tumorviruses Flashcards
what are the two models by which viruses could cause cancer?
1: provirus model: genes enter the cell at the time of infection carried by the tumor virus
2: oncogene model: genes for malignancy are already present in all cells of the body - tumor viruses can activate
what is the difference between cellular oncogenes and viral oncogenes?
cellular oncogenes have introns and exons whereas viral oncogenes don’t
what do human papillomaviruses cause?
papillomas
benign tumors of squamous cells
can develop to carcinoma
describe the DNA structure of papillomaviruses
double-stranded circular DNA
what type of virion structure do papilloma viruses have?
icosahedral nuclocapsid
are papillomaviruses enveloped?
nonenveloped
which genes in papillomaviruses are implicated in carcinogenesis? by what mechanism?
two of the early genes: E6 and E7
encode proteins that inactivate genes encoded by tumor suppressor genes (esp. p53 and RB) - interfere with the stability of these proteins
how many types of HPV are there? which ones infect which tissues?
over 100 types
HPV 1-4: skin warts
HPV-6 and HPV-11: genital warts, respiratory tract papillomas, especially laryngeal, in young children
HPV-16: E6 and E7 proteins bind more strongly to p53 and RB proteins than other types of HPV => more carcinogenic
also oral cancers
HPV-16 and HPV-18: intraepithelial neoplasia
about 30 types infect genital tract
in what kinds of cells does HPV replicate?
in terminally differentiated squamous cells
initially infects cells of basal layer of skin, but no virus produced in those cells
how does HPV DNA become malignant?
it’s incorporated into host cell DNA in vicinity of cellular proto-oncogenes
E6 and E7 are overexpressed
E6 can bind to the p53 protein and inactivate it
where is HPV DNA in latently infected cells?
episomally located
E6 and E7 are not overexpressed
why are E6 and E7 not overexpressed in the latent form of HPV?
another early gene, E2, controls E6 and E7 expression
E2 only functional when DNA is episomal
E2 inactivated when viral DNA is integrated, so now E6 and E7 can be overexpressed
how is HPV transmitted?
by skin-to-skin contact and by genital contact
can HPV be transmitted to neonates? if so, what would be the symptoms of neonatal HPV?
yes, during childbirth
causes warts in mouth and in respiratory tract, especially on larynx
what are koilocytes and what viral infection are they the hallmark of?
cytoplasmic vacuole that’s characteristic of infection by HPV
what type of immune reaction is induced by HPV? what is the result of this reaction?
both cell-mediated immunity and antibody
spontaneous regression of warts
what would be the clinical presentation of HPV infection?
most commonly, papillomas of various organs depending on the virus type
carcinoma of uterine cervix, penis and anus
intraepithelial neoplasia
what is intraepithelial neoplasia and what viruses cause it? how would you detect it?
HPV-16 and HPV-18
premalignant lesions on cervix (CIN) or penis (PIN)
detect by applying acetic acid to tissue
how would you diagnose HPV? (laboratory diagnosis)
usually diagnosed clinically - presence of koilocytes in lesions
PCR to detect DNA of 14 high-risk genotypes
how is HPV treated?
podophyllin for genital warts
alpha interferon also, and help prevent recurrences
liquid nitrogen for skin warts
plantar warts removed surgically or treated with salicyclic acid
cidofovir for severe infections
what are the vaccines for HPV? what types of vaccines are they?
1: Gardasil - recombinant vaccine against 4 types of HPV - has capsid proteins from types 6, 11, 16, and 18
2: cervarix - recombinant vaccine - proteins from 16 and 18 plus adjuvant AS04 that stimulates toll-like receptors and thereby enhances antibody production
how would you prevent HPV transmission?
vaccines
c-section uncertain in effectiveness
circumcision reduces risk of infection
steps of replicative cycle of hepatitis B
1: virion enters cell
2: uncoating
3: virion DNA polymerase synthesizes missing portion of DNA (remember that hep B has double-stranded circular DNA with two notches/gaps)
4: DNA serves as template for mRNA synthesis by cellular RNA polymerase
5: full-length positive-strand transcript made = template for minus-strand of progeny DNA
6: minus strand then template for plus-strand of genome DNA
catalyzed by reverse transcriptase
RNA-dependent DNA synthesis
some of progeny DNA integrates into host cell genome
where does replication of hep B DNA occur?
within the newly assembled virion core in the cytoplasm
what is malignant transformation of cells? what are morphogenic characteristics of these cells? (review of biochem last term)
when cells change in growth properties, shape and other features when becoming a tumor cell = altered morphology
lose characteristic differentiated shape
appear rounded and more retractile under microscope
rounding due to disaggregation of actin filaments
grow in disorganized, piled-up pattern
lose contact inhibition
grow in vitro at much lower concentrations of serum than non-malignant cells
grow well in suspension - non-malignant cells must be grown adherently
can be easily cloned
what happens to the life-span of cells in culture when infected by a tumor virus?
the cell becomes immortalized - able to continue growing long past the time when when its normal counterpart would have died
how do tumor viruses affect cell cycle phase?
tumor viruses can induce DNA synthesis and move cells that were in G1 phase to S phase
what is the effect of tumor viruses on karotype?
the karyotype of infected cells will change - changes in number and shape of chromosomes - deletions, duplications, translocations
what sorts of antigens will tumor virus infected cells display?
virus-encoded proteins
preexisting cellular proteins that have been modified
previously repressed cellular proteins that are now being synthesized
new antigens are recognition sites for immune surveillance against tumor cells
how will infection by tumor viruses affect agglutination by lectins?
agglutination will be enhanced
lectins = plant glycoproteins that bind specifically to certain sugars on the cell membrane surface
increased agglutination may be due to clustering of existing receptor sites rather than to the synthesis of new ones
what biochemical properties will change in tumor virus infected cells? (summary card)
1: reduced levels of cAMP
2: secrete more plasminogen activator
3: increased anaerobic glycolysis => increased lactic acid
4: loss of fibronectin
5: changes in membrane glycoproteins
how does infection with tumor virus affect cell cAMP production?
reduced production
addition of cAMP to malignant cells can cause reversion to non-malignant state
how does infection with tumor virus affect cell plasminogen activator secretion? (what is plasminogen activator?)
plasminogen activator is a protease that converts plasminogen to plasmin
plasmin dissolves the fibrin clot
malignant cells secrete more
how does infection with a tumor virus affect cell lactic acid production?
Warburg effect - cancer cells have increased anaerobic glycolysis and so increased lactic acid production
how does infection with a tumor virus affect fibronectin production?
fibronectin = high molecular weight glycoprotein
loss of fibronectin in malignant cells
effect of this unknown
how does infection with a tumor virus affect components of cell membranes?
changes in the components of glycoproteins and glycolipids in membranes
does viral material have to be present for the virus to transform a cell into its malignant form?
at least for some viruses
to determine this, rous sarcoma virus was made that is temperature sensitive - active at 35 degrees C but not at 39
infect chicken cells
at 35, these cells are transformed
when incubated to 39, regain normal morphology and behavior
return to 35 and go back to transformed morphology
therefore, continuous production of some functional virus-encoding protein is required for maintenance of the transformed state
how has the onc gene provided evidence that oncogenes exist in normal cells?
DNA copy of onc gene of chicken retrovirus Rous sarcoma was used as probe
DNA in normal embryonic cells hybridized to this probe - indicates that the cells contain a gene homologous to the viral gene
what are the hypothesized precursors to viral oncogenes?
cellular oncogenes
but although they’re similar, they’re not identical
cellular have exons and introns
viral don’t
but viral oncogenes likely acquired by incorporation of cellular oncogenes into retroviruses lacking these genes
therefore, retroviruses can be thought of as transducing agents
what types of proteins can viral oncogenes encode?
- protein kinases that phosphorylate tyrosine (most protein kinases in cells phosphorylate serine)
- cellular growth factors such as EGF
- proteins that have effect at cell membrane, such as G-proteins
- proteins that act in nucleus by binding to the DNA
describe the model of growth control that incorporates the variety of different types of proteins encoded by viral oncogenes.
growth factor binds receptor on cell membrane
membrane g-proteins and tyrosine kinases are activated
these interact with cytoplasmic proteins or produce second messengers
proteins or second messengers transported to nucleus
interact with nuclear factors
DNA synthesis activated
cell division occurs
overproduction or improper expression of any of the proteins in the above sequence can result in malignant transformation
what is the evidence that cellular oncogenes can cause malignant transformation? (5)
1: DNA-containing cellular oncogenes from tumor cells can transform normal cells in culture - genes have single base change
2: characteristic translocations of chromosomal segments can be seen in certain tumors
3: some tumors have amplification (multiple copies) of some oncogenes - results in overexpression of these genes
4: insertion of the DNA copy of the retroviral RNA (proviral DNA) near a cellular oncogene stimulates expression of c-one gene
5: some cellular oncogenes isolated from normal cells can cause malignant transformation if they have been modified to be over-expressed within the recipient cell
what is an example of when translocation is the apparent cause of tumorigenesis?
burkitt’s lymphoma cells
tranlocation moves cellular oncogene c-myc from its normal site on chromosome 8 to a new site adjacent to an immunoglobuilin heavy chain gene on chromosome 14
results in enhanced expression of c-myc
what are the two mechanisms that appear to be able to activate quiescent proto-oncogenes into functioning oncogenes?
mutation
increased expression
how can tumor suppressor genes contribute to oncogenesis? what viruses cause tumors by this mechanism
when mutated may not suppress tumor
like RB
called antioncogenes
HPV and SV40 viruses produce proteins that binds to and inactivates RB gene
HPV also produces protein that inactivates p53
must a virus replicate to have tumorigenic properties?
no. actually, in most cases DNA tumor viruses transform only cells in which they do not replicate
all that’s necessary for transformation is that one or a few of the viral proteins that are oncogenes are replicated
can occur in non-permissive cells
what is the essential step required for a DNA tumor virus to cause malignant transformation?
expression of the early genes of the virus
produce set of early proteins called T antogens
what are T antogens?
proteins produced by early genes of viruses
most located in cell nucleus
for SV40 gene, large T antigen is necessary and sufficient for tumorigenesis
has protein kinase and ATP activity
mostly located in nucleus, but some in outer cell membrane where can be detected by transplantation antigen tumor specific transplantation antigen (TSTA)
what do RNA tumor viruses usually do as opposed to DNA tumor viruses? (ie what genes does each type of virus affect?)
RNA tumor viruses affect proto-oncogenes by introducing, up-regulating or mutating them - affect on the DNA of the host cell - since acting on genes, is somewhat heritable
DNA viruses act on tumor suppressor genes by inactivating them - act at the protein level - sequester or degrade them
what are the 5 ways viruses can transform cells?
1: by inserting an oncogene into the host geneome - only happens in transducing retroviruses - no known human examples known
2: by random insertion into the host genome by altering expression of an existing cellular oncogene - only in non-transducing retroviruses
3: by altering growth properties of cells, rendering them more likely to enter a transformed state - done by non-transducing long latency retroviruses and DNA tumor viruses such as EBV and HHV-8
4: by infecting cells and altering activity/stability of cell cycle-related proteins with or without an integration effect - seen in DNA tumor viruses such as papilloma virus and hepadnaviruses such as hep b may fall into this category
5: other mechanisms in vivo, such as chronic tissue damage that causes regeneration of tissue - such as what hep C does in the liver
hep B may also fall in to this category
why are retroviruses particularly tumorigenic?
+stranded RNA viruses but must go through a DNA intermediate to make mRNA
this DNA integrates into the host cell genome
while some other viruses make DNA, retroviruses are the only ones for which integration into the genome is essential for replication and that can get out - only ones where integration is a viable concern
what is the structure of retrovirus genomes?
two identical strands of +sense RNA
what proteins do retroviruses carry in their nucleocapsids?
reverse transcriptase
integrase - integrates DNA into host genome
protease
what are the three basic categories of transforming viruses? (classified by mechanisms)
transducing
non-transducing
non-transducing, long latency
how do transducing retroviruses cause cancer? how did they become transducing retroviruses?
usually need coinfection with another virus to get integration of genome - lost one of their growth genes and acquired a cellular oncogene
insert their oncogene into the host DNA
How effective are transducing retroviruses at causing cancer? how quickly do they cause cancer?
almost 100% of infected cells will get tumors
tumor latency matter of days
non-transducing (summary card)
slow tumor latency
have not acquired a snark
insert into the wrong spot in genome - cause endogenous genes to be mistranslated
LTR gets inserted - it’s a downstream enhancer - orientation independent so can insert anywhere in genome and can act on genes far away as enhancers or can act as promoters to adjacent genes
how do transducing retroviruses capture oncogenes? (not going to be tested on)
viral DNA has LDRs on the side - strong promoters of transcription - where they insert into the genome, will induce high transcription rates for viral DNA
RNA that’s made and part of genome gets deleted - acquires piece of host DNA downstream from that deletion spot - if it was an oncogene, virus now carries oncogene
can also occur if virus just also replicates snark gene and so has an additional oncogene component
what is the cost of being a transducing retrovirus?
since the end sequence that was replaced by an oncogene is no longer there, the viral DNA can’t get out of the host DNA so can’t replicate unless there’s a helper virus also in the cell that contains the end-gene needed to get the DNA out of the genome
why is insertional inactivation of a tumor suppressor gene very rare?
we have two copies of these genes, so even if one is suppressed, it’s very unlikely that the virus would also suppress the other and so the gene will still be expressed
how could non-transducing RV be used theraputically?
could be used for gene therapy to insert a correct gene into the genome of those who have mutated versions
recently used to reverse SCID, but trial was halted due to the development of leukemia in 3 kids
what is the estimated worldwide incidence of HTLV-1?
20 million worldwide
where is the highest incidence of HTLV-1?
japan
what diseases does HTLV-1 cause?
no clinical manifestations but 2% of female carriers and 6% of male carriers develop adult T cell lymphoma/leukemia
how long is the latency of HTLV-1?
greater than 30 years often
how long do people usually carry HTLV-1?
usually lifelong, but there’s no clinical manifestations
how is HTLV-1 transmitted?
sexually and via IV drug use or transfusion
is there chromosomal location specificity with HTLV-1?
even though the tumor cells are clonal for insertion, there doesn’t seem to be any chromosomal location specificity
which genes does HTLV carry?
gag pol env LTRs (Long Terminal Repeats are not actually genes. They are structural elements in the viral genome that allow for the insertion of functional genes into the host genome.) and Tax
what is tax and what does it do? what virus carries it?
carried in HTLV
transcription factor
tax –> NF(kappa)B –> IL-2 –> proliferation
why does the presence of tax in HTLV-1 increase the likelihood of cancer?
tax leads to increase in IL-2, which leads to increase in T-cell proliferation
this rapid rate of proliferation leads to t cell malignancy
what does it mean for a cell to be clonal?
the virus will insert at the same insertion site in all of the cells from the same person
in what diseases is the xenotropic murine leukemia virus-related virus (XMRV) implicated?
prostate cancer - seen in 27% of prostate cancers
also found in blood of CFS patients
what type of virus is XMRV (of the types of tumor causing viruses)?
it’s unclear as to what group it belongs in
could be non-transducing or long latency
has no known oncogene
what are mechanisms by which DNA tumor viruses can cause cancer?
- interfere/inactivate tumor suppressors - p53 and RB are common targets
- stimulate/interfere with cell signaling
- produce cyclin homologs
- inhibit apoptosis
- inhibit immune response to tumors
how does the SV40 virus cause cancer?
it produces the LT protein, which sequesters p53
how is papillomavirus treated?
antivirals and vaccines
imiquimod (aldara) is a topical cream
potent agonist of TLR7 and IFNalpha
see resolution of lesions from 0-5 months
in what species does SV40 cause cancer?
in various animals but there’s no evidence that it causes cancer in humans
where does EBV usually cause cancer in the body?
Jaw, abdominal, and also in the heart
in what regions of the world does EBV usually cause cancer?
subsaharan/equatorial africa
what other diseases does cancer caused by EBV coinside with?
with highest rates of HIV and malaria
how does EBV cause cancer?
increases size and activity of germinal centers
these germinal centers are the sites of somatic mutation of Ig regions for maturation of Ab responses
LMP-1 protein drives B cells to proliferate - enters germinal centers
greater than 90% of cases of burkitt’s lymphoma have a reciprocal translocation that results in the movement of the proto-oncogene c-myc from chromosome 8 to the proximity of the antibody genes on chromosome 14 => overexpression of c-myc => transformation and cancer
what factors does HHV-8/KSHV rely on for growth?
exogenous cytokines
what does HHV-8/KSHV require to form tumors?
cofactors from HIV/immunosuppression
how does HHV-8/KSHV help tumor growth?
it’s highly angiogenic, so increases blood supply to tumors
viral IL-6 and v-GPCR homologs are essential for tumor growth - these cause cells to replicate
what are the most prevalent tumor causing viruses in the world?
hep B and C
what percentage of the world population are infected with Hep B and C
3-5%
how does hepatitis B cause cancer?
has an x protein that seems to target p53 and fos-myc
DNA may integrate into genome
causes chronic inflammation
how does hepatitis C cause cancer?
likely through chronic inflammation
non-cytolytic
does not have DNA intermediate, oncogene, or DNA integration
