Bleomycin-induced damage in prematurely condensed chromosomes and its relationship to cell cycle progression in CHO cells.

between the cytotoxic effects of these agents and the incidence of chromosome aberrations in the treated cells (2,4, 15). However, BLM treatment severely inhibits the progression of cells into mitosis. The probability that the extensively damaged cells might not be reaching mitosis makes it difficult to assay the chromosome damage accurately by the conven tional techniques. Recently, we have shown that chromosome damage in interphase cells can be effectively observed by the technique of premature chromosome condensation, which involves the fusion of mitotic with interphase cells (6, 7). The premature chromosome condensation technique has some advantages over the conventional mitotic chromosome studies in these respects: (a) it makes it possible to visualize chromosome damage in interphase cells immediately after exposure to mutagenic or clastogenic agents; (b) there is no need for the cells to progress to mitosis in order to evaluate chromosome damage; (c) the repair of chromosome aberrations as a function of time can be measured directly ; (d) the position of a cell in the cell cycle at the time of treatment becomes obvious by the morphology of its PCC. Taking advantage of this technique, in this study we tried to answer why many of the BLM-treated cells failed to reach mitosis and what is the extent of chromosomal damage suffered by these cells. The results obtained by the premature chromosome condensation technique in this study revealed a 5to 9-fold higher incidence of chromosome aberrations than the incidence observed by studying mitotic chromosomes. They also suggest that extensive damage to the genome may lead to the failure of cells to progress through the mitotic cycle.