Exploring the association of IL-10 polymorphisms in Behcet's disease: a systematic review and meta-analysis.

Background
Polymorphisms in the interleukin-10 (IL-10) gene have been studied in various ethnic groups for possible association with Behçet's disease (BD). This study aimed to perform a meta-analysis of eligible studies to calculate the association of IL-10 polymorphisms with BD.A systematic literature search was carried out in PubMed, Embase, Web of Science, and Scopus databases to identify relevant publications, and extracted the respective results. Pooled odds ratio (OR) with 95% confidence interval (CI) was used to evaluate the power of association with a random-effects model.


Results
A total of 19 articles, consisting of 10,626 patients and 13,592 controls were included in the meta-analysis. The meta-analysis revealed significant associations in allelic and genotypic test models of - 819 (C vs. T: OR = 0.691, P < 0.001; CC vs. TT: OR = 0.466, P < 0.001; CC + CT vs. TT: OR = 0.692, P < 0.001; and CC vs. CT + TT: OR = 0.557, P < 0.001), - 592 (C vs. A: OR = 0.779, P = 0.002; CC + AA vs. AA: OR = 0.713, P = 0.021; and CA vs. AA: OR = 0.716, P = 0.016), rs1518111 (G vs. A: OR = 0.738, P < 0.001; GG vs. AA: OR = 0.570, P < 0.001; GG + AG vs. AA: OR = 0.697, P < 0.001; GG vs. GA + AA: OR = 0.701, P < 0.001; and AG vs. GG: OR = 0.786, P = 0.004) and rs1554286 (C vs. T: OR = 0.582, P < 0.001; CC vs. TT: OR = 0.508, P < 0.001; CC + CT vs. TT: OR = 0.605, P < 0.001; CC vs. CT + TT: OR = 0.665, P = 0.012; and CT vs. TT: OR = 0.646, P = 0.001). However, we failed to find any association between - 1082 polymorphism and susceptibility of BD.


Conclusion
This meta-analysis demonstrated that the interleukin-10 -819, - 596, rs1518111 and rs1554286 polymorphisms could be responsible against BD susceptibility, and should probably be regarded as a protective factor for Behçet's disease.


Background
Behçet's disease (BD) is a chronic remitting systemic vasculitis of unrevealed etiology [1]. BD is heterogeneous in onset [2] that its common clinical manifestations are orogenital ulcers, skin lesions, and uveitis [3]. BD is frequently observed in the Far East and the Middle East; however, it has distributed along the Old Silk Route due to immigration [1,[4][5][6]. Although the exact cause of Behçet's disease is mostly indefinite, it is believed that the pivotal pathophysiologic event is an inflammatory reaction to environmental factors in a genetically predisposed host and presence of epigenetic changes [7][8][9][10][11]. Research has evidenced that various cytokine networks play a crucial role in the determination of the disease onset, course of the disease, and outcome [12]. Interleukin-10 (IL-10) [Gene ID: 3586] is a multifunctional cytokine that acts upon other cytokines' signaling pathways [13]. IL-10 is a Type II cytokine in a family, including IL-19, IL-20, IL-22, IL-26, and IL-29 [14]. IL-10 is expressed mostly by activated monocytes/macrophages, natural killer cells, dendritic cells, mast cells, T lymphocytes (mainly Th2 subsets), and B lymphocytes [15].
This immuno-regulatory cytokine was demonstrated to have an imperative cytokine-suppressing role in the autoimmunity and inflammatory responses as a result of its ability to the downregulate antigen presentation and macrophage activation [16][17][18][19][20]. The abundant evidence has revealed that low IL-10 expression is involved in the pathogenesis of inflammatory and autoimmune diseases such as BD [21]. Furthermore, polymorphisms in multiple immuno-regulatory genes have indicated as a risk predisposition for the developing of BD owing to their effect on the cytokine production [22]. The human IL-10 gene maps to 1q31-32 and contains five exons and four introns. The IL-10 promoter is highly polymorphic and these single nucleotide polymorphisms (SNPs) appear to be correlated to the expression level of IL-10. Three frequently investigated SNPs of IL-10 gene are − 1082 A to G (rs1800896) -lies within a putative Ets transcription factor binding site-, − 819 T to C (rs1800871) -placed within a putative positive regulatory region-, and − 592 A to C (rs1800872) -is located within a putative STAT-3 binding site and negative regulatory regionpolymorphisms [23,24]. Rs1518111 (2195 A to G) which is in linkage disequilibrium (LD) with rs1800896 and rs1554286 (2607 T to C) that is in a strong LD with rs1800571 are the intronic variant of second and third exons [25,26].
Considering the role of IL-10 in BD and the correlation between the IL-10 gene polymorphisms and IL-10 production, several studies have been carried out to investigate the association of the IL-10 gene polymorphisms with the BD susceptibility. However, contradictory reported results and/or small sample size could lead to low statistical power and false-positive results. Therefore, in order to deal with these ambiguities, and potential biases, such as publication bias and providing increased statistical power, we turned to meta-analysis. In the present study, a meta-analysis of all available published studies was carried out to clarify the association between the IL-10 gene polymorphisms and the BD susceptibility.

Literature search and study characteristics
The literature search filtered 2390 English and Persianlanguage potential articles related to the IL-10 polymorphisms and BD. After the removal of duplicates, other publications were reviewed based on the titles and abstracts. In the end, 24 eligible studies were selected with a full-text review where 5 studies were eliminated (two studies due to non-availability of the data, even no responses could be gotten after having sought the relevant data via email contacts, two were posters and one study was involving family members). A flowchart of the study selection process and its results is presented in Fig. 1.
We conduct the meta-analysis, involving 10,626 cases and 13,592 controls. Of the 19 articles, studies including different subpopulations were considered as separate studies. Consequently, these groups were independently analyzed. Ten studies reported on -1082G/A, nineteen studies on -819C/T, thirteen studies on −592C/A, thirteen studies on rs1518111G/A and three studies on rs1554286C/T. The baseline characteristics of the eligible studies are described in Table 1.

Association between rs1800896 polymorphism and risk of BD
Based on ten studies [26][27][28][29][30][31][32][33][34], including 1970 cases and 1930 controls, the association between IL-10 -1082G/A polymorphism and BD susceptibility was examined. The combined results showed that considering all the studies, the comparisons of allele and genotypes failed to detect any statistical association under the random-effects model ( Table 2 and Fig. 2).

Association between rs1800872 polymorphism and risk of BD
For IL-10 -592C/A polymorphism, thirteen studies [7, 28, 29, 31-33, 35, 39, 41, 42] were involved composed of 3132 BD patients and 3638 controls. The results of the combined analysis of the association between -592 polymorphism and BD are summarized in Table 2 and Fig. 2. There were significant associations for C vs. A allele, CC + CA vs. AA and CA vs. AA genotypes; however, other comparisons failed to obtain any significant association.

Association between rs1518111 polymorphism and risk of BD
Thirteen studies [8,26,35,37,38,40], including 5519 cases and 5643 controls focused on the relationship between rs1518111 polymorphism and BD risk. The combined results ( Table 2 and Fig. 2) showed significant differences in all models between BD cases and healthy controls.

Association between rs1554286 polymorphism and risk of BD
In total, three studies [26,28,36] which contained 2421 cases and 3429 controls were performed to assess the importance of rs1554286 polymorphism for BD susceptibility. As shown in Table 2 and Fig. 2, this polymorphism was found to be significantly associated with BD susceptibility under all models.

Heterogeneity and publication bias
A funnel plot was used to reveal any publication bias influencing the analysis. The funnel plots did not show a significant sign of asymmetry for IL-10 polymorphisms. For rs1554286, a funnel plot was not performed because it is useless when the number of studies is limited (Fig. 3). The Metareg's test and Egger's test were also conducted and the findings showed a lack of publication bias among all comparison models except heterozygous (CC vs. CT + TT) model of rs1800871.
To test heterogeneity among the selected studies, the I 2 statistic was employed. The heterogeneity was significant under the genetic models of CC vs. CT + TT and CT vs. TT of rs1800871. After excluding the study by Kirino [40], the heterogeneity was eliminated (I 2 = 44.3, association P < 0.001) and (I 2 = 19.6, association P < 0.001), respectively. Significant heterogeneity was detected in C vs. T model of rs1554286 polymorphism. It can be due to a small number of studies included in this analysis. Therefore, the random-effects model was applied to synthesize the data ( Table 2).

Discussion
In order to better understand the possible genetic association of the IL-10 gene polymorphisms and BD, we conducted a systematic review and meta-analysis using pooling the results of the independent usable studies to determine whether the IL-10 polymorphisms have a potential impact on the BD susceptibility.
Behçet's disease is a multi-system inflammatory disorder with complex etiology, therefore genes involved in the immune system and inflammatory responses are   [52]. Several studies have been conducted on the influence and impression of the interleukin-10 gene and its protein on the probability of the multiple autoimmune and inflammatory diseases, such as BD relying on its chromosomal location and functional relevance.
Research on the effect of IL-10 on various inflammatory models has led to different and sometimes contradictory results even in the same models, indicating that there are intricate influenced agents on the regulation of cellular responses by IL-10. In the experimental study by Rizzo et al. [53] indicated that IL-10 knockout mice had a susceptibility to develop the Experimental Autoimmune Uveoretinitis (EAU) in comparison to controls, also, IL-10-treatment in induced mice for the EAU showed a resistance to the disease. Other animal studies have also confirmed the protective effect of IL-10 on the ocular inflammation and CNS autoimmune [54][55][56]. Some studies on BD have observed a reduction in IL-10 protein level in BD patients' peripheral blood and aqueous humor [13,30,41,57], while in other studies, there has been an increase in the concentration of this protein in plasma and active ulcers of BD [58][59][60][61][62][63]. On the other hand, there is a study that has not recorded significant changes between patients and controls [64]. Similar to the protein concentration determination studies, the results of the mRNA expression level were varied among previous studies. Several investigations [13,41,64] have shown low expression of the IL-10 mRNA in B cells and serums of BD patients. In contrast, Ahmed Ben et al. [65] have reported a high expression of the IL-10 mRNA in oral/genital ulcers, pseudofolliculitis lesions and sites of positive pathergy test. In contrast, Balkan et al. [66] found no significant difference in the IL-10 mRNA expression.
So far, different factors have been studied in the production and regulatory the role of IL-10. One of these influential factors that have recently been studied on IL- Fig. 2 Forest plot of the association of IL-10 polymorphisms with BD. A random-effects model for the OR with 95% CI was used to detect an association between IL-10 polymorphisms with BD. (a) rs1800896 (b) rs1800871 (c) rs1800872 (d) rs1518111 (e) rs1554286 10 is epigenetic agents. The study of Alipour et al. [13] showed a hyper-methylation on IL-10 gene promoter region. Genetic variants are other factors that account for up to 75% of the IL-10 expression variance in interindividual human [67]. Investigation of the polymorphic region in the transcriptional regulation of IL-10 gene may be helpful in predicting inflammatory diseases such as BD. Three common polymorphisms of IL-10 (− 1082G/A, − 819C/T and -592C/A) have been evaluated in detail for their impacts on gene transcription. In vitro studies showed that the highest IL-10 expression has been observed in GCC (− 1082/− 819/− 592) haplotype and lowest expression in AT (− 1082/− 819) haplotype [68][69][70]. However, studies on BD have not confirmed the effects of the above alleles [30]. In addition to the promoter variants, the intronic variant of rs1518111 is also likely to influence transcription [66].
The present meta-analysis was performed to examine the association between the IL-10 polymorphisms (− 1082, − 819, − 592, rs1518111, and rs1554286) and BD susceptibility in different ethnic groups including East Asian, Middle Eastern, European, and African populations. The strongest associations were observed in the comparisons of allele and genotypes of − 819C/T promoter SNP, rs1518111G/A intronic SNP, and rs1554286C/T intronic SNP. The overall results indicated the protective impact of these polymorphisms against the disease. So that, individuals carrying C allele of −819 and rs1554286 have 31 and 42% lower risk of developing BD, respectively. We also achieved significant association in -592C/A promoter SNP, which showed a relatively protective role against BD. However, the comparisons of allele and genotypes of -1082G/A promoter SNP failed to demonstrate any significant association.
However, our results should be interpreted cautiously, with respect to the following limitation reasons. First, limited number of included studies in some studied SNPs; second, heterogeneity and publication bias, for instance, missing articles due to unpublished studies, possibly with negative results, and/or language bias due to writing in other languages (except English and Persian); third, lack of sufficient data on the characteristics and symptoms of the participants in most of the primary studies to perform subgroups analysis, such as based on age, gender and the disease activity; lastly, some genotype distributions of control groups do not follow the HWE.

Conclusions
Our data demonstrated significant associations of the IL-10 polymorphisms with BD. Accordingly, the polymorphisms of -819, − 592, rs1518111 and rs1554286 play relatively protective roles against Behcet's disease.

Search strategy
A comprehensive search for the IL-10 polymorphisms and BD was performed in PubMed, Embase, Web of Science, and Scopus databases for articles in English, and SID, Magiran, IranMedex, and IranDoc for articles in Persian up to February 2019 according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist and PICO approach [43]. The most relevant articles were identified by computerized searches using relevant text words and medical subject headings (MeSH) that included all spellings and combination of "Behcet's syndrome" or "Behcet's disease" and "Interleukin-10" or "IL-10". Besides, we handsearched reference lists of selected publications and previous meta-analysis study to ensure that all relevant studies and recent reviews were included. The search was limited to case-control studies and Genome-Wide Association Studies (GWAS). No restrictions were placed on race, ethnicity or geographic area publication date.

Inclusion and exclusion criteria
A study was included in this meta-analysis if (1) it was published by February 2019, (2) it was a case-control study or GWAS that determined the distributions of the IL-10 -1082G/A, −819C/T, −592C/A, rs1518111G/A, and rs1554286C/T polymorphisms, in patients with BD and controls, (3) it contained original data, and (4) it provided enough data to calculate odds ratios (ORs). Exclusion criteria were as follows: (1) Keywords/search terms/reviews, and abstract scanning criteria, (2) the study was a review or abstract and publications in duplicate, (3) unrelated to each of BD and these polymorphisms, (4) studies involving family members, because their analysis was based on linkage considerations, and (5) papers do not involve human subjects.

Data extraction
Two authors (E.S. and N.R.) independently extracted the following data from all studies selected by using a studyspecific data extraction form: first author's name, year of publication, ethnicity, mean age of participants, number of cases and controls and method for determination of genotype, studied polymorphisms, Hardy-Weinberg equilibrium (HWE) P-values. Any disagreement between authors was resolved by discussion until a consensus was reached. If they failed to reach an agreement, a third author (M.G.) was consulted to resolve the discrepancies. Due to the lack of the mean age and HWE P-values for some studies, we were not able to analyze the data based on this information. This study was conducted in the Liver and Gastrointestinal Disease Research Center, Tabriz University of Medical Sciences. . Z-test with P < 0.05 was used to authenticate the statistical significance of effect size. Sensitivity analysis was used to investigate the cause of dispersion. Then, outlier studies were removed and analysis was recomputed. In the case of a significant reduction in dispersion (I 2 index), this study was considered as a dispersion factor. With regard to the heterogeneity of the studies, Cochran's Q test (p-value [phet] < 0.10 was considered as statistically significant heterogeneity) and I 2 statistics (75 ≤ I 2 < 100 as extreme heterogeneity, 50 ≤ I 2 < 75 as high heterogeneity, 25 ≤ I 2 < 50 as moderate heterogeneity, and I 2 < 25 as no heterogeneity) were used to assess the degree of heterogeneity [71]. The random-effects model was used for meta-analysis because it accounts for random variability both within and among studies. Publication bias was investigated by Funnel plot and Egger's test. A P-value < 0.05 was considered statistically significant for all analyses.