Formation of γH2AX and pATM Foci in Human Mesenchymal Stem Cells Exposed to Low Dose-Rate Gamma-Radiation

Abstract

DNA double-strand breaks (DSB) are among the most harmful DNA lesions induced by ionizing radiation (IR). Although the induction and repair of radiation-induced DSB is well studied for acute irradiation, responses to DSB produced by chronic IR exposures are poorly understood, especially in human stem cells. The aim of this study was to examine the formation of DSB markers (γH2AX and phosphorylated kinase ATM, pATM, foci) in human mesenchymal stem cells (MSCs) exposed to chronic gamma-radiation (0.1 mGy/min) in comparison with acute irradiation (30 mGy/min) at cumulative doses of 30, 100, 160, 240 and 300 mGy. A linear dose-dependent increase in the number of both γH2AX and pATM foci, as well as co-localized γH2AX/pATM foci (“true” DSB), were observed after an acute radiation exposure. In contrast, the response of MSCs to a chronic low dose-rate IR exposure deviated from linearity towards a threshold model, for γH2AX, pATM foci and γH2AX/pATM foci, with an indication of a “plateau”. The state of equilibrium between newly formed DSB at a low rate during the protracted exposure time and the elimination of a fraction of DSB is proposed as a mechanistic explanation of the non-linear DSB responses following a low dose-rate irradiation. This notion is supported by the observation of the elimination of a substantial fraction of DSB 6 h after the cessation of the exposures. Our results demonstrate non-linear dose responses for γH2AX and pATM foci in human MSCs exposed to low dose-rate IR and showed the existence of a threshold, which may have implications for radiation protection in humans.

The p53–53BP1-Related Survival of A549 and H1299 Human Lung Cancer Cells after Multifractionated Radiotherapy Demonstrated Different Response to Additional Acute X-ray Exposure

“…The histone variant H2AX phosphorylated at DSB sites by several protein kinases (ATM, ATR, and DNA-PK) called γH2AX is the most widely used marker of DNA DSBs [35][36][37]. ATM (ataxia telangiectasia mutated) is a major kinase that phosphorylates H2AX in response to IR-induced DNA damage [38,39]. It is also involved in multiple pathways including DNA repair checkpoint activation, cell-cycle control regulation, apoptosis, senescence, and alterations in chromatin structure, transcription, and pre-mRNA splicing [40].…”

The Impact of Dose Rate on DNA Double-Strand Break Formation and Repair in Human Lymphocytes Exposed to Fast Neutron Irradiation

“…However, the effect was not so pronounced for the lowest dose of 0.125 Gy, possibly because the exposure to the lowest dose rate of 0.015 Gy/min for this radiation dose lasted only 8 min. These results provide a better understanding of the health effects related to low dose-rate neutron radiation, particularly at low doses, which are arguably the most common type of human exposure to neutrons [29].…”

“…To quantify the absolute number of DNA DSB per unit of radiation dose induced by LDR versus HDR exposure, the K coefficient (K, %) was calculated, as previously proposed by Ulyanenko et al [29]. The K coefficient for the HDR exposure was higher than that for the LDR exposure for all doses, which is in agreement with the previous study [29]. The maximum difference of 1.87 was observed between the K coefficients of the two dose rates at 0.250 Gy, which is also clear from the ratios presented in Table 2.…”

“…The maximum difference of 1.87 was observed between the K coefficients of the two dose rates at 0.250 Gy, which is also clear from the ratios presented in Table 2. At a similar absorbed dose of 0.240 Gy 137 Cs γ-radiation, Ulyanenko et al reported a difference of 2.43 between the low dose rate (0.0001 Gy/min) versus the high dose rate (0.030 Gy/min) [29]. Moreover, our results at 30 min postexposure agree with the data reported by Kotenko et al on Chinese hamster V79 cells, where the exposure to Co 60 γ-rays at a lower dose rate (0.001 Gy/min) led to a reduced number of γ-H2AX foci per cell compared to the HDR (0.4 Gy/min) exposure [28].…”

“…Low dose-rate effects are generally a characteristic of environmental (high radon exposures in buildings) and occupational (aircrew on long-distance flights or naturally occurring radioactive materials in the oil extraction industry) exposures and are also applicable to the space radiation environment, where astronauts are spending more and more time [22,27]. While there is a number of studies on DNA DSB formation after low dose-rate versus high dose-rate photon irradiation [28,29,30,31,32,33], there is currently limited information available on the dose-rate effects of high-linear energy transfer radiation qualities on DNA DSB formation and repair [34,35]. Due to an incomplete understanding of how biological systems respond to the low doses and low dose rates of cosmic radiation (including protons, helium nuclei, high-Z high-energy particles and secondary neutrons) and the limited human epidemiological data for these radiation qualities, it is currently difficult to accurately estimate the risk of carcinogenesis and other health effects due to long-term chronic IR exposure of astronauts in space.…”