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

Smoking causes induction of micronuclei and other nuclear anomalies in cervical cells

INTRODUCTION

Smoking is an independent cause of cervical cancer, which is the 4th most common malignancy in women. It is currently not known if tobacco consumption causes chromosomal damage (which is a hallmark of human cancer) in cervical cells and if age and the hormonal status have an impact on tobacco induced genetic instability in the cervix.

METHODS

We conducted a study with pre- and post-menopausal women smokers and never-smokers (25/group). Smokers consumed 30 light/medium cigarettes/day and were matched with the non-smoking group. Cervical cells were analyzed for induction of micronuclei (MN) which are caused by structural/numerical chromosomal aberrations; additionally, other nuclear anomalies reflecting genomic instability and cytotoxicity were scored. Furthermore, the frequencies of basal cells were recorded which reflect the mitotic activity of the mucosa.

RESULTS

MN and other abnormalities were increased in both groups of smokers. The effects were most pronounced in postmenopausal smokers (i.e. 2-fold higher) compared to premenopausal smokers. Also the number of basal cells (indicative for cell proliferation) was clearly enhanced in older women. Tar and nicotine had no detectable impact on chromosomal damage but a clear association with pack-years was observed.

CONCLUSIONS

Smoking increased chromosomal instability, cytotoxicity and induced cell divisions in cervical mucosa cells of pre- and post-menopausal women. The effects were more pronounced in the latter group indicating a higher risk for diseases (including cancer) that are causally related to DNA damage.

Role of active and passive smoking in high-risk human papillomavirus infection and cervical intraepithelial neoplasia grade 2 or worse

Objective

We performed a pooled analysis to examine cigarette smoking and household passive smoke exposure in relation to the risk of human papillomavirus (HPV) infection and cervical intraepithelial neoplasia grade 2+ (CIN2+).

Methods

Data were pooled from 12 cross-sectional studies for cervical cancer screenings from 10 provinces of China in 1999–2007. A total of 16,422 women were analyzed, along with 2,392 high-risk-HPV (hr-HPV) positive women and 381 CIN2+ cases. Pooled odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using logistic regression models controlling for sexual and non-sexual confounding factors.

Results

There was an excess risk between active smoking and hr-HPV infection and CIN2+. Adjusted OR for ever smokers vs. never smokers was 1.45 (95% CI=1.10–1.91), for hr-HPV infection and 1.89 (95% CI=1.03–3.44), for CIN2+. Passive smoking had a slightly increased risk on the hr-HPV infection with adjusted OR 1.11 (1.00–1.24), but no statistical association was observed between passive smoke exposure and CIN2+. Compared with the neither active nor passive smokers, both active and passive smokers had a 1.57-fold (95% CI=1.14–2.15) increased risk of HPV infection and a 1.99-fold (95% CI=1.02–3.88) risk of CIN2+.

Conclusion

Our large multi-center cross-sectional study found active smoking could increase the risk of overall hr-HPV infection and CIN2+ adjusted by passive smoking and other factors. Passive smoking mildly increased the risk of HPV infection but not the CIN2+. An interaction existed between passive tobacco exposure and active smoking for hr-HPV infection and the CIN2+.

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.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

Smoking increases the risk of high-grade vaginal intraepithelial neoplasia in women with oncogenic human papillomavirus

OBJECTIVES

In a large retrospective study, the association of smoking with human papillomavirus (HPV) genotype and vaginal intraepithelial neoplasia (VAIN) grade was analyzed.

METHODS

A SNOMED search was performed for vaginal biopsy or resection specimens diagnosed as VAIN over an 11-year period. The diagnosis of VAIN grade was confirmed by histological review. HPV genotype was determined by GP5+/6+ PCR and dot blot hybridization with type-specific oligonucleotide probes. Smoking history was obtained by chart review. Statistical analysis was performed using the chi-square test.

RESULTS

We identified specimens from 111 patients (age range 15-84); 64% (n=71) were diagnosed with high-grade VAIN (HGVAIN) and 36% (n=40) with low-grade VAIN (LGVAIN). High-risk (HR) HPV genotypes were identified in 83% of specimens (n=92), other types in 17% (n=19). Twenty-one different HPV genotypes were detected in total. Smoking history was available for 81% (n=90). Forty-one percent (n=37) had a positive smoking history. There was no significant difference in infection with HR vs. other types (p=0.92) among smokers when compared to non-smokers. In patients with HR HPV genotypes, smokers were at an increased risk for HGVAIN lesions when compared to patients who had never smoked (83% vs. 59%, p=0.02).

CONCLUSIONS

These data indicate an increased risk for HGVAIN in HR HPV positive women who smoke compared to HR HPV positive non-smokers.

Prospective study of urinary excretion of 7-methylguanine and the risk of lung cancer: Effect modification bymu class glutathione-S-transferases

Nitrosamines are mainly mutagenic through methylation of DNA. 7-Methylguanine (m(7)Gua) is a product of base excision repair and spontaneous depurination of such lesions in DNA and a metabolite from RNA. Associations between urinary excretion of m(7)Gua and risk of lung cancer were examined in a population-based cohort of 25,717 men and 27,972 women aged 50-64 years. During 3-7 years follow-up 260 cases with lung cancer were identified and a subcohort of 263 individuals matched on sex, age and smoking duration was selected for comparison. Urine collected at entry was analyzed for m(7)Gua by HPLC. Effect modification by glutathione-S-transferases GSTM1, GSTM3, GSTT1 and GSTP1 was investigated. We found higher excretion of m(7)Gua among current smokers than among former smokers. The IRR (incidence rate ratio) of lung cancer was 1.20 (95% CI: 1.00-1.43) per doubling of m(7)Gua excretion in unadjusted analysis and 1.12 (95% CI: 0.93-1.35) after adjustment for smoking status, intensity and duration at entry. This association was mainly present among current smokers. Comparing the highest with the lowest tertile of m(7)Gua excretion the IRR of lung cancer was 1.75 (95% CI: 1.04-2.95) irrespective of genotype and 2.75 (95% CI: 1.33-5.81) in subjects with GSTM1 null genotype. If not caused by residual confounding by smoking a possible association between m(7)Gua excretion and lung cancer supports the importance of methylation of guanine. The finding of an association between m(7)Gua excretion and lung cancer risk mainly among current smokers and subjects with GSTM1 null genotype supports causality in this respect.