Relationship between aberration yield and mitotic delay in human lymphocytes exposed to 200 MeV/u Fe-ions or X-rays

Chromatin and the Cellular Response to Particle Radiation-Induced Oxidative and Clustered DNA Damage

Exposure to environmental ionizing radiation is prevalent, with greatest lifetime doses typically from high Linear Energy Transfer (high-LET) alpha particles via the radioactive decay of radon gas in indoor air. Particle radiation is highly genotoxic, inducing DNA damage including oxidative base lesions and DNA double strand breaks. Due to the ionization density of high-LET radiation, the consequent damage is highly clustered wherein ≥2 distinct DNA lesions occur within 1–2 helical turns of one another. These multiply-damaged sites are difficult for eukaryotic cells to resolve either quickly or accurately, resulting in the persistence of DNA damage and/or the accumulation of mutations at a greater rate per absorbed dose, relative to lower LET radiation types. The proximity of the same and different types of DNA lesions to one another is challenging for DNA repair processes, with diverse pathways often confounding or interplaying with one another in complex ways. In this context, understanding the state of the higher order chromatin compaction and arrangements is essential, as it influences the density of damage produced by high-LET radiation and regulates the recruitment and activity of DNA repair factors. This review will summarize the latest research exploring the processes by which clustered DNA damage sites are induced, detected, and repaired in the context of chromatin.

Cytogenetically-based biodosimetry after high doses of radiation

Dosimetry is an important tool for triage and treatment planning following any radiation exposure accident, and biological dosimetry, which estimates exposure dose using a biological parameter, is a practical means of determining the specific dose an individual receives. The cytokinesis-blocked micronucleus assay (CBMN) is an established biodosimetric tool to measure chromosomal damage in mitogen-stimulated human lymphocytes. The CBMN method is especially valuable for biodosimetry in triage situations thanks to simplicity in scoring and adaptability to high-throughput automated sample processing systems. While this technique produces dose-response data which fit very well to a linear-quadratic model for exposures to low linear energy transfer (LET) radiation and for doses up for 5 Gy, limitations to the accuracy of this method arise at larger doses. Accuracy at higher doses is limited by the number of cells reaching mitosis. Whereas it would be expected that the yield of micronuclei increases with the dose, in many experiments it has been shown to actually decrease when normalized over the total number of cells. This variation from a monotonically increasing dose response poses a limitation for retrospective dose reconstruction. In this study we modified the standard CBMN assay to increase its accuracy following exposures to higher doses of photons or a mixed neutron-photon beam. The assay is modified either through inhibitions of the G2/M and spindle checkpoints with the addition of caffeine and/or ZM447439 (an Aurora kinase inhibitor), respectively to the blood cultures at select times during the assay. Our results showed that caffeine addition improved assay performance for photon up to 10 Gy. This was achieved by extending the assay time from the typical 70 h to just 74 h. Compared to micronuclei yields without inhibitors, addition of caffeine and ZM447439 resulted in improved accuracy in the detection of micronuclei yields up to 10 Gy from photons and 4 Gy of mixed neutrons-photons. When the dose-effect curves were fitted to take into account the turnover phenomenon observed at higher doses, best fitting was achieved when the combination of both inhibitors was used. These techniques permit reliable dose reconstruction after high doses of radiation with a method that can be adapted to high-throughput automated sample processing systems.

Dependence of micronuclei assay on the depth of absorbed dose

AIM

The purpose of the present study is to investigate the dependence of micronuclei response on the depth of absorbed dose.

BACKGROUND

One of the most common cytogenetic methods used for radiation dosimetry is micronuclei (MN). Being less complex and faster than other methods are two remarkable advantages of the MN method which make it suitable for monitoring of population. In biological dosimetry based on the micronuclei method, the investigation into the dependence of response on the depth in which dose is absorbed is significant, though has received less attention so far.

MATERIALS AND METHODS

Blood samples were poured in separate vials to be irradiated at different depths using a linear accelerator system.

RESULTS

According to the results, MN, as a function of the absorbed dose, had the best fitness with the linear-quadratic model at all depths. Furthermore, the results showed the dependence of MN response on the depth of absorbed dose. For doses up to 2 Gy, the maximum difference from the reference depth of 1.5 cm was related to the depth of 10 cm; however, by increasing the absorbed dose, the response associated with the depth of 20 cm showed the maximum deviation from the reference depth.

CONCLUSIONS

Consequently, it is necessary to apply a correction factor to the biological dosimetry. The correction factor is dependent on the depth and the absorbed dose.

Ionizing Particle Radiation as a Modulator of Endogenous Bone Marrow Cell Reprogramming: Implications for Hematological Cancers

Exposure of individuals to ionizing radiation (IR), as in the case of astronauts exploring space or radiotherapy cancer patients, increases their risk of developing secondary cancers and other health-related problems. Bone marrow (BM), the site in the body where hematopoietic stem cell (HSC) self-renewal and differentiation to mature blood cells occurs, is extremely sensitive to low-dose IR, including irradiation by high-charge and high-energy particles. Low-dose IR induces DNA damage and persistent oxidative stress in the BM hematopoietic cells. Inefficient DNA repair processes in HSC and early hematopoietic progenitors can lead to an accumulation of mutations whereas long-lasting oxidative stress can impair hematopoiesis itself, thereby causing long-term damage to hematopoietic cells in the BM niche. We report here that low-dose ¹H- and ⁵⁶Fe-IR significantly decreased the hematopoietic early and late multipotent progenitor (E- and L-MPP, respectively) cell numbers in mouse BM over a period of up to 10 months after exposure. Both ¹H- and ⁵⁶Fe-IR increased the expression of pluripotent stem cell markers Sox2, Nanog, and Oct4 in L-MPPs and 10 months post-IR exposure. We postulate that low doses of ¹H- and ⁵⁶Fe-IR may induce endogenous cellular reprogramming of BM hematopoietic progenitor cells to assume a more primitive pluripotent phenotype and that IR-induced oxidative DNA damage may lead to mutations in these BM progenitors. This could then be propagated to successive cell lineages. Persistent impairment of BM progenitor cell populations can disrupt hematopoietic homeostasis and lead to hematologic disorders, and these findings warrant further mechanistic studies into the effects of low-dose IR on the functional capacity of BM-derived hematopoietic cells including their self-renewal and pluripotency.

Modulation of radiation-induced cytogenetic damage in human peripheral blood lymphocytes by hypothermia

PURPOSE

Recent studies have shown that low temperature (hypothermia) at exposure can act in a radio-protective manner at the level of cytogenetic damage. The mechanisms of this phenomenon are not understood, but it was suggested to be due to hypothermia-induced perturbations of the cell cycle. The purpose of the present study was to detect whether a reduced frequency of micronuclei is observed in peripheral blood lymphocytes (PBL) irradiated at low temperature and harvested sequentially at 3 time points. Additionally, the level of apoptosis was estimated by microscopic analysis of the MN slides.

MATERIALS AND METHODS

Experiments were carried out with blood drawn from three donors at the Stockholm University and from three donors at the Jan Kochanowski University. Prior to irradiation, blood samples were incubated for 20min and irradiated at the respective temperature (0°C and 37°C) with gamma rays. Whole blood cultures were set up, cytochalasin B was added after 44h of irradiation and the samples were harvested after 72, 96 and 120h of incubation time.

RESULTS AND CONCLUSIONS

The frequency of micronuclei was markedly lower in PBL harvested at 72h, 96h and 120h following irradiation at 0°C as compared to 37°C. This indicates that the temperature effect observed in peripheral blood lymphocytes after irradiation is not related to a temporary perturbation of the cell cycle. Also, it is not due to selective elimination of damaged cells by apoptosis.

Relative proximity of chromosome territories influences chromosome exchange partners in radiation-induced chromosome rearrangements in primary human bronchial epithelial cells

It is well established that chromosomes exist in discrete territories (CTs) in interphase and are positioned in a cell-type specific probabilistic manner. The relative localisation of individual CTs within cell nuclei remains poorly understood, yet many cancers are associated with specific chromosome rearrangements and there is good evidence that relative territorial position influences their frequency of exchange. To examine this further, we characterised the complexity of radiation-induced chromosome exchanges in normal human bronchial epithelial (NHBE) cells by M-FISH analysis of PCC spreads and correlated the exchanges induced with their preferred interphase position, as determined by 1/2-colour 2D-FISH analysis, at the time of irradiation. We found that the frequency and complexity of aberrations induced were reduced in ellipsoid NHBE cells in comparison to previous observations in spherical cells, consistent with aberration complexity being dependent upon the number and proximity of damaged CTs, i.e. lesion proximity. To ask if particular chromosome neighbourhoods could be identified we analysed all radiation-induced pair-wise exchanges using SCHIP (statistics for chromosome interphase positioning) and found that exchanges between chromosomes (1;13), (9;17), (9;18), (12;18) and (16;21) all occurred more often than expected assuming randomness. All of these pairs were also found to be either sharing similar preferred positions in interphase and/or sharing neighbouring territory boundaries. We also analysed a human small cell lung cancer cell line, DMS53, by M-FISH observing the genome to be highly rearranged, yet possessing rearrangements also involving chromosomes (1;13) and (9;17). Our findings show evidence for the occurrence of non-random exchanges that may reflect the territorial organisation of chromosomes in interphase at time of damage and highlight the importance of cellular geometry for the induction of aberrations of varying complexity after exposure to both low and high-LET radiation.

Reduced chromosome aberration complexity in normal human bronchial epithelial cells exposed to low-LET γ-rays and high-LET α-particles

Purpose:

Cells of the lung are at risk from exposure to low and moderate doses of ionizing radiation from a range of environmental and medical sources. To help assess human health risks from such exposures, a better understanding of the frequency and types of chromosome aberration initially-induced in human lung cell types is required to link initial DNA damage and rearrangements with transmission potential and, to assess how this varies with radiation quality.

Materials and methods:

We exposed normal human bronchial lung epithelial (NHBE) cells in vitro to 0.5 and 1 Gy low-linear energy transfer (LET) γ-rays and a low fluence of high-LET α-particles and assayed for chromosome aberrations in premature chromosome condensation (PCC) spreads by 24-color multiplex-fluorescence in situ hybridization (M-FISH).

Results:

Both simple and complex aberrations were induced in a LET and dose-dependent manner; however, the frequency and complexity observed were reduced in comparison to that previously reported in spherical cell types after exposure to comparable doses or fluence of radiation. Approximately 1–2% of all exposed cells were categorized as being capable of transmitting radiation-induced chromosomal damage to future NHBE cell generations, irrespective of dose.

Conclusion:

One possible mechanistic explanation for this reduced complexity is the differing geometric organization of chromosome territories within ellipsoid nuclei compared to spherical nuclei. This study highlights the need to better understand the role of nuclear organization in the formation of exchange aberrations and, the influence three-dimensional (3D) tissue architecture may have on this in vivo.

Micronuclei in human peripheral blood lymphocytes exposed to mixed beams of X-rays and alpha particles

The purpose of this study was to analyse the cytogenetic effect of exposing human peripheral blood lymphocytes (PBL) to a mixed beam of alpha particles and X-rays. Whole blood collected from one donor was exposed to different doses of alpha particles ((241)Am), X-rays and a combination of both. All exposures were carried out at 37 °C. Three independent experiments were performed. Micronuclei (MN) in binucleated PBL were scored as the endpoint. Moreover, the size of MN was measured. The results show that exposure of PBL to a mixed beam of high and low linear energy transfer radiation led to significantly higher than expected frequencies of MN. The measurement of MN size did not reveal any differences between the effect of alpha particles and mixed beam. In conclusion, a combined exposure of PBL to alpha particles and X-rays leads to a synergistic effect as measured by the frequency of MN. From the analysis of MN distributions, we conclude that the increase was due to an impaired repair of X-ray-induced DNA damage.

Chromosome aberration measurements in mitotic and G2-PCC lymphocytes at the standard sampling time of 48 h underestimate the effectiveness of high-LET particles

The relationship between heavy-ion-induced cell cycle delay and the time-course of aberrations in first-cycle metaphases or prematurely condensed G(2)-cells (G(2)-PCC) was investigated. Lymphocytes of the same donor were irradiated with X-rays or various charged particles (carbon, iron, xenon, and chromium) covering an LET range of 2-3,160 keV/μm. Chromosome aberrations were measured in samples collected at 48, 60, 72, and 84 h postirradiation. Linear-quadratic functions were fitted to the data, and the fit parameters α and β were determined. At any sampling time, α values derived from G(2)-cells were higher than those from metaphases. The α value derived from metaphase analysis at 48 h increased with LET, reached a maximum around 155 keV/μm, and decreased with a further rise in LET. At the later time-points, higher α values were estimated for particles with LET > 30 keV/μm. Estimates of α values from G(2)-cells showed a similar LET dependence, yet the time-dependent increase was less pronounced. Altogether, our data demonstrate that heavily damaged lymphocytes suffer a prolonged G(2)-arrest that is clearly LET dependent. For this very reason, the standard analysis of aberrations in metaphase cells 48 h postirradiation will considerably underestimate the effectiveness of high-LET radiation. Scoring of aberrations in G(2)-PCC at 48 h as suggested by several authors will result in higher aberration yields. However, when particles with a very high-LET value (LET > 150 keV/μm) are applied, still a fraction of multiple damaged cells escape detection by G(2)-analysis 48 h postirradiation.

Complex exchanges are responsible for the increased effectiveness of C-ions compared to X-rays at the first post-irradiation mitosis

The purpose of the present study was to investigate as to what extent differences in the linear energy transfer (LET) are reflected at the chromosomal level. For this study human lymphocytes were exposed to 9.5 MeV/u C-ions (1 or 2 Gy, LET=175 keV/microm) or X-rays (1-6 Gy), harvested at 48, 72 or 96 h post-irradiation and aberrations were scored in first cycle metaphases using 24 color fluorescence in situ hybridization (mFISH). Additionally, in selected samples aberrations were measured in prematurely condensed G2-phase cells. Analysis of the time-course of aberrations in first cycle metaphases showed a stable yield of simple and complex exchanges after X-ray irradiation. In contrast, after C-ion exposure the yields profoundly increased with harvesting time complicating the estimation of the frequency of aberrations produced by high LET particles within the entire cell population. This is especially true for the yield of complex exchanges. Complex aberrations dominate the aberration spectrum produced by C-ions. Their fraction was about 50% for the two measured doses. In contrast, isodoses of X-rays induced smaller proportions of complex aberrations (i.e. 5% and 15%, respectively). For both radiation qualities the fraction of complexes did not change with harvesting time. As expected from the different dose deposition of high and low LET radiation, complex exchanges produced by high LET C-ions involved more breaks and more chromosomes than those induced by isodoses of X-rays. Noteworthy, C-ions but not X-rays induced a small number of complex chromatid-isochromatid exchanges that are not expected for cells exposed in the G0-phase. The results obtained so far for cells arrested in G2-phase confirm these patterns. Altogether our data show that the increased effectiveness of C-ions for the induction of aberrations in first cycle cells is determined by complex exchanges, whereas for simple exchanges the relative biological effectiveness is about one.

Cell cycle arrest and aberration yield in normal human fibroblasts. II: Effects of 11 MeV u−1C ions and 9.9 MeV u−1Ni ions

PURPOSE

To investigate further the relationship between high linear energy transfer (LET) induced cell cycle arrests and the yield of chromosome aberrations observable in normal human fibroblasts at the first post-irradiation mitosis.

MATERIALS AND METHODS

Normal human fibroblasts (AG01,522C) were exposed in G0/G1 to either 11 MeV u(-1) C ions (LET = 153.5 keV microm(-1)) or 9.9 MeV u(-1) Ni ions (LET = 2,455 keV microm(-1)), subcultured in medium containing 5-Bromo-2'-deoxyuridine (BrdU) and at multiple time-points post-irradiation the yield of chromosomal damage, the mitotic index and the cumulative BrdU-labelling index were determined. Furthermore, a mathematical approach was used to analyse the entire cell population.

RESULTS

Following high LET exposure normal fibroblasts suffer a transient delay into S-phase and into mitosis as well as a prolonged, probably permanent cell cycle arrest in the initial G0/G1-phase. Cells that reach the first mitosis at early times carried less aberrations than those collected at later times indicating a relationship between cell cycle delay and the number of aberrations. However, with respect to the whole cell population, only a few aberrant fibroblasts are able to progress to the first mitosis. For all endpoints studied the relative biological effectiveness (RBE) of C ions is in the range of 2 - 4, while for Ni ions RBE < 1 is estimated. In contrast, when compared on a per particle basis Ni ions with the higher ionization density were found to be more effective.

CONCLUSIONS

Detailed analysis of the data demonstrates that the number of fibroblasts at risk for neoplastic transformation is significantly reduced by a chronic cell cycle arrest in the initial G0/G1-phase and, for the first time, the LET-dependence of this effect has been shown.

Analysis of cell cycle in mouse bone marrow cells following acute in vivo exposure to 56Fe ions

A pilot study was conducted to examine the magnitude of cell-cycle delay and apoptosis in bone marrow (BM) cells collected at 18, 42 and 66 hr from radiosensitive CBA/CaJ mice and radioresistant C57BL/6J mice following a whole-body in vivo exposure to 1 GeV/amu (56)Fe ions or (137)Cs gamma rays. At each sacrifice, BM cells were collected from three mice of each strain per dose of (56)Fe ions (0, 10 and 100 cGy) and two mice of each strain per dose of (137)Cs gamma rays (0, 100 and 300 cGy). A significant G1-arrest (ANOVA, p < 0.05) was observed at 18 hr after exposure of mice to 100 cGy of (56)Fe ions or 300 cGy of (137)Cs gamma rays, relative to their corresponding sham-controls, resulting in a significant decrease in the percentage of cells cycling into S-phase in both strains. The percentage of S-phase cells subsequently increased and persisted up to 66 hr post-irradiation. Significant numbers of G2/M cells were found at 18 and 66 (but not at 42) hr post-irradiation, regardless of radiation-type or mouse-strain. It is likely that BM cells have undergone at least one cell cycle at 66 hr after exposure of mice to either 100 cGy (56)Fe ions or 300 cGy (137)Cs gamma rays. Our study is the first to investigate the in vivo effects of (56)Fe ions (1 GeV/amu) on the cell cycle of mouse BM cells using flow cytometry. The cell-cycle distribution (but not the number of apoptotic cells) was dependent on radiation-dose and harvest-time.

High yields of isochromatid breaks and successive formation of chromosome exchanges may lead to reproductive cell death following high-LET irradiation

To clarify the relationship between cell death and chromosomal aberrations following exposure to heavy-charged ion particles beams, exponentially growing Human Salivary Gland Tumor cells (HSG cells) were irradiated with various kinds of high energy heavy ions; 13 keV/μm carbon ions as a low-LET charged particle radiation source, 120 keV/μm carbon ions and 440 keV/μm iron ions as high-LET charged particle radiation sources. X-rays (200 kVp) were used as a reference. Reproductive cell death was evaluated by clonogenic assays, and the chromatid aberrations in G2/M phase and their repairing kinetics were analyzed by the calyculin A induced premature chromosome condensation (PCC) method. High-LET heavy-ion beams introduced much more severe and un-repairable chromatid breaks and isochromatid breaks in HSG cells than low-LET irradiation. In addition, the continuous increase of exchange aberrations after irradiation occurred in the high-LET irradiated cells. The cell death, initial production of isochromatid breaks and subsequent formation of chromosome exchange seemed to be depend similarly on LET with a maximum RBE peak around 100–200 keV/μm of LET value. Conversely, un-rejoined isochromatid breaks or chromatid breaks/gaps seemed to be less effective in reproductive cell death. These results suggest that the continuous yield of chromosome exchange aberrations induced by high-LET ionizing particles is a possible reason for the high RBE for cell death following high-LET irradiation, alongside other chromosomal aberrations additively or synergistically.

mFISH analysis of chromosomal damage in bone marrow cells collected from CBA/CaJ mice following whole body exposure to heavy ions (56Fe ions)

To date, there is scant information on in vivo induction of chromosomal damage by heavy ions found in space (i.e. 56Fe ions). For radiation-induced response to be useful for risk assessment, it must be established in in vivo systems especially in cells that are known to be at risk for health problems associated with radiation exposure (such as hematopoietic cells, the known target tissue for radiation-induced leukemia). In this study, the whole genome multicolor fluorescence in situ hybridization (mFISH) technique was used to examine the in vivo induction of chromosomal damage in hematopoietic tissues, i.e. bone marrow cells. These cells were collected from CBA/CaJ mice at day 7 following whole-body exposure to different doses of 1 GeV/amu 56Fe ions (0, 0.1, 0.5 and 1.0 Gy) or (137)Cs gamma rays as the reference radiation (0, 0.5, 1.0 and 3.0 Gy, at the dose rate of 0.72 Gy/min using a GammaCell40). These radiation doses were the average total-body doses. For each radiation type, there were four mice per dose. Several types of aberrations in bone marrow cells collected from mice exposed to either type of radiation were found. These were exchanges and breaks (both chromatid- and chromosome-types). Chromosomal exchanges included translocations (Robertsonian or centric fusion, reciprocal and incomplete types), and dicentrics. No evidence of a non-random involvement of specific chromosomes in any type of aberrations observed in mice exposed to 56Fe ions or 137Cs gamma rays was found. At the radiation dose range used in our in vivo study, the majority of exchanges were simple. Complex exchanges were detected in bone marrow cells collected from mice exposed to 1 Gy of 56Fe ions or 3 Gy of 137Cs gamma rays only, but their frequencies were low. Overall, our in vivo data indicate that the frequency of complex chromosome exchanges was not significantly different between bone marrow cells collected from mice exposed to 56Fe ions or 137Cs gamma rays. Each type of radiation induced significant dose-dependent increases (ANOVA, P < 0.01) in the frequencies of chromosomal damage, including the numbers of abnormal cells. Based upon the linear-terms of dose-response curves, 56Fe ions were 1.6 (all types of exchanges), 4.3 (abnormal cells) and 4.2 (breaks, both chromatid- and chromosome-types) times more effective than 137Cs gamma rays in inducing chromosomal damage.

Cell killing, nuclear damage and apoptosis in Chinese hamster V79 cells after irradiation with heavy-ion beams of (16)O, (12)C and (7)Li

Chinese hamster V79 cells were exposed to high LET (linear energy transfer) (16)O-beam (625keV/mum) radiation in the dose range of 0-9.83Gy. Cell survival, micronuclei (MN), chromosomal aberrations (CA) and induction of apoptosis were studied as a follow up of our earlier study on high LET radiations ((7)Li-beam of 60keV/mum and (12)C-beam of 295keV/mum) as well as (60)Co gamma-rays. Dose dependent decline in surviving fraction was noticed along with the increase of MN frequency, CA frequency as well as percentage of apoptosis as detected by nuclear fragmentation assay. The relative intensity of DNA ladder, which is a useful marker for the determination of the extent of apoptosis induction, was also increased in a dose dependent manner. Additionally, expression of tyrosine kinase lck-1 gene, which plays an important role in response to ionizing radiation induced apoptosis, was increased with the increase of radiation doses and also with incubation time. The present study showed that all the high LET radiations were generally more effective in cell killing and inflicting other cytogenetic damages than that of low LET gamma-rays. The dose response curves revealed that (7)Li-beam was most effective in cell killing as well as inducing other nuclear damages followed by (12)C, (16)O and (60)Co gamma-rays, in that order. The result of this study may have some application in biological dosimetry for assessment of genotoxicity in heavy ion exposed subjects and in determining suitable doses for radiotherapy in cancer patients where various species of heavy ions are now being generally used.

Measurements of metaphase and interphase chromosome aberrations transmitted through early cell replication rounds in human lymphocytes exposed to low-LET protons and high-LET 12C ions

Inheritable chromosome aberrations (CA) are of concern because cytogenetic damage may trigger the carcinogenic process. Moreover, stability of radiation-induced CA is a prerequisite for meaningful biological dosimetry. CA inheritability arguably depends on the aberration structure, with symmetrical exchanges being favoured over asymmetrical rearrangements, but it is also affected by radiation quality. CA induced by low-LET protons and high-LET 12C ions in G0 peripheral blood lymphocytes were measured in first- , second- and third-generation by combined FISH/harlequin staining of metaphase as well as prematurely condensed interphase chromosomes 1 and 2. As expected, the frequency of non-transmissible (NT) aberrations declined through replication rounds. A radiation-induced arrest occurred prior to first post-irradiation mitosis that prevalently affected aberrant cells. Aberrant cells incurred cycle delays also at subsequent cycles following proton-irradiation but not 12C ion-irradiation. As expected, the frequency of reciprocal translocations remained fairly stable while that of dicentrics was halved at each mitotic round. A significant fraction of complex-type exchanges was found in third-generation cells following both irradiations and appeared to be transmitted relatively more efficiently after protons than 12C ions. A low but stably transmitted frequency of transmissible (T)-type insertions were detected after 12C ions but not after low LET-irradiation. Our data support a differential ability by aberrant cells to progress through post-irradiation mitoses that is influenced by the aberration burden and radiation quality.

Alteration in the expression of signaling parameters following carbon ion irradiation

Ionizing radiation induces DNA damage, which generates a complex array of genotoxic responses. These responses depend on the type of DNA damage, which in turn can lead to unique cellular responses. High LET radiation results in clustered damages. This evokes specific signaling responses, which can be cytotoxic or cytoprotective in nature. In the present study the effect of carbon ion irradiation on p 44/42 MAPK and NF-kappaB, which are essentially survival factors, have been studied. Moreover, the effect of inhibition of DNA-PK, which is an important component of DNA repair mechanism, with wortmanin on these signaling factors has been studied. The expression of p 44/42 MAPK was different at 0.1 Gy and 1 Gy and wortmanin was found to inhibit its expression. NF-kappaB expression was higher at 1 Gy than at 0.1 Gy and its expression is unaffected by inhibition of DNA-PK. The notable findings of this study are that the responses to high and low dose of high LET radiation are essentially different and the 6 h time point post irradiation is crucial in deciding the response and needs further investigation.

Prediction of dose response for radiation induced exchange aberrations taking cell cycle delays into account

Chromosomal aberrations (CAs) are regarded as one of the most sensitive biological indicators of genetic alterations. The aberration frequency is routinely determined in the first metaphase. Yet, the data interpretation can be complicated due to radiation induced mitotic delays. To investigate the effect of delays on CA frequency in the first mitosis, human lymphocytes were irradiated with X rays and Giemsa detectable CAs were measured at different sampling times. Besides, a computer simulation was performed reproducing the main effects under investigation, that is, CA induction and cell progression through the mitotic cycle. The CA formation model takes into account the structural organisation of interphase chromosomes in a lymphocyte nucleus, DNA double-strand break (DSB) induction and their rejoining/misrejoining. Lymphocyte transition through the cell cycle was simulated by a Monte Carlo technique. The delay was proposed to result from DNA DSBs. The predicted ratios of first/second/third cycle metaphases agree with the experimental data for control and irradiated samples. Both experimental and calculated CA frequencies in the first mitosis were nearly time-independent. This was proposed to result from de-synchronisation of the lymphocyte population.

Influence of mitotic delay on the results of biological dosimetry for high doses of ionizing radiation

The purpose of this study was to systematically investigate how high doses of sparsely and densely ionizing radiations influence the proliferation time of lymphocytes in short-term cultures and, consequently, the observed frequencies of dicentric and centric ring chromosomes. Peripheral blood samples from five volunteers were irradiated with high doses of 200 kV X-rays and with neutrons with a mean energy of <En>or=2.1 MeV. First division metaphase cells were collected after different culture times of 48, 56, and 72 h and dicentrics, centric ring chromosomes, and acentric fragments were determined. The data hint at considerable mitotic delay. The main increase in the number of chromosome aberrations occurred between 48 and 72 h after an X-ray exposure and between 56 and 72 h after neutron exposure. When the data were used for a calibration of aberration frequency versus dose, subsequent dose estimations resulted, however, in comparable values. Thus, in spite of the influence of mitotic delay on observable chromosome aberrations, at least for the radiation types investigated here, a culture time of 48 h is acceptable for biological dosimetry.

Exploration of "over kill effect" of high-LET Ar- and Fe-ions by evaluating the fraction of non-hit cell and interphase death

The reason why RBE for cell killing fell to less than unity (1.0) with very high-LET heavy-ions ((40)Ar: 1,640 keV/microm; (56)Fe: 780, 1,200, 2,000 keV/microm) was explored by evaluating the fraction of non-hit cell (time-lapse observation) and cells undergoing interphase death (calculation based on our previous data). CHO cells were exposed to 4 Gy (30% survival dose) of Ar (1,640 keV/microm) or Fe-ions (2,000 keV/microm). About 20% of all cells were judged to be non-hit, and about 10% cells survived radiation damage. About 70% cells died after dividing at least once (reproductive death) or without dividing (interphase death). RBE for reproductive (RBE[R]) and interphase (RBE[I]) death showed a similar LET dependence with maximum around 200 keV/microm. In this LET region, at 30% survival level, about 10% non-survivors underwent interphase death. The corresponding value for very high-LET Fe-ions (2,000 keV/microm) was not particularly high (approximately 15%), whereas that for X-rays was less than 3%. However, reproductive death (67%) predominated over interphase death (33%) even in regard to rather severely damaged cells (1% survival level) after exposure to Fe-ions (2,000 keV/microm). These indicate that interphase death is a type of cell death characteristic for the cells exposed to high-LET radiation and is not caused by "cellular over kill effect". Both NHF37 (non-hit fraction at 37% survival) and inactivation cross-section for reproductive death (sigma[R]) began to increase when LET exceeded 100 keV/microm. The exclusion of non-hit fraction in the calculation of surviving fraction partially prevented the fall of RBE[R] when LET exceeded 200 keV/microm. On the other hand, the mean number of lethal damage per unit dose (NLD/Gy) showed the same LET-dependent pattern as RBE[R]. These suggest that the increase in non-hit fraction and sigma[R] with an increasing LET is caused by enhanced clustering of ionization and DNA damage which lowers the energy efficiency for producing damage and RBE.

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.

Chromosome aberration yields and apoptosis in human lymphocytes irradiated with Fe-ions of differing LET

In the present paper the relationship between cell cycle delays induced by Fe-ions of differing LET and the aberration yield observable in human lymphocytes at mitosis was examined. Cells of the same donor were irradiated with 990 MeV/n Fe-ions (LET=155 keV/micrometers), 200 MeV/n Fe-ions (LET=440 keV/micrometers) and X-rays and aberrations were measured in first cycle mitoses harvested at different times after 48-84 h in culture and in prematurely condensed G2-cells (PCCs) collected at 48 h using calyculin A. Analysis of the time-course of chromosomal damage in first cycle metaphases revealed that the aberration frequency was similar after X-ray irradiation, but increased two and seven fold after exposure to 990 and 200 MeV/n Fe-ions, respectively. Consequently, RBEs derived from late sampling times were significantly higher than those obtained at early times. The PCC-data suggest that the delayed entry of heavily damaged cells into mitosis results especially from a prolonged arrest in G2. Preliminary data obtained for 4.1 MeV/n Cr-ions (LET=3160 keV/micrometers) revealed, that these delays are even more pronounced for low energy Fe-like particles. Additionally, for the different radiation qualities, BrdU-labeling indices and apoptotic indices were determined at several time-points. Only the exposure to low energy Fe-like particles affected the entry of lymphocytes into S-phase and generated a significant apoptotic response indicating that under this particular exposure condition a large proportion of heavily damaged cells is rapidly eliminated from the cell population. The significance of this observation for the estimation of the health risk associated with space radiation remains to be elucidated.

Lethal sectoring, genomic instability, and delayed division delay in HeLa S3 cells surviving alpha- or X-irradiation

Lethal sectoring (LS) is the process for survival in which lethal damage remaining in irradiated cells is eliminated as lethal sector (offspring without reproductive integrity). This process occurs through the postirradiation 1st to 4th divisions with the accompanying appearance of a clonogenic progenitor (clonogen) (clonogenic sectoring: CS). The features of LS or CS and genomic instability (GI) were explored by analyzing the pedigrees of HeLa cells surviving alpha- (0.45 Gy) or X-irradiation (3 Gy) (20% survival dose). Most (approximately 70%) of the lethal latent damage was eliminated from alpha-particle survivors through the 1st to 2nd divisions, but it persisted in X-ray survivors until the 2nd generation. Although the frequency of CS was similar to that of LS for alpha-irradiation, CS was higher than LS for X-irradiation. Nonlethal damage remaining in the clonogens led to an elevated incidence of delayed cell death in their progeny. The mean incidence was higher for alpha-particle (16.3%) than X-ray survivors (8.3%), indicating the greater potentiality for GI by alpha-particles. Evidence is available to suggest the intrinsic difference in the mechanisms of GI induction by these two radiations: the association of misrepaired clustered DNA damage (CD) with alpha-particles and unrepaired PLD with X-rays. A novel phenomenon, "delayed division delay (DDD)" was noticed, though occasionally (approximately 10% per cell), with the progeny during the postirradiation 1st-3rd generations. DDD was much longer in alpha- (mean: approximately 11 h) than X-irradiated cells (approximately 4 h). Supposedly DDD was triggered by delayed chromosome breakage. However, a significant shortening of cell-cycle time at the postirradiation 1st generation was recognized with X-ray survivors.

Chromosome intrachanges and interchanges detected by multicolor banding in lymphocytes: searching for clastogen signatures in the human genome

Genomic fingerprints of mutagenic agents would have wide applications in the field of cancer biology, epidemiology and prevention. The differential spectra of chromosomal aberrations induced by different clastogens suggest that ratios of specific aberrations can be exploited as biomarkers of carcinogen exposure. We have tested this hypothesis using the novel technique of multicolor banding in situ hybridization (mBAND) in human peripheral blood lymphocytes exposed in vitro to X rays, neutrons, heavy ions, or the restriction endonuclease AluI. In the heavy-ion-irradiated cells, we further analyzed aberrations in chromosome 5 using multicolor FISH (mFISH). Contrary to the expectations of biophysical models, our results do not support the use of the ratios of inter-/intrachromosomal exchanges or intra-/interarm intrachanges as fingerprints of exposure to densely ionizing radiation. However, our data point to measurable differences in the ratio of complex/simple interchanges after exposure to different clastogens. These data should be considered in current biophysical models of radiation action in living cells.

Chromosome Aberrations Induced by High-LET Radiations

Measurements of chromosome aberrations in peripheral blood lymphocytes are currently the most sensitive and reliable indicator of radiation exposure that can be used for biological dosimetry. This technique has been implemented recently to study radiation exposures incurred by astronauts during space flight, where a significant proportion of the dose is delivered by high-LET particle exposure. Traditional methods for the assessing of cytogenetic damage in mitotic cells collected at one time point after exposure may not be suitable for measuring high-LET radiation effects due to the drastic cell cycle perturbations and interphase cell death induced by this type of exposure. In this manuscript we review the recent advances in methodology used to study high-LET induced cytogenetic effects and evaluate the use of chemically-induced Premature Chromosome Condensation (PCC) as an alternative to metaphase analysis. Published data on the cytogenetic effects of in vitro exposures of high-LET radiation is reviewed, along with biodosimetry results from astronauts after short or long space missions.