Chromosome-type exchange aberrations are induced by inhibiting repair of UVC-induced DNA lesions in quiescent human lymphocytes

Advances in the biophysical and molecular bases of radiation cytogenetics

PURPOSE

For more than 70 years radiation cytogenetics has continued to be a topic of major concern in relation to the action of radiation on living cells. To date, diverse cytogenetic findings have developed into orderly, quantitative interpretations and have stimulated numerous biophysical models. However, it is generally agreed that any one of the models used alone is still unable to explain all aspects of the observed chromosomal effects. In this review, a large number of radiation-induced chromosome aberration findings from the literature are reassessed with special attention given to the reaction kinetics and the relevant molecular processes.

CONCLUSION

It is now clear that DNA double-strand breaks (DSB) are an integral component of radiation-induced chromosome aberration. At the nexus of the maintenance of genome integrity, cells are equipped with excellent systems to repair DSB, notably non-homologous end-joining (NHEJ) and homologous recombination repair (HRR). These repair mechanisms are strictly regulated along with the DNA turnover cycle. NHEJ functions in all phases of the cell cycle, whereas HRR has a supplementary role specifically in S/G2 phase, where homologous DNA sequences are available in close proximity. The repair pathways are further regulated by a complex nuclear dynamism, where DSB are sensed and large numbers of repair proteins are recruited and assembled to form a repair complex involving multiple DSB. Considering such DSB repair dynamism, radiation-induced chromosome aberrations could be well understood as DSB-DSB pairwise interactions associated with the NHEJ pathway in all phases of the cell cycle and misrepair of a single DSB associated with the complementary HRR pathway in late S/G2 phase.

Cell cycle specificity of cytogenetic damage induced by 3,4-epoxy-1- butene

3,4-epoxy-1-butene (EB), a primary metabolite of butadiene, is a direct-acting "S-dependent" genotoxicant that can induce sister chromatid exchanges (SCEs) and chromosome aberrations (CAs) in cycling cells in vitro. However, EB is almost inactive when splenic or peripheral blood lymphocytes are exposed at the G(0) stage of the cell cycle. To investigate whether repair of DNA lesions is responsible for the lack of cytogenetic responses seen after G(0) treatments, we used cytosine arabinoside (ara-C) to inhibit DNA polymerization during DNA repair. If enough repairable lesions are present, double-strand breaks should accumulate and form chromosome-type ("S-independent") deletions and exchanges. This is exactly what occurred. EB induced chromosome deletions and dicentrics at the first division following treatment, when the EB exposure was followed by ara-C. Without ara-C treatment, there was no induction of CAs. These experiments indicate that the relatively low levels of damage induced by EB in G(0) lymphocytes are removed by DNA repair prior to DNA synthesis and thus, before the production of SCEs or chromatid-type aberrations.

Genotoxicological investigation of hospital nurses occupationally exposed to ethylene-oxide: I. Chromosome aberrations, sister-chromatid exchanges, cell cycle kinetics, and UV-induced DNA synthesis in peripheral blood lymphocytes

Structural, and numeric chromosome aberrations (CA), sister-chromatid exchange (SCE), phytohemagglutinin stimulation (LI), proliferative rate index (PRI), and UV light-induced unscheduled DNA-synthesis (UDS) were investigated in peripheral blood lymphocytes (PBL) of 48 historical controls ("Controls"); of 14 hospital controls in Budapest, Hungary ("Budapest controls"); of 9 nurses occupationally exposed to low-dose ethylene-oxide in Budapest ("Budapest exposed"); of 10 hospital controls in Eger, Hungary ("Eger controls"); and of 27 high dose ETO-exposed nurses in Eger ("Eger exposed"), where neoplasias, mainly breast cancers, were observed. ETO concentrations in the ambient air samples varied from 5 to 20 mg/m3 in Budapest; and from 5 to 100 mg/m3 in Eger. Bothe LI and PRI were depressed in Budapest exposed, indicating ETO-induced cytotoxicity and, however, normal in Eger exposed. SCE was slightly elevated in Budapest exposed, but significantly increased in Eger exposed. The yields of cells with high frequency SCE (HFC) were only increased in Eger exposed. The expected low CA frequencies were found in Controls and in Budapest controls. ETO exposures significantly increased the CA frequencies in Budapest and Eger exposed. In Budapest exposed, as expected, we found deletions; in a lesser extent chromatid exchanges and dicentrics; but no rings were detected. These results are in a good accordance to the published data of other investigations carried out on ETO-exposed human populations. However, in Eger exposed, beside the increased yields of deletions, the frequencies of dicentrics and rings showed a significant excess compared to the reviewed data. An unexpected, significant increase of dicentric and ring frequencies was also detected among the hospital controls in Eger controls without known clastogenic exposure. The role of confounding factors (age, smoking and drinking habit, total leukocyte count and hematocrit) was investigated by an analysis of variance on CA and SCE frequencies in Controls and in Eger exposed. Leukocyte count and mean age showed only significant effects on CA in Eger exposed and on SCE in Controls, respectively. A possible active confounding factor could be the temporary natural radioactivity of the local tap water. UDS in Budapest exposed and in Eger control were significantly higher then in the Controls and in Budapest controls. In Eger exposed UDS was significantly decreased compared to the Budapest exposed and Eger control groups. The explanation of the present results is difficult on the basis of the reviewed data on ETO-induced CA frequencies in exposed human populations, and it raises an issue of an independent genotoxic effect in Eger, which is common both in Eger controls and in Eger exposed, such as natural radioactivity.