By: Dr.Nooria Atta Page 1A significant percentage of human cancers worldwide are associated with infections due to known viruses,including human papillomaviruses (cervical cancer and other skin cancers), human lymphotropic viruses (adult T-cell leukemias and lymphomas in endemic areas), hepatitis B virus (liver cancer), and Epstein-Barr virus (Burkittlymphoma and nasopharyngeal carcinoma).(Arch Pathol Lab Med. 1999;123:1015–1022)Human Papilloma Virus as a carcinogenIntroductionCancer starts in our own cells, but several factors, both intrinsic and external to the body, whichinfluence our daily life, can add to the life time cancer risk. While canceris not infectious, someinfections can act as a stimulant and promoter of cancer. HPVs-Human Papilloma Viruses- area group of more than 100 related viruses (also is one of sexually transmitted infections) .Morethan half of sexually active people are infected with one or more HPV types at some point intheir lives. Recent research indicates that, at any point in time, 42.5 percent of women havegenital HPV infections, whereas less than 7 percent of adults have oral HPV infections1.Cervicalcancer is estimated to affect approximately 500 000 women each year, of whom 80% live indeveloping countries. Virtually all cervical cancer cases result from genital infection with humanpapillomavirus (HPV).3The ability of targeting the retinoblastoma (Rb) family of proteins andp53 and to induce telomerase is some of the critical events that contribute to oncogenicactivities of HPV in development of malignancies.World Health Organization has launched a campaign against cancer, with a three-fold strategy:prevent all the preventable cancers, cure all that can be cured, and reduce pain and discomfortwhere cure is not possible (WHO 1995).The epidemiology of HPV infectionHPV is a group of small DNA viruses that cause warts and certain cancers and precancers of theskin lining the lower genital tract, anus and mouth.Approximately more than 100 types of ithave been fully identified. All differ slightly from each other in their genetic structure.Thisdifference in genetic structure determines the location and the type of lesion that each type is
By: Dr.Nooria Atta Page 2likely to cause.23-30 types infect almost exclusively the skin of the lower genital tract. Theremaining types infect skin on other areas of the body, including the hands, feet, etc2.About half of HPV types belonging to the α-papillomavirus genus have been detected to infectthe epithelium and mucosa lining the anogenital tract. These are collectively known as mucosalHPV types and have been further classified based on their oncogenic potential as low- or high-risk HPV typesLow-risk HPV types (6, 11, 42, 43, 44, 54, 61, 70, 72, and 81) are virtually never found incancers. Therefore, they are also called non-carcinogenic HPV.High-risk HPV types (16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 68, 73, and 82) have beenidentified in cancers of the cervix, vagina, vulva, anus, and penis. Therefore, they are also calledcarcinogenic or oncogenic HPV.Low-risk HPV types (i.e. HPV6 and HPV11) cause common genital warts (≥ 90%), benignhyperproliferative lesions, with very limited tendency to malignant progression. Infection withhigh-risk HPV types is associated with the occurrence of high-grade dysplasias in differentanatomical locations: in the uterine cervix, they are known as high-grade cervical intraepithelialneoplasias (CIN 2 and 3), which are precursors of invasive cervical carcinoma. This is the secondmost common cancer among women worldwide and the leading female malignancy in severaldeveloping countries. The majority of cervical cancers (80%) are caused by just 4 HPV types (16,18, 31, and 45)2.Genital HPV infection is mostly transmitted by genital skin-to-skin contact, but not necessarilyduring sexual intercourse. HPV infection can occur at any age, however studies showed inverserelationship between age and human papillomavirus (HPV) prevalence in many countries. Stillin some of the poorest areas studied HPV prevalence was high across all age groups. In somecountries, cross-sectional and cohort studies have shown a U-shaped curve with a first peak inwomen under 30 years of age and a second peak in women aged 55–64 years.3About 80% ofcancer cases attributable to HPV were in developing countries (Table 1). The highest estimated
By: Dr.Nooria Atta Page 3incidence rates are in sub-Saharan Africa, Melanesia, Latin America and the Caribbean, south-central Asia and south-east Asia3.Table 1. HPV-infection attributable cancer in 2002: developed and developing countriesSiteAttributableto HPV (%)Developed countries Developing countriesTotalcancersAttributableto HPVTotalcancersAttributableto HPVCervix 100 83 400 83 400 409 400 409 400Penis 40 5 200 2 100 21 100 8 400Vulva,vagina40 18 300 7 300 21 700 8 700Anus 90 14 500 13 100 15 900 14 300Mouth > = 3 91 200 2 700 183 100 5 500Oro-pharynx> = 12 24 400 2 900 27 700 3 300Allcancers5 5 016 100 111 500 5 827 500 449 600Adapted from: Parkin et al., with permission from Elsevier Sciences3.HPV, Structure and GenomePapillomaviruses are small, non-enveloped, epitheliotropic, double-stranded DNA viruses thatinfect mucosal and cutaneous epithelia. More than 100 types of human papillomaviruses
By: Dr.Nooria Atta Page 4(HPVs) have beenidentified and approximately half of them infect the genital tract. The HPVviral particles have a length of 52-55nm and consist of a double-stranded DNA molecule ofabout 8000 base-pairs (bp) that is bound to cellular histones and contain a protein capsid4.AllHPV types contain approximately eight Open Reading Frames(ORFs), that are all transcribedfrom a single DNA strand.The genome of all HPV can be divided in three functional regions:Anon-coding region involved in the regulation of both HPV transcription and replication namedupstream regulatory region (URR) or long control region (LCR). The early and late coding regionscarry the early and late viral genes, respectively. Early genes (E1, E2, E4, E5, E6 andE7) regulatethe life cycle and productive phases of viral cycle. The late genes, L1 and L2, codify the majorand minor capsid proteins4, 5.(Fig & Table 2)The E1 and E2 proteins of HPV act as factors that recognize the origin of replication;E2 proteinis also the main regulator of viral gene transcription. E4, despite its name, isbelieved to beinvolved in the late stages of the life cycle of the virus and E5 may functionduring both earlyand late phases. The E6 and E7proteins (most important proteins in carcinogenic activities ofvirus) target a number of negativeregulators of the cell cycle, primarilyp53 and p105Rb,respectively. During the viral lifecycle, E6 and E7 facilitate stable maintenance of viral episomesand stimulate differentiating cells to re-enter the S phase. The L1 and L2 proteins assemble incapsomers, whichform icosahedral capsids around the viral genome during the generation ofprogenyvirions (Fehrmann&Laimins, 2003)4.Life cycle of HPV and Role of HPV in cancerPapillomaviruses are highly epitheliotropic; specifically, they establish productiveinfections onlywithin stratified epithelia of the skin, the anogenital tract and the oral cavity4. The life cycle ofvirus is initiated with infection of basal non differentiated cells through small wounds, known asmicrotraumasin mucosal surface(eg. in cervix) or skin.The viral life cycle is linked to thedifferentiation of the infected epithelial cell. Basal cells comprise the proliferating cellularcomponent of stratified epithelia, in which the viral genome is established. The ability of HPVsto establish their genome in basal cells relies upon the E1 (Hubert &Laimins, 2002), E2(Stubenrauchet al., 1998), E6 (Thomas et al., 1999) and in some cases E7 (Thomas et al., 1999;
By: Dr.Nooria Atta Page 5Floreset al., 2000) genes. The viral genome is replicated to a copy number of about 100 andmaintained for varyingperiods of time at this low copy number within the initially infected, butstill replicating cells.Normally, when basal cells undergo cell division, the daughter cellthat loses contact with thebasement membrane and migrates into the suprabasal layer initiates a programme of terminaldifferentiation4. However, HPV infected basal cell continue to divide and each form twodaughtercells, one of them remain in place and acting as repository for replication of the virus,which needs active cell division to maintain its life cycle. The other daughter cell continuesupward through suprabasal layer, where it differentiates and finally shads from epithelialsurface6. (Fig.2)Within this suprabasal compartment, cells support the amplification of the viralgenome, expression of capsid genes and assembly of progeny virus .Encapsidation of HPV DNAwithin capsids to generate progeny virus within the terminally differentiated cell compartmentis quantitatively dependent upon L2 ( the minor capsid protein). L2 is also needed forlocalization of viral DNA within the nucleus (Day et al., 2004)4. Since HPV rely on the cellularmachinery to replicate their genome, these viruses have to induce de novo DNA synthesis inotherwise quiescent cells. This is achieved by the concerted action of viral proteins expressedfrom specific HPV early genes.High-risk HPV types express two proteins, E6 and E7, with proved oncogenic potential. Theseare the two viral products always present in HPV-associated cervical tumor samples and derivedcell lines. Besides, their combined expression is sufficient to efficiently immortalize primaryhuman keratinocyte7. E6 is known for promoting the degradation of the tumor suppressor p53and activating telomerase expression and activity, whereas the best-described function ofoncogenic E7 is the binding and inactivation of members of the retinoblastoma protein (pRb)family.These interactions are considered central to the transforming capacity of these proteins.Besides, molecular studies show that these proteins operate by targeting a growing number ofother cellular factors.These operations affect keratinocytes transcription and differentiation,activate telomerase and inhibit apoptosis, extending the life span of the infected cell andallowing maximal HPV amplification.
By: Dr.Nooria Atta Page 6Figure 1. Schematic representation of abnormal epithelial differentiation induced by HPV infectionSimilarly, the interaction of E7 with several cell cycle regulators including cyclins, cyclin-dependent kinases, CDKs-inhibitors, histone deacetylase, AP1, E2F1 and E2F6 favors cell cycleprogression. Therefore, infected cells are capable of reentering S phase and sustain DNAsynthesis to serve the replicative interests of the virus6, 7.The high-risk HPV E6 and E7 oncoproteins also play a key role in the deregulation of innateimmunity by interfering with the expression of Toll-like receptors. These receptors recognizepathogen-associated molecular patterns to activate antigen-presenting cells and phagocytes,enhancing innate and adaptive responses against pathogens. Specifically, Toll-like receptors 9transcription is inhibited in cells expressing HPV16 E6 and E7. Impairment of these responses isconsidered an important step in HPV carcinogenesis. Furthermore, cervical cancer progressionhas been recently associated to upregulation of matrix metalloproteinases, which play asignificant role in tumor invasion and metastases in different cancers. Finally, multiple factorshave been reported that may have role in the fate of HPV-infected cells by interfering inperformance of the antiviral mechanisms of the host favoring HPV persistence and progressionof neoplasia7.
By: Dr.Nooria Atta Page 7Specifically, E6 and E7 are the HPV proteins associated with cancer. As previously mentionedthe HPV genome is composed of six early (E1, E2, E4, E5, E6, and E7) genes, two late (L1 and L2)genes, and a non-coding long control region (LCR). After the host cell is infected, first of all E1and E2 are expressed. High E2 levels before expression of the E6 and E7 oncogenic proteinshelps virus to cell DNA integration. After integration of the host and HPV genomes, E2 functiondecreases resulting in expression of E6/E7 genes.E6 is a 151 amino-acid peptide that is of particular interest because it appears to have multipleroles in granting immortality of the cell. To do so E6 mediates the degradation of p53, amajor tumor suppressor protein, reducing the cells ability to respond to DNA damage4. E6-associated protein concomitants to p53, leading toinhibition of the transcriptional activity ofp53 and the abrogation of p53-induced apoptosis, including apoptosis induced by E7 throughthe destabilization of p105Rb 4. In addition E6 binds to numerous other cellular proteins thatcan be divided into four broad classes:transcriptional co-activatorsproteins involved in cell polarity and motilitytumour suppressorsand inducers of apoptosisandDNA replication and repair factorsProteins that belong to the first class are p300 ,myc and interferon regulatory factor 3. Thosethat belong to the second are paxillin, the mammalian homologue of Drosophila discs largetumor-suppressor gene product, membrane-associated guanylate kinase with invertedorientation (MAGI-1) and multiple PD2 protein 1 (MUPP1); those that belong primarily to thethird group are p53 and Bak, and those belong to the fourth class are mcm7, XRCC1 and O6-methylguanine–DNA methyltransferase. Additional proteins that interact with E6 have beendescribed by Mantovani and Banks (2001)4. E6 has also been shown to target other cellularproteins, thereby altering several metabolic pathways. One of them is NFX1-91, which normallyrepresses production of telomerase (a protein that allows cells to divide an unlimited number
By: Dr.Nooria Atta Page 8of times). When NFX1-91 activity is altered by E6, telomerase levels increase leading toinactivating a major mechanism of cell growth and proliferation control4, 6.Through the interactions described above, E6 can affect transcriptional pathways, disrupt celladhesion and architecture, inhibit apoptosis, inhibite response to DNA damage, induce genomeinstability and immortalize cells.In most papillomavirus types, the primary function of the E7 protein is to inactivate members ofthepRb family of tumor suppressor proteins. Association of E7 with p105Rb causes itsdegradation and leads to the loss of p105Rb control over E2F transcription factors. In additionto binding p105Rb, E7 can bind to p107 and p130, two other members of the family of pocketproteins. The E7 interactions with above mentioned pocket proteins lead to cell immortality aswell as abrogation of normal responses to DNA damage. On the other hands,E7 complexeswith cyclinsand inactivates the cyclin-associated kinase inhibitors p21cip1 and p27kip1.Interaction with above mentioned negative cell cycle regulators leads to unscheduled cellproliferation. Another important aspect of the biology of E7, independent from p105Rb binding,is its ability to destabilize centrosomes, which causes mitotic defects and genomeinstability.These interactions contribute to the interference ( and disturbance) of E7 intranscription and signal transduction pathways and in DNA repair. A 2010 study has found thatE6 and E7 are involved in beta-catenin nuclear accumulation and activation of Wnt signaling inHPV-induced cancers4, 6.In the upper layers of the host epithelium, the late genes L1 and L2 are transcribed/translatedand serve as structural proteins that encapsidate the amplified viral genomes. Once thegenome is encapsidated, the capsid appears to undergo a redox-dependentassembly/maturation event, which is tied to a natural redox gradient that spans bothsuprabasal and cornified epithelial tissue layers. This assembly/maturation event stabilizesvirions, and increases their specific infectivity. Virions can then be sloughed off in thedead squames of the host epithelium and the viral lifecycle continues4.
By: Dr.Nooria Atta Page 9Fig &Table2: HPV-16 genome and transforming activity by the genes.Riemer A B et al. J. Biol. Chem. 2010;285:29608-29622ConclusionHPV is a group of small DNA viruses that cause warts and certain cancers and precancers of theskin lining the lower genital tract, anus and mouth. More than 100 types of it have been fullyidentified.Some HPV types are the causative agent of warts, while others have been shown tobe the causative agent of cervical cancer.HPV has also been implicated in other cancers such as oral and skin cancer. Those HPV typesthat are associated with cancer are referred to as High-risk, while those the cause benignwarts are Low-risk.The virus can be transmitted through skin to skin contact, and enters the body via minorabrasions. Those types that infect the genital region (approximately 30) can be transmittedthrough sexual activity - in fact, HPV is believed to be the most common sexually transmittedvirus.The ability of targeting the retinoblastoma (Rb) family of proteins and p53 and to inducetelomerase is some of the critical events that contribute to oncogenic activities of HPV indevelopment of malignancies.
By: Dr.Nooria Atta Page 10References:1. National cancer institute (http://www.cancer.gov/cancertopics/factsheet/Risk/HPV)2. American Society for Colposcopy and Cervical Pathology. Colposcopy:ColposcopicAppearance of High-Grade Lesions . Hagerstown, MD: American Society forColposcopy and Cervical Pathology. Retrieved January 4, 2012.3. WHO | Human papillomavirus and HPV vaccines: a review4. IARC MONOGRAPHS VOLUME 90, Human Papilloma Virus: (p45-75).5. Nubia Mu˜noz, Xavier Castellsagu´e, Amy Berrington de Gonz´alez, Lutz Gissmann, ScienceDirect, Elsevier,:(ch1, S3”1-10”)6. Cancer Biology & therapy 11:3,295-306; Feb1,2011,( Molecular pathogenesis of cervical cancer.)7. Oxford JournalsLife Sciences & MedicineCarcinogenesisVolume 31, Issue 11 (The role ofinflammation in HPV carcinogenesis)