Insights into the mechanisms of sister chromatid exchange formation

The DNA lesions responsible for the formation of sister chromatid exchanges (SCEs) have been the object of research for a long time. SCEs can be visualized by growing cells for either two rounds of replication in the presence of 5-bromo-2'-deoxyuridine (BrdU) or for one round with BrdU and the next without. If BrdU is added after cells were treated with a DNA-damaging agent, the effect on SCEs can only be analyzed in the second post-treatment mitosis. If one wishes to analyze the first post-treatment mitosis, cells unifilarily labeled with BrdU must be treated. Due to the highly reactive bromine atom, BrdU interacts with such agents like ionizing and UV radiation enhancing the frequency of SCEs. However, its precise role in this process was difficult to assess for a long time, because no alternative technique existed that allowed differential staining of chromatids. We have recently developed a method to differentially label sister chromatids with biotin-16-2'-deoxyuridine-5'-triphosphate (biotin-dUTP) circumventing the disadvantage of BrdU. This technique was applied to study the SCEs induced by ionizing and UV radiation as well as by mitomycin C, DNaseI and AluI. This article is a review of the results and conclusions of our previous studies.

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.

Sister chromatid differentiation with biotin-dUTP

The method of choice to differentiate sister chromatids is to incorporate BrdU in replicating DNA. The disadvantage of BrdU is that its spontaneous or induced radicalization may itself lead to sister chromatid exchanges. Biotin-labelled dUTP is a widely used thymidine analogon for labelling isolated DNA. Its chemical structure suggests that, in contrast to BrdU, it does not give rise to radical formation. We electroporated proliferating Chinese hamster ovary (CHO) cells in the presence of biotin-dUTP which was subsequently detected in metaphase cells with TRITC-conjugated avidin. Microscopic analysis of second mitoses after labelling revealed a clear differential staining of sister chromatids. Thus substitution of thymidine with biotin-dUTP is another method to analyse SCE.

The action of quartz in the presence of iron hydroxides in the human lung

In 1932 Kettle reported that the fibrogenic action of quartz particles is inhibited when they are coated with iron. Many authors subsequently confirmed this finding. Reif et al., who studied the effect of different iron-ore mine dusts, showed that, like aluminium hydroxide, iron hydroxide has a strong effect in inhibiting the fibrogenic action due to quartz. The findings of this paper, based on an X-ray survey of a group of 7735 iron ore miners, on autopsy examinations, tissue dust analysis and physiological examinations, confirm the view that FeO(OH) is a most powerful inhibiting substance.