Radiation Research Society Journal-based Historical Review of the Use of Biomarkers for Radiation Dose and Injury Assessment: Acute Health Effects Predictions

A multiple-parameter based approach using radiation-induced clinical signs and symptoms, hematology changes, cytogenetic chromosomal aberrations, and molecular biomarkers changes after radiation exposure is used for biodosimetry-based dose assessment. In the current article, relevant milestones from Radiation Research are documented that forms the basis of the current consensus approach for diagnostics after radiation exposure. For example, in 1962 the use of cytogenetic chromosomal aberration using the lymphocyte metaphase spread dicentric assay for biodosimetry applications was first published in Radiation Research. This assay is now complimented using other cytogenetic chromosomal aberration assays (i.e., chromosomal translocations, cytokinesis-blocked micronuclei, premature chromosome condensation, γ-H2AX foci, etc.). Changes in blood cell counts represent an early-phase biomarker for radiation exposures. Molecular biomarker changes have evolved to include panels of organ-specific plasma proteomic and blood-based gene expression biomarkers for radiation dose assessment. Maturation of these assays are shown by efforts for automated processing and scoring, development of point-of-care diagnostics devices, service laboratories inter-comparison exercises, and applications for dose and injury assessments in radiation accidents. An alternative and complementary approach has been advocated with the focus to de-emphasize "dose" and instead focus on predicting acute or delayed health effects. The same biomarkers used for dose estimation (e.g., lymphocyte counts) can be used to directly predict the later developing severity degree of acute health effects without performing dose estimation as an additional or intermediate step. This review illustrates contributing steps toward these developments published in Radiation Research.

Kaplan lecture 2023: lymphopenia in particle therapy

PURPOSE

Lymphopenia is now generally recognized as a negative prognostic factor in radiotherapy. Already at the beginning of the century we demonstrated that high-energy carbon ions induce less damage to the lymphocytes of radiotherapy patients than X-rays, even if heavy ions are more effective per unit dose in the induction of chromosomal aberrations in blood cells irradiated ex-vivo. The explanation was based on the volume effect, i.e. the sparing of larger volumes of normal tissue in Bragg peak therapy. Here we will review the current knowledge about the difference in lymphopenia between particle and photon therapy and the consequences.

CONCLUSIONS

There is nowadays an overwhelming evidence that particle therapy reduces significantly the radiotherapy-induced lymphopenia in several tumor sites. Because lymphopenia turns down the immune response to checkpoint inhibitors, it can be predicted that particle therapy may be the ideal partner for combined radiation and immunotherapy treatment and should be selected for patients where severe lymphopenia is expected after X-rays.

Simulated microgravity induces DNA damage concurrent with impairment of DNA repair and activation of cell-type specific DNA damage response in microglial and glioblastoma cells

Long-term spaceflights affect the structural changes in brain, alter motor or cognitive function and associated development of neuro-optic syndrome in astronauts. Studies addressing the impact of microgravity on brain cells are very limited. Herein, we employed microglial (CHME3) and glioblastoma (U87MG and A172) cells to study their molecular and functional adaptations under simulated microgravity (SMG) exposure. A reduction in cell viability and proliferation with decreased levels of PCNA were observed in these cells. SMG caused extensive DNA damage with an increase in γH2A.X (ser139) phosphorylation and differential activation/expression of DNA damage response (DDR) proteins including ATM, ATR, Chk1, Chk2 and p53 in all the three cell lines. Unlike CHME3, the ATM/Chk2-dependent DDR pathway was activated in glioblastoma cells suggesting a marked difference in the adaptation between normal and cancer cells to SMG. Five different classes of DNA repair pathways including BER, NER, MMR, NHEJ and HR were suppressed in both cell lines with the notable exception of NHEJ (Ku70/80 and DNA-PK) activation in U87MG cells. SMG induced mitochondrial apoptosis with increased expression of Bax, cleaved caspase-3 and cleaved poly-(ADP-ribose) polymerase, and reduced Bcl-2 level. SMG triggered apoptosis simultaneously via ERK1/2 and AKT activation, and inhibition of GSK3β activity which was reversed by MEK1 and PI3K inhibitors. Taken together, our study revealed that microgravity is a strong stressor to trigger DNA damage and apoptosis through activation of ERK1/2 and AKT, and impairment of DNA repair capacity, albeit with a cell-type difference in DDR and NHEJ regulation, in microglial and glioblastoma cells.

High throughput screen of small molecules as potential countermeasures to galactic cosmic radiation induced cellular dysfunction

Space travel increases galactic cosmic ray exposure to flight crews and this is significantly elevated once travel moves beyond low Earth orbit. This includes combinations of high energy protons and heavy ions such as ⁵⁶Fe or ¹⁶O. There are distinct differences in the biological response to low-energy transfer (x-rays) or high-energy transfer (High-LET). However, given the relatively low fluence rate of exposure during flight operations, it might be possible to manage these deleterious effects using small molecules currently available. Virtually all reports to date examining small molecule management of radiation exposure are based on low-LET challenges. To that end an FDA approved drug library (725 drugs) was used to perform a high throughput screen of cultured cells following exposure to galactic cosmic radiation. The H9c2 myoblasts, ES-D3 pluripotent cells, and Hy926 endothelial cell lines were exposed to a single exposure (75 cGy) using the 5-ion GCRsim protocol developed at the NASA Space Radiation Laboratory (NSRL). Following GCR exposure cells were maintained for up to two weeks. For each drug (@10µM), a hierarchical cumulative score was developed incorporating measures of mitochondrial and cellular function, oxidant stress and cell senescence. The top 160 scores were retested following a similar protocol using 1µM of each drug. Within the 160 drugs, 33 are considered to have an anti-inflammatory capacity, while others also indirectly suppressed pro-inflammatory pathways or had noted antioxidant capacity. Lead candidates came from different drug classes that included angiotensin converting enzyme inhibitors or AT1 antagonists, COX2 inhibitors, as well as drugs mediated by histamine receptors. Surprisingly, different classes of anti-diabetic medications were observed to be useful including sulfonylureas and metformin. Using a hierarchical decision structure, we have identified several lead candidates. That no one drug or even drug class was completely successful across all parameters tested suggests the complexity of managing the consequences of galactic cosmic radiation exposure.

Study of Rotary Cell Culture System-Induced Microgravity Effects on Cancer Biomarkers

Since humans started to explore the possibilities of life beyond earth, space missions began unmanned in the beginning and late with astronauts to explore the curiosity. There are several factors in space which are different from that of the earth's atmosphere, including gravity, cosmic radiation, extreme temperatures, and heavy ions. The space environment consists of very less gravitational force, which is often termed as microgravity. Microgravity, along with radiation in space, is known to suppress the immune system of astronauts, and hence, the chances of getting diagnosed with cancer could not be overlooked. Microgravity simulators are used to generate microgravity in laboratory conditions, and it could be a very beneficial tool to study the effect of altered gravity on various types of cells, including cancer cells. Cancer cells produce several biomarkers which are used for cancer diagnosis and prognosis. Protein biomarkers are indispensable for the ongoing fight against cancer. In this chapter, we describe the culture of both anchorage-dependent and suspension cell lines using rotary cell culture system (RCCS) to induce microgravity condition and its effect on protein biomarkers.

Dose-Effects Models for Space Radiobiology: An Overview on Dose-Effect Relationships

Space radiobiology is an interdisciplinary science that examines the biological effects of ionizing radiation on humans involved in aerospace missions. The dose-effect models are one of the relevant topics of space radiobiology. Their knowledge is crucial for optimizing radioprotection strategies (e.g., spaceship and lunar space station-shielding and lunar/Mars village design), the risk assessment of the health hazard related to human space exploration, and reducing damages induced to astronauts from galactic cosmic radiation. Dose-effect relationships describe the observed damages to normal tissues or cancer induction during and after space flights. They are developed for the various dose ranges and radiation qualities characterizing the actual and the forecast space missions [International Space Station (ISS) and solar system exploration]. Based on a Pubmed search including 53 papers reporting the collected dose-effect relationships after space missions or in ground simulations, 7 significant dose-effect relationships (e.g., eye flashes, cataract, central nervous systems, cardiovascular disease, cancer, chromosomal aberrations, and biomarkers) have been identified. For each considered effect, the absorbed dose thresholds and the uncertainties/limitations of the developed relationships are summarized and discussed. The current knowledge on this topic can benefit from further in vitro and in vivo radiobiological studies, an accurate characterization of the quality of space radiation, and the numerous experimental dose-effects data derived from the experience in the clinical use of ionizing radiation for diagnostic or treatments with doses similar to those foreseen for the future space missions. The growing number of pooled studies could improve the prediction ability of dose-effect relationships for space exposure and reduce their uncertainty level. Novel research in the field is of paramount importance to reduce damages to astronauts from cosmic radiation before Beyond Low Earth Orbit exploration in the next future. The study aims at providing an overview of the published dose-effect relationships and illustrates novel perspectives to inspire future research.

Predicting chromosome damage in astronauts participating in international space station missions

Space radiation consists of energetic protons and other heavier ions. During the International Space Station program, chromosome aberrations in lymphocytes of astronauts have been analyzed to estimate received biological doses of space radiation. More specifically, pre-flight blood samples were exposed ex vivo to varying doses of gamma rays, while post-flight blood samples were collected shortly and several months after landing. Here, in a study of 43 crew-missions, we investigated whether individual radiosensitivity, as determined by the ex vivo dose-response of the pre-flight chromosome aberration rate (CAR), contributes to the prediction of the post-flight CAR incurred from the radiation exposure during missions. Random-effects Poisson regression was used to estimate subject-specific radiosensitivities from the preflight dose-response data, which were in turn used to predict post-flight CAR and subject-specific relative biological effectiveness (RBEs) between space radiation and gamma radiation. Covariates age, gender were also considered. Results indicate that there is predictive value in background CAR as well as radiosensitivity determined preflight for explaining individual differences in post-flight CAR over and above that which could be explained by BFO dose alone. The in vivo RBE for space radiation was estimated to be approximately 3 relative to the ex vivo dose response to gamma irradiation. In addition, pre-flight radiosensitivity tended to be higher for individuals having a higher background CAR, suggesting that individuals with greater radiosensitivity can be more sensitive to other environmental stressors encountered in daily life. We also noted that both background CAR and radiosensitivity tend to increase with age, although both are highly variable. Finally, we observed no significant difference between the observed CAR shortly after mission and at > 6 months post-mission.

Cosmic-ray interaction data for designing biological experiments in space

There is growing interest in flying biological experiments beyond low-Earth orbit (LEO) to measure biological responses potentially relevant to those expected during a human mission to Mars. Such experiments could be payloads onboard precursor missions, including unmanned private-public partnerships, as well as small low-cost spacecraft (satellites) designed specifically for biosentinel-type missions. It is the purpose of this paper to provide physical cosmic-ray interaction data and related information useful to biologists who may be planning such experiments. It is not the objective here to actually design such experiments or provide radiobiological response functions, which would be specific for each experiment and biological endpoint. Nuclide-specific flux and dose rates were calculated using OLTARIS and these results were used to determine particle traversal rates and doses in hypothetical biological targets. Comparisons are provided between GCR in interplanetary space and inside the ISS. Calculated probabilistic estimates of dose from solar particle events are also presented. Although the focus here is on biological experiments, the information provided may be useful for designing other payloads as well if the space radiation environment is a factor to be considered.

Cellular responses and gene expression profile changes due to bleomycin-induced DNA damage in human fibroblasts in space

Living organisms in space are constantly exposed to radiation, toxic chemicals or reactive oxygen species generated due to increased levels of environmental and psychological stresses. Understanding the impact of spaceflight factors, microgravity in particular, on cellular responses to DNA damage is essential for assessing the radiation risk for astronauts and the mutation rate in microorganisms. In a study conducted on the International Space Station, confluent human fibroblasts in culture were treated with bleomycin for three hours in the true microgravity environment. The degree of DNA damage was quantified by immunofluorescence staining for γ-H2AX, which is manifested in three types of staining patterns. Although similar percentages of these types of patterns were found between flight and ground cells, there was a slight shift in the distribution of foci counts in the flown cells with countable numbers of γ-H2AX foci. Comparison of the cells in confluent and in exponential growth conditions indicated that the proliferation rate between flight and the ground may be responsible for such a shift. We also performed a microarray analysis of gene expressions in response to bleomycin treatment. A qualitative comparison of the responsive pathways between the flown and ground cells showed similar responses with the p53 network being the top upstream regulator. The microarray data was confirmed with a PCR array analysis containing a set of genes involved in DNA damage signaling; with BBC3, CDKN1A, PCNA and PPM1D being significantly upregulated in both flight and ground cells after bleomycin treatment. Our results suggest that whether microgravity affects DNA damage response in space can be dependent on the cell type and cell growth condition.

Impact of Particle Irradiation on the Immune System: From the Clinic to Mars

Despite the generalized use of photon-based radiation (i.e., gamma rays and X-rays) to treat different cancer types, particle radiotherapy (i.e., protons and carbon ions) is becoming a popular, and more effective tool to treat specific tumors due to the improved physical properties and biological effectiveness. Current scientific evidence indicates that conventional radiation therapy affects the tumor immunological profile in a particular manner, which in turn, might induce beneficial effects both at local and systemic (i.e., abscopal effects) levels. The interaction between radiotherapy and the immune system is being explored to combine immune and radiation (including particles) treatments, which in many cases have a greater clinical effect than any of the therapies alone. Contrary to localized, clinical irradiation, astronauts are exposed to whole body, chronic cosmic radiation, where protons and heavy ions are an important component. The effects of this extreme environment during long periods of time, e.g., a potential mission to Mars, will have an impact on the immune system that could jeopardize the health of the astronauts, hence the success of the mission. To this background, the purpose of this mini review is to briefly present the current knowledge in local and systemic immune alterations triggered by particle irradiation and to propose new lines of future research. Immune effects induced by particle radiation relevant to clinical applications will be covered, together with examples of combined radiotherapy and immunotherapy. Then, the focus will move to outer space, where the immune system alterations induced by cosmic radiation during spaceflight will be discussed.

Detection of DNA damage by space radiation in human fibroblasts flown on the International Space Station

Although charged particles in space have been detected with radiation detectors on board spacecraft since the discovery of the Van Allen Belts, reports on the effects of direct exposure to space radiation in biological systems have been limited. Measurement of biological effects of space radiation is challenging due to the low dose and low dose rate nature of the radiation environment, and due to the difficulty in distinguishing the radiation effects from microgravity and other space environmental factors. In astronauts, only a few changes, such as increased chromosome aberrations in their lymphocytes and early onset of cataracts, are attributed primarily to their exposure to space radiation. In this study, cultured human fibroblasts were flown on the International Space Station (ISS). Cells were kept at 37°C in space for 14 days before being fixed for analysis of DNA damage with the γ-H2AX assay. The 3-dimensional γ-H2AX foci were captured with a laser confocal microscope. Quantitative analysis revealed several foci that were larger and displayed a track pattern only in the Day 14 flight samples. To confirm that the foci data from the flight study was actually induced from space radiation exposure, cultured human fibroblasts were exposed to low dose rate γ rays at 37°C. Cells exposed to chronic γ rays showed similar foci size distribution in comparison to the non-exposed controls. The cells were also exposed to low- and high-LET protons, and high-LET Fe ions on the ground. Our results suggest that in G1 human fibroblasts under the normal culture condition, only a small fraction of large size foci can be attributed to high-LET radiation in space.

Cosmic radiation and cancer: is there a link?

Cosmic radiation can cause genetic and cytogenetic damage. Certain occupations including airline pilots and cabin crew are acknowledged to have a greater exposure to cosmic radiation. In a systematic search of MEDLINE, performed from 1990 to 2014, we analyzed clinical studies using the keywords: cosmic radiation, cancer, chromosome aberration, pilots and astronauts. Increased incidence of skin cancers among airline cabin crew has been reported in a number of studies and appears to be the most consistent finding. However, as with other cancers, it is unclear whether increased exposure to cosmic radiation is a factor in the increased incidence or whether this can be explained by lifestyle factors. Further research is needed to clarify the risk of cancer in relation to cosmic radiation.

Salivary biochemical markers as potential acute toxicity parameters for acute radiation injury: A study on small experimental animals

Researchers have been evaluating several biodosimetric/screening approaches to assess acute radiation injury, related to mass causality. Keeping in mind this background, we hypothesized that effect of whole-body irradiation in single fraction in graded doses can affect the secretion of various salivary components that could be used as acute radiation injury/toxicity marker, which can be used in screening of large population at the time of nuclear accidents/disaster. Thirty Sprague Dawley rats treated with whole-body cobalt-60 gamma irradiation of dose 1-5 Gy (dose rate: 0.95 Gy/min) were included in this study. Whole mixed saliva was collected from all animals before and after radiation up to 72 h postradiation. Saliva was analyzed for electrolytes, total protein, urea, and amylase. Intragroup comparison of salivary parameters at different radiation doses showed significant differences. Potassium was significantly increased as the dose increased from 1 Gy to 5 Gy (p < 0.01) with effect size of difference (r > 0.5). Sodium was significantly altered after 3-5 Gy (p < 0.01, r > 0.5), except 1 and 2 Gy, whereas changes in sodium level were nonsignificant (p > 0.5). Urea, total protein, and amylase levels were also significantly increased as the radiation dose increased (p < 0.01) with large effect size of difference (r > 0.5). This study suggests that salivary parameters were sensitive toward radiation even at low radiation dose which can be used as a predictor of radiation injury.

Comparison of space flight and heavy ion radiation induced genomic/epigenomic mutations in rice (Oryza sativa)

Rice seeds, after space flight and low dose heavy ion radiation treatment were cultured on ground. Leaves of the mature plants were obtained for examination of genomic/epigenomic mutations by using amplified fragment length polymorphism (AFLP) and methylation sensitive amplification polymorphism (MSAP) method, respectively. The mutation sites were identified by fragment recovery and sequencing. The heritability of the mutations was detected in the next generation. Results showed that both space flight and low dose heavy ion radiation can induce significant alterations on rice genome and epigenome (P<0.05). For both genetic and epigenetic assays, while there was no significant difference in mutation rates and their ability to be inherited to the next generation, the site of mutations differed between the space flight and radiation treated groups. More than 50% of the mutation sites were shared by two radiation treated groups, radiated with different LET value and dose, while only about 20% of the mutation sites were shared by space flight group and radiation treated group. Moreover, in space flight group, we found that DNA methylation changes were more prone to occur on CNG sequence than CG sequence. Sequencing results proved that both space flight and heavy ion radiation induced mutations were widely spread on rice genome including coding region and repeated region. Our study described and compared the characters of space flight and low dose heavy ion radiation induced genomic/epigenomic mutations. Our data revealed the mechanisms of application of space environment for mutagenesis and crop breeding. Furthermore, this work implicated that the nature of mutations induced under space flight conditions may involve factors beyond ion radiation.

ICRP, 123. Assessment of radiation exposure of astronauts in space. ICRP Publication 123

During their occupational activities in space, astronauts are exposed to ionising radiation from natural radiation sources present in this environment. They are, however, not usually classified as being occupationally exposed in the sense of the general ICRP system for radiation protection of workers applied on Earth. The exposure assessment and risk-related approach described in this report is clearly restricted to the special situation in space, and should not be applied to any other exposure situation on Earth. The report describes the terms and methods used to assess the radiation exposure of astronauts, and provides data for the assessment of organ doses. Chapter 1 describes the specific situation of astronauts in space, and the differences in the radiation fields compared with those on Earth. In Chapter 2, the radiation fields in space are described in detail, including galactic cosmic radiation, radiation from the Sun and its special solar particle events, and the radiation belts surrounding the Earth. Chapter 3 deals with the quantities used in radiological protection, describing the Publication 103 (ICRP, 2007) system of dose quantities, and subsequently presenting the special approach for applications in space; due to the strong contribution of heavy ions in the radiation field, radiation weighting is based on the radiation quality factor, Q, instead of the radiation weighting factor, wR. In Chapter 4, the methods of fluence and dose measurement in space are described, including instrumentation for fluence measurements, radiation spectrometry, and area and individual monitoring. The use of biomarkers for the assessment of mission doses is also described. The methods of determining quantities describing the radiation fields within a spacecraft are given in Chapter 5. Radiation transport calculations are the most important tool. Some physical data used in radiation transport codes are presented, and the various codes used for calculations in high-energy radiation fields in space are described. Results of calculations and measurements of radiation fields in spacecraft are given. Some data for shielding possibilities are also presented. Chapter 6 addresses methods of determining mean absorbed doses and dose equivalents in organs and tissues of the human body. Calculated conversion coefficients of fluence to mean absorbed dose in an organ or tissue are given for heavy ions up to Z=28 for energies from 10 MeV/u to 100 GeV/u. For the same set of ions and ion energies, mean quality factors in organs and tissues are presented using, on the one hand, the Q(L) function defined in Publication 60 (ICRP, 1991), and, on the other hand, a Q function proposed by the National Aeronautics and Space Administration. Doses in the body obtained by measurements are compared with results from calculations, and biodosimetric measurements for the assessment of mission doses are also presented. In Chapter 7, operational measures are considered for assessment of the exposure of astronauts during space missions. This includes preflight mission design, area and individual monitoring during flights in space, and dose recording. The importance of the magnitude of uncertainties in dose assessment is considered. Annex A shows conversion coefficients and mean quality factors for protons, charged pions, neutrons, alpha particles, and heavy ions(2 < Z ≤2 8), and particle energies up to 100 GeV/u.

Expression of p53-regulated proteins in human cultured lymphoblastoid TSCE5 and WTK1 cell lines during spaceflight

The aim of this study was to determine the biological effects of space radiations, microgravity, and the interaction of them on the expression of p53-regulated proteins. Space experiments were performed with two human cultured lymphoblastoid cell lines: one line (TSCE5) bears a wild-type p53 gene status, and another line (WTK1) bears a mutated p53 gene status. Under 1 gravity or microgravity conditions, the cells were grown in the cell biology experimental facility (CBEF) of the International Space Station for 8 days without experiencing the stress during launching and landing because the cells were frozen during these periods. Ground control samples were simultaneously cultured for 8 days in the CBEF on the ground for 8 days. After spaceflight, protein expression was analyzed using a Panorama(TM) Ab MicroArray protein chips. It was found that p53-dependent up-regulated proteins in response to space radiations and space environment were MeCP2 (methyl CpG binding protein 2), and Notch1 (Notch homolog 1), respectively. On the other hand, p53-dependent down-regulated proteins were TGF-β, TWEAKR (tumor necrosis factor-like weak inducer of apoptosis receptor), phosho-Pyk2 (Proline-rich tyrosine kinase 2), and 14-3-3θ/τ which were affected by microgravity, and DR4 (death receptor 4), PRMT1 (protein arginine methyltransferase 1) and ROCK-2 (Rho-associated, coiled-coil containing protein kinase 2) in response to space radiations. ROCK-2 was also suppressed in response to the space environment. The data provides the p53-dependent regulated proteins by exposure to space radiations and/or microgravity during spaceflight. Our expression data revealed proteins that might help to advance the basic space radiation biology.

Chromosomal aberrations in the bone marrow cells of mice induced by accelerated (12)C(6+) ions

The whole bodies of 6-week-old male Kun-Ming mice were exposed to different doses of (12)C(6+) ions or X-rays. Chromosomal aberrations of the bone marrow (gaps, terminal deletions and breaks, fragments, inter-chromosomal fusions and sister-chromatid union) were scored in metaphase 9h after exposure, corresponding to cells exposed in the G(2)-phase of the first mitosis cycle. Dose-response relationships for the frequency of chromosomal aberrations were plotted both by linear and linear-quadratic equations. The data showed that there was a dose-related increase in the frequency of chromosomal aberrations in all treated groups compared to controls. Linear-quadratic equations were a good fit for both radiation types. The compound theory of dual radiation action was applied to decipher the bigger curvature (D(2)) of the dose-response curves of X-rays compared to those of (12)C(6+) ions. Different distributions of the five types of aberrations and different degrees of homogeneity were found between (12)C(6+) ion and X-ray irradiation and the possible underlying mechanism for these phenomena were analyzed according to the differences in the spatial energy deposition of both types of radiation.

Frozen human cells can record radiation damage accumulated during space flight: mutation induction and radioadaptation

To estimate the space-radiation effects separately from other space-environmental effects such as microgravity, frozen human lymphoblastoid TK6 cells were sent to the "Kibo" module of the International Space Station (ISS), preserved under frozen condition during the mission and finally recovered to Earth (after a total of 134 days flight, 72 mSv). Biological assays were performed on the cells recovered to Earth. We observed a tendency of increase (2.3-fold) in thymidine kinase deficient (TK(-)) mutations over the ground control. Loss of heterozygosity (LOH) analysis on the mutants also demonstrated a tendency of increase in proportion of the large deletion (beyond the TK locus) events, 6/41 in the in-flight samples and 1/17 in the ground control. Furthermore, in-flight samples exhibited 48% of the ground-control level in TK(-) mutation frequency upon exposure to a subsequent 2 Gy dose of X-rays, suggesting a tendency of radioadaptation when compared with the ground-control samples. The tendency of radioadaptation was also supported by the post-flight assays on DNA double-strand break repair: a 1.8- and 1.7-fold higher efficiency of in-flight samples compared to ground control via non-homologous end-joining and homologous recombination, respectively. These observations suggest that this system can be used as a biodosimeter, because DNA damage generated by space radiation is considered to be accumulated in the cells preserved frozen during the mission, Furthermore, this system is also suggested to be applicable for evaluating various cellular responses to low-dose space radiation, providing a better understanding of biological space-radiation effects as well as estimation of health influences of future space explores.

Biological effects of space radiation on human cells: history, advances and outcomes

Exposure to radiation is one of the main concerns for space exploration by humans. By focusing deliberately on the works performed on human cells, we endeavored to review, decade by decade, the technological developments and conceptual advances of space radiation biology. Despite considerable efforts, the cancer and the toxicity risks remain to be quantified: 1) the nature and the frequency of secondary heavy ions need to be better characterized in order to estimate their contribution to the dose and to the final biological response; 2) the diversity of radiation history of each astronaut and the impact of individual susceptibility make very difficult any epidemiological analysis for estimating hazards specifically due to space radiation exposure. 3) Cytogenetic data undoubtedly revealed that space radiation exposure produce significant damage in cells. However, our knowledge of the basic mechanisms specific to low-dose, to repeated doses and to adaptive response is still poor. The application of new radiobiological techniques, like immunofluorescence, and the use of human tissue models different from blood, like skin fibroblasts, may help in clarifying all the above items.

NASA Radiation Biomarker Workshop, September 27-28, 2007

A summary is provided of presentations and discussions at the NASA Radiation Biomarker Workshop held September 27-28, 2007 at NASA Ames Research Center in Mountain View, CA. Invited speakers were distinguished scientists representing key sectors of the radiation research community. Speakers addressed recent developments in the biomarker and biotechnology fields that may provide new opportunities for health-related assessment of radiation-exposed individuals, including those exposed during long-duration space travel. Topics discussed included the space radiation environment, biomarkers of radiation sensitivity and individual susceptibility, molecular signatures of low-dose responses, multivariate analysis of gene expression, biomarkers in biodefense, biomarkers in radiation oncology, biomarkers and triage after large-scale radiological incidents, integrated and multiple biomarker approaches, advances in whole-genome tiling arrays, advances in mass spectrometry proteomics, radiation biodosimetry for estimation of cancer risk in a rat skin model, and confounding factors. A summary of conclusions is provided at the end of the report.

Physical and biological organ dosimetry analysis for international space station astronauts

In this study, we analyzed the biological and physical organ dose equivalents for International Space Station (ISS) astronauts. Individual physical dosimetry is difficult in space due to the complexity of the space radiation environment, which consists of protons, heavy ions and secondary neutrons, and the modification of these radiation types in tissue as well as limitations in dosimeter devices that can be worn for several months in outer space. Astronauts returning from missions to the ISS undergo biodosimetry assessment of chromosomal damage in lymphocyte cells using the multicolor fluorescence in situ hybridization (FISH) technique. Individual-based pre-flight dose responses for lymphocyte exposure in vitro to gamma rays were compared to those exposed to space radiation in vivo to determine an equivalent biological dose. We compared the ISS biodosimetry results, NASA's space radiation transport models of organ dose equivalents, and results from ISS and space shuttle phantom torso experiments. Physical and biological doses for 19 ISS astronauts yielded average effective doses and individual or population-based biological doses for the approximately 6-month missions of 72 mSv and 85 or 81 mGy-Eq, respectively. Analyses showed that 80% or more of organ dose equivalents on the ISS are from galactic cosmic rays and only a small contribution is from trapped protons and that GCR doses were decreased by the high level of solar activity in recent years. Comparisons of models to data showed that space radiation effective doses can be predicted to within about a +/-10% accuracy by space radiation transport models. Finally, effective dose estimates for all previous NASA missions are summarized.

mBAND Analysis of Chromosomal Aberrations in Human Epithelial Cells Exposed to Low- and High-LET Radiation

Energetic heavy ions pose a potential health risk to astronauts who have participated in extended space missions. High-LET radiation is much more effective than low-LET radiation in the induction of biological effects, including cell inactivation, genetic mutations, cataracts and cancer. Most of these biological end points are closely correlated with chromosomal damage, which can be used as a biomarker for radiation damage. Multicolor banding in situ hybridization (mBAND) has proven to be highly useful for the study of intrachromosomal aberrations, which have been suggested as a biomarker of exposure to high-LET radiation. To investigate biological signatures of radiation quality and the complexity of intrachromosomal aberrations, we exposed human epithelial cells in vitro to (137)Cs gamma rays or iron ions (600 MeV/nucleon) and collected chromosomes using a premature chromosome condensation technique. Aberrations in chromosome 3 were analyzed using mBAND probes. The results of our study confirmed the observation of a higher incidence of inversions for high-LET radiation. However, detailed analysis of the inversion type revealed that both iron ions and gamma rays induced a low incidence of simple inversions. Half of the inversions observed in the low-LET-irradiated samples were accompanied by other types of intrachromosome aberrations, but few inversions were accompanied by interchromosome aberrations. In contrast, iron ions induced a significant fraction of inversions that involved complex rearrangements of both inter- and intrachromosome exchanges.

Does reduced gravity alter cellular response to ionizing radiation?

This review addresses the purported interplay between actual or simulated weightlessness and cellular response to ionizing radiation. Although weightlessness is known to alter several cellular functions and to affect signaling pathways implicated in cell proliferation, differentiation and death, its influence on cellular radiosensitivity has so far proven elusive. Renewed controversy as to whether reduced gravity enhances long-term radiation risk is fueled by recently published data that claim either overall enhancement of genomic damage or no increase of radiation-induced clastogenicity by modeled microgravity in irradiated human cells. In elucidating this crucial aspect of space radiation protection, ground-based experiments, such as those based on rotating-wall bioreactors, will increasingly be used and represent a more reproducible alternative to in-flight experiments. These low-shear vessels also make three-dimensional cellular co-cultures possible and thus allow to study the gravisensitivity of radioresponse in a context that better mimics cell-to-cell communication and hence in vivo cellular behavior.

Space radiation does not induce a significant increase of intrachromosomal exchanges in astronauts' lymphocytes

Chromosome aberration analysis in astronauts has been used to provide direct, biologically motivated estimates of equivalent doses and risk associated to cosmic radiation exposure during space flight. However, the past studies concentrated on measurements of dicentrics and translocations, while chromosome intrachanges (inversions) have never been measured in astronauts' samples. Recent data reported in the literature suggest that densely ionizing radiation can induce a large fraction of intrachanges, thus leading to the suspicion that interchanges grossly underestimate the cosmic radiation-induced cytogenetic damage in astronauts. We have analyzed peripheral blood lymphocytes from 11 astronauts involved in short- or long-term space flights in low-Earth orbit using high-resolution multicolor banding to assess the frequency of intrachromosomal exchanges in both pre- and post-flight samples. We did not detect any inversions in chromosome 5 from a total of 2800 cells in astronauts' blood. In addition, no complex type exchanges were found in a total of 3590 astronauts' lymphocytes analyzed by multifluor fluorescence in situ hybridisation. We conclude that, within the statistical power of this study, the analysis of interchanges for biological dosimetry in astronauts does not significantly underestimate the space radiation-induced cytogenetic damage, and complex-type exchanges or intrachanges have limited practical use for biodosimetry at very low doses.

Biomarkers of space radiation risk

Radiation risk estimates are based on epidemiological data obtained on Earth for cohorts exposed predominantly to acute doses of gamma rays, and the extrapolation to the space environment is highly problematic and error-prone. The uncertainty can be reduced if risk estimates are compared directly to space radiation-induced biological alterations, i.e. by detecting biomarkers in astronauts. Chromosomal aberrations in peripheral blood lymphocytes are the only biomarker that can provide simultaneous information on dose, dose equivalent and risk, and they have been measured extensively in astronauts during the past 10 years. Individual relative risks calculated from chromosomal aberration measurements in crew members after single space missions in low-Earth orbit fall in the same range as the estimates derived from physical dosimetry, suggesting that the current system for radiogenic risk evaluation is essentially sound. However, the output of the biomarker test is dependent upon the sampling time. Recent results show a fast time-dependent decay of chromosomal aberrations in blood lymphocytes after space flight and a lack of correlation between translocations and cumulative dose in astronauts involved in two to five space missions. This "time factor" may reflect individual variability and time dependence in the risk produced by exposure to cosmic radiation during the flight. Biomarkers may be superior to dose in predicting space radiation risk, pending technical improvements in sensitivity, and validation by epidemiological studies.

Stability of chromosome aberrations in the blood lymphocytes of astronauts measured after space flight by FISH chromosome painting

Follow-up measurements of chromosome aberrations in the blood lymphocytes of astronauts were performed by FISH chromosome painting at various intervals from 5 months to more than 5 years after space flight and compared to preflight baseline measurements. For five of the six astronauts studied, the analysis of individual time courses for translocations revealed a temporal decline of yields with half-lives ranging from 10 to 58 months. The yield of exchanges remained unchanged for the sixth astronaut during an observation period of 5 months after flight. These results may indicate complications with the use of stable aberrations for retrospective dose reconstruction, and the differences in the decay time may reflect individual variability in risk from space radiation exposure.

Modelled microgravity does not modify the yield of chromosome aberrations induced by high-energy protons in human lymphocytes

The aim was to evaluate the effect of modelled microgravity on radiation-induced chromosome aberrations (CAs). G0 peripheral blood lymphocytes were exposed to 60 MeV protons or 250 kVp X-rays in the dose range 0-6 Gy, and allowed to repair DNA damage for 24 h under either normal gravity or microgravity modelled by the NASA-designed rotating-wall bioreactor. Cells were then stimulated to proliferate by phytohaemagglutinin (PHA) under normal gravity conditions and prematurely condensed chromosomes were harvested after 48 h. CAs were scored in chromosomes 1 and 2 by fluorescence in-situ hybridization. Proliferation gravisensitivity was examined by cell growth curves and by morphological evaluation of mitogen-induced activation. Cell replication rounds were monitored by bromodeoxyuridine labelling. Modelled microgravity markedly reduced PHA-mediated lymphocyte blastogenesis and cell growth. However, no significant differences between normal gravity and modelled microgravity were found in the dose-response curves for the induction of aberrant cells or total interchromosomal exchange frequency. Rotating-wall bioreactor-based microgravity reproduced space-related alterations of mitogen stimulation in human lymphocytes but did not affect the yield of CAs induced by low-linear energy transfer radiation.

Chromosomal intrachanges induced by swift iron ions

We measured the induction of structural aberrations in human chromosome 5 induced by iron ions using the novel technique of multicolor banding in situ hybridization (mBAND). Human lymphocytes isolated from whole blood were exposed in vitro to 500 MeV/n (LET=200 keV/micrometers, doses 1 or 4 Gy) Fe nuclei at the HIMAC accelerator in Chiba (Japan). Chromosomes were prematurely condensed by calyculin A after 48 h in culture and slides were painted by mBAND. We found a frequency of 0.11 and 0.57 residual breakpoints per chromosome 5 after 1 and 4 Gy Fe-ions, respectively. Inter-chromosomal exchanges were the prevalent aberration type measured at both doses, followed by terminal deletions, and by intra-chromosomal exchanges. Among intra-chromosomal exchanges, intra-arm events were more frequent than inter-arm, but a significant number of intra-changes was associated to inter-changes involving the same chromosome after 4 Gy of iron ions. These events show that the complexity of chromosomal exchanges induced by heavy ions can be higher than expected by previous FISH studies.

A model of chromosome aberration induction and chronic myeloid leukaemia incidence at low doses

Some chromosome aberration types, generally translocations, are correlated with specific cancers. An example is provided by chronic myeloid leukemia (CML) cells, most of which carry a translocation involving the ABL gene on chromosome 9 and the BCR gene on chromosome 22. The hypothesis of a causal relationship between CML and the chimeric protein product of the BCR-ABL translocation has recently received strong support. In this framework, a mechanistic model and Monte-Carlo code simulating radiation-induced chromosome aberrations in human lymphocytes will be presented. The current version of the model can predict dose-response curves for the main aberration types following acute irradiation with gamma rays and light ions of different energies. The model is based on the assumption that only clustered DNA lesions can lead to aberrations and that only lesion free ends in neighbouring chromosome territories can join and form exchanges. Such lesions are distributed within the cell nucleus according to the radiation track structure, i.e. randomly for low-LET radiation and along straight lines for high-LET light ions. Interphase chromosome territories are explicitly simulated and background aberrations are taken into account. Very good agreement was found with experimental data taken from the literature that provided a further validation of the model. As an application, yields of BCR-ABL translocations were calculated. Preliminary results led to a CML induction dose-response that is approximately quadratic below 0.1 Gy and essentially linear at higher doses up to 1 Gy. The numerical values obtained for the probability of CML induction are consistent with values obtained by other groups with different approaches.

Simultaneous measurement of multiple radiation-induced protein expression profiles using the Luminex(TM) system

Space flight results in the exposure of astronauts to a mixed field of radiation composed of energetic particles of varying energies, and biological indicators of space radiation exposure provides a better understanding of the associated long-term health risks. Current methods of biodosimetry have employed the use of cytogenetic analysis for biodosimetry, and more recently the advent of technological progression has led to advanced research in the use of genomic and proteomic expression profiling to simultaneously assess biomarkers of radiation exposure. We describe here the technical advantages of the Luminex(TM) 100 system relative to traditional methods and its potential as a tool to simultaneously profile multiple proteins induced by ionizing radiation. The development of such a bioassay would provide more relevant post-translational dynamics of stress response and will impart important implications in the advancement of space and other radiation contact monitoring.

Biological effectiveness of accelerated particles for the induction of chromosome damage measured in metaphase and interphase human lymphocytes

Chromosome aberrations were investigated in human lymphocytes after in vitro exposure to 1H-, 3He-, 12C-, 40Ar-, 28Si-, 56Fe-, or 197Au-ion beams, with LET ranging from approximately 0.4-1393 keV/microm in the dose range of 0.075-3 Gy. Dose-response curves for chromosome exchanges, measured at the first mitosis postirradiation using fluorescence in situ hybridization (FISH) with whole-chromosome probes, were fitted with linear or linear-quadratic functions. The relative biological effectiveness (RBE) was estimated from the initial slope of the dose-response curve for chromosomal damage with respect to low- or high-dose-rate gamma rays. Estimates of RBEmax values for mitotic spreads, which ranged from near 0.7 to 11.1 for total exchanges, increased with LET, reaching a maximum at about 150 keV/microm, and decreased with further increase in LET. RBEs for complex aberrations are undefined due to the lack of an initial slope for gamma rays. Additionally, the effect of mitotic delay on RBE values was investigated by measuring chromosome aberrations in interphase after chemically induced premature chromosome condensation (PCC), and values were up to threefold higher than for metaphase analysis.

Chromosome aberrations as biomarkers of radiation exposure: modelling basic mechanisms

The space radiation environment is a mixed field consisting of different particles having different energies, including high charge and energy (HZE) ions. Conventional measurements of absorbed doses may not be sufficient to completely characterise the radiation field and perform reliable estimates of health risks. Biological dosimetry, based on the observation of specific radiation-induced endpoints (typically chromosome aberrations), can be a helpful approach in case of monitored exposure to space radiation or other mixed fields, as well as in case of accidental exposure. Furthermore, various ratios of aberrations (e.g. dicentric chromosomes to centric rings and complex exchanges to simple exchanges) have been suggested as possible fingerprints of radiation quality, although all of them have been subjected to some criticisms. In this context a mechanistic model and a Monte Carlo code for the simulation of chromosome aberration induction were developed. The model, able to provide dose-responses for different aberrations (e.g. dicentrics, rings, fragments, translocations, insertions and other complex exchanges), was further developed to assess the dependence of various ratios of aberrations on radiation quality. The predictions of the model were compared with available data, whose experimental conditions were faithfully reproduced. Particular attention was devoted to the scoring criteria adopted in different laboratories and to possible biases introduced by interphase death and mitotic delay. This latter aspect was investigated by taking into account both metaphase data and data obtained with Premature Chromosome Condensation (PCC).

Radiation biodosimetry: applications for spaceflight

The multiparametric dosimetry system that we are developing for medical radiological defense applications could be adapted for spaceflight environments. The system complements the internationally accepted personnel dosimeters and cytogenetic analysis of chromosome aberrations, considered the best means of documenting radiation doses for health records. Our system consists of a portable hematology analyzer, molecular biodosimetry using nucleic acid and antigen-based diagnostic equipment, and a dose assessment management software application. A dry-capillary tube reagent-based centrifuge blood cell counter (QBC Autoread Plus, Becton [correction of Beckon] Dickinson Bioscience) measures peripheral blood lymphocytes and monocytes, which could determine radiation dose based on the kinetics of blood cell depletion. Molecular biomarkers for ionizing radiation exposure (gene expression changes, blood proteins) can be measured in real time using such diagnostic detection technologies as miniaturized nucleic acid sequences and antigen-based biosensors, but they require validation of dose-dependent targets and development of optimized protocols and analysis systems. The Biodosimetry Assessment Tool, a software application, calculates radiation dose based on a patient's physical signs and symptoms and blood cell count analysis. It also annotates location of personnel dosimeters, displays a summary of a patient's dosimetric information to healthcare professionals, and archives the data for further use. These radiation assessment diagnostic technologies can have dual-use applications supporting general medical-related care.

Adaptive response studies may help choose astronauts for long-term space travel

Long-term manned exploratory missions are planned for the future. Exposure to high-energy neutrons, protons and high charge and energy particles during a deep space mission, needs protection against the detrimental effects of space radiation. It has been suggested that exposure to unpredictable extremely large solar particle events would kill the astronauts without massive shielding. To reduce this risk to astronauts and to minimize the need for shielding, astronauts with highest significant adaptive responses should be chosen. It has been demonstrated that some humans living in very high natural radiation areas have acquired high adaptive responses to external radiation. Therefore, we suggest that for a deep space mission the adaptive response of all potential crew members be measured and only those with high adaptive response be chosen. We also proclaim that chronic exposure to elevated levels of radiation can considerably decrease radiation susceptibility and better protect astronauts against the unpredictable exposure to sudden and dramatic increase in flux due to solar flares and coronal mass ejections.

In vivo and in vitro measurements of complex-type chromosomal exchanges induced by heavy ions

Heavy ions are more efficient in producing complex-type chromosome exchanges than sparsely ionizing radiation, and this can potentially be used as a biomarker of radiation quality. We measured the induction of complex-type chromosomal aberrations in human peripheral blood lymphocytes exposed in vitro to accelerated H-, He-, C-, Ar-, Fe- and Au-ions in the LET range of approximately 0.4-1400 keV/micrometers. Chromosomes were analyzed either at the first post-irradiation mitosis, or in interphase, following premature condensation by phosphatase inhibitors. Selected chromosomes were then visualized after FISH-painting. The dose-response curve for the induction of complex-type exchanges by heavy ions was linear in the dose-range 0.2-1.5 Gy, while gamma-rays did not produce a significant increase in the yield of complex rearrangements in this dose range. The yield of complex aberrations after 1 Gy of heavy ions increased up to an LET around 100 keV/micrometers, and then declined at higher LET values. When mitotic cells were analyzed, the frequency of complex rearrangements after 1 Gy was about 10 times higher for Ar- or Fe- ions (the most effective ions, with LET around 100 keV/micrometers) than for 250 MeV protons, and values were about 35 times higher in prematurely condensed chromosomes. These results suggest that complex rearrangements may be detected in astronauts' blood lymphocytes after long-term space flight, because crews are exposed to HZE particles from galactic cosmic radiation. However, in a cytogenetic study of ten astronauts after long-term missions on the Mir or International Space Station, we found a very low frequency of complex rearrangements, and a significant post-flight increase was detected in only one out of the ten crewmembers. It appears that the use of complex-type exchanges as biomarker of radiation quality in vivo after low-dose chronic exposure in mixed radiation fields is hampered by statistical uncertainties.

Biological dosimetry in Russian and Italian astronauts

Large uncertainties are associated with estimates of equivalent dose and cancer risk for crews of long-term space missions. Biological dosimetry in astronauts is emerging as a useful technique to compare predictions based on quality factors and risk coefficients with actual measurements of biological damage in-flight. In the present study, chromosomal aberrations were analyzed in one Italian and eight Russian cosmonauts following missions of different duration on the MIR and the international space station (ISS). We used the technique of fluorescence in situ hybridization (FISH) to visualize translocations in chromosomes 1 and 2. In some cases, an increase in chromosome damage was observed after flight, but no correlation could be found between chromosome damage and flight history, in terms of number of flights at the time of sampling, duration in space and extra-vehicular activity. Blood samples from one of the cosmonauts were exposed in vitro to 6 MeV X-rays both before and after the flight. An enhancement in radiosensitivity induced by the spaceflight was observed.

Hypergravity modifies the signal transduction of ionizing radiation through p53

To determine the possible effect of hypergravity to modify the signal transduction of ionizing radiation, we analyzed the accumulation of p53 and the expression of p53-dependent genes, Waf-1 and Bax, using the western blot analysis. Hypergravity (20 x g) induced the accumulation of p53 in the human glioblastoma cell line A172 after 3 h of incubation. Low-dose (0.5 Gy) irradiation to the cells accumulated p53 1.5 h after irradiation, and induced Waf-1 and Bax. Under the condition of hypergravity (20 x g), the peak of p53 accumulation was shifted from 1.5 h to 3 h after irradiation, and the inductions of Waf-1 and Bax were suppressed entirely. These results indicate that hypergravity modifies the signal transduction of ionizing radiation through p53 in the cells.

Dose responses for chromosome aberrations produced in noncycling primary human fibroblasts by alpha particles, and by gamma rays delivered at sublimiting low dose rates

As the total dose of X or gamma rays is delivered at lower and lower rates, the yield of chromosome aberrations progressively diminishes. Simultaneously, the shape of the dose response changes from one exhibiting pronounced upward curvature at high dose rates to one approaching linearity at low dose rates. Although the maximum sparing effect caused by lowering the dose rate can be predicted from classical cytogenetic theory, it has yet to be verified experimentally. Here, noncycling normal human fibroblasts were exposed to graded doses of (137)Cs gamma rays at chronic dose rates of 6.3 and 2.8 cGy h(-1), dose rates that we reasoned should be lower than those required to achieve maximal sparing. This was indeed shown to be the case, after it was determined that the two chronic dose rates produced identical linear dose responses of 0.05 total aberrations per cell Gy(-1). Consistent with cytogenetic theory, this value was statistically indistinguishable from the linear coefficient derived from a fit to aberration frequencies produced by high-dose-rate exposure. Exposure to (238)Pu alpha particles also produced a linear dose response for total aberrations, whose slope-with respect to (137)Cs gamma rays as a reference radiation-implied a maximum RBE of 35 +/- 2.

Heavy-ion-induced mutations in the gpt delta transgenic mouse: comparison of mutation spectra induced by heavy-ion, X-ray, and gamma-ray radiation

Heavy-ion radiation accounts for the major component of absorbed cosmic radiation and is thus regarded as a significant risk during long-term manned space missions. To evaluate the genetic damage induced by heavy particle radiation, gpt delta transgenic mice were exposed to carbon particle irradiation and the induced mutations were compared with those induced by reference radiations, i.e., X-rays and gamma-rays. In the transgenic mouse model, deletions and point mutations were individually identified as Spi(-) and gpt mutations, respectively. Two days after 10 Gy of whole-body irradiation, the mutant frequencies (MFs) of Spi(-) and gpt were determined. Carbon particle irradiation significantly increased Spi(-) MF in the liver, spleen, and kidney but not in the testis, suggesting an organ-specific induction of mutations by heavy-ion irradiation. In the liver, the potency of inducing Spi(-) mutation was highest for carbon particles (3.3-fold increase) followed by X-rays (2.1-fold increase) and gamma-rays (1.3-fold increase), while the potency of inducing gpt mutations was highest for gamma-rays (3.3-fold increase) followed by X-rays (2.1-fold increase) and carbon particles (1.6-fold increase). DNA sequence analysis revealed that carbon particles induced deletions that were mainly more than 1,000 base pairs in size, whereas gamma-rays induced deletions of less than 100 base pairs and base substitutions. X-rays induced various-sized deletions and base substitutions. These results suggest that heavy-ion beam irradiation is effective at inducing deletions via DNA double-strand breaks but less effective than X-ray and gamma-ray irradiation at producing oxidative DNA damage by free radicals.

Chromosome aberrations in human lymphocytes induced by 250 MeV protons: effects of dose, dose rate and shielding

Although the space radiation environment consists predominantly of energetic protons, astronauts inside a spacecraft are chronically exposed to both primary particles as well as secondary particles that are generated when the primary particles penetrate the spacecraft shielding. Secondary neutrons and secondary charged particles can have an LET value that is greater than the primary protons and, therefore, produce a higher relative biological effectiveness (RBE). Using the accelerator facility at Loma Linda University, we exposed human lymphocytes in vitro to 250 MeV protons with doses ranging from 0 to 60 cGy at three different dose rates: a low dose rate of 7.5 cGy/h, an intermediate dose rate of 30 cGy/h and a high dose rate of 70 cGy/min. The effect of 15 g/cm2 aluminum shielding on the induction of chromosome aberrations was investigated for each dose rate. After exposure, lymphocytes were incubated in growth medium containing phytohemagglutinin (PHA) and chromosome spreads were collected using a chemical-induced premature chromosome condensation (PCC) technique. Aberrations were analyzed using the fluorescence in situ hybridization (FISH) technique with three different colored chromosome-painting probes. The frequency of reciprocal and complex-type chromosome exchanges were compared in shielded and unshielded samples.

Chromosome aberrations in the blood lymphocytes of astronauts after space flight

Cytogenetic analysis of the lymphocytes of astronauts provides a direct measurement of space radiation damage in vivo, which takes into account individual radiosensitivity and considers the influence of microgravity and other stress conditions. Chromosome exchanges were measured in the blood lymphocytes of eight crew members after their respective space missions, using fluorescence in situ hybridization (FISH) with chromosome painting probes. Significant increases in aberrations were observed after the long-duration missions. The in vivo dose was derived from the frequencies of translocations and total exchanges using calibration curves determined before flight, and the RBE was estimated by comparison with individually measured physical absorbed doses. The values for average RBE were compared to the average quality factor (Q) from direct measurements of the lineal energy spectra using a tissue-equivalent proportional counter (TEPC) and radiation transport codes. The ratio of aberrations identified as complex was slightly higher after flight, which is thought to be an indication of exposure to high-LET radiation. To determine whether the frequency of complex aberrations measured in metaphase spreads after exposure to high-LET radiation was influenced by a cell cycle delay, chromosome damage was analyzed in prematurely condensed chromosome samples collected from two crew members before and after a short-duration mission. The frequency of complex exchanges after flight was higher in prematurely condensed chromosomes than in metaphase cells for one crew member.

Risk estimation based on chromosomal aberrations induced by radiation

The presence of a causal association between the frequency of chromosomal aberrations in peripheral blood lymphocytes and the risk of cancer has been substantiated recently by epidemiological studies. Cytogenetic analyses of crew members of the Mir Space Station have shown that a significant increase in the frequency of chromosomal aberrations can be detected after flight, and that such an increase is likely to be attributed to the radiation exposure. The risk of cancer can be estimated directly from the yields of chromosomal aberrations, taking into account some aspects of individual susceptibility and other factors unrelated to radiation. However, the use of an appropriate technique for the collection and analysis of chromosomes and the choice of the structural aberrations to be measured are crucial in providing sound results. Based on the fraction of aberrant lymphocytes detected before and after flight, the relative risk after a long-term Mir mission is estimated to be about 1.2-1.3. The new technique of mFISH can provide useful insights into the quantification of risk on an individual basis.

Modelling chromosomal aberration induction by ionising radiation: the influence of interphase chromosome architecture

Several advances have been achieved in the knowledge of nuclear architecture and functions during the last decade, thus allowing the identification of interphase chromosome territories and sub-chromosomal domains (e.g. arm and band domains). This is an important step in the study of radiation-induced chromosome aberrations; indeed, the coupling between track-structure simulations and reliable descriptions of the geometrical properties of the target is one of the main tasks in modelling aberration induction by radiation, since it allows one to clarify the role of the initial positioning of two DNA lesions in determining their interaction probability. In the present paper, the main recent findings on nuclear and chromosomal architecture are summarised. A few examples of models based on different descriptions of interphase chromosome organisation (random-walk models, domain models and static models) are presented, focussing on how the approach adopted in modelling the target nuclei and chromosomes can influence the simulation of chromosomal aberration yields. Each model is discussed by taking into account available experimental data on chromosome aberration induction and/or interphase chromatin organisation. Preliminary results from a mechanistic model based on a coupling between radiation track-structure features and explicitly-modelled, non-overlapping chromosome territories are presented.

Biological effects of cosmic radiation: deterministic and stochastic

Our basic understanding of the biological responses to cosmic radiations comes in large part from an international series of ground-based laboratory studies, where accelerators have provided the source of representative charged particle radiations. Most of the experimental studies have been performed using acute exposures to a single radiation type at relatively high doses and dose rates. However, most exposures in flight occur from low doses of mixed radiation fields at low-dose rates. This paper provides a brief overview of existing pertinent clinical and biological radiation data and the limitations associated with data available from specific components of the radiation fields in airflight and space travel.