Structural characterization of a new variant of the CYP2A6 gene (CYP2A6*1B) apparently diagnosed as heterozygotes of CYP2A6*1A and CYP2A6*4C

Cigarette Smoking and Intracranial Aneurysms: A Pilot Analysis of SNPs in the CYP2A6 Gene in the Italian Population

BACKGROUND

Cigarette smoking is a modifiable risk factor associated with formation and rupture of intracranial aneurysms (IAs). Cytochrome P450 2A6 (CYP2A6) is the main enzyme implied in catabolism of nicotine and xenobiotics, giving rise to oxidative stress products. Our study investigated the associations between specific single-nucleotide polymorphisms (SNPs) in the CYP2A6 gene and the presence of sporadic IAs in a cluster of Italian patients, as well as their rupture regarding cigarette smoking habit.

METHODS

Three hundred and thirty-one Italian patients with sporadic IAs were recruited in a single institution. We recorded data on clinical onset with subarachnoid hemorrhage (SAH) and smoking habit. Genetic analysis was performed with a standard procedure on peripheral blood samples: CYP2A6 ∗1B2, CYP2A6 ∗2, and CYP2A6 ∗14 SNPs were analyzed in the study group along with 150 healthy control subjects. Statistical analysis was conducted according to genetic association study guidelines.

RESULTS

In the patient cohort, the frequency of aSAH was significantly higher in current smokers (P < 0.001; OR=17.45), regardless of the pattern of CYP2A6 SNPs. There was a correlation between IA rupture and cigarette smoking in patients with the heterozygous CYP2A6 ∗1B2 allele (P < 0.001; OR=15.47). All patients carrying the heterozygous CYP2A6 ∗14 allele had an aSAH event (100%), regardless of smoking habit, although this correlation was not statistically significant (P = 1).

CONCLUSIONS

According to our findings, a cigarette smoker carrying a fully active CYP2A6 enzyme (heterozygous ∗1B2 allele) may have an increased risk of IA rupture compared to those with functionally less active variants: further investigation on a larger sample is needed to verify this result. The role of the heterozygous CYP2A6 ∗14 allele in aSAH is yet to be clarified.

Effects of genetic polymorphism of drug-metabolizing enzymes on the plasma concentrations of antiepileptic drugs in Chinese population

As a chronic brain disease, epilepsy affects ~50 million people worldwide. The traditional antiepileptic drugs (AEDs) are widely applied but showing various problems. Although the new AEDs have partially solved the problems of traditional AEDs, the current clinical application of traditional AEDs are not completely replaced by new drugs, particularly due to the large individual differences in drug plasma concentrations and narrow therapeutic windows among patients. Therefore, it is still clinically important to continue to treat patients using traditional AEDs with individualized therapeutic plans. To date, our understanding of the molecular and genetic mechanisms regulating plasma concentrations of AEDs has advanced rapidly, expanding the knowledge on the effects of genetic polymorphisms of genes encoding drug-metabolizing enzymes on the plasma concentrations of AEDs. It is increasingly imperative to summarize and conceptualize the clinical significance of recent studies on individualized therapeutic regimens. In this review, we extensively summarize the critical effects of genetic polymorphisms of genes encoding drug-metabolizing enzymes on the plasma concentrations of several commonly used AEDs as well as the clinical significance of testing genotypes related to drug metabolism on individualized drug dosage. Our review provides solid experimental evidence and clinical guidance for the therapeutic applications of these AEDs.

Polymorphism in cytochrome P4502A6 reduces the risk to head and neck cancer and modifies the treatment outcome

The present case-control study consisting of 1300 cases of head and neck squamous cell carcinoma (HNSCC) and the equal number of controls aimed to investigate the association of functionally important polymorphisms in cytochrome P4502A6 (CYP2A6*1B, CYP2A6*4C, CYP2A6*9-rs28399433) with HNSCC and the treatment response in cases receiving a combination of chemotherapy/radiotherapy (CT/RT). A significant decrease in risk to HNSCC was observed in the cases with deletion (CYP2A6*4B and CYP2A6*4C) or reduced activity genotypes (CYP2A6*9) of CYP2A6. This risk to HNSCC was further reduced significantly in tobacco users among the cases when compared to nontobacco users among the cases. The risk was also reduced to a slightly greater extent in alcohol users among the cases when compared to nonalcohol users among the cases. In contrast with decreased risk to HNSCC, almost half of the cases with variant genotypes of CYP2A6 (CYP2A6*1A/*4C+*1B/*4C+*4C/*4C and *9/*9) did not respond to the treatment. Likewise, the survival rate in cases receiving the treatment, after 55 months of follow-up was significantly lower in cases with deletion (6.3%) or reduced activity (11.9%) allele than in the cases with common alleles (41%). The present study has shown that CYP2A6 polymorphism significantly reduces the risk to HNSCC. Our data further suggested that CYP2A6 polymorphism may worsen the treatment outcome in the cases receiving CT/RT.

Similarities in mRNA expression of peripheral blood drug metabolizing enzymes and cancer marker genes with biopsy samples of head and neck cancer patients

Purpose: To develop peripheral blood mRNA expression profiles of drug metabolizing enzymes (DMEs) as a surrogate to monitor tobacco induced head and neck squamous cell carcinoma (HNSCC), attempts were made to investigate (i) similarities in alterations with the cancer marker genes in biopsy samples and (ii) if alterations similar to that seen in biopsy samples are reflected in peripheral blood. Methods: Total RNA from eight soft gingival tissues and eight biopsy samples of HNSCC patients and total DNA and RNA from blood of healthy controls (n = 150) and HNSCC patients (n = 150) was processed for expression and genotyping studies. Blood from patients receiving chemo-radiotherapy was processed for follow-up study. Results: qRT-PCR revealed significant increase in mRNA expression of DMEs in biopsy and blood samples of HNSCC patients when compared to controls. Similar alterations were observed in cancer marker genes in these samples. Patients with variant genotypes of DMEs showed greater magnitude of alterations in mRNA expression when compared to wild type controls. Responders of chemo-radiotherapy showed significant decline in induction of mRNA expression of DMEs and cancer marker genes Conclusions: The data suggest that peripheral blood expression profiles could be used to monitor tobacco-induced HNSCC as well as the treatment response.

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.

Association of CYP2C9, CYP2A6, ACSM2A, and CPT1A gene polymorphisms with adverse effects of valproic acid in Chinese patients with epilepsy

OBJECTIVE

To explore the influence of CYP2C9, CYP2A6, ACSM2A, CPT1A gene polymorphisms on valproic acid (VPA) and its role in metabolism-related liver dysfunction in order to guide the clinical safety and rational use of VPA.

METHODS

One hundred two patients taking sodium valproate oral solution were genotyped. To assess the genotypes of relevant genes, the CYP2C9 gene was directly sequenced; for polymorphism classification, multiple Long-PCR electrophoresis was conducted for CYP2A6; and imLDR method was used for ACSM2A and CPT1A. GC-MS-SIM was used to determine the levels of VPA and 2-propyl-4-pentenoic acid (4-ene-VPA) in human plasma simultaneously.

RESULTS

CYP2C9 mutations had a significant impact on 4-ene-VPA concentration, in patients with wild-type CYP2C9 (CYP2C9*1), which has a greater capacity for VPA metabolism than the mutant type (CYP2C9*3), liver dysfunction was substantially higher. Patients with an ACSM2A polymorphism had higher levels of ALT and AST compared with wild-type (p<0.05), but the mutations had no effect on the VPA-related liver dysfunction (p>0.05). Among different CYP2A6 and CPT1A genotype groups, there was no significant correlation in the levels of VPA, 4-ene-VPA, ALT, AST or TB (p>0.05). The content of 4-ene-VPA had no direct correlation with the incidence of liver dysfunction.

CONCLUSIONS

Early detection of CYP2C9 gene polymorphisms may help to predict or prevent liver dysfunction caused by VPA. While the concentration of 4-ene-VPA was not suitable as an early warning index, the results provide clear theoretical guidance for the rational and safe clinical use of VPA.

Association of cytochrome P450 2A6 polymorphism, anxiety, and environmental factors with cigarette smoking by Thai adults

Background

The effects and associations of genetic variation, psychological, and environment factors associated with cigarette smoking and nicotine dependence remain largely unknown.

Objective

To determine the influence and association of functional genetic polymorphisms of cytochrome P450 2A6 (CYP2A6), anxiety, and environmental factors on cigarette smoking and nicotine dependence.

Method

A cross-sectional study was conducted at King Chulalongkorn Memorial Hospital, Thailand between October 2014 and June 2015. We recruited 127 Thai adult smokers when they visited for an annual physical check-up. Participants completed questionnaires regarding demographic characteristics, The Fagerstrom Test for Nicotine Dependence, and The Thai Hospital Anxiety and Depression Scale. Blood was collected for CYP2A6 genotyping to determine the enzyme metabolism level/group.

Results

Factors associated with significantly greater cigarette consumption were age and being ultrarapid/ extensive metabolizers (UM/EM). Anxiety and smoking by household family members were significantly associated with the degree of nicotine dependence. We observed associations between severe nicotine dependence and genotype (UM/EM) and age (b = 0.037; P = 0.005), intermediate metabolizers (IM) and age (b = 0.031; P = 0.43), UM/EM and anxiety (b = 0.258; P < 0.001), IM and anxiety (b = 0.285; P < 0.001), UM/EM and household smoking in the family members (b = 1.427; P = 0.003), and IM and smoking by household family members (b = 1.293; P = 0.024).

Conclusions

Information regarding the association between the gene encoding enzyme metabolism, anxiety, and their interactions may be beneficial for selecting treatment choices for smoking cessation for individual genotypic metabolizers.

Evaluation of Risk Factors for Nasopharyngeal Carcinoma in a High-risk Area of India, the Northeastern Region

Northeastern India is a major nasopharyngeal carcinoma (NPC) high risk-area although the rest of the country has very low incidence. A case-control study of 105 NPC cases and 115 controls was conducted to identify the potential risk factors for NPC development in this region. Information was collected by interviewer about socio-demographic characteristics, cigarette smoking, alcohol consumption, dietary history, occupational history, and a family history of cancer. Epstein-Barr viral load was assayed from the blood DNA by real time PCR. Associations between GSTs genotypes, cytochrome P450 family including CYP1A1, CYP2E1 and CYP2A6 polymorphisms and susceptibility to relationship between the diseases were studied using PCR-RFLP assay. Results indicate that Epstein-Barr virus load was significantly higher in patients compared to controls (p<0.0001). Furthermore, concentration of blood EBV-DNA was significantly higher in advanced stage disease (Stage III and IV) than in early stage disease (Stage I and II) (p<0.05). Presence of CYP2A6 variants that reduced the enzyme activity was significantly less frequent in cases than controls. Smoked meat consumption, exposure to smoke, living in poorly ventilated house and alcohol consumption were associated with NPC development among the population of Northeastern India. Thus, overall our study revealed that EBV viral load and genetic polymorphism of CYP2A6 along with living practices which include smoked meat consumption, exposure to smoke, living in poorly ventilated houses and alcohol consumption are the potential risk factors of NPC in north eastern region of India. Understanding of the risk factors and their role in the etiology of NPC are helpful forpreventive measures and screening.

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.

Genetic influence of dopamine receptor, dopamine transporter, and nicotine metabolism on smoking cessation and nicotine dependence in a Japanese population

Background

This study investigated whether polymorphisms of the ankyrin repeat and kinase domain containing 1 gene (ANKK1), which is adjacent to the dopamine D2 receptor gene (DRD2), and the dopamine transporter (SLC6A3) and cytochrome P450 2A6 (CYP2A6) genes influence smoking cessation and nicotine dependence in a Japanese population. In 96 current and former smokers, genotyping frequencies for the ANKK1/DRD2 TaqIA, SLC6A3 VNTR, and CYP2A6 polymorphisms were subjected to chi-square analysis, and regression analyses were used to determine the association of the genotypes of current smokers with a Heavy Smoking Index, in addition to evaluating the effect of the subjects’ smoking history on the association.

Results

Genotyping results suggested that nicotine dependence among current smokers homozygous for the SLC6A3 10r allele was lower than that of smokers carrying the minor alleles, and that the CYP2A6 polymorphism might mediate this association. Furthermore, the age at which current smokers began smoking might moderate the association between their genetic polymorphisms and nicotine dependence.

Conclusions

This study provides preliminary findings on the influence of genetic variants on the smoking phenotypes in a Japanese population.

Preliminary investigation of the contribution of CYP2A6, CYP2B6, and UGT1A9 polymorphisms on artesunate-mefloquine treatment response in Burmese patients with Plasmodium falciparum malaria

CYP2A6, CYP2B6, and UGT1A9 genetic polymorphisms and treatment response after a three-day course of artesunate-mefloquine was investigated in 71 Burmese patients with uncomplicated Plasmodium falciparum malaria. Results provide evidence for the possible link between CYP2A6 and CYP2B6 polymorphisms and plasma concentrations of artesunate/dihydroartemisinin and treatment response. In one patient who had the CYP2A6*1A/*4C genotype (decreased enzyme activity), plasma concentration of artesunate at one hour appeared to be higher, and the concentration of dihydroartemisinin was lower than for those carrying other genotypes (415 versus 320 ng/mL). The proportion of patients with adequate clinical and parasitologic response who had the CYP2B6*9/*9 genotype (mutant genotype) was significantly lower compared with those with late parasitologic failure (14.0% versus 19.0%). Confirmation through a larger study in various malaria-endemic areas is required before a definite conclusion on the role of genetic polymorphisms of these drug-metabolizing enzymes on treatment response after artesunate-based combination therapy can be made.

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.

A review of smoking behavior and smokers evidence (chemical modification, inducing nicotine metabolism, and individual variations by genotype: dopaminergic function and personality traits)

The nicotine metabolism of CYP2A6 (CYP2A6*1A,*1B, and *1C), and the cholecystokinin (CCK; which modulates the release of dopamine) and CCK-A receptor gene and personality traits for NEO-FFI, was investigated for the mechanism for elucidation of the smoking behavior in Japanese populations. The frequency of the CYP2A6*4C allele, which is a whole deleted allele of the human CYP2A6 gene, was higher, whereas that of CYP2A6*1A/*1B heterozygotes with higher nicotine metabolism activity was lower in nonsmokers than in smokers. There was also a significant difference between the current smoking and nonsmoking groups in the allele frequency of the CCK -45C/T polymorphism. It was also shown that the Openness (O) factor for smokers was significantly higher than that of nonsmokers; however, there were no significant differences in the Neuroticism (N), Extraversion (E), Agreeable (A), and Conscientiousness (C) scores among smokers than nonsmokers. It was suggested that the CYP2A6*4C allele may prevent the carrier from smoking, and being a CYP2A6*1A/*1B heterozygote and the CCK T allele may be risk factors for developing smoking behavior. Also, it is possible that persons with a low score in Openness may be refraining from smoking because they have a general negative impression toward smoking.

Polymorphism in cytochrome P450 2A6 and glutathione S-transferase P1 modifies head and neck cancer risk and treatment outcome

A case control study was carried out to investigate the association of functionally important polymorphism in cytochrome P450 2A6 (CYP2A6) and glutathione S-transferase P1 (GSTP1) genes with head and neck squamous cell carcinoma (HNSCC) and treatment response in cases receiving a combination of chemo-radiotherapy. The study group consisted of 350 males suffering from HNSCC and an equal number of male controls. Multivariate logistic regression analysis revealed statistically significant decrease in risk to HNSCC in cases with variant genotypes (CYP2A6*1B and CYP2A6*4C) of CYP2A6 (OR: 0.78; 95% CI: 0.43-1.22; P=0.04) or GSTP1 (OR: 0.71; 95% CI: 0.51-1.00; P=0.05). The risk associated with these variant genotypes was found to be further decreased in cases carrying a combination of variant genotypes of CYP2A6 and GSTP1 (OR: 0.40; 95% CI: 0.25-0.65; P=0.00). A similar decrease in risk was observed in cases with variant genotypes of CYP2A6 (OR: 0.59; 95% CI: 0.40-0.86; P=0.00) or GSTP1 (OR: 0.62; 95% CI: 0.42-0.91; P=0.01) and who were regular tobacco users (cigarette smokers or tobacco chewers). Interestingly, only 27% of the cases carrying the variant forms of CYP2A6 (*1A/*4C+*1B/*4C+*4C/*4C) responded to the treatment for HNSCC when compared to those with wild-type genotype (69%). However with GSTP1, cases with homozygous mutant genotype (Val/Val) showed a superior treatment response (75%) when compared to cases with wild-type genotype (25%). Further, cases carrying a combination of variant genotype of CYP2A6 and wild-type genotype of GSTP1 exhibited a very poor treatment response demonstrating that polymorphisms in CYP2A6 and GSTP1 not only modified the risk to HNSCC but also played a major role in determining the chemotherapeutic response.

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.

Interaction between cytochrome P450 1A2 genetic polymorphism and cigarette smoking on the risk of hepatocellular carcinoma in a Japanese population

Limited epidemiological evidence suggests that genetic polymorphisms of drug-metabolizing enzymes such as cytochrome P450 (CYP), glutathione S-transferase (GST) and N-acetyltransferase (NAT) may be involved in tobacco-related hepatocarcinogenesis. We conducted a case-control study, including 209 incident cases with hepatocellular carcinoma (HCC) and two different control groups [275 hospital controls and 381 patients with chronic liver disease (CLD) without HCC], to investigate whether CYP1A1, CYP1A2, CYP2A6, CYP2E1, GSTM1 and NAT2 polymorphisms are related to the risk of HCC with any interaction with cigarette smoking. Overall, no significant associations with HCC were observed for any genotypes against either control group. However, we found a significant interaction (P = 0.0045) between CYP1A2 -3860G>A polymorphism and current smoking on HCC risk when we compared HCC cases with CLD patients; adjusted odds ratios [ORs; and 95% confidence intervals (CIs)] for G/A and A/A genotypes relative to G/G genotype were 0.28 (0.12-0.66) and 0.18 (0.04-0.94), respectively, among current smokers (P trend = 0.002), as compared with 1.28 (0.80-2.06) and 0.76 (0.34-1.71), respectively, among never/former smokers (P trend = 0.96). Similarly, in CYP1A2 G/G genotype, significant risk increase was observed for current smoking (OR = 4.08, 95% CI = 2.02-8.25) or more recent cigarette use (e.g. pack-years during last 5 years, P trend = 0.0003) but not in G/A and A/A genotypes combined (OR for current smoking = 1.39, 95% CI = 0.63-3.03; P trend for pack-years during last 5 years = 0.40). These results suggest that the CYP1A2 -3860G>A polymorphism modifies the smoking-related HCC risk among CLD patients.

Cigarette smoking and glutathione S-transferase M1 polymorphism associated with risk for uterine cervical cancer

AIM

To elucidate the role of tobacco smoking and polymorphisms of carcinogen metabolism genes in cervical carcinogenesis.

METHODS

We analyzed genotypes of nine genes, 11 polymorphisms encoding carcinogen metabolizing enzymes, information on smoking, and the presence of human papillomavirus in 124 Japanese cervical cancer patients and 125 healthy controls.

RESULTS

The incidence of human papillomavirus infection (95.5% vs 9.9%; P < 0.001; odds ratio (OR), 231.98; 95% confidence interval [CI], 57.17-941.22), and smoking (41.1% vs 18.4%; P < 0.001; OR, 3.40; 95% CI, 1.88-6.17) were both significantly higher in patients than in controls. The genotype distributions of CYP1A1, CYP2E1, CYP2A6, NQO1, NAT2, mEH, MPO and GSTT1 genes were not statistically different; however, the ratio of the GSTM1 null genotype was significantly higher in patients than in controls (62.1% vs 47.2%; P = 0.019; OR, 1.83; 95% CI, 1.11-3.04). The incidence of GSTM1 null was significantly higher in the non-smoking group (63.0% vs 47.1%; P = 0.038; OR, 1.92; 95% CI, 1.04-3.54), and not in the smoking group (60.8% vs 47.8%; P = 0.300; OR, 1.69; 95% CI, 0.63-4.56).

CONCLUSIONS

In the current study, risk factors for developing cervical cancer were tobacco smoking and GSTM1 null; however, no association was observed between these two factors. We could not prove that smoking-GSTM1 null interaction was responsible for the increase in cervical cancer among young Japanese, and further studies with more detailed smoking status, not only active but passive smoking, will be required.

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.

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.

CYP2A6 gene polymorphism and personality traits for NEO-FFI on the smoking behavior of youths

We performed a survey on the relationship between nicotine dependence and psychological (the personality traits using neuroticism extroversion openess-five factor inventory (NEO-FFI)/nicotine metabolism (the CYP2A6 gene polymorphism) factors among Japanese young students to elucidate the mechanism of the development of nicotine dependence. As a result, the frequency of the CYP2A6*4C gene (enzyme activity deficit) was significantly (p<0.05) higher among nonsmokers than smokers, and the Openness score by NEO-FFI was higher among smokers than nonsmokers. However, there were no significant differences in the Neuroticism, Extraversion, Agreeableness, and Conscientiousness scores by NEO-FFI between smokers and nonsmokers. We conclude that the CYP2A6*4C gene and the Openness personality trait may affect the development of the smoking behavior of youth.

Identification of novel CYP2A6*1B variants: the CYP2A6*1B allele is associated with faster in vivo nicotine metabolism

Cytochrome P450 2A6 (CYP2A6) is the human enzyme responsible for the majority of nicotine's metabolism. CYP2A6 genetic variants contribute to the interindividual and interethnic variation in nicotine metabolism. We examined the association between the CYP2A6*1B variant and nicotine's in vivo metabolism. Intravenous infusions of deuterium-labeled nicotine were administered to 292 volunteers, 163 of whom were White and did not have common CYP2A6 variants, other than CYP2A6*1B. We discovered three novel CYP2A6*1B variants in the 3'-flanking region of the gene that can confound genotyping assays. We found significant differences between CYP2A6*1A/*1A, CYP2A6*1A/*1B, and CYP2A6*1B/*1B groups in total nicotine clearance (17.2+/-5.2, 19.0+/-6.4, and 20.4+/-5.9, P<0.02), non-renal nicotine clearance (16.4+/-5.0, 18.5+/-6.2, and 19.8+/-5.7, P<0.01), and the plasma trans-3'-hydroxycotinine/cotinine ratio (0.26+/-0.1, 0.26+/-0.1, and 0.34+/-0.1, P<0.001). There were also differences in total nicotine (29.4+/-12.9, 25.8+/-0.12.9, and 22.4+/-12.4, P<0.01), cotinine (29.2+/-8.1, 32.2+/-9.1, and 33.0+/-6.6, P<0.01) and trans-3'-hydroxycotinine (32.4+/-9.1, 34.2+/-12.3, and 41.3+/-11.3, P<0.001) excreted in the urine. We report evidence that CYP2A6*1B genotype is associated with faster nicotine clearance in vivo, which will be important to future CYP2A6 genotype association studies.

Growth hormone does not alter CYP2A6 activity in growth hormone-deficient children

A large number of metabolic alterations are increasingly being treated with growth hormone. Despite the fact that growth hormone is known to be the main regulator of several hepatic drug metabolizing enzymes in rodents, few studies deal with the effect of growth hormone on hepatic enzyme activities in human beings. The aim of this study was to determine the effects of growth hormone replacement therapy for 4 weeks on CYP2A6 activity in children, because changes in this enzyme activity may have important therapeutic and toxic consequences. A total of 31 growth hormone-deficient children (age range 4.1-13.1 years; mean age 9.88 +/- 2.89 years) participated. The genotypes of CYP2A6 gene, CYP2A6*1A, CYP2A6*1B, CYP2A6*4, CYP2A6*1x2 and CYP2A6*9, were determined by polymerase chain reaction. To assess the enzyme activity, we used caffeine as a probe drug at two points in time: before starting growth hormone therapy (Day 0) and after 4 weeks of growth hormone therapy (Day A). Caffeine and metabolite concentrations in urine were assayed by high-pressure liquid chromatography. The metabolite ratio 1,7-dimethilxanthine to 1,7-dimethylurate (17U/17X) served to indicate CYP2A6 activity. Median value and 95% confidence interval at baseline was 1.08 (0.98-1.24). The value after treatment was 1.08 (0.86-1.21). Data comparison between periods showed lack of statistically significant differences (P > 0.05). The relative change, measured by the ratio of medians and 90% confidence interval, was 1.02 (0.84-1.19). There were no significant differences when the ratio between genotype groups were compared. These results indicate that growth hormone replacement therapy of growth hormone-deficient children for 4 weeks does not modify the CYP2A6 activity and hence the efficacy or toxicity of the CYP2A6 substrate compounds.

Genetic variability in CYP2A6 and the pharmacokinetics of nicotine

Nicotine is the psychoactive substance responsible for tobacco dependence. It is also a therapeutic used to aid smoking cessation. Cytochrome P450 (CYP)2A6 is the human hepatic enzyme that mediates most of nicotine’s metabolic inactivation to cotinine. Genetic variation in the CYP2A6 gene can increase or decrease enzyme activity through altering the protein’s expression level or its structure and function. This article reviews CYP2A6 genetic variation and its impact on in vivo nicotine kinetics, including a description of the individual variants, different phenotyping approaches for assessing in vivo CYP2A6 activity and other sources of variation in nicotine metabolism such as gender. In addition, the effect of CYP2A6 polymorphisms on smoking behavior and tobacco-related lung cancer risk are briefly described. Furthering knowledge in this area will improve interpretation of studies examining smoking behavior, as well as those using nicotine as a therapeutic agent.

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.

A novel duplication type of CYP2A6 gene in African-American population

Human CYP2A6 is responsible for the metabolism of nicotine and its genetic polymorphisms affect smoking behavior and risk of lung cancer. In the present study, we identified a novel type of CYP2A6 gene duplication that is created through an unequal crossover event with the CYP2A7 gene at 5.2 to 5.6 kilobases downstream from the stop codon. The novel duplication type of CYP2A6 was found in African Americans (n = 176) at an allele frequency of 1.7%, but was not found in European-American (n = 187), Korean (n = 209), or Japanese (n = 184) populations. The plasma cotinine/nicotine ratio in subjects possessing the novel CYP2A6 gene duplication with the CYP2A6*1 allele (10.8 +/- 7.0, n = 4) was 1.4-fold higher than that in homozygotes of the wild type (8.0 +/- 5.0, n = 87), although the difference was not statistically significant. The findings in the present study suggested that the novel duplicated CYP2A6 allele, which is specific for African Americans, would increase nicotine metabolism and may affect smoking behavior.

[How I was enticed into molecular toxicology]

This paper introduces one of our projects performed at Hokkaido University. During the course of pharmacokinetic studies of SM-12502, which was under development as an anti-platelet-activating factor agent, we found three individuals who showed a slow metabolic phenotype in its pharmacokinetics. Analyzing the genes for CYP2A6 from the three, we discovered that they had the whole CYP2A6 gene deletion (CYP2A6*4C). Genetically engineered Salmonella YG7108 cells expressing human P450 were established to compare the mutagen-producing capacity of the P450 enzymes for various N-nitrosamines. We found that CYP2A6 was involved in the metabolic activation of N-nitrosamines with relatively bulky alkyl chains such as a tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), which has been known to cause lung tumors in rodents. Thus, to examine the hypothesis that individuals possessing the CYP2A6*4C have a reduced risk of cancer due to the lack of the metabolic activation of certain carcinogens in tobacco smoke, a case-control study was performed. The results clearly indicated a significant association between the CYP2A6 genotype and lung cancer risk in smokers. In contrast, there was no significant relationship between them in nonsmokers. In addition, our results showed that the reduced risk of cancer was caused by the reduced activity of CYP2A6. Thus it was expected that the inhibition of the enzyme would result in a reduced cancer risk caused by smoking. The results of experiments using mice which were treated with NNK, a carcinogenic nitrosamine contained in tobacco smoke, together with 8-methoxypsolaren, a strong inhibitor of CYP2A6, indicated that the inhibition of CYP2A6 completely abolished the occurrence of adenoma.

CYP2A7 polymorphic alleles confound the genotyping of CYP2A6*4A allele

Human cytochrome P450 (CYP) 2A6 metabolizes nicotine to cotinine. Genetic polymorphisms of CYP2A6 contribute to the interindividual variability of nicotine metabolism. We encountered some subjects possessing two copies of the CYP2A6 gene, although they were genotyped as heterozygotes of the CYP2A6*4A allele (entire CYP2A6 gene deleted allele). From the subjects, we found CYP2A7 polymorphic alleles (CYP2A7*1B, CYP2A7*1C, and CYP2A7*1D) in which the sequences in the 3'-flanking region were converted to the corresponding CYP2A6 sequences, being confused with the CYP2A6*4A. These allele frequencies in European-Americans (n=187) were 1.3, 2.1, 0.3%, respectively, but these were very rare in African-Americans (n=176), Japanese (n=184), and Koreans (n=209). By an improved genotyping method, the allele frequency of CYP2A6*4A of 3.7% in European-Americans was corrected to 0%. The comprehensible and reliable genotyping method developed in this study would be useful to evaluate associations between the genotype and phenotype.

Cytochrome P450 2A6 polymorphism in nasopharyngeal carcinoma

Nitrosamine has been identified as a carcinogen for nasopharyngeal carcinoma (NPC). Here, we investigated if a nitrosamine metabolizing gene, cytochrome P450 2A6 (CYP2A6) played an important role in NPC development. Relationships between the disease and the CYP2A6 were studied in 74 NPC patients and 137 age-matched healthy controls by using PCR-RFLP assay to distinguish between a wide type allele, *1A, and two mutant alleles, *1B and *4C. Overall, a significant association between CYP2A6 polymorphism and NPC development was observed (P<0.05). Individual with mutant alleles had an increased risk for NPC when compared to those with *1A/*1A (OR=2.37, 95% CI=1.27-4.46). In addition, males who carried mutant alleles of CYP2A6 had a fivefold increased risk for NPC when compared with those who carried *1A/*1A genotype (OR=5.02, 95% CI=1.82-14.14). It is thus suggested that CYP2A6 polymorphism may play a crucial role in NPC susceptibility and it may be used as a risk marker for NPC.

3′-UTR polymorphism in the human CYP2A6 gene affects mRNA stability and enzyme expression

Cytochrome P450 2A6 (CYP2A6) is the major nicotine C-oxidase in human and participates in the metabolism of drugs and precarcinogens. The CYP2A6 gene is highly polymorphic and more than 22 different alleles have been described. We here focused on the polymorphism in the 3'-UTR region, in particular the common CYP2A6*1B allele, carrying an unequal crossover element from the pseudogene CYP2A7. Analysis of CYP2A6 expression in a human liver bank (n=46) revealed that the protein level and catalytic activity using coumarin as a substrate were all higher, following a linear gene-dose relationship, in livers carrying one or two copies of CYP2A6*1B, as compared to other CYP2A6 allelic variants. Different variants of the CYP2A6 3'-UTR were cloned into a modified pGL3 plasmid downstream of the luciferase reporter gene. The plasmids, having the proximal promoter of CYP2A6 gene, were transfected into HeLa cells or injected into the tail veins of male CD1 mice. In both systems, the 3'-UTR CYP2A6*1B constructs caused higher reporter gene activity and the CYP2A7 3'-UTR construct lower activity, compared to the CYP2A6*1 3'-UTR constructs. Two SNPs differentiating the 3'-UTR between CYP2A7 and CYP2A6*1B were found to be of importance for the expression in both systems. Analysis of reporter enzyme degradation in HeLa cells showed that luciferase-3'-UTR-CYP2A6*1A had a half-life of approximately 4.9h as compared to 6.3h for luciferase-3'-UTR-CYP2A6*1B. In conclusion, we identified polymorphic motifs in the CYP2A6 3'-UTR of importance for CYP2A6 mRNA stabilization and enzyme expression. Such polymorphism has been described to influence the in vivo rate of nicotine elimination and possibly the cigarette consumption and risk of smoking induced lung cancer.

CYP2A6 polymorphisms are associated with nicotine dependence and influence withdrawal symptoms in smoking cessation

CYP2A6 is the main enzyme that catalyzes nicotine into cotinine. Interindividual differences in nicotine metabolism result at least partially from polymorphic variation of CYP2A6 gene. In this study, we evaluated the influence of CYP2A6 polymorphisms on clinical phenotypes of smoking, such as smoking habit and withdrawal symptoms. Japanese smokers (n = 107) were genotyped for CYP2A6*1, *4 and *9. Consistent with the previous reports, CYP2A6 genotypes have a tendency to correlate with the number of cigarettes per day and with daily intake of nicotine. Interestingly, CYP2A6 high-activity group (CYP2A6*1/*1, *1/*9, *1/*4, *9/*9) smoked the first cigarette of the day earlier than low-activity group (CYP2A6*4/*9, *4/*4), indicating more remarkable nicotine dependence. Furthermore, nicotine withdrawal symptoms were more serious in smoking cessation in CYP2A6 high-activity group. Collectively, CYP2A6 genotypes are related with nicotine dependence, influencing smoking habits and withdrawal symptoms in quitting smoking. It is proposed that individualized smoking cessation program could be designed based on CYP2A6 genotypes.

Genetic polymorphism of CYP2A6 as one of the potential determinants of tobacco-related cancer risk

Analyzing the CYP2A6 gene of subjects who showed a poor metabolic phenotype toward SM-12502, we discovered a novel mutant allele (CYP2A6*4C) lacking the whole CYP2A6 gene. Using genetically engineered Salmonella typhimurium expressing a human CYP, we found that CYP2A6 was involved in the metabolic activation of a variety of nitrosamines such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) contained in tobacco smoke. Taking these results into consideration, we hypothesized that the subjects carrying the CYP2A6*4C allele had lower risk of tobacco-related lung cancer. In accordance with our hypothesis, our epidemiological studies indicated that smokers homozygous for the CYP2A6*4C allele showed much lower odds ratios toward cancer risk. Other mutant alleles reducing the CYP2A6 activity, besides CYP2A6*4C, also reduced the risk of lung cancer in smokers, particularly of squamous-cell carcinoma and small-cell carcinoma, both smoking-related cancers. 8-Methoxypsoralen, an inhibitor of CYP2A6, efficiently prevented the occurrence of adenoma caused by NNK in A/J mice.

Three haplotypes associated with CYP2A6 phenotypes in Caucasians

The human cytochrome P450 2A6 (CYP2A6) enzyme metabolizes several xenobiotic compounds of clinical or toxicological importance. We aimed to identify genetic variants and major CYP2A6 haplotypes associated with CYP2A6 phenotypic variation. CYP2A6 mRNA level, protein level, activity and haplotypes were determined in Caucasian liver samples via real-time polymerase chain reaction, Western blot, coumarin 7-hydroxylation, DNA sequencing and genotyping, respectively. Phenotypes were then analyzed for associations with haplotypes. CYP2A6 transcript, protein and activity levels were correlated among each other. In 45 African-American, 156 Caucasian, 47 Chinese, 50 Japanese and 47 Korean DNA samples, we detected 95 different polymorphisms in the CYP2A6 gene, 49 of which had not been described previously. Caucasian variants formed 33 haplotypes which built four clades. Allele *9B and the CYP2A7/2A6 partial deletion allele CYP2A6*12B were both associated with decreased expression. The latter haplotype extends at least over 147 kb up into the CYP2B6 gene. A haplotype almost identical to allele *1A was associated with decreased expression and activity of CYP2A6 compared to all other haplotypes. In summary A CYP2A6*1A-like allele, *9B and *12B are major genetic determinants of CYP2A6 phenotype variation in Caucasians.

Genetic polymorphisms of cytochrome P450 in patients with hepatitis C virus-associated hepatocellular carcinoma

BACKGROUND

The carcinogenic process can be modulated by exposure to endogenous or environmental substance(s) acting as carcinogens or protocarcinogens. Polymorphic enzymes of cytochrome P450 (CYP) that play a role in detoxication/toxication of such substances via metabolization may account for the interpatient variability of clinical course in cancers such as hepatocellular carcinoma (HCC). Many CYP genetic polymorphisms, which may change enzyme activity, are known to exist in Japanese. The aim of the present study was to compare the frequencies of CYP polymorphisms between hepatitis C virus (HCV)-related HCC patients and healthy subjects.

METHODS

Seven mutant alleles and related genotypes of CYP in 44 HCV-positive HCC patients were chosen as follows: *1C heterozygous, *1C homozygous and *1F homozygous for CYP1A2, *4A homozygous for CYP2A6, *2A or *3 heterozygous, *2A or *3 homozygous and *2A and *3 heterozygous for CYP2C19, and *10/*5 homozygous for CYP2D6. These mutant alleles have been reported to change the CYP enzyme activity in Japanese. The frequencies of the mutant alleles and genotypes were then compared with those reported in healthy Japanese.

RESULTS AND CONCLUSION

There is no statistically significant difference in genetic mutant alleles between the two groups, except for the genotype of CYP2A6*4A homozygous. The frequency of this genotype in the HCC patients (0.144) is significantly higher than that in healthy Japanese (0.034; P < 0.05; odds ratio 3.36). The clinical significance related to HCC is unknown. Further evaluation of CYP2A6*4A (deletion type) in HCV-related HCC patients is required.

CYP2A6 IS A PRINCIPAL ENZYME INVOLVED IN HYDROXYLATION OF 1,7-DIMETHYLXANTHINE, A MAIN CAFFEINE METABOLITE, IN HUMANS

In a caffeine test previously performed with healthy Japanese volunteers, we found that the CYP1A2 index defined as urinary {5-acetylamino-6-amine-3-methyluracil (AAMU)+1-methylxanthine (1X)+1-methyluric acid (1 U)}/1,7-dimethyluric acid (17 U) was affected by the whole deleted allele of CYP2A6 (CYP2A6*4). Since the high value of the CYP1A2 index could be caused by a low urinary concentration of 17 U, we postulated that CYP2A6 was responsible for the 1,7-dimethylxanthine (17 X) metabolism to generate 17 U (17 X 8-hydroxylation). Thus, the role of CYP2A6 in the 17 X 8-hydroxylation was fully examined in the present study. Among 10 isoforms of human cytochrome P450 (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, or CYP3A5) expressed in Escherichia coli cells, CYP2A6 and CYP1A2 showed high catalytic activities for the 17 X 8-hydroxylation. The 17 X 8-hydroxylase activities significantly associated with coumarin 7-hydroxylase activities (r=0.67, p<0.01) in liver microsomes from 17 individuals, but not with ethoxyresorufin O-deethylase activities. Tranylcypromine, an inhibitor of CYP2A6, reduced the 17 X 8-hydroxylase activities of human liver microsomes. The 17 X 8-hydroxylase activities of CYP2A6.7, CYP2A6.10, and CYP2A6.11 expressed in E. coli cells were 12, 13, and 22% of that of CYP2A6.1, respectively. The 17 X 8-hydroxylase activities were found to be low in liver microsomes from individuals possessing the deletion or mutations in the CYP2A6 gene. Based on these data, we conclude that CYP2A6 is a main 17 X 8-hydroxylase and that the catalytic activities for the 17 X 8-hydroxylation are reduced by the genetic polymorphisms of the CYP2A6 gene.

Association of CYP2A6*1B genetic variant with the amount of smoking in French adults from the Stanislas cohort

This study was designed in order to investigate the influence of the genetic polymorphism of CYP2A6 on the amount of smoking. In all, 463 French adults included in the Stanislas cohort were studied and underwent two examinations at 5 years distance (t0 and t(+5) years). Information on their smoking habits was collected. They were genotyped by RFLP for the CYP2A6*1A, CYP2A6*1B and CYP2A6*4 alleles. CYP2A6*1B and CYP2A6*4 allele frequencies were 32 and 4%, respectively. The subjects carrying the CYP2A6*1B allele oxidize nicotine to cotinine faster than subjects with the CYP2A6*1A allele. The number of cigarettes smoked per day was significantly higher in the CYP2A6*1B/*1B group as compared to the CYP2A6*1A/*1A group (P = 0.01 at t0; P = 0.001 t(+5) years), with a larger increase in their daily cigarettes consumption over the 5-year period (P = 0.006). No significant difference in the smoking status was observed according to the CYP2A6 genotype. These data suggest that the CYP2A6*1B is associated with the number of cigarettes smoked per day.

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.

Metabolism and Disposition Kinetics of Nicotine

Nicotine is of importance as the addictive chemical in tobacco, pharmacotherapy for smoking cessation, a potential medication for several diseases, and a useful probe drug for phenotyping cytochrome P450 2A6 (CYP2A6). We review current knowledge about the metabolism and disposition kinetics of nicotine, some other naturally occurring tobacco alkaloids, and nicotine analogs that are under development as potential therapeutic agents. The focus is on studies in humans, but animal data are mentioned when relevant to the interpretation of human data. The pathways of nicotine metabolism are described in detail. Absorption, distribution, metabolism, and excretion of nicotine and related compounds are reviewed. Enzymes involved in nicotine metabolism including cytochrome P450 enzymes, aldehyde oxidase, flavin-containing monooxygenase 3, amine N-methyltransferase, and UDP-glucuronosyltransferases are represented, as well as factors affecting metabolism, such as genetic variations in metabolic enzymes, effects of diet, age, gender, pregnancy, liver and kidney diseases, and racial and ethnic differences. Also effects of smoking and various inhibitors and inducers, including oral contraceptives, on nicotine metabolism are discussed. Due to the significance of the CYP2A6 enzyme in nicotine clearance, special emphasis is given to the effects and population distributions of CYP2A6 alleles and the regulation of CYP2A6 enzyme.

Metabolic profile of nicotine in subjects whose CYP2A6 gene is deleted

Generally, 70-80% of absorbed nicotine is mainly metabolized to cotinine by cytochrome P450 (CYP) 2A6. There is genetic polymorphism in the human CYP2A6 gene. Among several mutated alleles, CYP2A6*4 allele is a whole deleted type. The purpose of the present study was to clarify the metabolic profile of nicotine in subjects whose CYP2A6 gene is deleted. We developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for nicotine and its nine metabolites. Excretion levels of nicotine and its metabolites in 24 h accumulated urine after the chewing of one piece of nicotine gum were evaluated in five Japanese subjects whose CYP2A6 genotype was determined. In three subjects with CYP2A6*1A/CYP2A6*1A, CYP2A6*1A/CYP2A6*1B, and CYP2A6*1A/CYP2A6*4 (group I), nicotine was mainly excreted as cotinine, trans-3'-hydroxycotinine, and their glucuronide (approximately 60%). In contrast, in two subjects with CYP2A6*4/CYP2A6*4 (group II), trace levels of cotinine, cotinine N-glucuronide, and cotinine 1'-N-oxide were detected. Trans-3'-hydroxycotinine and its O-glucuronide were not detected. The excretion levels of nicotine itself, nicotine N-glucuronide, and nicotine 1'-N-oxide were higher than those in the other three subjects. The total excretion levels of these three compounds were approximately 95% in group II versus 35% in group I. However, the sum of the excretion levels of nicotine and all metabolites was similar among these five subjects. This is the first report of the metabolic profile of nicotine in subjects whose CYP2A6 gene is deleted.

High prevalence of cytochrome P450 2A6*1A alleles in a black African population of Ghana

OBJECTIVE

We investigated the frequencies of the functionally important variants of the CYP2A6 gene in black African populations.

METHODS

Using genomic DNA sequencing, polymerase chain reaction (PCR)-restriction fragment length polymorphism and allele-specific PCR, the allele frequencies of CYP2A6 *1A, *1B, *2, *4A, *5, *6, *7, *8, *9, *10 and * 11 among 120 black Africans- including 105 Ghanaians, 12 Nigerians, 2 Ivorians and 1 Ugandan-were determined.

RESULTS

The allele frequencies were 80.5% for CYP2A6*1A, 11.9% for CYP2A6*1B, 1.9% for CYP2A6*4A and 5.7% for CYP2A6*9 in the Ghanaian subjects. No subject homozygous for the CYP2A6*4A allele, a whole gene deletion type of polymorphism prevalent among Orientals, was found. Furthermore, CYP2A6 variants such as *2, *5, *6, *7, *8, *10 and *11 were absent in these black African populations.

CONCLUSIONS

This study provides, for the first time, the results of the analysis of CYP2A6 allele frequency in black African populations and confirms large ethnic differences in the polymorphic CYP2A6 gene.

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.

Interindividual Variability in Nicotine Metabolism: C-Oxidation and Glucuronidation

Nicotine has roles in the addiction to smoking, replacement therapy for smoking cessation, as a potential medication for several diseases such as Parkinson's disease, Alzheimer's disease, and ulcerative colitis. The absorbed nicotine is rapidly and extensively metabolized and eliminated to urine. A major pathway of nicotine metabolism is C-oxidation to cotinine, which is catalyzed by CYP2A6 in human livers. Cotinine is subsequently metabolized to trans-3'-hydroxycotinine by CYP2A6. Nicotine and cotinine are glucuronidated to N-glucuronides mainly by UGT1A4 and partly by UGT1A9. Trans-3'-hydroxycotinine is glucuronidated to O-glucuronide mainly by UGT2B7 and partly by UGT1A9. Approximately 90% of the total nicotine uptake is eliminated as these metabolites and nicotine itself. The nicotine metabolism is an important determinant of the clearance of nicotine. Recently, advances in the understanding of the interindividual variability in nicotine metabolism have been made. There are substantial data suggesting that the large interindividual differences in cotinine formation are associated with genetic polymorphisms of the CYP2A6 gene. Interethnic differences have also been observed in the cotinine formation and the allele frequencies of the CYP2A6 alleles. Since the genetic polymorphisms of the CYP2A6 gene have a major impact on nicotine clearance, its relationships with smoking behavior or the risk of lung cancer have been suggested. The metabolic pathways of the glucuronidation of nicotine, cotinine, and trans-3'-hydroxycotinine in humans would be one of the causal factors for the interindividual differences in nicotine metabolism. This review mainly summarizes recent results from our studies.

Novel human CYP2A6 alleles confound gene deletion analysis

Cytochrome P450 (CYP) 2A6 metabolizes a number of drugs and a variety of procarcinogens. CYP2A6 also catalyzes nicotine C-oxidation leading to cotinine formation, a major metabolic pathway of nicotine in humans. There are genetic polymorphisms in the human CYP2A6 gene and a relationship between the CYP2A6 genotype and smoking habits as well as the incidence of lung cancer has been indicated. CYP2A6*4 alleles are the whole deleted type and are completely deficient in the enzymatic activity. An unequal crossover junction is located in the 3'-flanking region in the CYP2A6*4A allele, whereas the junction is located in either intron 8 or exon 9 in the CYP2A6*4D allele. In the present study, a novel genotyping method to distinguish between two different whole deleted alleles of CYP2A6*4A and CYP2A6*4D was established. In the process, two novel alleles, CYP2A6*1F and CYP2A6*1G, were found. The CYP2A6*1F has a single nucleotide polymorphism (SNP) of C5717T in exon 8, and the CYP2A6*1G has two SNPs, C5717T in exon 8 and A5825G in intron 8. The SNP of C5717T corresponds to C1224T on the cDNA sequence and is a synonymous mutation. Since the CYP2A6*1F produces a recognition site of the restriction enzymes that is the same as CYP2A6*4D, the presence of the CYP2A6*1F allele could cause a mistyping as the CYP2A6*4D allele. According to an improved genotyping method, the allele frequencies of CYP2A6*4A, CYP2A6*4D, CYP2A6*1F, and CYP2A6*1G in 165 Caucasians were 3.0%, 0%, 1.8%, and 1.2%, respectively. The allele frequencies of CYP2A6*4A, CYP2A6*4D, CYP2A6*1F, and CYP2A6*1G in 94 African-Americans were 0%, 0.5%, 0%, and 13.3%, respectively. This is the first report of a method that can distinguish between CYP2A6*4A, CYP2A6*4D, and CYP2A6*1F which could otherwise cause a mistyping as CYP2A6*4D.

Whole deletion of CYP2A6 gene (CYP2A6AST;4C) and smoking behavior

The relationship between nicotine metabolism of CYP2A6 and the smoking behavior in a Japanese population was investigated. The CYP2A6 genotypes were determined by the PCR method. There was a significant difference in the frequency of the CYP2A6AST;4C allele, which is a whole deleted allele for the human CYP2A6 gene, between smokers and nonsmokers. The frequency of the CYP2A6AST;4C allele was higher in nonsmokers than in smokers, whereas the frequency of CYP2A6AST;1A/AST;1B heterozygotes with a higher activity of nicotine metabolism was lower. In this study it was suggested that the CYP2A6AST;4C allele may prevent the carrier from smoking and the CYP2A6AST;1A/AST;1B heterozygote may be at risk for developing smoking behavior.

A nicotine C-oxidase gene (CYP2A6) polymorphism important for promoter activity

In humans, several polymorphic variants have been described for the gene encoding the major nicotine C-oxidase, cytochrome P450 2A6 (CYP2A6), which is to a great extent responsible for the large interindividual differences seen at the enzymatic and activity levels. Hitherto, mainly polymorphic variants in the open reading frame have been identified. In the present study, we identified a novel single nucleotide polymorphism (SNP) located in the 5' flanking region of the CYP2A6 gene. Sequencing of 1.4 kb of the 5'-upstream region of the CYP2A6 gene from eight individuals revealed a c.-1013A>G polymorphism defining two new alleles, CYP2A6*1D and CYP2A6*1E, lacking or having also the CYP2A7 3'-UTR. Analysis of genomic DNA from 32 Swedish and 109 Turkish subjects by dynamic allele-specific hybridization (DASH) showed that, in both groups, the variants carrying the c.-1013A>G SNP represent approximately 70% of the total number of alleles. Transfection of HepG2 cells with luciferase reporter constructs containing 1019 bp of the CYP2A6 5'-regulatory sequence showed that the region between c.-1005 and c.-1019 elicited a strong enhancer effect and that the CYP2A6*1D promoter had significantly reduced expression as compared to CYP2A6*1A carrying c.-1013A. Electrophoretic mobility shift assays (EMSA) showed that nuclear proteins from HepG2 and B16A2 cells exhibited a higher binding affinity to the probe harboring c.-1013A as compared to the c.-1013G probe, although the transcription factor(s) responsible for this binding could not be identified. In conclusion, our results indicate the presence of a strong enhancer or promoter responsive element between c.-1005 and c.-1019 in the CYP2A6 gene and that a c.-1013A>G polymorphism in this region affects CYP2A6 transcription.