The Effect of Modulators of Radiation-induced G2Arrest on the Repair of Radiation-induced DNA Damage Detectable by Neutral Filter Elution

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Abstract
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 (t½ approx. 7 min) was not modified by drug treatment; the slow component (t½ 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.