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.

A Novel TBK1 Variant (Lys694del) Presenting With Corticobasal Syndrome in a Family With FTD-ALS Spectrum Diseases: Case Report

Several variants of the TANK-Binding Kinase 1 (TBK1) gene have been associated with frontotemporal dementia - amyotrophic lateral sclerosis (FTD-ALS) spectrum diseases. Corticobasal syndrome (CBS) is characterized by asymmetric limb rigidity, dystonia or myoclonus, in association with speech or limb apraxia, cortical sensory deficit, and/or alien limb. It can result from a variety of underlying pathologies and although typically sporadic, it has been occasionally associated with MAPT and GRN variants. We describe here the proband of a family with multiple occurrences of FTD-ALS spectrum disease who developed an isolated right-sided primary asymmetric akinetic-rigid syndrome and subsequent speech and cognitive dysfunction associated with contralateral anterior temporal lobe atrophy on MRI and corresponding hypometabolism by FDG-PET. Genetic testing revealed a novel Lys694del variant of the TBK1 gene and Type A TDP-43 pathology in a predominantly frontotemporal distribution contralateral to the affected side. To our knowledge this is the first report of CBS as the initial expression of a TBK1 variant. This case emphasizes the importance of considering TBK1 genetic screening in patients with CBS, as this may be an underrepresented population on the spectrum of genetic FTD-ALS.

Generation of human chronic wasting disease in transgenic mice

Chronic wasting disease (CWD) is a cervid prion disease caused by the accumulation of an infectious misfolded conformer (PrP^Sc) of cellular prion protein (PrP^C). It has been spreading rapidly in North America and also found in Asia and Europe. Although bovine spongiform encephalopathy (i.e. mad cow disease) is the only animal prion disease known to be zoonotic, the transmissibility of CWD to humans remains uncertain. Here we report the generation of the first CWD-derived infectious human PrP^Sc by elk CWD PrP^Sc-seeded conversion of PrP^C in normal human brain homogenates using in vitro protein misfolding cyclic amplification (PMCA). Western blotting with human PrP selective antibody confirmed that the PMCA-generated protease-resistant PrP^Sc was derived from the human PrP^C substrate. Two lines of humanized transgenic mice expressing human PrP with either Val or Met at the polymorphic codon 129 developed clinical prion disease following intracerebral inoculation with the PMCA-generated CWD-derived human PrP^Sc. Diseased mice exhibited distinct PrP^Sc patterns and neuropathological changes in the brain. Our study, using PMCA and animal bioassays, provides the first evidence that CWD PrP^Sc can cross the species barrier to convert human PrP^C into infectious PrP^Sc that can produce bona fide prion disease when inoculated into humanized transgenic mice.

Impaired transmissibility of atypical prions from genetic CJDG114V

Objective

To describe the clinicopathologic, molecular, and transmissible characteristics of genetic prion disease in a young man carrying the PRNP-G114V variant.

Methods

We performed genetic, histologic, and molecular studies, combined with in vivo transmission studies and in vitro replication studies, to characterize this genetic prion disease.

Results

A 24-year-old American man of Polish descent developed progressive dementia, aphasia, and ataxia, leading to his death 5 years later. Histologic features included widespread spongiform degeneration, gliosis, and infrequent PrP plaque-like deposits within the cerebellum and putamen, best classifying this as a Creutzfeldt-Jakob disease (CJD) subtype. Molecular typing of proteinase K-resistant PrP (resPrP^Sc) revealed a mixture of type 1 (∼21 kDa) and type 2 (∼19 kDa) conformations with only 2, rather than the usual 3, PrP^Sc glycoforms. Brain homogenates from the proband failed to transmit prion disease to transgenic Tg(HuPrP) mice that overexpress human PrP and are typically susceptible to sporadic and genetic forms of CJD. When subjected to protein misfolding cyclic amplification, the PrP^Sc type 2 (∼19 kDa) was selectively amplified.

Conclusions

The features of genetic CJD^G114V suggest that residue 114 within the highly conserved palindromic region (113-AGAAAAGA-120) plays an important role in prion conformation and propagation.

IVIG Delays Onset in a Mouse Model of Gerstmann-Sträussler-Scheinker Disease

Our previous studies showed that intravenous immunoglobulin (IVIG) contained anti-Aβ autoantibodies that might be able to treat Alzheimer's disease (AD). Recently, we identified and characterized naturally occurring autoantibodies against PrP from IVIG. Although autoantibodies in IVIG blocked PrP fibril formation and PrP neurotoxicity in vitro, it remained unknown whether IVIG could reduce amyloid plaque pathology in vivo and be used to effectively treat animals with prion diseases. In this study, we used Gerstmann-Sträussler-Scheinker (GSS)-Tg (PrP-A116V) transgenic mice to test IVIG efficacy since amyloid plaque formation played an important role in GSS pathogenesis. Here, we provided strong evidence that demonstrates how IVIG could significantly delay disease onset, elongate survival, and improve clinical phenotype in Tg (PrP-A116V) mice. Additionally, in treated animals, IVIG could markedly inhibit PrP amyloid plaque formation and attenuate neuronal apoptosis at the age of 120 days in mice. Our results indicate that IVIG may be a potential, effective therapeutic treatment for GSS and other prion diseases.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

INTERPRETATION

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

Generation of a new form of human PrP(Sc) in vitro by interspecies transmission from cervid prions

Prion diseases are infectious neurodegenerative disorders that affect humans and animals and that result from the conversion of normal prion protein (PrP(C)) into the misfolded prion protein (PrP(Sc)). Chronic wasting disease (CWD) is a prion disorder of increasing prevalence within the United States that affects a large population of wild and captive deer and elk. Determining the risk of transmission of CWD to humans is of utmost importance, considering that people can be infected by animal prions, resulting in new fatal diseases. To study the possibility that human PrP(C) can be converted into the misfolded form by CWD PrP(Sc), we performed experiments using the protein misfolding cyclic amplification technique, which mimics in vitro the process of prion replication. Our results show that cervid PrP(Sc) can induce the conversion of human PrP(C) but only after the CWD prion strain has been stabilized by successive passages in vitro or in vivo. Interestingly, the newly generated human PrP(Sc) exhibits a distinct biochemical pattern that differs from that of any of the currently known forms of human PrP(Sc). Our results also have profound implications for understanding the mechanisms of the prion species barrier and indicate that the transmission barrier is a dynamic process that depends on the strain and moreover the degree of adaptation of the strain. If our findings are corroborated by infectivity assays, they will imply that CWD prions have the potential to infect humans and that this ability progressively increases with CWD spreading.

The prion diseases

The prion diseases are a family of rare neurodegenerative disorders that result from the accumulation of a misfolded isoform of the prion protein (PrP), a normal constituent of the neuronal membrane. Five subtypes constitute the known human prion diseases; kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), fatal insomnia (FI), and variant CJD (vCJD). These subtypes are distinguished, in part, by their clinical phenotype, but primarily by their associated brain histopathology. Evidence suggests these phenotypes are defined by differences in the pathogenic conformation of misfolded PrP. Although the vast majority of cases are sporadic, 10% to 15% result from an autosomal dominant mutation of the PrP gene (PRNP). General phenotype-genotype correlations can be made for the major subtypes of CJD, GSS, and FI. This paper will review some of the general background related to prion biology and detail the clinical and pathologic features of the major prion diseases, with a particular focus on the genetic aspects that result in prion disease or modification of its risk or phenotype.

The genetics of prion diseases

Prion diseases are a rare group of fatal neurodegenerative disorders of humans and animals that manifest primarily as progressive dementia and ataxia. Unique to these diseases is the prion, a misfolded isoform of the prion protein that can transmit disease from cell to cell or host to host by associating with, and transforming, normal prion protein into the misfolded isoform (the pathogenic scrapie-inducing form). Although the majority of cases occur on a sporadic basis, and rarely result from exposure to prions, such as mad cow disease, 10-15% are attributable to the presence of an autosomal dominant mutation of the prion protein gene (PRNP). Single base pair changes, or the insertion of one or more multiples of a 24 base pair repeat segment, make up the known sequence alterations of PRNP associated with genetic prion disease. The common polymorphic codon 129 of PRNP also plays an important and complex role in risk and phenotype of sporadic and genetic prion disease. This review will focus on the clinical and histopathologic features of the genetic prion diseases. Selected mutations will be highlighted as a way to illustrate general phenotype-genotype correlations.

Live cell fluorescence resonance energy transfer predicts an altered molecular association of heterologous PrPSc with PrPC

Prion diseases result from the accumulation of a misfolded isoform (PrP(Sc)) of the normal host prion protein (PrP(C)). PrP(Sc) propagates by templating its conformation onto resident PrP(C) to generate new PrP(Sc). Although the nature of the PrP(Sc)-PrP(C) complex is unresolved, certain segments or specific residues are thought to feature critically in its formation. The polymorphic residue 129 is one such site under considerable study. We combined transmission studies with a novel live cell yeast-based fluorescence resonance energy transfer (FRET) system that models the molecular association of PrP in a PrP(Sc)-like state, as a way to explore the role of residue 129 in this process. We show that a reduction in efficiency of prion transmission between donor PrP(Sc) and recipient PrP(C) that are mismatched at residue 129 correlates with a reduction in FRET between PrP-129M and PrP-129V in our yeast model. We further show that this effect depends on the different secondary structure propensities of Met and Val, rather than the specific amino acids. Finally, introduction of the disease-associated P101L mutation (mouse- equivalent) abolished FRET with wild-type mouse PrP, whereas mutant PrP-P101L displayed high FRET with homologous PrP-P101L, as long as residue 129 matched. These studies provide the first evidence for a physical alteration in the molecular association of PrP molecules differing in one or more residues, and they further predict that the different secondary structure propensities of Met and Val define the impaired association observed between PrP(Sc) and PrP(C) mismatched at residue 129.

A novel PRNP-P105S mutation associated with atypical prion disease and a rare PrPSc conformation

OBJECTIVE

To define the clinicopathologic, genetic, and pathogenic prion protein (PrP(Sc)) characteristics associated with a novel mutation of the prion protein gene (PRNP).

METHODS

The coding segment of PRNP from the proband and family members was sequenced and the brain of the proband was histologically studied. The Western blot profile of the proteinase K (PK) resistant fraction of PrP(Sc), an approximation of its conformation, or "PrP(Sc)-type," was determined.

RESULTS

We detected a novel mutation at codon 105 of PRNP that results in a serine (S) substitution of proline (P) (P105S), in a young woman who developed progressive aphasia, behavioral changes, dementia, and parkinsonism, lasting 10 years to her death. Histopathologic findings included an intense focus of multicentric PrP-plaques within the hippocampus, punctate plaques scattered throughout the cerebellum, and intense spongiform degeneration focally within the putamen, suggesting a variant of Gerstmann-Sträussler-Scheinker syndrome (GSS). However, PrP(Sc)-typing revealed two PK-resistant PrP(Sc) fragments (approximately 21 and 26 kDa), a pattern not previously detected in GSS.

CONCLUSIONS

This mutation is the third sequence variation at codon 105 of PRNP. The unusual phenotype and PrP(Sc)-type distinguishes this genetic prion disease from typical Gerstmann-Sträussler-Scheinker syndrome and other codon 105 substitutions, suggesting that, in addition to the loss of proline at this position, the PrP(Sc) conformation and phenotype is dependent on the specific amino acid substitution.

Identifying key components of the PrPC-PrPSc replicative interface

In prion disease, direct interaction between the cellular prion protein (PrP(C)) and its misfolded disease-associated conformer PrP(Sc) is a crucial, although poorly understood step promoting the formation of nascent PrP(Sc) and prion infectivity. Recently, we hypothesized that three regions of PrP (corresponding to amino acid residues 23-33, 98-110, and 136-158) interacting specifically and robustly with PrP(Sc), likely represent peptidic components of one flank of the prion replicative interface. In this study, we created epitope-tagged mouse PrP(C) molecules in which the PrP sequences 23-33, 98-110, and 136-158 were modified. These novel PrP molecules were individually expressed in the prion-infected neuroblastoma cell line (ScN2a) and the conversion of each mutated mouse PrP(C) substrate to PrP(Sc) compared with that of the epitope-tagged wild-type mouse PrP(C). Mutations within PrP 98-110, substituting all 4 wild-type lysine residues with alanine residues, prevented conversion to PrP(Sc). Furthermore, when residues within PrP 136-140 were collectively scrambled, changed to alanines, or amino acids at positions 136, 137, and 139 individually replaced by alanine, conversion to PrP(Sc) was similarly halted. However, other PrP molecules containing mutations within regions 23-33 and 101-104 were able to readily convert to PrP(Sc). These results suggest that PrP sequence comprising residues 98-110 and 136-140 not only participates in the specific binding interaction between PrP(C) and PrP(Sc), but also in the process leading to conversion of PrP(Sc)-sequestered PrP(C) into its disease-associated form.

A novel human disease with abnormal prion protein sensitive to protease

OBJECTIVE

To report a novel prion disease characterized by distinct histopathological and immunostaining features, and associated with an abnormal isoform of the prion protein (PrP) that, contrary to the common prion diseases, is predominantly sensitive to protease digestion.

METHODS

Eleven subjects were investigated at the National Prion Disease Pathology Surveillance Center for clinical, histopathological, immunohistochemical, genotypical, and PrP characteristics.

RESULTS

Patients presented with behavioral and psychiatric manifestations on average at 62 years, whereas mean disease duration was 20 months. The type of spongiform degeneration, the PrP immunostaining pattern, and the presence of microplaques distinguished these cases from those with known prion diseases. Typical protease-resistant PrP was undetectable in the cerebral neocortex with standard diagnostic procedures. After enrichment, abnormal PrP was detected at concentrations 16 times lower than common prion diseases; it included nearly 4 times less protease-resistant PrP, which formed a distinct electrophoretic profile. The subjects examined comprised about 3% of sporadic cases evaluated by the National Prion Disease Pathology Surveillance Center. Although several subjects had family histories of dementia, no mutations were found in the PrP gene open reading frame.

INTERPRETATION

The distinct histopathological, PrP immunohistochemical, and physicochemical features, together with the homogeneous genotype, indicate that this is a previously unidentified type of disease involving the PrP, which we designated "protease-sensitive prionopathy" (or PSPr). Protease-sensitive prionopathy is not rare among prion diseases, and it may be even more prevalent than our data indicate because protease-sensitive prionopathy cases are likely also to be classified within the group of non-Alzheimer's dementias.

Cytosolic prion protein toxicity is independent of cellular prion protein expression and prion propagation

Prion diseases are transmissible neurodegenerative diseases caused by a conformational isoform of the prion protein (PrP), a host-encoded cell surface sialoglycoprotein. Recent evidence suggests a cytosolic fraction of PrP (cyPrP) functions either as an initiating factor or toxic element of prion disease. When expressed in cultured cells, cyPrP acquires properties of the infectious conformation of PrP (PrP(Sc)), including insolubility, protease resistance, aggregation, and toxicity. Transgenic mice (2D1 and 1D4 lines) that coexpress cyPrP and PrP(C) exhibit focal cerebellar atrophy, scratching behavior, and gait abnormalities suggestive of prion disease, although they lack protease-resistant PrP. To determine if the coexpression of PrP(C) is necessary or inhibitory to the phenotype of these mice, we crossed Tg1D4(Prnp(+/+)) mice with PrP-ablated mice (TgPrnp(o/o)) to generate Tg1D4(Prnp(o/o)) mice and followed the development of disease and pathological phenotype. We found no difference in the onset of symptoms or the clinical or pathological phenotype of disease between Tg1D4(Prnp(+/+)) and Tg1D4(Prnp(o/o)) mice, suggesting that cyPrP and PrP(C) function independently in the disease state. Additionally, Tg1D4(Prnp(o/o)) mice were resistant to challenge with mouse-adapted scrapie (RML), suggesting cyPrP is inaccessible to PrP(Sc). We conclude that disease phenotype and cellular toxicity associated with the expression of cyPrP are independent of PrP(C) and the generation of typical prion disease.

The charge structure of helix 1 in the prion protein regulates conversion to pathogenic PrPSc

The prion diseases are transmissible neurodegenerative disorders linked to a pathogenic conformer (PrP(Sc)) of the normal prion protein (PrP(C)). Accumulation of PrP(Sc) occurs via a poorly defined process in which PrP(Sc) complexes with and converts endogenous PrP(C) to nascent PrP(Sc). Recent experiments have focused on the highly charged first alpha helix (H1) of PrP. It has been proposed that two putative asparagine-to-arginine intrahelical salt bridges stabilize H1 in PrP(C) yet form intermolecular ionic bonds with adjacent PrP molecules during conversion of PrP(C) to PrP(Sc) (M. P. Morrissey and E. I. Shakhnovich, Proc. Natl. Acad. Sci. USA 96:11293-11298, 1999). Subsequent work (J. O. Speare et al., J. Biol. Chem. 278:12522-12529, 2003 using a cell-free assay of PrP(Sc) conversion suggested that rather than promoting conversion, the salt bridges stabilize PrP(C) against it. However, the role of individual H1 charges in PrP(Sc) generation has not yet been investigated. To approach this question, we systematically reversed or neutralized each charged residue in H1 and tested the effect on conversion to PrP(Sc) in scrapie-infected murine neuroblastoma (ScN2a) cells. We find that replacements of charged H1 residues with like charges permit conversion, while charge reversals hinder it. Neutralization of charges in the N-terminal (amino acids 143 to 146) but not the C-terminal (amino acids 147 to 151) half of H1 permits conversion, while complete reversal of charge orientation of the putative salt bridges produces a nonconvertible PrP. Circular dichroism spectroscopy studies and confocal microscopy immunofluorescence localization studies indicated that charge substitutions did not alter the secondary structure or cell surface expression of PrP(C). These data support the necessity of specific charge orientations in H1 for a productive PrP(Sc)-PrP(C) complex.

Tracking the elusive prion

Prion diseases are infectious neurodegenerative fatal disorders. There are currently no treatments or cures. Considerable evidence suggests that the infectious agent is an abnormally folded protein that promotes or seeds its normal cellular isoform to fold into the infectious form. However, the precise mechanism and factors involved in this conversion remain unknown. A major stumbling block to further investigation has been the inability to seed the formation of new infectious material in vitro. Now, however, infectious material has been generated in a cell-free system. Although this system uses cell lysate rather than pure proteins, it nevertheless opens the door to the elucidation of targets of intervention and the development of useful diagnostic and therapeutic approaches.

Prion protein codon 129 genotype prevalence is altered in primary progressive aphasia

The prion protein (PrP) is central to the prion diseases, although a role in other neurodegenerative diseases has been postulated. A common polymorphism (Met or Val) at codon 129 of the PrP gene (PRNP) features prominently in the risk and phenotype, of prion disease, and an abnormality in its distribution frequency may signal a role for PrP in other diseases. We conducted a case-control study to compare the PRNP codon 129 genotype distribution in Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), and primary progressive aphasia (PPA), including 281 AD, 256 ALS, 39 PPA, and 415 healthy control subjects. Statistical analysis was applied to determine the presence or absence of disease-specific genotype associations. The distribution of codon 129 genotypes was similar among healthy control, AD, and ALS subjects, although the heterozygous state was significantly overrepresented (age-adjusted odds ratio, 8.47) in PPA, a rare condition of unknown cause. Although these findings do not entirely exclude a role for PrP in AD or ALS, they do not support the codon 129 genotype as a risk factor for either disease. However, the strong association between heterozygosity and PPA raises new questions about its cause and the role of PrP in other neurodegenerative diseases.

The AGAAAAGA palindrome in PrP is required to generate a productive PrPSc-PrPC complex that leads to prion propagation

The molecular hallmark of prion disease is the conversion of normal prion protein (PrPC) to an insoluble, proteinase K-resistant, pathogenic isoform (PrPSc). Once generated, PrPSc propagates by complexing with, and transferring its pathogenic conformation onto, PrPC. Defining the specific nature of this PrPSc-PrPC interaction is critical to understanding prion genesis. To begin to approach this question, we employed a prion-infected neuroblastoma cell line (ScN2a) combined with a heterologous yeast expression system to independently model PrPSc generation and propagation. We additionally applied fluorescence resonance energy transfer analysis to the latter to specifically study PrP-PrP interactions. In this report we focus on an N-terminal hydrophobic palindrome of PrP (112-AGAAAAGA-119) thought to feature intimately in prion generation via an unclear mechanism. We found that, in contrast to wild type (wt) PrP, PrP lacking the palindrome (PrPDelta112-119) neither converted to PrPSc when expressed in ScN2a cells nor generated proteinase K-resistant PrP when expressed in yeast. Furthermore, PrPDelta112-119 was a dominant-negative inhibitor of wtPrP in ScN2a cells. Both wtPrP and PrPDelta112-119 were highly insoluble when expressed in yeast and produced distinct cytosolic aggregates when expressed as fluorescent fusion proteins (PrP::YFP). Although self-aggregation was evident, fluorescence resonance energy transfer studies in live yeast co-expressing PrPSc-like protein and PrPDelta112-119 indicated altered interaction properties. These results suggest that the palindrome is required, not only for the attainment of the PrPSc conformation but also to facilitate the proper association of PrPSc with PrPC to effect prion propagation.

Inherited prion disease caused by the V210I mutation: transmission to transgenic mice

OBJECTIVE

To describe the clinical and neuropathologic profile and determine the strain characteristics of familial Creutzfeldt-Jakob disease (fCJD) caused by a point mutation of the PRNP gene at codon 210 that results in a valine-to-isoleucine substitution in the prion protein (PrP).

METHODS

The clinicopathologic features of four individuals from the United States who died of fCJD(V210I) were compared. Transgenic (Tg) mice expressing a chimeric human-mouse PrP transgene were inoculated with brain extracts from three fCJD(V210I) cases, sporadic CJD (sCJD), fCJD(E200K), and fatal familial insomnia (FFI), to compare prion strain characteristics.

RESULTS

The clinicopathologic profile of fCJD(V210I) was variable among cases but shared similarities with sCJD. The pattern of PrP(Sc) deposition in the brains of Tg mice was similar to that caused by sCJD but different from that associated with fCJD(E200K) or FFI.

CONCLUSIONS

Each of these prion diseases is characterized by a rapidly progressive dementia with myoclonus, periodic complexes on EEG, and spongiform change without PrP plaque deposition in the brain. The occurrence of a different PrP(Sc) phenotype with each PRNP mutation argues that each respective amino acid sequence substitution produces a different prion strain.

The prion diseases

The prion diseases constitute an unusual group of neurodegenerative disorders. Although they are similar in many ways to other more common diseases, such as Alzheimer disease and amyotrophic lateral sclerosis, they are set apart on the basis of their transmissible nature. In addition to the unique feature of transmissibility, the prion diseases demonstrate that the expression of diverse disease phenotypes is possible from a common etiologic factor. This review provides the reader with a basic understanding of the nature of prions and highlights the clinical and pathologic features of these fascinating diseases.

Amyotrophy in prion diseases

Amyotrophic lateral sclerosis was once thought to be caused by persistent viral infection, partly because some patients with transmissible Creutzfeldt-Jakob disease showed prominent amyotrophy. However, in the past 15 years there has been little interest in the amyotrophy in prion diseases, and the possible link to amyotrophic lateral sclerosis has been eschewed. We analyzed case reports of prion disease published after 1968 for evidence of amyotrophy. We defined amyotrophy as clinically evident fasciculation buttressed by electromyographic results in some cases. We sought evidence of motor neuron degeneration at autopsy. Prion disease was proved by transmissibility, immunohistochemistry demonstration of protease-resistant prion protein, or finding a mutation in the prion protein gene. Amyotrophy was noted in 27 patients: 13 with sporadic Creutzfeldt-Jakob disease, 2 with familial Creutzfeldt-Jakob disease, and 12 with Gerstmann-Sträussler-Scheinker disease. Of the 27, 23 showed clinical fasciculation and 10 had electromyographic evidence of denervation. The spinal cord was examined in 8 patients: 6 showed loss of motor neurons, 1 showed vacuolation of motor neurons, and 1 reported no abnormalities. Another 23 patients had typical histopathological characteristics but lacked molecular or biochemical proof of prion disease. The total number of patients with amyotrophy and proven prion disease that we identified was 50. This case review supports the belief that amyotrophy is occasionally a prominent feature of Creutzfeldt-Jakob disease and underscores the importance of documenting lower motor neuron function and the crucial role of examining the spinal cord at autopsy in cases of prion disease.

A familial case of Alzheimer's disease without tau pathology may be linked with chromosome 3 markers

Alzheimer's disease is the most common form of dementia that occurs in later years. The diagnosis is confirmed by the pathological findings of betaA4-amyloid-containing neuritic plaques and neurofibrillary tangles, the former being present in sufficient quantity commensurate with age. Other forms of dementia are more difficult to diagnose clinically; their pathology is noted for the lack of plaques and tangles. A patient with a family history of dementia presented with the clinical signs of Alzheimer's disease which lasted for 13 years. At autopsy the brain tissue had betaA4-amyloid-containing neuritic plaques, but no neurofibrillary tangles (i.e., the tissue was negative for staining with the tau antibody). Genetic analysis of DNA from family members revealed no linkage with chromosome 17 markers, indicating that this was not frontotemporal dementia. However, there was linkage with chromosome 3 markers. Thus, this form of Alzheimer's disease with a pathology of plaques only is linked with markers on chromosome 3.

The prion diseases: Creutzfeldt-Jakob, Gerstmann-Sträussler-Scheinker, and related disorders

The prion diseases are an interesting group of neurodegenerative disorders for a variety of reasons. The most obvious is their property of transmissibility, but beyond that they constitute a fascinating example of the diversity of disease expression possible from a common etiologic factor. Thought of as "strains" in animals and phenotypes in humans, these varied expressions of prion disease are most likely due to subtle conformational changes in the pathogenic form of the prion protein. These strain-like characteristics are best exemplified in the genetic varieties of human prion disease in which specific mutations are associated with specific phenotypic profiles. This review attempts to highlight the clinical and pathologic features of the prion diseases with a particular focus on the genetic determinants that define the various familial forms and that modify sporadic and iatrogenic forms of the disease.

Creutzfeldt-Jakob disease (CJD) after blood product transfusion from a donor with CJD

We report a second case of an association between an albumin transfusion and Creutzfeldt-Jakob disease. On balance, we believe our case represents a chance and not a causal relation.

A transmembrane form of the prion protein in neurodegenerative disease

At the endoplasmic reticulum membrane, the prion protein (PrP) can be synthesized in several topological forms. The role of these different forms was explored with transgenic mice expressing PrP mutations that alter the relative ratios of the topological forms. Expression of a particular transmembrane form (termed CtmPrP) produced neurodegenerative changes in mice similar to those of some genetic prion diseases. Brains from these mice contained CtmPrP but not PrPSc, the PrP isoform responsible for transmission of prion diseases. Furthermore, in one heritable prion disease of humans, brain tissue contained CtmPrP but not PrPSc. Thus, aberrant regulation of protein biogenesis and topology at the endoplasmic reticulum can result in neurodegeneration.

Changes of hippocampal N-acetyl aspartate and volume in Alzheimer's disease. A proton MR spectroscopic imaging and MRI study

Hippocampal atrophy detected by MRI is a prominent feature of early Alzheimer's disease (AD), but it is likely that MRI underestimates the degree of hippocampal neuron loss, because reactive gliosis attenuates atrophy. We tested the hypothesis that hippocampal N-acetyl aspartate (NAA: a neuronal marker) and volume used together provide greater discrimination between AD and normal elderly than does either measure alone. We used proton MR spectroscopic imaging (1H MRSI) and tissue segmented and volumetric MR images to measure atrophy-corrected hippocampal NAA and volumes in 12 AD patients (mild to moderate severity) and 17 control subjects of comparable age. In AD, atrophy-corrected NAA from the hippocampal region was reduced by 15.5% on the right and 16.2% on the left (both p < 0.003), and hippocampal volumes were smaller by 20.1% (p < 0.003) on the right and 21.8% (p < 0.001) on the left when compared with control subjects. The NAA reductions and volume losses made independent contributions to the discrimination of AD patients from control subjects. When used separately, neither hippocampal NAA nor volume achieved to classify correctly AD patients better than 80%. When used together, however, the two measures correctly classified 90% of AD patients and 94% of control subjects. In conclusion, hippocampal NAA measured by 1H MRSI combined with quantitative measurements of hippocampal atrophy by MRI may improve diagnosis of AD.

Mutation of the prion protein gene at codon 208 in familial Creutzfeldt-Jakob disease

Four point mutations and one insertion within the prion protein (PrP) gene have been tightly linked to the development of inherited prion disease. We developed a denaturing gradient gel electrophoresis system that allowed us to screen the entire open reading frame of the PrP gene. Using this system, we found a new mutation of the PrP gene in a patient with pathologically confirmed Creutzfeldt-Jakob disease and a negative family history for dementia. DNA sequencing revealed an adenine substitution for guanine at the second position of codon 208, which results in the nonconservative substitution of histidine for arginine. The same PrP mutation was identified in another younger member of the pedigree but was not present in more than 200 alleles tested. Such findings suggest that the frequency of inherited prion disease might be higher than ascertained by clinical history alone.

Prion disease (PrP-A117V) presenting with ataxia instead of dementia

Gerstmann-Sträussler-Scheinker disease (GSS) is caused by several different point mutations of the prion protein (PrP) gene, each of which generally produces a distinct clinical phenotype. An ataxic form of GSS is genetically linked to a mutation at codon 102 (CCG-->CTG) leading to the substitution of leucine for proline, while a "telencephalic" variant of GSS, in which dementia is the predominant symptom and ataxia is minimal, has been described in two kindreds with a mutation at codon 117 (GCA-->GTG) resulting in the substitution of valine for alanine. In this report, we present a family with ataxic GSS that has, however, the same mutation at codon 117 as is present in the telencephalic variant of GSS. Other than an additional silent mutation (GCA-->GCG) at codon 117 on the normal allele, there were no other mutations detected. At the polymorphic codon 129, valine was encoded by both alleles in the proband that we studied. Why this family with prion disease (PrP-A117V) should present with ataxia instead of dementia, which was found in two previously identified families with the same PrP gene mutation, remains to be established.

Complete penetrance of Creutzfeldt-Jakob disease in Libyan Jews carrying the E200K mutation in the prion protein gene

BACKGROUND

Creutzfeldt-Jakob disease (CJD) is a prion disease which is manifest as a sporadic, inherited, and transmissible neurodegenerative disorder. The mean age at onset of CJD is approximately 60 years, and as such, many people destined to succumb undoubtedly die of other illnesses first. The delayed onset of CJD has complicated the analysis of inherited forms of the illness and led to the suggestion that mutations in the prion protein (PrP) gene are necessary but not sufficient for prion disease despite genetic linkage; indeed, an environmental factor such as a ubiquitous virus has been proposed as a second necessary factor.

MATERIALS AND METHODS

To examine what appeared to be incomplete penetrance, we applied a life-table analysis to clinical and pedigree data from a cluster population of Libyan Jews in which the E200K mutation is prevalent. The study population included 42 affected and 44 unaffected members of 13 Libyan Jewish families, all of whom possessed the E200K mutation.

RESULTS

The calculated value using life table analysis is 0.77 at age 70 which increases to 0.89 if a mutation carrier survives to age 80 and 0.96 if age 80 is surpassed.

CONCLUSIONS

These data argue that the E200K mutation alone is sufficient to cause prion disease and does so in an age-dependent manner.

Isolated fascicular abducens nerve palsy and Lyme disease

A patient with Lyme disease developed an isolated sixth nerve palsy at the end of a 1 month course of oral antibiotics. Magnetic resonance imaging disclosed high-signal abnormality at the left pontomedullary junction, implicating involvement of the distal sixth nerve fascicle. Although facial numbness ensued during a subsequent course of intravenous antibiotics, corticosteroid therapy was associated with prompt improvement of neurologic signs, suggesting an immunologic mechanism for the central nervous system dysfunction.

Activation of central mu-opioid receptors is involved in clonidine analgesia in rats

The analgesic effect of clonidine in spontaneously hypertensive rats (SHR) and in normotensive Sprague-Dawley (SD) rats was assessed by using the formalin pain test. The analgesic response of SD rats to low doses (15-60 micrograms/kg i.p.) but not to a high dose (150 micrograms/kg i.p.) of clonidine was inhibited by naloxone, 2 mg/kg i.p., and similar interaction was noted in SHR. In both rat strains, the analgesic response to low i.p. doses of clonidine was also inhibited by injection of 5 micrograms of naloxone or 7 micrograms of beta-funaltrexamine, a mu-receptor antagonist, into the lateral cerebral ventricle. I.c.v. injection of 5 micrograms of ICI 174864, a delta-receptor antagonist, potentiated or did not influence clonidine analgesia in SD rats and SHR, respectively. It is concluded that the analgesic response to clonidine involves activation of central mu-opioid receptors in both SHR and SD rats, possibly by an endogenous opioid released by clonidine.

Activation of brainstem endorphinergic neurons causes cardiovascular depression and facilitates baroreflex bradycardia

The effects of electrical stimulation of the arcuate nucleus on blood pressure, heart rate and baroreflex sensitivity were studied in urethane-anesthetized Sprague-Dawley rats. Stimulation of the mid-anterior parts of the arcuate nucleus at 80 Hz, 0.8 ms and 50-200 microA caused a biphasic, depressor/pressor, response and moderate bradycardia. Intravenous administration of a vasopressin V1-receptor antagonist eliminated the pressor component and unmasked a pure depressor response. This depressor response could be inhibited by naltrexone, 2 mg/kg i.v., by an antiserum against beta-endorphin, 100 nl injected directly into the ipsilateral nucleus tractus solitarii, or by deafferentation of the dorsal vagal complex (nucleus tractus solitarii and dorsal vagal nucleus) by an ipsilateral, dorsolateral knife-cut of the medulla oblongata. Stimulation of the arcuate nucleus at currents of 20-40 microA did not influence basal blood pressure or heart rate but potentiated the reflex bradycardia induced by phenylephrine, and this effect was completely blocked by naltrexone. It is concluded that a beta-endorphin-containing pathway projecting from the arcuate nucleus to the ipsilateral dorsal vagal complex is involved in depressor cardiovascular regulation and in the facilitation of baroreflex bradycardia.

Endorphinergic mechanism in the central cardiovascular and analgesic effects of clonidine

In urethane-anesthetized male rats, injection of 5 nmol clonidine into the nucleus of the solitary tract (NTS) causes hypotension and bradycardia. These effects are greater in spontaneously hypertensive rats (SHR) and normotensive Sprague-Dawley (SD) rats than in normotensive Wistar-Kyoto (WKY) rats. The effects of clonidine are stereoselectively inhibited by 100 ng intra-NTS naloxone in SHR and SD but not in WKY rats. In SHR, the effects of clonidine are also inhibited by intra-NTS administration of ICI 174864 (a delta-receptor antagonist) but not by beta-funaltrexamine (a mu-receptor antagonist), while in SD rats only the mu- and not the delta-antagonist was effective. Neonatal treatment of SHR with monosodium glutamate (MSG) reduced the beta-endorphin content of the arcuate nucleus and the NTS, reduced the cardiovascular effects of clonidine, and abolished their naloxone sensitivity. MSG treatment of newborn WKY reduced the beta-endorphin content of the arcuate nucleus but not the NTS and did not affect the responses to clonidine. Measurement of pain sensitivity by the formalin test indicated that clonidine was more potent as an analgesic in SHR and SD than in WKY rats, and its effect was inhibited by naloxone (2 mg/kg i.p.) in the former two strains but not in WKY. It is proposed that a naloxone-sensitive component of the cardiovascular effects of clonidine is due to release of a beta-endorphin-like opioid from the NTS, and that this mechanism is present in SHR and SD but not in WKY rats.(ABSTRACT TRUNCATED AT 250 WORDS)

On the relationship between clonidine hypotension and brain beta-endorphin in the spontaneously hypertensive rat: studies with alpha adrenergic and opiate blockers

The relationship between the centrally mediated hypotensive and bradycardic effects of clonidine to central alpha-2 adrenergic receptor activation, brain beta-endorphin (BE) release and opiate receptor activation was studied in chloralose-anesthetized spontaneously hypertensive rats (SHRs) and Wistar-Kyoto rats, using a cerebroventricular perfusion system. Prior treatment of SHRs with i.v. naloxone (2 or 4 mg/kg) or i.c.v. yohimbine (10 or 20 micrograms/kg) reduced the hypotension and bradycardia induced by i.c.v. clonidine, but in Wistar-Kyoto rats naloxone had no similar blocking effects. Prazosin (20 micrograms/kg i.c.v.) reduced the clonidine bradycardia but not the hypotension in SHRs. Hypotension in the SHRs due to i.c.v. alpha-methylnorepinephrine (20 micrograms/kg) was reduced by both naloxone and yohimbine whereas alpha-methylnorepinephrine bradycardia was reduced by yohimbine but not by naloxone. Prior hypothalamic lesions in the SHRs reduced clonidine hypotension, but not bradycardia, and interfered with naloxone blockade of the residual clonidine hypotensive effect. Clonidine lowered immunoreactive BE levels in SHR hypothalamus, medulla and pituitary but did not change BE levels in the i.c.v. perfusate. The findings support the idea that in the SHRs, clonidine hypotension results from alpha-2 adrenergic stimulation of brain, causing BE release and central opiate receptor activation, and they suggest that the hypothalamus is involved in these interactions. Also, clonidine hypotension and bradycardia appear to involve different mechanisms in brain.

An intracerebroventricular perfusion system developed for the study of centrally acting antihypertensive drugs in the rat

A method for the complete construction and employment of an intracerebroventricular perfusion system for the rat brain is described. Rats maintained on the system for up to 7 hr displayed no detectable change in cardiovascular status. Penetration of intracerebroventricular-administered methylene blue dye into all areas of the cerebroventricles demonstrated the accessibility of the injection volume throughout the ventricular system. The hypotensive and bradycardic response produced by a 1.5-microgram/kg dose of clonidine (a centrally active antihypertensive agent) administered via the intracerebroventricular implant was of considerably greater magnitude and duration than an equivalent intravenous dose. Administration of [3H]-labeled clonidine via the intracerebroventricular implant resulted in relatively minor accumulation of the drug into peripheral organs, when compared to that found in the brain. These supportive data suggest the general applicability of this approach for studying the selective central effects of pharmacologic agents in a relatively inexpensive and easily procured laboratory animal.

Acute tolerance to clonidine hypotension and bradycardia in normotensive and hypertensive rats

Tolerance to the antihypertensive effect of clonidine has been observed in humans but there are few studies showing either acute or chronic tolerance to clonidine hypotension in experimental animals. We determined whether tachyphylaxis to acute clonidine hypotension could be demonstrated in normotensive Sprague Dawley rats by i.v. injections of 2 repetitive clonidine doses of either 1.5, 3.0, 5.0, or 10.0 micrograms/kg, or in SHR rats by repeated intracerebroventricular injections of 1.5 or 3.0 micrograms/kg, while monitoring arterial blood pressure. Second doses of clonidine were given 30 minutes after recovery of arterial pressure from the first dose. At 3.0 micrograms/kg and greater, the hypotension produced by the second dose was at least 50% less than that elicited by the first dose. By contrast, no acute tachyphylaxis was observed to the decrease in tension caused by clonidine in the transmurally stimulated isolated guinea pig ileum after 2 repeated exposures to clonidine, with intervening wash, at concentrations between 5 X 10(-10) M to 5 X 10(-8) M. Also, no tachyphylaxis occurred to the initial hypertensive phase of clonidine in Sprague Dawley rats.

A cryostat-freeze drying technique for enhanced visualization of catecholamine/indoleamine-containing neurons

A modification of the histofluorescent technique is described which allows for the enhanced visualization of catecholamine/indoleamine (CA/IN)-containing neurons and fibers in CNS and peripheral tissues. Utilizing freeze-dried, cryostat sections of various tissues, the cytoplasmic content of CA/IN-containing vesicles could be localized within the boundaries of identifiable neurons with minimal indication of bioamine dispersion. The present procedure is applicable for the histological assessment of rapid changes in neuronal CA content resulting from experimental manipulations and allows for the simultaneous biochemical measurement of endogenous bioamine content in cells without interference from pharmacological blocking agents.