Correlation between thermal radiosensitization and slowly rejoined DNA strand breaks in CHO cells.

The effect on the repair of slowly rejoined strand breaks was studied in CHO cells using the alkaline unwinding technique. Heat (45 degrees C, 20 min) combined with a X-ray dose of 9 Gy was found to result in an increased half-time of repair but also in an increased number of slowly rejoined strand breaks. When a time interval at 37 degrees C was inserted between irradiation and heat, the half-time of repair was not altered, whereas the number of slowly rejoined strand breaks as measured 300 min after irradiation decreased with increasing time interval between the two treatments. The half-time of 18 +/- 2 min suggested that the additionally formed, slowly rejoined strand breaks arise from a certain type of radiation-induced DNA base lesions with repair of which is modified by heat. The effect of X-irradiation combined with heat was also studied for cell survival. When irradiation and heat were separated by an incubation at 37 degrees C, cell survival increased with a half-time of 20 +/- 2 min, which is similar to that measured for the number of additional, slowly rejoined strand breaks. For a great variety of combined treatments, the reduction in cell survival correlates well with the enhanced number of slowly rejoined strand breaks measured 300 min after irradiation. This positive correlation and the similarity in the half-times mentioned above suggests that thermal radiosensitization results from the number of additional, slowly rejoined strand breaks formed when irradiation was combined with heat.