Aminoacid polymorphism in human prion protein and age at death in inherited prion disease

Met/Val129 polymorphism of the full-length human prion protein dictates distinct pathways of amyloid formation

Methionine/valine polymorphism at position 129 of the human prion protein, huPrP, is tightly associated with the pathogenic phenotype, disease progress, and age of onset of neurodegenerative diseases such as Creutzfeldt–Jakob disease or Fatal Familial Insomnia. This raises the question of whether and how the amino acid type at position 129 influences the structural properties of huPrP, affecting its folding, stability, and amyloid formation behavior. Here, our detailed biophysical characterization of the 129M and 129V variants of recombinant full-length huPrP(23–230) by amyloid formation kinetics, CD spectroscopy, molecular dynamics simulations, and sedimentation velocity analysis reveals differences in their aggregation propensity and oligomer content, leading to deviating pathways for the conversion into amyloid at acidic pH. We determined that the 129M variant exhibits less secondary structure content before amyloid formation and higher resistance to thermal denaturation compared to the 129V variant, whereas the amyloid conformation of both variants shows similar thermal stability. Additionally, our molecular dynamics simulations and rigidity analyses at the atomistic level identify intramolecular interactions responsible for the enhanced monomer stability of the 129M variant, involving more frequent minimum distances between E196 and R156, forming a salt bridge. Removal of the N-terminal half of the 129M full-length variant diminishes its differences compared to the 129V full-length variant and highlights the relevance of the flexible N terminus in huPrP. Taken together, our findings provide insight into structural properties of huPrP and the effects of the amino acid identity at position 129 on amyloid formation behavior.

Nascent β Structure in the Elongated Hydrophobic Region of a Gerstmann-Sträussler-Scheinker PrP Allele

Prion diseases are neurodegenerative disorders caused by the misfolding of the cellular prion protein (PrP^C). Gerstmann-Sträussler-Scheinker syndrome is an inherited prion disease with one early-onset allele (HRdup) containing an eight-amino-acid insertion; this LGGLGGYV insert is positioned after valine 129 (human PrP^C sequence) in a hydrophobic tract in the natively disordered region. Here we have characterized the structure and explored the molecular motions and dynamics of HRdup PrP and a control allele. High-resolution NMR data suggest that the core of HRdup has a canonical PrP^C structure, yet a nascent β-structure is observed in the flexible elongated hydrophobic region of HRdup. In addition, using mouse PrP^C sequence, we observed that a methionine/valine polymorphism at codon 128 (equivalent of methionine/valine 129 in human sequence) and oligomerization caused by high protein concentration affects conformational exchange dynamics at residue G130. We hypothesize that with the β-structure at the N-terminus, the hydrophobic region of HRdup can adopt a fully extended configuration and fold back to form an extended β-sheet with the existing β-sheet. We propose that these structures are early chemical events in disease pathogenesis.

Protective Effect of Val129-PrP against Bovine Spongiform Encephalopathy but not Variant Creutzfeldt-Jakob Disease

Bovine spongiform encephalopathy (BSE) is the only known zoonotic prion that causes variant Creutzfeldt-Jakob disease (vCJD) in humans. The major risk determinant for this disease is the polymorphic codon 129 of the human prion protein (Hu-PrP), where either methionine (Met₁₂₉) or valine (Val₁₂₉) can be encoded. To date, all clinical and neuropathologically confirmed vCJD cases have been Met₁₂₉ homozygous, with the exception of 1 recently reported Met/Val heterozygous case. Here, we found that transgenic mice homozygous for Val₁₂₉ Hu-PrP show severely restricted propagation of the BSE prion strain, but this constraint can be partially overcome by adaptation of the BSE agent to the Met₁₂₉ Hu-PrP. In addition, the transmission of vCJD to transgenic mice homozygous for Val₁₂₉ Hu-PrP resulted in a prion with distinct strain features. These observations may indicate increased risk for vCJD secondary transmission in Val₁₂₉ Hu-PrP–positive humans with the emergence of new strain features.

Insights into Mechanisms of Transmission and Pathogenesis from Transgenic Mouse Models of Prion Diseases

Prions represent a new paradigm of protein-mediated information transfer. In the case of mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, sometimes referred to as transmissible spongiform encephalopathies (TSEs), which frequently occur as epidemics. An increasing body of evidence indicates that the canonical mechanism of conformational corruption of cellular prion protein (PrP^C) by the pathogenic isoform (PrP^Sc) that is the basis of prion formation in TSEs is common to a spectrum of proteins associated with various additional human neurodegenerative disorders, including the more common Alzheimer's and Parkinson's diseases. The peerless infectious properties of TSE prions, and the unparalleled tools for their study, therefore enable elucidation of mechanisms of template-mediated conformational propagation that are generally applicable to these related disease states. Many unresolved issues remain including the exact molecular nature of the prion, the detailed cellular and molecular mechanisms of prion propagation, and the means by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological role of the normal form of the prion protein remains unclear and it is uncertain whether or not loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, advances in our understanding of prions have occurred because of their transmissibility to experimental animals, and the development of transgenic (Tg) mouse models has done much to further our understanding about various aspects of prion biology. In this review, we will focus on advances in our understanding of prion biology that occurred in the past 8 years since our last review of this topic.

Ascertainment bias causes false signal of anticipation in genetic prion disease

Anticipation is the phenomenon whereby age of onset in genetic disease decreases in successive generations. Three independent reports have claimed anticipation in Creutzfeldt-Jakob disease (CJD) caused by the c.598G > A mutation in PRNP encoding a p.Glu200Lys (E200K) substitution in the prion protein. If confirmed, this finding would carry clear implications for genetic counseling. We analyzed pedigrees with this mutation from four prion centers worldwide (n = 217 individuals with the mutation) to analyze age of onset and death in affected and censored individuals. We show through simulation that selective ascertainment of individuals whose onset falls within the historical window since the mutation's 1989 discovery is sufficient to create robust false signals both of anticipation and of heritability of age of onset. In our data set, the number of years of anticipation observed depends upon how strictly the data are limited by the ascertainment window. Among individuals whose disease was directly observed at a study center, a 28-year difference between parent and child age of onset is observed (p = 0.002), but including individuals ascertained retrospectively through family history reduces this figure to 7 years (p = 0.005). Applying survival analysis to the most thoroughly ascertained subset of data eliminates the signal of anticipation. Moreover, even non-CJD deaths exhibit 16 years anticipation (p = 0.002), indicating that ascertainment bias can entirely explain observed anticipation. We suggest that reports of anticipation in genetic prion disease are driven entirely by ascertainment bias. Guidelines for future studies claiming statistical evidence for anticipation are suggested.

Small-molecule theranostic probes: a promising future in neurodegenerative diseases

Prion diseases are fatal neurodegenerative illnesses, which include Creutzfeldt-Jakob disease in humans and scrapie, chronic wasting disease, and bovine spongiform encephalopathy in animals. They are caused by unconventional infectious agents consisting primarily of misfolded, aggregated, β -sheet-rich isoforms, denoted prions, of the physiological cellular prion protein (PrP(C)). Many lines of evidence suggest that prions (PrP(Sc)) act both as a template for this conversion and as a neurotoxic agent causing neuronal dysfunction and cell death. As such, PrP(Sc) may be considered as both a neuropathological hallmark of the disease and a therapeutic target. Several diagnostic imaging probes have been developed to monitor cerebral amyloid lesions in patients with neurodegenerative disorders (such as Alzheimer's disease, Parkinson's disease, and prion disease). Examples of these probes are Congo red, thioflavin T, and their derivatives. We synthesized a series of styryl derivatives, denoted theranostics, and studied their therapeutic and/or diagnostic potentials. Here we review the salient traits of these small molecules that are able to detect and modulate aggregated forms of several proteins involved in protein misfolding diseases. We then highlight the importance of further studies for their practical implications in therapy and diagnostics.

Dominant prion mutants induce curing through pathways that promote chaperone-mediated disaggregation

Protein misfolding underlies many neurodegenerative diseases, including the transmissible spongiform encephalopathies (prion diseases). Although cells typically recognize and process misfolded proteins, prion proteins evade protective measures by forming stable, self-replicating aggregates. However, coexpression of dominant-negative prion mutants can overcome aggregate accumulation and disease progression through currently unknown pathways. Here we determine the mechanisms by which two mutants of the Saccharomyces cerevisiae Sup35 protein cure the [PSI(+)] prion. We show that both mutants incorporate into wild-type aggregates and alter their physical properties in different ways, diminishing either their assembly rate or their thermodynamic stability. Whereas wild-type aggregates are recalcitrant to cellular intervention, mixed aggregates are disassembled by the molecular chaperone Hsp104. Thus, rather than simply blocking misfolding, dominant-negative prion mutants target multiple events in aggregate biogenesis to enhance their susceptibility to endogenous quality-control pathways.

Transgenic mouse models and prion strains

Here we review the known strain profiles of various prion diseases of animals and humans, and how transgenic mouse models are being used to elucidate basic molecular mechanisms of prion propagation and strain variation and for assessing the zoonotic potential of various animal prion strains.

Chronic wasting disease

Chronic wasting disease (CWD) is a prion disease of free-ranging and farmed ungulates (deer, elk, and moose) in North America and South Korea. First described by the late E.S. Williams and colleagues in northern Colorado and southern Wyoming in the 1970s, CWD has increased tremendously both in numerical and geographical distribution, reaching prevalence rates as high as 50% in free-ranging and >90% in captive deer herds in certain areas of USA and Canada. CWD is certainly the most contagious prion infection, with significant horizontal transmission of infectious prions by, e.g., urine, feces, and saliva. Dissemination and persistence of infectivity in the environment combined with the appearance in wild-living and migrating animals make CWD presently uncontrollable, and pose extreme challenges to wild-life disease management. Whereas CWD is extremely transmissible among cervids, its trans-species transmission seems to be restricted, although the possible involvement of rodent and carnivore species in environmental transmission has not been fully evaluated. Whether or not CWD has zoonotic potential as had Bovine spongiform encephalopathy (BSE) has yet to be answered. Of note, variant Creutzfeldt-Jakob disease (vCJD) was only detected because clinical presentation and age of patients were significantly different from classical CJD. Along with further understanding of the molecular biology and pathology of CWD, its transmissibility and species restrictions and development of methods for preclinical diagnosis and intervention will be crucial for effective containment of this highly contagious prion disease.

Prion protein misfolding

The crucial event in the development of transmissible spongiform encephalopathies (TSEs) is the conformational change of a host-encoded membrane protein - the cellular PrP^C - into a disease associated, fibril-forming isoform PrP^Sc. This conformational transition from the α-helix-rich cellular form into the mainly β-sheet containing counterpart initiates an ‘autocatalytic’ reaction which leads to the accumulation of amyloid fibrils in the central nervous system (CNS) and to neurodegeneration, a hallmark of TSEs.

The exact molecular mechanisms which lead to the conformational change are still unknown. It also remains to be brought to light how a polypeptide chain can adopt at least two stable conformations. This review focuses on structural aspects of the prion protein with regard to protein-protein interactions and the initiation of prion protein misfolding. It therefore highlights parts of the protein which might play a notable role in the conformational transition from PrP^C to PrP^Sc and consequently in inducing a fatal chain reaction of protein misfolding. Furthermore, features of different proteins, which are able to adopt insoluble fibrillar states under certain circumstances, are compared to PrP in an attempt to understand the unique characteristics of prion diseases.

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.

The elk PRNP codon 132 polymorphism controls cervid and scrapie prion propagation

The elk prion protein gene (PRNP) encodes either methionine (M) or leucine (L) at codon 132, the L132 allele apparently affording protection against chronic wasting disease (CWD). The corresponding human codon 129 polymorphism influences the host range of bovine spongiform encephalopathy (BSE) prions. To fully address the influence of this cervid polymorphism on CWD pathogenesis, we created transgenic (Tg) mice expressing cervid PrPC with L at residue 132, referred to as CerPrPC-L132, and compared the transmissibility of CWD prions from elk of defined PRNP genotypes, namely homozygous M/M or L/L or heterozygous M/L, in these Tg mice with previously described Tg mice expressing CerPrPC-M132, referred to as Tg(CerPrP) mice. While Tg(CerPrP) mice were consistently susceptible to CWD prions from elk of all three genotypes, Tg(CerPrP-L132) mice uniformly failed to develop disease following challenge with CWD prions. In contrast, SSBP/1 sheep scrapie prions transmitted efficiently to both Tg(CerPrP) and Tg(CerPrP-L132) mice. Our findings suggest that the elk 132 polymorphism controls prion susceptibility at the level of prion strain selection and that cervid PrP L132 severely restricts propagation of CWD prions. We speculate that the L132 polymorphism results in less efficient conversion of CerPrPC-L132 by CWD prions, an effect that is overcome by the SSBP/1 strain. Our studies show the accumulation of subclinical levels of CerPrPSc in aged asymptomatic CWD-inoculated Tg(CerPrP-L132) mice and also suggests the establishment of a latent infection state in apparently healthy elk expressing this seemingly protective allele.

Transgenic mouse models of prion diseases

Prions represent a new biological paradigm of protein-mediated information transfer. In mammals, prions are the cause of fatal, transmissible neurodegenerative diseases, often referred to as transmissible spongiform encephalopathies. Many unresolved issues remain, including the exact molecular nature of the prion, the detailed mechanism of prion propagation, and the mechanism by which prion diseases can be both genetic and infectious. In addition, we know little about the mechanism by which neurons degenerate during prion diseases. Tied to this, the physiological function of the normal form of the prion protein remains unclear, and it is uncertain whether loss of this function contributes to prion pathogenesis. The factors governing the transmission of prions between species remain unclear, in particular the means by which prion strains and PrP primary structure interact to affect interspecies prion transmission. Despite all these unknowns, dramatic advances in our understanding of prions have occurred because of their transmissibility to experimental animals and the development of transgenic mouse models has done much to further our understanding about various aspects of prion biology. In this chapter, I review recent advances in our understanding of prion biology that derive from this powerful and informative approach.

Defective retrotranslocation causes loss of anti-Bax function in human familial prion protein mutants

Prion protein (PrP) inhibits the activation of proapoptotic Bax in primary human neurons and MCF-7 cells. Because neuronal apoptosis occurs in human prion diseases, here we examine the anti-Bax function of familial PrP mutants. All Creutzfeldt-Jakob disease and fatal familial insomnia-associated prion protein mutations partially or completely lose the anti-Bax function in human neurons and, except for A117V and V203I, in MCF-7 cells. The ability of the mutants to protect against Bax-mediated cell death is divided into three groups: (1) group I, retention of anti-Bax function in both the Val129 and Met129 mutants; (2) group II, retention of anti-Bax function only in Val129 mutants; and (3) group III, reduction or no anti-Bax function in Val129 and Met129 mutants. The loss of anti-Bax function in these PrP mutants correlates completely with a significant decrease in the production of cytosolic PrP, a form of PrP shown previously to have anti-Bax function in human neurons. Cotransfection of the full-length PrP mutants with wild-type or mutant cytosolic PrP, but not with wild type full-length PrP, rescues the anti-Bax function of PrP. The results show that the failure of PrP mutants to produce cytosolic PrP is responsible for the loss of anti-Bax function and that the effect of the PrP mutants is dominant over wild-type PrP. Furthermore, these results imply that misfolded PrP that escapes retrotranslocation could accumulate at the cell surface and cause neuronal dysfunction.

Dissociation of pathological and molecular phenotype of variant Creutzfeldt-Jakob disease in transgenic human prion protein 129 heterozygous mice

All neuropathologically confirmed cases of variant Creutzfeldt-Jakob disease (vCJD), characterized by abundant florid plaques and type 4 disease-related prion protein (PrP(Sc)) in the brain, have been homozygous for methionine at polymorphic residue 129 of PRNP. The distinctive neuropathological and molecular phenotype of vCJD can be faithfully recapitulated in Prnp-null transgenic mice homozygous for human PrP M129 but not V129, where a distinct prion strain is propagated. Here we model susceptibility of 129MV heterozygotes, the most common PRNP genotype, in transgenic mice and show that, remarkably, propagation of type 4 PrP(Sc) was not associated with characteristic vCJD neuropathology. Depending on the source of the inoculum these mice can develop four distinct disease phenotypes after challenge with bovine spongiform encephalopathy (BSE) prions or vCJD (human-passaged BSE) prions. vCJD-challenged mice had higher attack rates of prion infection than BSE-challenged recipients. These data argue that human PRNP 129 heterozygotes will be more susceptible to infection with vCJD prions than to cattle BSE prions and may present with a neuropathological phenotype distinct from vCJD.

Clinical features of fatal familial insomnia: phenotypic variability in relation to a polymorphism at codon 129 of the prion protein gene

Fatal Familial Insomnia is a hereditary prion disease characterized by a mutation at codon 178 of the prion protein gene cosegregating with the methionine polymorphism at codon 129 of the mutated allele. It is characterized by disturbances of the wake-sleep cycle, dysautonomia and somatomotor manifestations (myoclonus, ataxia, dysarthria, spasticity). PET studies disclose severe thalamic and additionally cortical hypometabolism. Neuropathology shows marked neuronal loss and gliosis in the thalamus, especially the medio-dorsal and anterior-ventral nuclei, olivary hypertrophy and some spongiosis of the cerebral cortex. Detailed analysis of 14 cases from 5 unrelated families showed that patients ran either a short (9.1 +/- 1.1 months) or a prolonged (30.8 +/- 21.3 months) clinical course according to whether they were homozygote met/met or heterozygote met/val at codon 129. Moreover, homozygotes had more prominent oneiric episodes, insomnia and dysautonomia at onset, whereas heterozygotes showed ataxia and dysarthria at onset, earlier sphincter loss and epileptic Grand Mal seizures; they also displayed more extensive cortical involvement on PET and at postmortem examination. Our data suggest that the phenotype expression of Fatal Familial Insomnia is related, at least partly, to the polymorphism at codon 129 of the prion protein-gene.

Role of N-terminal Familial Mutations in Prion Protein Fibrillization and Prion Amyloid Propagation in Vitro*

A self-perpetuating conformational conversion of the prion protein (PrP) is believed to underlie pathology and transmission of prion diseases. Here we explore the effects of N-terminal pathogenic mutations (P102L, P105L, A117V) and the residue 129 polymorphism on amyloid fibril formation by the human PrP fragment 23-144, an in vitro conversion model that can reproduce certain characteristics of prion replication such as strains and species barriers. We find that these amino acid substitutions neither affect PrP23-144 amyloidogenicity nor introduce barriers to cross-seeding of soluble protein. However, the polymorphism strongly influences the conformation of the amyloid fibrils, as determined by infrared spectroscopy. Intriguingly, unlike conformational features governed by the critical amyloidogenic region of PrP23-144 (residues 138-139), the structural features distinguishing Met-129 and Val-129 PrP23-144 amyloid fibrils are not transmissible by cross-seeding. While based only on in vitro data, these findings provide fundamental insight into the mechanism of prion-based conformational transmission, indicating that only conformational features controlling seeding specificity (e.g. those in critical intermolecular contact sites of amyloid fibrils) are necessarily transmissible by cross-seeding; conformational traits in other parts of the PrP molecule may not be "heritable" from the amyloid template.

Polymorphism at residue 129 modulates the conformational conversion of the D178N variant of human prion protein 90-231

One of the arguments in favor of the protein-only hypothesis of transmissible spongiform encephalopathies is the link between inherited prion diseases and specific mutations in the PRNP gene. One such mutation (Asp178 --> Asn) is associated with two distinct disorders: fatal familial insomnia or familial Creutzfeldt-Jakob disease, depending upon the presence of Met or Val at position 129, respectively. In this study, we have characterized the biophysical properties of recombinant human prion proteins (huPrP90-231) corresponding to the polymorphic variants D178N/M129 and D178N/V129. In comparison to the wild-type protein, both polymorphic forms of D178N huPrP show a greatly increased propensity for a conversion to beta-sheet-rich oligomers (at acidic pH) and thioflavine T-positive amyloid fibrils (at neutral pH). Importantly, the conversion propensity for the D178N variant is strongly dependent upon the M/V polymorphism at position 129, whereas under identical experimental conditions, no such dependence is observed for the wild-type protein. Amyloid fibrils formed by wild-type huPrP90-231 and the D178N variant are characterized by different secondary structures, and these structures are further modulated by residue 129 polymorphism. Although on the basis of only in vitro data, this study strongly suggests that polymorphism-dependent phenotypic variability of familial prion diseases may be linked to differences in biophysical properties of prion protein variants.

Genetic influences on oxidative stress and their association with normal cognitive ageing

Oxidative stress is hypothesised to play a major role in ageing processes. Reactive oxygen species produced during normal aerobic metabolism damage cellular macromolecules. The brain is particularly susceptible to oxidative stress due to its high rate of aerobic metabolism. We hypothesised that polymorphisms in genes contributing to antioxidant defences are associated with variation in normal cognitive ageing in the absence of dementia. We examined associations between two SNPs (rs2073495 and rs743658) in Lactotransferrin (LTF), a gene involved in iron absorption, and the common M129V SNP in the prion protein gene, PRNP (rs1799990), with cognitive ability and cognitive ageing in a cohort of non-demented individuals born in 1921. All had cognitive ability measured at age 11 in the Scottish Mental Survey of 1932, and again at age 79. No association was identified with LTF. PRNP M129V was significantly related to Moray House Test (MHT) IQ scores at age 79, adjusted for sex and age 11 IQ (p=0.006). Individuals homozygous for the methionine allele performed significantly better than heterozygotes. This study supports the hypothesis that genetic variations in antioxidant defence genes, specifically PRNP, are important influences on the trajectory of normal cognitive ageing. An interaction between PRNP and klotho (KL) genotypes was also identified (p=0.015), highlighting the importance of analysing gene interactions when investigating associations with quantitative traits.

The presence of valine at residue 129 in human prion protein accelerates amyloid formation

The polymorphism at residue 129 of the human PRNP gene modulates disease susceptibility and the clinico-pathological phenotypes in human transmissible spongiform encephalopathies. The molecular mechanisms by which the effect of this polymorphism are mediated remain unclear. It has been shown that the folding, dynamics and stability of the physiological, alpha-helix-rich form of recombinant PrP are not affected by codon 129 polymorphism. Consistent with this, we have recently shown that the kinetics of amyloid formation do not differ between protein containing methionine at codon 129 and valine at codon 129 when the reaction is initiated from the alpha-monomeric PrP(C)-like state. In contrast, we have shown that the misfolding pathway leading to the formation of beta-sheet-rich, soluble oligomer was favoured by the presence of methionine, compared with valine, at position 129. In the present work, we examine the effect of this polymorphism on the kinetics of an alternative misfolding pathway, that of amyloid formation using partially folded PrP allelomorphs. We show that the valine 129 allelomorph forms amyloids with a considerably shorter lag phase than the methionine 129 allelomorph both under spontaneous conditions and when seeded with pre-formed amyloid fibres. Taken together, our studies demonstrate that the effect of the codon 129 polymorphism depends on the specific misfolding pathway and on the initial conformation of the protein. The inverse propensities of the two allelomorphs to misfold in vitro through the alternative oligomeric and amyloidogenic pathways could explain some aspects of prion diseases linked to this polymorphism such as age at onset and disease incubation time.

The residue 129 polymorphism in human prion protein does not confer susceptibility to Creutzfeldt-Jakob disease by altering the structure or global stability of PrPC

There are two common forms of prion protein (PrP) in humans, with either methionine or valine at position 129. This polymorphism is a powerful determinant of the genetic susceptibility of humans toward both sporadic and acquired forms of prion disease and restricts propagation of particular prion strains. Despite its key role, we have no information on the effect of this mutation on the structure, stability, folding, and dynamics of the cellular form of PrP (PrP(C)). Here, we show that the mutation has no measurable effect on the folding, dynamics, and stability of PrP(C). Our data indicate that the 129M/V polymorphism does not affect prion propagation through its effect on PrP(C); rather, its influence is likely to be downstream in the disease mechanism. We infer that the M/V effect is mediated through the conformation or stability of disease-related PrP (PrP(Sc)) or intermediates or on the kinetics of their formation.

Methionine 129 Variant of Human Prion Protein Oligomerizes More Rapidly than the Valine 129 Variant

The human PrP gene (PRNP) has two common alleles that encode either methionine or valine at codon 129. This polymorphism modulates disease susceptibility and phenotype of human transmissible spongiform encyphalopathies, but the molecular mechanism by which these effects are mediated remains unclear. Here, we compared the misfolding pathway that leads to the formation of beta-sheet-rich oligomeric isoforms of the methionine 129 variant of PrP to that of the valine 129 variant. We provide evidence for differences in the folding behavior between the two variants at the early stages of oligomer formation. We show that Met(129) has a higher propensity to form beta-sheet-rich oligomers, whereas Val(129) has a higher tendency to fold into alpha-helical-rich monomers. An equimolar mixture of both variants displayed an intermidate folding behavior. We show that the oligomers of both variants are initially a mixture of alpha- and beta-rich conformers that evolve with time to an increasingly homogeneous beta-rich form. This maturation process, which involves no further change in proteinase K resistance, occurs more rapidly in the Met(129) form than the Val(129) form. Although the involvement of such beta-rich oligomers in prion pathogenesis is speculative, the misfolding behavior could, in part, explain the higher susceptibility of individuals that are methionine homozygote to both sporadic and variant Creutzfeldt-Jakob disease.

Pathogenic human prion protein rescues PrP null phenotype in transgenic mice

Infectious prion diseases may be acquired, sporadic or inherited in their aetiology. Inherited prion diseases are caused by coding mutations in the prion protein (PrP) gene. We investigated whether one of the commonest of these mutations, E200K, results in a functionally inactive prion protein by expressing human PrP 200K in transgenic mice homozygous for murine PrP null alleles. We examined the intrinsic properties of hippocampal CA1 pyramidal cells in these mice by measuring the resting potential, time constants and amplitude of the slow after-hyperpolarisation (AHP). These mice show rescue of the reduced slow AHP electrophysiological phenotype found in PrP null mice. Using the AHP as a marker for PrP function, we conclude that this pathogenic PrP mutation, does not significantly affect the normal neuronal function of PrP.

Variant Creutzfeldt-Jakob disease: pathology, epidemiology, and public health implications

Prion diseases, or transmissible spongiform encephalopathies, include Creutzfeldt-Jakob disease (CJD) in humans and scrapie and bovine spongiform encephalopathy (BSE) in animals. These neurodegenerative diseases are invariably fatal and can be transmitted by inoculation or dietary exposure. They are associated with the accumulation of an altered, disease-associated form of the normal prion protein. Pathologically, prion diseases result in neuronal cell death and a characteristic spongiform appearance of the brain tissue. The emergence of a variant form of CJD (vCJD) in the United Kingdom in 1996 has been causally and experimentally linked to the UK BSE epidemic in the 1980s and early 1990s. The finding that BSE is transmissible to different animal species, unlike previously characterized prion diseases such as sheep scrapie, has raised enormous public health concerns worldwide. Although it is not yet possible to gauge the size of a potential vCJD epidemic, preliminary data indicate a significant dietary exposure to BSE-infected material in Britain and wider implications of the transmissibility of prion diseases. The threat to public health has intensified research efforts to understand the molecular basis of prion diseases, understand their transmission between species, improve methods of diagnosis, and develop therapeutic strategies for treatment and prevention of disease. In this review, we summarize current data on the pathology of BSE and vCJD and the epidemiology of vCJD, and we outline public health implications based on these data, emphasizing preventative measures and areas of research for screening and diagnosis.

Phenotypic variability in the brains of a family with a prion disease characterized by a 144-base pair insertion in the prion protein gene

The use of prion protein (PrP) immunohistochemistry in neuropathology has allowed identification of prion diseases with otherwise atypical histological features. The brains from family members with familial prion diseases can show marked histological variation. A histological and immunohistochemical study was performed on 10 brains of patients with a familial prion disease caused by a 144-base pair (bp) insertion in the prion protein gene. The histology from the cases showed variability in the severity of spongiform change and astrocytosis in both the cerebellum and the cerebrum. There was also variability in the density of microglial cells. The PrP immunohistochemistry revealed that in nine cases there was a similar patch-like deposition of PrP within the molecular layer of the cerebellum. Although in the cerebellum there did seem to be some correlation between the severity of spongiform change, astrocytosis and the density of microglial cells, there was no such correlation between any of these three parameters and the density of PrP staining. There was deposition of beta-amyloid precursor protein (beta-APP) in the cerebellum, suggesting that disrupted axonal transport had a possible role in the evolution of the disease. The cases illustrate the histological variability that can occur in familial prion diseases despite similarity in PrP staining. They also reveal that the relationship between PrP deposition and cerebral or cerebellar damage might be complex.

BSE prions propagate as either variant CJD-like or sporadic CJD-like prion strains in transgenic mice expressing human prion protein

Variant Creutzfeldt-Jakob disease (vCJD) has been recognized to date only in individuals homozygous for methionine at PRNP codon 129. Here we show that transgenic mice expressing human PrP methionine 129, inoculated with either bovine spongiform encephalopathy (BSE) or variant CJD prions, may develop the neuropathological and molecular phenotype of vCJD, consistent with these diseases being caused by the same prion strain. Surprisingly, however, BSE transmission to these transgenic mice, in addition to producing a vCJD-like phenotype, can also result in a distinct molecular phenotype that is indistinguishable from that of sporadic CJD with PrP(Sc) type 2. These data suggest that more than one BSE-derived prion strain might infect humans; it is therefore possible that some patients with a phenotype consistent with sporadic CJD may have a disease arising from BSE exposure.

Does the prion protein gene 129 codon polymorphism influence sleep? Evidence from a fatal familial insomnia kindred

OBJECTIVE

Experimental and clinical evidence in prion diseases suggests that the prion protein gene (PRNP) plays a role in regulating sleep.

METHODS

Seventeen healthy individuals belonging to a single fatal familial insomnia pedigree, 8 carriers and 9 non-carriers of the PRNP codon 178 mutation, underwent polysomnography and spectral electroencephalographic (EEG) analysis. All were also characterized with regard to the codon 129 polymorphism on both PRNP alleles.

RESULTS

PRNP codon 129 polymorphism exhibited influences on sleep-EEG activities. In particular, spindle frequency band power and balance between delta and spindle activity were found to correlate with the genotype of PRNP codon 129, irrespective of the mutation at codon 178.

CONCLUSIONS

Our data suggest that PRNP codon 129 polymorphism may also affect sleep in the healthy population and warrant further studies in the general population and other sleep disorders.

Familial Creutzfeldt-Jakob disease associated with a point mutation at codon 210 of the prion protein gene

Creutzfeldt-Jakob disease (CJD), the most known human prion disease, is usually sporadic but approximately 15% of the cases are familial. To date, seven CJD cases with codon 210 mutation (GTT to ATT) have been reported in the literature. We describe a case of a 57 year-old woman who presented gait disturbances and rapidly progressive dementia, leading to death four months after onset. Electroencephalogram revealed periodic activity, diffusion-weighted magnetic resonance imaging showed hypersignal in basal ganglia, and test for 14-3-3 protein was strongly positive in the CSF. The complete prion protein gene coding region was sequenced after PCR amplification, showing a point mutation in codon 210. This is the first case of CJD with codon 210 mutation diagnosed in Brazil. We emphasize the role of genetic search for prion protein gene mutation, even in patients presenting clinical features resembling sporadic CJD.

Sporadic--but not variant--Creutzfeldt-Jakob disease is associated with polymorphisms upstream of PRNP exon 1

Human prion diseases have inherited, sporadic, and acquired etiologies. The appearance of the novel acquired prion disease, variant Creutzfeldt-Jakob disease (vCJD), and the demonstration that it is caused by the same prion strain as that causing bovine spongiform encephalopathy, has led to fears of a major human epidemic. The etiology of classical (sporadic) CJD, which has a worldwide incidence, remains obscure. A common human prion-protein-gene (PRNP) polymorphism (encoding either methionine or valine at codon 129) is a strong susceptibility factor for sporadic and acquired prion disease. However, a quantitative-trait-locus study of prion incubation periods in mice has demonstrated an important factor that is close to Prnp but is independent of its coding sequence or that of the nearby prion-like doppel gene (Prnd). We have analyzed the PRNP locus for such tightly linked susceptibility factors. Fifty-six polymorphic sites have been identified within 25 kb of the PRNP open reading frame, including sites within the PRNP promoter and the PRNP 3' untranslated region. These have been characterized in 61 Centre d'Etude du Polymorphisme Humain (CEPH) families, demonstrating extensive linkage disequilibrium around PRNP and the existence of 11 major European PRNP haplotypes. Haplotype frequencies estimated in healthy U.K. control individuals were very similar to those deduced in the CEPH families. A common haplotype was overrepresented in patients with sporadic CJD (sCJD). Through use of a log-linear modeling approach to simultaneously model Hardy-Weinberg and linkage disequilibria, a significant independent association was found between sCJD and a polymorphism upstream of PRNP exon 1 (P=.005), in addition to the strong susceptibility conferred by codon 129 (P=2x10(-8)). However, although our sample size was necessarily small, no association was found between these polymorphisms and vCJD or iatrogenic CJD, in keeping with their having distinct disease mechanisms. In addition, there was no evidence of a PRNP founder effect in the first reported geographical cluster of vCJD.

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.

Identification of multiple quantitative trait loci linked to prion disease incubation period in mice

Polymorphisms in the prion protein gene are known to affect prion disease incubation times and susceptibility in humans and mice. However, studies with inbred lines of mice show that large differences in incubation times occur even with the same amino acid sequence of the prion protein, suggesting that other genes may contribute to the observed variation. To identify these loci we analyzed 1,009 animals from an F2 intercross between two strains of mice, CAST/Ei and NZW/OlaHSd, with significantly different incubation periods when challenged with RML scrapie prions. Interval mapping identified three highly significantly linked regions on chromosomes 2, 11, and 12; composite interval mapping suggests that each of these regions includes multiple linked quantitative trait loci. Suggestive evidence for linkage was obtained on chromosomes 6 and 7. The sequence conservation between the mouse and human genome suggests that identification of mouse prion susceptibility alleles may have direct relevance to understanding human susceptibility to bovine spongiform encephalopathy (BSE) infection, as well as identifying key factors in the molecular pathways of prion pathogenesis. However, the demonstration of other major genetic effects on incubation period suggests the need for extreme caution in interpreting estimates of variant Creutzfeldt-Jakob disease epidemic size utilizing existing epidemiological models.

The molecular biology of prion propagation

Prion diseases such as Creutzfeldt-Jakob disease (CJD) in humans and scrapie and bovine spongiform encephalopathy (BSE) in animals are associated with the accumulation in affected brains of a conformational isomer (PrP(Sc)) of host-derived prion protein (PrP(C)). According to the protein-only hypothesis, PrP(Sc) is the principal or sole component of transmissible prions. The conformational change known to be central to prion propagation, from a predominantly alpha-helical fold to one predominantly comprising beta structure, can now be reproduced in vitro, and the ability of beta-PrP to form fibrillar aggregates provides a plausible molecular mechanism for prion propagation. The existence of multiple prion strains has been difficult to explain in terms of a protein-only infectious agent but recent studies of human prion diseases suggest that strain-specific phenotypes can be encoded by different PrP conformations and glycosylation patterns. The experimental confirmation that a novel form of human prion disease, variant CJD, is caused by the same prion strain as cattle BSE, has highlighted the pressing need to understand the molecular basis of prion propagation and the transmission barriers that limit their passage between mammalian species. These and other advances in the fundamental biology of prion propagation are leading to strategies for the development of rational therapeutics.

Creutzfeldt-Jakob disease and the eye. I. Background and patient management

This article attempts to summarise our current understanding of TSEs as they affect man. Specific aspects relevant to ophthalmological practice, in particular the management of patients in day-to-day clinical practice and with respect to corneal transplantation, have been discussed. In the companion article we discuss the specific ophthalmic and neuro-ophthalmic features of these diseases.

Prominent psychiatric features and early onset in an inherited prion disease with a new insertional mutation in the prion protein gene

In five generations of the French M-E kindred, 11 members are now known to be or have been affected by a form of spongiform encephalopathy previously recorded as Gerstmann-Sträussler-Scheinker disease. Mean age at onset was 28 years (range 21-34 years). In six instances, these patients were hospitalized in psychiatric institutions with various diagnoses, the most frequent being mania or mania-like symptoms. Dementia occurred progressively after a lengthy course. Histological studies showed atrophy of the cerebellar molecular layer, which contained kuru and multicentric plaques labelled with anti-prion protein antibodies. Spongiosis was not prominent and remained largely limited to the periphery of plaques; it was more marked in the thalamus, where plaques were scarce. A 192 base pair (bp) insert (eight extra repeats of 24 bp) in the octapeptide coding region of the prion protein gene (PRNP) within a codon-129 methionine allele was found in four symptomatic subjects. Early age at onset, the prominence of psychiatric symptoms and the long course of the disease are noticeable clinical features in this family with an inherited prion disease due to a new insertional mutation in PRNP.

Mutation at codon 210 (V210I) of the prion protein gene in a North African patient with Creutzfeldt-Jakob disease

A point mutation at codon 210 of the prion protein gene (PRNP), resulting in the substitution of isoleucine for valine (V210I) has been found in a 54-year-old Moroccan patient affected with Creutzfeldt-Jakob disease (CJD). This patient is the first carrier of the PRNP V210I mutation reported from North Africa. The clinical presentation of the patient was rather similar to that seen in classical CJD, except that unusual early sensory symptoms were observed. The mother of the proband, aged 72, is a further example of an asymptomatic elderly carrier of the PRNP V210I mutation, suggesting an incomplete penetrance of the disease.

Variant Creutzfeldt-Jakob disease

It is clear that the prion strain causing bovine spongiform encephalopathy (BSE) in cattle has infected human beings, manifesting itself as a novel human prion disease, variant Creutzfeldt-Jakob disease (CjD). Studies of the incubation periods seen in previous epidemics of human prion disease and of the effect of transmission barriers limiting spread of these diseases between species, suggest that the early variant CJD cases may have been exposed during the preclinical phase of the BSE epidemic. It must therefore be considered that many cases may follow from later exposure in an epidemic that would be expected to evolve over decades. Since the number of people currently incubating this disease is unknown, there are concerns that prions might be transmitted iatrogenically via blood transfusion, tissue donation, and, since prions resist routine sterilisation, contamination of surgical instruments. Such risks remain unquantified. Although variant CJD can be diagnosed during life by tonsil biopsy, a prion-specific blood test is needed to assess and manage this potential threat to public health. The theoretical possibility that BSE prions might have transferred to other species and continue to present a risk to human health cannot be excluded at present.

Genetic classification of primary neurodegenerative disease

Review During the past 10 years (the "decade of the brain"), some of the genetic causes of many of the primary neurodegenerative diseases, which include Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, prion disease, and many ataxic syndromes, have been found. These breakthroughs mean that for many of these diseases we now know the initiating trigger as well as the final outcome. These diseases have many pathological mechanisms in common, and there may be relatively few pathways to neuronal death seen in these disorders. Thus, treatment strategies developed for a particular disease may be found to have efficacy in more than one disorder.

Molecular pathology of fatal familial insomnia

Fatal familial insomnia (FFI) is linked to a mutation at codon 178 of the prion protein gene, coupled with the methionine codon at position 129, the site of a methionine/valine polymorphism. The D178N mutation coupled with the 129 valine codon is linked to a subtype of Creutzfeldt-Jakob disease (CJD178) with a different phenotype. Two protease resistant fragments of the pathogenic PrP (PrPres), which differ in molecular mass, are associated with FFI and CJD178, respectively, suggesting that the two PrPres have different conformations and hence they produce different disease phenotypes. FFI transmission experiments, which show that the endogenous PrPres recovered in affected syngenic mice specifically replicates the molecular mass of the FFI PrPres inoculated and is associated with a phenotype distinct from that of the CJD178 inoculated mice, support this idea. The second distinctive feature of the FFI PrPres is the underrepresentation of the unglycosylated PrPres form. Cell models indicate that the underrepresentation of this PrPres form results from the PrP dysmetabolism caused by the D178N mutation and not from the preferential conversion of the glycosylated forms. Codon 129 on the normal allele further modifies the FFI phenotype determining patient subpopulations of 129 homozygotes and heterozygotes: disease duration is generally shorter, insomnia more severe and histopathology more restricted to the thalamus in the homozygotes than in the heterozygotes. The allelic origin of PrPres fails to explain this finding since in both cases FFI PrPres is expressed only by the mutant allele. Despite remarkable advances, many issues remain unsolved precluding full understanding of the FFI pathogenesis.

A wild-type prion protein does not acquire properties of the scrapie isoform when coexpressed with a mutant prion protein in cultured cells

Inherited prion diseases are linked to autosomal dominant mutations in the gene that encodes the prion protein (PrP). These mutations are thought to induce PrP to undergo a conformational alteration that converts it to a pathogenic form designated PrP(Sc). In patients who are heterozygous for PrP mutations, the protein encoded by the wild-type allele might influence the conversion of the mutant protein to the PrP(Sc) state, and might itself be converted into PrP(Sc). To test these possibilities, we have constructed stably transfected lines of CHO cells that express both wild-type mouse PrP and mouse PrP carrying an insertional mutation that is homologous to one associated with familial Creutzfeldt-Jakob disease. We find that wild-type PrP in these cells does not acquire any of four different biochemical properties characteristic of PrP(Sc) that we have previously documented in mutant PrPs expressed in CHO cells. We also observe that conversion of the mutant protein to a PrP(Sc)-like state is not impaired by coexpression of the wild-type protein. These results are consistent with the idea that sequence homology between PrP molecules has an important influence on PrP(Sc) generation, and they provide insight into the metabolism of PrP in patients who are heterozygous at the PrP locus.

Familial Creutzfeldt-Jakob disease. Codon 200 prion disease in Libyan Jews

Creutzfeldt-Jakob disease (CJD) is the most prevalent of the human prion diseases, a group of fatal neurodegenerative disorders afflicting both humans and animals. The unique characteristic of these diseases, whether sporadic, dominantly inherited, or acquired by transmission, is the accumulation in the brain of an abnormal isoform (PrPSc) of the cellular prion protein (PrPc). Progress has been made in understanding inherited prion diseases by genetically linking clusters of familial CJD (fCJD) to mutations of the PrP gene (PRNP). One of the largest clusters of fCJD exists among Jews of Libyan origin. The clinical and pathologic manifestations of CJD in this community resemble those seen with sporadic CJD (sCJD), but the incidence is about 100 times higher than in the general population. Initially, this high incidence was attributed to infection via consumption of sheep brains or eyeballs, but a mutation at codon 200 in PRNP resulting in the substitution of lysine (K) for glutamate (E), designated E200K, was identified in this population. The onset of fCJD (E200K) is age dependent and shows nearly complete penetrance by age 85 years. fCJD in Libyan Jews is invariably associated with accumulation of the pathologic isoform PrPSc in the central nervous system. Using mutation-specific antibodies, it was shown that most PrPSc in the brain of these patients originated from the mutant protein. Some studies suggest that mutant PrP may accumulate in brain and other organs due to an impaired degradation, and its accumulation has been postulated to promote conversion into PrPSc. fCJD (E200K) has been transmitted to primates and transgenic mice, highlighting the need to address ethical and public health issues surrounding the possibility of human to human transmission.

Molecular analysis of prion strain variation and the aetiology of 'new variant' CJD

Strains of transmissible spongiform encephalopathies are distinguished by differing physicochemical properties of PrPSc, the disease-related isoform of prion protein, which can be maintained on transmission to transgenic mice. 'New variant' Creutzfeldt-Jakob disease (CJD) has strain characteristics distinct from other types of CJD and which resemble those of BSE transmitted to mice, domestic cat and macaque, consistent with BSE being the source of this new disease. Strain characteristics revealed here suggest that the prion protein may itself encode disease phenotype.

Molecular basis of phenotypic variability in sporadic Creutzfeldt-Jakob disease

We sequenced the prion protein gene and studied the biochemical characteristics and the intracerebral distribution of protease-resistant prion protein with Western blot and immunohistochemistry in 19 cases of sporadic Creutzfeldt-Jakob disease. We identified four groups of subjects defined by the genotype at codon 129 of the prion protein gene, the site of a common methionine/valine polymorphism, and two types of protease-resistant prion proteins that differed in size and glycosylation. The four Creutzfeldt-Jakob disease groups showed distinct clinicopathological features that corresponded to previously described variants. The typical Creutzfeldt-Jakob disease phenotype or myoclonic variant and the Heidenhain variant were linked to methionine homozygosity at codon 129 and to "type 1" protease-resistant prion protein. The atypical and rarer variants such as that with dementia of long duration, the ataxic variant, and the variant with kuru plaques were linked to different genotypes at codon 129 and shared the "type 2" protease-resistant prion protein. Our data indicate that the sporadic form of Creutzfeldt-Jakob disease comprises a limited number of variants. The methionine/valine polymorphism at codon 129 of the prion protein gene and two types of protease-resistant prion proteins are the major determinants of these variants. These findings suggest the existence of prion strains in humans and provide the molecular basis for a novel classification of sporadic Creutzfeldt-Jakob disease.

What went wrong in BSE? From prion disease to public disaster

The recent report of 10 cases of a new variant of Creutzfeldt-Jakob disease (CJD) which could be related to bovine spongiform encephalopathy (BSE) has precipitated alarm throughout Europe. The beef trade in the UK has collapsed and the European beef market has been seriously damaged. What went wrong? Much of the difficulty of handling the BSE epidemic arose from the 4-5 year incubation period which made it difficult to ascertain whether measures taken to contain the epidemic had been effective. Public consternation and scientific equivocation arose because these prion diseases are unlike any other group of infectious diseases. Rather than being caused by a conventional micro-organism, the primary pathogenic event consists of the transformation of a normal protein (the prion protein) into an abnormal form, which can transmit disease. Prion disease is endemic in humans and sheep where it is associated with polymorphisms or mutations within the prion protein gene. Although the disease in these cases arises spontaneously, it produces an infectious prion protein. Under certain circumstances, abnormal prion protein contaminates other animals or humans resulting in epidemics of acquired prion disease. This review describes the events of the BSE epidemic and considers the difficulties in assessing the current risk to human health.