A structural basis for prion strain diversity.

Recent cryogenic electron microscopy (cryo-EM) studies of infectious, ex vivo, prion fibrils from hamster 263K and mouse RML prion strains revealed a similar, parallel in-register intermolecular ß-sheet (PIRIBS) amyloid architecture. Rungs of the fibrils are composed of individual prion protein (PrP) monomers that fold to create distinct N-terminal and C-terminal lobes. However, disparity in the hamster/mouse PrP sequence precludes understanding of how divergent prion strains emerge from an identical PrP substrate. In this study, we determined the near-atomic resolution cryo-EM structure of infectious, ex vivo mouse prion fibrils from the ME7 prion strain and compared this with the RML fibril structure. This structural comparison of two biologically distinct mouse-adapted prion strains suggests defined folding subdomains of PrP rungs and the way in which they are interrelated, providing a structural definition of intra-species prion strain-specific conformations.

2.7 Å cryo-EM structure of ex vivo RML prion fibrils.

Mammalian prions propagate as distinct strains and are composed of multichain assemblies of misfolded host-encoded prion protein (PrP). Here, we present a near-atomic resolution cryo-EM structure of PrP fibrils present in highly infectious prion rod preparations isolated from the brains of RML prion-infected mice. We found that prion rods comprise single-protofilament helical amyloid fibrils that coexist with twisted pairs of the same protofilaments. Each rung of the protofilament is formed by a single PrP monomer with the ordered core comprising PrP residues 94-225, which folds to create two asymmetric lobes with the N-linked glycans and the glycosylphosphatidylinositol anchor projecting from the C-terminal lobe. The overall architecture is comparable to that of recently reported PrP fibrils isolated from the brain of hamsters infected with the 263K prion strain. However, there are marked conformational variations that could result from differences in PrP sequence and/or represent distinguishing features of the distinct prion strains.

Humanized Transgenic Mice Are Resistant to Chronic Wasting Disease Prions From Norwegian Reindeer and Moose.

Chronic wasting disease (CWD) is the transmissible spongiform encephalopathy or prion disease affecting cervids. In 2016, the first cases of CWD were reported in Europe in Norwegian wild reindeer and moose. The origin and zoonotic potential of these new prion isolates remain unknown. In this study to investigate zoonotic potential we inoculated brain tissue from CWD-infected Norwegian reindeer and moose into transgenic mice overexpressing human prion protein. After prolonged postinoculation survival periods no evidence for prion transmission was seen, suggesting that the zoonotic potential of these isolates is low.

Structural features distinguishing infectious ex vivo mammalian prions from non-infectious fibrillar assemblies generated in vitro.

Seeded polymerisation of proteins forming amyloid fibres and their spread in tissues has been implicated in the pathogenesis of multiple neurodegenerative diseases: so called "prion-like" mechanisms. While ex vivo mammalian prions, composed of multichain assemblies of misfolded host-encoded prion protein (PrP), act as lethal infectious agents, PrP amyloid fibrils produced in vitro generally do not. The high-resolution structure of authentic infectious prions and the structural basis of prion strain diversity remain unknown. Here we use cryo-electron microscopy and atomic force microscopy to examine the structure of highly infectious PrP rods isolated from mouse brain in comparison to non-infectious recombinant PrP fibrils generated in vitro. Non-infectious recombinant PrP fibrils are 10 nm wide single fibres, with a double helical repeating substructure displaying small variations in adhesive force interactions across their width. In contrast, infectious PrP rods are 20 nm wide and contain two fibres, each with a double helical repeating substructure, separated by a central gap of 8-10 nm in width. This gap contains an irregularly structured material whose adhesive force properties are strikingly different to that of the fibres, suggestive of a distinct composition. The structure of the infectious PrP rods, which cause lethal neurodegeneration, readily differentiates them from all other protein assemblies so far characterised in other neurodegenerative diseases.

Evaluating the causality of novel sequence variants in the prion protein gene by example.

The estimation of pathogenicity and penetrance of novel prion protein gene (PRNP) variants presents significant challenges, particularly in the absence of family history, which precludes the application of Mendelian segregation. Moreover, the ambiguities of prion disease pathophysiology renders conventional in silico predictions inconclusive. Here, we describe 2 patients with rapid cognitive decline progressing to akinetic mutism and death within 10 weeks of symptom onset, both of whom possessed the novel T201S variant in PRNP. Clinically, both satisfied diagnostic criteria for probable sporadic Creutzfeldt-Jakob disease and in one, the diagnosis was confirmed by neuropathology. While computational analyses predicted that T201S was possibly deleterious, molecular strain typing, prion protein structural considerations, and calculations leveraging large-scale population data (gnomAD) indicate that T201S is at best either of low penetrance or nonpathogenic. Thus, we illustrate the utility of harnessing multiple lines of prion disease-specific evidence in the evaluation of the T201S variant, which may be similarly applied to assess other novel variants in PRNP.

Experimental sheep BSE prions generate the vCJD phenotype when serially passaged in transgenic mice expressing human prion protein.

The epizootic prion disease of cattle, bovine spongiform encephalopathy (BSE), causes variant Creutzfeldt-Jakob disease (vCJD) in humans following dietary exposure. While it is assumed that all cases of vCJD attributed to a dietary aetiology are related to cattle BSE, sheep and goats are susceptible to experimental oral challenge with cattle BSE prions and farmed animals in the UK were undoubtedly exposed to BSE-contaminated meat and bone meal during the late 1980s and early 1990s. Although no natural field cases of sheep BSE have been identified, it cannot be excluded that some BSE-infected sheep might have entered the European human food chain. Evaluation of the zoonotic potential of sheep BSE prions has been addressed by examining the transmission properties of experimental brain isolates in transgenic mice that express human prion protein, however to-date there have been relatively few studies. Here we report that serial passage of experimental sheep BSE prions in transgenic mice expressing human prion protein with methionine at residue 129 produces the vCJD phenotype that mirrors that seen when the same mice are challenged with vCJD prions from patient brain. These findings are congruent with those reported previously by another laboratory, and thereby strongly reinforce the view that sheep BSE prions could have acted as a causal agent of vCJD within Europe.

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.

Ex vivo mammalian prions are formed of paired double helical prion protein fibrils.

Mammalian prions are hypothesized to be fibrillar or amyloid forms of prion protein (PrP), but structures observed to date have not been definitively correlated with infectivity and the three-dimensional structure of infectious prions has remained obscure. Recently, we developed novel methods to obtain exceptionally pure preparations of prions from mouse brain and showed that pathogenic PrP in these high-titre preparations is assembled into rod-like assemblies. Here, we have used precise cell culture-based prion infectivity assays to define the physical relationship between the PrP rods and prion infectivity and have used electron tomography to define their architecture. We show that infectious PrP rods isolated from multiple prion strains have a common hierarchical assembly comprising twisted pairs of short fibres with repeating substructure. The architecture of the PrP rods provides a new structural basis for understanding prion infectivity and can explain the inability to systematically generate high-titre synthetic prions from recombinant PrP.

Transmission Properties of Human PrP 102L Prions Challenge the Relevance of Mouse Models of GSS.

Inherited prion disease (IPD) is caused by autosomal-dominant pathogenic mutations in the human prion protein (PrP) gene (PRNP). A proline to leucine substitution at PrP residue 102 (P102L) is classically associated with Gerstmann-Sträussler-Scheinker (GSS) disease but shows marked clinical and neuropathological variability within kindreds that may be caused by variable propagation of distinct prion strains generated from either PrP 102L or wild type PrP. To-date the transmission properties of prions propagated in P102L patients remain ill-defined. Multiple mouse models of GSS have focused on mutating the corresponding residue of murine PrP (P101L), however murine PrP 101L, a novel PrP primary structure, may not have the repertoire of pathogenic prion conformations necessary to accurately model the human disease. Here we describe the transmission properties of prions generated in human PrP 102L expressing transgenic mice that were generated after primary challenge with ex vivo human GSS P102L or classical CJD prions. We show that distinct strains of prions were generated in these mice dependent upon source of the inoculum (either GSS P102L or CJD brain) and have designated these GSS-102L and CJD-102L prions, respectively. GSS-102L prions have transmission properties distinct from all prion strains seen in sporadic and acquired human prion disease. Significantly, GSS-102L prions appear incapable of transmitting disease to conventional mice expressing wild type mouse PrP, which contrasts strikingly with the reported transmission properties of prions generated in GSS P102L-challenged mice expressing mouse PrP 101L. We conclude that future transgenic modeling of IPDs should focus exclusively on expression of mutant human PrP, as other approaches may generate novel experimental prion strains that are unrelated to human disease.

A novel and rapid method for obtaining high titre intact prion strains from mammalian brain.

Mammalian prions exist as multiple strains which produce characteristic and highly reproducible phenotypes in defined hosts. How this strain diversity is encoded by a protein-only agent remains one of the most interesting and challenging questions in biology with wide relevance to understanding other diseases involving the aggregation or polymerisation of misfolded host proteins. Progress in understanding mammalian prion strains has however been severely limited by the complexity and variability of the methods used for their isolation from infected tissue and no high resolution structures have yet been reported. Using high-throughput cell-based prion bioassay to re-examine prion purification from first principles we now report the isolation of prion strains to exceptional levels of purity from small quantities of infected brain and demonstrate faithful retention of biological and biochemical strain properties. The method's effectiveness and simplicity should facilitate its wide application and expedite structural studies of prions.

A novel prion disease associated with diarrhea and autonomic neuropathy.

BACKGROUND: Human prion diseases, although variable in clinicopathological phenotype, generally present as neurologic or neuropsychiatric conditions associated with rapid multifocal central nervous system degeneration that is usually dominated by dementia and cerebellar ataxia. Approximately 15% of cases of recognized prion disease are inherited and associated with coding mutations in the gene encoding prion protein (PRNP). The availability of genetic diagnosis has led to a progressive broadening of the recognized spectrum of disease. METHODS: We used longitudinal clinical assessments over a period of 20 years at one hospital combined with genealogical, neuropsychological, neurophysiological, neuroimaging, pathological, molecular genetic, and biochemical studies, as well as studies of animal transmission, to characterize a novel prion disease in a large British kindred. We studied 6 of 11 affected family members in detail, along with autopsy or biopsy samples obtained from 5 family members. RESULTS: We identified a PRNP Y163X truncation mutation and describe a distinct and consistent phenotype of chronic diarrhea with autonomic failure and a length-dependent axonal, predominantly sensory, peripheral polyneuropathy with an onset in early adulthood. Cognitive decline and seizures occurred when the patients were in their 40s or 50s. The deposition of prion protein amyloid was seen throughout peripheral organs, including the bowel and peripheral nerves. Neuropathological examination during end-stage disease showed the deposition of prion protein in the form of frequent cortical amyloid plaques, cerebral amyloid angiopathy, and tauopathy. A unique pattern of abnormal prion protein fragments was seen in brain tissue. Transmission studies in laboratory mice were negative. CONCLUSIONS: Abnormal forms of prion protein that were found in multiple peripheral tissues were associated with diarrhea, autonomic failure, and neuropathy. (Funded by the U.K. Medical Research Council and others.).

Atypical scrapie prions from sheep and lack of disease in transgenic mice overexpressing human prion protein.

Public and animal health controls to limit human exposure to animal prions are focused on bovine spongiform encephalopathy (BSE), but other prion strains in ruminants may also have zoonotic potential. One example is atypical/Nor98 scrapie, which evaded statutory diagnostic methods worldwide until the early 2000s. To investigate whether sheep infected with scrapie prions could be another source of infection, we inoculated transgenic mice that overexpressed human prion protein with brain tissue from sheep with natural field cases of classical and atypical scrapie, sheep with experimental BSE, and cattle with BSE. We found that these mice were susceptible to BSE prions, but disease did not develop after prolonged postinoculation periods when mice were inoculated with classical or atypical scrapie prions. These data are consistent with the conclusion that prion disease is less likely to develop in humans after exposure to naturally occurring prions of sheep than after exposure to epizootic BSE prions of ruminants.

Inherited prion disease with 4-octapeptide repeat insertion: disease requires the interaction of multiple genetic risk factors.

Genetic factors are implicated in the aetiology of sporadic late-onset neurodegenerative diseases. Whether these genetic variants are predominantly common or rare, and how multiple genetic factors interact with each other to cause disease is poorly understood. Inherited prion diseases are highly heterogeneous and may be clinically mistaken for sporadic Creutzfeldt-Jakob disease because of a negative family history. Here we report our investigation of patients from the UK with four extra octapeptide repeats, which suggest that the risk of clinical disease is increased by a combination of the mutation and a susceptibility haplotype on the wild-type chromosome. The predominant clinical syndrome is a progressive cortical dementia with pyramidal signs, myoclonus and cerebellar abnormalities that closely resemble sporadic Creutzfeldt-Jakob disease. Autopsy shows perpendicular deposits of prion protein in the molecular layer of the cerebellum. Identity testing, PRNP microsatellite haplotyping and genealogical work confirm no cryptic close family relationships and suggests multiple progenitor disease haplotypes. All patients were homozygous for methionine at polymorphic codon 129. In addition, at a single nucleotide polymorphism upstream of PRNP thought to confer susceptibility to sporadic Creutzfeldt-Jakob disease (rs1029273), all patients were homozygous for the risk allele (combined P=5.9×10(-5)). The haplotype identified may also be a risk factor in other partially penetrant inherited prion diseases although it does not modify age of onset. Blood expression of PRNP in healthy individuals was modestly higher in carriers of the risk haplotype. These findings may provide a precedent for understanding apparently sporadic neurodegenerative diseases caused by rare high-risk mutations.

Effect of fixation on brain and lymphoreticular vCJD prions and bioassay of key positive specimens from a retrospective vCJD prevalence study.

Anonymous screening of lymphoreticular tissues removed during routine surgery has been applied to estimate the UK population prevalence of asymptomatic vCJD prion infection. The retrospective study of Hilton et al (J Pathol 2004; 203: 733-739) found accumulation of abnormal prion protein in three formalin-fixed appendix specimens. This led to an estimated UK prevalence of vCJD infection of ∼1 in 4000, which remains the key evidence supporting current risk reduction measures to reduce iatrogenic transmission of vCJD prions in the UK. Confirmatory testing of these positives has been hampered by the inability to perform immunoblotting of formalin-fixed tissue. Animal transmission studies offer the potential for 'gold standard' confirmatory testing but are limited by both transmission barrier effects and known effects of fixation on scrapie prion titre in experimental models. Here we report the effects of fixation on brain and lymphoreticular human vCJD prions and comparative bioassay of two of the three prevalence study formalin-fixed, paraffin-embedded (FFPE) appendix specimens using transgenic mice expressing human prion protein (PrP). While transgenic mice expressing human PrP 129M readily reported vCJD prion infection after inoculation with frozen vCJD brain or appendix, and also FFPE vCJD brain, no infectivity was detected in FFPE vCJD spleen. No prion transmission was observed from either of the FFPE appendix specimens. The absence of detectable infectivity in fixed, known positive vCJD lymphoreticular tissue precludes interpreting negative transmissions from vCJD prevalence study appendix specimens. In this context, the Hilton et al study should continue to inform risk assessment pending the outcome of larger-scale studies on discarded surgical tissues and autopsy samples.

Isolation of proteinase K-sensitive prions using pronase E and phosphotungstic acid.

Disease-related prion protein, PrP(Sc), is classically distinguished from its normal cellular precursor, PrP(C), by its detergent insolubility and partial resistance to proteolysis. Molecular diagnosis of prion disease typically relies upon detection of protease-resistant fragments of PrP(Sc) using proteinase K, however it is now apparent that the majority of disease-related PrP and indeed prion infectivity may be destroyed by this treatment. Here we report that digestion of RML prion-infected mouse brain with pronase E, followed by precipitation with sodium phosphotungstic acid, eliminates the large majority of brain proteins, including PrP(C), while preserving >70% of infectious prion titre. This procedure now allows characterization of proteinase K-sensitive prions and investigation of their clinical relevance in human and animal prion disease without being confounded by contaminating PrP(C).

Heterozygosity at polymorphic codon 219 in variant creutzfeldt-jakob disease.

BACKGROUND: Genetic variants of the prion protein gene (PRNP) strongly determine susceptibility to prion diseases. All tested patients with definite variant Creutzfeldt-Jakob disease (vCJD) are homozygous for methionine at a common polymorphism at codon 129. A further genetic polymorphism at codon 219, a common variant in several Asian populations, is considered protective against sporadic CJD. OBJECTIVE: To report a finding of heterozygosity at codon 219 in 2 patients with vCJD. DESIGN: Case reports. SETTING: MRC (Medical Research Council) Prion Unit and Department of Neurodegenerative Disease, University College London Institute of Neurology, and National Prion Clinic, National Hospital for Neurology and Neurosurgery. Patients Two patients with clinical and investigation findings consistent with the diagnoses of probable vCJD. MAIN OUTCOME MEASURES: Clinical and genetic findings. RESULTS: A 34-year-old man had a 15-month history of behavioral change progressing to ataxia, dysarthria, involuntary choreiform movements, and severe cognitive impairment. Cerebrospinal fluid analysis was positive for 14-3-3 protein, electroencephalography showed generalized slowing, and magnetic resonance imaging revealed thalamic high signal bilaterally, typical of vCJD. A 31-year-old woman had a 16-month history of cognitive decline, ataxia, involuntary choreiform movements, and myoclonic jerks. Magnetic resonance imaging showed bilateral pulvinar high signal. The diagnosis was confirmed by a tonsillar biopsy demonstrating abnormal prion protein deposition in a typical pattern for vCJD. PRNP sequencing showed a methionine homozygous codon 129 genotype and an E219K polymorphism in both patients. CONCLUSIONS: The E219K polymorphism is neutral or may even confer susceptibility to vCJD. The observations are interpretable in the context of the conformational selection model of prion replication. A barrier to prion disease transmission depends on the degree to which permitted pathologic conformations of the prion protein overlap between the inoculum and the host.

Spontaneous generation of mammalian prions.

Prions are transmissible agents that cause lethal neurodegeneration in humans and other mammals. Prions bind avidly to metal surfaces such as steel wires and, when surface-bound, can initiate infection of brain or cultured cells with remarkable efficiency. While investigating the properties of metal-bound prions by using the scrapie cell assay to measure infectivity, we observed, at low frequency, positive assay results in control groups in which metal wires had been coated with uninfected mouse brain homogenate. This phenomenon proved to be reproducible in rigorous and exhaustive control experiments designed to exclude prion contamination. The infectivity generated in cell culture could be readily transferred to mice and had strain characteristics distinct from the mouse-adapted prion strains used in the laboratory. The apparent "spontaneous generation" of prions from normal brain tissue could result if the metal surface, possibly with bound cofactors, catalyzed de novo formation of prions from normal cellular prion protein. Alternatively, if prions were naturally present in the brain at levels not detectable by conventional methods, metal surfaces might concentrate them to the extent that they become quantifiable by the scrapie cell assay.

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.

Review. The origin of the prion agent of kuru: molecular and biological strain typing.

Kuru is an acquired human prion disease that primarily affected the Fore linguistic group of the Eastern Highlands of Papua New Guinea. The central clinical feature of kuru is progressive cerebellar ataxia and, in sharp contrast to most cases of sporadic Creutzfeldt-Jakob disease (CJD), dementia is a less prominent and usually late clinical feature. In this regard, kuru is more similar to variant CJD, which also has similar prodromal symptoms of sensory disturbance and joint pains in the legs and psychiatric and behavioural changes. Since a significant part of the clinicopathological diversity seen in human prion disease is likely to relate to the propagation of distinct human prion strains, we have compared the transmission properties of kuru prions with those isolated from patients with sporadic, iatrogenic and variant CJD in both transgenic and wild-type mice. These data have established that kuru prions have prion strain properties equivalent to those of classical (sporadic and iatrogenic) CJD prions but distinct from variant CJD prions. Here, we review these findings and discuss how peripheral routes of infection and other factors may be critical modifiers of the kuru phenotype.