Promoter methylation of tumor suppressor genes induced by human papillomavirus in cervical cancer

Cervical cancer is the most fourth common cancer in women worldwide. The E6 and E7 high-risk human papillomavirus (HPV) types are the main cause of this cancer. Several studies have revealed that promoter methylation of tumor suppressor genes is induced by HPV E7. Recently, it was found that HPV16-E7 and the DNA methyltransferase 1 complex could bind at the cyclin A1 (CCNA1) promoter, resulting inCCNA1promoter methylation. Therefore, there is a need to study other tumor suppressor genes for which HPV may induce promoter methylation. The present study investigated whether HPV induced cell adhesion molecule 1 (CADM1) and death associated protein kinase 1 (DAPK1) promoter methylation. C33a (no HPV infection) and SiHa (HPV 16 infection) cell lines were used for methylation status and expression observation. It was found thatCADM1andDAPK1promoter methylation, no expression ofCADM1and decreased expression ofDAPK1, was presented in SiHa cells. While no promoter methylation of these two genes was observed in C33a cells, with positive expression of the genes. It was subsequently investigated whether E6 and/or E7 could induce promoter methylation and decrease the expression of these two genes. Methylation-specific primer PCR and quantitative PCR were performed to elucidate the promoter methylation status and expression ofCADM1andDAPK1in C33a cells transfected with HPV16 E6-PCDNA3 or HPV16 E7-PCDNA3.1 myc-his, compared to empty vector-transfected cells. The results showed that HPV E7 could induceCADM1promoter methylation and decrease the gene expression in HPV E7 transfected C33a cells, while HPV E6 could induceDAPK1promoter methylation and decrease its expression in C33a cells transfected with HPV E6. Finally, the mechanism by which HPV E7 inducedCADM1promoter methylation was observed by performing chromatin immunoprecipitation; the data showed that E7 inducedCADM1methylation by the same mechanism as that forCCNA1, by binding at theCADM1promoter, resulting in the subsequent reduction of its expression in cervical cancer.