Role of polymorphic CYP2E1 and CYP2D6 genes in NNK-induced chromosome aberrations in cultured human lymphocytes

A critical review of advances in tumor metabolism abnormalities induced by nitrosamine disinfection by-products in drinking water

Intensified sanitation practices amid the recent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak might result in the increased release of chloramine disinfectants into surface water, significantly promoting the formation of nitrosamine disinfection by-products (DBPs) in drinking water. Unfortunately, these nitrosamine DBPs exhibit significant genotoxic, carcinogenic, and mutagenic properties, whereas chlorinating disinfectants remain in global practice. The current review provides valuable insights into the occurrence, identification, contamination status, exposure limits, and toxicity of the new unregulated disinfection by-products (nitrosamine DBPs) in drinking water. As a result, concentrations of nitrosamine DBPs far exceed allowable limits in drinking water, and prolonged exposure has the potential to cause metabolic disorders, a critical step in tumor initiation and progression. Importantly, based on recent research, we have concluded the role of nitrosamines DBPs in different metabolic pathways. Remarkably, nitrosamine DBPs can induce chronic inflammation and initiate tumors by activating sphingolipid and polyunsaturated fatty acid metabolism. Regarding amino acid and nucleotide metabolism, nitrosamine DBPs can inhibit tryptophan metabolism and de novo nucleotide synthesis. Moreover, inhibition of de novo nucleotide synthesis fails to repair DNA damage induced by nitrosamines. Additionally, the accumulation of lactate induced by nitrosamine DBPs may act as a pivotal signaling molecule in communication within the tumor microenvironment. However, with the advancement of tumor metabolomics, understanding the role of nitrosamine DBPs in causing cancer by inducing metabolic abnormalities significantly lags behind, and specific mechanisms of toxic effects are not clearly defined. Urgently, further studies exploring this promising area are needed.

The cellular basis of fetal endoplasmic reticulum stress and oxidative stress in drug-induced neurodevelopmental deficits

Prenatal substance exposure is a growing public health concern worldwide. Although the opioid crisis remains one of the most prevalent addiction problems in our society, abuse of cocaine, methamphetamines, and other illicit drugs, particularly amongst pregnant women, are nonetheless significant and widespread. Evidence demonstrates prenatal drug exposure can affect fetal brain development and thus can have long-lasting impact on neurobehavioral and cognitive performance later in life. In this review, we highlight research examining the most prevalent drugs of abuse and their effects on brain development with a focus on endoplasmic reticulum stress and oxidative stress signaling pathways. A thorough exploration of drug-induced cellular stress mechanisms during prenatal brain development may provide insight into therapeutic interventions to combat effects of prenatal drug exposure.

Procarcinogens - Determination and Evaluation by Yeast-Based Biosensor Transformed with Plasmids Incorporating RAD54 Reporter Construct and Cytochrome P450 Genes

In Vietnam, a great number of toxic substances, including carcinogens and procarcinogens, from industrial and agricultural activities, food production, and healthcare services are daily released into the environment. In the present study, we report the development of novel yeast-based biosensor systems to determine both genotoxic carcinogens and procarcinogens by cotransformation with two plasmids. One plasmid is carrying human CPR and CYP (CYP3A4, CYP2B6, or CYP2D6) genes, while the other contains the RAD54-GFP reporter construct. The three resulting coexpression systems bearing both CPR-CYP and RAD54-GFP expression cassettes were designated as CYP3A4/CYP2B6/CYP2D6 + RAD54 systems, respectively and used to detect and evaluate the genotoxic potential of carcinogens and procarcinogens by selective activation and induction of both CPR-CYP and RAD54-GFP expression cassettes in response to DNA damage. Procarcinogens were shown to be predominantly, moderately or not bioactivated by one of the CYP enzymes and thus selectively detected by the specific coexpression system. Aflatoxin B₁ and benzo(a)pyrene were predominantly detected by the CYP3A4 + RAD54 system, while N-nitrosodimethylamine only moderately activated the CYP2B6 + RAD54 reporter system and none of them was identified by the CYP2D6 + RAD54 system. In contrast, the genotoxic carcinogen, methyl methanesulfonate, was detected by all systems.

Our yeast-reporter system can be performed in 384-well microplates to provide efficient genotoxicity testing to identify various carcinogenic compounds and reduce chemical consumption to about 53% as compared with existing 96-well genotoxicity bioassays. In association with a liquid handling robot, this platform enables rapid, cost-effective, and high-throughput screening of numerous analytes in a fully automated and continuous manner without the need for user interaction.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

A genetic instrumental variables analysis of the effects of prenatal smoking on birth weight: evidence from two samples

There is a large literature showing the detrimental effects of prenatal smoking on birth and childhood health outcomes. It is somewhat unclear though, whether these effects are causal or reflect other characteristics and choices by mothers who choose to smoke that may also affect child health outcomes or biased reporting of smoking. In this paper we use genetic markers that predict smoking behaviors as instruments to address the endogeneity of smoking choices in the production of birth and childhood health outcomes. Our results indicate that prenatal smoking produces more dramatic declines in birth weight than estimates that ignore the endogeneity of prenatal smoking, which is consistent with previous studies with non-genetic instruments. We use data from two distinct samples from Norway and the United States with different measured instruments and find nearly identical results. The study provides a novel application that can be extended to study several behavioral impacts on health and social and economic outcomes.

A comprehensive haplotype analysis of the XPC genomic sequence reveals a cluster of genetic variants associated with sensitivity to tobacco-smoke mutagens

The impact of single-nucleotide polymorphisms (SNPs) of the DNA repair gene XPC on DNA repair capacity (DRC) and genotoxicity has not been comprehensively determined. We constructed a comprehensive haplotype map encompassing all common XPC SNPs and evaluated the effect of Bayesian-inferred haplotypes on DNA damage associated with tobacco smoking, using chromosome aberrations (CA) as a biomarker. We also used the mutagen-sensitivity assay, in which mutagen-induced CA in cultured lymphocytes are determined, to evaluate the haplotype effects on DRC. We hypothesized that if certain XPC haplotypes have functional effects, a correlation between these haplotypes and baseline and/or mutagen-induced CA would exist. Using HapMap and single nucleotide polymorphism (dbSNP) databases, we identified 92 SNPs, of which 35 had minor allele frequencies >or= 0.05. Bayesian inference and subsequent phylogenetic analysis identified 21 unique haplotypes, which segregated into six distinct phylogenetically grouped haplotypes (PGHs A-F). A SNP tagging approach used identified 11 tagSNPs representing these 35 SNPs (r(2) = 0.80). We utilized these tagSNPs to genotype a population of smokers matched to nonsmokers (n = 123). Haplotypes for each individual were reconstituted and PGH designations were assigned. Relationships between XPC haplotypes and baseline and/or mutagen-induced CA were then evaluated. We observed significant interaction among smoking and PGH-C (p = 0.046) for baseline CA where baseline CA was 3.5 times higher in smokers compared to nonsmokers. Significant interactions among smoking and PGH-D (p = 0.023) and PGH-F (p = 0.007) for mutagen-induced CA frequencies were also observed. These data indicate that certain XPC haplotypes significantly alter CA and DRC in smokers and, thus, can contribute to cancer risk.

Phenobarbital induces monkey brain CYP2E1 protein but not hepatic CYP2E1, in vitro or in vivo chlorzoxazone metabolism

Cytochrome P450 2E1 (CYP2E1) is expressed in the brain and liver, and can metabolize clinical drugs and activate toxins. The effect of phenobarbital on hepatic and brain CYP2E1 is unclear. We investigated the effect of chronic phenobarbital treatment on in vivo chlorzoxazone disposition (a CYP2E1 probe drug), in vitro chlorzoxazone metabolism, and hepatic and brain CYP2E1 protein levels in African Green monkeys (Cercopithecus aethiops). Monkeys were given oral saccharine or saccharine supplemented with 20 mg/kg phenobarbital (N = 6/group) for 22 days. Phenobarbital did not induce in vivo chlorzoxazone disposition, in vitro chlorzoxazone metabolism or hepatic CYP2E1 protein levels (all P > 0.05). However, phenobarbital induced brain CYP2E1 protein levels, using immunoblotting, by 1.26-fold in the cerebellum (P = 0.01) and 1.46-fold in the putamen (P = 0.04). Phenobarbital also increased cell-specific CYP2E1 expression, for example in the frontal cortical pyramidal neurons and cerebellar Purkinje cells. This data indicates that phenobarbital does not alter hepatic metabolism, but may alter metabolism of CYP2E1 substrates within the brain.

Cytochrome P450 interactions in human cancers: new aspects considering CYP1B1

Molecular epidemiological studies are now a powerful tool to determine differential genetic susceptibilities to cancer-causing agents, and to obtain information on potential mechanisms. Cytochrome P450 (CYP) allelic variants are considered biomarkers of susceptibility to cancer. Such variants have an influence on the bioactivation and thereby on the potency of chemical carcinogens. This is very much straight forward for tobacco smoke-related human cancers. A new aspect is the implication of CYP1B1 in tobacco smoke-related cancers at several organ sites. On this basis, the present review is focused on lung, breast, urinary bladder and head and neck cancer. The CYP profile of the human lung includes CYP1A1, -1B1, -2A6, -2A13, -2B6, -2C18, -2E1, -2F1, -3A5 and -4B1. Polycyclic aromatic hydrocarbons (PAHs) and nitrosamines, as active components of tobacco smoke, appear as primary chemical factors for lung malignancies. For human mammary cancer, the use of hormone replacement therapy (HRT) has been shown to be associated with an increase of breast cancer risk, and there seems to be a link between risks caused by HRT use and modifying polymorphisms of drug/xenobiotic enzymes. Specifically, an association of the CYP1B1*3/*3 genotype with increased breast cancer risks has been postulated. Cigarette smoking is a major cause of human urinary bladder cancer. Arylamines, PAHs and nitrosamines are locally activated within the urothelium. Important CYPs in the bladder epithelium of experimental animals and man are CYP1B1 and -4B1. Alcohol consumption and tobacco smoking are known as the major causes of head and neck cancers. Recently, it appears that a polymorphic variant CYP1B1*3/*3 relates significantly to the individual susceptibility of smokers to head and neck cancer, supporting the view that PAH are metabolically activated through CYP1B1. It appears that CYP1B1 plays a key role for the activation of carcinogens at several organ targets, with a likelihood of complex gene-environment interactions implying Phase II enzymes.

Cytokinesis-blocked micronucleus assay as a novel biomarker for lung cancer risk

In this case-control study, we modified the cytokinesis-block micronucleus (CBMN) assay, an established biomarker for genomic instability, to evaluate susceptibility to the nicotine-derived nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) by measuring the frequency of NNK-induced chromosomal damage endpoints (micronuclei, nucleoplasmic bridges, and nuclear buds) per 1,000 binucleated lymphocytes. Spontaneous and NNK-induced chromosomal damage were significantly higher in lung cancer patients compared with controls. Forty-seven percent of cases (versus 12% of controls) had >or=4 spontaneous micronuclei, 66% of cases (and no controls) had >or=4 spontaneous nucleoplasmic bridges, and 25% of cases (versus 5% of controls) had >or=1 spontaneous nuclear bud (P < 0.001). Similarly, 40% of cases (versus 6% of the controls) had >or=5 NNK-induced micronuclei, 89% of cases (and no controls) had >or=6 induced nucleoplasmic bridges, and 23% of cases (versus 2% of controls) had >or=2 induced nuclear buds (P < 0.001). When analyzed on a continuous scale, spontaneous micronuclei, nucleoplasmic bridges, and nuclear buds were associated with 2-, 29-, and 6-fold increases in cancer risk, respectively. Similarly, NNK-induced risks were 2.3-, 45.5-, and 10-fold, respectively. We evaluated the use of CBMN assay to predict cancer risk based on the numbers of micronuclei, nucleoplasmic bridges, and nuclear buds defined by percentile cut points in controls. Probabilities of being a cancer patient were 96%, 98%, and 100% when using the 95th percentiles of spontaneous and NNK-induced micronuclei, nucleoplasmic bridges, and nuclear buds, respectively. Our study indicates that the CBMN assay is extremely sensitive to NNK-induced genetic damage and may serve as a strong predictor of lung cancer risk.

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

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

Genotoxicity testing of four food preservatives and their combinations in the Drosophila wing spot test

In this study, four food preservatives (sodium nitrate, sodium nitrite, potassium nitrate and potassium nitrite) and there five combinations at a concentration of 25mM have been evaluated for genotoxicity in the somatic mutation and recombination test (SMART) of Drosophila melanogaster. Three-day-old larvae trans-heterozygous including two linked recessive wing hair mutations (multiple wing hairs and flare) were fed at different concentrations of the test compounds (25, 50, 75 and 100mM) in standard Drosophila Instant Medium. Wings of the emerging adult flies were scored for the presence of spots of mutant cells, which can result from either somatic mutation or mitotic recombination. Also lethal doses of food preservatives used were determined in the experiments. A positive correlation was observed between total mutations and the number of wings having mutation. In addition, the observed mutations in each wing were classified according to the size and type of the mutation. For the evaluation of genotoxic effects, the frequencies of spots per wing in the treated series were compared to the control group, which is distilled water. Chemicals used were ranked as sodium nitrite, potassium nitrite, sodium nitrate and potassium nitrate according to their genotoxic and toxic effects. Moreover, the genotoxic and toxic effects produced by the combined treatments were considerably increased, especially when the four chemicals were mixed. The present study shows that correct administration of food preservatives/additives may have a significant effect on human health.

The L84F and the I143V polymorphisms in the O6-methylguanine-DNA-methyltransferase (MGMT) gene increase human sensitivity to the genotoxic effects of the tobacco-specific nitrosamine carcinogen NNK

O-Methylguanine-DNA-methyltransferase (MGMT) is a direct-reversal DNA repair protein that removes DNA adducts formed by alkylating mutagens found in tobacco smoke. Several coding single nucleotide polymorphisms (cSNPs) in the MGMT gene have been reported. However, their effect on the levels and types of genetic damage induced by specific environmental carcinogens remains to be fully elucidated. We developed two novel genotyping techniques and used them, in conjunction with the mutagen-sensitivity assay, to test the hypothesis that the L84F and I143V cSNPs in the MGMT gene confer increased sensitivity to genetic damage induced by the alkylating tobacco-specific nitrosamine carcinogen NNK. Lymphocytes from 114 healthy volunteers were exposed in vitro to NNK, and the genotoxic response was assessed by measuring chromosome aberration (CA) frequencies. A significant (P<0.02) increase in NNK-induced CA was observed in cells from individuals with the 84F polymorphism compared to cells from individuals homozygous for the referent L84 allele. A significant positive interaction between this cSNP and smoking, gender and age was observed (P<0.03). In subjects with the variant 143V allele, significantly higher levels of NNK-induced CA were observed in males and in young subjects (<43 years old) compared to subjects homozygous for the referent I143 allele (P<0.02). Individuals who inherited two cSNPs had significantly higher levels of NNK-induced CA compared to individuals with none or with one cSNP (P<0.002). These new data suggest that the 84F and 143V cSNPs may alter the function characteristics of the MGMT protein, resulting in suboptimal repair of genetic damage induced by NNK.

Gender differences in genetic damage induced by the tobacco-specific nitrosamine NNK and the influence of the Thr241Met polymorphism in the XRCC3 gene

The rapid increase in adenocarcinoma of the lung and mortality amongst women strongly suggests that gender differences exist in sensitivity to certain tobacco carcinogens. In the current study, we performed the mutagen-sensitivity assay, with the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), to test the hypothesis that women are more sensitive to the genotoxic effects of NNK than men. Chromosome aberration (CA) frequencies in peripheral blood lymphocytes (PBLs) from 99 patients were evaluated before and after in vitro exposure to NNK. Because the Thr241Met polymorphism in the DNA-repair gene XRCC3 is associated with increased risk of tobacco-related cancers, especially among women, we also tested the hypothesis that individuals who inherit the homozygous variant 241Met allele are more sensitive to the genotoxic effects of NNK. CA frequency was significantly higher 1 hr after NNK treatment in women, compared with men (P = 0.02). When smoking and gender were considered together, a significant interaction was observed. PBLs from female smokers had significantly higher frequencies of NNK-induced CA, compared with female nonsmokers 1 hr after treatment (P = 0.02). We observed no overall effect of the Thr241Met polymorphism on NNK-induced CA in men, women, smokers, or nonsmokers. Overall, our data indicate that women are more sensitive to the genotoxic effects of NNK than men. Because in past years smoking among women has increased, and in view of the close correlation between NNK exposure and adenocarcinoma of the lung, our data provide a plausible explanation for the recent increase in the incidence of this cancer among women.

DNA repair capacity in lymphocytes of nasopharyngeal cancer patients

Possible hereditary factors in the tumorigenesis of nasopharyngeal cancer (NPC) have not yet been clearly identified. In the present study, the DNA repair capacity of lymphocytes after exposure to the nitrosamine NDEA was quantified in order to elucidate whether this measure may be a factor in susceptibility to NPC. The alkaline single-cell microgel electrophoresis (Comet) assay was used to quantify chemically induced DNA damage and repair capacity in lymphocytes of 30 NPC patients (NPC) and 29 non-tumor donors (NTD). The induction of DNA single strand breaks, alkali labile and incomplete excision repair sites after exposure of lymphocytes to NDEA was assessed as differences between repair intervals of 0 min, 15 min, 30 min and 60 min, respectively. A RC(total) was assessed using the difference between the OTMs of 0 min of repair time and the 60-min repair interval for both groups. Repair capacities (RC) were calculated for the intervals according to the Olive Tail Moment (OTM), a quantitative measure for DNA migration in the Comet assay for the group of NPC patients and the NTD, accordingly. RCs were compared between the two groups using the Mann-Whitney U-Test. RC(15 min), RC(30 min) RC(60 min) and the RC(total) after a 60-min repair interval demonstrated no significant difference between the two groups. Furthermore, when comparing grades of DNA migration (OTM<2, 2-5, 5-10, 10-20, 20-30 and >30), there were no differences evident. In this investigation, rejoining of DNA single strand breaks in lymphocytes of NPC and NTD appeared to be accomplished to an equal degree and in equal time periods. However, the applied method does not give evidence concerning the quality of the single strand break rejoining processes. In this group of patients, tumorigenesis in NPC could not be associated with a decreased DNA repair capacity.

Genotoxicity of tobacco smoke and tobacco smoke condensate: a review

This report reviews the literature on the genotoxicity of mainstream tobacco smoke and cigarette smoke condensate (CSC) published since 1985. CSC is genotoxic in nearly all systems in which it has been tested, with the base/neutral fractions being the most mutagenic. In rodents, cigarette smoke induces sister chromatid exchanges (SCEs) and micronuclei in bone marrow and lung cells. In humans, newborns of smoking mothers have elevated frequencies of HPRT mutants, translocations, and DNA strand breaks. Sperm of smokers have elevated frequencies of aneuploidy, DNA adducts, strand breaks, and oxidative damage. Smoking also produces mutagenic cervical mucus, micronuclei in cervical epithelial cells, and genotoxic amniotic fluid. These data suggest that tobacco smoke may be a human germ-cell mutagen. Tobacco smoke produces mutagenic urine, and it is a human somatic-cell mutagen, producing HPRT mutations, SCEs, microsatellite instability, and DNA damage in a variety of tissues. Of the 11 organ sites at which smoking causes cancer in humans, smoking-associated genotoxic effects have been found in all eight that have been examined thus far: oral/nasal, esophagus, pharynx/larynx, lung, pancreas, myeoloid organs, bladder/ureter, uterine cervix. Lung tumors of smokers contain a high frequency and unique spectrum of TP53 and KRAS mutations, reflective of the PAH (and possibly other) compounds in the smoke. Further studies are needed to clarify the modulation of the genotoxicity of tobacco smoke by various genetic polymorphisms. These data support a model of tobacco smoke carcinogenesis in which the components of tobacco smoke induce mutations that accumulate in a field of tissue that, through selection, drive the carcinogenic process. Most of the data reviewed here are from studies of human smokers. Thus, their relevance to humans cannot be denied, and their explanatory powers not easily dismissed. Tobacco smoke is now the most extreme example of a systemic human mutagen.

Beta-carotene: a cancer chemopreventive agent or a co-carcinogen?

Evidence from both epidemiological and experimental observations have fueled the belief that the high consumption of fruits and vegetables rich in carotenoids may help prevent cancer and heart disease in humans. Because of its well-documented antioxidant and antigenotoxic properties, the carotenoid beta-carotene (betaCT) gained most of the attention in the early 1980s and became one of the most extensively studied cancer chemopreventive agents in population-based trials supported by the National Cancer Institute. However, the results of three randomized lung cancer chemoprevention trials on betaCT supplementation unexpectedly contradicted the large body of epidemiological evidence relating to the potential benefits of dietary carotenoids. Not only did betaCT show no benefit, it was associated with significant increases in lung cancer incidence, cardiovascular diseases, and total mortality. These findings aroused widespread scientific debate that is still ongoing. It also raised the suspicion that betaCT may even possess co-carcinogenic properties. In this review, we summarize the current data on the co-carcinogenic properties of betaCT that is attributed to its role in the induction of carcinogen metabolizing enzymes and the over-generation of oxidative stress. The data presented provide convincing evidence of the harmful properties of this compound if given alone to smokers, or to individuals exposed to environmental carcinogens, as a micronutrient supplement. This has now been directly verified in a medium-term cancer transformation bioassay. In the context of public health policies, while the benefits of a diet rich in a variety of fruits and vegetables should continue to be emphasized, the data presented here point to the need for consideration of the possible detrimental effects of certain isolated dietary supplements, before mass cancer chemoprevention clinical trials are conducted on human subjects. This is especially important for genetically predisposed individuals who are environmentally or occupationally exposed to mutagens and carcinogens, such as those found in tobacco smoke and in industrial settings.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Polymorphic metabolizing genes and susceptibility to atherosclerosis among cigarette smokers

Atherosclerosis (AR) is the leading cause of morbidity and mortality in the US and cigarette smoking is a major contributing factor to the disease. Like cigarette smoking in lung cancer, genetic susceptibility may be an important factor in determining who is more likely to develop AR. However, the current emphasis has been on susceptibility based on altered cardiovascular homeostasis. In this investigation, we studied 120 AR patients and 90 matched controls to elucidate the association between polymorphisms in some metabolizing genes (GSTM1, GSTT1, CYP2E1, mEH, PON1, and MPO) and susceptibility to AR. We found that the GSTT1 null allele and the fast allele of mEH(*) (exon 4) are associated with risk for AR. Furthermore, the combined genotypes GSTM1 null/ CYP2E1(*)5B, GSTM1 null/mEH YY, and GSTT1 null/mEH YY are significantly associated with susceptibility to AR (OR = 15.42, 95% CI = 1.33-77.93, P = 0.021; OR = 3.48, 95% CI = 1.63-8.04, P = 0.0008; OR = 3.4; 95% CI = 0.99-17.38, P = 0.05; respectively). We have also conducted cytogenetic analysis to elucidate if induction of chromosome aberrations (CAs) is a biomarker of AR susceptibility. We found that among cigarette smokers (AR patients and smoker controls), individuals having the GSTM1 null allele had a significantly higher frequency of CAs compared to those with the normal allele (P < 0.05). This association was not found among nonsmokers. In addition, individuals who had inherited the CYP2E1(*)5B allele exhibited a significantly higher CA frequency (8.0 +/- 0.82) compared to those with the CYP2E1 wild-type genotype (4.31 +/- 0.35). Since the analysis of genetic susceptibility factors is still in its infancy, our study may stimulate additional investigations to understand the roles of genetic susceptibility and cigarette smoking in AR.

Genetic polymorphisms and chromosome damage

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

Validation of biomarkers as early predictors of disease

The development and validation of biomarkers that link environmental exposures to the pathogenesis of human disease is a leading priority in the field of environmental research. The validation of biomarkers as early predictors of clinical disease can enhance health risk assessment and contribute to effective new disease prevention policies in environmental and occupational settings. The process of validating biomarkers involves dealing with a range of characteristics that include the intrinsic qualities of the biomarker, its determinants, and the analytic procedure. We discuss here a three phase approach to validation. The final phase, consisting of longitudinal studies, is reached after the biomarker has been determined to be technically reliable and after the effect of external variables on the association with the outcome has been evaluated. We provide some examples of biomarkers reputed to be early predictors of cancer and cardiovascular disease (CVD). We then present original data to support the potential of DNA adducts to predict cancer and show, through re-evaluation of the Italian database on cytogenetic biomarkers, a lack of association between the frequency of chromosomal aberrations in circulating lymphocytes and CVD mortality rates -- a finding that should not be considered conclusive. In general, whenever a biomarker has been determined to be a valid predictor of disease, it should be used in risk assessment and public health policy.

Mutagen sensitivity of nasopharyngeal cancer patients

Primary nasopharyngeal carcinomas (NPCs) may be of various types, including squamous cell carcinomas, undifferentiated carcinomas, and lymphoepitheliomas. Tumor initiation has been linked to the Epstein-Barr virus and, in some geographical regions, to alimentary factors. Possible hereditary components for the appearance of NPCs have not yet been clearly identified. In this study, genetic sensitivity to the genotoxic effects of carcinogenic xenobiotics as an endogenous risk factor of tumor initiation was investigated. The single cell microgel electrophoresis assay was used to quantify chemically-induced DNA damage in lymphocytes of 30 NPC patients and 30 non-tumor donors. The xenobiotics investigated were N'-nitrosodiethylamine, sodium dichromate, and nickel sulphate, with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and dimethyl sulfoxide (DMSO) as positive and negative controls, respectively. The extent of DNA migration in the solvent control cultures was not significantly different between the two groups (1.2+/-0.5 mean Olive tail moment and standard deviation of 30 individuals for NPC patients; 1.1+/-0.4 for non-tumor donors). With constant exposure and electrophoretic conditions, genotoxic effects of varying degrees were induced by the different xenobiotics in tumor and non-tumor patients (nickel sulphate: 7.1+/-2.5 for NPC patients and 5.9+/-1.6 for non-tumor donors; sodium dichromate: 18.1+/-5.3 for NPC patients and 16.2+/-5.4 for non-tumor donors; MNNG: 47.8+/-13.3 for NPC patients and 52.7+/-13.6 for non-tumor donors). Only N'-nitrosodiethylamine proved to induce significantly more DNA migration in lymphocytes of tumor patients (9.8+/-3.1) as compared to non-tumor patients (8.2+/-2.3). Although for sodium dichromate the degree of DNA migration did not significantly differ, variability in migration patterns proved to be lower in the tumor group. Mutagen sensitivity of NPC patients was shown to be elevated for a selected xenobiotic, whereas a general elevation of DNA fragility was not present. Further studies on mutagen sensitivity as an endogenous risk factor influencing the susceptibility of patients at the time of first diagnosis of nasopharyngeal carcinomas are warranted.

Variant metabolizing gene alleles determine the genotoxicity of benzo[a]pyrene

Understanding the mechanisms involved with genetic susceptibility to environmental disease is of major interest to the scientific community. We have conducted an in vitro study to elucidate the involvement of polymorphic metabolizing genes on the genotoxicity of benzo[a]pyrene (BP). Blood samples from 38 donors were treated with BP and the induction of sister chromatid exchanges (SCE) and chromosome aberrations (CA) were evaluated. The latter is based on the tandem-probe fluorescence in situ hybridization (FISH) assay. The data indicate that the induction of genotoxicity was clearly determined by the inherited variant genotypes for glutathione-S-transferase (GSTM1) and microsomal epoxide hydrolase (EH). In a comparison of the two biomarkers, the CA biomarker shows a more definite association with the genotypes than does SCE. For example, the presence of the GSTM1 null genotype (GSTM1 0/0) is responsible for the highest level and significant induction of CA, irrespective of the presence of other genotypes in the different donors. This effect is further enhanced significantly by the presence of the excessive activation EH gene allele (EH4*) and decreased by the reduced activation EH gene allele (EH3*). Overall, the modulation of genotoxicity by the susceptibility genotypes provides support of their potential involvement in environmental cancer. Furthermore, the data indicate that the variant enzymes function independently by contributing their metabolic capability toward the expression of biologic activities. Therefore, studies like this one can be used to resolve the complexity of genetic susceptibility to environmental disease in human.

The 399Gln polymorphism in the DNA repair gene XRCC1 modulates the genotoxic response induced in human lymphocytes by the tobacco-specific nitrosamine NNK

DNA repair plays a critical role in protecting the genome of the cell from the insults of cancer-causing agents such as those found in tobacco smoke. Reduced DNA repair capacity would, therefore, constitute a significant risk factor for smoking-related cancers. Recently, a number of polymorphisms in several DNA repair genes have been discovered, and it is possible that these polymorphisms may affect DNA repair capacity and thus modulate cancer susceptibility in exposed populations. In the current study, we explored the relationship between two polymorphisms in the DNA repair gene XRCC1 (polymorphisms in codons 194 and 399) and the genotoxic response induced by the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The sister chromatid exchange (SCE) assay was used as a marker of genetic damage. Our results, using whole blood cultures from 47 volunteers, indicated that treatment of cells with 0.24, 0.72 and 1.44 mM of NNK induced a concentration-dependent increase in the mean number of SCE (P<0.001). There was a significant difference (P<0.05) in response to NNK treatment between cells from individuals with the 399Gln allele (either homozygous or heterozygous) and cells from individuals with the homozygous 399 Arg/Arg genotype. Treatment of cells that have the 399Gln allele with 0.24, 0.72 and 1.44 mM NNK resulted in 22.8, 35.8 and 52.8% increases in NNK-induced SCE, respectively. Treatment of cells with the 399 Arg/Arg genotype using the same NNK concentrations resulted in 16.0, 15.5 and 32.6% increases in NNK-induced SCE, respectively. In contrast, no significant difference in NNK-induced SCE was observed between cells with the codon 194 Arg/Arg genotype and cells with the codon 194 Arg/Trp genotype at all concentrations of NNK tested. These data suggest that the Arg399Gln amino acid change may alter the phenotype of the XRCC1 protein, resulting in deficient DNA repair. Our study underscores the important role of polymorphisms in DNA repair genes in influencing the genotoxic responses to environmental mutagens, and justifies additional studies to investigate their potential role in susceptibility to cancer.