Diagnosis of Gerstmann-Sträussler syndrome in familial dementia with prion protein gene analysis

Genetic Factors in Mammalian Prion Diseases

Mammalian prion diseases are a group of neurodegenerative conditions caused by infection of the central nervous system with proteinaceous agents called prions, including sporadic, variant, and iatrogenic Creutzfeldt-Jakob disease; kuru; inherited prion disease; sheep scrapie; bovine spongiform encephalopathy; and chronic wasting disease. Prions are composed of misfolded and multimeric forms of the normal cellular prion protein (PrP). Prion diseases require host expression of the prion protein gene (PRNP) and a range of other cellular functions to support their propagation and toxicity. Inherited forms of prion disease are caused by mutation of PRNP, whereas acquired and sporadically occurring mammalian prion diseases are controlled by powerful genetic risk and modifying factors. Whereas some PrP amino acid variants cause the disease, others confer protection, dramatically altered incubation times, or changes in the clinical phenotype. Multiple mechanisms, including interference with homotypic protein interactions and the selection of the permissible prion strains in a host, play a role. Several non-PRNP factors have now been uncovered that provide insights into pathways of disease susceptibility or neurotoxicity.

Methods for Molecular Diagnosis of Human Prion Disease

Human prion diseases are associated with a range of clinical presentations, and they are classified by both clinicopathological syndrome and etiology, with subclassification according to molecular criteria. Here, we describe updated procedures that are currently used within the MRC Prion Unit at UCL to determine a molecular diagnosis of human prion disease. Sequencing of the PRNP open reading frame to establish the presence of pathogenic mutations is described, together with detailed methods for immunoblot or immunohistochemical determination of the presence of abnormal prion protein in the brain or peripheral tissues.

Gerstmann-Sträussler-Scheinker disease

Gerstmann-Sträussler-Scheinker (GSS) is a slowly progressive hereditary autosomal dominant disease (OMIM: 137440) and the first human transmissible spongiform encephalopathy (TSE) in which a mutation in a gene encoding for prion protein (PrP) was discovered. The first "H" family had been known by the Viennese neuropsychiatrists since the XXth century and was reported by Gerstmann, Sträussler and Scheinker in 1936. In this chapter we present the clinical, neuropathological and molecular data on GSS with the mutations in the PRNP gene: at codons 102, 105, 117, 131, 145, 187, 198, 202, 212, 217 and 232. In several families with GSS the responsible mutations are unknown.

Human prion diseases: from Kuru to variant Creutzfeldt-Jakob disease

Transmissible spongiform encephalopathies (TSEs) or prion diseases are the names given to the group of fatal neurodegenerative disorders that includes kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker disease (GSS), fatal and sporadic familial insomnia and the novel prion disease variable protease-sensitive prionopathy (PSPr) in humans. Kuru was restricted to natives of the Foré linguistic group in Papua New Guinea and spread by ritualistic endocannibalism. CJD appears as sporadic, familial (genetic or hereditary) and infectious (iatrogenic) forms. Variant CJD is a zoonotic CJD type and of major public health importance, which resulted from transmission from bovine spongiform encephalopathy (BSE) through ingestion of contaminated meat products. GSS is a slowly progressive hereditary autosomal dominant disease and the first human TSE in which a mutation in a gene encoding for prion protein (PrP) was discovered. The rarest human prion disease is fatal insomnia, which may occur, in genetic and sporadic form. More recently a novel prion disease variable protease-sensitive prionopathy (PSPr) was described in humans.TSEs are caused by a still incompletely defined infectious agent known as a "prion" which is widely regarded to be an aggregate of a misfolded isoform (PrP(Sc)) of a normal cellular glycoprotein (PrP(c)). The conversion mechanism of PrP(c) into PrP(Sc) is still not certain.

Familial prion disease with Alzheimer disease-like tau pathology and clinical phenotype

OBJECTIVE

To describe the Alzheimer disease (AD)-like clinical and pathological features, including marked neurofibrillary tangle (NFT) pathology, of a familial prion disease due to a rare nonsense mutation of the prion gene (PRNP).

METHODS

Longitudinal clinical assessments were available for the proband and her mother. After death, both underwent neuropathological evaluation. PRNP was sequenced after failure to find immunopositive Aβ deposits in the proband and the documentation of prion protein (PrP) immunopositive pathology.

RESULTS

The proband presented at age 42 years with a 3-year history of progressive short-term memory impairment and depression. Neuropsychological testing found impaired memory performance, with relatively preserved attention and construction. She was diagnosed with AD and died at age 47 years. Neuropathologic evaluation revealed extensive limbic and neocortical NFT formation and neuritic plaques consistent with a Braak stage of VI. The NFTs were immunopositive, with multiple tau antibodies, and electron microscopy revealed paired helical filaments. However, the neuritic plaques were immunonegative for Aβ, whereas immunostaining for PrP was positive. The mother of the proband had a similar presentation, including depression, and had been diagnosed clinically and pathologically as AD. Reevaluation of her brain tissue confirmed similar tau and PrP immunostaining findings. Genetic analysis revealed that both the proband and her mother had a rare PRNP mutation (Q160X) that resulted in the production of truncated PrP.

INTERPRETATION

We suggest that PRNP mutations that result in a truncation of PrP lead to a prolonged clinical course consistent with a clinical diagnosis of AD and severe AD-like NFTs.

Review: contribution of transgenic models to understanding human prion disease

Transgenic mice expressing human prion protein in the absence of endogenous mouse prion protein faithfully replicate human prions. These models reproduce all of the key features of human disease, including long clinically silent incubation periods prior to fatal neurodegeneration with neuropathological phenotypes that mirror human prion strain diversity. Critical contributions to our understanding of human prion disease pathogenesis and aetiology have only been possible through the use of transgenic mice. These models have provided the basis for the conformational selection model of prion transmission barriers and have causally linked bovine spongiform encephalopathy with variant Creutzfeldt-Jakob disease. In the future these models will be essential for evaluating newly identified potentially zoonotic prion strains, for validating effective methods of prion decontamination and for developing effective therapeutic treatments for human prion disease.

Molecular pathology of human prion disease

Human prion diseases are associated with a range of clinical presentations and are classified by both clinicopathological syndrome and aetiology with sub-classification according to molecular criteria. Considerable experimental evidence suggests that phenotypic diversity in human prion disease relates in significant part to the existence of distinct human prion strains encoded by abnormal PrP isoforms with differing physicochemical properties. To date, however, the conformational repertoire of pathological isoforms of wild-type human PrP and the various forms of mutant human PrP has not been fully defined. Efforts to produce a unified international classification of human prion disease are still ongoing. The ability of genetic background to influence prion strain selection together with knowledge of numerous other factors that may influence clinical and neuropathological presentation strongly emphasises the requirement to identify distinct human prion strains in appropriate transgenic models, where host genetic variability and other modifiers of phenotype are removed. Defining how many human prion strains exist allied with transgenic modelling of potentially zoonotic prion strains will inform on how many human infections may have an animal origin. Understanding these relationships will have direct translation to protecting public health.

Characteristics of 263K scrapie agent in multiple hamster species

Transmissible spongiform encephalopathy (TSE) diseases are known to cross species barriers, but the pathologic and biochemical changes that occur during transmission are not well understood. To better understand these changes, we infected 6 hamster species with 263K hamster scrapie strain and, after each of 3 successive passages in the new species, analyzed abnormal proteinase K (PK)-resistant prion protein (PrPres) glycoform ratios, PrPres PK sensitivity, incubation periods, and lesion profiles. Unique 263K molecular and biochemical profiles evolved in each of the infected hamster species. Characteristics of 263K in the new hamster species seemed to correlate best with host factors rather than agent strain. Furthermore, 2 polymorphic regions of the prion protein amino acid sequence correlated with profile differences in these TSE-infected hamster species.

Absence of spontaneous disease and comparative prion susceptibility of transgenic mice expressing mutant human prion proteins

Approximately 15 % of human prion disease is associated with autosomal-dominant pathogenic mutations in the prion protein (PrP) gene. Previous attempts to model these diseases in mice have expressed human PrP mutations in murine PrP, but this may have different structural consequences. Here, we describe transgenic mice expressing human PrP with P102L or E200K mutations and methionine (M) at the polymorphic residue 129. Although no spontaneous disease developed in aged animals, these mice were readily susceptible to prion infection from patients with the homotypic pathogenic mutation. However, while variant Creutzfeldt–Jakob disease (CJD) prions transmitted infection efficiently to both lines of mice, markedly different susceptibilities to classical (sporadic and iatrogenic) CJD prions were observed. Prions from E200K and classical CJD M129 homozygous patients, transmitted disease with equivalent efficiencies and short incubation periods in human PrP 200K, 129M transgenic mice. However, mismatch at residue 129 between inoculum and host dramatically increased the incubation period. In human PrP 102L, 129M transgenic mice, short disease incubation periods were only observed with transmissions of prions from P102L patients, whereas classical CJD prions showed prolonged and variable incubation periods irrespective of the codon 129 genotype. Analysis of disease-related PrP (PrP^Sc) showed marked alteration in the PrP^Sc glycoform ratio propagated after transmission of classical CJD prions, consistent with the PrP point mutations directly influencing PrP^Sc assembly. These data indicate that P102L or E200K mutations of human PrP have differing effects on prion propagation that depend upon prion strain type and can be significantly influenced by mismatch at the polymorphic residue 129.

Can copper binding to the prion protein generate a misfolded form of the protein?

The native prion protein (PrP) has a two domain structure, with a globular folded alpha-helical C-terminal domain and a flexible extended N-terminal region. The latter can selectively bind Cu(2+) via four His residues in the octarepeat (OR) region, as well as two sites (His96 and His111) outside this region. In the disease state, the folded C-terminal domain of PrP undergoes a conformational change, forming amorphous aggregates high in beta-sheet content. Cu(2+) bound to the ORs can be redox active and has been shown to induce cleavage within the OR region, a process requiring conserved Trp residues. Using computational modeling, we have observed that electron transfer from Trp residues to copper can be favorable. These models also reveal that an indole-based radical cation or Cu(+) can initiate reactions leading to protein backbone cleavage. We have also demonstrated, by molecular dynamics simulations, that Cu(2+) binding to the His96 and His111 residues in the remaining PrP N-terminal fragment can induce localized beta-sheet structure, allowing us to suggest a potential mechanism for the initiation of beta-sheet misfolding in the C-terminal domain by Cu(2+).

A potential mechanism for Cu2+ reduction, beta-cleavage, and beta-sheet initiation within the N-terminal domain of the prion protein: insights from density functional theory and molecular dynamics calculations

The N-terminal region of the native human prion protein encompasses four highly conserved octarepeats that each contain a single His, Pro, Gln, and Trp residue as well as several Gly residues. At neutral pH these repeats are capable of individually binding copper (Cu(2+)) ions, involving the His side chain and the backbone amide of the Gly residues. In addition, the two His residues at positions 96 and 111 are also capable of binding Cu(2+). At low concentrations of the metal ion or at low pH, one Cu(2+) may be bound by multiple His residues of the four octarepeats. This complex is known to be redox active, while none of the other Cu(2+)-bound complexes are. Using density functional theory and molecular dynamics calculations data demonstrated how this form of the protein could reduce Cu(2+), through a process involving electron transfer from the Trp side chain. The reduced Cu gives rise to reactive oxygen species (ROS), which can lead to beta-cleavage of the prion protein chain at any of the Gly residues around position 90. Protein fragments of lengths similar to those arising from beta-cleavage are predominantly found in both healthy and Creutzfeldt-Jakob disease (CJD)-affected brains. Models of Cu binding to the His96 and His111 residues also indicate that different modes of Cu(2+) binding result in formation of stable beta-hairpin structures in this region of the protein. It is postulated that through interactions with the C-terminal part of the protein these hairpins may initiate misfolding and yield more stable beta-sheet structures that might associate in the same fashion with additional prion proteins.

Review. Lessons of kuru research: background to recent studies with some personal reflections

The widespread exposure of the UK population to bovine spongiform encephalopathy prions, and the potential consequences for public health, led to a renewed interest in kuru, the principal example of epidemic human prion disease. Kuru research in Papua New Guinea was expanded to study the range of incubation periods possible in human prion infection, to investigate maternal and other possible natural routes of transmission, to characterize genetic susceptibility and resistance factors and to gain insights into the peripheral pathogenesis of orally acquired prion disease in humans. Although now essentially over, the kuru epidemic continues to provide important lessons.

Lack of TAR-DNA binding protein-43 (TDP-43) pathology in human prion diseases

AIMS

TAR-DNA binding protein-43 (TDP-43) is the major ubiquitinated protein in the aggregates in frontotemporal dementia with ubiquitin-positive, tau-negative inclusions and motor neurone disease. Abnormal TDP-43 immunoreactivity has also been described in Alzheimer's disease, Lewy body diseases and Guam parkinsonism-dementia complex. We therefore aimed to determine whether there is TDP-43 pathology in human prion diseases, which are characterised by variable deposition of prion protein (PrP) aggregates in the brain as amyloid plaques or more diffuse deposits.

MATERIAL AND METHODS

TDP-43, ubiquitin and PrP were analysed by immunohistochemistry and double-labelling immunofluorescence, in sporadic, acquired and inherited forms of human prion disease.

RESULTS

Most PrP plaques contained ubiquitin, while synaptic PrP deposits were not associated with ubiquitin. No abnormal TDP-43 inclusions were identified in any type of prion disease case, and TDP-43 did not co-localize with ubiquitin-positive PrP plaques or with diffuse PrP aggregates.

CONCLUSIONS

These data do not support a role for TDP-43 in prion disease pathogenesis and argue that TDP-43 inclusions define a distinct group of neurodegenerative disorders.

Modeling by assembly and molecular dynamics simulations of the low Cu2+ occupancy form of the mammalian prion protein octarepeat region: gaining insight into Cu2+-mediated beta-cleavage

The prion protein has garnered considerable interest because of its involvement in prion disease as well as its unresolved cellular function. The octarepeat region in the flexible N-domain is capable of binding copper through multiple coordination modes. Under conditions of low pH and low Cu(2+) concentration, the four octarepeats (ORs) cooperatively coordinate a single copper ion. Based on the average structure of the PHGG and GWGQ portions of a copper-free OR(2) model from molecular dynamics simulations, the starting structures of the OR(4) complex could be constructed by assembling the repeating structure of PHGG and GWGQ fragments. The resulting model contains a preformed site suitable for Cu(2+) coordination. Molecular dynamics simulations of Cu(2+) bound to the assembled OR(4) model (Cu:OR(4)) reveal a close association of specific Trp and Gly residues with the Cu(2+) center. This low Cu(2+)-occupancy form of prion protein is redox-active and can readily initiate cleavage of the OR region, mediated by reactive oxygen species generated by Cu(+). The OR region is known to be required for beta-cleavage, as are the Trp residues within the OR region. The beta-cleaved form of the prion protein accumulates in amyloid fibrils. Hence, the close approach of Trp and Gly residues to the Cu(2+) coordination site in the low Cu(2+)-occupancy form of the OR region may signal an important interaction for the initiation of prion disease.

Implications for Creutzfeldt-Jakob Disease (CJD) in Dentistry: a Review of Current Knowledge

This review explores our current understanding of the risks of (variant) Creutzfeldt-Jakob disease transmission via dental practice, and whether they merit the rigorous enforcement of improved standards of instrument cleaning and decontamination. The recognition of prions as novel infectious agents in humans has caused significant concern among the public and medical professionals alike. Creutzfeldt-Jakob disease (CJD) in humans has been shown to be transmissible via several routes, including transplantation, contaminated medical products, and via neurosurgery. While the likelihood of transmission via dentistry is undoubtedly very low, this may be amplified considerably by unknown risk factors, such as disease prevalence (particularly in the UK), altered tissue distribution of vCJD, and the failure of decontamination processes to address the inactivation of prions adequately. Since current diagnostic techniques are unable to detect PrPSc in human dental tissues, there is limited evidence for the presence of infectivity. Given these uncertainties, the control of risk by reinforced and improved decontamination practices seems the most appropriate response.

PRNP M129V homozygosity in multiple system atrophy vs. Parkinson's disease

Multiple system atrophy (MSA) is a neurodegenerative disorder of unknown etiology characterized by extrapyramidal, pyramidal, cerebellar, and autonomic dysfunction in any combination. We report a patient with a 4-year history of MSA who developed dementia associated with sporadic Creutzfeldt-Jakob disease (CJD). Our proband was MM homozygous for the M129V polymorphism within the prion protein gene (PRNP), a known risk factor for CJD. We conducted a case-control study to test the hypothesis that homozygosity for the M129V polymorphism of PRNP occurs more frequently in MSA in comparison to Parkinson's disease and healthy volunteers. A total of 63 patients with MSA, 54 age-, race- and gendermatched controls with Parkinson's disease, and 126 matched healthy volunteers were studied. The genotype analysis revealed no significant difference in the codon 129 genotype distribution in MSA as compared to controls. Nonetheless, the frequencies of the MM and VV genotypes were higher in MSA than in Parkinson's disease. Thus, homozygosity, particularly VV homozygosity, at codon 129 of PRNP is associated with MSA compared to a clinically related but pathophysiologically distinct alpha-synucleinopathy. Considering the possibility that the prion protein contributes to the pathogenesis of MSA would require confirmation of these findings in an independent patient population.

Molecular diagnosis of human prion disease

Human prion diseases are associated with a range of clinical presentations, and they are classified by both clinicopathological syndrome and etiology, with subclassification according to molecular criteria. Here, we describe procedures that are used within the MRC Prion Unit to determine a molecular diagnosis of human prion disease. Sequencing of the PRNP open reading frame to establish the presence of pathogenic mutations is described, together with detailed methods for immunoblot or immunohistochemical determination of the presence of abnormal prion protein in brain or peripheral tissues.

Molecular dynamics simulations of two tandem octarepeats from the mammalian prion protein: fully Cu2+-bound and metal-free forms

Molecular dynamics simulations have been conducted on a model fragment (Ac-PHGGGWGQPHGGGW-NH(2)) of the prion protein octarepeat domain, both in the Cu(2+)-bound and metal-free forms. The copper-bound models are based on the consensus structure of the core Cu(2+)-binding site of an individual octarepeat, relevant to the fully Cu(2+)-occupied prion protein octarepeat region. The model peptides contain Cu(2+) bound through a His imidazole ring and two deprotonated amide N-atoms in the peptide backbone supplied by the following two Gly residues. Both the copper-bound and metal-free models have been simulated with the OPLS all-atom force field with the GROMACS molecular dynamics package. These simulations, with two tandem copper-binding sites, represent the minimum model necessary to observe potential structuring between the copper-binding sites in the octarepeat region. The GWGQ residues constitute a flexible linker region that predominantly adopts a turn, serving to bring adjacent His residues into close proximity. The consequent formation of stable structures demonstrates that the copper-bound octarepeat region allows the copper-coordinating sites to come into van der Waals contact, packing into particular orientations to further stabilize the bend in the GWGQ linker region.

Update on human prion disease

The recognition that variant Creutzfeldt-Jakob disease (vCJD) is caused by the same prion strain as bovine spongiform encephalopathy in cattle has dramatically highlighted the need for a precise understanding of the molecular biology of human prion diseases. Detailed clinical, pathological and molecular data from a large number of human prion disease patients indicate that phenotypic diversity in human prion disease relates in part to the propagation of disease-related PrP isoforms with distinct physicochemical properties. Incubation periods of prion infection in humans can exceed 50 years and therefore it will be some years before the extent of any human vCJD epidemic can be predicted with confidence.

Interaction of metals with prion protein: Possible role of divalent cations in the pathogenesis of prion diseases

Prion diseases are fatal neurodegenerative disorders that affect both humans and animals. The rapid clinical progression, change in protein conformation, cross-species transmission and massive neuronal degeneration are some key features of this devastating degenerative condition. Although the etiology is unknown, aberrant processing of cellular prion proteins is well established in the pathogenesis of prion diseases. Normal cellular prion protein (PrP(c)) is highly conserved in mammals and expressed predominantly in the brain. Nevertheless, the exact function of the normal prion protein in the CNS has not been fully elucidated. Prion proteins may function as a metal binding protein because divalent cations such as copper, zinc and manganese can bind to octapeptide repeat sequences in the N-terminus of PrP(c). Since the binding of these metals to the octapeptide has been proposed to influence both structural and functional properties of prion proteins, alterations in transition metal levels can alter the course of the disease. Furthermore, cellular antioxidant capacity is significantly compromised due to conversion of the normal prion protein (PrP(c)) to an abnormal scrapie prion (PrP(sc)) protein, suggesting that oxidative stress may play a role in the neurodegenerative process of prion diseases. The combination of imbalances in cellular transition metals and increased oxidative stress could further exacerbate the neurotoxic effect of PrP(sc). This review includes an overview of the structure and function of prion proteins, followed by the role of metals such as copper, manganese and iron in the physiological function of the PrP(c), and the possible role of transition metals in the pathogenesis of the prion disease.

Transgenic mice expressing bovine PrP with a four extra repeat octapeptide insert mutation show a spontaneous, non-transmissible, neurodegenerative disease and an expedited course of BSE infection

Transgenic (Tg) mice carrying four extra octapeptide repeats (OR) in the bovine PrP gene (10OR instead of 6) have been generated. In these mice, neuropathological changes were observed depending upon the level of transgene expression. These changes primarily involved a slowly advancing neurological disorder, characterized clinically by ataxia, and neuropathologically, by vacuolization in different brain areas, gliosis, and loss of cerebellar granule cells. Accumulation of insoluble bovine 10OR-PrP (bo10OR-PrP) was observed depending on the level of expression but no infectivity was found associated with this insoluble form. We also compared the behavior of bo6OR-PrP and bo10OR-PrP Tg mouse lines in response to BSE infection. BSE-inoculated bo10ORTg mice showed an altered course of BSE infection, reflected by reduced incubation times when compared to bo6ORTg mice expressing similar levels of the wild type 6OR-PrP. In BSE-inoculated mice, it was possible to detect PrP(res) in 100% of the animals. While insoluble bo10OR-PrP from non-inoculated bo10ORTg mice was non-infectious, brain homogenates from BSE-inoculated bo10ORTg mice were highly infectious in all the Tg mouse lines tested. This Tg mouse model constitutes a new way of understanding the pathobiology of bovine transmissible spongiform encephalopathy. Its potential applications include the assessment of new therapies against prion diseases.

Molecular neurology of prion disease

Prions are infectious pathogens principally composed of abnormal forms of a protein encoded in the host genome. They cause lethal neurodegenerative conditions including CJD, GSS, and kuru in humans and scrapie and bovine spongiform encephalopathy in domestic animals. Remarkably, distinct strains of prions occur despite absence of an agent-specific genome: misfolded proteins themselves may encode strain diversity--with wide implications in biology. The arrival of variant CJD, and the experimental confirmation that it is caused by infection with BSE-like prions, has focussed research on early diagnosis and treatment. Recent advances lead to considerable optimism that effective human therapies may now be developed. While several drugs have been tried in small numbers of patients, there is no clear evidence of efficacy of any agent and controlled clinical trials are urgently needed. Importantly, there is increasing recognition that fundamental processes involved in prion propagation--seeded aggregation of misfolded host proteins--are of far wider significance, not least in understanding the commoner neurodegenerative diseases that pose such a major and increasing challenge for healthcare in an ageing population.

Different behavior toward bovine spongiform encephalopathy infection of bovine prion protein transgenic mice with one extra repeat octapeptide insert mutation

In humans, insert mutations within the repetitive octapeptide region of the prion protein gene (Prnp) are often associated with familial spongiform encephalopathies. In this study, transgenic mice expressing bovine PrP (boTg mice) bearing an additional octapeptide insertion to the wild type (seven octapeptide repeats instead of six) showed an altered course of bovine spongiform encephalopathy (BSE) infection, reflected as reduced incubation times when compared with boTg mice expressing similar levels of the wild-type six-octapeptide protein. In both boTg mouse lines (bo6ORTg and bo7ORTg), incubation times were affected drastically depending on transgene expression levels and the inoculum used. In accordance with the lack of an interspecies barrier to BSE infection, we detected the typical signs of CNS spongiform degeneration by histopathological analysis and the presence of the bovine prion PrP(res) by Western blot or immunohistochemical analyses. When 7OR-PrP(res) was propagated in bo7ORTg mice, a similar earlier onset of clinical signs was observed compared with bo6ORTg mice. Proteins PrP(C) and PrP(res) containing seven octapeptides (7OR-PrP(C) and 7OR-PrP(res)) showed similar protease sensitivity and insolubility in nondenaturing detergents to homologous 6OR-PrP(C) and 6OR-PrP(res). In addition, bo7ORTg mice showed a higher sensitivity than bo6ORTg mice for detecting prion infection in specimens previously diagnosed as negative by conventional biochemical techniques. In the absence of clinical signs of disease, 7OR-PrP(res) could be detected as early as 120 d after inoculation by immunohistochemical and Western blot analyses. These findings may help us improve the current mouse bioassays and understand the role of the octapeptide repeat region in susceptibility to disease.

Molecular and clinical classification of human prion disease

While rare in humans, the prion diseases have become an area of intense clinical and scientific interest. The recognition that variant Creutzfeldt-Jakob disease is caused by the same prion strain as bovine spongiform encephalopathy in cattle has dramatically highlighted the need for a precise understanding of the molecular biology of human prion diseases. Detailed clinical, pathological and molecular data from a large number of human prion disease cases have shown that distinct abnormal isoforms of prion protein are associated with prion protein gene polymorphism and neuropathological phenotypes. A molecular classification of human prion diseases seems achievable through characterisation of structural differences of the infectious agent itself.

A critical review of the nature of the spongiform encephalopathy agent: protein theory versus virus theory

All spongiform encephalopathies (SEs) result in brain disorders brought about by a slow virus. Since the origin of bovine SE (BSE), the infectious nature of the disease has been firmly established. Tubulofilamentous particles/scrapie termed nemavirus (NVP) and scrapie-associated fibrils (SAF) are ultrastructural markers, whereas protease-resistant protein (PrP(sc)) is a protein marker. The PrP molecules aggregate to form SAF. Each NVP consists of three layers: an outer protein coat, an intermediate ssDNA layer, and inner PrP/SAF. Therefore, ssDNA and PrP/SAF are physically associated with each other. The existence of at least 20 stable strains of SEs implies that a nucleic acid molecule serves as the information molecule. Animals inoculated with PrP(sc) do not develop the clinical disease, however, ssDNA purified from scrapie-hamster brains by alkaline gel electrophoresis mixed with binding proteins before inoculation developed the clinical disease. It appears that an "accessory protein" coded by the ssDNA of the NVP interacts with normal PrP(c) molecules, resulting in their conversion to PrP(sc)/SAF. The pathogenesis process in the infected animal, with increasing incubation periods, reveals that larger amounts of normal PrP molecules are modified to form SAF. This interferes with the normal supply of PrP to cell membranes, which become disrupted and eventually fragment, resulting in the vacuoles typical of those found in the SEs. Critical review of scientific literature has demonstrated that the agent contains a DNA genome.

Prion diseases of humans and animals: their causes and molecular basis

Prion diseases are transmissible neurodegenerative conditions that include Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy (BSE) and scrapie in animals. Prions appear to be composed principally or entirely of abnormal isoforms of a host-encoded glycoprotein, prion protein. Prion propagation involves recruitment of host cellular prion protein, composed primarily of alpha-helical structure, into a disease specific isoform rich in beta-sheet structure. The existence of multiple prion strains has been difficult to explain in terms of a protein-only infections agent, but recent studies suggest that strain specific phenotypes can be encoded by different prion protein conformations and glycosylation patterns. The ability of a protein to encode phenotypic information has important biological implications. The appearance of a novel human prion disease, variant CJD, and the clear experimental evidence that it is caused by exposure to BSE has highlighted the need to understand the molecular basis of prion propagation, pathogenesis, and the barriers limiting intermammalian transmission. It is unclear if a large epidemic of variant CJD will occur in the years ahead.

Pathogenesis of prion diseases: a progress report

Almost 20 years have passed since Stanley Prusiner proposed that the agent causing transmissible spongiform encephalopathies consists exclusively of a protein and termed it prion. A mixed balance can be drawn from the enormous research efforts that have gone into prion research during this time. On the negative side, the protein-only hypothesis has not been conclusively proven yet. On the positive side, our understanding of spongiform encephalopathies has experienced tremendous advances, mostly through human genetics, mouse transgenetics, and biophysical methods. Perhaps the most astonishing development is the realization that many human neurodegenerative diseases for which transmissibility has been more or less stringently excluded, may follow pathogenetic principles similar to those of prion diseases. Also, the hypothesis that prion-like phenomena may underlie certain non-genetic traits observed in yeast has resulted in the surprising recognition that the instructional self-propagating changes in protein conformation may be much more prevalent in nature than previously thought. The latter developments have been astonishingly successful, and one could now argue that the prion principle is much more solidly established in yeast than in mammals.

Inherited prion disease with an alanine to valine mutation at codon 117 in the prion protein gene

A large English family with autosomal dominant segregation of presenile dementia, ataxia and other neuropsychiatric features is described. Diagnoses of demyelinating disease, Alzheimer's disease, Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker syndrome have been attributed to particular individuals at different times. An Irish family, likely to be part of the same kindred, is also described, in which diagnoses of multiple sclerosis, dementia, corticobasal degeneration and new variant CJD have been considered in affected individuals. Molecular genetic studies have enabled the classification of this disease at the molecular level as one of the group of inherited prion diseases, with the substitution of valine for alanine at codon 117 of the prion protein gene (PRNP). Only three other kindreds have been described world-wide with this mutation and only limited phenotypic information has been reported. Here we describe the phenotypic spectrum of inherited prion disease (PrPA117V). The diversity of phenotypic expression seen in this kindred emphasizes the logic of molecular classification of the inherited prion diseases rather than classification by specific clinicopathological syndrome. Indeed, inherited prion disease should be excluded by PRNP analysis in any individual presenting with atypical presenile dementia or neuropsychiatric features and ataxia, including suspected cases of new variant CJD.

SSCP analysis and sequencing of the human prion protein gene (PRNP) detects two different 24 bp deletions in an atypical Alzheimer's disease family

Alzheimer's disease (AD) is a progressive, degenerative neurological disorder of the central nervous system. AD is the fourth leading cause of death in elderly persons 65 years or older in Western industrialized societies. The etiology of AD is unknown, but clinical, pathological, epidemiological, and molecular investigations suggest it is etiologically heterogeneous. Mutations in the amyloid protein are rare and segregate with the disease in a few early-onset familial AD (FAD) families. Similarities between AD and the unconventional viral (UCV) diseases, and between the amyloid and prion proteins, implicate the human prion protein gene (PRNP) as another candidate gene. Single strand conformation polymorphism (SSCP) analysis was used to screen for mutations at this locus in 82 AD patients from 54 families (30 FAD), vs. 39 age-matched controls. A 24-bp deletion around codon 68 that codes for one of five Gly-Pro rich octarepeats was identified in two affected sibs and one offspring of one late-onset FAD family. Two other affected sibs, three unaffected sibs, and three offspring from this family, in addition to one sporadic AD patient and three age-matched controls, were heterozygous for another octarepeat deletion located around codon 82. Two of the four affected sibs had features of PD, including one who was autopsy-verified AD and PD. Although these deletions were found infrequently in other AD patients and controls, they appear to be a rare polymorphism that is segregating in this FAD family. It does not appear that mutations at the PRNP locus are frequently associated with AD in this population.

The nosology of Creutzfeldt-Jakob disease and conditions related to the accumulation of PrPCJD in the nervous system

Although typical cases of Creutzfeldt-Jakob disease are readily recognized pathologically and clinically, variant forms often pose a diagnostic challenge. From the 1920's, when this disease was first characterized, until quite recently diagnosis relied heavily on morphologic changes. New advances in immunoassays and PrP gene analysis now provide important adjuncts in recognizing the spectrum of disorders of PrP metabolism associated with these transmissible encephalopathies.

129/Ola mice carrying a null mutation in PrP that abolishes mRNA production are developmentally normal

The neural membrane glycoprotein PrP is implicated in the pathogenesis of the transmissible spongiform encephalopathies; however, the normal function of PrP and its precise role in disease are not understood. Recently, gene targeting has been used to produce mice with neo/PrP fusion transcripts, but no detectable PrP protein in the brain (1). Here we report the use of a different targeting strategy, to produce inbred mice with a complete absence of both PrP protein and mRNA sequences. At 7 mo of age, these mice show no overt phenotypic abnormalities despite the normal high levels of expression of PrP during mouse development. The mice are being used in experiments designed to address the role of PrP in the pathogenesis of scrapie and the replication of infectivity.

Human prion diseases

The prion diseases, sometimes referred to as the "transmissible spongiform encephalopathies," include kuru, Creutzfeldt-Jakob disease, and Gerstmann-Sträussler-Scheinker disease of humans as well as scrapie and bovine spongiform encephalopathy of animals. For many years, the prion diseases were thought to be caused by viruses despite intriguing evidence to the contrary. The unique characteristic common to all of these disorders, whether sporadic, dominantly inherited, or acquired by infection, is that they involve the aberrant metabolism of the prion protein (PrP). In many cases, the cellular prion protein is converted into the scrapie isoform by a posttranslational process that involves a conformational change. Often, the human prion diseases are transmissible to experimental animals and all of the inherited prion diseases segregate with PrP gene mutations.

Molecular genetics of human prion diseases

Human prion diseases occur in inherited, sporadic and acquired forms. The inherited forms are associated with coding mutations in the prion protein gene and the identification of one of these pathogenic mutations allows definitive diagnosis and has resulted in a widening of the previously recognized phenotypic spectrum of these diseases. Study of acquired prion disease provides evidence for genetic susceptibility to development of disease following treatment with contaminated pituitary hormones. Sporadic prion disease occurs predominantly in individuals homozygous with respect to a common PrP polymorphism at residue 129. The identification of pathogenic PrP alleles and the role of the codon 129 PrP gene polymorphism in determining susceptibility to prion disease provides strong support for the idea that an abnormal isoform of PrP, PrPSc, is the principal constituent of the prion and that its propagation involves direct PrP-PrP interactions which occur most readily between identical PrP molecules.

Prion protein (PrP) is not involved in the pathogenesis of spongiform encephalopathy in zitter rats

In order to elucidate the relationship between the prion protein (PrP) structure and the development of spongiform encephalopathy in zitter rats, we analyzed the nucleotide sequences and restriction fragment length variation (RFLV) of the Prn gene encoding PrP in zitter rats and inbred SD/J rats as a control. Prn genes from two strains had identical nucleotide sequences in their coding sequences. Obvious RFLV on the locus was not detected in zitter rats by a Southern blot hybridization. Consistently, zitter rat brains express the normal cellular PrP (PrPC), but do not accumulate the protease-resistant modified isoform (PrPSC). These results indicate that PrP is not involved in the pathogenesis of spongiform encephalopathy in zitter rats.

[Molecular basis of familial and sporadic forms of human prion diseases]

Prion diseases, also known as transmissible subacute spongiform encephalopathies (TSSE), are rare neurodegenerative disorders of both humans and animals. Their biochemical hallmark is an accumulation in the brain of an abnormal form of the host-encoded prion protein (PrP). This pathological accumulation could result from a protein conformational change under the influence of unknown factors. The normal function of PrP is unknown. The abnormal form is thought to induce neurodegeneration when experimentally or accidentally introduced in recipient hosts. Such a possibility would explain the transmissible character of these diseases illustrated in humans by iatrogenic contamination. Considerable attention has been focused on the host PrP gene and its relation with the genetic susceptibility of humans and animals. Mutations in PRNP, the gene which encodes PrP in humans, are present in 17% of the patients and might be causative. In patients without any PRNP mutation, a coding polymorphism (129 Met/Val) defines a predisposing factor. Since few years, important progress in the molecular genetics of TSSE in both humans and animals have been performed and point out that the development of different forms of these diseases, experimental, iatrogenic or spontaneous, are strongly dependent on the primary structure of the host PrP.