Effect of Heat on Induction and Repair of DNA Strand Breaks in X-irradiated CHO Cells

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Abstract
Chinese hamster ovary cells were exposed to various heat treatments followed by X-irradiation, and the induction and repair of DNA strand breaks was studied using the alkaline unwinding technique. Heat treatments alone were found to cause DNA strand breakage only for temperatures ≥43°C, whereas the number of radiation-induced strand breaks was unaffected by additional heating. Strand break repair was studied for irradiated cells preheated at temperatures ranging from 42°C to 45°C. The total repair curve could be separated into three phases, a fast (t = 0–15 min), an intermediate (t = 15–120 min) and a slow (t ≥ 120 min) phase. All phases were altered when cells were heated either prior to or after irradiation. The fast and the intermediate phase could be well irradiation. The fast and the intermediate phase could be well interpreted by the assumption that irradiation leads to both primary and secondary single-strand breaks, the latter being generated by enzymatic incision at sites of damaged bases. For irradiation alone, the ratio of all secondary strand breaks to all primary breaks was fsec = 1·5 ± 0·5. This ratio was not altered by preceding heat treatments (mean fsec = 1·7 ± 0·2). The main effect of heating on the repair kinetics of single-strand breaks was an increase in the repair half-time of primary and secondary breaks (maximum increase by a factor of 3·4), whereas the generation of secondary breaks was only slightly retarded (factor 1·3). The slow repair phase, which is assumed to represent the repair of DNA double-strand breaks, was best described by a single exponential component. The half-time of this component was found to increase from τslow = 170 ± 70 min for non-heated cells to τslow = 345 ± 80 min for cells heated at 45°C for 20 min, indicating that heat inhibited the repair of double-strand breaks. For irradiation alone, the initial fraction of the slow component was fslow = 0·065 ± 0·004. This fraction was enhanced by additional heating, with a maximum increase by a factor of 2·7 for cells heated at 45°C for 20 min. This elevation cannot be the result of an enhanced induction of double-strand breaks, but must be associated with an additional formation of slowly repaired strand breaks during repair incubation. These additional strand breaks must arise from stand breaks which in non-heated cells are repaired during the fast or intermediate phase. These findings suggest that thermal radiosensitization results from an impaired repair of double-strand breaks and also from the formation of strand breaks which are slowly repaired with time after irradiation.