Abstract
Life has evolved on Earth for about 4 billion years in the presence of the natural background of ionizing radiation. It is extremely likely that it contributed, and still contributes, to shaping present form of life. Today the natural background radiation is extremely small (few mSv/y), however it may be significant enough for living organisms to respond to it, perhaps keeping memory of this exposure. A better understanding of this response is relevant not only for improving our knowledge on life evolution, but also for assessing the robustness of the present radiation protection system at low doses, such as those typically encountered in everyday life. Given the large uncertainties in epidemiological data below 100 mSv, quantitative evaluation of these health risk is currently obtained with the aid of radiobiological models. These predict a health detriment, caused by radiation-induced genetic mutations, linearly related to the dose. However a number of studies challenged this paradigm by demonstrating the occurrence of non-linear responses at low doses, and of radioinduced epigenetic effects, i.e., heritable changes in genes expression not related to changes in DNA sequence. This review is focused on the role that epigenetic mechanisms, besides the genetic ones, can have in the responses to low dose and protracted exposures, particularly to natural background radiation. Many lines of evidence show that epigenetic modifications are involved in non-linear responses relevant to low doses, such as non-targeted effects and adaptive response, and that genetic and epigenetic effects share, in part, a common origin: the reactive oxygen species generated by ionizing radiation. Cell response to low doses of ionizing radiation appears more complex than that assumed for radiation protection purposes and that it is not always detrimental. Experiments conducted in underground laboratories with very low background radiation have even suggested positive effects of this background. Studying the changes occurring in various living organisms at reduced radiation background, besides giving information on the life evolution, have opened a new avenue to answer whether low doses are detrimental or beneficial, and to understand the relevance of radiobiological results to radiation protection.
Highlights
Living organisms have evolved on Earth for about 4 billion years in the presence of the natural background of ionizing radiation even if it was not always the same as today
The main contribution to radiation background exposure of the public comes from natural sources of both low- and highLET1, i.e., from cosmic radiation, external terrestrial radiation, Abbreviations: UNSCEAR, United Nations Scientific Committee on the Effects of Atomic Radiation; ICRP, International Commission on Radiological Protection; double strand breaks (DSB), Double strand break; LET, Linear energy transfer; LNT, Linear No-Threshold; non-targeted effects (NTE), Non-targeted effects; adaptive response (AR), Adaptive response; bystander effect (BE), Bystander effect; genomic instability (GI), Genomic instability; C, G, Cytosine, Guanine; CpG, 5’—C—phosphate—G−3’; DNMT, DNA methyltransferase; ncRNA, Non-coding RNA; miRNA, Micro RNA; reactive oxygen species (ROS), Reactive oxygen species; reactive nitrogen species (RNS), Reactive nitrogen species; TE, Transposable element; High Natural Background Radiation (HNBR), High natural background radiation. 1LET stands for Linear Energy Transfer, defined as the ratio between the energy E deposited by a charged particle in a very short track element, and its length x
It is worth noting here that: (i) the average annual dose from natural background of 2.4 mSv, corresponds to a low dose rate of ≈ 0.27 μSv/h; (ii) several areas of the world, such as Guarapan in Brazil, Ramsar in Iran, Yangjiang in China and Kerala in India, are found to have levels of natural background radiation that are in excess of those considered to be “normal background” [3]so that they are defined as High Natural Background Radiation (HNBR) areas [7]; (iii) the evaluation of inhalation and ingestion exposures are strictly related to human beings and may not hold for different organisms, certainly not for the cultured cells often used in radiobiology experiments
Summary
Living organisms have evolved on Earth for about 4 billion years in the presence of the natural background of ionizing radiation even if it was not always the same as today. The Earth’s crust contains radionuclides, and the Earth is continuously bombarded by high-energy particles originating in outer space and by the Sun (cosmic radiation). It should be considered that probably in the past the cosmic rays on Earth have experienced many fluctuations due to explosions of supernovae in the nearby interstellar space and to variations in solar wind. Understanding its role is important for improving our knowledge about life evolution on Earth and about the health effects of low dose ionizing radiation exposure, a hot topic in radiation protection. In this review the role of background radiation is considered in the perspective of the low dose issue in radiation protection
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