The p53-binding protein 1-Tudor-interacting repair regulator complex participates in the DNA damage response

The 53BP1-dependent end-joining pathway plays a critical role in double strand break repair and is uniquely responsible for cellular sensitivity to poly(ADP-ribose) polymerase inhibitors (PARPi) in BRCA1-deficient cancers. We and others have investigated the downstream effectors of 53BP1, including replication timing regulatory factor 1 (RIF1) and Pax transactivation domain-interacting protein (PTIP), in the past few years to elucidate how loss of the 53BP1-dependent repair pathway results in PARPi resistance in BRCA1 patients. However, questions regarding the upstream regulation of the 53BP1 pathway remain unanswered. In this study, we identified the Tudor-interacting repair regulator (TIRR) that specifically associates with the ionizing radiation-induced foci formation region of 53BP1. 53BP1 and TIRR form a stable complex, which is required for their expression. Moreover, the 53BP1-TIRR complex dissociates after DNA damage, and this dissociation may be ataxia telangiectasia mutated-dependent. Similar to 53BP1, loss of TIRR restores PARPi resistance in BRCA1-deficient cells. Collectively, our data identified a novel 53BP1-TIRR complex in DNA damage response. TIRR may play both positive and negative roles in 53BP1 regulation. On the one hand, it stabilizes 53BP1 and thus positively regulates 53BP1. On the other hand, its association with 53BP1 prevents 53BP1 localization to sites of DNA damage, and thus TIRR is also an inhibitor of 53BP1.