Analysis of linkage between scrapie incubation period and the prion protein gene in mice

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Abstract
A single gene is known to have a predominant influence on scrapie incubation period in mice. In crosses between strains that give a short incubation period, such as NZW mice, and those which give a long incubation period, such as I/LnJ mice, long incubation period was dominant using a Chandler scrapie agent isolate. Recently a close linkage was found between the incubation period gene and the prion protein (PrP) structural gene in I/LnJ mice crossed to NZW mice. Because this linkage suggested an important role for PrP in the pathogenesis of scrapie we sought to verify the linkage between these genes and extended the analysis to three additional mouse strains. All four of the mouse strains that we evaluated, I/LnJ, P/J, MA/MyJ, and RIIIS/J, had incubation periods longer than those of the NZW mice to which they were crossed. In addition, all four strains shared an XbaI restriction enzyme polymorphism, which suggested that all four strains might also exhibit linkage between the incubation period and the PrP structural gene. Very strong linkage between PrP and incubation period was found in I/LnJ and P/J mice crossed to NZW mice, whereas less obvious linkage was demonstrated for MA/MyJ mice crossed to NZW mice. In MA/MyJ mice genes other than PrP also had an obvious influence on incubation period. In RIIIS/J mice no linkage was shown. Although linkage between PrP and incubation period was very significant in I/LnJ and P/J mice, a few animals were identified in both crosses that represented potential recombinants in which PrP and incubation period did not segregate together. Therefore, although these phenotypes are certainly linked in I/LnJ and P/J mice, it is possible that PrP and incubation period are controlled by separate genes.