Depletion of BRD9-mediated R-loop accumulation inhibits leukemia cell growth via transcription-replication conflict.

Bromodomain-containing protein 9 (BRD9) has emerged as a promising therapeutic target for blood cancers, including acute myeloid leukemia, acute lymphoblastic leukemia, and multiple myeloma. PROTAC-based BRD9 degraders effectively hamper the growth and survival of leukemia cells; however, the underlying mechanism of these BRD9 degraders remains unclear. In this study, we demonstrated that depletion of BRD9 triggers DNA damage via R-loop accumulation, leading to conflicts between transcription and replication processes. Replication stress inhibits the proliferation of leukemia cells and promotes their differentiation. Mechanistically, BRD9 plays a pivotal role in recruiting BRD2 and BRD4 to chromatin through direct interactions, which is critical for preventing R-loop formation during transcription. Depletion of BRD9 in leukemia cells reduces the occupancy of BRD2 and BRD4 at R-loop-prone sites, thus promoting R-loop accumulation, transcription-replication collision, excessive DNA damage, and ultimately the demise of cancer cells. These findings provide valuable insights into the mechanisms by which BRD9 degraders function as effective therapies for leukemia mediated by the pathological accumulation of R-loops.