The important message of this study is that SNPs in ADAM33 are associated with the pathophysiology of COPD. Patients with the AA genotype for SNP ST+5 had more severe AHR, higher numbers of sputum inflammatory cells and higher numbers of CD8 cells in bronchial biopsies than those with the GG genotype. Moreover, individuals with the minor allele of SNP T_1 and T_2, and homozygotic individuals for the minor allele of SNP S_2 had significantly lower numbers of CD8 cells in bronchial biopsies, cells relevant to the pathology of COPD.
Prevalences of the ADAM33
SNPs found in our COPD population are comparable to those recently reported by van Diemen et al
We also confirm their findings that minor alleles for SNPs F+1, S_1 and S_2 are more prevalent in patients with COPD than in healthy participants. Regarding SNP Q‐1 we showed a significantly higher prevalence of the minor allele in patients with COPD compared with healthy controls, whereas van Diemen et al11
showed a trend in the same direction (see online data repository). In addition, we found a higher prevalence for the A‐allele in SNP ST+5 in patients with COPD. As far as we know, this is the first study suggesting that a COPD susceptibility gene is also associated with the pathophysiological process in COPD.
is a member of the ADAM family, a group of membrane anchored metalloproteases that contain a disintegrin and a metalloprotease domain and is expressed in multiple tissues, including lung tissue.24
As its function has not been unravelled yet, we can only speculate about its role in COPD. It is conceivable that ADAM33 has a role in both remodelling and inflammation by shedding growth factors, cytokines and their receptors from the cell surface. If a SNP alters ADAM33 function to increase protein production, this might result in enhanced airway inflammation. Alternatively, when ADAM33 has a diminishing effect on the release of proinflammatory cytokines, inflammation would increase by a decrease in ADAM33 production or function. The same could be true for the role of ADAM33 in airway remodelling, in case a change in ADAM33 results in an increase in growth factors resulting in proliferation of airway smooth muscle cells and lung fibroblasts. Whatever the functions of ADAM33 turn out to be, our data suggest that it is associated with inflammation and AHR.
We showed an association between ADAM33
and the severity of AHR in COPD. AHR is important to COPD given its association with accelerated FEV1
and increased risk of COPD mortality.26
The exact pathophysiology underlying AHR is unclear, but it is thought to result from an inflammatory process in the airways in addition to geometric changes owing to airway remodelling. As discussed above, it is possible that ADAM33 has a role in both these processes thereby contributing to the severity of AHR.
SNP ST+5 is associated with the total sputum cell count in our COPD population. When we replaced this by numbers of sputum neutrophils or macrophages, we found significant associations with SNPs S_2, F+1 and Q‐1 but not with the other SNPs. This suggests that not one specific cell type was predominantly accounting for the association of total cell count with SNP ST+5. Several studies have shown an increase in the number of neutrophils and macrophages and concentrations of proinflammatory cytokines like interleukin 8 and tumour necrosis factor α in induced sputum of patients with COPD.27,28,29
Intuitively, one would think that the severity of AHR is associated with the degree of inflammation and, in fact, we previously did show an independent, positive association between severity of AHR and total sputum cell counts in these patients with COPD30
; perhaps ADAM33
is the missing link.
O'Shaughnessy et al31
showed an increased number of neutrophils and CD8 lymphocytes in bronchial biopsies of smokers with air flow limitation, and this increase was inversely associated with the level of lung function. We, therefore, hypothesised that individuals with a genetic predisposition for a higher number of CD8 cells were more susceptible to a further increase in CD8 cells, which might finally result in air flow limitation. Interestingly, we found an association of SNP ST+5 with both the presence of COPD (see online data repository) and the number of CD8 cells in our patients with COPD.
has previously been associated with asthma in some,9,22,23
but not all studies.32,33
We would like to emphasise that we are confident that our patients with COPD genuinely have COPD and no asthma given the fact they all had moderate to severe air flow limitation after bronchodilation,
10 pack‐years of smoking, and no history of doctor‐diagnosed asthma. We selected eight SNPs in ADAM33
based on previous literature. One could argue that we did not genotype all known SNPs in ADAM33
. However, we did find consistent and significant associations between the genotyped SNPs and the severity of AHR and airway inflammation in our COPD population, indicating linkage of ADAM33
with pathophysiological features of COPD.
A potential criticism of our study is that the sample size of 111 patients with COPD is relatively small. From a practical point of view, biopsy studies are very demanding and the issue of sample size would especially be of concern in case we did not find any associations. However, we did find significant associations between ADAM33 and features of COPD. We set out with the a priori hypothesis that the same SNPs in ADAM33 previously shown to be associated with COPD are also associated with its pathophysiology. To investigate our hypothesis, we performed several analyses to assess the association of ADAM33 with hyper‐responsiveness and airway inflammation and found associations of moderate significance. This has affected the interpretation of the results. One could raise the issue of multiple testing being responsible for the current results and that we should have adjusted for this in our analyses. We do not agree with this for several reasons. Firstly, the independent variables in our analyses (eg, sputum total cell count and differential cell count) are mutually related, indicating that a rigid statistical procedure like a Bonferroni correction for multiple testing would not do justice to their biologically linked nature. Secondly, we did not randomly test for associations between ADAM33 and features of COPD but had a predefined hypothesis based on previous literature.
is a highly polymorphic gene containing at least 58 SNPs of which we investigated eight, based on previous literature.9,11,12,22,23
We found an association of SNP ST+5 with the severity of AHR and airway inflammation in induced sputum and bronchial biopsies in COPD. The ST+5 SNP is an intron SNP between the S‐exon (transmembrane region) and the T‐exon (which includes a SH3
domain and a phosphorylation site). This may have functional relevance, as non‐coding introns can exert their effect by influencing alternative splicing, splicing efficiency or messenger RNA turnover.
We furthermore found an association of SNP S_2 with the inflammatory cell profile in sputum and the number of inflammatory cells in bronchial biopsies. A higher number of airway wall CD8 cells is associated with more severe AHR,6
by itself a risk factor for accelerated lung function loss.25
van Diemen et al11
showed an association of the minor allele of SNP S_2 with accelerated lung function decline in a general population and of the minor allele of SNP T_2 with the presence of COPD. Yet, in this study the minor alleles of these SNPs were associated with a lower number of CD8 cells. How can we reconcile these seemingly discordant findings? SNP S_2, a silent mutation, is located in the S‐exon and SNP T_2 is located in the T‐exon. Both SNPs are in close proximity to SNP ST+5. The significant linkage disequilibrium between SNPs S_2, ST+5 and T_2 may suggest that the part of ADAM33
involved in genetic susceptibility and pathophysiology of COPD lies in the 3′ region. Alternatively, it may still be either at a different location within ADAM33
or at an adjacent gene. Our study was not designed to discuss this question and clearly, it needs further research.
In conclusion, our study confirms ADAM33 as a COPD susceptibility gene and is the first to extend this observation by revealing an association of ADAM33 with the severity of both AHR and airway inflammation in individuals affected with COPD. These findings constitute an important step forward in linking gene polymorphisms with COPD pathophysiology, thereby possibly contributing to better future treatments for this progressive and disabling disease.