Prions

Question 1

What are prions and prion diseases?

Answer 1

Prions are pathogenic proteins (proteinaceous infectious particle PrPsc) found in prion diseases, which are neurodegenerative diseases

Question 2

What are prion diseases also known as?

Answer 2

Transmissible Spongiform Encephalopathy (TSEs)

Question 3

Name three mammalian species where prions are found and their associated prion disease, which mammal was the first to be found with PrPscs?

Answer 3

1. First found PrPscs in sheep with scrapie
2. Mad Cow disease/ BSE
3. Chronic wasting disease in deer

Question 4

Name four prion diseases found in humans

Answer 4

1. Creutzfeldt-Jakob disease (CJD) AND variant Creutzfeldt-Jakob disease (vCJD)
2. Kuru
3. Fatal familiar Insomnia (FFI)
4. GSS

Question 5

Name five symptoms of prion diseases

Answer 5

1. Anxiety and depression
2. Ataxia; loss of physical coordination
3. Memory loss and loss of cognition (dementia symptoms)
4. Dystopia: muscle spasms
5. Incontinence; bowel and urinary

Question 6

What is the outcome of a prion disease and when does it usually occur?

Answer 6

Inevitably fatal and most die within a year of symptoms starting. NO cures, only treat the symptoms

Question 7

Name four pathological characteristics of the brain due to a prion diseases

Answer 7

1. Neuronal death (loss of neurones) leading to spongiform appearance of the brain (white gaps in the brain where the neurones were)
2. Proliferation of astrocytes and microglia: acting like the immune cells of the brain to clear up the dead cells and compartmentalize the problem - but are only high in a diseased brain
3. Build up of amyloid plaques (PrPscs) made of aggregated prions making protein lumps
4. Evidence of oxidative stress

Question 8

How common are prion diseases? List the three mechanisms in which they can occur from most-least common and when the onset of symptoms tends to occur for each

Answer 8

Very rare

1. Sporadic (spontaneous): symptoms develop between ages 60-65
2. Genetic: symptoms usually develop in early 50s and have longer duration
3. Acquired (transmitted/infectious); iatrogenic, Kuru (canabolism) and vCJD (likely via consumption of beef products)

Question 9

Name three examples of genetic prion diseases

Answer 9

1. Creutzfeldt-Jakob disease (CJD)
2. Fatal familiar Insomnia
3. GSS

Question 10

Different forms of the prion affects different parts of the brain and has slightly different characteristics. Specify where prions accumulate in the brain for the following prion diseases
A) CJD
B) FFI
C) Kuru

Answer 10

A) the cerebral cortex
B) thalamus
C) Cerebellum

Question 11

List 3 differences and 4 similarities between CJD and Alzheimer’s

Answer 11

Similarities:

1. Neurodegenerative diseases
2. Similar clinical symptoms including dementia
3. Linked high levels of oxidative stress
4. Both have buildup of misfolded proteins in amyloid fibres or plaques

Differences:

1. AD is much more common
2. Involve different proteins (PrP vs amyloid-beta)
3. Prions are transmissible

Question 12

Describe what composes an amyloid plaque in prion diseases. Name five other neurodegenerative diseases where protein aggregates occur in the brain and name the appropriate protein or defect for each. Which is similar to CWD?

Answer 12

Aggregates of proteins which stack up to make fibrils/fibres and clump together to make an amyloid plaque. All proteins that aggregate are beta sheet rich

1. Parkinson’s: a-synuclein
2. Huntington’s: huntingtin
3. Wilson’s: defects in copper metabolism (similar to CWD)
4. Amyotrophic lateral sclerosis: defects in superoxide dismutase
5. Alzheimer’s: amyloid beta proteins

Question 13

What differs prion diseases from other neurodegenerative diseases and what causes this?

Answer 13

They’re transmissible: Injections of diseased brain tissue of one animal into the brain of another of the same species transmits the disease. However, this is not a normal pathway for infection, which suggests that the infectious agent in the TSEs is a protein

Question 14

What age group was most affected by vCJD and what other prion disease was it related to?

Answer 14

Younger; median onset was 26, related to BSE/mad cow disease

Question 15

Why is it unlikely that prion diseases are transmitted through viruses?

Answer 15

Treating the brain tissue with nucleases which destroy viral nucleic acids does not reduce their infectiousness

Question 16

What does PRPres mean and what does it refer to?

Answer 16

Refers to the PRPsc because this protein is particularly resistant to conditions that would normally destroy proteins (i.e proteases, heat, strong alkaline conditions only have a limited affect)

Question 17

What are the two major routes of transmission? (Including the four iatrogenic routes)

Answer 17

1. Ingestion - through GI system (specifically the region of the gut with peyer’s patches/high immune function)
2. Iatrogenic: corneal grafts, dura mater grafts, human derived growth hormone injection (hGH), experimental transmission in animal models

Question 18

How does the immune system respond to prions and how do they reach the CNS?

Answer 18

They’re detected but don’t raise a large immune response. They evade the immune system’s response to break them down (resistant to proteolysis) and replicate in lymphoid tissue. A lot of lymphoid tissue is innervates by the ANS, and this provides a route for prions to enter the CNS.

Question 19

Which type of lymphoid tissue seems particularly able to harbour the prion?

Answer 19

Follicular dendritic cells

Question 20

Describe the process of how the prion protein replicates

*including

Answer 20

There’s a normal cellular form of the prion protein (PRPc), which every brain expresses highly at the synapses of neurones and in follicular dendritic cells - it’s coded by a gene on chromosome 20 and it has a largely unknown function. The PRPsc (prion protein) shares the same amino acid sequence as physiological PRPc, (it’s just a little shorter), and it can catalyze the conversion of PRPc to the prion. This results in exponential creation of PRPsc in the brain.

Question 21

Name the changes of a PRPc once it has been converted to a PRPsc

Answer 21

It becomes extracellular, protease resistant, aggregates (as it has a high beta sheet content), doesn’t bind to copper and is NOT easily decontaminated

Question 22

What are the two main proposed mechanisms of PRPsc catalyzed conversion? What are both the models based on and what would they both explain?

Answer 22

1. Temple directed refolding model
2. Nucleation dependent polymerization model
   Both are based on that you have the same amino acid sequence but a conformational change (change in the 3D structure)

Both would explain the long incubation period found in prion diseases

Question 23

What do the template refolding model and nucleation dependent polymerization models suggest?

Answer 23

Template refolding model: That there is a big energy barrier to overcome to convert PRPc to a prion, however once a small amount of prion is present it catalyzes the conversion with the formation of a heterodimer of the PRPc into more prion. From here, you only need a few prions to keep this going while the rest form amyloid plaques

Nucleation dependent polymerization model: suggests there is a low energy barrier and so a PRPsc can form more spontaneously, and slowly over time the pathogenic prion builds up to form amyloid fibres (individual monomers aren’t as important)

Question 24

Name three things that describe how the formation of PRPsc leads to neurodegeneration

Answer 24

1. PRPsc is cytotoxic
2. A proposed function of PRPc is an antioxidant and when its converted into PRPsc that protection is lost
3. Causes apoptosis and some necrosis of neurones (cleared by microglia) which leaves spongiform gaps

Question 25

How do PRPscs form in inherited diseases? What is the inheritance pattern?

Answer 25

Their prpn gene has polymorphism which predispose the conversation of PRPc -> PRPsc. The inheritance pattern is autosomal dominant

Question 26

Which gene is responsible for coding the PRPc protein?

Answer 26

Prpn

Question 27

Which type of prion disease is CJD, and how is the conversion of PRPc -> PRPsc explained?

Answer 27

Sporadic change which may be induced by a spontaneous somatic mutation in one of the prpn genes or by an environmental factor

Question 28

As a healthcare worker what should you do when you have a patient with a prion disease?

Answer 28

Minimize chances of transmission, advice on this is published by the government

Question 29

Name five precautions published by the government for patients with or at “increased risk” of CJD that undergoing surgical procedures

Answer 29

1. Perform it in operating theatre
2. Schedule to be end of the list to allow for thorough cleaning afterwards
3. Involve minimum number of personnel required
4. Protective clothing worn
5. If possible, use disposable instruments but if you must use reusable ones they must be tracked

Question 30

List four safety measures to reducer any possible risk of spreading vCJD through blood

Answer 30

1. Withdrawal and recall of any blood components donated by anyone who develops vCJD
2. Remove WBCs from all blood used transfusions (leucodepletion)
3. Donations from people who may have received a blood transfusion since 1980 not accepted
4. Promote appropriate use of blood, tissues and alternatives throughout NHS to reduce amount of blood transfused during and following surgery

Question 31

List four possible mechanisms that can be targeted for potential pharmaceutical interventions against prion diseases

Answer 31

1. Knock out the production of normal PRPc
2. Stabilize PRPc or the infectious prion to prevent the conversion
3. Prevent the PRPsc aggregation to stop formation of fibrils
4. Stabilize the fibrils so they don’t have a pathogenic effect

Question 32

Why are people infected with prion diseases largely asymptomatic?

Answer 32

Prions have a long incubation period as it takes time for the prions to build up and aggregate

Question 33

What is the structural difference between PRPc and PRPsc?

Answer 33

PRPc: primarily a-helical structure
PRPsc: primarily b-pleated sheeted structure

Question 34

Cam CJD be diagnosed, what makes it different from other neurodegenerative diseases?

Answer 34

Very hard to distinguish from other neurodegenerative diseases. The only definitive way to diagnose CJD is a brain biopsy, however this is high risk and is only used when it might rule out a treatable disorder. CJD progresses rapidly and is often diagnosed post-Morton.