Applicability of radiosurgery with heavy ion beams to inactivate specific organs in living organisms

It was the aim of the study to test the applicability of radiosurgery in inactivating a specific organ through local irradiation with heavy ion beams. Silkworms were exposed to whole-body or local irradiation with carbon ion beams ((12)C(5+), 18.3 MeV/u, range=1.1 mm). After irradiation at the wandering stage, no significant differences were observed regarding either survival or cocoon quality between locally irradiated larvae and controls. Only localized effects were seen, such as the deletion of wings and functional disorders of the reproduction primordium, depending on both irradiation dose and site. This observation was not true for whole-body irradiated larvae. After local irradiation of the hemopoietic organs at the 4th instar premolting stage, the hemocyte densities were clearly reduced and the hemopoietic organ capacity was disrupted. The change in hemocyte densities was accompanied by changes of hemolymph components. These results show that radiosurgery utilizing heavy ion beams can destroy a specific organ or tissue in a living organism.

PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis

During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis

During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

Amylase hyperproduction by deregulated mutants of the thermophilic fungus Thermomyces lanuginosus

Thermomyces lanuginosus was subjected to three cycles of mutagenesis (UV/NTG) and a selection procedure to develop amylase-hyperproducing, catabolite-repression-resistant and partially constitutive strains. One of the selected derepressed mutant strain III(51), produced approximately 7- and 3-fold higher specific activity of alpha-amylase (190 U/mg protein) and glucoamylase (105 U/mg protein), respectively, compared to a wild-type parental strain. Further, the effect of production parameters on mutant strain III(51) was studied using a Box-Behnken design. The regression models computed showed significantly high R(2) values of 96 and 97% for alpha-amylase and glucoamylase activities, respectively, indicating that they are appropriate for predicting relationships between corn flour, soybean meal and pH with alpha-amylase and glucoamylase production.

Chromosomal aberrations, DNA strand breaks and gene mutations in nasopharyngeal cancer patients undergoing radiation therapy

Nasopharyngeal cancer (NPC) is a common disease in the south part of China, and its incidence is increasing in the southwest of China in recent years. Radiation therapy is the main therapeutic method for NPC in China. In this study, genetic changes were assessed in randomly selected nine NPC patients receiving radiation therapy by different genotoxical screening methods, the cytokinesis-block micronucleus test (CB-MNT), the buccal mucosa cell micronucleus test (BMC-MNT), the undivided lymphocyte micronucleus test (UL-MNT), chromosomal aberration (CA) test, the comet assay and the hprt gene mutation test (HPRT). Patients were used as self-control before receiving radiation therapy. Apart from the UL-MNT, all the methods detected genetic damages in NPC patients, though with different sensitivities. CB-MNT is the best biological indicator for evaluating genetic damage induced by radiation therapy in NPC patients; followed by CA and HPRT, while the BMC-MNT is simplest method as a potential biological indicator.

A new mouse mutant, skijumper

Low blood sugar levels are a well-known cause of severe illness and often death in newborn humans, especially those that are small for age. Few of the causes of neonatal hypoglycemia are known, and many remain to be found. We describe a novel mouse mutant, skijumper (skimp), in which pups, despite feeding well, have low levels of glucose and develop opisthotonos, followed by death typically within a few days after birth. Genetic mapping studies have localized the lesion to a approximately 1 cM interval on mouse Chromosome (Chr) 7 between D7Mit318 and D7Mit93. We have carried out extensive analysis to define the phenotype and its likely cause. In addition to low blood glucose, affected skijumper mice have lowglycogen and ketone levels. Mass spectrometric analysis of blood samples has excluded major defects in amino acid metabolism. Initial biochemical analyses suggested a defect in ketogenesis as one possible cause of this phenotype. However, measurements of levels and activities of carnitine, carnitine palmitoyl transferases, and other enzymes involved in ketogenesis, along with studies of mitochondrial structure and function, did not demonstrate significant differences between skijumper, unaffected littermates, and control wild-type mice. These results indicate that abnormal enzyme activity in known pathways does not appear to be the primary biochemical lesion in skijumper. The skijumper may be a new valuable model for studying and understanding one type of neonatal morbidity and death.

Circling, deafness, and yellow coat displayed by yellow submarine (ysb) and light coat and circling (lcc) mice with mutations on chromosome 3

We describe here two mouse mutants, yellow submarine (Ysb) and light coat and circling (Lcc). Ysb arose as the result of insertions of a transgene, pAA2, into the genome. Lcc is an independent, radiation-induced mutation. Both mutants are characterized by recessive circling behavior and deafness, associated with a non-segregating, semi-dominant yellow coat color. Complementation tests showed that Ysb and Lcc are allelic. We attribute the yellow coat in Ysb and Lcc mice to the absence of black awl overhairs, increased agouti zigzag underhairs, and the presence of agouti awls with long subapical yellow pigment. Chromosomal mapping and genomic characterization showed the Ysb and Lcc mutations involve complex chromosomal rearrangements in overlapping regions of mouse chromosome 3, A2/A3-B/C and B-E1, respectively. Ysb and Lcc show for the first time, to our knowledge, the presence of genes in the B-C region of chromosome 3 important for balance and hearing and the pigmentation and specification of coat hair.

Persistent induction of somatic reversions of the pink-eyed unstable mutation in F1 mice born to fathers irradiated at the spermatozoa stage

Untargeted mutation and delayed mutation are features of radiation-induced genomic instability and have been studied extensively in tissue culture cells. The mouse pink-eyed unstable (p(un)) mutation is due to an intragenic duplication of the pink-eyed dilution locus and frequently reverts back to the wild type in germ cells as well as in somatic cells. The reversion event can be detected in the retinal pigment epithelium as a cluster of pigmented cells (eye spot). We have investigated the reversion p(um) in F1 mice born to irradiated males. Spermatogonia-stage irradiation did not affect the frequency of the reversion in F1 mice. However, 6 Gy irradiation at the spermatozoa stage resulted in an approximately twofold increase in the number of eye spots in the retinal pigment epithelium of F1 mice. Somatic reversion occurred for the paternally derived p(un) alleles. In addition, the reversion also occurred for the maternally derived, unirradiated p(un) alleles at a frequency equal to that for the paternally derived allele. Detailed analyses of the number of pigmented cells per eye spot indicated that the frequency of reversion was persistently elevated during the proliferation cycle of the cells in the retinal pigment epithelium when the male parents were irradiated at the spermatozoa stage. The present study demonstrates the presence of a long-lasting memory of DNA damage and the persistent up-regulation of recombinogenic activity in the retinal pigment epithelium of the developing fetus.

Intestinal stem cells protect their genome by selective segregation of template DNA strands

The stem cells in the crypts of the small intestinal mucosa divide about a thousand times during the lifespan of a laboratory mouse, and yet they show little evidence of any decline in proliferative potential and rarely develop carcinogenic mutations, suggesting that their genome is extremely well protected. Protection against DNA-replication-induced errors can be achieved by the selective sorting of old (template) and new DNA strands with all template strands retained in the stem cell line. The template strands in the stem cells can be labelled during development or during tissue regeneration using tritiated thymidine (3HTdR). Labelling newly synthesised strands with a different marker (bromodeoxyuridine, BrdUrd) allows segregation of the two markers to be studied. Template strand label is retained(3HTdR), whereas label in the newly synthesised strands (BrdUrd) is lost following the second division of the stem cell. Random errors may occur in the template strands owing to environmental elements. These are protected against by the altruistic cell suicide (apoptosis) of the cells incurring such errors. A final level of protection for the tissue compensates for excessive deletion of stem cells via the apoptosis pathway. This is achieved by a hierarchical age structure in the stem cell compartment, with some cells being able to efficiently repair DNA damage and hence being more radioresistant. The presence of these protective mechanisms ensures that the small intestine rarely develops cancer and that stem cells can sustain the extensive cell proliferation needed during life.

Convergence of the fanconi anemia and ataxia telangiectasia signaling pathways

Fanconi anemia (FA) and ataxia telangiectasia (AT) are clinically distinct autosomal recessive disorders characterized by spontaneous chromosome breakage and hematological cancers. FA cells are hypersensitive to mitomycin C (MMC), while AT cells are hypersensitive to ionizing radiation (IR). Here, we identify the Fanconi anemia protein, FANCD2, as a link between the FA and ATM damage response pathways. ATM phosphorylates FANCD2 on serine 222 in vitro. This site is also phosphorylated in vivo in an ATM-dependent manner following IR. Phosphorylation of FANCD2 is required for activation of an S phase checkpoint. The ATM-dependent phosphorylation of FANCD2 on S222 and the FA pathway-dependent monoubiquitination of FANCD2 on K561 are independent posttranslational modifications regulating discrete cellular signaling pathways. Biallelic disruption of FANCD2 results in both MMC and IR hypersensitivity.

Uneven distributions of naïve and memory T cells in the CD4 and CD8 T-cell populations derived from a single stem cell in an atomic bomb survivor: implications for the origins of the memory T-cell pools in adulthood

The processes that lead to the establishment and maintenance of memory T-cell pools in humans are not well understood. In this study, we examined the emergence of naïve and memory T cells in an adult male who was exposed to an atomic bomb radiation dose of approximately 2 Gy in 1945 at the age of 17. The analysis presented here was made possible by our earlier observation that this particular individual carries a hematopoietic stem cell mutation at the hypoxanthine phosphoribosyltransferase (HPRT) locus that is almost certainly a result of his exposure to A-bomb radiation. Our key finding is that we detected a very much higher HPRT mutant frequency in the naive (CD45RA(+)) cell component of this individual's CD4 and CD8 T-cell populations than in the memory (CD45RA(-)) cell component of his CD4 and CD8 T-cell populations. This stands in marked contrast to our finding that HPRT mutant frequencies are fairly similar in the naïve CD45RA(+) and memory CD45RA(-) components of the CD4 and CD8 T-cell populations of three unexposed individuals examined concurrently. In addition we found that the HPRT mutant frequencies were about 30-fold higher in the naïve (CD45RA(+)) CD4 T cells of the exposed individual than in his memory (CD45RA(-)) cell populations, but that the effect was a little less striking in his CD8 cell populations, where the HPRT mutant frequencies were only about 15-fold higher in his naïve T-cell pools than in his memory T-cell pools. We further found that 100% of the HPRT mutant cells in both his CD4 and CD8 naïve cell subsets appeared to have originated from repeated divisions of the initial HPRT mutant stem cell, whereas only 4 of 24 and 5 of 6 mutant cells in his CD4 and CD8 memory cell subsets appeared to have originated from that same stem cell. The most straightforward conclusion may be that the great majority of the T cells produced by this individual since he was 17 years old have remained as naïve-type T cells, rather than having become memory-type T cells. Thus the T cells that have been produced from the hematopoietic stem cells of this particular A-bomb-exposed individual seldom seem to enter and/or to remain in the memory T-cell pool for long periods. We speculate that this constraint on entry into memory T-cell pools may also apply to unirradiated individuals, but in the absence of genetic markers to assist us in obtaining evidential support, we must await clarifying information from radically different experimental approaches.

Detection of genetic alterations induced by low-dose X rays: analysis of loss of heterozygosity for TK mutation in human lymphoblastoid cells

To elucidate the genetic influence of low-dose ionizing radiation at the chromosome level, we exposed human lymphoblastoid TK6-20C cells to 10 cGy of X rays. The TK mutation frequency was 5.7 +/- 1.3 x 10(-6) at the background level and 6.9 +/- 2.8 x 10(-6) after X irradiation. Although this small increase was not statistically significant (P = 0.40), we applied multilocus analysis using 4 TK locus markers and 12 microsatellite loci spanning chromosome 17 for TK mutants exhibiting loss of heterozygosity (LOH). The analysis demonstrated a clear effect of low-dose ionizing radiation. We observed radiation-specific patterns in the extent of hemizygous LOH in 14 TK mutants among the 92 mutants analyzed. The deleted regions in these patterns were larger than they were in the control mutants, where those restricted to the TK locus. Surprisingly, the radiation-specific LOH patterns were not observed among the 110 nonirradiated TK mutants in this study. They were identified previously in TK6 cells exposed to 2 Gy of X rays. We consider these hemizygous LOH mutants to be a result of end-joining repair of X-ray-induced DNA double-strand breaks.

HSM2 (HMO1) gene participates in mutagenesis control in yeast Saccharomyces cerevisiae

We have previously reported about a new Saccharomyces cerevisiae mutation, hsm2-1, that results in increase of both spontaneous and UV-induced mutation frequencies but does not alter UV-sensitivity. Now HSM2 gene has been genetically and physically mapped and identified as a gene previously characterized as HMO1, a yeast homologue of human high mobility group genes HMG1/2. We found that hsm2 mutant is slightly deficient in plasmid-borne mismatch repair. We tested UV-induced mutagenesis in double mutants carrying hsm2-1 mutation and a mutation in a gene of principal damaged DNA repair pathways (rad2 and rev3) or in a mismatch repair gene (pms1 and recently characterized in our laboratory hsm3). The frequency of UV-induced mutations in hsm2 rev3 was not altered in comparison with single rev3 mutant. In contrast, the interaction of hsm2-1 with rad2 and pms1 was characterized by an increased frequency of UV-induced mutations in comparison with single rad2 and pms1 mutants. The UV-induced mutation frequency in double hsm2 hsm3 mutant was lower than in the single hsm2 and hsm3 mutants. The role of the HSM2 gene product in control of mutagenesis is discussed.

Detection of radiation and cyclophosphamide-induced mutations in individual mouse sperm at a human expanded trinucleotide repeat locus transgene

A method to measure the germline mutations induced by cancer treatment in humans is needed. To establish such a method we used a transgenic mouse model consisting of a human DNA repeat locus that has a high spontaneous mutation frequency as a biomarker. Alterations in repeat number were measured in individual sperm from mice hemizygous for an expanded (CTG)(162) human myotonic dystrophy type 1 (DM1) microsatellite repeat using single genome-equivalent (g.e.) PCR and detection by a DNA fragment analyzer. Mutation frequencies were measured in DNA from sperm from controls and sperm derived from stem spermatogonia, differentiating spermatogonia, and spermatocytes exposed to radiation and from spermatocytes of mice treated with cyclophosphamide. There was no increase above control levels in mutations, scored as >1 repeat changes, in any of the treated groups. However, moderately large deletion mutants (between 9 and 20 repeat changes) were observed at frequencies of 2.2% when spermatocytes were treated with cyclophosphamide and, 1.8 and 2.5% when spermatocytes and stem cells, respectively, were treated with radiation, which were significantly higher than the frequency of 0.3% in controls. Thus, radiation and cyclophosphamide induced deletions in the expanded DM1 trinucleotide repeat. PCR artifacts were characterized in sperm DNA from controls and from mice treated with radiation; all artifacts involved losses of more than 20 DM1 repeats, and surprisingly the artifact frequency was higher in treated sperm than in control sperm. The radiation-induced increase in the frequency of PCR artifacts might reflect alterations in sperm DNA that destabilize the genome not only during PCR amplification but also during early embryonic development.

Glycophorin A mutant frequency in radiation workers at the nuclear power plants and a hospital

We studied to assess the validity of the glycophorin A (GPA) mutant assay as a biological marker of the cumulative effects of chronic low doses of ionizing radiation. In 144 nuclear power plants workers and 32 hospital workers, information on confounding factors, such as age and cigarette smoking, was obtained through a self-administered questionnaire. The information on physical exposure doses was obtained from the registries for radiation exposure monitoring and control at each facility. The range of cumulative exposure doses were 0-12.02cGy. GPA mutant assay was performed by the BR6 method with modification using a FACScan flow cytometer. Potential confounders, such as, age and cigarette smoking habits showed increasing trends with GPA variants, but were not of statistical significance. The hospital workers showed higher frequency of the GPA NO variant than nuclear power plant workers. Significant dose-response relationships were found between cumulative exposure to radiation and variants levels by simple and multiple linear regression models. The slope of regression equation of the dose-response of nuclear power plants workers was much smaller than that of hospital workers. These findings suggest that there may be dose-rate effects. In a population exposed to chronic low-dose radiation, the GPA assay shows potential to be used as an effective biologic marker for assessing the cumulative exposure dose although it could not be able to see a dose relation below 10cGy of cumulative exposure dose.

Induction and characterization of mutations at the b locus of the medaka, Oryzias latipes

The b locus is one of the most familiar pigmentation loci in the medaka, but its biochemical function is still unknown. Here we report induction of new mutations at the b locus by radiation and ENU. We also characterized all these mutations and previously isolated spontaneous ones on the phenotypic basis. Unexpectively, all the 18 induced mutations reduced melanin contents in both eyes and skin correlatively, although degree of reduction was varied from mutations to mutations. Moreover, presumed null mutants (bs8, bg8, bc2, bd3, bd6, bg13, bg19, bg24) had slightly melanized (dark red) eyes. These results suggest that the b-locus product plays an important but not a critical role in melanogenesis. The spontaneous mutants were divided into two types: one (bdl2, bdl3, and bp) had similarities with the induced mutants in that they had slightly colored eyes and skin, the other (bv, B', bd, bdl1, and b) exhibited normally black eyes but lightly colored skin. The present study supports our recent results (Fukamachi et al., 2001) that mutational changes were found in the coding region of the b gene in some of the mutants which reduced both eyes and skin melanogenesis, while the mutational change for the b allele could not be found there. We speculate that the bv, B', bd, bdl1, and b alleles might arise by the mutations in the regulatory region for skin melanogenesis.

UV-A induces persistent genomic instability in human keratinocytes through an oxidative stress mechanism

Ultraviolet-A (UV-A, 320 to 400 nm) radiation comprises 95% of the solar ultraviolet radiation (UVR) reaching the earth's surface. It has been associated experimentally and epidemiologically with malignant melanoma. In this study we investigated whether UV-A radiation can induce a persistent, heritable hypermutability in mammalian cells similar to that observed following ionising radiation (IR). Using the immortalized human skin keratinocyte cell line HaCaT we found that UV-A radiation does lead to a continuing reduction in plating efficiency, an increased "spontaneous" mutant fraction, and an increase in micronucleus formation up to 21 d after initial exposure. Reversal of these effects using catalase may indicate a role for hydrogen peroxide in this phenomenon. These results add to the significance of UV-A radiation as a risk factor in skin carcinogenesis.

UV irradiation causes the loss of viable mitotic recombinants in Schizosaccharomyces pombe cells lacking the G(2)/M DNA damage checkpoint

Elevated mitotic recombination and cell cycle delays are two of the cellular responses to UV-induced DNA damage. Cell cycle delays in response to DNA damage are mediated via checkpoint proteins. Two distinct DNA damage checkpoints have been characterized in Schizosaccharomyces pombe: an intra-S-phase checkpoint slows replication and a G(2)/M checkpoint stops cells passing from G(2) into mitosis. In this study we have sought to determine whether UV damage-induced mitotic intrachromosomal recombination relies on damage-induced cell cycle delays. The spontaneous and UV-induced recombination phenotypes were determined for checkpoint mutants lacking the intra-S and/or the G(2)/M checkpoint. Spontaneous mitotic recombinants are thought to arise due to endogenous DNA damage and/or intrinsic stalling of replication forks. Cells lacking only the intra-S checkpoint exhibited no UV-induced increase in the frequency of recombinants above spontaneous levels. Mutants lacking the G(2)/M checkpoint exhibited a novel phenotype; following UV irradiation the recombinant frequency fell below the frequency of spontaneous recombinants. This implies that, as well as UV-induced recombinants, spontaneous recombinants are also lost in G(2)/M mutants after UV irradiation. Therefore, as well as lack of time for DNA repair, loss of spontaneous and damage-induced recombinants also contributes to cell death in UV-irradiated G(2)/M checkpoint mutants.

Tissue-specific deletion and discontinuous loss of heterozygosity are signatures for the mutagenic effects of ionizing radiation in solid tissues

The mouse Aprt locus on chromosome 8 was used as the selectable target for the study of spontaneous and ionizing radiation-induced mutations in kidney epithelia and ear fibroblasts. Fifty-two Aprt heterozygous mice were exposed to 7.5 Gy of (137)Cs-gamma radiation on their right sides, and Aprt-deficient clones were isolated from enzymatically digested tissues at times ranging from 1 day to 14 months after irradiation. A statistically significant increase in the mutant frequencies for the irradiated tissues was observed when compared with the spontaneous mutant frequencies for the nonirradiated tissues. A molecular analysis of spontaneous mutations observed for the nonirradiated tissues revealed tissue-specific differences; apparent chromosome loss was common in kidney mutants but infrequent in the ear mutants, whereas apparent deletions were common in the ear mutants but not detected in the kidney mutants. For the irradiated kidneys, apparent deletions were observed commonly demonstrating that these events are markers for ionizing radiation mutagenesis in this tissue. All of the loss of heterozygosity (LOH) tracts observed in the spontaneous mutants were continuous, but discontinuous LOH patterns were observed in 6--8% of ionizing radiation-induced ear and kidney cell mutants. Work with kidney-derived cell lines showed that discontinuous LOH is a novel signature for delayed ionizing radiation mutagenesis. Considered together, these results suggest that ionizing radiation-induced mutations in vivo can result from both direct and delayed mutagenic effects.

Effects of irradiated medium with or without cells on bystander cell responses

Recent studies have indicated that extranuclear or extracellular targets are important in mediating the bystander genotoxic effects of alpha-particles. In the present study, human-hamster hybrid (A(L)) cells were plated on either one or both sides of double-mylar dishes 2-4 days before irradiation, depending on the density requirement of experiments. One side (with or without cells) was irradiated with alpha-particles (from 0.1 to 100 Gy) using the track segment mode of a 4 MeV Van de Graaff accelerator. After irradiation, cells were kept in the dishes for either 1 or 48 h. The non-irradiated cells were then collected and assayed for both survival and mutation. When one side with cells was irradiated by alpha-particles (1, 10 and 100 Gy), the surviving fraction among the non-irradiated cells was significantly lower than that of control after 48 h co-culture. However, such a change was not detected after 1h co-culture or when medium alone was irradiated. Furthermore, co-cultivation with irradiated cells had no significant effect on the spontaneous mutagenic yield of non-irradiated cells collected from the other half of the double-mylar dishes. These results suggested that irradiated cells released certain cytotoxic factor(s) into the culture medium that killed the non-irradiated cells. However, such factor(s) had little effect on mutation induction. Our results suggest that different bystander end points may involve different mechanisms with different cell types.

Photocarcinogenesis in human adult skin grafts

It has been demonstrated previously that the exposure to 7,12-dimethyl[a]benzanthracene (DMBA) and UVB radiation leads to the development of epidermal cysts, squamous cell carcinomas (SCC), melanocytic hyperplasia and melanoma in human foreskins from newborns grafted to immunodeficient mice. Improved techniques in grafting full-thickness skin from adults have enabled us to study photocarcinogenesis in human skin from different body sites and from older donors. One hundred and fifty-five normal white skin specimens from the trunk and face of 53 adult individuals were grafted onto severe combined immunodeficient (SCID) and recombinase activating gene-1 (Rag-1) knockout mice and irradiated two to three times weekly with 40 mJ/cm(2) UVB or solar-simulated UV (SSUV) over a period of up to 10 months with or without one prior topical application of DMBA. Over an observation period of 2-22 months, histopathological and immunohistochemical analyses of 134 specimens revealed actinic keratoses in 30% of the DMBA- + UV-treated grafts, in 18% of the grafts exposed to SSUV only, and in 10% of the grafts exposed to UVB only. Actinic keratoses were absent in grafts treated with DMBA only. One SCC was found in an abdominal skin graft 3 months after exposure to DMBA followed by UVB. Point mutations in codon 61 of the human Ha-ras gene were detected in the SCC, five of six analyzed actinic keratoses and in non-lesional epidermis of DMBA- and UVB-treated grafts, indicating that DMBA as well as UVB alone can induce these mutations in human skin. In contrast to the previous experience with neonatal foreskin grafts, melanocytic lesions were not found except for mild hyperplasia in few cases. The data suggest that melanocytes from young individuals are more susceptible to the transforming effects of genotoxic agents than melanocytes from adults.

[Selection of beta-glucanase-producing Trichoderma köningii T199 and its fermentation conditions]

A mutant strain T199 producing about 8 times as much beta-glucanase as its parent strain trichoderma köningii T3 was obtained by treatment with ultraviolet light and N-methyl-N-nitro-N-nitrosoguanidine. Fermentation was conducted in 250 mL flask, each containing 30 mL of medium consisted of 5% corn cob powder, 3% wheat bran and 1.4% nitrogen source No. 10 ((NH4)2SO4 10%, Peptone 20%, Yeast extract 15%). The optima culture conditions were as below: initial pH 5.0, 30 degrees C, shaking speed 280 r/min, and cultivation time 5 d. Enzyme activity toward CMC-Na, lichenin, laminarin and barley beta-glucan at pH 5.0 and 60 degrees C for 10 min were 300, 1100, 12 and 1600 IU/mL, respectively. The optima pH and temperature for enzyme action toward barley beta-glucan were pH 5.0 and 70 degrees C, respectively. The enzyme was stable under below 50 degrees C and at pH 4.5-6.5.

Mutations in mammalian cells induced by heavy charged particles: an indicator for risk assessment in space

Induction of mutations at the HPRT locus in V79 Chinese hamster cells by heavy charged particles is reviewed. A unique dependence of mutation induction cross sections on LET or any other physical parameter of the particles cannot be found. There is, however, a general trend showing an increase up to about 100 keV/microm and an indication of a saturation with higher LET. An empirical expression is given approximating the data which may be useful for practical purposes. On the basis of this expression, it is suggested that the formula used by the ICRP for the quality factor overestimates the risk for very heavy particles. This may have implications for the assessment of the risk of exposure to iron ions in space.

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.

The mechanism of action of DNA photolyases

Structural analysis, biochemistry and model studies have provided new insights into the mechanism of action of photolyases. The light-driven electron and energy transfer events that lead to the photolyase-catalyzed repair of lethal, mutagenic and carcinogenic UV-light-induced DNA lesions have all been examined in the past few years.

Very high mutation rate in offspring of Chernobyl accident liquidators

Exposure to ionizing radiation has long been suspected to increase mutation load in humans. Nevertheless, such events as atomic bombing seem not to have yielded significant genetic defects. The Chernobyl accident created a different, long-term exposure to radiation. Clean-up teams (or 'liquidators') of the Chernobyl reactor are among those who received the highest doses, presumably in some combination of acute and chronic forms. In this study, children born to liquidator families (currently either in the Ukraine or Israel) conceived after (CA) parental exposure to radiation were screened for the appearance of new fragments using multi-site DNA fingerprinting. Their sibs conceived before (CB) exposure served as critical internal controls, in addition to external controls (non-exposed families). An unexpectedly high (sevenfold) increase in the number of new bands in CA individuals compared with the level seen in controls was recorded. A strong tendency for the number of new bands to decrease with elapsed time between exposure and offspring conception was established for the Ukrainian families. These results indicate that low doses of radiation can induce multiple changes in human germline DNA.

X-ray induction of microsatellite instability at autosomal loci in human lymphoblastoid WTK1 cells

Many models of carcinogenesis posit that multiple genetic events are required for a normal cell to become cancerous. As the mutation rate of a single gene is in the range of 10(-8) to 10(-5) per cell division, a central question remains, how does a single cell acquire multiple mutations? One hypothesis, originally articulated by Loeb [10], proposed that some mutations may not be isolated events, but are associated with a mutator phenotype that leads to the occurrence of additional mutations elsewhere in the cellular genome. To test this hypothesis, we utilized a human lymphoblastoid cell line (WTK1) that is known to be hypermutable at the autosomal thymidine kinase (TK) locus. We isolated 139 independent clones which were selected for new TK mutations that arose either spontaneously or as the result of a single X-ray exposure of 1.5Gy. These clones were examined for second-site alterations in several microsatellite loci scattered throughout the genome using polymerase chain reaction (PCR) amplification followed by both denaturing gel electrophoresis and single-stranded conformational polymorphism (SSCP) analysis. Of these clones, 21 exhibited second-site mutations primarily involving loss of heterozygosity, 17 arose from irradiated cells whereas the remaining four arose from non-irradiated cells. We further examined the 17 clones which exhibited alterations specifically at the D16S265 locus; alterations at this site were associated with an enhanced frequency of mutations at other loci in the same region of chromosome 16q, but were not associated with additional mutations at other sites in the genome. Furthermore, new mutations arose in loci on 16q when these clones were propagated for 6 months in culture. Overall, these results support the hypothesis that radiation can induce a type of genetic instability which may facilitate the occurrence of multiple mutations throughout the genome in a small population of exposed cells. Furthermore, some cells may possess localized regions in the genome which are highly sensitive to the induction of instability.

[The interaction of the rad201 gene with mei-9 and mei-41 genes in germline cells of Drosophila female]

The effect of Drosophila mutation rad201G1 together with mutations mei-41D5 and mei-9a on the sensitivity of oocytes to induction of dominant lethals (DLs) was studied. To this end, the frequencies of spontaneous and gamma-radiation-induced DLs in consecutive egg batches of females carrying double or single mutations were estimated. Since the effects of the mutations examined are expressed only at the previtellogenetic stages of oogenesis, only newly hatched (0-5-hour-old) females, whose oocytes did not develop farther than stage 7, were irradiated. The results obtained indicated that in intact and irradiated oocytes of double mutants mei-9a rad201G1 and mei-41D5 rad201G1, mutation rad201G1 epistatically suppresses the mutations of the both mei genes.

Risk of stillbirth in offspring of men exposed to ionising radiation

Radiation genetic risk models are employed to predict the frequency of radiation-related stillbirths to partners of occupationally exposed male workers, using the incidence data recently reported by Parker et al from an epidemiological study of Cumbrian births. Expanding on previously developed conservative risk estimates suggests that, of the 130 observed stillbirths to partners of male radiation workers, 0.3 cases would be attributable to paternal preconceptional irradiation, in contrast to the 17.5 (95% confidence interval: 3.1 to 31.9) cases predicted by Parker et al from their preferred dose-response model. The incompatibility of the results reported by Parker et al with those from other investigations, both epidemiological and experimental, and the inability of the study to consider a number of factors which might affect stillbirth rates, particularly those relating to the mother, make it difficult to accept that paternal irradiation received occupationally could have contributed to a detectable increase in stillbirths.

Cell killing and mutation induction by heavy ion beams

Carbon beam radiotherapy for cancer patients was initiated in Japan in June 1994. This study attempts to clarify the radiobiological effects of heavy ion beams. In this study, human cancer cell lines (RMG-1, MDA-MB231) and V79 cells were used. The cell killing was determined by colony forming assay, and mutation induction was determined by counting the number of 6-thioguanine resistant colonies (hprt locus mutation assay). The cell lines were irradiated with carbon (20 or 80 keV/microm) or neon beams (80 keV/microm). Carbon ions with a higher LET value (80 keV/microm) had an enhanced cytotoxic effect compared to those with a lower LET value (20 keV/microm). Carbon beams produced a slightly stronger cytotoxic effect than neon beams when irradiated at the same LET level (80 keV/microm), but the difference was not remarkable. The mutant fraction was significantly higher in all cell lines when they were irradiated with heavy ion beams, compared to the results for X-ray irradiation. The mutant fraction increased when the LET of the carbon beams increased. At equivalent LET values, the mutant fraction was lower for neon beams than for carbon beams. Fractionation of carbon beam irradiation had no effect on survival, but reduced the mutant fraction. Neon beams might be more appropriate for heavy ion therapy, especially when higher doses are being used. In addition, the fractionation of heavy ion beam administration might be appropriate for reducing the mutant fraction.

Mutagen sensitivity of human lymphoblastoid cells with a BRCA1 mutation in comparison to ataxia telangiectasia heterozygote cells

Our previous results indicated a close relationship between the presence of a BRCA1 mutation in lymphocytes and hypersensitivity for the induction of micronuclei by gamma irradiation and hydrogen peroxide (H(2)O(2)). Comparative investigations with the comet assay (single-cell gel electrophoresis) suggested a normal rate of damage removal and pointed to a disturbed fidelity of DNA repair as a direct or indirect consequence of a BRCA1 mutation. We now wanted to see whether similar results could be obtained with lymphoblastoid cell lines (LCLs) and whether such permanent cells are suitable as a model for the investigation of mechanisms involved in mutagen sensitivity. Our results show that LCLs with a BRCA1 mutation are also hypersensitive to the chromosome-damaging effects of gamma irradiation or H(2)O(2), as revealed by the micronucleus test. Interestingly, LCLs heterozygous for an ataxia telangiectasia (AT) mutation have similar characteristics as BRCA1 cells with respect to the induction and repair of DNA damage induced by either gamma irradiation or H(2)O(2). However, caffeine enhanced the induction of micronuclei by gamma irradiation only in normal and heterozygous AT cells but not in BRCA1 cells, thus indicating a difference in the pathways leading to mutagen sensitivity in cells with a BRCA1 or an AT mutation. Our results suggest that caffeine could be useful in discriminating AT heterozygotes from carriers of a BRCA1 mutation, as well as BRCA1 mutation carriers from normal individuals.

Fluorescence-based directed termination PCR: direct mutation characterization without sequencing

We describe a fluorescence-based directed termination PCR (fluorescent DT-PCR) that allows accurate determination of actual sequence changes without dideoxy DNA sequencing. This is achieved using near infrared dye-labeled primers and performing two PCR reactions under low and unbalanced dNTP concentrations. Visualization of resulting termination fragments is accomplished with a dual dye Li-cor DNA sequencer. As each DT-PCR reaction generates two sets of terminating fragments, a pair of complementary reactions with limiting dATP and dCTP collectively provide information on the entire sequence of a target DNA, allowing an accurate determination of any base change. Blind analysis of 78 mutants of the supF reporter gene using fluorescent DT-PCR not only correctly determined the nature and position of all types of substitution mutations in the supF gene, but also allowed rapid scanning of the signature sequences among identical mutations. The method provides simplicity in the generation of terminating fragments and 100% accuracy in mutation characterization. Fluorescent DT-PCR was successfully used to generate a UV-induced spectrum of mutations in the supF gene following replication on a single plate of human DNA repair-deficient cells. We anticipate that the automated DT-PCR method will serve as a cost-effective alternative to dideoxy sequencing in studies involving large-scale analysis for nucleotide sequence changes.

Homeotic evidence for the appendicular origin of the labrum in Tribolium castaneum

The ontogeny of the insect labrum, or upper lip, has been debated for nearly a century. Recent molecular data suggest a segmental appendage origin of this structure. Here we report the first arthropod mutation associated with a homeotic transformation of the labrum. Antennagalea-5 (Ag(5)) transforms both antennal and labral structures to resemble those of gnathal appendages in Tribolium castaneum. This labral transformation suggests that the labrum is a fused structure composed of two pairs of appendage endites, and is serially homologous to the gnathal appendages.

Evaluation of three somatic genetic biomarkers as indicators of low dose radiation effects in clean-up workers of the Chernobyl nuclear reactor accident

The goals of this study were to assess three biomarkers of genetic effect for their individual and collective ability to detect and estimate radiation exposure in Russian Chernobyl clean-up workers. Work assignments were planned to limit dose to 0.25 Gy. The three biomarkers employed were chromosome translocations detectcd in lynmphocytes by florescence in situ hybridisation (FISH), and mutation at two genes, glycophorin A (GPA) in red blood cells detected by flow cytometry and hypoxanthine phosphoribosyltransferase (HPRT) in lymphocytes detected by selective cell culture. Samples were Obtained from 1992 to 2000. The time between exposure at Chernobyl and sample acquisition was > or =5 years. The lymphocyte assays detected an elevation over controls in average outcomes it clean-up workers: translocation rates were 46% higher when adjusted for age and smoking and HPRT mutant frequencies were were 16% higher when adjusted for age. The G PA assay did not detect an exposure effect. The results indicate that measuring frequency of translocations by FISH is preferred for low dose radiation, retrospective biochemistry.

A genetic engineering approach to genetic algorithms

We present an extension to the standard genetic algorithm (GA), which is based on concepts of genetic engineering. The motivation is to discover useful and harmful genetic materials and then execute an evolutionary process in such a way that the population becomes increasingly composed of useful genetic material and increasingly free of the harmful genetic material. Compared to the standard GA, it provides some computational advantages as well as a tool for automatic generation of hierarchical genetic representations specifically tailored to suit certain classes of problems.

Benzo(a)pyrene and X-rays induce reversions of the pink-eyed unstable mutation in the retinal pigment epithelium of mice

The pink-eyed unstable (p(un)) mutation is the result of a 70kb tandem duplication within the murine p gene. Homologous deletion/recombination of the locus to wild-type occurs spontaneously in embryos and results in pigmented spots in the fur and eye that persist for life. Such deletion events are also inducible by a variety of DNA damaging agents, as we have observed previously with the fur spot assay. Here, we describe the use of the retinal pigment epithelium (RPE) of the eye to detect reversion events induced with two differently acting agents. Benzo(a)pyrene (B(a)P) induces a high frequency, and X-ray exposure a more modest increase, of p(un) reversion in both the fur and the eye. The eye-spot assay requires fewer mice for significant results than the fur spot assay. Previous work had elucidated the cell proliferation pattern in the RPE and a position effect variegation phenotype in the pattern of p(un) reversions, which we have confirmed. Acute exposure to B(a)P or X-rays resulted in an increased frequency of reversion events. The majority of the spontaneous reversions lie toward the periphery of the RPE whereas induced events are found more centrally, closer to the optic nerve head. The induced distribution corresponds to the major sites of cell proliferation in the RPE at the time of exposure, and further advocates the proposal that dividing cells are at highest risk to develop deletions.

A role for MHR1, a gene required for mitochondrial genetic recombination, in the repair of damage spontaneously introduced in yeast mtDNA

A nuclear recessive mutant in Saccharomyces cerevisiae, mhr1-1, is defective in mitochondrial genetic recombination at 30 degrees C and shows extensive vegetative petite induction by UV irradiation at 30 degrees C or when cultivated at a higher temperature (37 degrees C). It has been postulated that mitochondrial DNA (mtDNA) is oxidatively damaged by by-products of oxidative respiration. Since genetic recombination plays a critical role in DNA repair in various organisms, we tested the possibility that MHR1 plays a role in the repair of oxidatively damaged mtDNA using an enzyme assay. mtDNA isolated from cells grown under standard (aerobic) conditions contained a much higher level of DNA lesions compared with mtDNA isolated from anaerobically grown cells. Soon after a temperature shift from 30 to 37 degrees C the number of mtDNA lesions increased 2-fold in mhr1-1 mutant cells but not in MHR1 cells. Malonic acid, which decreased the oxidative stress in mitochondria, partially suppressed both petite induction and the temperature-induced increase in the amount of mtDNA damage in mhr1-1 cells at 37 degrees C. Thus, functional mitochondria require active MHR1, which keeps the extent of spontaneous oxidative damage in mtDNA within a tolerable level. These observations are consistent with MHR1 having a possible role in mtDNA repair.

The 26S proteasome negatively regulates the level of overall genomic nucleotide excision repair

Regulation of protein expression can be achieved through destruction of proteins by the 26S: proteasome. Cellular processes that are regulated by proteolysis include cell cycle progression, stress responses and differentiation. Several nucleotide excision repair proteins in yeast and humans, such as Rad23, Rad4 and XPB, have been shown to co-purify with Cim3 and Cim5, AAA ATPases of the 19S: proteasome regulatory subunit. However, it has not been determined if nucleotide excision repair is regulated through protein destruction. We measured nucleotide excision repair in yeast mutants that are defective in proteasome function and found that the repair of the transcribed and non-transcribed strands of an RNA polymerase II-transcribed reporter gene was increased in the absence of proteasome function. Additionally, overexpression of the Rad4 repair protein, which is bound to the repair/proteolytic factor Rad23, conferred higher rates of nucleotide excision repair. Based on our data we suggest that a protein (or proteins) involved in nucleotide excision repair or in regulation of repair is degraded by the 26S proteasome. We propose that decreased proteasome function enables increased DNA repair, due to the transient accumulation of a specific repair factor, perhaps Rad4.

Mutant alleles of Schizosaccharomyces pombe rad9(+) alter hydroxyurea resistance, radioresistance and checkpoint control

Schizosaccharomyces pombe rad9 mutations can render cells sensitive to hydroxyurea (HU), gamma-rays and UV light and eliminate associated checkpoint controls. In vitro mutagenesis was performed on S.pombe rad9 and altered alleles were transplaced into the genome to ascertain the functional significance of five groups of evolutionarily conserved amino acids. Most targeted regions were changed to alanines, whereas rad9-S3 encodes a protein devoid of 22 amino acids normally present in yeast but absent from mammalian Rad9 proteins. We examined whether these rad9 alleles confer radiation and HU sensitivity and whether the sensitivities correlate with checkpoint control deficiencies. One rad9 mutant allele was fully active, whereas four others demonstrated partial loss of function. rad9-S1, which contains alterations in a BH3-like domain, conferred HU resistance but increased sensitivity to gamma-rays and UV light, without affecting checkpoint controls. rad9-S2 reduced gamma-ray sensitivity marginally, without altering other phenotypes. Two alleles, rad9-S4 and rad9-S5, reduced HU sensitivity, radiosensitivity and caused aberrant checkpoint function. HU-induced checkpoint control could not be uncoupled from drug resistance. These results establish unique as well as overlapping functional domains within Rad9p and provide evidence that requirements of the protein for promoting resistance to radiation and HU are not identical.

Anaplastic changes associated with p53 gene mutation in differentiated thyroid carcinoma after insufficient radioactive iodine (131I) therapy

Thirty-two patients with differentiated thyroid carcinomas with distant metastasis were examined using a radioactive iodine (131I) tracer dose prior to 131I therapy and followed up for 10 years or until death (whichever occurred first). Nineteen patients who received 131I therapy had an accumulation of 131I in the metastases (group I) and 15 of those patients were alive more than 10 years after the first 131I treatment. In contrast, all 13 patients in whom the metastases did not show accumulation of 131I died within 10 years. Of the latter group, eight patients had received 131I therapy (group II), four of whom died with anaplastic changes within 5 years of treatment. p53 gene mutation was identified by immunohistochemistry in primary thyroid carcinoma tissue from patients with anaplastic changes that were evident during total thyroidectomy. Five patients did not receive 131I therapy (group III), of whom one, who also had a p53 gene mutation in the original tumor, died with anaplastic change 10 years after thyroidectomy. Seven patients in group I had p53 gene mutations in their thyroid carcinoma tissues, but none showed anaplastic changes. Our results suggest that 131I therapy may be useful for patients with distant metastases, with or without p53 gene mutations, which show accumulation of 131I from tracer and therapeutic doses. In contrast, 131I therapy is apparently not effective in patients who do not show sufficient accumulation of 131I, but rather, may cause early anaplastic changes with a p53 gene mutation.

Oxidative DNA base damage induced by singlet oxygen and photosensitization: recognition by repair endonucleases and mutagenicity

We have analyzed the recognition by various repair endonucleases of DNA base modifications induced by three oxidants, viz. [4-(tert-butyldioxycarbonyl)benzyl]triethylammonium chloride (BCBT), a photochemical source of tert-butoxyl radicals, disodium salt of 1,4-etheno-2,3-benzodioxin-1,4-dipropanoic acid (NDPO(2)), a chemical source of singlet oxygen, and riboflavin, a type-I photosensitizer. The base modifications induced by BCBT, which were previously shown to be mostly 7,8-dihydro-8-oxoguanine (8-oxoGua) residues, were recognized by Fpg and Ogg1 proteins, but not by endonuclease IIII, Ntg1 and Ntg2 proteins. In the case of singlet oxygen induced damage, 8-oxoGua accounted for only 35% of the base modifications recognized by Fpg protein. The remaining Fpg-sensitive modifications were not recognized by Ogg1 protein and relatively poor by endonuclease III, but they were relatively good substrates of Ntg1 and Ntg2. In the case of the damage induced by photoexcited riboflavin, the fraction of Fpg-sensitive base modifications identified as 8-oxoGua was only 23%. In contrast to the damage induced by singlet oxygen, the remaining lesions were not only recognized by Ntg1 and Ntg2 proteins and (relatively poor) by endonuclease III, but also by Ogg1 protein. The analysis of the mutations observed after transfection of modified plasmid pSV2gpt into Escherichia coli revealed that all agents induced near exclusively GC-->TA and GC-->CG transversions, the numbers of which were correlated with the numbers of 8-oxoGua residues and Ntg-sensitive modifications, respectively. In conclusion, both singlet oxygen and the type-I photosensitizer riboflavin induce predominantly oxidative guanine modifications other than 8-oxoGua, which most probably give rise to GC-->CG transversions and in which eukaryotic cells are substrates of Ntg1 and Ntg2 proteins.

Increased frequencies of gene and chromosome mutations after X-irradiation in mouse embryonal carcinoma cells transfected with the bcl-2 gene

Preimplantation stage mouse embryos are known to be highly sensitive to the killing effect of DNA-damaging agents such as radiation. Interestingly, however, this stage of development is well protected from radiation induction of malformation and carcinogenesis in postnatal life. In recent years, it has become clear that the stem cells of preimplantation stage embryos undergo extensive apoptosis after DNA damage. It has been postulated that this apoptosis is likely to be responsible for the resistance to malformation, by excluding cells carrying deleterious DNA damage. We have tested the possible role of apoptosis in elimination of gene and chromosome mutations in undifferentiated mouse embryonal carcinoma cell line, F9, transfected with human bcl-2 cDNA. The colony radiosensitivity of F9 cells was not affected by overexpression of the bcl-2 gene, but the apoptotic cell death was suppressed, as examined by DNA ladder assay and Hoechst staining. This suppression was accompanied by an increase in the frequencies of hprt mutation and micronucleus formation after X-irradiation. These results support the idea that maintenance of genomic integrity during early development is likely to be executed by apoptotic elimination of cells at risk.

Increase in X-ray-induced mutations by exposure to magnetic field (60 Hz, 5 mT) in NF-kappaB-inhibited cells

It is established that extremely low frequency magnetic fields (ELFMF) at the flux densities, i.e., 5 mT and less, are not mutagenic. However, exposure to ELFMF enhances mutations induced by X-rays. In this study, we examined the effects of long-term exposure to 5 mT ELFMF on mutation induction and X-ray-induced mutations in human malignant glioma cells (MO54) with different mutant IkappaB-alpha (a critical inhibitor of NF-kappaB) genes. Cells were exposed or sham-exposed to 5 mT ELFMF for up to 8 days with or without initial X-rays (4 Gy), and the mutant frequency of hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene was analyzed. An obvious increase in X-ray-induced mutations was observed after treatment with ELFMF in combination with X-irradiation in MO54 cells with tyrosine mutant IkappaB-alpha gene other than with serine mutant IkappaB-alpha gene or vector alone. Exposure to ELFMF alone increased mutations significantly in MO54 cells with tyrosine mutant IkappaB-alpha gene. In addition, X-ray-induced apoptoic cells were increased in MO54-V cells after exposure to ELFMF, while an anti-apoptotic effect of magnetic field was found in MO54-SY4 cells. Our data suggest that exposure to 5 mT ELFMF may induce mutations and enhance X-ray-induced mutations, resulting from the inactivation of NF-kappaB through the inhibition of tyrosine phosphorylation.

[Potential relationship between mutation process induced by low doses of ionizing radiation, and positional dynamics of chromosomes in nuclei of eukaryotic cells]

The mutation process has many stages. The information presented in this article suggest that a cell exposed to low LET radiation in the low-dose range (up to 1 cGy) must almost completely repair all spontaneous and radiation-induced DNA lesions. But reparation of DNA double-stranded breaks (DSB), which are the basis of genome instability has peculiarity. We have shown that the mechanisms of action of low doses (which initiate natural antimutagenic reactions of resting cells--an adaptive response) are associated with chromosome loci (centromere) movement in a cell nucleus. We suggest that the movement of chromosome loci in cell nucleus is the fundamental mechanism for repair of DSB and switching of the transcription of gene (it is known that in case of lymphoid cells Ikaros-complexes repressor is colocolizated with centromere loci); in particular, of nucleolar transcription activities because the latter is dependent on centromere arrangement. Because the movement of chromosome loci in both the mitotic cycle and under adapting dose on resting cells is much the same it could be assumed that in latter case the cells also lose their functional characteristic for differentiated resting cells. Under chronic exposure to low doses the functional changes can be the cause of organic changes if adapting dose affects the sufficient part of the cells. The role of cells of evolutional or ontogenetic reserve in mutation process is considered.

The role of nucleotide excision repair of Escherichia coli in repair of spontaneous and gamma-radiation-induced DNA damage in the lacZalpha gene

Base excision repair (BER) is a very important repair mechanism to remove oxidative DNA damage. A major oxidative DNA damage after exposure to ionizing radiation is 7,8-dihydro-8-oxoguanine (8oxoG). 8oxoG is a strong mutagenic lesion, which may cause G:C to T:A transversions if not repaired correctly. Formamidopyrimidine-DNA glycosylase (Fpg), a repair enzyme which is part of BER, is the most important enzyme to repair 8oxoG. In the past years, evidence evolved that nucleotide excision repair (NER), a repair system originally thought to repair only bulky DNA lesions, can also repair some oxidative DNA damages. Examples of DNA damages which are recognized by NER are thymine glycol and abasic sites (AP sites). The main objective of this study is to determine if NER can act as a backup system for the repair of spontaneous and gamma-radiation-induced damages when Fpg is deficient. For that purpose, the effect of a NER-deficiency on the spontaneous and gamma-radiation-induced mutation spectrum in the lacZ gene was determined, using double-stranded (ds) M13 DNA, with the lacZalpha gene inserted as mutational target sequence. Subsequently the DNA was transfected into a fpg(-)uvrA(-) Escherichia coli strain (BH420) and the mutational spectra were compared with the spectra of a fpg(-) E. coli strain (BH410) and a wild type E. coli strain (JM105), which were determined in an earlier study. Furthermore, to examine effects which are caused by UvrA-deficiency, and not by Fpg-deficiency, the spontaneous and gamma-radiation-induced mutation spectra of an E. coli strain in which only UvrA is deficient (BH430) were also determined and compared with a wild type E. coli strain (JM105). The results of this study indicate that if only UvrA is deficient, there is an increase in spontaneous G:C to T:A transversions as compared to JM105 and a decrease in A:T to G:C transitions. The gamma-radiation-induced mutation spectrum of BH420 (fpg(-)uvrA(-)) shows a significant decrease in G:C to A:T and G:C to T:A mutations, as compared to BH410 where only Fpg is deficient. Based on these results, we conclude that in our experiments NER is not acting as a backup system if Fpg is deficient. Instead, NER seems to make mistakes, leading to the formation of mutations.

[The evaluation of the role of recovery from potentially lethal and sublethal damages in the RBE of densely ionizing radiation]

The new data confirming the relations between RBE and recovery of cells are presented. The quantitative evaluation of the contribution of potentially lethal and sublethal damage recovery in radiosensitivity of cells of various origin after exposure to low- and high-LET ionizing radiation was carried out. The conclusion about the greater contribution of potentially lethal damage recovery in the magnitude of RBE in comparison with sublethal damage recovery was made.

Gamma-irradiation stimulates homology-directed DNA double-strand break repair in Drosophila embryo

To test the DNA double-strand break (DSB) repair activities present in Drosophila early embryos, we have analyzed the circularization of a microinjected linear plasmid. In order to study repair by homologous recombination, the linear plasmid was injected with an homologous fragment encompassing the break. After extraction from embryos, repair products were analyzed directly by PCR and after their cloning into bacteria. We demonstrate, in addition to the repair by homologous recombination, the presence of an efficient end-joining activity in embryos. Plasmid circularization by end-joining was accompanied by short deletions frequently associated with non-random insertions. Most importantly, pre-irradiation of embryos specifically enhanced the accurate repair by homologous recombination. Such a stimulation is described for the first time in the context of a whole higher organism.

Calmodulin antagonists and cAMP inhibit ionizing-radiation-enhancement of double-strand-break repair in human cells

Ionizing radiation (IR) enhances double-strand-break (DSB)-repair fidelity in plasmids processed in normal lymphoblasts but not in lymphoblasts from ataxia telangiectasia (A-T) patients. Putatively, signal-transduction pathways mediate this DNA-repair induction. Because IR inhibition of DNA synthesis is defective in A-T cells and is mediated by a calmodulin (caM)-dependent pathway, we evaluated the involvement of caM-dependent pathways in DSB-repair induction. Human lymphoblasts were gamma-irradiated with or without treatment with caM antagonists and the cells' abilities to repair shuttle pZ189 carrying a single DSB (linDNA) were assessed. In untreated controls, IR enhanced DSB-rejoining fidelity if transfection occurred promptly but diminished fidelity if transfection was delayed. Treatment with two caM antagonists, W-7 and W-13, prior to irradiation blocked this IR-enhancement of DSB-rejoining fidelity. Vinpocetine, a caM-dependent phosphodiesterase inhibitor, and 8-bromo-cAMP also inhibited IR enhancement of repair fidelity, but caM-dependent protein kinase II inhibitor KN62 had no effect. Other protein kinase inhibitors, staurosporine and genistein, also did not inhibit IR enhancement of DSB repair fidelity. However, staurosporine blocked the twofold reduction in DSB-repair fidelity seen if linDNA transfection was delayed 2 h after irradiation. These findings point to the involvement of caM/cAMP-dependent pathway(s) in mediating IR-enhancement of DSB-rejoining fidelity in mammalian cells.

CC to TT mutation in the mitochondrial DNA of normal skin: relationship to ultraviolet light exposure

We have previously reported that ultraviolet (UV)-specific (CC to TT) mutations in p53 gene can be detected in normal skin. This, however, cannot be used as a cumulative marker of UV exposure, since cells with the p53 mutation acquire a clonal growth advantage. Moreover, a large skin biopsy is necessary for each assay. In order to circumvent these problems, we have measured mitochondrial (Mt) DNA mutations; there are more than 1000 copies of the Mt genome per cell, and Mt genes are not directly involved in cell growth. We have established a sensitive allele-specific polymerase chain reaction (AS-PCR) assay capable of detecting one CC to TT mutation in Mt DNA among 10(7) wild-type genes using a mismatch allele-specific primer. With this assay, we found no mutation-positive samples from internal non-exposed tissue (stomach, colon, and blood) (0/50). In contrast, 17 out of 111 skin samples were positive: the mutation frequency in positive samples was around 10(7)-10(-6) (10-100 copies of mutant in 10(8) wild-type Mt DNA). In normal skin tissue, the prevalence of positive samples was higher in those from exposed sites (13/51) than in those from less-exposed sites (1/26) (p<0.05). However, a quantitative correlation between sunlight exposure and the accumulation of mutations was not found. We conclude that the UV exposure-associated CC to TT mutation in Mt DNA can be detected in normal skin, but that further studies are required to develop this as a quantitative marker for UV exposure.