Induction of apoptosis in relation to chromatin structure and inhibition of replication by DNA damage from ionizing radiation

Sammanfattning: The theme of this thesis has been chromatin organization ranging from methodological studies to involvement in apoptotic response. The aims have been: (i) to compare the information obtained by the AVTD method and comet assay concerning DNA-loop organization, (ii) to test the hypothesis that DNA-loop relaxation could be the triggering signal for induction of apoptosis in G0-lymphocytes, and (iii) to study the dose response for inhibition of the replication fork movement and pathways for the DNA repair process at the replication fork. AVTD was evaluated and compared with the established comet assay by studying changes in DNA-loop structure, induced by ethidium bromide. DNA-loops either relaxed or condensed in a dose dependent manner and changes in viscosity correlated with the length of comet tails. The dose response relations for induction of apoptosis in G0-lymphocytes were determined and compared with the dose response relations for relaxation of DNA-loops. Relaxation was shown to saturate at doses of 2-3 Gy after ?-irradiation, a dose in which approximately one SSB per chromatin loop of DNA was induced. Apoptotic markers such as chromatin condensation, p53 stabilization and DNA fragmentation also saturated at 2-3 Gy, suggesting that SSB dependent loop relaxation may trigger apoptosis. Radiation induced inhibition of replication fork movement was studied in proliferating Chinese hamster ovary cells. Doses over 100 Gy were needed to inhibit the fork elongation, as verified by both the ADU and by the DNA combing assay. Checkpoint signalling was shown not to be involved in this retarded elongation. On the other hand, the initiation of replication was sensitive to low doses of ionizing radiation. A dose of 12.5 Gy was enough to stop firing of new replicons and caffeine attenuated this inhibition. By measuring the speed of replication fork progression in repair deficient cell lines we concluded that replication forks are retarded by un-repaired DSBs, SSBs and/or base lesions.