Scrapie is the archetype of the transmissible spongiform encephalopathies (TSE), a closely related group of rare and fatal neurodegenerative diseases affecting humans and other mammals. They are of particular scientific interest because the transmissible infectious agent appears to be a single type of protein molecule, the prion protein, rather than an organism containing nucleic acid (1, 2). The diseases are also unique in that they are both hereditary and infectious. Their importance has been further highlighted in the past 10 years by the recognition that they can be transmitted across the species barrier, from cattle to humans, in contaminated foodstuffs. The characteristic signs of the disease include dementia, progressive loss of locomotion coordination (ataxia), and wasting, leading inevitably to death. Postmortem histological examination of the brain (3, 4) shows a spongiform appearance with amyloid plaques surrounding the voids. The incubation times of the diseases tend to be long; in mice it is about 1 year, in sheep about 5 years, and in humans it may be 10-50 years, depending on the disease strain. The spongiform encephalopathies can be transmitted between animals by a number of different routes. The most effective, and useful in laboratory studies, is by intracerebral inoculation of tissue homogenate. Other routes include feeding (5) and the inadvertent intramuscular injection (6) of infected material during medical treatment and corneal grafting (7). In the United Kingdom, scrapie may have been transferred from sheep to cattle (as bovine spongiform encephalopathy (BSE)) on a large scale, and thence to the human population (as new-variant Creutzfeldt-Jakob disease) (8) through infected beef products, so far on a small scale.
The spongiform encephalopathies include the following:
Scrapie: a disease of sheep and goats whose altered behavior prior to death includes the obsessive scraping of the fleece, hence the Scottish dialect name of "scrapie." It has been known for at least 250 years and is endemic in all countries except Australia and New Zealand. Experimental sheep-to-sheep transfer of scrapie by inoculation was first carried out 60 years ago, and it is thought that natural transmission occurs through pasture contaminated with placental tissue carrying the agent, but vertical transmission, ewe-to-lamb, is disputed. There is no evidence of direct transmission from sheep to humans. Scrapie can be bred out of flocks, indicating it has a genetic component. Similar diseases include transmissible mink encephalopathy in mink and chronic wasting disease in muledeer and elk. Spongiform encephalopathies have been reported in the United Kingdom in domestic cats and several types of deer, possibly from feeding with infected sheep or cattle remains.
Bovine spongiform encephalopathy (BSE), or "mad cow" disease: a disease of cattle first recognized in 1986 originating in, and largely confined to, the United Kingdom. The United Kingdom has reported 168,000 cases of BSE in the period 1986-1997 in an epidemic that peaked in 1992-1993 and is now diminishing rapidly. Other European countries have reported no more than a few hundred cases, often of cattle imported from the United Kingdom, but no cases of BSE have been found in the United States, which has nearly 100-fold more cattle than the United Kingdom. For commercial or political reasons, BSE cases are likely to be reported conservatively in every country. Epidemiological evidence suggests that BSE originated from the feeding (5) of inadequately rendered sheep remains from scrapie-containing flocks, but a spontaneous origin in cattle herds has not been entirely ruled out. The practice of using meat and bone meal in cattle feeds had been common for several decades, but reductions in the severity of the rendering conditions permitted by the U.K. government in the late 1970s and early 1980s apparently allowed the infectious scrapie agent to escape destruction and to pass into cattle. It is also known that BSE is transmissible from cow-to-calf (vertical transmission) at a very low rate.
1. Human Spongiform Encephalopathies
There are five recognized diseases in the human population associated with TSEs.
Creutzfeldt-Jakob Disease (CJD): a rare and fatal neurodegenerative disease in humans first described in Germany in the early 1920s independently by H. G. Creutzfeldt and A. Jakob. It normally affects people above 60 years of age and is presented as a rapidly progressive dementia, with a characteristic electroencephalogram signature. CJD may have one of three origins: hereditary transmission as an autosomal, dominant trait; acquisition by ingestion; or acquisition by an iatrogenic route, such as surgery, corneal transplant, or growth hormone injection. The majority of cases are sporadic (85%), 10-15% are familial, and the remainder are iatrogenic. The incidence of CJD is given as about 1 per million per year, with little geographic variation. It is possible, however, that CJD is rather more common than this because of its misdiagnosis as Alzheimer’s disease (9) (see Amyloid Precursor Protein), with which it shares a remarkable number of etiological and pathological similarities (10).
New-variant CJD, or v-CJD: appears to be distinct from normal CJD (8) and thus far limited to the United Kingdom. Cases have appeared from 1996 at a rate of about 10-20 per year. The new variant affects an age group below 40 years, it progresses more slowly than classic CJD, and the brain plaques are very similar to those of BSE in cattle brains. It is now thought to be a human form of BSE resulting from the ingestion of infectious cattle tissue. In view of the major epidemic of BSE in the U.K. cattle herds, and the prolonged incubation period, a parallel increase in cases of v-CJD at a later stage in the human population in the United Kingdom cannot be ruled out.
Kuru: most probably a sub-form of CJD, discovered in Papua, New Guinea, but now extinct. It is likely that its transmission was associated with ritual tribal practices of eating the brains of ancestors.
Gerstmann-Straussler-Scheinker disease: a very rare inherited midlife disease in humans manifest by ataxia.
Fatal familial insomnia (FFI): another very rare human midlife inherited disease in which dementia follows difficulty in sleeping.
It is evident that all these diseases are closely related and may represent essentially the same disease in different hosts. The nature of the transmissible agent has been very controversial. The initial assumption that it was a slow-acting virus has been eroded, as no virion could be isolated, no antibody reaction observed, and, most compelling of all, by the ability of the infectivity to survive irradiation, heat, and enzyme treatments that should destroy nucleic acids. In its place, the prion hypothesis was proposed by Prusiner in 1982 (2) that posited a proteinaceous infectious particle as the scrapie agent (see Prion). The essence of the prion hypothesis is that the prion protein (PrP) exists in two states, a normal, cellular form (PrPC), and a pathologic scrapie form (PrP). The normal and scrapie forms of the prion differ not covalently but in the three-dimensional fold of their polypeptide chains (11). Spectroscopic examination has demonstrated that the PrPC molecule is a predominantly alpha-helical protein, whereas PrPSc molecule has much more beta-sheet structure.
Moreover, PrPC is sensitive to proteolysis, but PrP has a large proteinase resistant core, which spontaneously forms insoluble amyloid fibrils. Scrapie diseases result from the ability of the scrapie form of the prion (PrP) to induce the structurally quite different cellular form (PrP C) to take up the structure of the PrPSc form in a chain reaction (12). In this way the scrapie form of the prion protein can effectively replicate itself, and also act as an infectious agent, capable of the induction of more scrapie prions in other hosts.
That a single protein molecule could be responsible for a group of inherited and communicable diseases was a radical idea naturally attracting much skepticism. However, a great deal of experimental verification of the prion hypothesis has been obtained (12), and it has recently received the imprimatur of the Nobel committee. It seems highly probable that the prion protein is indeed the source of the infectious scrapie agent, although the viral hypothesis has still not been entirely abandoned. One of the aspects that encourages retention of the viral origin of the scrapie diseases is the issue of "strains" (3, 4) (see Prion). It is known that scrapie prions appear as multiple strains that differ somewhat in their disease expression, for example, Gerstmann-Straussler-Scheinker disease, FFI, and different subtypes of CJD (see list above) are expressed by different point mutations in the prion gene. When these different prion strains are inoculated into mouse brain containing only the single type of mouse prion, the resultant scrapie prions exhibit the characteristics of the inoculated strain. In viral diseases, these different disease signatures would be encoded on the viral DNA/RNA, but in the prion hypothesis they are thought to be encrypted on scrapie prion proteins in the form of slightly different three-dimensional folds of their polypeptide chains. These different folds are assumed to be capable of being "imprinted" on the same cellular prion proteins in their conversion to the scrapie form (14), thereby expressing the particular strain of the infection. A similar line of argument has been used to explain why prions are able to cross some species barriers, but not others (15). The experimental evidence suggests that infection crosses species barriers more easily when the host prion proteins have a closer amino acid homology with the infecting prions. For example, transgenic mice carrying both mouse and hamster prion genes express more mouse prions when inoculated with mouse prions, and more hamster prions when inoculated with hamster prions. This suggests that prions preferentially recruit homologous prion proteins. Similarly, the spread of scrapie from sheep to cows may be facilitated because the sequences of sheep and cattle PrP differ at only seven positions. It is probable, however, that it is the locations as well as the number of differences that is crucial: how else to explain that scrapie can pass from sheep to cattle, and then to humans, while scrapie cannot apparently pass directly from sheep to humans?
The prion hypothesis has taken our understanding of the scrapie diseases a considerable way forward, and in doing so has added some radically new concepts about protein structure to molecular biology. The weight of evidence is quite strongly in its favor, but nevertheless some important aspects of the role of prions in molecular mechanisms of the scrapie diseases remain to be uncovered. For example:
1. What is the precise nature of the infectious scrapie agent?
2. In what molecular complex the does the structural conversion of the prion proteins take place, and how does the "imprinting" of prion strains occur within it?
3. How are the neurotoxic effects of scrapie prions expressed?
4. How does the agent pass from the gut to the brain?
5. What is its relation to Alzheimer’s disease?
More radical ideas will be needed to answer these questions—or to overturn the prion hypothesis.
