RNA Tumor Viruses
BMD 602 微生物學 – 病毒學 RNA Tumor Viruses Robert Yung-Liang Wang 王永樑 [email protected] 生物醫學研究所 微生物組 1 Teaching Objectives : ¾ To learn which viruses can cause cancer in humans ¾ To learn how cells become transformed by the virus ¾ To learn the differences between DNA and RNA tumor virus. ¾ To learn the group of Retrovirus. ¾ To understand the replication of Retrovirus ¾ To understand the discovery of cellular proto-oncogenes ¾ To understand how RNA viral oncogenes result in cell transformation ¾ HCV 2 ¾ WHAT IS CANCER? Cancer: The American Cancer Society Characteristics of Cancer : 9 9 9 9 4 Tumor Viruses For most viruses: Genome viral proteins Replication Progeny virions Lytic Life Cycle 5 Tumor Viruses Latent Life Cycle Virus Cell Integration (usually) Virus-specific proteins expressed Changes in the properties of host cell - 6 Tumor Viruses Transformation: Loss of Ability to form - viral genes interfere with control of cell replication 7 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 of the cell’s metabolism by viral genes and that confer on the infected cell certain properties characteristic of These changes often result from the integration of the viral genome into the host cell DNA 8 TRANSFORMATION Among the many altered properties of the TRANSFORMED CELL are: • Loss of in cultured cells) • (loss of contact inhibition formation • • Reduced • Transformed cells frequently exhibit chromosomal aberrations 9 Two Major Classes of Tumor Viruses DNA Tumor Viruses DNA viral genome Viral mRNA Viral protein 10 RNA Tumor Viruses Viral RNA genome (Virusencoded) Viral DNA genome (integrated) IMPORTANT (Host RNA pol II) Viral genomic RNA (Host splicing enzymes) messenger RNA viral protein Virus Important: Use RNA polymerase to make its genome An enzyme that normally makes mRNA 11 DNA Tumor Viruses DNA genome mRNA protein virus OR TRANSFORMATION In transformation usually only EARLY functions are expressed 12 How cells become transformed by the virus? If a virus takes up in a cell and alters the properties of that cell, the cell is said to be . Transformation by a virus may be defined as: changes in the biologic functions of a cell that result from regulation of the cell by viral genes and that confer on the infected cell certain properties characteristic of neoplasia. these changes often (but not always) result from integration of the viral genome into the host cell genome 13 RNA Tumor Viruses RNA Genome - Retroviruses RNA-dependent DNA Polymerase encoded by virus REVERSE TRANSCRIPTASE RNA genome Reverse transcriptase DNA genome Integrase Integrates Host RNA polymerase II RNA genome 14 RNA Tumor Viruses 15 RNA Tumor Viruses A normal retrovirus has: 3 genes : internal proteins : Envelope glycoproteins : Enzymes Reverse transcriptase Integrase Protease 16 RNA Tumor Viruses RNA is: • Diploid • Capped and polyadenylated sense (same as mRNA) Viral RNA cannot be read as mRNA New mRNA must be made Virus must make before proteins are made Therefore virus must carry into the cell 17 RNA Tumor Viruses 18 RNA Tumor Viruses Groups of Retroviruses • Oncovirinae Tumor viruses and similar • Lentiviruses Long latent period Progressive chronic disease Visna HIV • Spumavirinae 19 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? 20 HIV is a lentivirus, a class of retrovirus ¾ Lentiviruses integrate into the host cell genome as a . ¾ HIV can lie dormant ( ) in the proviral Form within a cell for many years, especially in resting (memory) lymphocytes. ¾ HIV can set up a lifelong infection. ¾ HIV is not transmitted through the germ line. 21 Provirus 22 What are the major mechanisms of HIV transmission in different regions of the world? 23 First report of hemophilia-related immune deficiency in July 1982 Kaposi's sarcoma (skin). Skin showing AIDS-associated Kaposi's sarcoma 24 Acute infection (acute retroviral syndrome) This period lasts for 6 to 12 weeks after initial infection until anti-HIV antibodies are detectable. cells are the major target been infected. 25 Virus titer, CD4 cell number and anti-gp120 titer during the course of HIV infection Rate of progression to AIDS associated with homosexuality and hemophilia HIV-HS: HIV negative homosexual. HIV+ HS: HIV positive homosexual Loss of the specific immune response to HIV ¾ responding CD4+ cells become infected. Thus, there is clonal deletion leading to tolerance. The cells that proliferate to respond to the virus are infected and killed by it. ¾ epitope variation can lead to escape of HIV from the immune response ¾ activated CD4+ T cells are susceptible to apoptosis. Spontaneous apoptosis of uninfected CD4+ and CD8+ T cells occurs in HIV-infected patients. Also there appears to be selective apoptosis of HIV-specific CD8+ cells ¾ the number of follicular dendritic cells falls over time, resulting in diminished capacity to stimulate CD4+ cells HIV LOAD BUT NOT CD4+ T CELL NUMBER IS A GOOD INDICATOR OF RAPID PROGRESSION TO AIDS Cells that are infected by HIV Entry of HIV via the mucosal route and transit via dendritic cells to the lymph nodes RNA Tumor Viruses Retrovirus Life Cycle Endocytosis Fusion of membranes Release of to cytoplasm Nucleus 35 The Structure of Retrovirus Retrovirus Structure 1. The envelope comes from the host cell plasma membrane 2. The envelope gene encodes the surface glycoprotein (SU) – transmembrane (TM) polyprotein. Retrovirus Structure 3. The gag (group specific antigen) gene encodes the viral matrix (MA) capsid (CA) and nucleoproteins (NC). Retrovirus Structure The pol gene products are: a) (a polymerase that copies RNA to DNA) b) (integrates the viral genome into the host genome) c) (cleaves the RNA as the DNA is transcribed so that reverse transcriptase can make the second complementary strand of DNA) d) (cleaves the polyproteins translated from mRNAs from the gag gene and the pol gene itself). This virally encoded protease is the target of a new generation of anti-viral drugs. Retrovirus Structure 1. The envelope comes from the host cell plasma membrane 2. The envelope gene encodes the surface glycoprotein (SU) – transmembrane (TM) polyprotein. 3. The gag (group specific antigen) gene encodes the viral matrix (MA) capsid (CA) and nucleoproteins (NC). 4. The pol gene products are: The replication of RNA virus : Video: HIV replication and life cycle !! The origin of viruses? 病毒的起源? 46 RNA Tumor Viruses Parental RNA RNA/DNA Hybrid Linear DNA/DNA duplex Circular Duplex DNA Integration cell) Replication (DNA genome in 47 RNA Tumor Viruses Drawback to this lifestyle Genomic RNA Reverse transcriptase DNA Genomic RNA Pol II is a host enzyme that, in the uninfected cell, makes mRNA When making mRNA, pol II entire gene to RNA 48 Problem of using RNA pol II to copy a gene RT primer Viral genomicRN Reverse A transcriptase dsDNA RNA synthesis initiation site promotor RNA pol II RNA synthesis termination site 49 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! 50 ¾ How can a retrovirus provide its own control promotors and enhancers The structure of the RNA genome of the mature retrovirus The genome structure of the DNA proviral form of a retrovirus 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 52 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 53 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 54 ¾ The discovery of cellular proto-oncogenes If the virus is to transform a cell, it must have sequences that and provide the other functions that are typical of a transformed cell. These are in addition to the gag/pol/env genome. Thus we also find an (onc) in the viral genome of many retroviruses that transform cells to neoplasia. It should be emphasized that the oncogene in RNA tumor viruses . It is an additional gene that gives the virus its capacity to transform the host cell. 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 56 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 57 RNA Tumor Viruses Viral Oncogene V-onc Cellular Proto-oncogene C-onc 58 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 • • 59 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 60 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? 61 Chronically transforming retroviruses do not have a v-onc Oncogenesis by Chronically transforming retroviruses do not have a v-onc Oncogenesis by 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 64 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 65 RNA Tumor Viruses Could C-oncs be involved in NON-VIRAL cancers? 66 RNA Tumor Viruses myb mos myc Genes can be assigned to sites on specific chromosomes mos and myc : chromosome 8 fes fes: chromosome 15 67 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 68 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) 69 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 70 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 71 Retinoblastoma 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 73 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 74 Anti-Oncogenes Retinoblastoma gene has normal regulatory function in many cells Involved in Retinoblastoma Lung carcinomas Breast carcinomas 75 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 76 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 77 Anti-Oncogenes Retinoblastoma Rb Gene Rb protein Rb Adenovirus E1A 105kD Rb Rb Stops replication Cell cycle continues 78 Anti-Oncogenes p53 P53 gene P53 gene Hepatitis C P53 P53 P53 gene Papilloma P53 Papilloma proteolysis P53 DNA Stops replication replication replication 79 HCV: Hepatitis C virus Characteristics of Hepatitis Viruses Virus A B C D E RNA DNA RNA RNA RNA Fecal-oral Parenteral; sexual; vertical Parenteral; sexual Like HBV Fecal-oral Acute Acute and chronic Mild acute, mostly chronic Acute or chronic Acute 2 – 6 weeks 3–5 month 2–9 weeks Like HBV 1 month Diagnostic tests Antibody Antibody, Antigen Antibody, Antigen Antibody, Antigen None Virus secretion: site, time after infection Stool, 2-3 weeks Blood, 3-4 weeks Blood, 3-4 weeks Blood, 3-4 weeks Stool, 1 month Genome Transmission route Disease course Latency period for disease HCV: Hepatitis C virus A positive-stranded RNA virus HCV life cycle HCV and Liver Cancer HCC (hepatocellular carcinoma) HCC (hepatocellular carcinoma) Hepatocellular carcinoma (HCC, also called malignant hepatoma) is a primary malignancy (cancer) of the liver. Figure. Pathogenesis of human hepatocellular carcinoma Lose of p53, pRb pathways leads to genomic instability Malignant Transformation The Wnt pathway in the HCV-related (Hepatocellular carcinomas) HCC Levrero M. 2006 Viral hepatitis and liver cancer
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