From Radiation-Induced DNA Damage to the Formation of Chromosomal Aberrations

摘要

There is considerable evidence suggesting that DNA DSBs play a major role in the formation of radiation-induced chromosomal aberrations. Furthermore, using the G_2-phase assay, it has been suggested that the enhanced formation of chromatid breaks seen at metaphase after G_2-phase irradiation of human tumour cells or cells from cancer-prone individuals results from deficient DNA repair processes during the G_2 phase. In view of the potential importance of this "G_2 assay" for correlating the enhanced formation of chromatid breaks with interindividual variability in radiosensitivity and cancer predisposition, we tested the following hypothesis: "The increased G_2-phase chromosomal radiosensitivity observed in a substantial proportion of cancer patients may not result necessarily from low-penetrance predisposing genes related to deficient DNA repair processes, but via mutations of genes directly related to cell cycle and feedback control mechanisms." For this purpose, emphasis was given to the complex of CDKl/cyclin B, which is a key regulator of the G_2 to M-phase cell cycle transition. To determine CDK1/cyclin B, activity the biochemical CDKl/cyclin B activity was measured as the capacity to phosphorylate histone H1. This end point is studied in relation to radiation-induced DNA damage and the development of chromatid breaks after irradiation of G_2-phase cells obtained from healthy individuals and cancer patients as well as from various cell lines such as TK6, MCF7, CHO and XRS-5. The chromatid breaks are visualised either in metaphase using conventional cytogenetics and the G_2-phase assay or alternatively directly in G_0 and G_2 phase by means of premature chromosome condensation (PCC). The results obtained suggest that after induction of DNA damage, cell cycle regulation is an important determinant in the formation of chromatid breaks and may underlie the expression of interindividual variability in G_2-phase chromosomal radiosensitivity and cancer susceptibility.