Time-Resolved Repair of Clustered DNA Damage in γ-Irradiated Yeast Cells

Background/Objectives: Exposure of cells to ionizing radiation induces isolated DNA lesions, including single-strand breaks, apurinic/apyrimidinic sites, and oxidized bases, as well as clustered damages of different complexity. The latter types of damage are difficult to repair, and the failure to process them accurately and efficiently is related to the induction of mutagenesis, genomic instability, cancer, and aging. Since various types of clustered lesions may occur simultaneously after radiation exposure, leading to a complex architecture of DNA damage, the study of the concomitant formation and the removal kinetics of clustered DNA damage is important to determine the mutagenic and, consequently, the carcinogenic potential of ionizing radiation. Methods: With the aim of capturing real-time coexisting lesion types and assessing the repair kinetics of clustered damages, the simultaneous determination of double-strand breaks, apurinic/apyrimidinic site clusters, and oxypurine clusters induced by γ-irradiation of Saccharomyces cerevisiae yeast cells was performed immediately after exposure and at time intervals during incubation in Liquid Holding Recovery conditions. Results: Ionizing radiation induced lethal and mutagenic events, leading to a dose-dependent linear increase in double-strand breaks, apurinic/apyrimidinic site clusters, and oxypurine clusters. The kinetic study showed that double-strand break frequencies declined during Liquid Holding Recovery, although a transient increase was detected at early time points. At 160 Gy, apurinic/apyrimidinic site clusters repair was evident, whereas at 400 Gy the frequency of damage increased before returning to the initial value at 24 h. In contrast, oxypurine clusters showed no net increase in repaired lesions over 24 h. Conclusions: The complex nature and topological characteristics of ionizing radiation-induced clustered DNA damage may influence lesion processing. Also, ionizing radiation may disrupt redox cellular homeostasis, leading to DNA damage and delayed effects.