DOI
https://doi.org/10.25772/RD8N-9N88
Defense Date
2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy
Department
Oral Health Research
First Advisor
Iain Morgan
Abstract
Human papillomaviruses (HPVs) are causative agents in around 5% of all cancers, including cervical and oropharyngeal. A feature of HPV cancers is their better clinical outcome compared with non-HPV anatomical counterparts. In turn, the presence of E2 predicts a better clinical outcome in HPV-positive cancers; the reason(s) for the better outcome of E2-positive patients is not fully understood.
Previously, we demonstrated that HPV16 E2 regulates host gene transcription that is relevant to the HPV16 lifecycle in N/Tert-1 cells. One of the genes repressed by E2 and the entire HPV16 genome in N/Tert-1 cells is TWIST1. In these studies, we demonstrate that TWIST1 RNA levels are reduced in HPV-positive versus HPV-negative head and neck cancer and that E2 and HPV16 downregulate both TWIST1 RNA and protein in our N/Tert-1 model; the HPV16 E6/E7 oncogenes cannot repress TWIST1. E2 represses the TWIST1 promoter in transient assays and is localized to the TWIST1 promoter; E2 also induces repressive epigenetic changes on the TWIST1 promoter. TWIST1 is a master transcriptional regulator of the epithelial to mesenchymal transition (EMT), and a high level of TWIST1 is a prognostic marker indicative of poor cancer outcomes. We demonstrate that TWIST1 target genes are also downregulated in E2-positive N/Tert-1 cells and that E2 promotes a failure in wound healing, a phenotype of reduced EMT.
E2 binds to a variety of host factors that help mediate its function, one of which is tumor suppressor p53. The tumor suppressor p53 primarily functions as a transcription regulatory factor during cellular stress and DNA-damage, leading to cell cycle arrest, senescence, and apoptosis. In HPV positive head and neck cancer, E6 degrades p53 in a cell-cycle specific manner, contributing to oncogenesis yet still allowing significant p53 expression under certain conditions. We found that p53 expression is retained in HPV-positive cell lines and patient derived xenograft models. While we found that TWIST1 repression by E2 is not dependent of the E2-p53 interaction, this interaction is independently important in sensitizing multiple cell lines to chemotherapy. Moreover, we demonstrated that this E2-p53 interaction is critical for the viral lifecycle and primary keratinocytes immortalized by a mutant deficient in E2-p53 interaction fail to proliferate or complete a viral lifecycle in organotypic raft culture models. We propose that TWIST1 repression, as well as E2 induced chemosensitivity may provide two independent mechanisms for better clinical response of E2-positive HPV tumors.
Rights
© Christian T. Fontan
Is Part Of
VCU University Archives
Is Part Of
VCU Theses and Dissertations
Date of Submission
4-1-2022