Differential induction of chromosomal instability by DNA strand-breaking agents

To investigate the role of DNA strand breakage as the molecular lesion responsible for initiating genomic instability, five different strand-breaking agents, bleomycin, neocarzinostatin, hydrogen peroxide, restriction endonucleases, and ionizing radiation, were examined for their capacity to induce delayed chromosomal instability. These studies used GM10115 human-hamster hybrid cells, which contain one copy of human chromosome 4 in a background of 20-24 hamster chromosomes. Chromosomal instability was investigated using fluorescence in situ hybridization to visualize chromosomal rearrangements involving the human chromosome. Rearrangements are detected multiple generations after treatment, in clonal populations derived from single progenitor cells surviving treatment of the specified DNA-damaging agents. Clastogenic and cytotoxic activities of all agents were tested by examining chromosome aberration yields in first-division metaphases and by clonogenic survival assays. Analysis of over 250 individual clones representing over 50,000 metaphases demonstrates that when compared at comparable levels of cell kill, ionizing radiation, bleomycin, and neocarzinostatin are equally effective at eliciting delayed genomic instability. These observations document, for the first time, the persistent destabilization of chromosomes following chemical treatment. In contrast, the analysis of nearly 300 clones and 60,000 metaphases, involving treatment with four different restriction endonucleases and/or hydrogen peroxide, did not show any delayed chromosomal instability. These data indicate that DNA strand breakage per se does not necessarily lead to chromosomal instability but that the complexity or quality of DNA strand breaks are important in initiating this phenotype.