IL-6 gene rs1800795 polymorphism and diabetes mellitus: a comprehensive analysis involving 39099 participants

Background Over the past two decades, many studies concentrated the association between a common polymorphism ( rs1800795 ) from interleukin-6 (IL-6) gene and Diabetes have been published, however, the results remain ambiguous and indefinite.Methods In current, we performed a comprehensive analysis to explore above relationship. A search was conducted in the PubMed, Embase, Chinese (CNKI and Wanfang) databases, covering all papers published until Sep 20, 2019. Odds ratios (OR) with 95% confidence intervals (CI) was applied to evaluate the strength of this association. Publication bias was assessed with both Begg and Egger’s tests.Results Overall, 26 case-control studies with 5973 T2DM patients and 13968 controls, and 11 case-control studies (10193 T1DM patients and 8965 health controls) were included for analysis in our study. Finally, significant decreased association was observed between the rs1800795 polymorphism and T2DM risk in overall sample, Asians and hospital-based subgroup (for example: C-allele vs. G-allele: OR = 0.65, 95%CI = 0.53-0.81, P < 0.05), however, increased associations were found from Mixed population and hospital-based subgroup between rs1800795 polymorphism and T1DM susceptibility (for example: CC vs. GG: OR = 2.45, 95%CI = 1.18-5.07, P = 0.016 for Mixed individuals).Conclusions In summary, there had a definite evidence to confirm that IL-6 rs1800795 polymorphism was associated with susceptibility of decreased T2DM and increased T1DM. study 39099 individuals. Our results indicated that IL-6 rs1800795 acted as a protective factor in T2DM, in other words, individuals carrying C-allele may have decreased association with T2DM, for Asians and HB source studies. IL-6 rs1800795 a risk factor in T1DM, there


Abstract
Background Over the past two decades, many studies concentrated the association between a common polymorphism ( rs1800795 ) from interleukin-6 (IL-6) gene and Diabetes Mellitus (DM) risk have been published, however, the results remain ambiguous and indefinite.Methods In current, we performed a comprehensive analysis to explore above relationship. A search was conducted in the PubMed, Embase, Chinese (CNKI and Wanfang) databases, covering all papers published until Sep 20, 2019. Odds ratios (OR) with 95% confidence intervals (CI) was applied to evaluate the strength of this association. Publication bias was assessed with both Begg and Egger's tests.Results Overall, 26 casecontrol studies with 5973 T2DM patients and 13968 controls, and 11 case-control studies (10193 T1DM patients and 8965 health controls) were included for analysis in our study. Finally, significant decreased association was observed between the rs1800795 polymorphism and T2DM risk in overall sample, Asians and hospital-based subgroup (for example: C-allele vs. G-allele: OR = 0.65, 95%CI = 0.53-0.81, P < 0.05), however, increased associations were found from Mixed population and hospitalbased subgroup between rs1800795 polymorphism and T1DM susceptibility (for example: CC vs. GG: OR = 2.45, 95%CI = 1.18-5.07, P = 0.016 for Mixed individuals).Conclusions In summary, there had a definite evidence to confirm that IL-6 rs1800795 polymorphism was associated with susceptibility of decreased T2DM and increased T1DM.

Background
Diabetes Mellitus (DM) is a chronic medical condition in which the body either produces too little insulin from pancreatic islet or has a lack of effective access to insulin [1]. Type 1 DM (T1DM) is most often diagnosed in children and adolescents about the development of islet function. Type 2 DM (T2DM) is due to insulin resistance, that the body can't use insulin effectively and may gradually mislay the production capacity [2][3][4]. To our current knowledge, age, obesity and a family history are risk factors for developing of DM [5]. However, the exact pathogenesis of DM is still not fully understood. Past genome-wide association studies (GWAS) already identified over 100 genetic sites, which supported that there have significantly associations between different sites and susceptibility of DM, it also means genetic factors may be crucial for its occurrence and development [6,7].
Interleukin-6 (IL-6), as a classic proinflammatory cytokine, plays a prominent role in inflammatory response, which is associated with insulin-resistant states and T2DM [8]. In addition, chronic lowgrade inflammation and activation of the innate immune system are closely involved in the pathogenesis of T1DM and its complications. Inflammatory cytokines, such as IL-6, are shown to be determinant in these pathogenic processes [9, 10].
The IL-6 gene is located at chromosome 7p21. The gene including seven exons covers approxi mately 12.8 kb of genomic DNA [11]. A common single nucleotide polymorphism (SNP) in the IL-6 promoter was first discovered and identified, named rs1800795 (also named -174G/C) in T2DM [12]. The rs1800795 polymorphism has been suggested to functionally affect IL-6 promoter activity, which showed the carried CC genotype individual is associated with lower plasma levels of IL-6 compared with individuals with GG genotype [13], in addition, whose G-allele in homozygous (GG genotype) was associated with higher concentrations of IL-6 increasing the immune response [14,15].

Document retrieval and data extraction
We made use of searches on databases, including Pubmed, Embase, CNKI and Wanfang, until on Sep 20, 2019, with keywords including 'Interleukin-6/IL-6', 'polymorphism/variant' and 'Diabetes Mellitus/DM/TIDM/T2DM'. Eligible studies must be according with the following criteria: @) the studies assess the association between TIDM or T2DM and rs1800795 variants; @) case/control studies; @) age-and gender-matched control subjects. The criteria for exclusion were @) not case/control studies; @) insufficient genotype frequency; @) duplicated studies and @) significantly biased articles. The information including name of first author, year of publication, origin, race, DM type, genotype methods, Hardy-Weinberg equilibrium (HWE) were collected.

Statistic analysis
The correlation between IL-6 rs1800795 polymorphism and the risk of TIDM/T2DM was measured by 95%CI and OR according to the genotype frequencies of cases and control groups. Ethnic group are divided into African, Mixed, Caucasian and Asian. Population-based (PB) and hospital-based (HB) control subgroups were collected.
The statistical significance of the summary is calculated by Z-test. In these studies, the heterogeneity hypothesis was assessed by Q-test based on chi-square [53]. If significant heterogeneity (< 0.1) is detected, the random model is carried out, otherwise the fixed effect model is selected [54,55]. For IL-6 rs1800795, we studied the relationship between variation and the risk of T2DM in C-allele vs. Gallele, CG vs. GG and CC+CG vs. GG models; and C-allele vs. G-allele, CC vs. GG, CC vs. CG+GG, CG vs. GG and CC+CG vs. GG models for TIDM risk. The asymmetry of funnel plot was evaluated by Begg's test, and the publication bias was evaluated by Egger's test, whose P-value < 0.05 was considered significant [56]. Pearson chi-square test was used in the control group (P < 0.05), and χ 2 test was used to evaluate the deviation of rs1800795 polymorphism from the expected frequency of HWE [57]. All above statistical tests were conducted using Stata (Version 11.0; Statacorp LP, College Station of Texas). The power of our meta-analysis was calculated online using the website http://www.power-analysis.com/.

Study selection and charac teristics
A total of 1134 articles were identified from main four databases (Pubmed, Embase, CNKI and Wanfang) based on above search criteria. Among them, 35 articles were excluded because of following reasons: systematic analysis/meta-analysis (8), just only case study (6), other polymorphisms in IL-6 gene (12), not sufficient data of each genotype (5) and duplication (4) ( Figure   1). Thus, a total of 40 different articles [13-18] accounting for a total of 16581 DM patients and 23799 healthy controls were included in our meta-analysis (29 case-control studies including 6388 T2DM patients and 10193 controls, and 11 case-control studies including 14834 T1DM and 8965 controls, respectively) ( Table 1). Additionally, we tried to compare the minor allele frequency (MAF) between our current study and data from 1000Genomes  Table 1). Genotyping of the SNPs of IL-6 gene rs1800795 polymorphism was conducted using some different genotype methods in Table 1.

Publication bias and sensitive analysis
Begg's and Egger's test were performed to assess publication bias, which was not found both for  Table 4) . To delete studies which may influence the power and stability of whole study, we applied the sensitive analysis, finally, also no 7 sensitive case-control studies were found (Figure 9,10, Table 4). articles about meta-analysis also shown the conclusions above association [21-24], However, lack of powerful and convincing conclusions remain exist. So, it is necessary to re-combine previous published studies to make a comprehensive meta-analysis to solve above association.

Discussion
To our best knowledge, meta-analysis is a powerful method to carry out inconsistent results based on large number of samples including different ethnicities or countries [24]. The conclusion obtained from meta-analysis is more reliable and persuasive than one single study [24]. To investigate the association between IL-6 rs1800795 and DM, our studies is a timely and comprehensive study including 39099 individuals. Our results indicated that IL-6 rs1800795 acted as a protective factor in T2DM, in other words, individuals carrying C-allele may have decreased association with T2DM, especially for Asians and HB source studies. However, IL-6 rs1800795 was a risk factor in T1DM, there 8 had a significantly increased association between this polymorphism and TIDM risk in four genetic models in Mixed population and HB source studies.
Above information indicated IL-6 rs1800795 polymorphism may have different effect in different types of DM, and also exist different influences for different ethnicities, such as Asians and Mixed population. Some potential reasons may be explained: the pathogenic mechanisms about T2DM and T1DM is different, many significant expressed genes are not exactly the same, on the other hand, the same gene may have different effects, even the opposite results, based on current results, IL-6 gene may have different way between T2DM and T1DM, so rs1800795 polymorphism affecting expression of IL-6 also have different roles between T2DM and T1DM. Different races includes heterogeneity, the same gene also may have different roles in different ethnicities [61,62]. Third, the heterogeneity about selection strategy also maybe exist, which should affect our results. Furthermore, to evaluate the stable and convince of current study, we applied the Power analysis. Finally, the power in T2DM was 1, however, in T1DM was 0.166, which means the conclusions from T2DM was powerful and persuasive, rather than for T1DM. That suggested more studies about rs1800795 and T1DM risk should be added in the future to obtain a trustworthy conclusion.
Although we made a comprehensive meta-analysis, several limitations also should be considered.

Conclusions
In summary, our present meta-analysis provided evidence that the IL-6 rs1800795 polymorphism was associated with significantly increased T1DM risk in Mixed population, in opposite, decreased 9 association were found in T2DM susceptibility in Asians. Consequently, further well-designed large studies, particularly those related to gene-gene and gene-environment interactions, are warranted.

Acknowledgements
Not applicable.

Authors' contributions
ZC and CZ designed and conceived this study. CZ contributed to literature searching. CZ were involved in data extraction. ZC analyzed the data. ZC wrote the manuscript. ZC revised the paper. All authors have approved the final edition of the manuscript.

Availability of data and materials
All the data generated in the present research is contained in this manuscript.

Ethics approval and consent to participate
Not applicable.

Consent for publication
Not applicable.       Forest plot of T2DM risk associated with IL-6 rs1800795 polymorphism (C-allele vs. G-allele) in the source of control subgroup. The squares and horizontal lines correspond to the studyspecific OR and 95% CI. The area of the squares reflects the weight (inverse of the variance). The diamond represents the summary OR and 95% CI. The area of the squares reflects the weight (inverse of the variance). The diamond represents the summary OR and 95% CI.   Sensitivity analysis between IL-6 rs1800795 polymorphism and T2DM risk (C-allele vs. Gallele).