A meta-analysis: association between Beta-2 adrenergic receptor Arg16Gly polymorphism and asthma in China

Asthma is a kind of allergic disease of the respiratory tract, and has become a global public health problem in recent years (To et al., 2012). The prevalence of asthma is 0.2-21.0% wordwide, which is still increased, especially in developing countries such as China (To et al., 2012; Pearce et al., 2007; Yangzong et al., 2012). So far, numerous reports demonstrated that multiple genetic variations are involved in asthma. Among these genetic variations, the polymorphisms in beta-2 adrenergic receptor (ADRB2) have been most evaluated in asthma. For instance, single nucleotide polymorphisms of ADRB2 in the coding gene region including Arg16Gly (A46G, rs1042713), Gln27Glu (C79G, rs1042714), Thr164Ile (C491T, rs1800888) and Arg19Cys (T-47C, rs1042711) have proven the influence of ADRB2 activation on the small airways, suggesting the effective therapy of ADRB2 agonists in asthma (Pignatti 2004; Gao et al., 2004). Notably, the mutation of the Arg16Gly located at the ADRB2 gene has been extensively examined in studies on ADRB2 polymorphisms in asthma. The potential relationships on ADRB2 Arg16Gly polymorphism and asthma risk among Chinese people being remain conflicting. To our knowledge, it had lower statistical effect on the result than using a meta-analysis while the sample sizes in the individual studies are small. Therefore, this meta-analysis was performed to understand the importance of ADRB2 Arg16Gly polymorphism for asthma risk in Chinese population.


Introduction
Asthma is a kind of allergic disease of the respiratory tract, and has become a global public health problem in recent years (To et al., 2012).The prevalence of asthma is 0.2-21.0%wordwide, which is still increased, especially in developing countries such as China (To et al., 2012;Pearce et al., 2007;Yangzong et al., 2012).So far, numerous reports demonstrated that multiple genetic variations are involved in asthma.Among these genetic variations, the polymorphisms in beta-2 adrenergic receptor (ADRB2) have been most evaluated in asthma.For instance, single nucleotide polymorphisms of ADRB2 in the coding gene region including Arg16Gly (A46G, rs1042713), Gln27Glu (C79G, rs1042714), Thr164Ile (C491T, rs1800888) and Arg19Cys (T-47C, rs1042711) have proven the influence of ADRB2 activation on the small airways, suggesting the effective therapy of ADRB2 agonists in asthma (Pignatti 2004;Gao et al., 2004).Notably, the mutation of the Arg16Gly located at the ADRB2 gene has been extensively examined in studies on ADRB2 polymorphisms in asthma.The potential relationships on ADRB2 Arg16Gly polymorphism and asthma risk among Chinese people being remain conflicting.To our knowledge, it had lower statistical effect on the result than using a meta-analysis while the sample sizes in the individual studies are small.Therefore, this meta-analysis was performed to understand the importance of ADRB2 Arg16Gly polymorphism for asthma risk in Chinese population.

Data collection
To evaluate the associations between ADRB2 Arg16Gly polymorphism and asthma risk, PubMed, Embase, Web of Science, the Cochrane Library, and Chinese National Knowledge Infrastructure (CNKI) prior to January 2019 were searched using terms: ("asthma" or "asthmatic") and ("beta-2 adrenergic receptor" or "ADRB2" or "b2-AR") and "polymorphism" and ("Chinese" or "China").The search had no restriction in language or publication status.
Inclusion standards: (1) studies examining the relationship between ADRB2 Arg16Gly polymorphism and asthma, (2) study design with the case-control, (3) sufficient data of genotype frequency, (3) Chinese individuals.Exclusion standards: (1) overlapped literatures, (2) unextractable data, (3) not a case-control design, (4) abstract or reviews.The potentially relevant studies and extracted data from the identified publications, including first author's name, publication year, geographic areas, age group, sample size, and available genotype information from ADRB2 Arg16Gly polymorphism were screened by the investigators.

Statistical analysis
Data were analyzed by STATA version 12 (StataCorp LP, College Station, TX, USA).Odds ratios (ORs) with 95% confidence intervals (CIs) were calculated and analyzed by Z-test.The model of G versus A, GG versus AA, GG versus (AA+AG) and (GG+AG) versus AA were examined with the asthma risk, respectively.Heterogeneity and Hardy-Weinberg equilibrium (HWE) were evaluated by I-squared based on Q and the df.When P heterogeneity < 0.1 or I 2 > 50%, a random-effects meta-analysis model was performed to estimate the pooled ORs; if not, a fixed-effects model was used.Pooled ORs were collected for the sensitivity assay.A p-value < 0.05 showed the significant change.

Meta-analysis
The overall estimated ORs for all studies combined were 0.99, 0.96, 1.02, and 0.95 for contrast of allele (G vs. A), contrast of homozygotes (GG vs. AA), recessive (GG vs. AG+AA) and dominant models (GG+AG vs. AA), respectively (Table 2, Figure 1), these associations were not statistically significant (P>0.05).Next, we performed subgroup analyses according to geographic areas, age group and HWE in controls.Only a borderline significant effect for Southern Chinese was found under the homozygotes genetic model (OR = 0.82, 95%CI = 0.68-1.00,P = 0.046).There was no publication bias for the meta-analyses (t = 1.69,P=0.101; Figures 2 and 3).Sensitivity analysis in Table 2 indicated that the data used in this study were relatively stable and credible based on the fixed-effects model and the random-effects model.

Discussion
Many articles had been published to analyze the relation in ADRB2 Arg16Gly polymorphism and asthma, however, no    comprehensive exact conclusion was obtained.In a previous study, ADRB2 Arg/Gly16 showed a significant association with asthma in Asian population (Xie et al., 2014).The subgroup analysis based on the age showed that, compared to the other genotypes, Gly16 had recessive protective functions for children (Thakkinstian et al., 2005).Individual study could generate diverse results as the variability in region and individual in different populations, or due to the limit of cases.Furthermore, a unique culture and lifestyle from diverse ethnic group resulted in the different genetic trait.To avoid these limits, we performed this study to further survey the relation about ADRB2 Arg16Gly polymorphism and asthma in Chinese populations.
Analyses of 34 publications including 6143 asthma cases and 5919 healthy controls provided the evidence that ADRB2 Arg16Gly polymorphism was not an important risk factor for asthma in Chinese.Subgroup analysis suggested that only a borderline significant effect for southern Chinese was found under the homozygotes genetic model.No association between ADRB2 Arg16Gly polymorphism and the risk of asthma was found in other subgroups.Therefore, this meta-analysis indicated that there may be no connection between ADRB2 Arg16Gly polymorphism and asthma in China.These findings were also found in other meta-analyses, which showed that ADRB2 Arg16Gly gene polymorphism was not related to asthma in the children and general populations (Guo et al, 2016;Liang et al., 2014;Khan et al., 2018).
Most importantly, ADRB2 Arg16Gly polymorphism may vary greatly between different races, but this study mainly focused on Chinese population.In the meta-analysis, heterogeneity is often observed among genetic association studies.Although subgroup analysis was done according to geographic areas, ages and HWE in controls, the heterogeneity was not effectively decreased.Therefore, other factors may be the potential reasons for their heterogeneity.Additionally, the following deficiencies need to be mentioned: First, significant heterogeneity still existed in the subgroup analysis, thus might introduce some bias.Secondly, we cannot perform some other subgroup analyses such as sex, and asthma phenotype due to the limitation of data.Thirdly, the risks for asthma are multifactorial and complicated, and only genetic variation is often difficult to determine the cause-effect relationships of asthma.

Conclusion
This meta-analysis proves that ADRB2 Arg16Gly polymorphism is not related to in development of asthma in China.However, due to some limitations exited in this study, further studies are needed to validate the interactions between ADRB2 Arg16Gly polymorphism and asthma susceptibility based on more information.

Figure 1 .
Figure 1.Forest plots of all selected studies regarding the association between ADRB2 Arg16Gly polymorphism and asthma risk within the Chinese population under the allele model.

Figure 2 .
Figure 2. Publication bias assessment of ADRB2 Arg16Gly polymorphism and asthma risk with Begg's funnel plot.

Table 1 .
Characteristics of studies included in the meta-analysis.

Table 2 .
Association of the ADRB2 Arg16Gly polymorphism and asthma risk.
ORr: Odd ratio for random-effect model; ORf: Odd ratio for fixed-effect model; P h P value for heterogeneity test.