For some time now, the general hypothesis has been that lesion formation begins with the infection of a basal stem cell (rather than a basal transiently amplifying cell) and that the longevity of BMS-354825 the stem cells is a key factor in the formation of a persistent lesion [3], [50], [91] and [92]. For the low-risk HPV types, which do not generally cause neoplasia and which do not massively stimulate basal cell proliferation, this is a plausible hypothesis, even though not yet formally proven. For the high-risk types, which can stimulate basal cell proliferation, it is less clear whether this is a necessity. The nature of the initially infected cell and how it relates

to disease outcome is thus still a matter of speculation. Irrespective of the nature of the infected basal cell, it is generally thought that infection is followed by an initial phase of genome amplification, and then by maintenance of the viral episome at low copy number [83], [93] and [94]. The copy number in the basal layer of lesions is often proposed as 200 or so copies per cell, based on the study of episomal cell lines derived from cervical lesions. In benign oral papillomas in Selleck MLN0128 animals, the basal copy number has been quantified using laser capture methods as 50 to 100 copies per cell [95], but it is likely that there will be variation

from lesion to lesion and between different sites. The viral replication proteins E1 and E2 are thought to be essential for this initial amplification phase, but may be dispensable for episomal maintenance-replication once the copy number has stabilised [96], [97] and [98]. The precise role of E1 and E2 in the epithelial basal layer during natural infection needs further clarification however, given the proposed role of E2 in genome partitioning (see below). E2 also regulates viral transcription, and has multiple binding sites in the viral LCR (long control region or upstream

regulatory region [URR]), and (during viral DNA replication) can recruit the viral E1 helicase to a specific E1 binding motif in the viral origin of replication. It has been speculated that the use of a viral DNA helicase (i.e., E1), Phosphatidylinositol diacylglycerol-lyase which is distinct from the cellular replication helicases (MCM proteins), allows viral DNA replication to be disconnected from cellular DNA replication during genome establishment and amplification [3] and [99]. Although the role of viral and cellular helicases in genome maintenance still needs some clarification, several studies have proposed a role for E2 in the regulation of accurate genome partitioning during basal cell division [94]. In bovine PV, this involves the cellular Brd4 protein, but in HPVs, other E2 binding proteins appear to be involved in the tethering of viral episomes to the cellular chromatin during cell division [93], [94], [100], [101] and [102].