Cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6: VII. Complementation analysis of X-irradiated wild-type CHO-K1 and xrs mutant cells using the premature chromosome condensation technique

Abstract

The complementation effect of wild-type CHO-K1 and xrs mutants after fusion, as judged by the frequencies of X-ray-induced G 1 and G 2 premature chromosome condensation (PCC), was studied. For induction of PCC, X-irradiated interphase cells (G 1 and G 2) were fused immediately with untreated mitotic cells of the same cell line or with mitotic cells of another line. The frequencies of breaks in G 1-PCC, or breals and chromatid exchanges in G 2-PCC were determined and the latter parameter was compared with the frequency of chromosomal aberrations in mitotic cells following G 2 irradiation. CHO-K1 cells were capable of complementing the X-ray sensitivity of both xrs 5 and xrs 6 cells. However, full restoration of the repair defect in xrs cells could never be accomplished. The mutants failed to complement each other. In CHO-K1 cells, the incidence of chromosomal aberrations was significantly higher in G 2-PCC (2.5-fold) than that observed in mitotic cells at 2.5 h after irradiation. The ratio of the induced frequency of aberrations in G 2-PCC to that in mitotic cells was correlated with the degree of repair of DNA double-strand breaks (dsb) and reached almost 1 in xrs 5 cells indicating no repair. In addition the data indicated that, during the period of recovery of CHO-K1 cells, X-ray-induced breaks decreased but exchanges remained at the same level. In contrast, due to a deficiency in rejoining of dsb in xrs mutants, breaks remained open for a longer period of time, allowing the formation of additional chromatid exchanges during recovery time.