A novel human disease with abnormal prion protein sensitive to protease

Authors


Abstract

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. Ann Neurol 2008;63:697–708

Human prion diseases or transmissible spongiform encephalopathies may be sporadic, inherited, or acquired by infection.1 Creutzfeldt–Jakob disease (CJD) is the most common phenotype and occurs in all three forms. In the sporadic form, CJD is classified into five subtypes, which can be readily distinguished based on clinical features, type and distribution of brain lesions, and pattern of prion protein (PrP) immunostaining.2, 3 Fatal insomnia, a much rarer phenotype, includes sporadic and inherited forms, and is characterized by loss of ability to sleep and preferential thalamic degeneration.4 Gerstmann–Sträussler–Scheinker disease (GSS), the third phenotype, occurs exclusively as a heritable disease invariably associated with a mutation in the PrP gene open reading frame (ORF) and is characterized by the presence of prion amyloid plaques.4

Despite their heterogeneity, all sporadic human prion diseases described to date have been associated with abnormal PrP (commonly called PrPSc but henceforth referred to as PrPr), which is resistant to treatment with proteases and is considered the diagnostic hallmark of these diseases.1 PrPr is derived from normal or cellular PrP (PrPC) via a posttranslational transition from α-helical to β-sheet–rich conformations. PrPC and PrPr are quite different. Whereas PrPC is soluble in nondenaturing detergents and is completely digested when exposed to appropriate concentrations of proteinase K (PK), PrPr is detergent insoluble and its C-terminal region resists PK treatment.5 Based on the size of their PK-resistant fragments, at least three major PrPr types are recognized, which codistribute with specific disease phenotypes: (1) PrPr type 1, which on PK treatment generates an approximately 21kDa fragment; (2) PrPr type 2, generating an approximately 19kDa fragment; and (3) PrP7-8, a PrP internal fragment of 7 to 8kDa.4–6 Both PrPr types 1 and 2 have been observed associated with distinct subtypes of CJD. To date, PrP7-8 has been consistently observed only in GSS. Therefore, the conformational changes, which render PrPr pathogenic and in many but not all cases infectious, may engender different species or strains of PrPr that can be recognized based on their distinct protease-resistant fragments and by their associated clinicopathological phenotype.5, 7–12

Studies mostly based on experimental models recently have shown that PK-resistant PrP (PrPr) is associated with varying quantities of a PrP isoform that, as PrPr, is detergent insoluble but sensitive to protease digestion (PrPs).11–15 The relation of PrPs with PrPr and the role that PrPs plays in the pathogenesis of prion diseases remains uncertain.16–18

Here we report 11 patients with a human disease characterized by the presence of detergent-insoluble PrP that is predominantly sensitive to protease digestion and forms unusual immunohistochemical patterns. Furthermore, the small amount of PrPr present generates a distinct profile on immunoblot. Several affected patients have family histories of dementia but lack mutations in the PrP gene ORF. We refer to this condition as protease-sensitive prionopathy (PSPr). PSPr broadens the spectrum of human prion diseases and raises several important issues related to the nature of these diseases in light of their association with different PrP isoforms. Among prion diseases, PSPr is not rare. Because the presenting clinical signs often suggest the diagnosis of non-Alzheimer's dementia, PSPr may be even more prevalent than our data indicate because many PSPr cases might currently be classified within this group of dementias. Parts of this study have been presented previously.19

Subjects and Methods

Subjects

The 11 (10 autopsy and 1 biopsy) patients and the control subjects were referred to the National Prion Disease Pathology Surveillance Center between May 2002 and January 2006. Consent was obtained to use tissues for research, including genetic analyses.

General Tissue Processing

Fixed and frozen brain tissues were obtained from all subjects and processed as described previously.20

Histopathology and Immunohistochemistry

Samples obtained from up to 18 brain regions were processed as described previously.2, 3 Lesion profiles were constructed using semiquantitative evaluation of spongiform degeneration (SD) and astrogliosis in 12 brain regions from 6 subjects, and 4 or 5 regions from 2 subjects. SD and astrogliosis were scored (Fig 1), and the scores from each of the brain regions were summed for each subject separately; values were averaged, and standard deviations were determined and plotted according to the brain region.2 Vacuoles with larger than 4μm diameter were measured individually on random photomicrographs of frontal neocortex (10/subject, ×180) using Spotsoftware version 4.6 after calibration (Diagnostic Instruments, Sterling Heights, MI). Sections from the frontal and occipital neocortices, hippocampus, basal ganglia, thalamus, cerebellar hemisphere, and midbrain were processed for PrP immunohistochemistry with the monoclonal antibody (Mab) 3F4 or 1E4 (Cell Sciences, Canton, MA).2, 20–23 Selected brain regions were also immunostained with the Mabs 4G8 to amyloid β.24

Figure 1.

Histopathology and lesion profile. The spongiform degeneration of protease-sensitive prionopathy (PSPr) (A) is characterized by a mixture of small and intermediate size vacuoles, whereas the vacuoles of two subtypes of sporadic Creutzfeldt–Jakob disease (CJD), sCJDMM1 (B) and sCJDMM2 (C), are mostly small (sCJDMM1) or much larger and confluent (sCJDMM2). (D) Eosinophilic microstructures surrounded by a pale halo (circle) in the cerebellar molecular layer; (A–D) Hematoxylin and eosin staining. (E) Lesion profiles of PSPr (diamonds), sCJDMM1 (squares), and sCJDVV2 (triangles). Vertical bars refer to standard deviations. In sCJDMM1 and sCJDVV2, for which data were adapted from Parchi and colleagues,2 standard deviations were omitted for clarity. Spongiform degeneration was scored on a 0 to 4 scale (0 = not detectable; 1 = mild; 2 = moderate; 3 = severe; 4 = confluent); astrogliosis was scored on a 0 to 3 scale (0 = not detectable; 1 = mild; 2 = moderate; 3 = severe). FC = front cortex; TC = temporal cortex; PC = parietal cortex; OC = occipital cortex; HI = CA1 of hippocampus; EC = entorhinal cortex; BG = basal ganglia; TH = thalamus mediodorsal nucleus; MB/ST = midbrain in PSPr, substantia nigra in sCJDMM1 and sCJDVV2; LC = pons; ME = medulla; CE = cerebellar cortex.

Electron Microscopy

Formalin-fixed postmortem brain tissue was processed for conventional electron microscopy and for PrP immunohistochemistry according to standard techniques using peroxidase-antiperoxidase Mab 3F4 to PrP.25

Molecular Genetics

The entire PrP ORF was amplified by polymerase chain reaction using genomic DNA extracted from unfixed brain tissue or blood and the primers PrPO-F [GTCATYATGGCGAACCTTGG (Y = C + T)] and PrPO-R [CTCATCCCACKATCAGGAAG (K = T + G)]; sequencing was done directly or after cloning into plasmid pSTBlue 1 (Novagen, Madison, WI) by automated sequencing.22

Prion Protein Characterization

CONVENTIONAL IMMUNOBLOT.

Five to 20μl 10% wt/vol brain homogenates with or without PK digestion (Sigma Chemical, St. Louis, MO) were loaded onto 15% Tris-HCl Criterion precast gels (Bio-Rad Laboratories, Hercules, CA) for sodium dodecyl sulfate polyacrylamide gel electrophoresis, and immunoblotted with 3F4 and 1E4 to human PrP residues 109 to 112 and 97 to 108, respectively.23 PrP was deglycosylated with PNGase F (New England Biolabs, Beverly, MA) following manufacturer's instructions.

ENRICHMENT OF THE ABNORMAL PRION PROTEIN.

Two procedures were utilized: (1) capture of the abnormal PrP with the gene 5 protein (g5p), as described previously13, 23; and (2) abnormal PrP precipitation with sodium phosphotungstate.26

SEDIMENTATION OF PRION PROTEIN IN SUCROSE GRADIENTS.

Brain homogenates were incubated with 2% Sarkosyl for 30 minutes on ice, loaded atop a 10 to 60% step sucrose gradient and centrifuged 1 hour at 200,000g in a SW55 rotor (Beckman Coulter, Fullerton, CA).16, 23, 27

Statistics

Analyses were performed with the two-tail Student's t test.

Results

Clinical Features

Mean age of onset and disease duration were 62 years (range, 48–71 years) and 20 months (range, 10–60 months), respectively (Table 1). Presentation and course were dominated by neurobehavioral and psychiatric signs, with progressive motor and cognitive decline. Seven patients were ataxic. Other consistent features included absence of periodic complexes on the electroencephalogram and nondiagnostic 14-3-3 protein test in the cerebrospinal fluid. Magnetic resonance imaging showed diffuse atrophy without restricted diffusion signals in all 10 patients examined. No subject had known history of prion exposure; probable familial occurrence of dementia was reported in 6 of 10 investigated patients (see Table 1).

Table 1. Clinical Findings
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Neurohistopathology

SD and astrogliosis of moderate severity were present in the cerebral cortex, basal ganglia, and thalamus of the PSPr cases without severe neuronal loss. SD comprised a mixture of fine vacuoles, comparable with those seen in sCJDMM1 (the most common sCJD subtype), and slightly larger vacuoles that resulted in a mean vacuolar diameter greater than that of sCJDMM1 (7.8 ± 2.7 vs 5.8 ± 1.2μm). But the “larger” vacuoles clearly were smaller than the “coarse” vacuoles characteristic of sCJDMM2 (see Figs 1A–C).2, 3 The hippocampal pyramidal cell layer appeared unaffected; the molecular layer of the dentate gyrus and the stratum lacunosum moleculare showed mild SD, which extended into the subiculum and the entorhinal and inferior temporal neocortices. No kuru plaques or multicore plaques were detected. In some subjects, structures suggestive of microplaques were observed in the molecular layer of the cerebellum (see Fig 1D). Lesion profiling identified the cerebral neocortex, basal ganglia, and thalamus as the regions most severely affected, whereas the brainstem and cerebellum were apparently spared (see Fig 1E). Congo red staining of selected cerebral and cerebellar cortices was negative.

Immunohistochemistry

PrP immunostaining with Mabs 3F4 and 1E4 of the cerebral cortex, basal ganglia, and thalamus from the PSPr cases was strong, and in the hippocampal formation was selective with strong immunoreactivity in the molecular layer of the dentate and stratum lacunosum moleculare, without pyramidal cell layer staining (Figs 2A, B). The staining pattern in the cerebrum was characterized by round, loose clusters of coarse granules quite evenly distributed over a background of smaller granules (see Fig 2C). The size of the cluster-forming granules often increased progressively toward the cluster's center, which generally contained a larger granule or a tight aggregate of small granules (see Fig 2D). Strongly immunostained globular structures were occasionally seen, rarely also in the white matter (see Fig 2D, inset). Immunoreactivity in cerebellum and brainstem was limited to minute, rounded structures or aggregates of a few granules in the cerebellar molecular layer and midbrain colliculi, except for one subject who displayed a large number of these structures (see Fig 2H). The immunostained clusters and globules could not be correlated with histologically detectable lesions except for the intense immunostaining of possible microplaques in the cerebellum of some cases (see Figs 1D and Fig 2H). The pattern of PrP immunostaining of cerebrum and cerebellum in the PSPr cases was readily distinguishable from those of sCJD subtypes and nonprion disease controls (see Figs 2E–J). Furthermore, on paraffin-embedded tissues, PrP immunoreactivity was virtually removed with PK treatment (50μg/ml, 37°C, 1 hour) in these cases, whereas it was only reduced in sCJD (data not shown). Amyloid-β immunostaining showed mostly diffuse plaques apparently compatible with the subject's age.

Figure 2.

Prion protein (PrP) immunohistochemistry. (A) Intense and widespread PrP immunostain of the cerebral cortex and (B) distinctive PrP immunostaining pattern in the hippocampal gyrus with staining of the molecular layers (arrows) but not of the pyramidal cell layer or of the end plate (asterisk). (C, D) The cortical staining consists of coarse granules forming loose clusters with larger granules or a tighter aggregate of granules at the center; (D, inset) heavily stained globular structures are also present. (A–D) Protease-sensitive prionopathy (PSPr). (E, F) Immunostaining patterns of the cerebral cortex in sCJDVV2 (E) and sCJDVV1 (F) showing laminar staining and occasional perineuronal staining in sCJDVV2 and weak and fine widespread staining in sCJDVV1. (G) No immunostaining is detectable in the cerebral cortex of a nonprion disease control. (H–J) Cerebellar immunostaining patterns in PSPr (H), sCJDVV2 (I), and sCJDVV1 (J). There is intense and exclusive staining of large granules in the molecular layers in PSPr (H), presumably corresponding to the eosinophilic microstructures surrounded by a pale halo shown in Figure 1D; staining of irregular deposit limited to the granule cell layer in sCJDVV2 (I); no detectable staining in sCJDVV1 (the staining of the granule cell nuclei is nonspecific) (J). (A–I) Monoclonal antibody 3F4.

Electron Microscopy

The ultrastructural examination of the cerebellar molecular layer from the case shown in Figure 1D showed poorly defined, rounded structures with barely detectable filament-like profiles that were embedded in an amorphous-granular matrix. These formations strongly reacted with antibodies to PrP and overall had the features of poorly formed or immature PrP microplaques (Figs 3A, B).

Figure 3.

Electron microscopy (EM) of brain microstructures of protease-sensitive prionopathy (PSPr). (A) EM of the eosinophilic microstructures observed at light microscopy (Fig 1D) shows plaquelike formations with fuzzy filamentous appearance (inset). These structures are strongly reactive with antibodies to prion protein (PrP) (B) consistent with PrP microplaques (peroxidase anti-peroxidase with 3F4).

Genetic Findings

All PSPr patients were homozygous for valine at codon 129 of the PrP gene, and none carried mutations in the PrP gene ORF; three subjects had silent polymorphisms (two at codon 117 and one at codon 122).

Prion Protein Characterization: Detergent-Insoluble, Protease-Resistant, and Protease-Sensitive Prion Protein

The total PrP immunoblot profile from all PSPr patients was indistinguishable from that of nonprion disease control subjects (Fig 4A). The glycoform ratios of the three PrP bands from the two groups were similar. Measured by densitometry in arbitrary units, the diglycosylated or upper band was 10.44 ± 1.78 (n = 3) in PSPr versus 7.83 ± 3.64 (n = 5) in nonprion disease control subjects (p = 0.30); the monoglycosylated or intermediate band was 4.40 ±1.88 (n = 3) in PSPr versus 3.40 ± 2.74 (n = 5) in control subjects (p = 0.79). Under our conditions, the unglycosylated or lower band was not measurable in both PSPr patients and control subjects (see Fig 4A). Furthermore, the mean amount of total PrP present in six subjects apparently did not significantly differ from that of the nonprion disease control subjects (n = 7) (1.69 ± 0.28 vs 1.57 ± 0.39; p = 0.53) and from that of cases with prion disease (n = 3) (1.69 ± 0.28 vs 2.03 ± 0.46; p = 0.20).

Figure 4.

Characterization of prion protein (PrP) in protease-sensitive prionopathy (PSPr). (A) On conventional immunoblots, proteinase K (PK)–resistant PrP is undetectable in nonprion disease control subjects (non-PrD) and PSPr patients, although it is prominent in sporadic Creutzfeldt–Jakob disease (sCJD). (B) PK-resistant PrP from non-PrD and PSPr is not detectable even after treatment with low PK concentrations, but only in sCJD control when probed with the monoclonal antibody 3F4. (C) Subcortical regions of three PSPr cases treated with PK at 50μg/ml before Western blot analysis with 3F4 showed various amounts of PK-resistant PrP in three PSPr cases. Samples from temporal cortex (Tc) were used as controls. (D) When the same samples used in (B) are probed with 1E4, moderately PK-resistant PrP fragments forming a ladder are observed. (A, B, D) Tissues are from the frontal cortex. BH = brain homogenate; Pu = putamen; SN = substantia nigra; T1 = PrPr type 1 control; T2 = PrPr type 2 control; Th = thalamus.

In conventional diagnostic immunoblot procedures using Mab 3F4, classic PrPr (PrP27-30) was undetectable in the brain homogenates from the frontal cortex of all 11 subjects, and from the occipital and cerebellar cortices of the 7 subjects in which these brain regions were tested (see Fig 4A). Treatment with various doses of PK showed no consistent difference between these subjects and nonprion disease control subjects in these brain regions (see Fig 4B). Barely detectable amounts of approximately 6kDa PK-resistant PrP (PrP∼6) were present in the temporal cortex of three of the eight tested subjects. Of the eight subjects for whom subcortical regions (substantia nigra, putamen, and thalamus) were available, significant quantity of PK-resistant PrP27-30 was found in one case, and minimal amounts in two others (one showed small amounts of PrP∼6 only), whereas no PrPr could be definitely detected in the other five subjects (see Fig 4C). In contrast, probing with Mab 1E4 demonstrated a ladder of PK-resistant PrP fragments ranging from approximately 29 to 6kDa in all PSPr cases examined (see Fig 4D). The ladder-like electrophoretic mobility of the PrPr fragments did not match those associated with common subtypes of CJD, except for an approximately 20kDa fragment, which, after deglycosylation, was tentatively identified as the unglycosylated form of PrPr (see Fig 4C; also data not shown).2 The approximately 6kDa fragment was also unglycosylated and was reminiscent of the PrP∼7 fragment of GSS.1 These fragments were most obvious at PK concentrations of 5 to 10μg/ml and decreased at greater PK concentrations. The ladder-like electrophoretic profile of PrP treated with PK was highly reproducible and was observed in all 11 PSPr cases examined. In contrast, the PrPr fragments from sCJD were clearly detectable with both 3F4 and 1E4 Mab only after treatment with more than 10μg/ml PK, and increased with greater PK concentrations (see Fig 4C; also data not shown). Therefore, a small amount of PrPr detectable with Mab 3F4 is present mostly in the subcortical regions of these subjects. Moreover, most of the PrPr appears to have a different conformation from that of typical PrPr because on PK digestion it generates a unique set of fragments that are detected by 1E4 but not by 3F4.

Total abnormal PrP and the PrPr conformers were further characterized in abnormal PrP-enriched preparations after the capture of the abnormal PrP with g5p, a single-stranded DNA binding protein with a high affinity for abnormal PrP regardless of its PK resistance.13, 23 The amount of PrP captured by g5p in the PSPr subjects was three times greater than the amount of PrP captured in nonprion disease control subjects (data not shown), but it was nearly 16 times less than the g5p-captured PrP in typical sCJD. As measured by densitometry in arbitrary units, the mean PrP captured by g5p in eight of PSPr subjects was 3.44 ± 2.8% of the total PrP detected by direct gel loading compared with 53.55 ± 24.6% in sCJD (n = 3; p = 0.00015; Fig 5A). Furthermore, although nearly 90% of the g5p-captured PrP was resistant to PK digestion in sCJD, the PrPr accounted for only 24% of the total abnormal PrP captured in the PSPr subjects (87.59 ± 26.8% in four sCJD cases vs 24.23 ± 14.9% in nine PSPr cases; p = 0.0001) (see Fig 5A). The PK-resistant PrP obtained after PrP enrichment from the subjects was distributed in three major bands of approximately 26, 20, and 6kDa, which were detected by both 3F4 and 1E4, and matched the major bands of the immunoblot ladder detected with 1E4 on direct loading (see Fig 5A; also data not shown). A similar PrP banding pattern was obtained after sodium phosphotungstate precipitation, another method of abnormal PrP enrichment.11, 26 It was detected by both 3F4 and 1E4, although the bands were much more prominent when probed with 1E4 (see Fig 5B). The abnormal PrP enrichment experiments confirm that, in PSPr patients, there is much less abnormal PrP than in sCJD, and that the proportion of abnormal PrP that is PK resistant is much smaller.

Figure 5.

Capture by g5p (A) and sodium phosphotungstate (NaPTA) (B) of prion protein (PrP) from protease-sensitive prionopathy (PSPr) and sCJDMM1 (sporadic Creutzfeldt–Jakob disease). Probing with 3F4 or 1E4 after stripping. The same ladder of proteinase K (PK)–resistant PrP as in Figure 4D is detectable in PSPr preparations after heavy loading of the gel. S1 = supernatant of brain homogenate obtained after low-speed centrifugation (1,000g for 10 minutes).

Prion Protein Sedimentation in Sucrose Gradients

After sucrose gradient sedimentation, 30% of the total PrP from the PSPr patients was recovered in fractions 7 to 11 containing large aggregates, whereas these fractions accounted for only 5% of the total PrP in nonprion disease subjects (Figs 6A, B, E). The same fractions contained about 24 and 58% of the total PrP in GSS patients with the A117V mutation and sCJDVV1, respectively (see Figs 6C–E). Also, the percentages of PrP recovered in fractions 2 and 3 differed significantly between PSPr and nonprion disease. PSPr differed from GSS in fractions 7 and 8, and from sCJD in fractions 1, 2, and 9 to 11 (see Fig 6E). In addition to the quantitative differences, the electrophoretic profiles of the high-molecular-weight aggregates from PSPr also differed from those of nonprion disease, GSS, and sCJDVV1 subjects: the lower band was double in PSPr but single in other conditions (see Figs 6A–D). Comparable data were obtained after gel filtration fractionation, which demonstrated that PrP aggregates exceeding 2,000kDa were more abundant in PSPr than in nonprion disease control subjects but much fewer than in sCJD (data not shown).

Figure 6.

Prion protein (PrP) profiles in sucrose gradient sedimentation. (A) Nonprion disease (non-PrD); (B) protease-sensitive prionopathy (PSPr); (C) Gerstmann–Sträussler–Scheinker disease (GSS) with the A117V mutation (GSSA117V); (D) sCJDVV1; (E) PrP distribution in the fractions plotted as percentages of the total PrP. Although the amounts of PrP from PSPr are similar to those of non-PrD subjects in fractions 1 and 4 to 6, they differ significantly in fractions 2, 3, and 7 to 11, and also clearly differ from GSSA117V in fractions 7 and 8 and sCJDVV1 in fractions 1, 2, and 9 to 11. PSPr fractions 8 to 11 also have a distinctive low double band (B, arrowheads) not present in the fractions from non-PrD, GSS, and sCJDVV1. n = 6 for non-PrD (green bars); n = 6 for PSPr (red bars); n = 3 for GSS (yellow bars); and n = 7 for sCJDVV1 (blue bars). Vertical bars refer to standard deviations. Asterisks denote PrP fractions from non-PrD, GSS, and sCJDVV1 that by statistical analysis are significantly different from corresponding PSPr fractions. *p < 0.05; **p < 0.01; ***p < 0.001.

Discussion

We report 11 patients affected by a disease that involves abnormal PrP and has homogeneous and distinctive features (Table 2). Based on several lines of evidence, we argue that these features allow for the separation of this condition from all known forms of human prion disease. First, the abnormal PrP associated with this disease is predominantly, and in several brain regions almost exclusively, sensitive to protease or PrPs, and the PK-resistant PrP isoform or PrPr has a distinctive electrophoretic profile. The high sensitivity to PK and the distinctive electrophoretic profile of the abnormal PrP clearly distinguish these cases from each of the five subtypes of sCJD and from sporadic fatal insomnia (sFI), the known human sporadic prion diseases.1 For example, compared with sCJDMM1, the most common and typical sCJD,2 these cases have 16 times less total abnormal PrP, and the fraction of the total abnormal PrP that is PK resistant is nearly 4 times less. Furthermore, the ladder-like electrophoretic profile of the PrPr associated with this condition has not been observed in either sCJD or sFI, all of which instead are characterized by the presence of the well-known PrPr type 1 or 2.1 When present, the traditional PrPr, commonly called PrP27-30, was located in subcortical regions and was of type 1, another combination not observed in sporadic human prion diseases.1 Second, these cases are also homogeneous as for the PrP coding genotype because they are all homozygous for valine at codon 129 of the PrP gene, the site of a common methionine/valine polymorphism.28 Valine homozygosity in white individuals is the rarest 129 genotype, being found only in 12% of people.28 The sCJD subtypes associated with valine homozygosity, sCJDVV1 and sCJDVV2, have been well characterized and differ from these cases phenotypically and for the characteristics of the abnormal PrP.1 Third, the pattern of PrP immunostaining and the presence of structures with the features of poorly formed plaques that we observed in the cerebellum are to our knowledge unprecedented. Lastly, the clinical presentation and initial course that prominently features relatively slow cognitive deterioration, occasional gait impairment, and incontinence has evoked the diagnoses of normal pressure hydrocephalus, diffuse Lewy body disease, or frontotemporal dementia, whereas prion disease was suspected only at a later stage based on the relatively short duration.

Table 2. Summary of Protease-Sensitive Prionopathy Common Features
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Although these cases can be easily distinguished from sporadic prion diseases, some of their features such as overrepresentation of PrPs and the multiple PK-resistant PrP fragments, have been reported in GSS.4 However, all cases of GSS reported to date are associated with a mutation in the coding region of the PrP gene or immediately adjacent to it.4 None of these cases carried such mutation. Moreover, the ladder-like, PK-resistant, PrP fragments observed in our cases are preferentially detected with 1E4 but not with 3F4, which obviously separates these cases from GSS carrying the multiple PK-resistant PrP fragments. In a recent study, we observed that although 1E4 and 3F4 have adjacent epitopes along human PrP residues 97-112, their accessibility to these epitopes is different because of different neighboring N-terminal residues.29 It is possible that the 1E4 selectively detected PK-resistant PrP fragments have N-terminal starting sites that are different from those of the well-characterized PrPr types 1 and 2. The earlier evidence clearly indicates that this condition differs from GSS, although the possibility that it represents the long-sought sporadic form of GSS remains to be excluded. Six of the 10 patients with obtainable pedigree had a family history of dementia that cannot be ignored, yet none carried a mutation in the PrP gene ORF. Therefore, at least in some cases, a causative mutation may be located outside the ORF of the PrP gene, a condition never observed in human prion diseases.1

All these considerations argue that the 11 patients were affected by a novel condition involving the PrP that cannot be classified within the spectrum of currently known human prion diseases. We suggest the designation of PSPr to emphasize a major distinctive feature (see Table 2).

Compared with other human prion diseases, PSPr is not exceedingly rare, because it accounts for about 3% of all sCJD and 16% of all valine homozygous CJD accessioned by the National Prion Disease Pathology Surveillance Center during the same time period as these 11 patients, making PSPr about as common as some of the well-known sporadic prion diseases (such as sCJDMM2, sFI, and sCJDVV1).2 Furthermore, because the clinical presentation and the duration of PSPr often do not point to the diagnosis of prion disease, some cases of PSPr may currently be classified within the group of non-Alzheimer's dementias and not be investigated further. Should this be the case, PSPr may be more common than this study suggests.

The small amount of PrPr associated with PSPr and the finding that about 76% of the detectable abnormal PrP is PK sensitive not only hinders the diagnosis but also has implications concerning origin, pathogenicity, infectivity, and classification of PSPr.

The discovery of PrPs has opened a new chapter in prion diseases.11–15 The demonstration that PrPs forms smaller aggregates than the PrPr counterpart,16 and that apparently it is competent to convert PrPC to PrPr in vitro, as well as to seed the polymerization of recombinant PrP into amyloid,17, 18 suggests that PrPs shares defining features with PrPr. However, the pathogenetic mechanisms of PrPs in the absence of PrPr and, therefore, the nature of the prion diseases associated with PrPs currently remain conjectural.

Prion diseases associated with PrPs, in the presence of minimal or no PrPr, have been modeled and studied in detail in a variety of transgenic (Tg) mouse lines carrying mouse homologues of human PrP gene mutants or overexpressing PrPC.12, 30–33 Two Tg mouse models appear relevant to these cases.

In the first model, Tg mice expressing high levels of mouse PrP carrying the P101L mutation, the mouse equivalent of the human P102L mutation associated with a GSS phenotype,4, 34, 35 spontaneously developed a neurodegenerative process characterized by SD and prion plaque formation. After inoculation, they transmitted a disease phenotypically similar to P101L-mutated Tg mice but not to wild-type mice. As in our cases, the affected mice had PrPs but no, or minimal amounts of, PrPr, indicating that PrPs can be associated with a prion disease that is under certain conditions transmissible and has a histopathological phenotype displaying general features of prion diseases.12

In the second model, Tg mice carrying the P101L mutation were inoculated with brain homogenate from patients affected by a subtype of GSS P102L characterized by the exclusive presence of an approximately 8kDa PK-resistant fragment reminiscent of the approximately 6kDa fragment observed in small amounts in our cases. The inoculated Tg mice remained largely asymptomatic, but at histological examination, they displayed PrP plaques and had minimal amounts of PrPr.33 They failed to transmit the disease to wild-type mice, but inoculation to P101L-mutated mice resulted in the formation of PrP plaques in the absence of clinical disease.

These mouse models and now our cases raise issues with the definition of prion diseases. Currently, it is unclear whether PSPr is transmissible because time-consuming transmissibility experiments to different lines of Tg mice and in vitro PrP replication are still ongoing. Should PSPr not be transmissible, the question is whether it is a prion disease. A similar question can be raised for GSS, of which to date only one subtype has been shown to be consistently transmissible.4 The issue is further compounded by the recent evidence that amyloid β, the pathogenic peptide of Alzheimer's disease, has the propensity to replicate after inoculation into susceptible Tg mice in a conformation-dependent fashion reminiscent of prions.36 These findings appear to blur the once tight association of prion diseases and transmissibility. It may be more practical to apply the label of prion diseases to all conditions in which the PrP is abnormal and appears to play a central role in the pathology, as in all prion diseases known to date and in PSPr.37 In contrast, one might reserve the qualification of transmissible to those prion diseases that can be transmitted to recipients expressing relatively normal amounts of wild-type PrP.36

The finding that several PSPr patients had first-degree relatives diagnosed with dementia necessitates a search for an underlying genetic cause. In AD, the discovery of mutations outside the gene of the amyloid precursor protein (the central protein in AD, as PrP is in prion diseases) has provided a wealth of information regarding pathogenetic mechanisms of AD.38 Similarly, the discovery of a mutation outside the PrP gene ORF capable of generating a prion disease may greatly expand our understanding of pathogenetic mechanisms and the role of PrP in prion diseases.

Acknowledgements

Supported by the NIH grants AG14359 and AG08702, Centers for Disease Control and Prevention (CCU 515004), and the Britton Fund to P.G.; NIH Grant NS049173 to C.S.; and the CJD Foundation to W.Q.Z. Drs J. McGeehan and G. Kneale kindly provided g5p. Drs J. Hedreen, L. S. Honig, C. S. Calder, L. P. Goldstick, and W. Longstreth helped in obtaining the cases. P. Scalzo and D. Kofskey provided skillful histological and immunohistochemical preparations. B. Chakraborty assisted in the preparation of the manuscript and illustrations.

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