Published ahead of print on April 19, 2007, doi:10.1164/rccm.200611-1723OC
© 2007 American Thoracic Society doi: 10.1164/rccm.200611-1723OC
The SERPINE2 Gene Is Associated with Chronic Obstructive Pulmonary Disease in Two Large Populations1 GlaxoSmithKline, Research Triangle Park, North Carolina; 2 University of Bergen, Bergen, Norway; 3 Cambridge Institute for Medical Research, Cambridge, United Kingdom; and 4 Brigham and Women's Hospital, Boston, Massachusetts Correspondence and requests for reprints should be addressed to Sreekumar G. Pillai, Ph.D., GlaxoSmithKline R&D, 5 Moore Drive, Research Triangle Park, NC 27709. E-mail: sreekumar.g.pillai{at}gsk.com
Rationale: Chronic obstructive pulmonary disease (COPD) is a complex disease influenced by multiple genes and environmental factors. A region on chromosome 2q has been shown to be linked to COPD. A positional candidate gene from the chromosome 2q region SERPINE2 (Serpin peptidase inhibitor, clade E [nexin, plasminogen activator inhibitor type 1], member 2), was previously evaluated as a susceptibility gene for COPD in two association studies, but the results were contradictory. Objectives: To identify the relationship between SERPINE2 polymorphisms and COPD-related phenotypes using family-based and case-control association studies. Methods: In the present study, we genotyped 25 single nucleotide polymorphisms (SNPs) from SERPINE2 and analyzed qualitative and quantitative COPD phenotypes in 635 pedigrees with 1,910 individuals and an independent case-control population that included 973 COPD cases and 956 control subjects. The family data were analyzed using family-based association tests. The case-control data were analyzed using logistic regression and linear models.
Measurements and Main Results: Six SNPs demonstrated significant associations with COPD phenotypes in the family-based association analysis (0.0016 Conclusions: These data provide further support for SERPINE2 as a COPD susceptibility gene.
Key Words: chronic obstructive pulmonary disease genetic association, SERPINE2 replication
Chronic obstructive pulmonary disease (COPD) is one of the leading causes of death and a major source of morbidity. Cigarette smoking is the most important risk factor for COPD, but only a minority of smokers develops the disease (1). Twin studies have demonstrated that pulmonary function is heritable (2, 3). Furthermore, familial aggregation studies suggested a genetic component of COPD susceptibility (4, 5). Genomewide linkage analysis of the Boston Early-Onset COPD Study families using pulmonary function phenotypes demonstrated a significant linkage peak on chromosome 2q (68). Linkage of spirometric phenotypes in general-population pedigrees to this region of chromosome 2q has also been reported (9). Using expression-array analysis of murine and human lung tissues, DeMeo and colleagues identified SERPINE2 (Serpin peptidase inhibitor, clade E [nexin, plasminogen activator inhibitor type 1], member 2) as a novel candidate COPD-susceptibility gene on chromosome 2q (10). Association analysis of the Boston Early-Onset COPD Study families showed significant association of multiple SERPINE2 single nucleotide polymorphisms (SNPs) with COPD-related phenotypes. DeMeo and colleagues also replicated these association results in a case-control study (10). However, Chappell and colleagues tested five SERPINE2 SNPs and were unable to replicate the SERPINE2 associations in a large case-control study (11). Because there are contradictory results regarding the association of SERPINE2 with COPD in these previous reports, we conducted genetic association analyses of SERPINE2 SNPs with both qualitative and quantitative COPD-related phenotypes in two large, independent datasets. We genotyped 25 SNPs in the SERPINE2 gene in 635 pedigrees with 1,910 individuals from the International COPD Genetics Network (ICGN), which is the largest family-based COPD collection reported to date. We also analyzed these SNPs in a large case-control collection from Bergen, Norway, which includes 973 cases with COPD and 956 control subjects.
Subject Recruitment and Clinical Evaluation In the multicenter ICGN study, subjects with known COPD were recruited as probands, and siblings and available parents were ascertained through the probands. Inclusion criteria for probands were airflow limitation (post-bronchodilator FEV1< 60% predicted and FEV1/VC < 90% predicted) at a relatively early age (45 to 65 yr), smoking history of 5 or more pack-years, and at least one eligible sibling (with 5 pack-year smoking history). COPD in siblings was defined by a post-bronchodilator FEV1 less than 80% of predicted and FEV1/VC less than 90% of predicted values. A total of 1,910 white individuals from 635 pedigrees were included in the family-based association analysis. Subjects for the case-control study were recruited from Bergen, Norway. The entry criteria for COPD cases were FEV1 of less than 80% predicted and FEV1/FVC of less than 0.7. The control subjects were selected based on FEV1 of more than 80% predicted and FEV1/FVC of more than 0.7. A total of 973 cases with COPD and 956 control subjects were included in the present analysis. Characteristics of the subjects are shown in Table 1. Additional details can be found in the online supplement.
SNP Selection and Genotyping Twenty-three SNPs within SERPINE2 and two more SNPs extending into the genes flanking SERPINE2 were selected based on their map positions. These 25 SNPs were genotyped in the ICGN family population and also in the Norwegian case-control population using the Illumina array-based custom SNP genotyping platform (Illumina, Inc., www.illumina.com). Hardy-Weinberg equilibrium for all SNPs was tested in control subjects by using the chi-square goodness-of-fit test to determine if the observed distribution of genotype frequencies was incompatible with the Hardy-Weinberg equilibrium distribution. Population stratification was assessed for the case-control population by genotyping 221 unlinked SNPs and estimating an inflation factor for genomic control (12, 13). The PedCheck program was used to detect Mendelian inconsistencies in the genotype data of COPD families (14). Additional details can be found in the online supplement.
Statistical Methods for Association Analysis In the case-control population, two models were used in the association analysis. A logistic regression model for the COPD binary phenotype and a linear regression model for the quantitative phenotypes (FEV1 and FEV1/FVC), with covariates including age, sex, and pack-years of smoking. For the quantitative trait analysis, only cases with COPD were included. The analyses were done using SAS software 8.2 (SAS Institute, Cary, NC) with an additive genetic model. Haplotype analyses were conducted using the HBAT function of the FBAT program with the use of Monte Carlo sampling for COPD (17), and using UNPHASED version 3.0 for FEV1 and FEV1/VC (18) in the family data. In the case-control data, haplotype analysis was performed using the expectation-maximization algorithm and score tests, implemented in Haplo.stats program, version 1.2.1 (19). The linkage disequilibrium (LD) structure in the SERPINE2 region was examined with the program Haploview, version 3.3 (20, 21). Additional details can be found in the online supplement.
Single Marker Association Analyses We evaluated 25 SNPs in the SERPINE2 region; the locations and characteristics of those SNPs are summarized in Table 2. In ICGN family-based association analysis, six SNPs in SERPINE2 demonstrated significant association with COPD and/or the quantitative spirometric phenotypes, FEV1 and FEV1/VC (p < 0.05) (Table 3). Five of these SNPsrs6734100, rs729631, rs975278, rs7583463, and rs6748795were significantly associated with COPD (0.0016 p 0.033); SNP rs6734100 showed the most significant association with COPD (p = 0.0016) and was significant even after a Bonferroni correction (p = 0.0016 x 25 = 0.04) for the number markers analyzed. Another SNP, rs16865390, was significantly associated with FEV1/VC (p = 0.042) (Table 3).
We attempted to replicate the results of the family-based association analysis in the case-control population. The results of the analyses to identify population stratification in the case-control study showed that the mean test statistic of the genomic control SNPs, inflation factor ( ), was 1.027; thus, no significant evidence of population stratification was found between the cases and control subjects. All SERPINE2 SNPs were in Hardy-Weinberg equilibrium in the control subjects (p > 0.05). In the COPD case-control data, six SNPs in SERPINE2 demonstrated significant association with COPD, FEV1, and/or FEV1/FVC (Table 4). Among them, rs16865421 was significantly associated with increased odds for COPD (p = 0.040; odds ratio, 1.33; 95% confidence interval, 1.011.74); rs6734100 was significantly associated with a reduction in FEV1 in cases with COPD (p = 0.008); and five SNPsrs6734100, rs729631, rs975278, rs7583463, and rs6748795were significantly associated with reductions in FEV1/FVC in cases with COPD (0.021 < p < 0.031) (Table 4). Thus, associations of five SNPs with COPD-related phenotypes were replicated in the ICGN family data and the case-control data.
Haplotype Block Structure and LD Figure 1 shows pairwise LD (r2) values for the 25 SNPs in the 74-kb sequence encompassing SERPINE2 and the flanking regions. Five haplotype blocks were revealed in ICGN family data and COPD case-control data, respectively. In ICGN family data, the significant SNPsrs6734100, rs729631, rs16865390, rs975278, and rs7583463were located in block 1, and rs6748795 was located in block 2. In the COPD case-control population, the significant SNPsrs6734100, rs729631, rs975278, rs7583463, and rs6748795were located in block 1, and rs16865421 in block 2. All of the replicated SNPs are in one single LD block in the case-control population. Because all significant SNPs are within the LD blocks in the SERPINE2 gene and not in the flanking regions, it is reasonable to speculate that the significant associations are from the SERPINE2 gene itself.
Haplotype Analyses Using a sliding-window approach, we performed adjacent 2- and 3-SNP haplotype analyses in both ICGN family data and COPD case-control data to identify the regions that most likely contribute directly to the observed associations. We listed significant results of haplotype-based association analysis in the two study populations in Table 5; both global (p-global) and individual (p-specific) haplotype results are reported. In the family data, 11 adjacent SNP combinations with a significant score test were detected (0.0078 p-specific 0.0226 and 0.0062 p-global 0.0423) with COPD; for 2-SNP haplotypes, the haplotype including the SNPs rs6734100 and rs729631 showed the strongest association (p-global = 0.0062 and p-specific = 0.0078); for 3-SNP haplotypes, haplotypes including rs6734100, rs729631, and rs16865390 showed the most significant association (p-global = 0.0165, p-specific = 0.0082). We did not detect significant haplotype associations with FEV1 and FEV1/VC in the family data. In the case-control data, two haplotypes demonstrated significant association (0.0149 p-specific 0.0257 and 0.0296 p-global 0.0398) with FEV1 among COPD cases; five haplotypes demonstrated significant association (0.0291 p-specific 0.0396 and 0.0383 p-global 0.0492) with FEV1/FVC among cases with COPD for 2-SNP haplotypes. Five haplotypes demonstrated significant association (0.0297 p-specific 0.0479 and 0.0409 p-global 0.0491) with FEV1/FVC for 3-SNP haplotypes. We did not detect significant haplotype associations with COPD in the case-control data. Significant results of haplotype analyses for FEV1/FVC in the case-control data were similar to those of associated haplotypes for COPD in the family data.
COPD is likely influenced by multiple genetic determinants, but severe 1-antitrypsin deficiency is the only proven genetic risk factor. In the present study, we investigated whether SNPs in the SERPINE2 gene were associated with COPD-related phenotypes in two independent, large datasets. Our study confirmed the previously reported association between SERPINE2 SNPs and COPD-related phenotypes (10). By testing 25 SNPs, including 13 novel ones that were not included by DeMeo and coworkers (10), we found that six SNPs were significantly associated with COPD-related phenotypes in ICGN family data, and six SNPs were associated in COPD case-control data. Five of these six significant SNPs were replicated in both populations, and three of these five SNPs (rs6734100, rs729631, and rs975278) were shown to be associated with COPD-related phenotypes by DeMeo and colleagues (10). SNP rs6734100 showed the most significant evidence of association with COPD. Our haplotype analyses supported the results of single SNP analysis.
Consisting of nine exons, the SERPINE2 gene encodes a 44-kD thrombin and urokinase inhibitor and is the most important physiological regulator of Association studies offer a potentially powerful approach to identify genetic variants that influence susceptibility to common complex diseases, but they are plagued by inconsistent results (27). The inconsistency may be due to false-positive studies, false-negative studies, population stratification, or true variability in genetic determinants among different populations (28). To address these issues, we have undertaken several strategies in the present study:
The lack of replication by Chappell and colleagues (11) of SERPINE2 associations could relate to several factors. They tested five SNPs from SERPINE2 in a large case-control population with 1,018 cases and 911 control subjects. Only limited information about their analysis is provided in their Letter to the Editor, but they did not find significant association results. Regarding the five SNPs that Chappell and colleagues studied, four of themrs1438831, rs920251, rs6747096, and rs3795879were also tested in the present analyses, but were not significantly associated with COPD-related phenotypes in the present study (Tables 3 and 4). Genetic heterogeneity between study populations and false-negative results (potentially related to population stratification in their samples) could have contributed to their lack of replication of SERPINE2 associations. The present study also has several weaknesses:
In conclusion, we conducted a robust genetic association study and found that variants in the SERPINE2 gene are likely to contribute to the development of COPD. Functional tests need to be performed to find the molecular mechanism that drives the genetic association between COPD phenotypes and SERPINE2.
The authors gratefully acknowledge the assistance of the site coordinators for the family and case-control collections. Mark Hall, Sandra Hammond, Rachel Taylor, Sara Alalouf, and Santhi Subramanian of GlaxoSmithKline for data management support. International COPD Genetics Network (ICGN) Investigators:Alvar Agusti, Son Dureta Hospital and Fundación Caubet-Cimera, Palma de Mallorca, Spain; Peter M. A. Calverley, University of Liverpool, Liverpool, UK; Claudio F. Donner, Division of Pulmonary Disease, S. Maugeri Foundation, Veruno (NO), Italy; Robert D. Levy, University of British Columbia, Vancouver, Canada; Barry J. Make, National Jewish Medical and Research Centre, Denver, Colorado; Peter D. Paré, University of British Columbia, Vancouver, Canada; Stephen I. Rennard, University of Nebraska, Omaha, Nebraska; Jørgen Vestbo, Department of Cardiology and Respiratory Medicine, Hvidovre Hospital, Copenhagen, Denmark; Emiel F. M. Wouters, University Hospital Maastricht, The Netherlands.
Supported by GlaxoSmithKline.
* The International COPD Genetics Network (ICGN) Investigators are listed at the end of this article. This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org Originally Published in Press as DOI: 10.1164/rccm.200611-1723OC on April 19, 2007 Conflict of Interest Statement: G.Z. is a postdoctoral fellow at GlaxoSmithKline. L.W. is a full-time employee of GlaxoSmithKline. J.A. is a full-time employee of GlaxoSmithKline. A.G. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. P.B. is acting as a principal investigator in a COPD study sponsored by GlaxoSmithKline. W.H.A. is a full-time employee of GlaxoSmithKline. D.A.L. received a research grant of $880,000 from Merck, Sharp, and Dohme, and $1,562,500 from GlaxoSmithKline to recruit patients in a multicenter study to identify biomarkers in COPD. E.K.S. received $7,400 in consulting fees from GlaxoSmithKline in 2005, $3,000 in lecture fees from GlaxoSmithKline in 2006, $500 in speaker's fees from Wyeth in 2004, and a honorarium of $1,500 from Bayer in 2005. E.K.S. also received $250,000 per year from 2001 to 2004 in research grants from GlaxoSmithKline, approximately $300,000 total in research grants from GlaxoSmithKline from 2005 to 2007, and an ECLIPSE grant of approximately $100,000 in 2007 from GlaxoSmithKline. S.G.P. is a full-time employee of GlaxoSmithKline. Received in original form November 28, 2006; accepted in final form April 19, 2007
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