The complexity of clustered DNA DSBs in human fibroblasts under the action of low and high-LET radiation

High-order clustered DNA lesions are the hallmark of the action of dense-ionizing radiation. It is defined as a combination of two or more individual lesions (single-strand breaks (SSB) and double-strand breaks (DSB), base damage, etc.) located within 1-2 DNA helical turns. Clustered DNA DSBs, containing DSBs and other DNA lesions, represent specific interest for investigation. To research the induction and repair of clustered DNA DSBs, human fibroblasts were irradiated with 60Co γ-rays (LET ≈ 0.3 keV/μm), 15N ions (LET = 181.4 keV/μm, E = 13 MeV/n) and protons in the expanded Bragg peak (LET = 2 – 100 keV/μm, E = 0.01 – 44 MeV/n). The dose for all types of irradiation was 1.25 Gy. The key proteins involved in the repair of base damage (OGG1) and DNA DSBs (53BP1) were visualized by immunocytochemical staining and fluorescence microscopy. To characterize the structure of clustered DNA lesions the detailed quantitative analysis of OGG1 and 53BP1 foci clusters was completed. The obtained results showed the formation of clustered DNA DSBs with higher complexity structure and low repair efficiency under the action of protons and 15N ions compare to γ-rays.