Relationship Between GSTT1 Gene Polymorphism and Hepatocellular Carcinoma in Patients from China

Hepatocellular carcinoma (HCC) is the sixth most common cancer and prevalent cancers in the human population, more than 50% of the world ́s HCC cases occur in China (Li & Jiang, 2011). It is well-documented that multiple risk factors contribute to hepatocarcinogenesis, including chronic hepatitis B virus (HBV) or hepatitis C virus (HCV) infections, cirrhosis, carcinogen exposure (such as aflatoxin B1), excessive alcohol drinking (Bayram et al., 2011). China is an important country in the AsiaPacific region, and the HBV is the mainly risk factor for the onset of HCC. The present evidences show that the factor of gene polymorphism is associated with the risk of HCC susceptibility (Yuan et al., 2011; Dong et al., 2012). Glutathione-S-transferases (GSTs) are an enzyme superfamily involved in the Phase II metabolism, acting as primary intracellular detoxifiers and contributing to a broad range of physiological processes including cellular defense (Masoudi et al., 2011; Wang et al., 2012). Glutathione-S-transferase T1 (GSTT1) is a most important sub-group type of GSTs, and its gene polymorphism takes part in the pathogenesis of cancers. There were some investigations reporting that GSTT1 deletion was associated with cancer susceptibility (Xu et al., 2011; Aguiar et al., 2012; Ramalhinho et al., 2012). In the past decades, most of the epidemiologic studies investigating the association of GSTT1 gene polymorphism with HCC susceptibility were conducted in Chinese populations. Unfortunately, the available


Introduction
Hepatocellular carcinoma (HCC) is the sixth most common cancer and prevalent cancers in the human population, more than 50% of the world´s HCC cases occur in China (Li & Jiang, 2011). It is well-documented that multiple risk factors contribute to hepatocarcinogenesis, including chronic hepatitis B virus (HBV) or hepatitis C virus (HCV) infections, cirrhosis, carcinogen exposure (such as aflatoxin B1), excessive alcohol drinking (Bayram et al., 2011). China is an important country in the Asia-Pacific region, and the HBV is the mainly risk factor for the onset of HCC. The present evidences show that the evidence is weak at present, due to sparseness of data or disagreements among the reported studies. The evidence from meta-analysis may be powerful than the individual investigation. This meta-analysis was performed to investigate whether the GSTT1 gene polymorphism was associated with the risk of HCC in Chinese population, by widely collect the reported investigations.

Search strategy for the association of GSTT1 gene polymorphism with HCC risk
The relevant studies were searched from the electronic databases of PubMed, Embase, Cochrane Library and CBM-disc (China Biological Medicine Database) on May 1, 2012. The retrieval strategy of (glutathione S-transferase T1 OR GSTT1) and (hepatocellular carcinoma OR liver cancer OR HCC) was entered into these databases mentioned above for search. The searches in Pubmed and Embase were limited in Human. Additional articles were identified through references cited in retrieved articles.

Inclusion and Exclusion Criteria
Inclusion criteria: (1) The outcome had to be HCC; (2) There had to be at least two comparison groups (HCC group vs control group); (3) Investigation should provide the data of GSTT1 genotype distribution.
Exclusion criteria: (1) Review articles and editorials; (2) Case reports; (3) Preliminary result not on GSTT1 gene polymorphism or outcome; (4) Investigating the role GSTT1 gene expression to disease; (5) If multiple publications for same data from the same study group occurred, we only recruited the later one into our final analysis.

Data extraction and synthesis
Two investigators independently extracted the following information from each eligible study: f first author's surname, year of publication, publication language, location of the study performed, control source of the control group and the number of cases and controls for GSTT1 genotypes. Frequencies of null genotype of GSTT1 were calculated for case group and control group, from the corresponding genotype distribution. The results were compared and disagreement was resolved by discussion.

Statistical Analysis
Cochrane Review Manager Version 5 (Cochrane Library, UK) was used to calculate the available data from each study. The pooled statistic was counted using the fixed effects model, but a random effects model was conducted when the P value of heterogeneity test was less than 0.1. Results were expressed with odds ratios (OR) for dichotomous data, and 95% confidence intervals (CI) were also calculated. P ﹤ 0.05 was required for the pooled OR to be statistically significant. I 2 was used to test the heterogeneity among the included studies. Sub-group analysis was also performed according to source of the controls (population vs hospital), sample size of case (﹤ 100 vs ≥ 100). Stata 11.0 was used to test the publication bias. The Begg adjusted rank correlation test (Begg & Mazumdar, 1994) was used for exploring publication bias (P﹤0.1 was considered significant), when the number of the included studies was more than ten.

Study characteristics for GSTT1 null genotype with HCC risk
Eighteen studies (Dong et al., 1997;Yu et al., 1999;Sun et al., 2001;Liu et al., 2002;Liu et al., 2003;McGlynn et al., 2003;Li et al., 2004;Chen et al., 2005;Guo et al., 2005;He et al., 2005;Long et al., 2005;Ma et al., 2005;Zhang et al., 2005;Long et al., 2006;Yang et al., 2009;Kao et al., 2010;Wei et al., 2010) were recruited into our investigation to study the relationship between GSTT1 null genotype and HCC risk. Eleven studies were published in Chinese and others were reported in English (Table 1). The data of our interest were extracted, and the frequencies of null genotype of GSTT1 for case group and control group were calculated (Table  1). Those 18 investigations contained 2693 case series and 4696 controls. The average distribution frequency of GSTT1 null genotype in HCC case was 52.04% and the average frequency in controls was 40.78%. The average distribution frequency of GSTT1 null genotype in cases was markedly increased when compared with that in control group (HCC/Control = 1.28).

Association of GSTT1 null genotype with HCC risk
In this meta-analysis, we found that GSTT1 null genotype was associated with HCC risk in Chinese (OR=1.53, 95%CI: 1.28-1.82; P﹤0.00001; Figure 1 and Table 2).

Sub-group analysis
Sub-group analysis for GSTT1 was also performed    according to the source of the controls (population vs hospital), sample size of case (﹤ 100 vs ≥ 100). We found that the results for GSTT1 from the sub-group analysis were consistent with the previous results ( Figure 2 for the population source and Figure 3 for the sample size of case ≥ 100; Table 2).

Evaluation of publication bias
No significant publication bias was showed for overall Chinese population (P=0.112; Figure 4). In the sub-group analysis, there was also no significant publication bias for the meta-analysis according to the population source of the controls (P=0.244), and for the meta-analysis according to the sample size of case more than 100 (P=0.436).

Discussion
In our study, we found that the null genotype of GSTT1 was associated with the HCC risk in Chinese. Our results indicated that GSTT1 null genotype was associated with the susceptibility of HCC in Chinese, and it might become a useful indicator to predict the risk of HCC in Chinese population. In our study, we found that the average distribution frequency of GSTT1 null genotype in cases have a 1.28-fold increase when compared with that in control group. In the sub-group study according to according to source of the controls (population vs hospital), sample size of case (﹤ 100 vs ≥ 100), we found that the results were consistent with the previous. There was no publication bias for overall Chinese population, the population source of the controls and the sample size of case more than 100. The conclusion in our meta-analysis might be robust to extent.
Three meta-analyses were performed to investigate the association of GSTT1 gene polymorphism and HCC risk. White et al. (2008) performed a meta-analysis and included 13 eligible studies to study the relationship between GSTT1 genetic variants and found that there was no statistical difference in the null genotype distribution of GSTT1 between the HCC group and control group, and they did not performed the analysis for Chinese population or Asians.  up-dated the meta-analysis from White et al. (2008) and included 18 studies for the association of GST gene polymorphism with HCC risk in Asians and found that null genotype of GSTT1 was associated with the risk of HCC. The conclusions were similar with ours. However, this meta-analysis not performed a sub-group study in Chinese population. Yu et al. (2011) conducted a meta-analysis in Chinese and included 16 studies for the relationship between GSTT1 gene polymorphism and HCC risk in Chinese, and they found that the null genotypes of GSTT1 was associated with increased risk of HCC. The number of included studies in our meta-analysis was larger than the previous meta-analyses. The conclusion from our study might be more robust.
GSTT1 null genotype might be an important factor for the morbidity and progression of cancers in Chinese population. Wang et al. (2012) conducted a prospective study in Chinese population to detect the association between GSTT1 gene polymorphisms and survival of gastric cancer, and found that individuals carrying null-GSTT1 had a moderate higher risk of death from gastric cancer. Liu et al. (2012) performed a meta-analysis to explore the association between GSTT1 null genotype and risk for cervical cancer, and reported a modification on the association between GSTT1 null genotype and cervical cancer.  conducted a meta-analysis to evaluate the association between polymorphism of GSTT1 and the risk of lung cancer in Chinese population, and this meta-analysis suggested that GSTT1 deletion polymorphisms might have an effect on the susceptibility of lung cancer in Chinese population. Those studies mentioned above might give us a message that GSTT1 null-genotype might be a risk factor to cause cancer in Chinese population. However, more studies should be performed in the future.
Our results indicated that there was an association between null genotypes of GSTT1 and HCC risk in Chinese population. The outcome might be robust to some extent. The GSTT1 null genotype might become a valuable indicator to predict the risk of HCC in Chinese population. Once it is confirmed, the early prevention would be conducted and the high mortality in Chinese would be improved. However, those findings should be regarded cautiously because many other ingredients, such as small sample size of the included report, limited statistical power, heterogeneity of enrolled cases, variable study designs and different interventions, were closely related to affect the results.
In conclusion, the results in our study support that null genotype of GSTT1 is associated with the risk of HCC in Chinese population. However, more association investigations are required to further clarify the role of the GSTT1 gene polymorphism in predicting the risk of HCC in Chinese population.