CONVERSION OF DNA-DAMAGE INTO CHROMOSOME-DAMAGE IN RESPONSE TO CELL-CYCLE REGULATION OF CHROMATIN CONDENSATION AFTER IRRADIATION

Cell fusion, premature chromosome condensation (PCC) and conventional cytogenetics were used to test whether the biochemical process of chro matin condensation-decondensation throughout the cell cycle, which dep ends on cyclin-regulated histone H1 kinase activity, affects the conve rsion of DNA damage into chromosome damage and determines intrinsic ce ll cycle-stage radiosensitivity, Results from three sets of experiment s are presented, Irradiated G(0) human lymphocytes were fused to expon entially growing hamster cells and time allowed for repair, while foll owing the hamster cells in their progress towards mitosis. Severe frag mentation was observed in the induced lymphocyte PCCs when hamster cel ls entered mitosis 13 h after irradiation, suggesting conversion of DN A damage into nonrepairable chromosome damage during G(1)/S transition , When PCC was used to analyse chromosome damage directly in G(0) and G(2) phase lymphocytes, the induction of breaks per cell per chromatid per Gy was found to be similar, suggesting that G(2) increased radios ensitivity is related to chromatin condensation occurring during G(2)/ M transition and not to an inherent chromatin structure at this phase, When chromatin condensation-decondensation at the G(1)/S and G(2)/M t ransitions was modified after irradiation by using conditioned media o r elevated temperature (40 degrees C), a dramatic change in the yield and the type of chromosomal aberrations was observed, All results obta ined were consistent with the proposed hypothesis, They may be also he lpful in the characterization of a DNA-chromosome damage conversion pr ocess which could give a biochemical explanation of the variability in radiosensitivity observed at the various stages of the cell cycle as well as among mutant cells and cells of different origin, The proposed conversion process is cell cycle-regulated and, therefore, subject to up-regulation or down-regulation following mutagen exposure and genet ic alterations.