Cytological characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6: I. Induction of chromosomal aberrations by X-irradiation and its modulation with 3-aminobenzamide and caffeine

Abstract

We have studied two X-ray-sensitive mutants xrs 5 and xrs 6 (derived from the CHO-K1 cell line), known to be defective in repair of double-strand breaks, for cell killing and frequency of the chromosomal aberrations induced by X-irradiation. The survival experiments showed that mutants are very sensitive to X-rays, the D 0, for the wild-type CHO-K1 was 6-fold higher than D 0 value for the mutants. The modal number of chromosomes (2 n = 23) and the frequency of spontaneously occurring chromosomal aberrations were similar in all 3 cell lines. X-Irradiation of synchronized mutant cells in G 1-phase significantly induced both chromosome- and chromatid-type of aberrations. The frequency of aberrations in xrs mutants was 12-fold more than in the wild-type CHO-K1 cells. X-Irradiation of G 2-phase cells also yielded higher frequency of aberrations in the mutants, namely 7–8 fold in xrs 5 and about 3.5-fold in xrs 6 compared to the wild-type CHO-K1 cells. There was a good correlation between relative inability to repair of DNA double-strand breaks and induction of aberrations. The effect of 3-aminobenzamide (3AB), an inhibitor of poly(ADP-ribose) synthetase on the frequency of X-ray-induced chromosomal aberrations in these 3 cell lines was also studied. 3AB potentiated the frequency of aberrations in G 1 and G 2 in all the cell types. In the mutants, 3AB had a potentiating effect on the frequency of X-ray-induced chromosomal aberrations only at low doses. X-Ray-induced G 2 arrest and its release by caffeine was studied by cytofluorometric methods. The relative speed with which irradiated S-G 2 cells progressed into mitosis in the presence of caffeine was CHO-K1 > xrs 5 > xrs 6. Caffeine could counteract G 2 delay induced by X-rays in CHO-K1 and xrs 5 but not in xrs 6. Large differences in potentiation by caffeine were observed among these cells subjected to X-rays and caffeine post-treatment for different durations. These responses and possible reasons for the increased radiosensitivity or xrs mutants are discussed and compared to ataxia telangiectasia (A - T) cells and a radiosensitive mutant mouse lymphoma cell line.