Radioresistance of chordoma cells is associated with the ATM/ATR pathway, in which RAD51 serves as an important downstream effector

Surgery followed by radiotherapy is the standard treatment for chordomas, which are a rare but low‑grade type of bone cancer arising from remnants of the embryonic notochord. However, disease recurrence following radiotherapy is common, most likely due to endogenous DNA repair mechanisms that promote cell survival upon radiation strikes. The ataxia telangiectasia mutated/ataxia telangiectasia mutated and Rad3 related (ATM/ATR)‑mediated pathway has a critical role in DNA repair mechanisms; however, it has rarely been investigated in chordomas. In the present study, the expression of signal molecules related to the ATM/ATR pathway in chordoma tissues and adjacent normal tissues were initially examined using immunohistochemistry and western blot analysis. Chordoma U‑CH1 and U‑CH2 cells were subsequently used to investigate cell responses to ionizing radiation and the potential protective actions mediated by the ATM/ATR pathway. Phosphorylated (p)‑ATM, p‑ATR, γ‑H2A histone family, member X (H2AX) and RAD51 were significantly upregulated in chordoma tissues relative to adjacent normal tissues (P<0.05). No significant reductions were observed in the viability of U‑CH1 and U‑CH2 cells following exposure to low‑dose (1 and 2 Gy) radiation. Radiation (1 and 2 Gy) triggered a significant upregulation in p‑ATM, γ‑H2AX and RAD51 expression in U‑CH1 cells (P<0.05), as well as a significant upregulation in p‑ATM, p‑ATR and RAD51 levels in U‑CH2 cells (P<0.05). RAD51 knockdown increased the responses of both U‑CH1 and U‑CH2 cells to 1 Gy radiation, as evidenced by the significantly decreased cell viability and increased apoptosis rate (P<0.05). Collectively, the results of the present study indicated that radioresistance of chordoma cells is associated with the ATM/ATR pathway, in which RAD51 serves as an important downstream effector. Thus, RAD51 presents a promising therapeutic target for improving the outcome of radiotherapy treatment in chordomas.