Matrix metalloproteinase gene polymorphisms and periodontitis susceptibility: a meta-analysis involving 6,162 individuals

We aimed to systematically investigate the potential association of matrix metalloproteinase (MMP)-9, -3, -2, and -8 gene polymorphisms with susceptibility to periodontitis using meta-analysis. A literature search in PubMed, Embase, and Web of Sciencewas conducted to obtain relevant publications. Finally a total of 16 articles with 24 case-control studies (nine on MMP-9-1562 C/T, seven on MMP-3-1171 A5/A6, four on MMP-2-753C/T, and four on MMP-8-799 C/T) were considered in this meta-analysis. The results based on 2,724 periodontitis patients and 3,438 controls showed that MMP-9-1562C/T, MMP-3-1171 A5/A6, and MMP-8-799C/T polymorphisms were associated with periodontitis susceptibility. No significant association was found between MMP-2-753 C/T and periodontitis susceptibility. Subgroup analyses suggested that the MMP-9-1562 C/T polymorphism reduced chronic periodontitis susceptibility and MMP-3-1171 A5/A6polymorphism increased chronic periodontitis susceptibility. In summary, current evidence demonstrated that MMP-9-753 C/Tpolymorphism reduced the risk of periodontitis, MMP-3-1171 5A/6A and MMP-8-799 C/Tpolymorphisms increased the risk of periodontitis, and MMP-2-753 C/T was not associated with risk of periodontitis.

epidemiological studies has been conducted to investigate the association between MMP-3-1171 A5/A6, MMP-8-799 C/T, and MMP-2-753 C/T polymorphisms and periodontitis susceptibility. Nevertheless, the results still remain inconsistency and individual studies based on small sample sizes have low statistical power to investigate the real association 22 . These facts warranted us to perform a meta-analysis to further investigate the role of these polymorphisms in the pathogenesis of periodontitis.

Methods
This study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statementin reporting this meta-analysis 23 . Eligible criteria. The following criteria were used for literature selection: 1) articles that explored the association between MMP gene polymorphisms and periodontitis (CP and/or AgP) susceptibility; 2) the study design was cohort or case-control study; 3) there are adequate data to estimate odds ratios (ORs) and corresponding 95% confidence intervals (CIs). Two investigators independently screened all papers by title or abstract and then by a full content evaluation. Any discrepancy between the two authors was solved by discussion with a third investigator.
Search strategy. A comprehensive electronic database search was conducted for studies that examined associations between MMP polymorphisms and periodontitis. We utilized the PubMed, Embase, and Web of Science citation index to identify papers in which MMP polymorphisms were examined in patients with periodontitis and controls (up to 20 September, 2015). In addition, bibliographies of all potentially relevant studies and recently reviews were reviewed to identify additional articles not indexed by the aforementioned databases. The following text words and MeSH terms were searched: "matrix metalloproteinase" (MeSH term and text word), "MMP" (text word), "genetic, polymorphism" (MeSH term), "polymorphism" (text word), "periodontal disease" (MeSH term and text word), and "periodontitis" (MeSH and text word). We limited the search to studies that were carried out on humans and were written in English.
Data extraction and quality assessment. Two authors independently extracted the data and assessed the methodological quality. The following data were extracted from each study: first author's surname, publication year, country, ethnicity, type of disease, source of control, sample size, number of genotype distribution, Hardy-Weinberg equilibrium (HWE) for controls, site of polymorphism, genotyping method, smoking percentile, and other factors for environment. Quality assessment of included studies was evaluated using the Newcastle-Ottawa scale (NOS) by two authors (P = 0.90 for κ test) independently 22 . Data Analysis. A χ 2 test was used to assess the deviation of genotype distribution from HWE among controls. We performed meta-analyses using allelic contrast, homozygote contrast, heterozygote contrast, recessive, and dominant models. The associations between the polymorphisms and periodontitis risk were assessed by ORs and corresponding 95% CIs. Cochran's Q metric was used to assess between tric was used to assess between-study heterogeneity. When the Q metric (P < 0.1) indicated significant heterogeneity among studies, the random-effects model (the Der-Simonian and Laird method) was applied to perform the meta-analysis. If the between study heterogeneity was not significant (P ≥ 0.1), the fixed-effects model (Mantel-Haenszel method) was used. We quantified the degree of heterogeneity using I 2 statistic 24 . I 2 ranges between 0 and 100%, and stands for the proportion of inter-study variability attributable to heterogeneity rather than random error. I 2 values of 75%, 50%, and 25% were defined as high, moderate, and low estimates, respectively. If the number of included studies was applicable, we conducted stratified analyses on the bias of type of disease, ethnicity, agreement with HWE for controls, source of control, severity of CP, smoking status, and genotyping method. Funnel plots are used to detect publication bias. In addition, we performed Egger's linear regression test to measure the asymmetry of funnel plot (P < 0.05) 25 . The meta-analysis was conducted using the Stata 12.0 software (Stata Corp LP, College Station, TX, USA), and the P value was two-sided.

Results
Study identification and characteristics. As shown in Fig. 1, we initially identified 142 articles. Finally we included 16 articles 26-41 with 24 case-control studies involving 2,724 cases and 3,438 controls. Among them, there were four polymorphisms of MMP gene included in our meta-analysis: nine on MMP-9-1562 C/T, seven on MMP-3-1171 A5/A6, four on MMP-2-753C/T, and four on MMP-8-799 C/T. Two of these studies 28,37 contained data on two different groups (CP and AgP), which were considered independently. One article contained two independent case-control studies in different countries and in this study only allele number was available 39 (Table 3. Stratification by disease type indicated that individuals were more susceptible to CP rather than AgP (Table 3).  (Table 4). Subgroup analyses according to disease type, ethnicity, and smoking status showed that the increased risk was predominant in CP, Asians, and non-smokers (Table 4).

Publication bias. Due to limitations of the quantity of included studies, we just test the publication bias for
MMP-9-1562 C/T and MMP-3-1171 A5/A6 polymorphisms. The funnel plots based on allele model for MMP-9-1562 C/T and MMP-3-1171 A5/A6 polymorphism were asymmetry and indicated that publication bias probably existed in the present study. The Egger's test showed there was some publication bias existed in the MMP-9-1562 C/T allele model (Table 2).

Discussion
In the present meta-analysis, we aggregated data from published studies to estimate genetic associations between MMP gene, namely MMP-2-753 C/T, MMP-3-1171 A5/A6, MMP-8-799 C/T, and MMP-9-1562 C/T polymorphisms, and periodontitis susceptibility. Our results provided some evidence to support an elevated risk between periodontitis susceptibility and MMP-3-1171 A5 allele and MMP-8-799 T allele, and a reduced risk between periodontitis susceptibility and MMP-9-1562 T allele. But our study provided no evidence to support an association between MMP-2-753 C/T polymorphism and periodontitis. Appreciable differences were identified in the etiology characteristic between CP and AgP 42 , indicating that there might be different genetic mechanism between them. In stratified analysis by disease type, their association with susceptibility of CP rather than AgP was observed. For MMP-9-1562 C/T polymorphism, we did not observe any meaningful associations in stratified analysis by ethnicity, severity of CP, and smoking status. The results were similar to MMP-3-1171 A5/A6 and MMP-2-753 C/T polymorphisms for periodontitis susceptibility. However, in stratified analysis by ethnicity and smoking status for MMP-8-799 C/T polymorphism indicated an elevated risk in Asian populations rather than Caucasian populations, and non-smokers rather than smokers.   To our knowledge, this is the first quantitative analysis that assessed the association between MMP-2-753 C/T, MMP-3-1171 A5/A6, and MMP-8-799 C/T polymorphisms and periodontitis susceptibility. With regard to MMP-9-1562 C/T polymorphism, two meta-analyses waspublished in 2013. Pan et al. 21 indicated that MMP-9-1562 C/T polymorphism might be involved in the development of periodontitis. However, Song et al. 19 suggested no association between MMP-9-1562 C/T polymorphism and periodontitis susceptibility. Compared with the previous meta-analyses, we had a lager sample size than them, which increased the statistical power, and we found   a reduced risk of MMP-9-1562 T allele with periodontitis susceptibility. Genetic association researches designed to investigate relations between gene polymorphisms and complex outcomes must be interpreted with caution, because many factors could potentially affect the results. Therefore, we assessed the association with severity of CP and smoking status even though we did not observe any significant difference among the moderate and severe CP and no association among non-smokers. This might be caused by small sample size that only two studies 29,33 presented the association between polymorphism and disease severity, two studies 31,32 investigate the interaction between polymorphism and smoking status. However, the present meta-analysis also has certain limitations that affect the interpretation of the results. First, the heterogeneity for MMP-9-1562 C/T polymorphism was high. Subgroup analyses suggested that the heterogeneity might come from the deviation of HWE, ethnicity, and genotyping method. Certainly, other clinical heterogeneity also might contribute to it, for instance different classification and diagnosis of periodontitis and differences on the oral examination by different clinicians. In addition, it is widely acknowledged that meta-analysis is a secondary analysis and we could not handle the problem of clinical heterogeneity in a meta-analysis. Therefore, we recommend that further studies should be designed as multi-center studies and utilizea unified criterion of disease. Second, although a comprehensive literature search was performed, it was likely that some publications were overlooked because of our language restriction for the literature search. In addition, the number of included studies for each polymorphism was limited. Therefore, the statistic power of present meta-analysis might be affected and the present results might be led to false positive or false negative rate. Third, we used genotype distributions and crude estimates of effect rather than adjusted estimates of association between polymorphism and periodontitis. Even though we investigated the interaction between polymorphism and disease severity and smoking status, the statistic power was limited due to most studies did not present the relevant data. Moreover, age, sex, and gene-gene interactions could not be assessed in our study due to insufficient data. Finally, the publication bias was of concern that small studies with negative results tend not to be published.
In conclusion, this meta-analysis with published data suggested that the MMP-2-753 C/T, MMP-3-1171 A5/ A6, and MMP-9-1562 C/T polymorphisms were associated with periodontitis susceptibility and there is lack of association between the MMP-8-799 C/T polymorphism and periodontitis. Further studies with large sample size, gene-gene, and gene-environment detailed information are needed to validate the present results.