Human Papillomavirus: E6 And E7 Oncogenes
The International Journal of Biochemistry & Cell Biology 39 (2007) 2006–2011Pathogens in focusHuman papillomavirus: E6 and E7 oncogenesGaëlle Boulet a,1 , Caroline Horvath a, ,1 , Davy Vanden Broeck b ,Shaira Sahebali a , Johannes Bogers aaAMBIOR, Laboratory for Cell Biology & Histology, University of Antwerp, Groenenborgerlaan 171, BE-2020 Antwerp, Belgiumb International Centre for Reproductive Health (ICRH), Ghent University, De Pintelaan 185, 9000 Ghent, BelgiumReceived 8 March 2007; received in revised form 6 July 2007; accepted 6 July 2007Available online 19 July 2007AbstractThe recognition of a causal relationship between human papillomaviruses and cancer almost 30 years ago led to a rapid expansionof knowledge in the field, resulting in the description of the main mediators of HPV-induced carcinogenesis, the viral proteins E6and E7. These oncoproteins show a remarkable pleiotropism in binding host-cell proteins, with the tumour suppressor genes p53and pRb as their major targets. These interactions induce proliferation, immortalization and malignant transformation of infectedcells.The link between HPV and cervical cancer led to the development of molecular methods, often based on the detection of E6 andE7, for screening and diagnosis. Therapeutic vaccines and gene therapy are primarily directed at E6 and E7. Although prophylacticvaccines are available, further understanding of the viral life cycle and the mechanisms underlying HPV-induced oncogenesis isnecessary to face the many challenges in the field of HPV and cancer. 2007 Elsevier Ltd. All rights reserved.Keywords: Human papillomavirus; E6 and E7 oncoproteins; Cancer1. IntroductionHuman papillomaviruses (HPVs) are small circulardouble-stranded DNA viruses that belong to the Papovaviridae family. The causal role of HPV in cancersof the uterine cervix has been firmly established biologically and epidemiologically. Most cancers of thevagina and anus are equally caused by HPV, as area fraction of cancers of the vulva, penis and oropharynx. Other malignancies causally linked to HPV arenon-melanoma skin cancer and cancer of the conjunctiva (Muñoz, Castellsagué, Berrington de González, &Gissman, 2006). 1Corresponding author. Tel.: 32 3 265 3300; fax: 32 3 265 3301.E-mail address: caroline.horvath@ua.ac.be (C. Horvath).Both the authors contributed equally to this work.1357-2725/ – see front matter 2007 Elsevier Ltd. All rights viruses were first described in 1907 byGiuseppe Ciuffo, but HPV remained largely unstudied until the advent of molecular virology in the 1970s(Ciuffo, 1907; Meisels & Fortin, 1976; Zur Hausen,1976). Of the more than 100 different HPV types identified, 40 are known to infect the genital tract (Woodman,Collins, & Young, 2007). These mucosal HPV types areclassified as “low-risk” and “high-risk” types based onthe prevalence ratio in cervical cancer and its precursors.Low-risk HPV types, such as 6 and 11, induce benignlesions with minimum risk of progression to malignancy.By contrast, high-risk HPVs have higher oncogenicpotential (Fehrmann & Laimins, 2003). Approximately99% of cervical cancers contain HPV DNA of highrisk types, with type HPV16 being the most prevalent,followed by types 18, 31, 33 and 45. Cervical HPV infection is one of the most common sexually transmitted
G. Boulet et al. / The International Journal of Biochemistry & Cell Biology 39 (2007) 2006–2011infections (Walboomers et al., 1999; Woodman et al.,2007).The discovery of the relationship between persistenthigh-risk HPV infection and cervical cancer initiateda rapid expansion of the field, which resulted in thedescription of the function of HPV E6 and E7 oncogenesin cervical carcinogenesis (Woodman et al., 2007).2. Overview of pathogenesisThe HPV genome is approximately 8000 bp in lengthand encodes eight open reading frames (ORFs), whichare transcribed as polycistronic mRNAs (Hebner &Laimins, 2006).Regulatory sequences required for viral replicationand transcription are concentrated in a non-coding regiontermed “upstream regulatory region” (URR) or “longcontrol region” (LCR). The gene products can be dividedinto “early” (E) and “late” (L) proteins depending on thetime of expression during the viral life cycle (Fehrmann& Laimins, 2003). The properties of these proteins aresummarized in Table 1.HPV is perfectly adapted to its natural host tissue,the differentiating epithelial cells of skin or mucosae. Itslife cycle is directly linked to epithelial cell differentiation and initiated when infectious particles enter cellsin the basal layer. Following entry, HPV genomes areestablished as episomes, which replicate in synchronywith the cellular DNA. Following cell division, daughter cells migrate towards the suprabasal compartment,where uninfected keratinocytes initiate terminal differentiation, while HPV-infected cells enter the S-phase ofthe cell cycle resulting in amplification of viral replicates (Muñoz et al., 2006). The subsequent release ofinfectious virions in the environment is facilitated bydisintegration of the epithelial cells occurring as a resultof natural turnover (Fehrmann & Laimins, 2003).The critical molecules in viral replication are E6and E7, which functionally inactivate the products of2007two important tumour suppressor genes, p53 and pRb,respectively. Both oncoproteins induce proliferation,immortalization and malignant transformation of theinfected cells.The differences between the E6 and E7 proteins ofhigh- and low-risk HPV types seem often of a morequantitative rather than a qualitative nature (Muñoz etal., 2006). E6 acts as repressor of apoptosis and mediatessurvival of severely damaged cells, while E7 functionsas promoter for replication and cell growth. Both canindependently immortalize human cells, but their jointfunction gives rise to an interesting complementary andsynergistic effect, inducing a marked increase in transforming activity (Fig. 1) (Zur Hausen, 2000).The physical state of HPV DNA within the cellhas been shown to predict the pathological courseof HPV infections. An important difference betweenhigh- and low-risk HPV types is integration in the hostgenome in high-grade lesions and cancer (Scheurer,Tortolero-Luna, & Adler-Storthz, 2005). High-risk HPVtypes show more tendency towards genomic integration,whereas low-risk types are preferentially maintained asextrachromosomal circular episomes (Arends, Buckley,& Wells, 1998). Integration occurs downstream of theearly genes E6 and E7, often in the E2 region. This resultsin an increased E6 and E7 expression due to loss of negative feedback control by the viral regulatory E2 protein.Moreover, integrant-derived transcripts are more stablethan those originating from episomal viral DNA. Thesubsequent increase in transforming E6 and E7 proteinsresults in a growth advantage for cells with integrantsover cells with episomal viral genes (Woodman et al.,2007).3. Structure of E6 and E7The E6 and E7 proteins of high-risk types are themain mediators of carcinogenesis due to their interactions with various cellular targets. E6 is one of the firstTable 1Overview of HPV gene productsGene productDescriptionE1E2E4E5E6E7Helicase function; essential for viral replication and control of gene transcriptionViral transcription factor; essential for viral replication and control of gene transcriptionInteraction with cytoskeleton proteins; viral assemblyGrowth stimulation by interaction with growth factor receptors; downregulation surface HLA class I moleculesCell immortalization; p53-degradation; telomerase activation; anti-apoptotic effect; induction of genomic instabilityCell immortalization; interaction with pRb and pRb-associated pocket proteins; transactivation of E2F-dependentpromoters; induction of genomic instabilityMajor capsid proteinMinor capsid protein; role in recruiting viral genomes for encapsidation; involvement in nuclear transport of viral DNAL1L2(E) early and (L) late.
2008G. Boulet et al. / The International Journal of Biochemistry & Cell Biology 39 (2007) 2006–2011Fig. 1. Cellular interactions of E6 and E7 oncoproteins and their synergy in induction of cell immortalization. E6 activates telomerase and SRCkinases, and inhibits p53 and BAK. E7 inhibits pRb, with consequent release of E2F and upregulation of p16, which is inactivated by E7. In addition,E7 stimulates cyclins A and E, inactivates CKIs p21 and p27 and induces centriole amplification. E6 and E7 synergize in cell immortalization (dottedlines); E6 prevents apoptosis induced by high E2F levels, while E7 shields E6 from inhibition by p16.genes expressed during HPV infection (Fehrmann &Laimins, 2003). The E6 ORF encodes a small protein ofapproximately 150 amino acids with a molecular weightof 16–18 kD. E6 proteins contain four C-x-x-C motifs,which are important in functions, such as transcriptionalactivation, transformation, immortalization and association with cellular proteins.In the case of high-risk mucosal HPV types, theE6 COOH-terminal region contains a PSD-95/Dlg/ZO1(PDZ) domain, involved in the interaction with severalPDZ domain-containing proteins (Fig. 2A) (Hebner &Laimins, 2006).The E7 oncogene encodes a low molecular weightprotein of approximately 100 amino acids. E7 proteins incorporate three conserved regions (CR), theNH2 -terminal CR1 domain, the CR2 region andthe COOH-terminal CR3 domain (Fig. 2B). TheNH2 -terminal CR1 domain is necessary for cellular transformation and pRb degradation, but does notdirectly contribute to pRb binding. The CR2 region contains a conserved pRb-binding core sequence LxCxEand a casein kinase II phosphorylation site (CKII). TheCOOH-terminal domain consists of two C-x-x-C motifsseparated by a 29/30-amino acid spacer. This region isimplicated in the association of pRb and other host cellular proteins, in metal binding and may function as adimerization domain (Hebner & Laimins, 2006; Müngeret al., 2004).Fig. 2. Structure of E6 and E7 oncoproteins. (A) Schematic representation of the E6 oncoprotein. The protein of approximately 150 aminoacids contains four C-x-x-C motifs (blue), which are important infunctions, such as transcriptional activation, transformation, immortalization and association with cellular proteins. The E6 COOH-terminuscontains a PDZ-binding motif (green) involved in the interaction withseveral PDZ domain-containing proteins. (B) Schematic representation of the E7 oncoprotein. This predominantly nuclear polypeptide ofapproximately 100 amino acids incorporates three conserved regions(CR). The COOH-terminal CR3 domain contains two copies of theC-x-x-C motif related to the E6 sequences (blue). This region is implicated in association of pRb and other host cellular proteins, in metalbinding and may function as a dimerization domain. The CR2 domain(red) contains the pRb-binding core sequence LxCxE and a phosphorylation site for casein kinase II (CKII). The NH2 -terminal CR1 domainis necessary for cellular transformation and pRb degradation, but doesnot directly contribute to pRb-binding (light blue).
G. Boulet et al. / The International Journal of Biochemistry & Cell Biology 39 (2007) 2006–201120094. Biological functionTable 2Cellular binding partners for E64.1. High-risk HPV E6 proteinBinding partnerConsequence of interactionBakBaxCBP/p300Anti-apoptotic effectAnti-apoptotic effectDownregulation of p53-dependent transcriptionc-mycPrevention of myc-induced apoptosisIncrease hTert transcription and telomeraseactivityE6APDeregulation signal transduction in proliferatingcellsEssential factor E6 degradation actionsE6TP1Inhibition Rap-mediated signallingE6BPInhibition of terminal cell differentiationp53-Independent anti-apoptotic effecthDlgDeregulation of cell cycleLoss of cell fluence on cell adhesion and polarityDecrease of interferon- transcriptionp53-Independent anti-apoptotic effectOverriding early G1-phase arrest pointDisruption of actin cytoskeleton and cell matrixinteractionsp53Deregulation of cell cycleAnti-apoptotic effectXRCC1Interference with DNA repair efficiencyThe most important function of high-risk E6 is binding of the tumour suppressor p53, a DNA-binding proteinexpressed in response to DNA damage or unscheduledinduction of DNA replication, resulting in cell cyclearrest or apoptosis. Since HPV depends on the cellularDNA synthesis machinery and must stimulate S-phaseprogression for the replication of its genome, overexpression of p53 inhibits viral replication (Fehrmann &Laimins, 2003). E6 binds the p53 protein through a cellular ubiquitin-ligase, the E6-associated protein (E6-AP),which recruits the ubiquitin complex of enzymes, ubiquitinating lysines on p53 and initiating its proteolysis.Degradation of p53 bypasses the normal growth arrestsignals at the G1/S and G2/M checkpoints and is theprime cause of chromosomal instability, with mutationalconsequences for HPV-positive cells and enhancementof integration of foreign DNA into the host-cell genome(Thomas, Pim, & Banks, 1999). Furthermore, E6 oncoproteins inhibit degradation of SRC-family kinases byE6-AP, stimulating mitotic activity (Zur Hausen, 2002).As well as their effects on p53 protein, high-risk E6proteins activate telomerase, an enzyme responsible forreplicating telomeric DNA at the ends of chromosomes.In normal somatic cells, telomerase activity is absent andtelomeres shorten through successive cell divisions, initiating the natural pathway that leads to senescence andcell death. E6 can upregulate telomerase activity by transcriptional activation of the human telomerase reversetranscriptase (hTert) gene, encoding the telomerase catalytic subunit (Fehrmann & Laimins, 2003).High-risk E6 proteins also interact with several PDZdomain-containing substrates, inducing their ubiquitinmediated degradation. These substrates are implicatedin the control of cell proliferation, cell polarity andadhesion, which further supports that E6 proteins contribute to HPV-induced malignancy (Massimi, Gammoh,Thomas, & Banks, 2004).E6 shows a remarkable pleiotropism in binding otherhost-cell proteins, leading to substantial functional consequences for E6-expressing cells, although they are notfully understood at present (Table 2) (Fig. 1).4.2. High-risk HPV E7 proteinE7 proteins are primarily localized in the nucleus,where they associate with retinoblastoma gene product (pRb) to facilitate progression into the S-phase ofthe cell cycle (Zur Hausen, 2002). In normal cells, pRbCBP, CREB-binding protein; E6-AP, E6-associated protein; E6TP1,E6-targeted protein; E6BP, E6-binding protein; hDlg, human homologue of the Drosophila disc large tumour-suppressor protein; hScrib,human homologue of the Drosophila Scribble tumour-suppressor protein; IRF3, interferon regulatory factor-3; MAGI, membrane associatedguanylate kinase with inverted structure domain; Mcm7, minichromosome maintenance protein 7.is hypophosphorylated in early G1 and bound to E2Ftranscription factors, forming complexes that functionas transcriptional repressors. Upon phosphorylation thecomplexes dissociate, allowing E2F to act as a transcriptional activator. By associating with hypophosphorylatedpRb, E7 prevents its binding with E2F, thereby promoting cell cycle progression. Additionally, the E7-inducedubiquitin-mediated pRb degradation appears to be essential in efficiently overcoming cell cycle arrest (Fehrmann& Laimins, 2003). Besides pRb, E7 interacts with twoother members of the pRb family, p107 and p130, whichalso negatively regulate E2F transcription.In addition, high-risk E7 stimulates the S-phasegenes cyclin E and cyclin A, interacts with cyclin-kinasecomplexes and abrogates the inhibitory activities ofcyclin-dependent kinase inhibitors (CKIs), such asp21CIP-1/WAF-1 and p27KIP-1 . These interactions are amajor factor in growth stimulation of HPV-infectedcells, uncoupling cyclin-dependent kinase activity from
2010G. Boulet et al. / The International Journal of Biochemistry & Cell Biology 39 (2007) 2006–2011Table 3Cellular binding partners for E7Binding partnerConsequence of interactionAP1 family membersAbrogation of IRF-1transcriptional activityEnhancement of c-myc-inducedtranscriptionActivation of cylins A/EInterference with G2/M cellcycle transitionc-mycCyclin A/E complexesHistone H1 kinaseIGFBP3Anti-apoptotic effectStimulation of cell proliferationMi2 Abrogation of IRF-1transcriptional activityE2F release with subsequentactivation of S-phase genesDeregulation of cell cycleActivation of genes for cellcycle progressionGrowth stimulation viaderegulation of cell cycleGrowth stimulation viaderegulation of cell cycleInhibition of interferon signalpathwayspRb degradation via proteasomeInterference with initiation oftranscriptionpRbpRb-associated pocket t4 ATPaseTBPAP1, activator protein 1; IRF-1, interferon regulatory factor-1;IGFBP3, insulin-growth factor-binding protein 3; TBP, TATA boxbinding protein.CKIs and interfering with the ability of p53 to induceG1 growth arrest following DNA damage (Zur Hausen,2000).Another consequence of high-risk E7 expression isthe induction of genomic instability. By inducing centriole amplification, E7 also induces aneuploidy of theE7-expressing cell, which contributes to tumorigenesis(Zur Hausen, 2002). Table 3 shows a variety of othercellular proteins that associate with E7 (Fig. 1).E6 and E7 are independently able to immortalizehuman cells, but their combined expression leads toa complementary and synergistic effect, which in turnis responsible for increased transforming efficiency.Whereas CKI INK4A counteracts the functions of E6,E7 bypasses this inhibition by directly activating cyclinsA and E. E6, in turn, prevents E7-induced apoptosisby degrading apoptosis-inducing proteins (Fehrmann &Laimins, 2003).5. Clinical implicationsThe identification of specific HPV types as causativeagents of cervical cancer and its precursor lesions ledto the development of molecular methods in screening and diagnosis. Accurate diagnosis of HPV infectionrelies on the detection of viral nucleic acids (Molijn,Kleter, Quint, & van Doorn, 2005). Presence of high-riskHPV DNA identifies women at particular risk of progression to cervical cancer. Polymerase chain reaction(PCR) is a widely used, highly sensitive method to detectHPV DNA. One approach is based on consensus primersthat target conserved regions within the L1/E1 regionand can therefore amplify a broad spectrum of HPVtypes. However, type-specific PCRs with primers thatbind to E6 and E7 are potentially more reliable becausethe L1/E1 region can be disrupted during viral integration and because the E6 and E7 nucleotide sequenceexhibits less variation (Morris, 2005). Nevertheless, thehigh prevalence of transient infections makes viral detection an incomplete means of identifying women at risk.HPV load and integration status have been suggested astype-dependent risk markers for cervical neoplastic progression. Quantitative real-time PCR methods targetingE6 or E7 allow the accurate determination of viral loadand screening for integration status based on the E2/E6ratio (Woodman et al., 2007).Another application of the strong etiological relation between HPV and cervical cancer is the preventionor treatment of this disease and other HPV-associatedmalignancies by HPV vaccines (Mahdavi & Monk,2005; Roden & Wu, 2006; Stanley, 2006). Prophylactic HPV vaccines prevent infections through inductionof capsid-specific neutralizing antibodies. Since themain tumorigenic effects of high-risk HPVs have beenattributed to E6 and E7, these oncoproteins are targets fortherapeutic vaccines that are presently in development,administering E6/E7 either in live vectors, as peptides,protein or nucleic acid form, or in cell-based vaccines(Mahdavi & Monk, 2005).E6 and E7 are also the primary targets in the development of gene therapy for HPV infections (Shillitoe,2006). Targe
E7 stimulates cyclins A and E, inactivates CKIs p21 and p27 and induces centriole amplification. E6 and E7 synergize in cell immortalization (dotted lines); E6 prevents apoptosis induced by high E2F levels, while E7 shields E6 from inhibition by p16. genes expressed during HPV infection (Fehr
Dec 21, 2020 · Pathologists should perform high-risk human papillomavirus (HR-HPV) testing on all patients with newly diagnosed oropharyngeal squamous cell carcinoma (OPSCC),. This testing may be performed on the primary tumor or on a regional lymph node metastasis when the clinical findings are consistent
examine cottontail rabbit and detached papillomavirus from warts (Shope & Hurst, 1933). As the result of this examination different types of papillomaviruses were isolated both from plants and animals. Infection in humans is caused by a group of Papillomavirus called Human papillomaviruses.
Bovine papillomavirus (BPV) is the etiological agent of bovine . sequences, transcripts, proteins and virus-like particles in sites before not recognized as permissive to productive infection due to the absence of cell differentiation, including peripheral blood [51,52] and placenta [53]. Similar results have been also described in humans, in .
05.1 Service Médical Rendu GARDASIL est un vaccin contre les papillomavirus humains 6, 11, 16 et 18 pour : - la prévention des lésions génitales précancéreuses (du col de l'utérus, de la vulve et du vagin) et du cancer du col de l'utérus dus à certains types oncogènes de Papillomavirus Humains
HPV Human papillomavirus HR High-risk HR Human Resources HSRC Human Sciences Research Council HSV Herpes simplex virus HUU HIV-unexposed and uninfected HVTN HIV Vaccine Trials Network IAEA International Atomic Energy Agency IANPHI International Association of National Public Health Institutes IARC International Agency for Research on Cancer IBBS Integrated HIV Bio-Behavioral Surveillance
AIDS, Viral Hepatitis, STD and TB Prevention (proposed), Kevin Fenton, and 11) can cause benign or low-grade cervical cell changes, MD, Director, and the Division of STD Prevention, John Douglas, MD, genital warts, and recurrent respiratory papillomatosis. High-Director. ris
needs. Recently, Dr. Hostetler and his team (Dr. James Beadle and Dr. Nadeida Valiaaeva) synthesized and evaluated a new family of anti-viral compounds. One of these compounds is an antiviral drug which NIH considers to be “highly active” against the human papillomavirus (HPV), including HPV-16 and HPV-18. These subtypes are high-
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