Background
Glaucoma is the second largest cause of bilateral blindness worldwide that affects approximately 60 million people [
1]. Primary open-angle glaucoma (POAG), the major type of glaucoma, is characterized by progressive damage of the optic nerve and retinal ganglion cells (RGC), and subsequent irrevocable vision loss [
2]. Although the etiology is not entirely clear, genetic factors are thought to be a potential risk to POAG patients, including both those with normal and elevated intraocular pressure (IOP). Recently, the relationship between genetic polymorphisms of the immune system and were not perceived as being related to glaucoma, however, their relationship has been highly suspected and investigated recently [
3].
Interleukin-1 (
IL-1), an important mediator of inflammation, has been suggested to play a crucial role in neuro-degeneration such as Alzheimer’s disease [
4]. In view of the potential similarities in cellular mechanism leading to neuro-degenerative disorder between glaucoma and Alzheimer’s disease (AD), some investigators speculated that the polymorphism in
IL-1 gene clusters may be a genetic predisposing factor for glaucoma. It has been reported that
IL-1 was expressed endogenously in trabecular meshwork (MT) cells that controls a stress response specific to the aqueous outflow pathway and confers a protective response against glaucoma [
5]. Moreover, experiments in vivo showed that
IL-1 promotes glutamate uptake by Müller cells and increases the number of surviving of RGCs [
6].
Recently, several single nucleotide polymorphisms (SNPs) in the IL-1 gene cluster [i.e.
IL-1β rs16944 (c. -511C > T),
IL-1α rs1800587 (c. -889C > T) and
IL-1β rs1143634 (c. +3953C > T)] have been investigated for association with POAG [
7‐
14]. However, the results remain controversial due to some studies identified there was an association between
IL-1 gene SPN and POA while others failed. Furthermore, to our knowledge, no relevant genome-wide association study (GWAS) or meta-analysis has been published on this subject. In the current study, we conducted a systematic review and meta-analysis to summarize the contradictory results from these relevant studies and to clarify the relationship between
IL-1 cluster polymorphisms and POAG.
Methods
This meta-analysis was followed the the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) statement [
15].
Literature search
To obtain eligible literature, a comprehensive search without a language restriction was conducted in the PubMed, EMBASE and Cochrane Library covering publications up to July 15, 2017, by using of the following keywords: (“glaucoma” or “open angle” or “POAG” or “intraocular hypertension”) AND (“interleukin-1” or “
IL-1” or “interleukin 1”) AND (“polymorphism” or “SNP” or “single nucleotide polymorphism” or “variation” or “mutation”) (Additional file
1). The titles, abstracts, and full texts were screened by two independent investigators (L.J.H., F.Y.F.) to determine inclusion and disagreements were resolved by discussion between all authors until a consensus was determined. Review articles and bibliographies of other relevant studies were searched manually to identify additional eligible studies.
Inclusion and exclusion criteria
Literature selection had to meet the the following criteria: (1) case-control or cohort studies on the relationship of the SNPs of IL-1 with POAG; (2) all patients in the eligible studies meeting the diagnostic criteria for POAG; (3) had sufficient information for estimating odds ratios (ORs) with corresponding 95% confidence interval (CI). The exclusion criteria were: (1) case only studies; (2) abstract, case report and review papers; (3) repeated or overlapped publication. Only the most recent or complete study was used in this meta-analysis in case of the same patient population was included in several publications.
Quality assessment
The methodological quality of eligible studies was evaluated independently by two authors (J.H.L. and Y.F.F.), according to a modified version of the Newcastle–Ottawa scale (NOS) for genetic association studies [
16] (Additional file
2). NOS quality scores ranged between 0 and 9 stars. Studies with a score of five stars or greater were considered high quality.
According to the PRISMA guidance, the necessary information from eligible studies was extracted independently by two reviewers (L.J.H. and S.M.S.) using a standardized form. The following data was retrieved: surname of first author, publication date, region, ethnicity, number of cases and controls, age, subtypes of POAG, genotyping method and genotypes frequency, evidence of Hardy-Weinberg equilibrium (HWE) in control group, etc. When the allele or genotype counts were not given specially in some articles, they were calculated from the frequencies and then rounded to the nearest integer. Disagreements were resolved by discussion between all authors and subsequent consensus.
Statistical analysis
Statistical analysis was undertaken using RevMan software (version 5.0; Cochrane Collaboration, Oxford, United Kingdom) and STATA 11.0 software (Stata Corporation, Texas, USA). The strength of association between IL-1 SNPs and POAG was estimated by ORs with 95% confidence intervals (CIs). The pooled ORs were performed for the allele model (T versus C), homozygote model (CC versus TT), heterozygote model (TC versus CC), dominant model (TT + CT versus CC) and recessive model (TT versus CT + CC), respectively.
I
2value was applied to evaluate the between-study heterogeneity, with <25%, 25%–50%, and >50% to represent low, moderate, and high degree of heterogeneity, respectively [
17]. In addition, a Q-statistic test was conducted and the heterogeneity was considered significant at
P < 0.10. The summary OR and 95% CI for each polymorphism were pooled by using the fixed-effect model (Mantel-Haenszel) when no significant heterogeneity was observed among studies. Otherwise, the random effect model (DerSimonian and Laird) was adopted.
In association analysis, a pooled
P value of less than 0.05 was considered as suggestive evidence for a genetic association. Aim to control false positive error rate, the Bonferroni method was used to adjust for multiple comparisons [
18]. We performed 20 times comparisons for
IL-1β rs16944, 15 times comparisons for
IL-1α rs1800587 and
IL-1β rs1143634 in POAG respectively, therefore, the
P values that were less than 0.05/20 in
IL-1β rs16944 and 0.05/15 in
IL-1α rs1800587 and
IL-1β rs1143634 showed significance.
Power analysis regarding the association of each
IL-1 SNP with POAG was performed using the statistical software Power and Sample Size Calculation (PS) version 3.1.2 (
http://biostat.mc.vanderbilt.edu/wiki/Main/PowerSampleSize) and a value greater than 0.8 meant high statistical power. The HWE was assessed by Fisher’s exact test. Subgroup analyses were performed based on ethnicity, as well as type of POAG including high tension glaucoma (HTG) (IOP > 21 mmHg) and non-HTG (IOP < 21 mmHg, with or without a strict diurnal testing). Sensitivity analysis was performed to assess the stability of individual studies, and the potential of publication bias was assessed by Begg’s funnel plot [
19] and further evaluated by Egger’s linear regression test [
20].
Discussion
This is the first meta-analysis to summarize the evidence of associations between IL-1 gene cluster polymorphisms and susceptibility of POAG. Our data revealed that the SNPs of IL-1β rs16944, IL-1α rs1800587 and IL-1β rs1143634 were not associated with the POAG risk. Stratification analyses showed that IL-1β rs1143634 has a suggestive associated with the risk of HTG, however, this association was feeble after Bonferroni adjustments.
Recently, polymorphisms in the
IL-1 gene clusters have been shown in chronic neurodegenerative disease, such as multiple sclerosis [
21], Parkinson’s disease [
22] and Alzheimer’s disease [
23]. The C allele of rs16944 and rs1800587, and the T allele of rs1143634 has been speculated to influence the risk of POAG by altering the expression level of the respective proteins, because the sites of rs1800587 and rs16944 are located within the transcriptional promoter regions of the
IL-1α and
IL-1β genes, respectively, and the site of rs1143634 is within the coding region of the
IL-1β gene [
24,
25].
IL-1 is a key mediator of immune and inflammatory responses, which act directly to mediate a number of cellular responses. Laboratory studies have showed that
IL-1 could promote ganglion cell loss and optic nerve damage by increasing matrix mellanoproteinase-9 (
MMP-9) synthesis in experimental models [
26,
27].
IL-1 has also been reported to induce nitric oxide synthesis and reactive oxygen species (ROS), which involved in RGC damage leading to neurodegeneration [
28]. However, some studies have put forward argument and their results pointed to certain neuro-protective role of
IL-1 [
5,
6]. Therefore, we undertake the present meta-analysis on all the available data to establish a more robust estimate of the association between
IL-1 gene clusters polymorphism and POAG.
Although the meta-analysis for overall studies showed that all the three
IL-1 clusters polymorphisms were not associated with the POAG risk, the results of subgroup analyses should arouse our attention. On the one hand, genetic heterogeneity for POAG may exist in different populations. Stratified analyses in the current meta-analysis based on ethnicity showed that there is no statistical evidence of significant association between the rs16944 and POAG in Asian population. However, one study in Caucasians showed that the SNP rs16944 is important risk factors associated with POAG [
7]. Moreover, of the included studies for the Asian population, one studies [
7] found a possible association of the SNP rs1143634 with POAG in Taiwan population but this association could not be further replicated in Singaporean [
9] Chinese and Indian subjects [
12]. Thus, more studies with POAG cohort from different ethnic background are needed to further define this association.
On the other hand, genetic heterogeneity for POAG may exist in different subtypes of this disease. For example, Wang et al. found an increased risk for individuals carrying T allele of
IL-1α rs1800587 in HTG patients with an IOP > 21 mmHg [
8] but not for normal-tension glaucoma (NTG) cases with IOP < 21 mmHg [
10]. Similarly, the SNP rs1143634 seem to be associated with HTG rather than NTG. Glaucomatous damage to the retina and optic nerve is often accompanied by pathological IOP elevation. These findings of our meta-analysis suggested that the pathogenesis and effect of
IL-1 may be different between HTG and NTG. Laboratory and clinical studies are needed to further clarify the molecular pathogenesis of the two subtypes of POAG as well as the role of
IL-1 on IOP elevation.
Publication bias should be considered as it is an important factor that affects the reliability of the results of meta-analyses. In the current study, both Begg’s and Egger’s test were used to assess the potential publication bias and failed to detected a significant bias in all genetic models (Fig.
5), demonstrating the robustness and credibility of the present meta-analysis. However, some limitations of this meta-analysis should be taken into careful consideration. Firstly, some unescapable bias may exist in the results as only published studies with full-text were included in our analysis. Secondly, haplotype analysis for the possibility of linkage disequilibrium between SNPs was not performed in the present meta-analysis. How et al. [
9] found that The TT haplotype of rs1143634 and rs16944 together and the TTT haplotype of rs16944, rs1800587 and rs1143634 was significantly more common in normal control subjects than in those with POAG. Further investigations of the haplotypic effect of a gene and the study of multiple polymorphisms in different genes are needed. Thirdly, the results of power calculations indicated that the combined sample sizes (overall and subgroups) in the current meta-analyses were still inadequate and underpowered to detect the association of
IL-1 gene SNPs with POAG, due to limited availability of published data. Finally, the strength of association between
IL-1 gene SNPs and POAG was carried out by unadjusted estimate, we didn’t adjust pool results by the factors like the age, gender, disease severity and genotyping procedure.