Cellular and Viral Factors that Control Human Papillomavirus Type 16 Late Gene Expression

Human papillomavirus type 16 (HPV-16) is the major cause of cervical cancer. We speculate that inhibition of HPV-16 late gene expression is a prerequisite for establishment of persistence and progression to cervical cancer. This is based on the findings that the late proteins are found only in the nuclei of terminally differentiated epithelium, and are never detected in human papillomavirus infected cervical cancer cells. It is therefore of great importance to understand how HPV-16 controls the onset of the immunogenic proteins L1 and L2 in an infected cancer cell. HPV-16 late gene expression is tightly regulated by differentiation-dependent transcription as well as by post-transcriptional mechanisms. The long-term goal of these studies was to understand how HPV late gene expression is regulated. The specific aim of this thesis was to identify cellular and viral factors that force the virus to switch on the late genes, and to determine the mechanism of action of these factors. This will help us to understand under which circumstances HPV establish persistent infections that could progress to cancer. We found three cellular factors; PTB, ASF/SF2 and SRp30c, and one viral factor; AdE4orf4, that in four distinctive ways were involved in the regulation of HPV-16 late gene expression. Interestingly, over-expression of PTB, AdE4orf4 or SRp30c produced different types of spliced late mRNAs. PTB induced the unspliced L2/L1 mRNA, while AdE4orf4 and SRp30c induced the spliced L1 and L1i mRNA, respectively. The three proteins had different mechanisms of action and different target sites within the HPV-16 genome, which revealed the many and complex pathways in HPV-16 gene regulation. These findings have contributed to a broader understanding of how the expression of HPV-16 late genes is controlled