Ionizing radiation and genetic risks. III. Nature of spontaneous and radiation-induced mutations in mammalian in vitro systems and mechanisms of induction of mutations by radiation

Development of Live Attenuated Salmonella Typhimurium Vaccine Strain Using Radiation Mutation Enhancement Technology (R-MET)

Salmonella enterica is a leading cause of food-borne diseases in humans worldwide, resulting in severe morbidity and mortality. They are carried asymptomatically in the intestine or gallbladder of livestock, and are transmitted predominantly from animals to humans via the fecal-oral route. Thus, the best preventive strategy is to preemptively prevent transmission to humans by vaccinating livestock. Live attenuated vaccines have been mostly favored because they elicit both cellular and humoral immunity and provide long-term protective immunity. However, developing these vaccines is a laborious and time-consuming process. Therefore, most live attenuated vaccines have been mainly used for phenotypic screening using the auxotrophic replica plate method, and new types of vaccines have not been sufficiently explored. In this study, we used Radiation-Mutation Enhancement Technology (R-MET) to introduce a wide variety of mutations and attenuate the virulence of Salmonella spp. to develop live vaccine strains. The Salmonella Typhimurium, ST454 strain (ST WT) was irradiated with Cobalt60 gamma-irradiator at 1.5 kGy for 1 h to maximize the mutation rate, and attenuated daughter colonies were screened using in vitro macrophage replication capacity and in vivo mouse infection assays. Among 30 candidates, ATOMSal-L6, with 9,961-fold lower virulence than the parent strain (ST454) in the mouse LD50 model, was chosen. This vaccine candidate was mutated at 71 sites, and in particular, lost one bacteriophage. As a vaccine, ATOMSal-L6 induced a Salmonella-specific IgG response to provide effective protective immunity upon intramuscular vaccination of mice. Furthermore, when mice and sows were orally immunized with ATOMSal-L6, we found a strong protective immune response, including multifunctional cellular immunity. These results indicate that ATOMSal-L6 is the first live vaccine candidate to be developed using R-MET, to the best of our knowledge. R-MET can be used as a fast and effective live vaccine development technology that can be used to develop vaccine strains against emerging or serotype-shifting pathogens.

The effects of short-term calorie restriction on mutations in the spleen cells of infant-irradiated mice

The risk of cancer due to exposure to ionizing radiation is higher in infants than in adults. In a previous study, the effect of adult-onset calorie restriction (CR) on carcinogenesis in mice after early-life exposure to X-rays was examined (Shang, Y, Kakinuma, S, Yamauchi, K, et al. Cancer prevention by adult-onset calorie restriction after infant exposure to ionizing radiation in B6C3F1 male mice. Int J Cancer. 2014; 135: 1038-47). The results showed that the tumor frequency was reduced in the CR group. However, the mechanism of tumor suppression by CR is not yet clear. In this study, we examined the effects of CR on radiation-induced mutations using gpt delta mice, which are useful to analyze mutations in various tissues throughout the whole body. Infant male mice (1-week old) were exposed to 3.8 Gy X-rays and fed a control (95 kcal/week/mouse) or CR (65 kcal/week/mouse) diet from adult stage (7-weeks old). Mice were sacrificed at the age of 7 weeks, 8 weeks and 100 days, and organs (spleen, liver, lung, thymus) were harvested. Mutations at the gpt gene in the DNA from the spleen were analyzed by using a gpt assay protocol that detects primarily point mutations in the gpt gene. The results showed that mutation frequencies were decreased in CR groups compared with non-CR groups. Sequence analysis of the gpt gene in mutants revealed a reduction in the G:C to T:A transversion in CR groups. Since it is known that 8-oxoguanine could result in this base substitution and that CR has an effect of reducing oxidative stress, these results indicate that the suppression of oxidative stress by CR is the cause of the reduction of this transversion.

Characterization of genome-wide variations induced by gamma-ray radiation in barley using RNA-Seq

Background

Artificial mutagenesis not only provides a new approach to increase the diversity of desirable traits for breeding new varieties but are also beneficial for characterizing the genetic basis of functional genes. In recent decades, many mutation genes have been identified which are responsible for phenotype changes in mutants in various species including Arabidopsis and rice. However, the mutation feature in induced mutants and the underlying mechanisms of various types of artificial mutagenesis remain unclear.

Results

In this study, we adopted a transcriptome sequencing strategy to characterize mutations in coding regions in a barley dwarf mutant induced by gamma-ray radiation. We detected 1193 genetic mutations in gene transcription regions introduced by gamma-ray radiation. Interestingly, up to 97% of the gamma irradiation mutations were concentrated in certain regions in chromosome 5H and chromosome 7H. Of the 26,745 expressed genes, 140 were affected by gamma-ray radiation; their biological functions included cellular and metabolic processes.

Conclusion

Our results indicate that mutations induced by gamma-ray radiation are not evenly distributed across the whole genome but located in several concentrated regions. Our study provides an overview of the feature of genetic mutations and the genes affected by gamma-ray radiation, which should contribute to a deeper understanding of the mechanisms of radiation mutation and their application in gene function analysis.

Overview of biological mechanisms of human carcinogens

This review summarizes the carcinogenic mechanisms for 109 Group 1 human carcinogens identified as causes of human cancer through Volume 106 of the IARC Monographs. The International Agency for Research on Cancer (IARC) evaluates human, experimental and mechanistic evidence on agents suspected of inducing cancer in humans, using a well-established weight of evidence approach. The monographs provide detailed mechanistic information about all carcinogens. Carcinogens with closely similar mechanisms of action (e.g. agents emitting alpha particles) were combined into groups for the review. A narrative synopsis of the mechanistic profiles for the 86 carcinogens or carcinogen groups is presented, based primarily on information in the IARC monographs, supplemented with a non-systematic review. Most carcinogens included a genotoxic mechanism.

Radiation-induced DNA-protein cross-links: Mechanisms and biological significance

Ionizing radiation produces various DNA lesions such as base damage, DNA single-strand breaks (SSBs), DNA double-strand breaks (DSBs), and DNA-protein cross-links (DPCs). Of these, the biological significance of DPCs remains elusive. In this article, we focus on radiation-induced DPCs and review the current understanding of their induction, properties, repair, and biological consequences. When cells are irradiated, the formation of base damage, SSBs, and DSBs are promoted in the presence of oxygen. Conversely, that of DPCs is promoted in the absence of oxygen, suggesting their importance in hypoxic cells, such as those present in tumors. DNA and protein radicals generated by hydroxyl radicals (i.e., indirect effect) are responsible for DPC formation. In addition, DPCs can also be formed from guanine radical cations generated by the direct effect. Actin, histones, and other proteins have been identified as cross-linked proteins. Also, covalent linkages between DNA and protein constituents such as thymine-lysine and guanine-lysine have been identified and their structures are proposed. In irradiated cells and tissues, DPCs are repaired in a biphasic manner, consisting of fast and slow components. The half-time for the fast component is 20min-2h and that for the slow component is 2-70h. Notably, radiation-induced DPCs are repaired more slowly than DSBs. Homologous recombination plays a pivotal role in the repair of radiation-induced DPCs as well as DSBs. Recently, a novel mechanism of DPC repair mediated by a DPC protease was reported, wherein the resulting DNA-peptide cross-links were bypassed by translesion synthesis. The replication and transcription of DPC-bearing reporter plasmids are inhibited in cells, suggesting that DPCs are potentially lethal lesions. However, whether DPCs are mutagenic and induce gross chromosomal alterations remains to be determined.

Can consumption of raw vegetables decrease the count of sister chromatid exchange? Results from a cross-sectional study in Krakow, Poland

Background

Sister chromatid exchange (SCE) is a widely used sensitive cytogenetic biomarker of exposure to genotoxic and cancerogenic agents. Results of human monitoring studies and cytogenetic damage have revealed that biological effects of genotoxic exposures are influenced by confounding factors related to life-style. Vegetable and fruit consumption may play a role, but available results are not consistent. The purpose of the study was to investigate the effect of consumption of raw and cooked vegetables and fruits on SCE frequency.

Methods

A total of 62 participants included colorectal cancer (CRC) patients, hospital-based controls and healthy laboratory workers. SCE frequency was assessed in blood lymphocytes. Frequency of vegetable and fruit consumption was gathered by structured semi-quantitative food frequency questionnaire.

Results

SCE frequency was lowest among hospital-based controls (4.4 ± 1.1), a bit higher in CRC patients (4.5 ± 1.0) and highest among laboratory workers (7.4 ± 1.2) (p < 0.05). Multivariable linear regression showed a significant inverse effect (b = −0.20) of raw vegetable consumption, but not so for intake of cooked vegetables and fruits.

Conclusions

The results of the study have shown the beneficial effect of consumption of raw vegetables on disrupted replication of DNA measured by SCE frequency, implying protection against genotoxic agents. Further effort is required to verify the role of cooked vegetables and fruits.

Ionizing radiation and genetic risks. XVII. Formation mechanisms underlying naturally occurring DNA deletions in the human genome and their potential relevance for bridging the gap between induced DNA double-strand breaks and deletions in irradiated germ cells

While much is known about radiation-induced DNA double-strand breaks (DSBs) and their repair, the question of how deletions of different sizes arise as a result of the processing of DSBs by the cell's repair systems has not been fully answered. In order to bridge this gap between DSBs and deletions, we critically reviewed published data on mechanisms pertaining to: (a) repair of DNA DSBs (from basic studies in this area); (b) formation of naturally occurring structural variation (SV) - especially of deletions - in the human genome (from genomic studies) and (c) radiation-induced mutations and structural chromosomal aberrations in mammalian somatic cells (from radiation mutagenesis and radiation cytogenetic studies). The specific aim was to assess the relative importance of the postulated mechanisms in generating deletions in the human genome and examine whether empirical data on radiation-induced deletions in mouse germ cells are consistent with predictions of these mechanisms. The mechanisms include (a) NHEJ, a DSB repair process that does not require any homology and which functions in all stages of the cell cycle (and is of particular relevance in G0/G1); (b) MMEJ, also a DSB repair process but which requires microhomology and which presumably functions in all cell cycle stages; (c) NAHR, a recombination-based DSB repair mechanism which operates in prophase I of meiosis in germ cells; (d) MMBIR, a microhomology-mediated, replication-based mechanism which operates in the S phase of the cell cycle, and (e) strand slippage during replication (involved in the origin of small insertions and deletions (INDELs). Our analysis permits the inference that, between them, these five mechanisms can explain nearly all naturally occurring deletions of different sizes identified in the human genome, NAHR and MMBIR being potentially more versatile in this regard. With respect to radiation-induced deletions, the basic studies suggest that those arising as a result of the operation of NHEJ/MMEJ processes, as currently formulated, are expected to be relatively small. However, data on induced mutations in mouse spermatogonial stem cells (irradiation in G0/G1 phase of the cell cycle and DSB repair presumed to be via NHEJ predominantly) show that most are associated with deletions of different sizes, some in the megabase range. There is thus a 'discrepancy' between what the basic studies suggest and the empirical observations in mutagenesis studies. This discrepancy, however, is only an apparent but not a real one. It can be resolved by considering the issue of deletions in the broader context of and in conjunction with the organization of chromatin in chromosomes and nuclear architecture, the conceptual framework for which already exists in studies carried out during the past fifteen years or so. In this paper, we specifically hypothesize that repair of DSBs induced in chromatin loops may offer a basis to explain the induction of deletions of different sizes and suggest an approach to test the hypothesis. We emphasize that the bridging of the gap between induced DSB and resulting deletions of different sizes is critical for current efforts in computational modeling of genetic risks.

Chronic exposure to the cytolethal distending toxins of Gram-negative bacteria promotes genomic instability and altered DNA damage response

Epidemiological evidence links chronic bacterial infections to the increased incidence of certain types of cancer but the molecular mechanisms by which bacteria contribute to tumour initiation and progression are still poorly characterized. Here we show that chronic exposure to the genotoxin cytolethal distending toxin (CDT) of Gram-negative bacteria promotes genomic instability and acquisition of phenotypic properties of malignancy in fibroblasts and colon epithelial cells. Cells grown for more than 30 weeks in the presence of sublethal doses of CDT showed increased mutation frequency, and accumulation of chromatin and chromosomal aberrations in the absence of significant alterations of cell cycle distribution, decreased viability or senescence. Cell survival was dependent on sustained activity of the p38 MAP kinase. The ongoing genomic instability was associated with impaired activation of the DNA damage response and failure to efficiently activate cell cycle checkpoints upon exposure to genotoxic stress. Independently selected sublines showed enhanced anchorage-independent growth as assessed by the formation of colonies in semisolid agarose. These findings support the notion that chronic infection by CDT-producing bacteria may promote malignant transformation, and point to the impairment of cellular control mechanisms associated with the detection and repair of DNA damage as critical events in the process.

Assessment of heritable genetic effects using new genetic tools and sentinels in an era of personalized medicine

The challenge of estimating human health effects from damage to the germ line may be met in the genomic era. Understanding the genetic, as opposed to postconception developmental basis of birth defects is critical to their use in monitoring heritable genetic damage. The causes of common birth defects are analyzed here: mendelian genetic, multigenic, developmental, inherited, or combinational. Only a small fraction of these (noninherited, mendelian genetic) are likely to be informative relative to germ cell mutagenesis, and these won't be discernible against the general background of birth defects. Targeted genetic testing as part of personalized medicine could be integrated into a strategy for assessing germ cell alterations in populations. Thus, "sentinel mutations," as originally proposed by Mulvihill and Ceizel, need not be restricted to X-linked or dominant mutations or conditions visible at birth. Several new sentinels related to personalized medicine are proposed, based on health impact (likelihood of monitoring), frequency, and genetic target suitability (responsiveness to diverse mutational mechanisms). Candidates could include CYP genes (related to metabolism of xenobiotics) important in optimizing drug doses and avoiding adverse reactions. High frequency LDLR mutations (related to familial high cholesterol) predict myocardial infarction in approximately50% of individuals. The more common recessive genetic diseases (cystic fibrosis, phenylketonuria, and others) monitored in newborn screening programs could be informative given parental analysis. New opportunities for genetic analyses need to be coupled with epidemiological studies on environmental exposures. These could focus on adverse outcomes related to tobacco, the mostubiquitous and potent environmental mutagen.

Radiation carcinogenesis: lessons from Chernobyl

Radiation is a carcinogen, interacting with DNA to produce a range of mutations. Irradiated cells also show genomic instability, as do adjacent non-irradiated cells (the bystander effect); the importance to carcinogenesis remains to be established. Current knowledge of radiation effects is largely dependent on evidence from exposure to atomic bomb whole body radiation, leading to increases in a wide range of malignancies. In contrast, millions of people were exposed to radioactive isotopes in the fallout from the Chernobyl accident, within the first 20 years there was a large increase in thyroid carcinoma incidence and a possible radiation-related increase in breast cancer, but as yet there is no general increase in malignancies. The increase in thyroid carcinoma, attributable to the very large amounts of iodine 131 released, was first noticed in children with a strong relationship between young age at exposure and risk of developing papillary thyroid carcinoma (PTC). The extent of the increase, the reasons for the relationship to age at exposure, the reduction in attributable fraction with increasing latency and the role of environmental factors are discussed. The large number of radiation-induced PTCs has allowed new observations. The subtype and molecular findings change with latency; most early cases were solid PTCs with RET-PTC3 rearrangements, later cases were classical PTCs with RET-PTC1 rearrangements. Small numbers of many other RET rearrangements have occurred in 'Chernobyl' PTCs, and also rearrangement of BRAF. Five of the N-terminal genes found in papillary carcinoma rearrangements are also involved in rearrangements in hematological malignancies; three are putative tumor suppressor genes, and two are further genes fused to RET in PTCs. Radiation causes double-strand breaks; the rearrangements common in these radiation-induced tumors reflect their etiology. It is suggested that oncogenic rearrangements may commonly involve both a tumor-suppressor gene (or a DNA repair gene) as well as an oncogene. Involvement of two relevant genes would give a greater chance of progression and a shorter latency than a single-gene mutation. More information is needed on germline mutations conferring susceptibility to radiation-induced PTCs, particularly DNA repair genes. The radiation exposure to the fallout after Chernobyl was very different from the whole body radiation after the atomic bombs. The type and molecular pathology of the thyroid tumors is changing with increasing latency, long latency tumors in other organs could occur in the future. A comprehensive follow up must continue for the lifetime of those exposed.

Implication of replicative stress-related stem cell ageing in radiation-induced murine leukaemia

BACKGROUND

The essential aetiology of radiation-induced acute myeloid leukaemia (AML) in mice is the downregulation of the transcription factor PU.1. The causative mutation of the PU.1-endocing Sfpi1 gene consists mostly of C:G to T:A transitions at a CpG site and is likely to be of spontaneous origin. To work out a mechanism underlying the association between radiation exposure and the AML induction, we have hypothesised that replicative stress after irradiation accelerates the ageing of haematopoietic stem cells (HSCs), and the ageing-related decline in DNA repair could affect the spontaneous mutation rates.

METHODS

Mathematical model analysis was conducted to examine whether and to what extent the cell kinetics of HSCs can be modified after irradiation. The haematopoietic differentiation process is expressed as a mathematical model and the cell-kinetics parameters were estimated by fitting the simulation result to the assay data.

RESULTS

The analysis revealed that HSCs cycle vigourously for more than a few months after irradiation. The estimated number of cell divisions per surviving HSC in 3 Gy-exposed mice reached as high as ten times that of the unexposed.

INTERPRETATION

The mitotic load after 3 Gy irradiation seems to be heavy enough to accelerate the ageing of HSCs and the hypothesis reasonably explains the leukaemogenic process.

EMS mutant spectra generated by multi-parameter flow cytometry

The CHO A(L) cell line contains a single copy of human chromosome 11 that encodes several cell surface proteins including glycosyl phosphatidylinositol (GPI) linked CD59 and CD90, as well as CD98, CD44 and CD151 which are not GPI-linked. The flow cytometry mutation assay (FCMA) measures mutations of the CD59 gene by the absence of fluorescence when stained with antibodies against the CD59 cell surface protein. We have measured simultaneous mutations in CD59, CD44, CD90, CD98 and CD151 to generate a mutant spectrum for ionizing radiation. After treatment with ethyl methanesulfonate (EMS) many cells have an intermediate level of CD59 staining. Single cells were sorted from CD59(-) regions with varying levels of fluorescence and the resulting clonal populations had a stable phenotype for CD59 expression. Mutant spectra were generated by flow cytometry using the isolated clones and nearly all clones were mutated in CD59 only. Interestingly, about 60% of the CD59 negative clones were actually GPI mutants determined by staining with the GPI specific fluorescently labeled bacterial toxin aerolysin (FLAER). The GPI negative cells are most likely caused by mutations in the X-linked pigA gene important in GPI biosynthesis. Small mutations of pigA and CD59 were expected for the alkylating agent EMS and the resulting spectra are significantly different than the large deletions found when analyzing radiation mutants. After analyzing the CD59(-) clonal populations we have adjusted the FCMA mutant regions from 1% to 10% of the mean of the CD59 positive peak to include the majority of CD59 mutants.

Numt-mediated double-strand break repair mitigates deletions during primate genome evolution

Non-homologous end joining (NHEJ) is the major mechanism of double-strand break repair (DSBR) in mammalian cells. NHEJ has traditionally been inferred from experimental systems involving induced double strand breaks (DSBs). Whether or not the spectrum of repair events observed in experimental NHEJ reflects the repair of natural breaks by NHEJ during chromosomal evolution is an unresolved issue. In primate phylogeny, nuclear DNA sequences of mitochondrial origin, numts, are inserted into naturally occurring chromosomal breaks via NHEJ. Thus, numt integration sites harbor evidence for the mechanisms that act on the genome over evolutionary timescales. We have identified 35 and 55 lineage-specific numts in the human and chimpanzee genomes, respectively, using the rhesus monkey genome as an outgroup. One hundred and fifty two numt-chromosome fusion points were classified based on their repair patterns. Repair involving microhomology and repair leading to nucleotide additions were detected. These repair patterns are within the experimentally determined spectrum of classical NHEJ, suggesting that information from experimental systems is representative of broader genetic loci and end configurations. However, in incompatible DSBR events, small deletions always occur, whereas in 54% of numt integration events examined, no deletions were detected. Numts show a statistically significant reduction in deletion frequency, even in comparison to DSBR involving filler DNA. Therefore, numts show a unique mechanism of integration via NHEJ. Since the deletion frequency during numt insertion is low, native overhangs of chromosome breaks are preserved, allowing us to determine that 24% of the analyzed breaks are cohesive with overhangs of up to 11 bases. These data represent, to the best of our knowledge, the most comprehensive description of the structure of naturally occurring DSBs. We suggest a model in which the sealing of DSBs by numts, and probably by other filler DNA, prevents nuclear processing of DSBs that could result in deleterious repair.

Changes to DNA sequence are the major source of variation in evolution. Those changes often arise from damage to DNA that is repaired in a way that fails to restore the original sequence. One type of DNA damage is a chromosomal double-strand break. Such breaks are mostly studied experimentally in model systems, because naturally occurring chromosomal breaks are hard to follow. Here, we used an evolutionary approach to study the repair of naturally occurring chromosomal breaks. Throughout evolutionary history, fragments of the mitochondrial genome, known as numts (nuclear sequences of mitochondrial origin), have been inserted into the nuclear genome. Numts are passively captured into random chromosomal breaks, leaving sequence traces in genomes. Humans and chimpanzees share a recent common ancestor and their genomes share high sequence similarity; therefore, their species-specific numts can be used to follow both some of the break structure and repair mechanisms. Comparing naturally occurring break and repair patterns with experimental repair patterns identified similarities but also highlighted a clear difference. Experimental breaks usually involve deletions, while deletions were significantly less frequent in the numt based repair system. We propose that extra-chromosomal DNA sequences, like numts, play a role in maintaining genome integrity by protecting naturally occurring chromosomal breaks from further deleterious processing.

Morphologic characteristics of Chernobyl-related childhood papillary thyroid carcinomas are independent of radiation exposure but vary with iodine intake

BACKGROUND

The Chernobyl accident caused an unprecedented increase in papillary thyroid carcinoma (PTC) incidence with a surprisingly short latency and unusual morphology. We have investigated whether unexpected features of the PTC incidence after Chernobyl were radiation specific or influenced by iodine deficiency.

METHODS

PTCs from children from Belarus, Ukraine, and the Russian Federation exposed to fallout from Chernobyl were compared with PTCs from children not exposed to radiation from the same countries, from England and Wales (E&W) and from Japan. The degree and type of differentiation, fibrosis, and invasion were quantified.

RESULTS

There were no significant differences between PTCs from radiation-exposed children from Belarus, Ukraine, and the Russian Federation and PTCs from children from the same countries who were not exposed to radiation. Childhood PTCs from Japan were much more highly differentiated (p < 0.001), showed more papillary differentiation (p < 0.001) and were less invasive (p < 0.01) than "Chernobyl" tumors, while tumors from E&W generally showed intermediate levels of degree and type of differentiation and invasion. There was a marked difference between the sex ratios of children with PTCs who were radiation exposed and those who were not exposed (F:M exposed vs. unexposed 1.5:1 vs. 4.2:1; chi(2) = 7.90, p < or = 0.01005).

CONCLUSIONS

The aggressiveness and morphological features of Chernobyl childhood PTCs are not associated with radiation exposure. The differences found between tumors from the Chernobyl area, E&W, and Japan could be influenced by many factors. We speculate that dietary iodine levels may have wide implications in radiation-induced thyroid carcinogenesis, and that iodine deficiency could increase incidence, reduce latency, and influence tumor morphology and aggressiveness.

Effects of organosulfurs, isothiocyanates and vitamin C towards hydrogen peroxide-induced oxidative DNA damage (strand breaks and oxidized purines/pyrimidines) in human hepatoma cells

The aim of this study was to evaluate the effects of organosulfurs, isothiocyanates and vitamin C towards hydrogen peroxide-induced DNA damage (DNA strand breaks and oxidized purines/pyrimidines) in human hepatoma cells (HepG2), using the Comet assay. Treatment with hydrogen peroxide (H(2)O(2)) increased the levels of DNA strand breaks and oxidized purine and pyrimidine bases, in a concentration and time dependent manner. Organosulfur compounds (OSCs) reduced DNA strand breaks induced by H(2)O(2). In addition, OSCs also decreased the levels of oxidized pyrimidines. However, none of the OSCs tested reduced the levels of oxidized purines. Isothiocyanates compounds (ITCs) and vitamin C showed protective effects towards H(2)O(2)-induced DNA strand breaks and oxidized purine and pyrimidine bases. The results indicate that removal of oxidized purine and pyrimidine bases by ITCs was more efficient than by OSCs and vitamin C. Our findings suggest that OSCs, ITCs and vitamin C could exert their protective effects towards H(2)O(2)-induced DNA strand breaks and oxidative DNA damage by the free radical-scavenging efficiency of these compounds.

Analysis of Common Deletion (CD) and a novel deletion of mitochondrial DNA induced by ionizing radiation

PURPOSE

In order to identify supportive evidence of radiation exposure to cells, we analyzed the relationship between exposure to ionizing radiation and the induction of deletions in mitochondrial DNA (mtDNA).

MATERIALS AND METHODS

Using human hepatoblastoma cell line, HepG2 and its derivatives, HepG2-A, -89 and -400, established after long term exposure to X-ray, mtDNA deletions were analyzed by polymerase chain reaction (PCR) and real-time PCR after cells were subjected to radiation and genotoxic treatments.

RESULTS

Common Deletion (CD), the most extensively studied deletion of mtDNA, was induced within 24 h after exposure to 5 Gray (Gy) of X-rays and was associated with replication of mtDNA. CD became undetectable several days after the exposure due to the death of cells containing mitochondria within which CD had been induced. Furthermore, we found a novel mtDNA deletion that consisted of a 4934 base-pair deletion (4934del) between nucleotide position 8435 and 13,368. A lower dose of ionizing radiation was required to induce the 4934del than for CD and this was independent of the quality of radiation used and was not induced by treatments with hydrogen peroxide (H(2)O(2)) and other genotoxic reagents including bleomycin.

CONCLUSION

CD is induced by ionizing radiation, however, the amount of CD detected at a certain point in time after radiation exposure is dependent on the initial frequency of CD induced and the death rate of cells with mtDNA containing CD. The novel mtDNA deletion found in this study, therefore, will be used to determine whether cells were exposed to ionizing radiation.

New approaches to assessing the effects of mutagenic agents on the integrity of the human genome

Heritable genetic alterations, although individually rare, have a substantial collective health impact. Approximately 20% of these are new mutations of unknown cause. Assessment of the effect of exposures to DNA damaging agents, i.e. mutagenic chemicals and radiations, on the integrity of the human genome and on the occurrence of genetic disease remains a daunting challenge. Recent insights may explain why previous examination of human exposures to ionizing radiation, as in Hiroshima and Nagasaki, failed to reveal heritable genetic effects. New opportunities to assess the heritable genetic damaging effects of environmental mutagens are afforded by: (1) integration of knowledge on the molecular nature of genetic disorders and the molecular effects of mutagens; (2) the development of more practical assays for germline mutagenesis; (3) the likely use of population-based genetic screening in personalized medicine.

Biochemical mechanisms of chromosomal translocations resulting from DNA double-strand breaks

Exposure of mammalian cells to agents that induce DNA double-strand breaks typically results in both reciprocal and nonreciprocal chromosome translocations. Over the past decade, breakpoint junctions of a significant number of translocations and other genomic rearrangements, both in clinical tumors and in experimental models, have been analyzed at the DNA sequence level. Based on these data, reasonable inferences regarding the biochemical mechanisms involved in translocations can be drawn. In a few cases, breakpoints have been shown to correlate with sites of double-strand cleavage by agents to which the cells or patients have been exposed, including exogenous rare-cutting endonucleases, radiomimetic compounds, and topoisomerase inhibitors. These results confirm that translocations primarily reflect misjoining of the exchanged ends of two or more double-strand breaks. Many junctions show significant loss of DNA sequence at the breakpoints, suggesting exonucleolytic degradation of DNA ends prior to joining. The size and frequency of these deletions varies widely, both between experimental systems, and among individual events in a single system. Homologous recombination between repetitive DNA sequences does not appear to be a major pathway for translocations associated with double-strand breaks. Rather, the general features of the junction sequences, particularly the high frequency small terminal deletions, the apparent splicing of DNA ends at microhomologies, and gap-filling on aligned double-strand break ends, are consistent with the known biochemical properties of the classical nonhomologous end joining pathway involving DNA-dependent protein kinase, XRCC4 and DNA ligase IV. Nevertheless, cells with deficiencies in this pathway still exhibit translocations, with grossly similar junction sequences, suggesting an alternative but less conservative end joining pathway. Although evidence for participation of specific DNA end processing enzymes in formation of translocations is largely circumstantial, likely candidates include DNA polymerases lambda and mu, Artemis nuclease, polynucleotide kinase/phosphatase, tyrosyl-DNA phosphodiesterase, DNase III, Werner syndrome protein, and the Mre11/Rad50/NBS1 complex.

Study of DNA damage induced by dental bleaching agents in vitro

Dental bleaching is a simple and conservative procedure for aesthetic restoration of vital and non-vital discolored teeth. Nevertheless, a number of studies have demonstrated the risk of tissue damage from the contact of these agents with the oral mucosa. In the current study, the genotoxic potential associated with exposure to dental bleaching agents was assessed by the single cell gel (comet) assay in vitro. Chinese hamster ovary (CHO) cells in vitro were exposed to six commercial dental bleaching agents (Clarigel Gold - Dentsply; Whitespeed - Discus Dental; Nite White - Discus Dental; Magic Bleaching - Vigodent; Whiteness HP - FGM and Lase Peroxide - DMC). The results pointed out that all dental bleaching agents tested contributed to DNA damage as depicted by the mean tail moment, being the strongest effect observed with the highest dose of hydrogen peroxide (Whiteness HP and Lase Peroxide, at a 35% concentration). On the other hand, Magic Bleaching (Vigodent) induced the lowest level of DNA breakage. Negative and positive controls displayed absence and presence of DNA-damaging, respectively. Taken together, these results suggest that dental bleaching agents may be a factor that increases the level of DNA damage. A higher concentration of hydrogen peroxide produced higher noxious activities in the genome as detected by single cell gel (comet) assay.

Assessment of genetic damage induced by dental bleaching agents on mouse lymphoma cells by single cell gel (comet) assay

Dental bleaching is a simple and conservative procedure for aesthetic restoration of vital discoloured teeth. However, dental bleaching agents may represent a hazard to human health, especially by causing DNA strand breaks. Genotoxicity tests form an important part of cancer research and risk assessment of potential carcinogens. In the current study, the genotoxic potential associated with exposure to dental bleaching agents was assessed by the single cell gel (comet) assay in vitro. Six commercial dental bleaching agents (Clarigel Gold - Dentsply; Whitespeed - Discus Dental; Nite White - Discus Dental; Magic Bleaching - Vigodent; Whiteness HP - FGM and Lase Peroxide - DMC) were exposed to mouse lymphoma cells in vitro. The results pointed out that all dental bleaching agents tested contributed to the DNA damage as depicted by the mean tail moment. Clear concentration-related effects were obtained for DNA damaging, being the strongest effect observed at the highest dose of the hydrogen peroxide (Whiteness HP and Lase Peroxide, at 35% concentration). On the contrary, Whitespeed (Discus Dental) induced the lowest level of DNA breakage. Taken together, these results suggest that dental bleaching agents may be a factor that increases the level of DNA damage as detected by the single cell gel (comet) assay.

Genetic and epigenetic features in radiation sensitivity

Recent progress especially in the field of gene identification and expression has attracted greater attention to genetic and epigenetic susceptibility to cancer, possibly enhanced by ionising radiation. It has been proposed that the occurrence and severity of the adverse reactions to radiation therapy are also influenced by such genetic susceptibility. This issue is especially important for radiation therapists since hypersensitive patients may suffer from adverse effects in normal tissues following standard radiation therapy, while normally sensitive patients could receive higher doses of radiation offering a better likelihood of cure for malignant tumours. This paper, the first of two parts, reviews the main mechanisms involved in cell response to ionising radiation. DNA repair machinery and cell signalling pathways are considered and their role in radiosensitivity is analysed. The implication of non-targeted and delayed effects in radiosensitivity is also discussed.

Prevention of oxidative injury in PC12 cells by a traditional Chinese medicine, Shengmai San, as a model of an antioxidant-based composite formula

The protective effect of Shengmai San (SMS) on oxidative damage in cultured PC12 cells was studied as a model of an antioxidant-based composite formula usable for the treatment of oxidative stress-related complex disorders. SMS, a traditional Chinese herbal medicine, has previously been shown to prevent cerebral oxidative injury in rats. Neuronal model PC12 cells were incubated with SMS for defined periods, chased with H(2)O(2) for 30 min at 37 degrees C, and subjected to an ELISA-based assay for determining the protein carbonyl content, and a Comet assay for DNA single strand breaks (SSBs). The results showed that both protein carbonyl content and DNA SSBs increased in PC12 cells after the H(2)O(2) chase in a concentration-dependent manner. Both H(2)O(2)-dependent carbonyl formation and DNA damage were markedly prevented in the cells pretreated with SMS, and the SMS effects were dependent on both the SMS concentration and the period of pre-incubation with SMS before the H(2)O(2) abuse. At the same time, cell viability was enhanced in the SMS-pretreated cells after the H(2)O(2) abuse compared to the control cells as determined by an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. It is concluded that SMS functions not only as a simple antioxidant but also as a modulator of cellular antioxidant defense.

Genetic susceptibility to thymic lymphomas and K-ras gene mutation in mice after exposure to X-rays and N-ethyl-N-nitrosourea

PURPOSE

Ras activation is one of the major mechanisms for the development of murine thymic lymphomas by radiation and chemical carcinogens. To gain insight into the relationship between genetic susceptibility and ras gene mutation, the frequency and spectrum of ras gene mutation was examined in thymic lymphomas from susceptible and resistant mice.

MATERIALS AND METHODS

K- and N-ras mutations in thymic lymphomas that arose in X-ray-irradiated and N-ethyl-N-nitrosourea (ENU)-treated mice of susceptible C57BL/6, rather resistant C3H and their hybrid B6C3F1 were analysed by polymerase chain reaction-single-strand conformation polymorphism and subsequent DNA sequencing.

RESULTS

C57BL/6 exhibited a higher incidence of thymic lymphomas after exposure to X-rays and ENU than C3H, with B6C3F1 being intermediate. K-ras gene mutations occurred frequently in the pathogenesis of ENU-induced thymic lymphomas in susceptible C57BL/6 as opposed to resistant C3H. The ras mutations were more frequent in ENU-induced thymic lymphomas than X-ray-induced thymic lymphomas, and with the latter, there was no clear evidence for strain differences, suggesting that the genetic susceptibility to X-rays was independent of ras activation. The mutations of K-ras in thymic lymphomas from C57BL/6 were predominantly GGT to GAT in codon 12, whereas this mutation type was never found in those from C3H. No strain difference was observed in the nucleotide sequence or expression levels of O(6)-alkylguanine alkyltransferase, indicating that this enzyme did not account for the genetic susceptibility to ras activation.

CONCLUSIONS

The results indicate that there is a clear strain and carcinogen dependency of K-ras mutation and that the frequency of ras mutation might determine the genetic susceptibility to ENU-induced lymphomagenesis, whereas pathways independent of ras activation might determine the susceptibility to X-ray-induced lymphomagenesis.

Acridine mutagenesis of zebrafish (Danio rerio)

Mutagenesis screening, in which heritable traits are isolated following damage to the genome, is a powerful approach for investigating gene function. Among vertebrate model organisms, the zebrafish (Danio rerio) is ideally suited to mutagenesis screens. The success of large-scale screens is dependent on the way in which changes are identified. The type of damage induced is also pivotal. Single base coding region deletions and insertions are suited to abolition of gene function whilst inducing a small physical alteration to the genome. Such mutations are not commonly found following mutagenesis schemes reported to date. Here, we show that an acridine mutagen, ICR191, which in other model organisms frequently induces single base deletions and insertions, is mutagenic in zebrafish. ICR191 induces hallmark phenotypes associated with genetic damage in treated embryos. Alterations are heritable. Offspring of mutagenised fish had mutations in a marker gene and were found to produce offspring with abnormal development. Using an adaptation of a molecular mutation detection method, fluorescent arbitrary primed PCR, we identified an induced alteration directly. The estimated frequency of induced mutations was sufficiently high to make it feasible to employ this approach for mutagenesis screening.

Base substitutions, targeted single-base deletions, and chromosomal translocations induced by bleomycin in plateau-phase mammary epithelial cells

Previous work showed that treatment of plateau-phase Chinese hamster ovary cells with the radiomimetic double-strand cleaving agent bleomycin induced very small deletions as well as interchromosomal reciprocal translocations, both of which could be ascribed to errors in end joining of DNA double-strand breaks. In an attempt to assess the possible role of TP53 in suppressing such repair errors, bleomycin-induced mutagenesis at the HPRT locus was examined in immortalized 184B5 human mammary epithelial cells (TP53(+)), and in a TP53-defective derivative, 184B5-E6tfxc6. For both cell lines, the most frequent bleomycin-induced mutations were base substitutions, with no apparent targeting to major bleomycin lesions. However, both lines also sustained single-base deletions that were targeted to expected sites of double-strand breaks, suggesting that they arose by end-joining repair of the breaks. Surprisingly, only a few large deletions or rearrangements, and no interchromosomal events involving the HPRT locus were detected among the mutants. The results suggest that in both cell lines, errors in double-strand break repair resulting in heritable large deletions and rearrangements are rare. Spectral karyotyping of bleomycin-treated 184B5 cells showed that a significant number of translocations were present shortly after bleomycin exposure, but their frequency decreased upon continued culture of the cells. Thus, for these cells, the lack of induced interchromosomal rearrangements can be explained in part by selection against such events as the cells proliferate.

Hydrogen peroxide-induced DNA damage and DNA repair in lymphocytes from malnourished children

The aim of this study was to assess DNA repair capacity in lymphocytes of children with protein calorie malnutrition using the single-cell gel electrophoresis (comet) assay. Repair capacity was assessed by estimating the relative decrease of DNA migration length 5, 15, 30, and 60 min after hydrogen peroxide treatment, in three groups of children: well-nourished (WN), well-nourished infected (WN-I), and malnourished infected (MN-I). In addition, the DNA migration length was evaluated in all groups before and after peroxide treatment. Comparison of mean migration lengths observed in WN and WN-I children showed significant differences at all times tested; between WN-I and MN-I differences were also observed, except after hydrogen peroxide exposure. This implies that lymphocytes of WN-I and MN-I children were equally sensitive to hydrogen peroxide. Nevertheless, the MN-I group clearly shows the greatest overall percentage of damaged cells at all times tested. In relation to repair capacity, at 5 min it was approximately 30% in both groups of well-nourished children, but only 20% in MN-I; 15 min after exposure, repair capacity increased to 51% in well-nourished children but only to 31% in MN-I; and at 60 min this capacity increased to 82% in well-nourished but only to 55% in MN-I. These data indicate that lymphocytes of malnourished children show a decreased capacity to repair hydrogen peroxide-induced DNA damage compared to that of well-nourished controls. This reflects that only malnutrition is associated with decreased DNA repair capacity. Additionally, the data confirm that severe infection and malnutrition are two factors clearly associated with increased DNA damage.

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.

Multiple manifestations of X-ray-induced genomic instability in Chinese hamster ovary (CHO) cells

Carcinogenesis is postulated to follow a multistep cascade in which the first genetic event may destabilize cells and thereby facilitate the induction of subsequent mutations within the same cell. It has recently been shown that exposure to ionizing radiation can in itself induce a persistent, heritable genetic instability in cells. To further investigate this phenomenon, we utilized a mutationally unstable population derived from a single Chinese hamster ovary (CHO) cell that survived X irradiation. We exposed these cells to a second dose of radiation, selected hypoxanthine phosphoribosyl transferase (HPRT) mutant subclones, and identified the type of mutations involved. We found complete deletions, continuous tract partial deletions, single-exon deletions, discontinuous-exon deletions ("skip mutations"), and point mutations (changes of less than 100 bp) among the isolated HPRT mutants. We hypothesized that the skip mutation clones might be more likely to demonstrate genomic instability. To test this hypothesis, mutant subclones were screened for three markers of genetic instability: alteration of minisatellite sequences, change in telomere length, and induction of chromosomal aberrations. Clones with skip mutations and single-exon deletions possessed elevated frequencies of minisatellite alterations and chromosomal aberrations, particularly rings and dicentrics. All mutant clones showed longer telomere terminal restriction fragment lengths than did wild-type cells. These results are consistent with the hypothesis that irradiation may induce a global instability phenotype, since the multiple alterations observed are mechanistically distinct, heritable cellular modifications that arose in the clonogenic progeny of the irradiated cells. Skip mutations may be one manifestation of this instability, but their presence was not specifically associated with the other genetic alterations.

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.

Analysis of mutational effects at the HPRT locus in human G(0) phase lymphocytes irradiated in vitro with gamma rays

The mutational effects of ionising radiation at the hypoxanthine-guanine phosphoribosyl transferase (HPRT) locus were studied in human peripheral blood G(0) phase lymphocytes irradiated in vitro with gamma rays. The presence of radiation induced mutants was assessed by selecting the HPRT mutants every week on the basis of 6-thioguanine resistance up to 1 month after irradiation. A dose-related increase of 14.25x10(-6) mutants/Gy was measured after an expression time of 7 days. After 2 weeks from culture starting the fraction of clonable cells in irradiated and control cell populations decreased, limiting the measurements of mutant frequency. The mutational spectrum of the HPRT gene was determined by PCR analyses in a total of 99 mutant clones derived from irradiated lymphocytes. The independent origin of mutant clones carrying the same mutation was assessed by analysing the TCR gamma gene rearrangements. The results showed a dose-related increase of deletion mutants up to 3Gy, whereas point mutation frequency increased only up to 2Gy. Two preferentially deleted regions were identified; one involving the HPRT exon 3, and another one the 3'-terminal and the 3'-flanking region of the gene. One complex mutation involving a non-contiguous deletion of exons 2-5 and 7/8 was observed among the mutants isolated after 3Gy irradiation.

Pooled analysis of p53 mutations in hematological malignancies

A computerized database is described that contains information about 507 mutations in the p53 gene of hematologic tumors and corresponding cell lines. Analysis of these mutations indicated the following findings: First, mutational spectrum analysis in these tumors was found to be similar to the pattern found for other solid tumors. However, when the patterns of base substitutions were examined separately according to the types of hematologic malignancies, followed by subgroup analysis, notable differences (in some cases of statistical significance) emerged. Second, mutational pattern analysis indicates that about 48% of base substitutions in hematologic tumors are suspected to be associated with carcinogen exposure. Third, deletions and insertions are localized mainly to exons 5-8 and repeated DNA sequences. However, the unusual profile of variations in frequency within each type of tumor suggests that, in addition to endogenous damage to template DNA, there is the factor of exposure to environmental physical and chemical carcinogens/mutagens. Fourth, p53 protein alterations analysis indicate that most of the changes in the amino acids are "semiconservative," presumably in order to avoid disrupting the structure of the p53 monomer. Consistent with this notion, structural mutations are more conservative than the binding mutations. Finally, molecular mechanisms that lead to p53 mutations, etiological factors that play a role in their formation, and the pathophysiological significance of consequent p53 protein alterations are discussed.

Dose-dependent changes in the spectrum of mutations induced by ionizing radiation

We examined the influence of dose on the spectrum of mutations induced at the hypoxanthine guanine phosphoribosyltransferase (Hprt) locus in Chinese hamster ovary (CHO) cells. Independent CHO-K1 cell mutants at the Hprt locus were isolated from cells exposed to 0, 0.5, 1.5, 3.0 and 6.0 Gy (137)Cs gamma rays, and the genetic changes responsible for the mutations were determined by multiplex polymerase chain reaction (PCR)-based exon deletion analysis. We observed dose-dependent changes in mutation spectra. At low doses, the principal radiation-induced mutations were point mutations. With increasing dose, multibase deletion mutations became the predominant mutation type such that by 6.0 Gy, there were almost three times more deletion mutations than point mutations. The dose response for induction of point mutations was linear while that for multibase deletions fit a linear-quadratic response. There was a biphasic distribution of deletion sizes, and different dose responses for small compared to large deletions. The frequency of large (>36 kb) total gene deletions increased exponentially, implying that they develop from the interaction between two independent events. In contrast, the dose response for deletion mutations of less than 10 kb was nearly linear, suggesting that these types of mutations develop mostly from single events and not the interactions between two independently produced lesions. The observation of dose-dependent changes in radiation-induced mutation spectra suggests that the types of alterations and therefore the risks from low-dose radiation exposure cannot be easily extrapolated from high-dose effects.

Gene rearrangement and Chernobyl related thyroid cancers

The increase in thyroid carcinoma post-Chernobyl has been largely confined to a specific subtype of papillary carcinoma (solid/follicular). This subtype is observed predominantly in children under 10 in unirradiated populations, but maintains a high frequency in those aged 10-15 from those areas exposed to fallout from the Chernobyl accident. The aim of this study was to link morphology with molecular biology. We examined 106 papillary carcinomas from children under the age of 15 at operation. All were examined for rearrangements of the RET oncogene by reverse transcription polymerase chain reaction (RT-PCR); a subset of these cases were also examined for mutations of the three ras oncogenes, exon 10 of the thyroid stimulating hormone receptor, associated more usually with a follicular rather than papillary morphology, and exons 5, 6, 7 and 8 of the p53 gene, commonly involved in undifferentiated thyroid carcinoma. Rearrangements of the REToncogene were found in 44% of papillary carcinomas in which we studied fresh material; none of the tumours examined showed mutation in any of the other genes. The two rearrangements resulting from inversion of part of chromosome 10 (PTC1 and PTC3) accounted for the majority of RET rearrangements identified, with PTC1 being associated with papillary carcinomas of the classic and diffuse sclerosing variants and PTC3 with the solid/follicular variant.

The protective activity of alpha-hederine against H2O2 genotoxicity in HepG2 cells by alkaline comet assay

This study was designed to evaluate the protective effect of alpha-hederine (alpha-hed) against H2O2-mediated DNA damage on HepG2 cell line by the alkaline comet assay. For the protective effect of alpha-hed study, cells were treated according to three protocols: pre-treatment, simultaneous treatment and post-treatment. The effect of alpha-hed on catalase activity was evaluated after treating the cells with 3.36 mg/ml of 3-amino-1,2,4-triazole (AMT) singly or in combination with alpha-hed (1.5 or 3 microg/ml) and H2O2 (8.8 microM) during 1 h. The catalase activity was also biochemically measured after treating cells with alpha-hed at 1.5, 3, or 15 microg/ml during 1 h. Additionally, the influence of alpha-hed on membrane RedOx potential, pool of reduced glutathione and total protein content was evaluated by flow cytometry. In the pre-treatment, the two concentrations of alpha-hed (1.5 and 3 microg/ml) decreased the lesions induced by H2O2 (8.8 microM) significantly. This decrease was about 57.2% and 66.1%, respectively. Similar results were observed when cells were treated with alpha-hed and H2O2 simultaneously. The decrease of H2O2-induced lesions was about 78.2% and 83.2% (alpha-hed 1.5 and 3 microg/ml, respectively). In the post-treatment protocol, this decrease was not significant. The combination of AMT and H2O2 induced more DNA damage than H2O2 alone (tail moment (TM) means was 31.4% and 21.8%, respectively). When alpha-hed was added to this mixture, TM means were reduced significantly (17.4% for alpha-hed 1. 5 microg/ml and 15.5% for alpha-hed 3 microg/ml). Up to 6.9 microg/ml, alpha-hed enhanced catalase activity (60.5%), followed by a decrease of the activity. Total protein content and membrane RedOx potential were slightly increased up to 11 microg/ml (14% and 3.6%, respectively) followed by a drop and a plateau. Pool of reduced glutathione remained unchanged up to 10 microg/ml then dropped and reached a plateau. In conclusion, alpha-hed could exert its protective effect against H2O2-mediated DNA damage by scavenging free radicals or by enhancing the catalase activity.

Ionizing radiation and genetic risks. X. The potential "disease phenotypes" of radiation-induced genetic damage in humans: perspectives from human molecular biology and radiation genetics

Estimates of genetic risks of radiation exposure of humans are traditionally expressed as expected increases in the frequencies of genetic diseases (single-gene, chromosomal and multifactorial) over and above those of naturally-occurring ones in the population. An important assumption in expressing risks in this manner is that gonadal radiation exposures can cause an increase in the frequency of mutations and that this would result in an increase in the frequency of genetic diseases under study. However, despite compelling evidence for radiation-induced mutations in experimental systems, no increases in the frequencies of genetic diseases of concern or other adverse effects (i.e., those which are not formally classified as genetic diseases), have been found in human studies involving parents who have sustained radiation exposures. The known differences between spontaneous mutations that underlie naturally-occurring single-gene diseases and radiation-induced mutations studied in experimental systems now permit us to address and resolve these issues to some extent. The fact that spontaneous mutations (among which are point mutations and DNA deletions generally restricted to the gene) originate through a number of different mechanisms and that the latter are intimately related to the DNA organization of the genes, are now well-documented. Further, spontaneous mutations include those that cause diseases through loss of function as well as gain of function of genes. In contrast, most radiation-induced mutations studied in experimental systems (although identified through the phenotypes of the marker genes) are predominantly multigene deletions which cause loss of function; the recoverability of an induced deletion in a livebirth seems dependent on whether the gene and the genomic region in which it is located can tolerate heterozygosity for the deletion and yet be compatible with viability. In retrospect, the successful mutation test systems (such as the mouse specific locus test) used in radiation studies have involved genes which are non-essential for survival and are also located in genomic regions, likewise non-essential for survival. In contrast, most of the human genes at which induced mutations have been looked for, do not seem to have these attributes. The inference therefore is that the failure to find induced germline mutations in humans is not due to the resistance of human genes to induced mutations but due to the structural and functional constraints associated with their recoverability in livebirths. Since the risk of inducible genetic diseases in humans is estimated using rates of "recovered" mutations in mice, there is a need to introduce appropriate correction factors to bridge the gap between these rates and the rates at which mutations causing diseases are potentially recoverable in humans. Since the whole genome is the "target" for radiation-induced genetic damage, the failure to find increases in the frequencies of specific single-gene diseases of societal concern does not imply that there are no genetic risks of radiation exposures: the problem lies in delineating the phenotypes of recoverable genetic damage that are recognizable in livebirths. Data from studies of naturally-occurring microdeletion syndromes in humans and those from mouse radiation studies are instructive in this regard. They (i) support the view that growth retardation, mental retardation and multisystem developmental abnormalities are likely to be among the quantitatively more important adverse effects of radiation-induced genetic damage than mutations in a few selected genes and (ii) underscore the need to expand the focus in risk estimation from known genetic diseases (as has been the case thus far) to include these induced adverse developmental effects although most of these are not formally classified as "genetic diseases". (ABSTRACT TRUNCATED)

Polymerase chain reaction-based deletion screening of bisulfite (sulfur dioxide)-enhanced gpt-mutants in CHO-AS52 cells

In this study, we have examined the mutagenicity of bisulfite (sulfur dioxide) at the xathine-guanine phosphoribosyl transferase locus (gpt) in the pSV2 gpt-transformed CHO cell line, AS52. Our results provide evidence for bisulfite as a weak gene mutagen because the chemical at high doses and at high cytotoxicity causes a 4-fold increase in mutant frequency (MF) and less than a doubling of the gpt gene deletion frequency compared to control. We suggest that the increase of MF in bisulfite-treated cells results from bisulfite activity,as a comutagen, enhancing the induction effect of unknown endogenous or exogenous factors on spontaneous mutagenesis of AS52 cells. For the spontaneous, 5 mM bisulfite- and 10 mM bisulfite-enhanced spontaneous mutants in AS52 cells, the percentage of total deletion mutations of the gpt gene is 36%, 44% and 65%, respectively

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.

DNA synthesis on discontinuous templates by human DNA polymerases: implications for non-homologous DNA recombination

DNA polymerases catalyze the synthesis of DNA using a continuous uninterrupted template strand. However, it has been shown that a 3'-->5' exonuclease-deficient form of the Klenow fragment of Escherichia coli DNA polymerase I as well as DNA polymerase of Thermus aquaticus can synthesize DNA across two unlinked DNA templates. In this study, we used an oligonucleotide-based assay to show that discontinuous DNA synthesis was present in HeLa cell extracts. DNA synthesis inhibitor studies as well as fractionation of the extracts revealed that most of the discontinuous DNA synthesis was attributable to DNA polymerase alpha. Additionally, discontinuous DNA synthesis could be eliminated by incubation with an antibody that specifically neutralized DNA polymerase alpha activity. To test the relative efficiency of each nuclear DNA polymerase for discontinuous synthesis, equal amounts (as measured by DNA polymerase activity) of DNA polymerases alpha, beta, delta (+/- PCNA) and straightepsilon (+/- PCNA) were used in the discontinuous DNA synthesis assay. DNA polymerase alpha showed the most discontinuous DNA synthesis activity, although small but detectable levels were seen for DNA polymerases delta (+PCNA) and straightepsilon (- PCNA). Klenow fragment and DNA polymerase beta showed no discontinuous DNA synthesis, although at much higher amounts of each enzyme, discontinuous synthesis was seen for both. Discontinuous DNA synthesis by DNA polymerase alpha was seen with substrates containing 3 and 4 bp single-strand stretches of complementarity; however, little synthesis was seen with blunt substrates or with 1 bp stretches. The products formed from these experiments are structurally similar to that seen in vivo for non-homologous end joining in eukaryotic cells. These data suggest that DNA polymerase alpha may be able to rejoin double-strand breaks in vivo during replication.

Quantitative and qualitative comparisons of spontaneous and radiation-induced specific-locus mutation in the ad-3 region of heterokaryon 12 of Neurospora crassa

The data from forward-mutation experiments to obtain specific-locus mutations at two closely linked loci in the adenine-3 (ad-3) region of heterokaryon 12 (H-12) of Neurospora crassa have been used to determine the relative frequencies and mutational spectra of ad-3 mutants occurring spontaneously and those induced by 7 different radiation treatments. Previous studies have demonstrated that specific-locus mutants at these two loci result from 5 major genotypic classes, namely two classes of gene/point mutations (ad-3AR and ad-3BR), and 3 classes of multilocus deletion mutations ([ad-3A]IR, [ad-3B]IR and [ad-3A ad-3B]IR). Two different approaches were used to compare spontaneous mutation in the ad-3 region with that induced by 7 different radiation treatments (UV, 32P, 447 MeV protons, 85Sr, 250 kVp X-rays, 39 MeV helium ions, and 101 MeV carbon ions). These comparisons included X2-tests on the numbers of ad-3 mutants resulting in the following two sets of ratios: (1) gene/point mutations and multilocus deletion mutations; and (2) complementing and non-complementing ad-3BR mutants. Combination of the data from these two methods of comparison has demonstrated that each of the 7 radiation treatments induced a spectrum of ad-3 mutants that is statistically different from the spontaneous spectrum. In addition, these same two methods of comparison have been used to compare the mutagenic effects of each of the 7 radiation treatments with each other. Combination of the data from these two methods of comparison demonstrated that the majority of radiation-induced specific-locus mutations: (90.5% (19/21 of the pairwise combinations)) are qualitatively different from each other. We conclude that the mechanisms by which various radiations modify DNA tend to exhibit fundamental differences from each other and from the processes involved in spontaneous mutation.

Quantitative and qualitative comparison of spontaneous and chemical-induced specific-locus mutation in the ad-3 region of heterokaryon 12 of Neurospora crassa

The data from forward-mutation experiments to obtain specific-locus mutations at two closely linked loci in the adenine-3 (ad-3) region of heterokaryon 12 (H-12) of Neurospora crassa have been tabulated to determine the relative frequencies and mutational spectra of ad-3 mutants occurring spontaneously and those induced by 22 different chemical treatments. Previous studies have demonstrated that specific-locus mutations at these two loci result from 5 major genotypic classes, namely two classes of gene/point mutations (ad-3AR and ad-3BR), and 3 classes of multilocus deletion mutations ([ad-3A]IR, [ad-3B]IR and [ad-3A ad-3B]IR). In addition, prior studies have demonstrated that some chemical mutagens induced ad-3 mutants exclusively, or almost exclusively, by gene/point mutation and other chemical mutagens by gene/point mutation and multilocus deletion mutation. In the latter cases, there was wide variation in the percentages of ad-3 mutants in these 5 major genotypic classes. Two comparative methods of analysis that also were used to compare spontaneous and chemical-induced ad-3 mutational spectra included X2-tests on the numbers of ad-3 mutants resulting in the following two sets of ratios: (1) gene/point mutations and multilocus deletion mutations; and (2) complementing and non-complementing ad-3BR, mutants. Combination of the p-values from X2-tests for these two methods of comparison demonstrated that all 22 chemicals induce a spectrum of ad-3 mutants that is qualitatively different from that occurring spontaneously. In addition, these same two methods of comparison have been used to compare the mutagenic effects of each of the 22 chemical treatments with each other. Combination of the data from these two methods of comparison has demonstrated that 93.1% (215/231) of the pairwise combinations of these 22 chemicals were different from each other. The implication of these experimental data on the induction of specific-locus mutations in somatic cells of Neurospora for genetic risk assessment exercises is discussed.

Study of DNA damage induction and repair capacity of fresh and cryopreserved lymphocytes exposed to H2O2 and gamma-irradiation with the alkaline comet assay

The alkaline SCGE assay was evaluated for use with cryopreserved lymphocytes in order to obtain results similar to the freshly isolated ones. The induction of DNA damage as well as the repair capacity of gamma-rays and H2O2 exposed cryopreserved human lymphocytes was found to be the same to that of the freshly isolated. Human lymphocytes (fresh or cryopreserved) responded differently to the effects of gamma-irradiation if compared to the H2O2 treatment. The distribution of DNA damage among gamma-irradiated lymphocytes was more homogeneous compared to H2O2, both in freshly isolated and in cryopreserved cells. 2.4 micrograms/ml phytohemagglutinin at the start of a 2-h incubation in RPMI of cryopreserved samples gave similar DNA repair and distribution patterns to the 2-h post-exposure incubation of freshly isolated lymphocytes. H2O2-induced DNA damage was not repaired completely. However, the repair of gamma-rays-induced DNA damage was more efficient. These findings confirm the different mode of action of the two agents on the induction of DNA damage, as well as, the different response of the lymphocytes' DNA repair system.

Polymerase chain reaction-based deletion analysis of spontaneous and arsenite-enhanced gpt mutants in CHO-AS52 cells

In this study, we have examined the mutagenicity of sodium arsenite at the xanthine-guanine phosphoribosyltransferase locus (ypt) in a pSV2 gpt-transformed CHO cell line, AS52. Our results provide very weak evidence for arsenite as a gene mutagen because the chemical at high doses and at high cytotoxicity enhances barely a doubling of mutant frequency (MF) and a doubling of the gpt gene deletion frequency compared to controls. We suggest that the increase in MF in arsenite-treated cells results from arsenic, as comutagen, enhancing the induction effect of any unknown endogenous or exogenous factors on the spontaneous mutagenesis of AS52 cells. Nested PCR analysis mutants has a total deletion of the gpt gene. For the spontaneous, 50 microM arsenite- and 100 microM arsenite-enhanced spontaneous mutants in AS52 cells, the percentages of total deletion of the gpt gene are 36.00%, 54.72% and 66.67%, respectively. We suggest that a high proportion of the gene deletion in arsenite-enhanced mutants may be due to the high cytotoxicity of the chemical.

The molecular nature of spontaneous mutations at the hprt locus in the radiosensitive CHO mutant xrs-5

The radiosensitive mutant xrs-5, a derivative of the Chinese hamster ovary (CHO) K1 cell, is defective in DNA double-strand break rejoining ability and in V(D)J recombination. The radiosensitivity and defective repair phenotype are complemented by the 80-kDa subunit of the Ku protein. We determined the nature of the mutations that develop spontaneously at the hprt locus in this cell line using both multiplex PCR deletion screening and DNA sequencing. Ninety-two independent spontaneous mutants were analyzed and the results were compared to the mutation spectrum of 64 previously analyzed hprt spontaneous mutants isolated from the parental CHO-K1 cell line. More than 50% of the spontaneous xrs-5 mutants had lost one or more exons while less than 25% of spontaneous CHO-K1 mutants had lost one or more exons. Most of the deletions in xrs-5 cells involved the loss of multiple exons while single exon deletions predominated in CHO-K1. There was also a nonrandom distribution of breakpoints in both CHO-K1 and xrs-5. Most of the deletion breakpoints were 3' to exon 9, around exons 4-6, or near exon 1. Although the frequency of base substitutions was lower in xrs-5, the spectrum of base substitutions was qualitatively similar to that of CHO-K1. There was no significant difference in the spontaneous mutant frequency in xrs-5 and CHO-K1. The results suggest that in certain regions of the hprt gene, base alterations can be converted to large deletions, and that alterations in the Ku protein complex can influence this process.

Mutation induction in gamma-irradiated primary human bronchial epithelial cells and molecular analysis of the HPRT- mutants

We have examined various radiobiological parameters using commercially-available primary normal human bronchial epithelial (NHBE) cells, which can be subcultured more than 20 population doublings, and have established the mutation system in order to characterize the molecular changes in gamma-irradiated primary cells. The survival curve, obtained after irradiation of cells with 137Cs gamma-rays, indicates that the D0, Dq, and n values are 1.34 Gy, 1.12 Gy, and 2.3, respectively. The induction of HPRT- mutation was dose-dependent and the mutant fraction increased in a non-linear fashion. Since the doubling number of NHBE cells is limited, DNA was extracted directly from the single mutant colonies and alteration in the HPRT gene locus was analyzed using multiplex PCR technique. Among spontaneous mutants, the proportion with total and partial deletions of the gene was 10.0% (2/20) and 60.0% (12/20), respectively, while 30.0% (6/20) did not have any detectable changes in the nine exons examined. On the other hand, the fraction of total deletion increased by more than 2-fold among mutants induced by gamma-rays in that 26.3% (10/38) of them showed the total gene deletions. Twenty-five out of 38 gamma-induced mutants (65.8%) had partial deletions and 3 mutants (7.9%) had no detectable alteration. The present results showed that gamma-irradiation efficiently induced HPRT gene mutation in primary human epithelial cells and that most of the induced mutants suffered larger deletions compared to that observed in spontaneous mutants. This system provides an useful tool for determination of mutagenicity and understanding the molecular mechanisms of environmental carcinogens in primary human bronchial cells.