Epstein-Barr virus and Cancer Flashcards
1
Q
What is Epstein-Barr virus (EBV)?
A
The fourth human herpesvirus (HHV4)
2
Q
How was EBV discovered?
A
- in 1958 Burkitt found lymphoma in children that as derived from viral particles discovered by Epstein and Barr
- It was similar structurally to other HHVs but has distinct biological activities
3
Q
What are the 3 subfamilies of HHV?
A
- alpha = HHV1/2, respiratory carcinomas
- beta = HHV5, Kaposi’s sarcoma
- gamma = EBV, Burkitt’s lymphoma/nasopharyngeal carcinoma
4
Q
What are some characteristics of EBV?
A
- member of the gamma subfamily of HHVs
- genome is a 172kb linear ds molecule that circulates in infected cells
- infects B cells and other certain epithelial cells
- efficiently immortalises resting B cells
5
Q
What are diseases associated with EBV?
A
- infectious mononucleosis (mono)
- Burkitt’s lymphoma
- nasopharyngeal carcinoma
- Hodgkin’s disease
- B cell lymphoma in immunocompromised hosts
6
Q
How does EBV infect B cells?
A
- infect via the viral glycoprotein gp350 which binds to the CD21 receptor found on B cells
- 2 regions of homology exist between the receptor and EBV which allows binding of the viral envelope and cell membrane
- EBV enters the B cell and the viral capsid dissolves to allow the viral genome to be transported to the nucleus
- can only infect B cells at certain times during their growth and maturation
7
Q
How does EBV infect epithelial cells?
A
- infect via viral protein BMRF-1 which binds to the cellular b1 integrins of the cell
- the viral protein gH/gL also binds with the integrins which triggers fusion of the viral envelope with the cell membrane
- EBV enters the B cell and the viral capsid dissolves to allow the viral genome to be transported to the nucleus
8
Q
How does EBV transmit?
A
- primary infection is usually asymptomatic in childhood
- virus is secreted in the saliva and is transmitted orally
- by the age of 40, almost ever everyone is infected
- could produce a lytic infection where <80 genes may be expressed
9
Q
How does EBV work latently?
A
- the initial lytic infection is followed by infected circulation of B cells
- results in persistent viral DNA in nucleus which establishes a latent infection
- latency depends on the differentiation stage of the cell
10
Q
How does EBV go from lytic to latent?
A
- lytic replication immediately follows viral entry (linear genome)
- once contact with CD21 receptor expressing B cells is made with EBV gp350, the virus enters the B cells and becomes latent (circular genome)
- the virus can then be reactivated back to lytic replication
11
Q
How can EBV lytic infection be treated?
A
- replication uses DNA polymerase
- Acyclovir can be phosphorylated by viral kinases that are only expressed in the lytic cycle
- it is converted into acyclovir triphosphate which can terminate viral DNA synthesis
12
Q
What can be seen from EBV infected cells placed in culture?
A
- EBV infected B cells give rise to a spontaneous out growth of EBV transformed cell line
- this is called lympoblastoid cell lines (LCL)
- LCLs can also be generated by infecting resting B cells with EBV which is useful for research
- every cell in LCLs express a limited set of viral gene products
13
Q
What latency type do EBV encoded genes belong in?
A
- Type l = EBNA1, EBER1/2
- Type ll = EBNA1, EBER1/2, LMP1/2
- Type lll = EBNA1/2/3, EBER1/2, LMP1/2
14
Q
Which latency types are found in different EBV infected cells?
A
- in epithelial cells, only ll is found
- in naive B cells, lll is found
- in GC infected cells, ll is found
15
Q
What is the function of EBV gene EBNA1?
A
- expressed in all EBV infected cells irrespective of latency type
- found in nucleus
- role in maintenance of genome through binding to origin of replication and allowing replication
- not recognised by cytotoxic T cells
- contains Gly-Ala repeat sequences to prevent degradation
- increases survival of infected B cells
- binds to p53 regulatory protein USP7 to lower p53 levels
- could be a target for future therapy
16
Q
What is the function of EBV gene EBNA2?
A
- found in nucleus
- transcriptional regulator and activator of several viral and cellular genes (eg LMP1/2 and CD23)
- interacts with DNA binding protein RBP-Jk to activate LMP and CD23
- deletion results in inability to transform B cells
- phosphorylation results in suppressed activity
17
Q
What is the function of EBV gene EBNA3?
A
- comprised of 3 related proteins (EBNA3A/B/C)
- found in nucleus
- function as transcriptional regulators
- 3A and 3C are essential for B cell transformation, 3B is dispensable
- EBNA3 proteins associate with RBP-Jk transcription factor
18
Q
What is the function of EBV’s LMP1?
A
- found in membrane
- main transforming protein of EBV
- major effector of virus induced cellular changes
- inhibits apoptosis by activating NFkB
19
Q
What is the function of EBV’s LMP2?
A
- found in intrinsic membrane
- prevents EBV infected B cells from entry into lytic cycle
20
Q
What is the function of EBER1/2?
A
- non-polyadenylated RNAs that are essential for EBV transformation of B cells
- induce IL-10 to suppress cytotoxic T cells
21
Q
What is Burkitt’s lymphoma?
A
A disease caused by EBV
22
Q
What are the symptoms of BL?
A
- abdominal inflammation
- swollen lymph nodes
- weakness/paralysis
- sweating
- weight loss
23
Q
What is endemic BL?
A
- type of BL caught by children in Africa who have reduced resistance due to chronic malaria infections
- effects 2-16 yrs old
- tumour doubles every 24 hours so needs urgent treatment or will be fatal
- can be treated with cyclophosphamide which induces apoptosis to shrink tumour
24
Q
What are the 3 types of BL?
A
- Endemic = children in Africa/New guinea)
- Sporadic = children/adults outside the endemic areas (15-20% EBV/30-40% HIV)
Immunodeficiency related = in association with HIV
25
Q
What latency type is mostly exhibited in BL?
A
Type l latency
26
Q
How does EBV cause BL?
A
- all BL cases carry 1 of 3 chromosomal translocations that place the MYC gene under regulation of the Ig heavy chain
- this deregulates c-Myc expression
- BL is associated with malaria/HIV which increases the chances of MYC translocation
- EBV provide a pool of B infected cells to aid c-MYC translocation
- EBNA1 aids cell survival and prevents apoptosis
27
Q
What is nasopharyngeal carcinoma (NPC)?
A
- a rare tumour arising from the epithelium of the nasopharynx
- more common in east Asia and Africa
- EBV and dietary/genetic factors are associated with NPC
- smoking/alcohol consumption and exposure to certain metals are also risk factors
- 50-70% of tumours show detectable LMP1 protein and 100% have LMP1 transcripts
28
Q
What are the symptoms of NPC?
A
- blood stained post nasal discharge
- impaired hearing
- tinnitus
29
Q
Which 3 ways do NPC tumours present?
A
- Proliferative growth causing nasal obstruction
- Ulcerative causing epistaxis
- Infiltrative causing cranial nerve involvement
30
Q
What treatments are there for NPC?
A
- external beam radiation therapy
- combination of chemo and radio therapy
- recurrent/persistent NPC is still a challenge to treat
31
Q
What is the structure of LMP1?
A
- 66kDa integral membrane protein comprising of short amino acid N-terminus, six hydrophobic alpha-helical transmembranes and a C-terminal tail
32
Q
What is the role of LMP1 and p53 in NPC?
A
- majority of NPC tumouors contain p53 genes and LMP1 can modulate its activity
- LMP1 has a growth advantage in tumours containing mutated p53 genes
- LMP1 can also repress p53 mediated DNA repair
33
Q
How is protein synthesis in NPC inhibited by elF2a
A
- PKR is activated through antiviral mechanisms
- active PKR phosphorylates polypeptide chain initiation factor elF2a which inhibits protein synthesis
- prolonged inhibition will lead to apoptosis
34
Q
How does EBER1 function in vivo?
A
- expression inhibits PKR and protects against IFN induced apoptosis
- it expresses growth factors IL-9 and IL-10
