Cervical cancer (CC) is the second most common type of cancer affecting the female population. The development of CC takes several years, and involves a precancerous stage known as cervical intraepithelial neoplasia (CIN). A key factor in the development of disease is the human papillomavirus (HPV) infection, which initiates carcinogenesis. Furthermore, CC is also impacted by epigenetic changes such as DNA methylation, which causes activation or exclusion of certain genes, and the hypermethylation of cytosines in promoters, thereby switching off previously active genes. The majority of DNA methylation events occur at cytosine-guanine nucleotides, which in the human genome are known as CpG islands. The aim of the present study was to investigate the methylation levels in intronic sequences of the two tumor suppressor genes cell adhesion molecule 1 (CADM1) and T-lymphocyte maturation associated protein (MAL) using cytological samples and to identify potential biomarkers involved in CIN by pyrosequencing. DNA was isolated from cervical smears from patients with CINs, with healthy patients serving as a control group. Samples were converted by treatment with sodium bisulfite and subsequent pyrosequencing to detect the methylation status of the selected genes. The presence of HPV DNA infection analyzed by the polymerase chain reaction, was detected in each sample. Of the total number of samples (n=91), the present study confirmed the presence of one or two high-risk subtypes of HPV in 39 cases (42.85%) and HPV infection was significantly associated with CIN2+ lesions. For the two genes (MAL and CADM1) the present study confirmed that the median methylation was significantly higher in HPV positive patients [P=0.0097, 95% confidence interval (CI): (-0.030, -0.003)/P=0.0024, 95% CI: (-0.06, -0.01)] when compared with patients negative for HPV DNA infection, and the average methylation was demonstrated to be increased with the degree of cervical lesion. The present study used logistical regression to model the dependence between the case/control statuses (control group vs. Dg. 1-4). The area under the curve values for MAL were: 84% for cervical inflammation, 71% for CIN1, 73.4% for CIN2+ and 77% for squamous cell carcinoma (SCC); and for CADM1 were: 88.6% for cervical inflammation, 68% for CIN1, 80% for CIN2+ and 89% for SCC. The present study confirmed that there were statistically significant differences between the methylation levels of individual CpGs and significantly higher median methylation in patients positive for HPV16/18. CADM1 exhibited higher levels of methylation in almost every study group when compared with MAL during the transition of CIN and appeared to be a promising biomarker for future study.
Keywords: DNA methylation; bio-markers; cervical cancer; human papillomavirus infection; pyrosequencing.