02/27d DNA Viruses I Flashcards
What are the six major families of DNA viruses that infect humans?
Parvoviruses Polyomaviruses Papillomaviruses Adenoviruses Herpesviruses Poxviruses
What is human parvovirus B19?
Single-stranded DNA virus
Autonomous
Causes a rash, aplastic crisis in patients with hemolytic disorders, and fetal infection
What does it mean if a virus is ‘autonomous’?
It does not require helper viruses to replicate
What is the replication strategy of parvoviruses?
Replicates in the nucleus of dividing cells
Uses host enzymes for viral gene expression and DNA replication
Does not have the ability to stimulate host DNA synthesis
Which human pathogens are polyomaviruses?
JC virus - leukoencephalopathy
BK virus - urinary tract infections
Merkel cell polyomavirus - Merkel cell skin cancers
What is the replication strategy of polyomaviruses?
Replicates in the nucleus using viral proteins
Activates the expression of several host genes encoding protein required for DNA replication
Also inactivates tumor suppressor proteins to stimulate replication
Capable of latency
In what circumstances might a latent polyomavirus reactivate?
Immunosuppression
Immunodeficiency diseases
Pregnancy
Diabetes
How do polyoma viruses typically cause their pathogenic effects?
Virus-induced cell death
Example - JCV replicates in oligodendrocytes; viral killing of these cells leads to demyelination
Which papillomaviruses are strongly associated with cervical and oral/pharyngeal carcinomas?
HPV 16 and 18
Which papillomaviruses are strongly associated with benign genital warts?
HPV 6 and 11
What is the replication strategy of papillomaviruses?
Virus is introduced into the basal layer of an epithelium (dividing cells)
Viral genome expressed differentiates as it moves from the basal layer to the topmost layer
Viral particles are only expressed and shed in the late epithelial layer, without an inflammatory response (helps the virus to escape immune detection)
How do papillomaviruses cause disease?
Virus induces cell proliferation and cell transformation
Viral genes E6 and E7 activate many promoters, induce DNA synthesis, and bind to tumor suppressor proteins such as Rb and p53
What are some common illnesses associated with adenovirus infection?
Respiratory diseases Pharyngitis Conjunctivitis Pneumonia Gastrointestinal disease
How do adenoviruses cause disease?
Lytic replication in host cell causes cell death
What are herpesviruses?
Large enveloped viruses with an icosahedral capsid
Capable of both lytic and latent infection
Replicate in the host cell nucleus
What virus causes chickenpox and shingles?
Varicella-Zoster virus
Why are the skin lesions of zoster restricted to one dermatome?
Result of reactivation of VZV in a single spinal nerve
What diseases are caused by Epstein-Barr virus?
Mononucleosis
B cell lymphomas
Name four pox viruses and their hosts
Variola virus (smallpox) - humans
Monkeypox virus - squirrels
Cowpox virus - rodents
Vaccinia virus - lab species
What is the significance of the capsid of DNA viruses? What does it mean for replication, infectivity, etc.?
Simplest way to package a genome
Makes the virus very hardy and robust
Fixed, rigid structure means that the virus can’t easily undergo antigenic change to avoid the immune system
Difficult to get a protein “box” through a cell membrane
What are the advantages of a viral envelope? Name three
Allows easier access through cell membranes
Different proteins can be placed on the surface to target different cells, without changing capsid structure
Adds extra volume to allow the virus to carry more proteins
What are the functions of the viral capsid?
Protective container for the viral genome
Delivery vehicle for the viral genome and viral proteins
Also functions in stability, resistance to immune defenses, host cell infection, uncoating, and delivery to the site of transcription
What other proteins are included in DNA viruses, besides the capsule?
Proteases that are important for viral production and assembly
Transcription factors
Transcriptional machinery
Countermeasures to host defenses
How do nonenveloped viruses enter host cells?
Taken up by endocytosis in clathrin-coated pits
Capsid is disaggregated in the endosome, lyses the endosome, and is released into the cytoplasm
Viral genome remains encapsulated in the cytoplasm, to protect it from host cell defense mechanisms
Transported to the nucleus along microtubules
What is included in the DNA viral genome?
1) Signals necessary for viral genome replication and packaging
2) Encodes proteins not provided by the cell
3) Provides a means of producing genetic diversity, usually by recombination
4) Regulatory elements that place the viral genome under the control of cell mechanisms (may be dependent on cell type or physiological status)
5) Accessory proteins - not essential for viral replication in vitro, but advantageous for virus replication in vivo
What is the purpose of the early and late classes of genes in DNA viruses?
Early phase genes are typically proteins and enzymes that required for DNA replication and cell division
Late phase genes are typically required for structural proteins and virus release
Temporal differential gene expression helps the virus to evade the immune system (more viral proteins means a higher chance of being recognized)
What are the three classes of temporally regulated genes in HSV-1? What do they encode?
Immediate early genes - transcriptional control factors
Delayed early genes - viral DNA replication
Late genes - major structural proteins
What is a productive or lytic infection?
One in which progeny virus is produced, and usually involves lysis and death of the host cell
What is a persistent infection?
Infection in which infected cells or virions are not cleared, and progeny viruses are continually released
What is a latent infection?
Persistent infection in which infectious virus particles are not present
