Interplay between Transcription and Homologous Recombination in the Presence of DNA Damage

Sammanfattning: The biochemical processes of DNA repair, replication and recombination compete for the same substrate, the DNA molecule. This competition is natural, as each process requires the same template. In order to resolve possible conflicts between these processes, when they take place on a particular stretch of DNA, certain crosstalk is expected. The complexity is additionally increased by the existence of another DNA dependent process, which occurs in all phases of the cell cycle: transcription.This thesis aims to investigate the link between transcription inhibition and homologous recombination, especially in the context of UV-induced DNA damage. The results show that the transcription inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) induces homologous recombination. The DNA damage caused by UVC irradiation is repaired mainly via nucleotide excision repair, however, it is also known to trigger recombinational repair. In the presence of UV-induced damage, transcription inhibition by DRB potentiates the induction of homologous recombination as a necessary mechanism of cell survival.The thesis also focuses on the toxicity mechanisms of the chemotherapeutic compound 6-thioguanine (6TG). The work in the thesis suggests application of 6TG as a treatment for hereditary forms of breast cancer, with genetically altered BRCA1 or BRCA2 functions. Most importantly, the treatment with 6TG is applicable to breast cancers, which have developed resistance to another class of chemotherapeutic drugs, poly-(ADP-ribose) polymerase (PARP) inhibitors. Repair of the DNA damage induced by 6TG treatment is investigated further with a particular focus the pathway of DNA damage avoidance involving DNA polymerase η. The function of DNA polymerase η seems to be an important factor for the outcome of DNA repair after 6TG exposure. The deficiency of DNA polymerase η is also investigated in connection with normal replication and the repair of UV-induced DNA damage.In summary, the work in the thesis sheds more light onto the fundamental connections between DNA replication, recombination, transcription, repair and damage avoidance. On a more practical side, the information obtained about these fundamental connections is used to suggest a possible therapy for several forms of breast cancer.