Atypical Creutzfeldt-Jakob disease with PrP-amyloid plaques in white matter: molecular characterization and transmission to bank voles show the M1 strain signature

Amyloid plaques formed by abnormal prion protein (PrPSc) aggregates occur with low frequency in Creutzfeldt-Jakob disease, but represent a pathological hallmark of three relatively rare disease histotypes, namely variant CJD, sporadic CJDMV2K (methionine/valine at PRNP codon 129, PrPSc type 2 and kuru-type amyloid plaques) and iatrogenic CJDMMiK (MM at codon 129, PrPSc of intermediate type and kuru plaques). According to recent studies, however, PrP-amyloid plaques involving the subcortical and deep nuclei white matter may also rarely occur in CJDMM1 (MM at codon 129 and PrPSc type 1), the most common CJD histotype. To further characterize the phenotype of atypical CJDMM1 with white matter plaques (p-CJDMM1) and unravel the basis of amyloid plaque formation in such cases, we compared clinical and histopathological features and PrPSc physico-chemical properties between 5 p-CJDMM1 and 8 typical CJDMM1 brains lacking plaques. Furthermore, transmission properties after bioassay in two genetic lines of bank voles were also explored in the two groups. All 5 p-CJDMM1 cases had a disease duration longer than one year. Three cases were classified as sporadic CJDMM1, one as sporadic CJDMM1 + 2C and one as genetic CJDE200K-MM1. Molecular mass, protease sensitivity and thermo-solubilization of PrPSc aggregates did not differ between p-CJDMM1 and classical CJDMM1 cases. Likewise, transmission properties such as incubation time, lesion profile and PrPSc properties in bank voles also matched in the two groups. The present data further define the clinical-pathologic phenotype of p-CJDMM1, definitely establish it as a distinctive CJD histotype and demonstrate that PrP-plaque formation in this histotype is not a strain-specific feature. Since cases lacking amyloid plaques may also manifest a prolonged (i.e. > than one year) disease course, unidentified, host-specific factors likely play a significant role, in addition to disease duration, in generating white matter PrP-amyloid plaques in p-CJDMM1.


Introduction
Prion diseases are a group of neurodegenerative disorders of humans and other mammals characterized by misfolding of the cellular prion protein (PrP C ). In the disease, PrP C is structurally converted into a pathogenic isoform, called scrapie prion protein (PrP Sc ), showing an increase in β-sheet content and a partial resistance to proteases in its C-terminal region [27]. As a consequence of PrP C conversion, oligomers and amyloid fibrils of aggregated PrP Sc accumulate in the CNS, leading to neurodegeneration.
Sporadic Creutzfeldt-Jakob disease (sCJD), the most common prion disease in humans, can be classified into 6 major phenotypic variants, according to molecular, histopathological, and clinical features [21,24,25]. These variants or histotypes largely correlate at molecular level with the genotype at the polymorphic PRNP codon 129, encoding for methionine (M) or valine (V), and the relative molecular mass of PrP Sc core fragment generated after proteolytic digestion, which can be 21 (type 1) or 19 kDa (type 2) [22]. These are C-terminal fragments that differ from each other for an epitope spanning residues 82-96, which is present in type 1 and removed in type 2. Other physico-chemical properties distinguishing PrP Sc aggregates among sCJD variants, associated with either type 1 or type 2, include the relative amount of the truncated C-terminal fragments, named CTF12-13 based on their molecular mass, and the socalled glycoform ratio, that is the ratio among the three differently glycosylated (e.g. di-, mono-, and unglycosylated) PrP Sc forms [20,22,32].
Five out of six of these major sCJD variants were shown to propagate in syngeneic hosts as distinct prion strains [2,17,23]. These are defined as natural isolates of infectious prions characterized by distinctive clinical and neuropathological features, which are faithfully recapitulated upon serial passage within the same host genotype [3,4]. As the only exception, sCJDVV2 and MV2K converged to a single phenotype/strain after experimental transmission [15,23], suggesting a host-genotypic effect determined by codon 129. Interestingly, the strain isolated from sCJDMV2K and VV2, currently designated as V2, has also been associated with kuru as well many iatrogenic cases of CJD secondary to contaminated growth hormone or dura mater grafts (d-CJD) [13,23,28]. Moreover, at variance with sCJD, iatrogenic CJD patients linked to the V2 strain include subjects carrying MM at codon 129 in addition to those carrying VV or MV [13,14,28].
PrP-amyloid plaques represent a distinctive histopathological feature in CJD since they show a strong correlation with both prion strain and PRNP genotype. The presence of florid plaques is a well-documented signature of vCJD (BSE strain) [31], while kuru-type plaques are the hallmark of the CJD V2 strain, although only in subjects carrying MV or MM at PRNP codon 129, since they are virtually lacking in those carrying VV despite the widespread focal PrP plaque-like deposits [24,28].
Experimental transmissions have linked sCJDMM1 to a distinctive prion strain, named M1 [2,23], which is typically associated with a diffuse, synaptic type of PrP deposition rather than with focal plaque-like protein aggregates. As a significant exception, however, Kobayashi et al. [12] described 3 sCJD cases, all with a relatively long disease duration and quite severe pathology, resembling the MM1 subtype in most features but the presence of PrP-amyloid plaques in both subcortical and deep nuclei white matter. This observation raises questions about the origin of this phenotype, namely the role of disease duration, prion strain and host genetic background in the formation of white matter PrP plaques.
To contribute to answering these questions, in this study we report the clinical, histopathological and PrP Sc biochemical characterization of five European MM1 cases with white matter plaques and the results of the experimental transmission to bank voles of one of these cases. Results are compared to those obtained in the typical MM1 subtype.

Patients and tissues
We studied 5 subjects affected by CJDMM1 associated with PrP Sc plaque-type deposits in white matter (hereafter indicated as p-CJDMM1) and 8 cases affected by typical CJDMM1 (hereafter indicated as np-CJDMM1). All cases were referred for diagnosis to the Laboratory of Neuropathology, University of Bologna, Italy between 2005 and 2016 as part of the National Surveillance program on CJD and related disorders or (one p-CJDMM1) in the context of a collaborative effort with the Dutch Surveillance Centre for Prion Diseases on the molecular characterization of autopsy confirmed prion cases [10]. The 8 selected np-CJDMM1 control cases were representative of the spectrum of clinical and histopathologic features of the sCJDMM1 subtype [21,25] including disease duration (range 1-14 months).
Brains were obtained at autopsy, one half, or tissue blocks from representative areas, were immediately frozen at −80°C, whereas the rest was fixed in formalin.

Clinical and diagnostic evaluation
We collected and reviewed all available medical information from hospital reports, including results of neurologic examination(s), cerebral magnetic resonance imaging (MRI) studies and electroencephalographic (EEG) recordings. We defined the date of disease onset as the time when unexplained progressive neurological or psychiatric symptoms first occurred, and as 'onset symptom(s)' the first neurological disturbance(s) complained by the patient. We measured total tau (t-tau) protein levels in the cerebrospinal fluid (CSF) by quantitative ELISA (INNOT-EST hTAU Ag, Innogenetics) according to the manufacturer's instructions, considering as an optimal cut-off value 1250 pg/mL on the basis of receiver operating characteristic curve analysis, as previously described [16]. Semi-quantitative detection of CSF 14-3-3 protein was performed by western blotting, as previously described [16].

Genetic analysis
Genomic DNA was extracted from blood or frozen brain tissue. Genotyping of the PRNP coding region was performed as described [10].

Neuropathology
We semi-quantitatively evaluated gray matter spongiform change and astrogliosis in 10 brain regions on hematoxylin and eosin stained sections, as reported [21].
For PrP immunohistochemistry, paraffin sections from formalin-fixed and formic acid treated blocks were processed using the monoclonal antibody (mAb) 3F4 (1:400, Signet Labs), according to published protocols [11,22], with some modifications. Briefly, after de-waxing and rehydration, sections were incubated for 15 min in 8% hydrogen peroxide solution in methanol to block endogenous peroxidase. Sections were then washed, immersed in 98% formic acid for 1 h, rewashed and microwaved in 1.5 mM HCl for 25 min, incubated with reagent A of Histostain-Plus IHC Kit (Thermo-Fisher Scientific) for 10 min and then probed overnight with mAb 3F4. After two sequential incubations with reagent B and C of Histostain-Plus IHC Kit interspersed with washing steps in TBS 1X, sections were treated with Romulin AEC Chromogen (Biocare Medical) for 5 min and Mayer's hematoxylin for 15 s before being dehydrated, cleared and coverslipped.
For LFB staining, slides were immersed overnight in LFB solution (final concentration, 0.1% solvent blue and 0.5% acetic acid in 95% alcohol) at 60°C. After immersion in 95% alcohol and washing, sections were immersed 5 s in 0.05% lithium carbonate and rewashed. The latter steps were repeated until suitable gray matter discoloration. The obtained sections were then processed for PAS staining through immersion in periodic acid for 10 min and, after a washing step in deionized water, incubation in dark condition with Schiff 's reagent for 15 min. Subsequently, slides were washed, incubated with Mayer's hematoxylin for 1 min, immersed in warm water and rewashed.

Transmission to bank voles
Brain tissue from the p-CJDMM1 index case (case #1 described below) and from 4 control cases without plaques (three sCJDMM1 and one sCJDMV1) were homogenized at 10% (w/v) concentration in phosphate buffered saline (PBS) and stored at −80°C. Two genetic lines of bank voles, Bv109M and Bv109I carrying methionine or isoleucine homozygosity at PRNP codon 109, were injected by the intracerebral route (20 μl) into the left cerebral hemisphere under ketamine anesthesia. Beginning one month after inoculation, voles were examined twice per week until the appearance of neurological signs, and evaluated daily thereafter. The animals were sacrificed with carbon dioxide when they reached the terminal stage of the disease. Survival time was calculated as the interval between inoculation and sacrifice, attack rate as the number of animals developing disease with respect to the total number of inoculated animals [18].
The lesion profile was based on the severity of vacuolation, with a score from 0 to 5, in nine grey-matter brain areas on hematoxylin and eosin-stained sections, as previously described [7]. Vacuolation scores derived from at least 6 individual voles per group and were reported as mean ± standard error (SEM).

Preparation of white matter total homogenates and PK digestion
Frontal and parietal cortical white matter was obtained from case #1 (p-CJDMM1), and one np-CJDMM1. PrP Sc was purified from 350 mg of white matter, following a previously published protocol [29] and re-suspended in 200 μl of lysis buffer at pH 6.9. PK digestion was carried out at a final concentration of 4 U/ml for 1 h at 37°C.

PrP deglycosylation
N-Linked glycans were removed by using a peptide-Nglycosidase F kit (New England Biolabs) according to the manufacturer's instructions.

PK titration curves
Grey matter tissues were homogenized (10% w/v) in lysis buffer at pH 8. Total protein concentration was measured by means of a standard colorimetric method based on bicinchoninic acid (Pierce) and then adjusted to a final value of 4200 μg/ml. Samples were digested using serial dilutions of PK activity ranging from 2 to 256 U/ ml, for 1 h at 37°C. Digested samples were treated as previously described.

Western blot
Samples were run in a 7 or 15 cm long separating gel and transferred to Immobilon-P membranes (Millipore). After blocking in 10% non-fat milk in Tween-Tris-buffered saline, membranes were probed overnight with the monoclonal antibody 3F4 with epitope at PrP residues 108-111 at 1:30000 working dilution (human samples). Immunoblots from bank voles samples were incubated overnight at 4°C with the monoclonal antibody 9A2 (1:8000, PrP residues 99-101) [26] instead of 3F4. In addition, all immunoblots were probed with the C-terminal antibody SAF60 (1:2000, PrP residues 157-161) [20] in order to detect the CTF13. After four washings in Tween-Tris-buffered saline, membranes were incubated for 1 h at room temperature with an anti-mouse secondary antibody conjugated to horseradish peroxidase (GE Healthcare; working dilution, 1:4000) and washed again four times in Tween-Tris-buffered saline. The immunoreactive signal was visualized by enhanced chemiluminescence (Immobilon Western, Millipore) on an LAS 3000 camera (Fujifilm).

Quantitative analysis of protein signal
Densitometric analysis was performed using the software AIDA (Image Data Analyzer v.4.15, Raytest GmbH). For PK titration, a semi-logarithmic curve was obtained by plotting the percentage of protein remaining after digestion (with respect to the sample digested with 2 U/ml) against the corresponding PK concentration. The ED 50 (i.e. the PK concentration needed to digest 50% of PrP Sc ) for each sample was calculated by means of the equation of the straight line that best fitted the linear portion of the curve (r 2 ≥ 0.95). For TSA, the percentage of protein solubilized after heating treatment (with respect to the sample treated at 95°C) was plotted against the corresponding heating temperature. The T 50 (i.e. the temperature needed to solubilize 50% of PrP Sc ) for each sample was calculated from the equation describing the sigmoidal curve that best fitted the data (r 2 ≥ 0.95).

Statistical analyses
All statistical analyses were performed with SigmaPlot 12.5 (Systat Software Inc.). Depending on the data distribution, Student's t test or Mann-Whitney test were used to detect differences between two groups, while one-way analysis of variance (ANOVA), followed by Dunn's or Holm-Sidak post hoc tests, was applied for three or more groups comparisons. P value <0.05 was considered statistically significant.

Clinical findings and diagnostic investigations
Results are summarized in Table 1. The mean age at disease onset in p-CJDMM1 was 58.2 ± 9.9 years (range 48-70) and the mean disease duration 22.0 ± 7.8 months (range 13-34). Clinical features were quite heterogeneous, reflecting the multifocal cerebral involvement, and included severe cognitive impairment, myoclonus along with cerebellar, pyramidal and visual symptoms and signs. Akinetic mutism appeared on average after 8.6 ± 3.8 months from onset. EEG examination revealed periodic sharp-wave complexes in 4 out of 5 subjects (80%), while brain MRI showed typical hyperintense cortical and/or basal ganglia abnormalities on fluid attenuated inversion recovery (FLAIR) or diffusion weighted (DW) sequences in 3 of the 5 cases. Western blot assay for 14-3-3 was positive in 4 out of 5 (80%) cases, while t-tau CSF levels were above the 1250 pg/ml threshold in all the 4 subjects that were tested.

Genetic analysis and PrP Sc typing
All 5 p-CJDMM1 cases carried MM at PRNP codon 129; in addition case #4 carried the E200K mutation while sequencing of the PRNP coding region excluded pathogenic mutations in the other four. PrP Sc typing showed type 1 in all cases with the co-occurrence of type 2 in case #5 (Fig. 1). Notably, there were no significant differences in PrP Sc electrophoretic mobility between p-CJDMM1 and np-CJDMM1 cases in both gray ( Fig. 1) and white matter (Additional file 1: Figure S1A).

Neuropathology
The histopathological features largely overlapped with those previously reported for the np-CJDMM1 histotype. Specifically, the pathological changes mainly involved the cerebral cortex, striatum, thalamus and cerebellum, whereas the hippocampus and brainstem were relatively spared (Additional file 2: Table S1). Consistently with the relatively long disease duration, the histopathological changes were, in most cases, rather severe. Indeed, marked atrophy and status spongiosus were the main findings in the most affected areas, whereas typical spongiform change with microvacuolation was best seen in less affected structures (i.e. the hippocampus) (Fig. 2  a,b). As the only exception, the case with the shortest duration (case #5) showed typical spongiform change and, overall, moderate rather than severe histopathological lesions in most affected gray matter areas (Additional file 2: Table S1). Immunohistochemical analysis of PrP Sc showed the typical synaptic deposition pattern in the molecular layer of cerebellum and in the cerebral cortex in all cases (Fig. 2c).
At variance with typical CJDMM1, however, all five cases also displayed a variable number of PrP Sc plaquelike deposits in the white matter, not correlating, overall, with the degree of white matter damage (Tables 2 and  3), in virtually all brain regions analyzed (Fig. 2 c,d and Table 2). Furthermore, with the only exception of the case carrying the E200K PRNP mutation (case #4), PAS staining revealed the presence of PrP-amyloid plaques (Fig. 2 e,f ), quantitatively reflecting the number of PrP Sc plaque-like deposits. While in three cases the plaques were immediately noticed given their number and the typical kuru-type morphology, they were much rarer and limited to a small core in the case with the longest duration and the most severe pathology (case #3). Notably, the three cases with the highest number of PrP plaques also showed numerous coarse focal PrP deposits and tract-like PrP deposits in gray matter areas delimiting the white matter (data not shown). These types of deposits were seen in the striatum in proximity of the internal capsule and in the lateral thalamus. Besides plaques, no other PrP deposits were seen in the white matter either intra-or extracellularly. Finally, the sections stained with the anti-APP and anti-synaptophysin antibodies revealed that the deposits of these proteins in the damaged white matter of p-CJDMM1 were, occasionally, co-localizing with PrPamyloid plaques (Fig. 2 e,f ).

Biochemical comparison between p-CJDMM1 and np-CJDMM1
Besides the presence of a typical type 1 fragment, the overlap of biochemical PrP Sc properties between p-CJDMM1 and np-CJDMM1 extended to the CTF13,  as revealed by antibody SAF60. As the only exception, in case #5, as expected given the mixed types 1 + 2 molecular phenotype [20], the ratio between PrP27-30 and CTF13 differed from that of the other cases (Additional file 3: Figure S2; Additional file 4: Table S3). Analysis of PrP Sc glycoform ratio demonstrated a slightly higher percentage of diglycosylated PrP Sc in p-CJDMM1 than in np-CJDMM1, whilst confirming a similar glycopattern (Additional file 5: Table S3).
As in grey matter, the analysis of CTF13 in white matter did not show any significant difference between p-CJDMM1 and np-CJDMM1 cases (Additional file 1: Figure S1B).
PK titration curves, performed on gray matter homogenates at pH 8 in both sporadic and genetic (E200K) cases, did not exhibit consistent differences between the two groups. P-CJDMM1 and np-CJDMM1 groups showed an ED 50 (expressed as mean ± standard deviation) of 7.87 ± 1.09 and 9.14 ± 2.92, respectively. No statistically significant differences were observed (Fig. 3 a,c). Case #5 was excluded from this analysis, since MM2-PrP Sc is associated with a higher PK-resistance than MM1-PrP Sc [29].
TSA, performed on THs at pH 6.9, also revealed comparable values among the analyzed samples. The calculated T 50 (expressed as mean ± standard deviation) was 79.68 ± 0.72 in p-CJDMM1 and 80.38 ± 2.90 in np-CJDMM1 (Fig. 3 b,d).

Transmission to bank voles
We have previously shown that sCJDMM(V)1 and gCJD E200K are transmissible to Bv109M with short incubation time and low or absent transmission barrier [18]. Case #1 (p-CJDMM1) and control case c (np-sCJDMM1) were thus inoculated in Bv109M and their results compared with those previously obtained from np-sCJDMM1 (case a), np-gCJD E200K-MM1 (case b) and np-sCJDMV1 (case d) transmissions. As the PRNP polymorphism at codon 109, coding for methionine or   Table 4. Overall, these experiments confirm and extend previous evidence of a very low or absent transmission barrier for CJDMM(V)1 in bank voles, which also applies to case #1 (p-CJDMM1). Interestingly, in both p-CJDMM1 and np-CJDMM(V)1 the survival time was shorter in Bv109M than in Bv109I. Furthermore, in both lines of bank voles the survival time was generally similar for p-CJDMM1 and np-CJDMM(V)1, although with some variations. Case #1 showed the shortest survival time in the 1st passage, and the longest one in the 2nd passage in both Bv109M and Bv109I. Statistically significant differences were sometimes observed in survival times between case #1 and various np-CJD cases (Table 4). However, given that the differences were generally not conserved between 1st and 2nd passage and, above all, that even the comparisons between np-CJDMM(V)1 cases were sometimes statistically significant, the reported differences more likely reflect the PrP Sc amount in the inoculum or other factors rather than a strainspecific feature. PrP Sc extracted from the brains of infected bank voles was subjected to Western blot analysis to detect possible differences induced by the two types of inocula. As for PrP Sc in the CJD brains, PrP Sc fragments were indistinguishable between bank voles inoculated with p-CJDMM1 or np-CJDMM(V)1 (Additional file 6: Figure  S3A). At variance with the human brain, vole PrP Sc was characterized by a predominance of the diglycosylated form, as previously reported [18]; however, likewise in Fig. 3 Analysis of PrP Sc PK-resistance (a, c) and thermo-solubility (b, d). Representative WBs of case #3 (upper) and a np-CJDMM1 case (lower) are shown in a and b. Membranes were incubated with the primary antibody 3F4. Relative molecular masses are expressed in kDa. c PK digestion profiles. ED 50 represents the PK concentration needed to digest 50% of PrP Sc (expressed as mean ± standard deviation). d Thermo-solubilization profiles. T 50 represents the temperature needed to solubilize 50% of PrP Sc (expressed as mean ± standard deviation). No statistically significant differences were observed in both ED 50 and T 50 between p-and np-CJDMM1 (ED 50 , Mann-Whitney Rank Sum Test, P = 0.570; T 50 , t-test, P = 0.306) CJD inoculated samples, PrP Sc glycoform ratio in voles did not show any statistically significant difference related to the inoculum (p-CJDMM1 or np-CJDMM(V)1) (Additional file 6: Figure S3B). Similarly, a comparable amount of CTF13 [26] was detectable, after sample deglycosylation, in voles infected with the two inocula (Additional file 7: Figure S4).

Neuropathology in bank voles
The lesion profiles of case #1 and inoculated controls showed an identical distribution of spongiform change (Fig. 4). In Bv109M, in both first and second passage, spongiform change was more prominent in superior colliculi, thalamus, hippocampus and retrosplenial and cingulate cortices. The other areas presented none or few scattered vacuoles. The Bv109I line displayed a slightly more pronounced spongiform change in the medulla, hypothalamus and septum, which was also maintained upon the two passages. Immunohistochemistry revealed a synaptic pattern of PrP deposition, while no plaquelike deposits were seen in the affected animals of both lines.

Discussion
The present data (i) add to previous studies reporting the rare occurrence of white matter PrP amyloid plaques in patients with an otherwise classic sCJDMM1 phenotype; (ii) originally report the occurrence of white matter PrP plaque-like deposits in genetic CJDMM1 and the results of the experimental transmission of p-sCJDMM1 to bank voles, and (iii) further address the issue of the molecular basis of amyloid plaque formation in CJD by providing an extensive characterization of the physicochemical properties of PrP Sc aggregates in p-CJDMM1.
No data are available on the relative frequency of this peculiar phenotype in the CJD population. Since the 5 cases described here were observed over a 15-20 yearperiod of diagnostic activity involving approximately 1000 CJD-affected brains from Italy and the Netherlands, we can estimate an incidence of p-CJDMM1 in western Europe around 0.5%.
A critical unsolved issue concerning the occurrence of white matter kuru-type amyloid plaques in sCJD carrying MM at codon 129 is whether or not this peculiar phenotype is linked to a specific prion strain. Our systematic analyses of PrP Sc properties combined with the results of the experimental transmissions strongly argue for both the classic np-CJDMM1 and the atypical p-CJDMM1 phenotypes being linked to the same (M1) prion strain. Accordingly, amyloid plaque formation in such cases represents a host-derived, likely genetic, effect. To consider an alternative possibility, one would postulate the unlikely scenario of the co-occurrence in p-CJDMM1 of a distinct prion strain besides M1, not inducing a distinctive cerebral grey matter pathology, not affecting PrP Sc properties, and not transmissible to bank voles.
Besides the presence of amyloid plaques, another interesting feature, distinguishing the p-CJDMM1 reported by us and Kobayashi et al. [12] from the np-CJDMM1 cases, is their significantly longer mean disease duration (22 months) in comparison to typical np-CJDMM1 cases (4 months). However, disease duration and the associated advanced pathology, although notoriously favoring the extent of plaque formation, cannot be the only causal factors since it is well established that most CJDMM1 patients with prolonged disease duration do not develop plaque-type depositions in the white matter. Moreover, the observations by Gelpi et al. and Berghoff et al. [1,9] in cases characterized by a short disease course, combined with our findings in case #5 and in a similar p-CJDMM1 case we recently obtained, also characterized by mild white matter changes (P. Parchi personal communication), clearly indicate that white matter amyloid plaques may develop early in the disease course and independently from a severe white matter damage. Interestingly, in our p-CJDMM1 cases, the onset and progression of clinical symptoms, including akinetic mutism, seem to be significantly delayed compared to np-CJDMM1 patients with similar disease duration. Taken together, these data support the hypothesis of a protective role of PrP amyloid, possibly by sequestering PrP Sc into large fibrils and partially preventing the molecular interaction between monomeric PrP C and PrP Sc , that is essential for conversion and prion propagation.
Since the mechanism of amyloid deposition seems to include the incorporation of lipid molecules into the aggregates [30], white matter appears even more suitable for PrP amyloid plaque formation than the grey matter. In this regard, it is noteworthy that plaque-like PrP deposition in sCJDVV2 and MV2K is often best observed at the boundaries between gray and white matter. Despite the intensive search, we failed to demonstrate a difference in the physico-chemical PrP Sc properties between p-CJDMM1 and np-CJDMM1 that would correlate with plaque formation. Similarly, PrP Sc properties did not differ between bank voles injected with the two CJD inocula. These data combined with the lack of PrP amyloid plaques or plaquelike deposits in the bank voles inoculated with p-CJDMM1 further point to a non-PrP factor of the host affecting PrP aggregation and fibrillation. It is well established that PrP Sc spread within the peripheral and central nervous systems by axonal transport although the cellular mechanism of prion transport in axons and into peripheral tissue is largely unresolved. Thus, one possibility would be a modified molecular interactome for PrP Sc during axonal transport favoring PrP Sc aggregation and amyloid plaque formation. Since PrP-amyloid plaques in p-CJDMM1 cases sometimes colocalize with APP, a well-established marker of axonal damage, PrP Sc deposition in white matter eventually disrupts axon integrity. The opposite scenario, namely axonal damage favoring PrP amyloid plaque formation, previously suggested by Kobayashi et al. [12] seems unlikely given the observation of plaque formation in cases  [18]. Brain-scoring positions include medulla (1), cerebellum (2), superior colliculus (3), hypothalamus (4), thalamus (5), hippocampus (6), septum (7), retrosplenial and adjacent motor cortex (8), and cingulate and adjacent motor cortex (9). Strong similarities characterize the lesion profiles of all these transmissions with short disease duration and/or lack of significant white matter damage [1,9] (and present cases #5).

Conclusions
The present study further establishes the existence of a rare CJD subtype, occurring in approximately 0.5% of CJD cases, designated as p-CJDMM1. The novel histotype largely overlaps with sCJDMM1 but shows, as a very distinctive feature, the presence of PrP-amyloid plaques of kuru-type in both subcortical and deep nuclei white matter. Likewise typical CJDMM1, p-CJDMM1 can also be observed in sCJD cases showing the cooccurrence of PrP Sc types 1 and 2. Moreover, plaque-like PrP deposits in the white matter can be a feature of genetic CJD. Most significantly, p-CJDMM1 share both PrP Sc and transmission properties with classic CJDMM1, strongly pointing to an host-dependent causal factor for amyloid plaque formation in this phenotype.