[Biology of non-conventional transmissible agents or prions]

Transmissible subacute spongiform encephalopathies (TSE) are a group of human and animal diseases which includes Creutzfeldt-Jakob disease, Gerstmann-Straüssler-Scheinker syndrome (GSS), Kuru, fatal familial insomnia (FFI), scrapie in sheep and goat, mink and feline transmissible encephalopathy, chronic wasting disease, and bovine spongiforme encephalopathy (BSE). TSE are transmissible among individuals of the same species and some of different species. These diseases stem from a specific category of agents that have biological and physiochemical characteristics unlike other micro-organisms; they are known as transmissible spongiform encephalopathy agent (TSA), prions, or virinos. So far, despite considerable progress made in the molecular biology toward the understanding of neurological injury, the nature of the TSA/prions remains unknown. TSE are characterised by the pathognomic accumulation, within the central nervous system of the infected individual, of a normal protein from the host organism, the PrP (prion protein). Differences between the PrP isolated from normal individuals (PrP-c) and PrP isolated from infected individuals (PrP-res) have been investigated. There are no differences in the sequence in amino acids, and the secondary structure seems identical, but since normal PrP is totally degraded by proteinase K pathological PrP resists to enzymatic digestion. One can therefore describe two PrP isoforms: a normal isoform, the PrP-c (c for cellular), sensitive to proteinase K and present in the normal individual and in the infected patients or animals: and a pathological isoform, the PrP-res, resistant to proteinase K and present in amount proportional to the infectivity in the brains of infected individuals. The presence of TSA/prions is detectable in the spleens of infected animals early after inoculation; it is then present in the CNS following a period not exceeding a half of the total length of the experimental disease. In the CNS, PrP-res is the origin of spongiosis and gliosis that are observed in infected individuals. A double causality, "infectious" and genetic, of these diseases can be derived from the various ideas currently accepted. Indeed the genetic basis of some familial forms has now been confirmed and transmissibility has been proven in natural situations such as the outbreak of CJD among children treated with extractive growth hormone and the recent surge of a new disease decimating British cattle, the bovine spongiform subacute encephalopathy, or mad cow disease. In TSE affected individuals, PrP has a key role in the incubation time and in the species barrier.