Creutzfeldt-Jakob Disease and the Link to Prions


~by Melissa Ehlert~

Unique as a pathogenic agent, the cause of Creutzfeldt-Jakob disease (CJD) is believed to be a protein. This "proteinaceous infectious agent" was termed a prion by S.B. Prusiner, the theoretical father of this recently discovered disease phenomenon (Prusiner, 1982) [see an article by the man himself]. The prion diseases include all of those in which there is a build-up of of the abnormal isoform of the prion protein, PrP-SC, which usually leads to spongiform encephalopathies. Human prion diseases include CJD, kuru, Gerstmann- Straussler-Scheinker disease (GSS), fatal familial insomnia (FFI) and the animal diseases believed to be caused by prions are scrapie in sheep, bovine spongiform encephalopathy (BSE or "mad cow disease)" and transmissible encephalopathy in mink. Scrapie was the first prion disease to be discovered, however this soon led to the recognition that kuru, an illness of progressive cerebral ataxia transmitted by ritualistic cannibalism within the Fore people of Papua New Guinea, was also a prion disease. Due to its resemblance of scrapie and kuru, CJD was soon also included in this unique catalog of disease agents. Although the human prion diseases are all very rare, there has been significant recent hype over BSE due to instances of disease transmission to humans from the consumption of infected cow carcases.

 

CREUTZFELDT-JAKOB DISEASE

CJD was first described in 1920 by Creutzfeldt and in 1921 by Jakob (Richardson, 1977). The disease appears in several different forms and is fairly rare, with approximately one per million people in the general population newly afflicted per year (DeArmond and Prusiner, 1995). It has been identified all over the world, with pockets of disease occurring more frequently in certain ethnic groups, including Libyan Jewish immigrants to Isreal, rural inhabitants of Czechoslovakia, and Northern African immigrants to France (Steelman, 1944). Men and women are affected to an equal extent. The majority of people afflicted are of adult age (mean age is 60 years at death) who present with progressive dementia and possibly ataxia due to spongiform degeneration of the cerebral cortex. Clinical photographs of such degeneration due to CJD can be viewed. Neuropathology is widespread in the cerebral cortex and subcortical nuclei. The mortality rate has been estimated at 80% within one year of diagnosis. However, definitive diagnosis requires antemortem brain biopsy in which the infectious agent is isolated.

 There are several different forms of CJD, which are summarized below:

 

1) Acquired CJD
a) peripheral route b) central route 2) Sporadic CJD 3) Familial CJD
(adapted from Ridley, 1994)

The most common prion disease in humans is sporadic CJD, of which less than 1% of these cases have been found to be infectious (Scott et  al., 1996). In fact, numerous attempts to prove that sporadic prion diseases are even caused by infection have failed;  hence, sporadic CJD is likely caused by spontaneous conversion of PrP-C to PrP-SC within an individual. Consistent data throughout many countries supports the incidence rate of one case per million population per annum, providing further proof for the liklihood that sporadic CJD is caused by a spontaneous mutation. Nonetheless, prion diseases may be inherited as well - between 10 and 15% of prion disesases are passed on as an autosomal dominant genetic trait (Scott et al., 1996).

 Although the liklihood of acquiring CJD is extremely rare, (Davanipour et al., 1985) attempted to identify risk factors for the disease. They concluded from case-control studies that head or neck trauma, surgery requiring stitches, opthalmic exams using a tonometer, contact with wild animals, and eating pork or rare meat could all increase one's chances of getting CJD. Moreover, those afflicted individuals had more than average exposure to wild animals such as deer and rabbits. Several cases of CJD have been traced to tissue transplants or injection of foreign body material (check out --advisory report to Canadian blood donors). Corneal and dura matter transplants as well as foreign pituitary growth hormone injection have all been cited as origins of transmission (Steelman, 1944). Of course there have also been cases of transmission of related neurological degenerative prion diseases to humans from ingestion of infected meat. In actuality, there is an extremely minimal threat of getting a prion disease via this route. the risk from eating beef less than driving car or having sex". CJD poses a serious epidemiological concern to hospitals, according to Steelman (1994) who highlights the need to protect hospital workers and other patients from the causative disease agent. It is resistive of routine sterilization, disinfection, and cleaning techniques and therefore requires increased precaution.
 
 

THE LINK TO PRIONS

Prions are very different from every other known infectious pathogen. Typical pathogens such as eukaryotic protists, bacteria, or viruses, are all made up of the essential building blocks of life: nucleic acids. However, the prion is the altered form of a protein particle built of amino acids (although it may contain small fragments of 10 - 80 nt in length of nucleic acid). PrP-C is the cellular form of the prion protein which has been found in all mammals and birds throughout life. PrP-C is attached to the external surface of cells by a glycolipid moiety and its function is unknown although it is believed to be involved in cell signalling and adhesion (Collinge et al., 1994). When this protein is produced in the abnormal form PrP-SC, it may lead to disease. Thus unlike viral pathogens, the essential protein that may lead to pathogenesis is encoded for by a chromosomal gene. In this light, Prusiner's definition of prions can be further understood as
"small proteinaceous infectious particles that resist inactivation by procedures modifying nucleic acids and contain an abnormal isoform of a cellular protein which is a major and necessary component."
 
 
 
 these diagrams are from an article by Huang, Prusiner, and Cohen, (1996) On the left is the normal prion protein, PrP-C. It consists of approximately 45% alpha-helix and lacks almost entirely any beta sheet. On the right is the abnormal conformer, PrP-SC. It consists of approximately 30% alpha-helix and 45% beta sheet (Pan et al., 1993). PrP-C appears to be post-translationally modified to PrP-SC only in conformation by the acquisition of increased beta sheet conformation and loss of some alpha-helix.

 A wide variety of studies have been dedicated to researching the nature of the infectious prion particle. They have included methods of molecular engineering of transgenic animals, neuropathological exams, as well as biochemistry, immunology and cell biology investigations. The bulk of research has led to the theory that either PrP is entirely responsible for disease pathogenesis or that as a result of the pathogenic process, PrP accumulates in infected material. This is an ongoing and current field of study; new variants of CJD are still being discovered.

 The phenomenon of prion replication has raised numerous previously unencountered questions. The Central Dogma of biological theory requires that nucleic acid act as a template for replication. However prions violate this theory if they are made up of negligable amounts of nucleic acid. Faithful believers in the power of prions have attempted to explain possible methods of prion replication -- the prion dimer hypothesis claims that an initial PrP-SC molecule combines with one wild-type PrP-C to form a heterodimer (Prusiner, 1982). This is then converted into a homodimer (of PrP-SC/PrP-SC) that could then dissociate and lead to further conversion to the pathogenic agent. From the initial presence of PrP-SC, there would be an exponential increase in the abnormal conformer. The gene to produce PrP-SC could therefore be genetically passed on or an individual could acquire some of the abnormal protein from an infected individual (such as the manner in which BSE was passed onto humans). Otherwise, the normal PrP-C gene could mutate to produce PrP-SC or the normal protein could spontaneously change to form the abnormal protein. Of course the liklihood of either event occurring is very low, but this is believed to be the cause of the rare sporadic CJD.

 There is also counter-evidence to the theory that CJD is caused by prions. Sklaviadis and collegues (1989) found that the CJD infectious agent is larger in size than what would be expected for a monomeric protein. From density gradient resolution, it also was shown to have a greater density than other proteins resolved and detected in the same gradient. This study concludes that the CJD pathogenic agent is more likely to be similiar to conventional animal viruses than solely a protein complex, due to the detection of a structure consisting of protein as well as significant nucleic acid.

 

UNDERSTANDING THE LINK

Within the human PrP gene on chromosome 20, 17 different point mutations have been found in families with inherited prion diseases. All of these mutations apparently aid in the conformational change of PrP-C to PrP-SC. In cases of familial CJD, mutations at codon 178 (D---&gtN) and at codon 129 (M---&gtV) of the PrP gene as well as at codons 180, 200, 210 and 232 in the region of the third and fourth helical domains have been identified (Huang, Prusiner, and Cohen, 1996). In addition, octapeptide repeats have been found as insertions in the N-terminal domain of PrP which segregate with inherited CJD. It appears that while in sporadic CJD cases there is a spontaneous conversion of PrP-C to PrP-SC, familial CJD is linked to a genetic mutation which likely enhances the ability of the normal PrP to convert to the rare conformation of the infectious agent by about one million fold (Gajdusek, 1991).

So just how does the abnormal PrP isoform cause the neurological degeration observed in individuals afflicted with CJD? This aspect of research has not been fully elucidated, although several interesting studies have investigated prion pathogenesis. It has been shown that in vivo, CNS neurons express the highest amount of PrP mRNA (Prusiner, 1982). In addition, when transgenic mice were constructed to overexpress PrP-C and then samples of their brains were removed and transmitted to hamsters, the hamsters developed scrapie-like brain infection (Prusiner, 1996). The titer of scrapie infectivity was found to be proportional to PrP-SC concentration. This evidence supports the hypothesis that PrP-C may be spontaneously converted to the pathogenic PrP-SC, which causes symptomatic neurological disorder. At this point, the increasing amount of PrP-SC complexes aggregate together and are resistant to normal degredation by proteases. PrP-C has been found to be completely digested by limited esposure to proteinase K, whereas PrP-SC is only partially digested and even still retains its infectious nature (Westaway et al., 1994). The resistance of PrP-SC to proteolysis may also enhance the rod formation and subsequent neurodegeneration. Apparently the presence of the beta sheet and the lower concentration of the alpha-helix conformation in PrP-SC compared to PrP-C increases the propensity of the protein to aggregate into rods. Amyloid plaques which stain with alpha-PrP antibodies serve as a diagnostic tool for CJD identification (Prusiner, 1982). Not all patients with CJD have these plaques; often familial CJD cases may just present with widespread spongiform degeneration.

 

THE PROGNOSIS FOR CJD

Further research is clearly needed on CJD and the nature of prions. It is not known exactly how the pathogenic agent is transmitted nor is it clear the method by which it replicates. Unfortunately there is no known treatment for prion diseases. A professional's answer's to questions on BSE and related prion diseases can be read. Oken and McGeer (1995) suggest that anti-inflammatory therapies may help to stop the build-up of amyloid plaques, although this has not been clinically investigated. In cases of familial CJD where there is an option of genetic counseling, transmisibility may be theoretically avoided. However due to the late onset and incomplete penetrance of some inherited prion diseases, it is nearly impossible to predict future disease progression in individuals. Research is still very promising and should provide hope for effective therapies some day soon.

 

Other sites on this subject include:

Q and A on Transmissible Spongiform Encephalopathies
The MAD COW page
More on the "mad cow" disease
Information on BSE with multiple links
On the evolution of the BSE and related disease agents
More on Prions
Reccomendations from vegetarians on the mad cow hype

References

Collinge, E. et al. (1994) Prion protein is necessary for normal synaptic function. Nature 370:295-297.

 Richardson, E.P.J. (1977) Myoclonic dementia: introduction IN Neurologic Classics in Modern Translation (ed. D.A. Rottenberg and F.H. Hochberg) New York: Hafner Press.

 Davanipour, Z. et al. (1985) Creutzfeldt-Jakob disease: possible transmission by consumption of wild animal brains. Neurology 35:1483-1486.

 DeArmond, S.J. and Prusiner, S.B. (1995) Am. J. Path. 146(4):785-811

 Gajdusek, D.C. (1991) The transmissible amyloidoses: genetic control of spontaneous generation of infectious amyloid proteins by nucleation of configurational change in host precursors: kuru-CJD-scrapie-BSE. Eur. J. Epidemiol. 7(5):567-577.

 Huang, Z., Prusiner, S.B. and Cohen, F.E. (1996) Structures of prion proteins and conformational models for prion diseases IN Prions Prions Prions (ed. S.B. Prusiner) Berlin: Springer-Verlag.

 Oken, R.J. and McGeer, P.L. (1995) Human prion diseases: possible new directions in prophylaxis and therapy. Medical Hypotheses 44:167-168.

Pan, K.M. et al. (1993) Conversion of alpha-helices into beta sheets features in the formation of scrapie prion proteins. Proc. Natl. Acad. Sci. USA 90:10962-10966.

Prusiner, S.B. (1991) Molecular biology of prion diseases. Science 252:1515-1552.

 Prusiner, S.B. (1982) Novel proteinaceous infectious particles cause scrapie. Science 216:136-144.

 Prusiner, S.B. (1996) Human prion disease and neurodegeneration IN Prions Prions Prions (ed. S.B. Prusiner) Berlin: Springer-Verlag.

 Scott, M.R.D. et al. (1996) Transgenetics and gene targeting in studies of prion diseases IN Prions Prions Prions (ed. S.B. Prusiner) Berlin: Springer-Verlag.

 Sklaviadis, T.K., Manuelidis, L. and Manuelidis, E.E. (1989) Physical properties of the Creutzfeldt-Jakob disease agent. J. Virol. 63(3):1212-1222.

Steelman, V.M. (1944) Creutzfeld-Jakob disease: Reccomendations for infection control. Am. J. Infect. Control 22(5):312-318.

Westaway et al., (1994) Degeration of skeletal muscle, peripheral nerves, and the central nervous system in transgenic mice overexpressing wild-type prion proteins. Cell 76:117-129.