Variant-specific prion interactions: Complicating factors.

Prions are protein conformations that "self-seed" the misfolding of their non-prion iso-forms into prion, often amyloid, conformations. The most famous prion is the mammalian PrP protein that in its prion form causes transmissible spongiform encephalopathy. Curiously there can be distinct conformational differences even between prions of the same protein propagated in the same host species. These are called prion strains or variants. For example, different PrP variants are faithfully transmitted during self-seeding and are associated with distinct disease characteristics. Variant-specific PrP prion differences include the length of the incubation period before the disease appears and the deposition of prion aggregates in distinct regions of the brain. Other more common neurodegenerative diseases (e.g., Alzheimer disease, Parkinson disease, type 2 diabetes and ALS) are likewise caused by the misfolding of a normal protein into a self-seeding aggregate. One of the most important unanswered questions is how the first prion-like seed arises de novo, resulting in the pathological cascade.