Non-random distribution of DNA double-strand breaks induced by particle irradiation

Efficient repair of bleomycin-induced double-strand breaks in barley ribosomal genes

Ability of barley ribosomal genes to cope with damage produced in vivo by the radiomimetic agent bleomycin was investigated. Repair kinetics of bleomycin-induced double-strand breaks in ribosomal and total genomic DNA was compared. Induction and repair of double-strand breaks in defined regions of the ribosomal genes was also analyzed. Preferential sensitivity of barley linker DNA towards bleomycin treatment in vivo was established. Relatively higher yield of initially induced double-strand breaks in genomic DNA in comparison to ribosomal DNA was also found. Fragments containing intergenic spacers of barley rRNA genes displayed higher sensitivity to bleomycin than the coding sequences. No heterogeneity in the repair of DSB between transcribed and non-transcribed regions of ribosomal genes was detected. Data indicate that DSB repair in barley rDNA, although more efficient than in genomic DNA, does not correlate with the activity of nucleolus organizer regions.

Modelling the probability distribution of the number of DNA double-strand breaks due to sporadic alkylation of nucleotide bases

Metabolites and certain chemical agents (for example methyl methanesulfonate) can induce nucleotide bases on chromosomal strands to become alkylated. These alkylated sites have the potential to become single-strand chromosomal breaks, a form of DNA damage, if they are exposed to a sufficient temperature in vitro. It has been proposed that a single-strand break (SSB) sufficiently close to another SSB on the opposite chromosomal strand will form a double-strand break (DSB). DNA repair mechanisms are less able to repair DSBs compared to SSBs. Because of the complex three-dimensional structure of DNA, some chromosomal regions are more susceptible to alkylation than others. A question of interest is therefore whether these alkylated bases are randomly distributed or tend to be clustered. Pulsed-field gel electrophoresis allows the number of DNA fragments (and hence the number of DSBs) to be observed directly. The randomness of alkylation events can therefore be tested using the standard statistical hypothesis-testing framework. Under the null hypothesis, that the SSBs are randomly distributed on each of the strands, we can calculate the probability of observing a number of DSBs at least as large as that observed and hence the associated p-value. Previously, the probability distribution of the number of DSBs has been determined by Monte Carlo simulations; when considering the whole genome this can be very time consuming. In this paper, we theoretically derive an approximation to the distribution enabling appropriate probabilities to be calculated quickly. Based on previous findings we assume that the number of breaks on each strand is small compared to the number of nucleotide bases. We show that our method can give the correct probability distribution when alkylation events are relatively rare, discuss how rare these events have to be and suggest potential extensions to the model when a greater proportion of bases are alkylated.

Apoptotic death of human lympholeukemia HL-60 cells resultant from combined effect of cobalt octa-4,5-carboxyphthalocyanine propylenglycol ether and ascorbate

Cobalt octa-4,5-carboxyphthalocyanine propylenglycol ether proposed for antitumor therapy potentiates the cytotoxic effect of ascorbate on HL-60 human leukemia cells. Combination of these substances caused the formation of H2O2 in the medium and initiated apoptotic death of cells. Catalase abolished the cytotoxic effect of this combination. The results indicate that binary catalytic system of this combination can be regarded as a potential antitumor agent.

Chromatin loops are responsible for higher counts of small DNA fragments induced by high-LET radiation, while chromosomal domains do not affect the fragment sizes

PURPOSE

To apply a polymer model of DNA damage induced by high-LET (linear energy transfer) radiation and determine the influence of chromosomal domains and loops on fragment length distribution.

MATERIALS AND METHODS

The yields of DSB (double-strand breaks) induced by high-LET radiation were calculated using a track structure model along with a polymer model of DNA packed in the cell nucleus. The cell nucleus was constructed to include the chromosomal domains and chromatin loops. The latter were generated by the random walk method.

RESULTS AND CONCLUSIONS

We present data for DSB yields per track per cell, DNA fragment sizes, the radial distribution of DSB with respect to the track center, and the distribution of 0, 1, 2, and more DSB from a single particle. Calculations were carried out for a range of particles including He (40 keV/microm), N (225 keV/microm), and Fe ions (150 keV/mum). Situations relevant to PFGE (pulsed-field gel electrophoresis) and microbeam experiments with direct irradiation of the cell nucleus were simulated to demonstrate the applicability of the model. Data show that chromosomal domains do not have a significant influence on fragment-size distribution, while the presence of DNA loops increases the frequencies of smaller fragments by nearly 30% for fragment sizes in the range from 2 kbp (bp = base pair) to 20 kbp.

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.

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.

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.

DNA-Incorporated125I Induces more than one Double-Strand Break per Decay in Mammalian Cells

The Auger-electron emitter 125I releases cascades of 20 electrons per decay that deposit a great amount of local energy, and for DNA-incorporated 125I, approximately one DNA double-strand break (DSB) is produced close to the decay site. To investigate the potential of 125I to induce additional DSBs within adjacent chromatin structures in mammalian cells, we applied DNA fragment-size analysis based on pulsed-field gel electrophoresis (PFGE) of hamster V79-379A cells exposed to DNA-incorporated 125IdU. After accumulation of decays at -70 degrees C in the presence of 10% DMSO, there was a non-random distribution of DNA fragments with an excess of fragments <0.5 Mbp and the measured yield was 1.6 DSBs/decay. However, since these experiments were performed under high scavenging conditions (DMSO) that reduce indirect effects, the yield in cells exposed to 125IdU under physiological conditions would most likely be even higher. In contrast, using a conventional low-resolution assay without measurement of smaller DNA fragments, the yield was close to one DSB/decay. We conclude that a large fraction of the DSBs induced by DNA-incorporated 125I are nonrandomly distributed and that significantly more than one DSB/decay is induced in an intact cell. Thus, in addition to DSBs produced close to the decay site, DSBs may also be induced within neighboring chromatin fibers, releasing smaller DNA fragments that are not detected by conventional DSB assays.

DNA DSB induced by iron ions in human fibroblasts: LET dependence and shielding efficiency

This paper reports on DNA DSB induction in human fibroblasts by iron ions of different energies, namely 5, 1 GeV/u, 414 and 115 MeV/u, in absence or presence of different shields (PMMA, Al and Pb). Measure of DNA DSB was performed by calibrated Pulsed Field Gel Electrophoresis using the fragment counting method. The RBE-LET relationships for unshielded and shielded beams were obtained both in terms of dose average LET and of track average LET. Weak dependence on these parameters was observed for DSB induction. The shielding efficiency, evaluated by the ratio between the cross sections for unshielded and shielded beams, depends not only on the shield type and thickness, but also on the beam energy. Protection is only observed at high iron ions energy, especially at 5 GeV/u, where PMMA shield gives higher protection compared to Al or Pb shields of the same thickness expressed in g/cm2.

Focus formation of DNA repair proteins in normal and repair-deficient cells irradiated with high-LET ions

To investigate the repair of clustered lesions within the DNA/chromatin, the focus formation and persistence of foci of the phosphorylated histone protein H2AX and the repair protein MRE11 were studied in normal cells and in cells lacking DNA-PKcs (M059J) or ATM (GM2052D) after irradiation with high-LET nitrogen ions or low-LET photons. There was a rapid formation of MRE11 and gamma-H2AX foci, and 0.5 h after high-LET irradiation, the number of foci in normal cells correlated well with the number of particle hits per cell nucleus. After 8 h of repair, there were significantly more gamma-H2AX foci than MRE11 foci remaining in the normal cells, independent of radiation quality. The difficulty in repairing clustered breaks was detected as slower rejoining of DSBs (measured by DNA fragmentation analysis), as quantification of the amount of gamma-H2AX over time, and as a larger fraction of repair foci remaining after 24 h in cells irradiated with high- LET ions. These data indicate that clustered lesions are repaired by a pathway involving the same proteins that repair sparsely distributed breaks. Further, for both low- and high- LET radiation, no reduction of the initial number of gamma-H2AX and MRE11 foci was detected in M059J cells up to 21 h after irradiation, which was in accordance with a complete absence of DSB rejoining in these cells. In the GM2052D cells there was also a higher level of foci remaining after 21 h; however, this was not accompanied by unrejoined DSBs, indicating that these foci not only represent DSBs but also may be a sign of persistent problems even when breaks are rejoined.

Chromatin organization contributes to non-randomly distributed double-strand breaks after exposure to high-LET radiation

The influence of higher-order chromatin structure on the non-random distribution of DNA double-strand breaks induced by high-LET radiation was investigated. Five different chromatin structures (intact cells, condensed and decondensed chromatin, nucleoids and naked genomic DNA) from GM5758 cells or K562 cells were irradiated with (137)Cs gamma-ray photons and 125 keV/microm nitrogen ions (16-25 MeV/nucleon). DNA was purified with a modified lysis procedure to avoid release of heat-labile sites, and fragment size distributions and double-strand break yields were analyzed by different pulsed-field gel electrophoresis protocols. Whereas double-strand breaks in photon-irradiated cells were randomly distributed, irradiation of intact K562 cells with high-LET nitrogen ions produced an excess of non-randomly distributed DNA fragments 10 kb-1 Mbp in size. Complete removal of proteins eliminated this non-random component. There was a gradual increase in the yield of double-strand breaks for each chromatin decondensation step, and compared to intact cells, the yields for naked DNA (in buffer without scavengers) increased 83 and 25 times after photon and nitrogen-ion irradiation, respectively. The corresponding relative biological effectiveness decreased from 1.6-1.8 for intact cells to 0.49 for the naked DNA. We conclude that the organization of DNA into chromatin fiber and higher-order structures is responsible for the majority of non-randomly distributed double-strand breaks induced by high-LET radiation. However, our data suggest a complex interaction between track structure and chromatin organization over several levels.

DNA fragmentation induced by Fe ions in human cells: shielding influence on spatially correlated damage

Outside the magnetic field of the Earth, high energy heavy ions constitute a relevant part of the biologically significant dose to astronauts during the very long travels through space. The typical pattern of energy deposition in the matter by heavy ions on the microscopic scale is believed to produce spatially correlated damage in the DNA which is critical for radiobiological effects. We have investigated the influence of a lucite shielding on the initial production of very small DNA fragments in human fibroblasts irradiated with 1 GeV/u iron (Fe) ions. We also used gamma rays as reference radiation. Our results show: (1) a lower effect per incident ion when the shielding is used; (2) an higher DNA Double Strand Breaks (DSB) induction by Fe ions than by gamma rays in the size range 1-23 kbp; (3) a non-random DNA DSB induction by Fe ions.

Induction of genomic instability in TK6 human lymphoblasts exposed to 137Cs gamma radiation: comparison to the induction by exposure to accelerated 56Fe particles

The induction of genomic instability in TK6 human lymphoblasts by exposure to (137)Cs gamma radiation was investigated by measuring the frequency and characteristics of unstable clones isolated approximately 36 generations after exposure. Clones surviving irradiation and control clones were analyzed for 17 characteristics including chromosomal aberrations, growth defects, alterations in response to a second irradiation, and mutant frequencies at the thymidine kinase and Na(+)/K(+) ATPase loci. Putative unstable clones were defined as those that exhibited a significant alteration in one or more characteristics compared to the controls. The frequency and characteristics of the unstable clones were compared in clones exposed to (137)Cs gamma rays or (56)Fe particles. The majority of the unstable clones isolated after exposure to either gamma rays or (56)Fe particles exhibited chromosomal instability. Alterations in growth characteristics, radiation response and mutant frequencies occurred much less often than cytogenetic alterations in these unstable clones. The frequency and complexity of the unstable clones were greater after exposure to (56)Fe particles than to gamma rays. Unstable clones that survived 36 generations after exposure to gamma rays exhibited increases in the incidence of dicentric chromosomes but not of chromatid breaks, whereas unstable clones that survived 36 generations after exposure to (56)Fe particles exhibited increases in both chromatid and chromosome aberrations.

X-ray-induced DNA double-strand breaks in mouse l1210 cells: a new computational method for analyzing neutral filter elution data

The aim of this article is to present a method for studying the shape of the dose and repair responses for X-ray-induced double-strand breaks (DSBs) as measured by neutral filter elution (NFE). The approach is closely related to a method we developed for the use of specific molecular size markers and used for determination of the absolute number of randomly distributed radiation-induced DSBs by pulsed-field gel electrophoresis (PFGE). Mouse leukemia L1210 cells were X-irradiated with 0-50 Gy. Samples were then evaluated both with PFGE and with NFE. Assuming that with both migration (PFGE) and elution (NFE), a heterogeneous population of double-stranded DNA fragments will start with the smallest fragments and proceed with increasingly larger fragments, it is possible to match the migration behavior of fractions of fragments smaller than a certain size to the fraction eluted at a specific time. This assumption does not exclude the possibility of DNA being sheared in the NFE filter. The yield, as determined by the size markers in PFGE, was used to find the corresponding elution times in the NFE experiment. These experimentally used elution times could then reversely be interpreted as size markers which finally were used to calculate DSBs/Mbp as a function of X-ray dose. The resulting lines were almost straight. The data were also plotted as relative elution and showed that, as expected, the dose response then appears with a more pronounced sigmoid shape.

Delayed cell cycle progression in human lymphoblastoid cells after exposure to high-LET radiation correlates with extremely localized DNA damage

To compare the genotoxic effects of high-LET ionizing radiation to those of low-LET radiation, we investigated the responses of human lymphoblastoid cells to DNA damage TK6 after treatment with either low-LET X rays or high-LET iron ions (1000 keV/microm). A highly localized distribution of gammaH2AX/RAD51 foci was observed in the nuclei of cells irradiated with iron ions, in sharp contrast to cells exposed to X rays, where the distribution of foci was much more uniform. This implied the occurrence of a relatively high frequency of closely spaced double-strand breaks, i.e. clustered DNA damage, after iron-ion exposure. Despite the well-established notion that clustered DNA damage is refractory to repair compared to isolated DNA lesions, there were no significant differences in the levels of clonogenic survival and apoptosis between cells treated with iron ions or X rays. Strikingly, however, cells accumulated in G(2)/M phase to a much lesser extent after iron-ion exposure than after X-ray exposure. This differential accumulation could be attributed to a much slower evacuation of the S-phase compartment in the case of cells irradiated with iron ions. Taken together, our results indicate that, relative to the situation for low-LET X rays, exposure to high-LET iron ions results in a substantially greater inhibition of S-phase progression as a result of a higher frequency of DNA replication-blocking clustered DNA damage.

Molecular targets in cellular response to ionizing radiation and implications in space radiation protection

DNA repair systems and cell cycle checkpoints closely co-operate in the attempt of maintaining the genomic integrity of cells damaged by ionizing radiation. DNA double-strand breaks (DSB) are considered as the most biologically important radiation-induced damage. Their spatial distribution and association with other types of damage depend on radiation quality. It is believed these features affect damage reparability, thus explaining the higher efficiency for cellular effects of densely ionizing radiation with respect to gamma-rays. DSB repair systems identified in mammalian cells are homologous recombination (HR), single-strand annealing (SSA) and non-homologous end-joining (NHEJ). Some enzymes may participate in more than one of these repair systems. DNA damage also triggers biochemical signals activating checkpoints responsible for delay in cell cycle progression that allows more time for repair. Those at G1/S and S phases prevent replication of damaged DNA and those at G2/M phase prevent segregation of changed chromosomes. Individuals with lack or alterations of genes involved in DNA DSB repair and cell cycle checkpoints exhibit syndromes characterized by genome instability and predisposition to cancer. Information reviewed in this paper on the basic mechanisms of cellular response to ionizing radiation indicates their importance for a number of issues relevant to protection of astronauts from space radiation.

Non-random radial arrangements of interphase chromosome territories: evolutionary considerations and functional implications

In the nucleus of animal and plant cells individual chromosomes maintain a compartmentalized structure. Chromosome territories (CTs), as these structures were named by Theodor Boveri, are essential components of the higher-order chromatin architecture. Recent studies in mammals and non-mammalian vertebrates indicate that the radial position of a given CT (or segments thereof) is correlated with its size, its gene-density and its replication timing. As a representative case, chicken cell nuclei show highly consistent radial chromatin arrangements: gene-rich, early replicating microchromosomes are clustered within the nuclear interior, while gene-poor, later replicating macrochromosomes are preferentially located at the nuclear periphery. In humans, chromosomes 18 and 19 (HSA18 and 19) territories that are of similar size show a distinctly different position in the cell nuclei of lymphocytes and lymphoblastoid cells: the gene-rich and early replicating HSA19 CTs are typically found close to the nuclear center, while the gene-poor and later replicating HSA18 CTs are preferentially located at the nuclear periphery. Recent comparative maps between human and chicken chromosomes revealed that the chicken macrochromosomes 2 and Z contain the genes homologous to HSA18, while the genes on HSA19 are located onto the chicken microchromosomes. These data lend tentative support to the hypothesis that differences in the radial nuclear positions of gene-rich, early replicating and gene-poor, later replicating chromatin have been evolutionarily conserved during a period of more than 300 million years irrespective of the evolution of highly divergent karyotypes between humans and chicken.

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.

Rejoining of radiation-induced DNA double-strand breaks: pulsed-field electrophoresis analysis of fragment size distributions after incubation for repair

The repair of radiation-induced DNA double-strand breaks (DSBs) is frequently investigated by measuring the time-dependent decrease in the fraction of fragmented DNA that is able to enter electrophoresis gels. When transformed into equivalent doses without repair, such measurements are thought to reflect the removal of DSBs, and they typically exhibit a fast initial component and a decreasing rate at longer repair intervals. This formalism, however, assumes that the spatial distribution of unrejoined breakage resembles the pattern of induction of DSBs. While the size distributions for initial fragmentation, such as that resolved by conventional pulsed-field gel electrophoresis (PFGE) (between about 10(5) and 10(7) bp), are well known to agree with the prediction of random breakage, no data are available from studies explicitly testing this relationship for residual breakage. Therefore, Chinese hamster V79 cells and MeWo (human melanoma) cells were irradiated with different doses (10-100 Gy) or were incubated for repair for up to 4 h after a single dose of 100 Gy (V79) or 90 Gy (MeWo) before being subjected to PFGE. Fragment size distributions were calculated by convolution of the PFGE profiles with an appropriately generated size calibration function. The results clearly demonstrate an over-representation of smaller fragments (below about 2-3 Mbp) compared to the prediction of randomness for residual breakage. In consequence, the time-dependent decrease of dose-equivalent values calculated from data on the fraction released may not directly reflect DSB rejoining rates. The present findings are compatible with an earlier suggestion of slow rejoining of breaks which have been induced as multiple breaks (two or more) in large chromosomal loops, thus also predicting an increase of the slowly rejoining DSB fraction with increasing dose.

p53-dependent S-phase damage checkpoint and pronuclear cross talk in mouse zygotes with X-irradiated sperm

One difficulty in analyzing the damage response is that the effect of damage itself and that of cellular response are hard to distinguish in irradiated cells. In mouse zygotes, damage can be introduced by irradiated sperm, while damage response can be studied in the unirradiated maternal pronucleus. We have analyzed the p53-dependent damage responses in irradiated-sperm mouse zygotes and found that a p53-responsive reporter was efficiently activated in the female pronucleus. [(3)H]thymidine labeling experiments indicated that irradiated-sperm zygotes were devoid of G(1)/S arrest, but pronuclear DNA synthesis was suppressed equally in male and female pronuclei. p53(-/-) zygotes lacked this suppression, which was corrected by microinjection of glutathione S-transferase-p53 fusion protein. In contrast, p21(-/-) zygotes exhibited the same level of suppression upon fertilization by irradiated sperm. About a half of the 6-Gy-irradiated-sperm zygotes managed to synthesize a full DNA content by prolonging S phase, while the other half failed to do so. Regardless of the DNA content, all the zygotes cleaved to become two-cell-stage embryos. These results revealed the presence of p53-dependent pronuclear cross talk and a novel function of p53 in the S-phase DNA damage checkpoint of mouse zygotes.

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.

DNA fragmentation by charged particle tracks

High-LET (linear energy transfer) charged particles induce DNA double-strand breaks (DSB) in a non-random fashion in mammalian cells. The clustering of DSB, probably determined by track structure as well as chromatin conformation, results in an excess of small- and intermediate-sized DNA fragments. DNA fragmentation in normal human fibroblasts (GM5758) was analyzed by pulsed-field gel electrophoresis after irradiation with photons (60Co) or 125 keV/micrometers nitrogen ions. Compared to conventional DSB analysis, i.e. assays only measuring the fraction of DNA smaller than a single threshold, the relative biological effectiveness (RBE) for DSB induction increased with 100%. Further, the size distribution of DNA fragments showed a significant dependence on radiation quality, with an excess of fragments up to 1 Mbp. Irradiation of naked genomic DNA without histone proteins increased the DSB yields 25 and 13 times for photons and nitrogen ions, respectively. The results suggest possible roles of both track structure and chromatin organization in the distribution of DNA double-strand breaks along the chromosome.

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

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

Extrapolation of the dna fragment-size distribution after high-dose irradiation to predict effects at low doses

The patterns of DSBs induced in the genome are different for sparsely and densely ionizing radiations: In the former case, the patterns are well described by a random-breakage model; in the latter, a more sophisticated tool is needed. We used a Monte Carlo algorithm with a random-walk geometry of chromatin, and a track structure defined by the radial distribution of energy deposition from an incident ion, to fit the PFGE data for fragment-size distribution after high-dose irradiation. These fits determined the unknown parameters of the model, enabling the extrapolation of data for high-dose irradiation to the low doses that are relevant for NASA space radiation research. The randomly-located-clusters formalism was used to speed the simulations. It was shown that only one adjustable parameter, Q, the track efficiency parameter, was necessary to predict DNA fragment sizes for wide ranges of doses. This parameter was determined for a variety of radiations and LETs and was used to predict the DSB patterns at the HPRT locus of the human X chromosome after low-dose irradiation. It was found that high-LET radiation would be more likely than low-LET radiation to induce additional DSBs within the HPRT gene if this gene already contained one DSB.

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.

Induction and Rejoining of DNA Double-Strand Breaks in Normal Human Skin Fibroblasts after Exposure to Radiation of Different Linear Energy Transfer: Possible Roles of Track Structure and Chromatin Organization

DNA double-strand breaks are nonrandomly induced by high-LET radiation. Differences in the induction and rejoining of DSBs after irradiation with ions having different LET were detected by fragment analysis. The data obtained indicate that the track structure of the traversing particle and its interaction with the different chromatin structures of the cellular DNA influence the yield as well as the distribution of the induced damage. The induction and rejoining of clustered DSBs induced by the same nitrogen ion fluence at LETs of 80-225 keV/microm were investigated by a detailed analysis of the DNA fragmentation patterns in normal human fibroblasts. The DSBs in the cells were allowed to rejoin during incubations for 0-20 h. Two separate pulsed-field gel electrophoresis protocols were used, optimized for separation of fragments in the size ranges 1-6 Mbp and 5 kbp-1.5 Mbp. A strong influence of LET on the level of DSB induction was evident. The DSB yield increased from 4.5 +/- 0.2 to 10.0 +/- 0.3 DSBs per particle traversal through the cell nucleus when LET increased from 80 to 225 keV/microm. Further, the size distribution of the DNA fragments showed a significant dependence on radiation quality, with an excess of fragments at 50-200 kbp and around 1 Mbp. Differences in repair kinetics were also evident, with slower rejoining for increasing LET, and the initial nonrandom fragment distributions were still present after 1 h of repair.

Simulation of exon deletion mutations induced by low-LET radiation at the HPRT locus

Friedland, W., Li, W. B., Jacob, P. and Paretzke, H. G. Simulation of Exon Deletion Mutations Induced by Low-LET Radiation at the HPRT Locus. Radiat. Res. 155, 703-715 (2001). The induction of HPRT mutants with exon deletions after irradiation with photons was simulated using the biophysical radiation track structure model PARTRAC. The exon-intron structure of the human HPRT gene was incorporated into the chromatin fiber model in PARTRAC. After gamma and X irradiation, simulated double-stranded DNA fragments that overlapped with exons were assumed to result in exon deletion mutations with a probability that depended on the genomic or the geometric distance between the breakpoints. The consequences of different assumptions about this probability of deletion formation were evaluated on the basis of the resulting fractions of total, terminal and intragenic deletions. Agreement with corresponding measurements was obtained assuming a constant probability of deletion formation for fragments smaller than about 0.1 Mbp, and a probability of deletion formation decreasing with increasing geometric or genomic distance between the end points for larger fragments. For these two assumptions, yields of mutants with exon deletions, size distributions of deletions, patterns of deleted exons, and patterns of deleted STS marker sites surrounding the gene were calculated and compared with experimental data. The yields, size distributions and exon deletion patterns were grossly consistent, whereas larger deviations were found for the STS marker deletion patterns in this comparison.

Untargeted mutation of the maternally derived mouse hypervariable minisatellite allele in F1 mice born to irradiated spermatozoa

Length change mutation at the Ms6hm hypervariable mouse minisatellite locus was analyzed in C57BL/6N x C3H/HeN F(1) mice and the F(1) of the reciprocal cross born to irradiated male parents. Spontaneous mutant frequencies were 8.4% and 9.8% for the paternally derived and maternally derived C3H/HeN alleles, respectively. The mutant frequencies for the paternally derived allele increased to 22% and 19% when the male parents were irradiated with 6 Gy at the postmeiotic spermatozoa stage and the spermatogonia stage, respectively. These increases in the mutant frequency were at least 10 to 100 times higher than those expected from the frequency of hits to the 3- to 4-kb allele, suggesting that the length change mutation at this minisatellite locus was not a targeted event due directly to DNA damage in the region. Further analysis demonstrated that the mutant frequency increased also at the maternally derived C3H/HeN allele to 20% when the male parents were irradiated at the spermatozoa stage. This increase in the maternal allele mutation was not observed in F(1) born to irradiated spermatogonia. The present study suggests that introduction of DNA damage by irradiated sperm triggers genomic instability in zygotes and in embryos of subsequent developmental stages, and this genomic instability induces untargeted mutation in cis at the paternally derived minisatellite allele and in trans at the maternally derived unirradiated allele. Untargeted mutation revealed in the present study defines a previously unnoticed genetic hazard to the maternally derived genome by the paternally introduced DNA damage.

Untargeted mutation of the maternally derived mouse hypervariable minisatellite allele in F1 mice born to irradiated spermatozoa

Length change mutation at the Ms6hm hypervariable mouse minisatellite locus was analyzed in C57BL/6N x C3H/HeN F(1) mice and the F(1) of the reciprocal cross born to irradiated male parents. Spontaneous mutant frequencies were 8.4% and 9.8% for the paternally derived and maternally derived C3H/HeN alleles, respectively. The mutant frequencies for the paternally derived allele increased to 22% and 19% when the male parents were irradiated with 6 Gy at the postmeiotic spermatozoa stage and the spermatogonia stage, respectively. These increases in the mutant frequency were at least 10 to 100 times higher than those expected from the frequency of hits to the 3- to 4-kb allele, suggesting that the length change mutation at this minisatellite locus was not a targeted event due directly to DNA damage in the region. Further analysis demonstrated that the mutant frequency increased also at the maternally derived C3H/HeN allele to 20% when the male parents were irradiated at the spermatozoa stage. This increase in the maternal allele mutation was not observed in F(1) born to irradiated spermatogonia. The present study suggests that introduction of DNA damage by irradiated sperm triggers genomic instability in zygotes and in embryos of subsequent developmental stages, and this genomic instability induces untargeted mutation in cis at the paternally derived minisatellite allele and in trans at the maternally derived unirradiated allele. Untargeted mutation revealed in the present study defines a previously unnoticed genetic hazard to the maternally derived genome by the paternally introduced DNA damage.

DNA fragmentation in mammalian cells exposed to various light ions

Elucidation of how effects of densely ionizing radiation at cellular level are linked to DNA damage is fundamental for a better understanding of the mechanisms leading to genomic damage (especially chromosome aberrations) and developing biophysical models to predict space radiation effects. We have investigated the DNA fragmentation patterns induced in Chinese hamster V79 cells by 31 keV/micrometer protons, 123 keV/micrometer helium-4 ions and gamma rays in the size range 0.023-5.7 Mbp, using calibrated Pulsed Field Gel Electrophoresis (PFGE). The frequency distributions of fragments induced by the charged particles were shifted towards smaller sizes with respect to that induced by comparable doses of gamma rays. The DSB yields, evaluated from the fragments induced in the size range studied, were higher for protons and helium ions than for gamma rays by a factor of about 1.9 and 1.2, respectively. However, these ratios do not adequately reflect the RBE observed on the same cells for inactivation and mutation induced by these beams. This is a further indication for the lack of correlation between the effects exerted at cellular level and the initial yield of DSB. The dependence on radiation quality of the fragmentation pattern suggests that it may have a role in damage repairability. We have analyzed these patterns with a "random breakage" model generalized in order to consider the initial non-random distribution of the DNA molecules. Our results suggest that a random breakage mechanism can describe with a reasonable approximation the DNA fragmentation induced by gamma rays, while the approximation is not so good for light ions, likely due to the interplay between ion tracks and chromatin organization at the loop level.

Rearrangements of the DNA in carbon ion-induced mutants of Arabidopsis thaliana

To elucidate the nature of structural alterations in plants, three carbon ion-induced mutations in Arabidopsis thaliana, gl1-3, tt4(C1), and ttg1-21, were analyzed. The gl1-3 mutation was found to be generated by an inversion of a fragment that contained GL1 and Atpk7 loci on chromosome 3. The size of the inverted fragment was a few hundred kilobase pairs. The inversion was found to accompany an insertion of a 107-bp fragment derived from chromosome 2. The tt4(C1) mutation was also found to be due to an inversion. The size of the intervening region between the breakpoints was also estimated to be a few hundred kilobase pairs. In the case of ttg1-21, it was found that a break occurred at the TTG1 locus on chromosome 5, and reciprocal translocation took place between it and chromosome 3. From the sequences flanking the breakpoints, the DNA strand breaks induced by carbon ions were found to be rejoined using, if present, only short homologous sequences. Small deletions were also observed around the breakpoints. These results suggest that the nonhomologous end-joining (NHEJ) pathway operates after plant cells are exposed to ion particles.

Biological effects of cosmic radiation: deterministic and stochastic

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

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.

Assessment of DNA damage induced by high-LET ions in human lymphocytes using the comet assay

The alkaline single-cell gel electrophoresis assay (comet assay) was used to analyze DNA damage induced in human lymphocytes by irradiation with high linear energy transfer (LET) ions. Our aim was to measure DNA breaks and to demonstrate the heterogeneity of the damage levels in a lymphocyte population irradiated with ions of different energies and LETs. Four experiments with heavy ions (Ar, C and U), as well as gamma-ray exposure, were conducted to enable comparisons. We demonstrated that the comet assay is able to assess the variability in DNA damage induced at the single cell level. The amount of DNA damage and its heterogeneity increased with particle fluence and LET, but saturated at high LETs. However, when expressed in terms of the mean dose, gamma-rays were more efficient than most of the ions used. The comet assay also allowed the detection of highly damaged cells (HDC), which were previously described as cells in late apoptotic stages. The rapid emergence of HDC in this study suggests that they were generated following ion irradiation-induced creation of DNA break clusters induced by ion exposure. Another clue was that the proportion of HDC increased with LET and fluence. We hypothesized that the LET threshold observed and the higher efficiency of low-LET radiation might be linked to the impossibility of measuring small DNA fragments in HDC.

Induction of DNA double-strand breaks in mammalian cells and yeast

Induction of DNA double-strand breaks (dsb) and their distribution are dependent on the energy deposition pattern within the cell nucleus (physical structure) and the ultrastructure of the chromosomes and its variation by the cell cycle and gene activities (biological structure). For electron radiation very similar RBE-values are observed for mammalian and yeast cells (AlK, 1.5 keV, 15 keV/micrometer: 2.6 in mammalian cells and 2.2 in yeast; CK 0.278 keV, 23 keV/micrometer: approx. 2.5 in mammalian cells and 3.8 in yeast). In contrast, the RBE-values for the induction of dsb of 4He2+ and light ions in the LET range from about 100 keV/micrometer up to 1000 keV/micrometer are significantly higher for yeast cells compared to mammalian cells. For example, the RBE-value of alpha-particles (120 keV/micrometer) is about 1.2 for mammalian cells whereas for yeast the RBE-value is about 2.5. The yeast chromatin has less condensed fibres compared with mammalian cells. Since a single CK photoelectron can induce only one dsb, the different condensation of the mammalian and yeast chromatin has no influence. However, particles may induce more than one dsb when traversing a chromatin fibre. The probability for the induction of closely neighboured dsb is higher the more condensed the chromatin fibres are. Since small DNA fragments (50 bp up to several kbp) are lost by standard methods of lysis, the underestimation of dsb yields increases with fibre condensation, which is in accordance with the observes dsb yields in mammalian cells and yeast. In order to obtain relevant yields of dsb (and corresponding RBE-values) the measurement of all DNA fragments down to about 50 bp are needed.

Locations of radiation-produced DNA double strand breaks along chromosomes: a stochastic cluster process formalism

Ionizing radiation produces DNA double strand breaks (DSBs) in chromosomes. For densely ionizing radiation, the DSBs are not spaced randomly along a chromosome: recent data for size distributions of DNA fragments indicate break clustering on kbp-Mbp scales. Different DSB clusters on a chromosome are typically made by different, statistically independent, stochastically structured radiation tracks, and the average number of tracks involved can be small. We therefore model DSB positions along a chromosome as a stationary Poisson cluster process, i.e. a stochastic process consisting of secondary point processes whose locations are determined by a primary point process that is Poisson. Each secondary process represents a break cluster, typically consisting of 1-10 DSBs in a comparatively localized stochastic pattern determined by chromatin geometry and radiation track structure. Using this Poisson cluster process model, which we call the randomly located clusters (RLC) formalism, theorems are derived for how the DNA fragment-size distribution depends on radiation dose. The RLC dose-response relations become non-linear when the dose becomes so high that DSB clusters from different tracks overlap or adjoin closely. The RLC formalism generalizes previous models, fits current data adequately and facilitates mechanistically based extrapolations from high-dose experiments to the much lower doses of interest for most applications.

Deletion-pattern analysis of alpha-particle and X-ray induced mutations at the HPRT locus of V79 Chinese hamster cells

To investigate the mutagenic mechanisms of low-energy alpha particles V79 Chinese hamster cells were irradiated with 241Am-alpha particles (mean LET of 112 keV/micron). Parallel experiments were performed using 300 kV X-rays. Cell inactivation and mutation induction cross sections were measured. At approximately 20%--survival level, DNA deletions were analysed at the HPRT locus by multiplex-PCR-analysis of all nine exons of 47 alpha-irradiated and 36 background mutants. 92 HPRT- mutants isolated after 300 kV-X-irradiation were analysed similarly for comparison, along with 15 corresponding background mutants. The resulting mutant deletion-pattern distributions were corrected for background mutations. alpha Particles induced a larger fraction of deletions than X-rays. Furthermore, non-contiguous partial deletions were present among the alpha-induced mutants, a type not found after X-irradiation.

Localization of chromosome breakpoints: implication of the chromatin structure and nuclear architecture

Restriction endonucleases and ionizing radiations have been extensively used to study the origin of chromosomal aberrations. Although a non-random distribution of chromosome breakpoints induced by these agents has been claimed by several authors, the significance of the chromatin structure and nuclear architecture in the localization of breakpoints is still not well understood. Breakpoint patterns produced by endonucleases targeted to specific genome sequences or by ionizing radiations could provide additional evidence to clarify this point. Results obtained from the localization of breakpoints induced by AluI, BamHI or DNase I as well as by neutrons or gamma-rays in G-banded Chinese hamster ovary (CHO) chromosomes are presented. AluI and BamHI were electroporated into CHO cells either during the G1 or S-phase of the cell cycle. A co-localization of breakpoints was found with a preferential occurrence in G-light bands independent of the cell cycle stage in which aberration production took place. Since AluI and BamHI recognition sequences are partitioned in the housekeeping and tissue-specific subgenomes respectively, we postulated that nuclease sensitive sites in active chromatin could be the main targets for the induction of breakpoints by these endonucleases. This assumption is supported by the finding that DNase I-induced breakpoint patterns in CHO cells are similar to those produced by AluI and BamHI. Digestion of fixed CHO chromosomes with these endonucleases induced G-banding suggesting a higher sensitivity of G-light chromatin. For comparison purposes, CHO cells were irradiated with neutrons or gamma-rays and breakpoints localized in G-banded chromosome aberrations. A higher occurrence of breakpoints in G-light bands was also observed. We detected seven breakage-prone G-light bands that were preferentially damaged by the three endonucleases and by both types of radiation. These results emphasize the possible implication of the chromatin structure and the nuclear architecture in the localization of chromosome breakpoints induced by endonucleases, neutrons and gamma-rays.

A formalism for analysing large-scale clustering of radiation-induced breaks along chromosomes

PURPOSE

To model intrachromosomal clustering of DSB (DNA double strand breaks) induced by ionizing radiation. That DSB are located non-randomly along chromosomes after high LET irradiation, with clustering even at extremely large scales, has been confirmed by recent pulsed field gel electrophoresis data for size distributions of DNA fragments. We therefore extend the standard random-breakage model for DNA fragment-size distributions to a more general 'clustered-breakage' formalism, which can take correlations of DSB locations along a chromosome into account.

METHODS

The new formalism is based mainly on a one-track probability distribution, describing the DNA fragment-size pattern due to a single primary high-energy particle, a pattern determined by track structure and chromatin geometry. Multi-track fragment-size distributions are derived mathematically from the one-track distribution, so that dose response relations are obtained.

RESULTS

The clustered-breakage formalism is applicable to any chromosomal geometry and any radiation track structure. It facilitates extrapolations of high-dose data to the much lower doses of interest for most applications. When applied to recently published data for irradiation of mammalian cells with ions of LET approximately 100 keV microm(-1) it indicates a pattern of Mbp-scale DSB clusters, each containing a number of DSB and corresponding to a very large-scale, multiply-damaged chromatin site. Although DSB are bunched, DSB clusters are scattered almost at random throughout the genome. Estimates of DSB yield are markedly increased by resolving such clusters into individual DSB. The dose response relation for fragments of a given size becomes non-linear when clusters from different tracks interlace or adjoin, as can occur for high doses and large sizes.

CONCLUSIONS

DSB clustering along chromosomes, which influences important radiobiological endpoints, is described quantitatively by the clustered-breakage formalism.

A review of dsb induction data for varying quality radiations

PURPOSE

This short review summarizes the data obtained with various techniques for measuring the yields of double strand breaks (dsb) produced by particle radiations of differing linear energy transfer (LET) in order to obtain relative biological effectiveness (RBE) values.

RESULTS AND CONCLUSIONS

Studies aimed at understanding the interactions of different types of radiation with cellular DNA have monitored the yields of DNA dsb versus radiation quality. Several techniques have been used to measure dsb yields in mammalian cells, and these include: neutral sedimentation gradients, filter elution and more recently pulsed field gel electrophoresis techniques (PFGE). Recent developments in PFGE have allowed the measurement of both the yields and the distribution of breaks within the genome, which go part of the way to explaining the RBE values close to 1.0 previously measured using other approaches with various radiation qualities. It is clear that future studies to determine the effectiveness of radiations of differing LET must use techniques that determine both yields and distributions of dsb, and assays need to be developed to allow these measurements at biologically relevant doses.

Size distributions of misrejoining DNA fragments in irradiated cells

When ionizing radiation strikes a cell it induces DNA double strand breaks (DSBs). Subsequently, some of the DSBs misrejoin and thus cause alterations in the size distribution of the DNA fragments. We derive a system of non-linear integro-differential equations describing the misrejoining interactions of five classes of DNA fragments, including rings and various types of linear fragments. The fragment classes are represented by density functions; the shape of a density function determines the probability that a fragment has a particular size and the amplitude (integral) equals the expected number of such fragments per cell. The equations are solved: analytically for exponentially distributed initial fragment sizes (corresponding to high doses) and numerically for arbitrary initial conditions. Computed final fragment size distributions are applied to situations representative of flow karyotypes and pulsed-field gel assays. For human flow karyotypes, the model can be used to obtain misrejoining estimates at doses too high for conventional methods of data analysis. For pulsed-field gel assays in which human chromosomes are digested with restriction endonucleases to form 'cut-somes' (restriction fragments), the model provides a means of misrejoining estimation when the cut-some sizes are non-random. The model suggests that if the cut-some size distribution for unirradiated cells is completely random, misrejoining of radiation-induced DSBs will not be detectable in the final size distribution.

Induction and repair of DNA double-strand breaks in human fibroblasts after particle irradiation

Two assay were employed to study the induction and repair of DNA double-strand breaks (dsbs) in normal human fibroblasts after exposure to particle radiation covering an LET range from 1 to 350 keV/micrometer. The hybridization assay allows measurement of absolute induction frequencies in defined regions of the genome and quantitates rejoining of correct DNA ends while the FAR assay determines all rejoining events, correct and incorrect. Assuming Poisson statistics for the number of breaks per DNA fragment investigated, and thus neglecting any clustering of breaks, we found the induction rate to decrease with increasing LET of the particles. RBE values compared to 225 kVp X-rays dropped to 0.48 for the highest LETs. Repair studies of X-ray-induced dsbs showed that almost all breaks (>95%) are rejoined after incubation times of 24 h while the frequency for correct rejoining is only 70%. Thus about 25% of the initially induced breaks are rejoined by the connection of incorrect DNA ends. Postirradiation incubation after particle irradiation showed less efficient total rejoining with increasing LET and an impaired ability for correct rejoining. The frequency for rejoining of incorrect DNA ends was found to be independent of LET. The possible biological significance of the different rejoining events is discussed.