Abstract B030: PARP-1 inhibition results in ovarian and endometrial cancer cell death by disrupting the nucleolar localization of the RNA helicase DDX21 which is associated with decreased survival outcomes

Abstract Cancer cells with DNA repair defects (e. g. , BRCA1/2 mutant cells) are vulnerable to PARP inhibitors (PARPi) due to induction of synthetic lethality. However, recent clinical evidence has shown that PARPi can prevent the growth of some cancers irrespective of their BRCA1/2 status, suggesting alternative mechanisms of action. One such mechanism involves DDX21, an RNA helicase that localizes to the nucleoli of cells. When ADP-ribosylated by PARP-1, DDX21 promotes rDNA transcription leading to ribosome biogenesis, protein translation, and ultimately the growth of breast cancer cells. Validating this pathway in other tumor types may lead to the expanded use of PARPi as a cancer therapeutic and the potential use of DDX21 nucleolar localization as a biomarker of clinical response to PARPi. For our first aim, we hypothesized that PARPi also blocks the DDX21/PARP-1/ribosome biogenesis pathway in ovarian and endometrial cancers and can thus be used to inhibit their growth. To test this, we are determining the effect of Niraparib, an inhibitor targeting PARP-1 activity, on DDX21 subcellular localization, ribosome biogenesis, and cell growth in different ovarian (OVCAR3, OVCAR4, and HCC5012) and endometrial (KLE, Ishikawa, and HEC-1-A) cancer cell lines. We are using Western blotting to assess the levels of DDX21 and ADP-ribosylation, and immunofluorescent staining to assess the localization of DDX21 to nucleoli before and after treatment with Niraparib. We are also using qPCR to determine the levels of rRNAs and Puromycin incorporation assays to monitor ribosome translation activity before and after treatment with Niraparib. Moreover, we are using cell growth assays to determine the effect of Niraparib on the proliferation of the ovarian and endometrial cancer cell lines. Our cell-based results indicate that the nucleolar localization of DDX21 is disrupted by Niraparib treatment, leading to a pan-nuclear distribution. Furthermore, Niraparib treatment in ovarian cancer cell lines causes decreased rRNA transcription, decreased protein translation, and decreased cell proliferation. For our second aim, we are assessing possible associations among DDX21 nucleolar localization, PARP-1 protein levels, and clinical outcomes in cancer samples from patients using immunohistochemical (IHC) staining for DDX21 and PARP-1. Analysis of endometrial cancer patient samples suggests a significant association among high DDX21 nucleolar localization, high PARP-1 expression, and decreased survival. Overall, our results suggest that Niraparib works, in part, by blocking the DDX21/PARP-1/ribosome biogenesis pathway leading to endometrial and ovarian cancer cell death. The potential use of DDX21 localization as a biomarker of response to PARPi and as a prognostic factor will be further explored by conducting additional laboratory and clinical studies. Citation Format: Marwa W. Aljardali, Kevin M. Kremer, Jessica Parker, Jayanthi S. Lea, W. Lee Kraus, Cristel V. Camacho. PARP-1 inhibition results in ovarian and endometrial cancer cell death by disrupting the nucleolar localization of the RNA helicase DDX21 which is associated with decreased survival outcomes 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 B030.