Role for RIF1-interacting partner DDX1 in BLM recruitment to DNA double-strand breaks

Human Rap1-interacting factor 1 (RIF1) is an important player in the repair of DNA double strand breaks (DSBs). RIF1 acts downstream of 53BP1, with well-documented roles in class switch recombination in B-cells and inhibition of end resection initiation in BRCA1-defective cells. Here, we report that DEAD Box 1 (DDX1), a RNA helicase also implicated in DSB repair, interacts with RIF1, with co-localization of DDX1 and RIF1 observed throughout interphase. Recruitment of DDX1 to DSBs is dependent on RIF1, with RIF1 depletion abolishing DDX1-mediated facilitation of homologous recombination at DSBs. As previously demonstrated for RIF1, DDX1 is also required for chromatin loading of Bloom syndrome helicase (BLM) to ionizing radiation-induced DSBs, a RIF1-related activity that is independent of 53BP1. We show that DDX1 and RIF1 have different nucleic acid requirements for accumulation at DSBs, with RNA-DNA hybrids required for DDX1 accrual at DSBs, and single-strand RNA required for accumulation of RIF1 at these sites. Our data suggest both convergent and divergent roles for DDX1 and RIF1 in DSB repair, and may help explain why RIF1 depletion does not fully mimic 53BP1 ablation in the restoration of homologous recombination defects in BRCA1-deficient cells.