Abstract 5578: Regulation and expression of transposable elements in ovarian cancer

Abstract Background: Transposable Elements (TEs) are highly repetitive DNA sequences that, when transcribed into RNA, bind immunogenic double-stranded RNA sensors. Through this, expression of TEs can induce type-I interferon (IFN) signaling which has shown potency in improving the response of tumors to immune-modulating therapies. We have published the R175H mutant p53 ovarian cancer cell lines activate chronic TE expression compared to wildtype p53 tumor cells. Despite this, activation of durable immune responses are not seen in R175H p53 tumors, indicating unknown tolerance mechanisms that dampen TE-induced IFN. To this end, we sought to characterize how p53 status alters the expression of TEs and influences downstream IFN signaling. Methods: To determine the effect that direct binding of p53 has on TE transcription, we correlated p53-targeted chromatin immunoprecipitation sequencing (chIP-seq) with bulk RNA-sequencing expression. Next, we used phosphorylation status of MAVS/MDA-5 signaling mediators, IRF3/7, to determine disruptions in kinase activity downstream of dsRNA sensing. Results: We found the majority of transcribed TEs did not display direct p53 binding, but rather were transcribed as a biproduct of intragenic insertion within downstream p53 target genes. Additionally, R175H mutant or null p53 bulk RNA sequencing results indicated the downregulation of many TE repressors including KRAB zinc finger proteins (ZNF43, ZNF93, and ZNF561) and DNA Methyltransferase 1 (DNMT1). This suggests TEs lose either DNA methylation and/or KRAB zinc finger silencing in mutant R175H or null p53 backgrounds. The assessment of IRF3/7 indicate R175H p53 cells have reduced phosphorylation levels as compared to wildtype p53 cells. We hypothesize that inactivation of TBK1 by the mutant R175H p53 protein prevents phosphorylation of IRF3 and thus prevents IFN stimulation despite higher TE expression. Discussion: Overall, these data suggest DNA methylation/expression of KRAB zinc finger proteins are indirect mechanisms by which p53 status dysregulates TE transcription. We aim to characterize DNA methylation status of TEs in wildtype, R175H, or null p53 ovarian cancer cells using whole genome bisulfite sequencing. Additionally, we hope to profile the activation of antiviral response mediators using phosphorylated western blot analyses to determine the disruption of effector signaling in these same cell lines. Citation Format: Reddick Russell Walker, Kevin Nestler, Melissa Hadley, Katherine B. Chiappinelli. Regulation and expression of transposable elements in ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5578.