NR AQBH
AU Heaply,S.
TI Prion Diseases
IA http://www-micro.msb.le.ac.uk/335/Prions.html vom Sep 1994
VT
Viroids and satellite elements (described elsewhere) have very small genomes of ~300-2000 nucleotides. Their replication and effects on host cell biology are largely understandable within a conventional scientific framework.
This document describes infectious agents which (almost certainly) do not have a nucleic acid genome. It seems that a protein alone is the infectious agent. The infectious agent has been called a prion. A prion has been defined as "small proteinaceous infectious particles which resist inactivation by procedures that modify nucleic acids". The discovery that proteins alone can transmit an infectious disease has come as a considerable surprise to the scientific community.
Prion diseases are often called spongiform encephalopathies because of the post mortem appearance of the brain with large vacuoles in the cortex and cerebellum. Probably most mammalian species develop these diseases. Specific examples include:
Scrapie: sheep
TME (transmissible mink encephalopathy): mink
CWD (chronic wasting disease): muledeer, elk
BSE (bovine spongiform encephalopathy): cows
Humans are also susceptible to several prion diseases:
CJD: Creutzfeld-Jacob Disease
GSS: Gerstmann-Sträussler-Scheinker syndrome
FFI: Fatal familial Insomnia
Kuru
Alpers Syndrome
These original classifications were based on a clinical evaluation of a patients family history symptoms and are still widely used, however more recent and accurate molecular diagnosis of the disease is gradually taking the place of this classification.
The incidence of sporadic CJD is about 1 per million per year. GSS occurs at about 2% of the rate of CJD.
It is estimated that 1 in 10,000 people are infected with CJD at the time of death.
These figures are likely to be underestimates since prion diseases may be misdiagnosed as other neurological disorders.
The diseases are characterised by loss of motor control, dementia, paralysis wasting and eventually death, typically following pneumonia. Fatal Familial Insomnia presents with an untreatable insomnia and dysautonomia. Details of pathogenesis are largely unknown.
Visible end results at post-mortem are non-inflammatory lesions, vacuoles, amyloid protein deposits and astrogliosis.
GSS is distinct from CJD, it occurs typically in the 4th-5th decade, characterised by cerebellar ataxia and concomitant motor problems, dementia less common and disease course lasts several years to death. (Originally thought to be familial, but now known to occur sporadically as well).
CJD typically occurs a decade later has cerebral involvement so dementia is more common and patient seldom survives a year (originally thought to be sporadic, but now known to be familial as well).
FFI pathology is characterised by severe selective atrophy of the thalamus.
Alpers syndrome is the name given to prion diseases in infants.
Scrapie was the first example of this type of disease to be noticed and has been known about for many hundreds of years. There are two likely methods of transmission in sheep:
1. Infection of pasture with placental tissue carrying the agent followed by ingestion, i.e. an acquired infection.
2. Parry showed considerable foresight and anticipated many years work by suggesting that it is not normally an infectious disease at all but a genetic disorder. He further suggested that selective breeding would get rid of the disease altogether.
Humans can be infected by prions in 2 ways:
1. Acquired infection (diet and following medical procedures such as surgery, growth hormone injections, corneal transplants) i.e. infectious agent implicated.
2. Hereditary mendelian transmission where it is an autosomal and dominant trait. This is not prima facie consistent with an infectious agent.
This is the feature that marks out prion diseases as unique. They are both infectious and hereditary diseases. They are also sporadic, in the sense that there are also cases in which there is no known risk factor although it seems likely that infection was acquired in one of the two ways listed above.
In fact prion diseases in humans are probably primarily a genetic neurotoxic disorder. Transmission of the disease to humans via infectious prions is likely to be rare.
Kuru is the condition which first brought prion diseases to prominence in the 1950s. Found in geographically isolated tribes in the Fore highlands of New Guinea. Established that ingesting brain tissue of dead relatives for religious reasons was likely to be the route of transmission. They ground up the brain into a pale grey soup, heated it and ate it. Clinically, the disease resembles CJD. Other tribes in the vicinity with same religious habit did not develop the disease. Speculated that at some point in the past a tribe member developed CJD, and as brain tissue is highly infectious this allowed the disease to spread. Afflicted tribes were encouraged not to ingest brain tissue and the incidence of disease rapidly declined and is now almost unknown.
Evidence suggests that a prion is a modified form of a normal cellular protein known as PrPc (for cellular), a normal host protein encoded by a single exon of a single copy gene. This protein is found predominantly on the surface of neurones attached by a glycoinositol phospholipid anchor, and is protease sensitive. Thought to be involved in synaptic function.
The modified form of PrPc which may cause disease i.e. the prion is known as PrPsc (for scrapie) which is relatively resistant to proteases and accumulates in cytoplasmic vesicles of diseased individuals.
It has been proposed that PrPsc when introduced into a normal cell causes the conversion of PrPc into PrPsc. Process is unknown but it could involve a chemical or conformational modification.
Several lines of evidence support protein only model of infection:
1. Nucleic acid is not necessary for infectivity:
unusually small target size for ultraviolet and ionising radiation:
the low ratio of nucleic acid to infectious material.
resistance of infectivity to agents which modify or damage nucleic acids but infectivity is susceptible to reagents which destroy proteins:
Stabilities of the scrapie agent and viriods (PSTV):
Chemical Treatment: Concentration PSTV Scrapie
Et2PC 10-20 mM (-) +
NH2OH 0.1-0.5 mM + -
Psoralen 10-500 µg/ml + -
Phenol Saturated - +
SDS 1-10% - +
Zn2+ 2 mM + -
Urea 3-8 M - +
Alkali pH 10 (-) +
KSCN 1 M - +
Enzymatic Treatment: Concentration PSTV Scrapie
RNAse A 0.1-100 µg/ml + -
DNAse 100 µg/ml - -
Proteinase K 100 µg/ml - +
Trypsin 100 µg/ml - +
+ = inactivated; - = no change in infectivity
failure to identify a scrapie specific nucleic acid either in prion preparations or infected brains using a variety of sophisticated techniques.
2. PrPsc is associated with scrapie infectivity:
purification of scrapie infectivity results in preparations highly enriched for PrPsc
purification of PrPsc results in enrichment of scrapie activity
purification of PrPsc by SDS-PAGE also recovers infectivity
PrPsc can be denatured and renatured without loss of infectivity
3. Susceptibility of a host to prion infection is co-determined by the prion inoculum and the PrP gene:
disease incubation time for a single prion isolate various between mouse strains, this variation depends on the Sinc gene, this is very closely linked to or coincident with the PrP gene itself, suggesting some forms of PrPc may be more easily converted to PrPsc than others.
when prions are transmitted from one species to another disease develops only after a very long incubation period, if at all, but on serial passage in the new species the incubation time often decreases dramatically and then stabilises. This species barrier can be overcome by introducing a PrP transgene from the prion donor i.e. hamster PrPc but not murine PrPc is a suitable substrate for conversion to hamster PrPsc by hamster prions and vice versa:
Bild: Prion2.gif
4. Mutated gene can give rise to disease without infection:
Homozygosity at the polymorphic amino acid position 129 of PrP protein predisposes an individual to acquired and sporadic CJD
2 unrelated GSS families have the same double mutation i.e. 178 D-N; 200 E-K
disease tightly linked to a P-L mutation at 102 in some familial GSS cases. 100 controls did not have this mutation nor did another 15 sporadic victims of CJD
other familial cases have been shown to carry this or several other mutations e.g. 117-A-V; 198 F-S; N178; V129 = CJD; N178 M129 = FFI
Presumably these mutations enhance the rate of spontaneous conversion of PrPc to PrPsc which permits disease manifestation within the lifetime of an individual. Suggests that some sporadic incidents can be accounted for by somatic mutation of the PrP gene.
5. Crucial experiment:
Mice carrying a murine transgene with the 102P-L GSS mutation spontaneously develop a lethal scrapie like disease. Brains also contain infectious prions because transmission to recipient animals has been demonstrated.
Mice lacking the PrPc gene develop normally, no evident physiological or behavioural problems. Suggests that loss of PrPc function is unlikely to be the cause of disease rather accumulation of PrPsc is responsible. When inoculated with prions they do not develop disease . There are obvious implications here for the livestock and pharmaceutical industries and treatment of familial cases. Animals expressing reduced levels of PrPc are also resistant to infection - perhaps more immediately relevant to human disease. Animals overexpressing PrPc are more likely to develop prion diseases.
Evidence against the prion model:
The existence of many different strains of scrapie (>15: latency, lesion patterns differ) which can be propagated in the same inbred mouse strain. These can transmit serially without changes in properties in the same mouse strain homozygous for a single PrP genotype. Cannot argue that there a distinct PrPc is converted to distinct PrPsc. Must mean that a common PrPc is corrupted in a different way, seems improbable but there is now experimental support for it.
So how can a protein be infectious?
PrPsc is a protease resistant form of PrPc, both are extensively post translationally modified. No chemical differences between the two forms of the protein have been detected. Clearly there must be some difference. One great problem is that infectivity ratio is about 100,000:1, so infectious agent even if derived from PrPc may not be PrPsc and it could be chemically modified.
However, a more likely possibility is that the difference between PrPc and PrPsc is conformational. 3D structure not known but other physical techniques suggest that PrPc contains almost no beta sheet whilst PrPsc has a lot. Proposed therefore that this protein can adopt 2 quite different stable conformations. The safe PrPc form is normally adopted but rarely it can switch to the PrPsc form. Mutations favour this switch. Propose that PrPsc is transdominant and converts PrPc to PrPsc in an exponential fashion. Precedents for this model do exist. There is a yeast mutant phenotype which doesn't correlate to any mutational difference in the gene structure but does correlate to a different protein structure. The Ure2p protein converts to an inactive conformation. Mutations in the tumour suppressor protein p53 which are associated with the onset of neoplastic disease have a different conformation to the normal protein. When normal protein is incubated with mutant protein its conformation is altered to the mutant form.
Concluding Remarks:
With an apparent incidence of less than 100 cases a year in the UK you might think it can hardly be considered an important medical problem. However, recent diagnostic procedures show that it is more common than we think. The pathology is similar to other more common neurological disorders with a similar late onset, such as Alzheimer's, Parkinson's and amyotrophic lateral sclerosis.
There is also, at least in the UK, BSE a grand experiment going on with us as the laboratory animals. Epidemiological studies show that BSE almost certainly arose through feeding cattle, a herbivore, with scrapie-infected sheep meet. In the early 1980s, industry changed their method of rendering offal. Presumably the agent which was previously inactivated or removed by this process no longer was. Recently about 100,000 cattle p.a. developed BSE. The rate may now be declining. We have already seen today how new scrapie strains arise when passaged through new hosts, as yet we do not fully know what the characteristics of the BSE strain are. It is clear that scrapie is not as infectious via the oral route as via intracerebral inoculation and there is no evidence that it is has been transmitted to WoMan by dietary means. One hopes that BSE is the same but agent has been transmitted to cats, pigs and other exotic ruminants via infected beef. So one cannot state with certainty that there is no risk to humans from infected meat. PrP is present in skeletal muscle (steak). PrP is also present on the surface of lymphocytes which are present in milk. Pasteurisation temperatures will not destroy this infective agent. Again there is no evidence that milk ingestion can cause disease.
We all have a long wait.
References:
Cann A.J: 'Principles of Molecular Virology.' Academic Press, 1993. Chapter 8.
"http://www.le.ac.uk/CWIS/AD/MI/ST/sh1.html">Dr Shaun Heaphy, September 1995.
AD Dr. Sharen Heaply
PO Internet
SP englisch
OR Prion-Krankheiten H