Prion Diseases

What Are Prion Diseases?

Prion diseases, also known as transmissible spongiform encephalopathies, are a group of rare progressive neurological infections affecting both humans and animals. Currently, several prion diseases have been identified, commonly including human Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker disease, sheep scrapie, bovine spongiform encephalopathy or mad cow disease, deer chronic wasting disease, and so forth. These prion diseases are manifested by a long incubation period with symptoms of no fever, no inflammation, and no specific immune response after infection. They usually present a series of neurological symptoms such as progressive ataxia, postural instability, perceptual allergy, dementia, etc. Human prion diseases are characterized by spongiform change, astrocytosis, neuronal loss, and amyloid plaque formation.

Risk Factors of Prion Diseases

Unlike other infections that are caused by an infectious virus or bacterium with a DNA/RNA genome, prion diseases are caused by the prion virus, a kind of infectious factor composed of only protein without nucleic acid.

Prion diseases can be caused by several risk factors, mainly as follows:

1. Genetic factor. The mutations in the human PrP encoded gene PRNP may lead to the misfolding of the prion protein (PrP), thereby causing human prion diseases.
2. Diet factor. Prion diseases also can be transmitted from animals to humans through directly or indirectly eating the prion-infected animal meat.
3. Aging factor. Some non-hereditary prion diseases tend to affect the elderly.
4. Medical contaminant factor. Corneal transplantation, electroencephalogram electrode implantation, medical equipment, or neural tissue contamination all may be risk factors for prion diseases.

Pathogenetic Role of Prion Protein in Prion Diseases

PrP is a glycosylphosphatidylinositol-anchored glycoprotein commonly found in human and animal tissues, which has a distinct and abnormal structure in prion disease individuals. PrP has two isoforms with different spatial conformations: one is normal cellular form PrP^C with a mainly α-helical structure, the other is pathogenic or scrapie form PrP^Sc with several β-sheet structures and α-helixes. The PrP^C and PrP^Sc share an identical primary sequence, but differ in secondary and tertiary structure, as well as in solubility and resistance to protease.

Fig.1 Normal and disease-causing prion protein. (Prusiner, 2004)

Although the specific physiological functions of the normal prion protein PrP^c are not so clear, it has been demonstrated that PrP^c also involves in the processes of apoptosis, long-term memory formation, neuronal development, synaptic plasticity, and innate immunity. However, the misfolded PrP^Sc exhibits extreme resistance to protease, and can’t be degraded by proteases in vivo. Thus, the abnormal PrP^Sc accumulates in the neural cells, which is the hallmark of prion diseases. The continuous accumulation of PrP^Sc further forms aggregated fibers until destroy the neural cells. The symptoms of prion diseases vary according to the region of damaged neural cells in the brain.

Fig.2 Models of PrP^C to PrP^Sc conversion. (Aguzzi, 2008)

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References

1. Prusiner, S.B. Detecting mad cow disease. Scientific America. 2004, 291(1): 86-93.
2. Aguzzi, A.; et al. Molecular Mechanisms of Prion Pathogenesis. Annual Review of Pathology. 2008,3: 11-40.