Analysis of bromodeoxyuridine-induced single and twin sister chromatid exchanges in tetraploid Chinese hamster ovary cells

Chromosome terminal deletion formation in Chinese hamster ovary cells

To investigate the fate of unrejoined DNA double-strand breaks, the frequency of 60Co gamma-ray- and restriction-enzyme-induced terminal chromosome deletions, a marker of unrejoined breaks, was determined in CHO-K1 and in xrs-5 cells. The xrs-5 cell is a DNA double-strand break repair-deficient derivative of CHO-K1. Terminal deletion frequency was small in both CHO-K1 and xrs-5 cells when cells were irradiated or treated with restriction enzyme while in the G1 phase of the cell cycle. In contrast, previous studies have shown that treatment of cells in G2 leads to large deletion frequencies, especially in xrs-5 cells. Cell cycle analyses show large G2 blocks in irradiated xrs-5 cells with only partial recovery over a 24-96-h period. These results suggest that most CHO cells with unrejoined breaks are blocked in G2 and, therefore, do not contribute to chromosome mutation frequencies. The small frequencies of terminal deletions that are found in these cells may reflect either an inefficiency in the G2 checkpoint mechanism or, perhaps, a modification of broken ends that allows passage through G2.

Abnormal sister-chromatid exchange induction by 3-aminobenzamide in an SV40-transformed Indian muntjac cell line: relationships with DNA maturation and DNA-strand breakage

In SVM cells, an SV40-transformed line of Indian muntjac fibroblasts, levels of sister-chromatid exchanges are known to be abnormally high after UV-irradiation or alkylation. The SVM line is also known to have a defect in the processing of DNA-strand breaks. Sister-chromatid exchange in other cells is known to be stimulated by the poly(ADP-ribose) transferase inhibitor, 3-aminobenzamide, which also retards DNA-break sealing. Sister-chromatid exchanges in SVM cells are found to be hypersensitive to 3-aminobenzamide, or to nicotinamide deprivation which similarly inhibits poly(ADP-ribosyl)ation; DNA-strand breaks are likewise induced by 3-aminobenzamide. Bromodeoxyuridine, needed to detect sister-chromatid exchanges, is more toxic to SVM cells and itself induces sister-chromatid exchanges to a greater extent than it does in normal muntjac cells. However, in contrast to the situation reported for other cell types prone to sister-chromatid exchange (the Chinese hamster ovary mutant EM9 and human Bloom's Syndrome cells), SVM cells do not show an abnormal delay in DNA maturation when, under the influence of bromodeoxyuridine and 3-aminobenzamide, they show a high level of sister-chromatid exchange. The mechanism by which BrdU exerts its effects can largely be explained in terms of familiar effects on deoxyribonucleotide pools and DNA integrity. 3-Aminobenzamide, however, induces sister-chromatid exchanges in SVM cells by another mechanism.

SCE variability in lymphocytes and fibroblasts. A controlled study

To determine whether the sister chromatid exchange (SCE) distributions obtained in lymphocytes and fibroblasts from different individuals are comparable, a controlled study was set up. Peripheral blood and skin biopsies were taken on the same day from five individuals living for years under the same environmental conditions. All samples were treated in the same fashion, and the SCEs were scored in 50 metaphases of peripheral blood lymphocytes and of skin fibroblasts in an early and in a late passage. A repeat blood sample was taken from the same five individuals 1 year later. Based on the results obtained in this first part of the study, five randomly chosen healthy blood donors were sampled at different times and studied in the same fashion. Each chromosome was identified, and the SCE scores were tabulated per chromosome over 50 metaphases. The statistical analysis consisted of fitting log linear models to these scores and examining the best fit by determining the exceedance probabilities (observed significance level). For lymphocytes, the results indicated that the SCE distributions depended only on the chromosome examined, and not on BrdU-exposure time, individuals, or time of sampling. Treatment with ethyl methane sulfonate (EMS) increased the number of SCEs proportionally on all chromosomes. Analysis of the SCE scores on lymphocytes and fibroblasts of the five individuals and on their low and high passage fibroblast cultures revealed the necessity of including higher order interactions in order to fit a suitable model to the data. Therefore comparison of the SCE scores of lymphocytes with those of fibroblasts or comparison of scores on fibroblasts from different individuals could not be done. In practice, to compare samples or individuals, it suffices to score the SCE on a limited number of chromosomes (e.G., the A group) of 50 metaphases.