Levels of the DNA adduct, N7-methyldeoxyguanosine, are associated with increased risk of failure of treatment of cervical intraepithelial neoplasia

Increased N7-methyldeoxyguanosine DNA adducts after occupational exposure to pesticides and influence of genetic polymorphisms of paraoxonase-1 and glutathione S-transferase M1 and T1

There are concerns about genetic risks associated with long-term exposure to pesticides as these compounds may damage DNA, resulting in mutations that eventually lead to cancer, neurological, and reproductive adverse health effects. This study assessed DNA damage in intensive agricultural workers exposed to pesticides by determining the levels of N7-methyldeoxyguanosine (N7-MedG), an adduct known to be a robust biomarker of recent exposure to chemical methylating agents. A cohort of 39 plastic greenhouse workers was assessed for changes in lymphocyte DNA N7-MedG levels between low level and high level exposures during the course of a spraying season. The contributions of genetic polymorphisms of the pesticide-metabolizing enzymes paraoxonase-1 (PON1) and the glutathione S-transferases, GSTM1 and GSTT1, on N7-MedG levels and other potential confounders were also assessed. N7-MedG increased in the period of high pesticide exposure as compared to the low exposure period (0.23 and 0.18 µmol N7-MedG/mol dG for the unadjusted and adjusted linear mixed models, P = 0.02 and 0.08, respectively). Significant decreased levels of erythrocyte acetylcholinesterase and plasma cholinesterase were observed in the high versus low exposure period in both the unadjusted (2.85 U/g hemoglobin and 213.13 U/L, respectively) and adjusted linear mixed models (2.99 U/g hemoglobin and 230.77 U/L, respectively), indicating pesticide intake. In intensive agriculture workers, higher pesticide exposure increased DNA alkylation levels, further demonstrating the genotoxicity of pesticides in man. In addition, pesticide-exposed individuals with inherited susceptible metabolic genotypes (particularly, null genotype for GSTM1 and the PON1 192R allele) appear to have an increased risk of genotoxic DNA damage. Environ. Mol. Mutagen. 56:437-445, 2015. © 2014 Wiley Periodicals, Inc.

Alkylation of sperm DNA is associated with male factor infertility and a reduction in the proportion of oocytes fertilised during assisted reproduction

Approximately one-third of IVF cases in the UK are attributed to male factor infertility and in the majority of cases the origin of male infertility is unknown. The integrity of sperm DNA is important both for the success of assisted reproduction and the implications for the off-spring. One type of DNA damage that has not been investigated with respect to fertility outcomes is the adduct N7-methyldeoxyguanosine (N7-MedG), a biomarker for exposure to alkylating agents. A prospective cohort of couples attending for IVF had their N7-MedG levels in sperm measured using an immunoslot blot technique to examine whether sperm N7-MedG levels are associated with male factor infertility, semen quality measures or assisted reproduction outcomes. Sufficient DNA for analysis was obtained from 67/97 couples and N7-MedG was detected in 94% of sperm samples analysed. Men diagnosed with male factor infertility had significantly higher mean levels of N7-MedG in their sperm DNA (P=0.03). Logistic regression analysis showed that N7-MedG levels were significantly negatively associated with the proportion of oocytes successfully fertilised irrespective of the method of fertilisation used (IVF or intra-cytoplasmic sperm injection; ICSI, P<0.001). Therefore exposure to DNA alkylating agents is significantly associated with male infertility and the proportion of oocytes fertilised during assisted reproduction. Reducing such exposure may improve male fertility but further work is required to determine the relative importance of exogenous and endogenous sources of exposure.

Bladder Tumor Contains Higher N7-Methylguanine Levels in DNA than Adjacent Normal Bladder Epithelium

Schistosoma haematobium-infected patients are more likely to develop bladder cancer and be more exposed to carcinogenic N-nitroso compounds than uninfected patients. As N7-methylguanine is a marker of exposure to methylating agents of this type, we have measured N7-methyldeoxyguanosine 3'-monophosphate (N7-MedGp) by (32)P postlabeling. DNA was isolated from 42 paired normal and tumor tissue of Egyptians with bladder cancer. N7-MedGp was detected in DNA from 93% of the tumors and 74% of the normal bladder tissue samples. Adduct levels were highly variable and ranged from 0.04 to 6.4 and from 0.02 to 0.72 micromol/mol deoxyguanosine 3'-monophosphate (dGp) in tumor and normal DNA, respectively. N7-MedGp levels in normal and tumor DNA were highly correlated with one another (P = 0.007). The mean difference (95% confidence interval) in adduct levels between tumor and normal DNA was 0.21 (0.13-0.32) micromol/mol dGp and this was statistically significant (P < 0.001). The adduct ratio (tumor DNA/normal DNA) varied between 0.2 and 136 (median, 4.6). N7-MedGp levels were not associated with gender, age, or the presence of schistosomiasis. However, lower N7-MedGp levels were found in normal DNA from individuals lacking the GSTM1 gene (P = 0.03) but not the GSTT1 gene or in subjects with the Ile105Val GSTP1 polymorphism. These results show that exposure to methylating agents is widespread and suggest that such exposure may play a role both in tumor initiation and progression.

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

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 micromol/mol dG) were significantly higher than those in nonsmokers (0.42, 0.20-0.91 micromol/mol dG: p = 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.