Progress in understanding the genetic contributions to COPD susceptibility and severity has been limited by lack of reproducibility of genetic association studies (1
). COPD is an inherently heterogeneous condition, and more exact phenotypic specification of the emphysema component of COPD through CT scanning has the potential to provide an important advance in phenotyping individuals for genetic association studies. More homogeneous COPD phenotypes may allow us to identify genes for emphysema and emphysema distribution; these genes can then be evaluated in a broader context as COPD susceptibility genes. Presently, genetic and nongenetic determinants of emphysema distribution are poorly understood. Given the implication for therapy associated with distributional features of emphysema, we sought to identify genes that are associated with emphysema distribution, especially upper lobe–predominant disease.
A well documented genetic cause of emphysema is severe AATD, and, classically, the distribution of emphysema in AAT-deficient individuals is predominantly in the basilar aspect of the lungs. Development of emphysema in this disease likely has a strong contribution from gene × smoking interactions (18
). We hypothesized that genetic susceptibility may be an important contributor to the distribution of emphysema in individuals not deficient in AAT.
In our study, we observed association of polymorphisms in the xenobiotic metabolizing enzymes, GSTP1 and EPHX1, with emphysema distribution, even after the exclusion of individuals with the PI MZ, MS, and SZ genotypes at the SERPINA1
locus. Oxidative stress, such as that induced by tobacco smoke, leads to the formation of reactive oxygen species and free radicals in the lung, which may lead to tissue and cellular damage that starts and perpetuates the lung injury process in COPD (19
). The ability to decrease oxidative stress is mediated in part through these pathway enzymes. Glutathione S-transferases are expressed in the lung and serve as antioxidants and hydroperoxidases. Microsomal epoxide hydrolase has high activity in the lung and is involved in the initial metabolism of reactive epoxide intermediates.
has been investigated by multiple groups as a potential COPD susceptibility gene. The majority of glutathione S-transferase activity in the lung is likely provided by GSTP1
). Of the two common polymorphic variants—Ile105Val (exon 5) and Ala114Val (exon 6)—the wild-type Ile105 variant was associated with densitometric apical percent emphysema and upper lobe–predominant disease by radiologist scores in our study. From a functional standpoint, enzymatic activity is altered by the variant at the 105 position (an adenine-to-guanine change at nucleotide 315, resulting in the isoleucine-to-valine substitution), with higher efficiency of metabolism of aromatic epoxides in the presence of the 105Val polymorphism (23
). Polymorphisms in GSTP1
at this functional site may influence regional detoxification of xenobiotic/oxidant stressors. The Ile105 variant has been demonstrated to be associated with COPD in a Japanese cohort (25
), and has also been associated with lung function decline in individuals with a family history of COPD in the Lung Health Study (26
). In asthma cohorts, children with the Val105/Val105 had higher FEV1
and FVC % predicted values (27
), and the presence of the Val105 genotype has been associated with an overall lower risk for asthma (28
). In individuals challenged with intranasal diesel particles and allergens, the Ile105 genotype was associated with increase in IgE and histamine, thus enhancing the allergic response (29
Despite the number of association studies that have demonstrated significant association of the Ile105 variant with obstructive lung disease phenotypes, the association of the Ile105Val variant has not been consistent across COPD case-control studies, with nonreplication in Boston Early-Onset COPD Study pedigrees (1
) and in a case-control study in Korea (30
). One contributor to nonreplication may be ethnic diversity between the case–control studies, but phenotypic heterogeneity may also contribute.
has also been investigated as a COPD susceptibility gene, and, in our current analysis, there is some evidence that the fast variant (His139Arg) is protective against upper lobe–predominant emphysema. Previous work by our group has demonstrated the protective effect of the fast allele (His139Arg, rs2234922; OR, 0.73; 95% CI, 0.56–0.96) in a case–control study of COPD using NETT cases (1
), along with association of other SNPs in the EPHX1
gene with DlCO
and exercise capacity phenotypes in NETT participants (13
). Similar to GSTP1
, microsomal epoxide hydrolase is important for the metabolism of by-products of cigarette smoke. The Tyr113His (rs1051740) “slow” variant in exon 3 has been associated with COPD (31
), and a haplotype that included the Tyr113His variant has been associated with rapid lung function decline in the Lung Health Study (33
). Associations with COPD of polymorphic variants in this gene have not been replicated in Japanese or Korean populations (34
The association of GSTP1 and EPHX1 polymorphisms with emphysema distribution suggests a potential role for xenobiotic metabolism in contributing to and/or directing upper lobe–predominant emphysema. From this observation, we hypothesize that there are COPD genes relevant to disease susceptibility and others genes relevant to disease severity and distribution. In our study, this would be consistent with EPHX1 being associated with COPD susceptibility (with a lower OR in the case-control study even with fewer cases when the narrower definition of COPD cases is based on those with upper lobe–predominant emphysema) and GSTP1 being more relevant as a determinant for emphysema distribution and severity.
There are several limitations of the present study. First is the concern that three different models of CT scanners were used during the NETT, and the results from different scanners may bias our results. However, all of the densitometry measures were performed by a single center, and we did include a variable for each study center to capture systematic differences in CT scanners and CT scan interpretation. We have not replicated these findings in an independent cohort given the paucity of CT phenotyping of patients in large-scale COPD genetics studies. In addition, and as mentioned in previous studies in this cohort (13
), a variable number of SNPs were tested for association in the genes of interest, so false-negative results for some genes might be observed due to inadequately capturing the linkage disequilibrium information across all of the genes. However, we used this initial SNP selection strategy to be able to compare our results—based on a more homogeneous emphysema phenotype—with results that have used only a spirometric definition of COPD. Given the number of comparisons performed, false-positive associations may be observed because of multiple testing. Many of the methods for controlling type I error in multiple testing are too conservative for correlated data (multiple SNPs in genes, or multiple CT scan phenotypes). Similar to Hersh and colleagues (13
), we used a test–replication approach within the NETT cohort to address the issue of false-positive results; despite the loss of power associated with limiting the size in each group, we still observed association of GSTP1
Ile105Val with emphysema distribution.
Genetic association studies with chest CT phenotypes have not been performed to date in as large a cohort as that presented here. Genetic contributions to emphysema distribution have been suggested by prior research. In addition to severe AATD, which is classically (but not always) associated with lower lobe emphysema, the MMP-9
(C-1562T) polymorphism was associated with upper lobe–dominant emphysema in a case–control study of 84 Japanese patients with COPD, but was not associated with COPD in a case–control analysis (9
). To date, no published studies have investigated a role for xenobiotic pathway enzymes and emphysema distribution. One possible explanation for our finding is that there are gradients of smoke metabolites and antioxidant protein expression in the lung from apex to base; if an imbalance of detoxification of smoke metabolites occurs in the upper part of the lung, then the major destructive effect of cigarette smoke in those with polymorphic variants would be in the upper lobes. Given the number of statistical comparisons performed in the current study, we recognize that replication is necessary in independent family-based and/or case-control studies.
In conclusion, we have demonstrated consistent association of the Ile105Val variant in GSTP1 with CT scan distribution of emphysema; there are no data to support an association with COPD susceptibility. Polymorphic variants in EPHX1 are associated with both emphysema distribution and COPD susceptibility. The role of the xenobiotic pathways and enzyme gradients in the lung should be investigated to elucidate the contribution to the development of specific distributional features of emphysema. Identification of genetic variants associated with distributional features of emphysema may have implications for selection for and outcome of lung volume reduction surgery.