Resistance to scrapie in PrP ARR/ARQ heterozygous sheep is not caused by preferential allelic use.

Abstract

BACKGROUND: In sheep, susceptibility to scrapie, which is similar to human prion diseases such as Kuru and variant Creutzfeldt-Jakob disease (vCJD), is determined by prion protein (PrP) gene (Prnp) polymorphisms. Sheep with genotype ARQ/ARQ, denoting polymorphisms at codons 136, 154, and 171, are susceptible, whereas those with genotypes ARR/ARQ and ARR/ARR are resistant, indicating dominance of ARR over the ARQ allele. AIMS: Based on familial CJD E200K, 129V, where preferential use of the 200E allele in EK heterozygous individuals confers resistance, heterozygous ARR/ARQ sheep were used to test the hypothesis that resistance is caused by preferential use of the ARR allele. METHODS: After assessment of equivalent PrP expression across genotypes, allele use was analysed by sequencing reverse transcription polymerase chain reaction derived DNA clones containing the Prnp gene coding sequence. RESULTS: The ARR to ARQ ratio was 1.1 in 133 clones, representing Prnp mRNA from three ARR/ARQ sheep, indicating equal use of both alleles. CONCLUSIONS: Dominance of the resistant associated allele in sheep scrapie involves mechanisms other than the absence of PrP derived from the disease associated ARQ allele.