1,3-Butadiene working group report

Genetic Determinants of 1,3-Butadiene Metabolism and Detoxification in Three Populations of Smokers with Different Risks of Lung Cancer

Background: 1,3-Butadiene (BD) is an important carcinogen in tobacco smoke that undergoes metabolic activation to DNA-reactive epoxides. These species can be detoxified via glutathione conjugation and excreted in urine as the corresponding N-acetylcysteine conjugates. We hypothesize that single nucleotide polymorphisms (SNPs) in BD-metabolizing genes may change the balance of BD bioactivation and detoxification in White, Japanese American, and African American smokers, potentially contributing to ethnic differences in lung cancer risk.Methods: We measured the levels of BD metabolites, 1- and 2-(N-acetyl-L-cysteine-S-yl)-1-hydroxybut-3-ene (MHBMA) and N-acetyl-S-(3,4-dihydroxybutyl)-L-cysteine (DHBMA), in urine samples from a total of 1,072 White, Japanese American, and African American smokers and adjusted these values for body mass index, age, batch, and total nicotine equivalents. We also conducted a genome-wide association study to identify genetic determinants of BD metabolism.Results: We found that mean urinary MHBMA concentrations differed significantly by ethnicity (P = 4.0 × 10-25). African Americans excreted the highest levels of MHBMA followed by Whites and Japanese Americans. MHBMA levels were affected by GSTT1 gene copy number (P < 0.0001); conditional on GSTT1, no other polymorphisms showed a significant association. Urinary DHBMA levels also differed between ethnic groups (P = 3.3 × 10-4), but were not affected by GSTT1 copy number (P = 0.226).Conclusions:GSTT1 gene deletion has a strong effect on urinary MHBMA levels, and therefore BD metabolism, in smokers.Impact: Our results show that the order of MHBMA levels among ethnic groups is consistent with their respective lung cancer risk and can be partially explained by GSTT1 genotype. Cancer Epidemiol Biomarkers Prev; 26(7); 1034-42. ©2017 AACR.

Alkyltransferase-mediated toxicity of bis-electrophiles in mammalian cells

The primary function of O(6)-alkylguanine-DNA alkyltransferase (AGT) is to maintain genomic integrity in the face of damage by both endogenous and exogenous alkylating agents. However, paradoxically, bacterial and mammalian AGTs have been shown to increase cytotoxicity and mutagenicity of dihaloalkanes and other bis-electrophiles when expressed in bacterial cells. We have extended these studies to mammalian cells using CHO cells that lack AGT expression and CHO cells stably transfected with a plasmid that expresses human AGT. The cytotoxicity of 1,2-dibromoethane, dibromomethane and epibromohydrin was significantly increased by the presence of AGT but cytotoxicity of butadiene diepoxide was not affected. Mutations caused by these agents were assessed using hypoxanthine-guanine phosphoribosyltransferase (HPRT) as a reporter gene. There was a small (c. 2-3-fold) but statistically significant AGT-mediated increase in mutations caused by 1,2-dibromoethane, dibromomethane and epibromohydrin. Analysis of the mutation spectrum induced by 1,2-dibromoethane showed that the presence of AGT also altered the types of mutations with an increase in total base substitution mutants due to a rise in transversions at both G:C and A:T sites. AGT expression also led to mutations arising from the transcribed strand, which were not seen in cells lacking AGT. Although the frequency of deletion mutations was decreased by AGT expression, the formation of large deletions (> or = 3 exons) was increased. This work demonstrates that interaction of AGT with some bis-electrophiles can cause mutagenicity and diminished cell survival in mammalian cells. It is consistent with the hypothesis that DNA-AGT cross-links, which have been characterized in experiments with purified AGT protein and such bis-electrophiles, can be formed in mammalian cells.

Alkyltransferase-mediated toxicity of 1,3-butadiene diepoxide

Human O(6)-alkylguanine-DNA alkyltransferase (hAGT) expression increases mutations and cytotoxicity following exposure to 1,3-butadiene diepoxide (BDO), and hAGT-DNA cross-links are formed in the presence of BDO. We have used hAGT mutants to investigate the mechanism of cross-link formation and genotoxicity. Formation of a hAGT-DNA conjugate in vitro was observed with C145S and C145A mutant proteins but was considerably diminished with the C145A/C150S double mutant confirming that cross-linking primarily involves either of these two cysteine residues, which are located in the active site pocket of the protein. Cross-link formation by BDO occurred both via (a) an initial reaction of BDO with hAGT followed by attack of the reactive hAGT complex on DNA, and (b) the initial reaction of BDO with DNA followed by a reaction between hAGT and the DNA adduct. These results differ from those with 1,2-dibromoethane (DBE) where Cys(145) is the only site of attachment and pathway (b) does not occur. The complex formed between hAGT at Cys(145) and BDO was very unstable in aqueous solution. However, the BDO-hAGT complex at Cys(150) exhibited stability for more than 1 h. The effect of hAGT and mutants on BDO-induced genotoxicity was studied in E. coli using the forward assay to rifampicin resistance. Both mutations and cell killing were greatly increased by wild type hAGT, and there was a smaller but significant effect with the C145A mutant. The R128A mutant and R128A/C145A and C145A/C150S double mutants were ineffective, supporting the hypothesis that the formation of hAGT-DNA cross-links is responsible for the enhanced genotoxicity detected in this biological system. In the absence of hAGT, there were equal proportions of G:C to A:T transitions, G:C to T:A transversions, and A:T to T:A transversions. Wild type hAGT expression yielded significantly greater G:C to A:T and A:T to G:C transitions, whereas C145A mutant expression resulted in more transitions and transversions at A:T base-pairs.

DNA adducts of 1,3-butadiene in humans: relationships to exposure, GST genotypes, single-strand breaks, and cytogenetic end points

The modulation of 1,3-butadiene (BD)-induced DNA adducts by occupational exposure, glutathione S-transferase (GST) genotypes, single-strand breaks, and cytogenetic end points was studied in 15 workers and 11 controls. The exposed group consisted of 8 smokers and 7 nonsmokers, whereas the control group consisted of 7 nonsmokers and 4 smokers. Among all subjects, the adduct levels in workers lacking GSTM1 were significantly higher than in those who were GSTM1 positive (P = 0.026), and individuals with combined GSTM1(-) and GSTT1(+) genotype had elevated level of adducts compared to that of persons with GSTM1(+) and GSTT1(+) (P = 0.049). The increase in BD-DNA adduct levels in all subjects was significantly related to BD exposure and GSTM1 genotype (linear multiple regression analysis, P = 0.001; P = 0.035). The results suggest that DNA adducts serve as a sensitive and specific biomarker, integrating exposure and host metabolic capacity, although the data are limited to a small number of subjects.

Micronuclei and gene mutations in transgenic big Blue((R)) mouse and rat fibroblasts after exposure to the epoxide metabolites of 1, 3-butadiene

1,3-Butadiene (BD) is a commodity compound and by-product in the manufacture of synthetic rubber that elicits a differential carcinogenic response in rodents after chronic exposure. Mice are up to approximately 1000-fold more sensitive to the tumorigenicity of inhaled BD than rats, thereby confounding human risk assessment analyses. Rodent transgenic in vivo and in vitro models have been recently utilized for generating genetic toxicology data in support of risk assessment studies. However, studies have not been extended to investigate multiple endpoints of genetic damage using in vitro transgenic models. The goal of this study was to evaluate possible differences in the production of genetic damage in transgenic Big Blue((R)) mouse (BBM1) and rat (BBR1) fibroblasts exposed to three predominant epoxide metabolites of BD. Analyses of cytotoxicity, micronucleus (MN) formation, cII mutant frequency (MF) and apoptosis were assessed after in vitro exposure of BBM1 and BBR1 cells exposed to various concentrations of butadiene monoepoxide (BMO), diepoxybutane (DEB) and butadiene diolepoxide (BDE). Both BMO and DEB reduced cell survival in BBM1 and BBR1 cells. However, BDE decreased cell survival only in BBM1 cells at the concentrations evaluated. Concentration-dependent increases in the formation of MN was observed in both BBM1 and BBR1 cells, with DEB being the most potent followed by BDE and then BMO. The dose-response for mutations induced at the cII locus was essentially equal after DEB exposure of BBM1 and BBR1 fibroblasts. In contrast, the cII MF was significantly increased only in BBM1 cells after exposure to either BMO or BDE. These data demonstrate a differential genetic response for gene mutations but not for MN formation in transgenic BBM1 and BBR1 fibroblasts and suggest a rodent species-specific difference in the persistence of DNA damage that results in gene mutations. In addition, apoptosis was observed in BBR1 cells but not in BBM1 cells when treated with any of the three BD epoxide metabolites. This response may partially explain the differential response to mutations induced by BMO and BDE. These data offer insight into specific differences in mouse and rat cells with respect to their response to BD epoxide metabolites. Such data may help to explain the different tumorigenicity results observed in rodent BD carcinogenicity studies.

The influence of GSTM1 and GSTT1 genotypes on the induction of sister chromatid exchanges and chromosome aberrations by 1,2:3,4-diepoxybutane

Cytogenetic tests - chromosome aberrations (CA), sister chromatid exchanges (SCE) and micronuclei (MN) - are most often applied in biomonitoring of the genotoxicity of potentially carcinogenic chemicals in human cells. One of the extensively studied genotoxins is diepoxybutane (DEB) - reactive biometabolite of butadiene (BD). Several studies showed a high SCE induction in human lymphocytes exposed in vitro to various concentrations of DEB. DEB also proved to be a potent inducer of chromosome aberrations and micronuclei. A bimodal distribution of SCE frequency after in vitro DEB treatment was observed. The aim of the present study was to examine the ability of DEB to induce different individual cytogenetic response measured by SCE and CA frequency. The possible influence of genetic polymorphism has also been taken into account, by including donors representing positive or null GSTM1 and GSTT1 genotypes. Our study supported the earlier results showing that DEB is an effective inducer of SCEs and CAs, causing also the decrease in replication index (RI). DEB bioactivity measured by SCE induction - but not by CA test - was significantly higher in GSTT1 negative than in GSTT1 positive donors. GSTM1 polymorphism had no influence on these endpoints. The donors GSTT1-/GSTM1+ were shown to be slightly more sensitive to DEB than GSTT1-/GSTM1- individuals. There was also observed a unimodal distribution of DEB-induced SCEs and CAs in the group, despite the fact that the experiment was performed on the lymphocytes obtained from both GSTT1 positive and negative donors.

The in vivo mutagenicity and mutational spectrum at the lacI transgene recovered from the spleens of B6C3F1 lacI transgenic mice following a 4-week inhalation exposure to 1,3-butadiene

1,3-Butadiene (BD) is carcinogenic and mutagenic in B6C3F1 mice. We determined the lacI mutant frequency and mutational spectrum in spleen following inhalation exposure to BD at levels that are known to induce tumors. B6C3F1 lacI transgenic mice were exposed to air or to 62.5, 625, or 1250 ppm BD for 4 weeks (6 h/day, 5 days/week) and euthanized 14 days after the last exposure. BD increased the lacI mutant frequency in spleen at all levels of BD examined. In BD-exposed mice, an increased frequency of G:C-->A:T transitions occurred at non-5'-CpG-3' sites. Exposure to BD in B6C3F1 lacI transgenic mice also increased the frequency of base substitution mutations that occurred at A:T base pairs when compared to air controls. The increased frequency of specific mutations at G:C base pairs in spleen was not observed in our previous studies in bone marrow and indicates tissue-specific differences in the BD-induced mutational spectrum. These data demonstrate that in vivo transgenic mouse mutagenicity assays can identify tissue-specific mutagenicity and mutational spectrum responses of genotoxic carcinogens at exposure levels that are known to induce tumors.

Micronucleus induction in germ and somatic cells of the mouse after exposure to the butadiene metabolites diepoxybutane and epoxybutene

The genotoxicity of diepoxibutane (DEB) and epoxybutene (EB), two of the main metabolites of 1,3-butadiene, was tested in the germ and somatic cells of the mouse by applying an MN assay in early spermatids, and in peripheral blood reticulocytes of a subgroup of the same animals. DEB (0.17 and 0.35 mmol/kg) and EB (0.35, 0.70 and 1.04 mmol/kg) were administered i.p. In the germ cell assay, significant increases of MN were observed after treatment of premeiotic S-phase cells with both butadiene metabolites, but DEB was shown to be more powerful than EB in the induction of chromosomal damage. A weak effect of the same compounds was also found after treatment of late spermatocytes, approaching the meiotic divisions. From the MN assay in peripheral blood reticulocytes, a statistically significant increase of the frequency of MN was detected at each dose tested for both chemicals. However, the results have again shown that DEB is much more efficient than EB in inducing chromosome damage.

Induction of sister chromatid exchange by 3,4-expoxybutane-1,2-diol in cultured human lymphocytes of different GSTT1 and GSTM1 genotypes

The induction of sister chromatid exchanges (SCEs) by a 48-h treatment with 3,4-epoxybutane-1,2-diol (EBD), a metabolite of 1,3-butadiene, was studied in whole-blood lymphocyte cultures of 22 human donors with known genotypes of two polymorphic glutathione S-transferases (GSTs), GSTT1 and GSTM1. For both genes, donors representing a homozygous 'null' genotype lacking the respective GST gene and isozyme and a 'positive' genotype with at least one intact gene and GST activity were included. The mean frequencies of SCE/cell were similar in all genotype groups: GSTT1 null (n = 10) (mean 22.0 for 250 microM and 32.9 for 500 [corrected] microM of EBD), GSTT1 positive (n = 14) (21.3 and 34.6, respectively), GSTM1 null (n = 10) (20.3 and 33.5) and GSTM1 positive donors (n = 15) (20.6 and 34.8). At 500 microM concentration of EBD, the lymphocyte cultures of all donors showed a significantly decreased replication index. No differences in EDB-induced SCEs or in replication index could be associated with the GSTM1 and GSTT1 genotypes either separately or in combination. When SCE induction by EBD was compared to that of two other known epoxide metabolites of butadiene, 1,2:3,4-diepoxybutane (DEB) was effective at concentrations over two orders of magnitude lower than EBD or 1,2-epoxy-3-butene (MEB). It is concluded that EBD is an efficient inducer of SEC in cultured human lymphocytes, although not quite as effective as MEB and clearly less effective than DEB. Contrary to previous findings with DEB and MEB, the polymorphic GSTM1 and GSTT1 do not appear to be involved in the detoxification of EBD in human lymphocytes.

Synthesis report of the step project detection of germ cell mutagens

The project 'Detection of Germ Cell Mutagens' was designed with three major goals: (1) Detection and characterization of germ-cell mutagens; (2) standardization and validation of new germ-cell tests; and (3) development of a data base on germ-cell mutagenicity. All three goals were achieved. The classical germ-cell tests were applied to characterize the genetic effects of acrylamide (AA), 1,3-butadiene (BD), trophosphamide (TP) and urethane (UR). All but UR were found to cause heritable genetic damage. The experimental data obtained for AA and BD were the basis for genetic risk evaluations during the EC/US Workshop on Risk Assessment 'Human Genetic Risk from Exposure to Chemicals, Focusing on the Feasibility of the Parallelogram Approach'. Nine chemicals were employed to validate the spermatid micronucleus assay with mice and rats: AA, BD and its metabolites 1,2-epoxybutene-3 and 1,2:3,4-diepoxybutane, chlorambucil, mitomycin C, methylnitrosourea, TP and UR. The spermatid micronucleus test was combined with micronucleus tests in somatic cells such as bone marrow or peripheral blood erythrocytes, and splenocytes which allowed a comparison of effects in somatic and germinal cells. Improvements of the spermatid micronucleus test included BrdU-labelling of premeiotic S-phase for the determination of stage sensitivity and fluorescence in situ hybridization with pancentromeric DNA-probes to distinguish between clastogenic and aneugenic events. The results indicate that the spermatid micronucleus test with its improvements is an adequate procedure to detect germ-cell clastogenicity and to compare the activity of chemicals in different tissues and between species, i.e., rats and mice. Other germ cell methods under study were the flow cytometric measurement of testicular sperm DNA and the cytogenetic analysis of preimplantation embryos for chromosomal aberrations and micronuclei. The collection of a reliable germ-cell data base was accomplished through a critical evaluation of the literature and with the data obtained in the present project. Remarkable concordance between responses of germ cell tests to chemical mutagens was the most striking conclusion to be drawn from the present data base.

Assessment of 1,3-butadiene mutagenicity in the bone marrow of B6C3F1 lacI transgenic mice (Big Blue): a review of mutational spectrum and lacI mutant frequency after a 5-day 625 ppm 1,3-butadiene exposure

1,3-Butadiene (BD) is a carcinogen that is bioactivated to at least two genotoxic metabolites. In the present article, we review briefly our previous studies on the in vivo mutagenicity and mutational spectra of BD in bone marrow and extend these studies to examine the effect of exposure time (5-days vs. 4-week exposure to 625 ppm BD used in previous studies) on the lacI mutant frequency in the bone marrow. Inhalation exposure to BD at 625 ppm and 1,250 ppm mutagenic in vivo, inducing an increase in the transgene mutant and mutation frequency in the bone marrow. Analysis of the mutational spectrum in BD-exposed and air control mice demonstrated that BD exposure induced an increased frequency of mutations at A:T base pairs. There was no difference in the lacI mutant frequency determined in the bone marrow between a short-term exposure to BD (5 days) and a longer-term exposure (4 weeks). These data taken together demonstrate that inhalation exposure to BD induces in vivo somatic cell mutation.