RPRM (Reprimo) triggers SCF(FBXW11)-mediated DNA-PKcs degradation to block non-homologous end joining and radiosensitize tumors

Targeting DNA damage repair pathways represents a promising strategy in cancer radiotherapy, however, the limited insight into the regulation of the non-homologous end joining (NHEJ) repair pathway in cancer cells severely restricts the development of precise radiosensitization approaches. Here, we identify Reprimo (RPRM), a p53-inducible tumor suppressor, is a potent inhibitor of NHEJ in which RPRM promotes proteasomal degradation of DNA-dependent protein kinase catalytic subunit (DNA-PKcs) phosphorylated at the T2609 site. Upon irradiation, RPRM quickly translocates into the nucleus to enhance the transcription of FBXW11, a F-box protein gene that encodes the substrate recognition component of SCF (SKP1-CUL1-F-box protein) E3 ubiquitin-protein ligase complex. In the nucleus, RPRM is also able to scaffold the assembly of the SCFFBXW11 E3 ligase complex by binding to both CUL1 and FBXW11. This complex can specifically ubiquitylate phosphorylated DNA-PKcs (T2609), leading to reduced levels of both phosphorylated DNA-PKcs (T2609) and total DNA-PKcs, thereby suppressing NHEJ-driven repair and enhancing tumor radiosensitivity. In consistence, inhibiting the SCFFBXW11 E3 ligase complex eliminates RPRM-mediated radiosensitization by protecting DNA-PKcs. Clinically, high RPRM expression in non-small cell lung cancer (NSCLC) predicts better patient prognosis. Moreover, RPRM expression correlates positively with FBXW11 and PRKDC (DNA-PKcs) in the tumor tissues of the NSCLC patient cohort. Significantly, high expression of RPRM protein is linked to enhanced radiosensitivity of patient-derived organoids (PDOs) from these NSCLC patients. These results establish RPRM as a key negative regulator in NHEJ repair pathway and the RPRM-SCFFBXW11-DNA-PKcs axis as a potential target to enhance cancer radiosensitivity.