The Influence of ACE Insertion/Deletion Gene Polymorphism on the Risk of IgA Nephropathy: A Debatable Topic

Background The connection between angiotensin-converting enzyme insertion/deletion (ACE I/D) gene polymorphisms and IgA nephropathy (IgAN) was conflicting. This pooled analysis was performed to explore this issue. Methods All eligible investigations were identified from various electronic databases, and the pooled analysis was evaluated using Stata software. Results 27 studies with 2538 IgAN cases and 3592 controls were included. In overall subjects, ACE D allele, DD, and II genotype were associated with IgAN susceptibility (D vs. I: OR = 1.21, 95% CI: 1.10–1.32, P < 0.001; DD vs. ID + II: OR = 1.38, 95% CI: 1.20–1.60, P < 0.001; and II vs. DD + ID: OR = 0.83, 95% CI: 0.73–0.95, P=0.007). In Asian and Chinese patients, ACE I/D gene polymorphism was also correlated with IgAN vulnerability. Moreover, ACE D allele, DD, and II genotype were correlated with the progression of IgAN (D vs. I: OR = 1.37, 95% CI: 1.09–1.73, P=0.008; DD vs. ID + II: OR = 1.57, 95% CI: 1.06–2.31, P=0.024; and II vs. DD + ID: OR = 0.69, 95% CI: 0.49–0.99, P=0.045). Conversely, in Caucasian subjects, there was no link between ACE I/D gene polymorphism and the risk of IgAN. Conclusion ACE I/D gene polymorphism was correlated with the vulnerability and progression of IgAN in Asian and Chinese patients, and ACE D allele and DD homozygote genotype could be adverse factors for IgAN, while the II homozygote genotype could be an advantage factor. But, no significant association was found between ACE I/D gene polymorphism and IgAN in Caucasians.


Introduction
IgA nephropathy (IgAN) is a common type of glomerulonephritis globally; in the Pacific Asian region, the prevalence rate of IgAN is even more high [1]. It is a serious public health problem with very high mortality and morbidity, and recent studies have showed that approximately 20% IgAN patients would progress to endstage renal disease (ESRD) if lacking effective treatment [2]. e presence of excessive immune complexes including IgA in the glomerulus was a key mechanism of IgAN. e current therapy was mainly on corticosteroids and immunosuppressors [3]. However, these measures were not fully effective. Recent research found that susceptibility to IgAN was influenced by a confluence of some genetic factors, and more importantly, some single-gene polymorphisms were associated with the susceptibility of IgAN [4].
Angiotensin-converting enzyme (ACE) was a key enzyme that converted angiotensin I to angiotensin II. e ACE gene consisted of 26 exons and located on chromosome 17q23 [5]. Single-nucleotide polymorphisms (SNPs) frequently occurred in the ACE gene; in the 16th intron of the ACE gene, a 287 bp fragment recognized as insertion/deletion (I/D) polymorphism has been found, and based on this I/D polymorphic marker locus, three genotypes could be defined: DD, II homozygotes, and ID heterozygote [6]. It has been found that serum ACE activity could be affected by ACE I/D gene polymorphisms, and these subjects with the D allele always have higher level of ACE activity [7]. Some research studies have demonstrated that ACE I/D polymorphism would affect the vulnerability of IgAN [8][9][10]. 3.17

Data Analysis.
In this study, we used the Stata software to perform this pooled analysis. An odds ratio (OR) was calculated to evaluate the influence of ACE I/D gene polymorphism on IgAN susceptibility, and four different genetic models were used: model 1, allele D vs. allele I; model 2, genotype DD vs. genotype ID + II; model 3, genotype II vs. genotype DD + ID; and model 4, genotype ID vs. genotype DD + II. When a P value was less than 0.05, it was considered as statistically significant for the pooled OR. e heterogeneity among various included studies was evaluated using a Q test. Additionally, Begg' s test was completed to evaluate the publication bias; when the P value of Begg' s test was less than 0.05, potential publication bias was most likely to exist [11].   Table 1, 27 studies were finally included in our study [9,, but 2 trials were excluded due to the fail to meet the HWE [32,35]. e main features of the included trials are described in Table 1. 6130 participants with 2538 cases and 3592 controls were included, and 9 of the studies were written in Chinese and 18 in English, from a total of 11 countries. e average age of participants ranged from 6 to    Table 1.

Publication Bias.
e publication bias has been assessed using Begg' s test and Funnel plots. Potential publication bias was found in the analysis for the link of genotype DD with IgAN risk (DD vs. ID + II genotype: Begg' s P � 0.044). Additionally, the funnel plot was asymmetrical (Table 2 and Figure 5).

Discussion
Our study demonstrated that the ACE I/D polymorphism was associated with the susceptibility and progression of IgAN. Specifically, it showed that the ACE I/D gene D allele and DD genotype were risk factors for IgAN, while the II genotype was an advantageous factor for IgAN. is study was not the first pooled analysis to explore the relationship between ACE I/D polymorphism and IgAN; a small amount of previous meta-analysis about this Evaluates of pooled OR for IgAN showed that the ACE gene locus ID genotype was not associated with IgAN. controversial issue has been performed. In 2001 and 2006, two meta-analyses including only 5 reports have been completed [38,39]; these two research studies found that the association between the genetic susceptibility of IgAN and ACE I/D gene polymorphism was not significant. However, another pooled analysis performed by Yong showed that the ACE I/D polymorphism DD genotype was associated with IgAN progression both in Asian and Caucasians patients and the DD genotype was associated with IgAN risk in Asians [40]. As the number of included studies in these pooled analyses was small, in order to gain a more credible result, we reexamined the related trials and included 21 studies. We found that the allele D and genotype DD were correlated with IgAN risk and progression only in Asian patients. Similarly, Qin et al. found that the allele D and genotype DD were associated with IgAN susceptibility only in Asians, but there was no significant association between the allele D or genotype DD and the progression of IgAN [10]. Due to the fact that we included much more trials in our analysis compared to the previous research, we believed that our findings were more credible. IgAN is a common type of glomerulonephritis globally, and it is a major cause of chronic renal failure among East Asian countries. e pathogenesis of IgAN is extremely complex, and immunological renal injury mediated by IgA is traditionally associated with IgAN [1,2]. It has been demonstrated that IgA immune complexes depositing in the glomerular mesangium could activate inflammatory response, profibrotic cell proliferation, and even the formation of a glomerular crescent [3]. But, more and more research suggested that renin-angiotensin-aldosterone system (RAAS) activation played a vital role in the development and occurrence of IgAN. Angiotensin II (AT II) was a pivotal factor of RAAS, and growing evidence indicated that AT II was a potential proinflammatory mediator associated with the renal tubule interstitial fibrosis [41]. e activation of AT II resulted in the production of various inflammatory cytokines, such as TNF-a, IL-6, IL-8, and TGF-β [42]. Consequently, these cytokines could greatly promote the occurrence and development of IgAN.
Recently, the researchers found that IgAN was also influenced by a confluence of environmental and genetic factors. Familial aggregation of IgAN indicated that genetic factor might participate in the pathogenesis of IgN [43]. With the technological development of genetic studies, especially the Genome Wide Association Study widely used, several SNPs within immune-or hypertension-related genes have been found significantly associated with IgAN. ACE was a pivotal factor of RAAS, the ACE gene contained 26 exons and 25 introns locating on 17q23 [6], and some polymorphic  genetic markers of ACE gene have been found. Among these polymorphic markers, insertion or deletion (I/D) polymorphism (rs4340) of a 287 bp Alu in the 16th intron was the most investigated [6]. As mentioned earlier, there was a close relationship between the occurrence of IgAN and RAAS activation, and ACE played a vital role in the activation of angiotensin II. It was worth noting that the ACE level was strongly linked with ACE I/D polymorphism, and in those patients with the ACE DD genotype, the level of ACE expression and activity both in renal tissue and in serum were all markedly increased [44,45]. e insertion or deletion of base in the ACE gene might alter the expression or stabilization of ACE via influencing the stability of ACE mRNA. us, it could be seen that ACE D allele carriers had higher ACE level and consequently possessed higher risk to IgAN vulnerability.
In this study, the pooled analysis results indicated that the relationship between I/D polymorphism of ACE gene locus and IgAN risk was inconsistent in different races.
In Asian and Chinese patients, we found that the genotype DD, II, and allele D were associated with the vulnerability and progression of IgAN, but there was no evident correlation in Caucasian patients. is might be due to the differences in the environment and genetic backgrounds. Also, ACE inhibitors could affect the analysis results, while some studies in Chinese subjects did not clearly state whether to exclude those patients using ACE inhibitors. In short, the understanding about the ACE I/D genetic susceptibility difference was acquainted scarcely, and future research is needed to explore the specific mechanisms.
Some potential limitations should be discussed in our study. Firstly, the sample sizes in some included trials were small. Secondly, public bias was existed in this meta-analysis. ird, the included trials were from various countries, but the trials were published only in English and Chinese, which would result in reporting bias. Fourth, some included studies did not clearly state whether to exclude those subjects using ACE inhibitors. Finally, our study was mainly focused on ACE I/D genetic alteration, and the relationship between many other genes SNPs and IgAN susceptibility needed to be further explored.

Conclusions
Our pooled analysis supported that ACE gene locus I/D polymorphisms were linked with the susceptibility and progression of IgAN in Asian and Chinese individuals. ACE gene locus allele D and genotype DD could be risk factors of IgAN vulnerability. Conversely, those subjects carrying the ACE II genotype would reduce the risk of IgAN susceptibility. However, no obviously correlation was found between ACE I/D gene polymorphism and IgAN in Caucasian patients. In future studies, much more high-quality trials are required to clarify it.

Data Availability
e data used to support the findings of this study are available from the first author and corresponding author upon request. Since this is a pooled analysis, data from previous published studies were used.

Conflicts of Interest
e authors declare no conflicts of interest.

Authors' Contributions
Shi-kun Yang and Fen-fen Chu analyzed the data for the manuscript and wrote the manuscript. Shi-kun Yang, Fenfen Chu, and Wen-li Zeng performed the literature search. Wen-li Zeng and Shi-kun Yang edited the manuscript.