Analyses of nicotine metabolism biomarker genetics stratified by sex in African and European Americans

Association of Urinary Biomarkers of Tobacco Exposure with Lung Cancer Risk in African American and White Cigarette Smokers in the Southern Community Cohort Study

BACKGROUND

After accounting for smoking history, lung cancer incidence is greater in African Americans than Whites. In the multiethnic cohort, total nicotine equivalents (TNE) are higher in African Americans than Whites at similar reported cigarettes per day. Greater toxicant uptake per cigarette may contribute to the greater lung cancer risk of African Americans.

METHODS

In a nested case-control lung cancer study within the Southern Community Cohort, smoking-related biomarkers were measured in 259 cases and 503 controls (40% White; 56% African American). TNE, the trans-3-hydroxycotinine/cotinine ratio, 4-(methylnitrosamino)-1-3-(pyridyl)-1-butanol (NNAL), mercapturic acid metabolites of volatile organic compounds, phenanthrene metabolites, cadmium (Cd), and (Z)-7-(1R,2R,3R,5S)-3,5-dihydroxy-2-[(E,3S)-3-hydroxyoct-1-enyl]cyclopenyl]hept-5-enoic acid were quantified in urine. Unconditional logistic regression was used to estimate the ORs and 95% confidence intervals (CI) for each biomarker and lung cancer risk.

RESULTS

TNE, NNAL, and Cd were higher in cases than controls (adjusted for age, race, sex, body mass index, and cigarettes per day). Among cases, these levels were higher in African Americans compared with Whites. After accounting for age, sex, body mass index, and pack-years, a one-SD increase in log-TNE (OR = 1.30; 95% CI, 1.10-1.54) and log-NNAL (OR = 1.27; 95% CI, 1.03-1.58 with TNE adjustment) was associated with lung cancer risk. In this study, in which NNAL concentration is relatively high, the association for log-TNE was attenuated after adjustment for log-NNAL.

CONCLUSIONS

Smoking-related biomarkers provide additional information for lung cancer risk in smokers beyond smoking pack-years.

IMPACT

Urinary NNAL, TNE, and Cd concentrations in current smokers, particularly African American smokers, may be useful for predicting lung cancer risk.

Genetic Prediction of Smoking Cessation Medication Side Effects: A Genome-Wide Investigation of Abnormal Dreams on Varenicline

Varenicline, the most efficacious smoking cessation monotherapy, produces abnormal dreams. Although genetic contributions to varenicline-associated nausea and cessation have been identified, the role of genetics in abnormal dreams is unknown. We conducted a genomewide association study (GWAS) of abnormal dreams in 188 European ancestry smokers treated with varenicline (NCT01314001). Additive genetic models examined the likelihood of experiencing abnormal dreams 2 weeks following varenicline initiation. For the top locus, we tested for selectivity to varenicline, effects on cessation, replication, and generalizability to African ancestry (AA) individuals. The top GWAS variant associated with abnormal dreams was rs901886, mapping to intron 2 of ICAM5 on chromosome 19. The prevalence of abnormal dreams in those with rs901886 CC, CT, and TT genotypes was 15%, 36%, and 62%, respectively (odds ratio = 2.94 for T vs. C, 95% confidence interval = 1.92-4.55, P = 2.03e-7; T allele frequency = 52%). This rs901886 association was selective to varenicline (P values > 0.05 on nicotine patch and placebo). There were also positive associations for rs901886 T (vs. C allele, P = 0.03) and for abnormal dreams (P = 0.06) with varenicline-aided cessation. Neither rs901886 (P = 0.40) nor abnormal dreams (P = 0.24) were associated with adherence. A similar direction of effect of rs901886 on abnormal dreams was observed in a second varenicline trial (NCT01836276). In AA individuals (n = 137), rs901886 was not associated with abnormal dreams (P = 0.41), but there was an association for a variant located ~ 74.4 kb 5' of ICAM5 (P = 2.56e-3). Variation in ICAM5 may influence abnormal dreams and cessation on varenicline. These findings provide additional support for genetically optimized smoking cessation approaches.

Sex Differences in Lung Cancer

Sex disparities in the incidence and mortality of lung cancer have been observed since cancer statistics have been recorded. Social and economic differences contribute to sex disparities in lung cancer incidence and mortality, but evidence suggests that there are also underlying biological differences that contribute to the disparity. This review summarizes biological differences which could contribute to the sex disparity. Sex hormones and other biologically active molecules, tumor cell genetic differences, and differences in the immune system and its response to lung cancer are highlighted. How some of these differences contribute to disparities in the response to therapies, including cytotoxic, targeted, and immuno-therapies, is also discussed. We end the study with a discussion of our perceived future directions to identify the key biological differences which could contribute to sex disparities in lung cancer and how these differences could be therapeutically leveraged to personalize lung cancer treatment to the individual sexes.

Influence of CYP2A6 Genetic Variation, Nicotine Dependence Severity, and Treatment on Smoking Cessation Success

INTRODUCTION

Genetic variation in Cytochrome P450 2A6 (CYP2A6), the major nicotine metabolizing enzyme, is associated with nicotine dependence and smoking cessation. Nicotine dependence severity also predicts smoking cessation. Our goals were to determine how CYP2A6 variation and nicotine dependence alter smoking cessation, and whether dependence could refine CYP2A6-based treatment recommendations.

AIMS AND METHODS

Adult smokers treated for 12 weeks with placebo, nicotine patch, or varenicline (NCT01314001) were grouped as CYP2A6 normal (n = 567) or slow (n = 432) nicotine metabolizers based on a CYP2A6 weighted genetic risk score. Fagerström test for nicotine dependence scores were measured at baseline and biochemically verified smoking cessation was assessed at end of treatment.

RESULTS

Dependence neither mediated nor moderated an association between CYP2A6 variation and smoking cessation overall, within any treatment arm, or after stratifying by ancestry (n = 591 European, n = 408 African ancestry) or sex (n = 444 women, n = 555 men). In within-treatment analyses, the mediation effect odds ratio (OR) ranged from 0.95 to 1.00 and the bias-corrected 95% confidence interval contained 1. Moderation (i.e. interaction) effect ORs ranged from 0.88 to 1.61 (p = .397-.828). For CYP2A6 normal metabolizers, quit rates on varenicline were similar for those with high (41.1%) and low (43.4%) dependence, while quit rates were lower for those with high versus low dependence on both patch (16.5 vs. 29.7%) and placebo (8.9 vs. 18.5%). CYP2A6 slow metabolizers with high versus low dependence had lower quit rates in all three treatment arms.

CONCLUSIONS

Although nicotine dependence severity neither mediated nor moderated CYP2A6 associations with smoking cessation, incorporating information on dependence may optimize the choice of smoking cessation treatment aid in CYP2A6 normal and slow metabolizers.

IMPLICATIONS

Variation in CYP2A6 and nicotine dependence severity alter smoking cessation success. Our findings suggest that while nicotine dependence severity is unlikely to mediate or moderate CYP2A6 associations with cessation, incorporating patient information on both CYP2A6 and nicotine dependence severity may lead to improved smoking cessation strategies.

The Use of Biomarkers to Guide Precision Treatment for Tobacco Use

This review summarizes the evidence to date on the development of biomarkers for personalizing the pharmacological treatment of combustible tobacco use. First, the latest evidence on FDA-approved medications is considered, demonstrating that, while these medications offer real benefits, they do not contribute to smoking cessation in approximately two-thirds of cases. Second, the case for using biomarkers to guide tobacco treatment is made based on the potential to increase medication effectiveness and uptake and reduce side effects. Next, the FDA framework of biomarker development is presented along with the state of science on biomarkers for tobacco treatment, including a review of the nicotine metabolite ratio, electroencephalographic event-related potentials, and other biomarkers utilized for risk feedback. We conclude with a discussion of the challenges and opportunities for the translation of biomarkers to guide tobacco treatment and propose priorities for future research.

Predicting nicotine metabolism across ancestries using genotypes

BACKGROUND

There is a need to match characteristics of tobacco users with cessation treatments and risks of tobacco attributable diseases such as lung cancer. The rate in which the body metabolizes nicotine has proven an important predictor of these outcomes. Nicotine metabolism is primarily catalyzed by the enzyme cytochrone P450 (CYP2A6) and CYP2A6 activity can be measured as the ratio of two nicotine metabolites: trans-3'-hydroxycotinine to cotinine (NMR). Measurements of these metabolites are only possible in current tobacco users and vary by biofluid source, timing of collection, and protocols; unfortunately, this has limited their use in clinical practice. The NMR depends highly on genetic variation near CYP2A6 on chromosome 19 as well as ancestry, environmental, and other genetic factors. Thus, we aimed to develop prediction models of nicotine metabolism using genotypes and basic individual characteristics (age, gender, height, and weight).

RESULTS

We identified four multiethnic studies with nicotine metabolites and DNA samples. We constructed a 263 marker panel from filtering genome-wide association scans of the NMR in each study. We then applied seven machine learning techniques to train models of nicotine metabolism on the largest and most ancestrally diverse dataset (N=2239). The models were then validated using the other three studies (total N=1415). Using cross-validation, we found the correlations between the observed and predicted NMR ranged from 0.69 to 0.97 depending on the model. When predictions were averaged in an ensemble model, the correlation was 0.81. The ensemble model generalizes well in the validation studies across ancestries, despite differences in the measurements of NMR between studies, with correlations of: 0.52 for African ancestry, 0.61 for Asian ancestry, and 0.46 for European ancestry. The most influential predictors of NMR identified in more than two models were rs56113850, rs11878604, and 21 other genetic variants near CYP2A6 as well as age and ancestry.

CONCLUSIONS

We have developed an ensemble of seven models for predicting the NMR across ancestries from genotypes and age, gender and BMI. These models were validated using three datasets and associate with nicotine dosages. The knowledge of how an individual metabolizes nicotine could be used to help select the optimal path to reducing or quitting tobacco use, as well as, evaluating risks of tobacco use.

"Genetic influences impacting nicotine use and abuse during adolescence: insights from human and rodent studies"

Nicotine use continues to be a major public health concern, with an alarming recent rise in electronic cigarette consumption. Heritability estimates of nicotine use and abuse range from 40-80%, providing strong evidence that genetic factors impact nicotine addiction-relevant phenotypes. Although nicotine use during adolescence is a key factor in the development of addiction, it remains unclear how genetic factors impact adolescent nicotine use and abuse. This review will discuss studies investigating genetic factors impacting nicotine use during adolescence. Evidence from both rodent and human studies will be summarized and integrated when possible. Human adolescent studies have largely included candidate gene studies for genes identified in adult populations, such as genes involved in nicotine metabolism, nicotinic acetylcholine receptor signaling, dopaminergic signaling, and other neurotransmitter signaling systems. Alternatively, rodent studies have largely taken a discovery-based approach identifying strain differences in adolescent nicotine addiction-relevant behaviors. Here, we aim to answer the following three questions by integrating human and rodent findings: 1) Are there genetic variants that uniquely impact nicotine use during adolescence? 2) Are there genetic variants that impact both adolescent and adult nicotine use? and 3) Do genetic factors in adolescence significantly impact long-term consequences of adolescent nicotine use? Determining answers for these three questions will be critical for the development of preventative measures and treatments for adolescent nicotine use and addiction.

Examining the role of mitochondrial genetic variation in nicotine dependence

Nicotine dependence (ND) has a heritability rate of ∼50%, suggesting genetic factors contribute to underlying mechanisms. Here, we aimed to examine variants within both mtDNA and the nuclear genome to determine if mitochondrial genes are associated with ND. A total of 129 mtDNA SNPs and 1136 nuclear-encoded mitochondrial genes in a sample of N = 374 Caucasians were selected for analysis. Age of onset of first, occasional, and daily smoking and Fagerström Test for Nicotine Dependence were used as outcomes for the analysis. Linear regression was used to test common variants. Gene analyses were performed using MAGMA. One nuclear mitochondrial SNP, rs78417112 found in the HSD17B4 gene, was significantly associated with the age of onset of occasional smoking. Additionally, one nuclear mitochondrial gene, PRKACA, was significantly associated with age of onset of both first and occasional smoking. Replication testing of the mtDNA m.1700T>C SNP, nominally associated with age of onset of daily smoking, was available in the PNAT2 clinical trial (N = 930 Caucasians). A meta-analysis showed this SNP was associated with age of onset of daily smoking (p-value = 0.004). Overall, the findings suggest mitochondrial genetic variation may contribute to variability in smoking phenotypes, although replication in larger samples is required.