1,3-Butadiene, isoprene and chloroprene: reviews by the IARC monographs programme, outstanding issues, and research priorities in epidemiology

Epidemiology and etiology of diffuse large B-cell lymphoma

As the most common non-Hodgkin lymphoma subtype, diffuse large B-cell lymphoma (DLBCL) incidence patterns generally parallel that for NHL overall. Globally, DLBCL accounts for a third of all NHLs, ranging between 20% and 50% by country. Based on United States (U.S.) cancer registry data, age-standardized incidence rate for DLBCL was 7.2 per 100,000. DLBCL incidence rises with age and is generally higher in males than females; in the U.S., incidence is highest among non-Hispanic whites (9.2/100,000). Like NHL incidence, DLBCL incidence rose in the first half of the 20th century but has largely plateaued. However, there is some evidence that incidence rates are rising in areas of historically low rates, such as Asia; there are also estimates for rising DLBCL incidence in the near future due to the changing demographics in developed countries whose aging population is growing. Established risk factors for DLBCL include those that result in severe immune deficiency such as HIV/AIDS, inherited immunodeficiency syndromes, and organ transplant recipients. Factors that lead to chronic immune dysregulations are also established risk factors, and include a number of autoimmune conditions (eg, Sjögren syndrome, systemic lupus erythematosus, rheumatoid arthritis), viral infections (eg, HIV, KSHV/HHV8, HCV, EBV), and obesity. Family history of NHL/DLBCL, personal history of cancer, and multiple genetic susceptibility loci are also well-established risk factors for DLBCL. There is strong evidence for multiple environmental exposures in DLBCL etiology, including exposure to trichloroethylene, benzene, and pesticides and herbicides, with recent associations noted with glyphosate. There is also strong evidence for associations with other viruses, such as HBV. Recent estimates suggest that obesity accounts for nearly a quarter of DLBCLs that develop, but despite recent gains in the understanding of DLBCL etiology, the majority of disease remain unexplained. An understanding of the host and environmental contributions to disease etiology, and concerted efforts to expand our understanding to multiple race/ethnic groups, will be essential for constructing clinically relevant risk prediction models and develop effective strategies for disease prevention.

Influence of the methyl group in isoprene epoxides on reactivity compared to butadiene epoxides: Biological significance

Reactions of the epoxides of 1,3-butadiene and isoprene (2-methyl-1,3-butadiene) with oxygen, nitrogen and sulfur nucleophiles have been compared to enable a better molecular understanding of the relative human toxicities of these epoxides. Hydrolysis of rac.-ethenyloxirane in (¹⁸O)water gave 77% (2-¹⁸O)but-3-ene-1,2-diol and 23% (1-¹⁸O)but-3-ene-1,2-diol. The R:S ratio for but-3-ene-1,2-diol from hydrolysis of (S)-ethenyloxirane was 75:25. Hence, hydrolysis of ethenyloxirane occurs by competing S_N2 attack at C-2 and C-3 in 3:1 ratio, with no S_N1 component. Hydrolysis of rac.-2-ethenyl-2-methyloxirane gave 2-hydroxy-2-methylbut-3-en-1-ol (73%) and 27% of a 2:1 mixture of the E- and Z-isomers of 4-hydroxy-2-methylbut-2-en-1-ol. In (¹⁸O)water (2-¹⁸O)2-hydroxy-2-methylbut-3-en-1-ol was obtained. Formation of these products occurs via S_N1 ionisation to resonance-stabilised allylic cations which are captured by water. Reaction of rac.-ethenyloxirane with l-valine methyl ester gave diastereoisomeric adducts from S_N2 attack of the valine amino at both C-2 (substituted position) and C-3 of the oxirane. The corresponding reaction of rac.-2-methyl-2-ethenyloxirane gave diastereoisomeric adducts, (R, S)- and (S, S)-N-(2-hydroxy-2-methyl-3-buten-1-yl)-l-valine methyl ester, from S_N2 attack of the valine amino solely at C-3. Reactions of rac.-2-ethenyl-2-methyloxirane with cysteine derivatives occurred at C-2 in neutral polar media (S_N1 reaction) or at C-3 in basic media (S_N2), whereas for ethenyloxirane products arose from attack at both C-2 and C-3. Reaction of meso-butadiene diepoxide (meso-2,2'-bioxirane) with l-valine methyl ester gave mainly 2:1 adducts, dimethyl 2,2'-(((2R,3S)-2,3-dihydroxybutane-1,4-diyl)bis(azanediyl))-(2S,2'S)-bis(3-methyl-butanoates), whereas 2-methyl-2,2'-bioxirane gave a mixture of 1:1 [methyl 2-(3,4-dihydroxy-3-methylpyrrolidin-1-yl)-3-methylbutanoates] and 2:1 adducts. Meso-2,2'-bioxirane reacted with N-acetylcysteine methyl ester in methanol to afford meso-thiolane-3,4-diol, by elimination of N-acetyldehydroalanine methyl ester from a precursor cyclic adduct. Similarly, 2-methyl-2,2'-bioxirane gave solely 3-methylthiolane-3,4-diols. Thus, the methyl group of isoprene has a subtle effect on the reactivity of its epoxides relative to those of butadiene and therefore, in the context of their toxicology, could abrogate crosslinking of nitrogen functions in biomolecules related to mutagenicity and carcinogenicity.

Mortality Patterns Among Industrial Workers Exposed to Chloroprene and Other Substances: Extended Follow-Up

OBJECTIVES

To update the U.S. portion of an historical cohort mortality study of workers with potential exposure to chloroprene (CD) and vinyl chloride (VC) with focus on lung and liver cancer.

METHODS

Subjects were 6864 workers from two sites with vital status determined through 2017 for 99% of subjects and cause of death for 97.2% of deaths. Historical exposures to CD and VC were estimated quantitatively. We performed external and internal mortality comparisons.

RESULTS

External comparisons revealed mostly deficits in deaths; internal comparisons revealed no consistent evidence of exposure-response relationships with CD or VC.

CONCLUSIONS

Our update continues to support the conclusion that the risk of death from lung or liver cancer is unrelated to exposure to CD or VC at levels experienced by workers in the two U.S. sites.

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."

Extended Analysis and Evidence Integration of Chloroprene as a Human Carcinogen

β-Chloroprene is used in the production of polychloroprene, a synthetic rubber. In 2010, Environmental Protection Agency (EPA) published the Integrated Risk Information System "Toxicological Review of Chloroprene," concluding that chloroprene was "likely to be carcinogenic to humans." This was based on findings from a 1998 National Toxicology Program (NTP) study showing multiple tumors within and across animal species; results from occupational epidemiological studies; a proposed mutagenic mode of action; and structural similarities with 1,3-butadiene and vinyl chloride. Using mouse data from the NTP study and assuming a mutagenic mode of action, EPA calculated an inhalation unit risk (IUR) for chloroprene of 5 × 10^-4 per µg/m³ . This is among the highest IURs for chemicals classified by IARC or EPA as known or probable human carcinogens and orders of magnitude higher than the IURs for carcinogens such as vinyl chloride, benzene, and 1,3-butadiene. Due to differences in pharmacokinetics, mice appear to be uniquely responsive to chloroprene exposure compared to other animals, including humans, which is consistent with the lack of evidence of carcinogenicity in robust occupational epidemiological studies. We evaluated and integrated all lines of evidence for chloroprene carcinogenicity to assess whether the 2010 EPA IUR could be scientifically substantiated. Due to clear interspecies differences in carcinogenic response to chloroprene, we applied a physiologically based pharmacokinetic model for chloroprene to calculate a species-specific internal dose (amount metabolized/gram of lung tissue) and derived an IUR that is over 100-fold lower than the 2010 EPA IUR. Therefore, we recommend that EPA's IUR be updated.

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.

The endogenous exposome

The concept of the Exposome is a compilation of diseases and one's lifetime exposure to chemicals, whether the exposure comes from environmental, dietary, or occupational exposures; or endogenous chemicals that are formed from normal metabolism, inflammation, oxidative stress, lipid peroxidation, infections, and other natural metabolic processes such as alteration of the gut microbiome. In this review, we have focused on the endogenous exposome, the DNA damage that arises from the production of endogenous electrophilic molecules in our cells. It provides quantitative data on endogenous DNA damage and its relationship to mutagenesis, with emphasis on when exogenous chemical exposures that produce identical DNA adducts to those arising from normal metabolism cause significant increases in total identical DNA adducts. We have utilized stable isotope labeled chemical exposures of animals and cells, so that accurate relationships between endogenous and exogenous exposures can be determined. Advances in mass spectrometry have vastly increased both the sensitivity and accuracy of such studies. Furthermore, we have clear evidence of which sources of exposure drive low dose biology that results in mutations and disease. These data provide much needed information to impact quantitative risk assessments, in the hope of moving towards the use of science, rather than default assumptions.

Conjugation of butadiene diepoxide with glutathione yields DNA adducts in vitro and in vivo

1,2,3,4-Diepoxybutane (DEB) is reported to be the most potent mutagenic metabolite of 1,3-butadiene, an important industrial chemical and environmental pollutant. DEB is capable of inducing the formation of monoalkylated DNA adducts and DNA-DNA and DNA-protein cross-links. We previously reported that DEB forms a conjugate with glutathione (GSH) and that the conjugate is considerably more mutagenic than several other butadiene-derived epoxides, including DEB, in the base pair tester strain Salmonella typhimurium TA1535 [Cho et al. (2010) Chem. Res. Toxicol. 23, 1544-1546]. In the present study, we determined steady-state kinetic parameters of the conjugation of the three DEB stereoisomers-R,R, S,S, and meso (all formed by butadiene oxidation)-with GSH by six GSH transferases. Only small differences (<3-fold) were found in the catalytic efficiency of conjugate formation (k(cat)/K(m)) with all three DEB stereoisomers and the six GSH transferases. The three stereochemical DEB-GSH conjugates had similar mutagenicity. Six DNA adducts (N(3)-adenyl, N(6)-adenyl, N(7)-guanyl, N(1)-guanyl, N(4)-cytidyl, and N(3)-thymidyl) were identified in the reactions of DEB-GSH conjugate with nucleosides and calf thymus DNA using LC-MS and UV and NMR spectroscopy. N(6)-Adenyl and N(7)-guanyl GSH adducts were identified and quantitated in vivo in the livers of mice and rats treated with DEB ip. These results indicate that such DNA adducts are formed from the DEB-GSH conjugate, are mutagenic regardless of sterochemistry, and are therefore expected to contribute to the carcinogenicity of DEB.

Cancer mortality and occupational exposure to aromatic amines and inhalable aerosols in rubber tire manufacturing in Poland

AIM

Most data on carcinogenic risk in the rubber industry are based on data from Western countries. This study assessed cancer risks in a retrospective cohort in a Polish tire manufacturing plant, relying on quantified exposure to inhalable aerosols and aromatic amines instead of job titles or external comparisons.

METHODS

Cumulative exposure for all exposures was assigned to cohort members based on estimates from a company-specific JEM. Cancer risks associated with cumulative exposure adjusted for co-exposures, gender and year of birth were calculated.

RESULTS

Exposure levels were higher for women than for men. Aromatic amine exposure was significantly associated with increased urinary bladder cancer risk (RR=7.32-8.27), depending on exposure level, and prostate cancer at low levels only (RR=5.86). In women, increased risks were found for all cancers (RR=2.50) and of the digestive organs and peritoneum (RR=4.54) at low level only, while an exposure-response association with breast cancer risk was found. Inhalable aerosol exposure was associated with cancers of the liver and intrahepatic bile ducts in a dose-dependent manner, while dose-dependent reduced risks were found for respiratory cancers (most notably the larynx) and cancer of the colon.

CONCLUSIONS

Increased risks for specific cancer sites in this rubber plant were similar to Western Europe and the US. However, several cancer risks were gender-specific which could relate to higher exposure levels in women or to differences in exposures to chemicals not assessed in this study.

The bis-electrophile diepoxybutane cross-links DNA to human histones but does not result in enhanced mutagenesis in recombinant systems

1,2-Dibromoethane and 1,3-butadiene are cancer suspects present in the environment and have been used widely in industry. The mutagenic properties of 1,2-dibromoethane and the 1,3-butadiene oxidation product diepoxybutane are thought to be related to the bis-electrophilic character of these chemicals. The discovery that overexpression of O(6)-alkylguanine alkyltransferase (AGT) enhances bis-electrophile-induced mutagenesis prompted a search for other proteins that may act by a similar mechanism. A human liver screen for nuclear proteins that cross-link with DNA in the presence of 1,2-dibromoethane identified histones H2b and H3 as candidate proteins. Treatment of isolated histones H2b and H3 with diepoxybutane resulted in DNA-protein cross-links and produced protein adducts, and DNA-histone H2b cross-links were identified (immunochemically) in Escherichia coli cells expressing histone H2b. However, heterologous expression of histone H2b in E. coli failed to enhance bis-electrophile-induced mutagenesis. These results are similar to those found with the cross-link candidate glyceraldehyde 3-phosphate dehydrogenase (GAPDH) [ Loecken , E. M. and Guengerich , F. P. ( 2008 ) Chem. Res. Toxicol. 21 , 453 - 458 ], but in contrast to GAPDH, histone H2b bound DNA with even higher affinity than AGT. The extent of DNA cross-linking of isolated histone H2b was similar to that of AGT, suggesting that differences in postcross-linking events explain the difference in mutagenesis.

Parental smoking and childhood leukemia

Childhood leukemia is the most common cancer among children, representing 31% of all cancer cases occurring in children younger than the age of 15 years in the USA. There are only few known risk factors of childhood leukemia (sex, age, race, exposure to ionizing radiation, and certain congenital diseases, such as Down syndrome and neurofibromatosis), which account for only 10% of the childhood leukemia cases. Several lines of evidence suggest that childhood leukemia may be more due to environmental rather than genetic factors, although genes may play modifying roles. Human and animal studies showed that the development of childhood leukemia is a two-step process that requires a prenatal initiating event(s) plus a postnatal promoting event(s). Despite a substantial public health effort to reduce cigarette smoking, a large proportion of the US and world population still smoke. Tobacco smoke contains at least 60 known human or animal carcinogens, with the major chemical classes being volatile hydrocarbons, aldehydes, aromatic amines, polycyclic aromatic hydrocarbons, and nitrosamines; among these chemicals, only benzene is an established leukemogen, although other chemicals in the tobacco could interact with one another in a complex way to jointly attain a significant carcinogenic effect on the development of leukemia. Although tobacco smoke is an established risk factor for adult myeloid leukemia, the studies of association between parental smoking and childhood leukemia have produced inconsistent results. The majority of the studies on maternal smoking and childhood leukemia did not find a significant positive association and some even reported an inverse association. In contrast to studies of maternal smoking, studies of paternal smoking and childhood leukemia reported more positive associations but only by less than half of the studies. Future directions to be considered for improving the study of parental smoking and childhood leukemia are: 1) consider all sources of benzene exposure in addition to smoking, including occupational exposure and traffic exhausts; 2) childhood leukemia is a heterogeneous disease and epidemiologic studies of childhood leukemia can be greatly improved by grouping childhood leukemia into more homogeneous groups by molecular techniques (e.g., structural and numerical chromosomal changes); and 3) assess gene-environment interaction. It is hoped that through the continual effort, more will be uncovered regarding the causes of childhood leukemia. In the meantime, more effort should be spent on educating the parents to quit smoking, because parental smoking is known to affect many childhood diseases (e.g., asthma, respiratory tract infection, and otitis media) that are much more prevalent than childhood leukemia.

Synthesis of DNA oligodeoxynucleotides containing structurally defined N6-(2-hydroxy-3-buten-1-yl)-adenine adducts of 3,4-epoxy-1-butene

3,4-Epoxy-1-butene (EB) is generated by cytochrome P450-mediated epoxidation of 1,3-butadiene (BD), an important environmental and industrial chemical classified as a probable human carcinogen. The ability of EB to induce point mutations at GC and AT base pairs has been attributed to its reactions with DNA to form covalent nucleobase adducts. Guanine alkylation is preferred at the endocyclic N7 nitrogen, while adenine can be modified at the N1-, N3-, N7-, and the N6 positions. For each of these sites, a pair of regioisomeric 2-hydroxy-3-buten-1-yl and 1-hydroxy-3-buten-2-yl adducts is produced as a result of epoxide ring opening at the terminal C-4 or the internal C-3 carbon position of EB, respectively. The N6-EB-adenine adducts are of particular interest because of their stability in DNA, potentially leading to their accumulation in vivo. In the present work, synthetic DNA oligomers containing structurally defined N6-(2-hydroxy-3-buten-1-yl)-dA (N6-HB-dA) adducts were prepared for the first time by a postoligomerization approach that involved coupling 6-chloropurine-containing DNA with synthetic 1-amino-3-buten-2-ol. N6-HB-dA-containing DNA oligomers were isolated by reversed phase HPLC, and the presence of N6-HB-dA in their structure was confirmed by molecular weight determination from HPLC-ESI- -MS of the intact strands and by HPLC-ESI+-MS/MS and MS/MS/MS analyses of the enzymatic digests using synthetic N6-HB-dA as an authentic standard. N6-HB-dA-containing oligomers generated in this study will be used for structural and biological studies.

Mortality patterns among industrial workers exposed to chloroprene and other substances

We conducted an historical cohort study to investigate the mortality experience of industrial workers potentially exposed to chloroprene (CD) and other substances, including vinyl chloride (VC), with emphasis on cancer mortality, including respiratory system (RSC) and liver. In 1999, the International Agency for Research on Cancer (IARC) classified CD as a possible carcinogen (Group 2B); VC was classified in 1987 as a known human carcinogen (Group 1). Subjects were 12,430 workers ever employed at one of two U.S. industrial sites (Louisville, KY (n=5507) and Pontchartrain, LA (n=1357)) or two European sites (Maydown, Northern Ireland (n=4849) and Grenoble, France (n=717)), with earliest CD production dates ranging from 1942 (L) to 1969 (P). Two sites (L and M) synthesized CD with the acetylene process that produced VC exposures. We determined vital status through 2000 for 95% of subjects and cause of death for 95% of the deaths. Historical exposures for individual workers were estimated quantitatively for CD and VC. Workers ever exposed to CD ranged from 92.3% (M) to 100% (G); to VC from 5.5% (M) to 22.7% (L). We computed standardized mortality ratios (SMRs) (using national and regional standard populations) in relation to selected demographic, work history and exposure factors. We used worker pay type (white or blue collar) as a rough surrogate for lifetime smoking history. For the combined cohort, SMRs (95% CIs) for all causes combined, all cancers combined, RSC and liver cancer were, respectively, 0.72 (0.69-0.74), 0.73 (0.68-0.78), 0.75 (0.67-0.84) and 0.72 (0.43-1.13). Site-specific (L, M, P and G, respectively) SMRs were: for all cancers combined: 0.75 (0.69-0.80), 0.68 (0.56-0.80), 0.68 (0.47-0.95) and 0.59 (0.36-0.91); for RSC: 0.75 (0.66-0.85), 0.79 (0.58-1.05), 0.62 (0.32-1.09) and 0.85 (0.41-1.56); for liver cancer: 0.90 (0.53-1.44) (17 deaths), 0.24 (0.01-1.34) (1 death), 0.0 (0-2.39) (no deaths) and 0.56 (0.01-3.12) (1 death). Among all workers ever exposed to CD, SMRs were: for all cancers combined: 0.71 (0.66-0.76); for RSC: 0.75 (0.67-0.84); for liver cancer: 0.71 (0.42-1.14). We also observed no increased mortality risks among cohort subgroups defined by race, gender, worker pay type, worker service type (short/long term), time period, year of hire, age at hire, duration of employment, the time since first employment, and CD or VC exposure status (never/ever exposed). In summary, our study has many strengths and is the most definitive study of the human carcinogenic potential of exposure to CD conducted to date. We conclude that persons exposed to chloroprene or vinyl chloride at the levels encountered in the four study sites did not have elevated risks of mortality from any of the causes of death examined, including all cancers combined and lung and liver cancer, the cancer sites of a priori interest. This conclusion is corroborated by our detailed analyses of mortality in relation to qualitative and quantitative exposures to CD and VC at each of the four study sites, reported in our companion paper (Marsh et al., submitted for publication).

Chemical process based reconstruction of exposures for an epidemiological study

As part of an historical cohort study to investigate the mortality experience of industrial workers exposed to chloroprene (beta-CD) and other substances, all available industrial hygiene exposure monitoring data were collected and summarized. From discussions with on-site industrial hygiene personnel, it was apparent that these data were not collected for epidemiological purposes and, therefore, their use in characterization of exposures was problematic as the data mostly pertained to samples collected to investigate the performance of specific tasks. These data were, however, informative for validating the exposure modeling process used to estimate historical exposures. The data summarized below clearly indicate that exposures to beta-CD were lowered across the time period of this study. Typically, the exposures recorded were less than the occupational exposure limits of the periods in which the exposures were recorded. Additionally, exposure measurements recorded in the recent past do not represent the exposure actually experienced by the worker as a strict personal protective equipment use program has been in place for the facilities studied since the mid-1980s.

Mortality patterns among industrial workers exposed to chloroprene and other substances

As part of an historical cohort study to investigate the mortality experience of industrial workers exposed to chloroprene (CD) and other substances, including vinyl chloride monomer (VC), we analyzed mortality from all cancers combined, respiratory system (RSC) and liver cancer in relation to CD and VC exposures. Subjects were 12,430 workers ever employed at one of two U.S. sites (Louisville, KY (n=5507) and Pontchartrain, LA (n=1357)) or two European sites (Maydown, Northern Ireland (n=4849) and Grenoble, France (n=717)). Historical exposures for individual workers were estimated quantitatively for CD and VC. For sites L, M, P and G, respectively, average intensity of CD exposures (median value of exposed workers in ppm) were 5.23, 0.16, 0.028 and 0.149 and median cumulative exposures (ppm years) were 18.35, 0.084, 0.133 and 1.01. For sites L and M, respectively, average intensity of VC exposures (median value of exposed workers in ppm) was 1.54 and 0.03 and median cumulative exposures (ppm years) were 1.54 and 0.094. We performed relative risk (RR) regression modeling to investigate the dependence of the internal cohort rates for all cancers combined, RSC and liver cancer on combinations of the categorical CD or VC exposure measures with adjustment for potential confounding factors. We categorized exposure measures into approximate quartiles based on the distribution of deaths from all cancers combined. We also considered 5- and 15-year lagged exposure measures and adjusted some RR models for worker pay type (white/blue collar) as a rough surrogate for lifetime smoking history. All modeling was site-specific to account for exposure heterogeneity. We also computed exposure category-specific standardized mortality ratios (SMRs) to assess absolute mortality rates. With the exception of a one statistically significant association with duration of exposure to CD and all cancers combined in plant M, we observed no evidence of a positive association with all cancers, RSC or liver cancer and exposure to CD and/or VC using both the unlagged and lagged exposure measures: duration, average intensity or cumulative exposure to CD or VC; time since first CD or VC exposure; and duration of CD exposure or time since first CD exposure in presence or absence of VC exposure. We observed elevated and statistically significantly elevated RRs for some analysis subgroups, but these were due to inordinately low death rates in the baseline categories. With the possible exception of all cancer mortality in plant G, our additional adjustment of RRs for pay type revealed no evidence of positive confounding by smoking. We conclude that exposures to CD or VC at the levels encountered in the four study sites do not elevate mortality risks from all cancers, RSC or liver cancer. This conclusion is corroborated by our analysis of general mortality patterns among the CD cohort reported in our companion paper [G. Marsh, A. Youk, J. Buchanich, M. Cunningham, N. Esmen, T. Hall, M. Phillips, Mortality patterns among industrial workers exposed to chloroprene and other substances. I. General mortality patterns, Chem.-Biol. Interact., submitted for publication].

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).

DNA-damaging ability of isoprene and isoprene mono-epoxide (EPOX I) in human cells evaluated with the comet assay

Isoprene is produced in combustion processes and is widely used as an industrial chemical. It is a natural product emitted by plants and endogenously produced by humans and other mammals. Therefore, exposure to isoprene from both endogenous and exogenous sources is unavoidable and occurs during the entire human life. Based on evaluations of the International Agency for Research on Cancer (IARC), isoprene has been classified in Group 2B (possibly carcinogenic to humans). In the present work, we have demonstrated, by use of the single-cell gel electrophoresis assay (SCGE or comet assay), that isoprene is able to induce DNA damage in peripheral blood mononuclear cells (PBMCs) in the presence of metabolic activation. In addition, treatment of cells with the main isoprene mono-epoxide (EPOX I) induced time- and dose- dependent DNA damage in both PBMCs and human leukaemia cells (HL60). The metabolic activation system, represented by rat liver post-mitochondrial fractions (S9), was obtained from rats that had been treated - or not - with inducing agents such as phenobarbital and ethanol. The inclusion of S9 fractions (4mg protein/mL) from non-induced or phenobarbital-induced rats resulted in a statistically significant enhancement of isoprene genotoxicity. A different pattern was obtained by the addition of ethanol-induced S9, which appeared highly genotoxic by itself even in the absence of isoprene. Reducing the concentration of ethanol-induced S9 to 0.25mg protein/mL resulted in a considerable enhancement of isoprene genotoxicity. In the absence of clear epidemiological evidence of the carcinogenicity of isoprene in humans, the results of this study seem to be particularly important since they add new findings to support the classification of this chemical as possibly carcinogenic to humans.

Chloroprene: overview of studies under consideration for the development of an IRIS assessment

Beta-chloroprene (C(4)H(5)Cl, chloroprene, 2-chloro-1,3-butadiene, CASRN 126-99-8) is a volatile, flammable liquid monomer utilized primarily in the manufacture of neoprene (polychloroprene) elastomer used in belts, hoses, gloves, wire coatings, and tubing. Absorption into the body occurs primarily via the respiratory system and may occur via the gastrointestinal tract or the skin. Once absorbed, chloroprene is widely distributed as evidenced by effects in several target organs including nose and lung, liver, and skin. Chloroprene metabolism is believed to include cytochrome P450 oxidation to a monoepoxide, hydrolysis by epoxide hydrolases, and glutathione conjugation. Similar to 1,3-butadiene, the epoxide is considered to be the toxic moiety, and species differences in metabolic capacity may influence the severity of effects as well as what tissues are affected. EPA has not previously developed an assessment of chloroprene's potential for human health effects. Existing human epidemiological studies offer little data on noncancer effects, and the associations of exposure with increased cancer (liver and lung) mortality reported are inconclusive. Recent epidemiological studies (submitted for publication) could offer information that may impact chloroprene's health assessment. Multiple-site tumors have been reported in rats and mice exposed to chloroprene by inhalation; nevertheless, there are marked differences in strain sensitivities (i.e., tumors in F344 rats versus no tumors in Wistar rats). Recently developed physiologically based toxicokinetic models may allow for the resolution of species and tissue differences and sensitivities as well as exposure-dose-response relationships relevant to humans. (This presentation does not necessarily reflect EPA policy.).

Cross-linking of the human DNA repair protein O6-alkylguanine DNA alkyltransferase to DNA in the presence of 1,2,3,4-diepoxybutane

1,2,3,4-Diepoxybutane (DEB) is a key carcinogenic metabolite of the important industrial chemical 1,3-butadiene. DEB is a bifunctional alkylating agent capable of reacting with DNA and proteins. Initial DNA alkylation by DEB produces N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-guanine monoadducts, which can react with another nucleophilic site to form cross-linked adducts. A recent report revealed a strong correlation between cellular expression of the DNA repair protein O6-alkylguanine DNA alkyltransferase (AGT) and the cytotoxic and mutagenic activity of DEB, suggesting that DEB induces AGT-DNA cross-links (Valadez, J. G., et al. (2004) Activation of bis-electrophiles to mutagenic conjugates by human O6-alkylguanine-DNA alkyltransferase. Chem. Res. Toxicol. 17, 972-982). The purpose of our study was to analyze the formation and structures of DEB-induced AGT-DNA conjugates and to identify specific amino acid residues within the protein involved in cross-linking. DNA-protein cross-link formation was detected by SDS-PAGE when 32P-labeled double-stranded oligodeoxynucleotides were exposed to DEB in the presence of either wild-type hAGT or a C145A hAGT mutant. Capillary HPLC-electrospray ionization mass spectrometry (ESI-MS) analysis of hAGT that had been treated with N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-deoxyguanosine (dG monoepoxide) revealed the ability of the protein to form either one or two butanediol-dG cross-links, corresponding to mass shifts of +353 and +706 Da, respectively. HPLC-ESI+ -MS/MS sequencing of the tryptic peptides obtained from dG monoepoxide-treated protein indicated that the two cross-linking sites were the alkyl acceptor site, Cys145, and a neighboring active site residue, Cys150. The same two amino acid residues of hAGT became covalently cross-linked to DNA following DEB treatment. Modification of Cys145 was further confirmed by HPLC-ESI+ -MS/MS analysis of dG monoepoxide-treated synthetic peptide GNPVPILIPCHR which represents the active site tryptic fragment of hAGT (C = Cys145). The replacement of the catalytic cysteine residue with alanine in the C145A hAGT mutant abolished DEB-induced cross-linking at this site, while the formation of conjugates via neighboring Cys150 was retained. The exact chemical structure of the cross-linked lesion was established as 1-(S-cysteinyl)-4-(guan-7-yl)-2,3-butanediol by HPLC-ESI+ -MS/MS analysis of the amino acids resulting from the total digestion of modified proteins analyzed in parallel with an authentic standard. AGT-DNA cross-linking is a likely mechanism of DEB-mediated cytotoxicity in cells expressing this important repair protein.

Some insights into the mode of action of butadiene by examining the genotoxicity of its metabolites

1,3-Butadiene (BTD) is an important commodity chemical and air pollutant that has been shown to be a potent carcinogen in mice, and to a lesser extent, a carcinogen in rats. To better assess butadiene's carcinogenic risk to humans, it is important to understand its mode of action and how this relates to differences in responses among species. In a series of in vitro experiments, lymphocytes from rats, mice, and humans were exposed to 3,4-epoxy-1-butene (EB) or 1,2:3,4-diepoxybutane (DEB) for 1h at the G(0) stage of the cell cycle, stimulated to divide, and cultured to assess the ability of these metabolites to induce sister chromatid exchange (SCE) and chromosome aberrations (CAs). EB induced no increases in SCEs or CAs in the cells from the three species. DEB was a potent SCE- and CA-inducer, with the results being similar in each rodent species. The response for SCEs seen in the human cells was more complex, with genetic polymorphism for glutathione-S-transferases (GST) possibly modulating the response. The single cell gel electrophoresis assay was used on genetically engineered V79 cell lines to investigate a possible influence of GST status. Experiments were also conducted to investigate the reason for EB's failure to induce SCEs or CAs in G(0) cells. The results indicate that EB-induced DNA damage was repaired before DNA synthesis in unstimulated lymphocytes, but EB caused a large increase in SCEs if actively cycling cells were treated. Thus, the results indicate that DEB damage is persistent in G(0) cells, and DEB is a much more potent genotoxicant than EB. The carcinogenic effect of butadiene will most likely depend on the degree to which DEB is produced and reaches target tissues, and to a lesser extent on the ability of EB to reach actively dividing or repair deficient cells.

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].

Childhood cancers and atmospheric carcinogens

STUDY OBJECTIVES

To retest previous findings that childhood cancers are probably initiated by prenatal exposures to combustion process gases and to volatile organic compounds (VOCs); and to identify specific chemical hazards.

DESIGN

Birth and death addresses of fatal child cancers in Great Britain between 1966 and 1980, were linked with high local atmospheric emissions of different chemical species. Among migrant children, distances from each address to the nearest emissions "hotspot" were compared. Excesses of outward over inward migrations show an increased prenatal or early infancy risk.

SETTING AND SUBJECTS

Maps of emissions of many different substances were published on the internet by the National Atmospheric Emissions Inventory and "hotspots" for 2001 were translated to map coordinates. Child cancer addresses were extracted from an earlier inquiry into the carcinogenic effects of obstetric radiographs; and their postcodes translated to map references.

MAIN RESULTS

Significant birth proximity relative risks were found within 1.0 km of hotspots for carbon monoxide, PM10 particles, VOCs, nitrogen oxides, benzene, dioxins, 1,3-butadiene, and benz(a)pyrene. Calculated attributable risks showed that most child cancers and leukaemias are probably initiated by such exposures.

CONCLUSIONS

Reported associations of cancer birth places with sites of industrial combustion, VOCs uses, and associated engine exhausts, are confirmed. Newly identified specific hazards include the known carcinogens 1,3-butadiene, dioxins, and benz(a)pyrene. The mother probably inhales these or related materials and passes them to the fetus across the placenta.