Induction and Repair of DNA Strand Breaks in X-irradiated Proliferating and Quiescent CHO Cells

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Abstract
Induction and repair of DNA strand breaks was studied in X-irradiated proliferating and quiescent CHO cells using the alkaline unwinding technique. The results showed that induction of strand breaks is identical for both states of proliferation, whereas repair is different. The decrease in the number of DNA strand breaks with incubation time at 37°C is best described by a sum of three exponential components I, II and III. The half-times of component I were similar (τIp = 1·73 min versus τIq = 1·66 min) whereas strand breaks comprising component II were repaired slightly faster (τIIp = 17·0 min versus τIIq = 14·2 min) and those comprising component III were repaired significantly faster (τIIIp = 218 min versus τIIIq = 113 min) in quiescent as compared with proliferating cells. In contrast, the initial fractions, f, of the three components were closely similar for both states of growth (proliferating cells: fI = 0·69, fII = 0·25, fIII = 0·06; quiescent cells: fI = 0·65, fII = 0·29, fIII = 0·06). Radiosensitivity as assayed by colony formation was found to be lower for quiescent cells than for proliferating cells. By fitting the survival data to the linear-quadratic equation, −ln(S/S0) = αD + βD2, the ratios αp/αq = 1·7 and βp/βq = 1·2 were obtained, which are similar to the ratios of the half-times of components III and II (τIIIp/τIIIq = 1·9; τIIp/τIIq = 1·2). This result indicates that the DNA damage represented by components II and III might be involved in cell killing.

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