12. Polyomavirus Flashcards
what family are polyomaviruses in?
polyomaviridae
why are polyomaviruses called polyomaviruses?
poly = multiple tumours
what is an example of polyomavirus?
Simian virus 40 (SV40)
3 characteristics of polyomavirus capsid
- non-enveloped
- 40-45nm
- icosahedral
describe the genome of polyomavirus
5 kbp circular dsDNA wrapped in histones
what baltimore class are polyomavirus?
Baltimore Class I
how are early and late transcripts of polyomaviruses encoded
early and late transcripts encoded on opposite DNA strand
how many viral proteins are there?
5-9
what are 3 types of hosts for polyomaviruses?
- mammals
- birds
- fish
describe SV40 contamination
SV40 was found to contaminate the poliovirus vaccine due to the monkey cells used to make the vaccine
how does SV40 affect monkeys, rodents, and humans?
MONKEYS - replicates without causing disease
RODENTS - causes cancer in laboratory conditions
HUMANS - no evidence for causing cancer
what happens when SV40 infects permissive cells?
virus goes thru full life cycle –> leads to lysis
what happens when SV40 infects non-permissive cells?
results in transformation/integration of DNA –> cancer
how do the early events differ when the virus infects non-permissive cells?
the early events of virus infection occur normally, until T antigens are expressed and trigger S-phase entry
what does integration of the viral genome cause?
leads to transformation of some cells –> only some bc integration is a rare event
which 2 proteins are generally needed for transformation?
LTAg and STAg
what were the first 2 polyomaviruses identified?
JC and BK
describe JC and BK infections (3 stages)
- primary infection in childhood (minor illness)
- latent infection thru life (no disease)
- reactivation with immunosuppression
what percent of people are persistently infected with JC/BK?
80-90%
What happens when the virus reactivates with immunosuppression?
causes lytic replication (massive replication) and serious disease –> often with mutations in genome that favours replication
what kind of people is BK virus common in?
common in kidney transplant patients
what can lytic viral replication of BK lead to? (3)
- kidney injury
- polyomavirus-associated nephropathy (PVAN)
- allograft loss
what are 2 early symptoms of PVAN?
VIRURIA –> virus in urine
VIREMIA –> virus in blood
what can BK virus cause in bone marrow transplant patients?
haemorrhagic cystitis
what is the treatment for PVAN?
no antiviral drug –> reduce immunosuppression
what are the decoy cells we see in urine of BK patient?
virally infected epithelial cells with intranuclear viral inclusion bodies that resemble cancer cells
what disease does JC virus cause?
progressive multifocal leukoencephalopathy (PML)
what 2 types of people is PML found in?
- AIDS patients
- MS patients treated with natalizumab (anti-integrin mAb)
describe the 4 steps of PML
- JC virus infects B cells
- JC virus reaches the brain and replicates in oligodendrocytes
- Oligodendrocytes lyse and become unable to myelinate neurons
- JC non-coding regulatory region is rearranged to allow viral replication in the brain (brain tropism)
what are the diff names for JC virus genome when it is normal and rearranged
non-rearranged = archetype
rearranged = prototype
what are the 4 polyomaviruses associated with cancer?
- BK
- JC
- MC
- TS
what is the only polyomavirus that is actually a human carcinogen?
merkel cell polyomavirus
what is merkel cell carcinoma?
what % of cases are caused by merkel cell polyomavirus
aggressive but rare type of skin cancer that affects merkel cells
80% caused by merkel cell polyomavirus
what are merkel cells/
neuroendocrine cells at the base of the epidermis that are close to nerve endings and allow us to sense light touch
what are the 2 aberrant events of MC polyomavirus that contribute to MC carcinoma?
- viral genome integrates into host DNA
- truncations/mutations in LTAg that impairs its DNA replication activity but maintains its transforming activity
what does truncated LTAg do in merkel cell polyomavirus
truncated so viral replication genes are not present –> by inhibiting Rb will work with STAg to transform merkel cells into MC carcinoma
where is the polyomavirus genome processed? –> what does this mean?
in the cell’s nucleus –> so it is wrapped in histones
what is the polyomavirus genome transcribed by?
RNA pol II
what are the 2 transcriptional units of polyomavirus genome?
- EARLY
- LATE
what does the early region encode?
early region encodes the alternatively spliced LTAg and STAg
what does the late region encode?
VP1, VP2, VP3
what is the third region of the genome called? what does it contain (2)
called Non-Coding Regulatory/Control Region (NCRR/NCCR)
has early/late promoters and origin of viral DNA replication
how are polyomaviruses classified? how can a new polyomavirus be assigned to a new species?
based on amino acid sequence of viral LTAg
if LTAg sequence differs by >15%, it is a new species
how many species are there?
how many genera are there?
> 100 species in 6 genera
which 3 genera contain viruses that infect mammals, including humans
alpha, beta, delta
what are the 3 roles of LTAg?
what do these functions rely on?
- oncogene
- hexameric DNA helicase
- transcription factor
rely on interaction of LTAg with host proteins
what is the oncogene function of LTAg involved in? (2)
- cellular proliferation (via entry in S-phase)
- transformation
what is the hexameric DNA helicase function of LTAg involved in?
DNA replication
what is the transcription factor function of LTAg involved in?
regulates switch from early to late gene transcription
what are 3 host proteins that LTAg interacts with?
- Hsc70 chaperone
- pRB
- p53
how does LTAg interact with Hsc70 and what does it lead to?
J-domain of LTAg binds Hsc70 to remodel the pRb-E2F complex to help release pRb from E2F
how does LTAg interact with Rb and what does it lead to?
Rb interacts with LxCxE Rb-binding motif to cause E2F to be released from Rb to promote S-phase entry for proliferation
how does LTAg interact with p53 and what does it lead to?
binds at helicase/p53 domain to block apoptosis
what are the 6 domains of LTAg and their roles?
- J-domain binds Hsc70
- LxCxE binds Rb and family members p107 and p130
- nuclear localization signal binds host importins
- origin binding domain binds ori
- helicase/p53 binds p53 and has DNA helicase activity
- host range region for viral replication incertain cell types
which domains of LTAg are required for DNA replication activity?
J-domain, NLS, OBD, and helicase domains
(most of the protein)
which domains of LTAg are required for transformation activity?
mainly N-terminal region: JD and LxCxE/Rb domain
and sometimes p53-binding domain
which viral proteins are required for genome replication?
only LTAg! everything else from host
3 functions of LTAg during REPLICATION
- finds origin of DNA replication by itself and binds origin in NCRR to initiate DNA synthesis
- LTAg assembles into double hexamer with helicase activity to unwind the DNA
- interacts with host DNA replication factors
how is viral DNA replication promoted?
LTAg drives the cells into S phase to promote synthesis of host replication machinery
what does STAg share with LTAg?
STAg shares an N-terminal J-domain with LTAg via overlapping coding sequence
is STAg essential?
not essential but can help improve viral proliferation in most polyomaviruses
2 roles of STAg
- activates expression of genes needed for S phase entry via enhancing LTAg activity
- in most viruses, STAg cannot transform cells on its own but increases transformation efficiency of LTAg
why is STAg of MC polyomavirus special?
STAg is the dominant oncogene for MC polyomavirus
what does the activity of STAg depend on?
depends on its interaction with host phosphatase PP2A
how does STAg interact with PP2A?
PP2A is a heterotrimer and STAg replaces the substrate-targeting subunit (that normally gives specificity to certain substrates) and inactivates the enzyme
i.e. STAg inhibits PP2A
describe the icosahedral symmetry of polyomavirus capsid
T=7 arrangement –> 72 pentamers of VP1
12 pentamers surrounded by 5 neighbours
60 pentamers surrounded by 6 neighbours
how do pentamers interact w each other and how are they stabilized?
pentamers interact via C-terminal portion of VP2
pentamers stabilized by Ca2+ ions and disulfide bonds
what do VP2 and VP3 and the N-terminus of VP1 interact with?
associate with pacakged genome
5 steps of SV40 entry into cell:
- SV40 binds GM1 gangliosides as entry receptors
- caveolae-dependent endocytosis targets virus to endosomes
- from endosomes, SV40 is targeted to ER
- SV40 leaves ER to cytosol
- from cytosol, enters nucleus and genome is released
what kind of receptors do polyomaviruses use for attachment?
all contain sialic acid-containing receptors for attachment
which entry mechanism does JC virus use?
clathrin-mediated endocytosis
which entry mechanism do SV40, BK, m, and MC use?
caveolin-mediated endocytosis and/or non-clathrin/non-caveolin lipid raft uptake mechanisms
what is special about JC and BK virus entry?
JC and BK may use extracellular vesicles for entry, bypassing the need for cellular attachment and entry factors
overall polyomavirus life cycle: (10 steps)
- VP1 attaches to cell and enters via diff receptors and entry mechanisms
- vesicle translocates along microtubules to reach ER
- in ER, host disulfide isomerases destabilize capsid structure
- then exported to cytoplasm with low Ca2+ which trigger loss of VP1 (uncoating)
- viral genome imported into nucleus and maintained as episome
- transcription of early genes
- expression of T antigen triggers cell to enter S-phase and promotes LTAg-dependent replication of viral DNA in nucleus
- transcription of late genes (capsid)
- new virions assembled
- virions released by lysis
