Abstract 1168: ADAR1 synthetic lethality in <i>BRCA1/2</i>-mutant tumors defines autocrine interferon poisoning as a targetable vulnerability of cancer cells

Abstract Background: ADAR1 is a double-stranded RNA editing enzyme that prevents autoimmunity against endogenous transcripts by blocking interferon responses triggered by cytosolic RNA sensors. ADAR1 has been proposed as a therapeutic target in immuno-oncology, but as yet, clinically-actionable predictive biomarkers for ADAR1 inhibition sensitivity are lacking. We used functional genomics to uncover and characterize a novel, mechanism-based genetic vulnerability of cancer cells which confers sensitivity to ADAR1 inhibition. Methods: A focused RNAi screen was conducted in BRCA1-isogenic TNBC cells to identify novel synthetic lethal effects among cancer-relevant pattern recognition receptors - including ADAR1. CRISPR-Cas9 and RNAi gene perturbation were used to validate these effects in a large panel of in vitro and in vivo models of BRCA1/2 deficiency comprising 13 human, mouse and zebrafish isogenic and non-isogenic systems. Mechanistic dissection was performed using functional genomics, imaging, DRIP-Seq, RNA-Seq, and genetic or pharmacological perturbation assays to evaluate notably markers of the DNA damage response, R-loop metabolism, and cell-autonomous innate immune signaling. Patient-derived tumors from a cohort of 63 BRCA1-selected TNBC cases, from published independent series of TNBC cases, and from BRCA2-isogenic prostate cancer PDXs, were further analyzed to assess ADAR1 activity using dedicated immunohistochemistry and bioinformatic RNA editing analyses. Results: We discovered that the loss of ADAR1 function confers profound synthetic lethality to BRCA1/2-mutant cancer cells, an effect which operates across cell types, cancer types, and species. Mechanistically, these effects are partly due to DNA damage caused by defective R-loop resolution upon ADAR1 loss and, strikingly, are fully abrogated by transcriptional silencing of RNA sensors or pharmacological inhibition of tumor cell-autonomous interferon responses. This implies autocrine interferon poisoning as the primary cause of BRCA1/2-dependent ADAR1 synthetic lethality, making this interaction distinct from all of the previously-described BRCA1/2-associated dependencies. We further show that BRCA1/2-mutant human tumors and PDXs exhibit increased RNA editing activity, supporting the biological relevance of our findings and suggesting an increased reliance of BRCA1/2-mutant cancers upon ADAR1 gene function. Conclusion: We uncover a penetrant synthetic lethality between ADAR1 and BRCA1/2 which primarily operates through autocrine interferon poisoning. While our work provides a preclinical rationale for assessing ADAR1-targeting agents in BRCA1/2-mutant cancers, this also represents the first report of a novel approach to synthetic lethal treatments, which exploits tumor cell-intrinsic cytosolic immunity as a targetable vulnerability in cancer. Citation Format: Roman M. Chabanon, Liudmila Shcherbakova, Magali Lacroix-Triki, Marine Aglave, Marlène Garrido, Jean Zeghondy, Victor Kriaa, Antoine Gougé, Elodie Edmond, Andrew N. Tutt, Fabrice Barlesi, Yohann Loriot, Suzette Delaloge, Marcel Tawk, Cindy Degerny, Yea-Lih Lin, Barbara Pistilli, Philippe Pasero, Christopher J. Lord, Sophie Postel-Vinay. ADAR1 synthetic lethality in BRCA1/2-mutant tumors defines autocrine interferon poisoning as a targetable vulnerability of cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1168.