Computational model of chromosome aberration yield induced by high- and low-LET radiation exposures

We present a computational model for calculating the yield of radiation-induced chromosomal aberrations in human cells based on a stochastic Monte Carlo approach and calibrated using the relative frequencies and distributions of chromosomal aberrations reported in the literature. A previously developed DNA-fragmentation model for high- and low-LET radiation called the NASARadiationTrackImage model was enhanced to simulate a stochastic process of the formation of chromosomal aberrations from DNA fragments. The current version of the model gives predictions of the yields and sizes of translocations, dicentrics, rings, and more complex-type aberrations formed in the G(0)/G(1) cell cycle phase during the first cell division after irradiation. As the model can predict smaller-sized deletions and rings (<3 Mbp) that are below the resolution limits of current cytogenetic analysis techniques, we present predictions of hypothesized small deletions that may be produced as a byproduct of properly repaired DNA double-strand breaks (DSB) by nonhomologous end-joining. Additionally, the model was used to scale chromosomal exchanges in two or three chromosomes that were obtained from whole-chromosome FISH painting analysis techniques to whole-genome equivalent values.     

Non-parametric estimation of thresholds for radiation effects in vertebrate species under chronic low-LET exposures

Databases on effects of chronic low-LET radiation exposure were analyzed by non-parametric statistical methods, to estimate the threshold dose rates above which radiation effects can be expected in vertebrate organisms. Data were grouped under three umbrella endpoints: effects on morbidity, reproduction, and life shortening. The data sets were compiled on a simple ‘yes’ or ‘no’ basis. Each data set included dose rates at which effects were reported without further details about the size or peculiarity of the effects. In total, the data sets include 84 values for endpoint “morbidity”, 77 values for reproduction, and 41 values for life shortening. The dose rates in each set were ranked from low to higher values. The threshold TDR5 for radiation effects of a given umbrella type was estimated as a dose rate below which only a small percentage (5%) of data reported statistically significant radiation effects. The statistical treatment of the data sets was performed using non-parametric order statistics, and the bootstrap method. The resulting thresholds estimated by the order statistics are for morbidity effects 8.1 × 10⁻⁴ Gy day⁻¹ (2.0 × 10⁻⁴–1.0 × 10⁻³), reproduction effects 6.0 × 10⁻⁴ Gy day⁻¹ (4.0 × 10⁻⁴–1.5 × 10⁻³), and life shortening 3.0 × 10⁻³ Gy day⁻¹ (1.0 × 10⁻³–6.0 × 10⁻³), respectively. The bootstrap method gave slightly lower values: 2.1 × 10⁻⁴ Gy day⁻¹ (1.4 × 10⁻⁴–3.2 × 10⁻⁴) (morbidity), 4.1 × 10⁻⁴ Gy day⁻¹ (3.0 × 10⁻⁴–5.7 × 10⁻⁴) (reproduction), and 1.1 × 10⁻³ Gy day⁻¹ (7.9 × 10⁻⁴–1.3 × 10⁻³) (life shortening), respectively. The generic threshold dose rate (based on all umbrella types of effects) was estimated at 1.0 × 10⁻³ Gy day⁻¹.     

Nontargeted effects of ionizing radiation: implications for low-dose exposures

The traditional thinking has been that the biological effects of ionizing radiation occur in irradiated cells as a consequence of the DNA damage they incur. This implies that: 1) biological effects occur only in irratiated cells, 2) radiation traversal through the nucleus of the cell is a prerequisite to produce a biological response, and 3) DNA is the target molecule in the cell. Evidence has been emerging, however, for non-DNA targeted effects of radiation; that is, effects including mutations, chromosomal aberrations, and changes in gene expression which occur in cells that in themselves receive no radiation exposure. Two of these phenomena will be described in this paper. The first is radiation-induced genomic instability whereby biological effects, including elevated frequencies of mutations and chromosomal aberrations, arise in the distant descendants of irradiated cells. The second phenomenon has been termed the "bystander effect", whereby in a mixed population of irradiated and nonirradiated cells, biological effects arise in those cells that receive no radiation exposure. The damage signals are transmitted from cell to cell through gap junction channels, and the genetic effects observed in bystander cells appear to result from an upregulation of oxidative stress. The possible influence of these non-targeted effects of radiation of the respounse to low-dose exposures is discussed.     

Time factors in combined exposures of mouse embryos to radiation and mercury

Summary There are situations in which the exposure to more than one agent results in an enhanced risk for the exposed organism, that is in which the observed effect exceeds the effect expected from the addition of the individual effects. Our knowledge of such hazards is rather limited, in particular for those agents that occur in the environment of man. When early mouse embryos in vitro were exposed to ionizing radiation and mercuric chloride, the observed risk was higher than expected from the individual effects. This increase in risk was due to an interaction between mechanisms induced by ionizing radiation and mercury. To gain some more insight into the mode of interaction, the time requirements of mercury exposure were studied. The amount of interaction did not depend on mercury exposure before or during irradiation. However, to achieve an enhanced risk, exposure had to start as soon as possible after irradiation and had to last as long as possible. This time dependence suggests that if inhibition of DNA-repair is involved in the mechanism of interaction at all, then there must be an additional late process that is also impaired by mercury.Dedicated to Prof. W. Jacobi on the occasion of his 60th birthday     

Changes in MicroRNA expression patterns in human fibroblasts after low-LET radiation

Exposure to radiation provokes cellular responses controlled in part by gene expression networks. MicroRNAs (miRNAs) are small non-coding RNAs which mostly regulate gene expression by degrading the messages or inhibiting translation. Here, we investigated changes in miRNA expression patterns after low (0.1 Gy) and high (2.0 Gy) doses of X-ray in human fibroblasts. At early (0.5 h) and late (6 and 24 h) time points, irradiation caused qualitative and quantitative differences in the down-regulation of miRNA levels, including miR-92b, 137, 660, and 656. A transient up-regulation of miRNAs was observed after 2 h post-irradiation following high doses of radiation, including miR-558 and 662. MicroRNA levels were inversely correlated with targets from mRNA and proteomic profiling after 2.0 Gy of radiation. MicroRNAs miR-579, 608, 548-3p, and 585 are noted for targeting genes involved in radioresponsive mechanisms, such as cell cycle checkpoint and apoptosis. We suggest here a model in which miRNAs may act as "hub" regulators of specific cellular responses, immediately down-regulated so as to stimulate DNA repair mechanisms, followed by up-regulation involved in suppressing apoptosis for cell survival. Taken together, miRNAs may mediate signaling pathways in sequential fashion in response to radiation, and may serve as biodosimetric markers of radiation exposure.     

Cell cycle delay in murine pre-osteoblasts is more pronounced after exposure to high-LET compared to low-LET radiation

Space radiation contains a complex mixture of particles comprised primarily of protons and high-energy heavy ions. Radiation risk is considered one of the major health risks for astronauts who embark on both orbital and interplanetary space missions. Ionizing radiation dose-dependently kills cells, damages genetic material, and disturbs cell differentiation and function. The immediate response to ionizing radiation-induced DNA damage is stimulation of DNA repair machinery and activation of cell cycle regulatory checkpoints. To date, little is known about cell cycle regulation after exposure to space-relevant radiation, especially regarding bone-forming osteoblasts. Here, we assessed cell cycle regulation in the osteoblastic cell line OCT-1 after exposure to various types of space-relevant radiation. The relative biological effectiveness (RBE) of ionizing radiation was investigated regarding the biological endpoint of cellular survival ability. Cell cycle progression was examined following radiation exposure resulting in different RBE values calculated for a cellular survival level of 1 %. Our findings indicate that radiation with a linear energy transfer (LET) of 150 keV/μm was most effective in inducing reproductive cell killing by causing cell cycle arrest. Expression analyses indicated that cells exposed to ionizing radiation exhibited significantly up-regulated p21(CDKN1A) gene expression. In conclusion, our findings suggest that cell cycle regulation is more sensitive to high-LET radiation than cell survival, which is not solely regulated through elevated CDKN1A expression.     

Adhesion of fungal spores and germlings to host plant surfaces

Summary Firm adhesion of fungal plant pathogens to their hosts is critical at several stages in the host-parasite interaction. Spores of many fungal species are capable of rapid, non-specific attachment to various surfaces. This early adhesion, which often occurs well before germ tube emergence, prevents spores from being blown or washed from the host surface before infection can take place. Adhesion is critical for proper sensing of topographic signals involved in thigmotropic responses and for differentiation and function of appressoria. Four fungal pathogens which exhibit a variety of adhesion mechanisms have been selected for discussion.Abbreviations EMC extracellular matrix - FSTEM freeze-substitution transmission electron microscopy - Con A concanavalin A - CryoSEM cryo scanning electron microscopy - MTM macroconidial tip mucilage - STM spore tip mucilage     

Sensitivity to low-dose/low-LET ionizing radiation in mammalian cells harboring mutations in succinate dehydrogenase subunit C is governed by mitochondria-derived reactive oxygen species

It has been hypothesized that ionizing radiation-induced disruptions in mitochondrial O? metabolism lead to persistent heritable increases in steady-state levels of intracellular superoxide (O?(?U+2212)) and hydrogen peroxide (H?O?) that contribute to the biological effects of radiation. Hamster fibroblasts (B9 cells) expressing a mutation in the gene coding for the mitochondrial electron transport chain protein succinate dehydrogenase subunit C (SDHC) demonstrate increases in steady-state levels of O??- and H?O?. When B9 cells were exposed to low-dose/low-LET radiation (5-50 cGy), they displayed significantly increased clonogenic cell killing compared with parental cells. Clones derived from B9 cells overexpressing a wild-type human SDHC (T4, T8) demonstrated significantly increased surviving fractions after exposure to 5-50 cGy relative to B9 vector controls. In addition, pretreatment with polyethylene glycol-conjugated CuZn superoxide dismutase and catalase as well as adenoviral-mediated overexpression of MnSOD and/or mitochondria-targeted catalase resulted in significantly increased survival of B9 cells exposed to 10 cGy ionizing radiation relative to vector controls. Adenoviral-mediated overexpression of either MnSOD or mitochondria-targeted catalase alone was equally as effective as when both were combined. These results show that mammalian cells over expressing mutations in SDHC demonstrate low-dose/low-LET radiation sensitization that is mediated by increased levels of O??- and H?O?. These results also support the hypothesis that mitochondrial O??- and H?O? originating from SDH are capable of playing a role in low-dose ionizing radiation-induced biological responses.     

Nuclear accumulation of cyclin D1 following long-term fractionated exposures to low-dose ionizing radiation in normal human diploid cells

Cyclin D1 is a mitogenic sensor that responds to growth signals from the extracellular environment and regulates the G₁-to-S cell cycle transition. When cells are acutely irradiated with a single dose of 10 Gy, cyclin D1 is degraded, causing cell cycle arrest at the G₁/S checkpoint. In contrast, cyclin D1 accumulates in human tumor cells that are exposed to long-term fractionated radiation (0.5 Gy/fraction of X-rays). In this study we investigated the effect of fractionated low-dose radiation exposure on cyclin D1 localization in 3 strains of normal human fibroblasts. To specifically examine the nuclear accumulation of cyclin D1, cells were treated with a hypotonic buffer containing detergent to remove cytoplasmic cyclin D1. Proliferating cell nuclear antigen (PCNA) immunofluorescence was used to identify cells in S phase. With this approach, we observed S-phase nuclear retention of cyclin D1 following low-dose fractionated exposures, and found that cyclin D1 nuclear retention increased with exposure time. Cells that retained nuclear cyclin D1 were more likely to have micronuclei than non-retaining cells, indicating that the accumulation of nuclear cyclin D1 was associated with genomic instability. Moreover, inhibition of the v-akt murine thymoma viral oncogene homolog (AKT) pathway facilitated cyclin D1 degradation and eliminated cyclin D1 nuclear retention in cells exposed to fractionated radiation. Thus, cyclin D1 may represent a useful marker for monitoring long-term effects associated with exposure to low levels of radiation.     

Non-malignant thyroid diseases after a wide range of radiation exposures

The thyroid gland is one of the most radiosensitive human organs. While it is well known that radiation exposure increases the risk of thyroid cancer, less is known about its effects in relation to non-malignant thyroid diseases. The aim of this review is to evaluate the effects of high- and low-dose radiation on benign structural and functional diseases of the thyroid. We examined the results of major studies from cancer patients treated with high-dose radiotherapy or thyrotoxicosis patients treated with high doses of iodine-131, patients treated with moderate- to high-dose radiotherapy for benign diseases, persons exposed to low doses from environmental radiation, and survivors of the atomic bombings who were exposed to a range of doses. We evaluated radiation effects on structural (tumors, nodules), functional (hyper- and hypothyroidism), and autoimmune thyroid diseases. After a wide range of doses of ionizing radiation, an increased risk of thyroid adenomas and nodules was observed in a variety of populations and settings. The dose response appeared to be linear at low to moderate doses, but in one study there was some suggestion of a reduction in risk above 5 Gy. The elevated risk for benign tumors continues for decades after exposure. Considerably less consistent findings are available regarding functional thyroid diseases including autoimmune diseases. In general, associations for these outcomes were fairly weak, and significant radiation effects were most often observed after high doses, particularly for hypothyroidism. A significant radiation dose-response relationship was demonstrated for benign nodules and follicular adenomas. The effects of radiation on functional thyroid diseases are less clear, partly due to the greater difficulties encountered in studying these diseases.     

Mitigation of lung injury after accidental exposure to radiation

There is a serious need to develop effective mitigators against accidental radiation exposures. In radiation accidents, many people may receive nonuniform whole-body or partial-body irradiation. The lung is one of the more radiosensitive organs, demonstrating pneumonitis and fibrosis that are believed to develop at least partially because of radiation-induced chronic inflammation. Here we addressed the crucial questions of how damage to the lung can be mitigated and whether the response is affected by irradiation to the rest of the body. We examined the widely used dietary supplement genistein given at two dietary levels (750 or 3750 mg/kg) to Fischer rats irradiated with 12 Gy to the lung or 8 Gy to the lung + 4 Gy to the whole body excluding the head and tail (whole torso). We found that genistein had promising mitigating effects on oxidative damage, pneumonitis and fibrosis even at late times (36 weeks) when drug treatment was initiated 1 week after irradiation and stopped at 28 weeks postirradiation. The higher dose of genistein showed no greater beneficial effect. Combined lung and whole-torso irradiation caused more lung-related severe morbidity resulting in euthanasia of the animals than lung irradiation alone.     

Cancer incidence and mortality after low-dose radiation exposure: epidemiological aspects

Current recommendations for limiting exposure to ionizing radiation are based on the linear-no-threshold (LNT) model for radiation carcinogenesis under which every dose, no matter how low, carries with it some cancer risk. In this review, epidemiological evidences are discussed that the LNT hypothesis is incorrect at low doses. A large set of data was accumulated that showed that cancer risk after ordinarily encountered radiation exposure (natural background radiation, medical X-rays, etc.) is much lower than projections based on the LNT model. The discovery of the low-level radiation hormesis (stimulating effect) implies a non-linear dose-response curve in the low-dose region. The further studies in this field will provide new insights about the mechanisms of radiation carcinogenesis.     

Thymocyte apoptosis in response to low-dose radiation

Thymocyte apoptosis was assessed by counting apoptotic bodies with flow cytometry (FCM) and measuring DNA fragmentation with fluorescence spectrophotometry (FSP). J-shaped dose-response curves were obtained after both whole-body irradiation (WBI) of mice and in vitro irradiation of EL4 cells with doses ranging from 0.025 to 4 Gy X-rays. There was a significant reduction of apoptosis rate to below control level with doses within 0.2 Gy, and a dose-dependent increase in apoptosis with doses above 0.5 Gy. When thymocytes were cultured 24 h after WBI with 75 mGy X-rays in complete RPMI 1640 medium, a reduction in apoptosis was observed in the course of incubation for 72 h, and the presence of Con A in the medium accentuated this reduction in a dose- and time-dependent manner. The implications of these observations and the possible molecular mechanisms for future studies are proposed.     

Effects of dexamethasone on late radiation injury following partial-body and local organ exposures.

Dexamethasone was evaluated as a treatment for radiation-induced lung, kidney, liver, and spinal cord injuries in rats. One experimental group was partial-body-irradiated (22.5 Gy) with the head, femur, and exteriorized intestine shielded to prevent acute mortality. Other animals received local irradiation to the kidney (20 Gy), liver (25 Gy), or a 1-cm segment of cervical spinal cord (18 to 40 Gy). Following irradiation half of the animals in each radiation group were given drinking water containing 188 micrograms/liter of dexamethasone. Tests were done to assess kidney function (hematocrit, plasma urea nitrogen, ethylenediaminetetraacetic acid clearance), liver function (rose bengal clearance, plasma glutamic oxaloacetic acid transaminase), or spinal cord injury (paralysis). The effectiveness of dexamethasone in preventing radiation injury was tissue specific. Dexamethasone eliminated lethal pleural fluid accumulation after partial-body irradiation and delayed development of kidney dysfunction after local kidney irradiation. As a result, dexamethasone increased the median survival time from 63 to 150 days after partial-body irradiation and from 126 to 175 days after local kidney irradiation. After whole-liver irradiation, development of hepatic functional injury was retarded by dexamethasone treatment but without significantly changing survival time. Dexamethasone had no effect on spinal cord tolerance but significantly shortened the latent period between radiation and paralysis.     

Application of Bayesian inference to characterize risks associated with low doses of low-LET radiation

Improved risk characterization for stochastic biological effects of low doses of low-LET radiation is important for protecting nuclear workers and the public from harm from radiation exposure. Here we present a Bayesian approach to characterize risks of stochastic effects from low doses of low-LET radiation. The stochastic effect considered is neoplastic transformation of cells because it relates closely to cancer induction. We have used a published model of neoplastic transformation called NEOTRANS1. It is based on two different classes of cellular sensitivity for asynchronous, exponentially growing populations (in vitro). One sensitivity class is the hypersensitive cell; the other is the resistant cell. NEOTRANS1 includes the effects of genomic damage accumulation, DNA repair during cell cycle arrest, and DNA misrepair (non-lethal repair errors). The model-associated differential equations are solved for conditions of in vitro irradiation at a fixed rate. Previously published solutions apply only to high dose rates and were incorrectly assumed to apply to only high-LET radiation. Solutions provided here apply to any fixed dose rate and to both high- and low-LET radiations. Markov chain Monte Carlo methods are used to carry out the Bayesian inference of the low-dose risk for neoplastic transformation of aneuploid C3H 10T1/2 cells for X-ray doses from 0 to 1000 mGy. We have assumed that for this low-dose range only the hypersensitive fraction of the cells are affected. Our results indicate that the initial slope of the risk vs dose relationship for neoplastic transformation is as follows: (1) directly proportional to the fraction, f1, of hypersensitive cells; (2) directly proportional to the radiosensitivity of the genomic target; and (3) inversely proportional to the rate at which hypersensitive cells with radiation-induced damage are committed to undergo correct repair of genomic damage. Further, our results indicate that very fast molecular events are associated with the commitment of cells to the correct repair pathway. Results also indicate a relatively large probability for misrepair that leads to genomic instability. Our results are consistent with the view that for very low doses, dose rate is not an important variable for characterizing low-LET radiation risks so long as age-related changes in sensitivity do not occur during irradiation.