Abstract

We observed inconsistent conclusions regarding the genetic role of glutathione S-transferase gene polymorphisms, including glutathione S-transferase M1 (GSTM1), glutathione S-transferase T1 (GSTT1) present/null, and glutathione S-transferase pi (GSTP1) Ile105Val polymorphisms, in the susceptibility to nasal or colorectal polyposis (NP or CP). Thus, we aimed to perform a meta-analysis to comprehensively evaluate this association by applying Stata/SE software. After the heterogeneity assumption, Mantel–Haenszel statistics were used to obtain the odds ratio (OR), 95% confidence interval (95% CI) and P-value of the association test (PA). We obtained a total of 235 articles by searching online databases. After screening, ten eligible case–control studies were finally enrolled in our meta-analysis. For the meta-analysis of the GSTT1 gene under present versus null, we observed a decreased risk of NP [OR = 0.65; PA=0.018], but not CP. In addition, we did not detect any evident association between the GSTM1 present/null polymorphism and NP or CP risk. For the meta-analysis of the GSTP1 Ile105Val polymorphism, compared with controls, an increased risk of NP cases was detected under the models of Val versus Ile (OR = 1.36; PA=0.027), Ile/Val versus Ile/Ile (OR = 1.70; PA=0.011) and Ile/Val+Val/Val versus Ile/Ile (OR = 1.65; PA=0.010). In conclusion, the null genotype of the GSTT1 polymorphism may be linked to an increased susceptibility to NP, whereas the Ile/Val genotype of the GSTP1 Ile105Val polymorphism may be associated with a decreased risk of NP.

Introduction

Polyposis refers to a chronic disorder characterized by the presence of polyps, which are neoplasms that grow on the mucosal surface of human organs or tissues, including the nasal cavity, vocal cord, stomach, or colorectal area [13]. The type of polyposis (e.g., nasal polyposis or colorectal polyposis [NP or CP], etc.) is named according to its location [13]. NP, a chronic and complicated inflammatory disorder with the formation of mostly benign polyps within bilateral nasal cavities, encompasses a series of pathological features, such as epithelial cell proliferation, eosinophil infiltration, and glandular changes [4,5]. Colorectal polyps are mostly regarded as a type of benign tumor, but some show malignant tendencies, and hence the removal of polyps is an effective approach to prevent the occurrence of cancer [6].

Although the exact etiology of polyposis remains to be elucidated, chronic stimulation and genetic factors may partly account for the presence of polyps [7,8]. Genetic variants reportedly involved in the complicated etiology or pathogenesis of NP or CP are increasingly reported [7,9,10].

The proteins glutathione S-transferase M1 (GSTM1), glutathione S-transferase T1 (GSTT1), and glutathione S-transferase pi (GSTP1), which are encoded by the GSTM1, GSTT1, and GSTP1 genes, respectively, are the three most common classes (mu, theta, and pi) of glutathione S-transferases in the human body [11,12]. The present/null polymorphism is the common variant of the GSTM1 and GSTT1 genes, while Ile105Val (rs1695) and Ala114Val (rs1138272) are the two common polymorphisms of the GSTP1 gene [11,12]. Conflicting conclusions on the associations between polymorphisms of GSTM1, GSTT1, and GSTP1 and the risk of NP or CP have been reported [1221]. No specific meta-analysis of this topic has been performed. Thus, we are interested in quantitatively examining such an association by pooling published related evidence together.

Materials and methods

Database search strategy

We retrieved articles from four electronic databases, PubMed, Web of Science (WOS), Embase, and China National Knowledge Infrastructure (CNKI), published up to January 2018. For example, the PubMed database was searched using the following medical subject heading (MeSH) terms: (((((((((((Polyps[MeSH Terms]) or Polyp) or Adenomatous Polyps) or Intestinal Polyps) or Colonic Polyps) or Nasal Polyps) or Intestinal Polyp) or Colonic Polyp) or Nasal Polyp) or Adenomatous Polyp)) and ((((((((((((((((Glutathione S-Transferase pi[MeSH Terms]) or Glutathione S Transferase pi) or GST Class-phi) or Class-phi, GST) or GST Class phi) or Glutathione Transferase P1-1) or Glutathione Transferase P1 1) or Transferase P1-1, Glutathione) or GSTP1 Glutathione D-Transferase) or D-Transferase, GSTP1 Glutathione) or GSTP1 Glutathione D Transferase) or Glutathione D-Transferase, GSTP1) or GSTP1)) or ((((((((Glutathione S-transferase M1[MeSH Terms]) or Gstm1 protein, mouse) or glutathione S-transferase, mu 1 protein, mouse) or Gstm1 protein, rat) or glutathione S-transferase, mu 1 protein, rat) or GSTM1 protein, human) or glutathione S-transferase M1, human) or GSTM1)) or ((((((((((glutathione S-transferase T1[MeSH Terms]) or glutathione S transferase theta 1) or glutathione S transferase GSTT1) or Gstt1 protein, mouse) or glutathione S-transferase, theta 1 protein, mouse) or Gstt1 protein, rat) or glutathione S-transferase theta 1, rat) or GSTT1 protein, human) or glutathione S-transferase theta 1 protein, human) or GSTT1)).

Study screening strategy

We designed our screening strategy according to the ‘Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)’ [22]. The main exclusion criteria were as follows: (1) review or meta-analysis; (2) meeting abstract; (3) other genes/diseases; (4) non-polymorphism data; (5) cell data; (6) data without genotype; and (7) duplicated studies. For eligible case–control studies, the complete genotype frequency data of GSTM1, GSTT1, and GSTP1 gene variants in both polyposis cases and negative controls were available.

Data extraction strategy

We thoroughly extracted the following basic information from each retrieved case–control study via a predesigned table: first author name, publication year, country, ethnicity, gene, present/null frequencies of the GSTM1 and GSTT1 genes, the Ile/IIe, Ile/Val, and Val/Val genotype frequencies of the GSTP1 Ile105Val polymorphism, disease type, genotyping method, source of control, and sample size. We also evaluated the study quality and asked for missing data by emailing the original authors.

Statistical analysis

We used Stata/SE software (StataCorp, U.S.A.) for our statistical analysis. A fixed-effect model was used for Mantel–Haenszel statistics when the P-value of heterogeneity from Cochran’s Q statistic >0.1 or I2 value <50.0%. We obtained the value of the summary odds ratio (OR), 95% confidence intervals (95% CIs), and P-value of the association test (PA). The PA of less than 0.05 was deemed statistically significant. The present versus null genetic model was used for the GSTM1 and GSTT1 genes, and the allele (Val versus Ile), homozygote (Val/Val versus Ile/Ile), heterozygote (Ile/Val versus Ile/Ile), dominant (Ile/Val+Val/Val versus Ile/Ile), and recessive (Val/Val versus Ile/Ile+Ile/Val) models were used for the GSTP1 gene.

We performed subgroup meta-analyses according to ethnicity (Caucasian/Asian), disease type (NP/CP), source of control (population/hospital-based), and country (The Netherlands/Germany). We also performed the Begg’s/Egger’s tests to assess the publication bias among the selected studies and sensitivity analyses to measure the statistical stability of the results.

Results

Case–control study inclusion

As per the flow diagram in Figure 1, we obtained eligible case–control studies for our meta-analysis. First, we retrieved a total of 235 records (24 records from PubMed, 26 records from WOS, 64 records from Embase, and 121 records from CNKI) by searching online databases. Second, we excluded 41 records because of duplication and another 194 records due to the exclusion criteria (details shown in Figure 1). Third, after assessing the 11 remaining full-text articles for eligibility, we further ruled out one article without genotype frequency data. Ultimately, ten eligible articles [1221] were collected. Table 1 shows the basic information of the case–control studies. The present/null polymorphisms of the GSTM1 and GSTT1 genes and the Ile105Val polymorphism of the GSTP1 gene were tested.
Figure 1

Flow chart for inclusion of eligible case–control studies

Figure 1

Flow chart for inclusion of eligible case–control studies

Table 1

Basic information of case–control studies

First author, year [REF] Country Ethnicity Gene Case Control Method 
    Total Present/null Disease Total Present/null Source  
Arbag, 2006 [18Turkey Asian GSTM1 98 55/43 NP 102 55/47 PB PCR 
   GSTT1 98 65/33 NP 102 78/24 PB PCR 
Berkhout, 2008 [19The Netherlands Caucasian GSTM1 85 38/47 CP 215 100/115 PB PCR 
   GSTT1 85 60/25 CP 214 165/49 PB PCR 
   GSTP1 85 31/39/15# CP 209 90/86/33# PB PCR 
Fruth, 2011 [15Germany Caucasian GSTM1 69 34/35 NP 49 23/26 HB1 PCR 
   GSTM1 69 34/35 NP 52 23/29 HB2 PCR 
   GSTT1 69 52/17 NP 49 36/13 HB1 PCR 
   GSTT1 69 52/17 NP 52 42/10 HB2 PCR 
   GSTP1 69 27/28/14# NP 49 28/16/5# HB1 PCR 
   GSTP1 69 27/28/14# NP 52 27/18/7# HB2 PCR 
Gawronska, 1999 [13Poland Caucasian GSTM1 27 10/17 CP 145 73/72 PB3 PCR 
   GSTM1 27 10/17 CP 17 8/9 HB4 PCR 
Hamachi, 2013 [21Japan Asian GSTM1 455 200/255 CP 1052 506/546 HB Multiplex PCR 
   GSTT1 455 258/197 CP 1052 552/500 HB Multiplex PCR 
Lamberti, 2002 [20Germany Caucasian GSTM1 402 185/217 CP 171 77/94 PB Multiplex PCR 
   GSTM1 402 185/217 CP 200 99/101 NR Multiplex PCR 
   GSTT1 406 336/70 CP 172 144/28 PB Multiplex PCR 
   GSTT1 406 336/70 CP 148 125/23 NR Multiplex PCR 
Lin, 1998 [17U.S.A. Mixed GSTM1 459 255/204 CP 507 258/249 HB PCR 
Ozcan, 2010 [12Turkey Asian GSTM1 75 45/30 NP 167 92/75 PB PCR 
   GSTT1 75 54/21 NP 167 142/25 PB PCR 
   GSTP1 75 28/37/10# NP 167 77/59/31# PB PCR 
Tiemersma, 2004 [14The Netherlands Caucasian GSTM1 431 203/228 CP 432 206/226 HB Multiplex PCR 
   GSTT1 431 370/61 CP 432 363/69 HB Multiplex PCR 
Tijhuis, 2005 [16The Netherlands Caucasian GSTM1 746 363/383 CP 698 320/378 HB Multiplex PCR 
   GSTT1 746 612/134 CP 698 571/127 HB Multiplex PCR 
First author, year [REF] Country Ethnicity Gene Case Control Method 
    Total Present/null Disease Total Present/null Source  
Arbag, 2006 [18Turkey Asian GSTM1 98 55/43 NP 102 55/47 PB PCR 
   GSTT1 98 65/33 NP 102 78/24 PB PCR 
Berkhout, 2008 [19The Netherlands Caucasian GSTM1 85 38/47 CP 215 100/115 PB PCR 
   GSTT1 85 60/25 CP 214 165/49 PB PCR 
   GSTP1 85 31/39/15# CP 209 90/86/33# PB PCR 
Fruth, 2011 [15Germany Caucasian GSTM1 69 34/35 NP 49 23/26 HB1 PCR 
   GSTM1 69 34/35 NP 52 23/29 HB2 PCR 
   GSTT1 69 52/17 NP 49 36/13 HB1 PCR 
   GSTT1 69 52/17 NP 52 42/10 HB2 PCR 
   GSTP1 69 27/28/14# NP 49 28/16/5# HB1 PCR 
   GSTP1 69 27/28/14# NP 52 27/18/7# HB2 PCR 
Gawronska, 1999 [13Poland Caucasian GSTM1 27 10/17 CP 145 73/72 PB3 PCR 
   GSTM1 27 10/17 CP 17 8/9 HB4 PCR 
Hamachi, 2013 [21Japan Asian GSTM1 455 200/255 CP 1052 506/546 HB Multiplex PCR 
   GSTT1 455 258/197 CP 1052 552/500 HB Multiplex PCR 
Lamberti, 2002 [20Germany Caucasian GSTM1 402 185/217 CP 171 77/94 PB Multiplex PCR 
   GSTM1 402 185/217 CP 200 99/101 NR Multiplex PCR 
   GSTT1 406 336/70 CP 172 144/28 PB Multiplex PCR 
   GSTT1 406 336/70 CP 148 125/23 NR Multiplex PCR 
Lin, 1998 [17U.S.A. Mixed GSTM1 459 255/204 CP 507 258/249 HB PCR 
Ozcan, 2010 [12Turkey Asian GSTM1 75 45/30 NP 167 92/75 PB PCR 
   GSTT1 75 54/21 NP 167 142/25 PB PCR 
   GSTP1 75 28/37/10# NP 167 77/59/31# PB PCR 
Tiemersma, 2004 [14The Netherlands Caucasian GSTM1 431 203/228 CP 432 206/226 HB Multiplex PCR 
   GSTT1 431 370/61 CP 432 363/69 HB Multiplex PCR 
Tijhuis, 2005 [16The Netherlands Caucasian GSTM1 746 363/383 CP 698 320/378 HB Multiplex PCR 
   GSTT1 746 612/134 CP 698 571/127 HB Multiplex PCR 
#

, genotype frequency of GSTP1 gene (IIe/IIe, Ile/Val, Val/Val); 1, chronic rhinosinusitis without nasal polyps; 2, healthy tissue controls of inferior turbinate; 3, healthy individuals; 4, hereditary non-polyposis colorectal cancer.Abbreviations: HB, hospital-based control; NR, not reported; PB, population-based control; REF, reference.

GSTM1 and GSTT1 polymorphism

We first performed a meta-analysis to study the genetic relationships between the present/null polymorphisms of the GSTM1 and GSTT1 genes and the risk of polyposis. As shown in Table 2, there was no high degree of heterogeneity in the present versus null model of GSTM1 (I2 value = 0.0%, P-value of Cochran’s Q statistic test [PH] =0.686, and GSTT1 [I2 value = 31.4%, PH=0.157]); consequently, a fixed-effect model was selected for the Mantel–Haenszel statistics. As shown in Table 2, 3345 cases and 3807 controls were included for the quantitative synthesis of the GSTM1 present/null polymorphism, while 2840 cases and 3086 controls were available for the GSTT1 polymorphism. In the pooled estimates of the overall meta-analysis, no difference between the case and control groups for the risk of polyposis was observed for GSTM1 [Table 2, PA=0.838] and GSTT1 (PA=0.914).

Table 2

Overall meta-analysis of the GSTP1, GSTM1, GSTT1 polymorphism and the risk of polyposis

Gene Genetic models I2 PH Fixed/random OR 95% CI PA PB PE Case/control 
GSTM1 Present vs null 0.0% 0.686 Fixed 1.01 0.92–1.11 0.838 0.583 0.612 3345/3807 
GSTT1 Present vs null 31.4% 0.157 Fixed 0.99 0.87–1.13 0.914 0.032 0.016 2840/3086 
GSTP1 Val vs Ile 3.9% 0.373 Fixed 1.30 1.04–1.62 0.019 0.308 0.094 298/477 
 Val/Val vs Ile/Ile 4.9% 0.368 Fixed 1.44 0.93–2.24 0.101 0.308 0.217 298/477 
 Ile/Val vs Ile/Ile 0.0% 0.897 Fixed 1.55 1.12–2.16 0.009 0.734 0.444 298/477 
 Ile/Val+Val/Val vs Ile/Ile 0.0% 0.784 Fixed 1.52 1.12–2.07 0.007 0.089 0.060 298/477 
 Val/Val vs Ile/Ile+Ile/Val 0.0% 0.775 Fixed 1.22 0.95–1.58 0.123 0.308 0.069 298/477 
Gene Genetic models I2 PH Fixed/random OR 95% CI PA PB PE Case/control 
GSTM1 Present vs null 0.0% 0.686 Fixed 1.01 0.92–1.11 0.838 0.583 0.612 3345/3807 
GSTT1 Present vs null 31.4% 0.157 Fixed 0.99 0.87–1.13 0.914 0.032 0.016 2840/3086 
GSTP1 Val vs Ile 3.9% 0.373 Fixed 1.30 1.04–1.62 0.019 0.308 0.094 298/477 
 Val/Val vs Ile/Ile 4.9% 0.368 Fixed 1.44 0.93–2.24 0.101 0.308 0.217 298/477 
 Ile/Val vs Ile/Ile 0.0% 0.897 Fixed 1.55 1.12–2.16 0.009 0.734 0.444 298/477 
 Ile/Val+Val/Val vs Ile/Ile 0.0% 0.784 Fixed 1.52 1.12–2.07 0.007 0.089 0.060 298/477 
 Val/Val vs Ile/Ile+Ile/Val 0.0% 0.775 Fixed 1.22 0.95–1.58 0.123 0.308 0.069 298/477 

Data in bold are PA<0.05.

Abbreviaition: PE, P-value of Egger’s test.

Table 3

Subgroup meta-analysis of the GSTM1, GSTT1 polymorphism and the risk of polyposis

Gene Genetic models Subgroup OR 95% CI PA Case/control 
GSTM1 Present vs null NP 1.16 0.85–1.58 0.356 311/370 
  CP 1.00 0.90–1.10 0.931 3034/3437 
  PB 1.00 0.81–1.26 0.936 687/800 
  HB 1.03 0.92–1.15 0.640 2256/2807 
  The Netherlands 1.05 0.90–1.23 0.537 1262/1345 
  Germany 0.98 0.79–1.23 0.883 942/472 
  Asian 0.91 0.75–1.11 0.351 628/1321 
  Caucasian 0.98 1.01–1.14 0.877 2258/1979 
GSTT1 Present vs null NP 0.65 0.45–0.93 0.018 311/370 
  CP 1.06 0.92–1.21 0.431 3034/3437 
  PB 0.70 0.53–0.93 0.014 687/800 
  HB 1.11 0.95–1.29 0.189 2256/2807 
  The Netherlands 1.01 0.82–1.24 0.934 1262/1345 
  Germany 0.91 0.67–1.23 0.534 942/472 
  Asian 1.02 0.83–1.214 0.870 628/1321 
  Caucasian 0.98 0.82–1.16 0.779 2258/1979 
Gene Genetic models Subgroup OR 95% CI PA Case/control 
GSTM1 Present vs null NP 1.16 0.85–1.58 0.356 311/370 
  CP 1.00 0.90–1.10 0.931 3034/3437 
  PB 1.00 0.81–1.26 0.936 687/800 
  HB 1.03 0.92–1.15 0.640 2256/2807 
  The Netherlands 1.05 0.90–1.23 0.537 1262/1345 
  Germany 0.98 0.79–1.23 0.883 942/472 
  Asian 0.91 0.75–1.11 0.351 628/1321 
  Caucasian 0.98 1.01–1.14 0.877 2258/1979 
GSTT1 Present vs null NP 0.65 0.45–0.93 0.018 311/370 
  CP 1.06 0.92–1.21 0.431 3034/3437 
  PB 0.70 0.53–0.93 0.014 687/800 
  HB 1.11 0.95–1.29 0.189 2256/2807 
  The Netherlands 1.01 0.82–1.24 0.934 1262/1345 
  Germany 0.91 0.67–1.23 0.534 942/472 
  Asian 1.02 0.83–1.214 0.870 628/1321 
  Caucasian 0.98 0.82–1.16 0.779 2258/1979 

Data in bold are PA<0.05.Abbreviations: PB, population-based control; HB, hospital-based control.

Furthermore, we performed subgroup meta-analyses by disease type (NP/CP), source of control (population/hospital-based), country (The Netherlands/Germany), and ethnicity (Caucasian/Asian) for both the GSTM1 and GSTT1 genes. As shown in Table 3, a decreased risk of NP was detected for the GSTT1 gene in the subgroups ‘NP", OR = 0.65, 95% CI = 0.45–0.93, PA=0.018) and ‘population-based’ (OR = 0.70, 95% CI = 0.53–0.93, PA=0.014) under the present versus null model. No differences between the cases and controls were observed for the other subgroups (all PA>0.05). We also constructed forest plots of the subgroup analysis by disease type for the GSTM1 (Figure 2) and GSTT1 (Figure 3) genes. These plots indicated that the GSTT1 null genotype may be associated with susceptibility toward NP.

Figure 2

Subgroup analysis by disease type for the GSTM1 polymorphism

Figure 2

Subgroup analysis by disease type for the GSTM1 polymorphism

Figure 3

Subgroup analysis by disease type for the GSTT1 polymorphism

Figure 3

Subgroup analysis by disease type for the GSTT1 polymorphism

GSTP1 polymorphism

Next, we performed the overall analysis involving 298 cases and 477 controls for the association between the GSTP1 Ile105Val polymorphism and polyposis susceptibility (Table 2). Mantel–Haenszel statistics with a fixed model was used (Table 2, all I2<50.0%, PH>0.1). We observed an increased polyposis risk in the models of Val versus Ile (OR = 1.30; 95% CI = 1.04–1.62; PA=0.019), Ile/Val versus Ile/Ile (OR = 1.55, 95% CI = 1.12–2.16, PA=0.009) and Ile/Val+Val/Val versus Ile/Ile (OR = 1.52, 95% CI = 1.12–2.07, PA=0.007) but not in the other genetic models (all PA>0.05). We obtained a similar conclusion for the subgroup ‘NP’ under the allele (Table 4, OR = 1.36, PA=0.027), heterozygote (OR = 1.70, PA=0.011) and dominant (OR = 1.65, PA=0.010) models. Moreover, we detected increased risk in the subgroups ‘P-value of Hardy–Weinberg Equilibrium test (PHWE) >0.05’ and ‘Caucasian’ only in the allele and homozygote models (Table 4, OR > 1, PA<0.05). Consequently, the Ile/Val genotype of GSTP1 Ile105Val polymorphism is more likely to be associated with an increased risk of NP.

Table 4

Subgroup meta-analysis of the GSTP1 Ile105Val polymorphism and the risk of polyposis

Genetic models Subgroup OR 95% CI PA Case/control 
Val vs Ile NP 1.36 1.04–1.80 0.027 213/268 
 PHWE>0.05 1.41 1.08–1.84 0.011 223/310 
 Caucasian 1.41 1.08–1.84 0.011 223/310 
Val/Val vs Ile/Ile NP 1.52 0.88–2.62 0.136 213/268 
 PHWE>0.05 1.76 1.04–2.98 0.034 223/310 
 Caucasian 1.76 1.04–2.98 0.034 223/310 
Ile/Val vs Ile/Ile NP 1.70 1.13–2.57 0.011 213/268 
 PHWE>0.05 1.48 1.00–2.21 0.052 223/310 
 Caucasian 1.48 1.00–2.21 0.052 223/310 
Ile/Val+Val/Val vs Ile/Ile NP 1.65 1.13–2.41 0.010 213/268 
 PHWE>0.05 1.13 0.94–1.35 0.191 223/310 
 Caucasian 1.13 0.94–1.35 0.191 223/310 
Val/Val vs Ile/Ile+Ile/Val NP 1.27 0.92–1.76 0.141 213/268 
 PHWE>0.05 1.29 0.95–1.76 0.109 223/310 
 Caucasian 1.29 0.95–1.76 0.109 223/310 
Genetic models Subgroup OR 95% CI PA Case/control 
Val vs Ile NP 1.36 1.04–1.80 0.027 213/268 
 PHWE>0.05 1.41 1.08–1.84 0.011 223/310 
 Caucasian 1.41 1.08–1.84 0.011 223/310 
Val/Val vs Ile/Ile NP 1.52 0.88–2.62 0.136 213/268 
 PHWE>0.05 1.76 1.04–2.98 0.034 223/310 
 Caucasian 1.76 1.04–2.98 0.034 223/310 
Ile/Val vs Ile/Ile NP 1.70 1.13–2.57 0.011 213/268 
 PHWE>0.05 1.48 1.00–2.21 0.052 223/310 
 Caucasian 1.48 1.00–2.21 0.052 223/310 
Ile/Val+Val/Val vs Ile/Ile NP 1.65 1.13–2.41 0.010 213/268 
 PHWE>0.05 1.13 0.94–1.35 0.191 223/310 
 Caucasian 1.13 0.94–1.35 0.191 223/310 
Val/Val vs Ile/Ile+Ile/Val NP 1.27 0.92–1.76 0.141 213/268 
 PHWE>0.05 1.29 0.95–1.76 0.109 223/310 
 Caucasian 1.29 0.95–1.76 0.109 223/310 

Data in bold are PA<0.05.Abbreviation: PHWE, P-value of Hardy–Weinberg Equilibrium test.

Publication bias and sensitivity analysis

We performed the Begg’s/Egger’s tests to evaluate publication bias. As shown in Table 2, we did not find evidence of a high degree of publication bias under all genetic models (P-value of Begg’s test [PB] >0.05; P-value of Egger’s test (PE) >0.05], with the exception of the present versus null model of the GSTT1 gene (PB=0.032, PE=0.016). We also constructed funnel plots of Begg’s test under the present versus null model of the GSTM1 (Figure 4A) and GSTT1 (Figure 4A) genes.

Figure 4

Begg’s test and sensitivity analysis for the GSTM1 and GSTT1 polymorphisms

(A and B) Funnel plot of Begg’s test. (C and D) Sensitivity analysis.

Figure 4

Begg’s test and sensitivity analysis for the GSTM1 and GSTT1 polymorphisms

(A and B) Funnel plot of Begg’s test. (C and D) Sensitivity analysis.

Furthermore, we performed a sensitivity analysis to observe the statistical stability of the above conclusions (Figure 4C for GSTM1, Figure 4D for GSTT1, others not shown).

Discussion

NP is thought to be closely associated with a group of clinical disorders characterized by chronic rhinosinusitis and shows differences in complexity and etiology [1,23,24]. A family-based genome-wide association study reported that the holocarboxylase synthetase (HLCS), major histocompatibility complex, class II, DR α (HLA-DRA), BICD cargo adaptor 2 (BICD2), V-set immunoregulatory receptor (VSIR), and solute carrier family 5 member 1 (SLC5A1) genes may be linked to the pathogenesis of chronic rhinosinusitis with nasal polyps [9]. Another study reported that NP risk is significantly associated with the present/null polymorphism of the GSTT1 gene but not the present/null polymorphism of the GSTM1 gene and the Ile105Val polymorphism of the GSTP1 gene in the Mersin region of Turkey [12]. Conversely, in the Konya region of Turkey, a lack of genetic impact of the GSTM1 and GSTT1 polymorphisms on the predisposition for NP was reported [18]. There is statistical correlation between the polymorphisms of the GSTT1, GSTM1, and GSTP1 genes and the genetic tendency of chronic rhinosinusitis with or without nasal polyps in Germany [15]. Accordingly, it would be of interest to evaluate the overall effect of the underlying roles of the GSTM1 present/null, GSTT1 present/null, and GSTP1 Ile105Val polymorphisms in the susceptibility towards NP or CP . Our findings suggest that the null genotype of the GSTT1 polymorphism and the GSTP1 Ile105Val polymorphism are more likely to be associated with the risk of NP. However, no association between the GSTM1 present/null polymorphism and the risk of NP or CP was observed.

Human glutathione S-transferases are a family of multifunctional enzymes with antioxidant activity that are involved in oxidative stress, cell differentiation, inflammatory responses, drug detoxification, and chemotherapy resistance [25]. Some meta-analyses have reported different conclusions regarding the association between these variants and clinical disease risk. For instance, the GSTP1 Ile105Val polymorphism and GSTM1 null genotype but not the GSTT1 null genotype seem to increase the risk of Alzheimer’s disease [11]. Similarly, the GSTM1 null genotype but not the GSTT1 null genotype may be linked to the risk of juvenile open-angle glaucoma (JOAG) [26]. However, the null genotypes of the GSTM1 and GSTT1 genes and the combined GSTM1/GSTT1 gene may be risk factors for endometriosis [27]. Our meta-analysis data supports a genetic role of the GSTT1 null genotype and the Ile/Val genotype of the GSTP1 Ile105Val polymorphism in the risk of NP. Individuals with the GSTT1 null genotype may show partial gene deletion, followed by enzymatic activity deficiency and decreased detoxification capacity. The Ile105Val polymorphism of the GSTP1 gene leads to an amino acid change from Ile to Val at residue 105 of the GSTP1 protein, which also may decrease the catalytic activity of the enzyme. Such a change may reduce the efficient detoxification of stress-induced intermediates, which has been implicated in the presence of an inflammatory response and nasal polyps in response to increased levels of electrophilic compounds or reactive oxygen species.

There are some limitations of the present study that should be fully discussed. We endeavoured to search relevant publications by using search terms for different polyps, such as adenomatous polyps, intestinal polyps, colonic polyps, nasal polyps, and intestinal polyps, without language or region restrictions. However, only the data on nasal and colorectal polyps permitted synthesis of the data. Detailed data on genotype frequencies in both cases and controls are required to perform the overall meta-analysis and subsequent subgroup analyses. Due to restrictions of data availability, only the Ile105Val polymorphism was analyzed in our meta-analysis of the GSTP1 gene, and we were unable to explore the roles of other variants within the GSTP1 gene, such as the Ala114Val polymorphism. The possible distinct effects of different haplotypes merits further evidence. In addition, the joint effects of GSTM1, GSTT1, GSTP1, and other genes, such as cytochrome P450 family 1 subfamily A member 1 (CYP1A1), is worthy of analysis upon publication of sufficient data.

No high degree of heterogeneity in the comparisons of the GSTM1, GSTT1, and GSTP1 polymorphisms was detected in our study. For the meta-analysis of the GSTP1 Ile105Val polymorphism, we included five case–control studies from four articles [12,15,16,19]. However, we observed obvious alteration of the pooled OR and 95% CI values when one study [16] was excluded during our sensitivity analysis. Therefore, we removed the present study and performed the meta-analysis again. We recognize that the data included in our meta-analysis were very limited.

Although we obtained a positive result for the role of the GSTT1 present/null polymorphism in the risk of NP, the limitation of small sample size weakened the statistical power of our analysis to some extent. In addition, hospital-based controls were included in some studies. The genotype distribution of the GSTP1 Ile105Val polymorphism in the control group of one article [12] did not agree with Hardy–Weinberg equilibrium. It would be of great value to thoroughly evaluate the influences of additional variables, such as clinical type, gender, age, environmental exposure or lifestyle, on the roles of the above variants in the risk of developing polyposis.

In summary, we pooled published relevant data and concluded that the GSTT1 null genotype may serve as a protective factor against NP, whereas the Ile/Val genotype of the GSTP1 Ile105Val polymorphism is more likely to be associated with an increased risk of NP. More genomic tests in the future are warranted to further determine the roles of glutathione S-transferase gene polymorphisms in the genetic susceptibility to different types of polyposis.

Author contribution

Y.Z. and G.Z. conceived and designed the meta-analysis. Y.Z., H.Z., and G.Z. performed the literature search. Y.Z., H.Z., P.L., and G.Z. analyzed the data. Y.Z. and G.Z. wrote the paper.

Competing interests

The authors declare that there are no competing interests associated with the manuscript.

Funding

The authors declare that there are no sources of funding to be acknowledged.

Abbreviations

     
  • BICD2

    BICD cargo adaptor 2

  •  
  • CI

    confidence interval

  •  
  • CNKI

    China National Knowledge Infrastructure

  •  
  • CP

    colorectal polyposis

  •  
  • CYP1A1

    cytochrome P450 family 1 subfamily A member 1

  •  
  • GSTM1

    glutathione S-transferase M1

  •  
  • GSTP1

    glutathione S-transferase pi

  •  
  • GSTT1

    glutathione S-transferase T1

  •  
  • HLA-DRA

    major histocompatibility complex, class II, DR α

  •  
  • HLCS

    holocarboxylase synthetase

  •  
  • HWE

    Hardy–Weinberg Equilibrium

  •  
  • JOAG

    juvenile open-angle glaucoma

  •  
  • MeSH

    medical subject heading

  •  
  • NP

    nasal polyposis

  •  
  • OR

    odds ratio

  •  
  • PA

    P-value of the association test

  •  
  • PB

    P-value of Begg’s test

  •  
  • PE

    P-value of Egger’s test

  •  
  • PH

    P-value of Cochran’s Q statistic test

  •  
  • PRISMA

    Preferred Reporting Items for Systematic Reviews and Meta-Analyses

  •  
  • SLC5A1

    solute carrier family 5 member 1

  •  
  • VSIR

    V-set immunoregulatory receptor

  •  
  • WOS

    Web of Science

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