Elsevier

Oral Oncology

Volume 37, Issue 7, October 2001, Pages 593-598
Oral Oncology

Genetic polymorphism of CYP1A1, GSTM1 and GSTT1 genes in Indian oral cancer

https://doi.org/10.1016/S1368-8375(01)00028-8Get rights and content

Abstract

Oral cancer ranks first among all cancers in males and is the third most common among females in India. Tobacco-derived carcinogens are involved in the development of oral cancer. Environment–gene interaction in oral carcinogenesis is well demonstrated by phase I and II enzymes that are involved in the metabolism of carcinogens. This study looked at the significance of genetic polymorphisms in CYP1A1, GSTM1 and GSTT1 genes in patients with oral cancer. The study included 98 oral cancer patients and 60 age and sex matched healthy controls. Genotypes of CYP1A1, GSTM1 and GSTT1 were determined by PCR-RFLP. GSTM1 null deletion was observed in 49% of oral cancer cases and 33% of control subjects. For GSTT1, 18% of carcinomas and 8% of controls had the null genotype. In the case of CYP1A1 m2 allele, 51% of oral cancers and 17% of normal controls, respectively, had one or both alleles with the isoleucine→valine substitution. Digestion of the PCR products with enzyme Nco1 revealed polymorphism for CYP1A1 m2 with bands at 263 bp. There was no association between genotypes with tumor size, stage, grade, and age. Since null genotype individuals may possibly be poor detoxifiers with reduced ability to neutralise the reactive carcinogenic intermediates, they may be a high risk category. The frequency distribution of CYP1A1 m2 (Ile/val) genotypes among oral cancer patients was significantly different that from normal controls. The risk of CYP1A1 can be supported by the functional difference between presence of valine and isoleucine; valine type has higher catalytic and mutagenic activity towards benzo[a] pyrene than the isoleucine type. In conclusion, our results suggest that polymorphism in CYP1A1 m2 gene and/or GSTM1 and GSTT1 null genotype may confer an increased risk for oral cancer.

Introduction

Oral cancer ranks first among all cancers in males and is the third most common among females in India [1]. The incidence rates from 7 to 17/100 000 person/year is higher than reported Western rates of 3–4 /100 000/year. Furthermore, about 7% of all cancer deaths in males and 4% in females are due to oral cancer [2]. Tobacco use is a well-known risk factor for multiple cancers including those of the head and neck, oesophagus, lung and bladder. Cofactors in several of these malignancies include alcohol, dietary factors and viral infection. Despite the risk of tobacco exposure, the majority of patients who smoke or chew tobacco do not get cancer. Factors that influence tobacco-exposed individuals developing a malignancy may thus include a combination of total tobacco exposure and genetic susceptibility.

The environment–gene interaction in carcinogenesis is well reflected by phase 1 and phase 2 enzymes that are involved in the metabolism of carcinogens. The superfamily of Cytochrome p450 (phase 1) enzymes catalyse the oxidative metabolism of most endogenous chemicals (e.g. hormones and fatty acids) and exogenous chemicals (e.g. polycyclic aromatic hydrocarbons, aromatic amines and mycotoxins). Many of the p450 genes are known to exist in variant forms that have different activities. Since many carcinogens require metabolic activation before binding to DNA, individuals with an elevated metabolic capacity to activate specific carcinogens may be at an increased risk of cancer. Two genetic polymorphisms have been demonstrated in the CYP1A1 gene, which may both result in increased enzyme activity [3], [4], [5], an isoleucine/valine substitution in exon 7 at the heme binding region (Nco1 restriction site) and a thymine/cytosine point mutation in the Msp1 restriction site [6], [7]. Glutathione S transferases (GST) are a group of Phase 2 enzymes that are primarily involved in detoxifying carcinogenic metabolites. The null genotype of GSTM1 has a decreased capability in detoxifying some carcinogens present in tobacco smoke. The function of GSTT1 is different from other GSTs in its slow metabolizing rate of 1-chloro-2,4-dinitrobenzene. The balance between the CYP and GST enzymes therefore may substantially influence cancer risk [8].

Previous studies on tobacco associated lung cancer have shown high CYP1A1 inducibility to be associated with increased risk of lung cancer [6]. Studies have also shown loss of GSTM1 gene to be associated with an increased risk of lung, bladder and breast cancer [9], [10], [11]. The null genotype of GSTT1 was reported to be associated with an increased risk of brain cancer and colorectal cancer [12]. Data also suggest that null genotypes of GSTM1 and GSTT1, both individually and in combination, are high penetrance genetic risk factors for developing oral leukoplakias and consequently oral cancer in habituated betal quid/tobacco chewers of Indian ethnicity [13]. In the present study we investigated genetic polymorphisms in CYP1A1, GSTM1 and GSTT1 in patients with oral cancer in relation to the normal controls (both tobacco users and nonusers) by using PCR-RFLP analysis.

Section snippets

Subjects

The study included 98 oral cancer patients (80 males and 18 females) selected from the Out Patient clinics of Regional Cancer Centre, Thiruvananthapuram, India and 60 age matched normal controls (45 males and 15 females). The study was approved by the Research Review and Advisory Committee of the Regional Cancer Centre. All oral cancer patients were habitual tobacco users while in the control group 18 of the 45 men were tobacco users.

DNA extraction and PCR analysis

DNA was isolated from peripheral blood using a Quick Clean

Results

Genomic DNA was checked using 1% agarose gel electrophoresis. The distribution of genotypes of CYP1A1 m2, GSTM1 and GSTT1 in the study populations are shown in Table 2. To identify the valine polymorphism of CYP1A1, the PCR products were digested with Nco1 restriction enzyme, yielding a 232 bp product for the wild type allele and 263 bp for the allele containing the polymorphism. The frequencies of the GSTM1 and GSTT1 null genotypes were significantly higher in oral cancer cases (Fig. 1, Fig. 2

Discussion

Epidemiologic studies have strongly implicated tobacco as an etiological agent in the development of oral cancer [14], [15], [16]. All carcinogens are lipophilic and have a tendency to be converted into water soluble hydrophillic compound and can be easily removed from the body through the excretory system. This conversion or detoxification of carcinogens is achieved by the addition of one atom of oxygen to the carcinogenic compound, brought about by the superfamily of Cytochrome P450 (CYP)

Acknowledgements

Study supported by Indian Council of Medical Research (Grant No. 5/13/21/98- NCD III).

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