Abstract 969: Pharmacologic induction of innate immune signaling via STING directly drives homologous recombination deficiency

Poly (ADP-ribose) polymerase inhibitors (PARPi) are FDA approved in a subset of patients with ovarian cancer or metastatic breast cancers who harbor BRCA gene mutations. Although these mutations, which generate homologous recombination deficiencies (HRD), have been the main predictor to PARPi sensitivity, responses to therapy have not been durable and have failed for the majority of sporadic triple negative breast cancers (TNBC). We previously reported that DNA methyltransferase inhibitor (DNMTi) azacytidine (Aza) improves the efficacy of a new generation of PARPi, Talazoparib (Tal), through increased trapping of cytotoxic PARP-DNA complexes in both BRCA-mutant and -proficient TNBC. These trapped complexes lead to increased and persistent levels of lethal double strand breaks (DSBs), suggesting that DSB repair may also be impaired with this treatment. Indeed, Aza/Tal treatment in BRCA-proficient TNBC cell lines significantly downregulates expression of HR and Fanconi Anemia (FA) genes, notably FANCD2, and decreases HR activity, thus generating HRD. DNMTi have also been established to induce a viral mimicry response which upregulate type I interferon (IFN) signaling and production of inflammatory cytokines. In the present study, we elucidate the link between Aza/Tal facilitated HRD and induction of innate immune and inflammatory related genes, mediated through a STING dependent mechanism, which we term as a pathogen mimicry response (PMR). Gene set enrichment analysis of RNA-Seq data derived from mono- and combination-treated TNBC cell line MDA-MB-231, reveals enrichment of innate immune and cytosolic DNA sensing pathways with significant increases of TNFα/NF-κB and IFNαβ gene sets. Overlap between HRD and immune related signaling was evaluated using the STRING database, which reveals a significant interaction specifically between FA and TNFα/NF-κB and IFNαβ pathway genes. Tal driven cytosolic DNA as well as an Aza augmentation in STING protein expression, emerges as the key node in Aza/Tal induced innate immune signaling. Transient knockdown of STING is able to significantly rescue Aza- and combination-induced repression of FANCD2. Critically, in the presence of FANCD2 rescue facilitated by STING-specific inhibition, Aza-induced HRD was also reversed. These STING perturbation data suggest that Aza-induced HRD is dependent on presence of competent STING signaling. This inverse relationship was validated in both METABRIC TNBC dataset and TCGA data sets including lung adenocarcinoma and lung squamous cell carcinoma, colon adenocarcinoma, and AML, thus suggesting broad applicability of this observed transcriptional program independent of pharmacologic intervention. Induction of PMR to drive HRD suggests that DNMTi-PARPi therapy strategies can expand the therapeutic scope of PARPi to encompass treatment of BRCA-proficient cancers. Citation Format: Lena J. McLaughlin, Aksinija A. Kogan, Stephen B. Baylin, Michael J. Topper, Feyruz V. Rassool. Pharmacologic induction of innate immune signaling via STING directly drives homologous recombination deficiency [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 969.