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Corresponding author at: Department of Clinical Laboratory, First Affiliated Hospital of Guangxi Medical University, 22 Shuangyong Road, Nanning 530021, People's Republic of China. Tel.: +86 13878801998.
In view of the controversies surrounding the association of glutathione S-transferases (GST) P1 with endometriosis, a meta-analysis of GSTP1 −313A/G polymorphism with endometriosis risk was performed.
Study design
The relevant studies were identified through a search of PubMed, Excerpta Medica Database (Embase), Elsevier Science Direct and Chinese Biomedical Literature Database (CBM) until March 2013. The association between GSTP1 −313A/G polymorphism and endometriosis risk was pooled by odds ratios (ORs) together with their 95% confidence intervals (95% CIs).
Results
A total of eight case–control studies were eventually identified. We found that GSTP1-313A/G polymorphism was not associated with endometriosis risk in the overall population (A vs. G: OR = 1.02, 95% CI = 0.97–1.07, P = 0.511; AA vs. GG: OR = 1.02, 95% CI = 0.98–1.06, P = 0.359; GA vs. GG: OR = 1.03, 95% CI = 0.98–1.08, P = 0.299; AA vs. GA/GG: OR = 1.01, 95% CI = 0.96–1.07, P = 0.621; AA/GA vs. GG: OR = 1.00, 95% CI = 0.97–1.03, P = 0.972). In the sub-group analysis based on ethnicity, a significant association was found in Caucasians under the recessive model (AA vs. GA/GG: OR = 1.28, 95% CI = 1.08–1.53, P = 0.006).
Conclusions
GSTP1 −313A/G polymorphism may not be associated with endometriosis risk, while the observed increase in risk of endometriosis may be due to small-study bias. Considering the limited sample size and ethnicity included in our meta-analysis, an updated meta-analysis will be urgently needed when further larger and well-designed studies are published.
Endometriosis is a common inflammatory disease that is characterized by the presence of endometrial tissue outside the uterine cavity. In industrialized countries, the incidence of endometriosis has been reported to be 6–10% of the general female population, and the frequency is 35–50% in women with pain or infertility [
]. Women's quality of life is influenced negatively by the pain, infertility and economic burden of the disease but the exact etiology of endometriosis remains unclear.
Several hypotheses have been raised to explain the origin of endometriosis. Of these, retrograde menstruation is the most widely accepted theory, but still fails to explain the etiopathogenesis of this disease considering the high prevalence of this event (90%), suggesting the involvement of additional genetic or non-genetic risk factors [
A great deal of evidence has indicated that endometriosis is likely to be a multifactorial disease. Several factors such as immunologic, hormonal, environmental, and genetic factors may confer susceptibility to endometriosis [
]. It has been reported that the first-degree relatives of women with endometriosis tend to have a 5–8-fold increase in risk of occurrence of this disease [
]. All these studies support the view that genetic factors and heritable tendencies play an important role in the development of endometriosis.
Because dioxins (dioxin and dioxin-like chemicals) have an adverse impact on growth factors, cytokines, hormones and the immune system, exposure to dioxins has been proposed to lead to the development of endometriosis. Thus, polymorphisms of the detoxification enzymes which cause a lack of function or decreased function of these enzymes may contribute to endometriosis. Like other members of the glutathione-s-transferase (GST) family, GSTP1, which locates at 11q13, participates in the detoxification of electrophilic compounds by glutathione conjugation [
]. Both exon 5 and exon 6 are related to the polymorphism and result in amino acid substitutions, but only exon 5 is linked to enzymatic activity. The polymorphism of exon 5 happens at the −313 site, where an adenosine-to-guanidine (A–G) transition leads to an Ile-to-Val substitution.
It has been confirmed that different GSTs have different conversion efficiency and enantio-selectivity for various toxic products [
An alpha class mouse glutathione S-transferase with exceptional catalytic efficiency in the conjugation of glutathione with 7b, 8a-dihydroxy-9a, 10a-oxy-7,8,9,10-tetrahydrobenzo[a] pyrene.
]. A number of studies have reported on the association of GSTM1 and GSTT1 with endometriosis, but studies on the association between GSTP1 and the endometriosis are rare. Furthermore, it has been reported that GSTP1 is a key substance which plays an important role in biotransformation of certain environmental pollutants [
Identification of genetic polymorphisms at the glutathione S-transferase Pi locus and association with susceptibility to bladder, testicular and prostate cancer.
]. This suggests that GSTP1 may be more important than other GST in the pathogenesis of endometriosis.
For this reason, several molecular epidemiological studies have been conducted to evaluate the association of GSTP1 −313A/G polymorphism with endometriosis risk, but the previous results remain controversial rather than consistent [
Glutathione transferase polymorphisms and risk of endometriosis associated with polychlorinated biphenyls exposure in Italian women: a gene–environment interaction.
Polymorphisms of dioxin receptor complex components and detoxification-related genes jointly confer susceptibility to advanced-stage endometriosis in the taiwanese han population.
]. Therefore, the aim of our study is to derive a more precise estimation of the association between GSTP1 −313A/G polymorphism and endometriosis risk by performing a meta-analysis of all eligible case–control studies. The present research may have clinical impact if we could demonstrate that the GSTP1 −313A/G polymorphism is significantly linked to endometriosis risk. Such a breakthrough may provide the introduction of predictive tests or preventive measures and even support the use of anti-oxidants in the medical treatment of endometriosis.
2. Materials and methods
2.1 Search strategy
We conducted an extensive search for studies that examined the association between the GSTP1 −313A/G polymorphisms and endometriosis risk. All eligible studies were identified by searching the PubMed, Embase, Elsevier Science Direct and Chinese Biomedical Literature Database (CBM) on March 20, 2013 using the terms as follows: (“GSTP1” or “glutathione-s-transferase P1”) in combination with (“polymorphism” or “polymorphisms” or “variant” or “mutation”) in combination with (“endometriosis”) for all publications on the association between GSTP1 −313A/G polymorphisms and endometriosis risk. No language or country restrictions were applied. Additional studies were identified by a hand search of the references of original studies. Review articles were also examined to find additional eligible studies.
2.2 Inclusion and exclusion criteria
The following inclusion criteria were applied for selecting papers: (a) the paper must be a case–control study; (b) articles evaluating the association between the GSTP1 polymorphisms and endometriosis risk; (c) the paper had to provide sufficient data to estimate an odds ratio (OR) and 95% confidence interval (95% CI). The following exclusion criteria were applied for excluding papers: (a) review articles and studies that contained overlapping data; (b) case reports; (c) if more than one study from the same group occurred, we only recruited the later study.
2.3 Data extraction
From each eligible study, the following information was extracted by two investigators independently with the standard protocol: the name of first author, year of publication, country of origin, ethnicity of the population, the sample size of cases and controls, source of the control group, genotyping methods and allele and genotype distribution in cases and controls. Different ethnicities of the population were categorized as Caucasians, Asian, African or Mixed. The results were reviewed by a third investigator and disagreement was resolved by discussion.
2.4 Statistical analysis
The possible association between GSTP1 313A/G polymorphisms and endometriosis risk was evaluated by OR and 95% CI under allele contrast (A vs. G), homozygote (AA vs. GG), heterozygote (GA vs. GG), recessive (AA vs. GA/GG), and dominant (AA/GA vs. GG) models. The heterogeneity between the studies was assessed by Q-statistic. If P < 0.10, between-study heterogeneity was considered to exist, and thus the random-effects model was used (DerSimonian and Laird method) [
]. The overall estimate of risk was calculated by the random-effects model when obvious heterogeneity existed (I2 value >50% or P < 0.10). Otherwise, the fixed-effects model was used when obvious heterogeneity was absent (I2 value <50% or P > 0.10). To explore sources of between-study heterogeneity, we did logistic meta-regression analyses. We examined the following study characteristics: ethnicity, source of control and study sample size.
Sensitivity analysis was used to assess the stability of the pooled studies by sequential omission of individual studies. The sensitivity analysis was also conducted if Hardy–Weinberg equilibrium (HWE) disequilibrium existed (P < 0.05 was considered statistically significant). HWE was tested using a professional web-based program (http://ihg2.helmholtz-muenchen.de/cgibin/hw/hwa1.pl). The funnel plot was used to visually observe the possible publication bias [
]. Begg's and Egger's tests were also used to statistically detect the publication bias (P < 0.05 was considered representative of statistically significant publication bias). STATA Software (version 9.0, Stata Corp) was applied to calculate all the available data for our meta-analysis.
3. Results
3.1 Study characteristics
According to the search criteria, a total of nine publications met our inclusion criteria. Of these articles, one [
] was excluded because it only investigated GSTM1 and GSTT1 polymorphism with endometriosis risk. As a result, a total of eight studies were used for our meta-analysis [
Glutathione transferase polymorphisms and risk of endometriosis associated with polychlorinated biphenyls exposure in Italian women: a gene–environment interaction.
Polymorphisms of dioxin receptor complex components and detoxification-related genes jointly confer susceptibility to advanced-stage endometriosis in the taiwanese han population.
]. The study characteristics of the included studies were presented in Table 1. In total, the eight case–control studies that examined the association between GSTP1 −313A/G polymorphism and endometriosis risk consisted of 1638 endometriosis patients and 1842 controls. There were six studies of Asians [
Polymorphisms of dioxin receptor complex components and detoxification-related genes jointly confer susceptibility to advanced-stage endometriosis in the taiwanese han population.
Glutathione transferase polymorphisms and risk of endometriosis associated with polychlorinated biphenyls exposure in Italian women: a gene–environment interaction.
Table 2 lists the main results of our meta-analysis. We found that GSTP1 −313A/G polymorphism was not associated with endometriosis risk in the overall population (A vs. G: OR = 1.02, 95% CI = 0.97–1.07, P = 0.482, Fig. 1 for A vs. G; AA vs. GG: OR = 1.02, 95% CI = 0.98–1.07, P = 0.299, Fig. 2 for AA vs. GG; GA vs. GG: OR = 1.04, 95% CI = 0.98–1.10, P = 0.249; Fig. 3 for GA vs. GG; AA vs. GA/GG: OR = 1.01, 95% CI = 0.96–1.07, P = 0.621, Fig. 4 for AA vs. GA/GG; AA/GA vs. GG: OR = 1.01, 95% CI = 0.99–1.04, P = 0.276; Fig. 5 for AA/GA vs. GG).
Table 2Results of meta-analysis for GSTP1 −313 A/G polymorphism and endometriosis risk.
Comparison
Population
N
Sample size
Test of association
Mode
Test of heterogeneity
Case
Control
OR
95% CI
P
χ2
P
I2
A vs. G
Overall
8
3276
3664
1.02
0.97–1.07
0.482
R
24.95
0.001
71.9
Asian
6
2614
3040
0.98
0.96–1.01
0.182
F
7.51
0.186
33.4
Caucasians
2
662
624
1.14
0.97–1.34
0.115
R
4.59
0.032
78.2
AA vs. GG
Overall
8
1055
1208
1.02
0.98–1.07
0.299
R
21.62
0.003
67.6
Asian
6
868
1047
1.00
0.98–1.02
0.981
F
1.82
0.874
0
Caucasians
2
187
161
1.19
0.89–1.57
0.237
R
7.84
0.005
87.2
GA vs. GG
Overall
8
660
745
1.04
0.98–1.10
0.249
R
13.96
0.052
49.9
Asian
6
495
555
1.01
0.96–1.05
0.721
F
0.38
0.996
0
Caucasians
2
165
190
1.11
0.85–1.44
0.434
R
8.31
0.004
88.0
AA vs. GA/GG
Overall
8
1638
1842
1.01
0.96–1.07
0.621
R
14.80
0.039
52.7
Asian
6
1307
1530
0.97
0.92–1.03
0.295
F
4.76
0.446
0
Caucasians
2
331
312
1.28
1.08–1.53
0.006
F
1.51
0.220
33.6
AA/GA vs. GG
Overall
8
1638
1842
1.01
0.99–1.04
0.276
R
18.53
0.010
62.2
Asian
6
1307
1530
1.00
0.99–1.02
0.878
F
1.25
0.940
0
Caucasians
2
331
312
1.08
0.93–1.24
0.317
R
8.18
0.004
87.8
OR, odds ratio; CI, confidence interval; F, fixed effects model; and R, random effects model.
In the sub-group analysis according to ethnicity, our meta-analysis suggested that GSTP1 A/G polymorphism was not associated with endometriosis risk in the Asian population (Table 2). Interestingly, a significant association was found in Caucasians under recessive model (AA vs. GA/GG: OR = 1.28, 95% CI = 1.08–1.53, P = 0.006, Fig. 4 and Table 2).
3.3 Heterogeneity analysis and sensitivity analysis
There was significant heterogeneity for all the genetic models in the overall population. To explore the source of heterogeneity, we assessed the dominant model (AA/GA vs. GG) by ethnicity (Asian or Caucasian), source of control (population-based or hospital-based) and sample size (≤400 subjects or >400 subjects). As a result, ethnicity (P = 0.007) but not source of control (P > 0.05) or sample size (P > 0.05) was found to contribute to substantial heterogeneity. However, some heterogeneity still existed in the Caucasian population and none of these variables could explain the heterogeneity (P > 0.05). Sensitivity analysis was used to assess the stability of pooled studies by sequential omission of individual studies. In our meta-analysis, no single study influenced the overall results qualitatively as indicated by sensitivity analysis. This indicates the robustness and reliability of our results.
3.4 Publication bias
Funnel plot and Begg's and Egger's tests were used to assess the publication bias of our included studies. The shape of the funnel plot did not suggest any evidence of obvious asymmetry (figures not shown). Begg's and Egger's tests were used to provide statistical evidence of symmetry. Similarly, the results revealed the absence of publication bias (allelic model: Begg's test P = 0.174, Egger's Test P = 0.057; homozygote model: Begg's test P = 0.108, Egger's Test P = 0.105; heterozygote model: Begg's test P = 0.536, Egger's Test P = 0.660; recessive model: Begg's test P = 0.060, Egger's test P = 0.174; dominant model: Begg's test P = 0.174, Egger's test P = 0.197).
4. Comments
Extensive epidemiological, experimental, and molecular evidence indicates that several factors such as immunologic, hormonal and environmental factors play an important role in the development of endometriosis. Since dioxins (dioxin and dioxin-like chemicals) have a negative effect on growth factors, cytokines, hormones and the immune system, exposure to dioxins has been considered to lead to the development of endometriosis. The role of dioxins which leads to endometriosis is neither directly damaging the cells nor combining complexes with proteins and nucleic acids but acting on the target genes in the cell nucleus through integrating with arylhydrocarbon nuclear translocator (ARNT). The GST gene is one of the target genes which regulate human metabolism, immune, endocrine and are closely related with the occurrence of endometriosis.
It has been confirmed that different GST have different conversion efficiency and enantio-selectivity for various toxic products which may lead to cancer [
An alpha class mouse glutathione S-transferase with exceptional catalytic efficiency in the conjugation of glutathione with 7b, 8a-dihydroxy-9a, 10a-oxy-7,8,9,10-tetrahydrobenzo[a] pyrene.
]. Because only GSTP1 −313 A/G polymorphism links to enzymatic activity, GSTP1 −313 A/G polymorphism may confer susceptibility to cancers.
Numerous studies have focused on GSTP1 −313 A/G polymorphism and the risk of various cancers, including lung, breast, colorectal, bladder, pancreatic, thyroid, prostate cancer, and B-cell lymphoma [
]. Up until now, several meta-analyses have been performed on GSTP1 −313 A/G polymorphism and cancers, such as colorectal cancer, prostate cancer, hepatocellular carcinoma, bladder cancer, gastric cancer, lung cancer and breast cancer [
]. This study was performed to investigate the association between GSTP1 −313 A/G polymorphism and endometriosis, a disease presenting similar biological behaviors to cancers [
GSTP1 is a key substance in the biotransformation and inactivation of certain environmental pollutant. Even more, GSTP1 may play an important role in the etiology of malignant disease [
Identification of genetic polymorphisms at the glutathione S-transferase Pi locus and association with susceptibility to bladder, testicular and prostate cancer.
]. Since the GSTP1 −313A/G polymorphism was identified; several studies have been conducted to evaluate the association of GSTP1 −313 A/G polymorphism with endometriosis risk, but the results remain inconclusive. Ertunc et al. concluded that GSTP1 −313 A/G polymorphism might modulate the risk of endometriosis with significantly decreased risk for GSTP1 val/val and marginally increased risk for GSTP1 ile/ile [
Glutathione transferase polymorphisms and risk of endometriosis associated with polychlorinated biphenyls exposure in Italian women: a gene–environment interaction.
Polymorphisms of dioxin receptor complex components and detoxification-related genes jointly confer susceptibility to advanced-stage endometriosis in the taiwanese han population.
]. To help explain the diversity of results from different studies, we conducted a meta-analysis to comprehensively explore the GSTP1 −313 A/G polymorphism and endometriosis risk. To the best of our knowledge, this is the first meta-analysis of the assessment for the association between GSTP1 −313 A/G polymorphism and endometriosis risk.
In the present study, an obvious association was found only in Caucasians under the recessive model (AA vs. GA/GG: OR = 1.28, 95% CI = 1.08–1.53, P = 0.006, Fig. 4 and Table 2). No obvious associations were found in other genetic models in Caucasians, and two studies with small sample size (331 cases and 312 controls) investigated the Caucasians. We concluded that GSTP1 −313 A/G polymorphism may not be associated with endometriosis risk, while the observed increase in risk of endometriosis may be due to small-study bias. Small-study bias in genetic association studies is not a new event when performing meta-analysis. For example, Li et al. published a meta-analysis to evaluate the association between ER-α gene PvuII T/C polymorphisms and endometriosis risk [
]. They concluded that the observed small nominal increase in risk of endometriosis associated with the ER-α PvuII T/C polymorphisms is due to small-study bias, which was similar with our results.
Obvious heterogeneity among studies was observed in the overall population. Although we found that ethnicity contributed substantial heterogeneity to our results by using the method of meta-regression, some heterogeneity cannot be explained by several possible source of heterogeneity such as ethnicity, sample size and source of controls. In the present meta-analysis, the eligible studies used several genotyping methods including PCR-RFLP, PCR-HRM, TaqMan and Direct Sequencing. All of these genotyping methods are accurate, rapid and commonly used in detecting gene polymorphism of molecular biology experiments. In previous meta-analyses, these genotyping methods were not considered to be the source of heterogeneity. Probably other limitations of included studies may partially contribute to the observed heterogeneity. Hence, we conducted analyses using the random effects model. Another important problem which may have a negative effect on the results is publication bias. In the present study, Funnel plot, and Begg's and Egger's tests were used to assess the publication bias of our included studies. Both the shape of funnel plot and statistical results did not show obvious publication bias. This suggests that the publication bias would not make substantial negative effect on our results and that results of our meta-analysis are relatively stable.
Endocrine disrupting chemicals (EDCs) have the capacity to interfere with normal signalling systems. EDCs may mimic, block or modulate the synthesis, release, transport and metabolism of natural hormones [
]. Many studies have demonstrated that EDC exposure is a risk factor for women's health and it is closely associated with many kinds of gynecological diseases, including endometriosis [
]. It is a pity that not all of the eligible studies included in our meta-analysis provide information about EDC exposure, so that we could not evaluate the impact of EDC exposure in endometriosis.
Although comprehensive analysis was conducted to demonstrate the association between GSTP1 −313A/G polymorphism and risk of endometriosis, there are still some limitations should be pointed out. First, only three of the eight included studies used controls that were population-based [
Glutathione transferase polymorphisms and risk of endometriosis associated with polychlorinated biphenyls exposure in Italian women: a gene–environment interaction.
Polymorphisms of dioxin receptor complex components and detoxification-related genes jointly confer susceptibility to advanced-stage endometriosis in the taiwanese han population.
]. Second, the number of studies and the number of subjects included in our meta-analysis were relatively small. Furthermore, there was no original study which investigated African or other ethnicities. Therefore, additional studies with larger sample size and providing detailed information should be conducted to assess the effect of GSTP1 −313A/G polymorphism on endometriosis risk in different ethnicities, especially in Africans. Third, as some studies included in our meta-analysis are based on unadjusted estimates, some risk factors such as age, family history and environment factors might cause confounding bias.
In spite of the above shortcomings, our meta-analysis also had some advantages. First, a meta-analysis of the association of GSTP1 −313A/G polymorphism on endometriosis risk is statistically more powerful than any single study. Second, a rigorous searching strategy which consisted of computer-assisted and manual searches made the eligible studies as inclusive as possible. Third, the quality of the case–control studies included in our meta-analysis was satisfactory and met our inclusion criteria, which leads to a more reliable result. Furthermore, our meta-analysis was well designed and conducted in the process of literature selection, data extraction and data analysis.
In conclusion, GSTP1 −313A/G polymorphism may not be associated with endometriosis risk, while the observed increase in risk of endometriosis may be due to small-study bias. Considering the limited sample size and ethnicities included in our meta-analysis, an updated meta-analysis will be urgently needed to confirm our results when further larger and well-designed studies are published.
Acknowledgements
This study has been supported by some students in acquisition of data and searching background information relevant to our study. We would like to thank them for their help which led to improvement of this article.
An alpha class mouse glutathione S-transferase with exceptional catalytic efficiency in the conjugation of glutathione with 7b, 8a-dihydroxy-9a, 10a-oxy-7,8,9,10-tetrahydrobenzo[a] pyrene.
Identification of genetic polymorphisms at the glutathione S-transferase Pi locus and association with susceptibility to bladder, testicular and prostate cancer.
Glutathione transferase polymorphisms and risk of endometriosis associated with polychlorinated biphenyls exposure in Italian women: a gene–environment interaction.
Polymorphisms of dioxin receptor complex components and detoxification-related genes jointly confer susceptibility to advanced-stage endometriosis in the taiwanese han population.
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