BET family BRD3 initiates DSB-induced chromatin remodeling with TIP60 to promote R-loop-mediated HR

Mechanisms for genome stability in actively transcribed regions are essential for cellular homeostasis; however, these mechanisms are poorly understood. Herein, we identify the bromodomain and extraterminal domain (BET) family BRD3 as the genome caretaker in actively transcribed chromatin. We identify the protein network between BRD3 and chromatin remodeler TIP60. During transcription, BRD3 localizes to actively transcribed chromatin through its N-terminal bromodomains. Following DNA double-strand breaks (DSBs) at the actively transcribed chromatin, the C-terminal extraterminal (ET) domain of BRD3 recruits CHD4 via its KIKL-like motifs to replace HP1 with the TIP60 (Tat-interactive protein, 60 kDa) complex, promoting H4K16 acetylation and MBTD1 recruitment, which creates chromatin barriers to 53BP1. This process recruits BRCA1 and R-loop-processing factors to promote R-loop-mediated homologous recombination (HR) and suppress 53BP1 and mutagenic non-homologous end-joining. Our study elucidates the mechanism by which BRD3 initiates DSB-induced chromatin remodeling by CHD4 and TIP60 to promote R-loop-mediated HR on actively transcribed chromatin to maintain genome stability.