Comprehensive analysis of interleukin-8 gene polymorphisms and periodontitis susceptibility

Background Associations between interleukin-8 (IL-8) gene polymorphisms and periodontitis susceptibility have been investigated in many published studies, but the conclusions are still inconsistent. Therefore, we performed this systematic review and meta-analysis to review which polymorphisms have been researched and to obtain a precise result of the same polymorphism from different studies. Results Finally 10 publications involving 1938 patients and 1569 controls were yielded, including 12 polymorphisms. Six studies investigated rs4073 polymorphism; two focused on rs2227306 and rs2227307; two referred to rs2227532 and T-738A; one detected rs2230054, rs1126579 and rs1126580; one inspected A2767T, T11722T2 and C1633T, and one for rs2234671 polymorphism. Of them, IL-8 C1633T and rs1126580 polymorphisms showed positive association while the other ten polymorphisms revealed negative results. Materials and methods A comprehensive literature search from PubMed, Web of Science, and Chinese National Knowledge Infrastructure was conducted for all potentially relevant studies published before January 2, 2017. Two authors selected the studies and extracted data. The pooled analysis was conducted using the RevMan 5.1 software if a polymorphism was reported by two or more studies. Conclusions Based on current evidence, the IL-8 rs4073, A2767T, T11722T2, rs2234671, rs2230054, rs1126579, rs2227306, rs2227307, rs2227532, and T-738A polymorphisms were not associated with periodontitis susceptibility; the IL-8 C1633T and rs1126580 polymorphisms were associated with increased risk of periodontitis.


INTRODUCTION
Interleukin (IL) plays an important role in mediating immune and inflammatory responses, and periodontitis is known as a chronic infectious disease. Hence, the genes and their variants (polymorphisms) of IL may be the important determinants of pathogenesis of periodontitis [1]. There are a large number of publications reporting the association between IL gene polymorphisms and periodontitis, some related meta-analyses have been performed for IL-1 polymorphisms [2][3][4][5][6], IL-4 polymorphisms [7], IL-6 polymorphisms [8], IL-10 polymorphisms [9][10], and IL-18 polymorphisms [11]. IL-8 gene is a component of IL genes with more than ten polymorphisms [12]. Numerous studies exploring the association between IL-8 gene polymorphisms and periodontitis have been published, but the results of previous studies of same polymorphism were inconsistent. Yang et al. [13] in 2016 performed a meta-analysis to investigate the effect of rs4073 (A251T/T-353A) on periodontitis susceptibility and found a positive association. The meta-analysis by Chen et al. [14] in 2015 focused on the rs4073 (A251T/T-353A) and rs2227532 (T-845C) polymorphisms which also revealed a positive association. Obviously, many other polymorphisms of IL-8 gene has not been explored, so we performed this analysis

Research Paper
Oncotarget 48997 www.impactjournals.com/oncotarget for further assessing the relationship between all IL-8 gene polymorphisms and periodontitis; additionally, we also reviewed which polymorphisms have been investigated. The present study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [15][16].

rs2227306 polymorphism
Meta-analysis of two studies [21][22] showed there was no significant association between rs2227306 (C781T) polymorphism and periodontitis risk. The specific results were presented in Table 3.

rs2227532 polymorphism
Two studies [18,25] reported the data for rs2227532 (T-845C) polymorphism, and the meta-analysis displayed this polymorphism was had no significant impact on periodontitis (Table 3).

T-738A polymorphism
The T-738A polymorphism was evaluated by two studies [18,25] and the pooled results uncovered there was non-significant association of T-738A polymorphism with periodontitis (Table 3).

rs1126579 polymorphism
The rs1126579 (T1208C) polymorphism was reported by one study [19]. The results indicated that this polymorphism was not implicated in periodontitis (C vs.
Compared with published two meta-analyses [13][14] on this topic, the major strength of our study is that it is the first comprehensive meta-analysis on IL-8 gene polymorphisms and periodontitis risk. We believe it gives a useful summary of current data regarding the relationship between IL-8 gene polymorphisms and periodontitis risk, and provides improved clinical clarity to obtain a solid evidence base for the diagnosis and treatment of periodontitis. As to the included studies, sample size for each polymorphism was very small, so the relevant researches are suggested to be conducted in the future. Since there are more than 15 polymorphisms of IL-8 gene have been characterized [29], the association between additional polymorphisms and risk of periodontitis should be investigated. The AgP and CP are  two different types of periodontitis and the former type is considered as a genetically inherited disease [2,32]. Hence, the further studies should report the data for CP and AgP separately.
The major limitation of our study was the numbers of included studies and sample size. Of these 12 polymorphisms, only IL-8 rs4073 polymorphism was involved in 6 studies while the others referred to one or two studies. Correspondingly, the sample size for each polymorphism was relatively small, which restrained the confidence of current results. Moreover, the subgroup analyses based on difference between ethnicity, smoking status, type of periodontitis, or other factors couldn't be conducted to investigate the source of heterogeneity, which might bias the results. Because of the limited numbers of included studies, we only assessed the publication bias of those investigating the IL-8 rs4073 polymorphism. Secondly, systematic review and metaanalysis is an observational study which was restricted by the quality of primary studies [6,16,[33][34][35]. Although we had conducted a more comprehensive search, our study could not escape from this limitation. Lastly, our results were based on unadjusted data and the original data were sufficient, hence the evaluation of the effects of the gene -gene or gene -environment interactions was neglected. These limitations mentioned above might affect our final conclusions.
In conclusion, our meta-analysis suggests that the IL-8 rs4073, A2767T, T 1 1722T 2 , rs2234671, rs2230054, rs1126579, rs2227306, rs2227307, rs2227532, and T-738A polymorphisms are not associated with periodontitis while the IL-8 C1633T and rs1126580 polymorphisms may elevate the susceptibility of periodontitis based on the currently available evidences. However, for considering that the studies included in our meta-analysis were based on small numbers, the current results should be treated with caution, and the results may change with the larger sample size in future. Due to these limitations, more well designed, studies with larger sample size, and adjusted risk factors are required to further validate our results.

Eligible criteria
We included the studies which met all of the following criteria: (1) the patients were clearly diagnosed as periodontitis (CP and/or AgP) and the controls were periodontitis-free or periodontal healthy; (2) at least one of the IL-8 polymorphisms and periodontitis susceptibility were evaluated, using a case-control or cohort study design; (3) the studies reported full data for necessary genotypes in each group or contained sufficient data to calculate them. If the same institute published two or more publications, we treated them as independent ones and chose the more comprehensive one.

Search strategy
A comprehensive literature search was performed in PubMed, Web of Science, and Chinese National Knowledge Infrastructure (CNKI) up to January 2, 2017. The following key words were used: IL-8, interleukin-8, interleukin 8, periodontal disease, periodontitis, variant, and polymorphism. Moreover, all listed references of included studies and recently reviews were retrieved for any additional relevant studies. No language restriction was applied in the search process.

Data extraction
Study selection and data extraction were performed by two authors and any discrepancy was resolved by discussion. The following data were extracted from each included study: surname of first author, year of publication, study design, country and ethnicity of study population, demographics, periodontitis type, smoking status, number of cases and controls, polymorphism, genotype distribution, source of controls, genotyping method, and Hardy-Weinberg equilibrium (HWE) for controls.

Data analysis
The OR and its 95% CI were calculated to estimate the relationship between the IL-8 polymorphisms and periodontitis risk. All studies used a allele genetic model and a dominant genetic model [5]. If a polymorphism was reported in two or more studies, a meta-analysis was conducted. Heterogeneity was assessed using the Cochran's Q statistic and I 2 statistic [15,36], with values of P ≥ 0.1 and I 2 < 50% indicating acceptable heterogeneity. If no significant heterogeneity existed, the fixed effect model was used; otherwise, the random-effects model was used. If the number of included studies was available, we conducted subgroup analyses on the ethnicity, smoking status, and periodontitis type. Publication bias was assessed by funnel plot [37]. All analyses were performed using RevMan 5.1 software [38][39].

CONFLICTS OF INTEREST
The authors declare no conflicts of interest.

GRANT SUPPORT
This research was supported (in part) by the Foundation of Evidence-based Medicine Nursery Fund of Taihe Hospital (EBM2013028 and EBM2014007), without commercial or not-for-profit sectors. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. No additional external funding received for this study. We also wish to thank relevant editors and peer-reviewers for their hard work and suggestions.