Characteristics of allele pool of the Romanov sheep breed for the prion protein gene associated with genetic sustainability to scrapie

The Romanov is a unique indigenous sheep breed of Russia, belonging to the group of Northern short-tailed sheep. The breed is known all over the world, due to out-of-season breeding ability, phenomenal fecundity (up to 10 lambs) and unsurpassed quality of sheepskins. Presently the gene pool of the breed is actively involved in creation of new types of modern prolific sheep and it is considered as an important genetic reserve for the sheep breeding of the future. Diseases resistance is the most important selection trait in sheep. One of the diseases which can cause serious economic loses is spongiform encephalopathy of sheep, also known as scrapie. Scrapie is a fatal neurodegenerative disease of sheep and goats, belonging to the class of transmissible spongiform encephalopathies (TSE), which also includes bovine spongiform encephalopathy (BSE). Three polymorphisms in amino acid codons 136 (A/V), 154 (R/H), and 171 (R/Q/H) of the PRPN gene are associated with resistance or susceptibility of sheep to classical scrapie. Depending on the PRNP genotype, there are five classes of genetic sustainability to Scrapie (G1-G5). The ARR allele is desirable regarding the resistance to scrapie. However, discovery of atypical scrapie (Nor98) showed a possibility of transmitting BSE to animals of different sustainability classes, including G1 (ARR/ARR genotype). It is shown, that L/F amino acid substitution at position 141 provides resistance to atypical scrapie. The aim of our work was to study the allele pool of the Romanov sheep by the PRNP gene, associated with sustainability to both classical and atypical forms of scrapie. The material for the work was tissue samples of 364 clinically healthy Romanov animals including three modern populations of the Yaroslavl region and one population introduced for breeding in the Kamchatka. Genomic DNA was isolated using the Nexttec columns (Nexttec Biotechnologie GmbH, Germany). Identification of the alleles in the codons 136 (A/T/V), 141 (L/F), 154 (R/H) and 171 (Q/R/H/K) was performed by pyrosequencing on the PSQ96MA device (Quiagen, USA). We found four alleles, 136/154/171 - ARR, ARQ, AHQ and VRQ, and nine haplotypes of PRNP as ARR/ARR, ARR/ARQ, ARR/AHQ, ARQ/ARQ, AHQ/ARQ, AHQ/AHQ, ARR/VRQ, VRQ/AHQ and ARQ/VRQ, relating to all five classes of genetic sustainability to the classical Scrapie. The allele of wild type ARQ (the frequency from 0.704 to 0.933) and the genotype ARQ/ARQ (sustainability class G3) were the prevalent. In all the studied groups, a desirable ARR allele was identified with frequencies varied from 0.022 to 0.089 and averaged 0.066. The undesirable VRQ allele was found in three of the four groups, while its frequency was relatively low - from 0.011 to 0.022. The study of the PRNP polymorphism by four codons 136/141/154/171 revealed the presence of five different alleles - ALRR, ALRQ, ALHQ, VLRQ, AFRQ and ten genotypes. We detected an animal carrying a sensitive to the atypical scrapie allele F at position 141 of PRNP (genotype VLRQ/AFRQ) with the allele frequency of 0.001. The results will be applied in the development of breeding programs for Romanovs, as well as in strategic planning of conservation of the genetic diversity of this unique Russian Northern short-tailed sheep.