Genotyping of human cytochrome P450 2A6 (CYP2A6), a nicotine C-oxidase

Mining of molecular insights of CYP2A6 and its variants complex with coumarin (CYP2A6*-coumarin) using molecular dynamics simulation

CYP2A6 is a very important enzyme that plays a crucial role in nicotine compounds and is responsible for the metabolism of more than 3% drugs of total metabolized drugs by the CYP family and reported as one of very important pharmacogenes. CYP2A6 is highly polymorphic in nature and reported with more than 40 variants, most of these variants are SNPs originated and population specific. It has been well observed and reported that the presence of these population-specific non-synonymous SNPs in CYP2A6 alters the rate of drug metabolism and as a functional consequence, drugs produce an abnormal response. Though genomics and pharmacogenomics studies are there, very less is known about the structural effects of these SNPs on molecular-interaction and folding of CYP2A6. To fill the knowledge gap, SNPs based four variants, i.e., CYP2A6*2, CYP2A6*18, CYP2A6*21, and CYP2A6*35, which are frequently reported in the South Asian population, were considered for the study. Coumarin (DB04665), a well reported drug, is considered as a model substance, and the effect of all four variants on 'CYP2A6*-coumarin' complex was studied. MD simulation-based analysis (at 200 ns) was performed and comparative analysis with respect to wild type 'CYP2A6-coumarin' complex was done. Though observation didn't find any global effect on complete complex but found some crucial minor-local alteration in interaction and folding process. It is assumed that the change due to SNPs in the single amino acid did not bring global change in physiochemical properties of CYP2A6* but caused local-trivial changes which are very crucial for its metabolic activity.Communicated by Ramaswamy H. Sarma.

Coumarin-Induced Hepatotoxicity: A Narrative Review

Coumarin is an effective treatment for primary lymphoedema, as well as lymphoedema related to breast cancer radiotherapy or surgery. However, its clinical use is limited in several countries due to the possible occurrence of hepatotoxicity, mainly in the form of mild to moderate transaminase elevation. It is worth noting that only a few cases of severe hepatotoxicity have been described in the literature, with no reported cases of liver failure. Data available on coumarin absorption, distribution, metabolism, and excretion have been reviewed, focusing on hepatotoxicity studies carried out in vitro and in vivo. Finally, safety and tolerability data from clinical trials have been thoroughly discussed. Based on these data, coumarin-induced hepatotoxicity is restricted to a small subset of patients, probably due to the activation in these individuals of alternative metabolic pathways involving specific CYP450s isoforms. The aim of this work is to stimulate research to clearly identify patients at risk of developing hepatotoxicity following coumarin treatment. Early identification of this subset of patients could open the possibility of more safely exploiting the therapeutical properties of coumarin, allowing patients suffering from lymphoedema to benefit from the anti-oedematous activity of the treatment.

How Can Drug Metabolism and Transporter Genetics Inform Psychotropic Prescribing?

Many genetic variants in drug metabolizing enzymes and transporters have been shown to be relevant for treating psychiatric disorders. Associations are strong enough to feature on drug labels and for prescribing guidelines based on such data. A range of commercial tests are available; however, there is variability in included genetic variants, methodology, and interpretation. We herein provide relevant background for understanding clinical associations with specific variants, other factors that are relevant to consider when interpreting such data (such as age, gender, drug-drug interactions), and summarize the data relevant to clinical utility of pharmacogenetic testing in psychiatry and the available prescribing guidelines. We also highlight areas for future research focus in this field.

Cancer Hazard Identification Integrating Human Variability: The Case of Coumarin

Coumarin is a naturally occurring sweet-smelling benzopyrone that may be extracted from plants or synthesized for commercial uses. Its uses include as a flavoring agent, fragrance enhancer, and odor-masking additive. We reviewed and evaluated the scientific evidence on the carcinogenicity of coumarin, integrating information from carcinogenicity studies in animals with mechanistic and other relevant data, including data from toxicogenomic, genotoxicity, and metabolism studies, and studies of human variability of a key enzyme, CYP2A6. Increases in tumors were observed in multiple studies in rats and mice in multiple tissues. Our functional pathway analysis identified several common cancer-related biological processes/pathways affected by coumarin in rat liver following in vivo exposure and in human primary hepatocytes exposed in vitro. When coumarin 7-hydroxylation by CYP2A6 is compromised, this can lead to a shift in metabolism to the 3,4-epoxidation pathway and increased generation of electrophilic metabolites. Mechanistic data align with 3 key characteristics of carcinogens, namely formation of electrophilic metabolites, genotoxicity, and induction of oxidative stress. Considerations of metabolism, human variability in CYP2A6 activity, and coumarin hepatotoxicity in susceptible individuals provide additional support for carcinogenicity concern. Our analysis illustrates the importance of integrating information on human variability in the cancer hazard identification process.

The pharmacogenetics of natural products: A pharmacokinetic and pharmacodynamic perspective

Natural products have represented attractive alternatives for disease prevention and treatment over the course of human history and have contributed to the development of modern drugs. These natural products possess beneficial efficacies as well as adverse efffects, which vary largely among individuals because of genetic variations in their pharmacokinetics and pharmacodynamics. As with other synthetic chemical drugs, the dosing of natural products can be optimized to improve efficacy and reduce toxicity according to the pharmacogenetic properties. With the emergence and development of pharmacogenomics, it is possible to discover and identify the targets/mechanisms of pharmacological effects and therapeutic responses of natural products effectively and efficiently on the whole genome level. This review covers the effects of genetic variations in drug metabolizing enzymes, drug transporters, and direct and indirect interactions with the pharmacological targets/pathways on the individual response to natural products, and provides suggestions on dosing regimen adjustments of natural products based on their pharmacokinetic and pharmacogenetic paratmeters. Finally, we provide our viewpoints on the importance and necessity of pharmacogenetic and pharmacogenomic research of natural products in natural medicine's rational development and clinical application of precision medicine.

Association of genetic polymorphisms CYP2A6*2 rs1801272 and CYP2A6*9 rs28399433 with tobacco-induced lung Cancer: case-control study in an Egyptian population

Background

Several studies have reported the role of CYP2A6 genetic polymorphisms in smoking and lung cancer risk with some contradictory results in different populations. The purpose of the current study is to assess the contribution of the CYP2A6*2 rs1801272 and CYP2A6*9 rs28399433 gene polymorphisms and tobacco smoking in the risk of lung cancer in an Egyptian population.

Methods

A case-control study was conducted on 150 lung cancer cases and 150 controls. All subjects were subjected to blood sampling for Extraction of genomic DNA and Genotyping of the CYP2A6 gene SNPs (CYP2A6*2 (1799 T > A) rs1801272 and CYP2A6*9 (− 48 T > G) rs28399433 by Real time PCR.

Results

AC and CC genotypes were detected in CYP2A6*9; and AT genotype in CYP2A6*2. The frequency of CYP2A6*2 and CYP2A6*9 were 0.7% and 3.7% respectively in the studied Egyptian population. All cancer cases with slow metabolizer variants were NSCLC. Non-smokers represented 71.4% of the CYP2A6 variants. There was no statistical significant association between risk of lung cancer, smoking habits, heaviness of smoking and the different polymorphisms of CYP2A6 genotypes.

Conclusion

The frequency of slow metabolizers CYP2A6*2 and CYP2A6*9 are poor in the studied Egyptian population. Our findings did not suggest any association between CYP2A6 genotypes and risk of lung cancer.

Variation in CYP2A6 Activity and Personalized Medicine

The cytochrome P450 2A6 (CYP2A6) enzyme metabolizes several clinically relevant substrates, including nicotine-the primary psychoactive component in cigarette smoke. The gene that encodes the CYP2A6 enzyme is highly polymorphic, resulting in extensive interindividual variation in CYP2A6 enzyme activity and the rate of metabolism of nicotine and other CYP2A6 substrates including cotinine, tegafur, letrozole, efavirenz, valproic acid, pilocarpine, artemisinin, artesunate, SM-12502, caffeine, and tyrosol. CYP2A6 expression and activity are also impacted by non-genetic factors, including induction or inhibition by pharmacological, endogenous, and dietary substances, as well as age-related changes, or interactions with other hepatic enzymes, co-enzymes, and co-factors. As variation in CYP2A6 activity is associated with smoking behavior, smoking cessation, tobacco-related lung cancer risk, and with altered metabolism and resulting clinical responses for several therapeutics, CYP2A6 expression and enzyme activity is an important clinical consideration. This review will discuss sources of variation in CYP2A6 enzyme activity, with a focus on the impact of CYP2A6 genetic variation on metabolism of the CYP2A6 substrates.

Distribution of polymorphic variants of CYP2A6 and their involvement in nicotine addiction

Tobacco consumption has become a major public health issue, which has motivated studies to identify and understand the biological processes involved in the smoking behavior for prevention and smoking cessation treatments. CYP2A6 has been identified as the main gene that codifies the enzyme that metabolizes nicotine. Many alleles have been identified after the discovery of CYP2A6, suggesting a wide interethnic variability and a diverse smoking behavior of the allele carrying individuals. The main purpose of this review is to update and highlight the effects of the CYP2A6 gene variability related to tobacco consumption reported from diverse human populations. The review further aims to consider CYP2A6 in future studies as a possible genetic marker for the prevention and treatment of nicotine addiction. Therefore, we analyzed several population studies and their importance at addressing and characterizing a population using specific parameters. Our efforts may contribute to a personalized system for detecting, preventing and treating populations at a higher risk of smoking to avoid diseases related to tobacco consumption.

CYP2A6 genotyping methods and strategies using real-time and end point PCR platforms

CYP2A6 genotyping is of clinical importance--CYP2A6 gene variants influence nicotine metabolism and are associated with nicotine dependence, cigarettes per day, smoking cessation and the risk for tobacco-associated cancers. CYP2A6 gene variants also influence the metabolism of therapeutic drugs, such as the anticancer agents, tegafur and letrozole. Over the years, CYP2A6 genotyping methods have evolved to incorporate novel gene variants and to circumvent genotyping errors resulting from the high degree of homology between CYP2A6 and neighboring CYP2A genes. Herein, CYP2A6 genotyping strategies are described for commonly genotyped functionally significant alleles including SNPs, small insertions/deletions and more complex structural variants. The methods presented utilize higher throughput SYBR green real-time PCR technology in addition to standard thermocycling.

Inhibitory effects of cytochrome P450 enzymes CYP1A2, CYP2A6, CYP2E1 and CYP3A4 by extracts and alkaloids of Gelsemium elegans roots

ETHNOPHARMACOLOGICAL RELEVANCE

Gelsemium elegans (GE), widely distributed in East Asia, South East Asia and Northern America, is a kind of well-known toxic plant throughout the world. Yet it has been used as a Chinese folk medicine for treatment of malignant tumors, pain, rheumatic arthritis, psoriasis and immune function.

AIM OF THE STUDY

The present study was to investigate the potential inhibitory effects of G. elegans (GE) roots on four major cytochrome P450 (CYP450) isoforms (CYP1A2, CYP2A6, CYP2E1 and CYP3A4) in vitro.

MATERIALS AND METHODS

Four extracts (petroleum ether, dichloromethane, EtOAc and aqueous) of GE and two commercially available alkaloids (koumine and humantenmine) were screened for their CYP isoforms inhibitory activity. Four enzyme inhibition assays were examined according to the method of the literature. Phenacetin, coumarin, chlorzoxazone and testosterone were used as probe substrates in order to determine CYP1A2, CYP2A6, CYP2E1 and CYP3A4 catalytic activity, respectively. Each probe substrate was incubated with or without each extract and active constituent for corresponding isoform, followed by determination of the kinetics parameters, IC50 and Ki, to characterize inhibitory effects.

RESULTS

GE dichloromethane extract selectively inhibited activities of CYP2E1 (IC50=29.04µg/ml) and CYP2A6 (IC50=46.84µg/ml), with Ki of 10.16 and 19.33µg/ml, respectively. In the case of alkaloids, koumine exhibited significant inhibitory effects on CYP2E1 while humantenmine showed more potent inhibition on CYP2E1 and CYP2A6 (IC50 of 47.44, 18.34 and 45.87µg/ml, Ki of 31.20, 35.06 and 52.06µg/ml, respectively). Because of their relatively high Ki values, the active constituents in GE dichloromethane extract were analyzed. The UPLC-DAD-ESI-MS/MS data showed that GE dichloromethane extract contains 6 kinds of indole alkaloids (koumine, humantenmine, humantenine, humantenirine, N-methoxytaberpsychine, and sempervirine). As for CYP1A2 and CYP3A4, the negligible inhibitions were observed.

CONCLUSION

G. elegans extracts inhibited several CYP450 enzyme activities with varying potency. Strong inhibition was observed in CYP2E1 and CYP2A6 isoforms by GE dichloromethane extract, koumine and humantenmine, inferring the involvement of alkaloids chemical constituents from GE dichloromethane extract in the effect.

Cytochrome p450 metabolism of betel quid-derived compounds: implications for the development of prevention strategies for oral and pharyngeal cancers

Betel quid (BQ) products, with or without tobacco, have been classified by the International Agency for Research on Cancer (IARC) as group I human carcinogens that are associated with an elevated risk of oral potentially malignant disorders (OPMDs) and cancers of the oral cavity and pharynx. There are estimated 600 million BQ users worldwide. In Taiwan alone there are 2 million habitual users (approximately 10% of the population). Oral and pharyngeal cancers result from interactions between genes and environmental factors (BQ exposure). Cytochrome p450 (CYP) families are implicated in the metabolic activation of BQ- and areca nut-specific nitrosamines. In this review, we summarize the current knowledge base regarding CYP genetic variants and related oral disorders. In clinical applications, we focus on cancers of the oral cavity and pharynx and OPMDs associated with CYP gene polymorphisms, including CYP1A1, CYP2A6, CYP2E1, and CYP26B1. Our discussion of CYP polymorphisms provides insight into the importance of screening tests in OPMDs patients for the prevention of oral and pharyngeal cancers. Future studies will establish a strong foundation for the development of chemoprevention strategies, polymorphism-based clinical diagnostic tools (e.g., specific single-nucleotide polymorphism (SNP) "barcodes"), and effective treatments for BQ-related oral disorders.

Advances in molecular modeling of human cytochrome P450 polymorphism

Cytochrome P450 (CYP) is a supergene family of metabolizing enzymes involved in the phase I metabolism of drugs and endogenous compounds. CYP oxidation often leads to inactive drug metabolites or to highly toxic or carcinogenic metabolites involved in adverse drug reactions (ADR). During the last decade, the impact of CYP polymorphism in various drug responses and ADR has been demonstrated. Of the drugs involved in ADR, 56% are metabolized by polymorphic phase I metabolizing enzymes, 86% among them being CYP. Here, we review the major CYP polymorphic forms, their impact for drug response and current advances in molecular modeling of CYP polymorphism. We focus on recent studies exploring CYP polymorphism performed by the use of sequence-based and/or protein-structure-based computational approaches. The importance of understanding the molecular mechanisms related to CYP polymorphism and drug response at the atomic level is outlined.

Association between CYP2A6 genetic polymorphisms and lung cancer: a meta-analysis of case-control studies

Cytochrome P450 2A6 (CYP2A6) is an enzyme responsible for the metabolism of nicotine and some tobacco-specific carcinogens (such as N-nitrosamines). CYP2A6 genetic variations are associated with the activity of the CYP2A6 enzyme, which affects smoking behavior and the rate at which some tobacco-specific carcinogens are metabolized, which in turn determines the incidence of lung cancer. Several studies have investigated the relationship between CYP2A6 genotypes and lung cancer; however, the results are controversial. In this meta-analysis, we searched for all studies on the association between CYP2A6 genotypes and lung cancer indexed in the MEDLINE, PubMed, Embase, China Biological Medicine, and Wanfang databases from January 1, 1966 to August 1, 2011. The pooled odds ratios (ORs) for one CYP2A6 mutant allele and two CYP2A6 mutant alleles, in comparison with the wild-type CYP2A6 gene, were 0.82 [95% confidence interval (CI) = 0.73-0.92] and 0.57 (95% CI = 0.48-0.68), respectively. Furthermore, in two studies of participants who were all smokers, the associations of one CYP2A6 mutant allele and two CYP2A6 mutant alleles with reduced risk of lung cancer were strengthened, and the pooled ORs were 0.71 (95% CI = 0.58-0.87) and 0.47 (95% CI = 0.35-0.62), respectively. However, we did not find statistically significant relationships between CYP2A6 genotypes and lung cancer in studies that included both never smokers and smokers (pooled OR(one CYP2A6 mutant allele) = 0.88, 95% CI = 0.76-1.01; pooled OR(two CYP2A6 mutant alleles) = 0.61, 95% CI = 0.35-1.06). The results of this meta-analysis suggest that the reduced-activity CYP2A6 genotype may decrease the risk of lung cancer in smokers only.

Institutional Profile: Karolinska Institutet

Research in pharmacogenomics has been intensive at Karolinska Institutet (KI) for approximately 25 years. Initial initiatives were focused on the identification and characterization of novel CYP2D6 alleles causing ultrarapid or defective drug metabolism. Such discoveries were possible owing to the early implementation of therapeutic drug monitoring and the access to individuals phenotyped with respect to drug metabolism. The translational work at KI has been of utmost importance for successful research, including functional characterization and clinical validation of allelic variants in drug metabolism, as well as discoveries of novel polymorphisms, recent examples being the CYP2C19 and UGT2B17 genes. The clinical pharmacology laboratory at KI campus Huddinge is one of the leading sites for therapeutic drug monitoring in northern Europe and obtains an increasing number of clinical requests, also important for pharmacogenetic research. Furthermore, the recently opened Center for Hematology and Regenerative Medicine, with a clear translational emphasis, offers an opportunity for studying drug metabolism and toxicity in vitro by use of human hepatocytes.

Polymorphic metabolism by functional alterations of human cytochrome P450 enzymes

The study of cytochrome P450 pharmacogenomics is of particular interest because of its promise in the development of rational means to optimize drug therapy with respect to patient's genotype to ensure maximum efficacy with minimal adverse effects. Drug metabolizing P450 enzymes are polymorphic and are the main phase I enzymes responsible for the metabolism of clinical drugs. Therefore, polymorphisms in the P450s have the most impact on the fate of clinical drugs in phase I metabolism since almost 80% of drugs in use today are metabolized by these enzymes. Predictive genotyping for P450 enzymes for a more effective therapy will be routine for specific drugs in the future. In this review, we discuss the current knowledge of polymorphic metabolism by functional alterations in nonsynonymous SNPs of P450 1A2, 2A6, 2C8, 2C9, 2C19, 2D6, and 3A4 enzymes.

Gene, ethnic and gender influences predisposition of adverse drug reactions to artesunate among Malaysians

CONTEXT

Artesunate (AS) and amodiaquine (AQ) are two prodrugs widely used as antimalarial agents and are metabolized by the CYP P450 2A6 (CYP 2A6) and CYP P450 2C8 (CYP 2C8) enzymes, respectively.

OBJECTIVE

In this study, we aim to investigate the association of both genes on AS and AQ's tolerabilities in the hope of identifying a pharmacogenetic approach that could be useful in prediction and prevention of adverse drug reactions (ADRs) among Malaysian population.

MATERIALS AND METHODS

In this randomized crossover study, loose and AS/AQ formulations were administered to normal healthy volunteers (n = 24) over two study phases. The drugs' tolerabilities (incidence of facial flushing, giddiness, headache, nausea, abdominal discomfort, progression of liver enzymes and neutrophil counts) were compared between the two treatment arms. Volunteers were also genotyped for the CYP2C8 and CYP2A6 variants.

RESULTS

The frequency of the CYP2A6*1B, CYP2A6*4, CYP2A6*8 and CYP2A6*9 alleles were 54.2%, 16.7%, 4.2% and 10.4%, respectively. No mutations for CYP2C8 gene were, however, detected. Most (96%) of the subjects were of the Malay ethnicity. Subjects having the CYP2A6*1B variants responsible for ultra rapid metabolism of AS suffered a significantly higher incidence of ADRs.

DISCUSSION

Our study is the first to report that CYP2A6 genotyping influences AS's ADR. Gender also plays a role where females reported more incidences of nausea (p < 0.05).

CONCLUSION

It is concluded that genetic polymorphisms of CYP2A6 as well as gender influence the side effect profiles of subjects receiving AS among this Malaysian population.

Associations of CYP2A6 genotype with smoking behaviors in southern China

AIMS

To investigate the association of CYP2A6 genetic polymorphisms with smoking-related phenotypes in Chinese smokers.

DESIGN

Case-only genetic association study.

SETTING

Southern China.

PARTICIPANTS

A total of 1328 Han Chinese smokers who participated in a community-based chronic disease screening project in Guangzhou and Zhuhai from 2006 to 2007.

MEASUREMENTS

All participants answered a structured questionnaire about socio-demographic status and smoking behaviors and informative alleles were genotyped for the cytochrome P450 2A6 (CYP2A6) gene (CYP2A6*4,*5,*7,*9 and *10).

FINDINGS

The frequencies of CYP2A6*4, *5, *7, *9 and *10 alleles were 8.5, 1.2, 6.3, 13.5 and 2.4%, which corresponded to 48.9, 15.4, 24.2 and 11.5% of participants being classified as normal, intermediate, slow and poor metabolizers, respectively. Multivariate analyses in male smokers demonstrated that compared with normal metabolizers, poor metabolizers reported smoking fewer cigarettes per day [adjusted odds ratio (OR) = 0.49; 95% confidence interval (CI): 0.32-0.76], started smoking regularly later in life (adjusted OR = 1.55; 95% CI: 1.06-2.26) and, among former smokers, reported smoking for a shorter duration prior to quitting (adjusted OR = 0.33; 95% CI: 0.12-0.94). However, poor metabolizers were less likely to quit smoking and remain abstinent than normal metabolizers (adjusted OR = 0.54; 95% CI: 0.34-0.86).

CONCLUSIONS

Reduced metabolism function of cytochrome P450 2A6 in smokers appears to be associated with fewer cigarettes smoked, later initiation of smoking regularly, shorter smoking duration and lower likelihood of smoking cessation.

Genetic polymorphisms of drug-metabolizing phase I enzymes CYP2E1, CYP2A6 and CYP3A5 in South Indian population

CYP2E1, CYP2A6 and CYP3A5 enzymes belong to phase I group of drug-metabolizing enzymes, which are involved in the metabolism of various compounds and xenobiotics. Presence of polymorphisms in the genes coding for these enzymes results in interindividual variations in drug metabolism, therapeutic response and susceptibility towards various diseases. The frequencies of these variants in genes differ considerably between ethnic groups. This study was carried out to estimate the allele and genotype frequencies of common variants in CYP2E1, CYP2A6 and CYP3A5 in South Indian population. Six hundred and fifty-two unrelated healthy volunteers of South Indian origin (Andhra Pradesh, Karnataka, Kerala and Tamil Nadu) were included in this study. Polymerase chain reaction-restriction fragment length polymorphism, allele-specific PCR, real-time PCR, SNaPshot and gene sequencing methods were used for the identification of gene polymorphisms. The frequencies of CYP2E1*1B, CYP2E1*5B and CYP2E1*6 alleles in South Indian population were 14.3, 1.3 and 22.4%, respectively. The frequencies of CYP2A6*2, CYP2A6*4A and CYP2A6*5 alleles were found to be 1, 8.9 and 0.7%, respectively. The distribution of CYP3A5*3 allele was 63.5%. There were no variant alleles of CYP3A5*2, CYP3A5*4 and CYP3A5*6 in South Indian population. The frequencies of CYP2E1, CYP2A6 and CYP3A5 in the South Indian population are distinct from Caucasians, Chinese, Japanese, African Americans and other compared populations. This is the first study conducted in the South Indian population with a larger sample size. The findings of our study provide the basic genetic information for further pharmacogenomic investigations in the population.

Association between nicotine metabolism and CYP2A6*1 and CYP2A6*4 genotypes in an Iranian population

The cytochrome P450 (CYP) family is the principal enzyme system involved in the metabolism of xenobiotics and endogenous compounds. Among this family, CYP2A6 is one of the most important enzymes for metabolism of nicotine. In this study, the linkage of CYP2A6*1 and CYP2A6*4 genotypes with nicotine metabolism was investigated. A single polymerase chain reaction-restriction fragment length polymorphism was used to resolve the genotypes into CYP2A6*1 (wild type), CYP2A6*2, or CYP2A6*3. The population studied consisted of 200 healthy smokers from Mashhad city, North East of Iran. The urinary cotinine as the principal metabolite of nicotine was analyzed for 12 subjects (7 subjects with CYP2A6*1 as controls and 5 subjects with CYP2A6*4). The results indicated that cumulative urinary cotinine excretion in CYP2A6*4 genotype was about one-eighth compared with the control group (wild type). Cotinine formation from nicotine has individual and ethnic variability that correlated with the level of CYP2A6 expression. Moreover, urinary cotinine level was drastically lower in CYP2A6*4 subjects than in CYP2A6*1 subjects.

The influence of CYP2A6 polymorphisms and cadmium on nicotine metabolism in Thai population

We investigated the influence of genetic, cadmium exposure and smoking status, on cytochrome P450-mediated nicotine metabolism (CYP2A6) in 182 Thai subjects after receiving 2mg of nicotine gum chewing for 30min. The urinary excretion of cotinine was normally distributed over a 2h period (logarithmically transformed). Individuals with urinary cotinine levels in the ranges of 0.01-0.21, and 0.52-94.99μg/2h were categorized as poor metabolizes (PMs: 6.5%), and extensive metabolizers (EMs: 93.5%), respectively. The majority of EMs (45%) carried homozygous wild-type genotypes (CYP2A6*1A/*1A, CYP2A6*1A/*1B and CYP2A6*1B/*1B), whereas only 1% of PMs carried these genotypes. Markedly higher frequencies of EMs were also observed in all heterozygous defective genotypes including the null genotype (*4C/*4C; 1 subject). A weak but significant positive correlation was observed between total amounts of urinary cadmium excretion and total cotinine excretion over 2h. Our study shows generally good agreement between CYP2A6 genotypes and phenotypes. Smokers accumulated about 3-4-fold higher mean total amounts of 2-h urinary cadmium excretion (127.5±218.2ng/2h) than that of non-smokers (40.5±78.4ng/2h). Among the smokers (n=16), homologous wild-type genotype *1/*1 was significantly the predominant genotype (6/16) compared with other defective allele including *4C/*4C. In addition, 2h urinary excretion of cotinine in smokers of all genotypes was significantly higher than non-smokers. The proportion of smokers who smoked more than 5 cigarettes/day was significantly higher in EMs in all CYP2A6 genotypes (n=14) than in PMs (n=0).

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Genetic polymorphism in cytochrome P450 2D6 (CYP2D6): Population distribution of CYP2D6 activity

Cytochrome P-450 2D6 (CYP2D6) is involved in the metabolism of many therapeutic drugs even though the enzyme represents a small proportion of the total CYP content of human liver. In vivo phenotyping with probe drug substrates such as debrisoquine and dextromethorphan showed a clear separation between poor metabolizers (PM) and extensive metabolizers (EM). This polymorphism may affect susceptibility to environmental disease, as suggested by molecular epidemiologic studies that found an association between CYP2D6 metabolizer phenotype and cancer risk; however, this association is not consistent. There are only a few examples of CYP2D6 involvement in toxicant mechanism of action, but this has not been extensively studied. Gene probe studies documented a number of genetic polymorphisms that underlie CYP2D6 metabolizer phenotypes. The EM group carries the wild-type (*1) or active (*2) variant alleles, while the PM group carries the *3, *4, *5, or *6 alleles, all of which code for a protein that has lower or null CYP2D6 activity. The current analysis characterizes (a) influence of genotype on phenotype based upon in vivo metabolism studies of probe drugs and (b) frequency of the major genotypes in different population groups is also characterized. These data were then incorporated into Monte Carlo modeling to simulate population distributions of CYP2D6 activity. This analysis reproduced the bimodal distributions commonly seen in phenotyping studies of Caucasians and found extensive population variability in enzyme activity, as indicated by the 9- to 56-fold difference between the PM modal median and the total population median CYP2D6 activity. This substantial degree of interindividual variability in CYP function indicates that assessments involving CYP2D6 substrates need to consider the full distribution of enzyme activity in refining estimates of internal dose in health assessments of xenobiotics.

CYP2A6 polymorphisms and risk for tobacco-related cancers

Tobacco consumption is the main identifiable risk to cancer, contributing to the majority of tumors in upper aerodigestive tissues. The psychoactive compound responsible for tobacco addiction, nicotine and the potent carcinogens present at high concentrations either in cigarette mainstream smoke or in smokeless tobacco products, 4-(methylnitrosamino)-1-(3-pyridyl)-butanone (NNK) and N-nitrosonornicotine (NNN) can be metabolized by CYP2A6. CYP2A6 is expressed in many aerodigestive tissues with high interindividual variability. The CYP2A6 gene is highly polymorphic and CYP2A6 alleles coding for enzymes with altered expression or metabolic capacity produce alterations in nicotine metabolism in vivo and seem to influence smoking behavior. These polymorphisms may change the rate of NNK and NNN activation and, therefore, may influence cancer risk associated with tobacco consumption. However, to date only a few and inconclusive studies have addressed the risk that a given CYP2A6 polymorphism presents for the development of tobacco-related tumors. Most, but not all, show a reduced risk associated with alleles that result in decreased enzyme activity. The overlapping substrate specificity and tissue expression between CYP2A6 and the highly similar CYP2A13 may add to the conflicting results observed. The intricate regulation of CYP2A6 and the variation of structurally different chemical compounds capable of inhibiting CYP2A enzymes also add to the complexity. Finally, the interaction between polymorphisms of genes that code for CYP2A6, CYP2A13 and other potent carcinogen-metabolizing CYP enzymes may help to determine individuals that are at higher risk of developing tumors associated with tobacco consumption.

CYP2A6 polymorphisms in Malays, Chinese and Indians

The genetically polymorphic cytochrome P450 (CYP) 2A6 is the major nicotine-oxidase in humans that may contribute to nicotine dependence and cancer susceptibility. The authors investigated the types and frequencies of CYP2A6 alleles in the three major ethnic groups in Malaysia and CYP2A6*1A, CYP2A6*1B, CYP2A6*1x2, CYP2A6*2, CYP2A6*3, CYP2A6*4, CYP2A6*5, CYP2A6*7, CYP2A6*8 and CYP2A6*10 were determined by allele-specific polymerase chain reaction (PCR) in 270 Malays, 172 Chinese and 174 Indians. Except for CYP2A6*2 and *3 that were not detected in the Malays and Chinese, all the other alleles were detected. Frequencies for the CYP2A6*4 allele were 7, 5 and 2%, respectively, in Malays, Chinese and Indians. A statistically significant high frequency of the duplicated CYP2A6*1x2 allele occurred among Chinese. Among Malays and Chinese, the most common allele was CYP2A6*1B, but it was CYP2A6*1A among Indians. These ethnic difference in frequencies suggested that further studies are required to investigate the implications on diseases such as cancer and smoking behaviour among these major ethnic groups in Malaysia.

The CYP2A6*4 allele is determinant of S-1 pharmacokinetics in Japanese patients with non-small-cell lung cancer

S-1 is an oral fluorouracil anticancer drug that contains the 5-FU prodrug tegafur. Tegafur has been shown to be converted enzymatically to 5-FU to exert its antitumor effect, and this conversion is principally catalyzed by CYP2A6. Forty-six non-small-cell lung cancer patients were enrolled. The frequencies of the CYP2A6*4C, CYP2A6*7, and CYP2A6*9 alleles were 17.4, 19.6, and 15.2%, respectively. In the S-1 pharmacokinetic analysis, the area under the concentration-time curve from 0 to 10 h (AUC(0-10)) ratios of 5-FU/tegafur showed large interindividual variabilities, ranging from 5.14 to 112.6. The AUC(0-10) for tegafur was 1.5-fold higher in patients with the CYP2A6*4C allele than in patients without the CYP2A6*4C allele P < 0.05). Furthermore, patients with the CYP2A6*4C allele had a significantly lower maximum plasma concentration (102.6 +/- 32.9 ng/ml) for 5-FU than patients without the CYP2A6*4C allele (157.0 +/- 65.5 ng/ml, P < 0.05). Genotyping of CYP2A6 polymorphisms may provide vital information for effective cancer therapy using S-1.

Artemisinin and CYP2A6 activity in healthy subjects

OBJECTIVE

To investigate whether the antimalarial drug artemisinin affects CYP2A6 activity in healthy subjects and to compare the utility of coumarin and nicotine as in vivo probe compounds for CYP2A6.

METHODS

Twelve healthy male Vietnamese subjects were given coumarin or nicotine in randomized sequence before and after 5 days of a repeated oral administration of artemisinin during two different treatment periods 1 month apart. Sequential blood samples were drawn at baseline 7 days prior to artemisinin treatment and on the first and fifth day of artemisinin treatment during both treatment periods. Plasma concentrations of 7-hydroxycoumarin glucuronide (7-OHCG), nicotine, cotinine and artemisinin were analysed by high-performance liquid chromatography and those of coumarin and 7-hydroxycoumarin (7-OHC) were determined by liquid chromatography-tandem mass spectrometry. Urine, collected in two time intervals on the days of coumarin intake, was treated with beta-glucuronidase and analysed for 7-OHC levels.

RESULTS

Artemisinin AUC(0-infinity) values decreased significantly to 23% [95% confidence interval (CI) 18%-28%] on the fifth day of artemisinin administration as compared with the first. The sum of renally excreted 7-OHC and 7-OHCG increased by 1.55-fold (adjusted 95% CI 1.08-2.23) in the 3- to 8-h interval compared to baseline 7 days before. The 7-OHCG/7-OHC plasma AUC(0-infinity) ratio increased by 1.72-fold (adjusted 95% CI 1.16-2.54) following 5 days of artemisinin intake. There was no significant change in the cotinine/nicotine AUC(0-11 hr) ratio between study days.

CONCLUSION

Artemisinin significantly increased the sum of renally excreted 7-OHC and 7-OHCG in one of the two collection intervals, suggesting an induction of CYP2A6. A significant increase in the 7-OHCG to 7-OHC AUC(0-infinity) ratio indicates artemisinin to be an inducer of glucuronidation.

Racially classified social group tobacco-related health disparities: what is the role of genetics?

Certain racially classified social groups suffer disproportionately from tobacco-caused morbidity and mortality. Recent advances in genetics are leading researchers to examine variables that may account for this. However, it is critical that investigators proceed with caution and utilize transdisciplinary approaches. A number of fundamental questions might be used to stimulate consensus building in this area of science. What is race and how should its complexity be operationalized? Is it possible/likely that pharmacogenetics will allow us to match smokers with cessation strategies based on a gene-psychological profile? What are the most important conceptual and methodological issues for a research agenda in this area?

In vivo evaluation of coumarin and nicotine as probe drugs to predict the metabolic capacity of CYP2A6 due to genetic polymorphism in Thais

The association between the distribution characteristics of CYP2A6 catalytic activities toward nicotine and coumarin, and the frequency distribution of CYP2A6 variant alleles reported was estimated in 120 healthy Thais. The distributions of the subjects as classified by the amounts of 7-hydroxycoumarin (7-OHC) excreted in the urine and by cotinine/nicotine ratio in the plasma were clearly bimodal. However, the numbers of apparently poor metabolizers for coumarin and nicotine were different. The inter-individual variability in the in vivo dispositions of coumarin and nicotine closely related to the CYP2A6 genetic polymorphism. There was a close correlation between the rate of 7-OHC excretion in the urine and cotinine/nicotine ratio in the plasma among subjects (R=0.92, p<0.001). The frequency of CYP2A6 allele found in the present study was: CYP2A6*1A=32% (95% CI, 22.1-39.4%), CYP2A6*1B=27% (95% CI, 19.4-33.5%), CYP2A6*9=20% (95% CI, 17.6-23.3%), CYP2A6*4=14% (95% CI, 9.6-17.8%), CYP2A6*7=5% (95% CI, 3.7-9.4%), CYP2A6*10=2% (95% CI, 0.8-5.1%). Subjects having CYP2A6*1A/*1B were found to have a higher rate of 7-OHC excretion, as well as a higher cotinine/nicotine ratio in the plasma compared with those of the other genotypes. In contrast, subjects with CYP2A6*4/*7 and CYP2A6*7/*7 almost lacked any cotinine formation, whereas urinary 7-OHC was still detectable. CYP2A6*9 allele clearly resulted in reduced enzyme activities. Despite the absence of the homozygote for CYP2A6*10 allele, the presence of CYP2A6*10 allele significantly decreased the enzyme activities. The results of the present study demonstrate that in vivo phenotyping of CYP2A6 using nicotine and coumarin are not metabolically equivalent. Nicotine is a better probe according to its specificity, while coumarin is still valuable to be used for a routine CYP2A6 phenotyping since the test employs a non-invasive method.

The genetics of nicotine dependence: Relationship to pancreatic cancer

Smoking of tobacco products continues to be a major cause of worldwide health problems. Epidemiological studies have shown that tobacco smoking is the greatest risk factor for the development of pancreatic cancer. Smokers who are able to quit smoking can reduce their risk of pancreatic cancer by nearly 50% within two years, however, their risk of developing pancreatic cancer remains higher than that of non-smokers for 10 years. Nicotine is the major psychoactive substance in tobacco, and is responsible for tobacco dependence and addiction. Recent evidence suggests that individuals have genetically based differences in their ability to metabolize nicotine, as well as genetic differences in the psychological reward pathways that may influence individual response to smoking initiation, dependence, addiction and cessation. Numerous associations have been reported between smoking behavior and genetic polymorphisms in genes that are responsible for nicotine metabolism. In addition, polymorphisms in genes that encode neurotransmitters and transporters that function in psychological reward pathways have been implicated in differences in smoking behavior. However, there is a large degree of between-study variability that demonstrates the need for larger, well-controlled case-control studies to identify target genes and deduce mechanisms that account for the genetic basis of inter-individual differences in smoking behavior. Understanding the genetic factors that increase susceptibility to tobacco addiction may result in more effective tobacco cessation programs which will, in turn, reduce the incidence of tobacco related disease, including pancreatic cancer.

Risk factors for tobacco dependence in adolescent smokers

OBJECTIVE

To study the incidence of conversion to tobacco dependence (TD) and the prevalence of the TD state in relation to several potential determinants in a sample of adolescent smokers.

METHODS

Questionnaires were administered every 3-4 months to document TD symptoms, amount of cigarette consumption, and depression symptoms in a prospective cohort of 1293 grade 7 students in a convenience sample of 10 schools.

RESULTS

Over 54 months of follow-up, 113 of 344 novice smokers converted to TD. The referent series for the analysis of incidence comprised 823 person-surveys. The prevalence series included 1673 person-surveys, contributed by 429 smokers. Conversion to TD and TD status were associated with the intensity of recent (that is, past 3-month) cigarette consumption (adjusted incidence rate ratio (aIRR) 1.63 (95% confidence interval (CI) 1.36 to 1.97) and adjusted prevalence odds ratio (aPOR) 1.35 (95% CI 1.23 to 2.48) per 100 cigarettes per month), slowest CYP2A6 activity (aIRR 4.19 (95% CI 1.38 to 12.76) and aPOR 2.30 (95% CI 1.29 to 4.09)), depression score (aIRR 1.61 (95% CI 1.17 to 2.21) and aPOR 1.47 (95% CI 1.22, 1.75) per 1-unit change). Additional determinants included, for conversion to TD, time since onset of cigarette use (aIRR 0.76 (95% CI 0.58 to 1.00) per year) and, for the TD state, positive TD status six months ago (aPOR 3.53 (95% CI 2.41 to 5.19)).

CONCLUSIONS

TD risk in adolescents is associated with intensity of recent cigarette consumption, while the role of more distant cigarette consumption appears small; subjects with slow nicotine metabolism and those with more depression symptoms are at increased risk of becoming tobacco dependent. The risk of being tobacco dependent is considerably higher in subjects who had previously developed the TD state.

CYP2A6 and CYP2E1 polymorphisms in a Brazilian population living in Rio de Janeiro

Cytochrome P450 (CYP) is a superfamily of enzymes involved in the metabolism of endogenous compounds and xenobiotics. CYP2A6 catalyzes the oxidation of nicotine and the activation of carcinogens such as aflatoxin B1 and nitrosamines. CYP2E1 metabolizes ethanol and other low-molecular weight compounds and can also activate nitrosamines. The CYP2A6 and CYP2E1 genes are polymorphic, altering their catalytic activities and susceptibility to cancer and other diseases. A number of polymorphisms described are ethnic-dependent. In the present study, we determined the genotype and allele frequencies of the main CYP2A6 and CYP2E1 polymorphisms in a group of 289 volunteers recruited at the Central Laboratory of Hospital Universitário Pedro Ernesto. They had been residing in the city of Rio de Janeiro for at least 6 months and were divided into two groups according to skin color (white and non-white). The alleles were determined by allele specific PCR (CYP2A6) or by PCR-RFLP (CYP2E1). The frequencies of the CYP2A6*1B and CYP2A6*2 alleles were 0.29 and 0.02 for white individuals and 0.24 and 0.01 for non-white individuals, respectively. The CYP2A6*5 allele was not found in the population studied. Regarding the CYP2E1*5B allele, we found a frequency of 0.07 in white individuals, which was statistically different (P < 0.05) from that present in non-white individuals (0.03). CYP2E1*6 allele frequency was the same (0.08) in both groups. The frequencies of CYP2A6*1B, CYP2A6*2 and CYP2E1*6 alleles in Brazilians are similar to those found in Caucasians and African-Americans, but the frequency of the CYP2E1*5B allele is higher in Brazilians.

CYP2A6, MAOA, DBH, DRD4, and 5HT2A genotypes, smoking behaviour and cotinine levels in 1518 UK adolescents

OBJECTIVES

Smoking is a major cause of death and often initiates in adolescence. Mutations in CYP2A6 slow metabolism of nicotine to cotinine. Haploinsufficiency in adults is associated with lower cigarette consumption, lower cotinine level and higher quit rates. Other genes are also implicated in smoking behaviour. We explored smoking behaviour and cotinine levels in relation to genotypes in adolescents.

METHODS

1518 subjects from the Ten Towns Heart Health Study were genotyped for CYP2A6 alleles *1A, *1B, *2, *4, *5, *9 and *12 to classify predicted nicotine metabolism rate. DBH(rs77905), MAOA(rs1801291+VNTR), DRD4(VNTR) and 5HT2A(rs6313) were also studied. Smoking status was established by questionnaire and salivary cotinine measurement at 13-15 and 18 years.

RESULTS

No significant associations were identified for DBH, MAOA, DRD4 and 5HT2A markers, with smoking status or cotinine level at either age. At age 18, haploinsufficiency (HI) for CYP2A6 was associated with a higher odds of being a current smoker compared with the *1B carriers (WT1B) (OR = 2.23 (1.16, 4.27) for current versus ex); *1A homozygotes (WT1A) were also at slightly higher risk (OR = 1.44 (1.01, 2.06)). Partial haploinsufficiency (PHI) was not associated with being a current smoker. There were no significant associations at age 13-15. PHI and HI were associated with higher cotinine levels amongst smokers at both 13-15 and at 18 years compared with WT1B and WT1A groups.

CONCLUSIONS

CYP2A6 haploinsufficiency increases likelihood of continuing smoking in teenagers. We hypothesize an explanatory 'occupancy' model to explain why haploinsufficiency results in faster progression to nicotine dependence, but lower subsequent consumption.

Characterization of common CYP1B1 variants with different capacity for benzo[a]pyrene-7,8-dihydrodiol epoxide formation from benzo[a]pyrene

Cytochrome P450 1B1 (CYP1B1), an extrahepatic enzyme inducible by smoking, is overexpressed in many tumors and catalyzes the metabolic activation of procarcinogens such as polycyclic aromatic hydrocarbons. In human, CYP1B1 is genetically polymorphic and five common missense mutations causing amino acid substitution have been identified. In this study, we have investigated CYP1B1 haplotypes present in a Spanish population and carried out functional analyses of the corresponding enzymes in yeast using benzo[a]pyrene as a substrate. CYP1B1*1, CYP1B1*2, CYP1B1*3, CYP1B1*4, CYP1B1*6, and CYP1B1*7, encoding combinations of the Arg48Gly, Ala119Ser, Leu432Val, Asn453Ser, and Ala443Gly amino acid substitutions, were present at frequencies of 14.3%, 25.5%, 38.8%, 18.1%, 0.4%, and 2.6%, respectively. The variant CYP1B1 forms were heterologously expressed with human reductase in Saccharomyces cerevisiae and kinetic analyses of benzo[a]pyrene metabolism were carried out. CYP1B1.7, having the amino acid substitutions Arg48Gly, Ala119Ser, Leu432Val, and Ala443Gly, exhibited a significantly decreased capacity (P < 0.001) for the formation of (+/-)-benzo[a]pyrene-trans-7,8-dihydrodiol from benzo[a]pyrene as indicated by lower intrinsic clearance (Vmax/Km). A somewhat decreased clearance was observed for CYP1B1.4, whereas no significant differences in kinetic properties among the remaining variant enzymes were observed as compared with CYP1B1.1. Thus, genetic polymorphism in the CYP1B1 gene, as defined by the haplotypes investigated, might cause interindividual differences in susceptibility (e.g., to lung cancer induced by smoking). The results indicate the necessity to make molecular epidemiologic investigations regarding the association of the specific CYP1B1 haplotypes and cancer risk.

Nicotine addiction through a neurogenomic prism: ethics, public health, and smoking

Studies are under way to examine the neurogenetic factors contributing to smoking behaviors. The combined approaches of genomics, molecular biology, neuroscience, and pharmacology are expected to fuel developments in pharmacogenetics, to create new genetic tests, and ultimately to provide the basis for innovative strategies for smoking cessation and prevention. The emergence of a neurogenomic understanding of nicotine addiction is likely to induce fundamental changes in popular, clinical, and public health views of smoking, which could significantly shape existing practices and policies to reduce tobacco use. Still a nascent area of research, nicotine addiction provides an excellent case study through which to anticipate key ethical and policy issues in both behavioral genetics and the neurogenomics of addictive behaviors.

Implications of CYP2A6 genetic variation for smoking behaviors and nicotine dependence

Nicotine is the primary addictive compound in tobacco smoke. In this review we summarize nicotine dependence and the genetics of smoking in brief before focusing on cytochrome P450 (CYP) 2A6. In humans nicotine is mainly inactivated to cotinine and CYP2A6 mediates approximately 90% of this conversion. Some, but not all, studies suggest that genetic variation in CYP2A6 may play a role in smoking. We review some of the recent findings on the influence of CYP2A6 genetic polymorphisms on nicotine kinetics, smoking behaviors, and how the gene appears to exert differential effects during various stages of smoking (eg, initiation, conversion to dependence, amount smoked during dependence, and quitting). These new findings will be put in the context of the discrepancies found in the literature. Implications of these recent findings on current and novel treatment approaches for smoking cessation and tobacco-related lung cancer will also be discussed.

Bases genéticas del hábito tabáquico

Dependence on tobacco is one of the most important health problems in our society due to the direct relationship with lung cancer. Early studies in twins revealed that genetic factors modify the susceptibility to develop tobacco dependence. Different studies are underway to try to find an association between polymorphisms of genes involved either in nicotine metabolization or in neural transmission and the initiation and maintenance of the dependence on tobacco. Here we review the studies performed so far and discuss new perspectives for future studies.

Transcriptional Regulation of the Human CYP2A6 Gene

Nicotine C-oxidation is primarily catalyzed by CYP2A6 in humans. This enzymatic activity exhibits a large interindividual variability, which to a great extent is caused by genetic polymorphisms in the CYP2A6 gene. There are large interindividual differences in CYP2A6 mRNA and protein levels, but little is known about the transcriptional regulation of CYP2A6, which can, e.g., explain such differences. Using transient transfections of 5'-deleted CYP2A6 promoter constructs in human hepatoma B16A2 cells, we show that maximal promoter activity was harbored in the sequence spanning from -112 to -61. Putative response elements for the transcription factors hepatocyte nuclear factor-4 (HNF-4)alpha, CCAAT-box/enhancer binding protein (C/EBP)alpha, C/EBPbeta, and octamer transcription factor-1 (Oct-1) were identified in this region, and electrophoretic mobility shift assays showed that these transcription factors bind to the predicted elements. To determine the relevance of these sites, expression vectors for these transcription factors were cotransfected with CYP2A6 promoter constructs in HepG2 cells. HNF-4alpha, C/EBPalpha, and Oct-1 exerted an activating effect, whereas overexpression of C/EBPbeta reduced CYP2A6 promoter activity. To confirm the importance of these sites in vivo, mutated CYP2A6 reporter constructs were injected into mouse liver. Mutation of either HNF-4 or C/EBP-Oct-1 motifs significantly decreased promoter activity, 52 and 26% of wildtype, respectively, whereas when both motifs were mutated the activity in mice decreased to 14% of wild type. In conclusion, the data indicate that the constitutive hepatic expression of CYP2A6 is governed by an interplay between the transcription factors HNF-4alpha, C/EBPalpha, C/EBPbeta, and Oct-1. These results will be important for the identification of new polymorphisms affecting CYP2A6 gene expression.

Differences in drug pharmacokinetics between East Asians and Caucasians and the role of genetic polymorphisms

Interethnic variability in pharmacokinetics can cause unexpected outcomes such as therapeutic failure, adverse effects, and toxicity in subjects of different ethnic origin undergoing medical treatment. It is important to realize that both genetic and environmental factors can lead to these differences among ethnic groups. The International Conference on Harmonization (ICH) published a guidance to facilitate the registration of drugs among ICH regions (European Union, Japan, the United States) by recommending a framework for evaluating the impact of ethnic factors on a drug's effect, as well as its efficacy and safety at a particular dosage and dosage regimen. This review focuses on the pharmacokinetic differences between East Asians and Caucasians. Differences in metabolism between East Asians and Caucasians are common, especially in the activity of several phase I enzymes such as CYP2D6 and the CYP2C subfamily. Before drug therapy, identification of either the genotype and/or the phenotype for these enzymes may be of therapeutic value, particularly for drugs with a narrow therapeutic index. Furthermore, these differences are relevant for international drug approval when regulatory agencies must decide if they accept results from clinical trials performed in other parts of the world.

The use of race variables in genetic studies of complex traits and the goal of reducing health disparities: A transdisciplinary perspective

The use of racial variables in genetic studies has become a matter of intense public debate, with implications for research design and translation into practice. Using research on smoking as a springboard, the authors examine the history of racial categories, current research practices, and arguments for and against using race variables in genetic analyses. The authors argue that the sociopolitical constructs appropriate for monitoring health disparities are not appropriate for use in genetic studies investigating the etiology of complex diseases. More powerful methods for addressing population structure exist, and race variables are unacceptable as gross proxies for numerous social/environmental factors that disproportionately affect minority populations. The authors conclude with recommendations for genetic researchers and policymakers, aimed at facilitating better science and producing new knowledge useful for reducing health disparities.

Progress in searching for susceptibility loci and genes for smoking-related behaviour

Smoking behaviour is influenced by both genetic and environmental factors. Many years of twin and adoption studies have demonstrated that heritability is at least 50% responsible for both smoking initiation and smoking persistence. Furthermore, the extent, to which genetic and environmental factors contribute to smoking behaviour, is significantly different in men and women. Linkage analyses from several independent studies provide evidences for suggested linkage of smoking behaviour to chromosomes 1, 2, 4, 5, 6, 9, 10, 11, 14, 17, 18 and 21. However, almost none of these loci have been replicated yet. Furthermore, numerous population-based association studies have been performed to examine the effects of a number of candidate genes, such as cytochrome P450, dopamine receptor (DR) and transporter, serotonin transporter and nicotinic acetylcholine receptor, on smoking behaviour. However, many of these reports have not yet received independent confirmation. Of these candidate genes, the D2 dopamine receptor (DRD2) gene has been extensively studied. Meta-analysis of 12 reported studies showed a significantly higher prevalence of the DRD2 TaqI A1 allele in smokers than that in non-smokers (p < 0.0001; pooled OR = 1.50; 95% CI = 1.33-1.70). For other candidate genes, insufficient published studies are available to allow a meta-analysis to be performed, or meta-analysis showed no significant difference between smokers and non-smokers. More studies are necessary to determine whether these genes play a significant role in smoking behaviour.

Polymorphic NF-Y dependent regulation of human nicotine C-oxidase (CYP2A6)

OBJECTIVES

In humans, cytochrome P450 2A6 (CYP2A6) constitutes the principal nicotine C-oxidase. Several different polymorphic CYP2A6 gene variants are known which contribute to the highly variable expression of this enzyme among individuals. In this study we report a novel polymorphism located in the 5' flanking region (-745A > G) of the CYP2A6 gene disrupting a CCAAT box.

METHODS AND RESULTS

Electrophoretic mobility shift assays (EMSA) indicated that NF-YA is part of this nuclear protein complex. Chromatin immunoprecipitation revealed that NF-Y recognizes a region of the CYP2A6 5' flanking region located between -932 and -606. EMSA showed that out of the three CCAAT boxes in the CYP2A6 promoter, with CCAAT core sequences located between -839/-835, -748/-744, and -689/-685, only the one at -748/-744 was able to compete with the nuclear protein complex binding to the -748/-744 CCAAT box. Cotransfection experiments indicated that NF-Y acts as a positive regulatory element on CYP2A6 gene regulation. EMSA demonstrated that an NF-Y consensus oligonucleotide but not the -745A > G oligonucleotide competed efficiently with binding of the protein complex to the -748/-744 CCAAT box. Promoter activity of the -745A > G variant was significantly reduced to 78% relative to the wild-type allele in HepG2 cells transfected with luciferase reporter plasmids. Finally, haplotype analysis was carried out comprising the -745A > G variant in combination with all known CYP2A6 3' and 5' flanking single nucleotide polymorphisms: -1013A > G, -48T > G, and the CYP2A6/CYP2A7 3' flank conversion.

CONCLUSION

A new haplotype, CYP2A6*1H was identified, with allele frequencies of 3.1% in Swedish and 5.2% in Turkish populations.

Metabolic Detoxification Determines Species Differences in Coumarin-Induced Hepatotoxicity

Hepatotoxicity of coumarin is attributed to metabolic activation to an epoxide intermediate, coumarin 3,4-epoxide (CE). However, whereas rats are most susceptible to coumarin-induced hepatotoxicity, formation of CE is greatest in mouse liver microsomes, a species showing little evidence of hepatotoxicity. Therefore, the present work was designed to test the hypothesis that detoxification of CE is a major determinant of coumarin hepatotoxicity. CE can either rearrange spontaneously to o-hydroxyphenylacetaldehyde (o-HPA) or be conjugated with gluatathione (GSH). o-HPA is hepatotoxic and is further detoxified by oxidation to o-hydroxyphenylacetic acid (o-HPAA). In vitro experiments were conducted using mouse liver microsomes to generate a constant amount of CE, and cytosols from F344 rats, B6C3F1 mice, and human liver were used to characterize CE detoxification. All metabolites were quantified by HPLC methods with UV detection. In rats and mice, GSH conjugation occurred non-enzymatically and through glutathione-S-transferases (GSTs), and the kinetics of GSH conjugation were similar in rats and mice. In rat liver cytosol, oxidation of o-HPA to o-HPAA was characterized with a high affinity K(m) of approximately 12 microM, and a V(max) of approximately 1.5 nmol/min/mg protein. In contrast, the K(m) and V(max) for o-HPA oxidation in mouse liver cytosol were approximately 1.7 microM and 5 nmol/min/mg protein, respectively, yielding a total intrinsic clearance through oxidation to o-HPAA that was 20 times higher in mouse than in rats. Human cytosols (two separate pools) detoxified CE through o-HPA oxidation with an apparent K(m) of 0.84 microM and a V(max) of 5.7 nmol/min/mg protein, for a net intrinsic clearance that was more than 50 times higher than the rat. All species also reduced o-HPA to o-hydroxyphenylethanol (o-HPE), but this was only a major reaction in rats. In the presence of a metabolic reaction replete with all necessary cofactors, GSH conjugation accounted for nearly half of all CE metabolites in rat and mouse, whereas the GSH conjugate represented only 10% of the metabolites in human cytosol. In mouse, o-HPAA represented the major ring-opened metabolite, accounting for the remaining 50% of metabolites, and in human cytosol, o-HPAA was the major metabolite, representing nearly 90% of all CE metabolites. In contrast, no o-HPAA was detected in rats, whereas o-HPE represented a major metabolite. Collectively, these in vitro data implicate o-HPA detoxification through oxidation to o-HPAA as the major determinant of species differences in coumarin-induced hepatotoxicity.

Polymorphisms of glutathione S-transferases M1 and T1 do not account for interindividual differences for smoking behavior

To identify whether the polymorphisms of glutathione S-transferase M1 (GSTM1) and GSTT1 genes predict a high-tended risk of using tobacco, the GSTM1 and GSTT1 genotypes of 369 Iranian males (254 nonsmokers and 115 smokers) and 314 Iranian females (245 nonsmokers and 69 smokers) were determined. The frequencies of GSTM1 (males: OR=0.98, 95% CI=0.62-1.57, P=.974; females: OR=1.34, 95% CI=0.75-2.39, P=.358) and GSTT1 (males: OR=1.25, 95% CI=0.76-2.04, P=.412; females: OR=0.84, 95% CI=0.46-1.51, P=.626) null genotypes were similar in nonsmokers and smokers. The risk of being a smoker was to be equally frequent in each combination of the genotypes. The present results revealed that there was no difference between smokers and nonsmokers for these two genetic polymorphisms.

Human variability for metabolic pathways with limited data (CYP2A6, CYP2C9, CYP2E1, ADH, esterases, glycine and sulphate conjugation)

Human variability in the kinetics of a number of phase I (CYP2A6, CYP2C9, CYP2E1, alcohol dehydrogenase and hydrolysis) and phase II enzymes (glycine and sulphate conjugation) was analysed using probe substrates metabolised extensively (>60%) by these routes. Published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and available data on subgroups of the population (effects of ethnicity, age and disease) were abstracted using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration time-curve (AUC)] and acute exposure (C(max)). Interindividual differences in kinetics for all these pathways were low in healthy adults ranging from 21 to 34%. Pathway-related uncertainty factors to cover the 95th, 97.5th and 99th centiles of healthy adults were derived for each metabolic route and were all below the 3.16 kinetic default uncertainty factor in healthy adults, with the possible exception of CYP2C9*3/*3 poor metabolisers (based on a very limited number of subjects). Previous analyses of other pathways have shown that neonates represent the most susceptible subgroup and this was true also for glycine conjugation for which an uncertainty factor of 29 would be required to cover 99% of this subgroup. Neonatal data were not available for any other pathway analysed.