Genetic Epidemiology of Cigarette Smoke–Induced Lung Disease

Abstract

Chronic obstructive pulmonary disease (COPD) and lung cancer represent two diseases that share a strong risk factor in smoking, and COPD increases risk of lung cancer even after adjusting for the effects of smoking. These diseases not only occur jointly within an individual but also there is evidence of shared occurrence within families. Understanding the genetic contributions to these diseases, both individually and jointly, is needed to identify the highest risk group for screening and targeted prevention, as well as aiding in the development of targeted treatments. The chromosomal regions that have been identified as being associated either jointly or independently with lung cancer, COPD, nicotine addiction, and lung function are presented. Studies jointly measuring genetic variation in lung cancer and COPD have been limited by the lack of detailed COPD diagnosis and severity data in lung cancer populations, the lack of lung cancer–specific phenotypes (histology and tumor markers) in COPD populations, and the lack of inclusion of minorities. African Americans, who smoke fewer cigarettes per day and have different linkage disequilibrium and disease patterns than whites, and Asians, also with different patterns of exposure to lung carcinogens and linkage patterns, will provide invaluable information to better understand shared and independent genetic contributions to lung cancer and COPD to more fully define the highest risk group of individuals who will most benefit from screening and to develop molecular signatures to aid in targeted treatment and prevention efforts.

Tobacco exposure, primarily in the form of cigarette smoking, is responsible for more deaths than any other preventable exposure, accounting for one in every five deaths (∼450,000) annually in the United States (1). Containing at least 250 chemicals that cause cancer or are toxic, tobacco use is associated with cancers of the lung, oral cavity, larynx, pancreas, bladder, and kidney, as well as cardiovascular diseases, chronic obstructive pulmonary disease (COPD), and a number of other diseases. Eighty to 90% of all lung cancer and COPD is attributable to smoking (2, 3). Lung cancer is the second leading form of cancer diagnosed in the United States, with an estimated 222,520 new cases and 157,300 deaths in 2010 (4). Likewise, COPD causes substantial morbidity and mortality; it is the fourth leading cause of mortality in adults in the United States and by 2030 is expected to be the third leading cause of death worldwide (5). Neither lung cancer, with a 5-year survival rate of just 15%, nor COPD is particularly responsive to treatment; therefore, both represent a major public health challenge. Although efforts to prevent initiation of smoking and promote smoking cessation have resulted in a substantial decline in the rate of smoking among adults, there remain almost 50 million former smokers and another 45 million current smokers who continue to be at risk of lung cancer and COPD (6), reinforcing the need to better understand the disease processes. Given that only 15% of smokers will develop lung cancer and/or COPD, understanding the genetic contributions to individual differences in susceptibility to these diseases is essential.

Copd History and Risk of Lung Cancer

Cigarette smoking contributes to lung cancer and COPD, but COPD itself is a risk factor for lung cancer. Once COPD is diagnosed, risk of developing lung cancer increases two- to threefold (7–12), even among never smokers (13). Lung cancer occurrence is also linked differentially to specific COPD phenotypes (i.e., emphysema and chronic bronchitis) (9, 14–19). In a metaanalysis, lung cancer was associated with a previous history of COPD (odds ratio [OR], 2.2; 95% confidence interval [CI], 1.7–3.0), chronic bronchitis (OR, 1.5; 95% CI, 1.3–1.8), and emphysema (OR, 2.0; 95% CI, 1.7–2.4) (12). In a large, population-based case-control study in women in Detroit, COPD was associated with a significantly increased risk of non–small cell lung cancer (NSCLC) (OR, 1.67; 95% CI, 1.15–2.41; in whites, OR, 1.85; 95% CI, 1.21–2.81) (19). No increase in risk was seen in African American women. A history of emphysema was associated with a threefold increased risk of lung cancer (OR, 3.21; 95% CI, 1.60–6.45). A history of chronic bronchitis was associated with a 1.7-fold increased risk of lung cancer (95% CI, 1.13–2.59). Lung cancer cases with a chronic obstructive lung disease history were more likely to be white, be heavy smokers, be exposed to environmental tobacco smoke, have childhood asthma, and have a history of asbestos exposure. Each of these exposures is a risk factor for both COPD and lung cancer. The question remains as to whether COPD is in the causal pathway in the development of lung cancer or a variation in manifestation from the same exposures.

There are some limitations to this work that need to be considered. The biggest limitation to the case-control studies, and all studies that rely on self-report, is the possibility of misclassification and lack of specificity of history of lung disease. The diagnosis of COPD includes a combination of two diseases that are frequently codiagnosed, chronic bronchitis and emphysema, making COPD a heterogeneous clinical phenotype. Disease presentation has been reported to vary by race, with COPD often diagnosed at an earlier age and at lower pack-years of exposure in African Americans than in whites (20, 21). One study also reports a different emphysema disease pattern seen on computed tomography (CT) between African Americans and whites (22). Given the clinical complexities, the COPD phenotype in these studies is unlikely to be uniformly accurate.

Three prospective studies have evaluated the association between CT evidence of emphysema and risk of lung cancer, reducing the potential for disease misclassification. de Torres and colleagues (23) report a 2.5-fold increased risk of lung cancer in the presence of emphysema, but no significant increased risk associated with airway obstruction. Risk was highest (relative risk, 4.3; 95% CI, 1.04–18.2) in the subset of participants with emphysema and no airway obstruction. Wilson and colleagues (11) report a 3.6-fold increased risk of lung cancer in smokers with CT-diagnosed emphysema and a 2.1-fold increased risk associated with airflow obstruction. In contrast, Maldonado and colleagues (24) reported no increased lung cancer risk among patients with CT-diagnosed emphysema. Risk of lung cancer has also been shown to increase with decreasing FEV₁; this was reported even in smokers with only minimal (10%) decreases in FEV₁ (18). Together these findings suggest that lung cancer occurrence is linked differentially to specific features of COPD; however, the pathologic mechanisms underlying this association are unknown.

Familial Aggregation of Lung Cancer and COPD

Not only do COPD and lung cancer exist jointly within individuals, these diseases aggregate individually and jointly within families. Familial aggregation of lung cancer was first described almost 60 years ago (25), and several studies since have shown that a first-degree family history of lung cancer confers an approximately twofold increased risk of lung cancer (26). Likewise, familial aggregation of COPD has been reported. FEV₁ and FEV₁/FVC values were shown to be significantly lower in first-degree relatives of probands with early-onset COPD as compared with controls (27). Probands were also 3.6 times more likely to have a parent who smoked and had a diagnosis of COPD. In addition to disease-specific aggregation, lung cancer and COPD jointly aggregate in families. First-degree relatives of patients with lung cancer and patients with COPD show impaired FEV₁ (28), and a family history of COPD increases risk of lung cancer development (29), suggesting a common underlying genetic contribution to these diseases.

Genetic Susceptibility to Lung Cancer and COPD

Several analytic approaches have been used to identify genes associated with lung cancer and/or COPD. Family linkage studies require large pedigrees including several affected relatives. This approach is focused on finding rare, highly penetrant genes demonstrating Mendelian inheritance. Under a model assuming common diseases are caused by common variants, different strategies are needed. Until recently, a candidate gene approach was used to focus on those genes most likely involved in disease pathogenesis. With technological advances, such as high-throughput genotyping, investigators have taken an agnostic approach to variant identification using whole genome association studies (GWAS) or admixture mapping. GWAS involves genotyping 1 million or more variants in very large numbers of individuals to identify regions containing more common (minor allele frequenciesc > 5%), low-penetrant variants associated with disease risk, whereas admixture mapping typically involves genotyping fewer ancestry informative markers in an admixed population. Most recently, whole genome or whole exome sequencing is being used to identify a third class of genetic variants, namely rare variants (minor allele frequenciesc < 5%) with low to moderate penetrance under a model assuming that common diseases are caused by rare variants. All of these approaches have been used in the search for disease susceptibility genes.

Family Linkage Studies

Family linkage studies have been used to identify genes for lung cancer, lung function, and COPD (30–32). The only linkage study in lung cancer successfully identified a region on 6q24 linked to familial lung cancer (30, 33). With fine mapping, the RGS17 gene was identified as a putative lung cancer susceptibility gene (34). Statistical modeling of the risk for lung cancer among those carrying a putative risk haplotype showed evidence for an interaction between exposure to smoking and inherited susceptibility to lung cancer (33). In addition to linkage on 6q, some evidence for linkage on chromosome 12 was also reported. For COPD, a region on 6q was most strongly linked to FEV₁ (31, 32). The region of tightest linkage was at the 6q terminus, just beyond the lung cancer linkage region and extending to the end of the chromosome. Evidence for linkage of lung function to moderate obstructive lung disease in smokers was reported on chromosome 12p (35, 36), and quantitative measures of FEV₁ and FVC have been linked to a region on 2q36 (37). The phenotype of postbronchodilator FEV₁ in these families was linked to a region on 8p23. These data provide some regions of potential overlap in areas linked to lung function, COPD, and lung cancer on chromosomes 6q and 12p.

Candidate Gene Studies

Pathways involved in the pathogenesis of COPD and lung cancer include inflammation, extracellular matrix proteolysis, and oxidative stress (38–40). A number of candidate gene single-nucleotide polymorphisms (SNPs) have been associated with susceptibility to lung cancer and COPD, including those coding for epoxide hydrolase 1 (EPHX1), matrix metalloproteinases, and IL-1β (41–45). Inflammatory pathway genes have been targeted for study because of the chronic inflammation caused by cigarette smoke exposure. IL1, IL1B, IL1RN, TNFA, IL8, COX2, and IL10 SNPs have been associated with risk of lung cancer (43, 46–53). In the Detroit lung cancer case-control study, associations between lung cancer risk and SNPs varied by race and by history of COPD (54). SNPs in IL7R, IL15, TNF, TNFRSF10A, IL1RN, and IL1A were associated with lung cancer risk in women with self-reported COPD but not among women without COPD. Engles and colleagues reported that IL1A SNPs were more strongly associated with lung cancer risk in those with emphysema (43). COPD is also characterized by chronic inflammation and has been associated with IL-1β, PGE-2, and transforming growth factor-β (55). TNFA and CXCR2 have been associated with spirometry measures of lung function (56), whereas IFNG has been associated with FEV₁% predicted in a series of smokers (57). Sadeghnejad and colleagues report that the association between pack-years of smoking and decline in mean percent predicted FEV₁ is modified by genotype at IL13 (58).

GWAS

Several GWAS have identified a region of 15q25.1, containing nicotinic acetylcholine receptor subunit genes CHRNA3 and CHRNA5, associated with lung cancer, nicotine addiction, cigarettes smoked per day, COPD, FEV₁, and emphysema (59–68). Additional regions of interest identified in lung cancer GWAS are on 3q29, 5p15, 6p21, 10q25, and 15q15 (69–71). The 5p15.33 region includes human telomerase reverse transcriptase (hTERT) and CLPTMIL, and the 6p21.33 region contains BAT3 and MSH5 genes involved in apoptosis and DNA repair, respectively (64). GWAS of lung function measures in the Framingham Heart Study showed IL-6R to be associated with forced expiratory flow midexpiratory phase, and GSTO2 was closely associated with mean FEV₁ and FVC measures (72). More recent GWAS report FEV₁ or FEV₁/FVC associated with regions on 4q31 (HHIP), 2q35 (TNS1), 4q24 (GSTCD), 4q22 (FAM13A), 5q33 (HTR4, ADAM19), 6p21 (AGER), 6q24 (GPR126), and 15q23 (THSD4) (73–76). A GWAS based on CT-diagnosed emphysema in patients with COPD reported an additional locus including BICD1 on chromosome 12p11.2, with potential functional significance in telomere shortening (77). Evaluating lung cancer and COPD jointly, Young and colleagues report that the 15q25 locus is associated with risk of both diseases, variants on 4q31 and 4q22 are associated with reduced risk of both diseases, loci on 6p21 are most strongly associated with lung cancer risk in smokers with COPD, and variants on 5p15 and 1q23 alter lung cancer risk when COPD is not present (78). The findings presented illustrate the complexities in teasing apart the genetic contributions to lung cancer and COPD jointly and separately.

Admixture Mapping

Admixture mapping has been used as an alternative approach for the identification of disease susceptibility regions in African Americans. This approach benefits from the linkage disequilibrium characteristics of admixed populations. In the only admixture study in lung cancer, regions on chromosomes 1q42, 3q25, 6p24, 10q23, and 15q12–13 were identified that need to be followed up with fine mapping. These regions contain potential candidate genes, including EPHX1, PTEN, GABA receptors, and CHRNA7. The 10q23 region has been reported to be associated with lung function (31). Redefining the outcome as self-reported COPD, several regions common to lung cancer and COPD were identified on chromosomes 1, 3, and 10 (Schwartz AG, unpublished data), where there is also evidence for association with lung function (72).

NextGen Sequencing

GWAS are designed to discover variation under a common disease–common variant hypothesis. To date, however, GWAS results explain only about 5% of the genetic variation in disease risk. Rare variants are not captured in GWAS studies. Whole genome or whole exome sequencing is a novel approach that has only recently become possible on a large scale at lower cost allowing the study of genetic variation to extend beyond common variants in the search for rare, moderately penetrant variants that confer greater risk than seen so far in GWAS. This approach results in the identification of novel SNPs, copy number variants, rearrangements, and insertions and deletions, and has been used successfully to follow up GWAS regions identified in prostate cancer (79). The most powerful way to conduct these studies is by enriching the sample for extreme phenotypes, such as individuals with a family history (80), as has been done successfully for other familial cancers, such as hereditary pancreatic (81) and breast (82) cancers. In addition, the approach is strengthened by the inclusion of sequencing in both germline and tumor DNA to identify novel causal tumor suppressor genes. These types of studies are just getting underway for lung cancer.

Summary

Lung cancer and COPD are responsible for hundreds of thousands of deaths each year in the United States. These diseases share a strong risk factor in smoking, and COPD increases risk of lung cancer even after adjusting for the effects of smoking. Uncoupling shared risk factors from causal pathways will be difficult. Although some of the biologic responses to tobacco smoke that underlie both COPD and lung cancer may be shared, genetic susceptibility underlying COPD and lung cancer may also be distinct. Which pathways are activated and/or suppressed is likely to determine the course of disease development. A number of chromosomal regions have been identified that are associated either jointly or independently with lung cancer, COPD, nicotine addiction, and lung function. Studies jointly measuring genetic variation in lung cancer and COPD have been limited by the lack of detailed COPD diagnosis and severity data in lung cancer populations, the lack of lung cancer–specific phenotypes (histology and tumor markers) in COPD populations, and the lack of inclusion of minorities. African Americans, who smoke fewer cigarettes per day and have different linkage disequilibrium and disease patterns than whites, and Asians, also with different patterns of exposure to lung carcinogens and linkage patterns, will provide invaluable information to better understand shared and independent genetic contributions to lung cancer and COPD. With the promise of new screening approaches for the early detection of lung cancer, defining the highest-risk group of individuals who will most benefit from screening will require a better understanding of the interactions between cigarette smoking, family history, genotype, lung function, and history of COPD. In addition, the development of molecular signatures for these diseases is needed to determine the common and disparate mechanisms in their pathogenesis to aid in targeted treatment and prevention efforts.