Increased G2 chromosomal radiosensitivity in cancer patients: the role of cdk1/cyclin-B activity level in the mechanisms involved

Purpose: To test the hypothesis that deficient DNA repair as measured by in creased G2 chromosomal radiosensitivity results from up-regulation of cdk1/ cyclinB and cell cycle control mechanisms during the G2 to hi transition. Materials and methods: A total of 185 cancer patients and 25 normal individ uals were tested for G2 chromosomal radiosensitivity. The chromatid breaks were analysed in metaphase using the G2 assay or directly in GO and G2 phas e using premature chromosome condensation (PCC). The activity of cdk1/cycli nB, a key regulator of the G2 to M-phase transition, was measured by histon e H1 kinase activity and correlated with the develop ment of chromatid brea ks after irradiation of cell lines in vitro. Results: Based on the G2 assay, cancer patients on average showed increased chromosomal radiosensitivity above controls. When the analysis was carried out directly in GO or G2 lymphocytes using PCC, no differences in the indu ction of chromosomal damage and its repair were observed between G2 assay-s ensitive and G2-normal donors. Using the G2 assay to test G2 radiosensitivi ty in various cell lines, it was found that the higher the cdk1/cyclinB act ivity level of the cell line tested, the higher the yield of chromatid brea ks scored. Furthermore, when mitotic cells from these cell lines were used for PCC induction in irradiated G2 lymphocytes it was observed that the hig her the cdk1/cyclinB activity level of mitotic cells used, the higher was t he induced yield of chromatid breaks. Conclusion: The cdk1/cyclin-B activity levels during the G2 to M transition impair DNA repair processes and play a major role in the yield of chromati d breaks induced after G2-irradiation. Regulation of cdk1/cyclinB complex a ctivity rather than deficient repair enzymes of DNA damage may underlie the mechanisms of G2 radiosensitivity.