Lack of Association Between GSTM 1 and GSTT 1 Polymorphisms and Brain Tumour Risk

Although they are rarely spread to other organs of the body, brain tumors are life-threatening because they can increase pressure in the brain, push the brain against the skull, and invade healthy brain tissue. It is estimated that 22,020 new cases and 13,140 deaths occurred in 2010 (Jemal et al., 2010). The etiology of brain tumors is poorly understood. However, growing evidence has shown that genetic and environmental characteristics play key roles in the development of brain tumors (Inskip et al., 1995; Fisher et al., 2007). Glutathione S-transferases (GSTs) are important enzymes that are involved in detoxification of varieties of environmental carcinogens (Hayes and Pulford 1995). The soluble GSTs are divided into four main classes: α (GSTA), μ (GSTM), π (GSTP), and θ (GSTT) (Hayes and Pulford 1995). Of the four families, GSTM1 and GSTT1 polymorphisms have been studied extensively in relation to brain tumors because the homozygous deletions (null genotypes) of the two variants have no enzymatic activity. The absence of enzymatic activity may decrease cell’s detoxification ability and increase individual’s susceptibility to tumorigenesis (Hayes and Strange, 2000; Norppa, 2004). Based on the biological function of GSTs in carcinogenesis, several molecular epidemiological studies have been done to investigate the association between GSTM1 and GSTT1 polymorphisms and brain tumors (Elexpuru-Camiruaga et al., 1995; Trizna et al., 1Department of Neurosurgery, West China Hospital of Sichuan University, Chengdu, 2Department of Neurosurgery, Shenzhen Children Hospital, Shenzhen, China *For correspondence: youchao1977@163.com Abstract


Introduction
Although they are rarely spread to other organs of the body, brain tumors are life-threatening because they can increase pressure in the brain, push the brain against the skull, and invade healthy brain tissue.It is estimated that 22,020 new cases and 13,140 deaths occurred in 2010 (Jemal et al., 2010).The etiology of brain tumors is poorly understood.However, growing evidence has shown that genetic and environmental characteristics play key roles in the development of brain tumors (Inskip et al., 1995;Fisher et al., 2007).
Glutathione S-transferases (GSTs) are important enzymes that are involved in detoxification of varieties of environmental carcinogens (Hayes and Pulford 1995).The soluble GSTs are divided into four main classes: α (GSTA), μ (GSTM), π (GSTP), and θ (GSTT) (Hayes and Pulford 1995).Of the four families, GSTM1 and GSTT1 polymorphisms have been studied extensively in relation to brain tumors because the homozygous deletions (null genotypes) of the two variants have no enzymatic activity.The absence of enzymatic activity may decrease cell's detoxification ability and increase individual's susceptibility to tumorigenesis (Hayes and Strange, 2000;Norppa, 2004).
Based on the biological function of GSTs in carcinogenesis, several molecular epidemiological studies have been done to investigate the association between GSTM1 and GSTT1 polymorphisms and brain tumors (Elexpuru-Camiruaga et al., 1995;Trizna et al.,

Lack of Association Between GSTM1 and GSTT1 Polymorphisms and Brain Tumour Risk
Xiu-Tian Sima 1 , Wei-Ying Zhong 1 , Jian-Gang Liu 2 , Chao You 1 * 1998; Kondratieva et al., 2000;Ezer et al., 2002;De Roos et al., 2003;Wrensch et al., 2004;Pinarbasi et al., 2005;Schwartzbaum et al., 2007;Custodio et al., 2010).However, the results were conflicting.Since an individual study may have lower power to detect the association between the GSTM1 and GSTT1 polymorphisms and susceptibility to brain tumors.In this study, we conducted a meta-analysis to increase the statistical power by pooling all the eligible data together.

Selection of published studies and data extraction
PubMed and Embase databases were searched for relevant reports on the association between GSTM1 and/ or GSTT1 polymorphisms and brain tumor (last search: Aug 12, 2011).The following terms were used: (GST or glutathione S-transferase) polymorphism and brain tumor.The inclusion criteria were as follows: (i) observational studies investigating the association between GSTM1 and/or GSTT1 polymorphisms and risk of brain tumors; (ii) case-control studies; and (iii) articles presentation of sufficient data for computing odds ratios (ORs).All languages were considered.The exclusion criteria were (i) investigations of association between GSTM1 and/or GSTT1 polymorphisms and survival in brain tumors; (ii) study of meta-analysis.
Two investigators (Sima and You) worked independently to abstract the following data into predetermined forms: the first author's name, the year of publication, the country (region), the year of sample collection, types of brain tumor, the number of cases and controls, matching criteria, and quality control for the genotyping techniques (Table 1).

Statistical analysis
Pooled ORs and their 95% confidence intervals (95% CIs) were calculated separately for GSTM1 and GSTT1 polymorphisms.We detected the heterogeneity among the studies using the Q-test and I 2 statistics (Higgins and Thompson 2002).If P > 0.10 and I 2 ≤ 50%, we chose the fixed effects model (Mantel-Haenszel) to pool data (Mantel and Haenszel 1959).In contrast, if P ≤ 0.10 or I 2 > 50%, we chose the random effects model (DerSimonian and Laird) to combine data (DerSimonian and Laird, 1986).Two subgroup analyses were addressed: types of brain tumors (glioma and meningioma), and source of controls (population and hospital).Publication bias was assessed using Begg's funnel plot asymmetry test (Begg and Mazumdar, 1994), and the level of statistical significance was set at 0.05.All statistical analyses were conducted using STATA statistical software, version 10.0 (STATA Corp., College Station, TX).

Study Characteristics
Totally, 96 records were retrieved through searching the PubMed and Embase databases.Among them, thirtyfour were excluded because of duplicate.The remaining records were reviewed, and forty-five were excluded because they were review articles or were absence of polymorphism, brain tumor, human studies and controls.We further excluded eight studies due to overlapped data (n=3), meta-analysis (n=1), no GSTM1 and/or GSTT1 polymorphism (n=2), no brain tumor (n=1) and no available data (n=1).Finally, 9 publications were included in this meta-analysis (for GSTM1 polymorphism: 2288 cases of brain tumor and 4251 controls; and for GSTT1 polymorphism: 2201 cases of brain tumor and 3345 controls) (Figure 1).The characteristics of the eligible studies are summarized in Table 1.All studies investigated the association between GSTM1 and/or GSTT1 polymorphism(s) and risk of brain tumors.For GSTM1 polymorphism, 9 studies examined the relationship in glioma, and 4 studies examined the relationship in meningioma.There were 3 populationbased studies, 5 hospital-based studies, and one study without available data.For GSTT1 polymorphism, 8 studies examined the relationship in glioma, and 4 studies examined the relationship in meningioma.Three studies recruited population-based controls, and 5 studies recruited hospital-based controls.All the brain tumors were diagnosed with histopathological confirmation.All the studies reported quality control measures to avoid genotyping error, including internal control, negative control, duplicate experiments and blindness to casecontrol status.Five studies mentioned matching variables, and the most common matching factors were age and sex.

Publication Bias
Publication bias was detected in the studies investigating the association between GSTM1 polymorphism and risk of glioma (P = 0.017).No evidence of publication bias in other studies was observed.

Discussion
In this study, we did not find any association between the GSTM1 and GSTT1 polymorphisms and risk of brain tumors, including glioma and meningioma.Similar negative results were also observed in both populationbased studies and hospital-based studies.These results provide evidence to support the data reported by Coles et al. who found that GST polymorphism was not a powerful predictor of tissue-specific GST expression (Coles and Kadlubar, 2003).Taken together, we may conclude that GSTM1 and GSTT1 homozygous deletion polymorphisms may not relate to the development of brain tumors.Some of our findings were similar to the results of a meta-analysis presented by Lai et al. in 2005(Lai et al., 2005).In that meta-analysis, the authors searched publications up to January 2005, and found no significant association between GSTM1 and GSTT1 polymorphisms and risk of glioma.In contrast, some of our findings were in disagreement with the results reported by Lai et al. who found that the GSTT1 null genotype was significantly associated with an increased risk of meningioma (OR =1.95; 95% CI, 1.02-3.76,P = 0.046) (Lai et al., 2005).The main reason for this discrepancy may be that the statistical power is not sufficient to provide accurate effect of GSTT1 polymorphism on the risk of meningioma.Only 3 studies with 242 cases and 1251 controls were available for assessing the relationship between GSTT1 polymorphism and meningioma susceptibility in that meta-analysis (Elexpuru-Camiruaga et al., 1995;De Roos et al., 2003;Pinarbasi et al., 2005).Moreover, the significance is marginal.When an additional study was included in this meta-analysis (Schwartzbaum et al., 2007), the power was increased with much more sample sizes, and the significant difference disappeared.
When stratified according to source of controls, we failed to find that the GSTM1 and GSTT1 polymorphisms could influence the risk of brain tumors in both populationbased studies and hospital-based studies.Further subgroup analyses (e.g, age, and ethnicity) were prevented due to lack of sufficient data.
Publication bias is very important in a meta-analysis.And thus we used Begg's funnel plot to assess this issue and found publication bias exhibiting in association studies between GSTM1 polymorphism and risk of glioma, indicating that there is a high risk of selective publication of reports because of unknown reasons.Some limitations should be addressed in this meta-analysis.Currently, it is widely recognized that the interaction of both environmental and genetic factors may contribute to the development of brain tumors (Inskip et al., 1995;Fisher et al., 2007).It is an important priority for understanding the effect of the interaction of GSTM1 and GSTT1 homozygous deletion polymorphisms and environmental exposure on the individual's susceptibility to brain tumors.However, the investigation of gene-environment interactions was not available in this meta-analysis because of absence of detailed information.Additionally, the lack of enough data precludes examination of gene-gene interactions.On the other hand, the power might not be sufficient owing to the relatively small sample size included in this study, which may be responsible for the negative results of the association between the GSTM1 and GSTT1 homozygous deletion polymorphisms and risk of brain tumors.
In conclusion, the results of this meta-analysis suggest that the GSTM1 and GSTT1 polymorphisms are not associated with the susceptibility to brain tumors.Further studies with larger sample size in diverse ethnic groups are warranted to confirm these findings.Additionally, association studies evaluating the effect of gene-gene and gene-environment interactions on the risk of brain tumors would be valuable.