The cell-killing effect of radiotherapy largely depends on unrepaired DNA double-stranded breaks (DSBs) or lethal chromosome aberrations induced by DSBs. Thus, the capability of DSB repair is critically important for the cancer-cell-killing effect of ionizing radiation. Here, we investigated the involvement of the DNA damage signaling factors ataxia telangiectasia mutated (ATM), ring finger protein 8 (RNF8), and RNF168 in quiescent G0/G1 cells, which are expressed in the majority of cell populations in tumors, after high linear energy transfer (LET) carbon-ion irradiation. Interestingly, ATM inhibition caused a substantial DSB repair defect after high-LET carbon-ion irradiation. Similarly, RNF8 or RNF168 depletion caused a substantial DSB repair defect. ATM inhibition did not exert an additive effect in RNF8-depleted cells, suggesting that ATM and RNF8 function in the same pathway. Importantly, we found that the RNF8 RING mutant showed a similar DSB repair defect, suggesting the requirement of ubiquitin ligase activity in this repair pathway. The RNF8 FHA domain was also required for DSB repair in this axis. Furthermore, the p53-binding protein 1 (53BP1), which is an important downstream factor in RNF8-dependent DSB repair, was also required for this repair. Importantly, either ATM inhibition or RNF8 depletion increased the frequency of chromosomal breaks, but reduced dicentric chromosome formation, demonstrating that ATM/RNF8 is required for the rejoining of DSB ends for the formation of dicentric chromosomes. Finally, we showed that RNF8 depletion augmented radiosensitivity after high-LET carbon-ion irradiation. This study suggests that the inhibition of RNF8 activity or its downstream pathway may augment the efficacy of high-LET carbon-ion therapy.
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