Abstract LB542: Overexpression of the short isoform of Brd4 in ovarian carcinoma leads to highly aggressive tumor phenotype

Abstract High-grade serous ovarian carcinoma (HGSOC) is one of the deadliest types of cancer in the US. Occurrence of sporadic HGSOC is associated with high genomic instability, which causes amplification and deletion of several different genes. BRD4 is one of the most frequently amplified genes in ovarian carcinoma patients. BRD4 amplification leads to overexpression of BRD4 proteins including long and short BRD4 isoforms (BRD4-L and BRD4-S, respectively), and is associated with poor patient prognosis. Recently, it has been shown that the short isoform BRD4-S contributes to oncogenic properties of several tumor types; however, little is known about its role in ovarian cancer. Therefore, in order to interrogate the function of different BRD4 isoforms in HGSOC, we generated ovarian cancer cell lines stably overexpressing BRD4 isoforms (BRD4-L and BRD-S). Our data revealed that overexpression of BRD4-S in ovarian carcinoma cell lines induces a highly aggressive tumor phenotype in vitro and in vivo. Ovcar4-BRD4-S cells showed significantly higher cell proliferation rate and increased anchorage-independent growth when compared with parental cell lines. In addition, Ovcar4-BRD4-S presents a high level of DNA damage reflected by increased pH2AX foci. We also observed a disruption of cell cycle in Ovcar4-BRD4-S cells associated with a higher number of cells arrested in G2/M phase and increased population of polyploid cells. In vivo experiments demonstrated that BRD4-S expressing tumors grow significantly faster than parental tumors. BRD4-S overexpressing tumors are also very resistant to DNA-damage agents including standard of care platinum drugs. Finally, our preliminary drug screening demonstrated that Ovcar4-BRD4-S cells are highly sensitive to Wee1 inhibitor MK-1775. Furthermore, the combination of MK-1775 with paclitaxel synergistically increased the antitumor effect. In summary, our studies indicate that amplification of the BRD4 gene in HGSOC patients might lead to an increase in splicing of BRD4-S transcript, which in consequence promotes highly aggressive and platinum-resistant tumors. Precision therapies targeting BRD4 amplified tumors, such as Wee1 inhibitors in combination with paclitaxel hold great promise to improve ovarian cancer treatment and reduce chemotherapy resistance, which is a critical problem for eradication of this aggressive tumor. Citation Format: Ana Luiza Drumond-Bock, Luyao Wang, Lin Wang, Magdalena Cybula, Maria Rostworowska, Magdalena Bieniasz. Overexpression of the short isoform of Brd4 in ovarian carcinoma leads to highly aggressive tumor phenotype [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 LB542.