Effect of DNA repair inhibitors on the induction and repair of bleomycin-induced chromosome damage

The phenomenon of premature chromosome condensation (PCC) was used to study the effects of 4 purported DNA repair inhibitors (hydroxyurea, hycanthone, cycloheximide, and streptovitacin A) on the induction and repair of chromosome aberrations after bleomycin treatment of Chinese hamster ovary cells. To test whether these repair inhibitors influenced the initial development of aberrations, exponentially growing populations of cells were treated with 25 μg/ml bleomycin for 30 min with or without the simultaneous presence of each inhibitor. Chromatid aberrations (gaps, breaks and exchanges) were then scored in the G₂ PCC-induced immediately after treatment. None of the purported DNA-repair inhibitors tested were found to alter the initial frequency of bleomycin-induced chromatid aberrations. To determine whether these same agents had an effect on the repair of chromosome damage, cells were first treated with bleomycin in the presence of inhibitor and then incubated for 1 h in bleomycin-free media with or without the presence of inhibitor prior to cell fusion. In all cases, exchanges, once formed, were not repaired. While hydroxyurea and hycanthone had no effect on the repair of chromosome damage, the protein synthesis inhibitors, cycloheximide and streptovitacin A, blocked the repair of bleomycin-induced chromatid breaks. These results suggest that protein synthesis inhibitors can prevent chromosome repair, however, the exact role of protein synthesis in the repair process remains to be elucidated. Since cycloheximide and streptovitacin A were found to inhibit the repair of chromatid breaks without altering the exchange frequency, these results suggest that chromatid exchanges are not simply the product of misrepair or recombinational repair of chromatid breaks.