Critical DNA target size model of ionizing radiation-induced mammalian cell death

Changes in activity and structure of lysosomes from liver of mouse irradiated in vivo

PURPOSE

Lysosomes may have an important role in response to ionizing radiation. Moreover, radiation could affect autophagy, which process involves the activity of lysosomal enzymes. In the present study, the effect of ionizing radiation on the lysosomal compartment of mouse liver was investigated after in vivo exposure.

MATERIALS AND METHODS

Morphology and ultrastructure of hepatocytes were assessed by light and electron microscopy, and activities of selected lysosomal enzymes were assessed in 12, 36 and 120 h after exposure to the mean dose of 1 Gy. The levels of autophagy-related proteins LC3-II and p62 were compared by Western blotting between untreated and irradiated animals (120 h after exposure).

RESULTS

Increased number of autophagic vacuoles in hepatocytes from exposed animals was documented in the ultrastructural study; destroyed mitochondria were the dominant component of such vacuoles. Moreover, an increased activity of lysosomal hydrolases was observed after exposure. However, levels of autophagy substrates LC3-II and p62 were barely affected in exposed animals 120 h after irradiation when the accumulation of autophagic vacuoles was observed.

CONCLUSION

Effects of irradiation included an increased number of autophagic vacuoles, especially of autophagosomes, and increased activity of lysosomal enzymes. However, putative markers of autophagic flux were not observed, which suggested suppression of the completion of the radiation-mediated autophagy pathway.

Evaluation of the in vivo genotoxic effects of gamma radiation on the peripheral blood leukocytes of head and neck cancer patients undergoing radiotherapy

The present study aimed to evaluate the genotoxic effects of ionizing radiation on non-target cells of Head and Neck Squamous Cell Carcinoma (HNSCC) patients exposed to various cumulative doses of gamma rays during radiotherapy. The ten patients (P1-P10) were treated with cobalt 60 gamma radiation (External Beam Radiotherapy) for a period of five to six weeks with a daily fraction of 2Gy for 5 days each week. The genotoxic effects of radiation (single strand breaks - SSBs) in these patients were analyzed using the alkaline single cell gel electrophoresis (SCGE) technique, with the Olive Tail Moment (OTM) as the critical parameter. A sample of each patient's peripheral blood before starting with radiotherapy (pre-therapy) served as the control, and blood collected at weekly time intervals during the course of the radiotherapy served as treated (10, 20, 30, 40, 50 and 60Gy) samples. In vivo radiosensitivity of these patients, as indicated by SSB's after the cumulative radiation doses at the various times, was assessed using Student's t-test. Significant DNA damage relative to the individual patient's pre-therapy baseline data was observed in all patients. Inter-individual variation of the genotoxic effects was analyzed using two-way ANOVA. The correlation between doses for the means of smoker and non-smoker patients was calculated using the Pearson test. The results of this study may indicate the need to reduce the daily radiotherapy dose further to prevent genotoxic effects on non-target cells, thus improving safety. Furthermore, these results may indicate that the estimation of DNA damage following exposure to a gamma radiation, as measured by the comet assay in whole blood leukocytes, can be used to screen human populations for radiation-induced genetic damage at the molecular level.

Retrospective Assessment of Radiation Exposure Using Biological Dosimetry: Chromosome Painting, Electron Paramagnetic Resonance and the Glycophorin A Mutation Assay

Biological monitoring of dose can contribute important, independent estimates of cumulative radiation exposure in epidemiological studies, especially in studies in which the physical dosimetry is lacking. Three biodosimeters that have been used in epidemiological studies to estimate past radiation exposure from external sources will be highlighted: chromosome painting or FISH (fluorescence in situ hybridization), the glycophorin A somatic mutation assay (GPA), and electron paramagnetic resonance (EPR) with teeth. All three biodosimeters have been applied to A-bomb survivors, Chernobyl clean-up workers, and radiation workers. Each biodosimeter has unique advantages and limitations depending upon the level and type of radiation exposure. Chromosome painting has been the most widely applied biodosimeter in epidemiological studies of past radiation exposure, and results of these studies provide evidence that dose-related translocations persist for decades. EPR tooth dosimetry has been used to validate dose models of acute and chronic radiation exposure, although the present requirement of extracted teeth has been a disadvantage. GPA has been correlated with physically based radiation dose after high-dose, acute exposures but not after low-dose, chronic exposures. Interindividual variability appears to be a limitation for both chromosome painting and GPA. Both of these techniques can be used to estimate the level of past radiation exposure to a population, whereas EPR can provide individual dose estimates of past exposure. This paper will review each of these three biodosimeters and compare their application in selected epidemiological studies.

Low-dose hypersensitivity: current status and possible mechanisms

PURPOSE

To retain cell viability, mammalian cells can increase damage repair in response to excessive radiation-induced injury. The adaptive response to small radiation doses is an example of this induced resistance and has been studied for many years, particularly in human lymphocytes. This review focuses on another manifestation of actively increased resistance that is of potential interest for developing improved radiotherapy, specifically the phenomenon in which cells die from excessive sensitivity to small single doses of ionizing radiation but remain more resistant (per unit dose) to larger single doses. In this paper, we propose possible mechanisms to explain this phenomenon based on our data accumulated over the last decade and a review of the literature.

CONCLUSION

Typically, most cell lines exhibit hyper-radiosensitivity (HRS) to very low radiation doses (<10 cGy) that is not predicted by back-extrapolating the cell survival response from higher doses. As the dose is increased above about 30 cGy, there is increased radioresistance (IRR) until at doses beyond about 1 Gy, radioresistance is maximal, and the cell survival follows the usual downward-bending curve with increasing dose. The precise operational and activational mechanism of the process is still unclear, but we propose two hypotheses. The greater amount of injury produced by larger doses either (1) is above a putative damage-sensing threshold for triggering faster or more efficient DNA repair or (2) causes changes in DNA structure or organization that facilitates constitutive repair. In both scenarios, this enhanced repair ability is decreased again on a similar time scale to the rate of removal of DNA damage.

Localization of radiation-induced chromosomal breakpoints along human chromosome 1 using a combination of G-banding and FISH

PURPOSE

To determine the exact location of radiation-induced chromosomal breakpoints along the euchromatic or heterochromatic regions: G-light and G-dark bands, respectively.

MATERIALS AND METHODS

The distribution of radiation-induced chromosomal breakpoints was scored in human lymphocytes irradiated in vitro with 3 Gy of gamma-radiation. Image analysis was applied to combine G-banded and FISH-painted images of the human chromosome 1.

RESULTS

A total of 195 chromosomal breakpoints in 176 cells with structural chromosomal aberrations was used for the present analysis. Radiation-induced breakpoints were found to be distributed randomly with respect to the p or q arms of chromosome 1 and specific band or band length, but more breakpoints were mapped to G-light than to G-dark bands, the difference being statistically significant.

CONCLUSIONS

The results can well be interpreted in terms of concepts of existing models of nuclear architecture, chromatin structure and transcriptional activities of the chromatin, which can influence the induction of primary chromosomal aberrations by gamma-rays. Differential repair of randomly produced primary aberrations may also explain the non-random distribution of radiation-induced breakpoints.

Oxidative DNA base damage in cancerous tissues of patients undergoing brachytherapy

This aim of this study was to measure the typical free radical-induced products of DNA bases in cellular DNA of cervical cancer tissues directly irradiated by applying brachytherapy to the patients. Significant increases in the amounts of modified bases over the control level were observed in the samples isolated after irradiation for all patients. These increases differed among patients and among products. The repair capacity and/or the amount of hypoxic cells inside the tumor may account for the different levels of modified bases. It is possible that the observed variabilities may account for the differences in clinical responses to brachytherapy.

A discrete cell survival model including repair after high dose-rate of ionizing radiation

A discrete cell survival model has been developed that is represented by six parameters (gamma, delta, alpha, epsilon, kappa, and t0). It is assumed that, linearly with the dose, two types of lesions are generated, with the number per unit dose described by the parameters gamma and delta. The two types of lesions, irreparable (lethal damage abbreviated as LD) damage and reparable (potentially lethal damage abbreviated as PLD) damage, follow a Poisson statistic. The PLD can be either repaired by an enzymatic process or converted into a lethal damage by a time- and dose-dependent process. For repair of PLD a Michaelis-Menten kinetics has been assumed, described by the maximum velocity alpha of the process and the Michaelis-Menten constant, kappa. The unrepaired PLD are fixed and become lethal after replating. The applicability of the model was tested by fitting 11 experimental data sets obtained with different cell lines and variable repair times simulated by delayed plating recovery or inhibition of repair processes by different agents. It has been concluded from the results that the repair process is rather totally than partially saturated. Therefore, the model was adapted by assuming a zero-order reaction for the repair process. Good agreement between model assumptions and molecular mechanisms is obtained by assuming PLD and LD to be double-strand breaks. The model and the results obtained are discussed and compared with published results and experimental data.

DNA base damage in lymphocytes of cancer patients undergoing radiation therapy

We investigated DNA base damage in genomic DNA of lymphocytes of cancer patients undergoing radiation therapy. Lymphocyte chromatin samples were analyzed by gas chromatography/isotope-dilution mass spectrometry for DNA base damage. The results provided evidence for formation of typical hydroxyl radical-induced base modifications in genomic DNA of lymphocytes. Different levels of DNA products in individuals were observed and, in the case of some patients, there was no significant product formation, possibly resulting from differences between individuals and between the types of radiation exposures. Decreases in product levels after an initial increase by radiation exposure were observed. This may indicate the removal of modified bases from lymphocyte DNA by cellular repair.

DNA damage, micronucleus formation, and cell death from 125I decays in DNA

CHO cells were pulse-labeled with 125I-iododeoxyuridine, harvested 30 min or 5 h after labeling, and stored at -196 degrees C for accumulation of 125I decays. The 30- min groups yielded low-LET survival curves (large shoulder, D0 136 decays/cell); 5-h groups showed a high-LET pattern of cell killing (no shoulder, D0 45 decay/cell). Surprisingly, the shift in 125I action was abolished in cells exposed to HAT medium; both 30-min and 5-h cell groups exhibited high-LET-type killing (no shoulder, D0 52 decays/cell). The striking difference in cell death was not accompanied by any change in induction or repair of DNA DSBs, but the pattern of micronucleus formation (and by implication chromosome damage) did parallel 125I-induced cell death. These findings suggest that cell killing may not be directly linked to the absolute number of DNA DSBs and that damage to higher-order genome structures may be an important factor in radiation-induced cell death.

Use of radiation quality as a probe for DNA lesion complexity

A number of studies have examined the possible relationships between either initial levels of DNA double-strand break (dsb) induction, their rejoining kinetics, or residual dsb and lethality in mammalian cells. With radiations of differing linear energy transfer (LET), the relative biological effectiveness (RBE) for dsb induction (20-100 keV/microns) has been lower than the RBEs measured for cell survival, and in many cases are around unity. Several studies have shown differences in the rejoining of dsb with less dsb rejoined after high than after low LET irradiation. These results suggest that there may be differences in the types of lesions being induced by different radiations and scored as DNA dsbs using current techniques. From modelling studies it is known that there is a range of energy deposition event sizes likely to occur in DNA, and there may also be uniquely large energy depositions associated with high LET radiations, particularly for large target sizes associated with the higher levels of chromatin structure. Many lesions induced will be clustered at the sites of these energy depositions. Assays need to be developed to measure complex lesions in both model DNA and cellular systems. Different levels of complexity need to be considered such as clustering of radicals close to DNA, localized areas of DNA damage (1-15 bp) and lesions which may be induced over larger distances related to higher-order structure. The use of radiations of differing LET will be an important probe in understanding DNA lesion complexity.

The development of chromosome-specific composite DNA probes for the mouse and their application to chromosome painting

The speed and ease of human cytogenetic analysis has been greatly enhanced by the technique of fluorescence in situ hybridization (FISH). Non-radioactive fluorescently tagged complex DNA probes specific for individual chromosomes can be hybridized to conventionally obtained metaphase chromosome spreads. Several chromosomes may be "painted" concurrently by using combinations of different labeled probes. Surveys of chromosome breakage and rearrangement may be performed very quickly by avoiding the time consuming process of GTG-banding. The application of FISH to mouse cytogenetics would allow large scale molecular toxicology studies to be conducted on the effects of such environmental insults as potential carcinogens, mutagens and radiation. Progress has been hampered, however, as the Mus musculus karyotype consists of 40 acrocentric chromosomes of approximately the same size, making the recognition and separation of individual chromosomes very difficult. We now describe the successful production and application of chromosome-specific composite DNA probes for M. musculus chromosomes 2 and 8. Stable Robertsonian translocated chromosomes were isolated on a flow sorter and their DNA subsequently amplified by degenerate oligonucleotide primer (DOP) PCR. Small pools (300 copies) of each chromosome were denatured at 94 degrees C then annealed with the primer at 30 degrees C for 15 cycles. This was followed by 20 cycles at an annealing temperature of 62 degrees C. Additional amplification was performed at an annealing temperature of 62 degrees C. The chromosome-specific DNA was labeled with biotin 11-dUTP by nick translation and used for FISH. The usefulness of the technique for translocation detection is demonstrated by analyzing chromosome exchanges induced in mice irradiated with 137Cs gamma rays.

DNA strand breaks and chromosomal aberrations induced by H2O2 and 60Co gamma-radiation

DNA strand breaks and chromosomal aberrations (CAs) were studied in human cells treated with hydrogen peroxide or with ionizing radiation. DNA strand breaks could be produced at dose levels of H2O2 much lower than those which induced CAs. Doses as low as 0.5 mM of H2O2 produced about as many DNA strand breaks as 2 Gy of 60Co gamma-radiation. On the other hand, as much as 20 mM H2O2 produced only half as many CAs as 1 Gy of 60Co gamma-radiation. The different mechanisms involved in the production of human genetic damage by H2O2 and gamma-radiation are discussed.

Targets for radiation-induced cell death: target replication during the cell cycle evaluated in cells exposed to X-rays or 125I decays

Chinese hamster ovary cells were labelled with 125I-iododeoxyuridine (1.15 x 10(3) Bq/ml) for 12 h, then synchronized by mitotic selection, plated for cell cycle traverse, and harvested during successive stages of the cell cycle for freezing and accumulation of 125I decays. Cell viability was evaluated by the colony-forming assay. Cells subjected to 125I decays during the G1 phase exhibited exponential survival curves with an N = 1 and a D0 = 38-41 decays/cell. A continuous increase in 125I resistance was observed as cells progressed through the S phase and cells in late-S/G2 yielded shouldered survival curves with a N = 2 and a D0 = 78-84 decays/cell. After mitosis, the radiation resistance of cells returned to G1 values. These findings suggest that the primary target for radiation-induced cell death is duplicated during S phase, with G1 cells containing one target and G2 cells two targets. Dual targets, although located within a single cell, act as independent entities as if already distributed between two separate daughter cells. Therefore, the colony-forming assay provides survival values representative of single cells/single targets only for cells irradiated during the G1 phase of the cell cycle. For cells irradiated in S or G2 phases, when intracellular target multiplicity > 1, the colony-forming assay systematically gives higher values of cell survival by up to 100% due to the target multiplicity. Experiments with external X-rays confirm these conclusions.

Model of mammalian cell reproductive death. II. Comparison with experimental data and discussion

A general equation for mammalian cell survival has been derived in the previous paper. This paper presents the results of comparison of theoretical evaluations with survival data available from the literature, including different cell lines, variations in linear energy transfer, dose rate and dose fractionation effects and the effects of ultrasoft X-rays and superheavy ions. Merits and demerits of the model are considered in comparison with other models of radiation-induced killing of mammalian cells published in the literature.

Model of mammalian cell reproductive death. I. Basic assumptions and general equations

A systemic model describing the major radiobiological effects of various types of radiation is proposed. The model base lines were substantiated, and general mathematical equations for cell survival developed. The model takes into consideration such physical and biological factors as linear energy transfer, ion track structure, and structural and functional organization of interphase chromatin. This paper presents the basic assumptions made and general equations for the cell killing.

Significance and measurement of DNA double strand breaks in mammalian cells

Techniques for measuring DNA double strand breaks in mammalian cells are being used increasingly by researchers studying both physiological processes, such as recombination, replication, and apoptosis, as well as pathological processes, such as clastogenesis induced by ionizing radiation, chemotherapeutic drugs, and chemical toxicants. In this review we evaluate commonly used assays for measuring DNA double strand breaks, focusing on neutral filter elution and pulsed field gel electrophoresis, and explore the advantages and limitations of applying these techniques to problems of current interest in carcinogenesis and genetic toxicology.

Mechanistic models

Several models and theories are reviewed that incorporate the idea of radiation-induced lesions (repairable and/or irreparable) that can be related to molecular lesions in the DNA molecule. Usually the DNA double-strand or chromatin break is suggested as the critical lesion. In the models, the shoulder on the low-LET survival curve is hypothesized as being due to one (or more) of the following three mechanisms: (i) "interaction" of lesions produced by statistically independent particle tracks, (ii) nonlinear (i.e., linear-quadratic) increase in the yield of initial lesions, and (iii) saturation of repair processes at high dose. Comparisons are made between the various approaches. Several significant advances in model development are discussed; in particular, a description of the matrix formulation of the Markov versions of the repair-misrepair (RMR) and lethal-potentially-lethal (LPL) models is given. The more advanced theories have incorporated statistical fluctuations in various aspects of the energy-loss and lesion-formation process. An important direction is the inclusion of physical and chemical processes into the formulations by incorporating relevant track structure theory (Monte Carlo track simulations) and chemical reactions of radiation-induced radicals. At the biological end, identification of repair genes and how they operate, as well as a better understanding of how DNA misjoinings lead to lethal chromosome aberrations, are needed for appropriate inclusion into the theories. More effort is necessary to model the complex end point of radiation-induced carcinogenesis.

Mechanisms of cell reproductive death and shapes of radiation dose--survival curves of mammalian cells

A 'paired dsb' mechanism of action for cell reproductive death by ionizing radiations is proposed, which allows interpretation of differences in shapes of survival curves caused by variation of linear energy transfer of the radiation, by the stage in the cell cycle, but cell culture conditions and by sensitizing and protecting compounds. It is based on the analysis of shapes of survival curves in terms of S(D)/S(O) = exp - (a1D + a2D2) and the suggestion that paired dsb in DNA, produced within distances of the order of 10 nm, are efficient in initiating the sequence of events causing cell reproductive death by individual particle tracks. Part of the lethality may result from two dsb's produced by single tracks at larger distances, and this might constitute potentially lethal damage which in favourable conditions can be repaired. Thus, the initial slope of a survival curve is not independent of the repair capacity of a cell, but indeed can be modified by cell conditions. The damage causing the quadratic term in the survival equation may be interpreted as a consequence of two dsb produced by two different ionizing particles, although other interactions cannot be excluded. The suggested mechanism of 'paired dsb' damage is consistent with information concerning the LET dependence of different effects in cells and their constituents.

Lysis solution composition and non-linear dose-response to ionizing radiation in the non-denaturing DNA filter elution technique

The suggestion by Okayasu and Iliakis (1989) that the non-linear dose-response curve, obtained with the non-denaturing filter elution technique for mammalian cells exposed to low-LET radiation, is the result of a technical artefact, was not confirmed.

DNA radiolysis by fast neutrons

The effects of fast neutron irradiation on DNA were studied using DNA of the pBR322 plasmid (4362 base pairs), and the results compared to those obtained with 60Co gamma rays. Irradiation of the plasmid DNA in solution with a neutrons beam (p34+Be) of the CERI (CNRS Orléans) cyclotron (with a flat energy spectrum from 34 MeV to low energies) results in half the yield of single-strand breaks (ssb), and 1.5 times higher yield of double-strand breaks (dsb) for neutrons as compared to gamma-rays. Possible specificity of the neutron-induced breaks was examined: the scavenging of OH. radicals by 0.1 mol dm-3 ethanol inhibits all neutron-induced ssb, but only 85 per cent of the dsb. For gamma-irradiation, both ssb and dsb are completely inhibited in these conditions. These results suggest at least three different origins for neutron-induced dsb. The occurrence of around 30 per cent of dsb can be explained by a radical transfer mechanism (proposed by Siddiqi and Bothe (1987) for gamma-irradiation). Around 55 per cent of dsb may be due to the non-random distribution of radicals in high-density tracks of the secondary particles of neutrons, which results in a simultaneous attack of the two strands by OH. radicals. These first two processes are both OH.-mediated and thus are sensitive to ethanol. The direct effect of fast neutrons and their secondaries (recoil protons, alpha-particles and recoil nuclei) can account for the remaining 15 per cent of dsb, not inhibited by 0.1 mol dm-3 ethanol.

On the nature of interactions leading to radiation-induced chromosomal exchange

Within the conceptual framework of so-called lesion-interaction models, chromosomal interchanges are believed to result from radiation damage to both chromosomes involved. More recently, models of radiation action have been proposed which suggest such exchanges arise from initial damage to only one chromosome, which then associates with an undamaged chromosome. The specific case of 'lesion-nonlesion' chromosomal interaction via telomere-break rejoining was examined through the use of a telomere-specific DNA probe. No evidence was found to support dicentric formation by this mechanism in normal human fibroblasts. To test the more general case (i.e. lesion-nonlesion interaction by some other mechanism) mitotic HeLa cells were fused together to determine whether exchanges would occur between the chromosomes of previously separate genomes, as seen in resulting cell syncytia at the next mitosis. The fusion of irradiated cells (with each other) produced a high frequency of such intergenomic exchanges. However, the frequency of these events was reduced 50-100-fold in syncytia resulting from the fusion of irradiated with unirradiated cells. These results strongly support the view that most radiation-induced exchange aberrations require initial damage to chromatin at both locations involved in the exchange--i.e. they are fundamentally two-hit in nature.