Prions are proteins that adopt an alternative conformation, which becomes self-propagating. Some prions cause degeneration of the central nervous system (CNS). Once relegated to causing a group of rare disorders of the CNS such as Creutzfeldt-Jakob disease (CJD), prions also appear to play a role in more common illnesses such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). While CJD is caused by the accumulation of PrPSc prions, recent investigations demonstrate unequivocally that α-synuclein prions cause multiple system atrophy (MSA). Infectious MSA prions have been recovered from human brain samples stored in formalin for up to 20 years. Similar resistance to formalin was demonstrated for brain samples from sheep with scrapie. Increasing data argue that Aβ prions contribute to AD, α-synuclein prions to PD, and tau prions to some types of frontotemporal dementia (FTD). In this chapter, we confine our discussion to CJD, which typically presents with a rapidly progressive dementia as well as motor abnormalities. The illness is relentlessly progressive and generally causes death within 9 months of onset. Most CJD patients are between 50 and 75 years of age; however, patients as young as 17 and as old as 83 have been recorded. The role of prions in the pathogenesis of neurodegenerative diseases is reviewed in Chap. 417.
CJD is one malady in a group of disorders caused by prions composed of the prion protein (PrP). PrP prions reproduce by binding to the normal, cellular isoform of the prion protein (PrPC) and stimulating conversion of PrPC into the disease-causing isoform PrPSc. PrPC is rich in α-helix and has little β-structure, whereas PrPSc has less α-helix and a high amount of β-structure (Fig. 430-1). This α-to-β structural transition in PrP is the fundamental event underlying this group of prion diseases (Table 430-1).
Structures of PrP prion proteins. A. Nuclear magnetic resonance structure of Syrian hamster recombinant (rec) PrP(90–231). Presumably, the structure of the α-helical form of recPrP(90–231) resembles that of PrPC. recPrP(90–231) is viewed from the interface where PrPSc is thought to bind to PrPC. Shown are α-helices A (residues 144–157), B (172–193), and C (200–227). Flat ribbons depict β-strands S1 (129–131) and S2 (161–163). B. Structural model of PrPSc. The 90–160 region has been modeled onto a β-helical architecture while the COOH terminal helices B and C are preserved as in PrPC.
TABLE 430-1Glossary of PrP Prion Terminology ||Download (.pdf) TABLE 430-1 Glossary of PrP Prion Terminology
|Prion ||Proteinaceous infectious particle that lacks nucleic acid. Prions are composed entirely of alternatively folded proteins that undergo self-propagation. Distinct strains of prions exhibit different biologic properties, which are epigenetically heritable. PrP prions ...|