Heat effects on the repair of DNA double-strand breaks in CHO cells

The effect of heat (43-45 degrees C) on the induction and the repair of DNA double-strand breaks (dsb) was studied in CHO cells after 60 Gy of X-rays using constant-field gel electrophoresis. Heat given prior to irradiation was found not to alter the number of dsb when measured immediately after irradiation. In non-heated cells, about 80% of all dsb were rapidly repaired with a half-time of 4 min, while 20% were repaired more slowly with T4 = 160 min. These kinetics were grossly altered by heat. Both the fast and the slow process were retarded. However, the main effect of heat was an increase in the number of slowly rejoined dsb. This increase was shown to result from the additional formation (up to 1.4-fold the initial number) of dsb early during the repair course. It is suggested that the additional dsb arose from base damage, the repair of which was unbalanced by heat. No evidence was found for apoptosis being involved in this process. The kinetics of the additional dsb was found to correlate with thermal radiosensitization.