CYP2A6 reduced activity gene variants confer reduction in lung cancer risk in African American smokers--findings from two independent populations.

We investigated genetic variation in CYP2A6 in relation to lung cancer risk among African American smokers, a high-risk population. Previously, we found that CYP2A6, a nicotine/nitrosamine metabolism gene, was associated with lung cancer risk in European Americans, but smoking habits, lung cancer risk and CYP2A6 gene variants differ significantly between European and African ancestry populations. Herein, African American ever-smokers, drawn from two independent lung cancer case-control studies, were genotyped for reduced activity CYP2A6 alleles and grouped by predicted metabolic activity. Lung cancer risk in the Southern Community Cohort Study (n = 494) was lower among CYP2A6 reduced versus normal metabolizers, as estimated by multivariate conditional logistic regression [odds ratio (OR) = 0.44; 95% confidence interval (CI) = 0.26-0.73] and by unconditional logistic regression (OR = 0.62; 95% CI = 0.41-0.94). The association was replicated in an independent study from MD Anderson Cancer Center (n = 407) (OR = 0.64; 95% CI = 0.42-0.98), and pooling the studies yielded an OR of 0.64 (95% CI = 0.48-0.86). Exploratory analyses revealed a significant interaction between CYP2A6 genotype and sex on the risk for lung cancer (Southern Community Cohort Study: P = 0.04; MD Anderson: P = 0.03; Pooled studies: P = 0.002) with a CYP2A6 effect in men only. These findings support a contribution of genetic variation in CYP2A6 to lung cancer risk among African American smokers, particularly men, whereby CYP2A6 genotypes associated with reduced metabolic activity confer a lower risk of developing lung cancer.

Pilot study of CYP2B6 genetic variation to explore the contribution of nitrosamine activation to lung carcinogenesis.

We explored the contribution of nitrosamine metabolism to lung cancer in a pilot investigation of genetic variation in CYP2B6, a high-affinity enzymatic activator of tobacco-specific nitrosamines with a negligible role in nicotine metabolism. Previously we found that variation in CYP2A6 and CHRNA5-CHRNA3-CHRNB4 combined to increase lung cancer risk in a case-control study in European American ever-smokers (n = 860). However, these genes are involved in the pharmacology of both nicotine, through which they alter smoking behaviours, and carcinogenic nitrosamines. Herein, we separated participants by CYP2B6 genotype into a high- vs. low-risk group (*1/*1 + *1/*6 vs. *6/*6). Odds ratios estimated through logistic regression modeling were 1.25 (95% CI 0.68-2.30), 1.27 (95% CI 0.89-1.79) and 1.56 (95% CI 1.04-2.31) for CYP2B6, CYP2A6 and CHRNA5-CHRNA3-CHRNB4, respectively, with negligible differences when all genes were evaluated concurrently. Modeling the combined impact of high-risk genotypes yielded odds ratios that rose from 2.05 (95% CI 0.39-10.9) to 2.43 (95% CI 0.47-12.7) to 3.94 (95% CI 0.72-21.5) for those with 1, 2 and 3 vs. 0 high-risk genotypes, respectively. Findings from this pilot point to genetic variation in CYP2B6 as a lung cancer risk factor supporting a role for nitrosamine metabolic activation in the molecular mechanism of lung carcinogenesis.

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.

Genome-Wide Meta-Analysis of Cotinine Levels in Cigarette Smokers Identifies Locus at 4q13.2.

Genome-wide association studies (GWAS) of complex behavioural phenotypes such as cigarette smoking typically employ self-report phenotypes. However, precise biomarker phenotypes may afford greater statistical power and identify novel variants. Here we report the results of a GWAS meta-analysis of levels of cotinine, the primary metabolite of nicotine, in 4,548 daily smokers of European ancestry. We identified a locus close to UGT2B10 at 4q13.2 (minimum p = 5.89 × 10(-10) for rs114612145), which was consequently replicated. This variant is in high linkage disequilibrium with a known functional variant in the UGT2B10 gene which is associated with reduced nicotine and cotinine glucuronidation activity, but intriguingly is not associated with nicotine intake. Additionally, we observed association between multiple variants within the 15q25.1 region and cotinine levels, all located within the CHRNA5-A3-B4 gene cluster or adjacent genes, consistent with previous much larger GWAS using self-report measures of smoking quantity. These results clearly illustrate the increase in power afforded by using precise biomarker measures in GWAS. Perhaps more importantly however, they also highlight that biomarkers do not always mark the phenotype of interest. The use of metabolite data as a proxy for environmental exposures should be carefully considered in the context of individual differences in metabolic pathways.

UGT1A and UGT2B genetic variation alters nicotine and nitrosamine glucuronidation in european and african american smokers.

BACKGROUND: Identifying sources of variation in the nicotine and nitrosamine metabolic inactivation pathways is important to understanding the relationship between smoking and cancer risk. Numerous UGT1A and UGT2B enzymes are implicated in nicotine and nitrosamine metabolism in vitro; however, little is known about their roles in vivo. METHODS: Within UGT1A1, UGT1A4, UGT1A9, UGT2B7, UGT2B10, and UGT2B17, 47 variants were genotyped, including UGT2B10*2 and UGT2B17*2. The association between variation in these UGTs and glucuronidation activity within European and African American current smokers (n = 128), quantified as urinary ratios of the glucuronide over unconjugated compound for nicotine, cotinine, trans-3'-hydroxycotinine, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), was investigated in regression models assuming a dominant effect of variant alleles. RESULTS: Correcting for multiple testing, three UGT2B10 variants were associated with cotinine glucuronidation, rs2331559 and rs11726322 in European Americans and rs835309 in African Americans (P ≤ 0.0002). Additional variants predominantly in UGT2B10 were nominally associated with nicotine (P = 0.008-0.04) and cotinine (P = <0.001-0.02) glucuronidation in both ethnicities in addition to UGT2B10*2 in European Americans (P = 0.01, P < 0.001). UGT2B17*2 (P = 0.03) in European Americans and UGT2B7 variants (P = 0.02-0.04) in African Americans were nominally associated with 3HC glucuronidation. UGT1A (P = 0.007-0.01), UGT2B10 (P = 0.02), and UGT2B7 (P = 0.02-0.03) variants in African Americans were nominally associated with NNAL glucuronidation. CONCLUSIONS: Findings from this initial in vivo study support a role for multiple UGTs in the glucuronidation of tobacco-related compounds in vivo, in particular UGT2B10 and cotinine glucuronidation. IMPACT: Findings also provide insight into ethnic differences in glucuronidation activity, which could be contributing to ethnic disparities in the risk for smoking-related cancers. Cancer Epidemiol Biomarkers Prev; 24(1); 94-104. ©2014 AACR.

Relationship between CYP2A6 and CHRNA5-CHRNA3-CHRNB4 variation and smoking behaviors and lung cancer risk.

Genetic variations in the CYP2A6 nicotine metabolic gene and the CHRNA5-CHRNA3-CHRNB4 (CHRNA5-A3-B4) nicotinic gene cluster have been independently associated with lung cancer. With genotype data from ever-smokers of European ancestry (417 lung cancer patients and 443 control subjects), we investigated the relative and combined associations of polymorphisms in these two genes with smoking behavior and lung cancer risk. Kruskal-Wallis tests were used to compare smoking variables among the different genotype groups, and odds ratios (ORs) for cancer risk were estimated using logistic regression analysis. All statistical tests were two-sided. Cigarette consumption (P < .001) and nicotine dependence (P = .036) were the highest in the combined CYP2A6 normal metabolizers and CHRNA5-A3-B4 AA (tag single-nucleotide polymorphism rs1051730 G>A) risk group. The combined risk group also exhibited the greatest lung cancer risk (OR = 2.03; 95% confidence interval [CI] = 1.21 to 3.40), which was even higher among those who smoked 20 or fewer cigarettes per day (OR = 3.03; 95% CI = 1.38 to 6.66). Variation in CYP2A6 and CHRNA5-A3-B4 was independently and additively associated with increased cigarette consumption, nicotine dependence, and lung cancer risk. CYP2A6 and CHRNA5-A3-B4 appear to be more strongly associated with smoking behaviors and lung cancer risk, respectively.