Biochemical, immunological, genetic and transgenetic experiments all indicate that the prion protein (PrP) is central to prion replication and disease susceptibility. Infectious prions are composed of a disease-specific PrP isoform designated PrPsc, and allelic forms of the PrP gene (Prnp) determine scrapie incubation time in mice. Mutant alleles of the human PrP gene (PRNP) are linked to familial prion diseases, and mice homozygous for a null allele of Prnp are resistant to prion disease and fail to support prion replication. However, additional factors impinge on prion replication and prion disease. Studies exploiting chimeric PrP transgenes and Prnp null mice suggest that efficient prion replication involves at least one auxiliary molecule, designated protein X. Both scrapie incubation time and the areas of the brain that are affected can differ dramatically among inbred strains of mice that express identical PrP molecules following infection with a single strain of scrapie. Treating prion incubation time as a quantitative trait led to the identification of at least two modifier genes. To identify the genes underlying these quantitative trait loci (QTLs), the QTL intervals will be isolated in congenic strains and the interval narrowed though high resolution mapping. The congenic strains also will be used to characterize the effects of each QTL on incubation time, pathology, distribution of PrPsc in the brain, PrP biochemistry, and concentration of PrPsc at onset of illness. Collaborative efforts with Dr. Hood (Project II) will take full advantage of advances in the Human Genome Project for candidate gene prioritization using oligonucleotide arrays and other technologies. BAC transgenesis and gene targeting will be used for identification of these and other prion disease modifiers that will be identified. QTL analysis is dependent on naturally occurring polymorphisms between mouse strains and thus samples only a small subset of potential modifier genes. Therefore, a screen will be established for chemically induced mutations that alter prion incubation time in transgenic mice so that all genes potentially are targeted. Finally, in collaboration with Drs. Hood (Project 11) and Prusiner (Project III), we will identify networks that are perturbed by misexpression of the PrP-related protein Dpl and screen for modifiers of Dpl-induced phenotypes. The combination of approaches will help define the genetic bases for susceptibility to prion disease and apply this new genetic knowledge to understand the underlying molecular processes.
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