Delayed chromosomal instability induced by DNA damage

The Response of Living Organisms to Low Radiation Environment and Its Implications in Radiation Protection

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.

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.

Small-Molecule Dual PLK1 and BRD4 Inhibitors are Active Against Preclinical Models of Pediatric Solid Tumors

Simultaneous inhibition of multiple molecular targets is an established strategy to improve the continuance of clinical response to therapy. Here, we screened 49 molecules with dual nanomolar inhibitory activity against BRD4 and PLK1, best classified as dual kinase-bromodomain inhibitors, in pediatric tumor cell lines for their antitumor activity. We identified two candidate dual kinase-bromodomain inhibitors with strong and tumor-specific activity against neuroblastoma, medulloblastoma, and rhabdomyosarcoma tumor cells. Dual PLK1 and BRD4 inhibitor treatment suppressed proliferation and induced apoptosis in pediatric tumor cell lines at low nanomolar concentrations. This was associated with reduced MYCN-driven gene expression as assessed by RNA sequencing. Treatment of patient-derived xenografts with dual inhibitor UMB103 led to significant tumor regression. We demonstrate that concurrent inhibition of two central regulators of MYC protein family of protooncogenes, BRD4, and PLK1, with single small molecules has strong and specific antitumor effects in preclinical pediatric cancer models.

Laser-Driven Ultrashort Pulsed Electron Beam Radiation at Doses of 0.5 and 1.0 Gy Induces Apoptosis in Human Fibroblasts

Rapidly evolving laser technologies have led to the development of laser-generated particle accelerators as an alternative to conventional facilities. However, the radiobiological characteristics need to be determined to enhance their applications in biology and medicine. In this study, the radiobiological effects of ultrashort pulsed electron beam (UPEB) and X-ray radiation in human lung fibroblasts (MRC-5 cell line) exposed to doses of 0.1, 0.5, and 1 Gy are compared. The changes of γH2AX foci number as a marker of DNA double-strand breaks (DSBs) were analyzed. In addition, the micronuclei induction and cell death via apoptosis were studied. We found that the biological action of UPEB-radiation compared to X-rays was characterized by significantly slower γH2AX foci elimination (with a dose of 1 Gy) and strong apoptosis induction (with doses of 0.5 and 1.0 Gy), accompanied by a slight increase in micronuclei formation (dose of 1 Gy). Our data suggest that UPEB radiation produces more complex DNA damage than X-ray radiation, leading to cell death rather than cytogenetic disturbance.

Radiation Biology and Its Role in the Canadian Radiation Protection Framework

The linear no-threshold (linear-non-threshold) model is a dose-response model that has long served as the foundation of the international radiation protection framework, which includes the Canadian regulatory framework. Its purpose is to inform the choice of appropriate dose limits and subsequent as low as reasonably achievable requirements, social and economic factors taken into account. The linear no-threshold model assumes that the risk of developing cancer increases proportionately with increasing radiation dose. The linear no-threshold model has historically been applied by extrapolating the risk of cancer at high doses (>1,000 mSv) down to low doses in a linear manner. As the health effects of radiation exposure at low doses remain ambiguous, reducing uncertainties found in cancer risk dose-response models can be achieved through in vitro and animal-based studies. The purpose of this critical review is to analyze whether the linear no-threshold model is still applicable for use by modern nuclear regulators for radiation protection purposes, or if there is sufficient scientific evidence supporting an alternate model from which to derive regulatory dose limits.

The CGL1 (HeLa × Normal Skin Fibroblast) Human Hybrid Cell Line: A History of Ionizing Radiation Induced Effects on Neoplastic Transformation and Novel Future Directions in SNOLAB

Cellular transformation assays have been utilized for many years as powerful in vitro methods for examining neoplastic transformation potential/frequency and mechanisms of carcinogenesis for both chemical and radiological carcinogens. These mouse and human cell based assays are labor intensive but do provide quantitative information on the numbers of neoplastically transformed foci produced after carcinogenic exposure and potential molecular mechanisms involved. Several mouse and human cell systems have been generated to undertake these studies, and they vary in experimental length and endpoint assessment. The CGL1 human cell hybrid neoplastic model is a non-tumorigenic pre-neoplastic cell that was derived from the fusion of HeLa cervical cancer cells and a normal human skin fibroblast. It has been utilized for the several decades to study the carcinogenic/neoplastic transformation potential of a variety of ionizing radiation doses, dose rates and radiation types, including UV, X ray, gamma ray, neutrons, protons and alpha particles. It is unique in that the CGL1 assay has a relatively short assay time of 18-21 days, and rather than relying on morphological endpoints to detect neoplastic transformation utilizes a simple staining method that detects the tumorigenic marker alkaline phosphatase on the neoplastically transformed cells cell surface. In addition to being of human origin, the CGL1 assay is able to detect and quantify the carcinogenic potential of very low doses of ionizing radiation (in the mGy range), and utilizes a neoplastic endpoint (re-expression of alkaline phosphatase) that can be detected on both viable and paraformaldehyde fixed cells. In this article, we review the history of the CGL1 neoplastic transformation model system from its initial development through the wide variety of studies examining the effects of all types of ionizing radiation on neoplastic transformation. In addition, we discuss the potential of the CGL1 model system to investigate the effects of near zero background radiation levels available within the radiation biology lab we have established in SNOLAB.

Paths from DNA damage and signaling to genome rearrangements via homologous recombination

DNA damage is a constant threat to genome integrity. DNA repair and damage signaling networks play a central role maintaining genome stability, suppressing tumorigenesis, and determining tumor response to common cancer chemotherapeutic agents and radiotherapy. DNA double-strand breaks (DSBs) are critical lesions induced by ionizing radiation and when replication forks encounter damage. DSBs can result in mutations and large-scale genome rearrangements reflecting mis-repair by non-homologous end joining or homologous recombination. Ionizing radiation induces genetic change immediately, and it also triggers delayed events weeks or even years after exposure, long after the initial damage has been repaired or diluted through cell division. This review covers DNA damage signaling and repair pathways and cell fate following genotoxic insult, including immediate and delayed genome instability and cell survival/cell death pathways.

Early and Delayed Induction of DSBs by Nontargeted Effects in ICR Mouse Lymphocytes after In Vivo X Irradiation

The goal of this study was to determine whether in vivo X irradiation induces nontargeted effects, such as delayed effects and bystander effects in ICR mouse lymphocytes. We first examined the generation of DNA double-strand breaks (DSBs) in lymphocytes, isolated from ICR mice exposed to 1 Gy X irradiation, by enumeration of p53 binding protein 1 (53BP1) foci, and observed that the number of 53BP1 foci reached their maximum 3 days postirradiation and decreased to background level 30 days postirradiation. However, the number of 53BP1 foci was significantly increased in lymphocytes isolated from ICR mice 90-365 days postirradiation. This result indicates that in vivo X irradiation induced delayed DSBs in ICR mouse lymphocytes. We next counted the number of 53BP1 foci in lymphocytes isolated from sham-irradiated ICR mice that had been co-cultured with lymphocytes isolated from 1 Gy X-irradiated ICR mice, and observed a significant increase in the number of 53BP1 foci 1-7 days postirradiation. This result indicates that in vivo X irradiation induced bystander effects in ICR mouse lymphocytes. These findings suggest that in vivo X irradiation induces early and delayed nontargeted effects in ICR mouse lymphocytes.

Non-targeted effects and radiation-induced carcinogenesis: a review

Exposure to ionising radiation is clearly associated with an increased risk of developing some types of cancer. However, the contribution of non-targeted effects to cancer development after exposure to ionising radiation is far less clear. The currently used cancer risk model by the international radiation protection community states that any increase in radiation exposure proportionately increases the risk of developing cancer. However, this stochastic cancer risk model does not take into account any contribution from non-targeted effects. Nor does it consider the possibility of a bystander mechanism in the induction of genomic instability. This paper reviews the available evidence to date for a possible role for non-targeted effects to contribute to cancer development after exposure to ionising radiation. An evolution in the understanding of the mechanisms driving non-targeted effects after exposure to ionising radiation is critical to determine the true contribution of non-targeted effects on the risk of developing cancer. Such an evolution will likely only be achievable through coordinated multidisciplinary teams combining several fields of study including: genomics, proteomics, cell biology, molecular epidemiology, and traditional epidemiology.

The role of oxidative stress on breast cancer development and therapy

Reactive oxygen species (ROS) are produced by both enzymatic and non-enzymatic systems within eukaryotic cells and play important roles in cellular physiology and pathophysiology. Although physiological concentrations are crucial for ensuring cell survival, ROS overproduction is detrimental to cells, and considered key-factors for the development of several diseases, such as neurodegenerative diseases, cardiovascular disorders, and cancer. Cancer cells are usually submitted to higher ROS levels that further stimulate malignant phenotype through stimulus to sustained proliferation, death evasion, angiogenesis, invasiveness, and metastasis. The role of ROS on breast cancer etiology and progression is being progressively elucidated. However, less attention has been given to the development of redox system-targeted strategies for breast cancer therapy. In this review, we address the basic mechanisms of ROS production and scavenging in breast tumor cells, and the emerging possibilities of breast cancer therapies targeting ROS homeostasis.

Theoretical considerations for thresholds in chemical carcinogenesis

There is increasing evidence for non-linear relationships for gene mutations, chromosomal aberrations and even tumor incidences in response to low doses of genotoxic carcinogens. To attain the biological relevance of such non-linear responses, there is a need to identify the underlying defense mechanisms that allow tolerance to low doses of genotoxicants. This communication discusses presumptive cancer prevention mechanisms that may contribute to thresholds, i.e. points of departure, for each endpoint, from initial DNA lesion to tumor formation. We discuss a sequential order of genome protection during carcinogenesis where genotoxicant scavenging, cellular efflux, DNA repair, elimination of damaged cells by apoptosis, autophagy, silencing by DNA damage-triggered replicative senescence, and finally, elimination of transformed (premalignant) cells by the immune system are thought to be responsible for a threshold in tumor formation. We highlight DNA repair, for which experimental evidence has been recently provided to dictate a role in PoDs. In conclusion, from a theoretical perspective it is reasonable to posit that tolerance to low dose levels exists for each requisite step of tumor formation and these tolerance mechanisms are critical in determining thresholds in chemical carcinogenesis.

DNA damage in cells exhibiting radiation-induced genomic instability

Cells exhibiting radiation-induced genomic instability exhibit varied spectra of genetic and chromosomal aberrations. Even so, oxidative stress remains a common theme in the initiation and/or perpetuation of this phenomenon. Isolated oxidatively modified bases, abasic sites, DNA single strand breaks and clustered DNA damage are induced in normal mammalian cultured cells and tissues due to endogenous reactive oxygen species generated during normal cellular metabolism in an aerobic environment. While sparse DNA damage may be easily repaired, clustered DNA damage may lead to persistent cytotoxic or mutagenic events that can lead to genomic instability. In this study, we tested the hypothesis that DNA damage signatures characterised by altered levels of endogenous, potentially mutagenic, types of DNA damage and chromosomal breakage are related to radiation-induced genomic instability and persistent oxidative stress phenotypes observed in the chromosomally unstable progeny of irradiated cells. The measurement of oxypurine, oxypyrimidine and abasic site endogenous DNA damage showed differences in non-double-strand breaks (DSB) clusters among the three of the four unstable clones evaluated as compared to genomically stable clones and the parental cell line. These three unstable clones also had increased levels of DSB clusters. The results of this study demonstrate that each unstable cell line has a unique spectrum of persistent damage and lead us to speculate that alterations in DNA damage signaling and repair may be related to the perpetuation of genomic instability.

Down-regulation of G9a triggers DNA damage response and inhibits colorectal cancer cells proliferation

G9a, a histone methyltransferase, is aberrantly expressed in some human tumor types. By comparing 182 paired colorectal cancer and peritumoral tissues, we found that G9a was highly expressed in colorectal cancer (CRC). Overexpression of G9a promoted CRC cells proliferation and colony formation, whereas knockdown of G9a inhibited CRC cells proliferation. Depletion of G9a increased the rate of chromosome aberration, induced DNA double strand breaks and CRC cells senescence. G9a inhibition synergistically increased γH2AX expression induced by topoisomerase I inhibitors and ultimately led to CRC cell death. The findings that down-regulation of G9a triggers DNA damage response and inhibits colorectal cancer cells proliferation may define G9a as potential oncotarget in CRC.

Advances in radiation biology: effect on nuclear medicine

Over the past 15 years and more, extensive research has been conducted on the responses of biological systems to radiation delivered at a low dose or low dose rate. This research has demonstrated that the molecular-, cellular-, and tissue-level responses are different following low doses than those observed after a single short-term high-dose radiation exposure. Following low-dose exposure, 3 unique responses were observed, these included bystander effects, adaptive protective responses, and genomic instability. Research on the mechanisms of action for each of these observations demonstrates that the molecular and cellular processes activated by low doses of radiation are often related to protective responses, whereas high-dose responses are often associated with extensive damage such as cell killing, tissue disruption, and inflammatory diseases. Thus, the mechanisms of action are unique for low-dose radiation exposure. When the dose is delivered at a low dose rate, the responses typically differ at all levels of biological organization. These data suggest that there must be a dose rate effectiveness factor that is greater than 1 and that the risk following low-dose rate exposure is likely less than that for single short-term exposures. All these observations indicate that using the linear no-threshold model for radiation protection purposes is conservative. Low-dose research therefore supports the current standards and practices. When a nuclear medical procedure is justified, it should be carried out with optimization (lowest radiation dose commensurate with diagnostic or therapeutic outcome).

Genetic and epigenetic changes in chromosomally stable and unstable progeny of irradiated cells

Radiation induced genomic instability is a well-studied phenomenon, the underlying mechanisms of which are poorly understood. Persistent oxidative stress, mitochondrial dysfunction, elevated cytokine levels and epigenetic changes are among the mechanisms invoked in the perpetuation of the phenotype. To determine whether epigenetic aberrations affect genomic instability we measured DNA methylation, mRNA and microRNA (miR) levels in well characterized chromosomally stable and unstable clonally expanded single cell survivors of irradiation. While no changes in DNA methylation were observed for the gene promoters evaluated, increased LINE-1 methylation was observed for two unstable clones (LS12 and CS9) and decreased Alu element methylation was observed for the other two unstable clones (115 and Fe5.0–8). These relationships also manifested for mRNA and miR expression. mRNA identified for the LS12 and CS9 clones were most similar to each other (261 mRNA), while the 115 and Fe5.0–8 clones were more similar to each other, and surprisingly also similar to the two stable clones, 114 and 118 (286 mRNA among these four clones). Pathway analysis showed enrichment for pathways involved in mitochondrial function and cellular redox, themes routinely invoked in genomic instability. The commonalities between the two subgroups of clones were also observed for miR. The number of miR for which anti-correlated mRNA were identified suggests that these miR exert functional effects in each clone. The results demonstrate significant genetic and epigenetic changes in unstable cells, but similar changes are almost as equally common in chromosomally stable cells. Possible conclusions might be that the chromosomally stable clones have some other form of instability, or that some of the observed changes represent a sort of radiation signature and that other changes are related to genomic instability. Irrespective, these findings again suggest that a spectrum of changes both drive genomic instability and permit unstable cells to persist and proliferate.

Implications for human and environmental health of low doses of ionising radiation

The last 20 years have seen a major paradigm shift in radiation biology. Several discoveries challenge the DNA centric view which holds that DNA damage is the critical effect of radiation irrespective of dose. This theory leads to the assumption that dose and effect are simply linked - the more energy deposition, the more DNA damage and the greater the biological effect. This is embodied in radiation protection (RP) regulations as the linear-non-threshold (LNT) model. However the science underlying the LNT model is being challenged particularly in relation to the environment because it is now clear that at low doses of concern in RP, cells, tissues and organisms respond to radiation by inducing responses which are not readily predictable by dose. These include adaptive responses, bystander effects, genomic instability and low dose hypersensitivity, and are commonly described as stress responses, while recognizing that "stress" can be good as well as bad. The phenomena contribute to observed radiation responses and appear to be influenced by genetic, epigenetic and environmental factors, meaning that dose and response are not simply related. The question is whether our discovery of these phenomena means that we need to re-evaluate RP approaches. The so-called "non-targeted" mechanisms mean that low dose radiobiology is very complex and supra linear or sub-linear (even hormetic) responses are possible but their occurrence is unpredictable for any given system level. Issues which may need consideration are synergistic or antagonistic effects of other pollutants. RP, at present, only looks at radiation dose but the new (NTE) radiobiology means that chemical or physical agents, which interfere with tissue responses to low doses of radiation, could critically modulate the predicted risk. Similarly, the "health" of the organism could determine the effect of a given low dose by enabling or disabling a critical response. These issues will be discussed.

Responses to ionizing radiation mediated by inflammatory mechanisms

Since the early years of the twentieth century, the biological consequences of exposure to ionizing radiation have been attributed solely to mutational DNA damage or cell death induced in irradiated cells at the time of exposure. However, numerous observations have been at variance with this dogma. In the 1950s, attention was drawn to abscopal effects in areas of the body not directly irradiated. In the 1960s reports began appearing that plasma factors induced by irradiation could affect unirradiated cells, and since 1990 a growing literature has documented an increased rate of DNA damage in the progeny of irradiated cells many cell generations after the initial exposure (radiation-induced genomic instability) and responses in non-irradiated cells neighbouring irradiated cells (radiation-induced bystander effects). All these studies have in common the induction of effects not in directly irradiated cells but in unirradiated cells as a consequence of intercellular signalling. Recently, it has become clear that all the various effects demonstrated in vivo may reflect an ongoing inflammatory response to the initial radiation-induced injury that, in a genotype-dependent manner, has the potential to contribute primary and/or ongoing damage displaced in time and/or space from the initial insult. Importantly, there is direct evidence that non-steroidal anti-inflammatory drug treatment reduces such damage in vivo. These new findings highlight the importance of tissue responses and indicate additional mechanisms of radiation action, including the likelihood that radiation effects are not restricted to the initiation stage of neoplastic diseases, but may also contribute to tumour promotion and progression. The various developments in understanding the responses to radiation exposures have implications not only for radiation pathology but also for therapeutic interventions.

Embryos of the zebrafish Danio rerio in studies of non-targeted effects of ionizing radiation

The use of embryos of the zebrafish Danio rerio as an in vivo tumor model for studying non-targeted effects of ionizing radiation was reviewed. The zebrafish embryo is an animal model, which enables convenient studies on non-targeted effects of both high-linear-energy-transfer (LET) and low-LET radiation by making use of both broad-beam and microbeam radiation. Zebrafish is also a convenient embryo model for studying radiobiological effects of ionizing radiation on tumors. The embryonic origin of tumors has been gaining ground in the past decades, and efforts to fight cancer from the perspective of developmental biology are underway. Evidence for the involvement of radiation-induced genomic instability (RIGI) and the radiation-induced bystander effect (RIBE) in zebrafish embryos were subsequently given. The results of RIGI were obtained for the irradiation of all two-cell stage cells, as well as 1.5 hpf zebrafish embryos by microbeam protons and broad-beam alpha particles, respectively. In contrast, the RIBE was observed through the radioadaptive response (RAR), which was developed against a subsequent challenging dose that was applied at 10 hpf when <0.2% and <0.3% of the cells of 5 hpf zebrafish embryos were exposed to a priming dose, which was provided by microbeam protons and broad-beam alpha particles, respectively. Finally, a perspective on the field, the need for future studies and the significance of such studies were discussed.

A framework for analysis of abortive colony size distributions using a model of branching processes in irradiated normal human fibroblasts

Background

Clonogenicity gives important information about the cellular reproductive potential following ionizing irradiation, but an abortive colony that fails to continue to grow remains poorly characterized. It was recently reported that the fraction of abortive colonies increases with increasing dose. Thus, we set out to investigate the production kinetics of abortive colonies using a model of branching processes.

Methodology/Principal Findings

We firstly plotted the experimentally determined colony size distribution of abortive colonies in irradiated normal human fibroblasts, and found the linear relationship on the log-linear or log-log plot. By applying the simple model of branching processes to the linear relationship, we found the persistent reproductive cell death (RCD) over several generations following irradiation. To verify the estimated probability of RCD, abortive colony size distribution (≤15 cells) and the surviving fraction were simulated by the Monte Carlo computational approach for colony expansion. Parameters estimated from the log-log fit demonstrated the good performance in both simulations than those from the log-linear fit. Radiation-induced RCD, i.e. excess probability, lasted over 16 generations and mainly consisted of two components in the early (<3 generations) and late phases. Intriguingly, the survival curve was sensitive to the excess probability over 5 generations, whereas abortive colony size distribution was robust against it. These results suggest that, whereas short-term RCD is critical to the abortive colony size distribution, long-lasting RCD is important for the dose response of the surviving fraction.

Conclusions/Significance

Our present model provides a single framework for understanding the behavior of primary cell colonies in culture following irradiation.

Alternative medicine techniques have non-linear effects on radiation response and can alter the expression of radiation induced bystander effects

Many so-called "alternative medicine" techniques such as Reiki and acupuncture produce very good outcomes for intractable pain and other chronic illnesses but the efficacy is often dismissed as being psychosomatic. However a plausible mechanism does exist i.e. that the treatments alter the electromagnetic fields in living organisms and thereby prevent or reduce activity of neurons which lead to the pain. Low doses of ionising radiation have similar effects on electromagnetic fields and are known to induce signaling cascades in tissues due to ion gradients. To test this hypothesis cell cultures were exposed to Reiki - like and to acupuncture - like treatments, both performed by qualified practitioners. The cells were exposed either before or after the treatment to x-rays and were monitored for production of direct damage or bystander signals. The data suggest that the alternative techniques altered the response of cells to direct irradiation and altered bystander signal mechanisms. We conclude that alternative medicine techniques involving electromagnetic perturbations may modify the response of cells to ionizing radiation. In addition to the obvious implications for mechanistic studies of low dose effects, this could provide a novel target to exploit in radiation protection and in optimizing therapeutic gain during radiotherapy.

Lack of genomic instability in bone marrow cells of SCID mice exposed whole-body to low-dose radiation

It is clear that high-dose radiation is harmful. However, despite extensive research, assessment of potential health-risks associated with exposure to low-dose radiation (at doses below or equal to 0.1 Gy) is still challenging. Recently, we reported that 0.05 Gy of 137Cs gamma rays (the existing limit for radiation-exposure in the workplace) was incapable of inducing significant in vivo genomic instability (measured by the presence of late-occurring chromosomal damage at 6 months post-irradiation) in bone marrow (BM) cells of two mouse strains, one with constitutively high and one with intermediate levels of the repair enzyme DNA-dependent protein-kinase catalytic-subunit (DNA-PKcs). In this study, we present evidence for a lack of genomic instability in BM cells of the severely combined-immunodeficiency (SCID/J) mouse (which has an extremely low-level of DNA-PKcs activity) exposed whole-body to low-dose radiation (0.05 Gy). Together with our previous report, the data indicate that low-dose radiation (0.05 Gy) is incapable of inducing genomic instability in vivo (regardless of the levels of DNA-PKcs activity of the exposed mice), yet higher doses of radiation (0.1 and 1 Gy) do induce genomic instability in mice with intermediate and extremely low-levels of DNA-PKcs activity (indicating an important role of DNA-PKcs in DNA repair).

No significant level of inheritable interchromosomal aberrations in the progeny of bystander primary human fibroblast after alpha particle irradiation

A major concern for bystander effects is the probability that normal healthy cells adjacent to the irradiated cells become genomically unstable and undergo further carcinogenesis after therapeutic irradiation or space mission where astronauts are exposed to low dose of heavy ions. Genomic instability is a hallmark of cancer cells. In the present study, two irradiation protocols were performed in order to ensure pure populations of bystander cells and the genomic instability in their progeny were investigated. After irradiation, chromosomal aberrations of cells were analyzed at designated time points using G₂ phase premature chromosome condensation (G₂-PCC) coupled with Giemsa staining and with multiplex fluorescent in situ hybridization (mFISH). Our Giemsa staining assay demonstrated that elevated yields of chromatid breaks were induced in the progeny of pure bystander primary fibroblasts up to 20 days after irradiation. MFISH assay showed no significant level of inheritable interchromosomal aberrations were induced in the progeny of the bystander cell groups, while the fractions of gross aberrations (chromatid breaks or chromosomal breaks) significantly increased in some bystander cell groups. These results suggest that genomic instability occurred in the progeny of the irradiation associated bystander normal fibroblasts exclude the inheritable interchromosomal aberration.

Chromosomal instability induced by mammography X-rays in primary human fibroblasts from BRCA1 and BRCA2 mutation carriers

PURPOSE

Mammography X-rays are known to induce DNA double-strand breaks (DSB) whose error-free recombinational repair requires the function of the tumour repressor genes BRCA1 (breast-cancer-associated gene 1) and BRCA2 (breast-cancer-associated gene 2). Since un- or misrepaired DSB lead to chromosomal anomalies which may promote the development of breast cancer, we have studied the potential of mammography X-rays for immediate and delayed induction of chromosomal anomalies in human primary fibroblasts from BRCA1 and BRCA2 mutation carriers.

MATERIALS AND METHODS

Primary human fibroblasts from three BRCA1, three BRCA2 mutation carriers, one BRCA2-deficient fanconi anemia (FA) patient and three normal individuals were exposed to various doses of mammography X-rays. Chromosomal anomalies at first mitosis and at several population doublings post-irradiation were assayed (Giemsa staining and Fish [fluorescence in situ hybridization]).

RESULTS

No effect of the BRCA mutation status was observed on survival curves after exposure to mammography X-rays and on the dose-dependent increase of chromosomal anomalies at first mitosis post-irradiation. In contrast, several population doublings after exposure to a low dose of only 0.5 Gy chromosomal instability, manifested as gross chromosomal rearrangements and aneuploidy, had developed in BRCA2-deficient FA fibroblasts and in some - but not all - BRCA heterozygous fibroblasts.

CONCLUSIONS

Low doses of mammography X-rays have the potential to induce chromosomal instability in fibroblasts from BRCA mutation carriers: Cells exhibit gross chromosomal rearrangements and aneuploidy similar to those observed in breast cancer cells. These results suggest that for women carrying a BRCA mutation early and frequent screening with mammography X-rays may not be the method of choice to detect breast cancer.

The in vivo Pig-a gene mutation assay is useful for evaluating the genotoxicity of ionizing radiation in mice

The in vivo Pig-a mutation assay has been adapted for measuring mutation in rats, mice, monkeys, and humans. To date, the assay has been used mainly to assess the mutagenicity of chemicals that are known to be powerful point mutagens. The assay has not been used to measure the biological effects associated with ionizing radiation. In this study, we modified the Pig-a gene mutation assay (Kimoto et al. [2011b]: Mutat Res 723:36-42) and used 3-color staining with fluorescently labeled anti-CD24, anti-TER-119, and anti-CD71 to detect the Pig-a mutant frequencies in total red blood cells (RBCs) and in reticulocytes (RETs) from X-irradiated mice. Single exposures to X-irradiation resulted in dose- and time-dependent increases in Pig-a mutant frequencies, and these subsequently declined over time returning to background frequencies. The same total amount of radiation, delivered either as a single dose or as four repeat doses at weekly intervals, increased Pig-a mutant frequencies to comparable levels, reaching maxima 2-3 weeks after the single dose or 2-3 weeks after the last of the repeat doses. These increased frequencies subsequently returned to background levels. Our results indicated that the 3-color Pig-a assay was useful for evaluating the in vivo genotoxicity of radiation.

The progression potential of peritoneal dissemination nodules from gastrointestinal tumors

It is necessary to examine the characteristics of the dissemination nodules to establish a therapeutic strategy for peritoneal dissemination from digestive malignancy. Ki-67 expression as a proliferation marker in peritoneal dissemination nodules was investigated. The subjects were 15 patients with gastrointestinal cancers who underwent resection of the primary tumor and disseminated nodules. The expression of Ki-67 in both primary tumor and peritoneal dissemination nodule from each patient was evaluated by immunohistochemistry. Ki-67 labeling index in the original tumor was higher than that in the disseminated nodule in 13 of 15 patients (P < 0.0001). The mean value of Ki-67 labeling index was 42.2% in the 15 original tumors and 18.7% in the 15 disseminated nodules. Proliferative activity in the disseminated nodules was lower than that in the primary tumors. Further examination about characteristics of cancer dissemination is needed to treat patients with peritoneal metastasis.

Streptonigrin induces delayed chromosomal instability involving interstitial telomeric sequences in Chinese hamster ovary cells

We analyzed the induction of chromosomal aberrations in Chinese hamster ovary (CHO) cells exposed to the radiomimetic compound streptonigrin (SN), in order to determine whether interstitial telomeric sequences (ITSs) are involved in the long-term clastogenic effect of this antibiotic. CHO cells were treated with a single concentration of SN (100ng/ml), and the frequency of unstable chromosomal aberrations was determined at three times after treatment (18h, and 6 and 15 days) by using PNA-FISH with a pan-telomeric probe. Cytogenetic analysis revealed a higher frequency of aberrations at 18h and 6 days after treatment in SN-exposed cultures vs. untreated cultures. The percentage of damaged cells and the yield of SN-induced aberrations at 6 days after treatment increased on average twofold compared with the ones at 18h after treatment. Moreover, a significant decrease in the frequency of aberrations was observed in SN-exposed cells at 15 days after treatment, resulting in a frequency of aberrations significantly lower than the frequency of aberrations observed in the corresponding control cultures. These data indicate that SN induces delayed chromosomal instability in CHO cells, and that the in vitro clastogenic effect of this compound persists for at least 6 days but less than 15 days after treatment. In addition, we found that SN induces delayed ITSs instability, cytogenetically detectable as additional FISH signals and centromeric breaks involving dissociation of the telomeric signal 6 days after treatment. We propose that the delayed effect of SN on ITSs results from breakage of heterochromatic centromeric ITSs blocks and further insertion of these sequences at the sites of mono- or isochromatid breaks occurring at G2 or G1-S phases of the cell cycle, respectively, since most of the additional FISH signals were present as single or double dots, and located at interstitial sites of the involved chromosomes.

Bleomycin induces delayed instability of interstitial telomeric sequences in Chinese hamster ovary cells

We analyzed the behavior of interstitial telomeric sequences (ITSs) in the progeny of Chinese Hamster Ovary (CHO) cells exposed to the radiomimetic compound bleomycin (BLM) in order to determine if ITSs play some role in the long-term clastogenic effect of this antibiotic. To this end, CHO cells were treated with a single concentration of BLM (2.5μg/ml), and the frequency of unstable chromosomal aberrations was determined at several times after treatment (18h, and 6, 15 and 34/36 days) by using PNA-FISH with a pan-telomeric probe [(TTAGGG)n repeats]. Cytogenetic analysis revealed a higher frequency of aberrations at 18h and 6 days after treatment in BLM-exposed cultures vs. untreated cultures, although the yield of BLM-induced aberrations decreased on average five times 6 days after treatment compared with the one induced 18h after treatment. Moreover, no significant differences in the frequency of aberrations were observed between untreated and BLM-exposed cells at 15 or 34/36 days after treatment. These data indicate that, in terms of unstable aberrations, the in vitro clastogenic effect of BLM on CHO cells persists for at least 6 days but less than 15 days after exposure. In addition, we found that BLM induces ITSs instability, cytogenetically detectable as acentric fragments (18h after treatment) or additional (new) FISH signals (6 days after treatment). We propose that the delayed effect of BLM on ITSs mainly results from breakage of heterochromatic ITSs blocks and further insertion of these sequences at the sites of monochromatid breaks occurring at G2 phase of the cell cycle, since most of the additional FISH signals were present as single dots and located at interstitial sites of the involved chromosomes.

No Evidence for the In Vivo Induction of Genomic Instability by Low Doses of CS Gamma Rays in Bone Marrow Cells of BALB/CJ and C57BL/6J Mice

In spite of extensive research, assessment of potential health risks associated with exposure to low-dose (≤ 0.1 Gy) radiation is still challenging. We evaluated the in vivo induction of genomic instability, expressed as late-occurring chromosome aberrations, in bone-marrow cells of two strains of mouse with different genetic background, i.e. the radiosensitive BALB/cJ and the radioresistant C57BL/6J strains following a whole-body exposure to varying doses of (137)Cs gamma rays (0, 0.05, 0.1, and 1.0 Gy). A total of five mice per dose per strain were sacrificed at various times post-irradiation up to 6 months for sample collections. Three-color fluorescence in situ hybridization for mouse chromosomes 1, 2, and 3 was used for the analysis of stable-aberrations in metaphase-cells. All other visible gross structural-abnormalities involving non-painted-chromosomes were also evaluated on the same metaphase-cells used for scoring the stable-aberrations of painted-chromosomes. Our new data demonstrated in bone-marrow cells from both strains that low doses of low LET-radiation (as low as 0.05 Gy) are incapable of inducing genomic instability but are capable of reducing specific aberration-types below the spontaneous rate with time post-irradiation. However, the results showed the induction of genomic instability by 1.0 Gy of (137)Cs gamma rays in the radiosensitive strain only.

Ionizing radiation is a potent inducer of mitotic recombination in mouse embryonic stem cells

Maintenance of genomic integrity in embryonic cells is pivotal to proper embryogenesis, organogenesis and to the continuity of species. Cultured mouse embryonic stem cells (mESCs), a model for early embryonic cells, differ from cultured somatic cells in their capacity to remodel chromatin, in their repertoire of DNA repair enzymes, and in the regulation of cell cycle checkpoints. Using 129XC3HF1 mESCs heterozygous for Aprt, we characterized loss of Aprt heterozygosity after exposure to ionizing radiation. We report here that the frequency of loss of heterozygosity mutants in mESCs can be induced several hundred-fold by exposure to 5-10Gy of X-rays. This induction is 50-100-fold higher than the induction reported for mouse adult or embryonic fibroblasts. The primary mechanism underlying the elevated loss of heterozygosity after irradiation is mitotic recombination, with lesser contributions from deletions and gene conversions that span Aprt. Aprt point mutations and epigenetic inactivation are very rare in mESCs compared to fibroblasts. Mouse ESCs, therefore, are distinctive in their response to ionizing radiation and studies of differentiated cells may underestimate the mutagenic effects of ionizing radiation on ESC or other stem cells. Our findings are important to understanding the biological effects of ionizing radiation on early development and carcinogenesis.

Detection of chromosomal instability in bystander cells after Si490-ion irradiation

There is increasing evidence that two of the biological effects associated with low-dose ionizing radiation, genomic instability and bystander responses, may be linked. To verify and validate the link between the two phenomena, the ability of Si490 ions (high-energy particles associated with radiation risk in space) to induce bystander responses and chromosomal instability in human bronchial epithelial (HBEC-3kt) cells was investigated. These studies were conducted at both the population and single cell level in irradiated and nonirradiated bystander cells receiving medium from the irradiated cultures. At the general population level, transfer of medium from silicon-ion (Si490)-irradiated cultures (at doses of 0.073 Gy, 1.2 Gy and 2 Gy) to nonirradiated bystander cells resulted in small increases in the levels of chromosomal aberrations at the first division. Subsequently, single cell clones isolated from irradiated and bystander populations were analyzed for the appearance of de novo chromosome-type aberrations after ∼50 population doublings using mFISH. Both irradiated and bystander clones demonstrated chromosomal instability (as seen by the de novo appearance of translocations and chromosomal fragments), albeit to different degrees, whereas sham-treated controls showed relatively stable chromosomal patterns. The results presented here highlight the importance of nontargeted effects of radiation on chromosomal instability in human epithelial cells and their potential relevance to human health.

Radiation-induced genomic instability

Radiation induced genomic instability can be observed in the progeny of irradiated cells multiple generations after irradiation of parental cells. The phenotype is well established both in vivo (Morgan 2003) and in vitro (Morgan 2003), and may be critical in radiation carcinogenesis (Little 2000, Huang et al. 2003). Instability can be induced by both the deposition of energy in irradiated cells as well as by signals transmitted by irradiated (targeted) cells to non-irradiated (non-targeted) cells (Kadhim et al. 1992, Lorimore et al. 1998). Thus both targeted and non-targeted cells can pass on the legacy of radiation to their progeny. However the radiation induced events and cellular processes that respond to both targeted and non-targeted radiation effects that lead to the unstable phenotype remain elusive.

Dose-response prediction for radiation-induced chromosomal instability

Radiation induces chromosome aberrations (CA) that are detected in the first post-irradiation cell cycle and in descendants of irradiated cells. Unstable aberrations in the progeny of exposed cells are referred to as one of the hallmarks of chromosomal instability (CIN). One of the important questions is what is the relationship between the dose response for radiation-induced CA and delayed CA, or CIN. To address this question, a mechanistic model for CIN was developed. Delayed CA are assumed to be formed both by transmission from previous mitotic cycles owing to chromosome breakage-fusion mechanism and by means of generation of DNA/chromosome breakage de novo in each cell cycle of survived cells. Monte Carlo simulation of DNA/chromosome breakage, CA production, cell death due to unstable CA and cell cycle kinetics was performed to predict the dose response for CIN. Different shapes of CIN dose-response curves were predicted for various time points after irradiation and under several assumptions on delayed DNA/chromosome breakage generation. For one of the scenarios studied, the pronounced dose dependence at early time points flattened or even turned into dose independence in a wide dose range after many rounds of replication where a stationary state between CA generation and elimination was achieved. This dose independence was shown to be in concert with the experimental data.

Application of dosimetry systems and cytogenetic status of the child population exposed to diagnostic X-rays by use of the cytokinesis-block micronucleus cytome assay

Low-dose ionizing radiation used for medical purposes is one of the definite risk factors for cancer development, and children exposed to ionizing radiation are at a relatively greater cancer risk as they have more rapidly dividing cells than adults and have longer life expectancy. Since cytokinesis-block micronucleus cytome (CBMN Cyt) assay has become one of the standard endpoints for radiation biological dosimetry, we used that assay in the present work for the assessment of different types of chromosomal damage in children exposed to diagnostic X-ray procedures. Twenty children all with pulmonary diseases between the ages of 4 and 14 years (11.30 ± 2.74) were evaluated. Absorbed dose measurements were conducted for posterior-anterior projection on the forehead, thyroid gland, gonads, chest and back. Doses were measured using thermoluminescence and radiophotoluminescent dosimetry systems. It was shown that, after diagnostic X-rays, the mean total number of CBMN Cyt assay parameters (micronucleus, nucleoplasmic bridges and nuclear buds) was significantly higher than prior to diagnostic procedure and that interindividual differences existed for each monitored child. For the nuclear division index counted prior and after examination, no significant differences were noted among mean group values. These data suggest that even low-dose diagnostic X-ray exposure may induce damaging effect in the somatic DNA of exposed children, indicating that immense care should be given in both minimizing and optimizing radiation exposure to diminish the radiation burden, especially in the youngest population.

Heavy ions, radioprotectors and genomic instability: implications for human space exploration

The risk associated with space radiation exposure is unique from terrestrial radiation exposures due to differences in radiation quality, including linear energy transfer (LET). Both high- and low-LET radiations are capable of inducing genomic instability in mammalian cells, and this instability is thought to be a driving force underlying radiation carcinogenesis. Unfortunately, during space exploration, flight crews cannot entirely avoid radiation exposure. As a result, chemical and biological countermeasures will be an important component of successful extended missions such as the exploration of Mars. There are currently several radioprotective agents (radioprotectors) in use; however, scientists continue to search for ideal radioprotective compounds-safe to use and effective in preventing and/or reducing acute and delayed effects of irradiation. This review discusses the agents that are currently available or being evaluated for their potential as radioprotectors. Further, this review discusses some implications of radioprotection for the induction and/or propagation of genomic instability in the progeny of irradiated cells.

Some important questions connected with non-targeted effects

This paper briefly reviews the highlights of experimental evidence that led to the adoption of the term "non-targeted" to describe new effects induced by ionising radiation that did not fit the classical radiobiological paradigm, principally genomic instability and bystander effect, identifying the reports that were most influential on the subsequent course of radiobiological research. The issue of appropriate terminology for the new effects is discussed. Particular emphasis is placed on the inheritance of genomic instability, where there are issues concerning which effects should be considered as transgenerational. Finally, in respect of the question as to whether these new effects are likely to have an impact on human health is addressed. It is concluded that there is a need for a clearer terminology to facilitate research progress, that real health effects cannot be ruled out and that therefore there is a need for new paradigms not only for radiobiology but also for risk assessment and radiological protection.