Independent Researcher, Rome, Italy.
Dipartimento di Diagnostica per Immagini, Radioterapia Oncologica ed Ematologia, Fondazione Policlinico Universitario A. Gemelli IRCCS, Rome, Italy.
Front Public Health. 2020 Dec 15;8:601711. doi: 10.3389/fpubh.2020.601711. eCollection 2020.
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.
生命在地球的自然背景电离辐射中已经进化了约 40 亿年。它极有可能塑造了现在的生命形式，并仍然在起作用。如今，自然背景辐射极小（每年几毫希沃特），但它可能对生物产生足够大的影响，或许使它们保持了对这种辐射的记忆。更好地理解这种反应不仅有助于提高我们对生命进化的认识，而且有助于评估目前低剂量辐射防护系统的稳健性，例如日常生活中通常遇到的剂量。鉴于 100 毫希沃特以下的流行病学数据存在较大不确定性，目前对这些健康风险的定量评估是借助放射生物学模型进行的。这些模型预测，由辐射引起的基因突变导致的健康危害与剂量呈线性相关。然而，许多研究通过证明低剂量下存在非线性反应以及放射性诱导的表观遗传效应（即与 DNA 序列变化无关的基因表达可遗传变化）挑战了这一范式。本综述重点关注除遗传机制外，表观遗传机制在低剂量和长时间暴露（特别是自然背景辐射）反应中的作用。许多证据表明，表观遗传修饰参与了与低剂量相关的非线性反应，如非靶向效应和适应性反应，并且遗传和表观遗传效应在部分上具有共同的起源：电离辐射产生的活性氧。细胞对低剂量电离辐射的反应比为辐射防护目的而假设的反应更为复杂，而且并不总是有害的。在背景辐射极低的地下实验室进行的实验甚至表明，这种背景具有积极影响。在降低的辐射背景下研究各种生物体发生的变化，除了提供有关生命进化的信息外，还开辟了一条新途径来回答低剂量是否有害或有益，以及理解放射生物学结果与辐射防护的相关性。
