HTATSF1-Nucleated PARylation and Phosphorylation Cascade Dictates Homologous Recombination and Modulates Chemotherapeutic Response of Breast Carcinoma

Homologous recombination (HR) is a high-fidelity, template-dependent repair pathway of DNA double-strand breaks (DSBs). The efficiency of HR system is tightly associated with tumor response to chemotherapy. Yet, how HR activity is regulated remains to be investigated. Here, we report that the RNA binding protein HTATSF1 is physically associated with Topoisomerase II-binding protein 1 (TOPBP1) in a cell-cycle and phosphorylation dependent manner. In specific, S-phase enriched HTATSF1 is phosphorylated by CK2 kinase at Ser748 and the phosphorylation mark is read by the N-terminal BRCT domains of TOPBP1. Importantly, the RNA recognition motifs (RRMs) of HTATSF1 interact with poly(ADP-ribosyl)ated RPA, and this allows the engagement of HTATSF1-TOPBP1 complex to post-resected DSB ends and facilitates the exchange of RPA and RAD51 to favor HR repair of DSBs. Furthermore, we showed CK2-HTATSF1-TOPBP1 axis is generally hyperactivated in multiple malignancies and imparts chemo-insensitivity of breast tumor, while loss-of-function mutation of HTATSF1 in clinical tumors predicts higher HR deficiency (HRD) scores and elicits vulnerabilities of tumor cells to PARP inhibitors (PARPis) or platinum drug. Together, our study reveals an S-phase specific HTATSF1-nucleated PARylation and phosphorylation cascade that directs the HR-promoting activity of TOPBP1, and suggests that the integrity of CK2-HTATSF1-TOPBP1 axis may serve as an indicator of tumor HR status and modulate chemotherapeutic response .