Pathogens and Cancer Flashcards
give examples of what pathogens can cause cancer:
EBV
Hep B/C virus
Kaposi’s sarcoma herpes virus
HPV
Merkel cell polyomavirus
Helicobacter pylori
how is HIV linked to cancer?
HIV is a cofactor for cancer as it causes immunosuppression:
- Virus replicates in CD4 cells – loss of CD4 immunity
- Patients can then develop AIDS
- Opportunistic infections likely in AIDS such as herpes virus, EBV, KSHV can cause cancer
HIV increases risk of cancer due to loss of immunity
what was the first virus found to be associated with cancer?
EBV first virus found to cause cancer
what does EBV infect?
Infects B cells
- Remains latent for lifetime in B cells
- Can transform B cells into immortalised proliferating cell lines using EBV in vitro
what cancers can EBV cause?
EBV can cause a whole range of cancers, not just one or 2 – , Burkitt’s, Hodgkin’s, B cell cancer, head and neck epithelial cancer, gastric carcinoma, NK/T cell lymphoma, leukaemias
what cancers do different viruses cause?
- HBV/HCV = hepatocellular carcinoma
- HPV = carcinoma of cervix, vagina, penis
- KSHV = Kaposi’s sarcoma, B cell lymphoma (primary effusion)
- MCV (Merkel Cell Polyomavirus) = Merkel cell carcinoma
how do viral-induced cancers differ across the world?
Viral-induced cancers have different global distribution:
- High prevalence in china, sub-Saharan Africa, south America
- These places have poor screening programs – high rates of cervical cancer caused by HPV
- Cofactors can drive virally-induced cancer e.g. food storage, nitrosamines
what are the features of polyomaviruses?
- enveloped
- circular dsDNA
- a small virus - 5000bp
- genome replication and virion assembly occurs in host nucleus
what is an example of a polyomavirus?
Merkel Cell Polyomavirus
what important proteins does MCV have?
- 3 capsid proteins VP1-3 to enable receptor-mediated entry
- capsid contains viral nucleic acids – dsDNA
- Nucleic acids contain late proteins (capsid proteins) and early, non-structural proteins (large T, small T, middle T and AGNO protein)
- Large T and Small T can manipulate cell cycle and drive cancer
what is contained within the genome of MCV?
- origin of replication - important for Large T to enable replication of viral genome
- regulatory region contains gene promoters and origin of replication
what does Large T control?
Large T controls replication of the viral genome
why does MCV depend on the host to replicate?
It is a small virus so doesn’t encode its own replicative enzymes
- manipulates host cell cycle before the cell can recognise that it has been infected
- they use the host DNA synthesis machinery to produce their viral proteins during S-phase
what genes of MCV function prior to DNA replication (early gene expression)?
Genes that function prior to DNA replication:
- Large T
- Middle T
- Small T
how does Large T induce replication?
Large T is transcribed and binds the replication origin to recruit host RNA pol II which recognises the regulatory region promoter of the viral genome
- Synthesis of T antigens followed by DNA replication
- Large T unwinds DNA drives the cell cycle
what occurs during late gene expression in MCV?
Occurs from new DNA template
- Capsid proteins synthesized in cytoplasm
- Capsid proteins bind DNA in nucleus
- Cell lysis - viral particles are released
what normally leads to cell cycle progression?
Mitogenic signal binds to surface receptor
- transmits signal to nucleus to upregulate cyclin D
- Cyclin D activates CDK4/6
- phosphorylation of Rb and release of E2Fs for S-phase gene transcription
how does MCV manipulate resting cells?
Polyomaviruses manipulate resting cells to enter the cell cycle
- This is mostly achieved through the action of Large T
- Large T directly binds Rb allowing entry into S phase without the need for mitogenic signals
what happens when Large T binds Rb?
Large T binds Rb
- This detaches the E2F TFs
- Phosphorylation of Rb is no longer needed to liberate E2Fs, meaning the cell enters cell cycle
- enables rapid replication of viral DNA
what happens when Large T promotes cell cycle progression?
Inappropriate entry into S-phase is detected, so p53 is activated
- virus needs to inactivate p53 to avoid arrest or apoptosis
how does MCV inhibit cell cycle control?
Large T can bind and inhibit Rb
Small T inhibits p53 by upregulating MDM2
what is MDM2?
an E3 ubiquitin ligase which can induce the degradation of p53 at the proteosome
how does small T function to inhibit p53?
Small T cannot bind to the host DNA itself, but can interact with TF complexes MycL and EP400
- these TFs will bind to the host DNA and upregulate MDM2
- MDM2 can then induce the degradation of p53 via ubiquitination
why is Large T indispensible for MCV replication?
- Large T binds to the origin of replication of the viral DNA.
- Acts a helicase to unwind the DNA.
- Recruits host DNA polymerase complex to this site and initiates DNA replication
what normally happens when a virus induces its replication in the host cell?
Virus antigens are replicated e.g. viral DNA, capsid
- Usually, new infectious virions will be released from the cell and kill the cell due to bursting
- So how does the host cell become cancerous?
what is Merkel Cell Carcinoma?
- rare but aggressive skin cancer of Merkel cells
- higher incidence in immunosupressed: 10x in AIDS, 5x in transplant patient
- present on exposed areas of body susceptible to UV - similar appearance to melanoma
- increased risk with age due to immunosenescence
- tumours mostly appear on head and neck
what is a merkel cell?
Light touch receptor –mechanoreceptor that synapses with somatosensory afferent nerves – can receive and signal fine contact
- Found just under the skin epidermis
how did scientists discover MCV?
digital transcriptome subtraction:
- Took tissue from healthy area and carcinoma region
- generated cDNA and sequenced the transcriptome
- subtracted the healthy transcriptome from the cancer transcriptome, with the remainder being the genome of the virus
- left 2400 sequences which showed homology to a monkey polyomavirus
- the viral DNA was found clonally integrated into the host cell genome
- this virus was found in >80% of merkel cell carcinoma
what are the key domains of Large T, identified by sequencing of MCV?
Large T has an Rb binding domain, origin binding domain and helicase domain to unwind DNA
- the helicase domain assembles on the origin and opens the origin sequence to allow initiation of cellular polymerase-mediated replication
- Enables replication of host cell genome
what is the model for MCV cause of cancer?
- most people are infected with the polyomavirus, but is kept under tight immune control so it doesn’t replicate
- If it does replicate, it destroys infected cells – no cancer
- If under immune suppression due to HIV, transplant or senescence, there is increased viral replication
- first insult may be UV, which induces dsDNA break in genome allowing virus to integrate into host genome via non-homologous recombination
what does sequencing of Large T in merkel cell carcinoma show? how does this affect its function?
Large T sequences from MCC samples showed that these consistently had mutations
- truncating mutations via stop codon insertion
- Rb-binding domain remains intact
- but helicase and origin binding domains are lost
Truncated large T can still bind Rb to induce progression into S-phase
- but Large T can’t unwind the DNA and transcribe the viral proteins, so virions are not generated and host cells don’t lyse
- mutations leads to excessive proliferation of infected cells - continual cell cycle where cells do not die, as viral replication has been disrupted
how was mutant large T shown to be crucial to maintain merkel cell carcinoma proliferation?
Ablated large T expression with siRNA:
- All MCV positive MCC cell lines underwent growth arrest and senescence when large T expression was blocked.
- Indicates large T is required to maintain MCC continuous growth
- Strongly suggests MCV is the causative agent of MCC.
what does human papilloma virus cause?
HPV can cause warts, verrucas, continual proliferation of keratinocytes without shedding, as well as cervical, head and neck cancer
what is the genome size of human papilloma virus (HPV)?
- small genome - 8000bp
how does HPV infect and replicate?
- HPV enters via microabrasions in the skin and infects basal epithelial cells
- HPVs replicate as extrachromosomal plasmids or episomes – integration into host DNA is not a normal part of the life cycle
why does the life cycle of HPV depend on basal epithelial cells?
The HPV life cycle critically depends upon the induction of terminal keratinocyte differentiation.
- Basal cells actively replicate to enable viral genome amplification, where the virions can assemble
- As basal cells differentiate and come out of cell cycle, they become keratinocytes at the top which are dead and shed off
- the differentiated, dead keratinocytes then release the virions
how does HPV use the host machinery for replication?
Small virus, so doesn’t encode own DNA replication machinery
- it requires host cell for replication
- Infects basal epithelial cells as these actively replicate and can provide replicative enzymes
what viral proteins does HPV express within basal epithelial cells?
During cell proliferation and viral genome amplification, HPV expresses of early viral proteins in basal cells e.g. E1, E2, E4, E5
What is the role of the HPV E2 protein?
E2 protein inhibits the expression of E6 and E7 in basal cells
- In the normal virus life cycle, expression of E6 and E7 is tightly regulated by E2 in replicating cells
- when basal cells begin to come out of cycle and differentiate, HPV expresses E6 and E7
how do low risk and high risk HPV differ?
low risk:
- E6 and E7 expression stimulates cell cycle entry in the upper epithelial cell layers allowing genome amplification
High risk:
- E6 and E7 expression stimulates cell cycle entry and cell proliferation in the lower and middle and upper epithelial layers to allow genome amplification and leading to neoplasia
how does E7 induce neoplasia?
Similar mechanism of polyomavirus and HPV
- E7 binds Rb and displaces E2Fs
- When cells begin to come out of cycle, E7 is expressed and causes cells to enter S-phase by inactivating Rb
- this activates p53
how does HPV inhibit p53 to enable cell cycle progression?
HPV encodes E6 to recruit E6AP ubiquitin ligase:
- Ubiqutinates and degrades p53 to avoid cell cycle arrest
- Lack of p53 leads to accumulation of DNA mutations which cant be detected – accumulation of mutations with no apoptosis = cancer
why does the expression of E6 and E7 become deregulated when HPV integrates into the host DNA?
Integration of HPV genome into host DNA disrupts E2 control of early promoter
- 4 cut sites in the viral genome block E2 activity once integrated, leading to upregulation of E6 and E7 oncoproteins
- continuous expression of E6 and E7 proteins retained in cancers = uncontrolled cell cycle progression
- necessary for malignant progression
what does loss of E6 and E7 induce in cancers?
knockdown of E6 and E7 in cancer cells leads to senescence/apoptosis
- important to maintain the transformed phenotype
what are the risk factors for cervical carcinogenesis?
- HPV persistence at the cervix - virus isn’t cleared
- HPV genome integration - deregulation of E6 and E7
- co-factors e.g. smoking
what are the features of herpesviruses?
- Larger viruses – 230kbp genome
- Encode a large array of enzymes involved in: nucleic acid metabolism, DNA synthesis and processing of proteins
- encodes sufficient enzymes for DNA replication - doesn’t rely on host cell polymerases
- relies on host cell for energy and nucleotides, not enzymes
- viral DNAs and capsid assembly occurs in nucleus - gain envelope as they bud from plasma membrane
- when they leave, they destroy the host cell
what are the 2 life cycles of herpesvirus?
Lytic cycle
latent cycle
what is the lytic cycle of herpesvirus?
viruses actively replicating
- Encodes its own enzymes to replicate DNA during lytic cycle – doesn’t rely on the host cell for replication enzymes at this point, just relies on host energy and nutrients
what is the latent cycle of herpesvirus?
Latent cycle - enter latency one they have infected a cell
- persists for entire lifetime and hides from immune system
- Downregulates viral protein expression – become less immunogenic so aren’t detected
what occurs during herpesvirus latency?
Upon infection of a cell the viral genome can circularise to form an episome
- Few if any viral genes are expressed from the viral episome
- Some viral genes may be expressed to facilitate establishment of latency
- No viral particles are produced in this state - no longer immunogenic
- However the virus has the capacity to reactivate from this form and produce new infectious particles when the B cell differentiates
what is an example of a herpesvirus?
Kaposi’s sarcoma herpes virus (KSHV)
what are the features of KSHV?
Encodes up to 86 genes, 22 of which have immunomodulatory capacity
- Even when the virus expresses multiple antigens, they can stop MHC trafficking, stop peptides shuttling through TAP, keep peptides from leaving ER
- overall reduces antigen presentation
how is KSHV transmitted and what does it infect?
Virus appears to be saliva transmitted but evidence for other routes also exists
KSHV infects B cells and endothelial cells
- can establish latency in B cells
what diseases can KSHV cause?
Kaposi’s sarcoma - endothelial tumours
Primary effusion lymphoma - B cell malignancy
Multicentric castleman disease - B cell pathology
how is KSHV distributed?
Seroprevalence correlates with disease incidence
- KSHV is necessary but not sufficient to cause KS
- highest rates in Africa and South America
what viral proteins does KSHV encode in Kaposi’s sarcoma?
Lana
V-cyclin
V-flip
kaposin
what is LANA in KSHV?
Latency-associated nuclear antigen
- expressed in all KS cancer cells
what is the function of LANA?
LANA tethers KSHV episome to host chromosomes:
- when cell replicates, LANA can bring host DNA machinery to KSHV origin
- when cells undergo mitosis, equal copies of viral genome are passed to daughter cells
LANA inhibits tumour suppressor function of p53 and Rb
LANA inhibits TGFb production – no CDKi activation – cell cycle dysregulation
what is the role of V-cyclin in KSHV?
V-cyclin is a cyclin D homologue:
- V-cyclin destabilises cell cycle control
- No need for mitogenic signal at cell surface, as virus already expresses V-cyclin to replace cyclin D
what is the role of v-FLIP in KSHV?
vFLIP is anti-apoptotic
- activates transcription factors associated with expression of pro-survival and proinflammatory genes
- e.g. BCL-2
what is the role of Kaposin in KSHV?
Complex of proteins which stabilise mRNA of pro-inflammatory cytokines
- Chronic inflammation can cause epigenetic changes to cells which silence tumour suppressor genes
what are the overall effects of KSHV on host cells?
KSHV can keep cell in cycle, inhibit CDKIs, can create inflammatory environment, can block apoptosis
what is primary effusion lymphoma?
Aggressive malignancy seen in late stage AIDS patients
- Cells are of B lymphocyte origin transformed by KSHV - cancer of plasmoblasts
- In many cases these cells are co-infected with Epstein-Barr virus
- B lymphocyte shows features of a differentiated B lymphocyte/plasma cell
- Usually present as an effusion in peritoneum or pericardial spaces
-Few treatment options
what viral proteins does KSHV encode in primary effusion lymphoma?
- LANA
- V-cyclin
- V-FLIP
- Kaposin
- v-IL6
- v- IRF3
what is the role of V-IL6 in KSHV primary effusion lymphoma?
Cytokine shows homology to human IL-6
- Binds to human IL-6 receptor but bypasses regulatory receptor
- Important for promoting B cell proliferation
- Has proinflammatory activity
what is the role of V-IRF3 in KSHV primary effusion lymphoma?
Viral interferon regulatory factor 3
- Inhibits components of the innate interferon anti-viral pathway
- Inhibits expression of class II MHC by interfering with transcriptional transactivators
- Promotes survival of primary effusion lymphomas
what is Helicobacter pylori?
- Colonizes the gastric mucosa of >50% population
- Can cause gastritis, gastric atrophy, gastric ulcer, gastric cancer, MALT-lymphoma
- Most infected people have an asymptomatic chronic active gastritis
- Can be treated with antibiotics but resistance is increasing
where is H. pylori most prevalent?
Predominately in developed countries
- Likely due to diet, and due to use of proton-pump inhibitors
in what stages of gastric cancer is H. pylori present?
H. pylori is in the stomach of people with early gastritis and at the intestinal metaplasia stage:
- H. pylori changes pH and microbiome of stomach as the cells change
- This causes the microbiome to outcompete helicobacter – so not seen in the gastric carcinoma itself
how does H. pylori induce gastric adenocarcinoma?
Helicobacter strains encoding the cytotoxin associated gene A (CagA) and VacA
- CagA strain are associated with increased risk of gastritis and gastric cancer
- Helicobacter can invade gastric glands and inject CagA into cells on the epithelium it colonizes
what are the functions of CagA produced by H. pylori?
- tethers to inner leaflet of plasma membrane via phosphatidylserine
- inactivates p53 to prevent apoptosis
- activates NF-KB to drive inflammation
- triggers MAP/ERK pathway to drive proliferation and change epithelial polarity, leading to loss of E-CAD to drive EMT and invasion
how does CagA drive EMT?
- Disrupts adherens E-CAD junctions – lose cell polarity
- Upregulates MMPs – detachment from ECM
- Drives N-cad expression
- epithelial cells become less polarised, more mesenchymal and migratory – can invade local tissue
how do T cells respond to H. pylori?
Tumour infiltration of T cells correlated with increased survival time in gastric cancer
T reg thought to decrease likelihood of inflammation however may prevent clearance
Achieving a balance appears crucial
how can H. pylori induction of gastric cancer be prevented?
Preventing infection may help or specifically targeting CagA expressing cells may be beneficial
- Antibiotics useful and can be screened for
