Abstract B009: Dual PARP and EHMT1/2 inhibition induces an interferon response and anti-tumor immunity in ovarian cancer

Abstract Ovarian cancer has a high propensity to develop therapy resistance. Epigenetic therapies can sensitize ovarian cancer cells to first-line treatments such as platinum-based chemotherapy and Poly (ADP-ribose) polymerase inhibitors (PARPi). Particularly, the reduction of histone 3 lysine 9 dimethylation (H3K9me2) via a euchromatin histone methyltransferase inhibitor (EHMTi) has been shown to sensitize PARPi-resistant cells to PARPi therapy. Additionally, cell growth defects were observed in PARPi-resistant cells treated with dual PARPi/EHMTi treatment compared to DMSO or single agent treatments. To determine the mechanism by which dual therapy sensitizes cells and reduces cell viability, bulk RNA-sequencing was performed on several PARPi-resistant cell lines treated with DMSO, PARPi, EHMTi, or dual PARPi/EHMTi. Transcriptomic analyses revealed that dual therapy increased interferon (IFN) signaling-related genes compared to DMSO or single agent treatments. Additionally, expression of transposable element (TE) transcripts, which can form double-stranded RNA (dsRNA) and induce an IFN response, was also increased in dual treated cells compared to controls. To determine if the reduction in cell viability is dependent on TEs and IFN signaling (i. e. , viral mimicry), CRISPRi technology was used to silence intracellular sensors of dsRNA, RIGI and MDA5. Cell growth assays of these silenced cells showed that the reduction in cell growth observed with dual treatment was partially dependent on MDA5 expression. Based on the observed increase in IFN signaling and T cell recruiting cytokines, we performed an animal study using an immunogenic, PARPi-resistant model of ovarian cancer to determine if this dual therapy has any anti-tumor effects. Indeed, we observed that single and dual EHMTi therapy significantly reduced tumor burden and that this reduction was partially dependent on CD8 T cells. Lastly, to determine if dual therapy also induces a viral mimicry response in the human tumor microenvironment, we created ex vivo cultures of tumors resected from patients with ovarian cancer. We then treated the ex vivo tissues with DMSO, PARPi, EHMTi, and dual PARPi/EHMTi, and performed bulk RNA-sequencing and multispectral immunohistochemistry. From these analyses, we found that dual therapy upregulated TE transcripts and multiple immune-related pathways, and increased T cell activity in the tumor microenvironment. Altogether, our data suggest that dual therapy activates viral mimicry and promotes anti-tumor immunity in both PARPi-resistant and -sensitive ovarian cancer. Citation Format: Lily Nguyen, Zachary L. Watson, Elizabeth R. Woodruff, Kimberly R. Jordan, Edward Chuong, Benjamin G. Bitler. Dual PARP and EHMT1/2 inhibition induces an interferon response and anti-tumor immunity in ovarian cancer abstract. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84 (5 Suppl₂): Abstract nr B009.