10. The Pathogenicity of Viruses - Papillomavirus

Question 1

What conditions are caused by the human papillomavirus?

Answer 1

Warts and some subtypes can cause cancer.

Question 2

What cancers have been associated with papilloma virus?

Answer 2

1. Cervical cancer
2. Tumours of the urogenital tract
3. Upper airway cancers

Question 3

How is the papillomavirus transmited?

Answer 3

Direct contact
Sexual contact
During birth from mother to child

Question 4

Describe the genome of the Papilloma virus.

Answer 4

Non-enveloped circular double stranded DNA, about 8000 bp in length enclosed by a capsid containing 8 different genes.

Question 5

Name the 8 Genes of HPV.

Answer 5

1. E4
2. E5
3. L2
4. L1
5. E6
6. E7
7. E1
8. E2

E1, E2, E4, E5, E6, E7, L1, L2

Question 6

How many types of papillomavirus are known?

Answer 6

Over 100

Question 7

What cell types or tissues structures does the papillomavirus infect?

Answer 7

Squamous epithelial cells of skin (keratinocytes) or mucous membranes.

Question 8

How does papillomavirus facilitate attachment to their target cells?

Answer 8

1. Initially papilloma virus binds to heparan sulfate proteoglycans (HSPGs) on the basement membrane exposed due to epithelial trauma or permeabilization.
2. This binding causes a conformational change in the capsid to expose an N-terminal L2 peptide motif.
3. This exposed L2 peptide motif is cleaved by furin or proprotein convertase 5/6.
4. Cleavage induces a conformational change that exposes a binding site on the capsid to be exposed to an unidentified host cell receptor.

Question 9

Describe the process of entry of the HPV once attachment has occured.

Answer 9

Internalisation of the PV occurs by endocytosis. It is thought to occur via clathrin-dependent and clathrin-independent endocytosis however recent studies have also implication caveolin-dependent uptake.

Question 10

What is particularly remarkable about the internalisation process of HPV?

Answer 10

It takes a significantly longer time than most other viruses, approximately 2-4 hours with some capsids never being internalised.

Question 11

How does the HPV genome escape the endosome after internalisation?

Answer 11

The capsid shedding occurs in the endosome.

The process of vDNA release is not very well understood however studies have shown the requirement of the protease ‘gamma-secretase’ for HPV infectivity and therefore it is suspected to play a part in endosomolysis.

Question 12

Once internalised where does the HPV migrate to?

Answer 12

The nucleus

Question 13

How is HPV trafficked to the nucleus?

Answer 13

It is not certain however theories suggest via microtubules facilitated by the motor protein dynein.

It is also suggested that nuclear entry requires nuclear envelope breakdown due to the requirement of cell division for infection to occur.

Question 14

How long does it take for HPV DNA to enter the nucleus?

Answer 14

~24 hours

Question 15

Once within the nucleus, where does the HPV DNA localise ?

Answer 15

Specific nuclear structures known as N10 bodies

Question 16

What is the effect of HPV localisation to N10 bodies?

Answer 16

The initiation of transcription.

Question 17

What genes are first expressed during transcription?

Answer 17

E6 and E7

Question 18

What is the role of E6 and E7?

Answer 18

E6 and E7 protein products result in non-scheduled cellular replication by reacting with the cell cycle control proteins p53 and pRB respectively.

E6-p53
E7-pRB

Question 19

What is the role of E1 protein?

Answer 19

E1 is a phosphoprotein which binds specifically to the origin of replication and aids in the initiation and elongation of viral DNA synthesis during the process of replication.

Question 20

What is the purpose of E2 transcription?

Answer 20

E2 protein regulates transcription by binding to E2 binding sites and initiating their transcription.

Also contributes to replication by binding to E1 and enhancing its ability to bind to the origin of replication.

Question 21

Which genes are associated with the HPV viral capsid?

Answer 21

L1, L2 and E4 genes.

Question 22

What occurs to HPV when unscheduled host cell replication occurs?

Answer 22

Replication of the HPV genome to form more viral particles which are past onto daughter cells.

Question 23

Describe the process of virion assembly

Answer 23

L1 and L2 proteins enter the nucleus mediated by karyopherins where the capsid is formed around copies of the genome.

Question 24

Describe the process of virion release.

Answer 24

Virions remain in the dividing cells until the host epithelial cell completes is differentiation into terminally differentiated squamous epithelium.
HPV virions are not thought to be lytic in nature and therefore release of the virion is suspected to occur as a result of normal loss of nuclear and cytoplasmic membrane integrity custom of terminally differentiated squamous cells, releasing virions into the environment.

Question 25

Which HPV genes are associated with oncogenesis?

Answer 25

E5, E6 and E7

Question 26

How does HPV alleviate the immune response?

Answer 26

Most forms of HPV exert no pathogenic consequence and so avoid an immune response to its presence.

Virion proteins are not expressed on the surface of living cells preventing infected cell detection.

Cancerous HPV infected cells are genetically unstable and rapidly evolve to evade immune recognition. Defects in MHC class I proteins are also common in these cells preventing their detection by cytotoxic t cells.