Effective spatio-temporal control of transcription and replication during S-phase is paramount to maintaining genomic integrity and cell survival. Dysregulation of these systems can lead to conflicts between the transcription and replication machinery, causing DNA damage and cell death. BRD4 allows efficient transcriptional elongation by stimulating phosphorylation of RNA polymerase II (RNAPII). We report that bromodomain and extra-terminal domain (BET) protein loss of function (LOF) causes RNAPII pausing on the chromatin and DNA damage affecting cells in S-phase. This persistent RNAPII-dependent pausing leads to an accumulation of RNA:DNA hybrids (R-loops) at sites of BRD4 occupancy, leading to transcription-replication conflicts (TRCs), DNA damage, and cell death. Finally, our data show that the BRD4 C-terminal domain, which interacts with P-TEFb, is required to prevent R-loop formation and DNA damage caused by BET protein LOF.

Effective spatio-temporal control of transcription and replication during S-phase is paramount to maintain genomic integrity and cell survival. Deregulation of these systems can lead to conflicts between the transcription and replication machinery leading to DNA damage. BRD4, a BET bromodomain protein and known transcriptional regulator, interacts with P-TEFb to ensure efficient transcriptional elongation by stimulating phosphorylation of RNA Polymerase II (RNAPII). Here we report that disruption of BET bromodomain protein function causes DNA damage that correlates with RNAPII-dependent transcript elongation and occurs preferentially in S-phase cells.BET bromodomain inhibition also causes accumulation of RNA:DNA hybrids (R-loops), which are known to lead to transcription-replication conflicts, DNA damage, and cell death. Furthermore, we show that resolution of R-loops abrogates BET-bromodomain inhibitor-induced DNA damage, and that BET-bromodomain inhibition induces both Rloops and DNA damage at sites of BRD4 occupancy. Finally, we see that the BRD4 Cterminal domain, which interacts with P-TEFb, is required to prevent R-loop formation and DNA damage caused by BET bromodomain inhibition. Together, these findings demonstrate that BET bromodomain inhibitors can damage DNA via induction of Rloops in highly replicative cells.

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