Abstract B006: P53 transcriptional regulation of transposable elements and the downstream immune response in ovarian cancer
Authors: Walker Reddick R. , Nestler Kevin , Hadley Melissa , Diab Noor , Chiappinelli Katherine 
Published: March 2024

Abstract
Ovarian cancers (OC) are characterized by an immunosuppressive tumor microenvironment and frequent mutations in the TP53 gene, which encodes for the p53 tumor suppressor protein. Low infiltration of tumor-fighting immune cells in these tumors contribute to ovarian cancer’s static 5-year survival rate and poor response to immunotherapy treatment. We and others have shown that using therapies that inhibit DNA methylation (DNMTis) can facilitate immune cell recruitment and activation to fight ovarian cancers. This response is mediated by interferon signaling that is initiated by the derepression of highly repetitive DNA elements called transposable elements (TEs). TEs are stretches of mobile noncoding DNA that transcribe virus-like double stranded RNA (dsRNA) that are recognized by cytosolic sensors and activate the type I interferon pathway. Downstream of type I interferon, tumors treated with DNMTis show increased recruitment of effector T cells, NK cells, and proinflammatory macrophages necessary for tumor-cell killing. Relevant to ovarian cancer, our lab has recently shown that baseline TE expression is altered by the mutational status of the TP53 gene. We showed the baseline expression of TEs is higher in OC cell lines with the p53 DNA binding domain mutation R175H. In addition, isogenic wildtype and R175H mutant p53 CRISPR modified OC cell lines recapitulate these differences in TE transcription. We are now characterizing the direct and indirect effects p53 has on TEs and downstream immune signaling. Western blot analysis of mutant and wildtype p53 OC cell lines show differences in baseline DNMT1 expression, the enzyme responsible for methylating cytosine bases on DNA known to be regulated by p53. Additionally, treatment with a p53 stabilizing drug, Nutlin-3a, shows wildtype p53 decreases DNMT1 expression while mutant p53 does not. The greater DNMT1 expression in the mutant p53 cell lines indicates that DNA methylation is not the driving regulator to TE expression, which we hypothesized was being blocked by increased p53 binding at TE loci. We sought to determine how p53 binds directly to TE loci using Cleavage Under Targets and Release Using Nuclease sequencing (CUT&RUN-seq), a method that precipitates protein-DNA complexes via an antibody guided nuclease system. Our data show that mutant p53 has increased occupancy of both the DNMT1 gene and specific TE classes like HERVH that we previously identified having a higher baseline expression in mutant OC cell lines. These new data support the conclusion that p53 binding activates the transcription of TEs and that “hotspot” mutant p53 creates an altered TE landscape. Future experiments will characterize this epigenetic regulation of TEs in a larger panel of mutant p53 cell lines and study the downstream immune signaling cascade of mutant versus wildtype p53 tumors treated with epigenetic therapies.
Citation Format: Reddick R. Walker, Kevin Nestler, Melissa Hadley, Noor Diab, Katherine Chiappinelli. P53 transcriptional regulation of transposable elements and the downstream immune response 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 Suppl_2):Abstract nr B006.