Abstract IA020: Targeting transposable elements with epigenetic modulators to reverse ovarian cancer immune evasion

Abstract Transposable elements (TEs) comprise the majority of the human genome. In most somatic tissues, TEs are silenced by DNA methylation and other epigenetic modifications. Tumors exhibit changes in DNA methylation including global loss of methylation at regions that are silenced for genome stability, like TEs, and gain of methylation at the promoter regions of tumor suppressor genes. We showed that low doses of DNMT inhibitors (DNMTis) upregulate immune signaling in solid tumor cell lines and patient samples. DNMTis activate type I interferon signaling by reducing methylation and increasing expression of endogenous retroviruses (ERVs), a type of TE, that activate dsRNA sensors. DNMTis plus HDACis (histone deacetylase inhibitors, which increase histone acetylation) increase TEs in a mouse model of ovarian cancer, activating interferon signaling and recruiting CD8+ T cells to kill cancer cells and sensitize to immune checkpoint blockade therapy. We performed a genome-wide analysis of the response to DNMTi, HDACi, and combination HDACi/DNMTi in four ovarian cancer cell lines (RNA-Seq, global methylome profiling, ATAC-Seq, and P53 chIP-Seq). The addition of HDACis to DNMTis augments the upregulation of specific ERVs and the resulting downstream interferon response in human ovarian cancer cell lines. Other TE species, including LINE1 and Alu elements, are increased by DNMTi and HDACi and may contribute to the interferon response. Alu elements, which can directly bind to the dsRNA sensor MDA5, are the most significantly demethylated TEs after DNMTi treatment. Importantly, we find considerable overlap between TEs upregulated by DNMTi in cell lines and TEs upregulated by DNMTi in ovarian cancer patients from a DNMTi clinical trial. We observed that ovarian cancer cell lines with TP53 mutations exhibited significantly fewer upregulated TEs with epigenetic therapy than wild-type TP53 cell lines. This observation was validated using isogenic cell lines; the CRISPR-edited TP53-mutant cell line had significantly higher baseline expression of TEs but upregulated fewer upon epigenetic treatment. p53 bound directly to TE DNA by chIP-Seq analysis and upregulated TEs in wild type but not mutant cell lines. In addition, our RNA-sequencing analysis indicated that A-to-I RNA editing of TEs by ADAR1 was increased after DNMTi treatment. This A-to-I editing makes TE RNA less immunogenic and inhibits the immune response to DNMTi. We find that combining ADAR1 knockdown and DNMTi treatment significantly increases type I interferon signaling, recruitment and activation of host immune cells, and survival in a murine model of ovarian cancer. These data thus give a comprehensive, genome-wide picture of TE chromatin and transcription-related changes in ovarian cancer after epigenetic treatment and implicate novel regulators (p53 and ADAR1) in response to epigenetic therapies for cancer. Citation Format: Katherine B. Chiappinelli. Targeting transposable elements with epigenetic modulators to reverse ovarian cancer immune evasion [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 Suppl_2):Abstract nr IA020.