A novel human disease with abnormal prion protein sensitive to protease

Enhanced Creutzfeldt-Jakob disease surveillance in the older population: Assessment of a protocol for screening brain tissue donations for prion disease

Human prion diseases, including Creutzfeldt-Jakob disease (CJD), occur in sporadic, genetic, and acquired forms. Variant Creutzfeldt-Jakob disease (vCJD) first reported in 1996 in the United Kingdom (UK), resulted from contamination of food with bovine spongiform encephalopathy. There is a concern that UK national surveillance mechanisms might miss some CJD cases (including vCJD), particularly in the older population where other neurodegenerative disorders are more prevalent. We developed a highly sensitive protocol for analysing autopsy brain tissue for the misfolded prion protein (PrPSc ) associated with prion disease, which could be used to screen for prion disease in the elderly. Brain tissue samples from 331 donors to the Edinburgh Brain and Tissue Bank (EBTB), from 2005 to 2022, were analysed, using immunohistochemical analysis on fixed tissue, and five biochemical tests on frozen specimens from six brain regions, based on different principles for detecting PrPSc . An algorithm was established for classifying the biochemical results. To test the effectiveness of the protocol, several neuropathologically confirmed prion disease controls, including vCJD, were included and blinded in the study cohort. On unblinding, all the positive control cases had been correctly identified. No other cases tested positive; our analysis uncovered no overlooked prion disease cases. Our algorithm for classifying cases was effective for handling anomalous biochemical results. An overall analysis suggested that a reduced biochemical protocol employing only three of the five tests on only two brain tissue regions gave sufficient sensitivity and specificity. We conclude that this protocol may be useful as a UK-wide screening programme for human prion disease in selected brains from autopsies in the elderly. Further improvements to the protocol were suggested by enhancements of the in vitro conversion assays made during the course of this study.

Variably protease-sensitive prionopathy with methionine homozygosity at codon 129 in the prion protein gene

Variably protease-sensitive prionopathy (VPSPr) is a recently characterised rare subtype of sporadic prion disease, mainly affecting individuals with valine homozygosity at codon 129 in the prion protein gene, with only seven methionine homozygote cases reported to date. This case presents clinical, neuropathological and biochemical features of the eighth VPSPr case worldwide with methionine homozygosity at codon 129 and compares the features with the formerly presented cases.The patient, a woman in her 70s, presented with cognitive decline, impaired balance and frequent falls. Medical history and clinical presentation were suggestive of a rapidly progressive dementia disorder. MRI showed bilateral thalamic hyperintensity. Cerebrospinal fluid real-time quaking-induced conversion was negative, and the electroencephalogram was unremarkable. The diagnosis was established through post-mortem pathological examinations. VPSPr should be suspected in rapidly progressive dementia lacking typical features or paraclinical results of protein misfolding diseases.

Large-scale validation of skin prion seeding activity as a biomarker for diagnosis of prion diseases

Definitive diagnosis of sporadic Creutzfeldt-Jakob disease (sCJD) relies on the examination of brain tissues for the pathological prion protein (PrPSc). Our previous study revealed that PrPSc-seeding activity (PrPSc-SA) is detectable in skin of sCJD patients by an ultrasensitive PrPSc seed amplification assay (PrPSc-SAA) known as real-time quaking-induced conversion (RT-QuIC). A total of 875 skin samples were collected from 2 cohorts (1 and 2) at autopsy from 2-3 body areas of 339 cases with neuropathologically confirmed prion diseases and non-sCJD controls. The skin samples were analyzed for PrPSc-SA by RT-QuIC assay. The results were compared with demographic information, clinical manifestations, cerebrospinal fluid (CSF) PrPSc-SA, other laboratory tests, subtypes of prion diseases defined by the methionine (M) or valine (V) polymorphism at residue 129 of PrP, PrPSc types (#1 or #2), and gene mutations in deceased patients. RT-QuIC assays of the cohort #1 by two independent laboratories gave 87.3% or 91.3% sensitivity and 94.7% or 100% specificity, respectively. The cohort #2 showed sensitivity of 89.4% and specificity of 95.5%. RT-QuIC of CSF available from 212 cases gave 89.7% sensitivity and 94.1% specificity. The sensitivity of skin RT-QuIC was subtype dependent, being highest in sCJDVV1-2 subtype, followed by VV2, MV1-2, MV1, MV2, MM1, MM1-2, MM2, and VV1. The skin area next to the ear gave highest sensitivity, followed by lower back and apex of the head. Although no difference in brain PrPSc-SA was detected between the cases with false negative and true positive skin RT-QuIC results, the disease duration was significantly longer with the false negatives [12.0 ± 13.3 (months, SD) vs. 6.5 ± 6.4, p < 0.001]. Our study validates skin PrPSc-SA as a biomarker for the detection of prion diseases, which is influenced by the PrPSc types, PRNP 129 polymorphisms, dermatome sampled, and disease duration.

An autopsy case of variably protease-sensitive prionopathy with Met/Met homogeneity at codon 129

The typical clinical manifestations of sporadic Creutzfeldt-Jakob disease (sCJD) are rapid-progressive dementia and myoclonus. However, the diagnosis of atypical sCJD can be challenging due to its wide phenotypic variations. We report an autopsy case of variably protease-sensitive prionopathy (VPSPr) with Met/Met homogeneity at codon 129. An 81-year-old woman presented with memory loss without motor symptoms. Seventeen months after the onset, her spontaneous language production almost disappeared. Diffusion-weighted images (DWI) showed hyperintensity in the cerebral cortex while electroencephalogram (EEG) showed nonspecific change. 14-3-3 protein and real-time qualing-induced conversion (RT-QuIC) of cerebrospinal fluid were negative. She died at age 85, 3.5 years after the onset. Pathological investigation revealed spongiform change, severe neuronal loss, and gliosis in the cerebral cortex. Mild to moderate neuronal loss and gliosis were observed in the basal ganglia. PrP immunostaining revealed plaque-like, dotlike, and synaptic structures in the cerebral cortex and small plaque-like structures in the molecular layer of the cerebellum. Analysis of PRNP showed no pathogenic mutations, and Western blot examination revealed the lack of a diglycosylated band consistent with VPSPr. The present case, which is the first report on a VPSPr case in Japan, supports previously published evidence that VPSPr cases can present variable and nonspecific clinical presentations. Because a small number of VPSPr cases can show typical magnetic resonance imaging (MRI) change in sCJD. We should investigate the possibility of VPSPr in a differential diagnosis with atypical dementia that presented DWIs of high intensity in the cortex, even though 14-3-3 proteins and RT-QuIC are both negative. In addition, VPSPr cases can take a longer clinical course compared to that of sCJD, and long-term follow-up is important.

NGS study in a sicilian case series with a genetic diagnosis for Gerstmann-Sträussler-Scheinker syndrome (PRNP, p.P102L)

BACKGROUND

Gerstmann Sträussler Scheinker (GSS) is an inherited, invariably fatal prion disease. Like other human prion diseases, GSS is caused by missense mutations in the prion protein (PrP) gene (PRNP), and by the formation and overtime accumulation of the misfolded, pathogenic scrapie PrP (PrPSc). The first mutation identified in the PRNP gene, and the one blamed as the main cause of the disease, is c.C305T:p.P102L.

METHODS AND RESULTS

The Sanger sequencing method was performed on the PRNP gene for the detection of c.C305T:p.P102L mutations in a cohort of 10 subjects; moreover, a study was carried out, using Next Generation Sequencing (NGS), by sequencing a group of genes related to amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), movement disorders and dementia which show a phenotypic profile similar to that of GSS. The results obtained from the study using NGS indicate the potential role of other genetic variants which could contribute to the various GSS phenotypes.

CONCLUSIONS

In conclusion, we highlight the large clinical variability in subjects presenting with GSS and p.P102L, as well as the hypothesis that the mutation in PrP codon 102 alone is not sufficient to trigger the cardinal clinical signs of the disease; furthermore, we do not exclude the possibility that further genetic variants play a decisive role in the aspects of the various phenotypes with which GSS manifests itself.

Novel histotypes of sporadic Creutzfeldt-Jakob disease linked to 129MV genotype

The MV1 and MV2 subtypes of sporadic Creutzfeldt-Jakob disease (sCJD) are linked to the heterozygous methionine (M)/valine (V) polymorphism at codon 129 of the prion protein (PrP) gene. MV2 is phenotypically heterogeneous, whereas MV1, due to its low prevalence, is one of the least well characterized subtypes. In this study, we investigated the biochemical properties of PrPSc and phenotypic expression of cases diagnosed as sCJD MV1 and MV2. We describe four MV2 histotypes: 2C, with cortical (C) coarse pathology; 2K, with kuru (K) plaque deposits; 2C-K, with co-existing C and K histotypic features; and the novel histotype 2C-PL that mimics 2C in the cerebral cortex and cerebellum, but exhibits plaque-like (PL) PrP deposits in subcortical regions (e.g., basal nuclei, thalamus and midbrain). Histotype prevalence is highest for 2C-K (55%), intermediate for 2C (31%), and lowest for 2C-PL and 2K (7%). Nearly every MV2 case expressed both PrPSc types, with T2 being the predominant type ("MV2-1"). MV1 cases typically show a rapid disease course (≤ 4 months), and feature the 1C histotype, phenotypically identical to sCJDMM1. Co-existing PrPSc types, with T1 significantly exceeding T2 ("MV1-2"), are detected in patients diagnosed as MV1 with longer disease courses. We observed four histotypes among MV1-2 cases, including two novel histotypes: 1V, reminiscent of sCJDVV1; 1C-2C, resembling sCJDMM1-2 with predominant MM1 histotypic component; and novel histotypes 1C-2PL and 1C-2K, overall mimicking 1C in the cerebral cortex, but harboring T2 and plaque-like PrP deposits in subcortical regions (1C-2PL), and T2 and kuru plaques in the cerebellum (1C-2K). Lesion profiles of 1C, 1V, and 1C-2C are similar, but differ from 1C-2PL and 1C-2K, as the latter two groups show prominent hippocampal and nigral degeneration. We believe that the novel "C-PL" histotypes are distinct entities rather than intermediate forms between "C" and "C-K" groups, and that 1C-2PL and 1C-2K histotypes may be characterized by different T1 variants of the same size.

Intrinsic determinants of prion protein neurotoxicity in Drosophila: from sequence to (dys)function

Prion diseases are fatal brain disorders characterized by deposition of insoluble isoforms of the prion protein (PrP). The normal and pathogenic structures of PrP are relatively well known after decades of studies. Yet our current understanding of the intrinsic determinants regulating PrP misfolding are largely missing. A 3D subdomain of PrP comprising the β2-α2 loop and helix 3 contains high sequence and structural variability among animals and has been proposed as a key domain regulating PrP misfolding. We combined in vivo work in Drosophila with molecular dynamics (MD) simulations, which provide additional insight to assess the impact of candidate substitutions in PrP from conformational dynamics. MD simulations revealed that in human PrP WT the β2-α2 loop explores multiple β-turn conformations, whereas the Y225A (rabbit PrP-like) substitution strongly favors a 310-turn conformation, a short right-handed helix. This shift in conformational diversity correlates with lower neurotoxicity in flies. We have identified additional conformational features and candidate amino acids regulating the high toxicity of human PrP and propose a new strategy for testing candidate modifiers first in MD simulations followed by functional experiments in flies. In this review we expand on these new results to provide additional insight into the structural and functional biology of PrP through the prism of the conformational dynamics of a 3D domain in the C-terminus. We propose that the conformational dynamics of this domain is a sensitive measure of the propensity of PrP to misfold and cause toxicity. This provides renewed opportunities to identify the intrinsic determinants of PrP misfolding through the contribution of key amino acids to different conformational states by MD simulations followed by experimental validation in transgenic flies.

A novel subtype of sporadic Creutzfeldt-Jakob disease with PRNP codon 129MM genotype and PrP plaques

The presence of amyloid kuru plaques is a pathological hallmark of sporadic Creutzfeldt-Jakob disease (sCJD) of the MV2K subtype. Recently, PrP plaques (p) have been described in the white matter of a small group of CJD (p-CJD) cases with the 129MM genotype and carrying resPrPD type 1 (T1). Despite the different histopathological phenotype, the gel mobility and molecular features of p-CJD resPrPD T1 mimic those of sCJDMM1, the most common human prion disease. Here, we describe the clinical features, histopathology, and molecular properties of two distinct PrP plaque phenotypes affecting the gray matter (pGM) or the white matter (pWM) of sCJD cases with the PrP 129MM genotype (sCJDMM). Prevalence of pGM- and pWM-CJD proved comparable and was estimated to be ~ 0.6% among sporadic prion diseases and ~ 1.1% among the sCJDMM group. Mean age at onset (61 and 68 years) and disease duration (~ 7 months) of pWM- and pGM-CJD did not differ significantly. PrP plaques were mostly confined to the cerebellar cortex in pGM-CJD, but were ubiquitous in pWM-CJD. Typing of resPrPD T1 showed an unglycosylated fragment of ~ 20 kDa (T120) in pGM-CJD and sCJDMM1 patients, while a doublet of ~ 21-20 kDa (T121-20) was a molecular signature of pWM-CJD in subcortical regions. In addition, conformational characteristics of pWM-CJD resPrPD T1 differed from those of pGM-CJD and sCJDMM1. Inoculation of pWM-CJD and sCJDMM1 brain extracts to transgenic mice expressing human PrP reproduced the histotype with PrP plaques only in mice challenged with pWM-CJD. Furthermore, T120 of pWM-CJD, but not T121, was propagated in mice. These data suggest that T121 and T120 of pWM-CJD, and T120 of sCJDMM1 are distinct prion strains. Further studies are required to shed light on the etiology of p-CJD cases, particularly those of T120 of the novel pGM-CJD subtype.

Pick's Disease, Seeding an Answer to the Clinical Diagnosis Conundrum

Pick's disease (PiD) is a devastating neurodegenerative disease that is characterized by dementia, frontotemporal lobar degeneration, and the aggregation of 3R tau in pathognomonic inclusions known as Pick bodies. The term PiD has adopted many meanings since its conception in 1926, but it is currently used as a strictly neuropathological term, since PiD patients cannot be diagnosed during life. Due to its rarity, PiD remains significantly understudied, and subsequently, the etiology and pathomechanisms of the disease remain to be elucidated. The study of PiD and the preferential 3R tau accumulation that is unique to PiD is imperative in order to expand the current understanding of the disease and inform future studies and therapeutic development, since the lack of intervention strategies for tauopathies remains an unmet need. Yet, the lack of an antemortem diagnostic test for the disease has further complicated the study of PiD. The development of a clinical diagnostic assay for PiD will be a vital step in the study of the disease that will greatly contribute to therapeutic research, clinical trial design and patient recruitment and ultimately improve patient outcomes. Seed aggregation assays have shown great promise for becoming ante mortem clinical diagnostic tools for many proteinopathies, including tauopathies. Future research on adapting and optimizing current seed aggregation assays to successfully detect 3R tau pathogenic forms from PiD samples will be critical in establishing a 3R tau specific seed aggregation assay that can be used for clinical diagnosis and treatment evaluation.

Development of an Automated Capillary Immunoassay to Detect Prion Glycotypes in Creutzfeldt-Jakob Disease

Creutzfeldt-Jakob disease (CJD) comprises a group of transmissible neurodegenerative diseases with vast phenotypic diversity. Sporadic CJD heterogeneity is predominantly influenced by the genotype at codon 129 of the prion-encoding gene and the molecular weight of PrP^Sc fragments after protease digestion, resulting in a classification of 6 subtypes of CJD (MM1, MM2, MV1, MV2, VV1, and VV2). The majority of cases with CJD can be distinguished using this classification system. However, a number of reported CJD cases are phenotypically unique from others within their same subtype, such as variably protease-sensitive prionopathies, or exist as a mixture of subtypes within the same patient. Western blotting of brain tissue, along with the genotyping of codon 129 of the prion-encoding gene, is considered the "gold standard" for the biochemical characterization of CJD. Western blotting requires a significant amount of prion protein for detection, is labor-intensive, and is also associated with high interassay variability. In addition to these limitations, a growing body of research suggests that unique subtypes of CJD are often undetected or misdiagnosed using standard diagnostic western blotting protocols. Consequently, we successfully optimized and developed a capillary-based western assay using the JESS Simple Western (ProteinSimple) to detect and characterize prion proteins from patients with CJD. We found that this novel assay consistently differentiated CJD type 1 and type 2 cases with a limit of detection 10 to 100× higher than traditional western blotting. Cases with CJD in which type 1 and type 2 coexist within the same brain region can be detected using type 1-specific and type 2-specific antibodies, and we found that there was remarkable specificity for the detection of cases with variably protease-sensitive prionopathy. The assay presented displays outstanding sensitivity, allowing for the preservation of valuable samples and enhancing current detection methods.

Sensitive microscopic quantification of surface-bound prion infectivity for the assessment of surgical instrument decontamination procedures

BACKGROUND

Pathogenic prions (PrP^Sc) are amyloid-rich hydrophobic proteins which bind avidly to surgical surfaces and represent some of the most difficult targets during the reprocessing of reusable surgical instruments. In-vitro methods to amplify and detect the presence of otherwise undetectable prion contamination are available, but they do not measure associated infectivity. Most of these methods rely on the use of proteinase K, however this can lead to the loss of a substantial portion of PrP^Sc, potentially producing false negatives.

AIM

To develop a sensitive in-situ method without proteinase treatment for the dynamic quantification of amyloid accumulation in N2a #58 cells following 22L-prion infection from infected tissues and spiked stainless-steel surfaces.

METHODS

We spiked cultures of N2a #58 cells with the 22L prion strain in solution or dried on stainless-steel wires and directly measured the accumulation of prion amyloid aggregates over several passages using highly sensitive fluorescence microscopy.

FINDINGS

We demonstrated a 10-log dynamic range using our method to test residual prion infectivity, that was validated to show variable decontamination efficacy against prions from commercially available cleaning chemistries.

CONCLUSIONS

The new cell-based infectivity method presented here avoids partial or possibly total proteinase K digestion of PrP^Sc in samples for greater sensitivity, in addition to low cost, no ethical concerns, and adaptability to detect different prion strains. This method can be used to test cleaning chemistries' efficacy with greater sensitivity than measuring total residual proteins, which may not correlate with residual prion infectivity.

Human prion diseases and the prion protein - what is the current state of knowledge?

Prion diseases and the prion protein are only partially understood so far in many aspects. This explains the continued research on this topic, calling for an overview on the current state of knowledge. The main objective of the present review article is to provide a comprehensive up-to-date presentation of all major features of human prion diseases bridging the gap between basic research and clinical aspects. Starting with the prion protein, current insights concerning its physiological functions and the process of pathological conversion will be highlighted. Diagnostic, molecular, and clinical aspects of all human prion diseases will be discussed, including information concerning rare diseases like prion-associated amyloidoses and Huntington disease-like 1, as well as the question about a potential human threat due to the transmission of prions from prion diseases of other species such as chronic wasting disease. Finally, recent attempts to develop future therapeutic strategies will be addressed.

Challenging Cases of Neurocognitive Disorders

Dementia is broadly defined by DSM-V as cognitive decline from a previous level that impacts the patient's functioning at work or play. This broad definition does not provide information about the underlying disease process, an aspect of clinical care that is of increasing importance, as therapeutic development inches closer to effective disease-modifying treatments. The most common neurodegenerative dementias include Alzheimer's disease, dementia with Lewy bodies, frontotemporal dementia, and Parkinson's disease dementia. Although rare, the prion diseases constitute an important group of dementias that should be routinely considered in the evaluation. Over the last two decades, advances in neuroimaging, biomarker development, and neurogenetics have not only led to a better understanding of the biology of these diseases, but they have improved our awareness of less common clinical subtypes of dementia. As such, to best define the disease process, the evaluation of a patient with cognitive decline requires attention to a myriad of disease aspects, such as the primary symptom at onset (memory, language, visual perception, praxis, etc.), the age at onset (younger or older than 65 years), the rate of disease progression (weeks to months or years), the cognitive and behavioral profile (neuropsychological assessment), and involvement of physical findings. We present here three cases that highlight the decision-making process in the evaluation of patients with atypical presentations of dementia.

Sporadic Creutzfeldt-Jakob disease VM1: phenotypic and molecular characterization of a novel subtype of human prion disease

The methionine (M)—valine (V) polymorphic codon 129 of the prion protein gene (PRNP) plays a central role in both susceptibility and phenotypic expression of sporadic Creutzfeldt-Jakob diseases (sCJD). Experimental transmissions of sCJD in humanized transgenic mice led to the isolation of five prion strains, named M1, M2C, M2T, V2, and V1, based on two major conformations of the pathological prion protein (PrP^Sc, type 1 and type 2), and the codon 129 genotype determining susceptibility and propagation efficiency. While the most frequent sCJD strains have been described in codon 129 homozygosis (MM1, MM2C, VV2) and heterozygosis (MV1, MV2K, and MV2C), the V1 strain has only been found in patients carrying VV. We identified six sCJD cases, 4 in Catalonia and 2 in Italy, carrying MV at PRNP codon 129 in combination with PrP^Sc type 1 and a new clinical and neuropathological profile reminiscent of the VV1 sCJD subtype rather than typical MM1/MV1. All patients had a relatively long duration (mean of 20.5 vs. 3.5 months of MM1/MV1 patients) and lacked electroencephalographic periodic sharp-wave complexes at diagnosis. Distinctive histopathological features included the spongiform change with vacuoles of larger size than those seen in sCJD MM1/MV1, the lesion profile with prominent cortical and striatal involvement, and the pattern of PrP^Sc deposition characterized by a dissociation between florid spongiform change and mild synaptic deposits associated with coarse, patch-like deposits in the cerebellar molecular layer. Western blot analysis of brain homogenates revealed a PrP^Sc type 1 profile with physicochemical properties reminiscent of the type 1 protein linked to the VV1 sCJD subtype. In summary, we have identified a new subtype of sCJD with distinctive clinicopathological features significantly overlapping with those of the VV1 subtype, possibly representing the missing evidence of V1 sCJD strain propagation in the 129MV host genotype.

A single amino acid residue in bank vole prion protein drives permissiveness to Nor98/atypical scrapie and the emergence of multiple strain variants

Prions are infectious agents that replicate through the autocatalytic misfolding of the cellular prion protein (PrP^C) into infectious aggregates (PrP^Sc) causing fatal neurodegenerative diseases in humans and animals. Prions exist as strains, which are encoded by conformational variants of PrP^Sc. The transmissibility of prions depends on the PrP^C sequence of the recipient host and on the incoming prion strain, so that some animal prion strains are more contagious than others or are transmissible to new species, including humans. Nor98/atypical scrapie (AS) is a prion disease of sheep and goats reported in several countries worldwide. At variance with classical scrapie (CS), AS is considered poorly contagious and is supposed to be spontaneous in origin. The zoonotic potential of AS, its strain variability and the relationships with the more contagious CS strains remain largely unknown. We characterized AS isolates from sheep and goats by transmission in ovinised transgenic mice (tg338) and in two genetic lines of bank voles, carrying either methionine (BvM) or isoleucine (BvI) at PrP residue 109. All AS isolates induced the same pathological phenotype in tg338 mice, thus proving that they encoded the same strain, irrespective of their geographical origin or source species. In bank voles, we found that the M109I polymorphism dictates the susceptibility to AS. BvI were susceptible and faithfully reproduced the AS strain, while the transmission in BvM was highly inefficient and was characterized by a conformational change towards a CS-like prion strain. Sub-passaging experiments revealed that the main strain component of AS is accompanied by minor CS-like strain components, which can be positively selected during replication in both AS-resistant or AS-susceptible animals. These findings add new clues for a better comprehension of strain selection dynamics in prion infections and have wider implications for understanding the origin of contagious prion strains, such as CS.

Prions are transmissible agents responsible for fatal neurodegenerative diseases in humans and animals. Prions exist as strains, exhibiting distinct disease phenotypes and transmission properties. Some prion diseases occur sporadically with a supposedly spontaneous origin, while others are contagious and give rise to epidemics, mainly in animals. We investigated the strain properties of Nor98/atypical scrapie (AS), a sporadic prion disease of small ruminants. We found that AS was faithfully reproduced not only in a homologous context, i.e. ovinised transgenic mice, but also in an unrelated animal species, the bank vole. A natural polymorphism of the bank vole prion protein, coding for methionine (BvM) or for isoleucine (BvI) at codon 109, dictated the susceptibility of voles to AS, with BvI being highly susceptible to AS and BvM rather resistant. Most importantly, the M109I polymorphism mediated the emergence of AS-derived mutant prion strains resembling classical scrapie (CS), a contagious prion disease. Finally, by sub-passages in bank voles, we found that the main strain component of AS is accompanied by minor CS-like strain components, which can be positively selected during replication in both AS-resistant or AS-susceptible vole lines. These findings allow a better understanding of strain selection dynamics and suggest a link between sporadic and contagious prion diseases.

Ultrastructural and biochemical classification of pathogenic tau, α-synuclein and TDP-43

Intracellular accumulation of abnormal proteins with conformational changes is the defining neuropathological feature of neurodegenerative diseases. The pathogenic proteins that accumulate in patients' brains adopt an amyloid-like fibrous structure and exhibit various ultrastructural features. The biochemical analysis of pathogenic proteins in sarkosyl-insoluble fractions extracted from patients' brains also shows disease-specific features. Intriguingly, these ultrastructural and biochemical features are common within the same disease group. These differences among the pathogenic proteins extracted from patients' brains have important implications for definitive diagnosis of the disease, and also suggest the existence of pathogenic protein strains that contribute to the heterogeneity of pathogenesis in neurodegenerative diseases. Recent experimental evidence has shown that prion-like propagation of these pathogenic proteins from host cells to recipient cells underlies the onset and progression of neurodegenerative diseases. The reproduction of the pathological features that characterize each disease in cellular and animal models of prion-like propagation also implies that the structural differences in the pathogenic proteins are inherited in a prion-like manner. In this review, we summarize the ultrastructural and biochemical features of pathogenic proteins extracted from the brains of patients with neurodegenerative diseases that accumulate abnormal forms of tau, α-synuclein, and TDP-43, and we discuss how these disease-specific properties are maintained in the brain, based on recent experimental insights.

Phenotypic Heterogeneity of Variably Protease-Sensitive Prionopathy: A Report of Three Cases Carrying Different Genotypes at PRNP Codon 129

Variably protease-sensitive prionopathy is an exceedingly rare, likely underestimated, sporadic prion disease that is characterized by heterogeneous and often non-specific clinical and pathological features posing diagnostic challenges. We report the results of a comprehensive analysis of three emblematic cases carrying different genotypes at the methionine (M)/valine (V) polymorphic codon 129 in the prion protein gene (PRNP). Clinical, biochemical, and neuropathological findings highlighted the prominent role of the host genetic background as a phenotypic modulator. In particular, the PRNP codon 129 showed a remarkable influence on the physicochemical properties of the pathological prion protein (PrPSc), especially on the sensitivity to proteinase K (PK) digestion (VV > MV > MM), which variably affected the three main fragments (i.e., of 19, 17, and 7 kDa, respectively) comprising the PrPSc profile after PK digestion and immunoblotting. This, in turn, correlated with significant differences in the ratio between the 19 kDa and the 7 kDa fragments which was highest in the MM case and lowest in the VV one. The relative amount of cerebral and cerebellar PrP mini-plaques immunohistochemistry showed a similar association with the codon 129 genotype (i.e., VV > MV > MM). Clinical manifestations and results of diagnostic investigations were non-specific, except for the detection of prion seeding activity by the real-time quaking-induced conversion assay in the only cerebrospinal fluid sample that we tested (from patient 129VV).

PMCA-Based Detection of Prions in the Olfactory Mucosa of Patients With Sporadic Creutzfeldt-Jakob Disease

Sporadic Creutzfeldt-Jakob disease (sCJD) is a rare neurodegenerative disorder caused by the conformational conversion of the prion protein (PrP^C) into an abnormally folded form, named prion (or PrP^Sc). The combination of the polymorphism at codon 129 of the PrP gene (coding either methionine or valine) with the biochemical feature of the proteinase-K resistant PrP (generating either PrP^Sc type 1 or 2) gives rise to different PrP^Sc strains, which cause variable phenotypes of sCJD. The definitive diagnosis of sCJD and its classification can be achieved only post-mortem after PrP^Sc identification and characterization in the brain. By exploiting the Real-Time Quaking-Induced Conversion (RT-QuIC) assay, traces of PrP^Sc were found in the olfactory mucosa (OM) of sCJD patients, thus demonstrating that PrP^Sc is not confined to the brain. Here, we have optimized another technique, named protein misfolding cyclic amplification (PMCA) for detecting PrP^Sc in OM samples of sCJD patients. OM samples were collected from 27 sCJD and 2 genetic CJD patients (E200K). Samples from 34 patients with other neurodegenerative disorders were included as controls. Brains were collected from 26 sCJD patients and 16 of them underwent OM collection. Brain and OM samples were subjected to PMCA using the brains of transgenic mice expressing human PrP^C with methionine at codon 129 as reaction substrates. The amplified products were analyzed by Western blot after proteinase K digestion. Quantitative PMCA was performed to estimate PrP^Sc concentration in OM. PMCA enabled the detection of prions in OM samples with 79.3% sensitivity and 100% specificity. Except for a few cases, a predominant type 1 PrP^Sc was generated, regardless of the tissues analyzed. Notably, all amplified PrP^Sc were less resistant to PK compared to the original strain. In conclusion, although the optimized PMCA did not consent to recognize sCJD subtypes from the analysis of OM collected from living patients, it enabled us to estimate for the first time the amount of prions accumulating in this biological tissue. Further assay optimizations are needed to faithfully amplify peripheral prions whose recognition could lead to a better diagnosis and selection of patients for future clinical trials.

Human Prion Disorders: Review of the Current Literature and a Twenty-Year Experience of the National Surveillance Center in the Czech Republic

Human prion disorders (transmissible spongiform encephalopathies, TSEs) are unique, progressive, and fatal neurodegenerative diseases caused by aggregation of misfolded prion protein in neuronal tissue. Due to the potential transmission, human TSEs are under active surveillance in a majority of countries; in the Czech Republic data are centralized at the National surveillance center (NRL) which has a clinical and a neuropathological subdivision. The aim of our article is to review current knowledge about human TSEs and summarize the experience of active surveillance of human prion diseases in the Czech Republic during the last 20 years. Possible or probable TSEs undergo a mandatory autopsy using a standardized protocol. From 2001 to 2020, 305 cases of sporadic and genetic TSEs including 8 rare cases of Gerstmann-Sträussler-Scheinker syndrome (GSS) were confirmed. Additionally, in the Czech Republic, brain samples from all corneal donors have been tested by the NRL immunology laboratory to increase the safety of corneal transplants since January 2007. All tested 6590 corneal donor brain tissue samples were negative for prion protein deposits. Moreover, the routine use of diagnostic criteria including biomarkers are robust enough, and not even the COVID-19 pandemic has negatively impacted TSEs surveillance in the Czech Republic.

Variably Protease-sensitive Prionopathy in a Middle-aged Man With Rapidly Progressive Dementia

Variably protease-sensitive prionopathy (VPSPr) is a recently described sporadic prion disease with distinctive clinical and histopathological features. We report the clinical, imaging, and neuropathological features of VPSPr in a 46-year-old right-handed man who presented with progressive cognitive decline, behavior disturbances, and a 50-pound weight loss over 6 months. The initial evaluation revealed severe cognitive impairment with no focal neurologic deficits. His cognitive, psychiatric, and behavior symptoms progressed rapidly, and he died 12 months after the initial visit. Throughout his disease course, workup for rapid progressive dementia was unremarkable except that brain MRI diffusion-weighted imaging showed persistent diffuse cortical and thalamic signal abnormalities. Sporadic Creutzfeldt-Jakob disease was highly suspected; however, two EEGs (8 months apart) demonstrated only nonspecific cerebral dysfunction. The patient's CSF 14-3-3 protein was negative at the initial visit and again 8 months later. His CSF real-time quaking-induced conversion and total tau level were normal. An autopsy of his brain was performed, and the neuropathological findings confirmed VPSPr. Our case underlines the importance of considering VPSPr in the spectrum of prion disease phenotypes when evaluating individuals with rapidly progressive dementia.

PART and ARTAG tauopathies at a relatively young age as a concomitant finding in sporadic Creutzfeldt-Jakob disease

Interactions between prion protein (PrP) and tau protein have long been discussed, especially in relation to the pathogenesis of neurodegenerative diseases. The presence of tauopathy in the genetic forms of Creutzfeldt-Jakob disease (CJD) brains is not uncommon. Molecular interactions between PrP and tau protein have been demonstrated in animal models; the role is attributed to the structural properties of misfolded isoform of the host-encoded prion protein (PrP^Sc) aggregates, especially amyloid, which contributes to the phosphorylation of tau protein, which is reflected in the frequent occurrence of tau pathology in inherited prion amyloidoses. The question is the relationship between PrP^Sc and hippocampal tau pathology without amyloid deposits (i.e. PART and ARTAG) in sporadic CJD (sCJD). The co-occurrence of these two proteinopathies in sCJD brains is quite rare. These pathological entities have been described in only a few cases of sCJD, all of them were older than 70 years. There have been speculations about the possibility of accelerating the course of pre-existing tauopathy or the possibility of accelerating the ageing process in the CJD brains. Here we present the clinical course and neuropathological findings of a patient with sCJD in whom the above mentioned tauopathies PART and ARTAG, considered to be typical for older age, were found as early as 58 years of age. According to the available information, this case represents an unusually early occurrence of age-related tauopathies not only in relation to sCJD, but also in general.

Cerebrospinal Fluid and Plasma Small Extracellular Vesicles and miRNAs as Biomarkers for Prion Diseases

Diagnosis of transmissible spongiform encephalopathies (TSEs), or prion diseases, is based on the detection of proteinase K (PK)-resistant PrP^Sc in post-mortem tissues as indication of infection and disease. Since PrP^Sc detection is not considered a reliable method for in vivo diagnosis in most TSEs, it is of crucial importance to identify an alternative source of biomarkers to provide useful alternatives for current diagnostic methodology. Ovine scrapie is the prototype of TSEs and has been known for a long time. Using this natural model of TSE, we investigated the presence of PrP^Sc in exosomes derived from plasma and cerebrospinal fluid (CSF) by protein misfolding cyclic amplification (PMCA) and the levels of candidate microRNAs (miRNAs) by quantitative PCR (qPCR). Significant scrapie-associated increase was found for miR-21-5p in plasma-derived but not in CSF-derived exosomes. However, miR-342-3p, miR-146a-5p, miR-128-3p and miR-21-5p displayed higher levels in total CSF from scrapie-infected sheep. The analysis of overexpressed miRNAs in this biofluid, together with plasma exosomal miR-21-5p, could help in scrapie diagnosis once the presence of the disease is suspected. In addition, we found the presence of PrP^Sc in most CSF-derived exosomes from clinically affected sheep, which may facilitate in vivo diagnosis of prion diseases, at least during the clinical stage.

Decrease in Skin Prion-Seeding Activity of Prion-Infected Mice Treated with a Compound Against Human and Animal Prions: a First Possible Biomarker for Prion Therapeutics

Previous studies have revealed that the infectious scrapie isoform of prion protein (PrP^Sc) harbored in the skin tissue of patients or animals with prion diseases can be amplified and detected through the serial protein misfolding cyclic amplification (sPMCA) or real-time quaking-induced conversion (RT-QuIC) assays. These findings suggest that skin PrP^Sc-seeding activity may serve as a biomarker for the diagnosis of prion diseases; however, its utility as a biomarker for prion therapeutics remains largely unknown. Cellulose ethers (CEs, such as TC-5RW), widely used as food and pharmaceutical additives, have recently been shown to prolong the lifespan of prion-infected mice and hamsters. Here we report that in transgenic (Tg) mice expressing hamster cellular prion protein (PrP^C) infected with the 263K prion, the prion-seeding activity becomes undetectable in the skin tissues of TC-5RW-treated Tg mice by both sPMCA and RT-QuIC assays, whereas such prion-seeding activity is readily detectable in the skin of untreated mice. Notably, TC-5RW exhibits an inhibitory effect on the in vitro amplification of PrP^Sc in both skin and brain tissues by sPMCA and RT-QuIC. Moreover, we reveal that TC-5RW is able to directly decrease protease-resistant PrP^Sc and inhibit the seeding activity of PrP^Sc from chronic wasting disease and various human prion diseases. Our results suggest that the level of prion-seeding activity in the skin may serve as a useful biomarker for assessing the therapeutic efficacy of compounds in a clinical trial of prion diseases and that TC-5RW may have the potential for the prevention/treatment of human prion diseases.

Clinical Use of Improved Diagnostic Testing for Detection of Prion Disease

Prion diseases are difficult to recognize as many symptoms are shared among other neurologic pathologies and the full spectra of symptoms usually do not appear until late in the disease course. Additionally, many commonly used laboratory markers are non-specific to prion disease. The recent introduction of second-generation real time quaking induced conversion (RT-QuIC) has revolutionized pre-mortem diagnosis of prion disease due to its extremely high sensitivity and specificity. However, RT-QuIC does not provide prognostic data and has decreased diagnostic accuracy in some rarer, atypical prion diseases. The objective of this review is to provide an overview of the current clinical utility of fluid-based biomarkers, neurodiagnostic testing, and brain imaging in the diagnosis of prion disease and to suggest guidelines for their clinical use, with a focus on rarer prion diseases with atypical features. Recent advancements in laboratory-based testing and imaging criteria have shown improved diagnostic accuracy and prognostic potential in prion disease, but because these diagnostic tests are not sensitive in some prion disease subtypes and diagnostic test sensitivities are unknown in the event that CWD transmits to humans, it is important to continue investigations into the clinical utility of various testing modalities.

Further Characterization of Glycoform-Selective Prions of Variably Protease-Sensitive Prionopathy

Prion is an infectious protein (PrP^Sc) that is derived from a cellular glycoprotein (PrP^C) through a conformational transition and associated with a group of prion diseases in animals and humans. Characterization of proteinase K (PK)-resistant PrP^Sc by western blotting has been critical to diagnosis and understanding of prion diseases including Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker (GSS) disease in humans. However, formation as well as biochemical and biological properties of the glycoform-selective PrP^Sc in variably protease-sensitive prionopathy (VPSPr) remain poorly understood. Here we reveal that formation of the ladder-like PrP^Sc in VPSPr is a PK-dependent two-step process, which is enhanced by basic pH. Two sets of PrP^Sc fragments can be identified with antibodies directed against an intermediate or a C-terminal domain of the protein. Moreover, antibodies directed against specific PrP glycoforms reveal faster electrophoretic migrations of PrP fragments mono-glycosylated at residue 181 and 197 in VPSPr than those in sporadic CJD (sCJD). Finally, RT-QuIC assay indicates that PrP^Sc-seeding activity is lower and its lag time is longer in VPSPr than in sCJD. Our results suggest that the glycoform-selective PrP^Sc in VPSPr is associated with altered glycosylation, resulting in different PK-truncation and aggregation seeding activity compared to PrP^Sc in sCJD.

[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.

Huntington's disease: lessons from prion disorders

Decades of research on the prion protein and its associated diseases have caused a paradigm shift in our understanding of infectious agents. More recent years have been marked by a surge of studies supporting the application of these findings to a broad array of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Here, we present evidence to suggest that Huntington's disease, a monogenic disorder of the central nervous system, shares features with prion disorders and that, it too, may be governed by similar mechanisms. We further posit that these similarities could suggest that, like other common neurodegenerative disorders, sporadic forms of Huntington's disease may exist.

Exploration of genetic factors resulting in abnormal disease in cattle experimentally challenged with bovine spongiform encephalopathy

Since the discovery of bovine spongiform encephalopathy (BSE), researchers have orally challenged cattle with infected brain material to study various aspects of disease pathogenesis. Unlike most other pathogens, oral BSE challenge does not always result in the expected clinical presentation and pathology. In a recent study, steers were challenged orally with BSE and all developed clinical signs and were sacrificed and tested. However, despite a similar incubation and clinical presentation, one of the steers did not have detectable PrP^Sc in its brain. Samples from this animal were analysed for genetic differences as well as for the presence of in vitro PrP^Sc seeding activity or infectivity to determine the BSE status of this animal and the potential reasons that it was different. Seeding activity was detected in the brainstem of the abnormal steer but it was approximately one million times less than that found in the normal BSE positive steers. Intra-cranial challenge of bovinized transgenic mice resulted in no transmission of disease. The abnormal steer had different genetic sequences in non-coding regions of the PRNP gene but detection of similar genotypes in Canadian BSE field cases, that showed the expected brain pathology, suggested these differences may not be the primary cause of the abnormal result. Breed composition analysis showed a higher Hereford content in the abnormal steer as well as in two Canadian atypical BSE field cases and several additional abnormal experimental animals. This study could point towards a possible impact of breed composition on BSE pathogenesis.

Understanding and exploiting interactions between cellular proteostasis pathways and infectious prion proteins for therapeutic benefit

Several neurodegenerative diseases of humans and animals are caused by the misfolded prion protein (PrP^Sc), a self-propagating protein infectious agent that aggregates into oligomeric, fibrillar structures and leads to cell death by incompletely understood mechanisms. Work in multiple biological model systems, from simple baker's yeast to transgenic mouse lines, as well as in vitro studies, has illuminated molecular and cellular modifiers of prion disease. In this review, we focus on intersections between PrP and the proteostasis network, including unfolded protein stress response pathways and roles played by the powerful regulators of protein folding known as protein chaperones. We close with analysis of promising therapeutic avenues for treatment enabled by these studies.

The utility of bank voles for studying prion disease

The transmission of prions between species is typically an inefficient process due to the species barrier, which represents incompatibility between prion seed and substrate molecules. Bank voles (Myodes glareolus) are an exception to this rule, as they are susceptible to a diverse range of prion strains from many different animal species. In particular, bank voles can be efficiently infected with most types of human prions and have played a critical role in validating variably protease-sensitive prionopathy (VPSPr) and certain forms of Gerstmann-Sträussler-Scheinker (GSS) disease as bona fide prion disorders rather than non-transmissible proteinopathies. The bank vole prion protein (BVPrP) confers a "universal prion acceptor" phenotype when expressed in mice and when used as a substrate for in vitro prion amplification assays, indicating that the unique prion transmission properties of bank voles are mediated by BVPrP. Over-expression of BVPrP in mice can also promote the spontaneous development of prion disease, indicating that BVPrP is intrinsically prone to both spontaneous and template-directed misfolding. Here, we discuss the utility of bank voles and BVPrP for prion research and how they have provided new tools for establishing rapid animal bioassays, modeling spontaneous prion disease, standardizing prion diagnostics, and understanding the molecular basis of the species barrier.

Review: Neuropathology of non-tau frontotemporal lobar degeneration

Frontotemporal dementia (FTD) is a heterogeneous clinical syndrome associated with frontotemporal lobar degeneration (FTLD) as a relatively consistent neuropathological hallmark feature. However, the discoveries in the past decade of many of the relevant pathological proteins aggregating in human FTD brains in addition to several new FTD causing gene mutations underlined that FTD is a diverse condition on the neuropathological and genetic basis. This resulted in a novel molecular classification of these conditions based on the predominant protein abnormality and allows most cases of FTD to be placed now into one of three broad molecular subgroups; FTLD with tau, TAR DNA-binding protein 43 or FET protein accumulation (FTLD-tau, FTLD-TDP and FTLD-FET respectively). This review will provide an overview of the molecular neuropathology of non-tau FTLD, insights into disease mechanisms gained from the study of human post mortem tissue as well as discussion of current controversies in the field.

Novel regulators of PrPC expression as potential therapeutic targets in prion diseases

INTRODUCTION

Prion diseases are rare and fatal neurodegenerative disorders. The key molecular event in these disorders is the misfolding of the physiological form of the cellular prion protein, PrP^C, leading to the accumulation of a pathological isoform, PrP^Sc, with unique features. Both isoforms share the same primary sequence, lacking detectable differences in posttranslational modification, a major hurdle for their biochemical or biophysical independent characterization. The mechanism underlying the conversion of PrP^C to PrP^Sc is not completely understood, so finding an effective therapy to cure prion disorders is extremely challenging.

AREAS COVERED

This review discusses the strategies for decreasing prion replication and throws a spotlight on the relevance of PrP^C in the prion accumulation process.

EXPERT OPINION

PrP^C is the key substrate for prion pathology; hence, the most promising therapeutic approach appears to be the targeting of PrP^C to block the production of the infectious isoform. The use of RNA interference and antisense oligonucleotide technologies may offer opportunities for treatment because of their success in clinical trials for other neurodegenerative diseases.

Proteinase K resistant cores of prions and amyloids

Amyloids and their infectious subset, prions, represent fibrillary aggregates with regular structure. They are formed by proteins that are soluble in their normal state. In amyloid form, all or part of the polypeptide sequence of the protein is resistant to treatment with proteinase K (PK). Amyloids can have structural variants, which can be distinguished by the patterns of their digestion by PK. In this review, we describe and compare studies of the resistant cores of various amyloids from different organisms. These data provide insight into the fine structure of amyloids and their variants as well as raise interesting questions, such as those concerning the differences between amyloids obtained ex vivo and in vitro, as well as the manner in which folding of one region of the amyloid can affect other regions.

Molecular pathology of neurodegenerative diseases: principles and practice

Neurodegenerative diseases are characterised by selective dysfunction and progressive loss of synapses and neurons associated with pathologically altered proteins that deposit primarily in the human brain and spinal cord. Recent discoveries have identified a spectrum of distinct immunohistochemically and biochemically detectable proteins, which serve as a basis for protein-based disease classification. Diagnostic criteria have been updated and disease staging procedures have been proposed. These are based on novel concepts which recognise that (1) most of these proteins follow a sequential distribution pattern in the brain suggesting a seeding mechanism and cell-to-cell propagation; (2) some of the neurodegeneration-associated proteins can be detected in peripheral organs; and (3) concomitant presence of neurodegeneration-associated proteins is more the rule than the exception. These concepts, together with the fact that the clinical symptoms do not unequivocally reflect the molecular pathological background, place the neuropathological examination at the centre of requirements for an accurate diagnosis. The need for quality control in biomarker development, clinical and neuroimaging studies, and evaluation of therapy trials, as well as an increasing demand for the general public to better understand human brain disorders, underlines the importance for a renaissance of postmortem neuropathological studies at this time. This review summarises recent advances in neuropathological diagnosis and reports novel aspects of relevance for general pathological practice.

Variable Protease-Sensitive Prionopathy Transmission to Bank Voles

Variably protease-sensitive prionopathy (VPSPr), a recently described human sporadic prion disease, features a protease-resistant, disease-related prion protein (resPrP^D) displaying 5 fragments reminiscent of Gerstmann-Sträussler-Scheinker disease. Experimental VPSPr transmission to human PrP-expressing transgenic mice, although replication of the VPSPr resPrP^D profile succeeded, has been incomplete because of second passage failure. We bioassayed VPSPr in bank voles, which are susceptible to human prion strains. Transmission was complete; first-passage attack rates were 5%-35%, and second-passage rates reached 100% and survival times were 50% shorter. We observed 3 distinct phenotypes and resPrP^D profiles; 2 imitated sporadic Creutzfeldt-Jakob disease resPrP^D, and 1 resembled Gerstmann-Sträussler-Scheinker disease resPrPD. The first 2 phenotypes may be related to the presence of minor PrP^D components in VPSPr. Full VPSPr transmission confirms permissiveness of bank voles to human prions and suggests that bank vole PrP may efficiently reveal an underrepresented native strain but does not replicate the complex VPSPr PrP^D profile.

Novel screening approaches for human prion diseases drug discovery

Introduction: Human prion diseases are rare fatal neurodegenerative diseases caused by the misfolding and aggregation of the prion protein in the form of infectious prions. So far, these diseases are incurable. One of the major difficulties in identifying suitable drugs is the availability of robust preclinical screening methods. All molecules identified have been screened using cell-based assays and in vivo murine models. The existence of a continuum of prion strains has hampered the identification of efficacious molecules modulating the progression of different forms of the disease. Areas covered: The advent of new in vitro screening methodologies is allowing for novel strategies to develop new compounds that could interfere with a broad range of diseases. In particular, two innovative techniques named Real Time Quaking Induced Conversion (RT-QuIC) and Protein Misfolding Cyclic Amplification (PMCA) have opened new venues for testing compounds in a rapid a reproducible way. These are discussed within. Expert opinion: For human prion diseases, one major hurdle has been a well-defined screening methodology. In other animal species, cell-based assays have been employed that could replicate animal prions indefinitely. Such a tool for human prion diseases is still missing. Therefore, the advent of RT-QuIC and PMCA has proven instrumental to overcome this limitation.

Variably protease-sensitive prionopathy: A differential diagnostic consideration for dementia

Purpose of review

Prion diseases are rare neurodegenerative diseases that are caused by abnormal pathogenic agents and can affect both humans and animals. These diseases are categorized as sporadic, inherited, or acquired by infection. Clinical manifestations include psychiatric symptoms, cognitive impairment, and parkinsonism, which are similar to those of other prion diseases and frontotemporal dementia variants.

Recent findings

More recently, scientists discovered a new sporadic prion disease called variably protease-sensitive prionopathy.

Summary

The following case discusses a patient presenting with sudden onset and rapid decline in cognitive, neurobehavioral, and motor functioning and his clinical journey including treatment interventions and diagnostic confirmation.

Variably protease-sensitive prionopathy mimicking frontotemporal dementia

Sporadic prion diseases are fatal neurodegenerative disorders characterized clinically by rapidly progressive dementia and myoclonus. Variably protease‐sensitive prionopathy (VPSPr) is a recently identified sporadic human prion disorder that may present with a lengthy atypical clinical history. Here, we describe a case of VPSPr in a patient with a long history of suspected frontotemporal dementia (FTD). A 61‐year‐old man presented with speech difficulties, including naming objects and constructing multipart sentences, while there was no difficulty in comprehension. Movement abnormalities included slightly jerky pursuit, minor dysmetria of saccades and brisk reflexes. There was no family history of dementia. Later he developed swallowing difficulties and the possibility of FTD with motor neuron disease was suspected. He died at the age of 71 and his brain was donated to the London Neurodegenerative Diseases Brain Bank. The brain (1004 g) showed mild to moderate atrophy, predominantly in the frontal lobe. Histology revealed moderate spongiform microvacuolation mostly affecting the frontal and parietal cortices, but also present focally in the basal ganglia and the cerebellum. Only mild Alzheimer pathology was found by extensive immunohistochemistry, in keeping with BrainNet Europe stage II. Trans‐activation response DNA‐binding protein 43 kDa and α‐synuclein immunostains were negative. Immunostaining for prion protein (PrP) showed granular/synaptic positivity in a patchy distribution, mainly within the deeper cortex, and also revealed microplaques in the cerebellum and basal ganglia. Western blotting confirmed a low molecular weight protease‐resistant PrP band with a faint ladder‐like pattern in the absence of types 1 and 2 isoforms. These features are diagnostic of VPSPr. VPSPr can mimic various neurodegenerative conditions; diagnosis requires both PrP immunohistochemistry and Western blotting. The presence of patchy spongiform change in the absence of other neurodegenerative pathology should raise suspicion of VPSPr, even in elderly patients with a lengthy clinical history.

Identification of anti-prion drugs and targets using toxicity-based assays

Prion diseases are untreatable and invariably fatal, making the discovery of effective therapeutic interventions a priority. Most candidate molecules have been discovered based on their ability to reduce the levels of PrP^Sc, the infectious form of the prion protein, in cultured neuroblastoma cells. We have employed an alternative assay, based on an abnormal cellular phenotype associated with a mutant prion protein, to discover a novel class of anti-prion compounds, the phenethyl piperidines. Using an assay that monitors the acute toxic effects of PrP^Sc on the synapses of cultured hippocampal neurons, we have identified p38 MAPK as a druggable pharmacological target that is already being pursued for the treatment of other human diseases. Organotypic brain slices, which can propagate prions and mimic several neuropathological features of the disease, have also been used to test inhibitory compounds. An effective anti-prion regimen will involve synergistic combination of drugs acting at multiple steps of the pathogenic process, resulting not only in reduction in prion levels but also suppression of neurotoxic signaling.

Recent advances in the histo-molecular pathology of human prion disease

Prion diseases are progressive neurodegenerative disorders affecting humans and other mammalian species. The term prion, originally put forward to propose the concept that a protein could be infectious, refers to PrP^Sc , a misfolded isoform of the cellular prion protein (PrP^C ) that represents the pathogenetic hallmark of these disorders. The discovery that other proteins characterized by misfolding and seeded aggregation can spread from cell to cell, similarly to PrP^Sc , has increased interest in prion diseases. Among neurodegenerative disorders, however, prion diseases distinguish themselves for the broader phenotypic spectrum, the fastest disease progression and the existence of infectious forms that can be transmitted through the exposure to diseased tissues via ingestion, injection or transplantation. The main clinicopathological phenotypes of human prion disease include Creutzfeldt-Jakob disease, by far the most common, fatal insomnia, variably protease-sensitive prionopathy, and Gerstmann-Sträussler-Scheinker disease. However, clinicopathological manifestations extend even beyond those predicted by this classification. Because of their transmissibility, the phenotypic diversity of prion diseases can also be propagated into syngenic hosts as prion strains with distinct characteristics, such as incubation period, pattern of PrP^Sc distribution and regional severity of histopathological changes in the brain. Increasing evidence indicates that different PrP^Sc conformers, forming distinct ordered aggregates, encipher the phenotypic variants related to prion strains. In this review, we summarize the most recent advances concerning the histo-molecular pathology of human prion disease focusing on the phenotypic spectrum of the disease including co-pathologies, the characterization of prion strains by experimental transmission and their correlation with the physicochemical properties of PrP^Sc aggregates.

In Vitro Seeding Activity of Glycoform-Deficient Prions from Variably Protease-Sensitive Prionopathy and Familial CJD Associated with PrPV180I Mutation

Both sporadic variably protease-sensitive prionopathy (VPSPr) and familial Creutzfeldt-Jakob disease linked to the prion protein (PrP) V180I mutation (fCJD^V180I) have been found to share a unique pathological prion protein (PrP^Sc) that lacks the protease-resistant PrP^Sc glycosylated at residue 181 because two of four PrP glycoforms are apparently not converted into the PrP^Sc from their cellular PrP (PrP^C). To investigate the seeding activity of these unique PrP^Sc molecules, we conducted in vitro prion conversion experiments using serial protein misfolding cyclic amplification (sPMCA) and real-time quaking-induced conversion (RT-QuIC) assays with different PrP^C substrates. We observed that the seeding of PrP^Sc from VPSPr or fCJD^V180I in the sPMCA reaction containing normal human or humanized transgenic (Tg) mouse brain homogenates generated PrP^Sc molecules that unexpectedly exhibited a dominant diglycosylated PrP isoform along with PrP monoglycosylated at residue 181. The efficiency of PrP^Sc amplification was significantly higher in non-CJDMM than in non-CJDVV human brain homogenate, whereas it was higher in normal TgVV than in TgMM mouse brain homogenate. PrP^C from the mixture of normal TgMM and Tg mouse brain expressing PrP^V180I mutation (Tg180) but not TgV180I alone was converted into PrP^Sc by seeding with the VPSPr or fCJD^V180I. The RT-QuIC seeding activity of PrP^Sc from VPSPr and fCJD^V180I was significantly lower than that of sCJD. Our results suggest that the formation of glycoform-selective prions may be associated with an unidentified factor in the affected brain and the glycoform-deficiency of PrP^Sc does not affect the glycoforms of in vitro newly amplified PrP^Sc.

Unexpected prion phenotypes in experimentally transfused animals: predictive models for humans?

The recently reevaluated high prevalence of healthy carriers (1/2,000 in UK) of variant Creutzfeldt-Jakob Disease (v-CJD), whose blood might be infectious, suggests that the evolution of this prion disease might not be under full control as expected. After experimental transfusion of macaques and conventional mice with blood derived from v-CJD exposed (human and animal) individuals, we confirmed in these both models the transmissibility of v-CJD, but we also observed unexpected neurological syndromes transmissible by transfusion: despite their prion etiology confirmed through transmission experiments, these original cases would escape classical prion diagnosis, notably in the absence of detectable abnormal PrP with current techniques. It is noteworthy that macaques developed an original, yet undescribed myelopathic syndrome associating demyelination and pseudo-necrotic lesions of spinal cord, brainstem and optical tract without affecting encephalon, which is rather evocative of spinal cord disease than prion disease in human medicine. These observations strongly suggest that the spectrum of human prion diseases may extend the current field restricted to the phenotypes associated to protease-resistant PrP, and may notably include spinal cord diseases.

The Structure of PrPSc Prions

PrP^Sc (scrapie isoform of the prion protein) prions are the infectious agent behind diseases such as Creutzfeldt–Jakob disease in humans, bovine spongiform encephalopathy in cattle, chronic wasting disease in cervids (deer, elk, moose, and reindeer), as well as goat and sheep scrapie. PrP^Sc is an alternatively folded variant of the cellular prion protein, PrP^C, which is a regular, GPI-anchored protein that is present on the cell surface of neurons and other cell types. While the structure of PrP^C is well studied, the structure of PrP^Sc resisted high-resolution determination due to its general insolubility and propensity to aggregate. Cryo-electron microscopy, X-ray fiber diffraction, and a variety of other approaches defined the structure of PrP^Sc as a four-rung β-solenoid. A high-resolution structure of PrP^Sc still remains to be solved, but the four-rung β-solenoid architecture provides a molecular framework for the autocatalytic propagation mechanism that gives rise to the alternative conformation of PrP^Sc. Here, we summarize the current knowledge regarding the structure of PrP^Sc and speculate about the molecular conversion mechanisms that leads from PrP^C to PrP^Sc.

Prion diseases

The human prion diseases comprise Creutzfeldt-Jakob disease, variably protease-sensitive prionopathy, Gerstmann-Sträussler-Scheinker disease, fatal familial insomnia, and kuru. Each is a uniformly fatal rare neurodegenerative disease in which conformational changes in the prion protein are thought to be the central pathophysiologic event. The majority of cases of human prion diseases occur worldwide in the form of sporadic Creutzfeldt-Jakob disease and a minority of around 10-15% are associated with mutations of the prion protein gene, termed PRNP, in the forms of genetic Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease, and fatal familial insomnia. Prion diseases are also transmissible and occur in iatrogenic and zoonotic forms (iatrogenic Creutzfeldt-Jakob disease and variant Creutzfeldt-Jakob disease respectively), adding a public health dimension to their management. Despite having a high public profile, human prion diseases are both rare and heterogeneous in their clinicopathologic phenotype, sometimes making a diagnosis challenging. A combined clinical, genetic, neuropathologic, and biochemical approach to diagnosis is therefore essential. The intensive study of these diseases continues to inform on neurodegenerative mechanisms and the role of protein misfolding in more common neurodegenerative diseases such as Parkinson disease and Alzheimer disease.

Variably protease-sensitive prionopathy

Variably protease-sensitive prionopathy (VPSPr), originally identified in 2008, was further characterized and renamed in 2010. Thirty-seven cases of VPSPr have been reported to date, consistent with estimated prevalence of 0.7-1.7% of all sporadic prion diseases. The lack of gene mutations establishes VPSPr as a sporadic form of human prion diseases, along with sporadic Creutzfeldt-Jakob disease (sCJD) and sporadic fatal insomnia. Like sCJD, VPSPr affects patients harboring any of the three genotypes, MM, MV, and VV at the prion protein (PrP) gene polymorphic codon 129, with VPSPr VV accounting for 65% of all VPSPr cases. Distinguishing clinical features include a median 2-year duration and presentation with psychiatric signs, speech/language impairment, or cognitive decline. Neuropathology comprises moderate spongiform degeneration, PrP amyloid miniplaques, and a target-like or plaque-like PrP deposition. The abnormal PrP associated with VPSPr typically forms an electrophoretic profile of five to seven bands (according to the antibody) presenting variable protease resistance depending on the 129 genotype. The familial prion disease associated with the V180I PrP gene mutation which harbors an abnormal PrP with similar electrophoretic profile might serve as a model for VPSPr. Transmission to animals has definitively established VPSPr as a prion disease. Because of its recent identification, rarity, and the elusiveness of its abnormal PrP, VPSPr remains largely understudied.

Human transmissible spongiform encephalopathies: historic view

The first of several pivotal moments leading to current understanding of human transmissible spongiform encephalopathies (TSEs) occurred in 1959 when veterinary pathologist W.J. Hadlow first recognized several similarities between scrapie-a slow infection of sheep caused by an unusual infectious agent-and kuru, a fatal exotic neurodegenerative disease affecting only people of a single language group in the remote mountainous interior of New Guinea, described two years earlier by D.C. Gajdusek and V. Zigas. Based on the knowledge of scrapie, Gajdusek, C.J. Gibbs, Jr., and M.P. Alpers soon initiated efforts to transmit kuru by inoculating kuru brain tissue into non-human primates, that-although requiring several years-ultimately proved successful. In the same year that Hadlow first proposed that kuru and scrapie might have similar etiology, I. Klatzo noted that kuru's histopathology resembled that of Creutzfeldt-Jakob disease (CJD), another progressive fatal neurodegenerative disease of unknown etiology that A.M. Jakob had first described in 1921. Gajdusek and colleagues went on to demonstrate that not only the more common sporadic form of CJD but also familial CJD and a generally similar familial brain disease (Gerstmann-Sträussler-Scheinker syndrome) were also transmissible, first to non-human primates and later to other animals. (Other investigators later transmitted an even rarer brain disease, fatal familial insomnia, to animals.) Iatrogenic CJD (spread by human pituitary-derived hormones and tissue grafts) was also transmitted to animals. Much later, in 1996, a new variant of CJD was attributed to human infection with the agent of bovine spongiform encephalopathy; vCJD itself caused an iatrogenic TSE spread by blood transfusion (and probably by a human-plasma-derived clotting factor). Starting in the 1930s, the scrapie agent was found to have a unique constellation of physical properties (marked resistance to inactivation by chemicals, heat and radiation), eventually interpreted as suggesting that it might be an unconventional self-replicating pathogen based on protein and containing no nucleic acid. The work of S.B. Prusiner led to the recognition in the early 1980s that a misfolded form of a ubiquitous normal host protein was usually if not always detectable in tissues containing TSE agents, greatly facilitating the diagnosis and TSEs and understanding their pathogenesis. Prusiner proposed that the TSE agent was likely to be composed partly if not entirely of the abnormal protein, for which he coined the term "prion" protein and "prion" for the agent. Expression of the prion protein by animals-while not essential for life-was later found to be obligatory to infect them with TSEs, and a variety of mutations in the protein clearly tracked with TSEs in families, explaining the autosomal dominant pattern of disease and confirming a central role for the protein in pathogenesis. Prusiner's terminology and the prion hypothesis came to be widely though not universally accepted. A popular corollary proposal, that prions arise by spontaneous misfolding of normal prion protein leading to sporadic cases of CJD, BSE, and scrapie, is more problematic and may serve to discourage continued search for environmental sources of exposure to TSE agents.

Characterization of mutations in PRNP (prion) gene and their possible roles in neurodegenerative diseases

Abnormal prion proteins are responsible for several fatal neurodegenerative diseases in humans and in animals, including Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker disease, and fatal familial insomnia. Genetics is important in prion diseases, but in the most cases, cause of diseases remained unknown. Several mutations were found to be causative for prion disorders, and the effect of mutations may be heterogeneous. In addition, different prion mutations were suggested to play a possible role in additional phenotypes, such as Alzheimer's type pathology, spongiform encephalopathy, or frontotemporal dementia. Pathogenic nature of several prion mutations remained unclear, such as M129V and E219K. These two polymorphic sites were suggested as either risk factors for different disorders, such as Alzheimer's disease (AD), variant CJD, or protease-sensitive prionopathy, and they can also be disease-modifying factors. Pathological overlap may also be possible with AD or progressive dementia, and several patients with prion mutations were initially diagnosed with AD. This review also introduces briefly the diagnosis of prion diseases and the issues with their diagnosis. Since prion diseases have quite heterogeneous phenotypes, a complex analysis, a combination of genetic screening, cerebrospinal fluid biomarker analysis and imaging technologies could improve the early disease diagnosis.