AN IONIZING RADIATION-SENSITIVE MUTANT OF CHO CELLS, IRS-20 .3. CHROMOSOME-ABERRATIONS, DNA BREAKS AND MITOTIC DELAY

The ionizing radiation-sensitive mutant of CHO cells, irs-20, showed a defect in the rate and extent of rejoining of gamma-ray-induced DNA d ouble-strand breaks (dsbs) compared with the parental CHO cells as mea sured using pulsed-field gel electrophoresis. Virtually all rejoined i n wild-type cells but some 20-30% failed to rejoin in the mutant cells during a 5-h period after irradiation. An increased level of chromoso me-type aberrations per unit dose was seen in irs-20 cells compared wi th wild-type cells irradiated during the G(1) phase. For the irs-20 ce lls, about half the dose was required to produce the same chromosome-t ype aberration frequency. Chromatid-type aberrations were induced in G (1)-irradiated irs-20 cells at frequencies nearly the same as for chro mosome types. For the parental wild-type CHO 10B2 cells, only chromoso me types were seen. The distribution of aberrations among cells was no t significantly different from Poisson for wild-type cells, but this w as not the case for irs-20 cells where the overdispersion was highly s ignificant. The mutant irs-20 cells displayed a much greater cell cycl e delay per unit dose (about five-fold) in reaching mitosis after irra diation in G(1) than the more radioresistant wild-type parental cells.