G2 repair and chromosomal damage in lymphocytes from workers occupationally exposed to low-level ionizing radiation

The effect of the G2 repair of chromosomal damage in lymphocytes from workers exposed to low levels of X- or gamma-rays was evaluated. Samples of peripheral blood were collected from 15 radiation workers, 20 subjects working in radiodiagnostics, and 30 healthy control donors. Chromosomal aberrations (CA) were evaluated by scoring the presence of chromatid and isochromatid breaks, dicentric and ring chromosomes in lymphocytes with/without 5 mM caffeine plus 3 mM-aminobenzamide (3-AB) treatment during G2. Our results showed that the mean value of basal aberrations in lymphocytes from exposed workers was higher than in control cells (p < 0.001). The chromosomal damage in G2, detected with caffeine plus 3-AB treatment was higher than the basal damage (untreated conditions), both in control and exposed populations (p < 0.05). In the exposed workers group, the mean value of chromosomal abnormalities in G2 was higher than in the control (p < 0.0001). No correlation was found between the frequency of chromosome type of aberrations (basal or in G2), and the absorbed dose. Nevertheless, significant correlation coefficients (p < 0.05) between absorbed dose and basal aberrations yield (r = 0.430) or in G2 (r = 0.448) were detected when chromatid breaks were included in the total aberrations yield. Under this latter condition no significant effect of age, years of employment or smoking habit on the chromosomal aberrations yield was detected. However, analysis of the relationship between basal aberrations yield and the efficiency of G2 repair mechanisms, defined as the percentage of chromosomal lesions repaired in G2, showed a significant correlation coefficient (r = -0.802; p < 0.001). These results suggest that in addition to the absorbed dose, the individual G2 repair efficiency may be another important factor affecting the chromosomal aberrations yield detected in workers exposed to low-level ionizing radiation.     

Coordinated action of the Fanconi anemia and ataxia telangiectasia pathways in response to oxidative damage

Fanconi anemia (FA) and ataxia telangiectasia (AT) share common traits such chromosomal instability and proneness to hematological cancers. Both AT and FA cell lines, and patients, are characterized by abnormally high levels of oxidative stress markers. The key FA protein FANCD2 is phosphorylated on Ser 222 by ATM after ionizing radiation (IR), thus allowing normal activation of the S-phase checkpoint, and ATM cells are known to be hypersensitive to oxidative damage. In this work we show that FANCD2 deficient cells have a defective S-phase checkpoint after Hydrogen Peroxide (H(2)O(2)) induced oxidative damage. ATM dependent phosphorylation of FANCD2 at the S222 residue is necessary for normal S-phase checkpoint activation after oxidative stress, while FANCD2 monoubiquitination at K561 is dispensable. We also show that FANCD2 is not required for base excision repair of 8-oxoG and other DNA lesions (abasic sites, uracils), while treatments that exclusively induce 8-oxoG, but not DNA double strand breaks, fail to activate FANCD2 monoubiquitination, thus indicating that the known accumulation of 8-oxoG in FA cells reflects an overproduction of ROS rather than defective processing of oxidized bases. We conclude that the handling of DNA damage after H(2)O(2)-induced oxidative stress requires the coordinated action of FANCD2 and ATM.     

Relationship of spontaneous chromosome instability and sister chromatid exchanges in Fanconi's anemia

The frequency of spontaneous instability of lymphocyte chromosomes of the first 2 mitoses, the rate of sister chromatid exchanges (SCEs), and the proliferative kinetics of lymphocytes were studied in a 6-year-old girl with Fanconi's anemia (FA) and in 4 healthy donors. The frequencies of aberrant cells and the total number of chromosome breaks in the FA patient decreased with cell transition from the first to the second mitosis. The FA lymphocytes had a slower proliferative kinetics and the level of SCEs was higher as compared with control. The probability of chromatid deletions at the sites of SCEs localization and in the dark and light stained chromatids was unequal. 33.8% of chromatid breaks were associated with SCEs. The data point to the relationship between SCEs and spontaneous chromosome instability in AF cells.     

Evaluation of vitamin B12 effects on DNA damage induced by pioglitazone

Pioglitazone is a prototype of thiazolidinediones, used for the treatment of type 2 diabetes mellitus. Previous studies suggest that pioglitazone might cause DNA damage by generation of oxidative species. In this study, we investigated the mutagenic effects of pioglitazone using sister chromatid exchanges (SCEs), and chromosomal aberrations (CAs) assays in cultured human lymphocytes. In addition, oxidative DNA damage was evaluated in cells culture by measuring 8-hydroxy-2'-deoxyguanosine (8-OH-dG) marker. We also investigated the possible protective effects of vitamin B12, which is associated with DNA repair, on DNA damage induced by pioglitazone. Treatment of the human lymphocytes with pioglitazone (100μM) significantly increases the frequency of SCEs and CAs (p<0.01). In addition, significant elevation in 8-OH-dG release from lymphocytes was observed after treatment with pioglitazone (p<0.01). On the other hand, pretreatment of cultures with vitamin B12 (13.5μg/ml) protected lymphocytes from the genotoxic effect of pioglitazone. Therefore, we conclude that pioglitazone is genotoxic, and it induces chromosomal and oxidative DNA damage in cultured lymphocytes and this toxicity is prevented by pretreatment with vitamin B12.     

Cytogenetical characterization of Chinese hamster ovary X-ray-sensitive mutant cells xrs 5 and xrs 6. VII. Complementation analysis of X-irradiated wild-type CHO-K1 and xrs mutant cells using the premature chromosome condensation technique

The complementation effect of wild-type CHO-K1 and xrs mutants after fusion, as judged by the frequencies of X-ray-induced G1 and G2 premature chromosome condensation (PCC), was studied. For induction of PCC, X-irradiated interphase cells (G1 and G2) were fused immediately with untreated mitotic cells of the same cell line or with mitotic cells of another line. The frequencies of breaks in G1-PCC, or breaks and chromatid exchanges in G2-PCC were determined and the latter parameter was compared with the frequency of chromosomal aberrations in mitotic cells following G2 irradiation. CHO-K1 cells were capable of complementing the X-ray sensitivity of both xrs 5 and xrs 6 cells. However, full restoration of the repair defect in xrs cells could never be accomplished. The mutants failed to complement each other. In CHO-K1 cells, the incidence of chromosomal aberrations was significantly higher in G2-PCC (2.5-fold) than that observed in mitotic cells at 2.5 h after irradiation. The ratio of the induced frequency of aberrations in G2-PCC to that in mitotic cells was correlated with the degree of repair of DNA double-strand breaks (dsb) and reached almost 1 in xrs 5 cells indicating no repair. In addition the data indicated that, during the period of recovery of CHO-K1 cells, X-ray-induced breaks decreased but exchanges remained at the same level. In contrast, due to a deficiency in rejoining of dsb in xrs mutants, breaks remained open for a long period of time, allowing the formation of additional chromatid exchanges during recovery time.     

Effects of G2 treatments with inhibitors of DNA synthesis and repair on chromosome damage induced by X-rays and chemical clastogens in root tips of Vicia faba. Comparison with corresponding effects in cultured human lymphocytes

The frequencies of chromatid aberrations produced in roots of Vicia faba by clastogenic (chromosome-damaging) agents were strongly enhanced by exposing the root-tip cells to inhibitors of DNA synthesis during the G2 phase. Chromosome damage produced by both S-dependent (maleic hydrazide, methyl methanesulfonate, thio-TEPA) and S-independent (X-rays, streptonigrin) mechanisms was enhanced by the inhibitor treatments. The types of aberrations affected by the inhibitors were mainly chromatid gaps and breaks and isochromatid breaks of the non-union type. Most effective among the inhibitors tested were hydroxyurea (HU) and 5-fluorodeoxyuridine (FdUrd). Post-treatments with caffeine were effective in enhancing clastogen-induced chromosome damage when given during the S phase. All types of aberrations, exchanges as well as breaks, were enhanced by the post-treatments. When given during the G2 phase, caffeine enhanced only the frequency of chromatid aberrations produced by X-rays. The enhancement was slight and obtained only when the cells were irradiated in the G2 phase and immediately post-treated with caffeine. Clastogen-treated cultures of human lymphocytes responded to post-treatments with inhibitors of DNA synthesis in very much the same way as clastogen-treated root-tip cells of Vicia faba. Thus, the frequencies of chromatid gaps and breaks and isochromatid breaks of the non-union type were strongly enhanced by exposing clastogen-treated lymphocytes to inhibitors of DNA synthesis during the G2 phase. The efficiency of the inhibitors, however, varied considerably in the two materials. On the whole, the number of inhibitors capable of enhancing induced chromosome damage was much larger in lymphocytes than in bean root tips. Only HU was equally effective in both materials. The most striking difference between the two materials was found when caffeine was given as a post-treatment. Thus, in human lymphocytes the frequencies of chromatid aberrations induced by most clastogenic agents were strongly enhanced when caffeine was given during the G2 phase, but little affected by post-treatments with caffeine during the S phase.     

Effects of caffeine and cycloheximide during G2 prophase in control and X-ray-irradiated human lymphocytes

The effect of caffeine and cycloheximide during the G2 phase on frequency of chromosomal aberrations and G2 duration was studied in control and X-ray-irradiated human lymphocytes in vitro. Caffeine treatments alone increase the frequencies of chromatid breakage and decrease the average G2 duration in control and X-ray-irradiated lymphocytes (40 R). Both caffeine effects are reversed by 0.5 micrograms/ml cycloheximide in combination treatments. Cycloheximide treatments alone prolong G2 duration in control as well as in X-ray-irradiated lymphocytes although no improvement in chromosome repairing by this inhibitor of protein synthesis was observed under the conditions of our experiments. We propose that the cycloheximide effect is associated with a low level of mitotic factors, required for the entrance into mitosis, which is maintained at a higher level in caffeine treatment alone. Finally, G2 delay has generally been associated with certain genome damage. The fact that the caffeine and cycloheximide effects on X-irradiated lymphocytes are also present in control lymphocytes (without X-rays) suggests that control of the G2 duration constitutes one of the mechanisms involved in DNA repair operating during the G2 phase.     

Chromatid damage induced by fluorescent light during G2 phase in normal and Gardner syndrome fibroblasts. Interpretation in terms of deficient DNA repair

Skin fibroblasts from Gardner syndrome (GS) compared with those from normal donors showed a significantly higher incidence of chromatid gaps and breaks following exposure to low-intensity, cool-white fluorescent light during G2 phase of the cell cycle. Considerable evidence supports the concept that chromatid gaps and breaks seen directly after exposure to DNA-damaging agents represent unrepaired DNA single- and double-strand breaks respectively. The changes in incidence of chromatid aberrations with time after light exposure are consistent with the sequence of events known to follow DNA damage and repair. Initially, the incidence of light-induced chromatid gaps was equivalent in GS and normal fibroblasts. In the normal cells, the chromatid gaps disappeared by 1 h post-exposure, presumably as a result of efficient repair of DNA single-strand breaks. In contrast, the incidence of gaps increased in GS cells by 0.5 h followed by a decrease at 1 h and concomitant increase in chromatid breaks. It appears from these findings that the increased incidence of chromatid damage in GS fibroblasts results from deficient repair of DNA single-strand breaks which arise from incomplete nucleotide excision of DNA damage during G2 phase.     

BRCA1 role in the mitigation of radiotoxicity and chromosomal instability through repair of clustered DNA lesions

Oxidatively-induced clustered DNA lesions are considered the signature of any ionizing radiation like the ones human beings are exposed daily from various environmental sources (medical X-rays, radon, etc.). To evaluate the role of BRCA1 deficiencies in the mitigation of radiation-induced toxicity and chromosomal instability we have used two human breast cancer cell lines, the BRCA1 deficient HCC1937 cells and as a control the BRCA1 wild-type MCF-7 cells. As an additional control for the DNA damage repair measurements, the HCC1937 cells with partially reconstituted BRCA1 expression were used. Since clustered DNA damage is considered the signature of ionizing radiation, we have measured the repair of double strand breaks (DSBs), non-DSB bistranded oxidative clustered DNA lesions (OCDLs) as well as single strand breaks (SSBs) in cells exposed to radiotherapy-relevant γ-ray doses. Parallel measurements were performed in the accumulation of chromatid and isochromatid breaks. For the measurement of OCDL repair, we have used a novel adaptation of the denaturing single cell gel electrophoresis (Comet assay) and pulsed field gel electrophoresis with Escherichia coli repair enzymes as DNA damage probes. Independent monitoring of the γ-H2AX foci was also performed while metaphase chromatid lesions were measured as an indicator of chromosomal instability. HCC1937 cells showed a significant accumulation of all types of DNA damage and chromatid breaks compared to MCF-7 while BRCA1 partial expression contributed significantly in the overall repair of OCDLs. These results further support the biological significance of repair resistant clustered DNA damage leading to chromosomal instability. The current results combined with previous findings on the minimized ability of base clusters to induce cell death (mainly induced by DSBs), enhance the potential association of OCDLs with breast cancer development especially in the case of a BRCA1 deficiency leading to the survival of breast cells carrying a high load of unrepaired DNA damage clusters.     

Clastogenic effects in human lymphocytes of power frequency electric fields: In vivo and in vitro studies

Summary In vivo and in vitro studies of the clastogenic effects of power frequency electric fields and transient electric currents have been performed. For the in vivo investigation peripheral lymphocytes from twenty switchyard workers were screened for chromosome anomalies. The rates of chromatid and chromosome breaks were found to be significantly increased compared to the rates in 17 controls.Exposure of human peripheral lymphocytes, in vitro, to a 50-Hz current with 1 mA/cm² current density did not induce any chromosome damage. Exposure to ten 3 µs-long spark discharge pulses with a peak field strength in the samples of 3.5 kV/cm, however, resulted in chromosome breaks at a frequency similar to that induced in lymphocytes in vitro by ionizing radiation at 0.75 Gy.The biological significance of chromosomal damage induced in somatic cells is discussed.     

Proliferative kinetics and mitomycin C-induced chromosome damage in Fanconi's anemia lymphocytes

Lymphocytes from two sisters with Fanconi's anemia (FA) were studied for cell cycle kinetics, sister chromatid exchanges (SCEs), and chromosomal aberrations when they had undergone one, two, or three or more divisions in mitomycin C (MMC)-treated cultures. Lymphocytes from the parents, another sister of the probands, and a healthy unrelated adult were examined as controls. Analyses of cell cycle kinetics by the sister chromatid differential staining method revealed that the relative frequency of metaphase cells at their third or subsequent divisions was much smaller in untreated FA cultures than in normal cultures fixed at 96 h after phytohemagglutinin stimulation. These data indicate that FA cells proliferate much more slowly than normal cells. MMC treatments of FA and normal cells led to a clearly dose-related delay in cell turnover times, the duration of delay being much longer in FA than in normal cells. FA cells had about 1.4 times higher frequencies of SCEs than normal cells in both MMC-treated and untreated cultures. FA cells also showed several times higher frequencies of chromosomal aberrations than normal cells, and the frequency of chromosomal aberrations decreased through subsequent mitoses by approximately 60% in both FA and normal cells.     

G2 phase repair of X-ray-induced chromosomal DNA damage in trichothiodystrophy cells

The repair of X-ray-induced DNA damage during G₂ cell-cycle phase has been examined in lines of skin fibroblasts from three patients with trichothiodystrophy (TTD), one with apparently normal and two with defective nucleotide excision repair (NER). These responses are compared with those of five lines from clinically normal controls, lines from xeroderma pigmentosum (XP), Cockayne syndrome (CS), Down syndrome (DS), and ataxia telangiectasia (AT) patients. Chromosomal DNA repair was measured as the chromatid aberration frequency (CAF) or total number of chromatid breaks and long gaps per 100 metaphase cells, determined 0.5–1.5 h after X-irradiation (53 rad). Chromatid breaks and gaps (as defined herein) represent unrepaired DNA strand breaks. Only one of the TTD lines, TTD 1BR, showed an abnormally high CAF. This line was shown subsequently to be of a different complementation group, representing a new nucleotide excision repair gene. An abnormally high CAF was also observed, as reported previously, in XP-C, AT and DS but not in CS skin fibroblasts. In addition, cell lines were examined for DNA incision activity by an indirect method in which chromatid aberrations were enumerated with or without ara-C, an inhibitor of repair synthesis, added after X-irradiation. All TTD lines had abnormally low incision activity.     

Analysis of X-ray induced aberrations in mammalian chromosomes by electrofusion induced premature chromosome condensation

Summary Premature chromosome condensation (PCC) was induced by electrofusion of metaphase cells of an Ehrlich ascites tumor cell line with interphase cells of a Muntjac cell line or of a Chinese Hamster subline. Electrofusion was performed by cell alignment in a weakly inhomogeneous a.c. field of 200 V/cm amplitude (peak-to-peak value) and of 1.7 MHz frequency, followed by the application of a series of breakdown (fusion) pulses of 5 kV/cm strength and 15 µs duration. Most of the PCC's were of the G2 type despite the large proportion of G1 and S cells in the suspension. The number of chromatid aberrations observed in electrofused cells which had not been subjected to irradiation was not significantly above the spontaneous level. This indicates that electrofusion, at least as used here, did not lead to lesions expressed as structural aberrations. When interphase cells were irradiated by X-ray doses below 3 Gy before electrofusion PCC analysis showed chromosome damage consisting mainly of breaks and gaps. The frequency of aberrations recorded by PCC was 6 to 40 fold larger than that seen in conventional metaphase analysis. This large increase probably arose because of an effective suppression of the G2 repair of chromosomal lesions by the fast condensation process which took place within about 30 min. This assumption was supported by PCC experiments in which the time between X-irradiation and fusion with subsequent chromosome condensation was varied. The results demonstrated that G2 repair of chromosomal lesions was not detectable until 20 min after fusion with a half-time of the repair kinetics of about 1.5 h. The selectivity of premature chromosome condensation in G2 cells is discussed in terms of the differences between electrofusion and chemically or virally induced fusion. It is assumed that the concentration and the transfer rate of the chromosome condensation factor from the metaphase to the interphase cell are the limiting factors in achieving PCC. This is because the localised permeabilisation of the membrane and the dominance of two-cell fusions are characteristic of electrofusion.     

Vanadate induces DNA strand breaks in cultured human fibroblasts at doses relevant to occupational exposure

To study possible genotoxic effects of occupational exposure to vanadium pentoxide, we determined DNA strand breaks (with alkaline comet assay), 8-hydroxy-2'deoxyguanosine (8-OHdG) and the frequency of sister chromatid exchange (SCE) in whole blood leukocytes or lymphocytes of 49 male workers employed in a vanadium factory in comparison to 12 non-exposed controls. In addition, vanadate has been tested in vitro to induce DNA strand breaks in whole blood cells, isolated lymphocytes and cultured human fibroblasts of healthy donors at concentrations comparable to the observed levels of vanadium in vivo. To investigate the impact of vanadate on the repair of damaged DNA, co-exposure to UV or bleomycin was used in fibroblasts, and DNA migration in the alkaline and neutral comet assay was determined. Although, exposed workers showed a significant vanadium uptake (serum: median 5.38microg/l, range 2.18-46.35microg/l) no increase in cytogenetic effects or oxidative DNA damage in leukocytes could be demonstrated. This was consistent with the observation that in vitro exposure of whole blood leukocytes and lymphocytes to vanadate caused no significant changes in DNA strand breaks below concentrations of 1microM (50microg/l). In contrast, vanadate clearly induced DNA fragmentation in cultured fibroblasts at relevant concentrations. Combined exposure of fibroblasts to vanadate/UV or vanadate/bleomycin resulted in non-repairable DNA double strand breaks (DSBs) as seen in the neutral comet assay. We conclude that exposure of human fibroblasts to vanadate effectively causes DNA strand breaks, and co-exposure of cells to other genotoxic agents may result in persistent DNA damage.     

Analysis of spontaneous and bleomycin-induced chromosome damage in peripheral lymphocytes of long-haul aircrew members from Argentina

Spontaneous and bleomycin (BLM)-induced chromosomal aberrations in G0 and G2 stages of the cell cycle have been analyzed in peripheral lymphocytes of 21 long-haul aircrew members from Argentina in order to assess BLM-induced clastogenesis as a first approach to determine the DNA repair capacity and thereby the susceptibility to environmental cancers in aircrew. The possibility that occupational exposure of flight personnel to cosmic radiation can induce an adaptive response in their peripheral lymphocytes that can be detected by a subsequent in vitro treatment with BLM was also investigated. For comparison, aberrations were also scored in the lymphocytes of 15 healthy volunteers matched by age, health, sex, drinking and smoking habits to the flight personnel group. Aircrew exhibited a higher frequency of spontaneous dicentrics and ring chromosomes than the control population (p<0.05). BLM sensitivity test showed that aircrew and controls are equally sensitive to BLM G2 clastogenic effects, since both groups exhibited a similar frequency of chromatid breaks per cell (p>0.05). However, the aircrew sampled population was almost two times more sensitive to BLM G0 clastogenic effects than controls (p<0.05). Therefore, our data suggest that chronic exposure of aircrew to cosmic radiation increases the in vitro chromosomal sensitivity of their peripheral lymphocytes to BLM (at least in the G0 stage of the cell cycle), and that occupational exposure of flight personnel to cosmic radiation does not induce an adaptive response to this radiomimetic compound. Our results justify further studies aimed at determine if those aircrew members hypersensitive to BLM are more prone to develop environmental cancer than BLM-insensitive individuals.     

Mitotic delay in lymphocytes from BRCA1 heterozygotes unable to reduce the radiation-induced chromosomal damage

Double strand breaks (DSB) are critical lesions involved in the formation of chromosomal aberrations. In response to DNA damage, the cell has mechanisms of repair and cell-cycle control to maintain the genome integrity in which BRCA1 gene is implicated. In the present study an evaluation of the radio-induced damage in G(2) phase of the cell cycle in lymphocytes from BRCA1 heterozygotes is presented. For this purpose Calyculin-A-based premature chromosome condensation (PCC) combined with mitotic arrest has been applied to examine with conventional cytogenetics the damage in G(2) and M phase cells, and to evaluate the G(2)-to-M phase transition. Irradiated peripheral blood lymphocytes from seven heterozygote females (BRCA1(+/-)) and seven control females (BRCA1(+/+)) have been analyzed. The mean proportion of G(2) cells in BRCA1(+/-) was significantly higher than in BRCA1(+/+), indicating a higher G(2) delay after IR exposure in cells from BRCA1(+/-) females. On the other hand, whereas the mean frequency of chromatid breaks (chtb) in G(2) cells was not statistically different between both groups, the mean frequency of chtb in M cells of the BRCA1(+/-) group was significantly higher than in the BRCA1(+/+) one. Moreover, the mean proportion of M cells with aberrations was significantly higher in BRCA1(+/-) than in BRCA1(+/+) suggesting that in spite of the higher G(2) delay of BRCA1(+/-) more damaged cells are able to pass the G(2)-to-M transition.     

Cytoprotective effect of honey against chromosomal breakage in fanconi anemia patients in vitro

BACKGROUND:

Natural honey is widely used all over the world as a complementary and alternative medicine in various disorders including Fanconi anemia (FA). FA is a rare genetic chromosomal instability syndrome caused by impairment of DNA repair and reactive oxygen species (ROS) imbalance. This disease is also related to bone marrow failure and cancer. The aim of this study was to evaluate the cytoprotective effect of honey on mitomycin C (MMC-) induced chromosomal damage in peripheral lymphocytes from FA patients.

MATERIALS AND METHODS:

Treatment of these complications with alkylation agents MMC may enhance chromosomal breakage. We have evaluated the effect of honey on MMC- induced chromosomal breakage in FA blood cells using chromosomal breakage assay. The basal chromosomal breakage count was higher among FA patients than healthy subjects.

RESULTS:

The addition of MMC alone gave a significantly higher of chromosomal breakage in FA patients than control group (P < 0.0001). Pre- treatment with honey significantly inhibited breakage induced by MMC in FA patients by its antioxidant effect.

CONCLUSION:

Honey can prevent MMC- induced chromosomal breakage by its antioxidant effect.     

Post-treatments with sodium arsenite during G2 enhance the frequency of chromosomal aberrations induced by S-dependent clastogens

Treatment with sodium arsenite during the G2 phase potentiated the chromatid breaks and chromatid exchanges induced by ultraviolet light or 4-nitroquinoline 1-oxide but not those induced by methyl methanesulfonate, ethyl methanesulfonate, mitomycin C or cisplatin in Chinese hamster ovary cells. A comparison was made between the effects of treatment during G2 with sodium arsenite, cytosine-β- -arabinofuranoside, aphidicolin, hydroxyurea, caffeine, 3-aminobenzamide and novobiocin on the frequency of chromosomal aberrations induced by the above-mentioned S-dependent clastogens. It was found that the effects varied considerably, both quantitatively and qlalitatively. However, potentiation was more often observed in the chromosomal aberrations induced by ultraviolet light and 4-nitroquinoline 1-oxide than by other S-dependent clastogens, and the frequency of chromatid exchanges was potentiated only in cells pretreated with ultraviolet light or 4-nitroquinoline 1-oxide. Furthermore, for all of the S-dependent clastogens studied, treatment with cytosine-β- -arabinofuranoside during the G2 phase potentiated the frequency of chromatid breaks but not the frequency of chromatid exchanges.     

Chromosomal aberrations in lymphocytes of employees in transformer and generator production exposed to electromagnetic fields and mineral oil

The objective was to study the risk of cytogenetic damage among high voltage laboratory workers exposed to electromagnetic fields and mineral oil. This is a cross sectional study of 24 exposed and 24 matched controls in a Norwegian transformer factory. The exposure group included employees in the high voltage laboratory and in the generator soldering department. Electric and magnetic fields and oil mist and vapor were measured. Blood samples were analyzed for chromosomal aberrations in cultured lymphocytes. In addition to conventional cultures, the lymphocytes were also treated with hydroxyurea and caffeine. This procedure inhibits DNA synthesis and repair in vitro, revealing in vivo genotoxic lesions that are repaired during conventional culturing. In conventional cultures, the exposure group and the controls showed similar values for all cytogenetic parameters. In the DNA synthesis- and repair-inhibited cultures, generator welders showed no differences compared to controls. Among high voltage laboratory testers, compared to the controls, the median number of chromatid breaks was doubled (5 vs. 2.5 per 50 cells; P<0.05) the median number of chromosome breaks was 2 vs. 0.5 (P>0.05) and the median number of aberrant cells was 5 vs. 3.5 (P<0.05). Further analysis of the inhibited culture data from this and a previous study indicated that years of exposure and smoking increase the risk of aberrations. We conclude that there was no increase in cytogenetic damage among exposed workers compared to controls in the conventional lymphocyte assay. In inhibited cultures, however, there were indications that electromagnetic fields in combination with mineral oil exposure may produce chromosomal aberrations.     

Occupational exposure to mercury vapour on genotoxicity and DNA repair

We have conducted a population study to investigate whether current occupational exposure to mercury can cause genotoxicity and can affect DNA repair efficiency. Blood samples from 25 exposed workers and 50 matched controls were investigated for the expression of genotoxicity. The data indicate that mercury exposure did not cause any significant differences between the workers and controls in the baseline levels of DNA strand breaks (as measured by the alkaline version of the single cell gel electrophoresis [SCGE] assay) or sister chromatid exchanges (SCE). However, the exposure produced elevated average DNA tails length in the SCGE assay and frequency of chromosome aberrations. In the studies, isolated lymphocytes were exposed to 6J/m2 UV-C light or 2 Gy dose of X-rays in a challenge assay and repair of the induced DNA damage was evaluated using the SCGE assay. Results from the UV-light challenge assay showed no difference between the workers and controls in the expression of DNA strand breaks after exposure followed by incubation in the absence or presence of the cellular mitogen (phytohemagglutinin, PHA). No difference in DNA strand breaks between the workers and controls was seen immediately after the X-ray challenge, either. However, significant differences were observed in cells that were incubated for 2h with and without phytohemagglutinin. Data from the X-rays challenge assay were further used to calculate indices that indicate DNA repair efficiency. Results show that the repair efficiencies for the workers (69.7% and 83.9% in un-stimulated and stimulated lymphocytes, respectively) were significantly lower than that of matched controls (85.7% and 90.4%, respectively). In addition, the repair efficiency showed a consistent and significant decrease with the duration of occupational exposure to mercury (from 75.7% for <10 years employment, to 65.1% for 11-20 years and to 64.1% for 21-35 years) associated with increase of cytogenetic damage. Our study suggests that the occupational exposure to mercury did not cause a direct genotoxicity but caused significant deficiency in DNA repair. Our observations are consistent with previous studies using the standard chromosome aberration assay to show that exposure to hazardous environmental agents can cause deficiency in DNA repair. Therefore, these affected individuals may have exposure-related increase of health risk from continued exposure and in combination with exposure to other genotoxic agents.