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The Nature of Tses Chris Bostock

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The Nature of Tses Chris Bostock The nature of TSEs Chris Bostock Scientists are Creutzfeld–Jacob disease (CJD) and bovine ● The nature of the infectious agent unsure of the ●spongiform encephalopathy (BSE or mad cow There are currently three theories. One says that the identity of the disease) both belong to a group of diseases infectious agent is somewhat like a small conventional causative infectious called transmissible spongiform encephalopathies virus, with genetic material coding for its own proteins. (TSEs) or prion diseases. BSE and the new variant of At present there is little evidence to support this theory. agent of TSEs, but CJD (vCJD) have been in the news recently and are new The second, and now most widely held theory states the properties are additions to the group, but, as a whole, TSEs have been that it is the altered physical state of normal host prion unlike those of around for centuries. Scrapie in sheep was first recorded protein, which is able to propagate itself by inducing conventional over two hundred years ago and sporadic CJD was first other normal prion protein molecules to adopt the pathogens. described in humans in the 1920s. Nevertheless, the abnormal conformation. This is commonly referred to TSEs remained a rather obscure group of diseases until as the prion hypothesis. The third idea, called the virino 1986 when BSE first appeared. In the years since the hypothesis, is most easily thought of as a hybrid of these emergence of BSE, new TSEs have also been found in hypotheses. It suggests that there is a very small piece of exotic species of ruminants in UK zoos, exotic and genetic material, which encodes only information for its domestic cats and, in 1996, vCJD was described in own survival and replication through interaction with a humans. host protein, perhaps the prion protein, using it also as The TSEs are fatal once clinical signs appear, but there a protective coat. The essential, but very important is a long, usually many years, incubation period between difference between the prion and virino hypotheses the time of first exposure to the infectious agent and the is that in the prion hypothesis, all the information to time of appearance of disease. A normal microbial determine the properties of the infectious agent is infection usually elicits an immune response in its target carried in the abnormal conformational state of the host, but in TSEs there is no conventional immune prion protein, whereas in the virino hypothesis, the BELOW: response, although the immune system plays an information is carried in an independent ‘genetic’ Fig. 1. Abnormal prion protein important part in the development of the disease, before molecule. As yet no-one has defined the precise can be detected as brown deposits the infectious agent gets into the central nervous system molecular state(s) of the normal and infectious forms of following staining with antibodies (CNS). A common feature of all TSEs is degeneration the prion protein, nor produced infectious ‘prions’ in vitro to prion protein in a section through part of the spinal cord in the brain and spinal cord and part of this process from normal prion protein, as would be predicted by the taken from a hamster infected involves a normal host protein, called PrP or prion prion hypothesis. But neither has anyone yet found any orally with the 263K strain of protein. During a TSE infection this protein is deposited molecule that might fit the role of the ‘virino’. scrapie (inset). The abnormal in an abnormal form and in excessive amounts in the accumulation of prion protein can brain, spinal cord and many peripheral nerves and tissues ● Prion protein genes be seen at higher magnification in individual neurons in the pathway (Fig. 1). The causative infectious agent has properties As with conventional microbes, there are different that connects the ear to the brain. unlike those of conventional pathogens and may be a new strains of TSEs, and the characteristics of an infection COURTESY TRICIA McBRIDE class of infectious agent. are determined by an interaction between a particular strain of infectious agent and a gene (or genes) in the host animal. The host gene that has the biggest effect in determining the outcome of an exposure to an infectious agent is the same gene that codes for the normal prion protein. In humans there are two common versions of the prion protein that differ in a single amino acid (whether it has valine or methionine at coding position 129), but there are also many rare mutant forms, most of which are associated with inherited susceptibility to prion disease. Sheep also have many different forms of the prion protein, none of which have so far been linked to ‘inherited’ scrapie and only some of which appear to affect the outcome of a scrapie infection. Two versions of the prion protein have been found in cattle but these do not appear to differ in their association with susceptibility of animals to BSE. Much of the work that has lead to an understanding of the role of the prion protein gene in TSEs was done in laboratory mice before anything was known about the existence of the prion protein or its gene. Different lines of inbred mice were found to differ in their response to infection with scrapie and a gene, called Sinc (for scrapie 164 MICROBIOLOGYTODAY VOL 27/NOV 00 Infection Incubation Period Disease 1 2 3 4 Natural 5 Scrapie 6 1 2 R3 3 C57BL 4 5 VM incubation period), was 6 C57BLxVM identified that controlled 7 the incubation period. BSE 8 There are two versions of Sinc, s7 and p7, which 1 FSE determine, respectively, 2 short and prolonged incubation after infection 1 of mice with a strain of 2 scrapie called ME7. Sinc vCJD 3 turns out to be the gene that encodes the prion 0 1 00 2 00 3 00 400 500 600 700 800 days protein. The difference between s7 and p7 Sinc genes is firm evidence to say how humans became infected. From ABOVE: now known to lie in the amino acids at two positions in a scientific point of view, one of the interesting observ- Fig. 2. Strains of TSE agents the protein; if it has leucine at position 108 and threonine ations to come out of this work is that the same strain of differ in the relative lengths of incubation period in different at 189 it is Sinc s7, whereas if it has proline at position agent (BSE) can be propagated by several species, each of inbred lines of mice. R3 and C57BL 108 and valine at 189 it is Sinc p7. which has a slightly different normal prion protein. are s7 mice, VM is a p7 mouse and Thus, if the conformation of the abnormal form of the C57BLxVM is the cross between ● Properties of different strains of infectious prion protein determines the biological properties of a the two lines. Several sources of BSE from cattle, FSE from domestic agents strain of infectious TSE agent, it must be independent of cats and vCJD from humans In addition to the different versions of the prion protein the structure of the normal prion protein in these produce very similar patterns, gene within a species there are many different strains of different species. Different prion proteins must be able to indicating that the same strain of the infectious agent that can infect that species. This is adopt the same abnormal conformation even though TSE agent is causing the infection. most clearly demonstrated by infecting the same inbred they have very different structures in their normal states. Different sources of scrapie on the other hand produce very different lines of Sinc s7 or Sinc p7 mice with different sources of Strains of TSEs differ in several other respects in patterns of incubation in the scrapie. Different sources of scrapie can produce widely addition to incubation periods and lesion profiles. They different mouse lines, indicating different incubation periods and result in differing show large differences in their resistance to inactivation that they are different from BSE as patterns of damage in the brain (called lesion profile). by heat or chemicals. Primary BSE and a mouse-passaged well as being different from each other. These two features, relative incubation periods in s7 and strain derived from it (called 301V) are the TSE agents BASED ON DATA PROVIDED BY MOIRA p7 mice and the lesion profile, are classically used to most resistant to inactivation by heat yet to be BRUCE AND COLLEAGUES define the different strains of infectious agent. Several discovered. Nevertheless, resistance to inactivation is a different strains of TSE agent can be propagated in the property of the ‘strain’ and not the primary structure of same inbred line of mouse and thus, if the abnormal the prion protein that carries it because the resistance conformation of the prion protein determines the properties of a strain remain the same when it is produced properties of the infectious agent, this means that the in mice differing in the type of prion protein they make same normal prion protein must be able to adopt (s7 v. p7). reproducibly several distinct abnormal disease- Different strains of TSEs are associated with different associated states. biochemical and biophysical properties of the abnormal Using the criteria of relative incubation period and prion protein. The prion protein has two sites at which it lesion profile it has been possible to characterize and can be glycosylated, i.e. amino acids at which sugar compare the infectious agents causing contemporary residues can be attached, but not all molecules of the scrapie, BSE, the spongiform encephalopathies of cats prion protein are fully glycosylated.
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