The RBE of 3.4 MeV alpha-particles and 0.565 MeV neutrons relative to 60Co gamma-rays for neoplastic transformation of human hybrid cells and the impact of culture conditions

The neoplastic transformation of human hybrid CGL1 cells is affected by perturbations from external influences such as serum batch and concentration, the number of medium changes during the 21-day expression period and cell seeding density. Nevertheless, for doses up to 1.5 Gy, published transformation frequencies for low linear energy transfer (LET) radiations (gamma-rays, MeV electrons or photons) are in good agreement, whereas for higher doses larger variations are reported. The (60)Co gamma-ray data here for doses up to 1.5 Gy, using a low-yield serum batch and only one medium change, are in agreement with published frequencies of neoplastic transformation of human hybrid cells. For 3.4 MeV alpha-particles (LET = 124 keV/mum) and 0.565 MeV monoenergetic neutrons relative to low doses of (60)Co gamma-rays, a maximum relative biological effectiveness (RBE(M)) of 2.8 +/- 0.2 and 1.5 +/- 0.2, respectively, was calculated. Surprisingly, at higher doses of (60)Co gamma-rays lower frequencies of neoplastic transformation were observed. This non-monotonic dose relationship for neoplastic transformation by (60)Co gamma-rays is likely due to the lack of a G2/M arrest observed at low doses resulting in higher transformation frequencies per dose, whereas the lower frequencies per dose observed for higher doses are likely related to the induction of a G2/M arrest.

DNA DSB induced in human cells by charged particles and gamma rays: Experimental results and theoretical approaches

PURPOSE

To quantify the role played by radiation track structure and background fragments in modulating DNA fragmentation in human cells exposed to gamma-rays and light ions.

MATERIALS AND METHODS

Human fibroblasts were exposed in vitro to different doses (in the range from 40 - 200 Gy) of (60)Co gamma-rays and 0.84 MeV protons (Linear Energy Transfer, LET, in tissue 28.5 keV/microm). The resulting DNA fragments were scored under two electrophoretic conditions, in order to optimize separation in the size ranges 0.023 - 1.0 Mbp and 1.0 - 5.7 Mbp. In parallel, DNA fragmentation was simulated both with a phenomenological approach based on the "generalized broken-stick" model, and with a mechanistic approach based on the PARTRAC (acronym of PARticle TRACk) Monte Carlo code (1.32 MeV photons were used for the simulation of (60)Co gamma-rays).

RESULTS

For both gamma-rays and protons, the experimental dose response in the range 0.023 - 5.7 Mbp could be approximated as a straight line, the slope of which provided a yield of (5.3 +/- 0.4) x 10(-9) Gy(-1) bp(-1) for gamma-rays and (7.1 +/- 0.6) x 10(-9) Gy(-1) bp(-1) for protons, leading to a Relative Biological Effectiveness (RBE) of 1.3 +/- 0.2. From both theoretical analyses it appeared that, while gamma-ray data were consistent with double-strand breaks (DSB) random induction, protons at low doses showed significant deviation from randomness, implying enhanced production of small fragments in the low molecular weight part of the experimental range. The theoretical analysis of fragment production was then extended to ranges where data were not available, i.e. to fragments larger than 5.7 Mbp and smaller than 23 kbp. The main outcome was that small fragments (<23 kbp) are produced almost exclusively via non-random processes, since their number is considerably higher than that produced by a random insertion of DSB. Furthermore, for protons the number of these small fragments is a significant fraction (about 20%) of the total number of fragments; these fragments remain undetected in these experiments. Calculations for 3.3 MeV alpha particle irradiation (for which no experimental data were available) were performed to further investigate the role of fragments smaller than 23 kbp; in this case, besides the non-random character of their production, their number resulted to be at least as much as half of the total number of fragments.

CONCLUSION

Comparison between experimental data and two different theoretical approaches provided further support to the hypothesis of an important role of track structure in modulating DNA damage. According to the theoretical approaches, non-randomness of fragment production was found for proton irradiation for the smaller fragments in the experimental size range and, in a significantly larger extent, for fragments of size less than 23 kbp, both for protons and alpha particles.

Single-strand annealing, conservative homologous recombination, nonhomologous DNA end joining, and the cell cycle-dependent repair of DNA double-strand breaks induced by sparsely or densely ionizing radiation

The cell cycle-dependent relative contributions of error-prone single-strand annealing (SSA), error-free conservative homologous recombination (HR), and potentially error-prone nonhomologous DNA end joining (NHEJ) to repair simple (induced by 200 kV X rays) or complex (induced by (241)Am alpha particles) DNA double-strand breaks (DSBs) in Chinese hamster ovary cells are reported for the first time. Cells of the parental cell line AA8 and its derivatives UV41 (SSA-deficient), irs1SF (HR-deficient) and V3 (NHEJ-deficient) were synchronized in G(1) or in S phase, and survival responses after exposure to either type of radiation were measured. It is demonstrated for the first time that in G(1)-phase SSA is negligible for the repair of DSBs of various complexities. HR-deficient cells exposed to X rays or alpha particles in G(1) phase show enhanced radiosensitivity, but this does not necessarily mean that HR is important in G(1) phase. NHEJ appears to be the most important (if not the only) mechanism in G(1) phase acting efficiently on simple DSBs, but complex DSBs are a less preferred target. In contrast to X rays, NHEJ-deficient cells show no cell cycle-dependent variation in sensitivity to alpha particles. Surprisingly, when these cells are exposed to X rays in G(1) phase, they are even more sensitive compared to alpha particles. It is also shown for the first time that in S phase all three mechanisms play a role in the repair of simple and complex DSBs. A defect in SSA confers radiosensitivity to cells in S phase, suggesting that the error-prone SSA mechanism is important for the repair of specific simple and complex DSBs that are not a substrate for HR or NHEJ. The most important mechanism in S phase for the repair of simple and complex DSBs is HR. This is also emphasized by the finding that irs1SF cells, after complementation of their HR defect by human XRCC3 cDNA, show a greater radioresistance than parental cells, whereas resistance to mitomycin C is only partially restored. Complementation confers a greater resistance to alpha particles than X rays, suggesting an important role of HR, especially for the repair of complex DSBs. In S phase, NHEJ is more important than SSA for the repair of simple DSBs, but SSA is more important than NHEJ for the repair of complex DSBs.

Advances in the biophysical and molecular bases of radiation cytogenetics

PURPOSE

For more than 70 years radiation cytogenetics has continued to be a topic of major concern in relation to the action of radiation on living cells. To date, diverse cytogenetic findings have developed into orderly, quantitative interpretations and have stimulated numerous biophysical models. However, it is generally agreed that any one of the models used alone is still unable to explain all aspects of the observed chromosomal effects. In this review, a large number of radiation-induced chromosome aberration findings from the literature are reassessed with special attention given to the reaction kinetics and the relevant molecular processes.

CONCLUSION

It is now clear that DNA double-strand breaks (DSB) are an integral component of radiation-induced chromosome aberration. At the nexus of the maintenance of genome integrity, cells are equipped with excellent systems to repair DSB, notably non-homologous end-joining (NHEJ) and homologous recombination repair (HRR). These repair mechanisms are strictly regulated along with the DNA turnover cycle. NHEJ functions in all phases of the cell cycle, whereas HRR has a supplementary role specifically in S/G2 phase, where homologous DNA sequences are available in close proximity. The repair pathways are further regulated by a complex nuclear dynamism, where DSB are sensed and large numbers of repair proteins are recruited and assembled to form a repair complex involving multiple DSB. Considering such DSB repair dynamism, radiation-induced chromosome aberrations could be well understood as DSB-DSB pairwise interactions associated with the NHEJ pathway in all phases of the cell cycle and misrepair of a single DSB associated with the complementary HRR pathway in late S/G2 phase.

BGRT: biologically guided radiation therapy-the future is fast approaching!

Rapid advances in functional and biological imaging, predictive assays, and our understanding of the molecular and cellular responses underpinning treatment outcomes herald the coming of the long-sought goal of implementing patient-specific biologically guided radiation therapy (BGRT) in the clinic. Biological imaging and predictive assays have the potential to provide patient-specific, three-dimensional information to characterize the radiation response characteristics of tumor and normal structures. Within the next decade, it will be possible to combine such information with advanced delivery technologies to design and deliver biologically conformed, individualized therapies in the clinic. The full implementation of BGRT in the clinic will require new technologies and additional research. However, even the partial implementation of BGRT treatment planning may have the potential to substantially impact clinical outcomes.

Clusters of DNA Double-Strand Breaks Induced by Different Doses of Nitrogen Ions for Various LETs: Experimental Measurements and Theoretical Analyses

The yields and clustering of DNA double-strand breaks (DSBs) were investigated in normal human skin fibroblasts exposed to gamma rays or to a wide range of doses of nitrogen ions with various linear energy transfers (LETs). Data obtained by pulsed-field gel electrophoresis on the dose and LET dependence of DNA fragmentation were analyzed with the randomly located clusters (RLC) formalism. The formalism considers stochastic clustering of DSBs along a chromosome due to chromatin structure, particle track structure, and multitrack action. The relative biological effectiveness (RBE) for the total DSB yield did not depend strongly on LET, but particles with higher LET produced higher fractions of small DNA fragments, corresponding in the formalism to an increase in the average number of DSBs per DSB cluster. The results are consistent with the idea that DSB clustering along chromosomes is what leads to large RBEs of high-LET radiations for major biological end points. At a given dose, large fragments are less affected by the variability in LET than small fragments, suggesting that the two free ends in large fragments are often produced by two different tracks. The formalism successfully described an extra increase in small DNA fragments as dose increases and a related decrease in large fragments, mainly due to interlacing of DSB clusters produced along a chromosome by different tracks, since interlacing cuts larger DNA fragments into smaller ones.

DNA Fragmentation Induced in Human Fibroblasts by Accelerated56Fe Ions of Differing Energies

DNA fragmentation was studied in the fragment size range 0.023-5.7 Mbp after irradiation of human fibroblasts with iron-ion beams of four different energies, i.e., 200 MeV/nucleon, 500 MeV/nucleon, 1 GeV/nucleon and 5 GeV/nucleon, with gamma rays used as the reference radiation. The double-strand break (DSB) yield (and thus the RBE for DNA DSB induction) of the four iron-ion beams, which have LETs ranging from 135 to 442 keV/mum, does not vary greatly as a function of LET. As a consequence, the variation of the cross section for DSB induction mainly reflects the variation in LET. However, when the fragmentation spectra were analyzed with a simple theoretical tool that we recently introduced, the results showed that spatially correlated DSBs, which are absent after gamma irradiation, increased markedly with LET for the iron-ion beams. This occurred because iron ions produce DNA fragments smaller than 0.75 Mbp with a higher probability than gamma rays (a probability that increases with LET). These sizes include those expected from fragmentation of the chromatin loops with Mbp dimensions. This result does not exclude a correlation at distances smaller than the lower size analyzed here, i.e. 23 kbp. Moreover, the DSB correlation is dependent on dose, decreasing when dose increases; this can be explained with the argument that at increasing dose there is an increasing fraction of fragments produced by DSBs caused by separate, uncorrelated tracks.

Microdosimetric Model for the Induction of Cell Killing through Medium‐Borne Signals

Microbeam, medium-transfer and low-dose experiments have demonstrated that intercellular signals can initiate many of the same biological events and processes as direct exposure to ionizing radiation. These phenomena cast doubt on cell-autonomous modes of action and the linear, no-threshold carcinogenesis paradigm. To account for the effects of intercellular signals, new approaches are needed to relate dosimetric quantities to the emission and processing of signals by irradiated and unirradiated cells. In this paper, microdosimetric principles are used to develop a stochastic model to relate absorbed dose to the emission and processing of cell death signals by unirradiated cells. Our analyses of published results of medium transfer experiments performed using HPV-G human keratinocytes suggest that the emission of death signals is a bi-exponential function of dose with a distinct plateau in the 5- to 100-mGy range. However, the emission of death signals by HPV-G cells may not become fully saturated until the absorbed dose becomes larger than 0.6 Gy. Similar saturation effects have been observed in microbeam and medium-transfer experiments with other mammalian cell lines. The model predicts that the cell-killing effect of medium-borne death signals decreases exponentially as the absorbed dose becomes small compared to the frequency-mean specific energy per radiation event.

Spectrum of complex DNA damages depends on the incident radiation

Ionizing radiation induces bistranded clustered damages--two or more abasic sites, oxidized bases and strand breaks on opposite DNA strands within a few helical turns. Since clusters are refractory to repair and are potential sources of double-strand breaks (DSBs), they are potentially lethal and mutagenic. Although induction of single-strand breaks (SSBs) and isolated lesions has been studied extensively, little is known about the factors affecting induction of clusters other than DSBs. To determine whether the type of incident radiation could affect the yields or spectra of specific clusters, we irradiated genomic T7 DNA, a simple 40-kbp linear, blunt-ended molecule, with ion beams [iron (970 MeV/nucleon), carbon (293 MeV/nucleon), titanium (980 MeV/nucleon), silicon (586 MeV/nucleon), protons (1 GeV/nucleon)] or 100 kVp X rays and then quantified DSBs, Fpg-oxypurine clusters and Nfo-abasic clusters using gel electrophoresis, electronic imaging and number average length analysis. The yields (damages/Mbp Gy(-1)) of all damages decreased with increasing linear energy transfer (LET) of the radiation. The relative frequencies of DSBs compared to abasic and oxybase clusters were higher for the charged particles-including the high-energy, low-LET protons-than for the ionizing photons.

Spatial distribution of radiation-induced double-strand breaks in plasmid DNA as resolved by atomic force microscopy

Atomic force microscopy (AFM) has been used to directly visualize, size and compare the DNA fragments resulting from exposure to low- and high-LET radiation. Double-stranded pUC-19 plasmid ("naked") DNA samples were irradiated by electron-beam or reactor neutron fluxes with doses ranging from 0.9 to 10 kGy. AFM scanning in the tapping mode was used to image and measure the DNA fragment lengths (ranging from a few bp up to 2864 bp long). Double-strand break (DSB) distributions resulting from high-LET neutron and lower-LET electron irradiation revealed a distinct difference between the effects of these two types of radiation: Low-LET radiation-induced DSBs are distributed more uniformly along the DNA, whereas a much larger proportion of neutron-induced DSBs are distributed locally and densely. Furthermore, comparisons with predictions of a random DSB model of radiation damage show that neutron-induced DSBs deviate more from the model than do electron-induced DSBs. In summary, our high-resolution AFM measurements of radiation-induced DNA fragment-length distributions reveal an increased number of very short fragments and hence clustering of DSBs induced by the high-LET neutron radiation compared with low-LET electron radiation and a random DSB model prediction.

Simulation of light ion induced DNA damage patterns

The biophysical simulation code PARTRAC was extended by a module to handle ions heavier than alpha particles. Cross sections for ion-electron interactions were taken from He(++) ions of the same velocity and scaled by Z(eff(2))/4. Calculated linear energy transfer values, radial dose distributions and secondary electron spectra were found in agreement with experimental results. DNA damage due to irradiation of human fibroblast cells by several light ions from H to S was calculated for various energies complemented by 220 kV(p) X rays as reference radiation. With increasing linear energy transfer, the calculated total yield of double-strand breaks per dose showed saturation behaviour at about twice the value for reference radiation. When data analysis methods for experimental double-strand break yield determination were applied to the simulated DNA damage patterns, the two data sets were found in accord. The calculated patterns of DNA damage clusters were analysed on local and regional scale finding regional clusters in closer correlation to experimental cell inactivation data.

Incorporation of microdosimetric concepts into a biologically-based model of radiation carcinogenesis

The generalised state-vector model of radiation carcinogenesis (SVM) simulates radiation induced biological effects by expressing the transition rates between the various initiation and promotion stages in terms of dose rate for low and high linear energy transfer (LET) particles. In the present work, the SVM has been reformulated to incorporate single track characteristics of particles with varying LET. Transition rates of the initiation phase were expressed as functions of LET by describing the complexity and clustering of DNA double strand breaks (DSBs) and its effect on repair kinetics, while the promotion phase was reformulated based on a multi-target single-hit hypothesis. Such an approach allows the consideration of hit frequencies and the variability of the specific energy and LET spectra of radon progeny alpha particles in bronchial target cells for different exposure conditions.

DNA Double-Strand Break Misrejoining after Exposure of Primary Human Fibroblasts to CKCharacteristic X Rays, 29 kVp X Rays and60Co γ Rays

The efficiency of ionizing photon radiation for inducing mutations, chromosome aberrations, neoplastic cell transformation, and cell killing depends on the photon energy. We investigated the induction and rejoining of DNA double-strand breaks (DSBs) as possible contributors for the varying efficiencies of different photon energies. A specialized pulsed-field gel electrophoresis assay based on Southern hybridization of single Mbp genomic restriction fragments was employed to assess DSB induction and rejoining by quantifying the restriction fragment band. Unrejoined and misrejoined DSBs were determined in dose fractionation protocols using doses per fraction of 2.2 and 4.4 Gy for CK characteristic X rays, 4 and 8 Gy for 29 kVp X rays, and 5, 10 and 20 Gy for 60Co gamma rays. DSB induction by CK characteristic X rays was about twofold higher than for 60Co gamma rays, whereas 29 kVp X rays showed only marginally elevated levels of induced DSBs compared with 60Co gamma rays (a factor of 1.15). Compared with these modest variations in DSB induction, the variations in the levels of unrejoined and misrejoined DSBs were more significant. Our results suggest that differences in the fidelity of DSB rejoining together with the different efficiencies for induction of DSBs can explain the varying biological effectiveness of different photon energies.

Influence of PMMA Shielding on DNA Fragmentation Induced in Human Fibroblasts by Iron and Titanium Ions

In the framework of a collaborative project on the influence of the shielding on the biological effectiveness of space radiation, we studied DNA fragmentation induced by 1 GeV/nucleon iron ions and titanium ions with and without a 197-mm-thick polymethylmethacrylate (PMMA) shield in AG1522 human fibroblasts. Pulsed- and constant-field gel electrophoresis were used to analyze DNA fragmentation in the size range 1-5700 kbp. The results show that, mainly owing to a higher production of small fragments (1-23 kbp), titanium ions are more effective than iron ions at inducing DNA double-strand breaks (DSBs), their RBE being 2.4 and 1.5, respectively. The insertion of a PMMA shield decreases DNA breakage, with shielding protection factors (ratio of the unshielded/shielded cross sections for DSB production) of about 1.6 for iron ions and 2.1 for titanium ions. However, the DSB yield (no. of DSBs per unit mass per unit dose) is almost unaffected by the presence of the shield, and the relative contributions of the fragments in the different size ranges are almost the same with or without shielding. This indicates that, under our conditions, the effect of shielding is mainly to reduce the dose per unit incident fluence, leaving radiation quality practically unaffected.

Measurement of the Initial Levels of DNA Damage in Human Lymphocytes Induced by 29 kV X Rays (Mammography X Rays) Relative to 220 kV X Rays and γ Rays

Experiments using the alkaline comet assay, which measures all single-strand breaks regardless of their origin, were performed to evaluate the biological effectiveness of photons with different energies in causing these breaks. The aim was to measure human lymphocytes directly for DNA damage and subsequent repair kinetics induced by mammography 29 kV X rays relative to 220 kV X rays, 137Cs gamma rays and 60Co gamma rays. The level of DNA damage, predominantly due to single-strand breaks, was computed as the Olive tail moment or percentage DNA in the tail for different air kerma doses (0.5, 0.75, 1, 1.5, 2 and 3 Gy). Fifty cells were analyzed per slide with a semiautomatic imaging system. Data from five independent experiments were transformed to natural logarithms and fitted using a multiple linear regression analysis. Irradiations with the different photon energies were performed simultaneously for each experiment to minimize interexperimental variation. Blood from only one male and one female was used. The interexperimental variation and the influence of donor gender were negligible. In addition, repair kinetics and residual DNA damage after exposure to a dose of 3 Gy were evaluated in three independent experiments for different repair times (10, 20, 30 and 60 min). Data for the fraction of remaining damage were fitted to the simple function F(d) = A/(t + A), where F(d) is the fraction of remaining damage, t is the time allowed for repair, and A (the only fit parameter) is the repair half-time. It was found that the comet assay data did not indicate any difference in the initial radiation damage produced by 29 kV X rays relative to the reference radiation types, 220 kV X rays and the gamma rays of 137Cs and 60Co, either for the total dose range or in the low-dose range. These results are, with some restrictions, consistent with physical examinations and predictions concerning, for example, the assessment of the possible difference in effectiveness in causing strand breaks between mammography X rays and conventional (150-250 kV) X rays, indicating that differences in biological effects must arise through downstream processing of the damage.

Mutations Induced by Heavy Charged Particles

The relative biological-effectiveness of radiation is increased when cells or tissue are exposed to densely ionizing (high-LET) radiation. A large number of studies focus on the following aspects of the biological effects of high-LET radiation: (i) basic understanding of radiation damage and repair; (ii) developing radiotherapy protocols for accelerated charged particles; and (iii) estimation of human risks from exposure to high-LET heavy charged particles. The increased lethal effectiveness (cell inactivation) of high-LET radiation contributes to new methods for using radiation therapy, but it is also necessary to study the enhanced mutagenic effect of high LET radiation, because higher frequencies of mutation can be expected to provide higher rates of carcinogenicity with human exposure. It is important to note that both measures of biological effectiveness (lethality and mutagenicity) depend on the quality of radiation, the dose, dose-rate effects, and the biological endpoints studied. This paper is intended to provide a review of current research on the mutagenic effects of high-LET radiation, and is organized into three sections. First, are descriptions of the induced mutations studied with various detection systems (section 1) because the detectable mutations induced by ionizing radiation, including heavy-ions, depend largely on the detection system used. Second is a discussion of the biological significance of the dependence of induced mutations on LET (section 2). This is related to the molecular nature of radiation lesions and to the repair mechanisms used to help cells recover from such damage. Finally, applications of mutation detection systems for studies in space (section 3) are described, in which the carcinogenic effects of space environmental radiation are considered.

Simulation of DNA damage after proton irradiation

The biophysical radiation track simulation model PARTRAC was improved by implementing new interaction cross sections for protons in water. Computer-simulated tracks of energy deposition events from protons and their secondary electrons were superimposed on a higher-order DNA target model describing the spatial coordinates of the whole genome inside a human cell. Induction of DNA double-strand breaks was simulated for proton irradiation with LET values between 1.6 and 70 keV/microm and various reference radiation qualities. The yield of DSBs after proton irradiation was found to rise continuously with increasing LET up to about 20 DSBs per Gbp and Gy, corresponding to an RBE up to 2.2. About half of this increase resulted from a higher yield of DSB clusters associated with small fragments below 10 kbp. Exclusion of experimentally unresolved multiple DSBs reduced the maximum DSB yield by 30% and shifted it to an LET of about 40 keV/microm. Simulated fragment size distributions deviated significantly from random breakage distributions over the whole size range after irradiation with protons with an LET above 10 keV/microm. Determination of DSB yields using equations derived for random breakage resulted in an underestimation by up to 20%. The inclusion of background fragments had only a minor influence on the distribution of the DNA fragments induced by radiation. Despite limited numerical agreement, the simulations reproduced the trends in proton-induced DNA DSBs and fragment induction found in recent experiments.

Sensitive detection of LOH events in a human cell line after C-ion beam exposure

A molecular analysis of the loss of heterozygosity (LOH) events in human cells after low-dose heavy-ion exposure could contribute to the sensitive detection of the genetic influences caused by high-LET radiation. We exposed human lymphoblastoid TK6-20C cells to 10 cGy of an accelerated C-ion (22 keV/microm) beam, and observed a 3.1-fold increase in the mutation frequency (MF) at the heterozygous thymidine kinase (TK) locus over the background level. This increase was due to the induction of TK mutants exhibiting hemizygous-type LOH. Surprisingly, the frequency of type-2 hemizygous LOHs (interstitial deletions) was about 23-fold, induced over the background level, and the LOH extent patterns of this type 2 induced after the irradiation were clearly different from that of the spontaneous background.. Since hemizygous-type LOH mutants are considered to be the result of the end-joining repair of DNA double-strand breaks (DSB), C-ions may more efficiently induce DSBs than X-rays in this low-dose region. In addition, an enhanced misrepair of C-ion-induced DSBs might also account for the induction of radiation-specific hemizygous-type LOH.

Detection of DNA damage in individual cells induced by heavy-ion irradiation with an non-denaturing comet assay

Investigating the biological effects of high-LET heavy-ion irradiation at low fluence is important to evaluate the risk of radiation in space. It is especially necessary to detect radiation damage induced by a precise number of heavy ions in individual cells. We thus compared the number of ions traversing a cell and the DNA damage produced by ion hits. We applied a comet assay to measure the DNA damage in individual cells. Cells attached on ion-track detector (CR-39) were irradiated with 17.3 MeV/u 12C, 15.7 MeV/u, 10.4 MeV/u 20Ne ion and 7.2 MeV/u 40Ar beams at TIARA, JAERI-Takasaki. After irradiation, CR-39 was covered with 1% agarose. The agarose was allowed to solidify on a glass slide, and then the electrophoresis was performed. Afterward, the CR-39 was taken off the glass slide. The agarose gel on the CR-39 was stained with ethidium bromide and the opposite side of the CR-39 was etched with a KOH-ethanol solution at 37 degrees C. We observed that heavy ions with higher LET values induced heavier DNA damage, even with the same number of ion hits within the irradiated cells. The result indicated that the amount of DNA damage induced by one particle depended on the LET value of the heavy ions.

Spatial distribution and yield of DNA double-strand breaks induced by 3-7 MeV helium ions in human fibroblasts

Accelerated helium ions with mean energies at the target location of 3-7 MeV were used to simulate alpha-particle radiation from radon daughters. The experimental setup and calibration procedure allowed determination of the helium-ion energy distribution and dose in the nuclei of irradiated cells. Using this system, the induction of DNA double-strand breaks and their spatial distributions along DNA were studied in irradiated human fibroblasts. It was found that the apparent number of double-strand breaks as measured by a standard pulsed-field gel assay (FAR assay) decreased with increasing LET in the range 67-120 keV/microm (corresponding to the energy of 7-3 MeV). On the other hand, the generation of small and intermediate-size DNA fragments (0.1-100 kbp) increased with LET, indicating an increased intratrack long-range clustering of breaks. The fragment size distribution was measured in several size classes down to the smallest class of 0.1-2 kbp. When the clustering was taken into account, the actual number of DNA double-strand breaks (separated by at least 0.1 kbp) could be calculated and was found to be in the range 0.010-0.012 breaks/Mbp Gy(-1). This is two- to threefold higher than the apparent yield obtained by the FAR assay. The measured yield of double-strand breaks as a function of LET is compared with theoretical Monte Carlo calculations that simulate the track structure of energy depositions from helium ions as they interact with the 30-nm chromatin fiber. When the calculation is performed to include fragments larger than 0.1 kbp (to correspond to the experimental measurements), there is good agreement between experiment and theory.

Heavy ion-induced DNA double-strand breaks in yeast

Induction of DSBs in the diploid yeast, Saccharomyces cerevisiae, was measured by pulsed-field gel electrophoresis (PFGE) after the cells had been exposed on membrane filters to a variety of energetic heavy ions with values of linear energy transfer (LET) ranging from about 2 to 11,500 keV/microm, (241)Am alpha particles, and 80 keV X rays. After irradiation, the cells were lysed, and the chromosomes were separated by PFGE. The gels were stained with ethidium bromide, placed on a UV transilluminator, and analyzed using a computer-coupled camera. The fluorescence intensities of the larger bands were found to decrease exponentially with dose or particle fluence. The slope of this line corresponds to the cross section for at least one double-strand break (DSB), but closely spaced multiple breaks cannot be discriminated. Based on the known size of the native DNA molecules, breakage cross sections per base pair were calculated. They increased with LET until they reached a transient plateau value of about 6 x 10(-7) microm(2) at about 300-2000 keV/microm; they then rose for the higher LETs, probably reflecting the influence of delta electrons. The relative biological effectiveness for DNA breakage displays a maximum of about 2.5 around 100-200 keV/microm and falls below unity for LET values above 10(3) keV/microm. For these yeast cells, comparison of the derived breakage cross sections with the corresponding cross section for inactivation derived from the terminal slope of the survival curves shows a strong linear relationship between these cross sections, extending over several orders of magnitude.

A review of studies of ionizing radiation-induced double-strand break clustering

Underpinning current models of the mechanisms of the action of radiation is a central role for DNA damage and in particular double-strand breaks (DSBs). For radiations of different LET, there is a need to know the exact yields and distributions of DSBs in human cells. Most measurements of DSB yields within cells now rely on pulsed-field gel electrophoresis as the technique of choice. Previous measurements of DSB yields have suggested that the yields are remarkably similar for different types of radiation with RBE values < or = 1.0. More recent studies in mammalian cells, however, have suggested that both the yield and the spatial distribution of DSBs are influenced by radiation quality. RBE values for DSBs induced by high-LET radiations are greater than 1.0, and the distributions are nonrandom. Underlying this is the interaction of particle tracks with the higher-order chromosomal structures within cell nuclei. Further studies are needed to relate nonrandom distributions of DSBs to their rejoining kinetics. At the molecular level, we need to determine the involvement of clustering of damaged bases with strand breakage, and the relationship between higher-order clustering over sizes of kilobase pairs and above to localized clustering at the DNA level. Overall, these studies will allow us to elucidate whether the nonrandom distributions of breaks produced by high-LET particle tracks have any consequences for their repair and biological effectiveness.

Computational approach for determining the spectrum of DNA damage induced by ionizing radiation

To study the characteristics of molecular damage induced by ionizing radiation at the DNA level, Monte Carlo track simulation of energetic electrons and ions in liquid water, a canonical model of B-DNA, and a comprehensive classification of DNA damage in terms of the origin and complexity of damage were used to calculate the frequencies of simple and complex strand breaks. A threshold energy of 17.5 eV was used to model the damage by direct energy deposition, and a probability of 0.13 was applied to model the induction of a single-strand break produced in DNA by OH radical reactions. For preliminary estimates, base damage was assumed to be induced by the same direct energy threshold deposition or by the reaction of an OH radical with the base, with a probability of 0.8. Computational data are given on the complexity of damage, including base damage by electrons with energies of 100-4500 eV and ions with energies of 0.3-4.0 MeV/nucleon (59-9 keV microm(-1) protons and 170-55 keV microm(-1) alpha particles). Computational data are presented on the frequencies of single- and double-strand breaks induced as a function of the LET of the particles, and on the relative frequencies of complex single- and double-strand breaks for electrons. The modeling and calculations of strand breaks show that: (1) The yield of strand breaks per unit absorbed dose is nearly constant over a wide range of LET. (2) The majority of DNA damage is of a simple type, but the majority of the simple single-strand breaks are accompanied by at least one base damage. (3) For low-energy electrons, nearly 20-30% of the double-strand breaks are of a complex type by virtue of additional breaks. The proportion of this locally clustered damage increases with LET, reaching about 70% for the highest-LET alpha particles modeled, with the complexity of damage increasing further, to about 90%, when base damage is considered. (4) The extent of damage in the local hit region of the DNA duplex is mostly limited to a length of a few base pairs. (5) The frequency of base damage when no strand breaks are present in the hit segment of DNA varies between 20-40% as a function of LET for protons and alpha particles.