Genetic Polymorphism of Angiotensin-Converting Enzyme and Chronic Obstructive Pulmonary Disease Risk: An Updated Meta-Analysis

The relationship between polymorphism of the angiotensin I converting enzyme (ACE) gene and chronic obstructive pulmonary disease (COPD) has been examined in many previous studies. However, their results were controversial. Therefore, we performed a meta-analysis to evaluate the relationship between the ACE gene and the risk of COPD. Fourteen case-control studies were included in this meta-analysis. The pooled p value, odds ratio (OR), and 95% confidence interval (95% CI) were used to investigate the strength of the association. The meta-analysis was performed using comprehensive meta-analysis software. Our meta-analysis results revealed that ACE polymorphisms were not related to the risk of COPD (p > 0.05 in each model). In further analyses based on ethnicity, we observed an association between insertion/deletion polymorphism of the ACE gene and risk of COPD in the Asian population (codominant 2, OR = 3.126, 95% CI = 1.919–5.093, p < 0.001; recessive, OR = 3.326, 95% CI = 2.190–5.050, p < 0.001) but not in the Caucasian population (p > 0.05 in each model). In conclusion, the present meta-analysis indicated that the insertion/deletion polymorphism of the ACE gene may be associated with susceptibility to COPD in the Asian population but not in the Caucasian population. However, the results of the present meta-analysis need to be confirmed in a larger sample.


Introduction
COPD is a serious disease which is characterized by destruction of the lung parenchyma and inflammation of the peripheral airways [1]. COPD is a global health problem due to its high prevalence, morbidity, mortality, and social care cost [2,3]. The Global Burden of Disease (GBD) study reported that COPD was the sixth leading cause of death in 1990 and the fourth leading cause of death in 2000 [4]. Also, an estimated 4.7 million people die due to COPD [5]. The cause of COPD is multifactorial. Cigarette smoking is known to be the major cause of COPD, but some COPD cases cannot be explained by smoking alone [6]. Environmental exposure such as occupational exposures [7] and indoor biomass fuel burning [8] is closely related to the development of COPD. In addition, many previous studies have reported the association between genetic factors and COPD susceptibility. Previous family and twin studies have shown the role of genetic factors in COPD susceptibility [9]. Recently, some authors have used genetic polymorphisms to explain the genetic contribution to the development of COPD, and several candidate genes such as proteinase-activated receptor-1 [10], plasminogen activator inhibitor-1 [11], and 2-adrenergic receptor [12] were reported to have an association with COPD susceptibility.
ACE has been one of the most studied candidate genes [13][14][15][16] due to its wide role in the development of various diseases such as systemic lupus erythematosus [17], hypertension, chronic kidney disease [18], and diabetic nephropathy [19]. The ACE gene is located on the chromosome 17q23 and it encodes angiotensin-converting enzyme (ACE) which plays a role in converting angiotensin I into angiotensin II. One of the well-known polymorphisms in the ACE gene is the insertion/deletion polymorphism, which is 287 bp long and results in three genotypes (II, ID, and DD). The genotypes have been shown to be associated with ACE activity and levels in plasma and tissues [20]. A previous study reported that ACE activity depends on the O2 concentration of the blood and increased ACE levels due to hypoxia could be associated with severe tissue damage [21]. In COPD patients, the ACE activity increased during exacerbation, while it decreased during remission [22]. Inhibition of ACE using an ACE inhibitor could improve the exercise capacity during pulmonary rehabilitation in COPD [23]. ACE has been known to have an association with COPD, and an ACE inhibitor has been used to treat COPD. Thus, many previous studies have attempted to clarify the association between ACE polymorphism and COPD susceptibility, but several studies have reported conflicting results. Therefore, the aim of this meta-analysis was to investigate the possible association between ACE polymorphism and COPD risk based on all available relevant studies.

Search Strategy.
All studies which were published from January 1, 1998, to March 1, 2016, and which examined the association between ACE insertion/deletion polymorphism and COPD risk were carefully searched. Case-control studies were searched in PubMed, Google, Embase, and Korean databases (KISS, KMbase, and RISS) up to March 2015. The following keywords were used: "angiotensin-converting enzyme" or "ACE" AND "polymorphisms" AND "chronic obstructive pulmonary disease or COPD." Only human studies were selected. Additional studies were identified by a manual search of the reference of the related original studies or review articles. If data or data subsets were published in more than one article, only the publication with the largest sample size was included.

Inclusion and Exclusion
Criteria. The inclusion criteria were as follows: (1) the studies should have evaluated the relationship between the ACE insertion/deletion polymorphism and COPD risk; (2) the study design should be a case-control study; and (3) the authors should have provided sufficient data on genotype distributions in the COPD group and the control group to estimate the odds ratio (OR) with a 95% confidence interval (CI). Studies were excluded if the genotype distribution in the control groups deviated from the Hardy-Weinberg equilibrium.

Data Extraction.
Two investigators independently extracted data and created an analysis. When the investigators differed in their conclusions, they rechecked the data and reached a consensus through discussion. Data extracted from the selected articles included the first author's name, the year of publication, country of origin, ethnicity of the study population, the number of cases and controls, and the genotype frequency of ACE insertion/deletion polymorphism.

Statistical Analysis.
Meta-analysis was performed using a comprehensive meta-analysis software program (Biostat Corporation, NJ, USA). The pooled value, OR, and 95% CI were used to investigate the association between the risk of COPD and ACE insertion/deletion polymorphism. In the present meta-analysis, we adopted the following genetic models [24,25]: codominant 1 (I/D genotype versus I/I genotype), codominant 2 (D/D genotype versus I/I genotype), dominant (D/D genotype + I/D genotype versus I/I genotype), and recessive (D/D genotype versus I/D genotype + I/I genotype) models and the allele model (D allele versus I allele). For codominant 1, codominant 2, and dominant models, we used I/I genotype as the reference group. For the recessive model, I/D genotype + I/I genotype was used as the reference group.
Firstly, we assessed the heterogeneity among studies to select the analysis model. The 2 -test-based statistic test and the 2 test were applied. The random-effects Mantel-Haenszel method was adopted if the result of the test was < 0.05 or the 2 statistic was >50%, which indicated a statistically significant degree of heterogeneity among studies. Otherwise, the fixed-effects Mantel-Haenszel method was adopted. Publication bias was evaluated by Egger's regression. A value less than 0.05 was considered statistically significant. Table 1 describes the characteristics of selected studies included in the metaanalysis. Briefly, the meta-analysis in the present study was performed with fourteen articles [13][14][15][16][26][27][28][29][30][31][32][33][34][35]. After pooling all data, these 14 articles included 977 patients with COPD and 1,092 control subjects.

Quantitative Synthesis.
Among these 14 studies, two studies [15,26] showed an error in the Hardy-Weinberg equilibrium in the control group ( < 0.001 and = 0.004, resp., Table 2). These two studies were excluded from the evaluation of the exact analysis. To conduct risk assessment in all COPD cases and control patients, we assessed the heterogeneity in each model, respectively (codominant, dominant, and recessive models and the allele model). The results of the heterogeneity test for meta-analysis are shown in Table 2. The random-effects method was applied if the result of the test was < 0.05 or the 2 statistic was >50%. Otherwise, the fixed-effects method was adopted. Insertion/ deletion polymorphism of the ACE gene did not show any significant association with susceptibility to COPD in each model, respectively ( > 0.05, Table 2).
We performed a meta-analysis in two subgroups based on ethnicity: Asian population and Caucasian population. Results of the analysis in the Asian subgroup and the Caucasian subgroup are presented in Table 2 and Figure 1. For Table 1: Information of eligible studies included in the meta-analysis.

Author
Year Country  COPD/control  COPD  Control  COPD  Control  HWE in control  I/I I/D D/D I/I I/D D/D  I  D  I  D  Simsek  2013  Turkey  66/40  15  20  31  8  19  13  50  82  35  45  0.823  Ulasli  2013  Turkey  80/49  16  17  49  6  10  33  49  111  22   The present results obtained from these genetic models suggested that insertion/deletion polymorphism of the ACE gene was significantly associated with a strong risk of COPD in the Asian population but not in the Caucasian population. Figures 1(a)-1(e) show the meta-analysis results for the codominant, dominant, and recessive models and the allele model in the Asian population, respectively.
To identify publication bias in the meta-analysis, funnel plots for all comparison models were generated to detect the presence of publication biases. The shapes of all of the funnels  were symmetrical. We also evaluated publication bias using Egger's regression. There was no publication bias ( > 0.05, Table 2).

Discussion
A high amount of ACE is located in the lung capillaries [36] and it is involved in catalyzing the conversion of angiotensin I and converts it to an active peptide, angiotensin II. Angiotensin II functions as a strong vasoconstrictor agent and controls pulmonary vascular tone [37]. ACE insertion/deletion polymorphism is associated with serum ACE levels [20] and D allele is related to increased formation of angiotensin II [38]. Thus, ACE insertion/deletion polymorphism has been reported to have an association with various diseases. The relationships between ACE polymorphism and autism [39], the pathogenesis of pulmonary hypertension [40], psoriasis [41], chronic kidney failure [42], and so on have been reported. As mentioned above, the deletion polymorphism, the D allele, of the ACE gene is associated with a higher ACE activity [20], and an elevation of the ACE activity was observed in COPD patients [43]. Therefore, the associations between ACE polymorphism and COPD have been investigated in many previous studies [13-16, 26, 28, 31, 44, 45]. However, the results of previous studies regarding the association between ACE polymorphism and COPD were conflicting and contradictory. Previous two meta-analysis studies that assessed the relationship between ACE polymorphism and COPD risk failed to verify the link in the Caucasian population, but they found an association in the Asian population [46,47]. Our metaanalysis results were in agreement with the previous results. A total of 977 COPD cases and 1,092 controls were included in the present study. We could not identify any statistically significant association between ACE polymorphism and COPD risk. But when we performed a meta-analysis in the subgroup based on ethnicity, we found a strong association between ACE polymorphism and COPD risk in the Asian population ( < 0.001) but not in the Caucasian population. A previous study examined the ratio of the frequencies of ACE II, ID, and DD genotypes among various ethnicities. The study showed that the ratio of the frequencies of ACE II, ID, and DD genotypes was 1 : 2 : 1 in the Europeans; there was a tendency towards a higher D allele frequency in the Nigerians, and there was a tendency towards a higher I allele frequency in the Samoans and the Yanomami Indians [48]. Although our study did not include Nigerians, Samoans, or Yanomami Indians, allele distribution between the Asian and Caucasian populations was different (all studies in the Asian population showed a higher I allele frequency). This might have led to a difference in the results between the two ethnicities.
According to GBD study, age-standardized death rates for COPD are higher in low-income countries such as South Asian and Sub-Saharan African countries [49]. But, the rate is similar in developed countries. In Korea, medical costs per person were US$ 2,803 ± 3,865 in 2009 and the total cost of COPD-related medications increased by 33.1% over 5 years [50]. Also, patients with high grade COPD impose three times higher economic burden ($3,744 versus $1,183) on the health care system in Korea [51]. In Singapore, the mean total cost was approximately $9.9 million per year and 42% of the total cost burden was incurred for the medical management of COPD [52]. Thus, the importance of prediction and early diagnosis of COPD, both clinically and economically, has increased.
In the present study, we collected previous studies that assessed the relationship between COPD risk and ACE polymorphism, but our study has some limitations. Our results showed the association between ACE polymorphism and COPD risk. However, we could not investigate various ethnic distributions because most of the studies included only the Caucasian and Asian populations. Also, environmental factors are key factors in COPD, but we could not consider the environmental factors in this meta-analysis. As mentioned above, our results showed no evidence of publication bias, but some results were influenced by the included articles.
Despite some limitations, our results showed a statistically significant difference in the allele and genotype distribution in the Asian population. If more results in various populations are obtained in further studies, the relationship between ACE polymorphism and the development of COPD can be clarified.