Chinese Medical Journal 2012;125(12):2213-2218
Effect of polymorphisms in the β2-adrenergic receptor on the susceptibility and pulmonary function of patients with chronic obstructive pulmonary disease: a meta analysis

Correspondence to:HE Bei,Department of Respiratory Diseases, Peking University Third Hospital, Beijing 100191, China (Tel: . Fax:. E-mail:puh3_hb@bjmu.edu.cn)
Keywords
meta-analysis; beta2-adrenergic receptor; chronic obstructive pulmonary disease; polymorphism; susceptibility; pulmonary function
Abstract
Background  Chronic obstructive pulmonary disease (COPD) is a heterogeneous and complex disease of which the pathogenesis remains largely unknown. Many factors could influence COPD development and progression. One of them is the genetic risk factor. A severe hereditary deficiency of alpha-1 antitrypsin is the best genetic proof. Four single nucleotide polymorphisms (SNPs) of beta2-adrenergic receptor (β2AR) result in single amino acid substitution. Two loci had been extensively studied and found that they could change the function of β2AR. Two SNPs consist of substitutions of glycine for arginine at amino acid position 16, glutamic acid for glutamine at position 27. Many studies proved that polymorphisms at position 16 and 27 altered the lung function of COPD patients or the patient’s susceptibility to the development of COPD. However, there was no exclusive conclusion. Therefore, a meta analysis was done to investigate the effect of polymorphisms in the β2-adrenergic receptor (ADRB2) gene on the risk of COPD and lung function.
Methods  Comprehensive searches of MEDLINE, Embase, Ovid, HighWire, Cochrane Library, and Chinese databases (CBMdisc, VIP, CNKI, and Wanfang data) from January 1980 to September 2011 were performed, using the keywords: COPD OR chronic obstructive pulmonary disease AND adrenoreceptor OR adrenergic receptor AND polymorphism OR mutation OR variation. Case-control research or cross sectional studies in which diagnosis of COPD met the Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines; all the studies reported the ADRB2 genotype at position 16 or 27. Outcomes measured were genotype frequency and forced expiratory volume in the first second (FEV1%) in both the case and control.
Results  Twelve case-control studies and eight cross-sectional studies were included. Compared to the control (n=1225), neither Gly/Gly (n=527) nor Arg/Arg (n=422) homozygotes at position 16 demonstrated increased susceptibility to COPD, with odds ratios (ORs) of 0.95 (95% CI (0.68, 1.31), z=0.33, P=0.740) and 0.82 (95% CI (0.52, 1.28), z=0.88, P=0.381), respectively. Similar results were obtained for position 27, with ORs of 0.97 (95% CI (0.77, 1.23), z=0.21, P=0.833) for Glu/Glu homozygotes (n=357) and 0.82 (95% CI (0.53, 1.29), z=0.85, P=0.393) for Gln/Gln homozygotes (n=704) (control=1183). In patients with COPD, Arg/Arg homozygotes (n=41) had a similar FEV1% compared with Gly/Gly homozygotes (n=102) (standardized mean difference (SMD)=0.88, 95% CI (–0.85, 2.62), z=1.00, P=0.319). The genotype distribution was different between Caucasian and Asian populations (all P <0.05 except the genotype Arg/Gly) for both position 16 and 27.
Conclusions  Polymorphisms of ADRB2 at positions 16 and 27 did not change the risk of COPD nor affect lung function or disease severity. The genotype distribution for these polymorphisms was different between Caucasian and Asian populations.
Chronic obstructive pulmonary disease (COPD) is a disease occurring worldwide and is characterized as incomplete reversible flow limitation. The β2-adrenergic receptor (ADRB2) is expressed in airway smooth muscle cells and is closely related to pulmonary ventilation function. This receptor is the target of β2 agonists, which function to induce bronchial relaxation. However, not all the patients with COPD have the same response to β2 agonists, suggesting that there is variance in the sensitivity of ADRB2 among these patients. Our previous research showed that ADRB2 is downregulated in airway smooth muscle cells and its sensitivity to β2 agonists is decreased in patients with COPD. The mechanism leading to these changes is not clear. So far, at least five single-nucleotide polymorphisms (SNPs) have been identified in ADRB2, including +46A/G, +79C/G, +100G/A, +491C/T, and –47 C/T, with the related amino acid positions of 16, 27, 34, 164, and 19, respectively. Most studies have focused on the polymorphisms at positions 16 and 27.1-3 In vitro studies have shown that Gly16Arg in ADRB2 induces agonist-promoted receptor downregulation. Some clinical studies demonstrated that the Gly16 polymorphism may be associated with susceptibility to the development of COPD, while the polymorphism at Gln27 may be related to the severity of COPD.4-6 However, the effect of these polymorphisms on COPD remains controversial. The objective of this meta analysis was to evaluate the effect of ADRB2 polymorphisms at positions 16 and 27 on susceptibility to COPD and respiratory function in patients with COPD, which was assessed by forced expiratory volume in the first second (FEV1%) of the normal predicted value. Another objective was to compare the frequencies of each genotype among different ethnicities.
 
METHODS
 
Search strategy
A search was performed to collect all relevant published articles about the effects of ADRB2 polymorphisms in patients with COPD. A comprehensive search of MEDLINE, Embase, Ovid, HighWire, Cochrane Library, and Chinese databases (CBMdisc, VIP, CNKI, Wanfang data) was performed for articles published between January 1980 and September 2011. Key words used in this search were COPD OR chronic obstructive pulmonary disease AND adrenoreceptor OR adrenergic receptor AND polymorphism OR mutation OR variation.
 
Study selection
Studies were included if they met the following criteria: (1) Case-control research or cross-sectional studies, (2) including only cases with COPD demonstrated by a postbronchodilator FEV1%/FVC <70% normal predictive value or as diagnosed by the Chronic Obstructive Lung Disease (GOLD) guidelines, and (3) reporting the ADRB2 genotype at position 16 or 27 in all the cases and controls.
 
Assessment of validity
The methodological quality of each case-control trial was assessed by NEWCASTLE-OTTAWA quality assessment, which included three sections: selection, comparability, and exposure. There were four questions in the selection section: Is the case definition adequate? Are the cases obviously representative or not? Is the selection of the controls from the community, a hospital, or is there no description? Is the definition of controls no history of diseases or no description of source? In the comparability section, whether the most important factor is matched between the cases and the controls should be determined, and also for the second most important factor. The exposure section assessed the methods as to how exposure was ascertained, whether the establishment of exposure was by the same method for cases and controls, and whether the nonresponse rate was the same for both groups.
 
Data extraction
Two reviewers independently extracted data from the selected studies. The extracted variables included the first author’s name, published journal, published year, country, ethnicity, study design, sample size, frequency of gene polymorphisms, and pulmonary spirometry.
 
Statistical analysis 
The odds ratio (OR) was used to measure the different exposure risks for each genotype between the patients with COPD and controls. The heterogeneity among studies was measured by I2 statistic. If the P >0.01, it was considered that there was no significant heterogeneity among the studies and a fixed-effect model was used to calculated the pooled OR; otherwise, a random effect model was used. Publication bias was tested by funnel plot analysis. Statistical analysis was performed using STATA v11.0 (StataCorp, College Station, TX, USA) or SPSS v16.0 (SPSS, Chicago, IL, USA) software.
 
RESULTS
 
Search results
Three hundred and forty-five potentially relevant articles were identified from the database searches, with sixty-nine of them published in Chinese. Three hundred and three articles were excluded because of duplicate publication, fundamental research, or because they were reviews. After screening the titles and abstracts, 16 articles were excluded because there were no normal controls in these studies or they were meeting abstracts or unpublished dissertations. After reviewing the full texts, there were 20 articles included in our pooled analysis;4-22 the other six were excluded for lack of important information (i.e., genotype frequency or pulmonary function tests; Figure 1). In the included articles, 12 were case-control studies and the others were cross sectional studies. All of the case-control studies were valid by NEWCASTLE-OTTAWA quality assessment.
 

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Figure 1. Flow chart indicating search results.
 
Polymorphisms at amino acid position 16 (Arg16Gly) and the morbidity risk of COPD
There were 10 studies on the polymorphism at position 16, including 912 cases and 1145 controls. Because the heterogeneity was significant among the studies, a random effect model was used. The pooled ORs for Gly/Gly and Arg/Arg homozygotes were 0.95 (95% confidence interval (CI) (0.68, 1.31), z=0.33, P=0.740, Figure 2) and 0.82 (95% CI (0.52, 1.28), z=0.88, P=0.381, Figure 3), respectively.
 

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Figure 2. The pooled OR for the Gly/Gly homozygote at position 16. DerSimonian and Laird method (D+L) pooled OR. Heterogeneity chi-squared=16.21 (d.f.=8), P=0.039, I-squared (variation in OR attributable to heterogeneity)=50.6%, estimate of between-study variance Tau-squared=0.1142. Test of OR=1: z=0.33, P=0.740.
Figure 3. The pooled OR for the Arg/Arg homozygote at position 16. D+L pooled OR. Heterogeneity chi-squared=25.56 (d.f.=9), P=0.002, I-squared (variation in OR attributable to heterogeneity)=64.8%, estimate of between-study variance Tau-squared=0.3103. Test of OR=1: z=0.88, P=0.381.
 
In the subgroup analysis, the participants were divided into Asian and Caucasian groups. For the Asian group, the pooled OR did not reach statistical significance for either homozygote (Gly/Gly: OR 1.10, 95% CI (0.75, 1.60), z=0.47, P=0.639; Arg/Arg: OR 0.61, 95% CI (0.30, 1.24), z=1.36, P=0.172). Similar results were obtained for the Caucasian group (Gly/Gly: OR 0.79, 95% CI (0.60, 1.03), z=1.75, P=0.080; Arg/Arg: OR 1.32, 95% CI (0.75, 2.33), z=0.98, P=0.328; not shown).
 
Polymorphisms at amino acid position 27 (Glu27Gln) and the morbidity risk of COPD
There were 10 studies on the polymorphism at position 27, including 994 cases and 1183 controls. The pooled ORs for Glu/Glu and Gln/Gln homozygotes were 0.97 (95% CI (0.77, 1.23), z=0.21, P=0.833, Figure 4) and 0.82 (95% CI (0.53, 1.29), z=0.85, P=0.393, Figure 5), respectively.
 

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Figure 4. The pooled OR for the Glu/Glu homozygote at position 27. Mantel-Haenszel method (M-H) pooled OR. Heterogeneity chi-squared=6.92 (d.f.=6), P=0.328. I-squared (variation in OR attributable to heterogeneity)=13.3%, Test of OR=1: z=0.21, P=0.832.
Figure 5. The pooled OR for the Gln/Gln homozygote at position 27. D+L pooled OR. Heterogeneity chi-squared=39.35 (d.f.=9), P=0.000. I-squared (variation in OR attributable to heterogeneity)=77.1%. Estimate of between-study variance Tau-squared=0.3713. Test of OR=1: z=0.85, P=0.393.
 
The participates were divided into Asian and Caucasian groups. In the Asian group, the ORs were 1.10 (95% CI (0.56, 2.15), z=0.28, P=0.777) and 0.81 (95% CI (0.36, 1.78), z=0.53, P=0.594) for Glu/Glu and Gln/Gln, respectively. The results for the Caucasian group were similar, with an OR 0.88 (95% CI (0.64, 1.21), z=0.78, P=0.435) for the Glu/Glu homozygote and 1.13 (95% CI (0.87, 1.47), z=0.92, P=0.356) for the Gln/Gln homozygote (not shown).
 
Comparison of pulmonary function among the different genotypes at position 16 in patients with COPD
Data from three studies included 143 patients with COPD, in which there were 41 cases with the Arg/Arg homozygote and 102 with the Gly/Gly homozygote. In the random effects analysis, the SMD statistic was 0.88 (95% CI (–0.85, 2.62), z=1.00, P=0.319, Figure 6).
 

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Figure 6. Comparison of FEV1% between Arg/Arg and Gly/Gly homozygotes. D+L pooled SMD. Heterogeneity chi-squared=23.86 (d.f.=2), P=0.000, I-squared (variation in SMD attributable to heterogeneity)=91.6%. Estimate of between-study variance Tau-squared=2.1286, Test of SMD=0: z=1.00, P=0.319.
 
Comparison of genotype frequencies between Caucasian and Asian populations
The genotype frequency at position 16 in patients with COPD was Arg/Gly>Gly/Gly>Arg/Arg for the Caucasian group and Arg/Gly>Arg/Arg>Gly/Gly for the Asian group. The frequencies of the two homozygotes were significantly different between the two ethnicities (P <0.01). The difference also appeared in the controls (P <0.01). The genotype frequency at position 27 in patients with COPD was Gln/Glu>Gln/Gln>Glu/Glu for the Cacasian group and Gln/Gln>Gln/Glu>Glu/Glu for the Asian group. The frequencies of the three genotypes at position 27 were all significantly different between the Caucasian and Asian groups (P <0.01) and in the controls (P <0.01). Regardless of ethnicity, the proportion of Gly-positive genotypes was about 70% (Table 1).
 

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Table 1. Genotype frequency distribution among the different ethnicities of patients with COPD and normal control subjects
 
Publication bias analysis
Publication bias analysis was performed based on funnel plots (Figure 7). The funnel plots were basically symmetrical, indicating that there was no significant publication bias in our study.
 

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Figure 7. Publication bias test by funnel plot. A: Studies about Gly/Gly homozygotes at position 16. B: Studies about Arg/Arg homozygotes at position 16. C: Studies about Glu/Glu homozygotes at position 27. D: Studies about Gln/Gln homozygotes at position 27.
 
DISCUSSION
 
Polymorphisms in ADRB2 have been shown to be associated with asthma;3,7,23,24 however, similar studies related to COPD are not as prevalent. In addition, the results of these studies have been inconsistent, especially in regard to the risk of COPD. One of the aims of this current study was to investigate whether the different genotypes at positions 16 and 27 could affect susceptibility to COPD.
 
In this study, it was shown that the Gly/Gly and Arg/Arg homozygotes at position 16 were not related to the risk of COPD, which was the same result obtained for the Glu/Glu and Gln/Gln homozygotes at position 27. Moreover, these results did not change when the patients were grouped by ethnicity. In our forest plot analysis of the Gly/Gly homozygote at position 16, none of the ORs in any of the articles were statistically significant except in the study by Matheson et al.7 Similarly, there were only three studies that indicted that the Arg/Arg homozygote at position 16 might play a role on the development of COPD, but the results were conflicting among these studies. In regard to the homozygote at position 27, none of the studies showed a significant relationship between the Glu/Glu homozygote and susceptibility to COPD. The above observations explain why the overall ORs did not achieve any statistical significance, indicating that polymorphisms at position 16 or 27 are not associated with a risk of morbidity for COPD.
 
Unlike COPD, polymorphisms in ADRB2 can modify the likelihood of the development of asthma, which has been demonstrated in numerous studies.2,23,24 Furthermore, different genotypes have different responses to the bronchodilators used for treating patients with asthma.3 The pathology and mechanism of COPD is different from asthma. Although in vitro studies suggest that polymorphisms at position 16 and 27 may result in variation in the function of ADRB2,25 there is no evidence indicating the effect of polymorphisms at these positions on the incidence of COPD.
 
Our data also showed that the FEV1% in patients with COPD harboring the Arg/Arg homozygote at position 16 was not significantly different from those with the Gly/Gly homozygote. Because FEV1% is more widely used and reflects the ventilation function and severity of disease more accurately, the articles using the actual value of FEV1, but not FEV1%, were not included in our study. Nevertheless, we found only three articles including FEV1% and evaluating polymorphisms at position 16 in patients with COPD and none evaluating polymorphisms at position 27.4 This is a limitation of this study. The above result was supported by a recent study that suggested the polymorphism at position 16 of ADRB2 does not predispose patients to increased disease severity. Joos et al19 also found that the polymorphism at position 16 does not contribute to a decrease in lung function in smokers. However, relevant studies were rare, indicating the need for a large-scale clinical trial to further investigate the effects of polymorphisms in ADRB2 on lung function and disease severity in patients with COPD.
In our study, it was observed that the distribution of genotypes in the Caucasian group was different from those in the Asian group, including both the patients with COPD and the controls. When we divided the participants according to race for subgroup analysis, the prevalence of COPD was still not changed by the polymorphisms in ADRB2, suggesting that race is related to the polymorphism distribution but not susceptibility to COPD.
 
In conclusion, polymorphisms at position 16 and 27 of ADRB2 did not change the risk of COPD or affect lung function or disease severity. However, the genotype distribution of these polymorphisms is different between the Caucasian and Asian populations.
 
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  1. National Natural Science Foundation of China,30770939; Doctoral Fund of Ministry of Education,20090001110093;