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1、 Academic Press,2000.,Pathogenesis,Part 1(click on this link),Academic Press,2000.,Cell Transformation by Viruses,Transformation is a change in the morphological,biochemical,or growth parameters of a cell.Transformation may or may not result in cells able to produce tumours in experimental animals,w
2、hich is properly known as neoplastic transformation.Therefore,transformed cells do not automatically result in the development of cancer.Carcinogenesis(or more properly,oncogenesis)is a complex,multi-step process in which cellular transformation may be only the first,although essential,step along th
3、e way.,Academic Press,2000.,Cell Transformation by Viruses,Transformed cells have an altered phenotype,which is displayed as one(or more)of the following characteristics:Loss of anchorage dependenceLoss of contact inhibitionColony formation in semi-solid mediaDecreased requirements for growth factor
4、s,Academic Press,2000.,Cell Transformation by Viruses,Cell transformation is a single-hit process,i.e.a single virus transforms a single cell(c.f.oncogenesis,i.e.the formation of tumours,which is a multi-step process).All or part of the virus genome persists in the transformed cell&is usually(but no
5、t always)integrated into the host cell chromatin.Transformation is usually accompanied by continued expression of a limited repertoire of virus genes,or rarely by productive infection.Virus genomes found in transformed cells are frequently replication-defective&contain substantial deletions.Transfor
6、mation is mediated by proteins encoded by oncogenes.Cell-transforming viruses may have RNA or DNA genomes,but all have at least a DNA stage in their replication cycle,i.e.the only RNA viruses directly capable of cell transformation are the retroviruses.,Academic Press,2000.,Oncogenes,Academic Press,
7、2000.,Oncogenes,Certain retroviruses carry homologues of c-oncs derived originally from the cellular genes&known as v-oncs.In contrast,the oncogenes of cell-transforming DNA viruses are unique to the virus genome-there are no homologous sequences present in normal cells.Genes involved in the formati
8、on of tumours can be grouped by their biochemical functions:Oncogenes&proto-oncogenes:oncogenes are mutated forms of proto-oncogenes,cellular genes whose normal functions is to promote the normal growth&division of cells.Tumour suppressor genes:genes which normally function to inhibit the cell cycle
9、&cell division.DNA repair genes:genes which ensure each strand of genetic information is accurately copied during cell division of the cell cycle.Mutations in these genes lead to an increase in the frequency of other mutations,e.g.in conditions such as ataxia-telangiectasia&xeroderma pigmentosum.,Ac
10、ademic Press,2000.,Oncogenes,Academic Press,2000.,Cell Transformation by Retroviruses,Not all retroviruses are capable of transforming cells,for example,lentiviruses such as HIV do not transform cells,although they are cytopathic.The retroviruses which can transform cells fall into three groups:tran
11、sducing,cis-activating&trans-activating.If oncogenes are present in all cells,why does transformation occur as a result of virus infection?The reason is that oncogenes may become activated in one of two ways,either by subtle changes to the normal structure of the gene,or by interruption of the norma
12、l control of expression.,Academic Press,2000.,Cell Transformation by Retroviruses,The transforming genes of the acutely transforming retroviruses(v-oncs)are derived from&are highly homologous to c-oncs&are believed to have been transduced by viruses.Most v-oncs possess slight alterations from their
13、c-onc progenitors-many contain minor sequence alterations which alter the structure&the function of the oncoprotein produced,others contain short deletions of part of the gene.Most oncoproteins from replication-defective,acutely transforming retroviruses are fusion proteins,containing additional seq
14、uences derived from virus genes,most commonly virus gag sequences at the amino terminus of the protein.These additional sequences may alter the function or the cellular localization of the protein&these abnormal attributes result in transformation.,Academic Press,2000.,Cell Transformation by Retrovi
15、ruses,Chronic transforming retrovirus genomes do not contain oncogenes-they activate c-oncs by a mechanism known as insertional activation.A provirus which integrates into the host cell genome close to a c-onc sequence may indirectly activate the expression of the gene in a way analogous to that in
16、which v-oncs have been activated by transduction.This can occur if the provirus is integrated upstream of the c-onc gene,which might be expressed via a read-through transcript of the virus genome plus downstream sequences.Insertional activation can also occur when a provirus integrates downstream of
17、 a c-onc sequence or upstream but in an inverted orientation.Activation results from enhancer elements in the virus promoter in mice,mouse mammary tumour virus(MMTV)insertion activates the int gene.,Academic Press,2000.,Insertional Activation,Academic Press,2000.,Trans-Activating Retroviruses,Human
18、T-cell leukaemia virus(HTLV)&related animal viruses encode a transcriptional activator protein in the virus tax gene.The tax protein acts in trans to stimulate transcription from the virus LTR.It is believed that the protein also activates transcription of many cellular genes by interacting with tra
19、nscription factors.However,HTLV oncogenesis,i.e.the formation of a leukaemic tumour,has a latent period of some 20-30 years.Therefore,cell transformation(which can be mimicked in vitro)&tumour formation(which cannot)are not one&the same-additional events are required for the development of leukaemia
20、.It is thought that chromosomal abnormalities which may occur in the population of HTLV-transformed cells are also required to produce a malignant tumour,although this is not completely understood.,Academic Press,2000.,Cell Transformation by DNA Viruses,In contrast to the oncogenes of retroviruses,t
21、he transforming genes of DNA tumour viruses have no cellular counterparts.Several families of DNA viruses are capable of transforming cells.In general terms,the functions of their oncoproteins are much less diverse than those encoded by retroviruses.They are mostly nuclear proteins involved in the c
22、ontrol of DNA replication which directly affect the cell cycle.They achieve their effects by interacting with cellular proteins which normally appear to have a negative regulatory role in cell proliferation.Two of the most important cellular proteins involved are known as p53&Rb.,Academic Press,2000
23、.,p53,p53 forms complexes with SV40 T-antigen&other DNA virus oncoproteins,including those of adenoviruses&papillomaviruses.The gene encoding p53 is mutated or altered in the majority of tumours,implying that loss of the normal gene product is associated with the emergence of malignantly transformed
24、 cells.p53 plays a central role in controlling the cell cycle.It is believed to be a tumour suppressor or anti-oncogene&has been called the guardian of the genome.p53 is a transcription factor which activates the expression of certain cellular genes,causing the cell cycle to arrest at the G1 phase.S
25、ince these viruses require ongoing cellular DNA replication for their own propagation,this explains why their transforming proteins target p53.,Academic Press,2000.,The Cell Cycle,Academic Press,2000.,Rb,Rb was discovered when it was noticed that the gene which encodes this protein is always damaged
26、 or deleted in a tumour of the optic nerve known as retinoblastoma.Therefore,the normal function of this gene is also thought to be that of a tumour suppressor.The Rb protein forms complexes with a transcription factor called E2F.This factor is required for the transcription of adenovirus genes,but
27、E2F is also involved in the transcription of cellular genes which drive quiescent cells into S phase.The formation of inactive E2F-Rb complexes thus has the same overall effect as the action of p53-arrest of the cell cycle at G1.Release of E2F by replacement of E2F-Rb complexes with E1A-Rb,T-antigen
28、-RB or E7-RB complexes therefore stimulates cellular&virus DNA replication.,Academic Press,2000.,SV40 T-Antigen,SV40 T-antigen is one of the known virus proteins which binds p53.Infection of cells by SV40 or other polyomaviruses can result in two possible outcomes:Productive(lytic)infectionNon-produ
29、ctive(abortive)infectionThe outcome of infection appears to be determined primarily by the cell type infected;e.g.mouse polyomavirus establishes a lytic infection of mouse cells but an abortive infection of rat or hamster cells,while SV40 shows lytic infection of monkey cells but abortive infection
30、of mouse cells.In addition to transcription,T-antigen is also involved in SV40 genome replication.SV40 DNA replication is initiated by binding of large T-antigen to the origin region of the genome.,Academic Press,2000.,SV40 T-Antigen,The SV40 genome is very small&does not encode all the information
31、necessary for DNA replication.Therefore,it is essential for the host cell to enter S phase,when cell DNA&the virus genome are replicated together.Protein-protein interactions between T-antigen&DNA polymerase a directly stimulate replication of the virus genome.The precise regions of the T-antigen in
32、volved in binding to DNA,DNA polymerase a,p53&Rb are all known.Inactivation of tumour suppressor proteins bound to T-antigen causes G1-arrested cells to enter S phase÷&this is the mechanism which results in transformation.The function of T-antigen is to alter the cellular environment to permit
33、 virus DNA replication.The frequency with which abortively infected cells are transformed is low(about 1x10-5)-transformation is a rare&accidental consequence of the sequestration of tumour suppressor proteins.,Academic Press,2000.,Protein-Binding Domains ofSV40 T-Antigen,Academic Press,2000.,Cell T
34、ransformation by Adenoviruses,The immediate early proteins of adenoviruses are analogous in many ways to SV40 T-antigen.E1A is a trans-acting transcriptional regulator of the adenovirus early genes.Like T-antigen,the E1A protein binds to Rb,inactivating the regulatory effect of this protein,permitti
35、ng virus DNA replication&accidentally stimulating cellular DNA replication.E1B binds p53&reinforces the effects of E1A.The combined effect of the two proteins can be seen in the phenotype of cells transfected with DNA containing these genes.However,the interaction of these transforming proteins with
36、 the cell is more complex than simple induction of DNA synthesis.Expression of E1A alone causes cells to undergo apoptosis.Expression of E1A&E1B together overcomes this response&permits transformed cells to survive&grow.,Academic Press,2000.,Cell Transformation by Papillomaviruses,Human papillomavir
37、us(HPV)genital infections are very common,occurring in more than 50%of young,sexually active adults,&are usually asymptomatic.Certain serotypes of HPV appear to be associated with a low risk of subsequent development of anogenital cancers such as cervical carcinoma,after an incubation period of seve
38、ral decades.500,000 new cases of cervical neoplasia are diagnosed every year,making this one of the three most common causes of cancer death in women globally.HPV is a primary cause of cervical cancer,93%of all cervical cancers test positive for one or more high risk type HPV.Of the 60 HPV types cur
39、rently recognized,only four seem to be associated with a high risk of tumour formation:HPV-16,18,31,45.Once again,transformation is mediated by the early gene products of the virus.The transforming proteins appear to vary from one type of papillomavirus to another.It appears that two or more early p
40、roteins often cooperate to give a transformed phenotype.,Academic Press,2000.,Cell Transformation by Papillomaviruses,Although some papillomaviruses can transform cells on their own(e.g.BPV-1),others appear to require the cooperation of an activated cellular oncogene(e.g.HPV-16/ras).More confusingly
41、,in most cases,all or part of the papillomavirus genome,including the putative transforming genes,is maintained in the tumour cells,whereas in some cases(e.g.BPV-4),the virus DNA may be lost after transformation,which may indicate a possible hit-and-run mechanism of transformation.Different papillom
42、aviruses appear to use slightly different mechanisms to achieve genome replication&consequently,cell transformation may proceed via a slightly different route.There is no positive evidence that adenoviruses or polyomaviruses are involved in the formation of human tumours.In contrast,the evidence tha
43、t papillomaviruses are commonly involved in the formation of malignant penile&cervical carcinomas is now very strong.,Academic Press,2000.,Viruses&Cancer,There are numerous examples of viruses which cause tumours in experimental animals.This stimulated a long search for viruses which might be the ca
44、use of cancer in humans.For many years,this search was unsuccessful,so much so that a few scientists categorically stated that viruses did not cause human tumours.We currently know of at least six viruses which are associated with the formation of tumours in infected humans(HHV4/EBV,HBV,HCV,HHV-8,HP
45、Vs,HTLV).However,the relationship between virus infection&tumourigenesis is indirect&complex.,Academic Press,2000.,Epstein Barr Virus(HHV-4),In 1962,Dennis Burkitt described a highly malignant lymphoma whose distribution in Africa paralleled that of malaria.Burkitt recognized that this tumour was ra
46、re in India,but occurred in Indian children living in Africa,&therefore looked for an environmental cause.EBV was first identified in 1964 in a lymphoblastoid cell line derived from an African patient with Burkitts lymphoma.The association between EBV&Burkitts lymphoma is not entirely clear cut:EBV
47、is widely distributed worldwide but Burkitts lymphoma is rare.EBV is found in many cell types in Burkitts lymphoma patients,not just in the tumour cells.Rare cases of EBV-negative Burkitts lymphoma are sometimes seen in countries where malaria is not present,suggesting there may be more than one rou
48、te to this tumour.,Academic Press,2000.,Epstein Barr Virus(HHV-4),EBV has a dual cell tropism for human B-lymphocytes(generally a non-productive infection)&epithelial cells,in which a productive infection occurs.The usual outcome of EBV infection is polyclonal B-cell activation&a benign proliferatio
49、n of these cells which is frequently asymptomatic but sometimes produces a relatively mild disease known as infectious mononucleosis or glandular fever.In 1968,it was shown that EBV could efficiently transform(i.e.immortalize)human B-lymphocytes in vitro.This observation clearly strengthens the case
50、 that EBV is involved in the formation of tumours.There is now epidemiological and/or molecular evidence that EBV infection is associated with at least five human tumours:Burkitts lymphoma,nasopharyngeal carcinoma,B-cell lymphomas in immunosuppressed individuals,some clonal forms of Hodgkins disease
