Viral Oncogenesis Flashcards
Common traits of human viral oncogenesis
- Oncoviruses are necessary but not sufficient for cancer development - most people who are infected do not develop cancer from the virus
- Viral cancers appear in the context of persistent infections and occur many years to decades after acute infection
- The immune system can play a deleterious or a protective role in virally induced oncogenesis
How do viruses cause cancer INDIRECTLY
Chronic inefction causes cirrhosis, inflammation, tissue damage, high levels of cell division. This cycle vastly increases the probability that hepatocytes will develop mutations and chromosomal aberrations that derail their growth control
How do viruses cause cancer DIRECTLY
Upon infection, some DNA viruses stimulate the cells to enter S phase of the cell cycle and ready themselves for DNA synthesis. The virus needs this environment for its own replication. If the infected cell is not killed, the same viral proteins can continue to direct the cell to override normal controls on cell growth
Viruses most clearly associated with human cancer
Hepatitis B: hepatocellular carcinoma
Hepatitis C: hepatocellular carcinoma
EBV: lymphoma; nasopharyngeal carcinoma
HHV8: Kaposi’s sarcoma KSHV
HTLV: adult T cell leukemia
HPV: cervical cancer; head and neck cancer
Merkel Cell Carcinoma
Aggrressive skin cancer in elderly and immunosuppressed patients
Papillomavirus characteristics
- Member of the papovavirus family
- Infect cutaneous and mucosal epithelia hands, feet, anogenital tract
- Small circular, double stranded DNA genome
- Many subtypes (about 100)
- 1/3 subtypes infect genital tract (sexually transmitted)
Early and Late genes encoded by virus
- E1 and E2 mediate the replication and transcription of the viral DNA
- E4 disrupts cytokeratins to facilitate virus egress
- L1 and L2 compose the capsid
- E5 stimulates constitutive growth factor receptor signaling
- E6 and E7 neutralize the major “brakes” that regulate the cell cycle - p53 and Rb - and hence uncouples cell division from key regulatory controls
Which viral encoded genes are associated with oncogenesis
E5, E6, E7
Course of Papillomavirus infections
Papillomavirus ⇒ Inoculation of epithelium ⇒ Hand, foot, throat, or cervix ⇒ Local multiplication ⇒ Wart ⇒ Resolution (latency)/Cell transformation
How does HPV affect the epithelium
HPV stimulation of cell cycle causes cells in the stratum spinosum to replicate. Normally, only the stratum basale cells replicate
What are CDKs and how are they regulated
Cyclin dependent kinases move the cell cycle ahead by phosphorylating key substrates
Regulated in many ways:
- Temporally regulated synthesis
- Proteasome-mediated degredation of their cyclin subunits
- Stimulatory and inhibitory phosphorylation events
- Stoichiometric inhibitors
Rb protein
A key “brake” that blocks progression into the S phase
When sufficient Cdk/cyclin has accumulated in G1, Rb becomes phosphoryalted and inactivated and the cell can move into S phase
How is E7 a key regulator of cell cycle progression
The E7 protein binds to Rb, targets it for proteosomal degradation, and so prevents it from blocking the progression to S phase
How is E6 a key regulator of cell “accuracy’’ and why is it important in cellular immortality
- E6 protein recruits a ubiquitin ligase that targets p53 for degradation and therefore prevents it from blocking the progression to S phase or inducing apoptosis
- E6 protein induces the expression of telomerase, which enables cells to maintain their chromosomal telomeres and avoid senescence
What does p53 do?
“guardian of the genome”
Activated by innapropriate entry into cell cycle
Induces the production of Cdk/cyclin inhibitors, and stops the cell cycle - major inducer of apoptosis
What data support the hypothesis that HPV causes cervical cancer
- Sexually active populations have a higher risk of cervical cancer; sexually inactive populations have an extremely low risk
- Greater than 99% of all cervical cancer specimens show evidence of infection with “high risk” strains of HPV strains
- In vitro analysis show a correlation between “high risk” strains and the activity of the E6 and E7 proteins in in-vitro assays of cellular transformation
p53 and Rb are called ______ ______ ______
tumor suppressor genes
During earlier stages of acute and persistent infection, the viral DNA is maintained as an ________
episome
What is the significance of integration?
- Development of carcinoma is associated with integration of DNA into the host chromosome
- Integration disrupts the expression of the E2 gene which is a transcriptional repressor, and allows increased levels of E6 and E7 expression
___ and ___ are always retained in cancer cells
E6, E7
Where does the virus enter the epithelium?
Basal layer
LSIL:
HSIL:
CIN:
LSIL: Low grade squamous intraepithelial lesion
HSIL: High grade
CIN: Cervical intraepithelial neoplasia
Tests to diagnose those patients whose HPV lesions are likely to progress to cervical cancer?
Pap test
HPV DNA
E6/E7 mRNA
L1 protein
p16INK4A protein
Pap smear for CIN I, CIN II, and Cervical carcinoma
Cytological evidense of displasia or neoplasia; detection of koilocytotic cells which are rounded and appear in clumps
Hybrid Capture Assay for detection, Strain analysis, and quantification of HPV DNA
- Release Nucleic Acids - base solution releases target DNA
- Hybridise RNA probe with target DNA
- Capture Hybrids - universal capture antibodies specific for RNA:DNA hybrids
- Label for detection - conjugated with alkaline phosphatase
- Detect, Read and Interpret results - chemiluminescent dioxetane substrate
What are the issues associated with the development of a vaccine that prevents genital infection with HPV?
- Need for mucosal infection due to route of infection
- Need to recognize multiple high risk strains
- Administration of a vaccine that included oncogenes not possible
- Societal issues - is this a vaccine to prevent cancer or STD?
Capsid Protein L1
Induces protective humoral immunity
Can be produced in culture
Self-assembles into virus-like particles (VLPs) that resemble virions and induce appropriate immunological response
Vaccine containing purified VLPs: what does it do?
Induces protective humoral immunity
Antibodies present at mucosal surface prevent incoming HPV from establishing infection
Currently Licensed Vaccines
Gardasil
Cervarix
True Or False? THe HPV vaccine should only be used in women
FALSE!
Vaccination of boys/men will benecessary to also protect men and deplete the high risk HPV strains from the population
