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Cervical cancer is the second most common cancer in women and each year about 440,000 new cases are diagnosed. Infection with the human papillomavirus (HPV) is the established major risk factor for this cancer, but only a small proportion of women with HPV infection and pre-invasive lesions progress to invasive cancer. Other factors determine whether HPV infection leads to invasive cervical cancer and these are likely to be associated with genetic modifications.
A cross-sectional study was carried out to examine the hypothesis that increasing severity of abnormality in cervical cells, from normal to invasive cancer, is associated with an increase in the promoter methylation of selected tumour suppressor genes. Additionally, it was hypothesised that gene methylation profile would associate with folate status.
A total of 308 women were recruited to the study, including women with a diagnosis of normal, low grade cervical abnormality (cervical intraepithelial neoplasia grades 1 and 2; CIN1 & 2), high grade abnormality (CIN 3), and invasive cancer.
Blood was collected for folate status measurements and for genotyping for the C677T polymorphism in MTHFR, and cervical cells were collected for measures of DNA methylation, and for testing of human papillomavirus (HPV) infection. Cervical biopsies were collected for diagnostic purposes.
The prevalence of high risk HPV infection increased as histological classification became more severe (P < 0.001). Increasing severity of cervical abnormality was associated with a fall in folate status. Thus, women diagnosed with cervical intraepithelial neoplasia grades 1,2, or 3 or cancer had a significantly lower red cell folate status than those with normal cervical histology (P = 0.041, 0.046, 0.026, and 0.012 respectively).
Of great importance was the finding that red blood cell folate status was significantly lower in women with HR-HPV infection than those without (P = 0.031). There was no interaction between HR-HPV and histological grade. DNA global hypomethylation was greater in women with invasive cancer than for any other group (P <0.001), but showed no significant relation to folate status. Of the seven tumour suppressor genes investigated three showed a significant increase in promoter methylation with increasingly severe histology, and factor analysis confirmed that these genes formed a distinct group.
These were death-associated protein kinase; DAPK, E-cadherin; CDH1, and hypermethylated in cancer-1; HIC1. Promoter hypermethylation of these three tumour suppressor genes may lead to gene silencing and increased risk of progression of precancerous lesions to cancer.
The results of the study described here support a role for folate in determining risk of cervical cancer, possibly through an influence over HR-HPV infection. The gene cluster might be useful in predicting which women with HPV infection are most at risk of developing cervical cancer.