<<

. 2
( 45 .)



>>

is not even clear that the placentas were the source of the disease, since the
only sheep that can be guaranteed to be scrapie-free prior to exposure to
infected tissue come from flocks that are genetically resistant to experimental
infection and that therefore are largely unsuitable as recipients in this type of
experiment. The recent demonstration that scrapie develops in genetically sus-
ceptible sheep born to unaffected, genetically unsusceptible dams followmg
embryo transfer (13) greatly reduces the probable relevance of perinatal events
m natural scrapie. In the origmal experiment, however, the embryo donors had
been challenged with scrapie, therefore further experiments are required to
establish whether this is relevant or whether genetically susceptible animals
resultmg from embryo transfer will develop scrapie without exposure of the
embryo donor to the agent. The time is clearly right for a reevaluation of theo-
ries about the occurrence of natural sheep scrapie in the light of the finding of
dtsease-specific polymorphisms in the PrP gene of sheep and other species and
the demonstration of pathogenic mechanisms that do not require exposure to
the infectious agent. “Naturally occurring scrapie m sheep is not the same as
experimental scrapie in any species. It should be studied, m depth, as a unique
disease” (14).
2.4.2. Transmissible Disease in Genetic Cases
The recent demonstration that genetically manipulated mice spontaneously
can develop neurological disease, which then can be experimentally transmit-
ted to other rodents, has bridged the gap between the two opposing mecha-
msms of “inheritance” and “infection.”
It is well established that diseasecan be transmitted experimentally to animals
by mtracerebral injection of brain tissue homogenate from affected members
of those families in which typical prion disease 1sinherited as an autosomal
8 Ridley and Baker

dommant. It is also the case that in each family disease is associated with one
of a number of different pathogenic mutations within the open reading frame of
the PrP gene. Currently about 17 such mutations have been described, most of
which appear to be fully penetrant. That an apparently genetic dtsease can
involve a transmissible agent raises the question of whether the gene mutations
are causing disease, sui generis, or whether they are predisposing Individuals
to develop disease as a result of mfection with an exogenous agent. The argu-
ment that the genetic mutations are causing disease receives support from the
work on transgemc maceexpressing mutant prion protein. These animals spon-
taneously develop neurological disease in adulthood despite bemg protected
from possible Infection with exogenous agents. Furthermore, disease 1s then
transmissible from these animals by serial passage to other animals (1). These
expertments suggest that attempts to classify at least some casesof priori dis-
ease as genetic or infectious may be inappropriate.
ConversIon of Prp to Prpc
2.4.3.
Recent experimental findmgs have suggested that the fundamental feature of
pathogenesis in this group of diseasesconsistsof the conversion of a pre-existmg
molecule to a new shape or conformation rather than to replzcation of an agent.
2 4 3.1. THE RELATIONSHIP BETWEEN PRP˜ AND PRP˜˜
The most important aspect of molecular pathogenesis is the relationship
between PrPC and PrPSC. number of important features of this relationship
A
have been established.
1 PrPCand PrPscare both encodedby the samegeneand have the sameammo acid
sequenceand relatrve mol wt of 33-35 kDa.
2. PrPCand PrPSC antrgenically mdtstmguishable.
are
3 PrPSC probably formed by the posttranslational modrficatron of PrPC.
IS
4. PrPC is destroyed completely by proteolytic enzymes but PrPSC reduced to a
IS
protease-resistantcore of mol wt 27-30 kDa, termed PrP27-30.
5 PrPC IS soluble m the presence of detergent, whereas PrPSCforms msoluble
aggregates
6 Turnover is rapid for PrPc but very slow for PrPSC
7 PrPC 1sfound m brain and someother tissuesof all mammals,whereas PrPSC is
found only m the brain and, to a lesserextent, other tissuesof animalsor humans
with prron disease
8 PrPClevels are constant throughout disease,but PrPSC levels m brain rise during
diseaseprogresstonin casesof prton disease.
9. PrPSC aggregateshave a fibrrllar structure with the tmctorral properties of amy-
loid, whereas PrPCdoesnot aggregatein this way.
10. Levels of PrPSC associated
are wtth mfectlvrty titer, whereasno suchrelationshtp
is seenfor PrPc.
The Paradox of Prion Disease 9

2.4.3.2. WHY IS PRP˜ CONVERTED TO PRP˜˜?
Since the conversion of PrPC to PrPSC irreversible, it would seem that thts
is
conversion consists of moving from a higher to lower energy state and it is only
the intervening higher activation threshold that prevents the process happening
more frequently. Four factors may contribute to the crossing of this barrier:
1 The presenceof PrP sc PrP seems to belong to a small group of protems that can
have one of several conformations, the presence of one form predlsposmg the
other molecules to take up that shape (15-17). Such a hypothesis can explain:
a Transmission of disease in the absence of agent-specific nuclerc acid,
b The exponential accumulation of PrPSCdurmg disease,
c The observation that, on transmission, the PrPSCin the recipient 1shost coded;
d. The posslbihty that there is more than one self-perpetuating conformation
allowmg for the observation of “strains of agent,” and
e That where the PrP of the donor and host are different, the ability of the donor
PrP to interact with the host PrP will be lessened, leading to the observation
of a “species barrier.”
2. The amino acid sequence of PrP. Mutations in the PrP gene may produce dlffer-
ences m the stability of the secondary structure of the PrP protein such that a
spontaneous increase in the P-pleated sheet content 1smore probable. This could
lead to the occurrence of genetic cases
3 Availability of PrPC Transgenic mice with multiple copies of the normal PrP
gene can develop spontaneous prion disease, those with only one copy show pro-
longed mcubatlon times, and mice with no PrP gene cannot be infected Factors
such as an increase in general metabolic rate, which may inter &a affect the rate
of production and breakdown of PrPC, could occasionally tip the balance toward
the production of PrPsc and therefore contrlbute to the occurrence of prlon dls-
ease m sporadic cases
4 Time. In a dynamic equlllbrium, the probability that a rare event will have
occurred increases with time. The amount of PrPSC required to imtlate the mas-
sive production of PrPsc is not known, but it is likely to consist of a threshold
quantity. Since PrPSC IS not disposed of, the total amount present m one area will
accrue with time. Naturally occurring prlon disease has a characterlstlc age at
onset, which usually IS later m sporadic than inherited cases, but which is con-
fined to middle age and beyond. Whether this reflects the effect of real time or
the effect of age-related changes m the availability of PrPC is not yet clear

2.4.4. Prion Disease and Amyloidosis
The recognition that PrPSC a propensity to take on the structure described
has
as “amyloid” has allowed prion diseasesto be compared to other amyloldoses
rather than other mfectlous diseases.
PrPc ISa soluble protem containing about 40% a-helical domains and almost
no P-pleated structure PrPSC largely insoluble with reduced a-helix content
is
Ridley and Baker
10

(30%) and increased P-pleated structure (43%). The P-pleated content of
PrP27-30 IS even higher (>50%). Pathogemc mutations m the PrP gene are
thought to alter the stability of the tertiary structural relations between the alpha
hellcal structures m the PrP molecule, thereby altering the probablllty that this
structural relationship will break down into a P-pleated sheet conformation
(IS). Protems with high P-pleated content have a propensity to form fibrlls that
themselves aggregate to form large deposits that have the tmctorlal properties
of “amylold.” For example, they exhibit blrefrmgence when stamed with
Congo red and are viewed under polarized light
The most Important CNS amyloldosis occurs m Alzhelmer disease (AD),
which IS characterized by the presence in brain of P-amylold plaques, neu-
rofibrlllary tangles (NFTs), and often cerebral amylold angiopathy (CAA), the
deposition of P-amyloid m cerebral blood vessels Similarities between AD
and prlon diseases include:
1 Approximately 15% of cases of AD and prlon disease occur with an autosomal
dominant pattern of mherttance. In famlhal cases, AD may be associated with
one of several mutations m the gene that makes the precursor protein (amylold
precursor protein, APP) whose modlficatlon produces P-amylold, whereas prlon
disease 1s associated with mutations m the PrP gene (AD also may be associated
with mutations m other genes that interact with the APP gene )
2 AD and prlon disease both have an age at onset largely confined to later middle
age and beyond with onset being earher In familial than m sporadic cases
3 j3-amylold plaques are found in AD, whereas PrP-amylold plaques are seen in
about 20% of cases of prlon disease Even where PrP-amylold plaques are not
visible by light microscopy, electron microscopy reveals that the PrPSC exists m
free fibrlls m the neuropll, suggesting that prlon disease involves amyloldosls
even m cases where plaques are absent (19).
4. In a few cases of prlon disease the degree of CAA and P-amyloid either m sepa-
rate plaques or integrated into PrP plaques IS greater than would be expected by
chance association, suggesting an interaction m the pathogenesls of both types of
amyloldosls.
5 In AD, P-amylold plaques are formed when one of two alternative processing
pathways of APP results m the production of a truncated protem sequence that
takes on a P-pleated sheet conformation. These molecules then polymerize into
fibrlls that aggregate into the characterlstlc form of a plaque In prlon disease,
PrP-amyloid may consist of a truncated form of PrPSC (20) It is, however, not yet
clear whether the crucial difference between PrPC and PrPSC that associates the
latter with mfectlvlty 1sthe formatlon of PrPsc into amylold fibrlls or whether PrPSC
exists m a pathogemc form prior to Its subsequent breakdown mto PrP-amylold.
6 Although AD IS not a transmissible disease, the laminar distribution of P-plaques
in the cortex 1s consistent with the spread of pathology around the brain by a self-
sustaining mechanism (21). The mJectlon of P-amylold containing bram into the
brains of monkeys has been found to result m the formation of P-amylold plaques
II
The Paradox of Prion Disease
in cortex (22), suggesting that P-amyloidosis may be transmrssrble under experi-
mental conditions

3. Treatment and Prevention
Perhaps because prion disease in humans is so rare, the amount of effort
devoted to constderatron of treatment for prion disease is but a fraction of the
interest and effort expended m understanding the nature of the disease. How-
ever, several strategies might be considered, some of which will be discussed
in subsequent chapters.
3.7. Genetic Methods
1. The identification of mutations in the PrP gene opens up the possrbihty of genetic
counselmg for affected famrhes, although counselmg for adult-onset disorders 1s
a complex issue Where affected famrhes request it, prenatal genetic testmg can
eliminate the disease from all subsequent progeny
2. Selecting sheep that are resistant to scrapre on the basis of polymorphrsms m the
PrP gene can radically alter the incidence of scrapie m flocks
3 Since mice that lack a PrP gene are not noticeably sick and are resistant to prron
disease, a technique (if rt existed) that blocked PrP gene expression might not
have seriously deleterious effects and would prevent disease in disease-allele
carriers and m those known to be at risk from acquired infection. Such a tech-
nique might also prevent disease progression but would not be expected to reverse
the accumulated damage

3.2. Biochemical Treatments
1. It might be speculated that treatments that decrease the availability of PrPc, either
by reducing PrP synthesis or by speeding up its metabolism, could slow down the
progress of the disease. Alternatively, rt should be borne m mind that centrally
acting drugs that might be intended to relieve some of the symptoms of disease
inadvertently might increase the availability of PrPC and so increase the rate of
disease progression
2 Heterologous PrP molecules can interfere with the process of PrP polymerrzatron
and with the productron of PrPSC in scrapre-infected cell cultures so, in theory,
such molecules might be used to arrest the disease. Such a strategy, however,
only is a remote possibrhty since tt presents considerable difficulties of drug
delivery and, rf an inappropriate PrP molecule was used, it could accelerate the
disease process
3.3. Prevention
Acquired disease can be prevented by an understanding of the procedures
that lead to transmission. For example, kuru has been controlled by a change in
cultural habits. BSE has been reduced by banning the feeding of specified offals
of all species and all ruminant-derived protein to ruminants, although the final
12 Ridley and Baker

eradication of the disease and prevention of its reoccurrence may requn-e
changes m the rendering process, reverting to a sttuatton in which the agent
from any source is destroyed. Iatrogenic human disease may be prevented by a
better appreciation of the risk from brain and related tissue and the recogmtton
that tissue from casesof human prmn disease cannot be excluded from use on
the basis of diagnosis alone because of the posstble msidious onset and atypi-
cal presentatton of the disease.
References
1 Hstao, K K , Groth, D , Scott, M , Yang, S.-L., Serban, H., Rapp, D , Foster, D ,
et al ( 1994) Serial transmtsston m rodents of neurodegeneratron from transgemc
mice expressing mutant priori protein. Proc Nat1 Acad Scl USA 91, 9126
9130.
2 Kocisko, D A , Come, J H., Prtola, S. A , Chesebro, B , Raymond, G J ,
Lansbury, P T , et al. (1994) Cell-free formation of protease-resistant prton pro-
tem Nature 370,47 1474.
3 Glasse, R and Lmdenbaum, S (1992) Fteldwork m the South Fore: the process of
ethnographic mquny, m Przon Diseases of Humans and Animals (Prusmer, S. B ,
Collmge, J., Powell, J., and Anderton, B., eds.), Ellis Horwood, London, pp 77-91
4 Hstao, K , Baker, H F , Crow, T J , Poulter, M., Owen, F , Terwtlhger, J D , et
al. (1989) Linkage of a prion protem missense variant to Gerstmann-Straussler
syndrome Nature 338,342-345
5. Poulter, M., Baker, H. F., Froth, C D , Leach, M., Lofthouse, R., Ridley, R M., et
al (1992) Inherited prton disease with 144 base pan gene msertton 1 Genealogi-
cal and molecular studtes Brain 115, 675-685
6 Palmer, M S , Dryden, A J , Hughes, J T , and Collinge, J (199 1) Homozygous
prton protein genotype predtsposes to sporadic Creutzfeldt-Jakob disease Nature
352,340-342.
7 Parry, H B. (1983) Scrapie Disease m Sheep. Hlstoncal, Clinical, Epldemlologl-
cal and Practical Aspects of the Natural Disease, Academtc, London.
8 Goldman, W., Hunter, N., Foster, J. D., Salbaum, J. M., Beyreuther, K , and Hope,
J (1990) Two alleles of a neural protem gene linked to scrapte m sheep Proc
Nat1 Acad Scz USA 87,2476-2480.
9. Westaway, D , DeArmond, S. J , Cayetanocanlas, J , Groth, D., Foster, D., Yang,
S. L , et al (1994) Degeneration of skeletal muscle, pertpheral nerves, and the
central nervous system m transgemc mice overexpressmg wild-type prion pro-
teins. Cell 76, 117-l 29.
10 Patttson, I H (1974) Scrapie m sheep selectively bred for high suscepttbthty
Nature 248,594-595
11 Dtckmson, A. G , Stamp, J T., and Renwtck, C. C (1974) Maternal and lateral
transmission of scrapie in sheep J Comp Path01 84, 19-25
12. Patttson, I H , Hoare, M. N., Jebbett, J N., and Watson, W. A. (1972) Spread of
scrapie to sheep and goats by oral dosing with foetal membranes from scrapte-
affected sheep Vet. Ret 90,465468
The Paradox of Prlon Disease 13
13. Foster, J D., McKelvey, W. A. C., Mylne, M J. A , Wllllams, A , Hunter, N ,
Hope, J , et al (1992) Studies on maternal transmission of scraple m sheep by
embryo transfer Vet Ret 130,341-343.
14. Pattison, I. H. (1992) A sideways look at the scraple saga, in Przon Drseases of
Humans and Anzmals (Prusmer, S. B , Collmge, J , Powell, J , and Anderton, B.,
eds ), Ellis Horwood, London, pp. 16-22.
15 Caspar, D L D (1991) Self-control of self-assembly. Nature 31533 l-333
16. Cheng, M. Y , Hart& F -U , and Horwich, A L (1990) The mltochondrlal
chaperomn hsp60 is required for its own assembly. Nature 348,455-458
17 Mllner, J. and Medcalf, E. A. (1991) Cotranslatlon of activated mutant ˜53 with
wild type drives the wild type ˜53 protein mto the mutant conformation Cell 65,
766-774.
18 Huang, Z., Gabriel, J.-M , Baldwin, M A., Fletterxk, R J , Prusiner, S B , and
Cohen, F E (1994) Proposed three-dimensional structure for the cellular prlon

<<

. 2
( 45 .)



>>