarcinoma (PDA) can be characterized by two distinct transcriptional subtypes: Classical and Basal-like. The basal PDA subtype is more aggressive and has the worst overall survival. Previous work has shown that Sirtuin 6 histone deacetylase (SIRT6) acts as a tumor suppressor and cooperates with oncogenic KRAS in GEMM models of PDA. Our study identifies SIRT6 as a biomarker for basal PDA and investigates the underlying sensitivity of basal PDA to inhibition of transcriptional cyclin dependent kinases (CDKs). Through analysis of data from hundreds of human PDA tumors, we identified that low SIRT6 expression correlates with basal PDA while the converse is true for classical PDA. Using many in vitro drug assays, genetic manipulation experiments, and in vivo drug studies in human patient-derived xenografts of PDA, we also discovered that basal PDA is uniquely sensitive to the covalent CDK7/12/13 inhibitor THZ1 as well as other inhibitors of transcriptional CDKs. Importantly, removal of SIRT6 can sensitize SIRT6 high/classical PDA to THZ1. Further investigation identified that SIRT6 low/basal PDA has an inactivated Integrated Stress Response (ISR), which is primarily driven by ATF4. To determine how loss of SIRT6 leads to an inability to activate the ISR, we have integrated use of human PDA cell lines, and organoids as well as murine PDA GEMMs where SIRT6 has been deleted or downregulated. We uncovered that SIRT6 regulates the ISR through control of ATF4 translation, specifically through a 3’ UTR open reading frame (uORF) dependent mechanism. To determine whether ISR activation could protect basal PDA cells from transcriptional CDK inhibition, we manipulated expression of ISR pathway components. We found that activation of the ISR in SIRT6 low/basal PDA reduces sensitivity to THZ1, while inactivation of the ISR sensitizes SIRT6 high/classical PDA to this inhibitor. Interestingly, both basal and classical PDA exhibit increased phosphorylation of eIF2a upon inhibition of transcriptional CDKs, which typically indicates activation of the ISR in response to stress. However, this activation of the ISR remains incomplete since ATF4 translation is not upregulated in basal PDA in the absence of SIRT6. Therefore, we discovered that loss of SIRT6 sensitizes basal PDA to inhibition of transcriptional CDKs through an inability to launch a functional ISR response. Furthermore, we have uncovered an important biomarker which controls a stress-induced transcriptional program that may be exploited with targeted therapies in a particularly aggressive PDA subtype. Citation Format: Jessica Gianopulos, Nithya Kartha, Zachary Schrank, Stephanie Dobersch, Sarah Cavender, Bryan Kynnap, Adrianne Wallace-Povirk, Cynthia Wladyka, Juan Santana, Jaeseung C. Kim, Angela Yu, Caroline Bridgwater, Kathrin Fuchs, Sarah Dysinger, Aaron Lampano, Faiyaz Notta, David Price, Andrew Hsieh, Sunil Hingorani, Sita Kugel. Sirtuin 6 histone deacetylase is required for the integrated stress response and resistance to inhibition of transcriptional cyclin dependent kinases in Pancreatic Ductal Adenocarcinoma [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B075.