RETROVIRIDAE

• STRUCTURE: ss (+) RNA, cap and tail, diploid (two copies), enveloped.

  • The two copies of RNA are base-paired to each other, and are hooked to an accessory tRNA which is essential for replication.

  • Reverse Transcriptase: RNA-Dependent DNA Polymerase. It is encoded by Pol gene and has three different functions.

    • RNA-Dependent DNA Polymerase (make ds-DNA out of RNA)

    • RNAse-H: This activity degrades the RNA portion of the RNA:DNA hybrid during replication. Located on the same protein as the RT action.

    • Integrase: This activity cleaves host and viral DNA in preparation for integration of virus into host genome. Separate protein cleaved from the pol transcript.

  • Long Terminal Repeats (LTR'S): Genome contains LTR's at each end, which have some sequences in them unique to each virus.

    • R: Redundant sequences at both ends of genome.

    • U5: Unique sequence at 5' end.

    • U3: Unique sequence at 3' end.

  • THREE RETROVIRAL GENES: These genes are required by all retroviruses for successful replication.

    • Gag: Group-Antigen. Encodes a structural precursor protein.

    • Pol: Polymerase. Encodes the Reverse Transcriptase.

    • Env: Envelope-protein. Encodes glycoproteins that become a part of the viral envelope.

• ONCOVIRUS PARTICLES: Retroviruses become proviruses and can cause transformation by several mechanisms. They can induce host-genome mutation (insertional mutagenesis), and they can act as transposable elements.

  • Four major types recognized.

    • A-Type: Non-infectious

    • B-Type: Budding through plasma membrane with eccentric core. MMTV.

    • C-Type: Most Common. Budding through plasma membrane with central core. Includes HTLV, RSV (Rous-Sarcoma), MuV.

    • D-Type: Some primate viruses.

  • Two Categories:

    • Acute: v-onc+ viruses have genes homologous to host proto-oncogenes. They are defective in that they can't replicate -- replication-deficient transformation-competent v-onc+ viruses.

    • Chronic: Not in themselves v-onc+ genes, but rather they cause transformation long after infection by insertional mutagenesis. Usually cause leukemia.

  • Specific Viral Oncogenes: v-onc genes.

    • arc: Role in Rous Sarcoma. It encodes a Tyrosine Kinase.

    • erbB: Encodes a receptor for Epidermal Growth Factor (EGF-R).

    • sis: Encodes PDGF.

    • ras: Encodes GTPase activity. v-ras (oncogene) differs from c-ras (protooncogene) by one codon.

    • myc: In Burkitt's Lymphoma, it is involved with an Ig gene in a 8:14 translocation.

    • abl: Involved in 9:22 translocation with bcr to form the Philadelphia chromosome, in Chronic Myelocytic Leukemia (CML).

    • erbA: A thyroid-hormone receptor in birds. It blocks the differentiation of proliferating erythroid precursor cells.

  • Tumor-Suppressor Genes:

    • Retinoblastoma (Rb) Gene

    • Wilms Tumor Gene

    • p53

• REPLICATION: Via reverse transcriptase

  • Cytoplasmic Stage: Particle is not immediately uncoated. RNA genome utilizes Reverse Transcriptase to make a double-stranded copy of DNA (RNA ------> cDNA ------> dsDNA). Then the RNA is degraded.

    • It utilizes a hydrogen-bonded host-cell tRNA as a primer.

  • Nuclear Stage: ds-DNA copy enters nucleus. It becomes integrated as a provirus, with the aid of the nuclease activity of Reverse Transcriptase. If integration fails, then the virus can't survive. Thus it's a potential drug target.

    • Transcription then ensues and viral mRNA is made. The unspliced transcripts serve as progeny genome.

    • Gag, Pol, and Env proteins are translated, glycosylated, and incorporated with the genome as a new virion particle is formed.

• Endogenous Proviruses: Human have a few silent endogenous proviruses: EREV1, and ERV3. They are both defective and produce no symptoms.

• HUMAN T-LYMPHOMA VIRUS (HTLV-1): (ONCOVIRINAE)

  • STRUCTURE: Type-C Oncovirus particle.

  • MANIFESTATIONS: Adult T-Cell Leukemia / Lymphoma (ATL). The virus infects CD4+ T-Cells, causing proliferation.

    • tax/rex gene is the single viral gene that is implicated in oncogenesis. HTLV virus is integrated into the infected cell as a provirus (no replication occurs). The mechanism of transformation is unknown, but they think it is acute and that the virus has v-onc+ genes.

    • Symptoms: Skin manifestations, hypercalcemia, leukocytosis, hepatosplenomegaly.

    • Bad prognosis.

• Rous Sarcoma Virus (RSV): (Oncovirinae). Causes sarcoma in birds.

• HUMAN IMMUNODEFICIENCY VIRUS (HIV): (LENTIVIRINAE)

  • SUBTYPES:

    • HIV-1: Human HIV.

    • HIV-2: More closely related to SIV and infects monkeys.

  • STRUCTURE:

    • Gag, Pol, and Env proteins.

    • Accessory Proteins:

      • vpu protein is required for maturation and budding.

      • tat (transactivator of transcription): Activates transcription of the major viral mRNA's.

    • Glycoproteins: Encoded by the Env genes. gp160 is the precursor glycoprotein, which is then cut in two:

      • gp120: Also called the 'Secretory Subunit." High antigenic variability.

        • High degree of antigenic shift. We make antibodies to gp120, but it is always changing its antigenic specificity.

          • Has V1-V5 Variable Regions. V3 is responsible for M-Tropism. The CD4 binding domain lies in between V4 and V5.

        • It is responsible for the binding of the virus to CD4⁺ receptors.

      • gp41: Transmembrane component. Less antigenic variability.

        • Responsible for the fusion of viral and cell membranes.

        • Also confers the tropism on the cell -- L-Tropic or M-Tropic.

      • Protease Inhibitors inhibit this cleavage.

    • Structural Proteins: Capsid proteins encoded by the Gag gene.

      • P55 is cleaved by protease into 5 substituents, including P24.

      • These structural proteins are required for viral replication.

      • Protease Inhibitors inhibit this cleavage.

  • VIRUS-TROPISM: Conferred by gp41 glycoprotein.

    • L-Tropic: Attracted to lymphocytes (CD4⁺ cells)

      • Infection of T_H cells accounts for depressed CD4 count.

    • M-Tropic: Attracted to macrophages or cells in macrophage lineage.

      • Infection of Alveolar Macrophages: accounts for HIV interstitial pneumonia.

      • Infection of CNS microglial cells: accounts for neurological symptoms and HIV encephalopathy.

  • DIAGNOSTIC TESTS:

    • ELISA Test: Detects antibodies to P24. It is highly sensitive but not specific. If positive, must confirm with a Western Blot.

    • Western Blot: Required confirmatory test. Highly specific. Must reveal antibodies to two viral proteins to be confirmatory.

    • Immunofluorescence, Flow Cytometry: Can be used to detect P24 viral antigen and thereby monitor disease and measure viral burden.

  • MANIFESTATIONS:

    • Primary Infection: A mononucleosis-like infection of fever, pharyngitis, lymphadenopathy. Leukopenia is present, but CD4 count returns to normal after initial infection.

    • Asymptomatic Phase: Can last from 1 to 15 years.

      • The virus continues to replicate throughout -- so it is not proper to call it a latent infection.

      • About 5% of CD4 cells are infected at any point in time.

    • AIDS / AIDS-Related Complex (ARC): Happens when CD4 count gets below 500, and then below 200.

      • What makes AIDS emerge?

        • Antigenic stimulation of the infected CD4 cell may cause it to undergo apoptosis, thereby setting off the symptomatic phase of the disease.

        • May need to wait for cytopathic variants of HIV to arise, by random mutations.

      • OPPORTUNISTIC INFECTIONS: Fungal, mycobacterial, viral, and parasitic infections that require CMI. They show up at 500 CD4-count, and then others show up at 200 CD4-count.

      • MALIGNANCIES: Kaposi Sarcoma, B-Cell Lymphomas in brain, invasive cervical cancer in women.

        • Kaposi Sarcoma: Probably sexually transmitted, and caused by HHV-8.

      • HIV Neurological Disease: Cognitive symptoms, behavioral symptoms, motor symptoms, due to infection of microglial cells by M-Tropic viruses.

      • HIV-Associated Nephropathy: 10-40% of infected patients. Renal failure and mesangial hyperplasia, due to infection of mesangial cells (nephritic macrophages) by M-Tropic viruses.

      • HIV Interstitial Pneumonia: Due to infection of alveolar macrophages. It is a giant-cell pneumonia.

  • TREATMENT:

    • Nucleoside-Analog Inhibitors of Reverse Transcriptase: AZT. It inhibits Reverse Transcriptase.

      • SIDE-EFFECTS: Bone marrow toxicity, nausea, myalgia, headache.

      • PROBLEM: Reverse Transcriptase acquires resistance by mutation. Resistance arises more rapidly in patients with advanced disease.

      • ddC and ddI are other analogs that are useful in combination with AZT. They also ameliorate side-effects.

    • Non-Nucleoside Inhibitors of Reverse Transcriptase:

    • Protease Inhibitors: They inhibit the cleavage of structural protein P55 into P24, thereby inhibiting replication. Promising drug.

      • The protease is an Aspartate Protease, thus any drug blocking that mechanism has promise.

  • VACCINES: Three types have been tried, all disappointing.

    • Humoral Immunity against gp120: it was unsuccessful.

    • Whole killed virus

    • Live attenuated virus: low success-rate and too risky.

• HUMAN FOAMY VIRUS (HFV): (SPUMAVIRINAE). Causes foamy vacuolation of many different cell types. Apparently benign.

Category: Microbiology Notes