Discussion
In this study, we described the relationships among nicotine dependence, a proven genetic susceptibility locus for nicotine dependence and COPD, and structural measures of COPD, including severity of emphysema and air-trapping on chest CT in COPD and non-COPD smoking controls. Although recently there was a report on the association of a SNP in the nAChR gene with emphysema severity [
18], to our knowledge, this is the first analysis of the relationship among these three conditions in current smokers. Compatible with previous reports, FTND scores in our study population were correlated with the cumulative intensity of smoking in pack-years, daily amount of smoking, and younger age of smoking initiation [
27,
28]. In addition, the cumulative intensity of smoking in pack-years was correlated with lower lung function and emphysema. Nevertheless, contrary to our initial hypothesis, FTND score was negatively correlated with emphysema severity in both COPD and control subjects. In addition, FTND score decreased as COPD severity, assessed by GOLD stage, increased. We observed significant associations of rs8034191 and rs1051730 in the CHRNA3/5 locus with FTND score, but we found differential evidence for association of SNPs related to nicotine dependence with emphysema severity according to current smoking status.
Although nicotine was reported to inhibit apoptosis through the muscarinic acetylcholine receptor in some cell lines [
29‐
31], any beneficial effect of current smoking on emphysema is extremely unlikely. There are several possible explanations for our finding of less quantitative radiographic emphysema and less severe COPD in subjects with greater nicotine addiction. The first potential explanation is selection bias in the study population, namely the 'healthy smoker effect' and/or 'survivor effect'. Cigarette smoking causes functional impairments such as troublesome sputum and cough, dyspnea, decrease in exercise capacity, and increased risk of mortality in subjects with COPD. These negative effects provide incentive to quit smoking, which is greater in more severely affected individuals. Since FTND score could be measured only in current smokers, affected subjects who have quit smoking are not included in our analysis. The development of COPD and the progression to more severe COPD could also lead to a reduction in daily smoking intensity among those individuals that continue to smoke; since the number of cigarettes currently smoked per day is part of the FTND, efforts to taper smoking could appear as a reduction in nicotine dependence. The observation that COPD subjects had greater average cigarettes smoked per day but similar current cigarettes smoked per day compared to control subjects (Table
1) suggests that some reduction in smoking intensity among COPD subjects has occurred with disease development. Since the daily cigarette usage is only one component of the FTND score, however, the impact of reducing cigarettes smoked per day on our results is uncertain.
Premature death may also lead to the exclusion of advanced COPD cases with severe emphysema or gas trapping from this study. This possibility is supported by the fact that the proportion of advanced stages of COPD was relatively low in this study population, and ex-smokers showed more severe obstruction on spirometry (Additional file
1; Table S2). The FTND may not reflect every aspect of nicotine dependence [
32] and does likely have limitations in assessing nicotine dependence cross-sectionally in a smoking group of COPD subjects. Nevertheless, FTND has been validated for its usefulness in general population samples [
20,
21,
32] and is widely used in studies including COPD populations [
33,
34]. Furthermore, in this study, the negative correlation between FTND and emphysema severity was found even in controls without airflow obstruction and in cases with mild-to-moderate COPD. Therefore, selection bias and the limitations of applying the FTND in COPD patients, likely do not explain all of our results.
Another plausible explanation for the negative association of FTND score with emphysema is interference in measuring the radiographic outcome variable of percent emphysema on quantitative chest CT scan analysis. Smoking induces airway irritation and inflammation and results in accumulation of mucus and inflammatory cells including neutrophils, macrophages, and lymphocytes in small airways even in subjects without airflow obstruction [
35‐
37]. A previous study has shown that the count of inflammatory cells within small airway walls is correlated with the smoking intensity in pack-years and is higher in current than ex-smokers [
35]. Camiciottoli
et al. also reported that emphysema severity was higher in former smokers than in current smokers [
38] as we observed in our study. Therefore, since subjects with higher dependence on nicotine tended to have greater current smoking exposures, those with higher FTND likely had more inflammatory changes in peripheral airways and alveoli which could be associated with increased lung density. Supporting this hypothesis, the effect of FTND on emphysema severity in multivariate analysis decreased and the effect on gas trapping disappeared after adjusting for the current number of cigarettes smoked per day. In addition, the association of nicotine addiction risk alleles with emphysema severity changed toward significant relationships in ex-smokers. Therefore, the hypothesis that measuring emphysema severity on chest CT might have limitations in current smokers may explain the differential genotypic association of SNPs with emphysema severity on chest CT according to their smoking status. The detection of significant genetic associations to quantitative CT emphysema phenotypes may be limited in current smokers due to this potential effect of current smoking on lung inflammation.
In smokers' lungs, emphysema is not the only possible phenotype and other pathological processes may increase lung density, including interstitial lung disease or other subclinical parenchymal diseases [
39,
40]. Lederer
et al reported an increase of high attenuation areas with an increasing of amount of smoking even in healthy smokers [
39]. This may also be the case in our study population. Considering our findings and previous reports related to the effects of smoking on CT imaging, more studies are needed to clarify the clinical significance of measuring low lung attenuation in populations that include current smokers.
In this study, we failed to show an association of nicotine dependence candidate SNPs with the severity of emphysema in current smokers while a significant association of candidate SNPs with nicotine dependence was found. Contrary to our findings in current smokers, Lambrechts
et al. reported recently that rs1051730, one of the SNPs that we tested in this study, was associated with the presence and severity of emphysema while they did not show association of these SNPs with nicotine dependence measured by the number of pack-years smoked instead of FTND [
18]. Among the subjects in Lambrechts' study, current smokers comprised only 45.6% and 50.7% of their two cohorts, which may decrease the overall effect of measuring emphysema in current smokers. They also used visual estimation of emphysema in one population[
18]. These factors may lead to the differences from the results of our study, which is supported by the finding that the association of candidate SNPs with emphysema severity tended to be significant in our study when genetic association analysis was applied only in ex-smokers.
The association of SNPs in the CHRNA3/5 locus with smoking behavior has been was widely reported [
41‐
43], and our results confirmed it again. We also analyzed genotypes in only white control subjects and COPD subjects with definite airflow obstruction (GOLD stage II-IV) to limit population heterogeneity.
Despite interesting findings, our study has limitations. As mentioned above, the population of advanced COPD cases was small, and the analysis across GOLD stages was limited. Our study sample was relatively small for genetic association analysis, and we did not include a replication population. We performed some genetic association analyses in a combined set of cases and controls. The appropriate adjustment for potential bias in such analyses is uncertain [
44,
45]. We performed adjustment for case-control status for FTND, but not for traits directly related to COPD pathophysiology (e.g. FEV1, emphysema) since this would likely have been an overadjustment. Although these are limitations, they are mitigated by the large body of evidence supporting a role for these SNPs in nicotine dependence in other populations.
In summary, FTND score was negatively associated with the severity of emphysema in COPD and healthy current smokers, and the FTND score decreased with increasing GOLD stage. Genetic variants in CHRNA3/5 (rs8034191 and rs1051730) were significantly associated with nicotine dependence. However, in a relatively small group of current smokers, an association of genetic variants in CHRNA3/5 (rs8034191 and rs1051730) with severity of emphysema or air trapping on CT was not found; the impact of current smoking on CT-measured emphysema may limit detection of significant genetic associations.
Acknowledgements
Source of support: Supported by National Institutes of Health grants U01HL089856 (E.K.S.), U01HL089897 (J.D.C.), K08HL080242 (C.P.H.), R01HL094635 (C.P.H.), and a grant from the Alpha-1 Foundation (C.P.H.).
COPDGene® Investigators:
Ann Arbor VA: Jeffrey Curtis, MD (PI), Ella Kazerooni, MD (RAD)
Baylor College of Medicine, Houston, TX: Nicola Hanania, MD, MS (PI), Philip Alapat, MD, Venkata Bandi, MD, Kalpalatha Guntupalli, MD, Elizabeth Guy, MD, Antara Mallampalli, MD, Charles Trinh, MD (RAD), Mustafa Atik, MD
Brigham and Women's Hospital, Boston, MA: Dawn DeMeo, MD, MPH (Co-PI), Craig Hersh, MD, MPH (Co-PI), George Washko, MD, Francine Jacobson, MD, MPH (RAD)
Columbia University, New York, NY: R. Graham Barr, MD, DrPH (PI), Byron Thomashow, MD, John Austin, MD (RAD)
Duke University Medical Center, Durham, NC: Neil MacIntyre, Jr., MD (PI), Lacey Washington, MD (RAD), H Page McAdams, MD (RAD)
Fallon Clinic, Worcester, MA: Richard Rosiello, MD (PI), Timothy Bresnahan, MD (RAD)
Health Partners Research Foundation, Minneapolis, MN: Charlene McEvoy, MD, MPH (PI), Joseph Tashjian, MD (RAD)
Johns Hopkins University, Baltimore, MD: Robert Wise, MD (PI), Nadia Hansel, MD, MPH, Robert Brown, MD (RAD), Gregory Diette, MD
Los Angeles Biomedical Research Institute at Harbor UCLA Medical Center, Los Angeles, CA: Richard Casaburi, MD (PI), Janos Porszasz, MD, PhD, Hans Fischer, MD, PhD (RAD), Matt Budoff, MD
Michael E. DeBakey VAMC, Houston, TX: Amir Sharafkhaneh, MD (PI), Charles Trinh, MD (RAD), Hirani Kamal, MD, Roham Darvishi, MD
Minneapolis VA: Dennis Niewoehner, MD (PI), Tadashi Allen, MD (RAD), Quentin Anderson, MD (RAD), Kathryn Rice, MD
Morehouse School of Medicine, Atlanta, GA: Marilyn Foreman, MD, MS (PI), Gloria Westney, MD, MS, Eugene Berkowitz, MD, PhD (RAD)
National Jewish Health, Denver, CO: Russell Bowler, MD, PhD (PI), Adam Friedlander, MD, David Lynch, MB (RAD), Joyce Schroeder, MD (RAD), John Newell, Jr., MD (RAD)
Temple University, Philadelphia, PA: Gerard Criner, MD (PI), Victor Kim, MD, Nathaniel Marchetti, DO, Aditi Satti, MD, A. James Mamary, MD, Robert Steiner, MD (RAD), Chandra Dass, MD (RAD)
University of Alabama, Birmingham, AL: William Bailey, MD (PI), Mark Dransfield, MD (Co-PI), Hrudaya Nath, MD (RAD)
University of California, San Diego, CA: Joe Ramsdell, MD (PI), Paul Friedman, MD (RAD)
University of Iowa, Iowa City, IA: Geoffrey McLennan, MD, PhD (PI), Edwin JR van Beek, MD, PhD (RAD), Brad Thompson, MD (RAD), Dwight Look, MD
University of Michigan, Ann Arbor, MI: Fernando Martinez, MD (PI), MeiLan Han, MD, Ella Kazerooni, MD (RAD)
University of Minnesota, Minneapolis, MN: Christine Wendt, MD (PI), Tadashi Allen, MD (RAD)
University of Pittsburgh, Pittsburgh, PA: Frank Sciurba, MD (PI), Joel Weissfeld, MD, MPH, Carl Fuhrman, MD (RAD), Jessica Bon, MD
University of Texas Health Science Center at San Antonio, San Antonio, TX: Antonio Anzueto, MD (PI), Sandra Adams, MD, Carlos Orozco, MD, Mario Ruiz, MD (RAD)
Administrative Core: James Crapo, MD (PI), Edwin Silverman, MD, PhD (PI), Barry Make, MD, Elizabeth Regan, MD, Jonathan Samet, MD, Sarah Moyle, MS, Douglas Stinson
Genetic Analysis Core: Terri Beaty, PhD, Barbara Klanderman, PhD, Nan Laird, PhD, Christoph Lange, PhD, Michael Cho, MD, Stephanie Santorico, PhD, John Hokanson, MPH, PhD, Dawn DeMeo, MD, MPH, Nadia Hansel, MD, MPH, Jacqueline Hetmanski, MS, Tanda Murray, Edwin Silverman, MD, PhD
Imaging Core: David Lynch, MB, Joyce Schroeder, MD, John Newell, Jr., MD, John Reilly, MD, Harvey Coxson, PhD, Philip Judy, PhD, Eric Hoffman, PhD, Raul San Jose Estepar, PhD, James Ross, MSc, Rebecca Leek, Jordan Zach, Alex Kluiber, Jered Sieren, Heather Baumhauer, Verity McArthur, Dzimitry Kazlouski, Andrew Allen, Tanya Mann
PFT QA Core, LDS Hospital, Salt Lake City, UT: Robert Jensen, PhD
Biological Repository, Johns Hopkins University, Baltimore, MD: Homayoon Farzadegan, PhD, Stacey Meyerer, Shivam Chandan, Samantha Bragan
Data Coordinating Center and Biostatistics, National Jewish Health, Denver, CO: James Murphy, PhD, Douglas Everett, PhD, Ruthie Knowles, Amber Powell, Carla Wilson
Epidemiology Core, University of Colorado School of Public Health, Denver, CO: John Hokanson, MPH, PhD, Jennifer Black-Shinn, MPH, Gregory Kinney, MPH