Abstract 3340: Small molecule mediated stabilization of PP2A modulates the homologous recombination pathway and potentiates DNA damage-induced cell death

High Grade Serous Carcinoma (HGSC) is the most lethal ovarian cancer subtype and accounts for approximately 60% of all ovarian tumors. Despite recent advances in drug development and increased understanding of genetic alterations that drive HGSC progression, mortality has not declined, highlighting the need for novel therapies. PARP inhibitors (PARPi) have become the mainstay of HGSC targeted therapy research given that these tumors are driven by a high degree of genomic instability resulting from the combination of fast DNA replication rates and numerous defects in the DNA-damage response (DDR) pathway. Nonetheless, only ~25% of these patients initially respond to treatment and a significant percentage eventually relapses with resistant disease. Here, we discovered that a Small Molecule Activator of Protein Phosphatase 2A (PP2A) (SMAP-061) induces apoptosis in both established and patient-derived HGSC cell lines as well as in genetically distinct Patient-Derived Xenograft (PDX) mouse models. Interestingly, we also uncovered that several genes that make-up the heterotrimer PP2A tumor suppressor protein are heterozygously lost in more than 95% of HGSC tumors, second only to p53. Mechanistically, we show that stabilization of PP2A protein by SMAP-061 inhibits the Homologous Recombination (HR) pathway via the direct inhibition of RAD51, ultimately leading to chronic accumulation of DNA damage and thus programmed cell death. Furthermore, we found that SMAP-061’s ability to inhibit HR potentiated the effects of PARP inhibition and resulted in synergistic cell death in both HR proficient and deficient models. These studies emphasize the potential of PP2A activators to expand the patient population that can benefit from PARPi therapies and possibly overcome PARPi resistance. In sum, our data highlights a new role of PP2A in regulating the DDR pathway in HGSC and supports the use of SMAPs in both HR proficient and deficient HGSC tumors. Citation Format: Rita A. Avelar, Amy Armstrong, Goutham Narla, Analisa DiFeo. Small molecule mediated stabilization of PP2A modulates the homologous recombination pathway and potentiates DNA damage-induced cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3340.