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					          Tumor Viruses
              For most viruses:
      Genome                viral proteins

Replication         Lysis             Progeny virions



              Lytic Life Cycle


                                                        1
              Tumor Viruses
                 Latent Life Cycle
                           Virus


                            Cell


                    Integration (usually)


                      Transformation

   Virus-specific proteins expressed - No mature virus
Changes in the properties of host cell - TRANSFORMATION
                                                      2
                   Tumor Viruses
Transformation:
     Loss of growth control
     Ability to form tumors - viral genes interfere with
control of cell replication




                                                       3
                TRANSFORMATION
    Both DNA and RNA tumor viruses can transform cells
                  Integration occurs (usually)
                      Similar mechanisms

            VIRAL TRANSFORMATION
The changes in the biological functions of a cell that result from
                         REGULATION
 of the cell’s metabolism by viral genes and that confer on the
        infected cell certain properties characteristic of
                          NEOPLASIA
  These changes often result from the integration of the viral
              genome into the host cell DNA                       4
             TRANSFORMATION
    Among the many altered properties of the
         TRANSFORMED CELL are:
• Loss of growth control (loss of contact inhibition
in cultured cells)
• Tumor formation
• Mobility
• Reduced adhesion
• Transformed cells frequently exhibit chromosomal
aberrations                                     5
Two Major Classes of Tumor Viruses
                DNA Tumor Viruses
                    DNA viral genome
 DNA-dependent
 DNA polymerase                        Host RNA
  (Host or viral)                     polymerase

                      Viral mRNA




                      Viral protein


                                                   6
        RNA Tumor Viruses
                 Viral RNA genome

                                Reverse transcriptase (Virus-encoded)

            Viral DNA genome (integrated)
                                DNA-dependent RNA polymerase (Host
IMPORTANT                        RNA pol II)
                 Viral genomic RNA

                                Splicing (Host splicing enzymes)

                  messenger RNA
                                        Important: Use HOST
                                          RNA polymerase
                    viral protein        to make its genome
                                            An enzyme that
                                               normally
                        Virus                                      7
                                             makes mRNA
           DNA Tumor Viruses
                        DNA genome
                                       Host RNA
                                      polymerase II
                           mRNA
                                     Host enzymes

                           protein


                            virus

                   OR TRANSFORMATION
In transformation usually only EARLY functions are expressed
                                                          8
  DNA Tumor Viruses In
  Human Cancer

                         Papilloma Viruses
• cause natural cancers in animals
• cause benign warts
• ubiquitous
• epitheliotropic - most human tumors are malignancies of epithelial
cells

                                                                  9
  DNA Tumor Viruses In
  Human Cancer

                        Papilloma Viruses
• epidermodysplasia verruciformis
             wart         malignant
                          squamous cell carcinoma




                                                    10
DNA Tumor Viruses In
  Human Cancer

         Epidermodysplasia
            verruciformis

          Papilloma virus



                             11
     DNA Tumor Viruses In
       Human Cancer
                           Papilloma Viruses
urogenital cancer
             wart            malignant squamous cell carcinoma
       Papilloma viruses are found in 91% of women with cervical cancer

     Squamous cell carcinoma:
     Larynx
     Esophagus                               All histologically similar
     Lung

             10% of human cancers may be HPV-linked                       12
          DNA Tumor Viruses In
            Human Cancer
                      Papilloma Viruses
• 51 types identified - most common are types 6 and 11
• most cervical, vulvar and penile cancers are ASSOCIATED with
types 16 and 18 (70% of penile cancers)


             EPIDEMIOLOGIAL STUDIES BUT:
     HPV 16 and HPV 18 do transform human keratinocytes




                                                             13
          DNA Tumor Viruses In
            Human Cancer
                 Polyoma Viruses
• Simian virus 40 - juvenile hamster sarcomas, transformation
• Polyoma - mouse leukemia, in vitro transformation
• Human polyomas (JC and BK) - monkey sarcoma, transformation
PROGRESSIVE MULTIFOCAL LEUKOENCEPHALOPATHY
Polyoma virus transforms cells when the genome is incomplete
Early functions are necessary

                                                            14
DNA Tumor Viruses In
  Human Cancer
       Adenoviruses
  Highly oncogenic in animals
  Only part of virus integrated
     Always the same part
        Early functions
   E1A region: 2 T antigens
    E1B region: 1 T antigen
                                  15
 E1A and E1B = Oncogenes
          DNA Tumor Viruses In
            Human Cancer
                     ONCOGENE
A gene that codes for a protein that potentially can transform
             a normal cell into a malignant cell
An oncogene may be transmitted by a virus in which case it is
            known as a VIRAL ONCOGENE


                        v-onc
                                                         16
          DNA Tumor Viruses In
            Human Cancer
                        Herpes Viruses
Considerable evidence for role in human cancer
• Some very tumorigenic in animals
• Viral DNA found in small proportion of tumor cells: “hit and run”
             • Epstein-Barr Virus
              • Burkitt’s Lymphoma
            • Nasopharyngeal cancer
           • Infectious mononucleosis
   • Transforms human B-lymphocytes in vitro                  17
DNA Tumor Viruses In
  Human Cancer
    Hepatitis B Virus
       DNA genome
             RNA polymerase II   Host enzyme

       RNA Provirus
             Reverse transcriptase
                              Viral enzyme
       DNA genome


                                       18
            DNA Tumor Viruses In
              Human Cancer
                        Hepatitis B continued



                    • Vast public health problem
• 10% of population in underdeveloped countries are chronic carriers


                          •Long latency




                                                              19
              DNA Tumor Viruses In
                Human Cancer
                             Hepatitis B continued
        Epidemiology:
Strong correlation between HBV and
hepatocellular carcinoma
China: 500,000 - 1 million new cases of
hepatocellular carcinoma per year
Taiwan: Relative risk of getting HCC is 217
x risk of non-carriers




                                                     20
      DNA Tumor Viruses In
        Human Cancer
                    Summary

  • Can transform cells or have lytic life cycle


        • Often integrate into host genome



• In transformation ONLY early genes are transcribed


                                                   21
        RNA Tumor Viruses
     RNA Genome - Retroviruses
RNA-dependent DNA Polymerase encoded by virus
          REVERSE TRANSCRIPTASE
                 RNA genome
                          Reverse transcriptase     virus


                 DNA genome
                           Integrase                virus


                  Integrates
                           Host RNA polymerase II     host


                 RNA genome                           22
RNA Tumor
 Viruses




        23
RNA Tumor Viruses
  A normal retrovirus has:
           3 genes
   GAG : internal proteins
ENV: Envelope glycoproteins
        POL: Enzymes
       Reverse transcriptase
             Integrase
                               24
             Protease
           RNA Tumor Viruses
                        RNA is:
      • Diploid        Capped and polyadenylated
           • Positive sense (same as mRNA)

          Viral RNA cannot be read as mRNA
                  New mRNA must be made
Virus must make negative sense DNA before proteins are
                        made
Therefore virus must carry REVERSE TRANSCRIPTASE
                                             25
                     into the cell
RNA Tumor Viruses




                    26
             RNA Tumor Viruses
                   Groups of Retroviruses
• Oncovirinae          important



     Tumor viruses and similar
• Lentiviruses         important



     Long latent period
     Progressive chronic disease
     Visna       HIV
• Spumavirinae                              27
       RNA Tumor Viruses
Retroviruses known to cause human cancer
  • Human T cell lymphotropic virus -1 (HTLV-1)
     Adult T cell leukemia, Sezary T-cell leukemia
        Africa, Caribbean, Some Japanese Islands

  • Human T cell lymphotropic virus -2 (HTLV-2)

                  Hairy cell leukemia

                    • HIV?                        28
   RNA Tumor Viruses
     Retrovirus Life Cycle
            Endocytosis


       Fusion of membranes


Release of nucleocapsid to cytoplasm


              Nucleus                  29
                   RNA Tumor Viruses
Parental RNA
         Reverse transcriptase

RNA/DNA Hybrid
                   Reverse
                transcriptase
Linear DNA/DNA duplex


Circular Duplex DNA
         Integrase        Host DNA
                         polymerase
Integration                                Replication (DNA genome in cell)
                                              Host splicing
              Host RNA pol II                   enzymes
                                                                    30
Transcription                   Viral RNA genome       mRNA      protein
            RNA Tumor Viruses
                   Drawback to this lifestyle


                        Genomic RNA
                                   Reverse transcriptase
                             DNA
                                  Host RNA pol II
                        Genomic RNA
Pol II is a host enzyme that, in the uninfected cell, makes mRNA
 When making mRNA, pol II does not copy entire gene to RNA
                                                            31
         Problem of using RNA pol II to copy a gene


                  RT                                      Viral
               primer                                 genomicRNA
                                            Reverse
                                         transcriptase
                                                     dsDNA

                      RNA synthesis
                       initiation site

           promotor



                  RNA pol II
                                            RNA synthesis termination
                                                      site



                                                               32
Result: New copy of viral RNA is shorter - lacks control sequences
              RNA Tumor Viruses
                RNA polymerase II will not copy
     Upstream sequences from transcription initiation site
                   • Promotors / Enhancers
  Down stream sequences from transcription termination site
          • Enhancers / Poly A site / termination site


                            ?
Perhaps virus could integrate downstream of a promotor etc so
               that the cell provides sequences

                               OR
             Virus provides its own promotors etc
                                                             33
                        BUT not copied!
               RNA Tumor Viruses
     Repeat                                         Repeat
     region                                         region
                Clue: Difference in the two forms
                              RNA

        R      U5    GAG     POL      ENV    U3      R

                              DNA



U3       R      U5    GAG      POL      ENV U3      R        U5

         LTR                                                      34
               R          U5             Viral RNA    U3     R

                                                                Reverse
                                                             transcriptase




      U3           R        U5                          U3        R     U5
                       Long terminal repeats are formed
promotor
           POLII                                                      POLII


                   RNA initiation site               RNA termination site
                                                                       35
Retroviruses can have only one
           promotor
                                Contained in U3



         LTR                                               LTR

 POLII                                                              POLII


          RNA initiation site                     RNA termination site

         Therefore only one long RNA can be made
                                                                         U5
            Therefore mRNA requires processing
         Explains why RNA has to be positive sense                  36
        Some retroviruses have an
               extra gene


               “typical retrovirus”


    R    U5    GAG     POL      ENV   U3    R


              Rous Sarcoma Virus

R       U5    GAG     POL       ENV   SRC       U3   R
                                                     37
Some retroviruses have an oncogene
   instead of their regular genes
             Avian Myeloblastosis
                    Virus

   R   U5          GAG     POL    MYB        U3       R

            Feline Sarcoma Virus (FSV)

  R    U5   dGAG         FMS     dENV   U3        R

       Avian Myelocytoma Virus (MC29)

                                                          38
   R   U5   dGAG     MYC         dENV   U3        R
RNA Tumor Viruses

    Viral Oncogene
         V-onc


Cellular Proto-oncogene
         C-onc

                          39
           RNA Tumor Viruses
               Proto-oncogene
A cellular (host) gene that is homologous with a
similar gene that is found in a transforming virus
      A cellular oncogene can only induce
               transformation after
                   • mutation
   • some other change in the cell’s genome   40
          RNA Tumor Viruses
    The discovery of the acutely transforming
            retroviruses that contain
v-oncs explains how cancers may arise as a result
                   of infection


 These viruses cause rapid cancer in animals in
                 the laboratory


                                             41
               RNA Tumor Viruses
                          In contrast:
            Chronically transforming retroviruses
cause tumors inefficiently after prolonged period of time
        Avian Leukosis Virus (causes lymphomas)


        R      U5   GAG      POL     ENV   U3     R

      No oncogene! – How does it cause a
                   tumor?                             42
           RNA Tumor Viruses
 ALV can integrate into the host cell genome at
               MANY locations
  but in tumor it is always at the SAME site (or
           restricted number of sites)
       Suggest tumor arose from one cell
• Something must be important about this site for
               transformation
          • Crucial event must be rare
                                              43
        RNA Tumor Viruses
        What is special about this site?
Myelocytoma tumors from several birds all have
        the oncogene close to this site

              It is close to
                  C-myc!
 Oncogenesis by promotor insertion         44
          RNA Tumor Viruses


Could C-oncs be involved in NON-VIRAL cancers?




                                          45
            RNA Tumor Viruses
                                Genes can be
                                 assigned to
                                   sites on
                                   specific
      myb        mos
                                chromosomes
                 myc

                                mos and myc :
                                chromosome 8


fes                         fes: chromosome 15
                                                46
            Cancers often result from gene
Burkitt’s Lymphoma
                   translocations
 8:14 translocation
Break in chromosome
      14 at q32



                      myc




                                Acute myelocytic leukemia
                                          7:15
                                          9:18      47
                                        11:15:17
       Oncogenesis by rearrangement

Tumor                        c-onc         new promotor
Burkitt’s lymphoma           myc     (8)   Ig heavy (8 to 14)
                                           Ig light (8 to 2)
T cell chronic lymphocytic   myc           T cell receptor (8 to 14)
leukemia
B-cell chronic lymphocytic   bcl-1         Ig heavy (11 to 14)
leukemia                     bcl-2         Ig heavy (18 to 14)
T cell chronic lymphocytic   tcl-1         T cell receptor
leukemia                                   (14 inversion)
                                                                 48
   RNA Tumor Viruses
 What do oncogenes encode?
Proteins that are involved in growth control and
                 differentiation



                                Growth factors
                           Growth factor receptors
                         Signal transduction proteins
                            Transcription factors


                                                        49
                Oncogenes
Mutations in a proto-oncogene are dominant “gain
              of function” mutations
However other oncogenic genes show recessive
                 mutations
              Anti-Oncogenes
          • Loss of function mutations
                • Retinoblastoma
                     • p53                  50
                Proto-oncogenes
                           Dominant
      Heterozygote         mutations    Homozygote

 Allele 1       Allele 2           Allele 1       Allele 2
 Normal         Mutant             Mutant         Mutant



 Binds under     Mutant           Mutant         Mutant
    special      always           always         always
circumstances     binds            binds          binds

            Always binds                       Always binds

                                                              51
     Function gained                   Function gained
                     Anti-Oncogenes
                            Recessive mutations
                            Mutation    growth

     Rb Gene         Mutant Rb         Mutant Rb         Mutant Rb


      Rb
Rb protein

             Heterozygote                     Homozygote


     Rb                                       Function lost

    Binds and controls cell cycle       No binding - Growth continues
                                                                    52
  Anti-Oncogenes
Retinoblastoma gene has normal
regulatory function in many cells


           Involved in
        Retinoblastoma
       Lung carcinomas
       Breast carcinomas            53
       Anti-Oncogenes
                    P53
               Inactivated by
                 • deletion
              • point mutation
In a series of colorectal cancers all showed:
   • Allele 1: partial or complete deletion
         • Allele 2: Point mutation           54
             DNA Tumor Viruses
                Oncogenes
• Adenovirus                         E1A region 2
• SV 40                              Large T
• Polyoma                            Large T
• BK virus                           Large T
• Lymphotropic virus                 Large T
• Human papilloma Virus-16           E7


All have a sequence in common
Mutations in this region abolish transformation capacity   55
                    Anti-Oncogenes
                         Retinoblastoma
          Rb Gene                              Adenovirus E1A



  Rb        Rb
protein          105kD


                                          Rb


     Rb
                                                           56
    Stops replication               Cell cycle continues
              Anti-Oncogenes
                        p53
      P53 gene           P53 gene    P53 gene
                       Hepatitis C   Papilloma


        P53           P53             P53




                                            Papilloma
                                            proteolysis
P53      DNA

                                                    57
  Stops replication   replication     replication

				
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posted:11/11/2011
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