Induction of sister chromatid exchange by 3-aminobenzamide is independent of bromodeoxyuridine.

The poly(ADP-ribose) polymerase inhibitor, 3-aminobenzamide (3AB), significantly increases sister chromatid exchange (SCE) frequency without causing apparent damage to cellular DNA. A previous study has suggested that the increase of SCEs by 3AB results from DNA replication on a template strand containing bromodeoxyuridine (BrdU), which is used to visualize SCEs. Therefore, to study the importance of BrdU incorporation on the induction of SCEs by 3AB, we analyzed exchanges induced during the first round of replication (twin SCEs) and those induced during the second (single SCEs). 3AB increased the formation of SCEs in both replication cycles, but significantly more exchanges were induced in the second cycle, when BrdU was present in the template DNA. These data are consistent with the suggestion that the presence of BrdU in the template strand of DNA plays an important role in SCE induction by 3AB. However, we also studied 3AB-induced SCEs by autoradiography of cells cultured with 3H-thymidine (3H-dT) instead of BrdU. A significant increase in SCE frequency was also observed in cells from these cultures. Furthermore, the analysis of twin and single SCEs showed that with 3H-dT too, there was a greater increase in SCEs in the second cycle than in the first. Thus, SCE induction by 3AB in the second cycle is not dependent on the presence of BrdU in template DNA. Incubation of cells with deoxycytidine was found to have no effect on the frequency of SCEs induced by 3AB, suggesting that an imbalance in the deoxycytidine precursor pool did not account for the effect.