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Introduction

Coronary artery disease (CAD), a frequently encountered illness, poses a public health concern. CAD is considered to be a polygenic illness resulting from both environmental and hereditary factors (1). Genomic research is regarded as one of the four epoch-making revolutions and has become a study hotspot in exploring CAD pathogenesis from the molecular hereditary perspective (2).

Connexin 37 (Cx37), a gap junction protein expressed in vascular endothelial cells, regulates and controls the growth, proliferation, senium, and regeneration of the vascular endothelium by mediating the signal transduction pathway between smooth muscle and endothelial cells (3). In 2009, Derouette et al found Cx37 gene knockout mice to have the tendency to develop endarterium atheromatous plaques, suggesting that Cx37 is involved in the generation and progression of atherosclerosis as a protective landmark in the plaque formation process following endothelial injury (4). Thus, Cx37 affects the development and progression of CAD and myocardial infarction (MI).

The Cx37 gene, located in 1p35.1, spans 2.727 kb and contains two exons and one intron encoding 333 amino acids (5). The variation of thymine (T base) substituting for cytosine (C base) in the 1,019th base of the Cx37 gene contributes to the supplantation of proline by serine in the 319th base of the corresponding amino acid sequence. This rs1764391 mutation affects the tertiary structure formation of the gap junction protein. Moreover, the altered amino acid transforms the phosphorylation locus and further affects protein synthesis, transport, fabrication and signal transduction. Therefore, the function of vascular endothelial cells is affected to different extents, thus contributing to individual differences in CAD susceptibility (6).

Results also differed among the studies carried out in China. Zhang et al reported the T allele to be the hereditary risk factor for CAD (9). In contrast with Zhang’s work, Han et al found that the C allele was probably correlated with CAD occurrence in the Chinese northern Han population (10). However, Zhang et al observed no association between Cx37 C1019T gene polymorphism and CAD (11).

Considering the results of controversial studies on Cx37 C1019T gene polymorphism and CAD, the present meta-analysis involving 2,206 subjects was conducted to deduce a reasonable conclusion on the association between Cx37 C1019T gene polymorphism and CAD in the Chinese population.

Materials and methods

Publication search and inclusion criteria

Selected studies published between 2001 and 2010 were obtained using the MeSH terms ‘coronary artery disease’ or ‘coronary heart disease’, ‘polymorphism’, ‘connexin 37’, ‘gene,’ and ‘Chinese’ in searching the electronic databases of PubMed, Embase, and Web of Science, as well as the China Biological Medicine Database and the China National Knowledge Infrastructure (last research updated on November 20, 2012).

The relevant publications were required to meet the following inclusion criteria: i) evaluation of Cx37 C1019T gene polymorphism and CAD in the Chinese population and ii) CAD diagnosis based on the examination results of coronary arteriography, clinical symptoms with electrocardiogram, echocardiography, treadmill exercise test, and myocardial perfusion imaging in emission computed tomography.

Data extraction

The data were extracted using a standard protocol. In the present meta-analysis, studies considered to be repeated publications, studies violating the inclusion criteria, or research providing little information were excluded. If the same data appeared in different manuscripts, only one study result was used. The drawn data comprised the following items: the first author’s name, publication year, region, number of genotypes, genotyping method, study design, matching criteria, and total number of cases and controls.

Statistical analysis

The allelic (distribution of T allelic frequency of Cx37 C1019T gene polymorphism), recessive (TT vs. CC + CT), dominant (CC vs. CT + TT), additive (T vs. C), homozygous (TT vs. CC), and heterozygous (CT vs. CC) genetic models were used. An odds ratio (OR) corresponding to a 95% confidence interval (CI) was used to compare the association of Cx37 C1019T and CAD. The Chi-square-based Q-test was applied to determine heterogeneity among studies (significance was set at the P<0.05 level) (12). The variation attributed to heterogeneity was assessed by calculating the inconsistency index I2. As there was significant heterogeneity for all of the genetic models, the random-effect model (DerSimonian and Laird method) was applied for the above-mentioned models (13). The fixed-effect model was not used (the Mantel-Haenszel method) in the current meta-analysis (14). The pooled OR was determined by the Z test, and significance was set at P<0.05.

The Hardy-Weinberg equilibrium (HWE) was assessed by Fisher’s exact test, and significance was set as P<0.05. The funnel plot was adopted to assess potential publication bias. The funnel plot asymmetry on the natural logarithm scale of the OR was assessed by Egger’s linear regression test (significance was set at the P<0.05 level) (15). Stata 11.0 software (StataCorp, College Station, TX, USA) was used for statistical analysis.

Results

Studies and populations

A total of 17 publications were derived from the literature search, from which 6 complied with the research inclusion criteria. Of the 11 excluded studies, 2 were published repeatedly, 2 were reviews, 3 were unassociated with Cx37 C1019T gene polymorphism, and 3 had foreign subjects. One study was excluded for deviating from HWE. Data were collected from a total of 1,244 CAD patients and 962 controls of the Han ethnicity (Table I) (9–11,16–18). The studies were conducted in 6 investigated regions: Taiwan, Zhejiang, Liaoning, Anhui, Shanxi and Guangdong.

Table I Characteristics of the investigated studies of the association between connexin 37 C1019T gene polymorphism and CAD.

Table I

Characteristics of the investigated studies of the association between connexin 37 C1019T gene polymorphism and CAD.

		CAD			Control
Author (Year)	Region	CC	CT	TT	CC	CT	TT	Matching criteria	Sample size (CAD/control)	Refs.
Yeh, et al(2001)	Taiwan	141	34	2	66	31	5	Age, gender, ethnicity, BMI, smoker (%), DM (%)	177/102	(16)
Xie, et al(2006)	Shanxi	89	47	14	75	34	8	Age, ethnicity	150/117	(17)
Zhang, et al(2007)	Zhejiang	78	68	27	90	46	12	Age, gender, ethnicity, BMI, smoker, BP	173/148	(9)
Han, et al(2008)	Liaoning	110	268	124	76	187	147	Ethnicity, BMI	502/410	(10)
Zhang, et al(2008)	Guangdong	60	54	22	96	42	9	Ethnicity	136/147	(11)
Feng, et al(2010)	Anhui	41	42	23	6	16	16	Gender, ethnicity, BMI, DM (%)	106/38	(18)

[i] Polymerase chain reaction-restriction fragment length polymorphism geno/typing method and case-control study design have been adopted in all of the studies. The ethnicity in all of the studies was Han. CAD, coronary artery disease; BMI, body mass index; DM, diabetes mellitus; BP, blood pressure.

Combined analyses

No significant association was observed between Cx37 C1019T gene polymorphism and CAD in the Chinese population under the allelic (OR=0.96, 95% CI=0.59–1.56, P=0.87, Pheterogeneity<0.00001, I2=91.1%); recessive (OR=0.77, 95% CI=0.28–2.08, P=0.60, Pheterogeneity<0.00001, I2=90.1%); dominant (OR=0.990, 95% CI=0.773–1.266, P=0.934, Pheterogeneity=1.0×10−10, I2=84.7%); additive (OR=1.000, 95% CI= 0.736–1.359, P=1.000, Pheterogeneity=1.0×10−10, I2=91.0%); homozygous (OR=1.062, 95% CI=0.598–1.887, P=0.836, Pheterogeneity=1.0×10−10, I2=86.0%) and heterozygous genetic models (OR=1.017, 95% CI=0.802–1.291, P=0.888, Pheterogeneity=1.0×10−10, I2=77.9%) (Table II, Figs. 1 and 2).

Figure 1 Forest plot of coronary artery disease (CAD) associated with connexin 37 C1019T gene polymorphism under an allelic genetic model (distribution frequency of T allelic of Cx37 C1019T gene).

Figure 2 Forest plot of coronary artery disease (CAD) associated with connexin 37 C1019T gene polymorphism under a recessive genetic model (TT vs. CC + CT).

Table II Summary of meta-analysis of association of connexin 37 (Cx37) C1019T gene polymorphism and CAD risk in the Chinese population.

Table II

Summary of meta-analysis of association of connexin 37 (Cx37) C1019T gene polymorphism and CAD risk in the Chinese population.

Genetic model	Outcome	Pooled OR (95% CI)	P-value	Literature number	CAD size	Control size	Pheterogeneity
Allelic	Distribution frequency of T allele	0.96 (0.59–1.56)	0.87	6	1,244	962	P<0.00001a
Recessive	TT vs. CT + CC	0.77 (0.28–2.08)	0.60	6	1,244	962	P<0.00001a
Dominant	CC vs. CT + TT	0.990 (0.773–1.266)	0.934	6	1,244	962	1.0×10−10a
Additive	T vs. C	1.000 (0.736–1.359)	1.000	6	1,244	962	1.0×10−10a
Homozygous	TT vs. CC	1.062 (0.598–1.887)	0.836	6	1,244	962	1.0×10−10a
Heterozygous	CT vs. CC	1.017 (0.802–1.291)	0.888	6	1,244	962	1.0×10−10a

a P<0.05. OR, odds ratio; CI, confidence interval; CAD, coronary artery disease; CAD size, total number of CAD cases; control size, total number of control group.

Bias diagnostics

The funnel plot and Egger’s test were adopted to assess publication bias. No visual publication bias was found in the funnel plot (Fig. 3). The difference was not statistically significant in the Egger’s test, suggesting low publication bias in this meta-analysis (allelic genetic model, T=1.36, P=0.247).

Figure 3 Funnel plot for studies of the association of coronary artery disease (CAD) and connexin 37 C1019T gene polymorphism under an allelic genetic model (distribution of T allelic frequency of connexin 37 C1019T gene polymorphism). The horizontal and vertical axis correspond to the OR and confidence limits. OR, odds ratio; SE, standard error.

Discussion

In the present meta-analysis, Cx37 C1019T gene polymorphism was not found to be associated with CAD susceptibility under the allelic (OR=0.96), recessive (OR=0.77), dominant (OR=0.990), additive (OR=1.000), homozygous (OR=1.062), and heterozygous genetic models (OR=1.017).

Atherosclerosis refers to the chronic inflammation of the arterial wall. Endothelial dysfunction is crucial in the early stages of atherosclerosis (19). Cell-to-cell interactions are required to maintain endothelial cell integrity. The cell gap junction is an important channel for message exchange between cells (20). In 2006, Wong et al found that Cx37 knockout mononuclear phagocytes adhered to the endothelium more strongly than Cx37-deficient cells (21). This characteristic was associated with the adenosine triphosphate (ATP) release between cells. Extracellular ATP release reduced the normally expressed mononuclear phagocytes of Cx37. By contrast, Cx37 knockout and inhibition of the Cx37 semi-pathway decreased extracellular ATP release causing mononuclear phagocytes to adhere to the matrix. This condition suggests that the Cx37 semi-pathway releases ATP, consequently prohibiting monocyte adhesion. Alteration of the cell adhesion characteristic contributes to mononuclear phagocyte aggregation in the atherosclerotic plaque and accelerates atherosclerotic progression (22). Therefore, Cx37 plays an important role in maintaining the structural and functional integrity of the vascular endothelium, preventing thrombosis, and inhibiting foam cell generation, which are closely correlated with the suppression of atherosclerotic progression.

In 1998, Yeh et al noted that Cx37 existed in coronary artery endothelial cells through immunological detection with a Cx37 antibody (23). Moreover, the mutation of Cx37 C1019T was associated with carotid artery atherosclerosis, acute MI, and CAD (16,24,25). No consensus exists on the relationship between Cx37 C1019T polymorphism and CAD. In 2001, Yeh et al found that the C allele of the CX37 gene potentially plays a role in the manifestation of coronary atherosclerosis in Taiwan (16). In 2010, Feng et al reported a similar result in male subjects from Anhui Province, China (18). By contrast, Zhang SR et al and Zhang CL et al both reported different results showing that the T allele was a risk factor for CAD (9,11).

The number of meta-analyses on gene polymorphism and CAD increases annually (26–28). However, no meta-analysis on the relationship between Cx37 C1019T gene polymorphism and CAD was retrieved. The current meta-analysis analyzed the independent studies on this subject comprehensively and quantitatively. The meta-analysis was used to analyze and summarize the collected research data using a statistical method and to provide a method to quantify the average effect to address the research question. The advantage of a meta-analysis involves its capacity to increase conclusion reliability and resolve inconsistency among study results by increasing the sample size. Therefore, the drawn conclusion should be more objective and reasonable than that of individual studies.

The present meta-analysis has a number of limitations. Sample non-uniformity in terms of gender, age and location existed among the individual studies. The case selection criteria and statistical methods differed among the independent studies. The illness phenotype was diverse. Large-scale studies on CAD and Cx37 C1019T were inadequate. Impact factors, such as diet and lifestyle, genetic background as well as pharmaceutical intervention, were considered distinct and require further investigation.

In conclusion, the present meta-analysis suggested no association between the Cx37 C1019T gene polymorphism and CAD susceptibility in the Chinese population. However, considering the above limitations, further studies are required to confirm these results.

Acknowledgements

This study was funded by the National Natural Science Foundation of China (NSFC 81100073 to Dr Yan-Yan Li), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Excellent Young and Middle-Aged Teachers Assistance Program of Nanjing Medical University for Dr Yan-yan Li. We would like to thank all our colleagues working in the Department of Geriatrics, the First Affiliated Hospital of Nanjing Medical University.

References