Open Access

Association between glutathione S-transferase M1/T1 gene polymorphisms and susceptibility to endometriosis: A systematic review and meta-analysis

  • Authors:
    • Xiaoyan Xin
    • Zhishan Jin
    • Huajian Gu
    • Yuanyue Li
    • Tingting Wu
    • Teng Hua
    • Hongbo Wang
  • View Affiliations

  • Published online on: February 26, 2016     https://doi.org/10.3892/etm.2016.3110
  • Pages: 1633-1646
  • Copyright: © Xin et al. This is an open access article distributed under the terms of Creative Commons Attribution License.

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Abstract

Endometriosis is a polygenic/multifactorial disease caused by interactions between multiple genes and the environment. Findings from studies evaluating the association between the glutathione S‑transferase (GST) M1/T1 null genotype and susceptibility to endometriosis are inconsistent. This meta‑analysis updated and reevaluated the possible associations between GSTM1, GSTT1 and combined GSTM1/GSTT1 (null genotype versus wild‑type) gene polymorphisms and susceptibility to endometriosis. The PubMed, Embase and Chinese BioMedical Literature databases and Google Scholar were searched for case‑control genetic association studies on GSTM1/GSTT1 (null genotype versus wild‑type) gene polymorphisms and endometriosis in comparison with non‑endometriosis or healthy controls. Fixed‑effect and random‑effect meta‑analytical techniques were conducted for the outcome measure and subgroup analyses. The meta‑analysis demonstrated significant associations between the GSTM1 [odds ratio (OR)=1.56; 95% confidence interval (CI): 1.25‑1.95; P<0.0001), GSTT1 (OR=1.31; 95% CI: 1.02-1.68; P=0.037) and GSTM1/GSTT1 (OR=1.68; 95% CI: 1.29-2.17; P<0.0001) null genotypes and increased risk for endometriosis. The results suggest that the GSTM1, GSTT1, and combined GSTM1/GSTT1 null genotypes increase susceptibility to endometriosis. Additional well-designed studies and precise analyses are warranted to confirm these findings.

Introduction

Endometriosis manifests as ectopic endometrial cells outside the uterus. It is an intractable disease that causes infertility, dysmenorrhea and pelvic pain. Endometriosis occurs in 10% of women of childbearing age. Notably, the incidence of endometriosis has been rising in recent years (1). The pathogenesis of endometriosis remains to be elucidated.

Published reports indicate that endometriosis is a polygenic/multifactorial disease caused by interactions between multiple genes and the environment (2,3). In particular, a correlation has been identified between endometriosis and exposure to environmental toxins such as dioxin (4); dioxin and dioxin-like compounds have been implicated in the development of endometriosis (5,6).

The phase II conjugation enzymes usually function to inactivate environmental toxins. Among these, glutathione S-transferase (GST) may be critical for the detoxification of dioxins. Human GSTs are classified into two distinct categories: Soluble or cytosolic and membrane-bound microsomal. The soluble or cytosolic GSTs are subdivided into seven families named α, µ, ω, π, σ, θ and ζ (7). Genes in several of these families are polymorphic, including: GSTA2 in the α family, GSTM1 and GSTM3 in the µ family, GSTP1 in the π family, GSTO, GSTT1, and GSTT2 in the θ family, and GSTZ1 in the ζ family. Heritable allelic differences in GSTM1, GSTM3, GSTT1 and GSTP1 may have marked relevance for individual susceptibility to disease.

GSTM1 and GSTT1 are two candidate genes that may play an important role in the development of endometriosis. GSTM1 and GSTT1 are located on chromosomes 1p13.3 and 22q11.23, respectively. They are critical in the detoxification of the products of oxidative stress produced during the repair of the ovarian epithelium. GSTM1 and GSTT1 null alleles have reduced enzyme activity, a state that may contribute to inefficient detoxification of intermediates produced during stress. This may increase damage to various host genes and contribute to the pathogenesis of endometriosis (8,9).

A meta-analysis summarizing the literature up to the year 2005 suggested that the GSTT1 null genotype, but not the GSTM1 null genotype, was associated with an increased risk for endometriosis (7). In the years since 2005, additional reports investigating this topic have been published. The objective of the present study was to update the existing meta-analysis and reevaluate the possible associations between GSTM1, GSTT1 and combined GSTM1/GSTT1 (null genotype vs. wild-type) gene polymorphisms and susceptibility to endometriosis.

Materials and methods

Searches

For this systematic review and meta-analysis, PubMed (from January 1996 to January 2014), Embase (from January 1996 to January 2014), Chinese BioMedical Literature database (from January 1996 to January 2014) and Google Scholar (from January 1996 to January 2014) were searched. The following keywords were used: ‘endometriosis’, ‘polymorphisms’, ‘glutathione S-transferases’, ‘GSTM1’ and ‘GSTT1’ or their combinations.

Reference lists from articles identified by the electronic search were searched by hand. This process was performed iteratively until no additional articles could be identified.

Inclusion and exclusion criteria

Articles published in English or Chinese were included if they reported quantitative outcomes from case-control genetic association studies on GSTM1, GSTT1 or combined GSTM1/GSTT1 (null genotype vs. wild-type) gene polymorphisms and endometriosis versus non-endometriosis or healthy controls.

Studies were excluded if they were case reports, case-only studies, letters, reviews or meta-analyses; included subjects who were related; included cases of adenomyosis, which has unknown etiology (10); reported insufficient data; or were duplicate studies.

Selection of studies

Two reviewers (XYX and ZSJ) independently examined titles and abstracts to select eligible studies. Records were removed that were ongoing or unpublished studies, or were published as abstracts or conference proceedings. Where data sets were overlapping or duplicated, only the most recent information was included. The full text of potentially relevant studies was retrieved. Two reviewers (XYX and HJG) independently examined the full text records to determine which studies met the inclusion criteria. Disagreement about the selection of studies was resolved by discussion and consensus.

Data extraction and management

Two reviewers (XYX and ZSJ) independently extracted data from eligible studies including the first author's last name, publication year, study location, ethnicity, matching variability, diagnostic criteria, stages of disease, source of controls, numbers of cases and controls, and numbers and/or percentages of null genotypes. Disagreement about data extraction was resolved by discussion and consensus.

Assessment of quality of evidence in included studies

Two reviewers (YYL and HJG) independently assessed quality of evidence in the included studies using the 9-star Newcastle-Ottawa Scale, which considers selection, comparability and outcome evaluation criteria.

Assessment of heterogeneity

Heterogeneity was assessed using the χ2 test and I2 test. The I2 statistic was interpreted as follows: I2=0–40%, heterogeneity may not be important; I2= 30–60%, heterogeneity may be moderate; I2=50–90%, heterogeneity may be substantial; and I2=75–100%, considerable heterogeneity (11). If heterogeneity was present, meta-regression was used to find the source.

Assessment of reporting biases

A funnel plot of effect estimates against their standard errors (SEs) was created to assess possible reporting bias between studies. Funnel plot asymmetry was assessed using Egger's linear regression test and Begg's rank correlation test; P<0.05 suggested publication bias.

GSTM1/GSTT1 and risk for endometriosis

Two reviewers (XYX and HJG) independently combined data from trials using a fixed-effect model (DerSimonian and Laird method) when there was no significant heterogeneity in populations (I2<50%) and a random-effect model (Mantel-Haenszel method) when there was considerable heterogeneity. Variables were synthesized using odds ratios (ORs). A P-value of 0.05 was used as the cut-off value to determine statistical significance, and data are presented as the estimated OR with 95% confidence intervals (CIs). All statistical analyses were performed using STATA software, version 12.0 (StataCorp, College Station, TX, USA). Inconsistencies in data analysis were resolved through consensus and discussion with a third reviewer (ZSJ).

Sensitivity and subgroup analyses

Sensitivity analyses were performed to explore the impact of excluding outlying results. Subgroup analyses were performed by stratifying patients according to ethnicity (Caucasian, Asian or mixed), characteristics of controls (hospital patients or healthy individuals), and quality of evidence (high-quality or low-quality).

Results

Screening and selection

The searches identified 120 articles. Titles and abstracts were screened, and 36 studies were identified as potentially eligible for inclusion. The full text articles for these studies were retrieved. Following analysis of the full text articles, four studies were excluded and 32 studies were found to be eligible for inclusion according to the criteria used for considering studies in this review (Fig. 1).

Included studies

The characteristics of the included studies are shown in Table I. There were 32 case-control genetic association studies involving 3,990 cases of endometriosis and 4,625 controls. One publication addressed two groups of subjects with different ethnicities and was considered as two case-control genetic association studies (12); thus, the total number of studies was considered to be 33. Studies included data relevant to the GSTM1 genotype, GSTT1 genotype or the combined GSTM1/GSTT1 genotype. Of the 32 eligible studies, 20 were conducted in Asia (1231), eight in Europe (3239), two in North America (40,41), and two in South America (42,43). The evidence reported in 23 studies was identified as high-quality, and that in 10 studies was identified as low-quality.

Table I.

Characteristics of included studies on the GSTM1, GSTT1 and combined GSTM1/GSTT1 gene polymorphisms.

Table I.

Characteristics of included studies on the GSTM1, GSTT1 and combined GSTM1/GSTT1 gene polymorphisms.

GSTM1 (n)GSTT1 (n)GSTM1+GSTT1 (n)



First author, yearEthnicityCountriesSource of controlsQualityCases/nullControls/nullCases/nullControls/nullCases/nullControls/nullRefs.
Baranov, 1996CaucasianRussiaHealthy individualsLow42/3467/26 (33)
Baranova, 1999CaucasianRussia, FranceHospital patientsHigh65/5072/3365/1372/7 (32)
Baranov, 1999CaucasianRussiaHealthy individualsLow150/8899/42 (34)
Hadfield, 2001CaucasianUKHospital patientsHigh132/5952/27116/2950/14 (35)
Baxter, 2001CaucasianEnglandHealthy individualsHigh84/40219/107 (36)
Bischoff, 2002CaucasianUSAHospital patientsLow62/1336/20 (40)
Ivaschenko, 2003CaucasianRussiaHospital patientsHigh74/4240/1774/2740/674/1640/2(37)
Arvanitis, 2003CaucasianGreeceHealthy individualsHigh275/161346/181275/24346/31275/11346/16(38)
Peng, 2003AsianChinaHospital patientsHigh76/5080/37 (15)
Lin, 2003AsianChinaHospital patientsHigh68/4928/1268/5328/9 (16)
Morizane, 2004AsianJapanHealthy individualsLow108/57173/89108/52173/71108/30173/43(17)
Hsieh, 2004AsianChinaHospital patientsHigh150/95159/8 (18)
De Carvalho, 2004MixedBrazilHospital patientsLow61/2132/17 (42)
Ding, 2004AsianChinaHealthy individualsHigh80/46105/5580/59105/4780/34105/24(12)
Ding, 2004AsianChinaHealthy individualsHigh41/21107/5741/15107/3241/10107/14(12)
Babu, 2005CaucasianIndiaHospital patientsHigh310/121215/64310/42215/34310/14215/11(19)
Hur, 2005AsianKoreaHospital patientsLow194/112259/145194/104259/125 (20)
Aban, 2007CaucasianTurkeyHospital patientsHigh150/88150/65150/59150/44 (21)
Chang, 2007AsianChinaHospital patientsHigh74/4865/3074/4665/3274/2765/13(30)
Kim, 2007AsianKoreaHospital patientsHigh316/183256/146316/178256/124 (22)
Rozati, 2009CaucasianIndiaHospital patientsHigh97/26102/15 (13)
Yang, 2009AsianChinaHospital patientsLow216/134216/100 (14)
Cao, 2009AsianChinaHospital patientsHigh51/33102/6151/22102/39 (23)
Wu, 2009AsianChinaHospital patientsHigh96/6385/40 (24)
Huang, 2010AsianChinaHospital patientsHigh28/1229/10 (25)
Trabert, 2011CaucasianUSAHealthy individualsHigh254/137567/268 (41)
Hosseinzadeh, 2011CaucasianIranHealthy individualsHigh120/87200/80 (26)
Wu, 2012AsianChinaHealthy individualsLow121/57171/52121/40171/33121/23171/15(27)
Seifati, 2012CaucasianIranHospital patientsHigh101/51142/74 (28)
Vichi, 2012CaucasianItalyHospital patientsHigh181/104162/85181/20162/32 (39)
Matsuzaka, 2012AsianJapanHospital patientsHigh97/43143/6797/38143/56 (29)
Frare, 2013MixedBrazilHealthy individualsLow50/2546/3450/1646/27 (43)
Sachan, 2013CaucasianIranHealthy peopleLow66/27100/16 (31)

[i] Null genotype vs. wild type gene polymorphisms and susceptibility to endometriosis. GSTM1, glutathione S-transferase µ1; GSTT1, glutathione S-transferase θ1.

Excluded studies

Of the 36 studies that were relevant to the GSTM1/GSTT1 genotype and endometriosis, four were excluded. Of these, three were duplicates (13,14,32), and one included subjects who were related (44).

GSTM1/GSTT1 and risk for endometriosis
GSTM1genotype

Data reporting on the GSTM1 gene polymorphism are described in 33 case-control studies (3,990 cases of endometriosis and 4,625 controls). The meta-analysis demonstrated that there was a significant association between the GSTM1 null genotype and an increased risk for endometriosis (OR=1.56; 95% CI: 1.25–1.95; P<0.0001; Fig. 2A).

Subgroup analyses stratified by ethnicity (Caucasian: OR=1.599; 95% CI: 1.205–2.122; P=0.001; Asian: OR=1.772; 95% CI: 1.242–2.528, P=0.002), source of controls (hospital patients: OR=1.561; 95% CI: 1.151–2.117; P=0.004; healthy individuals: OR=1.569; 95% CI: 1.131–2.176; P=0.007), and quality of evidence (high-quality: OR=1.563; 95% CI: 1.253–1.949; P<0.0001) confirmed this finding.

Subgroup analysis stratified for mixed ethnicity (two case control studies involving 111 cases of endometriosis and 78 controls) demonstrated a significant association between the GSTM1 null genotype and a decreased risk for endometriosis (OR=0.404; 95% CI: 0.219–0.745; P=0.004; Table II). Compared with individual Caucasian and Asian populations, the difference was statistically significant (P<0.001; data shown in Table III).

Table II.

Meta-analysis of the association between GSTM1, GSTT1 and combined GSTM1/GSTT1 (null genotype vs. wild-type) gene polymorphisms and susceptibility to endometriosis.

Table II.

Meta-analysis of the association between GSTM1, GSTT1 and combined GSTM1/GSTT1 (null genotype vs. wild-type) gene polymorphisms and susceptibility to endometriosis.

GroupNo. of studiesNo. of subjects (cases/controls)OR [95%CI]P-value
Total studies
  GSTM1 genotype333,990/4,6251.563 [1.253–1.949]<0.001
  GSTT1 genotype182,371/2,4901.345 [1.044–1.733]0.022
  GSTM1+GSTT1 genotype  81,083/1,2221.672 [1.291–2.166]0.005
Caucasian
  GSTM1 genotype162,163/2,5691.599 [1.205–2.122]0.001
  GSTT1 genotype  71,171/1,0351.124 [0.745–1.697]0.577
  GSTM1+GSTT1 genotype  3659/6011.185 [0.717–1.961]0.508
Asian
  GSTM1 genotype151,716/1,9781.772 [1.242–2.528]0.002
  GSTT1 genotype101,150/1,4091.573 [1.186–2.085]0.002
  GSTM1+GSTT1 genotype  5424/6211.898 [1.404–2.565]<0.001
Mixed
  GSTM1 genotype  2111/780.404 [0.219–0.745]0.004
  GSTT1 genotype  150/46
  GSTM1+GSTT1 genotype  0
Controls from hospital patients
  GSTM1 genotype212,599/2,4251.561 [1.151–2.117]0.004
  GSTT1 genotype121,696/1,5421.284 [0.963–1.712]0.089
  GSTM1+GSTT1 genotype  3458/3201.797 [1.081–2.989]0.024
Controls from healthy individuals
  GSTM1 genotype121,391/2,2001.569 [1.131–2.176]0.007
  GSTT1 genotype  6675/9481.315 [0.767–2.254]0.320
  GSTM1+GSTT1 genotype  5625/9021.657 [1.085–2.532]0.001
High quality
  GSTM1 genotype232,920/3,4261.563 [1.253–1.949]<0.001
  GSTT1 genotype141,898/1,8411.376 [1.020–1.858]0.037
  GSTM1+GSTT1 genotype  6854/8781.753 [1.265–2.430]0.001
Low quality
  GSTM1 genotype101,070/1,1991.259 [0.785–2.020]0.340
  GSTT1 genotype  4473/6491.121 [0.646–1.944]0.684
  GSTM1+GSTT1 genotype  2229/3441.542 [1.009–2.356]0.045

[i] OR, odds ratio; CI, confidence interval; GSTM1, glutathione S-transferase µ1; GSTT1, glutathione S-transferase θ1.

Table III.

Comparisons of subgroup analyses for GSTM1, GSTT1 and combined GSTM1/GSTT1 studies.

Table III.

Comparisons of subgroup analyses for GSTM1, GSTT1 and combined GSTM1/GSTT1 studies.

A, Analysis of the GSTM1 gene

GSTM1 (n)

SubgroupSubjectsNullNormalχ2P-value
Ethnicity 3.2450.72a
  CaucasianCases1,1281,035
  Controls1,1201,449
  AsianCases1,003713
  Controls9091,069
  MixedCases466518.737 <0.001b
Controls512723.467 <0.001c
Source of controls 0.1300.718
  Hospital patientsCases1,3971,202
Controls1,0731,352
  Healthy individualsCases780611
Controls1,0071,193
Quality 0.8250.364
  High qualityCases1,6091,311
Controls1,5391,887
  Low qualityCases568502
Controls541658

B, Analysis of the GSTT1 gene

GSTT1 (n)

SubgroupSubjectsNullNormalχ2P-value

Ethnicity 6.7660.009
  CaucasianCases214957
Controls168867
  AsianCases607543
Controls568841
Source of controls 0.6380.425
  Hospital patientsCases6311,065
Controls5221,020
  Healthy individualsCases206469
Controls241707
Quality 0.0620.803
  High qualityCases6251,273
  Controls5071,334
  Low qualityCases212261
Controls256393

C, Analysis of GSTM1+GSTT1 genes

GSTM1+GSTT1 (n)

SubgroupSubjectsNullNormalχ2P-value

Ethnicity 7.6420.006
  CaucasianCases41618
Controls29572
  AsianCases124300
Controls109152
Source of controls 0.0910.763
  Hospital patientsCases57401
Controls26294
  Healthy individualsCases108517
Controls112790
Quality 0.0220.882
  High qualityCases112542
Controls80598
  Low qualityCases53176
Controls58286

a Caucasians vs..Asians

b Caucasians vs. mixed

c Asians vs. mixed.

GSTT1 genotype

Data reporting on the GSTT1 gene polymorphism are described in 18 case-control studies (2,371 cases of endometriosis and 2,490 controls). The meta-analysis demonstrated a significant association between the GSTT1 null genotype and an increased risk for endometriosis (OR=1.31; 95% CI: 1.02–1.68; P=0.037; Fig. 2B).

Subgroup analysis stratified by ethnicity demonstrated a significant association between the GSTT1 null genotype and an increased risk for endometriosis among Asians (OR=1.573; 95% CI: 1.186–2.085; P=0.002), but not among Caucasians (OR=1.124; 95% CI: 0.745–1.697; P=0.577).

Subgroup analyses stratified by the source of controls found no significant association between the GSTT1 null genotype and an increased risk for endometriosis among hospital-based studies (OR=1.284; 95% CI: 0.963–1.712; P=0.089) or among healthy individuals (OR=1.315; 95% CI: 0.767–2.254; P=0.320).

Subgroup analyses stratified by quality of evidence demonstrated a significant association between the GSTT1 null genotype and an increased risk for endometriosis among studies considered high-quality evidence (OR=1.376; 95% CI: 1.020–1.858; P=0.037), but not among studies considered low-quality evidence (OR=1.121, 95% CI: 0.646–1.944; P=0.684; Table II).

Combined GSTM1/GSTT1 genotype

Data reporting on the combined GSTM1/GSTT1 gene polymorphism are described in eight case-control studies (1,083 cases of endometriosis and 1,222 controls). The meta-analysis demonstrated a significant association between the combined GSTM1/GSTT1 null genotype and an increased risk for endometriosis (OR=1.68, 95% CI: 1.29–2.17; P<0.0001; Fig. 2C).

This association was unchanged by subgroup analyses stratified by source of controls (hospital-based studies: OR=1.797; 95% CI: 1.081–2.989; P=0.024; healthy individuals: OR=1.657; 95% CI: 1.085–2.532; P=0.001) or quality of evidence (high-quality evidence: OR=1.753; 95% CI: 1.265–2.430; P=0.001; low-quality evidence: OR=1.542; 95% CI: 1.009–2.356, P=0.045; Table II).

Subgroup analysis stratified by ethnicity demonstrated a significant association between the combined GSTM1/GSTT1 null genotype and an increased risk for endometriosis among Asian populations (OR=1.898; 95% CI: 1.404–2.565; P<0.001), but not among Caucasian populations (OR=1.185; 95% CI: 0.717–1.961; P=0.508).

Publication bias

Visual inspection of a Funnel plot, Egger's test and Begg's rank correlation test revealed no significant publication bias for the GSTM1, GSTT1 and combined GSTM1/GSTT1 studies (Fig. 3; Table IV).

Table IV.

Heterogeneity and publication bias of GSTM1, GSTT1 and combined GSTM1/GSTT1 studies.

Table IV.

Heterogeneity and publication bias of GSTM1, GSTT1 and combined GSTM1/GSTT1 studies.

HeterogeneityPublication bias (P-value)


GroupI2 value (%)P-valueEgger's testBegg's funnel plot
Total studies
  GSTM1 genotype81.8<0.0010.3130.412
  GSTT1 genotype69.9<0.0010.5570.705
  GSTM1+GSTT1 genotype44.70.0810.1701.000
Caucasian
  GSTM1 genotype79.2<0.0010.4540.322
  GSTT1 genotype64.70.0090.3390.764
  GSTM1+GSTT1 genotype58.50.0900.0210.296
Asian
  GSTM1 genotype83.8<0.0010.0980.083
  GSTT1 genotype62.40.0040.1600.210
  GSTM1+GSTT1 genotype13.60.0810.3400.806
Mixed
  GSTM1 genotype0.00.664<0.0010.317
Controls from hospital patients
  GSTM1 genotype83.6<0.0010.3900.506
  GSTT1 genotype65.90.0010.3350.451
  GSTM1+GSTT1 genotype62.20.0710.5851.000
Controls from healthy individuals
  GSTM1 genotype79.4<0.0010.5980.784
  GSTT1 genotype78.4<0.0010.4310.707
  GSTM1+GSTT1 genotype45.90.1160.5311.000
High quality
  GSTM1 genotype80.9<0.0010.0420.068
  GSTT1 genotype69.9<0.0010.5300.189
  GSTM1+GSTT1 genotype49.00.0810.6411.000
Low quality
  GSTM1 genotype85.1<0.0010.7880.516
  GSTT1 genotype77.20.0040.3471.000
  GSTM1+GSTT1 genotype62.90.101

[i] GSTM1, glutathione S-transferase µ1;. GSTT1, glutathione S-transferase θ1.

Heterogeneity analysis

There was evidence of significant heterogeneity (I2>50%) between studies of GSTM1 and GSTT1, and those used in subgroup analyses, although not among studies of GSTM1/GSTT1 combined (Table IV). Therefore, the random-effect model was used in all analyses with the exception of the analysis of combined GSTM1/GSTT1 gene polymorphisms. For the GSTM1 and GSTT1 gene polymorphisms, a meta-regression was conducted in which publication year, ethnicity, source of controls, sample size, and quality of evidence were covariates. All the covariates were entered into the meta-regression model simultaneously, and the covariates that had the highest P-values were omitted one at a time in order to identify any sources of heterogeneity among them. However, the meta-regression analysis did not identify any of these covariates as a significant source of heterogeneity (Figs. 4 and 5).

Sensitivity analysis

To explore the effects of individual studies on the pooled OR estimates, a sensitivity analysis was performed, with the omission of one study at a time. The OR estimates for the GSTM1 polymorphism were not notably altered (Fig. 6A). The OR estimates for the GSTT1 and combined GSTM1/GSTT1 polymorphisms were altered when studies were excluded (Fig. 6B and C).

Discussion

In the present study, a meta-analysis of data from 33 studies was conducted to examine the associations between the GSTM1, GSTT1 and combined GSTM1/GSTT1 null genotypes and susceptibility to endometriosis. The risk for endometriosis was significantly increased in the presence of the GSTM1, GSTT1 and combined GSTM1/GSTT1 null genotypes compared with the wild-type. Subgroup analyses stratified by ethnicity, source of controls and quality of evidence confirmed this finding among several subgroups, but particularly among studies considered high-quality evidence. Notably, among patients of mixed ethnicity, the GSTM1 null genotype was significantly associated with a decreased risk for endometriosis compared with the wild-type.

A similar meta-analysis of 23 studies performed in 2005 demonstrated an increased risk for endometriosis in women with the GSTT1 null genotype (8). However, the authors requested that their findings be interpreted with caution as asymmetry in the funnel plot was evident, which was likely due to publication bias (8). This previous study did not include subgroup analyses or an evaluation of the combined GSTM1/GSTT1 null genotype-endometriosis association.

Previous meta-analyses have found that the GSTM1/GSTT1 gene polymorphism is associated with cervical cancer (45), breast cancer (46), bladder cancer (47), gastric cancer (48,49) and acute leukemia (50). In accordance with the observations of the present study, several studies have shown that the GSTM1 (OR=32.6, 95% CI: 15.07–70.32, P<0.0001) (18) and GSTT1 (OR>3; P<0.0001) null genotypes (12,16) are associated with an increased risk for endometriosis. However, other reports suggest the GSTM1 (OR=0.21, 95% CI: 0.09–0.52, P<0.0001; OR=0.35, 95% CI: 0.15–0.83, P<0.0001) (40,43), GSTT1 (OR≥5; P<0.0001) (16) and combined GSTM1/GSTT1 (OR=0.38; P<0.001) (17) null genotypes are associated with a decreased risk for endometriosis. These divergent results may be explained by differences in GSTM1/GSTT1 null genotype frequencies and study locations. The frequency of the GSTM1/GSTT1 null genotype may vary from 10 to 65% depending on the region and population studied (51). Different study locations may introduce confounding variables associated with variations in lifestyles and exposures to toxic substances of the study populations.

The results of the present study must be interpreted with caution due to the presence of substantial heterogeneity. Among analyses of the studies of GSTM1 and GSTT1, the cause of heterogeneity remains unclear, despite meta-regression analyses being conducted. Among the analyses of combined GSTM1/GSTT1 studies, subgroup and sensitivity analyses suggested that studies that included patients with advanced stage endometriosis caused most of the variability. Publication bias was unlikely to have influenced the findings.

In addition to the heterogeneity, there were several limitations to this study. Firstly, the composition of the endometriosis patient and control populations varied between studies. For instance, some studies included only patients with advanced endometriosis (1720,22,27,35), while control populations consisted of a mixture of infertile (29), postmenopausal (43) and premenopausal (18,35) women, and newborn babies that had not been exposed to the environment (17). Furthermore, patients and controls were not always accurately matched by age or environmental exposures. Secondly, gene-gene or gene-environment interactions may jointly increase the risk for endometriosis; therefore, different lifestyle and environmental factors may contribute to differential genotypic frequencies in cases and controls. Attempts were made to mitigate inaccuracies associated with this limitation through a subgroup analysis stratified according to ethnicity. Thirdly, this study was based on published articles. As a positive result is more likely to be published, publication bias is an inherent limitation of all meta-analyses irrespective of the outcomes of the Egger's linear regression test and Begg's rank correlation test.

In conclusion, the present meta-analysis shows the GSTM1, GSTT1 and combined GSTM1/GSTT1 null genotypes are likely associated with increased susceptibility to endometriosis. These data are in contrast to those reported previously. Therefore, further studies reporting higher quality evidence are necessary to verify these conclusions.

Acknowledgements

The study was supported by Hubei Provincial Natural Science Foundation of China (Grant No.2013060602010236).

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Spandidos Publications style
Xin X, Jin Z, Gu H, Li Y, Wu T, Hua T and Wang H: Association between glutathione S-transferase M1/T1 gene polymorphisms and susceptibility to endometriosis: A systematic review and meta-analysis. Exp Ther Med 11: 1633-1646, 2016.
APA
Xin, X., Jin, Z., Gu, H., Li, Y., Wu, T., Hua, T., & Wang, H. (2016). Association between glutathione S-transferase M1/T1 gene polymorphisms and susceptibility to endometriosis: A systematic review and meta-analysis. Experimental and Therapeutic Medicine, 11, 1633-1646. https://doi.org/10.3892/etm.2016.3110
MLA
Xin, X., Jin, Z., Gu, H., Li, Y., Wu, T., Hua, T., Wang, H."Association between glutathione S-transferase M1/T1 gene polymorphisms and susceptibility to endometriosis: A systematic review and meta-analysis". Experimental and Therapeutic Medicine 11.5 (2016): 1633-1646.
Chicago
Xin, X., Jin, Z., Gu, H., Li, Y., Wu, T., Hua, T., Wang, H."Association between glutathione S-transferase M1/T1 gene polymorphisms and susceptibility to endometriosis: A systematic review and meta-analysis". Experimental and Therapeutic Medicine 11, no. 5 (2016): 1633-1646. https://doi.org/10.3892/etm.2016.3110