N7‐methyldeoxyguanosine levels in DNA isolated from cervical cytology samples are associated with smoking

Abstract

Smoking has been associated, in epidemiological studies, with an increased risk of cervical neoplasia. This may be in part due to the presence of tobacco-specific nitrosamines in cervical mucous of smokers, which may result in carcinogenic DNA damage. We have thus examined whether cervical DNA contains alkylation damage arising from exposure to methylating agents (N7-methyldeoxyguanosine, N7-MedG). DNA was extracted from cervical cytology samples and N7-MedG levels were measured using an immunoslotblot assay. Ninety percentage of the DNA samples were alkylated and N7-MedG levels (mean, 95% CI) in ever-smokers (1.27, 0.90-1.81 lmol/mol dG) were significantly higher than those in nonsmokers (0.42, 0.20-0.91 lmol/mol dG: p 5 0.005). N7-MedG adduct levels were significantly correlated with number of cigarettes smoked per day and pack years of cigarette smoking in current smokers. There was no association with N7-MedG levels and cervical intraepithelial neoplasia status, age, parity or contraception use. Our study suggests that cervical DNA contains alkylation damage that can arise from exposure to cigarette smoke. ' 2006 Wiley-Liss, Inc.Key words: alkylation; smoking; cervical cancer; N7-methylguanine Cervical cancer is one of the most common neoplastic diseases affecting women with a combined worldwide incidence of almost half a million new cases annually.1 Laboratory and epidemiological research has provided overwhelming evidence that some subtypes of human papillomavirus (HPV) are a ''necessary but insufficient cause'' for cervical cancer.2-5 Therefore, other risk factors are also needed for the initiation and promotion of cervical carcinogenesis.A number of epidemiological studies suggest that cigarette smoking may be an aetiological risk factor for cervical cancer. 6-10The risk for neoplasia appears to increase with longer duration and intensity of smoking. 11,12 The cervix is known to contain enzymes that can metabolise the carcinogens that are present in cigarette smoke, 13,14 and expression of these enzymes may be modified by HPV infection. 14,15 Cigarette-derived chemicals, nicotine and its major metabolite cotinine, have been found in cervical mucus at levels higher than those present in blood serum, [16][17][18][19] and the tobacco-specific nitrosamine 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK) is more prevalent in cervical mucus samples from smokers than those from nonsmokers.20 NNK alkylates DNA forming a range of DNA adducts including N7-methyldeoxyguanosine (N7-MedG), a poorly repaired adduct that can act as a biomarker of exposure to alkylating agents. 21 DNA adducts have been detected in human cervical tissues. [22][23][24][25][26][27] In most, but not all, of these studies, adduct levels were higher in smokers with the detected adducts, probably arising from exposure to polycyclic aromatic hydrocarbons present in cigarette smoke. However, other DNA adducts, in particular alkylation-induced lesions, may also be present, but would not have been detected by the procedures used. Our recent r...

The formation and biological significance of N7-guanine adducts

“…Similar to the data from animal studies, the presence of N 7-Me-Gua and other N 7-guanine adducts has been demonstrated ubiquitously in DNA from humans not known to be exposed to alkylating agents (Table 3) [146, 187-196]. Most of these reports investigated DNA from blood samples; either white blood cells (WBC) or individual blood cell types, and measured N 7-Me-Gua by 32 P-postlabeling or HPLC-ECD.…”

“…, formed from potential environmental sources due to their indistinguishable chemical identity. Interestingly, N 7-Me-Gua amounts were 3-fold and 5-fold higher in tumor tissue compared to normal tissues from cervical or bladder tissues, respectively [187, 194]. In contrast, N7-Me-Gua amounts were lower in colon tumor DNA compared to normal colon tissue [192].…”

Optimizing immunoslot blot assays and application to low DNA adduct levels using an amplification approach

“…The immunoslot blot (ISB) assay was first developed by Nehls and coworkers in 1984 (1). Since then the assay has been applied to the detection of several DNA adducts formed both in vitro (2)(3)(4)(5) and in vivo (6)(7)(8)(9)(10)(11)(12) and the use of both monoclonal (13,14) and polyclonal (2) antibodies has been described.…”

Prospective study of urinary excretion of 7‐methylguanine and the risk of lung cancer: Effect modification by mu class glutathione‐S‐transferases

“…5 Upon metabolic activation TSNA can form DNA adducts of which N 7 -methylguanine (m 7 Gua) is by far the most abundant, representing 70-90% of methylation products. [6][7][8][9][10] Whereas m 7 Gua is innocuous in DNA, the levels increase dose-dependently in DNA upon administration of methylating agents, correlating with pro-mutagenic and carcinogenic methyl-adducts, such as O 6 -methylguanine and methyladenine, [11][12][13] and may thus serve as biomarker of exposure to methylating agents. Indeed, a physiologically based pharmacokinetic model for interspecies dose extrapolation of dimethyl sulphate has been based on m 7 Gua adducts in the nasal cavity.…”