E7 Oncoprotein of Novel Human Papillomavirus Type 108 Lacking the E6 Gene Induces Dysplasia in Organotypic Keratinocyte Cultures

Abstract

The genome organization of the novel human papillomavirus type 108 (HPV108), isolated from a low-grade cervical lesion, deviates from those of other HPVs in lacking an E6 gene. The three related HPV types HPV103, HPV108, and HPV101 were isolated from cervicovaginal cells taken from normal genital mucosa (HPV103) and low-grade (HPV108) and high-grade cervical (HPV101) intraepithelial neoplasia (Z. Chen, M. Schiffman, R. Herrero, R. DeSalle, and R. D. Burk, Virology 360:447-453, 2007, and this report). Their unusual genome organization, against the background of considerable phylogenetic distance from the other HPV types usually associated with lesions of the genital tract, prompted us to investigate whether HPV108 E7 per se is sufficient to induce the above-mentioned clinical lesions. Expression of HPV108 E7 in organotypic keratinocyte cultures increases proliferation and apoptosis, focal nuclear polymorphism, and polychromasia. This is associated with irregular intra-and extracellular lipid accumulation and loss of the epithelial barrier. These alterations are linked to HPV108 E7 binding to pRb and inducing its decrease, an increase in PCNA expression, and BrdU incorporation, as well as increased p53 and p21 CIP1 protein levels. A delay in keratin K10 expression, increased expression of keratins K14 and K16, and loss of the corneal proteins involucrin and loricrin have also been noted. These modifications are suggestive of infection by a high-risk papillomavirus.A large number of human papillomavirus (HPV) types have been associated with benign and malignant lesions of the genital tract. High-risk papillomaviruses encode two oncoproteins, E6 and E7, which independently immortalize human keratinocytes, although their combined actions have complementary and synergistic effects (5,22,23,35,43). High-risk E7 protein binds to a multitude of cellular proteins in vitro. Probably its most important action is targeting the pocket protein pRb for degradation and thereby allowing uncontrolled cell cycle progression (30,34). More recent studies, however, have indicated that the pRb-independent functions of E7 are sufficient to induce disruption of terminal differentiation and mild hyperplasia (3). The key role of high-risk E6, on the other hand, is to inactivate the oncosuppressive protein p53 functionally by inducing its degradation through the ubiquitin-proteosome pathway (31). Expression of high-risk E7 alone stabilizes the p53 protein, leading to increased levels (12), although its transcriptional activity is disturbed (32).The genome organizations of the majority of human-pathogenic papillomaviruses are characterized by an early region containing five genes (E1, E2, E4, E6, and E7) and two genes (L1 and L2) in the late region. The HPV types of the genus Alphapapillomavirus usually harbor an E5 gene in the region between the early and late genes, whereas this gene is absent in the HPV types commonly associated with cutaneous lesions and grouped in the genera Betapapillomavirus, Gammapapillomavirus, Deltapapillomavirus, ...

Role of the host restriction factor APOBEC3 on papillomavirus evolution

“…3C). This further supports an effect of tropism on TC dinucleotide content (Chen et al 2007; Nobre et al 2009). This analysis was based on available clinical data (de Villiers et al 2004) and the tropism of many PVs is not yet clear.…”

Molecular archeological evidence in support of the repeated loss of a papillomavirus gene

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