Abstract B21: Brd4 modulates S-phase checkpoint response and sensitizes ovarian cancer cells to ATR inhibition

The BET (bromodomain and extra terminal domain) family proteins function as chromatin readers through interactions with acetylated lysine residues on both histones and transcription factors. BRD4 is the most well-studied member of this group that promotes tumorigenesis by regulating expression of core transcriptional programs; however, the role of Brd4 in other cellular processes is not entirely clear. Here, we show using small molecule inhibition of the BET proteins that Brd4 is required to maintain S-phase checkpoint signaling. Treatment of high-grade serous ovarian and osteosarcoma cancer cell lines with BET inhibitors, AZD5153 and JQ1, results in the accumulation of gH2AX and 53BP1 foci formation, which is suggestive of elevated DNA damage levels. The induction of DNA damage caused by both inhibitors corresponds with cell cycle arrest at the G1/S boundary. Displacement of Brd4 in cell lines under intrinsic or exogenous replication stress led to a time-dependent reduction in phospho-Chk1 first detected within 30 minutes of AZD5153 treatment in U2OS cells. The decrease in Chk1 phosphorylation was observed without a concomitant decrease in total Chk1, and this was not replicated by treatment with pan-transcriptional inhibitors, suggesting a non-transcriptional mechanism linked to Brd4. Furthermore, Brd4 interacts with core DNA replication machinery during S-phase and inhibition of Brd4 leads to aberrant DNA replication without decreasing ATR phosphorylation. Consistent with a role in S-phase signaling, Brd4 inhibition sensitizes replicating cells to replication stress-inducing agents. Finally, we observed that ovarian cancer cell lines are highly-sensitive to the combination of Brd4 and ATR (AZD6738) inhibitors. Concurrent treatment with AZD5153 and AZD6738 induces DNA damage and apoptosis in a synergistic manner. Together, these findings highlight a novel function for Brd4 and provide evidence to support further evaluation of Brd4i combinations with DNA damage repair agents. Citation Format: Jingwen Zhang, Huawei Chen, Austin Dulak. Brd4 modulates S-phase checkpoint response and sensitizes ovarian cancer cells to ATR inhibition. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr B21.