1. Abira Khan

2.   Cause cancer by inducing changes that affect cell proliferation  ~ 20% of all human cancers causes by one of 5 viruses” 1.Epstein-Barr virus 2.Hepatitis B 3.Hepatitis C 4.HTLV I 5.Human Papillomaviruses Oncogenic Viruses

3.   DNA Viruses 1.Human papilomaviruses (HPV) 2.Epstein Barr Virus (EBV) 3.Hepatitis B Virus (HBV)  RNA Viruses 1.Retroviruses 2.Hepatitis C Virus  RNA tumor viruses introduce a transforming gene into the cell (??HCV)  DNA tumor viruses induce or alter the expression of a pre existing cellular gene/s (proto-oncogene) Oncogenic Viruses

4.   Virus infection provides a “hit” towards the genesis of cancer 1.Act as a “mutagen” 2.Other cofactors (genetic, immunological, or enviromental) may be needed for development of cancer  Cell transformation is accompanied by the persistence of all or part of the viral genome and continual expression of a limited number of viral genes.  Viral oncogenes are expressed that alter normal cellular gene expression and signal transduction pathways. How do Viruses Transform Cells?

5.   RNA viruses activate oncogenes  DNA viruses negate tumor suppressors  Both kind of tumor viruses integrate their genome into host DNA; at least a few copies of gene  Permissive cells are productively infected. The virus proceeds through a lytic cycle that is divided into the usual early and late stages. The cycle ends with release of progeny viruses and (ultimately) cell death  Nonpermissive cells cannot be productively infected, and viral replication is abortive. Some of the infected cells are transformed; in this case, the phenotype of the individual cell changes and the culture is perpetuated in an unrestrained manner. General Theme

6.   Some induce tumors in natural host 1.Papilloma 2.EBV, KSHV 3.Hepatitis B  Others induce tumors in experimental systems: 1.Adenovirus 2.Polyomaviruses, SV40 DNA Tumor Viruses Papillomavirus

7.   DNA tumor Viruses do not carry any transduced oncogene but, most of the RNA tumor viruses do  The oncogenes of DNA transforming viruses carry out early viral functions.  The oncogene becomes integrated into the host cell genome and is expressed constitutively  Transformation occurs ONLY in “aborted” viral life cycle (early genes expressed but replication, which is cytocidal, does not occur)  The oncogenes of polyomaviruses are T antigens, which are expressed by alternative splicing from a single locus.  Adenoviruses express several E1A and E1B proteins from two genes Transformation by DNA Viruses

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9.   Retroviruses (family Retroviridae) are enveloped, single stranded (+) RNA viruses that replicate through a DNA intermediate using reverse transcriptase.  This large and diverse family includes members that are oncogenic, are associated with a variety of immune system disorders, and cause degenerative and neurological syndromes  RNA tumor viruses “create” oncogenes by acquiring, modifying, deregulating cellular genes (proto-oncogenes)  v-onc not essential viral gene & unrelated to strategy of viral replication  Replication of RNA viruses is not cytocidal nor is it required for tumorigenesis RNA Tumor Viruses

10.   Transmission may be either: 1.Horizontal- By infectious virus (exogenous virus) 2.Vertical- By proviruses integrated in germ cells (endogenous virus)  Can transmit either as free viral particle or (for some retroviruses) through cell-cell contact  Mechanisms: 1.Retroviral transduction of oncogene (transducing retrovirus) 2.Oncogene activation by retroviral insertion (cis-acting / nontransducing retrovirus) 3.Oncogenesis mediated by essential retrovirus proteins (trans-activating / nontransducing long-latency Transformation by RNA Viruses

11.   Rapid tumor formation: eg RSV; 2 weeks 1.RSV has activated dominant oncogene in genome (v-SRC) 2.Protein produced immediately when virus replicates  Intermediate kinetics of tumor formation: eg ALV; months 1.ALV carries no dominant v-ONC gene 2.Cis-activation: Provirus turns on expression of endogenous oncogene  Slow kinetics of tumor formation; eg HTLV; years 1.HTLV carries no dominant v-ONC gene 2.Does not cause cis-acIvation of local oncogenes 3.A viral regulatory protein activates oncogenes by trans- activation Retroviral transformation

12.   Viral transformation by activation of cellular signal transduction pathways  Viral transformation via cell cycle control pathways Mechanism of Oncogenesis

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14.   Viral mimics cellular signal transduction pathway 1.Transduction of cellular genes by acutely transforming retroviruses 2.Other virus homologous cellular genes  Viral specific signal transduction molecule  Alteration of expression and activity of cellular signal transduction proteins 1.Insertion activation by simple non transuding retroviruses 2.Alteration in activity of cellular signal transduction molecules Activation of Cellular Signal Transduction Pathways

15.   Retroviruses can transfer genetic information both horizontally and vertically.  Horizontal transfer is accomplished by the normal process of viral infection, in which increasing numbers of cells become infected in the same host  Vertical transfer results whenever a virus becomes integrated in the germ line of an organism as an endogenous provirus; like a lysogenic Bacteriophage  Nondefective viruses follow the usual retroviral life cycle. They provide infectious agents that have a long latent period, and often are associated with the induction of leukemias. Two classic models are FeLV (feline leukemia virus) and MMTV (mouse mammary tumor virus).  Tumorigenicity does not rely upon an individual viral oncogene, but upon the ability of the virus to activate a cellular proto-oncogene (s). Retroviruses Activate/Incorporate Cellular Genes

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17.   Acute transforming viruses have gained new genetic information in the form of an oncogene. This gene is not present in the ancestral (nontransforming virus); it originated as a cellular gene that was captured by the virus by means of a transduction event during an infective cycle. These viruses usually induce tumor formation in vivo rather rapidly, and they can transform cultured cells in vitro. Reflecting the fact that each acute transforming virus has specificity toward a particular type of target cell, these viruses are divided into classes  Acute transforming retroviruses have oncogenes that are derived from cellular genes.  Nondefective transforming viruses do not have oncogenes, but activate an equivalent gene(s) in the host genome.  When a retrovirus captures a cellular gene by exchanging part of its own sequence for a cellular sequence, some of the original retro -viral sequences are replaced by a cellular sequence, creates a transducing virus that has two important properties,  Usually these viruses are replication-defective, cannot replicate by itself, but they can propagate with a wild-type "helper" virus  During an infection, their expression may alter the phenotype of the infected cell Retroviruses Activate/Incorporate Cellular Genes

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19.   Cytoplasmic tyrosine kinase  The protein product of v-src was the first retroviral transforming protein to be identified  Posses protein tyrosine kinases activity  Myristoylation of the N-terminus enables Src to associate with the plasma membrane. Src Proto-oncogene 416 SH3 SH2 214250516 527 Mutation alter morphology of transformed cells; some also confers transforming activity Sequence homology to other kinase catalytic domains; tyr-416 is autophosphorylated Phosphorylated at Tyr- 527 in c-src inhibits kinase Myris tylati on Membranebinding domain ModulatoryCatalyticSuppressor SH2= Phosphotyrosine SH3= proline rich sequence SH4 =Myristoylation

20.   Src autophosphorylates and its activity is controlled by the state of phosphorylation at two Tyr residues.  Oncogenic variants are derived from c-Src by mutations that cause decreased phosphorylation at Tyr-527 and increased phosphorylation at Tyr-416.  v-Src lacks Tyr-527 and is constitutively active.  Src was the protein in which the SH2 and SH3 motifs were originally identified  Two sites in Src control its kinase activity.  It is inactivated by phosphorylation  at tyrosine residue 527, which is part of the C-terminal sequence of 19 amino acids that is missing from v-Src.  The c-Src protein is phosphorylated in vivo at this position by the kinase Csk, which maintains it in an inactive state.  Src is activated by phosphorylation at Tyr-416, which is located in the activation loop of the kinase domain. Src Activity- Control

21.   The c-Src and v-Src proteins are similar, share N-terminal modification, cellular location, protein tyrosine kinase activity  c-Src is expressed at high level in seveal types of cells, a large number of signaling proteins are target for src kinase.  c-Src is activated by growth actor receptors, e.g PDGF receptor.  SH2 and SH3 are modulatory domains;  Mutation in SH2 reduce transforming activity, ie. Activate c-Src  Mutation in SH3 domain increase transforming activity, ie. has a negative regulatory role  In the inactive state, Tyr-527 is phosphorylated and enables c- terminal regions of Src to bind to SH2 domain  Auto phosphorylation creates binding site for SH2 that displaces Tyr-527  This leads to its dephosphorylation and conformational change and allow Tyr-416 to be phosphorylated Src Oncogene

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23.   Genome of some large DNA viruses contain some coding region related to cellular genes  Human Herpes Virus 8 contains GPCR which mimics cellular GPCR receptors. In absence of ligands V-GPCR fully activated.  Kaposis Sarcoma an extensive level of angiogenesis occurs due to excessive level of vascular endothelial growth factor. Virus Homologous Cellular Genes

24.   EBV protein (LMP-1) act as signal transduction molecule  LMP-1 can 1.Immortalize human B lymphocytes 2.Inhibit differentiation of epithelial cell lines in culture 3.Induce transformation in rodent fibroblast  LMP is an integral protein of cell membrane  Functions as a constitutively active receptor  In absence of any ligand, LMP-1 oligomerizes and activates the cellular transcriptional regulator Nf- κ b  LMP-1 activates the kinase cascade that normally releases this transcriptional regulator from association with cytoplasmic inhibitor Signal Transduction by Viral Protein

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26.  Cell surface receptor  Signal transduction initiated by binding of growth factors to the receptor (TK)  Ligand binding induce the oligomerisation of receptor molecules and autophosphoylation of tyrosine kinase  Activated receptor internalized by endocytosis  In the endosome the ligand is released and the receptor is degraded  The E5 proteins of bovine and human papillomaviruses binds to vacuolar ATPase and interfere with the degradation of the receptor  E5 protein also increase concentration of activated receptor to the cell surface Alteration of Cellular Signal Transduction Molecules

27.   Members of Parvoviridae and Herpesviridae have protein which permanently activate cellular ST pathways by binding to src tyrosine kinase  SV40 mT protein binds to src protein, does not increase the concentration of src, but increase its catalytic activity Src-SV40

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29.   Insertional activation by retroviruses  Promoter insertion: produce chimeric RNA, proviral LTR linked to cellular protooncogene sequences 1.Transcription from LH LTR include viral coding sequences 2.Transcription from RH LTR Left end LTR is deleted  Enhancer insertion: Viral and cellular transcripts are not fused, enhancer increase the activity of cellular promoter Insertional Activation

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31.   Inhibition of Rb function by viral proteins  Inhibition of p53 Function by binding to viral protein  Production of Viral Specific Cyclins Cell Cycle Control Pathways

32.   Negative regulator of cell growth, also known as anti- oncogene  Transformation of normal cells also accompanied by loss of function of one or more tumor suppressor genes  Rb protein inhibit entry of cell into S phase by binding with transcription factor E2f  Loss of Rb function cause retinoblastoma, an ocular cancer of children Tumor Suppressor Genes

33.   Retinoblastoma is a human childhood disease, involving a tumor of the retina  It occurs both as a heritable trait and sporadically (by somatic mutation)  Retinoblastoma arises when both copies of the RB gene are inactivated  In the inherited form of the disease, one parental chromosome carries a change in this position  A somatic event in retinal cells that causes loss of the other copy of the RB gene causes a tumor  In the sporadic form of the disease, the parental chromosomes are normal, and both RB alleles are lost by somatic events RB is a Recessive Trait

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35.   RB is a nuclear phosphoprotein that influences the cell cycle  Non-phosphorylated RB prevents cell proliferation  In the normal cell cycle, phosphorylation of RB by cdk- cyclin kinases is necessary to proceed into S phase  In resting (G0/G1) cells, RB is not phosphorylated. RB is phosphorylated during the cell cycle by cyclin/cdk complexes, most particularly at the end of Gl; it is dephosphorylated during mitosis  The nonphosphorylated form of RB specifically binds several proteins, and these interactions therefore occur only during part of the cell cycle (prior to S phase). Phosphorylation releases these proteins  Certain tumor antigens suppress the inhibitory action by sequestering the nonphosphorylated form of RB.  Several tumor suppressors act by blocking the cdk-kinase complexes that phosphorylate RB. Rb

36.   Adenoviral E1A, SV40 LT, and E7 protein of HPV disrupt Rb-E2f complex and sequester inhibitory form of Rb Rb Inhibition

37.   Guardian of the genome  p53 is a tumor suppressor that is lost or inactivated in >50% of all human cancers  p53 is a tetramer  p53 is a DNA-binding protein. The ability to bind to its specific target sequences is conferred by the central domain  p53 activates transcription at promoters  The N-terminal region provides the transactivator domain.  p53 may repress other genes; the mechanism is unknown.  p53 also has the ability to bind to damaged DNA  Induce apoptosis  Wild-type p53 is activated by damage to DNA.  The response may be to block cell cycle progression or to cause apoptosis depending on the circumstances. p53

38.   p53 activates various pathways through its role as a transcription factor  The major pathway is inhibition of the cell cycle at Gl is mediated via activation of p21, which is a CKI (cell cycle inhibitor)  CKI is involved with preventing cells from proceeding through Gl  Activation of GADD45 is involved with maintaining genome stability. p53

39.   More than half of all human cancers either have lost p53 protein or have mutations in the gene.  The survival curves for wild type mice (p53+/+), heterozygotes who have lost one allele (p53+/~), and homozygotes who have lost both alleles (p53~/_).  The frequency of tumors is increased from 45% to 80% by loss of the first allele, causing the mice to die sooner; and loss of both alleles shortens the life span dramatically due to the occurrence of tumors in virtually 100% of the mice.  Same phenotype is produced either by the deletion of both alleles or by a missense point mutation in one allele.  Both situations are found in human cancers.  Mutations in p53 accumulate in many types of human cancer, probably because loss of p53 provides a growth advantage to cells p53 Mutation

40.   Different DNA viruses block p53 function by distinct mechanisms  HPV type 16 and 18 E6 proteins bind to p53 and a cellular E6 associated ubiquitin protein ligase, cause ubiquitination and protease mediate destruction of p53  SV40 LT stabilizes the p53 but sequesters in inactive complex  Adenoviral E1B 55kd binds to N-terminal of p53 and convert it from activator to repressor of transcription  Adenoviral E4 ORF 6 protein binds to p53 C-terminal sequences, which increases the turn over rate of p53 and decreases the p53 dependent transcription and induction of apoptosis p53-Virus

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42.   Some viruses can able to produce v-cyclins which show 31% identity and 58% similarity with cyclin D2 and binds predominantly with Cdk6  This Cdk 6 associated with V-cyclin cannot be inhibited by Ink4/cip/kip  Cells are always in proliferating state Production of Viral Specific Cyclins

43.   Genes VIII- Lewin, Chapter 30  Principles of Virology- Flint, Chapter References