In these HPV types, the role of the wound healing response in driving the initial proliferation of the infected cell(s) may well be critical [103], with signalling from the local microenvironment influencing viral gene expression [104] and/or protein functions. In the case of the high-risk types that cause neoplasia, there is a clear role of the viral E6 and E7 proteins in driving cell proliferation in the basal and parabasal DNA Damage inhibitor cell

layers, especially at cervical sites where neoplasia can occur [3]. It is also clear that there are many functional differences between the high and low-risk E6 and E7 proteins (see Fig. 4A and [105]), and that these contribute, along with differences in promoter activity and patterns of gene expression, to the different HPV-associated pathologies seen in vivo. Indeed, recent studies have suggested that the deregulation of E6/E7 expression, even in the absence of genome integration,

is a critical event in determining neoplastic grade [106], which is classified according to the extent to which basal-like cells extend into suprabasal epithelial layers [107]. The E6/E7-mediated proliferation 3-MA in vivo of the basal and parabasal cells following infection by the high-risk HPV types facilitates an expansion in lesion size, which is thought in part to be linked to specific functions of the high-risk E6 and E7 proteins (Fig. 4A). Functional differences between the high- and low-risk E7 proteins centre to a large extent on their differential ability to associate with members of the Retinoblastoma (Rb) protein (pRb) family, with the high-risk E7 proteins being able to bind and degrade both p105 and p107, which control cell cycle entry in the basal layer, as well as p130, which is involved in cell cycle re-entry in the upper epithelial layers ([48] and [108] and Figure 4 and Figure 5). The low-risk E7 proteins generally appear to have a lower affinity for p105 out and p107 than the high-risk types, but can associate with and degrade p130 in order to create a replication-competent environment in

the mid-epithelial layers that is suitable for genome amplification [105] and [109] (Fig. 5). An unfortunate characteristic of the high-risk E7 proteins however is their ability to stimulate host genome instability, particularly through deregulation of the centrosome cycle in the proliferating basal cells [110], [111], [112], [113], [114] and [115]. The PDZ–domain-binding motif, which is located at the C-terminus of all the high-risk E6 proteins, provides another key difference between high- and low-risk PVs. High-risk E6 proteins are able to interact with a several PDZ targets through this motif, many of which are involved in the regulation of cell polarity, cell proliferation and cell signalling [116] and [117].