The disease characteristics of different strains of scrapie in Sinc congenic mouse lines: implications for the nature of the agent and host control of pathogenesis.

Mouse lines which are congenic for Sinc, the major gene controlling scrapie incubation period, have been produced by selective breeding from the inbred C57BL(Sincs7) and VM(Sincp7) strains; the s7 allele of Sinc has been introduced into a VM background by 18 serial backcrosses, at each generation selecting on the basis of the incubation period with the ME7 scrapie strain. The characteristics of the disease produced by seven scrapie strains have been compared in Sincs7 and Sincp7 congenic mice and in the F1 cross between them. As previously found in non-congenic mice, each scrapie strain has a characteristic, precisely reproducible incubation period pattern in the three Sinc genotypes. The Sinc gene controls the incubation period for all scrapie strains tested but the direction of allelic action and the apparent dominance pattern differs between scrapie strains. Comparison with non-congenic mice shows that other genes also have a minor effect on incubation period. The distribution of vacuolar degeneration in the brain depends mainly on the scrapie strain but is also influenced by Sinc and other unspecified mouse genes. Restriction fragment length polymorphism analysis has already shown that the close linkage between Sinc and the gene encoding PrP has been maintained in the Sinc congenic lines, strengthening the possibility that PrP is the Sinc gene product. The present study confirms that scrapie strains carry information which is independent of the host but nevertheless suggests that host PrP protein interacts with this information to regulate the progression of the disease.