Stanley Prusiner, MD
Molecular Biological, Genetic and Protein Structural Studies of Prion Disease
My research is focused on infectious proteins called prions that cause neurodegenerative diseases. Nascent prions are created either spontaneously by mutation of a host protein or by exposure to an exogenous source. Prions are composed largely, if not entirely, of a modified form of the prion protein (PrP) designated PrPSc. Like other infectious pathogens, they multiply but prions do not have a nucleic acid genome to direct the synthesis of their progeny. A post-translational, conformational change features in the conversion of cellular PrP (PrPC) into PrPSc during which alpha-helices are transformed into beta-sheets. Since this structural transition in PrP underlies both the replication of prions and the pathogenesis of CNS degeneration, much of the effort in the laboratory is devoted to elucidating the molecular events responsible for this process. Indeed, prion diseases seem to be disorders of protein conformation. After demonstrating genetic linkage between the PrP gene and the control of scrapie incubation times, we established linkage between a point mutation in the human PrP gene and development of the fatal, familial disease Gerstmann-Straussler-Scheinker (GSS). Like humans with GSS, transgenic mice expressing mutant PrP develop neurodegeneration and produce prions de novo as demonstrated by transmission of disease to inoculated recipients. These studies argue that prion diseases can be both inherited and infectious. Investigations of prion strains led to the conclusions that variations in disease phenotype are determined by the conformation of PrPSc. Since prion strains replicate, this argues that PrPSc must act as a template for the refolding of PrPC into a second molecule of PrPSc. Recent investigations argue that the length of the incubation time specified by prion strains is determined by the rate of PrPSc clearance. This discovery is being extended using inducible transgenes that are regulated by tetracycline derivatives in bigenic mice. Several studies are directed at elucidating the structure of PrPSc: (1) synthetic PrP peptides carrying pathologic mutations, (2) small redacted PrP molecules supporting PrPSc formation, (3) EM image analysis of 2-D paracrystalline arrays of PrPSc, and (4) epitope mapping of PrPSc using numerous recombinant antibody fragments (Fab). During prion replication, an as yet to be identified factor that we have provisionally designated protein X binds to PrPC. The PrPC/protein X complex then binds PrPSc; by an unknown process, PrPC is transformed into a second molecule of PrPSc. We are attempting to isolate protein X in order to develop an in vitro system for studying the mechanism by which nascent PrPSc is formed. Since PrPC molecules carrying basic residues within the epitope that binds to protein X act as dominant negatives in preventing PrPSc formation, we are developing drugs based on these findings. From an examination of several hundred thousand compounds, we have identified several lead compounds that effectively block PrPSc formation in cultured cells. Once more effective analogs of these drugs are discovered, we plan to initiate studies in mice and hamsters.