Genetic polymorphisms influence variability in benzene metabolism in humans

The role of genetic polymorphism in modulating urinary excretion of two benzene metabolites, i.e. trans,trans-muconic acid (t,t-MA) and S-phenylmercapturic acid (PMA), has been investigated in 59 non-smoking city bus drivers, professionally exposed to benzene via vehicle exhausts. Exposure to benzene was determined by personal passive samplers (mean +/- SD = 82.2 +/- 25.6 micrograms/m3), while internal dose and metabolic rate were evaluated by measuring urinary excretion of unmodified benzene (mean +/- SD = 361 +/- 246 ng/l), t,t-MA (mean +/- SD = 602 +/- 625 micrograms/g creatinine), and PMA (mean +/- SD = 5.88 +/- 4.76 micrograms/g creatinine). Genetic polymorphism at six loci encoding cytochrome-P450-dependent monooxygenases (CYP2E1 and CYP2D6), glutathione-S-transferases (GSTT1, GSTP1 and GSTM1) and NAD(P)H:quinone oxidoreductase (NQOR) was determined by polymerase chain reaction-based methods. No evidence emerged for a possible role of CYP2E1, GSTM1 and GSTP1 polymorphisms in determining the wide differences observed in the rate of benzene biotransformation. Conversely, a significantly higher t,t-MA urinary excretion was found to be correlated to, GSTT1 null genotype, and a significantly lower PMA excretion was detected in the subjects lacking NQOR activity and in the CYP2D6 extensive-metabolizers. Many biological (i.e. age and body burden) or lifestyle factors (i.e. rural or urban residence, use of paints and solvents, medication, alcohol and coffee intake), also taken into account as potential confounders, did not influence the correlations found. These findings suggest that CYP2D6, GSTT1 and NQOR polymorphisms contribute in explaining the metabolic variability observed in our sample. Therefore, these polymorphisms should be regarded as potential risk factors for benzene-induced adverse health effects.     

Genetic susceptibility to benzene toxicity in humans

Human metabolism of benzene involves pathways coded for by polymorphic genes. To determine whether the genotype at these loci might influence susceptibility to the adverse effects of benzene exposure, 208 Bulgarian petrochemical workers and controls, whose exposure to benzene was determined by active personal sampling, were studied. The frequency of DNA single-strand breaks (DNA-SSB) was determined by alkaline elution, and genotype analysis was performed for five metabolic loci. Individuals carrying the NAD(P)H:quinone oxidoreductase 1 (NQO1) variant had significantly twofold increased DNA-SSB levels compared to wild-type individuals. The same result was observed for subjects with microsomal epoxide hydrolase (EPHX) genotypes that predict the fast catalytic phenotype. Deletion of the glutathione S-transferase T1 (GSTT1) gene also showed a consistent quantitative 35-40% rise in DNA-SSB levels. Neither glutathione S-transferase M1 (GSTM1) nor myeloperoxidase (MPO) genetic variants exerted any effect on DNA-SSB levels. Combinations of two genetic polymorphisms showed the same effects on DNA-SSB as expected from the data on single genotypes. The three locus genotype predicted to produce the highest level of toxicity, based on metabolic pathways, produced a significant 5.5-fold higher level of DNA-SSB than did the genotype predicted to yield the least genotoxicity.     

The nature of chromosomal aberrations detected in humans exposed to benzene

Benzene is an established cause of human leukemia that is thought to act by producing chromosomal aberrations and altered in cell differentiation. In several recent studies increased levels of chromosomal aberrations in peripheral blood lymphocytes were correlated with a heightened risk of cancer, especially hematological malignancies. Thus, chromosomal aberrations may be a predictor of future leukemia risk. Previous studies exploring whether benzene exposure induces chromosomal aberrations have yielded mostly positive results. However, it remains unclear whether the chromosomal aberrations induced by benzene occur in a distinct pattern. Here, we thoroughly review the major chromosome studies published to date in benzene-exposed workers, benzene-poisoned and preleukemia patients, and leukemia cases associated with benzene expose. Although three cytogenetic markers (chromosomal aberrations, sister chromatid exchanges, and micronuclei) are commonly examined, our primary focus is on studies of chromosomal aberrations, because only this marker has so far been correlated with increased cancer risk. This review surveys the published literature, analyzes the study results, and discusses the characteristics of effects reported. In most studies of currently exposed workers, increases in chromosomal aberrations were observed. However, due to the relatively small number of affected individuals and variability in the reported aberrations, firm conclusions cannot be made about the involvement of specific chromosomes or chromosome regions. Further, in leukemia cases associated with benzene exposure, there is no evidence of a unique pattern of benzene-induced chromosomal aberrations in humans. Leukemia cases associated with benzene exposure are, however, more likely to contain clonal chromosome aberrations then those arising de novo in the general population.     

毒物代谢酶的基因多态性与慢性苯中毒易感性

目的探讨毒物代谢酶CYP2E1、MPO、NQO1、GSTT1和GSTM1基因多态性与慢性苯中毒遗传易感性之间的关系。方法选择100名慢性苯中毒工人为病例组及90名同期接苯但无苯中毒表现的同工种工人为对照组,应用PCR-RFLP及多重PCR方法判定CYP2E1、MPO、NQO1、GSTT1和GSTM1基因基因型。结果携带NQO1 C609T T/T基因型(纯合突变型)个体发生苯中毒的危险性是具有C/T基因型(杂合型)和C/C基因型(野生型)个体的2.82倍(95%CI:1.42~5.58,P<0.05),是具有C/C基因型(野生型)个体的2.94倍(95%CI:1.25~6.90,P<0.05);携带GSTT1缺失型(null)基因型个体发生苯中毒的危险性是具GSTT1非缺失型(non-null)基因型个体的1.91倍(95%CI:1.05~3.45,P<0.05)。未发现CYP2E1、MPO、GSTM1基因型与苯中毒之间的关系。同时携带NQO1 C609T纯合突变型(T/T)、GSTT1缺失型(null)与GSTM1缺失型(null)个体接苯时发生苯中毒的危险性最高,是NQO1 C609T杂合型(C/T)和野生型(C/C)、GSTT1非缺失型(non-null)与GSTM1非缺失型(non-null)个体的20.41倍(95%CI:3.79~111.11,P<0.01)。结论基因之间的交互作用在苯中毒的发生中起重要作用。同时携带NQO1 C609T纯合突变基因型(T/T)、GSTT1缺失基因型(null)和GSTM1缺失基因型(null)个体发生苯中毒的风险最大,可考虑作为苯中毒的重要生物标志物。     

Genetic polymorphisms involved in toxicant-metabolizing enzymes and the risk of chronic benzene poisoning in Chinese occupationally exposed populations

Benzene is a recognized haematotoxin and leukaemogen, but its mechanism of action and the role of genetic susceptibility are still unclear. Cytochrome P450 2E1 (CYP2E1) and myeloperoxidase (MPO) are involved in benzene activation; and NAD (P)H:quinine oxidoreductase 1 (NQO1), glutathione S-transferase theta 1 (GSTT1) and glutathione S-transferase mu 1 (GSTM1) participate in benzene detoxification. The common, well-studied single-nucleotide polymorphisms (SNPs) were analysed in these genes drawn from the toxicant-metabolizing pathways. A total of 100 workers with chronic benzene poisoning (CBP) and 90 controls were enrolled in China. There was a 2.82-fold (95% CI = 1.42-5.58) increased risk of CBP in the subjects with the NQO1 609C > T mutation genotype (T/T) compared with those carrying heterozygous (C/T) and wild-type (C/C). The subjects with the GSTT1 null genotype had a 1.91-fold (95% CI = 1.05-3.45) increased risk of CBP compared with those with GSTT1 non-null genotype. There was no association of CYP2E1 and MPO genotype with CBP. A three genes' interaction showed that there was a 20.41-fold (95% CI = 3.79-111.11) increased risk of CBP in subjects with the NQO1 609C > T T/T genotype and with the GSTT1 null genotype and the GSTM1 null genotype compared with those carrying the NQO1 609C > T C/T and C/C genotype, GSTT1 non-null genotype, and GSTM1 non-null genotype. The study provides evidence of an association of a gene-gene interaction with the risk of CBP.     

Depressions in B- and T-lymphocyte mitogen-induced blastogenesis in mice exposed to low concentrations of benzene

In a short-term (6 h/day × 6 days) benzene inhalation dose-response study, mitogen-induced blastogenesis of both B- and T-lymphocytes in male, C57B1 mice was observed to be significantly depressed at relatively low levels of benzene. Exposure to 10 ppm benzene resulted in a significant depression in femoral lipopolysaccharide (LPS)-induced B-colony-forming ability, while total numbers of B-lymphocytes at this concentration were not significantly depressed. Similarly, splenic phytohemag-glutinin (PHA)-induced blastogenesis was significantly depressed at 31 ppm, without a concomitant significant depression in numbers of T-lymphocytes. These data indicate that concentrations of benzene at or near the current standard for occupational exposure (10 ppm) can affect certain immune-associated processes.     

Chromosomal aberrations in workers exposed to low levels of benzene: association with genetic polymorphisms

Benzene and its metabolites damage human lymphocytes, resulting in chromosomal aberrations and aneuploidy. Polymorphisms in the genes for benzene-metabolizing enzymes have been implicated in benzene-associated haematotoxicity. In this study, we examined the specificity of benzene-induced aneuploidy and the influence of genetic polymorphisms (GSTM1, GSTT1, GSTP1, NAT2, NQO1 and CYP2E1) on chromosomal aberrations. In total, 82 benzene-exposed workers from a coke oven plant and 76 matched controls were examined. The benzene concentration in the work-place air ranged from 0.014-0.743 p.p.m. (geometric mean 0.557 p.p.m.). Benzene exposure was associated with significant increases in both monosomy and trisomy of chromosomes 8 and 21. Translocations between chromosomes 8 and 21 [t(8:21)] were eight-fold more frequent in the high-level exposure group compared to the control group. Multiple regression analysis indicated that the frequencies of chromosome aberrations were significantly associated with benzene exposure and polymorphisms in the metabolic enzyme genes. A particular subset of genotypes, which included the GSTM1-null and GSTT1-null genotypes, the slow acetylator type of NAT2, a variant of the NQO1 genotype and the CYP2E1 DraI and RsaI genotypes, were either separately, or in combination, associated with increased frequencies of aneuploidy among the benzene-exposed individuals after adjustments for age, alcohol consumption and smoking. These results suggest that polymorphisms in the genes for benzene-metabolizing enzymes influence the susceptibility of individuals to chromosomal aberrations in relation to benzene exposure.     

Mice exposed in utero to low concentrations of benzene exhibit enduring changes in their colony forming hematopoietic cells

Pregnant Swiss-Webster mice were exposed to either 0, 5, 10, or 20 ppm benzene from days 6 through 15 of gestation. Hematopoietic progenitor cell assays were performed on fetal, neonatal and adult progeny of the exposed dams. All concentrations of benzene employed induced marked changes in the numbers of the more differentiated erythroid colony forming cells, the CFU-E. Granulocytic colony forming cells (GM-CFU-C) were affected by the 2 higher exposure concentrations. When mice previously exposed in utero to 10 ppm benzene were re-exposed as adults, marked depressions in the numbers of CFU-E and GM-CFU-C were observed. Thus, in utero exposures to concentrations of benzene at the current occupational exposure limit induce alterations of the murine hematopoietic system which persist into adulthood.     

Specific immune responses in workers exposed to benzene

The aim of this study was to elaborate a method for detection of specific IgG antibodies (Abs) to the haptenes p-benzoquinone (p-BQ) and hydroquinone (HQ) for assessment of specific humoral immune responses. Plasma and urine, collected from petrochemical plant workers have been analyzed. The workers were divided into three professional groups in ascending order of benzene exposure. The concentration of benzene in the air was determined by gas chromatography with mass-spectrometry and trans,trans-muconic acid (biomarker of benzene exposure) in urine-by liquid chromatography with UV-detection. Specific IgG Abs to haptenes p-BQ and HQ in plasma were determined with newly developed ELISA. The relationships "exposure-effect," revealed increased levels of specific IgG to haptens correlating with the benzene exposure. The "exposure-response" relationships demonstrated that workers with value of OD over X+2SD were 62% low exposure group, 68% in group with level of exposure on Threshold Limit Value (TLV) and 91% in the highest exposure group. The data obtained show that there is a good correlation between antibody production and the biomarker of exposure t,t-muconic acid. CONCLUSION: The newly developed method is applicable for assessment of specific humoral immune responses in workers exposed to benzene. There was a good correlation between benzene exposure and formation of antibodies against benzene metabolites.     

Genetic polymorphisms in heterocyclic amine metabolism and risk of colorectal adenomas

High red meat intake has been linked with an increased risk of colorectal cancer and adenomas. During high temperature cooking of red meats, heterocyclic amines (HCAs) are generated; however, to be carcinogenic, they must be metabolized by enzymes including cytochrome P450 1A2 (CYP1A2) and N-acetyltransferase 1 (NAT1) and/or N-acetyltransferase 2 (NAT2). We have conducted a clinic-based case-control study of colorectal adenomas that focused on assessment of exposure to HCAs (estimated by use of a HCA database and meat cooking module) and modification of these exposures by genetic factors. We have previously reported that intake of MeIQx was associated with an increased risk of colorectal adenomas [overall association at 80th percentile, > 27.00 ng/day: odds ratio (OR) = 2.68, 95% confidence interval (CI) 1.58-4.55]. Here, we report our evaluation of whether variation in CYP1A2, NAT1 and/or NAT2 modify the association between HCAs and colorectal adenoma formation in 146 cases and 228 frequency-matched controls. The NAT1*10 allele was associated with a nonsignificant increased risk of colorectal adenomas (OR = 1.43; 95% CI 0.86-2.36). Further, when we analysed 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) intake as a categorical variable, we observed a six-fold increase in adenoma risk among rapid NAT1 acetylators who consumed more than 27 ng a day (OR = 6.50; 95% CI 2.16-19.7), whereas among slow NAT1 acetylators, the increase in risk was two-fold (OR = 2.32; 95% CI 1.12-4.81). While suggestive, the results were not significantly different from each other on either an additive or multiplicative scale. In contrast, NAT2 genotype and CYP1A2 and NAT2 hepatic activity measured by caffeine urinary metabolites were not associated with adenoma risk, although an increase in risk with rapid CYP1A2 activity could not be ruled out (OR = 1.46; 95% CI 0.76-2.81). Moreover, there was no evidence that the effect of MeIQx was enhanced among subjects in any subgroup defined by variation in these measures. These results are compatible with the hypothesis that high HCA exposure is associated with an increased risk of colorectal adenomas, particularly in genetically susceptible subgroups. Further study of larger populations is needed to confirm and extend these observations.     

Genetic polymorphisms of cytochrome P450 enzymes and antidepressant metabolism

INTRODUCTION: The cytochrome P450 (CYP) enzymes are the major enzymes responsible for Phase I reactions in the metabolism of several substances, including antidepressant medications. Thus, it has been hypothesized that variants in the CYP network may influence antidepressant efficacy and safety. Nonetheless, data on this field are still contradictory. The authors aim to give an overview of the published studies analyzing the influence of CYP highly polymorphic loci on antidepressant treatment in order to translate the acquired knowledge to a clinical level. AREAS COVERED: The authors collected and compared experimental works and reviews published from the 1980s to the present and included in the Medline database. The included studies pertain to the effects of CYP gene polymorphisms on antidepressant pharmacokinetic parameters and clinical outcomes (response and drug-related adverse effects), with a focus on applications in clinical practice. The authors focused mainly on in vivo studies in humans (patients or healthy volunteers). EXPERT OPINION: Great variability in antidepressant metabolism among individuals has been demonstrated. Thus, with the current interest in individualized medicine, several genetic tests to detect CYP variants have been produced. They provide a potentially useful way to anticipate some clinical outcomes of antidepressant treatment, although they will only be extensively used in clinical practice if precise and specific treatment options and guidelines based on genetic tests can be provided.     

Airborne concentrations of benzene due to diesel locomotive exhaust in a roundhouse

Concentrations of airborne benzene due to diesel exhaust from a locomotive were measured during a worst-case exposure scenario in a roundhouse. To understand the upper bound human health risk due to benzene, an electromotive diesel and a General Electric four-cycle turbo locomotive were allowed to run for four 30-min intervals during an 8-h workshift in a roundhouse. Full-shift and 1-h airborne concentrations of benzene were measured in the breathing zone of surrogate locomotive repairmen over the 8-h workshift on 2 consecutive days. In addition, carbon monoxide was measured continuously; elemental carbon (surrogate for diesel exhaust) was sampled with full-shift area samples; and nitrogen dioxide/nitric oxide was sampled using full-shift and 15-min (nitrogen dioxide only) area samples. Peak concentrations of carbon monoxide ranged from 22.5 to 93 ppm. The average concentration of elemental carbon for each day of the roundhouse study was 0.0543 and 0.0552 microg/m(3 )for an 8-h workshift. These were considered "worst-case" conditions since the work environment was intolerably irritating to the eyes, nose, and throat. Short-term nitrogen dioxide concentrations ranged from 0.81 to 2.63 ppm during the diesel emission events with the doors closed. One-hour airborne benzene concentrations ranged from 0.001 to 0.015 ppm with 45% of the measurements below the detection limit of 0.002-0.004 ppm. Results indicated that the 8-h time-weighted average for benzene in the roundhouse was approximately 100-fold less than the current threshold limit value (TLV) of 0.5 ppm. These data are consistent with other studies, which have indicated that benzene concentrations due to diesel emissions, even in a confined environment, are quite low.     

Genetic polymorphisms in hMTH1, hOGG1 and hMYH and risk of chronic benzene poisoning in a Chinese occupational population

Oxidative damage to DNA induced by benzene is an important mechanism of its genotoxicity, which leads to chronic benzene poisoning (CBP). Therefore, genetic variation in DNA repair genes may contribute to susceptibility to CBP in the exposed population. We hypothesized that single nucleotide polymorphisms (SNPs) in hMTH1, hOGG1 and hMYH genes are associated with risk of CBP. We genotyped SNPs at codon 83 of hMTH1, codon 326 of hOGG1, and codon 324 of hMYH in 152 CBP patients and 152 healthy workers occupationally exposed to benzene without poisoning manifestations. The genotypes were determined by polymerase chain reaction-restrained fragment length polymorphism (PCR-RFLP) technique. There were 2.51-fold [adjusted odds ratio (OR_adj), 2.51; 95% CI, 1.14-5.49; P = 0.02] and 2.49-fold (OR_adj, 2.49; 95% CI: 1.52-4.07; P < 0.01) increased risk of CBP for individuals carrying genotypes of hMTH1 83Val/Met + Met/Met and hOGG1 326Cys/Cys, respectively. Compared with individuals carrying genotypes of hOGG1 326Cys/Cys and hMYH 324His/His at the same time, there was a 0.33-fold (OR_adj, 0.33; 95% CI: 0.15-0.72; P < 0.05) decreased risk of CBP for those with genotypes of hOGG1 326Ser/Cys + Ser/Ser and hMYH 324His/Gln + Gln/Gln. In the smoking group, there was a 0.15-fold (OR_adj, 0.15; 95% CI, 0.03-0.68; P = 0.01) decreased risk of CBP for subjects carrying genotypes of hMYH 324His/Gln + Gln/Gln compared with those of genotype of hMYH 324His/His. Therefore, our results suggested that polymorphisms at codons 83 of hMTH1 and codon 326 of hOGG1 might contribute to CBP in a Chinese occupational population.     

Personal air exposures and breath concentrations of benzene and other volatile hydrocarbons for smokers and nonsmokers

Personal air exposures and exhaled breath concentrations of 20 volatile organic compounds were measured for 198 smokers and 322 non-smokers in 5 U.S. cities (1980-84). Smokers showed significantly increased breath concentrations of six hydrocarbons: benzene, styrene, ethylbenzene, o-xylene, m + p-xylene, and octane. Homes with smokers had higher indoor air concentrations of the same compounds than homes without smokers during fall and winter. Passive smokers exposed at work had significantly higher levels of benzene and other aromatics in their breath than unexposed non-smokers.     

Urinary arsenic metabolism in a Western Chinese population exposed to high-dose inorganic arsenic in drinking water: Influence of ethnicity and genetic polymorphisms

To investigate the differences in urinary arsenic metabolism patterns of individuals exposed to a high concentration of inorganic arsenic (iAs) in drinking water, an epidemiological investigation was conducted with 155 individuals living in a village where the arsenic concentration in the drinking water was 969 μg/L. Blood and urine samples were collected from 66 individuals including 51 cases with skin lesions and 15 controls without skin lesions. The results showed that monomethylated arsenic (MMA), the percentage of MMA (%MMA) and the ratio of MMA to iAs (MMA/iAs) were significantly increased in patients with skin lesions as compared to controls, while dimethylated arsenic (DMA), the percentage of DMA (%DMA) and the ratio of DMA to MMA (DMA/MMA) were significantly reduced. The percent DMA of individuals with the Ala/Asp genotype of glutathione S-transferase omega 1 (GSTO1) was significantly lower than those with Ala/Ala. The percent MMA of individuals with the A2B/A2B genotype of arsenic (+ 3 oxidation state) methyltransferase (AS3MT) was significantly lower than those with AB/A2B. The iAs and total arsenic (tAs) content in the urine of a Tibetan population were significantly higher than that of Han and Hui ethnicities, whereas MMA/iAs was significantly lower than that of Han and Hui ethnicities. Our results showed that when exposed to the same arsenic environment, different individuals exhibited different urinary arsenic metabolism patterns. Gender and ethnicity affect these differences and above polymorphisms may be effectors too.     

Mechanism of microsomal metabolism of benzene to phenol

The mechanism of microsomal hydroxylation of benzene to phenol has been studied by examining the microsomal metabolism of the specifically deuterated derivative 1,3,5-[2H3]benzene. Evidence for the formation of the following four products was obtained: 2,3,5-[2H3]phenol, 3,5-[2H2]phenol, 2,4,6-[2H3]phenol, and 2,4-[2H2]phenol. The presence of 2,3,5-[2H3]phenol and 2,4-[2H2]phenol shows that, in the microsomal metabolism of benzene to phenol, a NIH shift had occurred. A deuterium isotope effect (kH/kD) of approximately 4 was detected in both the meta- and para-deuterated phenols. This finding indicates that cyclohexadienone, formed either by isomerization of the epoxide or directly from the enzyme-substrate complex, is a major intermediate in the metabolism of benzene to phenol.     

Paraoxonase and acetylcholinesterase activities in humans exposed to organophosphorous compounds

Different kinds of organophosphorous compounds (OP) are used as pesticides in Turkish agriculture. Suicidal, accidental, or occupational exposure may occur in developing countries. OP inhibit acetylcholinesterase (AChE) activities; on the other hand, serum paraoxonase (PON1) hydrolyzes the toxic metabolites of a variety of OP. In recent years, some studies have shown that PON1 activity is an important marker in individuals who are exposed to OP. Both serum cholinesterase and PON1 activities were measured spectrophotometrically from 18 male agricultural workers who were chronically exposed to azinphos methyl, chlorpyriphos, or malathion and other pesticides during cereal spraying, transportation, and storage. The individuals were classified according to PON1 phenotypes using the antimode 60% stimulation method to determine the dividing point between non-salt-stimulated, A type (homozygotes for the low-activity allele), and salt-stimulated AB (heterozygotes) and B types (homozygotes for the high-activity allele). A positive correlation was found between AChE activities and percent of PON1 stimulation. The individuals with phenotype A had the lowest enzyme activities. This study suggests that individuals with phenotype A might be more sensitive to OP-induced toxicity.