Radiation response in human cells : DNA damage formation, repair and signaling

Abstract: Ionizing radiation induces a range of different DNA lesions. In terms of mutation frequency and mammalian cell survival, the most critical of these lesions is the DNA double-strand break (DSB). DSB left unrepaired or mis-repaired may result in chromosomal aberrations that can lead to permanent genetic changes or cell death. The complexity of the DNA damage and the capacity to repair the DSB will determine the fate of the cell. This thesis focuses on the DNA damage formation, repair and signaling after irradiation of human cells.Radiation with high linear energy transfer (LET) produces clustered damaged sites in the DNA that are difficult for the cell to repair. Within these clustered sites, non-DSB lesions are formed that can be converted into a DSB and add to the damage complexity and affect DSB repair and the measurement. Heat-labile sites in DNA are converted into DSB at elevated temperatures. We show that heat-released DSB are formed post-irradiation with high-LET ions and increase the initial yield of DSB by 30%-40%, which is similar to yields induced by low-LET radiation.DNA-PKcs, a central player in non-homologous end-joining (NHEJ), the major mammalian DSB repair pathway, has been found to be both up- and downregulated in different tumor types. In Paper II we show that low levels of DNA-PKcs lead to extreme radiosensitivity but, surprisingly, had no effect on the DSB repair. However, the fraction of cells in G2/M phase increased two-fold in cells with low levels of DNA-PKcs. The study continued in Paper IV, where cells were synchronized to unmask potential roles of DNA-PKcs in specific cell cycle phases. Irradiation of DNA-PKcs suppressed cells in the G1/S phase caused a delay in cell cycle progression and an increase in accumulation of G2 cells. Further, these cells showed defects in DNA repair, where a significant amount of 53BP1 foci remained after 72 h. This further strengthens the hypothesis that DNA-PKcs has a role in regulation of mitotic progression.Several cellular signaling pathways are initiated in response to radiation. One of these downstream signaling proteins is AKT. We identified an interaction between DNA-PKcs and AKT. Knockouts of both AKT1 and AKT2 impaired DSB rejoining after radiation and low levels of DNA-PKcs increased radiosensitivity and decreased DNA repair further.