Tumor Viruses
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Tumor Viruses For most viruses: Genome Replication viral proteins 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 - TRANSFORMATION2 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 or viral) Host RNA 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 RNA pol II) IMPORTANT Viral genomic RNA Splicing (Host splicing enzymes) messenger RNA viral protein Virus Important: Use HOST RNA polymerase to make its genome An enzyme that normally makes mRNA 7 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 Lung All histologically similar 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 E1A and E1B = Oncogenes 15 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 DNA genome Viral enzyme 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 RNA genome host 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 Protease 24 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 RNA pol II Transcription Host splicing enzymes Viral RNA genome mRNA 30 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 primer Viral 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 BUT not copied! 33 RNA Tumor Viruses Repeat region Clue: Difference in the two forms Repeat region RNA R U5 GAG POL ENV U3 R DNA U3 R LTR U5 GAG POL ENV U3 R U5 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) R U5 dGAG MYC dENV U3 R 38 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 myb mos myc Genes can be assigned to sites on specific chromosomes mos and myc : chromosome 8 fes fes: chromosome 15 46 Cancers often result from gene translocations Burkitt’s Lymphoma 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 Burkitt’s lymphoma myc (8) new promotor Ig heavy (8 to 14) Ig light (8 to 2) T cell chronic lymphocytic myc T cell receptor (8 to 14) 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 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 Heterozygote Dominant mutations Homozygote Allele 1 Allele 2 Allele 1 Allele 2 Normal Mutant Mutant Mutant Binds under special circumstances Mutant always binds Always binds Mutant always binds Mutant always binds Always binds 51 Function gained Function gained Anti-Oncogenes Recessive mutations Mutation Rb Gene Mutant Rb growth Mutant Rb Mutant Rb Rb Rb protein Heterozygote Rb Binds and controls cell cycle Homozygote Function lost 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 Adenovirus E1A Rb Gene Rb protein Rb 105kD Rb Rb Stops replication Cell cycle continues 56 Anti-Oncogenes p53 P53 gene P53 gene Hepatitis C P53 P53 P53 gene Papilloma P53 Papilloma proteolysis P53 DNA Stops replication replication replication 57
