hprt mutant lymphocyte frequencies in workers at a 1,3-butadiene production plant

Synthesis and characterization of the trans- and cis-isomers of N-acetyl-S-(4-hydroxy-2-buten-1-yl)-L-cysteine and their attempted detection in human urine

1,3-Butadiene (BD) is a carcinogenic air pollutant. N-acetyl-S-(4-hydroxy-2-buten-1-yl)-L-cysteine (MHBMA3 or 4HBeMA), an urinary BD metabolite with unspecified configuration, is considered the most sensitive BD biomarker and has been used in routine biomonitoring since 2012. However, two issues remain unaddressed: why its concentrations are unusually high relative to other urinary BD biomarkers and why some authors reported no detection of the biomarker whereas other authors readily quantitated it. To address the issues, we synthesized and structurally characterized the authentic trans- and cis-isomers of MHBMA3 (designated NE and NZ, respectively), developed an isotope-dilution LC-MS/MS method for their quantification, and examined 67 urine samples from barbecue restaurant personnel (n = 47) and hotel administrative staff (n = 20). The restaurant personnel were exposed to barbecue fumes, which contain relatively high concentrations of BD. The results showed that NE and NZ had highly similar NMR spectra, and were difficult to be well separated chromatographically. The NMR data showed that the MHBMA3 isomer investigated in most previous studies was NE. We did not detect NE and NZ in any samples; however, an interfering peak with varying heights was observed in most samples. Notably, under the chromatographic conditions used in the literature, the peak exhibited indistinguishable retention time from that of NE. Thus, it is highly likely that the interfering peak has been mis-identified as NE in previous studies, providing a reasonable explanation for the high MHBMA3 concentration in urine. The contradiction in the presence of MHBMA3 in urine was also caused by the mis-identification, because the researchers who reported the absence of MHBMA3 were actually detecting NZ. Thus, we clarified the confusion on MHBMA3 in previous studies through correctly identifying the two MHBMA3 isomers. The presence of NE and NZ in human urine warrants further investigations.

Mutagenicity monitoring in humans: Global versus specific origin of mutations

An underappreciated aspect of human mutagenicity biomonitoring is tissue specificity reflected in different assays, especially those that measure events that can only occur in developing bone marrow (BM) cells. Reviewed here are 9 currently-employed human mutagenicity biomonitoring assays. Several assays measure chromosome-level events in circulating T-lymphocytes (T-cells), i.e., traditional analyses of aberrations, translocation studies involving chromosome painting and fluorescence in situ hybridization (FISH) and determinations of micronuclei (MN). Other T-cell assays measure gene mutations. i.e., hypoxanthine-guanine phosphoriboslytransferase (HPRT) and phosphoribosylinositol glycan class A (PIGA). In addition to the T-cell assays, also reviewed are those assays that measure events in peripheral blood cells that necessarily arose in BM cells, i.e., MN in reticulocytes; glycophorin A (GPA) gene mutations in red blood cells (RBCs), and PIGA gene mutations in RBC or granulocytes. This review considers only cell culture- or cytometry-based assays to describe endpoints measured, methods, optimal sampling times, and sample summaries of typical quantitative and qualitative results. However, to achieve its intended focus on the target cells where events occur, kinetics of the cells of peripheral blood that derive at some point from precursor cells are reviewed to identify body sites and tissues where the genotoxic events originate. Kinetics indicate that in normal adults, measured events in T-cells afford global assessments of in vivo mutagenicity but are not specific for BM effects. Therefore, an agent's capacity for inducing mutations in BM cells cannot be reliably inferred from T-cell assays as the magnitude of effect in BM, if any, is unknown. By contrast, chromosome or gene level mutations measured in RBCs/reticulocytes or granulocytes must originate in BM cells, i.e. in RBC or granulocyte precursors, thereby making them specific indicators for effects in BM. Assays of mutations arising directly in BM cells may quantitatively reflect the mutagenicity of potential leukemogenic agents.

Applying Tobacco, Environmental, and Dietary-Related Biomarkers to Understand Cancer Etiology and Evaluate Prevention Strategies

Many human cancers are caused by environmental and lifestyle factors. Biomarkers of exposure and risk developed by our team have provided critical data on internal exposure to toxic and genotoxic chemicals and their connection to cancer in humans. This review highlights our research using biomarkers to identify key factors influencing cancer risk as well as their application to assess the effectiveness of exposure intervention and chemoprevention protocols. The use of these biomarkers to understand individual susceptibility to the harmful effects of tobacco products is a powerful example of the value of this type of research and has provided key data confirming the link between tobacco smoke exposure and cancer risk. Furthermore, this information has led to policy changes that have reduced tobacco use and consequently, the tobacco-related cancer burden. Recent technological advances in mass spectrometry led to the ability to detect DNA damage in human tissues as well as the development of adductomic approaches. These new methods allowed for the detection of DNA adducts in tissues from patients with cancer, providing key evidence that exposure to carcinogens leads to DNA damage in the target tissue. These advances will provide valuable insights into the etiologic causes of cancer that are not tobacco-related.See all articles in this CEBP Focus section, "Environmental Carcinogenesis: Pathways to Prevention."

Base Excision Repair of N6-Deoxyadenosine Adducts of 1,3-Butadiene

The important industrial and environmental carcinogen 1,3-butadiene (BD) forms a range of adenine adducts in DNA, including N⁶-(2-hydroxy-3-buten-1-yl)-2'-deoxyadenosine (N⁶-HB-dA), 1,N⁶-(2-hydroxy-3-hydroxymethylpropan-1,3-diyl)-2'-deoxyadenosine (1,N⁶-HMHP-dA), and N⁶,N⁶-(2,3-dihydroxybutan-1,4-diyl)-2'-deoxyadenosine (N⁶,N⁶-DHB-dA). If not removed prior to DNA replication, these lesions can contribute to A → T and A → G mutations commonly observed following exposure to BD and its metabolites. In this study, base excision repair of BD-induced 2'-deoxyadenosine (BD-dA) lesions was investigated. Synthetic DNA duplexes containing site-specific and stereospecific (S)-N⁶-HB-dA, (R,S)-1,N⁶-HMHP-dA, and (R,R)-N⁶,N⁶-DHB-dA adducts were prepared by a postoligomerization strategy. Incision assays with nuclear extracts from human fibrosarcoma (HT1080) cells have revealed that BD-dA adducts were recognized and cleaved by a BER mechanism, with the relative excision efficiency decreasing in the following order: (S)-N⁶-HB-dA > (R,R)-N⁶,N⁶-DHB-dA > (R,S)-1,N⁶-HMHP-dA. The extent of strand cleavage at the adduct site was decreased in the presence of BER inhibitor methoxyamine and by competitor duplexes containing known BER substrates. Similar strand cleavage assays conducted using several eukaryotic DNA glycosylases/lyases (AAG, Mutyh, hNEIL1, and hOGG1) have failed to observe correct incision products at the BD-dA lesion sites, suggesting that a different BER enzyme may be involved in the removal of BD-dA adducts in human cells.

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.

Genotoxic risk in rubber manufacturing industry: a systematic review

A large body of evidence from epidemiological studies among workers employed in the rubber manufacturing industry has indicated a significant excess cancer risk in a variety of sites. The International Agency for Research on Cancer has recently classified the "Occupational exposures in the rubber-manufacturing industry" as carcinogenic to humans (Group 1). A genotoxic mechanism for the increased cancer risk was suggested on the basis of the evidence from the scientific literature. Exposure assessment studies have shown that workers in the rubber manufacturing industry may be exposed to different airborne carcinogenic and/or genotoxic chemicals, such as certain aromatic amines, polycyclic aromatic hydrocarbons, N-nitrosamines, although the available information does not allow to establish a causal association of cancer or genotoxic risk with particular substances/classes of chemicals or specific jobs. The aim of this paper is to critically evaluate, by conducting a systematic review, the available biomonitoring studies using genotoxicity biomarkers in rubber manufacturing industry. This systematic review suggests that a genotoxic hazard may still be present in certain rubber manufacturing industries. A quantitative risk assessment needs further studies addressing the different, processes and chemicals in the rubber manufacturing industries.

(32)P-postlabelling analysis of 1,3-butadiene-induced DNA adducts in vivo and in vitro

Butadiene monoepoxide (BMO), epoxybutanediol (EBD) and diepoxybutane (DEB) are reactive metabolites of 1,3-butadiene (BD), an important industrial chemical classified as a probable human carcinogen. The covalent interactions of these metabolites with DNA lead to the formation of DNA adducts which may induce mutations or other types of DNA damage, resulting in tumour formation. In the present study, two pairs of diastereomeric N-1-BMO-adenine adducts were identified in the reaction of BMO with 2´-deoxyadenosine-5´-monophosphate (5´-dAMP). The major products formed by reacting EBD with 2´-deoxyguanosine-5´-monophosphate (5´-dGMP) were characterized as diastereomeric N-7-(2´,3´,4´-trihydroxybut-1´-yl)-5´-dGMP by UV and electrospray mass spectrometry. The formation of N-7-BMO-guanine adducts (1´-carbon, 60; 2´carbon, 54/10(4) nucleotides) in BMO-treated DNA was about four times higher than that of N-1-BMO-adenine adducts (1´-carbon, 20; 2´-carbon, 8.7/10(4) nucleotides). However, the recovery of N-1-BMO-adenine adducts in DNA (45 ± 5%) was two times higher than that of N-7-guanine adducts (20 ± 4%) by 32P-postlabelling analysis. Using the 32P-postlabelling/ HPLC assay, N-1-BMO-adenine, N-7-BMO-guanine and N-7-EBDguanine adducts were detected in BMO- or DEB-treated DNA and in liver DNA of rats exposed to BD by inhalation. The amount of N-7-EBD-guanine adducts (11/10(8) nucleotides) in rat liver was about three-fold higher than N-7-BMO-guanine adducts (4.0/10(8) nucleotides). The novel finding of N-1-BMO-adenine adducts formed in vivo may contribute to the understanding of the mechanisms of BD carcinogenic action.

Persistence and repair of bifunctional DNA adducts in tissues of laboratory animals exposed to 1,3-butadiene by inhalation

1,3-Butadiene (BD) is an important industrial and environmental chemical classified as a human carcinogen. The mechanism of BD-mediated cancer is of significant interest because of the widespread exposure of humans to BD from cigarette smoke and urban air. BD is metabolically activated to 1,2,3,4-diepoxybutane (DEB), which is a highly genotoxic and mutagenic bis-alkylating agent believed to be the ultimate carcinogenic species of BD. We have previously identified several types of DEB-specific DNA adducts, including bis-N7-guanine cross-links (bis-N7-BD), N(6)-adenine-N7-guanine cross-links (N(6)A-N7G-BD), and 1,N(6)-dA exocyclic adducts. These lesions were detected in tissues of laboratory rodents exposed to BD by inhalation ( Goggin et al. (2009) Cancer Res. 69 , 2479 -2486 ). In the present work, persistence and repair of bifunctional DEB-DNA adducts in tissues of mice and rats exposed to BD by inhalation were investigated. The half-lives of the most abundant cross-links, bis-N7G-BD, in mouse liver, kidney, and lungs were 2.3-2.4 days, 4.6-5.7 days, and 4.9 days, respectively. The in vitro half-lives of bis-N7G-BD were 3.5 days (S,S isomer) and 4.0 days (meso isomer) due to their spontaneous depurination. In contrast, tissue concentrations of the minor DEB adducts, N7G-N1A-BD and 1,N(6)-HMHP-dA, remained essentially unchanged during the course of the experiment, with an estimated t(1/2) of 36-42 days. No differences were observed between DEB-DNA adduct levels in BD-treated wild type mice and the corresponding animals deficient in methyl purine glycosylase or the Xpa gene. Our results indicate that DEB-induced N7G-N1A-BD and 1,N(6)-HMHP-dA adducts persist in vivo, potentially contributing to mutations and cancer observed as a result of BD exposure.

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.

Hemoglobin adducts in 1,3-butadiene exposed Czech workers: female-male comparisons

We previously reported results of a molecular epidemiological study of female and male 1,3-butadiene (BD) exposed Czech workers showing that females appeared to absorb or metabolize less BD per unit exposure concentration than did males, based on metabolite concentrations in urine (Chem. Biol. Interact. 166 (2007) 63-77). However, that unexpected observation could not be verified at the time because the only additional BD metabolite measurement attempted was for 1,2,3,4-diepoxybutane (DEB) as reflected in specific N,N[2,3-dihydroxy-1,4-butyl]valine (pyr-Val) hemoglobin adduct concentrations, which were not quantifiable in any subject with the method then employed. Neither somatic gene mutations nor chromosome aberrations were associated with BD exposure levels in that study, consistent with findings in an earlier Czech study of males only. We have since measured production and accumulation of the 1,2-dihydroxy-3,4-epoxybutane (EBD) metabolite as reflected in N-[2,3,4-trihydroxy-butyl]valine (THB-Val) hemoglobin adduct concentrations. The mean THB-Val concentration was significantly higher in exposed males than in control males (922.3pmol/g and 275.5pmol/g, respectively), but exposed and control females did not differ significantly (224.5pmol/g and 181.1pmol/g, respectively). In both the control and exposed groups mean THB-Val concentrations were significantly higher for males than females. THB-Val concentrations were significantly correlated with mean 8-h TWA exposures for both males and females, but the rate of increase with increasing BD exposure was significantly lower for females. THB-Val concentrations also increased with increasing urine M2 metabolite [isomeric mixture of 1-hydroxy-2-{N-actylcysteinyl}-3-butene and 2-hydroxy-1-{N-acetylcysteinyl}-3-butene] concentrations in both sexes but the rate of increase was also lower in females than in males. There were no significant correlations between THB-Val concentrations and either somatic gene mutations or chromosome aberrations in either males or females. These results using another biomarker to measure a second metabolite of BD support the original conclusion that females absorb or metabolize less BD than males per unit exposure and indicate that the size of the difference increases with exposure. This observation in humans differs from findings in rodents where at prolonged exposures to high BD levels the females form higher amounts of hemoglobin adducts than do males, a difference that disappears at shorter duration lower exposure levels, while female susceptibility to BD induced mutations and tumorgenesis in rodents appears to persist at all BD exposure levels.

Molecular dosimetry of 1,2,3,4-diepoxybutane-induced DNA-DNA cross-links in B6C3F1 mice and F344 rats exposed to 1,3-butadiene by inhalation

1,3-Butadiene (BD) is an important industrial and environmental chemical classified as a human carcinogen based on epidemiologic studies in occupationally exposed workers and animal studies in laboratory rats and mice. BD is metabolically activated to three epoxides that can react with nucleophilic sites in biomolecules. Among these, 1,2,3,4-diepoxybutane (DEB) is considered the ultimate carcinogen due to its high genotoxicity and mutagenicity attributed to its ability to form DNA-DNA cross-links. Our laboratory has developed quantitative high-performance liquid chromatography-muESI(+)-tandem mass spectrometry methods for two DEB-specific DNA-DNA cross-links, 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) and 1-(guan-7-yl)-4-(aden-1-yl)-2,3-butanediol (N7G-N1A-BD). This report describes molecular dosimetry analysis of these adducts in tissues of B6C3F1 mice and F344 rats exposed to a range of BD concentrations (0-625 ppm). Much higher (4- to 10-fold) levels of DEB-DNA cross-links were observed in mice compared with rats exposed to the same BD concentrations. In both species, bis-N7G-BD levels were 1.5- to 4-fold higher in the liver than in other tissues examined. Interestingly, tissues of female animals exposed to BD contained higher concentrations of bis-N7G-BD adducts than tissues of male animals, which is in accord with previously reported differences in tumor incidence. The molecular dosimetry data presented herein suggest that species and gender differences observed in BD-induced cancer are directly related to differences in the extent of BD metabolism to DEB. Furthermore, a rat model of sensitivity to BD may be more appropriate than a mouse model for assessing human risk associated with BD exposure, because rats and humans seem to be similar with respect to DEB formation.

Quantitative high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry analysis of the adenine-guanine cross-links of 1,2,3,4-diepoxybutane in tissues of butadiene-exposed B6C3F1 mice

1,3-Butadiene (BD) is an important industrial chemical used in the manufacture of rubber and plastics as well as an environmental pollutant present in automobile exhaust and cigarette smoke. It is classified as a known human carcinogen based on the epidemiological evidence in occupationally exposed workers and its ability to induce tumors in laboratory animals. BD is metabolically activated to several reactive species, including 1,2,3,4-diepoxybutane (DEB), which is hypothesized to be the ultimate carcinogenic species due to its bifunctional electrophilic nature and its ability to form DNA-DNA and DNA-protein cross-links. While 1,4- bis-(guan-7-yl)-2,3,-butanediol ( bis-N7G-BD) is the only type of DEB-specific DNA adduct previously quantified in vivo, four regioisomeric guanine-adenine (G-A) cross-links have been observed in vitro: 1-(guan-7-yl)-4-(aden-1-yl)-2,3-butanediol (N7G-N1A-BD), 1-(guan-7-yl)-4-(aden-3-yl)-2,3-butanediol (N7G-N3A-BD), 1-(guan-7-yl)-4-(aden-7-yl)-2,3-butanediol (N7G-N7A-BD), and 1-(guan-7-yl)-4-(aden-6-yl)-2,3-butanediol (N7G-N (6)A-BD) ( Park ( 2004) Chem. Res. Toxicol. 17, 1638- 1651 ). The goal of the present work was to develop an isotope dilution HPLC-positive mode electrospray ionization-tandem mass spectrometry (HPLC-ESI (+)-MS/MS) method for the quantitative analysis of G-A DEB cross-links in DNA extracted from BD-exposed laboratory animals. In our approach, G-A butanediol conjugates are released from the DNA backbone by thermal or mild acid hydrolysis. Following solid-phase extraction, samples are subjected to capillary HPLC-ESI (+)-MS/MS analysis with (15)N 3, (13)C 1-labeled internal standards. The detection limit of our current method is 0.6-1.5 adducts per 10 (8) normal nucleotides. The new method was validated by spiking G-A cross-link standards (10 fmol each) into control mouse DNA (0.1 mg), followed by sample processing and HPLC-ESI (+)-MS/MS analysis. The accuracy and precision were calculated as 105 +/- 17% for N7G-N3A-BD, 102 +/- 25% for N7G-N7A-BD, and 79 +/- 11% for N7G-N (6)A-BD. The regioisomeric G-A DEB adducts were formed in a concentration-dependent manner in DEB-treated calf thymus DNA, with N7G-N1A-BD found in the highest amounts. Under physiological conditions, N7G-N1A-BD underwent Dimroth rearrangement to N7G-N (6)A-BD ( t 1/2 = 114 h), while hydrolytic deamination of N7G-N1A-BD to the corresponding hypoxanthine lesion was insignificant. We found that for in vivo samples, a greater sensitivity could be achieved if N7G-N1A-BD adducts were converted to the corresponding N7G-N (6)A-BD lesions by forced Dimroth rearrangement. Liver DNA extracted from female B6C3F1 mice that underwent inhalation exposure to 625 ppm BD for 2 weeks contained 3.1 +/- 0.6 N7G-N1A-BD adducts per 10 (8) nucleotides ( n = 5) (quantified as N7G-N (6)A-BD following base-induced Dimroth rearrangement), while the amounts of N7G-N3A-BD and N7G-N7A-BD were below the detection limit of our method. None of the G-A cross-links was present in control animals. The formation of N7G-N1A-BD cross-links may contribute to the induction of AT base pair mutations following exposure to BD. Quantitative methods presented here may be used not only for studies of biological significance in animal models but potentially to predict risk associated with human exposure to BD.

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.

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).

Toxicology of 1,3-butadiene, chloroprene, and isoprene

The diene monomers, 1,3-butadiene, chloroprene, and isoprene, respectively, differ only in substitution of a hydrogen, a chlorine, or a methyl group at the second of the four unsaturated carbon atoms in these linear molecules. Literature reviewed in the preceding sections indicates that these chemicals have important uses in synthesis of polymers, which offer significant benefits within modern society. Additionally, studies document that these monomers can increase the tumor formation rate in various organs of rats and mice during chronic cancer bioassays. The extent of tumor formation versus animal exposure to these monomers varies significantly across species, as well among strains within species. These studies approach, but do not resolve, important questions of human risk from inhalation exposure. Each of these diene monomers can be activated to electrophilic epoxide metabolites through microsomal oxidation reactions in mammals. These epoxide metabolites are genotoxic through reactions with nucleic acids. Some of these reactions cause mutations and subsequent cancers, as noted in animal experiments. Significant differences exist among the compounds, particularly in the extent of formation of highly mutagenic diepoxide metabolites, when animals are exposed. These metabolites are detoxified through hydrolysis by epoxide hydrolase enzymes and through conjugation with glutathione with the aid of glutathione S-transferase. Different strains and species perform these reactions with varying efficacy. Mice produce these electrophilic epoxides more rapidly and appear to have less adequate detoxification mechanisms than rats or humans. The weight of evidence from many studies suggests that the balance of activation versus detoxification offers explanation of differing sensitivities of animals to these carcinogenic actions. Other aspects, including molecular biology of the many processes that lead through specific mutations to cancer, are yet to be understood. Melnick and Sills (2001) compared the carcinogenic potentials of these three dienes, along with that of ethylene oxide, which also acts through an epoxide intermediate. From the number of tissue sites where experimental animal tumors were detected, butadiene offers greatest potential for carcinogenicity of these dienes. Chloroprene and then isoprene appear to follow in this order. Comparisons among these chemicals based on responses to external exposures are complicated by differences among studies and of species and tissue susceptibilities. Physiologically based pharmacokinetic models offer promise to overcome these impediments to interpretation. Mechanistic studies at the molecular level offer promise for understanding the relationships among electrophilic metabolites and vital genetic components. Significant improvements in minimization of industrial worker exposures to carcinogenic chemicals have been accomplished after realization that vinyl chloride caused hepatic angiosarcoma in polymer production workers (Creech and Johnson 1974; Falk et al. 1974). Efforts continue to minimize disease, particularly cancer, from exposures to chemicals such as these dienes. Industry has responded to significant challenges that affect the health of workers through efforts that minimize plant exposures and by sponsorship of research, including animal and epidemiological studies. Governmental agencies provide oversight and have developed facilities that accomplish studies of continuing scientific excellence. These entities grapple with differences in perspective, objectives, and interpretation as synthesis of knowledge develops through mutual work. A major challenge remains, however, in assessment of significance of environmental human exposures to these dienes. Such exposure levels are orders of magnitude less than exposures studied in experimental or epidemiological settings, but exposures may persist much longer and may involve unknown but potentially significant sensitivities in the general population. New paradigms likely will be needed for toxicological evaluation of these human exposures, which are ongoing but as yet are not interpreted.

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.

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.

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.

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.

Assessment of 1,3-butadiene exposure in polymer production workers using HPRT mutations in lymphocytes as a biomarker

1,3-Butadiene (BD), which is used to make styrene-butadiene rubber, is a potent carcinogen in mice and a probable carcinogen, associated with leukemia, in humans. We have previously used HPRT mutation as a biomarker to evaluate exposures to BD in a monomer production plant. We now report on a study of 49 workers in a styrene-butadiene rubber plant in which we used the concentration of the BD metabolite 1,2-dihydroxy-4-(N-acetylcysteinyl-S)-butane (M1) in urine as a biomarker of exposure and the frequency of HPRT variant (mutant) lymphocytes (Vf) as a biomarker of effect. Workers were assigned to high- and low-exposure groups based on historical information about work areas and jobs. Personal exposure to BD for one work shift was measured using a passive badge dosimeter. Each participant provided a urine specimen and blood sample at the end of the work shift and completed a questionnaire providing information on lifestyle, health, and work activities. The average BD exposures in the high- and low-exposure groups were significantly different, even after excluding two extreme values, (high 1.48 ppm; low 0.15 ppm, p < 0.002). This study was done in 1994 and 1995 before the establishment, in 1996, of the new permissible exposure limit of 1 ppm. Both the mean M1 and the HPRT Vf were more than three times greater in the high-exposure group than in the low-exposure group (p < 0.0005). The three end points correlated with each other, with sample correlation coefficients between 0.4 and 0.6. The correlations among BD exposure and the biomarkers of internal exposure and genotoxicity suggest that occupational exposure to BD, in the range of 1-3 ppm, may be associated with adverse biological effects.

Health risk assessment of 1,3-butadiene as a Priority Substance in Canada

1,3-Butadiene was included in the second list of Priority Substances to be assessed under the Canadian Environmental Protection Act. Potential hazards to human health were characterized on the basis of critical examination of available data on health effects in experimental animals and occupationally exposed human populations, as well as information on mode of action. Based on consideration of all relevant data identified as of April 1998, butadiene was considered highly likely to be carcinogenic to humans, and likely to be a somatic and germ cell genotoxicant in humans. In addition, butadiene may also be a reproductive toxicant in humans. Estimates of the potency of butadiene to induce these effects have been derived on the basis of quantitation of observed exposure-response relationships for the purposes of characterization of risk to the general population in Canada exposed to butadiene in the ambient environment.

Biomarkers for assessing occupational exposures to 1,3-butadiene

The overall objective of this study was to evaluate a continuum of biomarkers in blood and urine for their sensitivities as indicators of low level occupational exposures to 1,3 butadiene (BD). The study design was largely cross-sectional, with biological samples collected within a short timeframe. Personal 8-h BD exposure measures were made on several occasions over a 60-day period for each potentially exposed worker in order provide maximum accuracy for this independent variable and to accommodate the different expression intervals of the several biomarkers. Co-exposures to styrene, toluene and benzene were also measured. The study included 24 BD monomer production workers (mean BD exposure=0.642 mg/m(3)), 34 polymerization workers (mean BD exposure=1.794 mg/m(3)) and 25 controls (mean BD exposure=0.023 mg/m(3)). The several biomarkers were measured by a consortium of investigators at different locations in the US and Europe. These biomarkers included: (1) metabolic genotypes (CYP2E1, EH, GST M1, GST T1, ADH2, ADH3), determined in Prague and Burlington, VT; (2) urinary M1 and M2 metabolites (1,2-dihydroxy-4-[N-acetylcysteinyl]-butane and 1-hydroxy-2-[N-acetylcysteinyl]-3-butene, respectively), determined in Albuquerque, NM and Leiden; (3) hemoglobin adducts (N-[2-dihydroxy-3-butenyl]valine=HBVal and N-[2,3,4-trihydroxybutyl]valine=THBVal), determined in Amsterdam and Chapel Hill, NC, respectively; (4) HPRT mutations determined by autoradiographic assay in Galveston, TX, with slides re-read in Burlington, VT; (6) hypoxanthine-guanine phosphoribosyltransferase (HPRT) mutations determined by cloning assay in Leiden with mutational spectra characterized in Burlington, VT; (7) sister chromatid exchanges and chromosome aberrations determined by standard methods and FISH analysis in Prague. Urinary M1 and M2 metabolites and HBVal and THBVal hemoglobin adducts were all significantly correlated with BD exposure levels, with adducts being the most highly associated. No significant relationships were observed between BD exposures and HPRT mutations or any of the cytogenetic endpoints, regardless of method of assay.

Markers for carcinogenicity among butadiene-polymer workers in China

We examined a spectrum of genotoxic and other outcomes in 41 butadiene-polymer production workers and 38 nonexposed controls, in China, to explore the role of butadiene in human carcinogenesis. Among butadiene-exposed workers, median air exposure was 2 ppm (6-h TWA), due largely to intermittent high-level exposures. Compared to unexposed subjects, butadiene-exposed workers had greater levels of hemoglobin N-(2,3,4-trihydroxybutyl)valine (THBVal) adducts (P<0.0001), and adduct levels tended to correlate, among butadiene-exposed workers, with air measures (P=0.03). Butadiene-exposed workers did not differ, however, from unexposed workers with respect to frequency of uninduced or diepoxybutane-induced sister chromatid exchanges, aneuploidy as measured by fluorescence in situ hybridization of chromosomes 1, 7, 8 and 12, glycophorin A variants or lymphocyte hprt somatic mutation. Also among the exposed, greater THBVal levels were not associated with increases in uninduced sister chromatid exchanges, aneuploidy, glycophorin A, or hprt mutations. Butadiene-exposed workers had greater lymphocyte (P=0.002) and platelet counts (P=0.07) and lymphocytes as a percent of white blood cells were moderately correlated with greater THBVal levels (Spearman's rho=0.32, P=0.07). Among butadiene-exposed workers, several serum cytokines correlated with THBVal adduct levels. Overall, the study demonstrated exposure to butadiene in these workers, by a variety of short-term and long-term measures, but did not show specific genotoxic effects, at the chromosomal or gene levels, related to that exposure.

Assessment of butadiene exposure in synthetic rubber manufacturing workers in Texas using frequencies of hprt mutant lymphocytes as a biomarker

1,3-Butadiene (BD), which is used to manufacture synthetic rubber, is a mutagen and carcinogen. Because past occupational exposures have been associated with an increased risk of leukemia, there has been a dramatic reduction in workplace exposure standards. The health benefits of these reduced levels of occupational exposure to BD will be difficult to evaluate using relatively insensitive traditional epidemiological studies; however, biomarkers can be used to determine whether there are genotoxic effects associated with recent exposures to BD. In past studies of BD-exposed workers in Southeast Texas, we observed an increase in the frequency of lymphocytes with mutations in a reporter gene, hprt. Frequencies of hprt mutant cells correlated with air levels of BD and with the concentration of a BD metabolite in urine. Average exposures to 1-3 parts per million (p.p.m.) of BD were associated with a threefold increase in hprt variant (mutant) frequencies (Vfs). We now report results from a follow-up study of workers in a synthetic rubber plant in Southeast Texas. Thirty-seven workers were evaluated on three occasions over a 2-week period for exposure to BD by the use of personal organic vapor monitors and by determining the concentration of a BD metabolite in urine. The frequency of hprt mutants was determined, by autoradiography, with lymphocyte samples collected 2 weeks after the final exposure measurement. Based on their work locations, the study participants were assigned to high-exposure (N=22) or low-exposure (N=15) groups. The BD exposure, +/-standard error, of the workers in the high-exposure group (1.65+/-0.52 p.p.m.) was significantly greater than the low-exposure group (0.07+/-0.03 p.p.m.; P<0.01). The frequency of hprt mutant lymphocytes was also significantly different in the two groups (high, 10.67+/-1.5 x 10(-6); low, 3.54+/-0.6 x 10(-6); P<0.001). The concentration of the urine metabolite was greater in the high-exposure group, but the difference was not significant. The correlation coefficient between hprt Vf and BD exposure levels was r=0.44 (CI(95), 0.11-0.69; P=0.011). This study reproduced the findings from a previous study at this plant. Although studies of butadiene-exposed workers in other countries have not detected an effect of exposure on frequencies of hprt mutant lymphocytes, we have repeatedly observed this result in our studies in Texas.

Point mutations induced by 1,2-epoxy-3-butene N1 deoxyinosine adducts

The National Toxicology Program has recently classified 1,3-butadiene (BD) as a human carcinogen. BD is metabolized to the intermediates 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB), and 1,2-dihydroxy-3,4-epoxybutane. All three metabolites have been implicated in producing specific types of DNA damage and as genotoxic agents in mice, rat, and human cells. This study has focused on EB-induced N1 deoxyinosine lesions that are formed by deamination of deoxyadenosine following reaction of the epoxide at the N(1) position. The R and S stereoisomers of this lesion were incorporated site-specifically within the context of an 11-mer oligodeoxynucleotide, incorporated into M13mp7L2 single-stranded DNA, and transfected into E. coli. Both stereoisomers modestly reduced plaque-forming ability, indicating that neither lesion presents a base modification that cannot be bypassed. The resulting plaques were assessed for point mutations using differential hybridization and DNA sequence analyses. The overall mutagenic spectrum revealed that the N1 adducts were highly mutagenic (approximately 90% per replication cycle), causing a predominance of A --> G transitions.

A review of the genetic and related effects of 1,3-butadiene in rodents and humans

In this paper, the metabolism and genetic toxicity of 1,3-butadiene (BD) and its oxidative metabolites in humans and rodents is reviewed with attention to newer data that have been published since the latest evaluation of BD by the International Agency for Research on Cancer (IARC). The oxidative metabolism of BD in mice, rats and humans is compared with emphasis on the major pathways leading to the reactive intermediates 1,2-epoxy-3-butene (EB), 1,2:3, 4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBdiol). Results from recent studies of DNA and hemoglobin adducts indicate that EBdiol may play a more significant role in the toxicity of BD than previously thought. All three metabolites are capable of reacting with macromolecules, such as DNA and hemoglobin, and have been shown to induce a variety of genotoxic effects in mice and rats as well as in human cells in vitro. DEB is clearly the most potent of these genotoxins followed by EB, which in turn is more potent than EBdiol. Studies of mutations in lacI and lacZ mice and of the Hprt mutational spectrum in rodents and humans show that mutations at G:C base pairs are critical events in the mutagenicity of BD. In-depth analyses of the mutational spectra induced by BD and/or its oxidative metabolites should help to clarify which metabolite(s) are associated with specific mutations in each animal species and which mutational events contribute to BD-induced carcinogenicity. While the quantitative relationship between exposure to BD, its genotoxicity, and the induction of cancer in occupationally exposed humans remains to be fully established, there is sufficient data currently available to demonstrate that 1,3-butadiene is a probable human carcinogen.

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

The association of occupational exposure to 1,3-butadiene (BD) and induction of cytogenetic damage in peripheral lymphocytes was studied in 19 male workers from a monomer production unit and 19 control subjects from a heat production unit. The exposure to BD was measured by passive personal monitors. The following biomarkers were used: chromosomal aberrations (CA), sister chromatid exchanges (SCE), cells with a high frequency of SCE (HFC), micronuclei, comet assay parameters like tail length (TL) and percentage of DNA in tail [T (%)] and polymorphisms of GSTM1 and GSTT1 genotypes. BD exposure with a median value of 0.53 mg/m3 (range: 0.024-23.0) significantly increased (a) the percentage of cells with chromosomal aberrations in exposed vs. control groups (3.11% vs. 2.03%, P<0.01), (b) the frequency of SCE per cell (6.96 vs. 4.87, P<0.001), and (c) the percentage of HFC (19.9% vs. 4.1%, P<0.001). BD exposure had no significant effects on formation of micronuclei and on comet assay parameters. Effect of smoking was observed only for HFC in BD-exposed group. GSTM1 genotype affected chromosomal aberrations in exposed group, while GSTT1 genotype affected chromosomal aberrations in controls. No effect of GSTM1 or GSTT1 genotypes was observed on any other biomarkers used.

Frequencies of hprt mutant lymphocytes in marijuana-smoking mothers and their newborns

Reports of increases in the prevalence of marijuana smoking, especially among young people, have led to concerns about possible genotoxic effects from marijuana use due to exposure to the mutagenic and carcinogenic agents present in marijuana smoke. Prior studies of the adverse health consequences of marijuana smoking, using disease outcomes, have sometimes been confounded by the fact that most marijuana smokers also smoke tobacco. In the present study, the potential mutagenic effects of marijuana smoking were investigated with a somatic cell mutation assay that detects mutations occurring in vivo in the hprt gene. Subjects were volunteers recruited from a prenatal clinic that performs urine drug screens on all consenting patients. Blood samples were collected from 17 subjects whose drug screens indicated marijuana use, but who did not smoke tobacco or use cocaine or opiates, and 17 non-smokers with negative drug screens. Absence of tobacco use was confirmed by plasma cotinine tests. Cord blood samples were collected from newborns of 5 of the marijuana smokers and 5 non-smokers. Lymphocytes were isolated, cryopreserved, and later thawed and assayed with the autoradiographic hprt assay. The frequency of variant (mutant) lymphocytes (Vf) in the 17 non-smokers (+/- standard error) was 1.93 (+/- 0.17) per million evaluatable cells. The Vf of 17 marijuana smokers was more than three-fold higher, 6.48 (+/- 0.48) x 10(-6), a significant difference, p < 0.001. Cord blood lymphocytes from 5 newborns of non-smokers had a Vf of 0.85 (+/- 0.23) x 10(-6), compared to 2.55 (+/- 0.60) x 10(-6) for 5 newborns of marijuana smokers, significantly higher, p < 0.05. Because of the known association between increases in somatic mutations and the development of malignancies, this study indicates that marijuana smokers may have an elevated risk of cancer. For pregnant marijuana smokers, there is also concern for the possibility of genotoxic effects on the fetus, resulting in heightened risk of birth defects or childhood cancer.

Genetic effects of 1,3-butadiene and associated risk for heritable damage

A summary of the results of the studies conducted in the EU Project "Multi-endpoint analysis of genetic damage induced by 1,3-butadiene and its major metabolites in somatic and germ cells of mice, rats and man" is presented. Results of the project are summarized on the detection of DNA and hemoglobin adducts, on the cytotoxic and clastogenic effects in somatic and germinal cells of mice and rats, on the induction of somatic mutations at the hprt locus of experimental rodents and occupationally exposed workers, on the induction of dominant lethal mutations in mice and rats, and on heritable translocations induced in mice, after exposure to butadiene (BD) or its major metabolites, butadiene monoepoxide (BMO), diepoxybutane (DEB) and butadiene diolepoxide (BDE). The primary goal of this project was to collect experimental data on the genetic effects of BD in order to estimate the germ cell genetic risk to humans of exposure to BD. To achieve this, the butadiene exposure are based on data for heritable translocations and bone marrow micronuclei induced in mice and chromosome aberrations observed in lymphocytes of exposed workers. A doubling dose for heritable translocations in human germ cells of 4900 ppm/h is estimated, which, assuming cumulative BD exposure over the sensitive period of spermatogenesis, corresponds to 5-6 weeks of continuous exposure at the workplace to 20-25 ppm. Alternatively, the rate of heritable translocation induction per ppm/h of BD exposure is estimated to be approximately 0.8 per million live born, compared to a spontaneous incidence of balanced translocations in humans of approximately 800 per million live born. These estimates have large confidence intervals and are only intended to indicate orders of magnitude of human genetic risk. These risk estimates are based on data from germ cells of BD-exposed male mice. The demonstration that clastogenic damage was induced by DEB in preovulatory oocytes at doses which were not ovotoxic implies that additional studies on the response of mammalian female germ cells to BD and its metabolites are needed. The basic assumption of the above genetic risk estimates is that experimental mouse data obtained after BD exposure can be extrapolated to humans. Several points exist in the present report and in the literature which contradict this assumption: (1) the level of BMO-hemoglobin adducts was significantly elevated in BD-exposed workers; however, it was considerably lower than would have been predicted from comparable rat and mouse exposures; (2) the concentrations of the metabolites DEB and BMO were significantly higher in mouse than in rat blood after BD exposure. Thus, while metabolism of BD is qualitatively similar in the two species, it is quantitatively different; (3) no increase of HPRT mutations was shown in 19 workers exposed on average to 1.8 ppm of BD, while in a different population of workers from a US plant exposed on average to 3.5 ppm of BD, a significant increase of HPRT variants was detected; and (4) data from cancer bioassays and cancer epidemiology suggest that rat is a more appropriate model than mouse for human cancer risk from BD exposure. However, the dominant lethal study in rats gave a negative result. At present, we do not know which BD metabolite(s) may be responsible for the genetic effects even though the bifunctional alkylating agent DEB is the most likely candidate for the induction of clastogenic events. Unfortunately, methods to measure DEB adducts in hemoglobin or DNA are only presently being developed. Despite these several uncertainties the use of the mouse genetic data is regarded as a justifiable and conservative approach to human genetic risk estimation given the considerable heterogeneity observed in the biotransformation of BD in humans.

Abnormal DNA repair activities in lymphocytes of workers exposed to 1,3-butadiene

Exposure to high concentrations of butadiene has been shown to cause cancer among exposed workers. We have conducted a biomarker study to elucidate whether current butadiene exposure conditions are hazardous to workers. Twenty-four workers exposed consistently to butadiene were matched with 19 co-workers who had much less contact with butadiene and who served as our controls. In the standard cytogenetic assay, there was no difference in chromosome aberration frequencies between the exposed and control groups. In the challenge assay, the exposed group shows a consistent, but non-significant, increase in chromosome aberrations indicating some abnormality in DNA repair response. The observed dicentric frequency in the challenge assay (indicative of abnormal repair of damaged chromosomes) is significantly correlated with a butadiene metabolite, 1,2-dihydroxy-4-(N-acetylcysteinyl)butane, in urine (r = 0.52; p = 0.04). Furthermore, cigarette smokers had consistently abnormal repair response compared with non-smokers for both the control and exposed groups. A small subset of the studied workers were evaluated for toxicant-induced DNA repair deficiency using an independent cat-host cell reactivation (CAT-HCR) assay. When cigarette smokers and non-smokers were combined in our analysis, we observed that the exposed group (n = 9) had a significant reduction of DNA repair activities (p = 0.009) compared with the control group (n = 6). Cigarette smoking contributed significantly to the effect as exposed smokers (n = 4) had a significant reduction in DNA repair activities (p = 0.04) compared with exposed non-smokers. The results from the two independently conducted assays support each other and confirm the previously reported abnormal DNA repair response in another group of butadiene workers. In conclusion, our data indicates that exposure to environmental toxicants, such as butadiene, can cause DNA repair defects. Therefore, the current butadiene exposure conditions are still hazardous to workers. However, our data indicates that butadiene is not a potent genotoxic agent. Furthermore, the butadiene-induced effect is significantly enhanced by the cigarette smoking habit.

Monophosphate 32P-postlabeling assay of DNA adducts from 1,2:3,4-diepoxybutane, the most genotoxic metabolite of 1,3-butadiene: in vitro methodological studies and in vivo dosimetry

Among the main DNA-reactive metabolites of 1,3-butadiene (BD), both 1,2:3,4-butadiene diepoxide (BDE) and 1,2-epoxy-3-butene (BME) have been reported in mice and rats exposed to BD, but blood and tissue levels of these metabolites are much higher in mice than in rats under similar exposure conditions. BDE, being more reactive and genotoxic than BME, is thought to be responsible for the greater susceptibility of mice to BD carcinogenicity. While BDE is a DNA-alkylating agent and some BDE adducts have been characterized, no sufficiently sensitive method has been reported for studying BDE-DNA binding in vivo. In the present investigation, a modified dinucleotide/monophosphate version of the 32P-postlabeling assay was applied to detect BDE-DNA adducts, which were prepared by reacting BDE with calf thymus DNA or deoxyribooligonucleotides [(AC)10, (AG)10, (CCT)7 and (GGT)7] in vitro or with skin DNA of mice in vivo upon topical treatment. Optimal resolution by 2-D PEI-cellulose TLC of the highly polar 5'-monophosphate adducts was achieved at +4 degrees C using 0.3 M LiCI (DI) and 0.4 M NaCl, 0.04 M H3BO3, pH 7.6 (D2). The profiles of the 32P-postlabeled adducts were similar for calf thymus and skin DNA, with 3 major spots being detected. Adducts obtained in in vitro and in vivo experiments were compared by re- and cochromatography in 4 or 5 different solvents, and these experiments provided evidence that corresponding BDE adducts, for the most part, were identical and represented adenine derivatives. Guanine adducts were not detected by this method although literature data indicate their formation. Quantitatively, the assay responded linearly to adduct concentration, as shown in an experiment where BDE-modified skin DNA was serially diluted up to 81-fold with control DNA. The limit of detection was approximately 1 adduct in 10(8) normal nucleotides. Further, in an in vivo dosimetry study, skin DNA from groups of 8 individual mice treated with different doses of BDE (1.9, 5.7, 17, 51 and 153 mumol/mouse) for 3 days exhibited a linear relationship (r > or = 0.992) between adduct levels and dose. The results suggest that the 32P-postlabeling assay described herein will have utility in mechanistic studies and biomonitoring of DNA adduct formation from BDE and possibly other polar epoxides.

hprt mutation frequency among workers exposed to 1,3-butadiene in China

Hypoxanthine-guanine phosphoribosyl transferase (hprt) mutation frequency (M(f)) was studied in workers at a polybutadiene rubber production facility in Yanshan, China. Exposed workers included for study were active either as process analysts, who sampled butadiene production process lines and analyzed product by gas chromatography, or as process operators, who did routine process control, minor maintenance and, as needed, major repair operations. For process analysts at the polymer and dimethyl formamide (DMF) facilities, the median air levels of BD were 1.0 and 3.5 ppm, respectively. Among process operators, air levels of 1.1 ppm were found during routine activities, while the median air level during pump repair and related operations was 45 ppm (6-h time-weighted average). Overall, M(f) was similar in unexposed (mean M(f) = 20.2 x 10(-6)) and butadiene-exposed (mean M(f) = 21.6 x 10(-6)) workers (P = 0.68). M(f) decreased with cloning efficiency, increased with age, and was moderately greater in women than in men. After adjustment by multiple regression analysis for mean age, sex, and cloning efficiency, the adjusted mean M(f)(Xadj) was 13.6 x 10(-6) in unexposed and 18.0 x 10(-6) in butadiene-exposed. This 32% difference was, however, not statistically significant (P = 0.13). Butadiene exposure was associated with a modest, if any, increase in hprt M(f) in this population of Chinese workers.

Biological monitoring for mutagenic effects of occupational exposure to butadiene

The use of biological markers in the evaluation of human exposure to hazardous agents has increased rapidly in recent years. Because 1,3-butadiene is a mutagenic carcinogen, existing occupational levels of exposure may be appropriately evaluated using somatic cell mutation as a biomarker. Previously, we have described a biomarker study of workers in a butadiene monomer plant (Ward et al., 1994). We now report results from a second study of the same group of workers, conducted after plant modernization, and present preliminary results from a study of exposures in a styrene butadiene rubber (SBR) plant. Air levels of butadiene were determined using either charcoal tubes with air pumps or passive badge dosimeters. The quantity of a butadiene metabolite in the urine was used as a biomarker of exposure and the mutagenic effects of exposure were measured using the autoradiographic hprt mutant lymphocyte assay. In all three studies, the frequencies of hprt mutants were significantly elevated in workers from the areas of highest exposure when compared to workers from lower exposure areas or non-exposed subjects. The concentration of the urinary metabolite was significantly increased in high-exposed workers in the first study of monomer plant workers but not in the second. In the first monomer plant study, historical air concentrations of butadiene were higher in the production units than in the central control unit. While concurrent determined air concentrations were not elevated in the second monomer plant study, they were elevated in high exposure areas in the SBR plant study. Mutant frequencies in the lower-exposure and the non-exposed groups were consistent with historical values for non-smoking individuals who were not exposed to known mutagens. The use of biomarkers, including the hprt mutant lymphocyte assay, may be of great value in determining an appropriate occupational exposure limit for butadiene.

Future research needs for non-cancer and cancer effects among populations exposed to 1,3-butadiene

In the last decade there has been in-depth research into understanding the health effects of 1,3-butadiene in humans and in animals. With increasing knowledge of metabolism, pharmacokinetics and mechanism of action studied in animals, the uncertainties in risk assessment will be lessened. Still, some data gaps exist which, if filled, will be useful for meaningful risk assessments for the general population. This paper discusses the future needs for research in both non-cancer and cancer effects.

Biochemistry of 1,3-butadiene metabolism and its relevance to 1,3-butadiene-induced carcinogenicity

Recently, the roles of specific P450 isoforms, myeloperoxidase (MPO), GSH-S-transferase and epoxide hydrolase in the metabolism of 1,3-butadiene, and its major oxidative metabolite, butadiene monoxide (BM), were investigated. The results provided evidence for P450s 2A6 and 2E1 being major catalysts of 1,3-butadiene oxidation in human liver microsomes. cDNA-expressed human P450s 2E1, 2A6, and 2C9 catalyzed BM oxidation to meso- and (+/-)-diepoxybutane (DEB), but the rates of BM oxidation in mouse, rat, or human liver microsomes were much lower than the rates of 1,3-butadiene oxidation in these tissues. Human MPO catalyzed 1,3-butadiene oxidation to BM, but MPO incubations with BM did not yield DEB. Rates of BM formation in mouse and human liver microsomes were similar and were nearly 3.4-fold higher than that obtained with rat liver microsomes. However, rat liver epoxide hydrolase activity was nearly 2-fold higher than that of mouse liver microsomes. Rat and mouse liver GSH-S-transferases exhibited similar BM conjugation kinetics, but rats excreted more BM-mercapturic acids compared to mice given low equimolar doses of BM. BM reacted with guanosine and adenosine to yield N7-, N2-, and N1-guanosinyl and N6-adenosinyl adducts, respectively. These results may contribute to a better understanding of the biochemical basis of 1,3-butadiene-induced carcinogenicity.

Biological effect monitoring in industrial workers from the Czech Republic exposed to low levels of butadiene

Blood samples were collected twice (in 1993 and 1994) from 19 workers exposed to 1,3-butadiene and 19 matched controls. Three exposed and three control subjects were the same in 1993 and 1994. Personal passive dosimetry was performed in 1993 and twice in 1994 on the day preceding blood sampling. Mean exposure level in 1994 was 1.76 +/- 4.20 ppm (S.D.) and individual exposure levels ranged between 0.012 ppm (detection limit) and 19.77 ppm. Using the clonal assay, geometric mean of hprt mutant frequencies adjusted for cloning efficiency, age and smoking were, respectively, 7.85 (+/- 7.09) x 10(-6) and 10.14 (+/- 9.16) x 10(-6) in pooled (1993 plus 1994) exposed and control subjects. The difference was not statistically significant indicating that 1,3-butadiene did not induce a detectable increase in mutations at the hprt locus. A similar result was obtained for the 1994 subjects alone. There was no difference between adjusted geometric mean mutant frequencies of exposed and unexposed non-smokers or between exposed and unexposed smokers. Analysis of chromosomal aberrations in lymphocytes from 1994 subjects indicated that the percentage of aberrant cells was significantly enhanced in exposed subjects. In 1993 (data not shown), it was impossible to demonstrate a significant increase of aberrant cells in subjects exposed to 1,3-butadiene. Frequencies of micronuclei in cytochalasin-B blocked binucleate lymphocytes in exposed and unexposed 1994 subjects were not significantly different. This was also the case for earlier samples analyzed in the same plant. Using the comet assay for 1994 subjects, no statistically significant difference was found between the whole group of exposed and unexposed subjects. This was true for both the comet tail length and the percentage of DNA in the tail. In exposed smokers, however, the comet tail length was significantly longer than in unexposed smokers. Unexpectedly, in unexposed smokers the tail length was significantly shorter than in unexposed non-smokers. It was also unexpected that the percentage of DNA in the comet tail was significantly lower in exposed non-smokers than in unexposed non-smokers.

Biomonitoring of 1,3-butadiene and related compounds

The 1990 Clean Air Act Amendments list several volatile organic chemicals as hazardous air pollutants, including ethylene oxide, butadiene, styrene, and acrylonitrile. The toxicology of many of these compounds shares several common elements such as carcinogenicity in laboratory animals, genotoxicity of the epoxide intermediates, involvement of cytochrome P450 for metabolic activation (except ethylene oxide), and involvement of at least two enzymes for detoxication of the epoxides (e.g., hydrolysis or conjugation with glutathione). These similarities facilitate research strategies for identifying and developing biomarkers of exposure. This article reviews the current knowledge about biomarkers of butadiene. Butadiene is carcinogenic in mice and rats, which raises concern for potential carcinogenicity in humans. Butadiene is metabolized to DNA-reactive metabolites, including 1,2-epoxy-3-butene and diepoxybutane. These epoxides are thought to play a critical role in butadiene carcinogenicity. Butadiene and some of its metabolites (e.g., epoxybutene) are volatile. Exhalation of unchanged butadiene and excretion of butadiene metabolites in urine represent major routes of elimination. Therefore, biomonitoring of butadiene exposure could be based on chemical analysis of butadiene in exhaled breath, blood levels of butadiene epoxides, excretion of butadiene metabolites in urine, or adducts of butadiene epoxides with DNA or blood proteins. Mutation induction in specific genes (e.g., HPRT) following butadiene exposure can be potentially used as a biomarker. Excretion of 1,2-dihydroxy-4-(N-acetylcysteinyl-S)butane or the product of epoxybutene with N-7 in guanine in urine, epoxybutene-hemoglobin adducts, and HPRT mutation have been used as biomarkers in recent studies of occupational exposure to butadiene. Data in laboratory animals suggest that diepoxybutane may be a more important genotoxic metabolite than epoxybutene. Biomonitoring methods need to be developed for diepoxybutane and other putative reactive butadiene metabolites. With butadiene and related compounds, the ultimate challenge is to identify useful biomarkers of exposure in which quantitative linkages between exposure and internal dose of the important DNA-reactive metabolites are established.

Identification of needs in biomarker research

Interest in the use of biological markers to evaluate future disease risk has increased greatly in recent years. Biomarkers are observable end points in a continuum of events leading from exposure to toxic agents to diseases that ultimately result from exposure. Because many significant diseases develop over long periods of time, methods for detecting early events that can predict risk are important for disease prevention. Biomarkers are generally categorized as detecting exposure, effects of exposure, or individual susceptibility to exposure. Although there has been significant progress in the technical development of biomarkers, implementation of their use in human populations has progressed much more slowly. We discuss four major needs in the development of biomarkers. First, new biomarkers need to be developed to fill gaps in our ability to observe steps in the continuum from exposure to disease. Second, the relationships between biomarker responses and disease pathology needs to be better understood. Third, the sensitivity, specificity, and variability of biomarkers need to be better characterized and they must be better validated as predictors of disease risk. Fourth, there are several societal impediments to the practical implementation of biomarker studies as public health tools. A common agreement among employers, employees, regulators, and the legal community must be established regarding appropriate and ethical uses and interpretation of biomarker data.

Monitoring populations for DNA repair deficiency and for cancer susceptibility

The induction of a mutator phenotype has been hypothesized to cause the accumulation of multiple mutations in the development of cancer. Recent evidence suggests that the mutator phenotype is associated with DNA repair deficiencies. We have been using a challenge assay to study exposed populations to test our hypothesis that exposure to environmental toxicants induce DNA repair deficiency in somatic cells. In this assay, lymphocytes were irradiated in vitro to challenge cells to repair the radiation-induction DNA strand breaks. An increase of chromosome aberrations in the challenged cells from toxicant-exposed populations compared to nonexposed populations is used to indicate abnormal DNA repair response. From studies of cigarette smokers, butadiene-exposed workers, and uranium-exposed residents, the assay showed that these exposed populations had mutagen-induced abnormal DNA repair response. The phenomenon was also demonstrated using experimental animals. Mice were exposed in vivo to two different doses of N-methyl-N'-nitro-N-nitroso-guanidine (MNNG) and their lymphocytes were challenged with one dose of a radiomimetic chemical, bleomycin, in vitro. These challenged lymphocytes showed an MNNG dose-dependent increase of abnormal DNA repair response. In a population that was potentially exposed to teratogens--mothers having children with neural tube defects--lymphocytes from these mothers did not have the abnormal response in our assay. In studies with patients, we reported that lymphocytes from Down's syndrome patients have the abnormal DNA repair response. Lymphocytes from skin cancer-prone patients (epidermodysplasia verruciformis) have normal response to gamma-ray challenge but abnormal response to UV-light challenge. These patient studies also indicate that the challenge assay is useful in documenting the radiosensitivity of Down's syndrome and the UV sensitivity in EV patients. In most cases, the challenge assay is more sensitive in detecting biological effects than the standard chromosome aberration assay. Our series of studies indicates that the challenge assay can be used to document biological effects from exposure to mutagens and that the effect is an abnormal DNA repair response. This abnormality can increase the risk for development of cancer. The repair deficiency is currently being validated using a plasmid transfection (host-reactivation) assay. The need to integrate chromosome aberration and the challenge assays with other relevant assays for better documentation of biological effects and for more precise prediction of health risk will be presented. Our experience in using genetic polymorphism and host-reactivation assays will be discussed.

Molecular and genetic toxicology of 1,3-butadiene

During the last 9 years, there have been many studies published concerning the mutagenic potential of butadiene in mammalian systems, including alterations at the molecular level. Butadiene has tested positive in several mouse in vivo and in vitro assays, but has generally tested negative in rat studies. Most of these studies are cytogenetic and include positive data in mice for chromosomal aberrations, micronucleus formation, and sister chromatid exchanges. Butadiene also induces mutations in lung, spleen, and bone marrow of transgenic mice. The positive bone marrow cytogenetic and transgenic data may be significant in view of the increased lymphohematopoietic malignancies observed in mice and probably in humans. In addition, butadiene causes mutations in the K-ras protooncogene and in the p53 tumor suppressor gene in mouse studies. Mutations in these genes are associated with oncogenesis in humans as well as in rodents. Also, positive mutagenicity data have been obtained in a pilot study of workers exposed to butadiene. Positive dominant lethal studies in rodents suggest that exposure to butadiene can result in germ cell mutation and heritable risk. These mutagenicity and molecular data suggest that butadiene is both a somatic and germ cell mutagen in mammals, possibly including humans.

Chromosome aberrations and response to gamma-ray challenge in lymphocytes of workers exposed to 1,3-butadiene

An integrated population monitoring study was initiated to investigate whether occupational exposure to current low levels of butadiene is mutagenic to workers. Ten exposed workers (mean production area concentration of 3.5 ppm) and 10 matched plant controls (mean exposure to 0.03 ppm) were selected and blood samples were collected for our study. The standard cytogenetic assay was used to determine chromosome aberration frequencies. In addition, a challenge assay was used to determine response to gamma-rays as an indication of DNA repair deficiencies. In the latter assay, cells were exposed to gamma-rays at the G1 phase of the cell cycle in vitro and the frequencies of chromosome aberrations in the first post-irradiation metaphase cells were quantitated. Based on results of the cytogenetic assay, the exposed group had a higher frequency of cells with chromosome aberrations and higher chromatid breaks per 100 cells compared with the control. However, the difference was not significant (p > 0.1). With the challenge assay, the exposed group had a higher frequency of aberrant cells (p < 0.04), chromatid breaks (p < 0.05), deletions (p < 0.07), and dicentrics (p < 0.02) than the controls. In addition, the dicentric frequencies from workers were significantly correlated with the presence of a butadiene metabolite [1,2-dihydroxy-4-(N-acetylcysteinyl-S)butane] in urine with a correlation of coefficient of 0.6 (p < 0.01). Two outliers were identified and our interpretation of their responses will be discussed. This study indicates that the workers had exposure-induced mutagenic effects. Together with the observation of gene mutation in a subset of the present population, this study indicates that the current occupational exposure to butadiene may not be safe to workers.

Application of integrated genetic monitoring: the optimal approach for detecting environmental carcinogens

Short-term in vitro genetic toxicity assays have not fulfilled their anticipated role in predicting the carcinogenicity of environmental agents reliably and economically. A reduction in emphasis from nonanimal systems to relevant animal assays and population monitoring will help to reestablish the credibility of this field. An analysis of the various steps in the carcinogenic process indicates the biological responses occurring during these stages can be utilized for early detection of environmental carcinogens. Emphasis should be placed on using the earliest significant response that indicates genetic damage (e.g., gene mutations and chromosome alterations). Assays that detect pregenomic damage (e.g., adduct formation), without evidence of subsequent heritable genetic alterations, may produce misleading results for risk assessment and should not be considered as stand-alone monitoring procedures. Late biological responses may occur in tissues or organs where genetic damage may be difficult to measure, and the opportunity for intervention diminishes as we approach the clinical outcome. For example, analyzing localized cells that contain activated protooncogenes and inactivated tumor suppressor genes, although they further document adverse response from exposure to carcinogens, may be of greater value for indicating clinical outcome than for genetic monitoring. With few notable exceptions, the window of opportunity for genetic monitoring is the period after exposure where genetic damage is evident and where circulating lymphocytes can faithfully record this damage. An ongoing study of butadiene-exposed workers illustrates an optimum protocol, where multiple assays can be carried out and correlated with both external and internal measurements of exposure.(ABSTRACT TRUNCATED AT 250 WORDS)