Scrapie SNPs

Scrapie SNPs Veterinary screening test for: Scrapie sheep Scrapie is a fatal infective neurodegenerative disease which affect both sheeps and goats and belong to the transmissible spongiform encephalopathies (TSEs) group, or prion diseases, which affects humans and animals. The pathogenetic event common to all TSE is the accumulation of the pathologic isoform (PrPsc) of a normal sialglycoprotein coded from the host (cellular prion protein PrPc) both at the level of the nervous system and in many peripheral districts. In the animal scope, the most known of these diseases are little ruminant (ovine and caprine) scrapie, which is the “prototype” of TSE, and the bovine spongiform encephalopathies (BSE) (Pocchiari, 1994; Prusiner, 1998). PrPc localization into synaptosomes and its tight association with copper in presynaptic area suggest that it can carry out an important role in normal nervous transmission, taking part also in nitric oxide (NO) metabolism (Sorenson, 2001). The infective protein PrPsc has not a different aminoacidic sequence compared to the host cellular protein PrPc, but has a different glycosylation and intramolecular disulfur bounds. When PrPsc enters into the host, it converts the normal protein PrPc, produced by the cellular gene, into the infective form PrPsc, which is responsible for neurological symptomatology in the animal.

The disease can be transmitted to other animals, both horizontally (by infection) and vertically (by maternal transmission). In many animal species sensible to TSE, like human, sheep, mouse and, more recently, goat and deer, it’s also been documented the key influence of host genotype in modulating susceptibility or resistance to the disease. This genetic component is characterized by a series “non silent” polymorphisms located in particular positions, or codons, depending on the species, into the cellular PrPc gene. This gene is responsible for the production of a protein PrPc bearing one or more aminoacidic substitutions in the mentioned codons (Pocchiari, 1994; Prusiner, 1998). Based on this, the pathogenic phenotype observed from time to time would be, in the above mentioned species, the product of the interaction between the aetiological agent and the host genotype (Di Guardo e Marcato, 2002). As regards ovins, there are three PrPc polymorphic sites able to condition the higher or lower susceptibility of the host towards the disease. The polymorphisms are in codon 136 (A/V, Ala/Val), codon 154 (R/H, Arg/His) and codon 171 (R/Q, Arg/Gln or rarely H/Q, His/Gln) (Agrimi et al., 2003). Therefore the three codons can give rise to different possible combinations, each called allele.

Each allele is represented with the letters that identify the aminoacid coded by the three codons. ARR allele (Ala/Arg/Arg) confers resistance to the disease, while VRQ allele (Val/Arg/Gln) determines susceptibility to the disease. Since each individual inherits one allele of PrP gene from one parent and the other allele from the other parent, in the same individual there will be two alleles that could be identical (homozygos), or different (heterozygosis). The combination of the two alleles is defined as genotype. 15 different PrP genotypes have been observed and each has been associated to a specific risk for scrapie. VRQ/VRQ genotype is associated to the maximum susceptibility for scrapie, while ARR/ARR genotype is associated to the highest resistance (Vaccari G, 2001, Westaway 1994). European Commission has emanated a directive (2003/100/EC) to motivate member States to implement plans of genetic selection in ovine farms, based on genomic genotyping test, to increase the frequency of the resistance allele ARR in ovine population. The kit is available in two versions: Classic scrapie (Scrapie Genotype Kit, which allows the study of codons A136V, R154H, R171H, R171Q), and allows the execution of 100 reactions for each codon (total of 400 reactions). Atypical scrapie (Scrapie Genotype Plus Kit, which allows in addition to the above the study of the codon L141F, involved in atypical scrapie (total 500 reactions).

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