Environmentally Relevant Chronic Low-Dose Tritium and Gamma Exposures do not Increase Somatic Intrachromosomal Recombination in pKZ1 Mouse Spleen.

The toxicity of tritium is a public health concern given its presence and mobility in the environment. For risk predictions using radiological protection models, it is essential to allocate an appropriate radiation weighting factor (WR). This in turn should be consistent with the observed relative biological effectiveness (RBE) of tritium beta radiation. Although the International Commission on Radiological Protection (ICRP) currently recommends a WR of 1 for the calculation of committed effective dose for X rays, gamma rays and electrons of all energies, including tritium energies, there are concerns that tritium health risks are underestimated and that current regulatory tritium drinking water standards need revision. In this study, we investigated potential cytotoxic and genotoxic effects in mouse spleen after one month and eight months of chronic exposure to low-dose tritiated water (HTO). The dose regimes studied were designed to mimic human chronic consumption of HTO at levels of 10 kBq/l, 1 MBq/l and 20 MBq/l. The total doses from these radiation exposures ranged from 0.01 to 180 mGy. We also compared the biological effects of exposure to HTO with equivalent exposure to external whole-body 60Co gamma rays. Changes in spleen weight and somatic intrachromosomal recombination (DNA inversions) in spleen tissue of pKZ1Tg/+ mice were monitored. Our results showed no overall changes in either spleen organ weights and no increase mouse splenic intrachromosomal recombination frequencies, indicating that current drinking water standards for tritium exposure in the form of HTO are likely to be adequately protective against cytotoxic and genotoxic damage in spleen. These results demonstrate no evidence for cytotoxicity or genotoxicity in mouse spleen following chronic exposures to HTO activities (or equivalent gamma doses) up to 20 MBq/L.

Sensitive Detection of Radiation-Induced Medulloblastomas after Acute or Protracted Gamma-Ray Exposures in Ptch1 Heterozygous Mice Using a Radiation-Specific Molecular Signature.

Recently reported studies have led to a heightened awareness of the risks of cancer induced by diagnostic radiological imaging, and in particular, the risk of brain cancer after childhood CT scans. One feature of Ptch1+/- mice is their sensitivity to radiation-induced medulloblastomas (an embryonic cerebellar tumor) during a narrow window of time centered on the days around birth. Little is known about the dynamics of how dose protraction interacts with such narrow windows of sensitivity in individual tissues. Using medulloblastomas from irradiated Ptch1+/- mice with a hybrid C3H × C57BL/6 F1 genetic background, we previously showed that the alleles retained on chromosome 13 (which harbors the Ptch1 gene) reveal two major mechanisms of loss of the wild-type allele. The loss of parental alleles from the telomere extending up to or past the Ptch1 locus by recombination (spontaneous type) accounts for almost all medulloblastomas in nonirradiated mice, while tumors in irradiated mice often exhibited interstitial deletions, which start downstream of the wild-type Ptch1 and extend up varying lengths towards the centromere (radiation type). In this study, Ptch1+/- mice were exposed to an acute dose of either 100 or 500 mGy gamma rays in utero or postnatally, or the same radiation doses protracted over a four-day period, and were monitored for medulloblastoma development. The results showed dose- and age-dependent radiation-induced type tumors. Furthermore, the size of the radiation-induced deletion differed with the dose rate. The results of this work suggest that tumor latency may be related to the size of the deletion. In this study, 500 mGy exposure produced radiation-induced type tumors at all ages and dose rates, while 100 mGy exposure did not significantly produce radiation-induced type tumors. The radiation signature allows for unique mechanistic insight into the action of radiation to induce DNA lesions with known causal relationship to a specific tumor type, particularly for doses and dose rates that are relevant to both diagnostic and accidental radiological exposures.

Sub-micrometer 20MeV protons or 45MeV lithium spot irradiation enhances yields of dicentric chromosomes due to clustering of DNA double-strand breaks.

In conventional experiments on biological effects of radiation types of diverse quality, micrometer-scale double-strand break (DSB) clustering is inherently interlinked with clustering of energy deposition events on nanometer scale relevant for DSB induction. Due to this limitation, the role of the micrometer and nanometer scales in diverse biological endpoints cannot be fully separated. To address this issue, hybrid human-hamster AL cells have been irradiated with 45MeV (60keV/µm) lithium ions or 20MeV (2.6keV/µm) protons quasi-homogeneously distributed or focused to 0.5×1µm(2) spots on regular matrix patterns (point distances up to 10.6×10.6µm), with pre-defined particle numbers per spot to provide the same mean dose of 1.7Gy. The yields of dicentrics and their distribution among cells have been scored. In parallel, track-structure based simulations of DSB induction and chromosome aberration formation with PARTRAC have been performed. The results show that the sub-micrometer beam focusing does not enhance DSB yields, but significantly affects the DSB distribution within the nucleus and increases the chance to form DSB pairs in close proximity, which may lead to increased yields of chromosome aberrations. Indeed, the experiments show that focusing 20 lithium ions or 451 protons per spot on a 10.6µm grid induces two or three times more dicentrics, respectively, than a quasi-homogenous irradiation. The simulations reproduce the data in part, but in part suggest more complex behavior such as saturation or overkill not seen in the experiments. The direct experimental demonstration that sub-micrometer clustering of DSB plays a critical role in the induction of dicentrics improves the knowledge on the mechanisms by which these lethal lesions arise, and indicates how the assumptions of the biophysical model could be improved. It also provides a better understanding of the increased biological effectiveness of high-LET radiation.

Influence of radiation quality on mouse chromosome 2 deletions in radiation-induced acute myeloid leukaemia.

Leukaemia is the prevailing neoplastic disorder of the hematopoietic system. Epidemiological analyses of the survivors of the Japanese atomic bombings show that exposure to ionising radiation (IR) can cause leukaemia. Although a clear association between radiation exposure and leukaemia development is acknowledged, the underlying mechanisms remain incompletely understood. A hemizygous deletion on mouse chromosome 2 (del2) is a common feature in several mouse strains susceptible to radiation-induced acute myeloid leukaemia (rAML). The deletion is an early event detectable 24h after exposure in bone marrow cells. Ultimately, 15-25% of exposed animals develop AML with 80-90% of cases carrying del2. Molecular mapping of leukaemic cell genomes identified a minimal deleted region (MDR) on chromosome 2 (chr2) in which a tumour suppressor gene, Sfpi1 is located, encoding the transcription factor PU.1, essential in haematopoiesis. The remaining copy of Sfpi1 has a point mutation in the coding sequence for the DNA-binding domain of the protein in 70% of rAML, which alters a single CpG sequence in the codon for arginine residue R235. In order to identify chr2 deletions and Sfpi.1/PU.1 loss, we performed array comparative genomic hybridization (aCGH) on a unique panel of 79rAMLs. Using a custom made CGH array specifically designed for mouse chr2, we analysed at unprecedentedly high resolution (1.4M array- 148bp resolution) the size of the MDR in low LET and high-LET induced rAMLs (32 X-ray- and 47 neutron-induced). Sequencing of Sfpi1/PU.1DNA binding domain identified the presence of R235 point mutations, showing no influence of radiation quality on R235 type or frequency. We identified for the first time rAML cases with complex del2 in a subset of neutron-induced AMLs. This study allowed us to re-define the MDR to a much smaller 5.5Mb region (still including Sfpi1/PU.1), identical regardless of radiation quality.

The impact of homologous recombination repair deficiency on depleted uranium clastogenicity in Chinese hamster ovary cells: XRCC3 protects cells from chromosome aberrations, but increases chromosome fragmentation.

Depleted uranium (DU) is extensively used in both industry and military applications. The potential for civilian and military personnel exposure to DU is rising, but there are limited data on the potential health hazards of DU exposure. Previous laboratory research indicates DU is a potential carcinogen, but epidemiological studies remain inconclusive. DU is genotoxic, inducing DNA double strand breaks, chromosome damage and mutations, but the mechanisms of genotoxicity or repair pathways involved in protecting cells against DU-induced damage remain unknown. The purpose of this study was to investigate the effects of homologous recombination repair deficiency on DU-induced genotoxicity using RAD51D and XRCC3-deficient Chinese hamster ovary (CHO) cell lines. Cells deficient in XRCC3 (irs1SF) exhibited similar cytotoxicity after DU exposure compared to wild-type (AA8) and XRCC3-complemented (1SFwt8) cells, but DU induced more break-type and fusion-type lesions in XRCC3-deficient cells compared to wild-type and XRCC3-complemented cells. Surprisingly, loss of RAD51D did not affect DU-induced cytotoxicity or genotoxicity. DU induced selective X-chromosome fragmentation irrespective of RAD51D status, but loss of XRCC3 nearly eliminated fragmentation observed after DU exposure in wild-type and XRCC3-complemented cells. Thus, XRCC3, but not RAD51D, protects cells from DU-induced breaks and fusions and also plays a role in DU-induced chromosome fragmentation.

Autosomal mutations in mouse kidney epithelial cells exposed to high-energy protons in vivo or in culture.

Proton exposure induces mutations and cancer, which are presumably linked. Because protons are abundant in the space environment and significant uncertainties exist for the effects of space travel on human health, the purpose of this study was to identify the types of mutations induced by exposure of mammalian cells to 4-5 Gy of 1 GeV protons. We used an assay that selects for mutations affecting the chromosome 8-encoded Aprt locus in mouse kidney cells and selected mutants after proton exposure both in vivo and in cell culture. A loss of heterozygosity (LOH) assay for DNA preparations from the in vivo-derived kidney mutants revealed that protons readily induced large mutational events. Fluorescent in situ hybridization painting for chromosome 8 showed that >70% of proton-induced LOH patterns resembling mitotic recombination were in fact the result of nonreciprocal chromosome translocations, thereby demonstrating an important role for DNA double-strand breaks in proton mutagenesis. Large interstitial deletions, which also require the formation and resolution of double-strand breaks, were significantly induced in the cell culture environment (14% of all mutants), but to a lesser extend in vivo (2% of all mutants) suggesting that the resolution of proton-induced double-strand breaks can differ between the intact tissue and cell culture microenvironments. In total, the results demonstrate that double-strand break formation is a primary determinant for proton mutagenesis in epithelial cell types and suggest that resultant LOH for significant genomic regions play a critical role in proton-induced cancers.

Comparative analysis of cell killing and autosomal mutation in mouse kidney epithelium exposed to 1 GeV protons in vitro or in vivo.

Human exposure to high-energy protons occurs in space flight scenarios or, where necessary, during radiotherapy for cancer or benign conditions. However, few studies have assessed the mutagenic effectiveness of high-energy protons, which may contribute to cancer risk. Mutations cause cancer and most cancer-associated mutations occur at autosomal loci. This study addresses the cytotoxic and mutagenic effects of 1 GeV protons in mouse kidney epithelium. Mutant fractions were measured for an endogenous autosomal locus (Aprt) that detects all types of mutagenic events. Results for kidneys irradiated in vivo are compared with the results for kidney cells from the same strain exposed in vitro. The results demonstrate dose-dependent cell killing in vitro and for cells explanted 3-4 months postirradiation in vivo. Incubation in vivo for longer periods (8-9 months) further attenuates proton-induced cell killing. Protons are mutagenic to cells in vitro and for in vivo irradiated kidneys. The dose-response for Aprt mutation is curvilinear after in vitro or in vivo exposure, bending upward at the higher doses. While the absolute mutant fractions are higher in vivo, the fold-increase over background is similar for both in vitro and in situ exposures. Results are also presented for a limited study on the effect of dose fractionation on the induction of Aprt mutations in kidney epithelial cells. Dose-fractionation reduces the fraction of proton-induced Aprt mutants in vitro and in vivo and also results in less cell killing. Taken together, the mutation burden in the epithelium is slightly reduced by dose-fractionation. Autosomal mutations accumulated during clinical exposure to high-energy protons may contribute to the risk of treatment-associated neoplasms, thereby highlighting the need for rigorous treatment planning to reduce the dose to normal tissues. For low dose exposures that occur during most space flight scenarios, the mutagenic effects of protons appear to be modest.

Comparison of the bromodeoxyuridine-mediated sensitization effects between low-LET and high-LET ionizing radiation on DNA double-strand breaks.

The incorporation of halogenated pyrmidines such as bromo- and iodo-deoxyuridines (BrdU, IdU) into DNA as thymidine analogs enhances cellular radiosensitivity when high-linear energy transfer (LET) radiation is not used. Although it is known that high-LET ionizing radiation confers fewer biological effects resulting from halogenated pyrimidine incorporation, the exact mechanisms of reduced radiosensitivity with high-LET radiation are not clear. We investigated the radiosensitization effects of halogenated pyrimidines with high-LET radiation using accelerated carbon and iron ions. Cells synchronized into the G1 phase after unifilar (1 cell cycle) and bifilar (2 cell cycles) substitution with 10 µM BrdU were exposed to various degrees of LET with heavy ions and X-rays. We then carried out a colony formation assay to measure cell survival. The γ-H2AX focus formation assay provided a measure of DNA double-strand break (DSB) formation and repair kinetics. Chromosomal aberration formations for the first post-irradiation metaphase were also scored. For both low-LET X-rays and carbon ions (13 keV/µm), BrdU incorporation led to impaired DNA repair kinetics, a larger initial number of DNA DSBs more frequent chromosomal aberrations at the first post-irradiated metaphase, and increased radiosensitivity for cell lethality. The enhancement ratio was higher after bifilar substitution. In contrast, no such synergistic enhancements were observed after high-LET irradiation with carbon and iron ions (70 and 200 keV/µm, respectively), even after bifilar substitution. Our results suggest that BrdU substitution did not modify the number and quality of DNA DSBs produced by high-LET radiation. The incorporation of halogenated pyrimidines may produce more complex/clustered DNA damage along with radicals formed by low-LET ionizing radiation. In contrast, the severity of damage produced by high-LET radiation may undermine the effects of BrdU and account for the observed minimal radiosensitization effects.

[Genotoxic effects of radiation and hyperthermia in inbred mice strains with different radiation sensitivity].

Peculiarities of repair of DNA injuries induced by radiation and hyperthermia and their realization on chromosomal level in bone marrow cells ofexperimental animals with different radiation sensitivity were studied. It is shown that radiomodification efficiency of mild hyperthermia is higher for radioresistant animals. More intensive elimination of chromosomal injuries the level of which in remote terms of examination corresponds to the control value, is observed. In the group of animals with higher radiation sensitivity prolonged thermal potentiation of radiation effects on chromosomal level is determined.

Radioprotective effect of rhizome extract of Zingiber montanum in Rattus norvegicus.

The present study aims at determining the ability of 60% ethanol extract of the rhizome of Zingiber montanum (J. König) A. Dietr. to protect bone marrow cells in vivo from radiation-induced chromosomal aberrations. Albino rats (Rattus norvegicus, 2n = 42) were used to carry out investigations on the radioprotective properties of Z. montanum. Acute toxicity of the extract was determined, and a suitable injectable dose was selected for intra-peritoneal administration. The LD(50) of the extract calculated for 72 h was 2.9 g/kg, and the calculated LD(10) dose was 1.7 g/kg. The calculated maximum tolerated dose of the rhizome extract was 1.3 g/kg. Rats were divided into 12 groups (with or without the administration of extract) and exposed to different radiation doses from 1 to 5 Gy. Whole-body irradiation of rats showed a significant dose-dependent increase in different types of chromosomal aberrations. The most common chromosomal aberrations were breaks, fragments, gaps, rings, endoreduplications and dicentric chromosomes. Ethanol extract of rhizome at a dose of 0.5 g/kg did not show any significant increase in chromosomal aberrations in unirradiated animals as compared to that of the control group. Intra-peritoneal administration of the extract at a dose of 0.5 g/kg considerably reduced the frequency of the aberrations stated above in irradiated animals with DMF value of 1.36 at 1 to 5 Gy dose range of gamma radiation. The incidence of micronucleated polychromatic erythrocytes and micronucleated normochromatic erythrocytes due to the radiation exposure was considerably reduced in extract-treated groups of animals with DMFs 1.34 and 1.17, respectively, as compared to that of the extract-untreated groups. Our results suggest that rhizome extract of Z. montanum may have a potential in protecting normal hematopoietic cells from radiation-induced damage.

Chromosome shattering: a mitotic catastrophe due to chromosome condensation failure.

Chromosome shattering has been described as a special form of mitotic catastrophe, which occurs in cells with unrepaired DNA damage. The shattered chromosome phenotype was detected after application of a methanol/acetic acid (MAA) fixation protocol routinely used for the preparation of metaphase spreads. The corresponding phenotype in the living cell and the mechanism leading to this mitotic catastrophe have remained speculative so far. In the present study, we used V79 Chinese hamster cells, stably transfected with histone H2BmRFP for live-cell observations, and induced generalized chromosome shattering (GCS) by the synergistic effect of UV irradiation and caffeine posttreatment. We demonstrate that GCS can be derived from abnormal mitotic cells with a parachute-like chromatin configuration (PALCC) consisting of a bulky chromatin mass and extended chromatin fibers that tether centromeres at a remote, yet normally shaped spindle apparatus. This result hints at a chromosome condensation failure, yielding a "shattered" chromosome complement after MAA fixation. Live mitotic cells with PALCCs proceeded to interphase within a period similar to normal mitotic cells but did not divide. Instead they formed cells with highly abnormal nuclear configurations subject to apoptosis after several hours. We propose a factor depletion model where a limited pool of proteins is involved both in DNA repair and chromatin condensation. Chromosome condensation failure occurs when this pool becomes depleted.

The current knowledge on radiosensitivity of ovarian follicle development stages.

BACKGROUND: The aim of this paper is to review the available information on ovarian radiation sensitivity and the genetic hazard of ionizing radiation in female mammals including humans. METHODS: The literature present in the author's laboratories (international papers from the 1970s) was complemented by a Medline literature search using the keywords 'ionizing radiation genetic effects', 'oocyte radiosensitivity' and 'oocyte DNA repair' (1990-2008). Further articles were acquired from citations in the research papers and reports. RESULTS: Animal data show that oocyte radiosensitivity varies widely according to the follicle/oocyte stage and the species. Oocytes near ovulation show the highest susceptibility to radiation induction of mutational events. Congenital anomalies have been observed after exposure to high doses (1-5 Gy), but extrapolation of these data to humans requires caution. In humans, the dose required to induce permanent ovarian failure would vary from 20.3 Gy at birth to 14.3 Gy at 30 years. Most epidemiological studies found little evidence of genetic diseases at the doses at which medical, occupational or accidental exposure occurred. CONCLUSIONS: The fact that genetic effects were observed in irradiated animals suggests that these could also occur in humans. The probability of such events remains low compared with the 'spontaneous' risks of genetic effects.

Comparison of the induction and disappearance of DNA double strand breaks and gamma-H2AX foci after irradiation of chromosomes in G1-phase or in condensed metaphase cells.

The induction and disappearance of DNA double strand breaks (DSBs) after irradiation of G1 and mitotic cells were compared with the gamma-H2AX foci assay and a gel electrophoresis assay. This is to determine whether cell cycle related changes in chromatin structure might influence the gamma-H2AX assay which depends on extensive phosphorylation and dephosphorylation of the H2AX histone variant surrounding DSBs. The disappearance of gamma-H2AX foci after irradiation was much slower for mitotic than for G1 cells. On the other hand, no difference was seen for the gel electrophoresis assay. Our data may suggest the limited accessibility of dephosphorylation enzyme in irradiated metaphase cells or trapped gamma-H2AX in condensed chromatin.

Genomic instability in rat: breakpoints induced by ionising radiation and interstitial telomeric-like sequences.

The Norwegian rat (Rattus norvegicus) is the most widely studied experimental species in biomedical research although little is known about its chromosomal structure. The characterisation of possible unstable regions of the karyotype of this species would contribute to the better understanding of its genomic architecture. The cytogenetic effects of ionising radiation have been widely used for the study of genomic instability, and the importance of interstitial telomeric-like sequences (ITSs) in instability of the genome has also been reported in previous studies in vertebrates. In order to describe the unstable chromosomal regions of R. norvegicus, the distribution of breakpoints induced by X-irradiation and ITSs in its karyotype were analysed in this work. For the X-irradiation analysis, 52 foetuses (from 14 irradiated rats) were studied, 4803 metaphases were analysed, and a total of 456 breakpoints induced by X-rays were detected, located in 114 chromosomal bands, with 25 of them significantly affected by X-irradiation (hot spots). For the analysis of ITSs, three foetuses (from three rats) were studied, 305 metaphases were analysed and 121 ITSs were detected, widely distributed in the karyotype of this species. Seventy-six percent of all hot spots analysed in this study were co-localised with ITSs.

Chromosomal aberrations induced by (12)C6+ ions and 60Co gamma-rays in mouse immature oocytes.

The ovaries of Kun-Ming strain mice (3 weeks) were irradiated with different doses of (12)C6+ ion or (60)Co gamma-ray. Chromosomal aberrations were analyzed in metaphase II oocytes at 7 weeks after irradiation. The relative biological effectiveness (RBE) of (12)C6+ ion was calculated with respect to 60Co gamma-ray for the induction of chromosomal aberrations. The (12)C6+ ion and 60Co gamma-ray dose-response relationships for chromosomal aberrations were plotted by linear quadratic models. The data showed that there was a dose-related increase in frequency of chromosomal aberrations in all the treated groups compared to controls. The RBE values for (12)C6+ ions relative to 60Co gamma-rays were 2.49, 2.29, 1.57, 1.42 or 1.32 for the doses of 0.5, 1.0, 2.0, 4.0 or 6.0 Gy, respectively. Moreover, a different distribution of the various types of aberrations has been found for (12)C6+ ion and 60Co gamma-ray irradiations. The dose-response relationships for (12)C6+ ion and 60Co gamma-ray exhibited positive correlations. The results from the present study may be helpful for assessing genetic damage following exposure of immature oocytes to ionizing radiation.

X-ray-induced deletion complexes in embryonic stem cells on mouse chromosome 15.

Chromosomal deletions have long been used as genetic tools in dissecting the functions of complex genomes, and new methodologies are still being developed to achieve the maximum coverage. In the mouse, where the chromosomal deletion coverage is far less extensive than that in Drosophila, substantial coverage of the genome with deletions is strongly desirable. This article reports the generation of three deletion complexes in the distal part of mouse Chromosome (Chr) 15. Chromosomal deletions were efficiently induced by X rays in embryonic stem (ES) cells around the Otoconin 90 (Oc 90), SRY-box-containing gene 10 (Sox 10), and carnitine palmitoyltransferase 1b (Cpt 1 b) loci. Deletions encompassing the Oc 90 and Sox 10 loci were transmitted to the offspring of the chimeric mice that were generated from deletion-bearing ES cells. Whereas deletion complexes encompassing the Sox 10 and the Cpt 1 b loci overlap each other, no overlap of the Oc 90 complex with the Sox 10 complex was found, possibly indicating the existence of a haploinsufficient gene located between Oc 90 and Sox 10. Deletion frequency and size induced by X rays depend on the selective locus, possibly reflecting the existence of haplolethal genes in the vicinity of these loci that yield fewer and smaller deletions. Deletions induced in ES cells by X rays vary in size and location of breakpoints, which makes them desirable for mapping and for functional genomics studies.

Replicated anterior zeugopod (raz): a polydactylous mouse mutant with lowered Shh signaling in the limb bud.

A unique limb phenotype is described in a radiation-induced mutant mouse resulting from an inversion of a proximal segment of chromosome 5. The limb phenotype in the homozygous mutant presents with two anterior skeletal elements in the zeugopod but no posterior bone, hence the name replicated anterior zeugopod, raz. The zeugopod phenotype is accompanied by symmetrical central polydactyly of hand and foot. The chromosomal inversion includes the Shh gene and the regulatory locus, located approximately 1 Mb away, within the Lmbr1 gene. In homozygous mutants, the expression of Shh mRNA and Shh protein is severely downregulated to about 20% of wild-type limb buds, but Shh expression appears normal throughout the remainder of the embryo. Correspondingly, Gli3 expression is upregulated and posteriorly expanded in the raz/raz limb bud. We propose that the double anterior zeugopod and symmetrical central polydactyly are due to an increased and uniform concentration of the Gli3 repressor form because of lowered Shh signaling.

Induction of delayed mutations and chromosomal instability in fibroblasts after UVA-, UVB-, and X-radiation.

Mutations in critical genes are believed to be a necessary part of cancer induction. The conventional view of radiation mutagenesis is that radiation induces most mutations in cells shortly after irradiation, because of false repair or lack of repair of DNA damage before or during DNA replication. In contrast, we here show that delayed mutations in the hypoxanthine phosphoribosyltransferase locus of Chinese hamster fibroblasts (V79) arise many cell generations after three types of carcinogenic irradiation: (a). UVA-; (b). UVB-; or (c). X-radiation. The frequency of mutations at the hypoxanthine phosphoribosyltransferase locus was measured in clones 14 days after irradiation with doses killing 80% of the cells. The proportion of unstable clones, as indicated by mutant fractions 10-7500-fold above background, was higher for the cells treated with UVA (13.2%) than for cells treated with UVB (9.2%) and X-radiation (9.6%). In contrast, UVA produces few immediate mutations compared with UVB and X-radiation. Thus, UVA-radiation, which is suspected to cause melanomas, produces few immediate mutations but more delayed mutations than UVB or X-radiation. Clones of cells that developed delayed mutations were examined for markers of chromosome instability, such as increased numbers of centrosomes, DNA content, and variability in the number of chromosomes. All radiation types increased the variability in the number of chromosomes in unstable clones. Although UVB and X-radiation, which damages DNA by direct interaction, resulted in an increased number of centrosomes in cell clones, the oxidative UVA-radiation did not. Thus, the mechanism of UVA-induced chromosomal instability is apparently different from that of UVB and X-radiation.

A model for interphase chromosomes and evaluation of radiation-induced aberrations.

We have developed a theoretical model for evaluating radiation-induced chromosomal exchanges by explicitly taking into account interphase (G(0)/G(1)) chromosome structure, nuclear organization of chromosomes, the production of double-strand breaks (DSBs), and the subsequent rejoinings in a faithful or unfaithful manner. Each of the 46 chromosomes for human lymphocytes (40 chromosomes for mouse lymphocytes) is modeled as a random polymer inside a spherical volume. The chromosome spheres are packed randomly inside a spherical nucleus with an allowed overlap controlled by a parameter Omega. The rejoining of DSBs is determined by a Monte Carlo procedure using a Gaussian proximity function with an interaction range parameter sigma. Values of Omega and sigma have been found which yield calculated results of interchromosomal aberration frequencies that agree with a wide range of experimental data. Our preferred solution is one with an interaction range of 0.5 microm coupled with a relatively small overlap parameter of 0.675 microm, which more or less confirms previous estimates. We have used our model with these parameter values and with resolution or detectability limits to calculate yields of translocations and dicentrics for human lymphocytes exposed to low-LET radiation that agree with experiments in the dose range 0.09 to 4 Gy. Five different experimental data sets have been compared with the theoretical results. Essentially all of the experimental data fall between theoretical curves corresponding to resolution limits of 1 Mbp and 20 Mbp, which may reflect the fact that different investigators use different limits for sensitivity or detectability. Translocation yields for mouse lymphocytes have also been calculated and are in good agreement with experimental data from 1 cGy to 10 cGy. There is also good agreement with recent data on complex aberrations. Our model is expected to be applicable to both low- and high-LET radiation, and we include a sample prediction of the yield of interchromosomal rejoining in the dose range 0.22 Gy to 2 Gy of 1000 MeV/nucleon iron particles. This dose range corresponds to average particle traversals per nucleus ranging from 1.0 to 9.12.