O2-04 Cigarette smoking, genetic polymorphisms and colorectal cancer - PDF document

O2-04 Cigarette smoking, genetic polymorphisms and colorectal cancer risk Hoirun Nisa, 銀光、 古野純典（ 九州大学・ 大学院医学研究院・ 予防医学分野 ) Background: It is uncertain whether smoking is related to colorectal cancer risk whereas smoking has consistently associated with increased risk of colorectal adenomas. Different types of tobacco carcinogens are activated and detoxified through different metabolic pathways. Cytochrome P-450 (CYP)1A1, glutathione-S-transferase (GST), quinone oxidoreductase (NQO1) and epoxide hydrolase (EPHX1) are important enzymes in the metabolism of tobacco carcinogens, and functional genetic polymorphisms are known of these enzymes. We investigated the relation of cigarette smoking and related genetic polymorphisms to colorectal cancer risk, with special reference to the interaction between smoking and genetic polymorphism. Methods: The subjects were 685 cases of colorectal cancer and 778 community controls who gave informed consent to genetic analysis in the Fukuoka Colorectal Cancer Study, a population-based case control study. Lifestyle factors were ascertained by interview, and DNA was extracted from buffy coat. Genotyping was done for CYP1A1 6235T>A (MspI) and 4889A>G (Ile462Val), GSTM1, GSTT1, NQO1 609C>T (Pro187Ser) and EPHX1 Ex4+52A>G (His139Arg). Logistic regression analysis was used to control for sex, age, area, and other factors. Likelihood ratio test was used to assess the interaction. Results: Cigarette smoking was not associated with an increased risk of colorectal cancer. None of the individual polymorphisms under study was related to colorectal cancer risk and showed a measurable effect modification on the relation between smoking and colorectal cancer. Of the gene-gene interactions studied, GSTT1 and CYP1A1 Ile462Val polymorphisms showed a statistically significant interaction (P = 0.02). As compared with those having Ile/Ile genotype and GSTT1 non-null genotype, individuals with CYP1A1 462Val variant allele showed an odds ratio (OR) of 1.23 (95% CI: 0.90–1.69) when they had GSTT1 null genotype, and an OR of 0.76 (95% CI 0.56–1.01) when they had GSTT1 non-null genotype. The composite genotypes of these two polymorphisms, however, showed no measurable interaction with cigarette smoking in relation to colorectal cancer risk. Conclusion: The present study showed no clear association of cigarette smoking and genetic polymorphisms related to the metabolism of tobacco carcinogens with colorectal cancer risk. The observed interaction between GSTT1 and CYP1A1 Ile462Val polymorphisms may be a chance finding and needs further confirmation.

Introduction Both environmental and genetic factors are thought to play an important role in colorectal carcinogenesis [1]. The role of high-penetrance single genes in the etiology of colorectal cancer is estimated to be less than 10% [2,3] and the combination of low penetrance genes and environmental factors seem to contribute substantially to the occurrence of colorectal cancer. It has been a matter of interest whether smoking is related to increase risk of CRC [4-11]. A large number of studies have consistently found that cigarette smoking is associated with increased risk of colorectal adenoma, as reviewed extensively elsewhere [12, 13]. The results on smoking and CRC are, however, inconsistent [4-11]. Several studies suggested a modest increase in the risk of colorectal cancer associated with smoking. Even in the former studies, the findings were disparate with respect to sex or site-specific colorectal cancer. These inconsistent findings may be due to differences in study design and genetic susceptibility in study populations. Tobacco smoke contains various types of carcinogens such as polycyclic aromatic hydrocarbons (PAHs), heterocyclic amines, aromatic amines, and N-nitrosamines which require metabolic activation and detoxification by different enzymatic pathways. PAHs are metabolized through complex phase I and phase II metabolic enzymes such as cytochrome P-450 (CYP), glutathione-S- transferases (GSTs), quinone oxidoreductase (NQO1) and epoxide hydrolase (EPHX1). Cytochrome P-450 CYP1A1 is a phase I, predominantly extrahepatic, microsomal enzyme involved in the bioactivation of PAHs including benzo(a)pyrene. Two genetic polymorphisms in CYP1A1, the T3801C substitution creating a Msp I restriction site in the 3’-flanking region and the A2445G substitution resulting in an amino acid change in the heme-binding region of exon 7 at codon 462 (Ile462Val), appear to be associated with higher

levels of biomarkers for PAH exposure [14, 15]. An increased risk of in situ colorectal cancer associated with CYP1A1 T3801C was reported in a case-control study in Hawaii, but no association between CYP1A1 T3801C and colorectal cancer was observed in subsequent studies. CYP1A1 Ile462Val was found to be unrelated to colorectal cancer risk those studies. GSTs are a superfamily of detoxification enzymes that facilitate the inactivation of chemical carcinogens and environmental toxic compounds. GSTs consist of several classes of genes and the relation of genetic polymorphisms for GSTM1 and GSTT1 to cancer risk has been rather extensively studied. The “null” alleles of these polymorphisms result in a complete loss of enzyme function and thus may be at increased risk of tobacco related cancers. Results on GSTM1 and GSTT1 in relation to colorectal cancer are inconsistent as reviewed elsewhere. The NQO1 primarily involved in detoxification through their two electron reduction to hydroquinones, thereby inhibiting the DNA. The functional C609T polymorphism (rs1800566) causing amino acid change (Pro187Ser) resulting in loss of NQO1 activity and therefore may increase susceptibility to the risk of cancer especially of tobacco-related cancers [25]. The effect of the NQO1 polymorphism seems to be more evident among smoking-related cancers as common environmental exposure to quinone arises from the oxidative metabolite of benzo(a) pyrene [benzo(a)pyrene 3,6-quinones] present in the tobacco smoke [24]. Hamajima et al. have demonstrated that the NQO1 Pro187Ser polymorphism had an association with the risk of lung cancer, and may modify the effect of smoking for cancers of the lung and esophagus [26]. Two studies have also reported a positive relation of the NQO1 Pro187Ser polymorphism to the risk of bladder cancer among smokers [27-28]. Results are inconsistent regarding the NQO1 Pro187Ser polymorphism and colorectal neoplasia [21,26,29-32]. Mitrou et al. showed an evidence for the association between NQO1 Pro187Ser polymorphism and colorectal adenoma [29], and Hou et al. reported only a marginal association [21]. The role of the NQO1 Pro187Ser polymorphism in relation to an

increase risk of CRC has been positively observed in two studies [30,31], but there is no relation found in one study [32]. EPHX1 also plays an important role in determining the susceptibility of an individual to cancer. In exon 4 of the EPHX1 gene, the functional A415G polymorphism resulting a histidine to arginine change at codon 139 (His139Arg) and this change enhances enzyme activity by 25% through an increase in EPHX1 protein stability [33]. None of the previous studies examining the relationship between EPHX1 His139Arg polymorphism and risk of either colorectal adenoma [29,34-36] or carcinoma [37-38] have showed significant association. However, some studies have shown that an association between EPHX1 His139Arg polymorphism and risks for colorectal adenoma [29,34,36] and for CRC [37] tended to be highest among smokers. Recently many studies have focused on the relationship between genetic polymorphisms and risks of CRC. However, none of the studies have considered the cumulative effect of polymorphisms in five metabolic genes, CYP1A1, GSTM1, GSTT1, NQO1 and EPHX1, on the pathway to tobacco smoke metabolism in CRC. In light of this, we conducted a case control study to determine whether functionally characterized variants of the polymorphisms in CYP1A1, GSTM1, GSTT1, NQO1 and EPHX1 are associated with the risk of CRC. In addition, we assessed the interaction between smoking, genotype, the possibly combined genotypes and CRC risk. Materials and Methods The Fukuoka Colorectal Cancer Study is a case-control study of incident cases and community controls, with Fukuoka city and three adjacent areas as the study area. Details of methodological issues have been described elsewhere [39]. The study protocol was approved by the ethical committees of the Faculty of Medical Sciences, Kyushu University, but two of the participating hospitals which had no ethical committees at the time of survey obtained