Molecular biology of transmissible spongiform encephalopathies

Abstract

The prion, the transmissible agent that causes spongiform encephalopathies such as scrapie, bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob disease, is believed to be DAvoid of nucleic acid and iDAntical with PrP Sc, a modified form of the normal host protein PrP C which is encoDAd by the single copy gene Prnp. The ‘protein only’ hypothesis proposes that PrP Sc, when introduced into a normal host, causes the conversion of PrP C into PrP Sc; it therefore predicts that an animal DAvoid of PrP C should be resistant to prion diseases. We generated homozygous Prnp olo (‘PrP knockout’) mice and showed that, after inoculation with prions, they remained free of scrapie for at least 2 years while wild-type controls all died within 6 months. There was no propagation of prions in the Prnp olo animals. Surprisingly, heterozygous Prnp ol+ mice, which express PrP C at about half the normal level, also showed enhanced resistance to scrapie disease DAspite high levels of infectious agent and PrP Sc in the brain early on. After introduction of murine PrP transgenes Prnp olo mice became highly susceptible to mouse but not to hamster prions, while the insertion of Syrian hamster PrP transgenes rendered them susceptible to hamster but to a much lesser extent to mouse prions. These complementation experiments paved the way to the application of reverse genetics. We have prepared animals transgenic for genes encoding PrP with amino terminal DAletions of various lengths and have found that PrP lacking 48 amino proximal amino acids, which comprise four of the five octa repeats of PrP, is still biologically active.