A Study On The Association Between MPO G-463A Polymorphism And Susceptibility To Gastrointestinal Cancer

Gastrointestinal cancer, mainly including stomach and colorectal cancer, is the common malignant tumor of digestive tract. Reported by International Agency for Research on Cancer in 2005, gastric cancer and colorectal cancer is the fourth and third most common cancer, and the second and fourth most fatal malignancy in the world respectively. Although the incidence and mortality of gastrointestinal cancer have been decreasing in western countries over the past few decades, high rates are still present in the developing countries. In china, gastric cancer and colorectal cancer is in the third and fifth place respectively among the five leading causes of death from cancer. Gastrointestinal cancer is considered to be a result of gene-environment interactions, which is a complex, multi-step and multi-factorial process. Epidemiological studies have found evidence that the risk of gastrointestinal cancer was influenced by polymorphisms of many genes including inflammatory response, DNA repair, metabolic enzymes and oxidative damage.Oxidative stress has been confirmed to be involved in the pathogenesis of various diseases. Reactive oxygen species play important roles in initiation and progression of carcinogenesis by oxidizing bases, abasic sites and strand breaks. Oxidative stress sometimes can be utilized therapeutically. Potential anticancer drugs acting by this mechanism may prove novel in future. Antioxidants have been confirmed to be effective and safe of cancer treatment. The addition of antioxidants to cancer treatment regimens would improve the survival of patients.As an oxidative stress-related enzyme, myeloperoxidase (MPO) is a lysosomal hemoprotein located in polymorphonuclear neutrophils and monocytes. It can catalyze a reaction that produces hypochlorous acid (HOCI), which may cause host DNA damage and lead to the mutation of oncogenes and tumor suppressor genes. As a phaseⅠmetabolic enzyme, MPO also has been known to convert metabolites of the tobacco smoke procarcinogen benzo[a]pyrene [B(a)P] into highly carcinogenic intermediates benzo(a)pyrene diol-epoxide (BPDE) which is capable of forming DNA adducts and causing sister chromatid exchanges. The G-463A polymorphism (G to A substitution) in the promoter region of the MPO gene has been confirmed to influence the expression of MPO. The A allele is associated with lower MPO enzyme levels by the loss of a SP1 transcription binding site located in an Alu hormone-responsive element (HRE). Several epidemiological studies have examined the role of the MPO G-463A polymorphism in many human malignancies. A number of studies have shown the MPO AA/GA genotype to be associated with decreased susceptibility to lung cancer. And a protective effect of the MPO A allele was observed in breast cancer, bladder cancer, esophageal cancer, hepatoblastoma and laryngeal cancer.However, to our knowledge, there is no study concerning association between the MPO polymorphism and susceptibility to gastrointestinal cancer. In this hospital-based, case-control study, we examined the association between the risk of gastrointestinal cancer and the MPO G-463A polymorphism by univariate and multivariate analysis.PartⅠA Study on the Association between MPO G-463A Polymorphism and Susceptibility to Gastric CancerThe atrophic gastritis subsequent to Helicobacter pylori infection, which is the precursor of gastric cancer, is characterized by extensive infiltration of neutrophils. MPO protein released by neutrophils into the extracellular milieu amplifies the oxidative potential of hydrogen peroxides that induce gastric mucosal injury. In fact, a strong positive correlation between the levels of neutrophil infiltration and Helicobacter pylori-induced atrophic gastritis was found in the MPO GG genotype but not in the MPO GA genotype, and Helicobacter pylori water extract can activate neutrophils and enhance the secretion of MPO. In addition, MPO levels of neutrophils in gastric cancer patients were found to be significantly higher than in healthy controls. In this hospital-based, case-control study, we used PCR-RFLP protocols to examine the prevalence ofMPO G-463A polymorphism in gastric cancer.1 The distribution of genotypic characteristics in the cases and controls and the risk estimates for the variant MPO genotypes.The genotype frequencies in cases were 75.6% for wild type GG, 23.6% for GA and 0.8% for AA, while in controls were 62.6%, 32.4% and 5.0% respectively. The genotype distribution were in Hardy-Weinberg equilibrium in the control population (x~2=0.14, P=0.71), indicating that there was no genetic drift or any selective advantage for particular MPO alleles. A significantly different distribution of the MPO -463G/A genotype was demonstrated among the cases and controls (x~2=7.42, P=0.03). A lower frequency of A alleles was found in cases (12.6%) compared with the controls (21.2%). Only one AA genotype was identified in case group.Among total cases and controls, the crude OR for subjects with the variant genotypes (the sum of GA and AA) was 0.54 (95% CI=0.32-0.92) relative to GG carriers. And we noted a more decreased risk for gastric cancer in subjects with the AA genotype (crude OR=0.14, 95% CI=0.02-1.05), although not statistically significant. When adjusted for age, sex, smoking status, hypertension, diabetes and residence, the OR for subjects with AA genotype and the OR for those with the variant genotypes (the sum of GA and AA) was 0.56 (95% CI=0.32-0.97).2 Stratified analyses for the variant MPO genotype in cases and controlsStratified analyses were conducted by the median age of controls (58 years), sex, smoking status and residence. In statistical analyses stratified by age, the decreased risk associated with the mutant genotypes tended to be more evident in younger subjects (age＜58 years)(adjusted OR=0.42, 95% CI=0.18-0.94). But in older subjects (age≥58 years), the association between the MPO polymorphism and gastric cancer risk was not statistically significant (P=0.39). Compared with the GG genotype, the adjusted OR for the mutant genotypes was 0.68 (95% CI=0.24-1.94) among female subjects and 0.51 (95% CI=0.26-0.98) among male subjects. We did not note a statistically significant inverse association with gastric cancer risk in both non-smokers (adjusted OR=0.59, 95% CI=0.33-1.08) and smokers (adjusted OR=0.50, 95% CI=0.12-2.05). In rural subjects, possession of the variant genotypes was associated with an approximately 60% reduced risk of gastric cancer (adjusted OR=0.41, 95% CI=0.18-0.95), while the association was not statistically significant in urban subjects (P=0.46).Further stratified analyses by tumors differentiation showed no significant association between the MPO genotype and tumor differentiation.PartⅡA Study on the Association between MPO G-463A Polymorphism and Susceptibility to Colorectal CancerChronic intestinal inflammation is a known risk factor for developing colorectal cancer and oxidative stress may play an important role on the inflammation-driven carcinogenesis process. MPO is the major enzymatic catalyst for initiation of lipid peroxidation at sites of inflammation. Actually, MPO activity was found to be increased significantly in colorectal cancerous tissues compared with macroscopically normal tissues. Combining the association of -463 genotype and the expression of MPO, we hypothesized that MPO polymorphism may influence the risk of colorectal cancer. In this study, we used PCR-RFLP protocols to examine the association between the risk of colorectal cancer and the MPO G-463A polymorphism.1 The distribution of genotypic characteristics in the cases and controls and the risk estimates for the variant MPO genotypes.The genotype frequencies in cases were 75.6% for wild type GG, 23.6% for GA and 0.8% for AA, while in controls were 62.6%, 32.4% and 5.0% respectively. The genotype frequencies in controls met the Hardy-Weinberg equilibrium (x~2=0.07, P=0.79). The distribution of MPO -463 genotype was significantly different between colorectal cancer cases and controls (x~2=6.76, P=0.034). The A alleles frequency was 10.6% in colorectal cancer cases and 18.6% in controls.Subjects with the variant genotypes (the sum of GA and AA) showed a 49% reduced risk of colorectal cancer relative to GG carriers (crude OR=0.51, 95% CI=0.29-0.90). Unconditional logistic regression analysis was used calculating the ORs and 95% CIs which were adjusted for age, sex, smoking status, hypertension, diabetes and residence. Adjustment did not change the risk estimates (GA+AA: adjusted OR=0.52, 95% CI=0.29-0.90).2 Stratified analyses for the variant MPO genotype in colorectal cancer cases and controlsStratified analyses were also conducted by the median age of controls (60 years), sex, smoking status and residence. A statistically significant association between reduced colorectal cancer risk and MPO -463 GA/AA genotype was observed in younger (age＜60 years) subjects (adjusted OR=0.44, 95% CI=0.20-0.99)but not in older (age≥60 years) subjects (P=0.22). Among male subjects, the adjusted OR of variant genotypes was 0.41 (95% CI=0.19-0.87). But the association was not statistically significant in female subjects (P=0.34). The decreased risk associated with the variant genotypes tended to be more evident in non-smokers (adjusted OR=0.50, 95% CI=0.27-0.92) than in smokers (P=0.65). In statistical analyses stratified by residence, no significant association was observed in rural subjects or urban subjects.In further analyses, we found a 57% reduced risk of rectal cancer in subjects with GA+AA genotypes compared to those with GG (adjusted OR=0.43, 95% CI=0.20-0.89). The distribution of MPO genotype was not different between stage A-B and stage C-D case group. ConclusionWe concluded that:1. The distribution of MPO -463 genotype was significantly different between gastric cancer cases and controls. Subjects with the variant genotypes (the sum of GA and AA) had a 44% reduced risk of gastric cancer relative to those with GG.2. The protective effect of A allele on gastric cancer was significant in male or younger (age＜58 years) subjects, but not in female or older subjects. In addition, there was also a significantly reduced risk in subjects residing in rural areas but not in urban areas.3. The distribution of MPO -463 genotype was significantly different between colorectal cancer cases and controls. Subjects with the variant genotypes (sum of GA and AA) showed an overall reduced risk of colorectal cancer relative to GG carriers.4. The protective effect of A allele on colorectal cancer risk was significant in younger (age＜60 years), male or non-smoking subjects, but not in older, female or smoking subjects.5. The variant genotype was associated with reduced risk of rectal cancer but not colon cancer. Colorectal tumor characteristic was not found to be associated with the MPO genotype.