Introduction

Prostate cancer is one of the most common cancers among men in developed countries, which has become a major public health challenge. Traditionally considered as a disease of elderly men, an increasing proportion of prostate cancer cases now occur in men of pre-retirement ages. Established risk factors include age, ethnicity, and family history. In addition, a few low-penetrance susceptibility genes, including E-cadherin with a higher population frequency, may be relevant to prostate cancer risk in combination with environmental factors. Prostate cancer is the second most prevalent cancer and the sixth leading cause of death in males [1, 2]. More prostate cancer cases now occur in younger men [3, 4], not only in elderly men. New markers for identifying high-risk populations as well as novel strategies for early detection and preventive care are urgently needed. Due to the high incidence and low survival rate, novel biomarkers for early diagnosis of prostate cancer are urgently needed. The mechanism of prostatic tumorigenesis is still not fully understood. Although the casual mechanisms of prostate cancer are still not fully understood, several risk factors have been suggested to be associated with prostate cancer, including ethnic variance, environmental factors, family history, and lifestyles [5–7]. It has been suggested that low-penetrance susceptibility genes combining with environmental factors may be important in the development of cancer. Emerging evidence has suggested potential genetic risks for prostate cancer [8, 9]. It has also been reported that promoter methylation of related genes may also play an important role in prostate carcinogenesis [10, 11].

In recent years, several common low-penetrate genes have been identified as potential prostate cancer susceptibility genes. An important one is E-cadherin gene, which locates on chromosome 16q22.1 and consists of 16 coding exons [12, 13]. E-cadherin, which is encoded by E-cadherin gene in epithelial cells [14, 15], plays an important role in the establishment and maintenance of intercellular adhesion, cell polarity, and tissue architecture [16, 17]. But its expression is largely reduced in undifferentiated cancers. The E-cadherin/catenin complex is important for cellular polarity and maintenance of normal tissue morphology and cellular differentiation. Decreased E-cadherin expression is supposed to be associated with various malignancies including prostate cancer [14, 18–20]. In addition, disruption of E-cadherin with rare mutations may also be involved in carcinogenesis through a modified Wnt signaling pathway. Several polymorphisms and somatic mutations have been identified in E-cadherin. The −160C/A polymorphism in the promoter region of the E-cadherin gene has been reported to have a direct effect on its transcriptional regulation and therefore may influence susceptibility to cancers.

E-cadherin −160C/A polymorphism has been identified in the promoter region related to the transcriptional start site [21]. It has been shown that the A allele decreased transcriptional efficiency by about 10–68 % compared with the wild-type C allele. It was also observed that the C allele showed much higher binding affinity to transcriptional factor than the mutant allele, indicating that the −160C/A variant may alter transcriptional activity of the E-cadherin gene and be responsible for decreased E-cadherin expression and increased susceptibility to epithelial cancers. Furthermore, aberrant E-cadherin functions have been reported to be associated with malignant transformation of prostatic epithelium as well as metastasis and poor prognosis of prostate cancer.

A number of studies have reported the function of E-cadherin −160C/A polymorphism in prostate cancer risk, but the results are inconclusive, partially because of the possible small effect of the polymorphism on prostate cancer risk and the relatively small sample size in each of published studies. Li et al. first reported that the −160C/A polymorphism directly affects the E-cadherin gene transcriptional regulation [22]. Kallakury et al. [23] found that promoter methylation of E-cadherin gene is more frequently in prostate carcinoma and may play a role in the decreased expression of the E-cadherin protein. Thereafter, accumulating epidemiological and molecular studies have reported results about the association of E-cadherin −160C/A polymorphism and promoter methylation with prostate cancer risk, although their results have been conflicting. Thus, these observations raised quite a controversial question regarding the significance of −160C/A in prostate cancer pathogenesis. Obviously, statistical power of an individual study could be very limited for efficient assessment of the −160A allele. Integration of these data sets may provide improved statistical power to detect the significance. Therefore, we performed the current meta-analysis to derive a relatively comprehensive assessment of the relationship between −160C/A polymorphism and promoter methylation of E-cadherin gene and the risk of prostate cancer.

Materials and methods

Literature search

To identify relevant studies, we conducted a literature search in PubMed, Embase, Web of Science, Cochrane Library, and Chinese National Knowledge Infrastructure (CNKI) databases without language limitation. The last retrieval was conducted on April 1, 2014. The search strategy was formulated using the following keywords: (“E-cadherin” or “CDH1” or “−160C/A” or “rs16260”) and (“single nucleotide polymorphism” or “SNP”) and (“methylation” and “hypermethylation”) and (“prostate cancer” or “prostatic neoplasm”). References of relevant studies were also manually searched to explore additional eligible studies.

Selection criteria

Included candidate studies had to be original and had to report the genotype frequencies in cases and controls or their estimated odds ratios (ORs) with 95 % confidence intervals (95 % CIs). Moreover, all patients had to meet the diagnostic criteria for prostate cancer and control subjects should be cancer- or disease-free. When duplicate publications from the same study were found, only the one with the most complete data was included in this meta-analysis.

Data extraction

Two investigators independently extracted the following data using a standardized table: surname of first author, year of publication, country of origin, ethnicity, basic characteristics of subjects (number, age, and sex), genotype frequency, genotyping method, methylation frequency, detection method, clinical characteristics, etc. For data not available in the publications, required information was obtained by contacting corresponding authors.

Quality assessment

The strengthening the reporting of genetic association studies (STREGA) quality score system was used to assess the qualities of all included studies [24]. The STREGA system includes 22 assessment aspects, with STREGA scores higher than 14 indicating a moderate high quality. Two investigators independently evaluated the quality of included studies. Discrepancies about the quality score were resolved by consensus among all the authors.

Statistical analysis

All analyses were conducted using STATA 12.0 software. Hardy–Weinberg equilibrium (HWE) in the control group was tested for each data set to ensure that genotype distribution was not significantly deviated from HWE [25]. The association between E-cadherin −160C/A polymorphism and promoter methylation and the risk of prostate cancer was evaluated using the pooled ORs with their corresponding 95 % CIs. The Z test was used to assess the significance of the pooled estimates, with a P value less than 0.05 as a cut-off value. Between-study heterogeneity was tested using the Cochran’s Q statistic and the I 2 metric [26, 27]. When P > 0.05 for Q statistic or I 2 < 50 % indicates no significant heterogeneity, the fixed-effect model was applied to calculate the pooled ORs and 95 %CIs. Otherwise, a random-effect model was used. In addition to an overall analysis, subgroups analyses were performed based on ethnicity, source of control, detection method, Gleason score (GS), and tumor grade (TG), where applicable. Pathologic grade was determined according to the Gleason pattern and classified as low GS (6 or lower) and high GS (7 or higher). Localized cancer was defined as low TG, advanced or metastasis cancer as high TG. One-way sensitivity analyses were also conducted by omitting individual studies in turn to reflect the influence of individual data sets on the pooled results [28]. Potential publication bias was tested by Begg’s funnel plot and Egger’s publication plot [29, 30].

Results

Baseline characteristics of included studies

The review process of the literature research is illustrated in Fig. 1. The initial screening identified 84 potentially relevant articles and six were excluded as duplicate publications. After the title and abstract review, 48 were excluded, among which 22 were letters or reviews, 13 were not human studies, and 11 were not related to our research topic, leaving 30 articles available for further full text review. In accordance with the inclusion criteria, 22 articles were selected for this meta-analysis, 11 for E-cadherin −160C/A polymorphism [21, 31–40], and another 11 for promoter methylation of E-cadherin gene [23, 41–50]. The publication year of included studies ranged from 2000 to 2013. There were 14 studies from 11 articles that investigated the association between E-cadherin −160C/A polymorphism and prostate cancer risk, with eight in Caucasians, three in Asians, and the other three in Africans. Eight studies used normal controls, whereas four employed the benign prostatic hyperplasia (BPH) as controls. Eight studies differentiated prostate cancer patients based on low Gleason score (GS) or high GS, while ten classified the patients as low tumor grade (TG) and high TG. Among the 11 studies considering the association between promoter methylation of E-cadherin gene and the risk of prostate cancer, seven compared the methylation frequency of prostate cancer tissues with that of normal tissues, and six investigated the methylation frequency difference between low GS and high GS patients, while seven focused on the variance between low TG and high TG. The qualities of all included studies were moderately high, with STREGA scores higher than 14. Further details on the main characteristics of each study can be found in Table 1 and Table 2.

Fig. 1
figure 1

Flow diagram of the selection of studies and specific reasons for exclusion from the present meta-analysis

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Table 1 Main characteristics of all eligible studies for the E-cadherin −160C/A polymorphism
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Table 2 Main characteristics and methylation data of all eligible studies for the promoter methylation of E-cadherin gene
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Association between E-cadherin −160C/A polymorphism and prostate cancer risk

Since between-study heterogeneity was obvious (I 2 > 50 % and P < 0.01), random-effect model was applied for the overall analysis. As shown in Table 3, the overall analysis suggested that E-cadherin −160C/A was significantly associated with an increased risk of prostate cancer (AA vs. CC: OR = 1.21, 95 % CI = 1.05–1.70, P = 0.011; CA vs. CC: OR = 1.28, 95 % CI = 1.05–1.55, P = 0.013; AA + CA vs. CC: OR = 1.29, 95 % CI = 1.07–1.56, P = 0.009). Stratified analysis by ethnicity found a significant association for Caucasians and Asians (Table 3; Fig. 2a). In addition, subgroup analysis based on source of control suggested that the comparison between prostate cancer patients and BPH controls was more significant than normal controls (Fig. 2b). It is worth noting that the risk allele of E-cadherin −160C/A was only associated with high pathologic grade of prostate cancer, including high GS (Fig. 2c) and high TG (Fig. 2d).

Table 3 Meta-analysis of the association between the E-cadherin −160C/A polymorphism and the risk of prostate cancer
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Fig. 2
figure 2

Forest plots of ORs for the association between E-cadherin −160C/A polymorphism and prostate cancer risk (a ethnicity, b source of control, c Gleason score, d tumor grade)

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Association between promoter methylation of E-cadherin gene and the risk of prostate cancer

Random-effect model was applied for the analysis of the association between promoter methylation of E-cadherin gene and the risk of prostate cancer, since significant heterogeneity was also observed. The comparison between prostate cancer tissues and normal tissues suggested that the frequencies of promoter methylation in prostate cancer tissues were significantly higher than normal tissues (OR = 4.85, 95 % CI = 1.58–14.95, P = 0.006), either for Caucasians (OR = 4.25, 95 % CI = 1.15–15.71, P = 0.030) or Asians (OR = 8.87, 95 % CI = 1.48–53.06, P = 0.017) (Fig. 3a). Furthermore, we observed a significant association in subgroups based on methylation-specific PCR (MSP), but not in non-MSP subgroups, indicating MSP may be a more promising method to detect methylation status (Fig. 3b). However, no significant results were found in the comparison between low GS and high GS tissues (Fig. 3c), as well as the comparison between low TG and high TG tissues (Fig. 3d).

Fig. 3
figure 3

Forest plots of ORs for the association between promoter methylation of E-cadherin gene and the risk of prostate cancer (a ethnicity, b detection method, c Gleason score, d tumor grade)

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Sensitivity analyses and publication bias

One-way sensitivity analyses were performed to determine the influence of individual data sets on the pooled ORs by sequentially removing each eligible study. With the omission of each study, pooled estimates remained virtually the same, indicating that no single study heavily influenced summary ORs, either for polymorphism analysis (Fig. 4a) or methylation analysis (Fig. 4b). Begg’s funnel plots and Egger’s publication plots were used to assess the potential for publication bias in included studies. The shapes of the plots did not reveal any evidence of obvious asymmetry, with the P value larger than 0.10 for each plot (Fig. 5). The above tests indicated a promising level of robustness and accuracy for the results of this meta-analysis.

Fig. 4
figure 4

One-way sensitivity analysis for the robustness of the results in the meta-analysis (a E-cadherin −160C/A polymorphism, b promoter methylation of E-cadherin gene)

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Fig. 5
figure 5

Begg’s funnel plots and Egger’s publication plots for the association between E-cadherin gene and susceptibility to prostate cancer (a Begg’s funnel plot on the E-cadherin −160C/A polymorphism, b Egger’s publication plot on the E-cadherin −160C/A polymorphism, c Begg’s funnel plot on the promoter methylation of E-cadherin gene, d Egger’s publication plot on the promoter methylation of E-cadherin gene)

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Discussion

Recently, genetic variants of the E-cadherin gene in the etiology of several cancers have drawn increasing attention. Growing number of studies have suggested that −160A in the promoter region of the E-cadherin gene was emerging as a low penetrance tumor susceptibility allele in the development of several kinds of cancer, such as prostate cancer, urothelial cancer, and gastric cancer. Prostate cancer is still one of the most common male malignancies [1, 2]. Heredity factors are supposed to play important roles in the tumorigenesis of prostate cancer [51, 52]. It has been generally accepted that gene mutation and promoter methylation can suppress transcriptional expression of tumor suppressor gene and result in the development of malignant tumors including prostate cancer. A number of studies have reported a function of the E-cadherin −160 C/A polymorphism in prostate cancer risk with inconclusive results. Emerging evidence has reported the role of E-cadherin −160C/A polymorphism and promoter methylation in the risk of prostate cancer. However, their results have been inconclusive, possibly partially due to the small effects of these risk factors on prostate carcinoma and limited statistical power resulting from the relatively small sample sizes of individual studies. To our knowledge, the present meta-analysis is the most comprehensive overview of the association between E-cadherin −160C/A polymorphism and promoter methylation and the risk of prostate cancer.

For the polymorphism of E-cadherin gene, the overall analysis of our meta-analysis showed that E-cadherin −160C/A was significantly associated with an increased risk of prostate cancer. Since the same polymorphism may have different roles in cancer susceptibility for different ethnicities, subgroup analyses based on ethnicity were performed. Our results indicated that risk appeared to be significant in Europeans and Asians but not in Africans, suggesting a possible role of ethnic differences in their heredity background and living environment. The influence of the risk allele in Africans might be masked by the presence of other unidentified causal genes involved in prostate tumorigenesis. In addition, this variance may be just due to chance since small sample size studies lack sufficient statistical power to detect a slight effect. Considering the limited number and sample sizes of studies on Africans included in this meta-analysis, more large-scale studies are warranted to confirm our results. Interestingly, we also found that the risk allele of E-cadherin −160C/A was only associated with high pathologic grade of prostate cancer. A potential explanation may be that this genetic mutation may begin to play a role only at the advanced stage of prostate cancer and thus further aggravate the progression of this disease.

Our analyses of the association between promoter methylation of E-cadherin gene and the risk of prostate cancer suggested that the frequencies of promoter methylation in prostate cancer tissues were significantly higher than normal tissues. It is reasonable to consider that transcriptional inactivation attributed to promoter methylation may lead to the malignant proliferation of prostate tissues. No ethnicity variance was found for the methylation analysis, suggesting that promoter methylation of E-cadherin gene may threaten the health of men all over the world. Our results suggested that MSP appears to be superior in comparison to the other methods compared in this meta-analysis, which also provide evidence for the extensive use of this method in clinical context. Unfortunately, we did not find any significant difference when comparing different pathologic grade tissues of prostate cancer, although we are expecting to find a correlation between the clinical stage or pathologic grade of prostate cancer and promoter methylation of E-cadherin gene. A possible explanation to this result may be that promoter methylation status of E-cadherin gene is irrelevant to the progression of prostate cancer, but just related to its tumorigenesis. However, it should be mentioned that classification for prostate cancer based on Gleason score or tumor stage as low and high may be not accurate to detect a correlation between clinical stage and promoter methylation of E-cadherin. Hence, further investigations should be conducted to differentiate prostate cancer more specifically.

Although several meta-analyses about this association between the E-cadherin −160C/A polymorphism and prostate cancer have been conducted previously, it is the most comprehensive systematic review of this association. For instance, Wang et al. [53] conducted the first general overview to assess susceptibility of E-cadherin −160C > A to seven types of cancers. For prostate cancer, eight studies were included in their meta-analysis. Since then, four new studies have been conducted and should be included in the updated meta-analysis. Wang et al. [54] further performed a meta-analysis of 47 case–control studies on the association between E-cadherin −160C > A and 16 types of cancers. Still, a study conducted by Forszt et al. [33] was left out, which should be included according to inclusion criteria. Although two more meta-analyses focusing on the association between E-cadherin 160C/A polymorphism and prostate cancer risk were conducted, their included studies are far from comprehensive. Qiu et al. [55] missed three eligible studies, while the most recent meta-analysis conducted by Deng et al. [56] even left out four eligible ones. Our meta-analysis comprehensively searched all related studies, and 14 studies from 11 publications were involved.

Our study has several methodological advantages. This study is the first meta-analysis attempt to explore the interaction between promoter methylation of E-cadherin gene and the risk of prostate cancer, although there are some systematic reviews considering the role of E-cadherin −160C/A polymorphism in prostate cancer susceptibility. We also are the first to investigate variances in source of control, genotyping method, and pathologic grade on this association in our meta-analysis. Furthermore, sensitivity analyses and publication bias were used to confirm the robustness of our results. Several potential limitations of our study also warrant mention. First, although we tried to incorporate all relevant studies, we may still have left out useful publications during screening process. Second, in the subgroup analysis by ethnicity, the number of Africans was relatively small and did not have enough statistical power to analyze the association. Last, our results were based on unadjusted estimates, while a more precise analysis should be conducted if individual data were available, which would allow for adjustment by other covariates including age, environmental factors, and lifestyle. Hence, further investigations are warranted to combine these covariates into analysis.

Despite these limitations, the present meta-analysis supports growing evidence of E-cadherin −160C/A polymorphism and promoter methylation in the risk of prostate cancer, especially in Europeans and Asians. We believe that identifying these risk factors would be informative in prostate cancer progression. Moreover, gene–gene and gene–environment interactions should also be considered in further investigations. Such studies taking these factors into account may eventually lead to a more comprehensive understanding of this association.