Evolving paradigms for the biological response to low dose ionizing radiation; the role of epigenetics

Factors Influencing Effects of Low-dose Radiation Exposure

ABSTRACT

It is now well accepted that the mechanisms induced by low-dose exposures to ionizing radiation (LDR) are different from those occurring after high-dose exposures. However, the downstream effects of these mechanisms are unclear as are the quantitative relationships between exposure, effect, harm, and risk. In this paper, we will discuss the mechanisms known to be important with an overall emphasis on how so-called "non-targeted effects" (NTE) communicate and coordinate responses to LDR. Targeted deposition of ionizing radiation energy in cells causing DNA damage is still regarded as the dominant trigger leading to all downstream events whether targeted or non-targeted. We regard this as an over-simplification dating back to formal target theory. It ignores that last 100 y of biological research into stress responses and signaling mechanisms in organisms exposed to toxic substances, including ionizing radiation. We will provide evidence for situations where energy deposition in cellular targets alone cannot be plausible as a mechanism for LDR effects. An example is where the energy deposition takes place in an organism not receiving the radiation dose. We will also discuss how effects after LDR depend more on dose rate and radiation quality rather than actual dose, which appears rather irrelevant. Finally, we will use recent evidence from studies of cataract and melanoma induction to suggest that after LDR, post-translational effects, such as protein misfolding or defects in energy metabolism or mitochondrial function, may dominate the etiology and progression of the disease. A focus on such novel pathways may open the way to successful prophylaxis and development of new biomarkers for better risk assessment after low dose exposures.

Persons chronically exposed to low doses of ionizing radiation: A cytogenetic dosimetry study

The dosimetry and control of exposure for individuals chronically exposed to ionizing radiation are important and complex issues. Assessment may be optimized by evaluating individual adaptation and radiosensitivity, but it is not possible for a single model to account for all relevant parameters. Our goal was to develop approaches for the calculation of doses for persons chronically exposed to ionizing radiation, taking their radiosensitivities into consideration. On the basis of ex vivo radiation of blood samples, dose-effect models were constructed for dose ranges 0.01-2.0 and 0.01-0.4 Gy, using different cytogenetic criteria. The frequencies of "dicentric chromosomes and rings" at low doses are too low to have predictive value. The different responses of subjects to radiation made it possible to categorize them according to their radiosensitivities and to generate separate dose-effect curves for radiosensitive, average, and radioresistant individuals, reducing the amount of error in retrospective dosimetry.

Radiation dose rate effects: what is new and what is needed?

Despite decades of research to understand the biological effects of ionising radiation, there is still much uncertainty over the role of dose rate. Motivated by a virtual workshop on the "Effects of spatial and temporal variation in dose delivery" organised in November 2020 by the Multidisciplinary Low Dose Initiative (MELODI), here, we review studies to date exploring dose rate effects, highlighting significant findings, recent advances and to provide perspective and recommendations for requirements and direction of future work. A comprehensive range of studies is considered, including molecular, cellular, animal, and human studies, with a focus on low linear-energy-transfer radiation exposure. Limits and advantages of each type of study are discussed, and a focus is made on future research needs.

Contribution of transposable elements to transgenerational effects of chronic radioactive exposure of natural populations of Drosophila melanogaster living for a long time in the zone of the Chernobyl nuclear disaster

The accident at the Chernobyl Nuclear Power Plant (ChNPP) led to the negative impact of chronic radioactive contamination on populations of organisms associated with the transgenerational transmission of genome instability. When the destabilization of genome, different genetic damages occur, the accumulation of which leads to the formation of mutations, morphological anomalies, and mortality in the offspring. The mechanisms underlying the manifestation of transgenerational events in the offspring of irradiated parents are not well understood. In this study, for the first time, the features of the influence of transposable elements (TEs) on the long-term biological consequences of the ChNPP are considered. In this work, specimens of D. melanogaster obtained from natural populations in 2007 in the areas of the ChNPP with heterogeneous radioactive contamination were studied. The descendants from these populations were maintained in laboratory (inbred) conditions for 160 generations. A stable transgenerational transmission of dominant lethal mutations (DLMs) to the offspring of all studied populations was shown. The DLM frequencies strongly were correlated with the level of survival of offspring. The mean frequencies of recessive sex-linked lethal mutations varied at the level of spontaneous point mutations. The simultaneous presence of P, hobo and I elements indicates that the studied populations do not have a definite cytotype, their phenotypic status is unstable. The behavior of TEs in the genomes of offspring depends not only on parental exposure, but also on origin of population, distance to the ChNPP, and inbred conditions. The obtained results confirm the hypothesis that TEs are involved in transgenerational transmission and accumulation of mutations by the offspring of irradiated parents. The TEs pattern present in the Chernobyl genomes of D. melanogaster is a peculiar of epigenetic mechanism for the regulation of plasticity and adaptation of populations living for many generations under conditions of a technogenically caused radiation background.

Radiobiological features in offspring of natural populations of Drosophila melanogaster after Chernobyl accident

In their natural habitats, populations of organisms are faced with different levels of chronic low-intensity radiation, causing a wide range of radiobiological effects (from radiosensitivity to radioadaptive response and hormesis). In this study, specimens of Drosophila melanogaster were selected from territories of the Chernobyl nuclear power plant with different levels of radioactive contamination. The isogenic stocks derived from these specimens represent the genetic systems of current populations and make it possible to study radioresistance and its mechanisms in future generations under controlled laboratory conditions. Previous studies have shown that transgenerational radiation effects at the level of lethal mutations and survival rate are unstable and depend not only on the level of chronic low-intensity irradiation, but also on other factors. A single acute irradiation exposure of offspring whose parents inhabited a site with a higher level of chronic irradiation made it possible to reveal pronounced radioresistant features in the offspring. And the offspring whose parents were exposed to radiation levels close to the natural radiation background, on the contrary, acquired radiosensitive features. Their response to acute exposure includes a high-frequency of lethal mutations and a short lifespan. The differential response to different levels of chronic parental exposure is caused by differences in the activities of certain transposons that destabilize the genome. Our data contribute to the understanding of genetic and epigenetic mechanisms (via transposon activity) of the effect of parental radiation exposure on the health and adaptive potential of populations affected by the technogenically increased radiation background.

Radiobiological and social considerations following a radiological terrorist attack; mechanisms, detection and mitigation: review of new research developments

PURPOSE

This review focuses on recent research in understanding the different aspects of what society should expect from a radiological attack. Although some scenarios of a radiologic event can be impossible to be prepared for, the effort put toward educating and better preparing for these types of events can help minimize some of the issues. The different areas discussed in this review include radioisotopes of concern, detection of radiation dose, biological effects of ionizing radiation exposures, low dose effects, targeted and non-targeted effects (NTE), psychological effects, mitigations, with a brief mention of other considerations such as medical preparedness, communication, policy implications and ethical issues. This review also discusses solutions to rectify the issues faced at hand that may come up in the event of a radiologic terrorist attack.

CONCLUSIONS

A review of recent work in the area shows that a multi-layered and interdisciplinary approach is needed to prepare for a radiological terrorist attack. As well as medical preparedness, the approach needs to include sociological and psychological planning as well as an understanding of ethical issues. Since the likely 'dirty bomb' scenarios may involve low dose exposures to high numbers of people, a much better theoretical and practical understanding of low dose radiobiology and the development of robust low dose exposure biomarkers is needed as part of an integrated plan.

[Prospects for using low-dose radiation in the complex therapy for COVID-19]

This review presents the literature data of new approaches for the treatment of COVID-19 with low doses of radiation (LDR). In addition, data on the use of LDR for the treatment of various disorders, in particular pneumonia, a number of inflammatory processes of various etiology, as well as Alzheimer's disease are discussed. The mechanisms of LDR action are briefly described, associated with the activation of the immune system and antiinflammatory response due to the effect on the processes of oxidative stress, which is reflected in an increase in the activity of cytokines (interleukin- (IL-) 6), changes in the expression of a number of genes (such as P53 and NF-κB (p65)) and long non-coding RNAs (ncRNAs) (the authors' own data are presented). Based on the analysis of the material presented, it can be assumed that further clinical trials of the effect of MDR (5-10 cGy) on patients with COVID-19, who are at different stages of the disease, will reveal the optimal conditions for the development and use of an effective treatment regimen.

First transcriptome profiling of D. melanogaster after development in a deep underground low radiation background laboratory

Natural background radiation is a permanent multicomponent factor. It has an influence on biological organisms, but effects of its deprivation still remain unclear. The aim of our work was to study for the first time responses of D. melanogaster to conditions of the Deep Underground Low-Background Laboratory DULB-4900 (BNO, INR, RAS, Russia) at the transcriptome level by RNA-seq profiling. Overall 77 transcripts demonstrated differential abundance between flies exposed to low and natural background radiation. Enriched biological process functional categories were established for all genes with differential expression. The results showed down-regulation of primary metabolic processes and up-regulation of both the immune system process and the response to stimuli. The comparative analysis of our data and publicly available transcriptome data on D. melanogaster exposed to low and high doses of ionizing radiation did not reveal common DEGs in them. We hypothesize that the observed changes in gene expression can be explained by the influence of the underground conditions in DULB-4900, in particular, by the lack of stimuli. Thus, our study challenges the validity of the LNT model for the region of background radiation doses below a certain level (~16.4 nGy h-1) and the presence of a dose threshold for D. melanogaster.

Genome wide methylation profiling of selected matched soft tissue sarcomas identifies methylation changes in metastatic and recurrent disease

In this study we used the Illumina Infinium Methylation array to investigate in a cohort of matched archival human tissue samples (n = 32) from 14 individuals with soft tissue sarcomas if genome-wide methylation changes occur during metastatic and recurrent (Met/Rec) disease. A range of sarcoma types were selected for this study: leiomyosarcoma (LMS), myxofibrosarcoma (MFS), rhabdomyosarcoma (RMS) and synovial sarcoma (SS). We identified differential methylation in all Met/Rec matched samples, demonstrating that epigenomic differences develop during the clonal evolution of sarcomas. Differentially methylated regions and genes were detected, not been previously implicated in sarcoma progression, including at PTPRN2 and DAXX in LMS, WT1-AS and TNXB in SS, VENTX and NTRK3 in pleomorphic RMS and MEST and the C14MC / miR-379/miR-656 in MFS. Our overall findings indicate the presence of objective epigenetic differences across primary and Met/Rec human tissue samples not previously reported.

Anti-aging effect of DL-β-hydroxybutyrate against hepatic cellular senescence induced by D-galactose or γ-irradiation via autophagic flux stimulation in male rats

The present study aims to shed new light on anti-aging effect of DL-β-hydroxybutyrate (βOHB) against hepatic cellular senescence induced by d-galactose or γ-irradiation. The rats divided into 6 groups. Group 1, control, group 2, exposed to γ-ray (5 GY), group 3, injected by d-galactose (150 mg/kg) daily for consecutive 6 weeks, which regarded to induce the aging, group 4, injected intraperitoneal by β-hydroxybutyrate (βOHB) (72.8 mg/kg) daily for consecutive 14 days, group 5, exposed to γ-ray then treated with βOHB daily for consecutive 14 days, group 6, injected daily with d-galactose for consecutive 6 weeks, then treated with βOHB daily at the last two weeks of d-galactose. Aspartate amino transferase (AST), alanine amino transferase (ALT), Insulin, interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) were estimated in serum. Moreover, protein expression of Microtubule-associated proteins 1A/1B light chain 3B (LC3-II/LC3-I) ratio, mechanistic target of rapamycin (mTOR), pAMPK, mRNA gene expression of 5' AMP-activated protein kinase (AMPK), Nucleoporin p62 (p62), cyclin-dependent kinase inhibitor 1(P21^CIP1), cyclin-dependent kinase inhibitor 2A (p16^INK4a) and DNA fragmentation percentage were measured in liver tissue as a biomarker of cellular senescence. The results confirmed that βOHB modulated serum level of AST, ALT, insulin, IL-6 and TNF-α, protein expression of mTOR and LC3-II/LC3-I ratio, pAMPK and p62 in liver aging model induced by d-galactose or γ-irradiation. Histopathological examination results of liver tissue indicated coincidence with those recorded by molecular biochemical inspection. Taken together, these findings suggest that βOHB may be useful in combating hepatic cellular senescence induced by d-galactose or γ-irradiation via autophagy dependent mechanisms.

Ionizing Radiation-Induced Epigenetic Modifications and Their Relevance to Radiation Protection

The present system of radiation protection assumes that exposure at low doses and/or low dose-rates leads to health risks linearly related to the dose. They are evaluated by a combination of epidemiological data and radiobiological models. The latter imply that radiation induces deleterious effects via genetic mutation caused by DNA damage with a linear dose-dependence. This picture is challenged by the observation of radiation-induced epigenetic effects (changes in gene expression without altering the DNA sequence) and of non-linear responses, such as non-targeted and adaptive responses, that in turn can be controlled by gene expression networks. Here, we review important aspects of the biological response to ionizing radiation in which epigenetic mechanisms are, or could be, involved, focusing on the possible implications to the low dose issue in radiation protection. We examine in particular radiation-induced cancer, non-cancer diseases and transgenerational (hereditary) effects. We conclude that more realistic models of radiation-induced cancer should include epigenetic contribution, particularly in the initiation and progression phases, while the impact on hereditary risk evaluation is expected to be low. Epigenetic effects are also relevant in the dispute about possible "beneficial" effects at low dose and/or low dose-rate exposures, including those given by the natural background radiation.

Establishing mechanisms affecting the individual response to ionizing radiation

Purpose: Humans are increasingly exposed to ionizing radiation (IR). Both low (<100 mGy) and high doses can cause stochastic effects, including cancer; whereas doses above 100 mGy are needed to promote tissue or cell damage. 10-15% of radiotherapy (RT) patients suffer adverse reactions, described as displaying radiosensitivity (RS). Sensitivity to IR's stochastic effects is termed radiosusceptibility (RSu). To optimize radiation protection we need to understand the range of individual variability and underlying mechanisms. We review the potential mechanisms contributing to RS/RSu focusing on RS following RT, the most tractable RS group.Conclusions: The IR-induced DNA damage response (DDR) has been well characterized. Patients with mutations in the DDR have been identified and display marked RS but they represent only a small percentage of the RT patients with adverse reactions. We review the impacting mechanisms and additional factors influencing RS/RSu. We discuss whether RS/RSu might be genetically determined. As a recommendation, we propose that a prospective study be established to assess RS following RT. The study should detail tumor site and encompass a well-defined grading system. Predictive assays should be independently validated. Detailed analysis of the inflammatory, stress and immune responses, mitochondrial function and life style factors should be included. Existing cohorts should also be optimally exploited.

Relevance of Non-Targeted Effects for Radiotherapy and Diagnostic Radiology; A Historical and Conceptual Analysis of Key Players

Non-targeted effects (NTE) such as bystander effects or genomic instability have been known for many years but their significance for radiotherapy or medical diagnostic radiology are far from clear. Central to the issue are reported differences in the response of normal and tumour tissues to signals from directly irradiated cells. This review will discuss possible mechanisms and implications of these different responses and will then discuss possible new therapeutic avenues suggested by the analysis. Finally, the importance of NTE for diagnostic radiology and nuclear medicine which stems from the dominance of NTE in the low-dose region of the dose–response curve will be presented. Areas such as second cancer induction and microenvironment plasticity will be discussed.

Polygenic scores: Are they a public health hazard?

I argue here that polygenic scores are a public health hazard because the underlying methodology, genome wide association, from which they are derived, incorrectly assumes that the information encoded in the genomic DNA sequence is causal in terms of the cellular phenotype. This is not so when the cell is viewed from the perspective of a) fundamental physics, b) the protein chemistry that characterises the cellular cytoplasm and c) the fundamental requirement for evolution to yield unlimited species diversity.

Current evidence for a role of epigenetic mechanisms in response to ionizing radiation in an ecotoxicological context

The issue of potential long-term or hereditary effects for both humans and wildlife exposed to low doses (or dose rates) of ionising radiation is a major concern. Chronic exposure to ionising radiation, defined as an exposure over a large fraction of the organism's lifespan or even over several generations, can possibly have consequences in the progeny. Recent work has begun to show that epigenetics plays an important role in adaptation of organisms challenged to environmental stimulae. Changes to so-called epigenetic marks such as histone modifications, DNA methylation and non-coding RNAs result in altered transcriptomes and proteomes, without directly changing the DNA sequence. Moreover, some of these environmentally-induced epigenetic changes tend to persist over generations, and thus, epigenetic modifications are regarded as the conduits for environmental influence on the genome. Here, we review the current knowledge of possible involvement of epigenetics in the cascade of responses resulting from environmental exposure to ionising radiation. In addition, from a comparison of lab and field obtained data, we investigate evidence on radiation-induced changes in the epigenome and in particular the total or locus specific levels of DNA methylation. The challenges for future research and possible use of changes as an early warning (biomarker) of radiosensitivity and individual exposure is discussed. Such a biomarker could be used to detect and better understand the mechanisms of toxic action and inter/intra-species susceptibility to radiation within an environmental risk assessment and management context.

Residential exposure to radon and DNA methylation across the lifecourse: an exploratory study in the ALSPAC birth cohort

Background: Radon (and its decay products) is a known human carcinogen and the leading cause of lung cancer in never-smokers and the second in ever-smokers. The carcinogenic mechanism from radiation is a combination of genetic and epigenetic processes, but compared to the genetic mechanisms, epigenetic processes remain understudied in humans. This study aimed to explore associations between residential radon exposure and DNA methylation in the general population. Methods: Potential residential radon exposure for 75-metre area buffers was linked to genome-wide DNA methylation measured in peripheral blood from children and mothers of the Accessible Resource for Integrated Epigenomic Studies subsample of the ALSPAC birth cohort. Associations with DNA methylation were tested at over 450,000 CpG sites at ages 0, 7 and 17 years (children) and antenatally and during middle-age (mothers). Analyses were adjusted for potential residential and lifestyle confounding factors and were determined for participants with complete data (n = 786 to 980). Results: Average potential exposure to radon was associated in an exposure-dependent manner with methylation at cg25422346 in mothers during pregnancy, with no associations at middle age. For children, radon potential exposure was associated in an exposure-dependent manner with methylation of cg16451995 at birth, cg01864468 at age 7, and cg04912984, cg16105117, cg23988964, cg04945076, cg08601898, cg16260355 and cg26056703 in adolescence. Conclusions: Residential radon exposure was associated with DNA methylation in an exposure-dependent manner. Although chance and residual confounding cannot be excluded, the identified associations may show biological mechanisms involved in early biological effects from radon exposure.

Re-evaluation of the linear no-threshold (LNT) model using new paradigms and modern molecular studies

The linear no-threshold (LNT) model is currently used to estimate low dose radiation (LDR) induced health risks. This model lacks safety thresholds and postulates that health risks caused by ionizing radiation is directly proportional to dose. Therefore even the smallest radiation dose has the potential to cause an increase in cancer risk. Advances in LDR biology and cell molecular techniques demonstrate that the LNT model does not appropriately reflect the biology or the health effects at the low dose range. The main pitfall of the LNT model is due to the extrapolation of mutation and DNA damage studies that were conducted at high radiation doses delivered at a high dose-rate. These studies formed the basis of several outdated paradigms that are either incorrect or do not hold for LDR doses. Thus, the goal of this review is to summarize the modern cellular and molecular literature in LDR biology and provide new paradigms that better represent the biological effects in the low dose range. We demonstrate that LDR activates a variety of cellular defense mechanisms including DNA repair systems, programmed cell death (apoptosis), cell cycle arrest, senescence, adaptive memory, bystander effects, epigenetics, immune stimulation, and tumor suppression. The evidence presented in this review reveals that there are minimal health risks (cancer) with LDR exposure, and that a dose higher than some threshold value is necessary to achieve the harmful effects classically observed with high doses of radiation. Knowledge gained from this review can help the radiation protection community in making informed decisions regarding radiation policy and limits.

When a duck is not a duck; a new interdisciplinary synthesis for environmental radiation protection

This consensus paper presents the results of a workshop held in Essen, Germany in September 2017, called to examine critically the current approach to radiological environmental protection. The meeting brought together participants from the field of low dose radiobiology and those working in radioecology. Both groups have a common aim of identifying radiation exposures and protecting populations and individuals from harmful effects of ionising radiation exposure, but rarely work closely together. A key question in radiobiology is to understand mechanisms triggered by low doses or dose rates, leading to adverse outcomes of individuals while in radioecology a key objective is to recognise when harm is occurring at the level of the ecosystem. The discussion provided a total of six strategic recommendations which would help to address these questions.