In this study, we examined SNPs in 6 COPD candidate genes including the ADRB2 gene, which has been shown to be associated with bronchodilator responsiveness, and found associations between bronchodilator responsiveness in NETT subjects and SNPs in the EPHX1, SERPINE2, ADRB2 genes. The association between one SNP in EPHX1 and bronchodilator responsiveness was replicated in extended pedigrees from the Boston Early-Onset COPD Study.
The product of EPHX1
is microsomal epoxide hydrolase, which is important for the metabolism of cigarette smoke by-products. In previous studies, the fast allele of EPHX1
(His139Arg) was found to be protective against COPD in a case-control study comparing NETT cases to control smokers (9
), protective against upper lobe predominant emphysema in NETT (17
), and associated with improved response to lung volume reduction surgery (LVRS), as measured by BODE score (22
). The slow allele of EPHX1
(Tyr113His) has been associated with emphysema susceptibility (26
) and reduced lung function (27
). Haplotype analysis of His139Arg and Tyr113His revealed associations with rapid lung function decline in the Lung Health Study (28
). In NETT, other SNPs in EPHX1
have been associated with exercise capacity, DLCO and response to LVRS (21
The replicated SNP, rs1009668, was chosen because it is downstream of EPHX1
, but is actually located in exon 1 of an adjacent gene, KIAA0792
. The function of this gene product is not known, but the gene has been shown to be expressed in adenocarcinoma of the lung (29
). This SNP results in replacement of valine with methionine at amino acid position 622. In COPD patients, lower BDR may be related to more severe emphysema (4
). This SNP (rs1009668), which was associated with lower BDR, was also associated with increased emphysema and may have a role in COPD pathogenesis. However, it is not clear whether this SNP association is due to linkage disequilibrium with a functional variant in EPHX1
or in KIAA0792
A SNP in intron1 of SERPINE2
(rs7588220) was associated with BDR in both NETT subjects and Boston Early-Onset COPD Study families, although the directionality of association was not consistent. This gene was associated with COPD susceptibility in several COPD populations (11
). However, the role of this gene in pathophysiology of COPD has yet to be identified. It is possible that SERPINE2
variants could define a subset of COPD patients with differential bronchodilator responsiveness.
Two previous studies have investigated the association between BDR and genetic polymorphisms in COPD. A recent publication reported an association of BDR and polymorphisms of the β2 adrenergic receptor gene (ADRB2
) in 246 Japanese patients with COPD (8
). Arg16 and Arg16-Gln27 haplotype was associated with decreased BDR. This gene has been studied as susceptibility gene for asthma (31
) and COPD (12
), and also for direct role in drug response (32
). Studies in asthma suggest the Arg16 allele is associated with greater acute bronchodilator responsiveness but decreased long-term response to regular use of short-acting β2 agonists (14
In our study, there was association only with haplotypes of the codon 16 and 27 polymorphisms in NETT subjects, but this was not replicated in EOCOPD families. In the EOCOPD families, the codon 16 polymorphism was associated with BDR only in the subgroup of subjects with a history of physician-diagnosed asthma. This suggests that ADRB2 is not a major determinant of BDR in COPD, but may be relevant in patients with an asthmatic phenotype.
In another study (7
), a polymorphism in the hemopoietic cell kinase (HCK
) gene was associated with differential expression of Hck protein and myeloperoxidase release from polymorphonuclear leukocytes in 60 COPD patients. This polymorphism was associated with BDR in 487 subjects from the Lung Health Study, suggesting that this gene contributes to COPD pathogenesis and modifies BDR. Because this gene was not associated with COPD susceptibility, we did not include it in our candidate gene panel.
The present study has several limitations. First, because there is not a single optimal measurement for bronchodilator responsiveness, we analyzed bronchodilator phenotypes expressed in three ways. All three measurements have their advantages and limitations, so we investigated all three phenotypes and used a replication strategy in this study to limit the possibility of false positive results. Second, the replicated SNP association in EPHX1
was for a different, but highly correlated, BDR phenotype in our two study populations. Third, intra-individual variability in response to bronchodilator (33
) may reduce power to detect genetic associations, yet we were still able to identify SNP-level replication for association between EPHX1
and BDR. Fourth, the LD-tagging approach yielded SNPs that may not be the specific functional variants. The associated SNPs were intronic or in an exon of a gene with unknown function. The actual causal variant or variants remain to be discovered. Fifth, multiple statistical comparisons are a potential concern in any complex disease genetics study. Though the optimal approach to adjust for multiple testing is not clear, we used replication of the results in another study sample to guard against false positive results.
The NETT subjects and the Boston Early-Onset COPD probands all have severe airflow obstruction. Thus the applicability of our findings to mild-to-moderate COPD patients may be limited. However, family members in the Boston Early-Onset COPD Study had a broad range of lung function values. Early-onset COPD subjects likely represent a unique subgroup of COPD patients. This could contribute to lack of replication of our associations in NETT.
In conclusion, polymorphisms of EPHX1, SERPINE2, and ADRB2 were associated with bronchodilator responsiveness phenotypes in one population of subjects with severe COPD. BDR in COPD patients may depend upon different disease subtypes, differences in drug metabolism, or other pharmacogenetic effects. This study revealed that EPHX1 -- which has been previously associated with COPD susceptibility, lung function and CT phenotypes – demonstrated replication in a second population. BDR may be partly explained by genetic factors. Future directions will include replication in additional populations, identification of the functional variant or variants, and determination whether there are subtypes of COPD with differential bronchodilator responsiveness.