Solute Carrier Family 30 Member 8 Gene 807C/T Polymorphism and Type 2 Diabetes Mellitus in the Chinese Population: A Meta-Analysis Including 6,942 Subjects

Background Although solute carrier family 30 (zinc transporter) member 8 (SLC30A8) gene 807C/T polymorphism is associated with an increased risk of type 2 diabetes mellitus (T2DM) risk, there remains some inconsistency between individual studies. Objective The aim of the study is to explore the relationship between SLC30A8 gene 807C/T polymorphism and T2DM in the Chinese population. Methods The current meta-analysis compiles and analyzes the data of 6,942 participants from 10 independent studies. Either a fixed or random-effects model was adopted to evaluate the pooled odds ratio (ORs) and the corresponding 95% confidence interval (95% CI). Results A significant association between SLC30A8 gene 807C/T polymorphism and T2DM was found in the Chinese population under allelic (OR: 0.85, 95% CI: 0.80–0.91, P = 7.42 × 10−7), recessive (OR: 0.52, 95% CI: 0.38–0.72, P = 8.49 × 10−5), dominant (OR: 2.40, 95% CI: 1.68–3.41, P = 1.30 × 10−6), homozygous (OR: 0.52, 95% CI: 0.40–0.67, P = 2.90 × 10−7), heterozygous (OR: 0.79, 95% CI: 0.71–0.88, P = 1.63 × 10−5), and additive genetic models (OR: 0.73, 95% CI: 0.64–0.83, P = 7.05 × 10−7). Conclusion SLC30A8 gene 807C/T polymorphism was significantly associated with an increased T2DM risk in the Chinese population. Therefore, individuals of Chinese descent with the C allele of SLC30A8 gene 807C/T polymorphism may be more susceptible to developing T2DM, while individuals with the T allele may be protected against T2DM.


methods:
The current meta-analysis compiles and analyzes the data of 6,942 participants from 10 independent studies. Either a fixed or random-effects model was adopted to evaluate the pooled odds ratio (ORs) and the corresponding 95% confidence interval (95% CI).
conclusion: SLC30A8 gene 807C/T polymorphism was significantly associated with an increased T2DM risk in the Chinese population. Therefore, individuals of Chinese descent with the C allele of SLC30A8 gene 807C/T polymorphism may be more susceptible to developing T2DM, while individuals with the T allele may be protected against T2DM.
Keywords: solute carrier family 30 (zinc transporter) member 8, 807c/t, polymorphism, chinese, type 2 diabetes mellitus iNtRODUctiON The latest data from International Diabetes Federation (IDF) that 1 in 11 adults has diabetes worldwide (total up to 4.15 hundred million). In addition, there are many prediabetic patients. Morbidity associated with prediabetes and diabetes is as high as 6.7 and 8.8%, respectively. If current trends persist, the IDF forecasts that by 2040, 10% of world population will have diabetes with an additional 7.5% of the population categorized as prediabetic.
The prevalence and the complexity of this disease process require further research into these conditions. Type 2 diabetes mellitus (T2DM), also called adult-onset diabetes, usually occurs between the ages of 35-40 and accounts for the vast majority of diabetic disease. Alongside behavioral and environmental factors, genetic factors play a significant role in the pathogenesis of T2DM. The gene of interest in this study is the solute carrier family 30 member 8 (SLC30A8) gene. This gene encodes for the zinc transporter-8 (ZnT-8) protein which is embedded into the vesicular membrane of pancreatic β-cells. These vesicles store insulin granular. ZnT-8 could facilitate the binding of zinc and insulin in the cytoplasm of the β-cells and play an important role in the synthesis, maturation, storage, and secretion of insulin.
The SLC30A8 gene, located on the chromosome 8q24.11, spans about 37 kb and contains eight exons encoding 369 amino acids. The SLC30A8 gene 807C/T polymorphism (rs13266634) involves the substitution of the cytosine (C) in the 807th position with thymine (T). This changes the arginine (R) in the 325th position to tryptophan (W) (1). This 807C allele would make the protein function change and influence the post-transcription mechanism (2). This mutation result in abnormal Zn + accumulation and reduce the insulin secretion. In 2006, Chimienti et al. found that increased expression of the SLC30A8 gene increased insulin secretion in response to high plasma glucose (3). Hence, SLC30A8 gene could play a role in the pathogenesis of T2DM by influencing the pancreatic islet β-cells function.
Although many studies have been performed on the association of SLC30A8 gene 807C/T polymorphism and increased T2DM risk, the individual studies fail to present a consistent result. In 2016, Su et al. found that SLC30A8 gene 807C/T polymorphism was significantly associated with T2DM risk and the C allele was the risk allele in a Xinjiang T2DM population (4). Analogously, in 2011, Wang et al. also found that the SLC30A8 gene rs13266634 polymorphism was associated with T2DM risk and the C allele is the T2DM susceptible allele in a Hunan population (5). On the other hand, Zhong et al. found that there was no significant association between SLC30A8 gene 807C/T polymorphism and T2DM in another Xinjiang population (6).
In the present meta-analysis, the data from 3,458 T2DM patients and 3,484 was compiled to give insight on the relationship between SLC30A8 gene 807C/T polymorphism and T2DM in the Chinese population (Table S1 in Supplementary Material).

Publication Search and inclusion criteria
For the initial search, we used the keywords "solute carrier family 30 (zinc transporter) member 8, " "SLC30A8, " "SLC30A8, 807C/T, Chinese and type 2 diabetes mellitus, " "rs13266634, Chinese and type 2 diabetes mellitus" in the following databases: PubMed, the

Data extraction
The information was extracted out on the basis of a standard protocol. Three investigators were in charge of gathering the data from the retrieved studies. Two independently searched for duplicate studies and a third was responsible for resolving any disagreements. Duplicate studies and those that either deviated from the inclusion criteria or could not supply sufficient data were removed from the current meta-analysis. Similar datasets presented in different publications by the similar author group were incorporated into the analysis only once. The extracted data, including the first author's name, publication year, region, genotyping method, matching criteria, genotype number, and total number of cases and controls, are displayed in the Table 1.

Statistical analysis
Six genetic models were used in this analysis: allelic (T allele distribution frequency of SLC30A8 gene 807C/T polymorphism), recessive (TT vs. CT + CC), dominant (CC vs. TT + TC), homozygous (TT vs. CC), heterozygous (CT vs. CC), and additive (total T vs. total C). We used odds ratio (OR) and its corresponding to its 95% confidence interval (CI) to compare the association of SLC30A8 gene 807C/T polymorphism and T2DM. But the model we would use to calculate the OR is dependent on the heterogeneity of the dataset. We calculated the heterogeneity between the individual studies by using the Chi-square-based Q-test with the significance level set at P < 0.05 (7). If heterogeneity was detected, the fixed-effects model, or the Mantel-Haenszel method, but if heterogeneity was not detected we would use the random-effect model, or DerSimonian and Laird method (8,9). Z-test was used to calculate the pooled OR with the significance set at P < 0.05 level. Fisher's exact test was used to evaluate the HWE with the significance set at P < 0.05 level. We looked for potential publication bias by using a funnel plot and assessed for asymmetry using Egger's linear regression test (P < 0.05) (10). Revman 5.0 and STATA 12.0 software (StataCorp, College Station, TX, USA) was used to conduct the statistical analyses in this meta-analysis.

Bias Diagnostics
The Egger's test result has shown that there was no significant publication bias under the allele genetic model in this metaanalysis (T = −0.93, P = 0.379) (Figure 7). Moreover, no publication bias was visualized in the funnel plot under the recessive genetic model (Figure 8).

DiScUSSiON
In this meta-analysis, a significant association was observed between SLC30A8 gene 807C/T polymorphism and T2DM in the Chinese population under the allele (OR: 0.85), recessive (OR: 0.52), dominant (OR: 2.40), homozygous (OR: 0.52), heterozygous (OR: 0.79), and additive (OR: 0.73) genetic models. Hence, in the present meta-analysis we concluded that Chinese carriers of the C allele of the SLC30A8 gene 807C/T polymorphism may be at an increased risk of developing T2DM. Since the heterogeneity was detected under recessive, dominant, homozygous, and additive genetic models, the random genetic model was used. Furthermore, meta-regression was subsequently performed to explore the heterogeneity source. TT1 was the main heterogeneity source under the recessive, dominant, homozygous, and additive genetic models. The heterogeneity was significantly reduced in the subgroup analysis stratified by TT1 which suggested that TT1 was indeed the main heterogeneity source under these genetic models. In addition, as no heterogeneity was detected under the allele and heterozygous genetic models, the fixed genetic model was used.
Although insulin resistance of peripheral tissue was major investigative target of early research into T2DM, researchers subsequently found that defective secretion of insulin is required for normal glucose tolerance to progress toward hyperglycemia (19). This is reinforced by prospective observations that pancreatic beta cells of newly diagnosed diabetic patients have half the functionality of a normal healthy individual with a predicted decline of 4.5% annually. If we were to follow the curve backwards, however, researchers estimate that these patients had 100% of beta cell functioning 12 years before diagnosis (21).
In 2010, Li et al. explored the relationship between SLC30A8 gene rs13266634C/T polymorphism and T2DM in a Hengyang population and found significantly decreased scores on the homeostasis model assessment-β in patients with the TT, CT, and CC genotype (22). This suggested a potential link between the 807C allele and defective insulin secretion. Cohort studies of other non-Chinese populations also corroborated his result. Boesgaard et al. found that among non-diabetic (NDM) descendants of T2DM population, patients with the 807C allele had a reduced acute insulin response (AIR) during an intravenous    glucose tolerance test (23). More specifically, AIR in CC genotype patients was 19% lower than that of TT genotype patients. This demonstrated the impaired pancreatic islet β cell function in patients with the 807C allele even though they were not yet diagnosed with T2DM. In 2008, Cauchi et al. analyzed novel risk loci for T2DM in a general French population with normal blood glucose level and found that the fasting insulin level in the rs13266634 C allele carriers already significantly decreased than that in the non rs13266634 C allele carriers at baseline (24). Wang et al. found that the insulin sensitivity in rs13266634 C allele carriers is higher than that in the non rs13266634 C allele carriers in the NGT persons. It is also the reason for they could maintain the normal tolerance with the low serum insulin level (14).
The mechanism underlying the relationship between SLC30A8 gene rs13266634C/T polymorphism, T2DM, and pancreas islet β cells dysfunction is likely associated with its molecular action. The ZnT-8 protein encoded by SLC30A8 gene is located in insulincontaining vesicle membrane in pancreatic islet β cells and is implicated in the zinc transport into the vesicle (25). Insulin then binds with two Zn 2+ ions to form a stable hexameric structure. The insulin will only be secreted out when the pancreas islet β cells are stimulated (26). Hence, SLC30A8 is the dominant sector participating the insulin synthesis, storage, and secretion. When released, Zn may also inhibit the neighboring pancreas islet α cells from releasing glucagon and attenuate the pancreas islet β cells apoptosis (27,28). The SLC30A8 gene rs13266634 C allele would lead to the abnormal ZnT-8 structure and function, and cause decreased insulin secretion and increased glucagon secretion, pancreas islet β cells apoptosis, blood glucose levels. The current meta-analysis results confirmed this conclusion. In 2007, Zeggini et al. performed a genome-wide association study on the SLC30A8 gene rs13266634C/T polymorphism and T2DM. They genotyped rs13266634 independently and obtained replication of this finding in the French scan (29).
In 2003, Chen et al. have performed a meta-analysis on the association of SLC30A8 gene rs13266634C/T polymorphism and T2DM (30). In 2015, Cheng et al. also conducted this metaanalysis and replicated the same conclusion (31). In 2016, Fan et al. also reported that SLC30A8 gene polymorphism was associated with T2DM increased risk in African and European populations as well as Asian groups (32). However, in all of the above-mentioned meta-analyses, the authors ignored the ethnicity difference and mixed the Chinese population with other populations to analyze the association. Furthermore, the study by Huang (13) deviating from the HWE was still included in Fan's meta-analysis which was not consistent with their exclusion criteria. Hence, their results may not be as objective or credible as the current meta-analysis.
The large-scale studies on the association of T2DM and SLC30A8 gene rs13266634C/T polymorphism are still needed to further explore this connection. As T2DM is a disease of multifactorial inheritance and other gene polymorphisms as SUMO4 gene M55V polymorphism (33), LEPR gene Gln223Arg polymorphism (34), and TCF7L2 gene rs12255372G/T polymorphism (35) are also associated with T2DM susceptibility. The potential effects of these polymorphisms, independently and synergistically, were not incorporated into the analysis.
In conclusion, our meta-analysis found a significant association between SLC30A8 gene rs13266634C/T polymorphism and an increased risk of T2DM in the Chinese population. Although large-scale studies are still required to test our result, this finding may present a small step toward developing a personalized therapy to T2DM in the Chinese population.