Elizaveta Yashkina

Although ionizing radiation quality shapes the spatial structure and complexity of primary DNA lesions, its contribution relative to absorbed dose in the late DNA damage response (DDR) remains poorly quantified. We analyzed γH2AX and pATM foci kinetics in human mesenchymal stromal/stem cells exposed to ⁶⁰Co γ-rays, 150 MeV protons, and 14.1 MeV neutrons at doses of 0.25, 0.5, 1.0, and 2.0 Gy, delivered at dose rates of 0.5 Gy/min for γ-rays, 1 Gy/min for protons, and 46 mGy/min for neutrons, respectively. An effective kinetic model was used to describe the time course of the DDR and incorporated an ultraslow component as a model-based index of long-lasting repair burden. Radiation types differed less in early signal amplitude than in decay rate and late persistence. At every investigated dose, the normalized ultraslow fraction followed the order γ-rays < protons < neutrons. This ordering was preserved in per-dose fits, while parametric bootstrapping supported the γ–neutron separation, although the absolute values of the internal fractions were only partially identifiable. RAD51, analyzed independently from model fitting, showed the same qualitative ordering at late time points, supporting the interpretation that radiation quality is more associated with persistent DDR signaling than absorbed dose within the investigated range. These findings suggest that absorbed dose alone may not fully characterize the late cellular response to different radiation qualities and highlight the importance of radiation quality in comparative radiobiology, radiation protection, and the interpretation of normal-tissue responses to therapeutic irradiation.

The effects of low-dose radiation exposure remain a controversial topic in radiation biology. This study compares early (0.5, 4, 24, 48, and 72 h) and late (5, 10, and 15 cell passages) post-irradiation changes in γH2AX, 53BP1, pATM, and p-p53 (Ser-15) foci, proliferation, autophagy, and senescence in primary fibroblasts exposed to 100 and 2000 mGy X-ray radiation. The results show that exposure to 100 mGy significantly increased γH2AX, 53BP1, and pATM foci only at 0.5 and 4 h post irradiation. There were no changes in p-p53 (Ser-15) foci, proliferation, autophagy, or senescence up to 15 passages post irradiation at the low dose.

DNA repair (DNA damage) foci observed 24 h and later after irradiation are called “residual” in the literature. They are believed to be the repair sites for complex, potentially lethal DNA double strand breaks. However, the features of their post-radiation dose-dependent quantitative changes and their role in the processes of cell death and senescence are still insufficiently studied. For the first time in one work, a simultaneous study of the association of changes in the number of residual foci of key DNA damage response (DDR) proteins (γH2AX, pATM, 53BP1, p-p53), the proportion of caspase-3 positive, LC-3 II autophagic and SA-β-gal senescent cells was carried out 24–72 h after fibroblast irradiation with X-rays at doses of 1–10 Gy. It was shown that with an increase in time after irradiation from 24 h to 72 h, the number of residual foci and the proportion of caspase-3 positive cells decrease, while the proportion of senescent cells, on the contrary, increases. The highest number of autophagic cells was noted 48 h after irradiation. In general, the results obtained provide important information for understanding the dynamics of the development of a dose-dependent cellular response in populations of irradiated fibroblasts.

To assess the prospects for using intense femtosecond laser radiation in biomedicine, it is necessary to understand the mechanisms of its action on biological macromolecules, especially on the informational macromolecule—DNA. The aim of this work was to study the immunocytochemical localization of DNA repair protein foci (XRCC1 and γH2AX) induced by tightly focused femtosecond laser radiation in human cancer A549 cells. The results showed that no XRCC1 or γH2AX foci tracks were observed 30 min after cell irradiation with femtosecond pulses of 10¹¹ W∙cm⁻² peak power density. An increase in the pulse power density to 2 × 10¹¹ W∙cm⁻² led to the formation of linear tracks consisting both of XRCC1 and γH2AX protein foci localized in the places where the laser beam passed through the cell nuclei. A further increase in the pulse power density to 4 × 10¹¹ W∙cm⁻² led to the appearance of nuclei with total immunocytochemical staining for XRCC1 and γH2AX on the path of the laser beam. Thus, femtosecond laser radiation can be considered as a tool for local ionization of biological material, and this ionization will lead to similar effects obtained using ionizing radiation.