Resolution of DNA damage at the single-cell level shows largely different actions of X-rays and bleomycin.

Both X-rays and the radiomimetic agent bleomycin (BLM) induce DNA strand breaks, predominantly via reactive radicals. To compare the induction of breaks with the two agents in Chinese hamster (CHO-K1) cells, two different alkaline unwinding methods, a 3H tracer-based analysis of large cell populations and an optical adaption allowing measurement of single cells, were applied. Radiation and BLM show qualitatively similar dose responses when the average number of DNA strand breaks is measured in a large cell population. However, the breakage pattern at the single-cell level indicates large discrepancies between the actions of the two agents. Irradiated cells show a uniform distribution of DNA strand breaks over the cell population. Effects of treatment with 30 micrograms x ml-1 BLM for 2 hr vary from practically zero in some cells to high levels of DNA strand breakage in others. Unlike the repair of radiation-induced DNA breaks, the repair efficiency of BLM-induced DNA strand breaks, as measured at the single-cell level, varies strongly among cells of the same population. Such heterogeneity at the cellular level potentially reduces BLM's usefulness for tumor therapy because the appearance of BLM-resistant subpopulations may critically impair treatment outcome.