CLINICAL ASPECTS OF BIOCHEMISTRY

NEURODEGENERATIVE DISEASES

Prion diseases

Alzheimer's disease

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

SPONGIFORM BRAIN TISSUE

From Prusiner (1998)

HOW CAN PROTEINACEOUS PARTICLE BE INFECTIOUS?

(a) May contain "shielded" nucleic acid?

(b) Proteins specify own aa sequence?

(c) Normal cells carry a gene that encodes PrP, and the product is changed to a different conformation by the infectious particle?

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.

1. The nature of PrPc and PrPsc

2. Conversion of PrPc to PrPsc

3. Inherited TSEs4. Species specificity and species barriers5. Strains6. Normal function; role in disease7. Doppel8. BSE and nvCJD9. Therapies?

TSEs - MAIN TOPICS

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-helical, N-terminal end unordered

PrPsc

•Same sequence and postranslational modifications as PrPc

•Different conformation - more -sheet•Tends to form insoluble aggregates•More resistant to proteolysis than PrPc •Insoluble PrPsc (in amyloid plaques) not infectious?

PrPCPrPSc PrPCPrPSc

-PK +PK

PrPSc IS RESISTANT TO PROTEOLYSIS

Based on Priola (2001)

STRUCTURE OF THE HUMAN PRION PROTEIN

Globular domain

Based on Rivera-Milla et al (2003)

HYPOTHETICAL MODELS FOR PrPc AND PrPsc

Based on Prusiner (1998)

beta sheet

alpha helix

PrPSc CAN CONVERT PrPC TO PrPSc IN VITRO

Based on Priola (2001)

TWO MODELS FOR CONVERSION OF PrPC TO PrPSc

(a)

PrPC PrPSc

(b)

Etc Etc~6

Very slow fast fast fast

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?

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)

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

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)

DIFFERENT PrPSc STRAINS - DIFFERENT CONFORMATIONS

Based on Priola (2001)

-PK +PK

WHY DOES PrPSc CAUSE DISEASE?

Possible explanations include:

• Neurotoxic

• Deposits disrupt cells

• Deposits disrupt intercellular contacts (synapses etc)

• Loss of PrPC

NORMAL FUNCTION OF PRPC

Not clear; knockout mice lacking PrP are not seriously abnormalPossible roles in cell signalling and in processing copper ions have been suggested

DOPPEL (Dpl)

A PrP-like protein (~25% sequence identity but shorter).

Gene close to PrP gene - could explain variable effect of PrP knockouts

Involvement in prion diseases?

Based on Behrens & Aguzzi (2002)

ANNUAL INCIDENCE OF BSE IN THE UK

40,000

30,000

20,000

10,000

Num

ber

of B

SE

cas

es

85 86 87 88 89 90 91 92 93 94 95 96

16 83 663

Year of epidemic

nvCJD incidence

0

5

10

15

20

25

30

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

nvCJD incidence

0

5

10

15

20

25

30

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004

40,000

30,000

20,000

10,000Num

ber

of B

SE

cas

es

85 86 87 88 89 90 91 92 93 94 95 96Year of epidemic

CJD - POSSIBLE THERAPIES

• Drugs that stabilise PrPC (stabilise helical conformation)

• Drugs that inhibit aggregation & amyloid ( sheet) formation

• Immunization against PrPC or PrPSc