Large DNA viruses: Herpesviruses, Poxviruses, Baculoviruses and Giant viruses Viruses are the only obstacles to the domination of the Earth by mankind. -Joshua Lederberg
Recommended reading: Field s Virology Chapter 66: The Family Herpesviridae: A Brief Introduction Chapter 67: Herpes Simplex Virus Chapter: Poxviridae: The Viruses and Their Replication
What do I mean by big DNA viruses? DNA Genomes >100 kbp->1,000 kbp For examples: Herpesviruses: ~125-250 kbp Baculoviruses: ~130-160 kbp Poxviruses: ~130-360 kbp Mimivirus: ~1,200 kbp And now, Pandorovirus: 2.8 Mbp! 1 um
Why are these genomes so big? Provides large coding capacity (100s-1000s of proteins) Each virus encodes an array of enzymes involved in DNA synthesis and nucleotide metabolism Highly controlled gene expression occuring in 3 waves
Herpesviruses Enveloped, icosahedral virion Linear, dsdna genome in the virion ~125-240 kbp Genome replicated and capsid formed in the nucleus
Poxviruses Enveloped, brick shaped virion Closed, linear, mostly dsdna genome ~130-360 kbp Genome replicated and capsid assembled in the cytoplasm What unique challenges does this present?
Baculoviruses Enveloped, rod-shaped virion Circular, dsdna genome ~130-160 kbp Genome replicated and capsid formed in the nucleus
Mimiviruses Enveloped icosahedral virion Circular, dsdna genome ~1.2 Mbp Genome replicated and capsid assembled in the cytoplasm Genome encodes trnas, amino-acyl trna synthetases, RNA translation factors Is it alive?
Herpes simplex (HSV) Herpesvirus Varicella-zoster virus (VZV) Herpes labialis Varicella Shedding of VZV Cytomegalovirus (CMV) Epstein-Barr virus (EBV) Cytomegalic bodies Kaposi s Saroma-associated herpesvirus (KSHV) EBV-infected lymphocytes KSHV infected cells Human herpesvirus 6 (HHV6) Human herpesvirus 7 (HHV7)
HERPESVIRUSES Virus family originally defined by virion architecture Virus types originally defined by disease Now family and virus type defined by genomics Herpesviruses are found in mammals, birds, reptiles, and amphibians -ancient viral family that co-evolved with vertebrates Herpesviruses are everywhere! Ubiquitous in nature and in humans (with one exception)
7 common properties of herpesviruses: 1. Large, linear dsdna genome 130-235,000 bp 2. Encode an array of enzymes involved in nucleic acid metabolism Nucleotide metabolism: Thymidine kinase (TK), thymidylate synthetase (TS), dihydrofolate reductase (DHFR), dutpase, ribonucleotide reductase (RR) Nucleic acid metabolism: DNA polymerase, ssdna binding protein, helicase, primase, gyrase 3. Immediate early (IE), early (E) and late (L) patterned gene expression 4. Viral DNA synthesis and capsid assembly in the nucleus 5. Production of infectious progeny results in cell death 6. Latency Ability to reactivate in the presence of humoral immunity Implies an ability to evade immuno-surveillance Viral DNA is circular during latency 7. Like diamonds, herpesviruses are forever.
Three herpesvirus subfamilies Three subfamilies: α, β, γ herpesviruses α herpesviruses: broad host range short replication cycle in vitro rapid spread in culture Infects: epithelial cells, neurons Latency: in ganglia Herpes simplex 1 (HSV-1, HHV1) Herpes simplex 2 (HSV-2, HHV2) Varicella-zoster virus (VZV, HHV3)
Three herpesvirus subfamilies β herpesviruses restricted host range long replication cycle in vitro cytomegaly Infects: Epithelial cells, monocytes, secretory glands, kidneys, bone marrow, etc Latency: Myeloid lineages, elsewhere? Cytomegalovirus (CMV, HHV5) Human Herpesvirus 6 (HHV6) Human Herpesvirus 7 (HHV7)
Three herpesvirus subfamilies γ herpesviruses restricted host range long replication cycle in vitro associated with cancer Infects: epithelia cells, B cells Latency: B lymphocytes Epstein-Barr Virus (EBV, HHV4) Kaposi Sarcoma Associated Herpesvirus (KSHV, HHV8)
Herpesviruses undergo latency in vivo Latency: the ability to reactivate in the presence of humoral immunity What makes herpesviruses so special?
Structure of Herpesviruses Similar structure for all herpesviruses HSV1, 2 30-35 virion proteins Capsid--icosahedral Core--torus (DNA & proteins) Tegument Kinases Transcription factors VHS--virus host shutoff (only HSV1, 2) Envelope
Viral Gene Expression HSV genome Linear dsdna ~170,000 bp Productive infection (same for all herpesviruses) ~80 ORF Viral mirnas IE genes (functional) Transcription factors Regulate E genes expression Influence L gene expression Alter host gene expression Modulate host immune response IE E E genes (genome replication) Nucleotide metabolism DNA synthesis vtk L Messy nomenclature L genes (structural) Virion structural components Some genes are diploid Lytic factors
Herpesviruses and mirnas It appears that all herpesviruses encode mirnas Rhesus lymphocryptovirus (rlcv) encodes ~68 mirnas Viral mirnas are expressed during latency and productive infections Viral mirnas target viral transcripts Some downregulate the expression of IE proteins responsible for productive infections Viral mirnas target host transcripts Some function in immune evasion Herpesviruses also alter the expression of host mirnas
Genome architecture of herpesviruses
Replication Productive infection (similar for all herpesviruses) IE=α E=β L=γ Tegument proteins & cell death DNA replication
Pathogenesis: HSV-1 Virus must contact mucosal surface or abraded skin Initial infection in epithelia cells Attachment via glycoproteins (gb, gh, others) Replication at site of infection Gingivostomatitis Latency Latency Associated Transcripts (LATs) Noncoding Function? Lasts for days/weeks/months/years Then something happens Infection of sensory neurons Retrograde transport to neuron cell body DNA circularizes in nucleus (episome) Why?
Pathogenesis: HSV-1 Reactivation Replication cycle similar to productive infection at the point of initiating DNA replication Ganglion (TG, SC) Anterograde transport along axon Replication at or near site of primary infection End results: Lysis of neuron Infection & lysis of epithelium Vesicle formation infected cell juice Disruption of vesicles Release of evil humors (i.e., virus shedding) Infection of nearby epithelium Infection of sensory neurons Latency
Examples of diseases associated with herpesviruses Herpes labialis (HSV-1, -2) Varicella or chicken pox (VZV) Zoster or shingles (VZV) Interstitial pneumonia (CMV) Roseola infantum (HHV-6) Infectious mononucleosis (EBV, CMV)
Herpesviruses and cancer γ herpesviruses (primarily associated with latency) Epstein-Barr virus (EBV) Lymphoblastoid cell lines (LCLs) B cell lymphomas including Burkitt s lymphoma, Hodkin s disease Nasopharyngeal carcinoma Kaposi s sarcoma-associated herpesvirus (KHSV) Kaposi s sarcoma Primary effusion lymphoma (PEL)
Herpesviruses and cancer EBV proteins/transcripts of significance: EBNA LMP EBERs mirnas KSHV proteins/transcripts of significance: LANA V-cyclin mirnas
Poxviridae Viruses associated with humans Vaccina virus (VV) Variola virus (small pox virus) Mulluscom contagiosum virus Defined by type (disease)
Structure/Replication Large, brick-shaped virion (350 nm x 270 nm) Double membrane Closed, linear, dsdna genome ~130-360 kbp 3 waves of gene expression: Early, Intermediate, Late Functions?
Structure/Replication Cytoplasmic replication Challenges Gene expression: Genome replication: Effect on host gene expression Immune evasion
Diseases associated with poxviruses Concept of vaccine Jenner Vaccinia Small pox Mulluscom contagiousum
Baculovirus Nucleopolyhedrosis virus (NPV) Autographica californica NPV (AcNPV) Used as expression vector in insect and human cells to express high levels of proteins