FBXW7 Promotes Nonhomologous End-Joining Repair and Radioresistance

DNA double-strand breaks (DSB), which can cause genomic instability and tumorigenesis if uncorrected, are primarily repaired in eukaryotic cells by one of two mechanisms, homologous recombination (HR) or nonhomologous end-joining (NHEJ). Loss-of-function mutations of the tumor suppressor F-box and WD repeat domain containing 7 (FBXW7), which encodes the substrate recognition component of the SKP1–Cullin1–F-box protein (SCF^FBXW7) E3 ubiquitin ligase, result in increased genomic instability by an unknown mechanism. To elucidate the role of FBXW7 in DSB repair, Zhang and colleagues analyzed the effects of ionizing radiation (IR), the major exogenous cause of DSBs, on FBXW7. IR resulted in the rapid recruitment of FBXW7 to DNA damage sites after phosphorylation of the serine 26 (S26) residue on FBXW7 by ATM. Depletion of FBXW7 in both cancer and normal cells had no effect on HR but significantly inhibited NHEJ and increased radiosensitization. Immunoprecipitation analyses revealed that FBXW7 bound to X-ray repair cross-complementing protein 4 (XRCC4), a component of the complex that mediates NHEJ, and that this interaction was dependent on S325 and S326 within the putative FBXW7 binding motif on XRCC4. Moreover, FBXW7 promoted lysine 63 (K63)–linked polyubiquitylation of XRCC4 after IR, leading to enhanced interaction of XRCC4 with the Ku70/80 heterodimer to facilitate NHEJ. Inactivation of DNA-dependent protein kinase catalytic subunit (DNA-PKcs), which is known to phosphorylate S325 and S326 on XRCC4, or ATM ablated FBXW7-mediated polyubiquitylation of XRCC4. K296 was found to be the ubiquitylated lysine residue on XRCC4, and ubiquitylation at K296 increased chromatin binding of XRCC4, NHEJ repair, and radioresistance. Pretreatment with MLN4924, a small-molecule inhibitor of SCF^FBXW7 currently in clinical trials, ablated FBXW7-driven polyubiquitylation of XRCC4 and inhibited NHEJ in response to IR. Together, these results show that FBXW7 promotes NHEJ repair of DSBs and cancer cell survival following radiation, and identify FBXW7 as a potential therapeutic target to enhance tumor radiosensitivity.

Zhang Q, Karnak D, Tan M, Lawrence TS, Morgan MA, and Sun Y. FBXW7 facilitates nonhomologous end-joining via K63-linked polyubiquitylation of XRCC4. Mol Cell 2016;61:419–33.

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