Central and peripheral pathology of kuru: pathological analysis of a recent case and comparison with other forms of human prion disease

Detection of prions in the urine of patients affected by sporadic Creutzfeldt-Jakob disease

OBJECTIVE

Currently, it is unknown whether infectious prions are present in peripheral tissues and biological fluids of patients affected by sporadic Creutzfeldt-Jakob disease (sCJD), the most common prion disorder in humans. This represents a potential risk for inter-individual prion infection. The main goal of this study was to evaluate the presence of prions in urine of patients suffering from the major subtypes of sCJD.

METHODS

Urine samples from sCJD patients spanning the six major subtypes were tested. As controls, we used urine samples from people affected by other neurological or neurodegenerative diseases as well as healthy controls. These samples were analyzed blinded. The presence of prions was detected by a modified version of the PMCA technology, specifically optimized for high sensitive detection of sCJD prions.

RESULTS

The PMCA assay was first optimized to detect low quantities of prions in diluted brain homogenates from patients affected by all subtypes of sCJD spiked into healthy urine. Twenty-nine of the 81 patients affected by sCJD analyzed in this study were positive by PMCA testing, whereas none of the 160 controls showed any signal. These results indicate a 36% sensitivity and 100% specificity. The subtypes with the highest positivity rate were VV1 and VV2, which combined account for about 15-20% of all sCJD cases, and no detection was observed in MV1 and MM2.

INTERPRETATION

Our findings indicate that potentially infectious prions are secreted in urine of some sCJD patients, suggesting a possible risk for inter-individual transmission. Prion detection in urine might be used as a noninvasive preliminary screening test to detect sCJD.

Prion strains: shining new light on old concepts

Prion diseases are a group of inevitably fatal neurodegenerative disorders affecting numerous mammalian species, including humans. The existence of heritable phenotypes of disease in the natural host suggested that prions exist as distinct strains. Transmission of sheep scrapie to rodent models accelerated prion research, resulting in the isolation and characterization of numerous strains with distinct characteristics. These strains are grouped into categories based on the incubation period of disease in different strains of mice and also by how stable the strain properties were upon serial passage. These classical studies defined the host and agent parameters that affected strain properties, and, prior to the advent of the prion hypothesis, strain properties were hypothesized to be the result of mutations in a nucleic acid genome of a conventional pathogen. The development of the prion hypothesis challenged the paradigm of infectious agents, and, initially, the existence of strains was difficult to reconcile with a protein-only agent. In the decades since, much evidence has revealed how a protein-only infectious agent can perform complex biological functions. The prevailing hypothesis is that strain-specific conformations of PrPSc encode prion strain diversity. This hypothesis can provide a mechanism to explain the observed strain-specific differences in incubation period of disease, biochemical properties of PrPSc, tissue tropism, and subcellular patterns of pathology. This hypothesis also explains how prion strains mutate, evolve, and adapt to new species. These concepts are applicable to prion-like diseases such as Parkinson's and Alzheimer's disease, where evidence of strain diversity is beginning to emerge.

Differential Accumulation of Misfolded Prion Strains in Natural Hosts of Prion Diseases

Prion diseases, also known as transmissible spongiform encephalopathies (TSEs), are a group of neurodegenerative protein misfolding diseases that invariably cause death. TSEs occur when the endogenous cellular prion protein (PrP^C) misfolds to form the pathological prion protein (PrP^Sc), which templates further conversion of PrP^C to PrP^Sc, accumulates, and initiates a cascade of pathologic processes in cells and tissues. Different strains of prion disease within a species are thought to arise from the differential misfolding of the prion protein and have different clinical phenotypes. Different strains of prion disease may also result in differential accumulation of PrP^Sc in brain regions and tissues of natural hosts. Here, we review differential accumulation that occurs in the retinal ganglion cells, cerebellar cortex and white matter, and plexuses of the enteric nervous system in cattle with bovine spongiform encephalopathy, sheep and goats with scrapie, cervids with chronic wasting disease, and humans with prion diseases. By characterizing TSEs in their natural host, we can better understand the pathogenesis of different prion strains. This information is valuable in the pursuit of evaluating and discovering potential biomarkers and therapeutics for prion diseases.

[Prion diseases and the biosecurity problems.]

The review presents the current state of the problem of prions and prion diseases with an emphasis on theepidemiological and epizootological risks of pathogens that cause fatal neurodegenerative diseases in humans and animals. The results of molecular genetic studies of the conversion of normal PrP^c prion protein molecules to infectious forms of PrP^d, resistance to physical disinfection methods, in particular exceptional thermal stability, and their ability to overcome interspecific barriers, while increasing virulence, are described. The possibility of infection not only by nutrition, when eating even heat-treated meat of sick animals, but also due to surgical interventions, especially neurosurgical and ophthalmic, as well as the use of immunobiological preparations, are emphasized. Since there are currently no means for the effective treatment of prion diseases in the world, attention is drawn to the high degree of relevance for the biosafety of the country to develop domestic highly sensitive test systems that can effectively detect prion infectious protein in vivo at the preclinical stage of the disease. The latest methods of automatic protein amplification and identification of prion proteins are briefly described as the most promising areas of research in the field of diagnosis of prion diseases.

Prevalence in Britain of abnormal prion protein in human appendices before and after exposure to the cattle BSE epizootic

Widespread dietary exposure of the population of Britain to bovine spongiform encephalopathy (BSE) prions in the 1980s and 1990s led to the emergence of variant Creutzfeldt-Jakob Disease (vCJD) in humans. Two previous appendectomy sample surveys (Appendix-1 and -2) estimated the prevalence of abnormal prion protein (PrP) in the British population exposed to BSE to be 237 per million and 493 per million, respectively. The Appendix-3 survey was recommended to measure the prevalence of abnormal PrP in population groups thought to have been unexposed to BSE. Immunohistochemistry for abnormal PrP was performed on 29,516 samples from appendices removed between 1962 and 1979 from persons born between 1891 through 1965, and from those born after 1996 that had been operated on from 2000 through 2014. Seven appendices were positive for abnormal PrP, of which two were from the pre-BSE-exposure era and five from the post BSE-exposure period. None of the seven positive samples were from appendices removed before 1977, or in patients born after 2000 and none came from individuals diagnosed with vCJD. There was no statistical difference in the prevalence of abnormal PrP across birth and exposure cohorts. Two interpretations are possible. Either there is a low background prevalence of abnormal PrP in human lymphoid tissues that may not progress to vCJD. Alternatively, all positive specimens are attributable to BSE exposure, a finding that would necessitate human exposure having begun in the late 1970s and continuing through the late 1990s.

Kuru, the First Human Prion Disease

Kuru, the first human prion disease was transmitted to chimpanzees by D. Carleton Gajdusek (1923–2008). In this review, we summarize the history of this seminal discovery, its anthropological background, epidemiology, clinical picture, neuropathology, and molecular genetics. We provide descriptions of electron microscopy and confocal microscopy of kuru amyloid plaques retrieved from a paraffin-embedded block of an old kuru case, named Kupenota. The discovery of kuru opened new vistas of human medicine and was pivotal in the subsequent transmission of Creutzfeldt–Jakob disease, as well as the relevance that bovine spongiform encephalopathy had for transmission to humans. The transmission of kuru was one of the greatest contributions to biomedical sciences of the 20th century.

A Quick Method to Evaluate the Effect of the Amino Acid Sequence in the Misfolding Proneness of the Prion Protein

Prion diseases or transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases where the misfolding of the prion protein (PrP) is a crucial event. Based on studies in TSE-affected humans and the generation of transgenic mouse models overexpressing different mutated versions of the PrP, we conclude that both wild-type and mutated PrPs exhibit differential propensity to misfold in vivo. Here, we describe a new method in vitro to assess and quantify the PrP misfolding phenomenon in order to better understand the molecular mechanisms involved in this process.

Prion Strains and Transmission Barrier Phenomena

Several experimental evidences show that prions are non-conventional pathogens, which physical support consists only in proteins. This finding raised questions regarding the observed prion strain-to-strain variations and the species barrier that happened to be crossed with dramatic consequences on human health and veterinary policies during the last 3 decades. This review presents a focus on a few advances in the field of prion structure and prion strains characterization: from the historical approaches that allowed the concept of prion strains to emerge, to the last results demonstrating that a prion strain may in fact be a combination of a few quasi species with subtle biophysical specificities. Then, we will focus on the current knowledge on the factors that impact species barrier strength and species barrier crossing. Finally, we present probable scenarios on how the interaction of strain properties with host characteristics may account for differential selection of new conformer variants and eventually species barrier crossing.

Sporadic and Infectious Human Prion Diseases

Human prion diseases are rare neurodegenerative diseases that have become the subject of public and scientific interest because of concerns about interspecies transmission and the unusual biological properties of the causal agents: prions. These diseases are unique in that they occur in sporadic, hereditary, and infectious forms that are characterized by an extended incubation period between exposure to infection and the development of clinical illness. Silent infection can be present in peripheral tissues during the incubation period, which poses a challenge to public health, especially because prions are relatively resistant to standard decontamination procedures. Despite intense research efforts, no effective treatment has been developed for human prion diseases, which remain uniformly fatal.

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.

Mammalian prions and their wider relevance in neurodegenerative diseases

Prions are notorious protein-only infectious agents that cause invariably fatal brain diseases following silent incubation periods that can span a lifetime. These diseases can arise spontaneously, through infection or be inherited. Remarkably, prions are composed of self-propagating assemblies of a misfolded cellular protein that encode information, generate neurotoxicity and evolve and adapt in vivo. Although parallels have been drawn with Alzheimer's disease and other neurodegenerative conditions involving the deposition of assemblies of misfolded proteins in the brain, insights are now being provided into the usefulness and limitations of prion analogies and their aetiological and therapeutic relevance.

Prion diseases: New considerations

The transmissible spongiform encephalopathies, which include Creutzfeldt-Jakob disease, are fatal neurodegenerative disorders caused by the pathological accumulation of abnormal prion protein. The diagnosis of Creutzfeldt-Jakob disease is complex. The electroencephalogram, magnetic resonance imaging, lumbar puncture and genetic testing findings can help in the differential diagnosis of rapidly progressive dementia. There has recently been considerable debate as to whether proteins involved in the development of neurodegenerative diseases should be regarded as prions or only share prion-like mechanisms. Two recent reports described the detection of abnormal prion protein in the nasal mucosa and urine of patients with Creutzfeldt-Jakob disease. These findings raise major health concerns regarding the transmissibility of human prion diseases. We set out to address this neurological hot topic and to draw conclusions on the basis of what is known in the literature thus far.

Evidence for human transmission of amyloid-β pathology and cerebral amyloid angiopathy

More than two hundred individuals developed Creutzfeldt-Jakob disease (CJD) worldwide as a result of treatment, typically in childhood, with human cadaveric pituitary-derived growth hormone contaminated with prions. Although such treatment ceased in 1985, iatrogenic CJD (iCJD) continues to emerge because of the prolonged incubation periods seen in human prion infections. Unexpectedly, in an autopsy study of eight individuals with iCJD, aged 36-51 years, in four we found moderate to severe grey matter and vascular amyloid-β (Aβ) pathology. The Aβ deposition in the grey matter was typical of that seen in Alzheimer's disease and Aβ in the blood vessel walls was characteristic of cerebral amyloid angiopathy and did not co-localize with prion protein deposition. None of these patients had pathogenic mutations, APOE ε4 or other high-risk alleles associated with early-onset Alzheimer's disease. Examination of a series of 116 patients with other prion diseases from a prospective observational cohort study showed minimal or no Aβ pathology in cases of similar age range, or a decade older, without APOE ε4 risk alleles. We also analysed pituitary glands from individuals with Aβ pathology and found marked Aβ deposition in multiple cases. Experimental seeding of Aβ pathology has been previously demonstrated in primates and transgenic mice by central nervous system or peripheral inoculation with Alzheimer's disease brain homogenate. The marked deposition of parenchymal and vascular Aβ in these relatively young patients with iCJD, in contrast with other prion disease patients and population controls, is consistent with iatrogenic transmission of Aβ pathology in addition to CJD and suggests that healthy exposed individuals may also be at risk of iatrogenic Alzheimer's disease and cerebral amyloid angiopathy. These findings should also prompt investigation of whether other known iatrogenic routes of prion transmission may also be relevant to Aβ and other proteopathic seeds associated with neurodegenerative and other human diseases.

Sporadic Creutzfeldt-Jakob Disease: Prion Pathology in Medulla Oblongata-Possible Routes of Infection and Host Susceptibility

Sporadic Creutzfeldt-Jakob disease (sCJD), the most frequent human prion disorder, is characterized by remarkable phenotypic variability, which is influenced by the conformation of the pathologic prion protein and the methionine/valine polymorphic codon 129 of the prion protein gene. While the etiology of sCJD remains unknown, it has been hypothesized that environmental exposure to prions might occur through conjunctival/mucosal contact, oral ingestion, inhalation, or simultaneous involvement of the olfactory and enteric systems. We studied 21 subjects with definite sCJD to assess neuropathological involvement of the dorsal motor nucleus of the vagus and other medullary nuclei and to evaluate possible associations with codon 129 genotype and prion protein conformation. The present data show that prion protein deposition was detected in medullary nuclei of distinct sCJD subtypes, either valine homozygous or heterozygous at codon 129. These findings suggest that an "environmental exposure" might occur, supporting the hypothesis that external sources of contamination could contribute to sCJD in susceptible hosts. Furthermore, these novel data could shed the light on possible causes of sCJD through a "triple match" hypothesis that identify environmental exposure, host genotype, and direct exposure of specific anatomical regions as possible pathogenetic factors.

Prions in the urine of patients with variant Creutzfeldt-Jakob disease

BACKGROUND

Prions, the infectious agents responsible for transmissible spongiform encephalopathies, consist mainly of the misfolded prion protein (PrP(Sc)). The unique mechanism of transmission and the appearance of a variant form of Creutzfeldt-Jakob disease, which has been linked to consumption of prion-contaminated cattle meat, have raised concerns about public health. Evidence suggests that variant Creutzfeldt-Jakob disease prions circulate in body fluids from people in whom the disease is silently incubating.

METHODS

To investigate whether PrP(Sc) can be detected in the urine of patients with variant Creutzfeldt-Jakob disease, we used the protein misfolding cyclic amplification (PMCA) technique to amplify minute quantities of PrP(Sc), enabling highly sensitive detection of the protein. We analyzed urine samples from several patients with various transmissible spongiform encephalopathies (variant and sporadic Creutzfeldt-Jakob disease and genetic forms of prion disease), patients with other degenerative or nondegenerative neurologic disorders, and healthy persons.

RESULTS

PrP(Sc) was detectable only in the urine of patients with variant Creutzfeldt-Jakob disease and had the typical electrophoretic profile associated with this disease. PrP(Sc) was detected in 13 of 14 urine samples obtained from patients with variant Creutzfeldt-Jakob disease and in none of the 224 urine samples obtained from patients with other neurologic diseases and from healthy controls, resulting in an estimated sensitivity of 92.9% (95% confidence interval [CI], 66.1 to 99.8) and a specificity of 100.0% (95% CI, 98.4 to 100.0). The PrP(Sc) concentration in urine calculated by means of quantitative PMCA was estimated at 1×10(-16) g per milliliter, or 3×10(-21) mol per milliliter, which extrapolates to approximately 40 to 100 oligomeric particles of PrP(Sc) per milliliter of urine.

CONCLUSIONS

Urine samples obtained from patients with variant Creutzfeldt-Jakob disease contained minute quantities of PrP(Sc). (Funded by the National Institutes of Health and others.).

Infectious prion diseases in humans: cannibalism, iatrogenicity and zoonoses

In contrast with other neurodegenerative disorders associated to protein misfolding, human prion diseases include infectious forms (also called transmitted forms) such as kuru, iatrogenic Creutzfeldt-Jakob disease and variant Creutzfeldt-Jakob disease. The transmissible agent is thought to be solely composed of the abnormal isoform (PrP(Sc)) of the host-encoded prion protein that accumulated in the central nervous system of affected individuals. Compared to its normal counterpart, PrP(Sc) is β-sheet enriched and aggregated and its propagation is based on an autocatalytic conversion process. Increasing evidence supports the view that conformational variations of PrP(Sc) encoded the biological properties of the various prion strains that have been isolated by transmission studies in experimental models. Infectious forms of human prion diseases played a pivotal role in the emergence of the prion concept and in the characterization of the very unconventional properties of prions. They provide a unique model to understand how prion strains are selected and propagate in humans. Here, we review and discuss how genetic factors interplay with strain properties and route of transmission to influence disease susceptibility, incubation period and phenotypic expression in the light of the kuru epidemics due to ritual endocannibalism, the various series iatrogenic diseases secondary to extractive growth hormone treatment or dura mater graft and the epidemics of variant Creutzfeldt-Jakob disease linked to dietary exposure to the agent of bovine spongiform encephalopathy.

Kuru: a journey back in time from papua new Guinea to the neanderthals' extinction

Kuru, the first human transmissible spongiform encephalopathy was transmitted to chimpanzees by D. Carleton Gajdusek (1923-2008). In this review, I briefly summarize the history of this seminal discovery along its epidemiology, clinical picture, neuropathology and molecular genetics. The discovery of kuru opened new windows into the realms of human medicine and was instrumental in the later transmission of Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker disease as well as the relevance that bovine spongiform encephalopathy had for transmission to humans. The transmission of kuru was one of the greatest contributions to biomedical sciences of the 20th century.

Prion disease: a tale of folds and strains

Research on prions, the infectious agents of devastating neurological diseases in humans and animals, has been in the forefront of developing the concept of protein aggregation diseases. Prion diseases are distinguished from other neurodegenerative diseases by three peculiarities. First, prion diseases, in addition to being sporadic or genetic like all other neurodegenerative diseases, are infectious diseases. Animal models were developed early on (a long time before the advent of transgenic technology), and this has made possible the discovery of the prion protein as the infectious agent. Second, human prion diseases have true equivalents in animals, such as scrapie, which has been the subject of experimental research for many years. Variant Creutzfeldt-Jakob disease (vCJD) is a zoonosis caused by bovine spongiform encephalopathy (BSE) prions. Third, they show a wide variety of phenotypes in humans and animals, much wider than the variants of any other sporadic or genetic neurodegenerative disease. It has now become firmly established that particular PrP(Sc) isoforms are closely related to specific human prion strains. The variety of human prion diseases, still an enigma in its own right, is a focus of this article. Recently, a series of experiments has shown that the concept of aberrant protein folding and templating, first developed for prions, may apply to a variety of neurodegenerative diseases. In the wake of these discoveries, the term prion has come to be used for Aβ, α-synuclein, tau and possibly others. The self-propagation of alternative conformations seems to be the common denominator of these "prions," which in future, in order to avoid confusion, may have to be specified either as "neurodegenerative prions" or "infectious prions."

Kuru: genes, cannibals and neuropathology

Kuru was the first human transmissible spongiform encephalopathy (TSE) or prion disease identified, occurring in the Fore linguistic group of Papua New Guinea. Kuru was a uniformly fatal cerebellar ataxic syndrome, usually followed by choreiform and athetoid movements. Kuru imposed a strong balancing selection on the Fore population, with individuals homozygous for the 129 Met allele of the gene (PRNP) encoding for prion protein (PrP) being the most susceptible. The decline in the incidence of kuru in the Fore has been attributed to the exhaustion of the susceptible genotype and ultimately by discontinuation of exposure via cannibalism. Neuropathologically, kuru-affected brains were characterized by widespread degeneration of neurons, astroglial and microglial proliferation, and the presence of amyloid plaques. These early findings have been confirmed and extended by recent immunohistochemical studies for the detection of the TSE-specific PrP (PrP). Confocal laser microscopy also showed the concentration of glial fibrillary acidic protein-positive astrocytic processes at the plaque periphery. The fine structure of plaques corresponds to that described earlier by light microscopy. The successful experimental transmission of kuru led to the awareness of its similarity to Creutzfeldt-Jakob disease and Gerstmann-Sträussler-Scheinker disease and formed a background against which the recent epidemics of iatrogenic and variant Creutzfeldt-Jakob disease could be studied.

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.

An overview of human prion diseases

Prion diseases are transmissible, progressive and invariably fatal neurodegenerative conditions associated with misfolding and aggregation of a host-encoded cellular prion protein, PrP^C. They have occurred in a wide range of mammalian species including human. Human prion diseases can arise sporadically, be hereditary or be acquired. Sporadic human prion diseases include Cruetzfeldt-Jacob disease (CJD), fatal insomnia and variably protease-sensitive prionopathy. Genetic or familial prion diseases are caused by autosomal dominantly inherited mutations in the gene encoding for PrP^C and include familial or genetic CJD, fatal familial insomnia and Gerstmann-Sträussler-Scheinker syndrome. Acquired human prion diseases account for only 5% of cases of human prion disease. They include kuru, iatrogenic CJD and a new variant form of CJD that was transmitted to humans from affected cattle via meat consumption especially brain. This review presents information on the epidemiology, etiology, clinical assessment, neuropathology and public health concerns of human prion diseases. The role of the PrP encoding gene (PRNP) in conferring susceptibility to human prion diseases is also discussed.

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.

Atypical prion diseases in humans and animals

Although prion diseases, such as Creutzfeldt-Jakob disease (CJD) in humans and scrapie in sheep, have long been recognized, our understanding of their epidemiology and pathogenesis is still in its early stages. Progress is hampered by the lengthy incubation periods and the lack of effective ways of monitoring and characterizing these agents. Protease-resistant conformers of the prion protein (PrP), known as the "scrapie form" (PrP(Sc)), are used as disease markers, and for taxonomic purposes, in correlation with clinical, pathological, and genetic data. In humans, prion diseases can arise sporadically (sCJD) or genetically (gCJD and others), caused by mutations in the PrP-gene (PRNP), or as a foodborne infection, with the agent of bovine spongiform encephalopathy (BSE) causing variant CJD (vCJD). Person-to-person spread of human prion disease has only been known to occur following cannibalism (kuru disease in Papua New Guinea) or through medical or surgical treatment (iatrogenic CJD, iCJD). In contrast, scrapie in small ruminants and chronic wasting disease (CWD) in cervids behave as infectious diseases within these species. Recently, however, so-called atypical forms of prion diseases have been discovered in sheep (atypical/Nor98 scrapie) and in cattle, BSE-H and BSE-L. These maladies resemble sporadic or genetic human prion diseases and might be their animal equivalents. This hypothesis also raises the significant public health question of possible epidemiological links between these diseases and their counterparts in humans.

Tau, prions and Aβ: the triad of neurodegeneration

This article highlights the features that connect prion diseases with other cerebral amyloidoses and how these relate to neurodegeneration, with focus on tau phosphorylation. It also discusses similarities between prion disease and Alzheimer's disease: mechanisms of amyloid formation, neurotoxicity, pathways involved in triggering tau phosphorylation, links to cell cycle pathways and neuronal apoptosis. We review previous evidence of prion diseases triggering hyperphosphorylation of tau, and complement these findings with cases from our collection of genetic, sporadic and transmitted forms of prion diseases. This includes the novel finding that tau phosphorylation consistently occurs in sporadic CJD, in the absence of amyloid plaques.

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.

Agent strain variation in human prion disease: insights from a molecular and pathological review of the National Institutes of Health series of experimentally transmitted disease

Six clinico-pathological phenotypes of sporadic Creutzfeldt-Jakob disease have been characterized which correlate at the molecular level with the type (1 or 2) of the abnormal prion protein, PrP(TSE), present in the brain and with the genotype of polymorphic (methionine or valine) codon 129 of the prion protein gene. However, to what extent these phenotypes with their corresponding molecular combinations (i.e. MM1, MM2, VV1 etc.) encipher distinct prion strains upon transmission remains uncertain. We studied the PrP(TSE) type and the prion protein gene in archival brain tissues from the National Institutes of Health series of transmitted Creutzfeldt-Jakob disease and kuru cases, and characterized the molecular and pathological phenotype in the affected non-human primates, including squirrel, spider, capuchin and African green monkeys. We found that the transmission properties of prions from the common sporadic Creutzfeldt-Jakob disease MM1 phenotype are homogeneous and significantly differ from those of sporadic Creutzfeldt-Jakob disease VV2 or MV2 prions. Animals injected with iatrogenic Creutzfeldt-Jakob disease MM1 and genetic Creutzfeldt-Jakob disease MM1 linked to the E200K mutation showed the same phenotypic features as those infected with sporadic Creutzfeldt-Jakob disease MM1 prions, whereas kuru most closely resembled the sporadic Creutzfeldt-Jakob disease VV2 or MV2 prion signature and neuropathology. The findings indicate that two distinct prion strains are linked to the three most common Creutzfeldt-Jakob disease clinico-pathological and molecular subtypes and kuru, and suggest that kuru may have originated from cannibalistic transmission of a sporadic Creutzfeldt-Jakob disease of the VV2 or MV2 subtype.

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.

A clinical study of kuru patients with long incubation periods at the end of the epidemic in Papua New Guinea

Kuru is so far the principal human epidemic prion disease. While its incidence has steadily declined since the cessation of its route of transmission, endocannibalism, in Papua New Guinea in the 1950s, the arrival of variant Creutzfeldt–Jakob disease (vCJD), also thought to be transmitted by dietary prion exposure, has given kuru a new global relevance. We investigated all suspected cases of kuru from July 1996 to June 2004 and identified 11 kuru patients. There were four females and seven males, with an age range of 46–63 years at the onset of disease, in marked contrast to the age and sex distribution when kuru was first investigated 50 years ago. We obtained detailed histories of residence and exposure to mortuary feasts and performed serial neurological examination and genetic studies where possible. All patients were born a significant period before the mortuary practice of transumption ceased and their estimated incubation periods in some cases exceeded 50 years. The principal clinical features of kuru in the studied patients showed the same progressive cerebellar syndrome that had been previously described. Two patients showed marked cognitive impairment well before preterminal stages, in contrast to earlier clinical descriptions. In these patients, the mean clinical duration of 17 months was longer than the overall average in kuru but similar to that previously reported for the same age group, and this may relate to the effects of both patient age and PRNP codon 129 genotype. Importantly, no evidence for lymphoreticular colonization with prions, seen uniformly in vCJD, was observed in a patient with kuru at tonsil biopsy.