Human papillomavirus molecular biology and disease association

Abstract

Human papillomaviruses (HPVs) have evolved over millions of years to propagate themselves in a range of different animal species including humans. Viruses that have co-evolved slowly in this way typically cause chronic inapparent infections, with virion production in the absence of apparent disease. This is the case for many Beta and Gamma HPV types. The Alpha papillomavirus types have however evolved immunoevasion strategies that allow them to cause persistent visible papillomas. These viruses activate the cell cycle as the infected epithelial cell differentiates in order to create a replication competent environment that allows viral genome amplification and packaging into infectious particles. This is mediated by the viral E6, E7, and E5 proteins. High-risk E6 and E7 proteins differ from their low-risk counterparts however in being able to drive cell cycle entry in the upper epithelial layers and also to stimulate cell proliferation in the basal and parabasal layers. Deregulated expression of these cell cycle regulators underlies neoplasia and the eventual progression to cancer in individuals who cannot resolve high-risk HPV infection. Most work to date has focused on the study of high-risk HPV types such as HPV 16 and 18, which has led to an understanding of the molecular pathways subverted by these viruses. Such approaches will lead to the development of better strategies for disease treatment, including targeted antivirals and immunotherapeutics. Priorities are now focused toward understanding HPV neoplasias at sites other than the cervix (e.g. tonsils, other transformation zones) and toward understanding the mechanisms by which low-risk HPV types can sometimes give rise to papillomatosis and under certain situations even cancers.

Figure 1

(A) Evolutionary tree showing the proposed appearance of an ancestral “papillomavirus” between the branch point leading to amphibians and reptiles. It is thought that virus/host co‐evolution has occurred during speciation, and that this has led to the widespread distribution of papillomaviruses in organisms as diverse as snakes, birds, and mammals, (B) The human papillomaviruses types found in humans fall into five genera, with the Alpha and the Beta/Gamma genera representing the largest groups. Human papillomaviruses types from the Alpha genus are often classified as low‐risk cutaneous (gray), low‐risk mucosal (orange), or high‐risk (pink). The high‐risk types identified using red text are confirmed as “human carcinogens” on the basis of epidemiological data. The remaining high‐risk types are “probable” or “possible” carcinogens. The evolutionary tree is based on alignment of the E1, E2, L1, and L2 genes 6, (C) Percentage of cervical cancers that are causally attributed to infection with members of the Alpha genus. Members of the Alpha 9 and 7 species have been studied most thoroughly

Figure 2

(A) Typical genome organization of the high‐risk Alpha, Mu, and Beta HPV genomes. Although all share a common genetic organization, the size and position of the major ORFs can vary, with Beta HPV types lacking an E5 ORF. The positions of the major promoters are marked with arrows on the high‐risk Alpha HPV genome map, with early and late polyadenylation sites marked as polyadenylation late and polyadenylation early, (B) Electron micrograph of negatively stained papillomavirus particles. Individual capsomeres within the capsid structure can just be visualized. Papillomavirus particles are approximately 55 nm diameter and are non‐enveloped.

Figure 3

(A) The E6 and E7 proteins of the high‐risk and low‐risk HPV types have different functions, which reflect their different biologies. The ability of the high‐risk HPV types to drive cell division in neoplasia is thought to reflect the ability of their E7 protein to bind and degrade multiple members of the pRb protein family, as well as the ability of E6 to efficiently degrade p53 and to compromise the function of PDZ‐domain proteins that regulate cell contact and signaling pathways, (B) High‐risk HPV infection can lead to a “silent” or asymptomatic infection in which viral genomes persist in the basal layer without the development of disease, or alternatively to the development of a productive lesion such as CIN1 in which viral gene expression is regulated as the infected cells differentiate. In some instances, infection can lead to higher‐grade neoplasia, with deregulated viral gene expression leading to secondary genetic changes in the host cell and possible integration of the viral genome into the cellular chromosome. The deregulated gene expression seen in CIN2 and 3, which are considered to be precancerous lesions, predispose to the development of cancer

Figure 4

High‐risk human papillomaviruses (HPV) infection disrupts the molecular pathways that regulate epithelial differentiation and cell proliferation. Cell cycle progression is regulated in the different epithelial layers by members of the pRb (retinoblastoma) family of proteins. The E7 proteins of high‐risk HPV types can target members of this protein family for degradation (shown in B). This releases members of the E2F transcription factor family, which allows basal and parabasal cells to enter S‐phase. In uninfected epithelium (shown in A), the release of E2F is dependent on external growth factors, which stimulate cyclinD/cdk activity to allow pRb phosphorylation and E2F release. The expression of cellular proteins involved in cell cycle progression is regulated by p16^INK4A, which is involved in a negative feedback loop by suppressing the activity of the cyclinD/cdk. The inability of low‐risk HPV types to drive robust basal cell proliferation is thought to be because these types can only efficiently target the p130 retinoblastoma family member, which controls suprabasal, but not basal cell cycle entry. The high‐risk E7 proteins are thought to target all members of the pRb family. In addition to E7, high‐risk HPVs encode a second protein involved in cell cycle entry. This is the E6 protein, which acts to suppress the rise in p53 that would otherwise occur following E7‐mediated elevation in p14 levels. (shown in B) Elevated p14 leads to inactivation of the MDM protein that is normally involved in degrading p53. High‐risk E6 proteins directly regulate p53 levels in the cell by mediating its ubiquitination and degradation via the proteasome pathway. In uninfected cells (shown in A), p53 levels are maintained at a low level, partly as a result of the normal activity of MDM

Figure 5

Lesion formation is thought to be facilitated by the presence of microwounds, which allows the virus to infect epithelial basal cells (e.g. an epithelial stem cell (1)). At particular sites, such as the squamocolumnar junction of the cervical transformation zone, basal cells, reserve cells, and stem‐like/stem cells are close to the epithelial surface and may be more prone to infection. At other sites, the development of a lesion may be facilitated by a wound repair (2). Once a lesion has become established, basal and parabasal epithelial cells can be driven into the cell cycle, either to mediate basal cell division (i.e. cell proliferation) or to drive cell cycle re‐entry (but not mitosis) in the upper epithelial layers in order to support viral genome amplification (3). Clearance of disease involves activation of a cell‐mediated immune response and a suppression of viral gene expression as activated T‐cells accumulate in the vicinity of the lesion. It is thought that viral genomes can persist in the basal epithelial cells with very limited gene expression, allowing possible reactivation under some circumstances, such as it can occur following immunosuppression 245