E7, Ptpn14, And Yap1: How Papillomaviruses Retain Cells In The Basal Epithelial Layer With Implications For Persistence And Carcinogenesis

Persistent human papillomavirus (HPV) infections cause about 4.5% of the human cancer burden. The HPV E7 oncoprotein is one of the primary drivers of HPV-mediated carcinogenesis and facilitates HPV replication. The most well-studied HPV E7 activity is inactivation of the retinoblastoma tumor suppressor (RB1), which dysregulates the cell cycle. However, there is substantial evidence to suggest that HPV E7 must have other activities in addition to RB1 inactivation. HPV E7 proteins from diverse HPV genotypes interact with the host cell tumor suppressor protein tyrosine phosphatase non-receptor 14 (PTPN14). Many HPV E7 proteins also direct PTPN14 for proteosome-mediated degradation. Through my dissertation work, I sought to identify the role of PTPN14 degradation by HPV E7 in the HPV replicative cycle and carcinogenesis. To initially characterize PTPN14 function in keratinocytes, we used mutant HPV E7 proteins that cannot degrade PTPN14, CRISPR knockout of PTPN14, and high throughput RNA sequencing to assess differential gene expression. We found that PTPN14 degradation by HPV E7 broadly represses genes related to keratinocyte differentiation. In a subsequent study, we used three-dimensional organotypic epithelial cultures to study the mechanism downstream of PTPN14 degradation. We found that PTPN14 loss in keratinocytes promotes basal cell-specific activation of the HIPPO pathway transcriptional regulator YAP1. PTPN14 degradation and YAP1 activation by HPV E7 did not prevent terminal differentiation in these cultures. Instead, the repression of differentiation observed in monolayer culture translated to delayed commitment to differentiation and promotion of basal cell identity in the three-dimensional system. This indicated that PTPN14 degradation by HPV E7 may help HPV resist epithelial turnover and facilitate persistent infection. YAP1 and its paralogue TAZ are oncogenes when dysregulated. Accordingly, through these studies we also found that PTPN14 degradation and YAP1/TAZ transcriptional activity contribute to the ability of HPV E7 proteins from oncogenic genotypes to extend the lifespans of primary keratinocytes, a metric of E7 carcinogenic activity. Overall, the experiments described in this dissertation outline the effects of PTPN14 degradation by HPV E7 in keratinocytes and stratified squamous epithelia which have implications for HPV persistence and HPV-mediated carcinogenesis