Because hyperthermia enhances X-ray cell killing, the effect of incubation at 41 and 42°C on the repair of DNA in X-irradiated Chinese hamster cells was investigated. It was found that: (1) At 42°C, cells can repair X-ray induced single-strand breaks and do so even more rapidly than at 37°C. (2) Following large X-ray doses (10 krad and above), pronounced DNA degradation is observed when cells are incubated for more than 2 hr at 42°C. (3) Following small X-ray doses, doses which are sufficient to affect the integrity of a DNA complex but not sufficient to induce an easily detectable number of single-strand breaks, incubation at 41°C slows the repair of the complex. At 42°C, the repair of the complex is atypical with some concomitant DNA degradation. Relative to thermally enhanced cell killing, it is suggested that inhibition of repair of sublethal damage may be related to the slower and/or atypical repair of the complex. The enhancement of lethal damage expression could reflect enhanced DNA degradation.

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