Animal Viruses

Question 1

What family does poliovirus belong to?

Answer 1

Picornavirus

Question 2

Which virus although rarely does it, can infect the CNS and cause paralysis?

Answer 2

Poliovirus

Question 3

Which virus does not require elongation factors and how does it work?

Answer 3

Poliovirus mRNA contains an internal ribosome entry site which is the secondary structure that binds to the ribosome on its own without the host proteins.
Normally elf4E and elf4G recruit the rest of the complex and the 40S ribosomal subunit on the mRNA. Having found the first AUG, it then recruits the 60S subunit.
The IRES binds to the 40S subunit and places the AUG in the correct position for initiation.

Question 4

How is poliovirus mRNA translated?

Answer 4

3CD proteins accumulate on the clover leaf which recruits a polymerase to the cis-acting response element. This polymerase adds U residues to the VPg protein which already has a few nucleotides attached to it and acts like a primer. The primer then attaches to the polyA tail at the 3’ end as is elongated to make the (-) strand. The two strands are separated by a helicase, a second uridylation reaction is carried out for the (+) strand to be made. The + strand forms its secondary structure immediately which causes it to dissociate.

Question 5

Give two examples of picornaviruses

Answer 5

Poliovirus and hepatitis A

Question 6

Describe the Salk vaccine for polio

Answer 6

Developed in 1955. Inactivated by chemical or physical procedures but still generates a response because the capsid is still there. No risk of reversion BUT it is expensive, only delivered through injection and has little intestinal immunity.

In the Cutter incident 260 vaccinated children had poliomyelitis and left 192 children paralysed because the virus was not inactivated properly. Vaccine then withdrawn.

Question 7

Describe the Sabin vaccine for polio

Answer 7

Developed in 1963. Mutations are attenuated. These vaccines are the viruses that propagate in different conditions to those in the normal host so that they are temperature sensitive and cold adapted. This limits their reproduction & makes them less pathogenic in warm blooded animals.
Advantages - can be ingested orally (mimicking natural route of infection) and making it easier to administer, cheaper, replicates in intestine so easier to achieve herd immunity as it can be passed on through stools.

Question 8

What are the differences between the three types of influenza?

Answer 8

Influenza A and B are extremely similar except B is seasonal and does not cause a pandemic. Influenza C has very mild symptoms so does not cause any disease at all most of the time.

Question 9

What drugs are used against influenza and how do they work?

Answer 9

Amantadine blocks the M2 ion channel. Tamil blocks neuraminidase. Relenza is another NA inhibitor. Both of these two mimic siliac acid.
No drugs for HA because they have such a high affinity binding.

Question 10

What is the function of neuraminidase?

Answer 10

It is a membrane protein with sialisidase activity. It prevents viruses from binding to each other and prevents HA from binding to an infected cell.

Question 11

Describe the process of RNA transcription in influenza

Answer 11

RNA molecules have a nuclear localisation sequence that takes them from cytosol to nucleus. No cap or polyA tail so PB2 binds a host cap, PB1 cleaves it off and uses it as a primer. The polyA tail is made by stuttering through polyU in the template.

Question 12

Describe the morphologies of the two filoviruses

Answer 12

Both are threadlike, Marburg is rod shaped whereas ebola is more elongated

Question 13

What category is HIV in?

Answer 13

Category 3 because even though people rarely die from it, it has no cure.

Question 14

What category is Marburg?

Answer 14

Biosafety 4 (highest). Due to its high infectivity, how easy it is to spread and its high mortality.

Question 15

How are filoviruses transmitted?

Answer 15

Through contact with body fluids such as blood, breast milk and semen.

Question 16

What treatment is currently used for marburg/ebola?

Answer 16

Anti-clotting agents try to keep the patient stable for long enough until the immune system has time to kick in and kill the virus off. Anti-coagulants stop bleeding from the wrong places. Steroids used to keep patients from shock.

Question 17

What is the GP protein in ebola?

Answer 17

Major antigenic peptide, 80% of them become soluble glycoproteins whereas the other 20% are membrane bound. This 20% is made by stuttering which adds an extra A residue resulting in a full length protein.

Question 18

How does ebolavirus enter the cell?

Answer 18

Virus binds to the cell through its GP. The TIM1 receptor on cell surface binds phosphatidyl serine on virion membrane. This cause macropinocytosis (an actin-dependent process) and the virus is engulfed into an endosome.
In some cells the host cathespin L and cathespin B process the GP glycoprotein. The GP glycoprotein then mediates fusion between the virion and host cell membrane.
Ribonucleocapsid is released into the cytoplasm.

Question 19

Why was the outbreak in West Africa a lot worse than those in East and Central Africa?

Answer 19

Was a new disease so locals did not understand it as much and take preventative action. Poverty meant that health system is poor - not enough bed spaces to treat people, doctors and nurses also became infected.

Question 20

How does Rous Sarcoma Virus cause cancer?

Answer 20

It is the only virus which has gained an oncogene and has had its gag, pol and env genes remain intact. It contains an altered version of the src gene. This encodes tyrosine kinase which when activated by dephosphorylation, it increases cell mobility, causes the breakdown of extracellular matrix & deregulates p53.
The src encoded by RSV cannot be turned off.

Question 21

How does HaMLV transform a cell?

Answer 21

The captured oncogene is ras. Ras proteins have GTPase activity and link extracellular and intracellular signals. For example is a growth factor binds to the receptor it may cause ras to become active. The amount of ras must be controlled as too much can cause cancer. The ras encoded by HaMLV is under the viral promoter so cannot regulate its expression.

Question 22

How can cells be chronically transformed?

Answer 22

The retrovirus itself does not contain an oncogene but it inserts upstream of a cellular proto-oncogene. This is rarer as the retrovirus must insert in a particular place and this takes longer to occur.
For example, the oncogene may now be under the control of the viral promoter so you have direct expression of the oncogene.
Or the oncogene may be under the control of a viral enhancer and this can occur over a longer distance, increasing the expression of c-onc.

Question 23

Give an example of a chronically transforming virus

Answer 23

MMTV, if this inserts upstream of int-1 it causes tumours in mammary tissues. This is because this gene is only expressed in embryos and switched off in adults. This is an example of expression at the wrong time.

Question 24

What is the third way of a virus transforming a cell?

Answer 24

Retroviruses with transacting functions

Question 25

Give an example of transformation via virally encoded transactivators

Answer 25

ATLV (aka HTLV1) encodes protein called Tax. Tax not only upregulates viral RNA expression but also of IL2 and IL2R. These cause upregulation of T cells -> T cell leukaemia.

Question 26

Name DNA viruses that also transform cells,

Answer 26

Papillomavirus, Herpesviruses and Hepatitis viruses.

Papillomavirus accidentally integrates into the genome by the integrase of something else - it does not benefit from integrating. The integration causes elevated expression of:
E5 - interferes with apoptosis of the cell
E6 - binds to p53 and causes its degradation
E7 - regulates another oncogene.

Question 27

Which receptors does HIV bind to?

Answer 27

CD4+ receptors and chemokine receptors CCR5 and CTXR4.

Question 28

Give 7 responses of the immune system to viruses

Answer 28

1. Neutralising antibodies
2. APOBEC incorporated into virion particles
3. Mutation in CCR5 receptors makes it more difficult to infect
4. Interferon a & b production to warn other cells
5. Tetherin restricts retroviruses from leaving the cell
6. Rhesus macaque TRIM5alpha is a protein that blocks the uncoating of the virus and stops the capsid from breaking down.
7. NK cells kill cells lacking MHC I.

Question 29

How does HIV try and evade host defences?

Answer 29

1. Avoids neutralising antibodies by spreading via syncytia.
2. Nef protein shunts MHC I into degradation by lysosomes so it is not killed by T cells
3. vif protein triggers degradation of APOBEC so it never gets into the virion.
4. Positive selection of the V3 loop of HIV env which is a major antigenic peptide. It results in rapid evolution and antibody escape mutants.

Question 30

What are prions?

Answer 30

They are proteins NOT viruses which produce protein aggregates resistant to proteases.

Question 31

What is the role of interferons and how do they work?

Answer 31

Interferon binds to the receptors causing the dimerization of the JAKs and causes them to phosphorylate each others tyrosine molecules. This allows the STATs to bind which then become phosphorylated themselves. The STATs dissociate, dimerize and then migrate into the nucleus where they bind to DNA and drive transcription of specific genes such as TIP. It also upregulates MHC I expression.

Question 32

Which cells are responsible for secreting each type of interferon and what are the triggers?

Answer 32

IFNa - tumour or virus infection
By dendritic cells, macrophages and B cells.

IFNb - dsRNA
By epithelial cells and fibroblasts.

Question 33

Which hepatitis virus is the only dsDNA one?

Answer 33

Hepatitis B

Question 34

How are each of the hepatitis viruses most commonly spread?

Answer 34

A and E - faecal and oral mainly (food-borne) + percutaneous

B, C, D - blood transfusions, intravenous drug use, haemodialysis, mother to baby