Integration of Human Papillomavirus is not a Necessary Mechanism in Cervical Cancer Development

Cervical cancer is a major cause of cancer-related death in women worldwide. Human papillomavirus (HPV) is essential, though not sufficient, to cause cervical cancer. HPV16 and HPV18 are the most prevalent high-risk types worldwide, whereas, HPV52 and HPV58 also show a notable higher prevalence in East Asia than in other parts of the world.;Studies have suggested that HPV viral load is an important determinant for the development of high-grade lesions. While some studies observed a positive correlation between viral load and disease severity, others have reported no association. The HPV genome can exist in two physical forms, episomal or integrated. The E2 gene, encoded by HPV has an important role in the regulation of E6 and E7 viral oncogenes. When HPV integrates into the host chromosome, it may result in disruption of the E2 gene thereby its control on the expression of the E6 and E7.;The hypothesis for this study was that high viral load and disruption of E2 gene associated with integration of HPV into the host genome was not the only pathway leading to cervical cancer development. In this study, the viral load and integration profile for HPV types 16, 18, 52 and 58 among different severity of cervical lesions were analyzed. Further detailed studies were performed on HPV16 with emphases on E6/E7 mRNA transcript levels, E2 and LCR sequence variation and the methylation status of two E2 binding sites. The ultimate aim was to determine what other alternative mechanisms exist apart from viral integration to drive the oncogenicity of HPV that lead to the development of cervical cancer.;The results showed that the relationship between viral load and disease varied between different HPV types and that normalization of cellular DNA input using a housekeeping gene was crucial for accurate interpretation among different cervical lesion grades. A key finding from this study was that a substantial proportion of invasive cervical carcinomas were found to contain the purely episomal form of the HPV genome. The levels of the three E6/E7 mRNA transcripts species in invasive cervical carcinomas containing the pure episomal form of the viral genome were found to be similar to those with pure integrated forms. This observation suggested that invasive cervical carcinoma samples containing the episomal form of the HPV genome were also mediated by the up-regulated E6/E7 mRNA expression. More importantly, this up-regulation in E6/E7 mRNA expression did not depend on integration and disruption of the E2 gene.;The alternative mechanism that up-regulated of the expression of E6 and E7 oncogene found in invasive cervical carcinoma samples harbouring the episomal form of the viral genome was likely to be a consequence of methylation of CpG sites in the two E2 binding sites at the promoter region of HPV16. This observation explained and supported that the repressive role of E2 on E6 and E7 transcriptional regulation was abolished due to methylation of the E2 binding sites, and that a sustained level of the E6 and E7 oncoproteins was maintained, working in synergy in cell transformation and in carcinogenesis. These observations confirmed the hypothesis that HPV integration was not the only mechanism leading to the development of cervical cancer.