Identification of a second bovine amyloidotic spongiform encephalopathy: molecular similarities with sporadic Creutzfeldt-Jakob disease

Abstract

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are mammalian neurodegenerative disorders characterized by a posttranslational conversion and brain accumulation of an insoluble, protease-resistant isoform (PrP(Sc)) of the host-encoded cellular prion protein (PrP(C)). Human and animal TSE agents exist as different phenotypes that can be biochemically differentiated on the basis of the molecular mass of the protease-resistant PrP(Sc) fragments and the degree of glycosylation. Epidemiological, molecular, and transmission studies strongly suggest that the single strain of agent responsible for bovine spongiform encephalopathy (BSE) has infected humans, causing variant Creutzfeldt-Jakob disease. The unprecedented biological properties of the BSE agent, which circumvents the so-called "species barrier" between cattle and humans and adapts to different mammalian species, has raised considerable concern for human health. To date, it is unknown whether more than one strain might be responsible for cattle TSE or whether the BSE agent undergoes phenotypic variation after natural transmission. Here we provide evidence of a second cattle TSE. The disorder was pathologically characterized by the presence of PrP-immunopositive amyloid plaques, as opposed to the lack of amyloid deposition in typical BSE cases, and by a different pattern of regional distribution and topology of brain PrP(Sc) accumulation. In addition, Western blot analysis showed a PrP(Sc) type with predominance of the low molecular mass glycoform and a protease-resistant fragment of lower molecular mass than BSE-PrP(Sc). Strikingly, the molecular signature of this previously undescribed bovine PrP(Sc) was similar to that encountered in a distinct subtype of sporadic Creutzfeldt-Jakob disease.

Fig. 1.

PrP deposition in the brains of group 1 and group 2 cattle. Immunohistochemistry showing the glial and granular patterns of PrP deposition observed in the dorsal nucleus of vagus nerve (a, ×210), thalamus (c, ×210), pyriform cortex (e, ×220), and olfactory bulb (g, ×150) of an animal representative of group 1. In group 2 cattle, the dorsal nucleus of the vagus nerve is unstained (b, ×210), whereas PrP-positive plaques are observed in the thalamus (d, ×210), pyriform cortex (f, ×210), and olfactory bulb (h, ×80).

Fig. 2.

PrP-positive amyloid plaques in group 2 animals. PrP-immunostaining of the pyriform cortex from group 2 cattle, showing the presence of kuru-like amyloid plaques (a and b, ×450). At ultrastructural examination amyloid deposits are composed of aggregates and bundles of unbranched fibrils (c, ×12,550; d, ×60,000).

Fig. 3.

Biochemical analysis and regional distribution of PrP^Sc in group 1 and group 2 cattle. (a) Immunoblot with 6H4 monoclonal antibody of proteinase K-treated brain homogenates from the thalamus of group 1 (odd lanes) and group 2 animals (even lanes), before (lanes 1-4) and after (lanes 5-8) enzymatic deglycosylation. (b and c) Regional distribution of brain PrP^Sc in group 1 (b) and group 2 (c) cattle; values of PrP^Sc are reported below each gel as the percentage of the highest value obtained. Molecular size markers are shown on the right as M_r × 10^-3.

Fig. 4.

Electrophoretic analysis of PrP^Sc in cattle TSE and sCJD. (a) Western blot detection of PrP^Sc in brains of group 1 animals (lanes 1 and 5); subject with sCJD and type 1 PrP^Sc, methionine/methionine at codon 129 (lane 2); subject with sCJD and type 2 PrP^Sc, methionine/valine at codon 129 (lane 3); and group 2 cattle (lane 4). (b) Relative proportions of the three PrP^Sc glycoforms in group 1 and group 2 cattle compared with glycoform profiles obtained in nine sCJD patients, methionine/valine at codon 129 and with type 2 PrP^Sc. Mean ± standard deviation is shown. Upper band, diglycosylated form; middle band, monoglycosylated form; and lower band, unglycosylated form.