BRD4 Is an Endogenous Inhibitor of the DNA Damage Response

The DNA damage response (DDR) must be initiated in the context of chromatin, but the role of chromatin structure in regulating the DDR is incompletely understood. To identify proteins that link chromatin modulation to the DDR, Floyd and colleagues coupled a focused RNA interference screen of genes that encode chromatin modifiers with a high-throughput multiplexed microscopy-based assay for indicators of early DDR signaling, cell-cycle progression, mitosis, and apoptosis. Knockdown of BRD4 isoform B, a splice variant of a bromodomain-containing protein implicated in transcriptional regulation, significantly increased the number, size, and intensity of phosphorylated histone H2AX (γH2AX) foci after ionizing radiation. BRD4 bromodomain mutations that abrogated acetyl-lysine binding had the same effect as BRD4 isoform B knockdown, indicating that BRD4 inhibits the DDR by binding to acetylated histones. BRD4 isoform B knockdown did not induce significant changes in DDR gene expression or affect the generation or repair of DNA double-strand breaks, but the DNA of BRD4-deficient cells was more susceptible to micrococcal nuclease digestion, suggesting that BRD4 normally promotes a closed chromatin structure that suppresses H2AX phosphorylation and attenuates DDR signaling. Consistent with this model, upon irradiation, BRD4 associated more tightly with chromatin and bound components of the condensin II complex, which has known roles in chromatin compaction. Importantly, knockdown or inhibition of BRD4 isoform B with the small-molecule inhibitor JQ1 promoted cancer cell survival following ionizing radiation in association with increased H2AX phosphorylation and more rapid and efficient recovery from DNA damage checkpoints, raising the possibility that induction of the DDR by BRD4 inhibitors might promote radioresistance. Together, these findings reveal a transcription-independent role for BRD4 in the DDR and may have implications for the clinical use of BRD4 inhibitors, particularly in conjunction with radiotherapy.

Floyd SR, Pacold ME, Huang Q, Clarke SM, Lam FC, Cannell IG, et al. The bromodomain protein Brd4 insulates chromatin from DNA damage signalling. Nature 2013;498:246–50.

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