Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis
GPX1 Pro198Leu polymorphism, erythrocyte GPX activity, interaction with alcohol consumption and smoking, and risk of colorectal cancer
Introduction
Reactive oxygen species (ROS), such as hydrogen peroxide, are constantly generated in vivo from cell metabolism and multiple exposure-driven processes. Oxidative damage to DNA, including oxidized bases, formation of DNA adducts and DNA strand breaks [1], may be induced in the cell, when ROS are formed in amounts that exceed the capacity of the antioxidant defence system. Higher levels of ROS [2] and an increased lipid peroxidation [3] have been observed in colorectal cancer tissue compared to the levels in adjacent healthy tissue. This indicates that oxidative stress plays an important role in the pathogenesis of colorectal cancer. Oxidative DNA damage may be counteracted by the enzymes glutathione peroxidases (GPX) and 8-oxoguanine glycosylase 1 (hOGG1) in humans.
GPX are selenium-dependent antioxidant enzymes that reduce H2O2 and lipid peroxides/hydroperoxides by oxidizing glutathione. Four isotypes have been characterized: GPX1-GPX4, of which GPX1 and GPX2 are expressed in colon tissue [4]. GPX1 is considered to be fairly ubiquitous [4]. The isozymes have similar properties and substrate specificity and both GPX1 and GPX2 efficiently metabolise hydrogen peroxide. A higher mutation load are observed among GPX1/2-double knockout mice compared with non-double knockout mice [5], and knockdown of GPX1 is associated with a higher frequency of DNA damage in human prostate cells [6]. In addition, in vitro studies have shown that GPX1-overexpression are associated with lower levels of UV-induced micronuclei in human breast cancer cells [7]. Previous findings suggest that the GPX1 Pro198Leu polymorphism is associated with risk of lung cancer and breast cancer, but the literature is not consistent [8], [9], [10], [11]. No previous study has linked the GPX1 Pro198Leu polymorphism with risk of colorectal cancer, but a tendency for protection was observed among homozygous carriers of the variant allele with an OR of 0.62 (CI: 0.24–1.60) [12].
The level of GPX activity are observed to be higher in breast [13] and colorectal cancer tissue [14] compared to adjacent healthy tissue or tissue from healthy individuals, respectively, whereas the enzyme activity was lower in erythrocytes from patients with gastrointestinal [15] or cervical cancer [16] compared to healthy individuals. In a Danish prospective breast cancer study based on pre-diagnostic blood samples the GPX activity was lower among carriers of the GPX1 198Leu allele [17], and in vitro studies have shown that the GPX1 198Leu enzyme has lower activity than the wild type enzyme [9]. GPX enzyme activities are influenced by intake of selenium [18], [19]. Selenium supplementation are associated with lower levels of UV-induced micronuclei in human breast cancer cells in vitro [7] possibly mediated through the stimulation of selenoproteins as GPX. Additionally, GPX enzyme activity may be influenced by tobacco smoking [20], alcohol consumption [21], and intake of fruit and vegetables [22]. Thus, the amino acid substitution and a variety of lifestyle factors may influence the enzyme activity of the antioxidant GPX1 and thereby the level of oxidative stress with potential of accumulation of mutagenic oxidative damage to DNA.
An increased load of ROS may cause higher levels of 8-oxo-7,8-dihydroguanine (8-oxoG) in human colorectal carcinoma compared with non-malignant tissue [23]. 8-oxoG is a strongly mutagenic lesion due to the ability to mispair with adenine during DNA replication, leading to G:C to T:A mutations. The OGG1 gene encodes the protein OGG1, a bifunctional glycosylase involved in the DNA base excision repair pathway that removes 8-oxoG directly from the oxidatively damaged DNA [24], [25], [26]. No association are observed between OGG1 Ser326Cys polymorphism and colorectal cancer [12], [27] or colorectal adenomas [12].
We hypothesized, that the two single nucleotide polymorphisms (SNPs) GPX1 Pro198Leu and OGG1 Ser326Cys are associated with risk of colorectal cancer. We studied, whether erythrocyte GPX enzyme activity in pre-diagnostic blood samples was associated to risk of colorectal cancer, and whether the GPX1 Pro198Leu genotype and several lifestyle factors as smoking, alcohol consumption, diet, and intake of antioxidants predicted GPX enzyme activity in erythrocytes. Additionally, interaction between lifestyle factors and the two polymorphisms and GPX enzyme activity were studied in relation to colorectal cancer risk.
Section snippets
Study group
Diet, Cancer and Health (DCH) is a Danish prospective follow-up study. A population of 160,725 individuals aged 50–64 years and born in Denmark, were invited to participate, of which 57,053 individuals with no previous cancer diagnosis were recruited [28], [29]. At enrolment in 1993–1997, detailed information on diet, smoking habits, weight, height, reproduction, medical treatment, socio-economic characteristics, and lifestyle exposures were collected. The participants reported their average
Results
Blood samples for genotyping and measurement of enzyme activity were available for 1156 participants. Forty-seven percent of the cases were diagnosed with rectal cancer, 50% with colon cancer, and for 3% the topography was not specified. Among cases, 96% had a diagnosis of adenocarcinomas, 2% of carcinoid tumor and 2% of various other histological subtypes. The distribution of cancer location was comparable for men and women (data not shown). The median follow-up time after enrolment was 6.7
Discussion
In a large prospective study we found that alcohol consumption and smoking were associated with higher risk of colorectal cancer among homozygous carriers of the variant GPX1 allele. To our knowledge, the present study is the first to report possible interaction between the two lifestyle factors and the GPX1 Pro198Leu polymorphism in relation to colorectal cancer risk. Erythrocyte GPX activity was modulated by the GPX1 Pro198Leu polymorphism. This is in agreement with results from our previous
Conflict of interest statement
None.
Acknowledgements
We thank Anne-Karin Jensen and Lourdes M. Pedersen for excellent technical assistance. And our thanks goes to the students Sana Ebdah and Xu Jin from Roskilde University, Denmark, who genotyped the GPX1 5′UTR polymorphism. This study was supported by Danish Research Council (Grant number 2052-03-0016) and a scholarship from the Danish Graduate School in Public Health Science and from the Faculty of Medicine at University of Copenhagen.
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