Consensus classification of human prion disease histotypes allows reliable identification of molecular subtypes: an inter-rater study among surveillance centres in Europe and USA

Abstract

The current classification of human sporadic prion diseases recognizes six major phenotypic subtypes with distinctive clinicopathological features, which largely correlate at the molecular level with the genotype at the polymorphic codon 129 (methionine, M, or valine, V) in the prion protein gene and with the size of the protease-resistant core of the abnormal prion protein, PrP(Sc) (i.e. type 1 migrating at 21 kDa and type 2 at 19 kDa). We previously demonstrated that PrP(Sc) typing by Western blotting is a reliable means of strain typing and disease classification. Limitations of this approach, however, particularly in the interlaboratory setting, are the association of PrP(Sc) types 1 or 2 with more than one clinicopathological phenotype, which precludes definitive case classification if not supported by further analysis, and the difficulty of fully recognizing cases with mixed phenotypic features. In this study, we tested the inter-rater reliability of disease classification based only on histopathological criteria. Slides from 21 cases covering the whole phenotypic spectrum of human sporadic prion diseases, and also including two cases of variant Creutzfeldt-Jakob disease (CJD), were distributed blindly to 13 assessors for classification according to given instructions. The results showed good-to-excellent agreement between assessors in the classification of cases. In particular, there was full agreement (100 %) for the two most common sporadic CJD subtypes and variant CJD, and very high concordance in general for all pure phenotypes and the most common subtype with mixed phenotypic features. The present data fully support the basis for the current classification of sporadic human prion diseases and indicate that, besides molecular PrP(Sc) typing, histopathological analysis permits reliable disease classification with high interlaboratory accuracy.

Fig. 1

Recommended diagnostic flowchart for diagnosing human sporadic prion disease histotypes and vCJD

Fig. 2

Distinctive histopathological features of sporadic human prion disease subtypes. a Typical spongiform change characterized by small, fine, microvacuoles (H&E stain, sCJD MM1, cerebral cortex); b spongiform change characterized by medium-sized vacuoles (H&E stain, sCJD VV1, cerebral cortex); c spongiform change characterized by relatively large confluent vacuoles (H&E stain, sCJD MM 2C cerebral cortex); d unicentric amyloid plaque of the kuru type (H&E stain, sCJD MV 2K, cerebellum); e, f florid plaques in the cerebral cortex (PAS stain and PrP IHC, vCJD, cerebral cortex); g synaptic pattern of PrP deposition in the cerebellum of sCJD MM1. A delicate diffuse staining is seen in the molecular layer, while the cerebellar glomeruli are stained in the granular cell layers. h Perivacuolar and i coarse PrP^Sc staining as typically seen in MM 2C and MM/MV 1+2C mixed sCJD cases. The cerebellum in MM 2C is either PrP negative or shows a focal patchy/coarse staining (j). k Plaque-like PrP deposits in the granular and molecular layers of the cerebellum in MV 2K. l Smaller plaque-like PrP deposits in the granular and molecular layers of the cerebellum in sCJD VV2. These plaque-like deposits are not visible in routine H&E or PAS-stained sections. m Perineuronal PrP^Sc staining in the deep cortical layers of sCJD VV2. All pictures in the panel have the same magnification (×200), except for d (×400) and g (×100)

Fig. 3

Distinctive regional distribution of histopathological changes in sCJD histotypes MM 2T, VV1, and VV2. Severe neuronal loss and gliosis in the medial thalamic (a) and inferior olivary nuclei (b) and relative sparing of anterior striatum (c) in sCJD MM 2T. Moderate to severe spongiform change (small to medium-sized vacuoles), gliosis, and neuronal loss in the cerebral cortex (d) and anterior striatum (e), and relative sparing of the cerebellum (f) in sCJD VV1; laminar distribution of spongiform change, with relative sparing of superficial (g) compared to the deep (h) layers in the cerebral neocortex of sCJD VV2. Severe spongiform change in the CA1/subiculum areas of the hippocampus (i) despite the virtually complete sparing of the occipital cortex (j) in a typical VV2 case with a relative short duration (five months). All pictures in the panel have the same magnification (×100) except for a and b (×200)