Association Between IL-6 -174G/C Polymorphism and Risk of Multiple Sclerosis: A Meta-Analysis

Abstract

Objective:

Interleukin-6 (IL-6) is a pleiotropic cytokine and important mediator of many inflammatory processes, which might affect susceptibility to multiple sclerosis (MS). The aim of this study was to assess the effect of IL-6-174G/C polymorphism on the risk of MS using a meta-analysis.

Materials and Methods:

The Pubmed, ISI Web of Science, Wanfang, VIP, and China National Knowledge Infrastructure databases were screened and six studies were included in the meta-analysis. Pooled odds ratios (ORs) with corresponding 95% confidence intervals (CI) were calculated to evaluate the association between the IL-6-174G/C polymorphism and risk of MS.

Results:

No significant association between the IL-6-174G/C polymorphism and risk of MS was observed in overall analyses. With stratification according to ethnicity, we found that carriers with the IL-6-174CC genotype had a 1.87-fold increased risk for the development of MS in Asians (recessive model: OR=1.87, 95% CI, 1.08-3.24), but not in Caucasians.

Conclusion:

This meta-analysis provides evidence that the IL-6-174G/C polymorphism may be a risk factor for the development of MS in Asians. Further association studies with a larger sample size are required to confirm the result in different populations.

Introduction

Multiple sclerosis (MS) is an inflammatory disorder of the central nervous system resulting in demyelination and axonal damage (Pender and Greer, 2007; Compston and Coles, 2008). Although the exact mechanism of MS is still unknown, it is generally believed that environmental and genetic factors contribute to the disease susceptibility and severity.

Previous work has shown that cytokines are the key components of the immune system, which play critical roles in oligodendrocyte death and axonal degeneration (Bjartmar et al., 2003; Matsushita et al., 2013; Stepien et al., 2013). Interleukin-6 (IL-6) is a pleiotropic cytokine and important mediator of many inflammatory processes, which might affect susceptibility to MS (Giralt et al., 2013; Malpass, 2013). The IL-6 gene is located on the long arm of chromosome 7 in humans (7p21). A single-nucleotide polymorphism at position −174 in the promoter region of IL-6 (rs1800795 G/C) has been identified to suppress IL-6 transcription (Fishman et al., 1998). Based on the important roles of IL-6 playing on MS risk, the relationship between the IL-6-174G/C polymorphism and MS susceptibility has been examined extensively.

In spite of large numbers of studies investigating the effect of the IL-6-174G/C polymorphism on MS risk, the association is far from being identified due to conflicting results. Mirowska-Guzel et al. (2011) reported that the frequency of the IL-6-174 CC genotype was more than twice as often for MS patients as for controls. In contrast, Makarycheva et al. (2010) reported that no significant difference in the distribution of the IL-6-174G/C polymorphism between cases and controls. Therefore, it is of great value to provide strong evidence by combining all the data together. In this study, we performed a meta-analysis to ascertain whether the IL-6-174G/C polymorphism is related to the risk of MS.

Materials and Methods

Selection of published studies

We searched the Pubmed, ISI Web of Science, Wanfang, VIP, and China National Knowledge Infrastructure databases by using the following terms: “interleukin-6 or IL-6” and “polymorphism or variant” and “multiple sclerosis” (last search update 28 May, 2013). The inclusion criteria were as follows: (1) the design is a case-control study to examine the association between the IL-6-174G/C polymorphism and MS risk, and (2) sufficient data to calculate odds ratios (ORs) and 95% confidence intervals (CI).

Data extraction

The following information was extracted: the first author, year of publication, country of origin, ethnicity, matching variables, genotyping method, the IL-6-174G/C genotype frequency of cases and controls, total number of cases and controls, and Hardy-Weinberg equilibrium (HWE).

Statistical analysis

The association between the IL-6-174G/C polymorphism and risk of MS was evaluated using heterozygote comparison (GG vs. CG), homozygote comparison (GG vs. CC), dominant model (GG vs. CC/CG), and recessive model (CC vs. CG/GG), respectively. ORs and 95% CI were calculated to estimate the association. The Q-test and I² statistics were used to assess the between study heterogeneity (Higgins and Thompson, 2002). First, we used the random-effects model for the evaluation. If the p-value is more than 0.10 in heterogeneity analysis, the fixed-effects model was used (Mantel and Haenszel, 1959; DerSimonian and Laird, 1986). Meta-regression was used to check the sources of heterogeneity. The subgroup analyses were carried out according to ethnicity (Asian vs. Caucasian) and HWE. A funnel plot and Egger's regression asymmetry test were carried out to evaluate publication bias (Egger et al., 1997). All analyses were performed using STATA software, version 10.0 (STATA Corp., College Station, TX).

Results

Studies included in the meta-analysis

By searching the databases, 65 publications were identified. Seventeen of them were excluded for duplicated data. The remaining 48 full-text studies were read, and 42 records were excluded because of not IL-6-174G/C polymorphism (n=19), not MS (n=14), no available data (n=5), review articles (n=3), and overlapped data (n=1). Finally, a total of six studies were included in the meta-analysis. The characteristics of the included studies are shown in Table 1.

Table 1.

Characteristics of Literature Included in the Meta-Analysis

References	Year of publication	Country (region)	Ethnicity	Sample size (cases/controls)	Genotyping methods	Matching criteria	HWE
Yan et al.	2012	Australia (Brisbane)	Caucasian	509/279	DNA sequencing	-	Yes
Mihailova et al.	2005	Bulgaria (Sofia)	Caucasian	55/86	PCR-SSP	Ethnicity	Yes
Izad et al.	2010	Iran (Tehran)	Asian	98/92	PCR-SSP	Ethnicity	Yes
Shahbazi et al.	2010	Iran (Gorgan)	Asian	345/426	PCR-SSP	Sex, ethnicity, and age	No
Mirowska-Guzel et al.	2011	Poland (Warsaw)	Caucasian	230/195	PCR-RFLP	Age	Yes
Makarycheva et al.	2010	Russia (Moscow)	Caucasian	104/208	PCR-RFLP	-	Yes

PCR-SSP, polymerase chain reaction-sequence-specific primer; PCR-RFLP, polymerase chain reaction-restriction fragment length polymorphism; HWE, Hardy-Weinberg equilibrium.

Meta-analysis

The pooled results of all the included studies are shown in Table 2. No significant association between the IL-6-174G/C polymorphism and risk of MS was observed (heterozygote comparison: OR=1.21, 95% CI, 0.80-1.84; homozygote comparison: OR=0.86, 95% CI, 0.46-1.62; dominant model: OR=1.15, 95% CI, 0.74-1.76; and recessive model: OR=1.23, 95% CI, 0.70-2.14, respectively). After exclusion of the study that has deviated from HWE (Shahbazi et al., 2010), no significant association was also found. In the subgroup analysis according to ethnicity, the same negative result was also detected in Caucasians (heterozygote comparison: OR=1.02, 95% CI, 0.69-1.51; homozygote comparison: OR=0.98, 95% CI, 0.40-2.42; dominant model: OR=1.03, 95% CI, 0.61-1.73; and recessive model: OR=1.03, 95% CI, 0.49-2.15, respectively). However, a significant association was found in Asians in the recessive model (OR=1.87, 95% CI, 1.08-3.24).

Table 2.

Meta-Analysis of Association Between IL-6-174G/C Polymorphism and Multiple Sclerosis Risk

			GG vs. CG		GG vs. CC		GG vs. CC/CG (dominant)		CC vs. CG/GG (recessive)
	n	Cases/controls	OR (95% CI)	p^a	OR (95% CI)	p^a	OR (95% CI)	p^a	OR (95% CI)	p^a
Total	6	1339/1281	1.21 (0.80-1.84)^b	<0.001	0.86 (0.46-1.62)^b	<0.001	1.15 (0.74-1.76)^b	<0.001	1.23 (0.70-2.14)^b	<0.001
Ethnicity
Asian	2	443/518	1.61 (0.81-3.22)^b	0.04	0.69 (0.39-1.20)	0.77	1.42 (0.71-2.85)^b	0.001	1.87 (1.08-3.24)	0.81
Caucasian	4	896/763	1.02 (0.69-1.51)^b	0.05	0.98 (0.40-2.42)^b	<0.001	1.03 (0.61-1.73)^b	0.03	1.03 (0.49-2.15)^b	<0.001
HWE	5	994/855	1.00 (0.81-1.23)	0.10	0.90 (0.42-1.94)^b	<0.001	1.00 (0.66-1.53)^b	0.004	1.11 (0.59-2.11)^b	<0.001

p-Value for heterogeneity test.

Random-effects model was used.

OR, odds ratio; CI, confidence interval.

Heterogeneity analysis and publication bias

Obvious heterogeneity was observed in overall comparisons and subgroup analyses. Meta-regression was done to explore the sources of heterogeneity by considering the following variables: ethnicity, HWE, genotyping methods, and sample size. However, none of the possible variables could account for the heterogeneity between studies. A funnel plot and Egger's regression asymmetry test were used to evaluate publication bias of the studies reporting the effect of the IL-6-174G/C polymorphism on MS risk, and no evidence of publication bias was detected.

Discussion

It is well known that inflammatory cytokines may influence the outcome of immunological disorder. IL-6 is an inflammatory cytokine that is relevant to several neurological diseases, including MS. Increased levels of IL-6 mRNA and protein have been detected in the blood and cerebrospinal fluid in MS patients (Navikas et al., 1996; Maimone et al., 1997; Schonrock et al., 2000; Stelmasiak et al., 2000; Stelmasiak et al., 2001). Evidence from animal model studies also showed that IL-6 is involved in the MS pathological process (Campbell et al., 1997; Okuda et al., 2000). IL-6-deficient mice revealed resistance to myelin oligodendrocyte glycoprotein induced experimental autoimmune encephalomyelitis (EAE), whereas administration of IL-6 caused typical EAE (Okuda et al., 2000). Transgenic mice that constitutively overexpress IL-6 in the intact central nervous system resulted in unique chronic-progressive neurological disorders with neurodegeneration and cognitive impairment (Campbell et al., 1997). These findings suggest that IL-6 may be a candidate gene for the development of MS.

Previously, a functional polymorphism in the promoter region of IL-6 (rs1800795 G/C) has been reported to regulate IL-6 transcription activity (Fishman et al., 1998). Given the functional importance of IL-6 in the pathogenesis of MS, extensive studies have been performed to examine the relationship between the IL-6-174G/C polymorphism and the risk of MS. Conflicting results, however, have been obtained. Therefore, it is necessary to clarify this issue by conducting a quantitative meta-analysis.

To the best our knowledge, this is the first study to investigate the association between the IL-6-174G/C polymorphism and the risk of MS using a meta-analysis. After pooling all the eligible publications together, no significant association was detected between the IL-6-174G/C polymorphism and the risk of MS in overall analyses. With stratification according to ethnicity, we found that carriers with the IL-6-174CC genotype had a 1.87-fold increased risk for the development of MS in Asians, but not in Caucasians. Nevertheless, obvious heterogeneity between studies was found in both the overall analyses and subgroup analyses. To explore the heterogeneity, some potential reasons were considered, such as ethnicity, HWE, genotyping methods, and sample size. Unfortunately, none of the variables can account for the heterogeneity.

Even though we found a positive result of the effect of the IL-6-174G/C polymorphism on MS risk in Asians, this finding should be considered with caution because only two reports of Asians were included in the meta-analysis. The relatively small sample size might provide limited power to assess the relationship between the IL-6-174G/C polymorphism and MS risk. Moreover, obvious heterogeneity occurred in the study, which may lead to misleading results. Although the limitations existed in the study, no evidence of publication bias was detected, indicating that the result is robust.

To conclude, this meta-analysis provides evidence that the IL-6-174G/C polymorphism may be a risk factor for the development of MS in Asians. Further association studies with a larger sample size are required to confirm genetic variants regarding MS susceptibility in different populations. Additionally, gene-gene and gene-environment interaction analyses would also be of great value.

Footnotes

Author Disclosure Statement

The authors declare they have no conflicts of interest.

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