X-irradiation of G1 CHO cells induces SCE which are both true and false in BrdU-substituted cells but only false in biotin-dUTP-substituted cells.

The SCE-test is widely used in genetic toxicology and therefore knowledge of the contribution of BrdU to the formation of spontaneous and induced SCE is of great importance. The present study was undertaken to analyse the role of BrdU in X-ray-induced SCE. If SCE resulted from inversions, rings and double minutes (RDM) would be the asymmetrical counterparts of SCE and should therefore have the same frequencies. Dose-effect relationships of SCE and RDM show that the frequencies of SCE are much higher than those of RDM. We conclude that only a few SCE may represent inversions. In a second set of experiments, endoreduplications were induced in cells irradiated either before or after labelling with BrdU. Analysis of SCE in endoreduplicated chromosomes allows the discrimination of the cell cycle in which they originated. The results show that SCE are only induced in the first cell cycle following irradiation of BrdU-substituted cells, indicating that labelling with BrdU is a necessary prerequisite for the formation of SCE. In order to test this directly, radiation-induced SCE frequencies were studied in cells prelabelled with BrdU or biotin-dUTP in a third set of experiments. The structure of biotin-dUTP suggests that, in contrast to BrdU, it does not give rise to radicals during irradiation. Significantly lower frequencies of SCE were observed in biotin-dUTP-substituted cells than in BrdU-labelled cells. Calculations show that nearly all SCE induced in biotin-dUTP-labelled chromosomes can be explained by chromosomal aberrations (false SCE). In contrast to this, most SCE induced by X-rays in BrdU-labelled cells are not due to chromosomal aberrations, but result from S-dependent lesions (true SCE). This clearly points towards radiation damage in BrdU-moieties as the source of DNA lesions leading to SCE.