Background: Persistent infections with carcinogenic human papillomavirus (HPV) types are the necessary cause of cervical cancer. We recently demonstrated that the HPV16 genome is strongly methylated in cervical precancer compared with transient infections. However, the extent of methylation in other HPV types and its role in progression to cancer is poorly understood.
Methods: We analyzed whole-genome methylation patterns of the three next most carcinogenic HPV genotypes: HPV31 (closely related to HPV16), and two other closely related types, HPV18 and HPV45. DNA was extracted from cervical cytology specimens from 92 women with precancer and 96 women infected with HPV31, HPV18, or HPV45, but who had no cytological or histological abnormalities. After bisulfite modification, genome-wide pyrosequencing was performed covering 80-106 sites. We calculated differences in median methylation, odds ratios, areas under the curve, and Spearman rank correlation coefficients for methylation levels between different sites. All statistical tests were two-sided.
Results: For all three HPV types, we observed strongly elevated methylation levels at multiple CpG sites in the E2, L2, and L1 regions among women with cervical intraepithelial neoplasia grade 3 compared with women with transient infections. We observed high correlation of methylation patterns between phylogenetically related types. The highest areas under the curve were 0.81 for HPV31, 0.85 for HPV18, and 0.98 for HPV45. Differential methylation patterns in cervical intraepithelial neoplasia grade 3 patients with multiple infections suggest that methylation can clarify which of the infections is causal.
Conclusions: Carcinogenic HPV DNA methylation indicates transforming HPV infections. Our findings show that methylation of carcinogenic HPV types is a general phenomenon that warrants development of diagnostic assays.