According to this hypothesis, the relatively low levels of E6 and E7 present in CIN1 do not compromise the functions of their cellular targets sufficiently to facilitate cancer progression. The viral deregulation seen in CIN2/3+ is also thought to facilitate integration of the viral episome into the host cell chromosome, which can further deregulate the expression

of E6 and E7; genes which are often referred to as viral oncogenes. Although it is not clear exactly how gene expression from the viral episome can become deregulated in early CIN, data from the vaccine trials has indicated that CIN2+ can occur in young women soon after infection [163], [164], [165] and [166]. In these instances, deregulated gene expression may IWR-1 cell line be driven by changes in cell signalling

as can be brought about by hormonal Cobimetinib molecular weight changes [58], or epigenetic modifications such as viral DNA methylation, which may depend on the nature of the infected epithelial cell [167]. The HPV16 LCR contains hormone response elements that can be stimulated by estrogen, and there is ample evidence of cooperation between estrogen and HPV in the development of cervical cancer in both humans and in model systems [58], [168], [169] and [170]. In CIN, it has been reported that the LCR is differentially methylated according to disease severity, which suggests that epigenetic changes may also regulate gene expression [171] (and thus disease [106]). It is also thought that for HPV16 at least, the E7 protein not can induce epigenetic changes that may contribute to changes in cellular gene expression [172], [173] and [174].

Although common fragile sites (CFS) in the host cell genome are hot spots where integration is more likely to occur [53], integration is, in general, a chance event that can sometimes result in the disruption of viral genes that regulate normal transcription from the LCR. Key amongst these is E2, which is a virally-encoded transcription factor that normally regulates E6/E7 abundance by binding to sites within the viral long control region (LCR). The majority of cervical cancers contain one or many copies of HPV, integrated more or less randomly into the host chromosome, with the viral integration site frequently lying within the regulatory E1 or E2 genes [55] and [175]. Integration and the loss of E6/E7 regulation can facilitate persistent high-level expression of these genes [176] and [177] and the accumulation of genetic errors that eventually lead to cancer [178]. In recent years, there has been much debate as to whether early integration events in CIN1 drive progression through CIN2 and CIN3 to cancer, or whether some degree of viral gene expression de-regulation underlies the early CIN2 and CIN3 phenotypes, and whether it is this initial deregulation that causes chromosome instability and thus facilitates integration (Figure 6 and Figure 7).