CLINICAL ASPECTS OF BIOCHEMISTRY

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CLINICAL ASPECTS OF BIOCHEMISTRY NEURODEGENERATIV
E DISEASES Prion diseases Alzheimer's disease
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SOME PRION DISEASES TRANSMISSIBLE SPONGIFORM
ENCEPHALOPATHIES
Species Transmission
Scrapie Sheep Infection
Bovine spongiform encephalopathy (BSE) Cow Infection (contaminated feed)
Kuru Human Infection (cannibalism)
Creutzfeldt-Jakob disease Iatrogenic Familial Sporadic New variant Human Infection (growth hormone etc.) Germline mutations of PrP gene Somatic mutations of PrP gene or spontaneous conversion Infection (eating beef?)
Gertmann-Straussler-Scheinker disease (GSS) human Germline mutations of PrP gene
Fatal familial insomnia (FFI) Human Germline mutations of PrP gene
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SPONGIFORM BRAIN TISSUE
From Prusiner (1998)
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HOW CAN PROTEINACEOUS PARTICLE BE INFECTIOUS?

1. May contain "shielded" nucleic acid?
2. Proteins specify own aa sequence?
3. Normal cells carry a gene that encodes PrP, and
   the product is changed to a different
   conformation by the infectious particle?

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THE PRION HYPOTHESIS
TSEs occur when the normal cellular form of the
prion protein (PrPc) is converted to the abnormal
form (PrPsc). PrPc and PrPsc differ in
conformation. The conversion is autocatalytic
- PrPsc facilitates the conversion of more PrPc
to PrPsc.
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TSEs - MAIN TOPICS

• 1. The nature of PrPc and PrPsc
• Conversion of PrPc to PrPsc
• Inherited TSEs
• Species specificity and species barriers
• Strains
• Normal function role in disease
• Doppel
• BSE and nvCJD
• Therapies?

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Normal (PrPc) and abnormal (PrPs) forms of prion
protein

• PrPc
• Precursor 250 amino acids. Mature PrPc 210
  aas
• Hydrophobic glycoprotein
• GPI anchor (glucosyl phosphatidyl inositol)
• NMR structure - C-terminal end a-helical,
  N-terminal end unordered
• PrPsc
• Same sequence and postranslational modifications
  as PrPc
• Different conformation - more b-sheet
• Tends to form insoluble aggregates
• More resistant to proteolysis than PrPc
• Insoluble PrPsc (in amyloid plaques) not
  infectious?

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PrPSc IS RESISTANT TO PROTEOLYSIS
-PK
PK
PrPSc
PrPC
PrPSc
PrPC
Based on Priola (2001)
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STRUCTURE OF THE HUMAN PRION PROTEIN
Globular domain
Based on Rivera-Milla et al (2003)
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HYPOTHETICAL MODELS FOR PrPc AND PrPsc
Based on Prusiner (1998)
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alpha helix
beta sheet
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PrPSc CAN CONVERT PrPC TO PrPSc IN VITRO
Based on Priola (2001)
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TWO MODELS FOR CONVERSION OF PrPC TO PrPSc
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INHERITED FORMS OF CJD, GSS, FFI etc
Mutations may stabilise PrPsc conformation e.g.
P102L in GSS when this was engineered into mice
they developed 'scrapie') Met/Val129
polymorphism in man - Val homozygotes more
susceptible to CJD? Met homozygotes more
susceptible to nvCJD?
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SOME POINT MUTATIONS IN THE PrP GENE THAT CAUSE
HUMAN PRION DISEASE
Position Normal Mutant
102 Pro Leu
105 Pro Leu
145 Ala Stop
178 Asp Asn
180 Val Ile
200 Glu Lys
Based on Priola (2001)
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SPECIES BARRIERS TO TRANSFER OF PrPSc
Sequence differences between PrP from different
species may provide (and explain?) some barrier
to infection - but incomplete. E.g. Mouse ?
mouse transfer gives more rapid infection than
mouse ? hamster etc. But, mouse ? hamster ?
hamster gives faster infection, Homologous PrPSc
is better at converting PrPC than heterologous
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STRAINS OF PRION DISEASES
Scrapie occurs as about 20 different strains
(differentiated by time taken to infect mice and
different behavioural effects). CJD occurs as
2-4 different strains. BSE only one. May be
explicable in terms of different conformations,
but the more strains the more far-fetched this
explanation. The biggest problem with the
Prusiner model? For 2 CJD strains - evidence for
different conformations (pattern of proteolysis)
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DIFFERENT PrPSc STRAINS - DIFFERENT CONFORMATIONS
Based on Priola (2001)
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NORMAL FUNCTION OF PRPC
Not clear knockout mice lacking PrP are not
seriously abnormal Possible roles in cell
signalling and in processing copper ions have
been suggested
WHY DOES PrPSc CAUSE DISEASE?

• Possible explanations include
• Neurotoxic
• Deposits disrupt cells
• Deposits disrupt intercellular contacts
  (synapses etc)
• Loss of PrPC

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DOPPEL (Dpl)
A PrP-like protein (25 sequence identity but
shorter). Gene close to PrP gene - could
explain variable effect of PrP knockouts Involvem
ent in prion diseases?
Based on Behrens Aguzzi (2002)
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ANNUAL INCIDENCE OF BSE IN THE UK
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nvCJD incidence
30
25
20
15
10
5
0
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
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CJD - POSSIBLE THERAPIES

• Drugs that stabilise PrPC (stabilise helical
  conformation)
• Drugs that inhibit aggregation amyloid (b
  sheet) formation
• Immunization against PrPC or PrPSc