Chromosomal aberrations, sister-chromatid exchanges, cells with high frequency of SCE, micronuclei and comet assay parameters in 1, 3-butadiene-exposed workers

Environmental Tobacco Smoke in Occupational Settings: Effect and Susceptibility Biomarkers in Workers From Lisbon Restaurants and Bars

Environmental tobacco smoke (ETS) has been recognized as a major health hazard by environmental and public health authorities worldwide. In Portugal, smoke-free laws are in force for some years, banning smoking in most indoor public spaces. However, in hospitality venues such as restaurants and bars, owners can still choose between a total smoke-free policy or a partial smoking restriction with designated smoking areas, if adequate reinforced ventilation systems are implemented. Despite that, a previous study showed that workers remained continuously exposed to higher ETS pollution in Lisbon restaurants and bars where smoking was still allowed, comparatively to total smoke-free venues. This was assessed by measurements of indoor PM2.5 and urinary cotinine, a biomarkers of tobacco smoke exposure, demonstrating that partial smoking restrictions do not effectively protect workers from ETS. The aim of the present work was to characterize effect and susceptibility biomarkers in non-smokers from those hospitality venues occupationally exposed to ETS comparatively to non-exposed ones. A group of smokers was also included for comparison. The sister chromatid exchange (SCE), micronucleus (MN) and comet assays in whole peripheral blood lymphocytes (PBLs) and the micronucleus assay in exfoliated buccal cells, were used as biomarkers of genotoxicity. Furthermore, a comet assay after ex vivo challenge of leukocytes with an alkylating agent, ethyl methanesulfonate (EMS), was used to analyze the repair capacity of those cells. Genetic polymorphisms in genes associated with metabolism and DNA repair were also included. The results showed no clear association between occupational exposure to ETS and the induction of genotoxicity. Interestingly, the leukocytes from non-smoking ETS-exposed individuals displayed lower DNA damage levels in response to the ex vivo EMS challenge, in comparison to those from non-exposed workers, suggesting a possible adaptive response. The contribution of individual susceptibility to the effect biomarkers studied was unclear, deserving further investigation.

Challenge-comet assay, a functional and genomic biomarker for precision risk assessment and disease prevention among exposed workers

Advancements in genomic technologies have ushered application of innovative changes into biomedical sciences and clinical medicine. Consequently, these changes have created enormous opportunities to implement precision population/occupational disease prevention and target-specific disease intervention (or personalized medicine). To capture the opportunities, however, it is necessary is to develop novel, especially genomic-based, biomarkers which can provide precise and individualized health risk assessment. In this review, development of the Challenge-comet assay is used as an example to demonstrate how assays need to be validated for its sensitivity, specificity, and functional and quantitative features, and how assays can be used to provide individualized health risk assessment for precision prevention and intervention. Other examples of genomic-based novel biomarkers will also be discussed. Furthermore, no biomarkers can be used alone therefore their integrated usage with other biomarkers and with personal data bases will be discussed.

Population-Based Analysis of DNA Damage and Epigenetic Effects of 1,3-Butadiene in the Mouse

Metabolism of 1,3-butadiene, a known human and rodent carcinogen, results in formation of reactive epoxides, a key event in its carcinogenicity. Although mice exposed to 1,3-butadiene present DNA adducts in all tested tissues, carcinogenicity is limited to liver, lung, and lymphoid tissues. Previous studies demonstrated that strain- and tissue-specific epigenetic effects in response to 1,3-butadiene exposure may influence susceptibly to DNA damage and serve as a potential mechanism of tissue-specific carcinogenicity. This study aimed to investigate interindividual variability in the effects of 1,3-butadiene using a population-based mouse model. Male mice from 20 Collaborative Cross strains were exposed to 0 or 635 ppm 1,3-butadiene by inhalation (6 h/day, 5 days/week) for 2 weeks. We evaluated DNA damage and epigenetic effects in target (lung and liver) and nontarget (kidney) tissues of 1,3-butadiene-induced carcinogenesis. DNA damage was assessed by measuring N-7-(2,3,4-trihydroxybut-1-yl)-guanine (THB-Gua) adducts. To investigate global histone modification alterations, we evaluated the trimethylation and acetylation of histones H3 and H4 across tissues. Changes in global cytosine DNA methylation were evaluated from the levels of methylation of LINE-1 and SINE B1 retrotransposons. We quantified the degree of variation across strains, deriving a chemical-specific human variability factor to address population variability in carcinogenic risk, which is largely ignored in current cancer risk assessment practice. Quantitative trait locus mapping identified four candidate genes related to chromatin remodeling whose variation was associated with interstrain susceptibility. Overall, this study uses 1,3-butadiene to demonstrate how the Collaborative Cross mouse population can be used to identify the mechanisms for and quantify the degree of interindividual variability in tissue-specific effects that are relevant to chemically induced carcinogenesis.

Major groove orientation of the (2S)-N(6)-(2-hydroxy-3-buten-1-yl)-2'-deoxyadenosine DNA adduct induced by 1,2-epoxy-3-butene

1,3-Butadiene (BD) is an environmental and occupational toxicant classified as a human carcinogen. It is oxidized by cytochrome P450 monooxygenases to 1,2-epoxy-3-butene (EB), which alkylates DNA. BD exposures lead to large numbers of mutations at A:T base pairs even though alkylation of guanines is more prevalent, suggesting that one or more adenine adducts of BD play a role in BD-mediated genotoxicity. However, the etiology of BD-mediated genotoxicity at adenine remains poorly understood. EB alkylates the N(6) exocyclic nitrogen of adenine to form N(6)-(hydroxy-3-buten-1-yl)-2'-dA ((2S)-N(6)-HB-dA) adducts ( Tretyakova , N. , Lin , Y. , Sangaiah , R. , Upton , P. B. , and Swenberg , J. A. ( 1997 ) Carcinogenesis 18 , 137 - 147 ). The structure of the (2S)-N(6)-HB-dA adduct has been determined in the 5'-d(C(1)G(2)G(3)A(4)C(5)Y(6)A(7)G(8)A(9)A(10)G(11))-3':5'-d(C(12)T(13)T(14)C(15)T(16)T(17)G(18)T(19) C(20)C(21)G(22))-3' duplex [Y = (2S)-N(6)-HB-dA] containing codon 61 (underlined) of the human N-ras protooncogene, from NMR spectroscopy. The (2S)-N(6)-HB-dA adduct was positioned in the major groove, such that the butadiene moiety was oriented in the 3' direction. At the Cα carbon, the methylene protons of the modified nucleobase Y(6) faced the 5' direction, which placed the Cβ carbon in the 3' direction. The Cβ hydroxyl group faced toward the solvent, as did carbons Cγ and Cδ. The Cβ hydroxyl group did not form hydrogen bonds with either T(16) O(4) or T(17) O(4). The (2S)-N(6)-HB-dA nucleoside maintained the anti conformation about the glycosyl bond, and the modified base retained Watson-Crick base pairing with the complementary base (T(17)). The adduct perturbed stacking interactions at base pairs C(5):G(18), Y(6):T(17), and A(7):T(16) such that the Y(6) base did not stack with its 5' neighbor C(5), but it did with its 3' neighbor A(7). The complementary thymine T(17) stacked well with both 5' and 3' neighbors T(16) and G(18). The presence of the (2S)-N(6)-HB-dA resulted in a 5 °C reduction in the Tm of the duplex, which is attributed to less favorable stacking interactions and adduct accommodation in the major groove.

Cytogenetic methods in human biomonitoring: principles and uses

Cellular phenotypes can be applied as biomarkers to differentiate normal from abnormal biological -conditions. Several cytogenetic methods have been developed and allow the accurate detection of such phenotypic changes.Based on their mechanisms of formation, cellular phenotypes may be used either as biomarkers of exposure or as biomarkers of effect. Therefore, it is important that cytogenetic methods implemented in human biomonitoring should be based on a good knowledge of these mechanisms.In this chapter, we aim to review the mechanistic basis, the methodology, and the use in human biomonitoring studies of four major cytogenetic endpoints: sister chromatid exchanges (SCEs), high frequency cells (HFCs), chromosomal aberrations (CAs), and micronuclei (MN). In addition, an overview of potential confounding factors on the induction of these cytogenetic makers is presented. Furthermore, the combination of cytogenetics with molecular methods, which allows chromosome and gene identification on metaphase as well as in interphase cells with high resolution, is discussed. Finally, practical recommendations for an efficient application of these cytogenetic assays and a correct interpretation of the results on the basis of cellular phenotype(s) assessment in human biomonitoring are highlighted.

Genotoxicity of mercury: contributing for the analysis of Amazonian populations

Mercury is an important source of environmental contamination affecting human beings throughout the world and especially in the Amazon. Riverside populations have been chronically exposed to relatively high levels of methylmercury for many years. Long-term effects of mercury exposure are not well known, but human genotoxicity was already showed in both in vitro and epidemiological studies. However, to date, only two studies were carried out in Amazonian populations with conflicting results and without comparing to a non-exposed population. Aiming to highlight this question and avoid interference factors, this work analyzed in vitro genotoxicity of mercury in blood lymphocytes of Amazonian individuals by two methods (micronucleus and chromosomal aberrations). Deleterious effects of low levels (1-500 μg/l or 0,004-2 μM) of methylmercury were only detected with the method to detect chromosomal aberrations. Mitotic index (proportion of cells in metaphase) was the parameter most sensible. Thus, this technique was applied for the analysis of an Amazonian non-exposed population (Panacauera) with similar social-economical characteristics of the exposed populations studied elsewhere. The mean of the mitotic index for Panacauera population was 0.0814 ± 0.0097. Inter-individual variability of this index had no relation with sex or age. This value was above those registered for some groups of exposed populations. This fact points to mercury as the main responsible for inhibiting the cell cycle and/or the loss of proliferative capacity of the cells. These results already support mitotic index as an essential parameter for the early diagnose of mercury genotoxicity in humans, and especially in Amazonian populations.

1,3-Butadiene: II. Genotoxicity profile

1,3-Butadiene’s (BD’s) major electrophilic metabolites 1,2-epoxy-3-butene (EB), 1,2-dihydroxy-3,4-epoxybutane (EBD), and 1,2,3,4-diepoxybutane (DEB) are responsible for both its mutagenicity and carcinogenicity. EB, EBD, and DEB are DNA reactive, forming a variety of adducts. All three metabolites are genotoxic in vitro and in vivo, with relative mutagenic potencies of DEB >> EB > EBD. DEB also effectively produces gene deletions and chromosome aberrations. BD’s greater mutagenicity and carcinogenicity in mice over rats as well as its failure to induce chromosome-level mutations in vivo in rats appear to be due to greater production of DEB in mice. Concentrations of EB and DEB in vivo in humans are even lower than in rats. Although most studies of BD-exposed humans have failed to find increases in gene mutations, one group has reported positive findings. Reasons for these discordant results are examined. BD-related chromosome aberrations have never been demonstrated in humans except for the possible production of micronuclei in lymphocytes of workers exposed to extremely high levels of BD in the workplace. The relative potencies of the BD metabolites, their relative abundance in the different species, and the kinds of mutations they can induce are major considerations in BD’s overall genotoxicity profile.

Biomonitoring of benzene and 1,3-butadiene exposure and early biological effects in traffic policemen

The objective of this study was to determine benzene and 1,3-butadiene exposure through ambient air and personal air monitoring, as well as through biomarkers of exposure, and to evaluate the potential health risk of exposure through the use of biomarkers of early biological effects in central Bangkok traffic policemen. Ambient air concentrations of benzene and 1,3-butadiene at the roadsides were significantly higher than in police offices used as control sites (p<0.001). Traffic policemen had a significantly higher exposure to benzene (median 38.62 microg/m(3)) and 1,3-butadiene (median 3.08 microg/m(3)) than office policemen (median 6.17 microg/m(3) for benzene and 0.37 microg/m(3) for 1,3-butadiene) (p<0.001). Biomarkers of benzene exposure, blood benzene, and urinary metabolite, trans, trans-muconic acid were significantly higher in traffic policemen than office policemen (p<0.001). No significant difference between traffic and office policemen was found in urinary benzene metabolite, S-phenyl mercapturic acid, or in urinary 1,3-butadiene metabolite, monohydroxy-butenyl mercapturic acid. Biomarkers of early biological effects, 8-hydroxy-2'-deoxyguanosine in leukocytes (8-OHdG), DNA-strand breaks, and DNA-repair capacity, measured as an increase in gamma ray-induced chromosome aberrations were significantly higher in traffic policemen than controls (p<0.001 for 8-OHdG, p<0.01 for tail length, p<0.001 for olive tail moment, p<0.05 for dicentrics and p<0.01 for deletions). Multiple regression model including individual exposure, biomarkers of exposure, ages and years of work as independent variables showed that only the levels of individual 1,3-butadiene exposure were significantly associated with 8-OHdG and olive tail moment at p<0.0001 indicating more influence of 1,3-butadiene on DNA damage. These results indicated that traffic policemen, who are exposed to benzene and 1,3-butadiene at the roadside in central Bangkok, are potentially at a higher risk for development of diseases such as cancer than office policemen.

Biomonitoring of oral epithelial cells in petrol station attendants: comparison between buccal mucosa and lateral border of the tongue

Owing to the influence of geno- and cytotoxicity on chemical carcinogenesis, studies have demonstrated that petroleum derivatives are able to induce genetic damage and cellular death with conflicting results so far. The aim of the present study was to comparatively evaluate DNA damage (micronucleus) and cellular death (pyknosis, karyolysis and karyorrhexis) in exfoliated oral mucosa cells from gas petrol attendants using two different anatomic buccal sites: cheek mucosa and lateral border of the tongue. A total of 23 gas petrol attendants and 23 health controls (non-exposed individuals) were included in this setting. Individuals had epithelial cells from cheek and lateral border of the tongue mechanically exfoliated, placed in fixative and dropped in clean slides which were checked for the above nuclear phenotypes. The results pointed out significant statistical differences (p<0.05) of micronucleated oral mucosa cells from gas petrol attendants for both oral sites evaluated. In the same way, petroleum derivate exposure was able to increase other nuclear alterations closely related to cytotoxicity such as karyorrhexis, pyknosis and karyolysis, being the most pronunciated effects as those found in the lateral border of the tongue. No interaction was observed between smoking and petroleum exposure. In summary, these data indicate that gas petrol attendants comprise a high risk group for DNA damage and cellular death. It seems that the lateral border of the tongue is a more sensitive site to geno- and cytotoxic insult induced by petroleum derivates.

Investigation on the mechanisms of genotoxicity of butadiene, styrene and their combination in human lymphocytes using the Comet assay

The toxicity of butadiene and styrene is exerted by their metabolites. Such metabolites have been extensively scrutinized at the in vitro level demonstrating evident genotoxic properties. In monitoring, a diverse range of outcomes has been produced. Additionally, epidemiological studies in rubber workers face difficulties of data interpretation due to the changeability and multiple exposures of the workers as well as to confounding factors inherent to the cohorts. Nevertheless, toxicity has been associated with a significant trend of increasing the risk of leukaemia in employees at the styrene-butadiene rubber industry. Thus, further effort must be made to distinguish the exposures to each chemical over time and to characterize their interrelationships. The present investigation focuses on the effects and mechanisms of damage of the mixture styrene-butadiene by examining its metabolites: styrene oxide (SO), butadiene monoepoxide (BME) and butadiene diepoxide (BDE) respectively. The in vitro Comet assay on frozen lymphocytes has been employed to ascertain the DNA damage patterns for the styrene-butadiene metabolites combined and on their own. Different patterns were observed for the mixture and each of its components. This study has also led to determining the mechanism of damage of the mixture and the compounds. With regard to the presence of reactive oxygen species (ROS), co-treatment with catalase does not modulate the genotoxicity of the mixture but it does modulate its components. The outcomes also indicate that the mixture induces cross-links and this is due to the influence of BDE in the mixture, being more evident as the concentration of BDE increases. An investigation on the sensitivity of lymphocytes from occupationally un/exposed subjects to in vitro exposure of the mixture and its components revealed that occupationally exposed subjects had a substantially higher background of DNA damage and a lower sensitivity to the metabolites of styrene, 1,3-butadiene and its mixture.

Cytogenetic biomonitoring of oral mucosa cells from adults exposed to dental X-rays

PURPOSE

Although it has been clearly demonstrated that X-rays play a key role in diagnosing medical and dental problems, this type of ionizing radiation is also able to induce noxious activities, such as genetic damage. The aim of the present study was to evaluate DNA damage (micronucleus) and cellular death in exfoliated buccal mucosa cells from healthy individuals (smokers and nonsmokers) following dental X-ray exposure.

MATERIAL AND METHODS

A total of 39 healthy people who had submitted to panoramic dental radiography were included in the study: 9 smokers and 30 nonsmokers.

RESULTS

The results indicated no significant statistically differences (P>0.05) in micronucleated oral mucosa cells before and after dental X-ray exposure. On the other hand, X-ray exposure did increase other nuclear alterations closely related to cytotoxicity, such as karyorrhexis, pyknosis, and karyolysis. It seems that cigarette smoke did not affect X-ray outcomes induced in buccal cells.

CONCLUSION

These data indicate that dental panoramic radiography may not induce chromosomal damage, but it is able to promote cytotoxicity. Because cellular death is considered a prime mechanism in nongenotoxic mechanisms of carcinogenesis, dental X-ray should be used only when necessary.

Chromosomal aberrations in tire plant workers and interaction with polymorphisms of biotransformation and DNA repair genes

We evaluated chromosomal aberrations in lymphocytes of 177 workers exposed to xenobiotics in a tire plant and in 172 controls, in relation to their genetic background. Nine polymorphisms in genes encoding biotransformation enzymes and nine polymorphisms in genes involved in main DNA repair pathways were investigated for possible modulation of chromosomal damage. Chromosomal aberration frequencies were the highest among exposed smokers and the lowest in non-smoking unexposed individuals (2.5+/-1.8% vs. 1.7+/-1.2%, respectively). The differences between groups (ANOVA) were borderline significant (F=2.6, P=0.055). Chromosomal aberrations were higher in subjects with GSTT1-null (2.4+/-1.7%) than in those with GSTT1-plus genotype (1.8+/-1.4%; F=7.2, P=0.008). Considering individual groups, this association was significant in smoking exposed workers (F=4.4, P=0.040). Individuals with low activity EPHX1 genotype exhibited significantly higher chromosomal aberrations (2.3+/-1.6%) in comparison with those bearing medium (1.7+/-1.2%) and high activity genotype (1.5+/-1.2%; F=4.7, P=0.010). Both chromatid- and chromosome-type aberration frequencies were mainly affected by exposure and smoking status. Binary logistic regression analysis revealed that frequencies of chromatid-type aberrations were modulated by NBS1 Glu185Gln (OR 4.26, 95%CI 1.38-13.14, P=0.012), and to a moderate extent, by XPD Lys751Gln (OR 0.16, 95%CI 0.02-1.25, P=0.081) polymorphisms. Chromosome-type aberrations were lowest in individuals bearing the EPHX1 genotype conferring the high activity (OR 0.38, 95%CI 0.15-0.98, P=0.045). Present results show that exposed individuals in the tire production, who smoke, exhibit higher chromosomal aberrations frequencies, and the extent of chromosomal damage may additionally be modified by relevant polymorphisms.

HPRT mutations in lymphocytes from 1,3-butadiene-exposed workers in China

BACKGROUND

1,3-Butadiene (BD) is an important industrial chemical and an environmental and occupational pollutant. The carcinogenicity of BD in rodents has been proved, but its carcinogenic and mutagenic molecular mechanism(s) are not fully elucidated in humans.

OBJECTIVES

In the present study, we compared the mutation frequencies and exon deletions of BD-exposed workers with that of control subjects in China to identify the characteristic mutations associated with BD exposure in the human HPRT (hypoxanthine-guanine-phosphoribosyltransferase) gene.

METHODS

Seventy-four workers exposed to BD via inhalation and 157 matched controls were evaluated in Nanjing, China. Molecular analysis of HPRT mutant T lymphocytes from BD-exposed workers and nonexposed control subjects was conducted to identify changes in the structure of the HPRT gene. A total of 783 HPRT mutants were analyzed by multiplex polymerase chain reaction, in which 368 HPRT mutants were isolated from BD-exposed workers and 415 mutants from control subjects.

RESULTS

The BD-exposed workers showed a higher mutation frequency (18.2 +/- 9.4 x 10(-6)) than the control subjects (12.7 +/- 7.3 x 10(-6)), but the difference was not significant (p > 0.05). The frequency of exon deletions in BD-exposed workers (27.4%) was significantly higher than that in control subjects (12.5%) (p < 0.05), which mainly included multiplex exon deletions (2-8 exons).

CONCLUSIONS

The results of the present study suggest that BD should increase the frequency of large deletions of HPRT gene in human lymphocytes This change confirms and supports the previous findings in BD-exposed workers.

Chromosomal aberration frequencies determined by conventional methods: Parallel increases over time in the region of a petrochemical industry and throughout the Czech Republic

The rationale for cytogenetic monitoring to determine if safe maximum allowable concentrations (MAC) of genotoxic chemicals are being maintained in a workplace is that exposure levels that do not increase chromosomal aberration frequencies are without harmful effects. Such monitoring, widely used in occupational health programs in the Czech Republic (CR), includes workers exposed to 1,3-butadiene (BD) or other chemicals. Studies of BD exposed workers in the years 1992, 1993, 1994, 1998, and 2004 compared mean frequencies of cells carrying chromosomal aberrations (frequency of aberrant cells=%AB.C.) in exposed workers with those in non-exposed matched controls in the same plant or in other individuals living in the region of the same petrochemical industry. Workers potentially exposed to acrylonitrile at this site were also evaluated in 2000, along with another unexposed matched control group. The %AB.C. values of exposed workers and their controls were also compared with reference values determined for normal individuals (ages 20-59 years) throughout the CR. Substantial discrepancies were noted between subjects in the region of the petrochemical industry (exposed workers and controls) for the years 2000 and 2004 and the reference CR-wide normal values that had been determined during an earlier time period. The matched non-exposed controls at the petrochemical industry site showed a mean %AB.C. value of 1.56+/-1.23% (N=25) in 1998; this rose to a mean of 2.65+/-2.29% (N=33) in 2000. In 2004, values for non-exposed matched controls at the industry site were 2.64+/-1.75% for males (N=25) and 2.38+/-1.74% (N=26) for females. However, the earlier determined CR-wide %AB.C. mean reference values for normal individuals were 1.77+/-1.16% (N=1305) for the interval 1977-1988 and 1.45+/-1.17% (N=2140) for the interval 1991-1999. As both reference values are substantially lower than those determined in 2000 and 2004 for the non-exposed matched controls at the petrochemical industry site, an analysis of the CR-wide mean normal individual reference values for this same 2000-2004 period was conducted. Unexpectedly, it was found that this reference value too had risen to 1.95+/-1.36% (N=1045) and was comparable to the concurrent matched control values at the petrochemical industry site where the monitoring studies were conducted. This substantial increase in %AB.C. values in 2000 and 2004, therefore, has occurred throughout the CR and is probably unrelated to chemicals uniquely present at the petrochemical industry site.

Structure of the 1,4-Bis(2'-deoxyadenosin-N(6)-yl)-2S,3S-butanediol intrastrand DNA cross-link arising from butadiene diepoxide in the human N-ras codon 61 sequence

The 1,4-bis(2'-deoxyadenosin-N(6)-yl)-2S,3S-butanediol intrastrand DNA cross-link arises from the bis-alkylation of tandem N(6)-dA sites in DNA by R,R-butadiene diepoxide (BDO(2)). The oligodeoxynucleotide 5'-d(C(1)G(2)G(3)A(4)C(5)X(6)Y(7)G(8)A(9)A(10)G(11))-3'.5'-d(C(12)T(13)T(14)C(15)T(16)T(17)G(18)T(19)C(20)C(21)G(22))-3' contains the BDO(2) cross-link between the second and third adenines of the codon 61 sequence (underlined) of the human N-ras protooncogene and is named the (S,S)-BD-(61-2,3) cross-link (X,Y = cross-linked adenines). NMR analysis reveals that the cross-link is oriented in the major groove of duplex DNA. Watson-Crick base pairing is perturbed at base pair X(6).T(17), whereas base pairing is intact at base pair Y(7).T(16). The cross-link appears to exist in two conformations, in rapid exchange on the NMR time scale. In the first conformation, the beta-OH is predicted to form a hydrogen bond with T(16) O(4), whereas in the second, the beta-OH is predicted to form a hydrogen bond with T(17) O(4). In contrast to the (R,R)-BD-(61-2,3) cross-link in the same sequence (Merritt, W. K., Nechev, L. V., Scholdberg, T. A., Dean, S. M., Kiehna, S. E., Chang, J. C., Harris, T. M., Harris, C. M., Lloyd, R. S., and Stone, M. P. (2005) Biochemistry 44, 10081-10092), the anti-conformation of the two hydroxyl groups at C(beta) and C(gamma) with respect to the C(beta)-C(gamma) bond results in a decreased twist between base pairs X(6).T(17) and Y(7).T(16), and an approximate 10 degrees bending of the duplex. These conformational differences may account for the differential mutagenicity of the (S,S)- and (R,R)-BD-(61-2,3) cross-links and suggest that stereochemistry plays a role in modulating biological responses to these cross-links (Kanuri, M., Nechev, L. V., Tamura, P. J., Harris, C. M., Harris, T. M., and Lloyd, R. S. (2002) Chem. Res. Toxicol. 15, 1572-1580).

Evaluation of level of DNA damage in blood leukocytes of non-diabetic and diabetic rat exposed to cigarette smoke

The objective of the present study was to use the comet assay to evaluate the steady-state level of DNA damage in peripheral blood leukocytes from diabetic and non-diabetic female Wistar rats exposed to air or to cigarette smoke. A total of 20 rats were distributed into four experimental groups (n=5 rats/group): non-diabetic (control) and diabetic exposed to filtered air; non-diabetic and diabetic exposed to cigarette smoke. A pancreatic beta (beta)-cytotoxic agent, streptozotocin (40 mg/kg b.w.) was used to induce experimental diabetes in rats. Rats placed into whole-body exposure chambers were exposed for 30 min to filtered air (control) or to tobacco smoke generated from 10 cigarettes, twice a day, for 2 months. At the end of the 2-month exposure period, each rat was anesthetized and humanely killed to obtain blood samples for genotoxicity analysis using the alkaline comet assay. Blood leukocytes sampled from diabetic rats presented higher DNA damage values (tail moment=0.57+/-0.05; tail length=19.92+/-0.41, p<0.05) compared to control rats (tail moment=0.34+/-0.02; tail length=17.42+/-0.33). Non-diabetic (tail moment=0.43+/-0.04, p>0.05) and diabetic rats (tail moment=0.41+/-0.03, p>0.05) exposed to cigarette smoke presented non-significant increases in DNA damage levels compared to control group. In conclusion, our data show that the exposure of diabetic rats to cigarette smoke produced no additional genotoxicity in peripheral blood cells of female Wistar rats.

Micronuclei, DNA single-strand breaks and DNA-repair activity in mice exposed to 1,3-butadiene by inhalation

We investigated single-strand breaks and endonuclease III-sensitive sites in DNA along with gamma-irradiation-specific DNA-repair activity in hepatocytes and frequencies of micronuclei in polychromatic bone-marrow erythrocytes of male NMRI mice (2 months old, weight 30-35 g) during sub-acute inhalation exposure to 1,3-butadiene (28 days, 500 mg/m3) and up to 28 days after the exposure. Concentrations of 1,3-butadiene in blood, an indicator of internal exposure, moderately increased during the exposure period. The most interesting finding was that gamma-irradiation-specific DNA-repair activity gradually increased during exposure, being significantly higher compared with control levels on days 7 and 28 of exposure (P = 0.005 and 0.035, respectively), reaching a maximum on day 1 after the termination of exposure (P = 0.003) and then returning to control levels. A significant correlation between gamma-irradiation-specific DNA-repair activity and the concentration of 1,3-butadiene in blood (R = 0.866, P = 0.050) supports a possible induction of DNA-repair activity by the exposure to 1,3-butadiene and formation of its metabolites. The initial increase in micronucleus frequency (micronuclei per 1000 cells) in the exposed mice continuously decreased from 20.4 +/- 5.1 (day 3) to 15.1 +/- 3.2 (day 28) within the exposure period, and subsequently from 12.4 +/- 5.1 to 4.6 +/- 1.6 in the period following termination of the 1,3-butadiene exposure, while micronucleus frequencies in control animals were significantly lower (from 1.7 +/- 1.5 to 4.2 +/- 0.8).

Chromosomal changes: induction, detection methods and applicability in human biomonitoring

The objective of this state of the art paper is to review the mechanisms of induction, the fate, the methodology, the sensitivity/specificity and predictivity of two major cytogenetic endpoints applied for genotoxicity studies and biomonitoring purposes: chromosome aberrations and micronuclei. Chromosomal aberrations (CAs) are changes in normal chromosome structure or number that can occur spontaneously or as a result of chemical/radiation treatment. Structural CAs in peripheral blood lymphocytes (PBLs), as assessed by the chromosome aberration (CA) assay, have been used for over 30 years in occupational and environmental settings as a biomarker of early effects of genotoxic carcinogens. A high frequency of structural CAs in lymphocytes (reporter tissue) is predictive of increased cancer risk, irrespective of the cause of the initial CA increase. Micronuclei (MN) are small, extranuclear bodies that arise in dividing cells from acentric chromosome/chromatid fragments or whole chromosomes/chromatids that lag behind in anaphase and are not included in the daughter nuclei in telophase. The cytokinesis-block micronucleus (CBMN) assay is the most extensively used method for measuring MN in human lymphocytes, and can be considered as a "cytome" assay covering cell proliferation, cell death and chromosomal changes. The key advantages of the CBMN assay lie in its ability to detect both clastogenic and aneugenic events and to identify cells which divided once in culture. Evaluation of the mechanistic origin of individual MN by centromere and kinetochore identification contributes to the high sensitivity of the method. A number of findings support the hypothesis of a predictive association between the frequency of MN in cytokinesis-blocked lymphocytes and cancer development. Recent advances in fluorescence in situ hybridization (FISH) and microarray technologies are modifying the nature of cytogenetics, allowing chromosome and gene identification on metaphase as well as in interphase. Automated scoring by flow cytometry and/or image analysis will enhance their applicability.

The genotoxic risk of underground coal miners from Turkey

A cytogenetic monitoring study was carried out on a group of workers from a bituminous coal mine in Zonguldak province of Turkey, to investigate the genotoxic risk of occupational exposure to coal mine dust. Cytogenetic analysis, namely sister chromatid exchanges (SCEs), chromosomal aberrations (CAs) and micronucleus (MN) tests were performed on a strictly selected group of 39 workers and compared to 34 controls matched for gender, age, and habit. Smoking and age were considered as modulating factors. Both SCE and CA frequencies in coal miners appeared significantly higher than in controls. Similarly, there was a significant increase in the frequency of total micronuclei in exposed group as compared to control group. The effect of smoking on the level of SCE and MN was significant in the control group. A positive correlation between the age and the level of SCE was also found in controls. The frequencies of both SCE and CA were significantly enhanced with the years of exposure. The results of this study demonstrated that occupational exposure to coal mine dust leads to a significant induction of cytogenetic damage in peripheral lymphocytes of workers engaged in underground coal mining.

Lack of genotoxic effect in workers exposed to very low doses of 1,3-butadiene

1,3-Butadiene (BD), a probable carcinogen to humans, has been shown to have an ill-defined genotoxicity in occupationally exposed workers. In the present study, the influence of exposure to very low doses of BD and to cigarette smoking was investigated on some cytogenetic endpoints, namely, sister chromatid exchanges (SCE), chromosomal aberrations (CA) and cells with a high frequency of SCE (HFC), in peripheral blood lymphocytes. Twenty-seven male workers employed in a petrochemical plant and 26 matched controls were included in the study. As regards the airborne BD values, there was a significant difference between exposed (median BD value 1.5, min-max 0.2-69.0 microg/m3) and non-exposed workers (median BD value 0.4, min-max <0.1-3.8 microg/m3). Genotoxic biomarkers were not able to distinguish between the two groups. The frequency of SCE was higher in smokers than in non-smokers (p=0.001), with a positive correlation between the number of cigarettes smoked per day and both SCE (r=0.4; p=0.004) and HFC frequency (r=0.3; p=0.04). Multiple regression analysis confirmed the influence of cigarette smoking on the level of SCE and HFC, while these parameters were not affected by personal exposure to BD. Overall, the biomarkers of genotoxic effect investigated in our study were not able to discriminate between workers with a very low exposure to BD and controls, while it was possible to distinguish between smokers and non-smokers on the basis of SCE.

The effect of smoking on DNA effects in the comet assay: a meta-analysis

The comet assay (alkaline single-cell gel electrophoresis, SCG or SCGE) is frequently used in biomonitoring to detect genotoxic effects in humans exposed at the workplace or in their environment. Because of its ready accessibility, blood is most frequently used in such studies. Many studies investigated cigarette smoking either as a genotoxic exposure itself or as a potential confounding factor in occupational studies. However, although smoking is considered to be a relevant exposure towards various genotoxins, conflicting results have been reported in the comet assay studies. The actual reasons for this discrepancy are not known. To further evaluate evidence for smoking-related DNA effects in the comet assay, we now used a meta-analysis approach based on a literature search. We identified 38 studies from 37 publications which were suited for a formal meta-analysis based on the standardized mean difference (SMD) between the study groups. The evaluation of these 38 studies indicated higher levels of DNA damage in smokers than in non-smokers [under a random effects model, SMD = 0.55, 95% confidence interval = (0.16-0.93)]. Subdividing these studies into studies investigating the effect of smoking as a genotoxic exposure (Type A studies, n = 12) and studies investigating smoking as a potential confounder in occupational studies (Type B, n = 26) indicated a significant difference only in Type A studies but not in Type B studies. Furthermore, studies using image analysis or image length measurements (n = 23) only indicated a tendency for a genotoxic effect of smoking, whereas studies using an arbitrary score (n = 15) found a significantly higher level of DNA damage in smokers.

Differences in DNA-damage in non-smoking men and women exposed to environmental tobacco smoke (ETS)

There is much data implicating environmental tobacco smoke (ETS) in the development and progression of disease, notably cancer, yet the mechanisms for this remain unclear. As ETS is both a pro-oxidant stressor and carcinogen, we investigated the relationship of ETS exposure to intracellular and serum levels of DNA-damage, both oxidative 8-hydroxy-2-deoxyguanosine (8OHdG) and general, in non-smokers from non-smoking households, occupationally exposed to ETS. General DNA-damage consisting of single and double strand breaks, alkali-labile sites and incomplete base-excision repair, increased significantly in a dose-dependent manner with ETS exposure in men (P=0.015, n=32, Pearson) but not women (P=0.736, n=17). Intracellular 8OHdG-DNA-damage and general DNA-damage were both greater in men than women (P=0.0005 and 0.016, respectively) but 8OHdG serum levels did not differ between the genders. Neither 8OHdG-DNA-damage nor serum levels correlated with increasing ETS exposure. This is the first study to demonstrate dose-dependent increases in DNA-damage from workplace ETS exposure. Perhaps most interesting was that despite equivalent ETS exposure, significantly greater DNA-damage occurred in men than women. These data may begin to provide a mechanistic rationale for the generally higher incidence of some diseases in males due to tobacco smoke and/or other genotoxic stressors.

Structure of the 1,4-bis(2'-deoxyadenosin-N6-yl)-2R,3R-butanediol cross-link arising from alkylation of the human N-ras codon 61 by butadiene diepoxide

The solution structure of the 1,4-bis(2'-deoxyadenosin-N(6)-yl)-2R,3R-butanediol cross-link arising from N(6)-dA alkylation of nearest-neighbor adenines by butadiene diepoxide (BDO(2)) was determined in the oligodeoxynucleotide 5'-d(CGGACXYGAAG)-3'.5'-d(CTTCTTGTCCG)-3'. This oligodeoxynucleotide contained codon 61 (underlined) of the human N-ras protooncogene. The cross-link was accommodated in the major groove of duplex DNA. At the 5'-side of the cross-link there was a break in Watson-Crick base pairing at base pair X(6).T(17), whereas at the 3'-side of the cross-link at base pair Y(7).T(16), base pairing was intact. Molecular dynamics calculations carried out using a simulated annealing protocol, and restrained by a combination of 338 interproton distance restraints obtained from (1)H NOESY data and 151 torsion angle restraints obtained from (1)H and (31)P COSY data, yielded ensembles of structures with good convergence. Helicoidal analysis indicated an increase in base pair opening at base pair X(6).T(17), accompanied by a shift in the phosphodiester backbone torsion angle beta P5'-O5'-C5'-C4' at nucleotide X(6). The rMD calculations predicted that the DNA helix was not significantly bent by the presence of the four-carbon cross-link. This was corroborated by gel mobility assays of multimers containing nonhydroxylated four-carbon N(6),N(6)-dA cross-links, which did not predict DNA bending. The rMD calculations suggested the presence of hydrogen bonding between the hydroxyl group located on the beta-carbon of the four-carbon cross-link and T(17) O(4), which perhaps stabilized the base pair opening at X(6).T(17) and protected the T(17) imino proton from solvent exchange. The opening of base pair X(6).T(17) altered base stacking patterns at the cross-link site and induced slight unwinding of the DNA duplex. The structural data are interpreted in terms of biochemical data suggesting that this cross-link is bypassed by a variety of DNA polymerases, yet is significantly mutagenic [Kanuri, M., Nechev, L. V., Tamura, P. J., Harris, C. M., Harris, T. M., and Lloyd, R. S. (2002) Chem. Res. Toxicol. 15, 1572-1580].

Dual roles of glycosyl torsion angle conformation and stereochemical configuration in butadiene oxide-derived N1 beta-hydroxyalkyl deoxyinosine adducts: a structural perspective

The solution structure of the N1-[1-hydroxy-3-buten-2(R)-yl]-2'-deoxyinosine adduct arising from the alkylation of adenine N1 by butadiene epoxide (BDO), followed by deamination to deoxyinosine, was determined in the oligodeoxynucleotide 5'-d(CGGACXAGAAG)-3'.5'-d(CTTCTTGTCCG)-3'. This oligodeoxynucleotide contained the BDO adduct at the second position of codon 61 of the human N-ras protooncogene (underlined) and was named the ras61 R-N1-BDO-(61,2) adduct. 1H NMR revealed a weak C5 H1' to X6 H8 nuclear Overhauser effects (NOE), followed by an intense X6 H8 to X6 H1' NOE. Simultaneously, the X6 H8 to X6 H3' NOE was weak. The resonances arising from the T16 and T17 imino protons were not observed. 1H NOEs between the butadiene moiety and the DNA positioned the adduct in the major groove. Structural refinement based upon a total of 394 NOE-derived distance restraints and 151 torsion angle restraints yielded a structure in which the modified deoxyinosine was in the syn conformation about the glycosyl bond, with a glycosyl bond angle of 83 degrees , and T17, the complementary nucleotide, was stacked into the helix but not hydrogen bonded with the adducted inosine. The refined structure provides a plausible hypothesis as to why these N1 deoxyinosine adducts strongly code for the incorporation of dCTP during trans lesion DNA replication, irrespective of stereochemistry, both in Escherichia coli [Rodriguez, D. A., Kowalczyk, A., Ward, J. B. J., Harris, C. M., Harris, T. M., and Lloyd, R. S. (2001) Environ. Mol. Mutagen. 38, 292-296] and in mammalian cells [Kanuri, M., Nechev, L. N., Tamura, P. J., Harris, C. M., Harris, T. M., and Lloyd, R. S. (2002) Chem. Res. Toxicol. 15, 1572-1580]. Rotation of the N1 deoxyinosine adduct into the syn conformation may facilitate incorporation of dCTP via Hoogsteen type templating with deoxyinosine, generating A to G mutations. However, conformational differences between the R- and the S-N1-BDO-(61,2) adducts, involving the positioning of the butenyl moiety in the major groove of DNA, suggest that adduct stereochemistry plays a secondary role in modulating the biological response to these adducts.

Human population studies with cytogenetic biomarkers: review of the literature and future prospectives

Cytogenetic biomarkers are by far the most frequently used endpoints in human population studies. Their sensitivity for measuring exposure to genotoxic agents and their role as early predictors of cancer risk have contributed to this success. In this article, we present an overview of the last 25 years of population studies with cytogenetic biomarkers, describing the evolution of this research and addressing the most promising innovations for the future. The evaluation has been restricted to the most popular assays, i.e., chromosomal aberrations (CAs) and micronucleus (MN), which are considered to be causally related to early stages of chronic diseases, especially cancer, and may therefore play a major role in prevention. An extensive literature search covering the period 1 January 1980 to 31 December 2003 was performed using the Medline/PubMed database. A total of 833 population studies using CAs and 434 using matched MN inclusion criteria were included in the analysis. We report the distribution of selected papers by year of publication, country, language, agents investigated, and methods employed. The state of the art and future prospects regarding cytogenetic techniques and epidemiologic and statistical methods are discussed. The role of susceptibility and its potential impact on genotoxic damage are discussed with special attention to the effect of major genetic polymorphisms on the baseline frequency of CAs and micronuclei.

Variability in human sensitivity to 1,3-butadiene: influence of polymorphisms in the 5'-flanking region of the microsomal epoxide hydrolase gene (EPHX1)

The carcinogenic effects of 1,3-butadiene (BD), a mutagenic chemical widely used in the manufacture of synthetic rubber, are likely initiated through its epoxide metabolites. In humans, these epoxides are detoxified predominantly by hydrolysis, a reaction mediated by the microsomal epoxide hydrolase (mEH; EPHX1) enzyme. It appears reasonable to hypothesize that BD-exposed individuals possessing lower mEH detoxification capacity may have elevated risk of adverse health effects. The interindividual levels of mEH enzymatic activity vary considerably, and polymorphisms in the mEH gene may contribute to this variability. In addition to the well-studied coding region polymorphisms encoding Tyr113His and His139Arg substitutions, seven other polymorphic sites in the 5'-flanking region of the mEH gene have been reported. These polymorphisms appear to differentially affect mEH gene transcriptional activities. The 5'-flanking region polymorphisms exist in two linkages, the -200 linkage (-200C/T, -259C/T, -290T/G) and the -600 linkage (-362A/G, -613T/C, -699T/C), whereas the -399T/C polymorphism exists as an independent site. Because these polymorphisms may affect total mEH enzymatic activity, we hypothesized that they influence the mutagenic response associated with occupational exposure to BD. We genotyped the 5'-region of the mEH gene in 49 non-smoking workers from two styrene-butadiene rubber facilities in southeast Texas and evaluated the linkage patterns against results obtained from an autoradiographic HPRT mutant lymphocyte assay, used as a biomarker of genotoxic effect. In the study population, 67% were exposed to low BD levels, <150 parts per billion, and 33% were exposed to >150 ppb. We used the observed HPRT mutant (variant) frequency (VF) in the studied population and a 4-way first-order interaction statistical model to estimate parameters that describe the influence of exposure, genotypes and the interaction between the two on the HPRT VF in the target population. The background (baseline) VF, defined as the VF (x 10(-6)) +/- S.E.M. at low levels of BD exposure (<150 ppb) where all the genotypes under study are homozygous wild-type, was estimated to be 4.02 +/- 1.32. Exposure to >150 ppb of BD alone resulted in an estimated increase in VF of 3.42 +/- 2.47 above the baseline level. Inheritance of the variant ATT allele in the -600 linkages resulted in an estimated increase in VF of 3.39 +/- 1.67 above the baseline level. When the interaction between BD exposure and the ATT allele in the -600 linkage group was considered, a statistically significant positive interaction was observed, with an estimated increase in the VF of 10.89 +/- 2.16 (95% CI = 6.56-15.20; p = 0.0027) above baseline. These new data confirm and extend our previous findings that sensitivity to the genotoxic effects of BD is inversely correlated with predicted mEH activity.

Genetic polymorphisms of DNA repair and xenobiotic-metabolizing enzymes: effects on levels of sister chromatid exchanges and chromosomal aberrations

Elevated levels of chromosomal aberrations (CAs) in peripheral blood lymphocytes, widely used as a cytogenetic biomarker of genotoxic effects, have been linked to cancer predisposition. However, tobacco smoking, occupational carcinogen exposure, or time since CA analysis do not appear to explain the cancer predictivity of CAs. Alternatively, the observed CA-cancer association could reflect unidentified exposures or individual susceptibility. We assessed the effects of genetic polymorphisms of DNA repair proteins and xenobiotic-metabolizing enzymes (XMEs) on the levels of CAs and sister chromatid exchanges (SCEs) in peripheral lymphocytes of 145 (CAs) and 60 (SCEs) healthy Caucasians. Genotypes of DNA repair genes X-ray repair cross-complementation group 1 (XRCC1 codons 194, 280, 399) and 3 (XRCC3 codon 241 [corrected]), and XME genes glutathione-S-transferase (GST) M1 and T1 and N-acetyl transferase 2 (NAT2) were determined using polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP)-based methods. After Poisson regression adjustment for age, sex, smoking, country, and genotypes, a higher frequency of chromosome-type breaks was observed for NAT2 slow acetylators (in nonsmokers) and GSTT1 null subjects (in smokers). Individuals carrying variant alleles for XRCC1 codons 280 and 194 showed a decreased level of chromosome-type breaks. The effect of GSTM1 null and XRCC1 codon 399 genotypes on the frequency of CAs was modified by smoking. In linear regression models adjusting for age, sex, smoking, and genotypes, none of the polymorphisms significantly affected SCE frequency, although GSTT1 null subjects had a slightly elevated SCE level. Our results are in line with earlier findings on the influence of NAT2, GSTT1, and GSTM1 polymorphisms on the level of lymphocyte chromosome damage and suggest that also XRCC1 polymorphism affects CA frequencies, thus apparently influencing DNA repair phenotype. It remains to be examined whether these or other genetic polymorphisms could explain the observed cancer risk predictivity of high CA frequency.

Monitoring DNA damage following radiation exposure using cytokinesis–block micronucleus method and alkaline single-cell gel electrophoresis

The structure of DNA can be damaged as a result of exposure to ionizing radiation. Determining the frequency of chromosome aberrations is a well-known method to estimate the dose of radiation received in acute and chronic exposures. In the past few years, cytogenetic analysis has benefited from the development of new techniques, such as the micronucleus (MN) and comet assays, which provide additional information concerning repair capacity after exposure. The present article discusses the use of peripheral blood lymphocytes for the assessment of populations exposed to ionizing radiation. Also discussed are individual factors that interfere with the frequency of mutations and their impact in the selection of control individuals for the monitoring of radiation exposure and in the interpretation of results.

Lack of increased genetic damage in 1,3-butadiene-exposed Chinese workers studied in relation to EPHX1 and GST genotypes

1,3-Butadiene (BD) is an important industrial chemical and pollutant. Its ability to induce genetic damage and cause hematological malignancies in humans is controversial. We have examined chromosome damage by fluorescence in situ hybridization (FISH) and mutations in the HPRT gene in the blood of Chinese workers exposed to BD. Peripheral blood samples were collected and cultured from 39 workers exposed to BD (median level 2 ppm, 6 h time-weighted average) and 38 matched controls in Yanshan, China. No difference in the level of aneuploidy or structural changes in chromosomes 1, 7, 8, and 12 was detected in metaphase cells from exposed subjects in comparison with matched controls, nor was there an increase in the frequency of HPRT mutations in the BD-exposed workers. Because genetic polymorphisms in glutathione S-transferase (GST) enzymes and microsomal epoxide hydrolase (EPHX1) may affect the genotoxic effects of BD and its metabolites, we also related chromosome alterations and gene mutations to GSTT1, GSTM1 and EPHX1 genotypes. Overall, there was no effect of variants in these genotypes on numerical or structural changes in chromosomes 1, 7, 8 and 12 or on HPRT mutant frequency in relation to BD exposure, but the GST genotypes did influence background levels of both hyperdiploidy and HPRT mutant frequency. In conclusion, our data show no increase in chromosomal aberrations or HPRT mutations among workers exposed to BD, even in potentially susceptible genetic subgroups. The study is, however, quite small and the levels of BD exposure are not extremely high, but our findings in China do support those from a similar study conducted in the Czech Republic. Together, these studies suggest that low levels of occupational BD exposure do not pose a significant risk of genetic damage.

Biological monitoring in occupational exposure to low levels of 1,3-butadiene

Exposure to 1,3-butadiene (BD), a probable carcinogen to humans, was investigated in two groups of subjects working in a petrochemical plant where BD is produced and used to prepare polymers: 42 occupationally exposed workers and 43 internal non-occupationally exposed controls. BD personal exposure was very low but significantly different in the two groups (median airborne BD 1.5 and 0.4 microg/m(3) in exposed and controls, respectively). Similarly, BD in blood and urine, but not in exhaled air, was higher in the exposed workers than in controls (blood BD 3.7 ng/l versus <1.8 ng/l, urinary BD 2.4 ng/l versus <1.0 ng/l). These three biomarkers correlated significantly with personal exposure ( 0.283 < or = Pearson's r < or = 0.383) and between them (0.780 < or = r < or = 0.896). Excretion of urinary mercapturic acids N-acetyl-S-(3,4-hydroxybutyl)-l-cysteine (MI), N-acetyl-S-(1-hydroxymethyl-2-propenyl)-l-cysteine and N-acetyl-S-(2-hydroxy-3-butenyl)-l-cysteine (MII), chromosomal aberrations (CA), and sister chromatid exchanges (SCE) in peripheral blood lymphocytes were not influenced by occupational exposure. Our results show that unmetabolised BD in biological fluids, and particularly urinary BD, represents the biomarker of choice for assessing occupational exposure to low airborne concentrations of BD.

Investigations on the effect of cigarette smoking in the comet assay

The comet assay (single-cell gel electrophoresis, SCG) is widely accepted as an in vitro and in vivo genotoxicity test. Because of its demonstrated ability to detect various kinds of DNA damage and its ease of application, the technique is being increasingly used in human biomonitoring. However, the assessment of small genotoxic effects as typically obtained in biomonitoring may be limited by the different sources of assay variability and the lack of an optimal protocol with high sensitivity. To better characterize the suitability of the comet assay for biomonitoring, we are performing a comprehensive investigation on blood samples from smokers and non-smokers. Because tobacco smoke is a well-documented source of a variety of potentially mutagenic and carcinogenic compounds, smokers should be a suitable study group with relevant mutagen exposure. Here, we report our results for the first sample of 20 healthy male smokers and 20 healthy male non-smokers. Baseline and benzo[a]pyrene diolepoxide (BPDE)-induced effects were analysed by two investigators using two image analysis systems. The study was repeated within 4 months. Furthermore, the influence of a repair inhibitor (aphidicolin, APC) on baseline and BPDE-induced DNA damage was comparatively analysed. In all experiments, a reference standard (untreated V79 cells) was included to correct for assay variability. None of these approaches revealed significant differences between smokers and non-smokers. Although more data is needed for a final conclusion, this study indicates some limitations of the comet assay with regard to the detection of DNA damage induced by environmental mutagens in peripheral blood cells.

Markers of individual susceptibility and DNA repair rate in workers exposed to xenobiotics in a tire plant

Workers employed in tire plants are exposed to a variety of xenobiotics, such as 1,3-butadiene (BD), soots containing polycyclic aromatic hydrocarbons, and other organic chemicals (e.g., styrene). In the present study, we investigated markers of genotoxicity [chromosomal aberrations (CAs) and single-strand breaks (SSBs)] in a cohort of 110 tire plant workers engaged in jobs with different levels of xenobiotic exposure in relation to various polymorphisms in genes coding for biotransformation enzymes (CYP1A1, CYP2E1, EPHX1, GSTM1, GSTP1, and GSTT1) and in genes involved in DNA repair (XPD exon 23, XPG exon 15, XPC exon 15, XRCC1 exon 10, and XRCC3 exon 7). In addition, the expression of CYP2E1, a gene playing a key role in BD metabolism, was determined by real-time PCR in peripheral blood lymphocytes, and the capacity of lymphocytes to repair gamma-ray-induced SSBs and to convert 8-oxoguanine in HeLa cell DNA into SSBs was assessed using in vitro assays. No positive associations were detected between the CA frequency or SSB induction and levels of workplace exposure; however, a nonsignificant twofold higher irradiation-specific DNA repair rate was found among highly exposed workers. In evaluations conducted with the markers of individual susceptibility, workers with low-EPHX1-activity genotypes exhibited a significantly higher CA frequency as compared to those with medium and high-EPHX1-activity genotypes (P = 0.050). CA frequencies were significantly lower in individuals homozygous for the XPD exon 23 variant allele in comparison to those with the wild-type CC genotype (P = 0.003). Interestingly, CAs were higher in individuals with higher CYP2E1 expression levels, but the association was nonsignificant (P = 0.097). The results from this study suggest the importance of evaluating markers of individual susceptibility, since they may modulate genotoxic effects induced by occupational exposure to xenobiotics.

Genetic susceptibility, biomarker respones, and cancer

A large number of studies have reported associations between polymorphisms of xenobiotic-metabolizing enzymes (XMEs) and various cancers. However, the carcinogenic exposures behind such findings have usually been unclear. Information on susceptibility to specific carcinogens could better be obtained by examining situations where the exposure and the endpoint studied are nearer in time, i.e., by studying biomarkers of carcinogen exposure and early (genotoxic) effect in exposed humans. For example, analyses of DNA adducts and cytogenetic endpoints have indicated an increased susceptibility of glutathione S-transferase M1 (GSTM1) null genotype to genotoxicity of tobacco smoking, supporting the view that the associations of the GSTM1 null genotype with bladder and lung cancer are partly related to smoking. In vitro genotoxicity studies with human cells offer an experimental tool that can be used, within the limits of the cell systems, to predict individual sensitivity and genotype-carcinogen interactions. In vitro sensitivity to the genotoxicity of 1,2:3,4-diepoxybutane, an epoxide metabolite of 1,3-butadiene has clearly been shown to depend on GSTT1 genotype, which has also been implicated to modify, along with GSTM1 genotype, the in vitro genotoxicity of 1,2-epoxy-3-butene, another epoxide metabolite of 1,3-butadiene. These genotypes appear to modulate the excretion of 1,3-butadiene-specific mercapturic acids, and influence genotoxicity biomarker levels in 1,3-butadiene-exposed workers. The excretion of specific mercapturic acids (PHEMA) in workers exposed to styrene has clearly been shown to depend on GSTM1 genotype, and GSTT1 genotype seems to modulate the excretion of one PHEMA diastereoisomer. These genotypes have also been implicated to modulate the in vitro genotoxicity of styrene. In general, the genetic polymorphisms potentially important for biomarker response largely depend on the exposing agent, biological material examined, and ethnicity of the population under study. Individual exposure level may vary a lot, and a reliable estimate of the exposure is essential for correct interpretation of genotype-exposure interaction. Besides XME polymorphisms, any polymorphisms that affect cellular response to DNA damage could, in principle, modify individual sensitivity to genotoxins. For instance, those concerning DNA repair proteins are presently being studied by many laboratories.

Comet assay on gill cells and hemocytes from the blue mussel Mytilus edulis

Gill cells and hemocytes from the blue mussel Mytilus edulis were examined for DNA damage using the comet assay after laboratory exposure in vitro and in vivo to methyl methansulfonate (MMS). Hydrogen peroxide and UV radiation were used as positive control. Comet assay was also carried out on hemocytes from blue mussels sampled at polluted and unpolluted coastal areas. After 60 min in vitro exposure of gill cells to MMS, the highest response, a tail moment of 6.70+/-4.25, was obtained at 1.0mg/L. At higher doses the response decreased. After 2 days in vivo exposure a dose response was seen at concentrations between 1.0 and 33.0mg/L MMS for both gill cells and hemocytes. However, after 4 days in vivo exposure using the same concentrations of MMS, a maximum effect was seen at a 10 times lower concentration of 3.3mg/L. At the higher doses, the effect decreased. Hemocytes from blue mussels sampled at four polluted sites in Køge Bay had a great variation in tail moments with the highest value of 5.38+/-4.39. The average of all samples from Køge Bay had tail moments of 2.75+/-1.00(n=19), which was significantly higher (P<0.05) than the average, 1.72+/-1.16(n=10), of samples from unpolluted coastal waters.

The use of non-tumor data in cancer risk assessment: reflections on butadiene, vinyl chloride, and benzene

The estimation and characterization of a cancer risk is grounded in the observation of tumors in humans and/or experimental animals. Increasingly, however, other kinds of data (non-tumor data) are finding application in cancer risk assessment. Metabolism and kinetics, adduct formation, genetic damage, mode of action, and biomarkers of exposure, susceptibility, and effects are examples. While these and other parameters have been studied for many important chemicals over the past 30-40 years, their use in risk assessments is more recent, and new insights and opportunities are continuing to unfold. To provide some perspective on this field, the ILSI Risk Science Institute asked a select working group to characterize the pertinent non-tumor data available for 1,3-butadiene, benzene, and vinyl chloride and to comment on the utility of these data in characterizing cancer risks. This paper presents the findings of that working group and concludes with 15 simple principles for the use of non-tumor data in cancer risk assessment.

1,3-Butadiene: exposure estimation, hazard characterization, and exposure-response analysis

1,3-Butadiene has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to 1,3-butadiene primarily through ambient air. Inhaled 1,3-butadiene is carcinogenic in both mice and rats, inducing tumors at multiple sites at all concentrations tested in all identified studies. In addition, 1,3-butadiene is genotoxic in both somatic and germ cells of rodents. It also induces adverse effects in the reproductive organs of female mice at relatively low concentrations. The greater sensitivity in mice than in rats to induction of these effects by 1,3-butadiene is likely related to species differences in metabolism to active epoxide metabolites. Exposure to 1,3-butadiene in the occupational environment has been associated with the induction of leukemia; there is also some limited evidence that 1,3-butadiene is genotoxic in exposed workers. Therefore, in view of the weight of evidence of available epidemiological and toxicological data, 1,3-butadiene is considered highly likely to be carcinogenic, and likely to be genotoxic, in humans. Estimates of the potency of butadiene to induce cancer have been derived on the basis of both epidemiological investigation and bioassays in mice and rats. Potencies to induce ovarian effects have been estimated on the basis of studies in mice. Uncertainties have been delineated, and, while there are clear species differences in metabolism, estimates of potency to induce effects are considered justifiably conservative in view of the likely variability in metabolism across the population related to genetic polymorphism for enzymes for the critical metabolic pathway.

Variability in human sensitivity to 1,3-butadiene: Influence of the allelic variants of the microsomal epoxide hydrolase gene

The carcinogenic effects of 1,3-butadiene (BD), a chemical widely used in the rubber industry, are thought to be due to its epoxide metabolites. In humans, these epoxides are detoxified predominantly by hydrolysis, a reaction mediated by the microsomal epoxide hydrolase (mEH) enzyme. The mEH gene is polymorphic and the most common mEH coding-region variants detected in human populations are the two amino acid polymorphisms Tyr113His and His139Arg. Polymorphic amino acid substitutions at residues 113 and 139 in the human mEH protein can associate in four distinct combinations: Tyr113/His139, Tyr113/Arg139, His113/His139, and His113/Arg139. In vitro studies have shown that each of these genotypes has a unique mEH protein level that can affect net mEH enzymatic activity. In the current study, we examined the relationships among the genotypes involving these two polymorphisms and the mutagenic responses associated with occupational exposure to BD. We studied 49 nonsmoking workers from two styrene-butadiene rubber facilities in southeast Texas using the autoradiographic HPRT mutant lymphocyte assay as a biomarker of genotoxic effect. We genotyped the study participants simultaneously for both polymorphisms, using a multiplex PCR assay developed in our laboratory, and the subjects were assigned to a specific group based on the predicted mEH activity associated with their genotypes (low, intermediate, and high). In the study population, 67% were exposed to low BD levels of <150 ppb (measured by personal badge dosimeters) and 33% were exposed to >150 ppb (mean 2,244 ppb). In the BD low-exposure group, the mEH genotypes had no significant effect on the HPRT variant (mutant) frequency (Vf). In the high-exposure group (BD > 150 ppb), individuals with genotypes associated with low mEH activity had a significant (P < 0.05) 3-fold increase in HPRT Vf (Vf +/- SEM = 13.95 +/- 2.15 x 10(-6)) compared to high-activity individuals (4.41 +/- 1.19 x 10(-6)), and a 2-fold increase in Vf compared to intermediate-activity individuals (6.44 +/- 2.09 x 10(-6)). Our results indicate that mEH genotypes may play a significant role in human sensitivity to the genotoxic effects of exposure to BD.

Biomonitoring of exposure to urban air pollutants: analysis of sister chromatid exchanges and DNA lesions in peripheral lymphocytes of traffic policemen

In order to elucidate the health effects of occupational exposure to traffic fumes, a few biomarkers of early genetic effect were investigated in Rome traffic policemen. One hundred and ninety healthy subjects engaged in traffic control (133 subjects) or in office work (57 subjects) participated the study. For all subjects, detailed information on smoking habits and other potential confounders were recorded by questionnaires. Average exposure of the study groups to benzene and other aromatic hydrocarbons was evaluated in a parallel exposure survey. All workers were genotyped for the following metabolic polymorphisms: CYP1A1 (m1, m2, and m4 variants), CYP2E1 (PstI and RsaI), NQO1 (Hinf1), GSTM1 and GSTT1 (null variants). In this paper, the results of the analysis of sister chromatid exchanges (SCE) in peripheral lymphocytes, and DNA damage by alkaline (pH 13) comet assay in mononuclear blood cells are reported. No statistically significant difference in the frequency of SCE or high frequency cells (HFC) was observed between traffic wardens and office workers (controls), despite the significantly higher exposure to benzene of the former (average group exposure 9.5 versus 3.8microg/m(3), 7h TWA). Conversely, both SCE per cell and HFC were highly significantly (P<0.001) increased in smokers compared to nonsmokers, showing a significant correlation (P<0.001) with the number of cigarettes per day. Multiple regression analyses of data, with metabolic polymorphisms, smoking habits, alcohol consumption, age, gender, and family history of cancer as independent variables, showed that smoking habits, and possibly the CYP2E1 variant genotypes, were the main factors explaining the variance of both SCE and HFC. Within smokers, an association of borderline significance between the CYP1A1 variant genotypes and increased SCE (P=0.050) and HFC (P=0.090) was found. This effect was mainly observed in light smokers (<15 cigarettes per day). The analysis of DNA damage by comet assay did not highlight any statistically significant difference between the exposed and control workers. Moreover, no significant model explaining tail moment variance was obtained by multiple regression analysis using the independent variables shown above. On the whole, these results indicate that exposure to moderate air pollution levels does not result in a detectable increase of genetic damage in blood cells. This evidence does not rule out any possibility of adverse effects, but strongly suggests that in urban residents life-style related factors, such as tobacco smoking, give the prevailing contribution to individual genotoxic burden.

Chromosomal damage in peripheral blood lymphocytes of traffic policemen and taxi drivers exposed to urban air pollution

Urban air contains a diversity of chemical compounds, some of which are genotoxins. An increased risk of cancer has also been reported in occupations with heavy exposure to traffic-related pollution. The aim of this study was to assess the cytogenetic effects of urban air pollution by analyzing the chromosomal aberration (CA) frequencies in lymphocytes and to estimate the polycyclic aromatic hydrocarbons (PAHs) exposure by measuring urinary 1-hydroxypyrene (1-OHP) levels. A total of 15 traffic policemen and 17 taxi drivers working in the city of Ankara were the exposed groups and 23 healthy men working in the office departments were the control group. The overall mean +/- S.D. values of 1-OHP excretions of traffic policemen, taxi drivers and control subjects were 0.59 +/- 0.40 micromol/mol creatinine, 0.32 +/- 0.25 micromol/mol creatinine and 0.57 +/- 0.36 micromol/mol creatinine, respectively. Urinary 1-OHP levels of non-smoking policemen were significantly greater than those of nonsmoking control subjects (p < 0.05). The overall mean +/- S.D. values for CA frequencies (%) from policemen, taxi drivers and control group were 1.29 +/- 1.59, 1.81 +/- 1.79, and 0.26 +/- 0.73, respectively. There was a significantly greater frequency of CAs in exposed groups relative to the matched control population (p < 0.05; p < 0.01). Age, sex and smoking habits have not influenced the cytogenetic end-point in this study. Our results demonstrate that occupational exposure to urban air pollutants leads to a significant induction of cytogenetic damage in peripheral lymphocytes of traffic policemen and taxi drivers.

Genetic polymorphisms and chromosome damage

Genetic polymorphisms that affect xenobiotic metabolism or cellular response to DNA damage can modulate individual sensitivity to genotoxins. Information on the effects of such polymorphisms on the level of chromosome damage may facilitate the identification of risk groups and increase the sensitivity of cytogenetic endpoints as biomarkers of genotoxic exposure and effect. Glutathione S-transferase M1 (GSTM1) is an important detoxification enzyme which, due to a homozygous gene deletion (null genotype), is lacking from about 50% of Caucasians. A higher level of DNA adducts and chromosome damage has been detected in lymphocytes of tobacco smokers and bus drivers who lack the GSTM1 gene. Other polymorphic glutathione S-transferases include GSTM3, GSTP1, and GSTT1. The GSTT1 null genotype (10-20% of Caucasians) has been associated with an increased "baseline" level of sister chromatid exchanges (SCEs) in lymphocytes. N-acetyltransferase 2 (NAT2), metabolizing xenobiotics with primary aromatic amine and hydrazine structures, is another important polymorphic phase II enzyme. Subjects having the NAT2 slow acetylator genotype appear to show an increased baseline frequency of lymphocyte CAs in the absence of identified environmental exposure. Besides human biomonitoring studies, genetic polymorphisms may be important in explaining individual variation in genotoxic response observed in genetic toxicology tests with human cells. Several studies have suggested that blood cultures from GSTT1 null and GSTM1 null individuals have increased in vitro sensitivity to various genotoxins. The best-known example is probably the diepoxybutane sensitivity of GSTT1 null donors. Recently discovered polymorphisms affecting DNA repair may be expected to be of special importance in modulating genotoxic effects; the first available studies have suggested that the exon 10 Arg399Gln polymorphism of XRCC1 gene (X-ray repair cross-complementing group 1) could affect individual genotoxic response. In conclusion, the genetic polymorphism of GSTM1 influences the frequency of chromosome damage in exposed humans, while that of GSTT1 and NAT2 affect the "baseline" level of such damage. Both GSTM1 and GSTT1 genotypes may shape the in vitro genotoxic response of human lymphocytes. The significance of DNA repair polymorphisms is presently unclear.