The Effect of Modulators of Radiation-induced G₂ Arrest on the Repair of Radiation-induced DNA Damage Detectable by Neutral Filter Elution

Summary

It has been suggested that radiation-induced G₂-arrest is an extension of interphase to allow repair of DNA double-strand breaks (dsb) prior to mitosis. Cycloheximide blocks the repair of the lesions which result in radiation-induced G₂ arrest. Caffeine and cordycepin reduce the duration of G₂ arrest. All of these agents should therefore reduce the cell's ability to repair dsb. The influence of cycloheximide (50 µg/ml), caffeine (5 mm) and cordycepin (0·15 mm) on the repair of the damage detectable in DNA by neutral filter elution was determined. Chinese hamster ovary cells (CHO) were irradiated with X-ray doses of 20, 60 and 100 Gy then allowed to repair without drug treatment or in the presence of each drug for intervals up to 6 h. DNA damage repair proceeded in two phases. The fast component of the repair process (t_½ approx. 7 min) was not modified by drug treatment; the slow component (t_½ approx. 170 min) was unaffected by cycloheximide or cordycepin, but appeared to be inhibited by caffeine. It was concluded that: (a) the lesion which results in radiation-induced G₂ arrest is not the lesion which is detectable by neutral filter elution, and (b) the influence of caffeine on dsb repair is specific to caffeine and is not mediated by a reduction in the duration of G₂ arrest.