Polymorphism of glutathione S-transferase loci GSTM1 and GSTT1 and susceptibility to colorectal cancer in Egypt

Individual and combined effects of GSTM1 and GSTT1 polymorphisms on colorectal cancer risk: an updated meta-analysis

BACKGROUND

The presence or absence of glutathione S-transferase M1 gene (GSTM1) and glutathione S-transferase T1 gene (GSTT1) polymorphisms, and their combined effects have been suggested as a risk factor for colorectal cancer (CRC). However, the results are inconsistent.

OBJECTIVES

An updated meta-analysis was performed to solve the controversy.

METHODS

Meta-analyses of Observational Studies in Epidemiology (MOOSE) guidelines were used.

RESULTS

Overall, the GSTM1 null genotype was associated with an increased CRC risk in Caucasians (odds ratio (OR) = 1.14, 95% confidence interval (CI): 1.05-1.23), Asians (OR = 1.19, 95% CI: 1.08-1.32), high-quality studies (OR = 1.12, 95% CI: 1.06-1.18). Moreover, the GSTM1 null genotype was also associated with an increased colon cancer risk (OR = 1.32, 95% CI: 1.16-1.51). The GSTT1 null genotype was also associated with an increased CRC risk in Asians (OR = 1.08, 95% CI: 1.02-1.15) and Caucasians (OR = 1.24, 95% CI: 1.09-1.41). Moreover, The GSTT1 null genotype was associated with an increased rectal cancer risk (OR = 1.13, 95% CI: 1.01-1.27, I2 = 8.3%) in subgroup analysis by tumor location. Last, the GSTM1 null/GSTT1 null genotype was associated with an increased CRC risk in Asians.

CONCLUSION

This meta-analysis indicates that the GSTM1 and GSTT1 null genotypes are associated with increased CRC risk in Asians and Caucasians, and the GSTM1 null/GSTT1 null genotype was associated with increased CRC risk in Asians.

Glutathione S-transferase T1 gene polymorphism and colorectal cancer risk: an updated analysis

BACKGROUND AND OBJECTIVE

The association between glutathione S-transferase T1 (GSTT1) gene polymorphisms and colorectal cancer (CRC) susceptibility is still controversial. In order to clarify the effect of GSTT1 genotype on the CRC risk, we carried out an updated meta-analysis of published case-control studies to provide more precise evidence.

METHODS

Two investigators independently searched the databases of Pubmed, EMBASE and China National Knowledge Infrastructure (CNKI) up to October 15, 2012. Crude odds ratios (OR) and 95% confidence intervals (CI) were calculated to investigate the strength of the association in a fixed- or random-effects model depending on statistical heterogeneity.

RESULTS

Forty-six case-control studies with 15,373 colorectal cancer cases and 21,238 controls were included. Overall, the pooled results indicated that GSTT1 null genotype was significantly associated with increased CRC risk (OR=1.21, 95% CI=1.10-1.33). When stratifying for ethnicity and control sources, we also observed positive association between GSTT1 null genotype and increased risk of CRC. When stratifying by the location, we found there was a statistically significant association in the rectal cancer (OR=1.28, 95% CI=1.01-1.64), but not in colon cancer (OR=1.27, 95% CI=0.94-1.73). Subgroup analyses for Dukes stage, histological differentiation of CRC and smoking habit did not reveal any significant differences in genotype distribution. In addition, we observed a strong correlation between increased CRC risk and the combined GSTM1 and GSTT1 null genotype.

CONCLUSIONS

This meta-analysis suggests that the GSTT1 null genotype may contribute to increased risk of colorectal cancer. More well-designed studies based on larger population are needed to confirm our results.

Frequencies of glutathione s-transferase (GSTM1, GSTM3 AND GSTT1) polymorphisms in a Malaysian population

INTRODUCTION

Glutathione S-transferase (GST) is a xenobiotic metabolising enzyme (XME), which may modify susceptibility in certain ethnic groups, showing ethnic dependent polymorphism. The aim of this study was to determine GSTM1, GSTM3 and GSTT1 gene polymorphisms in a Malaysian population in Kuala Lumpur.

MATERIAL AND METHODS

Blood or buccal swab samples were collected from 137 Form II students from three schools in Wilayah Persekutuan Kuala Lumpur. Genotyping was done by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP).

RESULTS

Glutathione-S-transferase GSTM3 gene frequencies were 89% for AA, 10% for AB and 1% for BB. The gene frequencies for deleted GSTM1 and GSTT1 were 66% and 18% respectively.

CONCLUSIONS

This study suggested that the Malay population is at risk for environmental diseases and provides the basis for gene-environment association studies to be carried out.

Glutathione S-transferase M1 polymorphism and sporadic colorectal cancer risk: An updating meta-analysis and HuGE review of 36 case-control studies

PURPOSE

Sporadic colorectal cancer (CRC) is considered to be a multifactorial disease, in which multiple exposures to endogenous factors interact with individual genetic background in a complex manner, resulting in modulation of the risk. The glutathione S-transferase M1 gene (GSTM1) is a particularly attractive candidate for CRC susceptibility because it codes an enzyme involved in the metabolism of environmental carcinogens. However, the epidemiological findings have been inconsistent.

METHODS

To evaluate this association, we performed an extensive meta-analysis of 36 case-control studies (including 10,009 cases and 15,070 controls).

RESULTS

Overall, the combined data showed that GSTM1 deficiency is associated with a marginal effect on CRC risk (odds ratio [OR] = 1.13; 95% confidence interval [CI]: 1.03-1.23; P for heterogeneity <0.001). When stratified by race and tumor site, significant results were only observed in Caucasians (OR = 1.14, 95% CI: 1.01-1.27; P for heterogeneity <0.001), whereas no increased risk was detected in other subgroups.

CONCLUSIONS

The findings of our study support the suggestion that GSTM1 polymorphism is associated with an increased risk of CRC, especially in the Caucasian population. Further investigation into the association between GSTM1 polymorphism and the risk of CRC is warranted and should include larger sample sizes and other genetic polymorphisms in metabolism of environmental carcinogens.

Lifestyle, occupational, and reproductive factors and risk of colorectal cancer

PURPOSE

Lifestyle factors and environmental exposures might help explain the risk of colorectal carcinoma in countries where the incidence is low, but unique patterns of young onset and a high proportion of rectal cancer exist.

METHODS

We obtained detailed lifestyle information from 421 patients with colorectal cancer and 439 hospital-controls in Egypt. Logistic regression models were computed to evaluate the risk factors of colorectal carcinoma.

RESULTS

A history of pesticide exposure and more frequently eating food directly from farms were significantly associated with a higher risk of colorectal carcinoma (odds ratio = 2.6; 95% CI = 1.1-5.9, and odds ratio = 4.6; 95% CI = 1.5-14.6, respectively). Parous women who reported 7 or more live births or breastfed for 19 months or longer per live birth had a significantly lower risk for colorectal carcinoma (odds ratio = 0.3; 95% CI = 0.2-0.7, and odds ratio = 0.2; 95% CI = 0.1-0.4, respectively). Compared with patients aged 40 years or older, industrial exposures were more common in younger patients (P = .05).

CONCLUSIONS

Agricultural and industrial exposures were associated with increased risk of colorectal carcinoma, whereas prolonged lactation and increased parity were inversely associated with colorectal carcinoma in women. Further research to elucidate the biological role of intense environmental and industrial exposures and reproductive factors including lactation may further clarify the etiology of colorectal cancer.

GSTM1, GSTT1, GSTP1, GSTA1 and colorectal cancer risk: a comprehensive meta-analysis

Glutathione S-transferases (GSTs) catalyse reactions between glutathione and lipophilic compounds with electrophilic centres, leading to neutralisation of toxic compounds, xenobiotics and products of oxidative stress. Controversy exists about whether GST polymorphisms (GSTM1 null/present genotype, GSTT1 null/present genotype, GSTP1 Ile105Val and GSTA1 *A/*B) represent risk factors for colorectal cancer. This meta-analysis aims to examine the associations between the above-mentioned polymorphisms and colorectal cancer risk. Forty-four studies were eligible for GSTM1 (11,998 colorectal cancer cases, 17,552 controls), 34 studies for GSTT1 (8596 cases, 13,589 controls), 19 studies for GSTP1 (5421 cases, 7671 controls) and four studies for GSTA1 polymorphism (1648 cases, 2039 controls). Pooled odds ratios (ORs) were appropriately derived from fixed-effects or random-effects models. Separate analyses were conducted on Caucasian and Chinese populations. Where appropriate, sensitivity analysis concerning the deviation of genotype frequencies in controls from the Hardy-Weinberg equilibrium was performed. GSTM1 null allele carriers exhibited increased colorectal cancer risk in Caucasian populations (pooled OR=1.150, 95% confidence interval (CI): 1.060-1.248, random effects); no significant association was detected for Chinese subjects (pooled OR=1.025, 95% CI: 0.903-1.163, fixed effects). Similarly, GSTT1 null allele carriers exhibited increased colorectal cancer risk in Caucasian populations (pooled OR=1.312, 95% CI: 1.119-1.538, random effects); the association in Chinese subjects was not significant (pooled OR=1.068, 95% CI: 0.788-1.449, random effects). Concerning GSTP1 Ile105Val no significant associations were demonstrated in either race. GSTA1 *A/*B polymorphism was not associated with colorectal cancer risk. GSTM1 and GSTT1 null genotypes confer additional risk for colorectal cancer in Caucasian populations.

An updating meta-analysis of the glutathione S-transferase T1 polymorphisms and colorectal cancer risk: a HuGE review

INTRODUCTION

GSTT1 status has been extensively studied as a colorectal cancer risk factor. However, the results are inconsistent. To examine this controversy, we performed a meta-analysis to evaluate the relationship between GSTT1 polymorphism and colorectal cancer.

MATERIALS AND METHODS

We performed a literature search using PUBMED, EMBASE, Cochrane Library, and HuGNet database to February 2009, with no restrictions. All articles were independent and contained the minimum information necessary to estimate the colorectal cancer risk associated with GSTT1 null. Summary odds ratio (ORs) and 95% confidence intervals (CIs) were calculated using random-effect or fixed-effect models based on the heterogeneity of included studies.

RESULTS

A total of 23 case-control studies, including a total of 11,057 subjects (5,058 cases and 5,999 controls), that related to GSTT1 polymorphism and risk of colorectal cancer were identified and included for analysis. The random-effect meta-analyses of all the 23 studies suggested that there was a small increased risk of colorectal cancer for individuals with GSTT1 null (OR was 1.23; 95% CI 1.02-1.49; I (2) = 76.9%, P for heterogeneity <0.001). The fixed-effect meta-analyses reached a similar results in Caucasians populations of ten studies (OR = 1.39; 95% CI 1.21-1.59; I (2) = 29.8%, P for heterogeneity = 0.171) and Asians populations of five studies (OR = 1.23; 95% CI 1.04-1.45; I (2) = 0.0%, P for heterogeneity = 0.428), with as inversely association in the other ethnic populations from four studies (OR = 0.69; 95% CI 0.54-0.877; I (2) = 0.0%, P for heterogeneity = 0.58).

CONCLUSION

There was a small increased risk of colorectal cancer for individuals with GSTT1 null, especially for Caucasians populations and Asian populations.

Glutathione S-transferase P1 Ile105Val polymorphism and colorectal cancer risk: a meta-analysis and HuGE review

Colorectal cancer is the third most common form of cancer and the fourth most frequent cause of cancer deaths worldwide. Its development is influenced by both environmental and genetic factors. The glutathione S-transferase P1 gene (GSTP1) is a particularly attractive candidate for colorectal cancer susceptibility because it codes the enzyme involved in the metabolism of environmental carcinogens such as polycyclic aromatic hydrocarbons (PAHs). However, epidemiologic findings have been inconsistent. To investigate a putative association of GSTP1 Ile105Val polymorphism with the risk of colorectal cancer, we performed a meta-analysis and HuGE review of 16 published case-control studies (involving a total of 4386 colorectal cancer patients and 7127 controls). We used odds ratios (ORs) with 95% confidence intervals (CIs) to assess the strength of the association. Overall, the comparison of Val versus Ile allele showed no differential susceptibility to colorectal cancer (OR=0.98, 95% CI: 0.92-1.04). A protective effect was found in recessive, with an OR of 0.86 (95% CI: 0.76-0.98). Whereas no significant association was observed in either dominant or codominant model. In stratified subgroup analysis, no effect of Val allele was seen in subjects of Caucasian and Asian descent, and in healthy and hospital controls. In conclusion, the meta-analysis suggests that the GSTP1 Ile105Val polymorphism is unlikely to increase considerably the risk of sporadic colorectal cancer, and it should be confirmed in further studies.

Microsomal glutathione S-transferase gene polymorphisms and colorectal cancer risk in a Han Chinese population

BACKGROUND AND AIMS

Glutathione S-transferases (GSTs) are phase II detoxification enzymes. Human GSTs have been classified into cytosolic, mitochondrial, and microsomal families. Several studies reported the association of colorectal cancer (CRC) risk with the genetic polymorphisms of cytosolic GSTs. The microsomal GSTs are structurally distinct but functionally similar to cytosolic GSTs; their association with CRC has not been reported. In this report, we summarized the result of a case-control study aimed at investigating the association of MGST1 gene locus polymorphisms with CRC risk among Han Chinese.

PATIENT/METHODS

Three hundred and seventy-two healthy controls and 238 sporadic CRC patients participated in this study. DNA resequencing was conducted for the 3.4 kb genomic DNA region containing the promoter, exons, exon-intron junctions, and the 5' and 3' untranslated regions.

RESULTS

We detected 13 single nucleotide polymorphisms (SNPs) including four novel SNPs not reported in database/literature. The gene shows a much higher nucleotide diversity than most human genes. The linkage and recombination analysis revealed 24 common haplotypes (13% > or = freq > or = 1%) and identified extensive intragenic recombination throughout the MGST1 locus (R = 81.8). Significant CRC association (P < or = 0.005) was not detected for each individual SNP. However, SNPs 102G>A and 16416G>A reached a marginal level of statistical significance with P values of 0.016 and 0.078, respectively. A combined genotype analysis detected a statistically significant CRC association for individuals carrying 102G>A/16416G>A (GG/GG) genotype (adjusted OR, 1.682; 95% confidence interval (CI), 1.177-2.404; P = 0.004). Consistent with the results of genotype analysis, the GG haplotype (102G>A/16416G>A) with two risk alleles was associated with a significantly higher CRC risk comparing with the haplotypes with one or no risk allele (adjusted OR 1.744; 95% CI 1.309-2.322; P = 0.0001).

CONCLUSION

The results suggest that MGST1 polymorphisms may contribute to CRC risk among Han Chinese.

GSTM1 and GSTT1 Polymorphisms, Cigarette Smoking, and Risk of Colon Cancer: A Population-based Case-control Study in North Carolina (United States)

Cigarette smoke is a risk factor for colon cancer, but the importance of dose and interaction with genetic susceptibility remain poorly understood. We used data from a population-based case control study, to examine the association between cigarette smoking and colon cancer in African Americans and whites, and colon cancer and polymorphisms in GSTM1 and GSTT1. A total of 554 cases of primary colon cancer and 874 controls were included in this analysis. We found no association between cigarette smoking (ever versus never) and colon cancer in African Americans (odds ratio (OR)=0.93, 95% confidence interval (CI)=0.65-1.33). In contrast, there was an increased risk of cigarette smoking in whites (OR=1.43, CI=1.05-1.94). There was a small increased risk of colon cancer for individuals with GSTM1 null (African Americans, OR=1.43, CI, 0.98-2.09; whites, OR=1.19, CI, 0.90-1.58) and a decreased risk of colon cancer for individuals with GSTT1 null (African Americans, OR=0.59, CI: 0.40-0.86; whites, OR=0.72, CI: 0.53-1.00). There were weak interactions between GSTT1 null and cigarette smoking in whites, and GSTM1 null genotype and cigarette smoking in African Americans. GSTT1 and GSTM1 polymorphisms may be weakly related to colon cancer risk and there may be racial differences in gene-smoking interactions.

Glutathione S-Transferase M1, T1, P1 Genotypes and Risk for Development of Colorectal Cancer

The glutathione S-transferase (GST) supergene family is an important part of cellular enzyme defense against endogenous and exogenous chemicals, many of which have carcinogenic potential. The present investigation was conducted to detect a possible association between polymorphisms at the GSTM1, GSTT1, and GSTP1 genes and the interaction with cigarette smoking and colorectal cancer incidence. We examined 181 patients with colorectal cancer and 204 controls. DNA was extracted from whole blood, and the GSTM1, GSTT1, and GSTP1 polymorphisms were determined using a real-time polymerase chain reaction and fluorescence resonance energy transfer with a Light-Cycler instrument. Associations between specific genotypes and the development of colorectal cancer were examined by use of logistic regression analysis to calculate odds ratios (OR) and 95% confidence intervals (CI). The GSTM1 polymorphism was associated with an increased risk of developing colorectal cancer (OR = 1.62, 95% CI: 1.06-2.46). Also the risk of colorectal cancer associated with the GSTT1 null genotype was 1.64 (95% CI: 1.10-2.59). Statistically no differences were found between patients with colorectal cancer and control groups for the GSTP1 Ile/Ile, Ile/Val and Val/Val genotypes. In addition, the frequencies of the GSTM1 and GSTT1 deletion genotypes differed significantly between the cases and controls for current smokers; the GSTT1 null genotype especially is associated with a greater risk of colorectal cancer (OR = 2.44, 95% CI: 1.24-4.81). The GSTM1 and GSTT1 deletions were associated with an increased risk of developing a transverse or rectal tumor (OR = 1.86, 95% CI: 1.15-3.00; OR = 1.70, 95% CI: 1.02-2.84; respectively). The glutathione S-transferase polymorphisms were not associated with risk in patients stratified by age. The risk of colorectal cancer increased as putative high-risk genotypes increased for the combined genotypes of GSTM1 null, GSTT1 null, and either GSTP1 valine heterozygosity or GSTP1 valine homozygosity (OR = 2.69, 95% CI: 1.02-7.11). In conclusion, the results obtained in this study clearly suggest that those susceptibility factors related to different GST polymorphic enzymes are predisposing for colorectal cancer.

Relationship between metabolic enzyme polymorphism and colorectal cancer

AIM: To clarify the influence of genetic polymorphisms on colorectal cancer.

METHODS: The results of 42 related studies from 1990 to 2001 were analyzed by meta-analysis. Mantel-Haenzel fixed-effect model or Dersimonian-Laird random-effect model and ReviewManager 4.1 statistical program were applied in processing the data.

RESULTS: Meta analysis of these studies showed that GSTT1 deletion (pooled OR = 1.42), N-acetyltransferase 2 (NAT2)-rapid acetylator phenotype and genotye (pooled OR = 1.08) and NAT2-rapid acetylator phenotype (pooled OR = 1.15) had a significantly increased risk for colorectal cancer (P<0.05), other genotypes like GSTM1 deletion, GSTP1 1le105Val, NAT1*10, NAT2-rapid acetylator genotype CYP1A1 L1e462Val, CYP1A1 MspI*C, MTHFR C677T and MTR A2759G had no significant relationship with colorectal cancer (P>0.05).

CONCLUSION: Risks for colorectal cancer are significantly associated with the genetic polymorphisms of GSTT1 deletion, NAT2-rapid acetylator phenotype and genotye and NAT2-rapid acetylator phenotype.

Glutathione S-transferase M1 and T1 null genotypes and the risk of gastric and colorectal cancers

Several polymorphic glutathione S-transferase (GST) enzymes are involved in the detoxification of active metabolites of many potential carcinogens and may therefore be important in modulating susceptibility to cancers. GSTM1 and GSTT1 are polymorphic, and the null alleles result in a lack of corresponding enzyme activities. Previous studies demonstrated that the GSTM1 and GSTT1 null genotypes correlated with an increased risk of developing some cancers. In this study, we determined GSTM1 and GSTT1 polymorphisms in a population of 131 healthy controls from the south of Iran, 46 patients with colorectal cancers, and 42 patients with gastric cancer. The gastric cancer risk statistically increased due to the GSTM1 null genotype (odds ratio (OR)=2.3, 95% confidence interval (CI): 1.15--4.95). On the other hand, the GSTT1 null genotype in gastric cancer and null genotypes of GSTM1 and GSTT1 in colorectal cancer were not statistically significant. Moreover, individuals showing the GSTM1 and GSTT1 null genotypes might exhibit a greater predisposition to gastric (OR=3.31, 95% CI: 1.14--9.57) and colorectal (OR=2.73, 95% CI: 0.94--7.95, P=0.07) cancers.

Polymorphisms and colorectal tumor risk

BACKGROUND & AIMS

Increasingly, studies of the relationship between common genetic variants and colorectal tumor risk are being proposed. To assess the evidence that any of these confers a risk, a systematic review and meta-analysis of published studies was undertaken.

METHODS

Fifty studies of the effect of common alleles of 13 genes on risk were identified. To clarify the impact of individual polymorphisms on risk, pooled analyses were performed.

RESULTS

Of the 50 studies identified, significant associations were seen in 16, but only 3 were reported in more than one study. Pooling studies, significant associations were only seen for 3 of the polymorphisms: adenomatosis polyposis coli (APC)-I1307K (odds ratio [OR] = 1.58, 95% confidence interval [CI]: 1.21-2.07); Harvey ras-1 variable number tandem repeat polymorphism (HRAS1-VNTR; OR = 2.50, 95% CI: 1.54-4.05); and methylenetetrahydrofolate reductase (MTHFR)(Val/Val) (OR = 0.76, 95% CI: 0.62-0.92). For tumor protein 53 (TP53), N-acetyl transferase 1 (NAT1), NAT2, glutathione-S transferase Mu (GSTM1), glutathione-S transferase Theta (GSTT1), and glutathione-S transferase Pi (GSTP1) polymorphisms, the best estimates are sufficient to exclude a 1.7-fold increase in risk of colorectal cancer.

CONCLUSIONS

APC-I1307K, HRAS1-VNTR, and MTHFR variants represent the strongest candidates for low penetrance susceptibility alleles identified to date. Although their genotypic risks are modest, their high frequency in the population implies that they may well have considerable impact on colorectal cancer incidence. Determining precise risk estimates associated with other variants and gene-gene and gene-environment interactions will be contingent on further studies with sample sizes larger than typically used to date.

Inheritance of the 194Trp and the 399Gln variant alleles of the DNA repair gene XRCC1 are associated with increased risk of early-onset colorectal carcinoma in Egypt

Patients under age 40 constitute 35.6% of all colorectal cancer cases in Egypt, an unusual disease pattern to which both environmental exposures and inefficient DNA repair may contribute. While a number of polymorphisms in DNA repair genes have been recently identified, their role as cancer risk modifiers is yet to be determined. In a pilot case-control study, we tested the hypothesis that polymorphisms in the gene for the DNA repair enzyme XRCC1 are associated with increased risk of colorectal cancer among Egyptians. Using a multiplex polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) methodology, allelic variants of the XRCC1 gene at codons 194 (Arg-->Trp) (194Trp) and 399 (Arg-->Gln) (399Gln), were analyzed in DNA from lymphocytes of 48 newly-diagnosed colorectal cancer cases and 48 age- and sex-matched controls. Overall, the inheritance of 194Trp allele (Arg/Trp genotype) and 399Gln allele (combined Arg/Gln and Gln/Gln genotypes) was associated with increased colorectal cancer risk (odds ratio (OR)=2.56, 95% confidence limits (CL) 0.73-9.40, and P=0. 08 for 194Trp allele and OR=3.98, 95% CL 1.50-10.6, and P<0.001 for 399Gln allele). Interestingly, the frequencies of 194Trp and 399Gln genotypes were higher in colorectal cancer cases under age 40 than in corresponding controls, and an association between both polymorphisms and early age of disease onset was observed (OR=3.33, 95% CL 0.48-35.90, and P=0.16 for 194Trp and OR=11.90, 95% CL 2.30-51.50, and P=0.0003 for 399Gln). Analysis of the data after adjustment for place of residence indicated that the frequencies of the genotypes with the 194Trp and the 399Gln alleles were higher among urban residents (OR=3.33, 95% CL 0.48-35.90, and P=0.16 for 194Trp and OR=9.97, 95% CL 1.98-43.76, and P<0.001 for 399Gln) than among rural residents (OR=2.00, 95% CL 0.36-26.00, and P=0.30 for 194Trp and OR=1.90, 95% CL 0.50-7.53, and P=0.20 for 399Gln). These findings support our hypothesis and suggest that polymorphisms in the XRCC1 gene, in conjunction with place of residence, may modify disease risk. This first demonstration that polymorphisms in DNA repair genes may contribute to colorectal cancer susceptibility and may increase the risk of early onset of the disease opens the door for future studies in that direction.

Mammalian class theta GST and differential susceptibility to carcinogens: a review

Glutathione S-transferases (GSTs) are an important part of the cellular detoxification system and, perhaps, evolved to protect cells against reactive oxygen metabolites. Theta is considered the most ancient among the GSTs and theta-like GSTs are found in mammals, fish, insects, plants, unicellular algae, and bacteria. It is thought that an ancestral theta-gene underwent an early duplication before the divergence of fungi and animals and further duplications generated the variety of the other classes of GSTs (alpha, mu, phi, etc.). The comparison of the aminoacidic homologies among mammals suggests that a duplication of an ancient GST theta occurred before the speciation of mammals and resulted in the subunits GSTT1 and GSTT2. The ancestral GST theta has a dehalogenase activity towards several halogenated compounds, such as the dichloromethane. In fact, some aerobic and anaerobic methylotrophic bacteria can use these molecules as the sole carbon and energy source. The mammalian GST theta cannot sustain the growth of bacteria but still retains the dehalogenating activity. Therefore, although mammalian GST theta behaves as a scavenger towards electrophiles, such as epoxides, it acts also as metabolic activator for halogenated compounds, producing a variety of intermediates potentially dangerous for DNA and cells. For example, mice exposed to dichloromethane show a dose-dependent incidence of cancer via the GSTT1-1 pathway. Because GSTT1-1 is polymorphic in humans, with about 20% of Caucasians and 80% of Asians lacking the enzyme, the relationship between the phenotype and the incidence of cancer has been investigated extensively in order to detect GSTT1-1-associated differential susceptibility towards endogenous or exogenous carcinogens. The lack of the enzyme is related to a slightly increased risk of cancer of the bladder, gastro-intestinal tract, and for tobacco-related tumors (lung or oral cavity). More pronounced risks were found in males with the GSTT1-null genotype for brain diseases and skin basal cell carcinomas not related to sunlight exposures. Moreover, there was an increased risk of kidney and liver tumors in humans with the GSTT1-1 positive genotype following exposures to halogenated solvents. Interestingly, the liver and kidney are two organs that express the highest level of GST theta in the human body. Thus, the GSTT1-1 genotype is suspected to confer decreased or increased risk of cancer in relation to the source of exposure; in vitro studies, mostly conducted on metabolites of butadiene, confirm the protective action of GSTT1-1, whereas, thus far, experimental studies prove that the increasing risk is limited.