Contributions of DNA double strand break repair pathways to DNA crosslink repair.

DNA crosslink-inducing drugs are widely used in clinical settings for treatment of solid tumors. Double strand breaks (DSBs) that arise during interstrand crosslink (ICL) repair are crucial determinants of the therapeutic response, as they lead to cell death if not repaired. DSBs can be repaired through non-homologous end joining (NHEJ), theta-mediated end joining (TMEJ), and homologous recombination (HR). HR is considered a major pathway for repairing DSBs induced during ICL repair. In this study, we examine the roles of NHEJ, TMEJ, and HR in ICL repair using mouse embryonic stem (mES) cells. We show that DNA-PKcs-deficient mES cells are resistant to the crosslinkers mitomycin C (MMC), cisplatin and carboplatin, contrasting with the increased sensitivity observed in mES cells lacking Rad54. Furthermore, the absence of DNA-PKcs correlates with enhanced HR activity, as evidenced by an increased number of Rad54 foci following MMC treatment. The combined knock-outof DNA-PKcs and Rad54 reduces sensitivity to crosslinkers compared to cells lacking only Rad54, suggesting the involvement of another DSB repair pathway besides HR. We found that TMEJ deficiency can sensitize cells to cisplatin, particularly in those lacking NHEJ and HR repair. This suggests that TMEJ contributes to cell survival following cisplatin treatment. In clinical settings, higher PRKDC expression correlates with poorer survival, while elevated RAD54L and POLQ expression correlates with better survival in cisplatin-treated cervical and head and neck cancers. These findings reflect the opposing roles of NHEJ versus HR and TMEJ in replication-associated DSB repair, as observed in vitro.