Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, 45122 Essen, Germany.
Nucleic Acids Res. 2011 Oct;39(19):8416-29. doi: 10.1093/nar/gkr463. Epub 2011 Jul 10.
In cells exposed to ionizing radiation (IR), double-strand breaks (DSBs) form within clustered-damage sites from lesions disrupting the DNA sugar-phosphate backbone. It is commonly assumed that these DSBs form promptly and are immediately detected and processed by the cellular DNA damage response (DDR) apparatus. This assumption is questioned by the observation that after irradiation of naked DNA, a fraction of DSBs forms minutes to hours after exposure as a result of temperature dependent, chemical processing of labile sugar lesions. Excess DSBs also form when IR-exposed cells are processed at 50°C, but have been hitherto considered method-related artifact. Thus, it remains unknown whether DSBs actually develop in cells after IR exposure from chemically labile damage. Here, we show that irradiation of 'naked' or chromatin-organized mammalian DNA produces lesions, which evolve to DSBs and add to those promptly induced, after 8-24 h in vitro incubation at 37°C or 50°C. The conversion is more efficient in chromatin-associated DNA, completed within 1 h in cells and delayed in a reducing environment. We conclude that IR generates sugar lesions within clustered-damage sites contributing to DSB formation only after chemical processing, which occurs efficiently at 37°C. This subset of delayed DSBs may challenge DDR, may affect the perceived repair kinetics and requires further characterization.
在暴露于电离辐射（IR）的细胞中，双链断裂（DSB）会在破坏 DNA 糖磷酸主链的损伤部位形成簇状损伤。人们通常认为这些 DSB 会迅速形成，并立即被细胞 DNA 损伤反应（DDR）机制检测和处理。然而，裸露 DNA 经照射后，有一部分 DSB 会在暴露数分钟到数小时后形成，这是由于温度依赖性的、对不稳定糖损伤的化学处理所致，这一观察结果对上述假设提出了质疑。当 IR 暴露的细胞在 50°C 下进行处理时，也会形成过多的 DSB，但迄今为止，这些 DSB 被认为是与方法相关的人工产物。因此，目前尚不清楚 IR 暴露后细胞中是否会因化学不稳定损伤而实际形成 DSB。在这里，我们表明，对“裸露”或染色质组织化的哺乳动物 DNA 进行照射会产生损伤，这些损伤会在 37°C 或 50°C 体外孵育 8-24 小时后，演变为 DSB，并与迅速诱导的 DSB 一起增加。在染色质相关 DNA 中，这种转化更为有效，在细胞中 1 小时内完成，在还原环境中则会延迟。我们得出结论，IR 会在簇状损伤部位内产生糖损伤，只有在化学处理后才会导致 DSB 形成，而这种化学处理在 37°C 时效率更高。这部分迟发性 DSB 可能会对 DDR 构成挑战，可能会影响到人们对修复动力学的感知，因此需要进一步研究。
