ReviewNew markers for cervical dysplasia to visualise the genomic chaos created by aberrant oncogenic papillomavirus infections
Introduction
Although population wide screening in most Western countries has led to a remarkable reduction in the incidence and mortality of cervical cancer [1], with approximately 400 000 new cases and 250 000 cancer-related deaths each year, and remains a major global health burden. Since its description by George Papanicolaou in 1941 [2], the technical approach of the screening assay has remained essentially the same. It is still based on the morphological interpretation of cells in smears taken from the surface of the cervix. Scoring criteria include the assessment of cell size/nuclear size, the nuclear shape and nuclear staining intensity, nuclear and chromatin architecture, shape of the nuclear membrane and the ratio between cytoplasm and nuclear volume [3], most of these being signs of acute or persistent papillomavirus infections. Due to the ambiguity of these morphological criteria, screening results are affected by a high degree of inter- and intra-observer variability. In particular, ‘low grade’ changes are often prone to misinterpretation 3, 4. In addition to the insecurity and distress for the affected women, this also causes significant costs for healthcare providers 5, 6, 7. In view of the tremendous scientific achievements which have been made to unravel the molecular pathogenesis of cervical cancer, it is now time to translate these significant improvements into new screening concepts and technologies. The purpose of this review is to summarise the major pathogenic features of persistent oncogenic papillomavirus infections in cervical carcinogenesis. Based on this knowledge, new biomarkers for dysplastic cervical cells can be delineated. The basic concepts and potential clinical applications in modified screening programmes will also be discussed.
Section snippets
Prevalence of human papillomavirus infection and the association with cervical cancer
The human papillomaviruses (HPV) infect epithelial cells in both mucosal surfaces and the skin. A subgroup of the mucosa-associated HPV types, the high-risk papillomaviruses (HR-HPVs), infect primarily mucosal epithelia of the anogenital tract and are the major cause of cervical cancer (Table 1, Table 2). Women infected with HR-HPVs have an at least 100-fold increased risk of developing a high grade cervical lesion compared with non-infected individuals; nucleic acids of HR-HPVs can be readily
The natural history of HR-HPV infections
Most HPV-related dysplastic lesions develop within the anogenital tract of females. Therefore, most epidemiological studies on the incidence and prevalence of HPV infections have been performed in female cohorts. Penile lesions are very rare, although a similar percentage of males are likely to be infected and seem to represent the major reservoir for transmission of the oncogenic viruses [19]. This suggests that besides the simple infection, a particular interaction with specific host cells is
The intracellular control of viral gene expression and replication in epithelial stem cells
How expression of viral genes suddenly occurs in the non-senescent, not yet terminally differentiated, epithelial stem cells is largely unknown. Several lines of evidence point to loss of intracellular transcriptional control by specific cellular surveillance factors referred to as cellular interferrin, factor CIF (reviewed in [27]). This permits ‘aberrant’ expression of the E6 and E7 genes in replicating epithelial stem cells (Fig. 3c and d). This concept nicely explains how infections with
Molecular functions of the E6 and E7 oncogene products
As described above, replication of the HR-HPV genomes is restricted to terminally differentiated cells, which have left the cell cycle and switched off their cellular DNA replication machinery (Fig. 3a–d). The viral genome therefore contributes to the cell's genetic functions, compensating for the missing cellular functions required for the replication of viral DNA. One major player here is the E7 gene product, which has been shown to mediate the initiation of DNA synthesis as a consequence of
Diagnostic exploitation of molecular pathways induced by the viral oncogenes: E7-mediated overexpression of p16ink4a identifies HPV-transformed cells
In searching for new diagnostic concepts one aspect of the interaction of E7 and the pocket proteins, in particular pRb, seems particularly worthwhile to consider further. As discussed above, the transcriptional inhibitory activity of the complex of pRB and E2F seems to be degraded, particularly due to E7-enhanced ubiquitination and proteasome-mediated decay. pRB and its complex with E2F, as discussed earlier, controls the initiation of the G1/S phase progression at least in terms of activation
Integration of viral sequences into the cellular genome is a progression marker for cervical precancer
Disturbances of the spindle pole apparatus in proliferating cells generates a tremendous potential for conflicts on ‘where to go’ for individual chromosomes not tied simply to two poles, but to a complex of additional aberrant centromeres. This might be a strong promoter for chromosomal breakage and recombination, a well described hallmark of HPV-related carcinogenesis (reviewed in [81]). Interestingly, recombination of viral and cellular genes also seems to be promoted by this mode of E6- and
Conclusions
Detailed analysis of the biochemical pathways contributing to the HPV-mediated transformation of epithelial cells has the potential to predict new biomarkers for the detection of dysplastic cells in clinical samples. In particular, marked overexpression of the cyclin dependent kinase inhibitor p16INK4a was regularly observed in HR-HPV induced cervical lesions with deregulated viral oncogene expression in replicating basal or para-basal cells. Implementing staining with p16INK4a-specific
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