Effect of Q211 and K222 PRNP Polymorphic Variants in the Susceptibility of Goats to Oral Infection With Goat Bovine Spongiform Encephalopathy

Bona fide atypical scrapie faithfully reproduced for the first time in a rodent model

Atypical Scrapie, which is not linked to epidemics, is assumed to be an idiopathic spontaneous prion disease in small ruminants. Therefore, its occurrence is unlikely to be controlled through selective breeding or other strategies as it is done for classical scrapie outbreaks. Its spontaneous nature and its sporadic incidence worldwide is reminiscent of the incidence of idiopathic spontaneous prion diseases in humans, which account for more than 85% of the cases in humans. Hence, developing animal models that consistently reproduce this phenomenon of spontaneous PrP misfolding, is of importance to study the pathobiology of idiopathic spontaneous prion disorders. Transgenic mice overexpressing sheep PrP^C with I112 polymorphism (TgShI112, 1-2 × PrP levels compared to sheep brain) manifest clinical signs of a spongiform encephalopathy spontaneously as early as 380 days of age. The brains of these animals show the neuropathological hallmarks of prion disease and biochemical analyses of the misfolded prion protein show a ladder-like PrP^res pattern with a predominant 7-10 kDa band. Brain homogenates from spontaneously diseased transgenic mice were inoculated in several models to assess their transmissibility and characterize the prion strain generated: TgShI112 (ovine I112 ARQ PrP^C), Tg338 (ovine VRQ PrP^C), Tg501 (ovine ARQ PrP^C), Tg340 (human M129 PrP^C), Tg361 (human V129 PrP^C), TgVole (bank vole I109 PrP^C), bank vole (I109I PrP^C), and sheep (AHQ/ARR and AHQ/AHQ churra-tensina breeds). Our analysis of the results of these bioassays concludes that the strain generated in this model is indistinguishable to that causing atypical scrapie (Nor98). Thus, we present the first faithful model for a bona fide, transmissible, ovine, atypical scrapie prion disease.

Classical scrapie in small ruminants is caused by at least four different prion strains

The diversity of goat scrapie strains in Europe has recently been studied using bioassays in a wide collection of rodent models, resulting in the classification of classical scrapie into four different categories. However, the sole use of the first passage does not lead to isolate adaptation and identification of the strains involved and might therefore lead to misclassification of some scrapie isolates. Therefore, this work reports the complete transmission study of a wide collection of goat transmissible spongiform encephalopathy (TSE) isolates by intracranial inoculation in two transgenic mouse lines overexpressing either small ruminant (TgGoat-ARQ) or bovine (TgBov) PrP^C. To compare scrapie strains in sheep and goats, sheep scrapie isolates from different European countries were also included in the study. Once the species barrier phenomenon was overcome, an accurate classification of the isolates was attained. Thus, the use of just two rodent models allowed us to fully differentiate at least four different classical scrapie strains in small ruminants and to identify isolates containing mixtures of strains. This work reinforces the idea that classical scrapie in small ruminants is a prion disease caused by multiple different prion strains and not by a single strain, as is the case for epidemic classical bovine spongiform encephalopathy (BSE-C). In addition, the clear dissimilarity between the different scrapie strains and BSE-C does not support the idea that classical scrapie is the origin of epidemic BSE-C.

Exploration of genetic factors resulting in abnormal disease in cattle experimentally challenged with bovine spongiform encephalopathy

Since the discovery of bovine spongiform encephalopathy (BSE), researchers have orally challenged cattle with infected brain material to study various aspects of disease pathogenesis. Unlike most other pathogens, oral BSE challenge does not always result in the expected clinical presentation and pathology. In a recent study, steers were challenged orally with BSE and all developed clinical signs and were sacrificed and tested. However, despite a similar incubation and clinical presentation, one of the steers did not have detectable PrP^Sc in its brain. Samples from this animal were analysed for genetic differences as well as for the presence of in vitro PrP^Sc seeding activity or infectivity to determine the BSE status of this animal and the potential reasons that it was different. Seeding activity was detected in the brainstem of the abnormal steer but it was approximately one million times less than that found in the normal BSE positive steers. Intra-cranial challenge of bovinized transgenic mice resulted in no transmission of disease. The abnormal steer had different genetic sequences in non-coding regions of the PRNP gene but detection of similar genotypes in Canadian BSE field cases, that showed the expected brain pathology, suggested these differences may not be the primary cause of the abnormal result. Breed composition analysis showed a higher Hereford content in the abnormal steer as well as in two Canadian atypical BSE field cases and several additional abnormal experimental animals. This study could point towards a possible impact of breed composition on BSE pathogenesis.

Transgenic mouse models for the study of prion diseases

Prions are unique agents that challenge the molecular biology dogma by transmitting information on the protein level. They cause neurodegenerative diseases that lack of any cure or treatment called transmissible spongiform encephalopathies. The function of the normal form of the prion protein, the exact mechanism of prion propagation between species as well as at the cellular level and neuron degeneration remains elusive. However, great amount of information known for all these aspects has been achieved thanks to the use of animal models and more precisely to transgenic mouse models. In this chapter, the main contributions of these powerful research tools in the prion field are revised.

Identification and futuristic role of novel polymorphism of caprine PrP gene

The Caprine Prion Protein (PrP) gene polymorphism in three different native Indian goat populations of Southern Odisha, namely Ganjam (a registered breed of India), Ghumusari and Raighar was studied. The 876 bp amplified segment of PrP gene contains full length coding sequence of 771 bp. In Ganjam and Ghumusari goats, any difference of nucleotide sequence was not identified. However, the comparison of nucleotide sequences of Raighar goats and goats of other locality revealed a change in nucleotide at five different positions (G190A, G724A, A727T, C775G and C800T) which includes two non-synonymous nucleotide changes. The non-synonymous nucleotide change resulted a change in amino acid at two different positions (Ser234Cys and Lys246Phe) in mature polypeptide which were not reported earlier and therefore, considered as novel. On the basis of these variants of PrP gene phylogenetic tree was constructed which showed that Ganjam and Raighar goats appeared in different clade. Since any occurrence of Scrapie infection in goats of Odisha was not reported, it can be proposed that these changes in amino acid may be responsible as resistance allele.

Novel Variations in Native Ethiopian Goat breeds PRNP Gene and Their Potential Effect on Prion Protein Stability

Scrapie is a lethal neurodegenerative disease of sheep and goats caused by the misfolding of the prion protein. Variants such as M142, D145, S146, H154, Q211, and K222 were experimentally found to increase resistance or extend scrapie incubation period in goats. We aimed to identify polymorphisms in the Afar and Arsi-Bale goat breeds of Ethiopia and computationally assess the effect of variants on prion protein stability. In the present study, four non-synonymous novel polymorphisms G67S, W68R, G69D, and R159H in the first octapeptide repeat and the highly conserved C-terminus globular domain of goat PrP were detected. The resistant genotype, S146, was detected in >50% of the present population. The current study population showed a genetic diversity in Ethiopian goat breeds. In the insilico analysis, the R68 variant was predicted to increase stability while S67, D69, and H159 decrease the stability of prion protein. The new variants in the octapeptide repeat motif were predicted to decrease amyloidogenicity but H159 increased the hotspot sequence amyloidogenic propensity. These novel variants could be the source of conformational flexibility that may trigger the gain or loss of function by prion protein. Further experimental study is required to depict the actual effects of variants on prion protein stability.

Potential scrapie-associated polymorphisms of the prion protein gene (PRNP) in Korean native black goats

Small ruminants, including sheep and goats are natural hosts of scrapie, and the progression of scrapie pathogenesis is strongly influenced by polymorphisms in the prion protein gene (PRNP). Although Korean native goats have been consumed as meat and health food, the evaluation of the susceptibility to scrapie in these goats has not been performed thus far. Therefore, we investigated the genotype and allele frequencies of PRNP polymorphisms in 211 Korean native goats and compared them with those in scrapie-affected animals from previous studies. We found a total of 12 single nucleotide polymorphisms (SNPs) including 10 nonsynonymous and 2 synonymous SNPs in Korean native goats. Significant differences in allele frequencies of PRNP codons 143 and 146 were found between scrapie-affected goats and Korean native goats (p < 0.01). By contrast, in PRNP codons 168, 211 and 222, there were no significant differences in the genotype and allele frequencies between scrapie-affected animals and Korean native goats. To evaluate structural changes caused by nonsynonymous SNPs, PolyPhen-2, PROVEAN and AMYCO analyses were performed. PolyPhen-2 predicted “possibly damaging” for W102G and R154H, “probably damaging” for G127S. AMYCO predicted relatively low for amyloid propensity of prion protein in Korean native black goats. This is the first study to evaluate the scrapie sensitivity and the first in silico evaluation of nonsynonymous SNPs in Korean native black goats.

Thermostability as a highly dependent prion strain feature

Prion diseases are caused by the conversion of physiological PrP^C into the pathogenic misfolded protein PrP^Sc, conferring new properties to PrP^Sc that vary upon prion strains. In this work, we analyze the thermostability of three prion strains (BSE, RML and 22L) that were heated at 98 °C for 2 hours. PrP^Sc resistance to proteinase K (PrP^res), residual infectivity by mouse bioassay and in vitro templating activity by protein misfolding cyclic amplification (PMCA) were studied. Heated strains showed a huge loss of PrP^res and a radically different infectivity loss: RML was the most thermolabile strain (6 to 7 log10 infectivity loss), followed by 22L (5 log10) while BSE was the most thermostable strain with low or null infectivity reduction showing a clear dissociation between PrP^res and infectivity. These results indicate that thermostability is a strain-specific feature, measurable by PMCA and mouse bioassay, and a great tool to distinguish prion strains.

Variation in the prion protein gene (PRNP) sequence of wild deer in Great Britain and mainland Europe

Susceptibility to prion diseases is largely determined by the sequence of the prion protein gene (PRNP), which encodes the prion protein (PrP). The recent emergence of chronic wasting disease (CWD) in Europe has highlighted the need to investigate PRNP gene diversity in European deer species, to better predict their susceptibility to CWD. Here we report a large genetic survey of six British deer species, including red (Cervus elaphus), sika (Cervus nippon), roe (Capreolus capreolus), fallow (Dama dama), muntjac (Muntiacus reevesii), and Chinese water deer (Hydropotes inermis), which establishes PRNP haplotype and genotype frequencies. Two smaller data sets from red deer in Norway and the Czech Republic are also included for comparison. Overall red deer show the most PRNP variation, with non-synonymous/coding polymorphisms at codons 98, 168, 226 and 247, which vary markedly in frequency between different regions. Polymorphisms P168S and I247L were only found in Scottish and Czech populations, respectively. T98A was found in all populations except Norway and the south of England. Significant regional differences in genotype frequencies were observed within both British and European red deer populations. Other deer species showed less variation, particularly roe and fallow deer, in which identical PRNP gene sequences were found in all individuals analysed. Based on comparison with PRNP sequences of North American cervids affected by CWD and limited experimental challenge data, these results suggest that a high proportion of wild deer in Great Britain may be susceptible to CWD.

Genome-wide definition of selective sweeps reveals molecular evidence of trait-driven domestication among elite goat (Capra species) breeds for the production of dairy, cashmere, and meat

Background

The domestication of wild goats and subsequent intensive trait-driven crossing, inbreeding, and selection have led to dramatic phenotypic purification and intermediate breeds for the high-quality production of dairy, cashmere wool, and meat. Genomic resequencing provides a powerful means for the direct identification of trait-associated sequence variations that underlie molecular mechanisms of domestication.

Results

Here, we report our effort to define such variations based on data from domestic goat breeds (Capra aegagrus hircus; five each) selected for dairy, cashmere, and meat production in reference to their wild ancestors, the Sindh ibex (Capra aegagrus blythi; two) and the Markhor (Capra falconeri; two). Using ∼24 million high-quality single nucleotide polymorphisms (SNPs), ∼1.9 million insertions/deletions, and 2,317 copy number variations, we define SNP-desert-associated genes (SAGs), domestic-associated genes (DAGs), and trait-associated genes (TAGs) and attempt to associate them with quantitative trait loci (QTL), domestication, and agronomic traits. A greater majority of SAGs shared by all domestic breeds are classified into Gene Ontology categories of metabolism and cell cycle. DAGs, together with some SAGs, are most relevant to behavior, immunity, and trait specificity. Whereas, TAGs such as growth differentiation factor 5 and fibroblast growth factor 5 for bone and hair growth, respectively, appear to be directly involved in growth regulation.

Conclusions

When investigating the divergence of Capra populations, the sequence variations and candidate function-associated genes we have identified provide valuable molecular markers for trait-driven genetic mapping and breeding.

Protective Effect of Val129-PrP against Bovine Spongiform Encephalopathy but not Variant Creutzfeldt-Jakob Disease

Bovine spongiform encephalopathy (BSE) is the only known zoonotic prion that causes variant Creutzfeldt-Jakob disease (vCJD) in humans. The major risk determinant for this disease is the polymorphic codon 129 of the human prion protein (Hu-PrP), where either methionine (Met₁₂₉) or valine (Val₁₂₉) can be encoded. To date, all clinical and neuropathologically confirmed vCJD cases have been Met₁₂₉ homozygous, with the exception of 1 recently reported Met/Val heterozygous case. Here, we found that transgenic mice homozygous for Val₁₂₉ Hu-PrP show severely restricted propagation of the BSE prion strain, but this constraint can be partially overcome by adaptation of the BSE agent to the Met₁₂₉ Hu-PrP. In addition, the transmission of vCJD to transgenic mice homozygous for Val₁₂₉ Hu-PrP resulted in a prion with distinct strain features. These observations may indicate increased risk for vCJD secondary transmission in Val₁₂₉ Hu-PrP–positive humans with the emergence of new strain features.

Classic and atypical scrapie - a genetic perspective

Scrapie was the first prion disease to be recognised and the study of this disease in sheep and goats has provided a wealth of information not only for scrapie but also for the other prion diseases. All prion diseases are under strong genetic control of the prion gene PRNP, independent of whether they are typical or atypical scrapie and which of the different prion strains is causing infection. Decades of studies using experimental disease challenges and field surveys have established disease association models, in which species-specific amino acid variations in the prion or PrP protein, encoded by the PRNP gene, can predict disease susceptibility or resistance. PRNP genetics represents an important and successful basis for implementing scrapie eradication strategies in sheep and goats. In general terms these studies have revealed that there appear to be many more amino acid changes in PrP leading to increased resistance than to higher susceptibility. Most changes are in the globular part of PrP protein and three regions appear to have major influence. This knowledge can be transferred into prion diseases of other species to facilitate genetic control strategies. However, an obstacle remains with the lack of fully understanding the underlying molecular mechanism, impeding our ability to deal with the difference in the genetic control between typical and atypical forms of scrapie or to predict association in newly infected species. This chapter will discuss the advances in both typical and atypical scrapie from a genetic perspective.

Effect of Polymorphisms at Codon 146 of the Goat PRNP Gene on Susceptibility to Challenge with Classical Scrapie by Different Routes

This report presents the results of experimental challenges of goats with scrapie by both the intracerebral (i.c.) and oral routes, exploring the effects of polymorphisms at codon 146 of the goat PRNP gene on resistance to disease. The results of these studies illustrate that while goats of all genotypes can be infected by i.c. challenge, the survival distribution of the animals homozygous for asparagine at codon 146 was significantly shorter than those of animals of all other genotypes (chi-square value, 10.8; P = 0.001). In contrast, only those animals homozygous for asparagine at codon 146 (NN animals) succumbed to oral challenge. The results also indicate that any cases of infection in non-NN animals can be detected by the current confirmatory test (immunohistochemistry), although successful detection with the rapid enzyme-linked immunosorbent assay (ELISA) was more variable and dependent on the polymorphism. Together with data from previous studies of goats exposed to infection in the field, these data support the previously reported observations that polymorphisms at this codon have a profound effect on susceptibility to disease. It is concluded that only animals homozygous for asparagine at codon 146 succumb to scrapie under natural conditions.IMPORTANCE In goats, like in sheep, there are PRNP polymorphisms that are associated with susceptibility or resistance to scrapie. However, in contrast to the polymorphisms in sheep, they are more numerous in goats and may be restricted to certain breeds or geographical regions. Therefore, eradication programs must be specifically designed depending on the identification of suitable polymorphisms. An initial analysis of surveillance data suggested that such a polymorphism in Cypriot goats may lie in codon 146. In this study, we demonstrate experimentally that NN animals are highly susceptible after i.c. inoculation. The presence of a D or S residue prolonged incubation periods significantly, and prions were detected in peripheral tissues only in NN animals. In oral challenges, prions were detected only in NN animals, and the presence of a D or S residue at this position conferred resistance to the disease. This study provides an experimental transmission model for assessing the genetic susceptibility of goats to scrapie.

Protecting effect of PrP codons M142 and K222 in goats orally challenged with bovine spongiform encephalopathy prions

Breeding towards genetic resistance to prion disease is effective in eliminating scrapie. In sheep, classical forms of scrapie have been eradicated almost completely in several countries by breeding programs using a prion protein (PrP) gene (PRNP) amino acid polymorphism. For goats, field and experimental studies have provided evidence for several amino acid polymorphisms that are associated with resistance to scrapie, but only limited data are available concerning the susceptibility of caprine PRNP genotypes to BSE. In this study, goat kids representing five PRNP genotypes based on three polymorphisms (M142, Q211 and K222 and the wild type I142, R211 and Q222) were orally challenged with bovine or goat BSE. Wild type goats were killed with clinical signs between 24-28 months post inoculation (mpi) to both challenges, and goats with genotype R/Q211 succumbed between 29-36 mpi. I/M142 goats developed clinical signs at 44-45 mpi and M/M142 goats remained healthy until euthanasia at 48 mpi. None of the Q/K222 goats showed definite clinical signs. Taken together the highest attack ratios were seen in wild type and R/Q211 goats, and the lowest in I/M142, M/M142 and Q/K222. In all genotype groups, one or more goats remained healthy within the incubation period in both challenges and without detectable PrP deposition in the tissues. Our data show that both the K222 and M142 polymorphisms lengthen the incubation period significantly compared to wild type animals, but only K222 was associated with a significant increase in resistance to BSE infection after oral exposure to both BSE sources.

Genetic resistance to transmissible spongiform encephalopathies (TSE) in goats

Breeding programmes to promote resistance to classical scrapie, similar to those for sheep in existing transmissible spongiform encephalopathies (TSE) regulations, have not been established in goats. The European Commission requested a scientific opinion from EFSA on the current knowledge of genetic resistance to TSE in goats. An evaluation tool, which considers both the weight of evidence and strength of resistance to classical scrapie of alleles in the goat PRNP gene, was developed and applied to nine selected alleles of interest. Using the tool, the quality and certainty of the field and experimental data are considered robust enough to conclude that the K222, D146 and S146 alleles both confer genetic resistance against classical scrapie strains known to occur naturally in the EU goat population, with which they have been challenged both experimentally and under field conditions. The weight of evidence for K222 is greater than that currently available for the D146 and S146 alleles and for the ARR allele in sheep in 2001. Breeding for resistance can be an effective tool for controlling classical scrapie in goats and it could be an option available to member states, both at herd and population levels. There is insufficient evidence to assess the impact of K222, D146 and S146 alleles on susceptibility to atypical scrapie and bovine spongiform encephalopathy (BSE), or on health and production traits. These alleles are heterogeneously distributed across the EU Member States and goat breeds, but often at low frequencies (< 10%). Given these low frequencies, high selection pressure may have an adverse effect on genetic diversity so any breeding for resistance programmes should be developed at Member States, rather than EU level and their impact monitored, with particular attention to the potential for any negative impact in rare or small population breeds.

Transmission and Replication of Prions

Transmissible spongiform encephalopathies (TSEs) are a group of progressive, invariably fatal diseases that affect the nervous system of many mammals including humans. The key molecular event in the pathogenesis of TSEs is the conversion of the cellular prion protein PrP^C into a disease-associated isoform PrP^Sc. The "protein-only hypothesis" argues that PrP^Sc itself is the infectious agent. In effect, PrP^Sc can adopt several structures that represent different prion strains. The interspecies transmission of TSEs is difficult because of differences between the host and donor primary PrP sequence. However, transmission is not impossible as this occurred when bovine spongiform encephalopathy spread to humans causing variant Creutzfeldt-Jakob disease (vCJD). This event determined a need for a thorough understanding of prion replication and transmission so that we could be one step ahead of further threats for human health. This chapter focuses on these concepts and on new insights gained into prion propagation mechanisms.

A cross-sectional study of PRNP gene in two native Sicilian goat populations in Italy: a relation between prion gene polymorphisms and scrapie incidence

Transmissible spongiform encephalopathies (TSEs) are a group of neurodegenerative diseases affecting humans and animals, and scrapie in small ruminants is considered the archetype of TSEs. Derivata di Siria is a native dairy goat of Sicily (south Italy), which is related to Syrian goat breeds. Scrapie disease is considered endemic in Sicily since 1997, following the administration of an infected vaccine.Derivata di Siria goatswere involved in six of 66 scrapie-infected flocks in Sicily. Prion protein gene (PRNP) analysis revealed that none of the scrapie cases carried the p.Gln222Lys variant. Sequencing of PRNP in this goat population showed a high frequency (15%) of p.Gln222Lys variant confirming its association with scrapie resistance. PRNP polymorphisms were also analysed in the population of Pantelleria, a small Sicilian Island, where scrapie has never been reported. The native goat breed 'Pantesca' was maintained up to almost 80 years and the size of the sheep population on this island has historically been very low. Currently, a crossbreed goat population of 253 heads is present on the island. PRNP genotyping of Pantelleria goats showed genetic variation, with low presence of wild-type goats and the lack of protective alleles. These data reinforce the association between PRNP polymorphisms in small ruminants and scrapie incidence.

Prion protein gene polymorphisms in Turkish native goat breeds

Susceptibility to 'scrapie' disease in goats is influenced by polymorphisms of the prion protein (PRNP) gene. The aim of this study was to identify PRNP gene polymorphisms in a total of 356 scrapie disease-free goats from 10 Turkish native breeds. Eighteen single-nucleotide polymorphisms were detected in the caprine PRNP open-reading frame. Ten previously described amino acid substitutions (I142M, H143R, N146S, N146D, R151H, R154H, P168Q, R211Q, Q222K and P240S) and two novel dimorphisms (G134E and Q163P) were identified. The strongest association between caprine PRNP and relative resistance to scrapie disease has been reported previously for polymorphisms at codons 146 (S/D) and 222 (K). In the present study, these three PrP variants were relatively rare with 6.3%. This is the first report on PRNP gene variation in Turkish native goat breeds and our knowledge of these polymorphisms will assist goat breeding programmes to reduce the risk of scrapie.

Goat K222-PrPC polymorphic variant does not provide resistance to atypical scrapie in transgenic mice

Host prion (PrP^C) genotype is a major determinant for the susceptibility to prion diseases. The Q/K₂₂₂-PrP^C polymorphic variant provides goats and mice with high resistance against classical scrapie and bovine spongiform encephalopathy (BSE); yet its effect against atypical scrapie is unknown. Here, transgenic mice expressing the goat wild-type (wt) or the K₂₂₂-PrP^C variant were intracerebrally inoculated with several natural cases of atypical scrapie from sheep and goat and their susceptibility to the prion disease was determined. Goat wt and K₂₂₂-PrP^C transgenic mice were 100% susceptible to all the atypical scrapie isolates, showing similar survival times and almost identical disease phenotypes. The capacity of the K₂₂₂-PrP^C variant to replicate specifically the atypical scrapie strain as efficiently as the goat wt PrP^C, but not the classical scrapie or cattle-BSE as previously reported, further suggests the involvement of concrete areas of the host PrP^C in the strain-dependent replication of prions.

Scrapie incidence and PRNP polymorphisms: rare small ruminant breeds of Sicily with TSE protecting genetic reservoirs

Background

Transmissible spongiform encephalopathies (TSE) are fatal neurodegenerative diseases of several mammalian species, including humans. In Italy, the active surveillance through rapid tests on brain stem from small ruminants started in 2002 on randomly selected samples of healthy slaughtered animals. Sampling number was proportionally related to the regional small ruminant population. Of the twenty Italian regions, Sicily has the second largest population of small ruminants which is mainly constituted by crossbreed animals (>70 %). Sicily contains also three native sheep breeds Pinzirita, Comisana and Valle del Belice. Native goat breeds are Girgentana, Messinese, Argentata dell’Etna, Maltese and Rossa Mediterranea. The polymorphisms of prion protein gene (PRNP) may influence disease susceptibility and breeding programs for genetic TSE resistance are being applied in sheep. Protective alleles have been recently reported for goats also. These differ from those in sheep and may allow breeding programs in the near future.

In this paper the data of active surveillance for scrapie control in general population of small ruminants in Sicily are reported together with the analysis on the polymorphism of PRNP in a number of Sicilian autochthonous breeds. The evaluation of the frequency of protective alleles is fundamental for the implementation of a TSE resistance breeding program.

Results

TSE surveillance in small ruminants in Sicily showed a of total fifty seven scrapie outbreaks from 1997 to 2014 involving mainly crossbreed animals. The PRNP polymorphism analysis in autochthonous breeds showed protective allele frequencies of 30–40 % ARR in sheep and 12–18 % K222 in three of the four goat breeds; these breeds are distributed over limited areas of the island.

Conclusion

The study on PRNP polymorphisms in Sicilian small ruminant population showed higher frequency of the protective alleles compared to most other European breeds. Our results suggest that PRNP genetic variety in Sicilian sheep and goats can be a resource for TSE resistance breeding programmes while maintaining the conservation of endangered breeds and valorisation of their typical food products.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-016-0766-9) contains supplementary material, which is available to authorized users.

Review: A review on classical and atypical scrapie in caprine: Prion protein gene polymorphisms and their role in the disease

Scrapie is a naturally occurring transmissible spongiform encephalopathy in sheep and goat. It has been known for ~250 years and is characterised by the accumulation of an abnormal isoform of a host-encoded prion protein that leads to progressive neurodegeneration and death. Scrapie is recognised in two forms, classical and atypical scrapie. The susceptibility to both types of scrapie is influenced by polymorphisms of the prion protein gene (PRNP). Sheep susceptibility or resistance to classical scrapie is strongly regulated by the polymorphisms at codons 136, 154 and 171 of the PRNP. The genetic role in atypical scrapie in sheep has been defined by polymorphisms at codons 141, 154 and 171, which are associated with different degrees of risk in the occurrence of the ovine disease. Progress has been achieved in the prevention of scrapie in sheep due to efficient genetic breeding programmes based on eradication and control of the disease. In Europe, the success of these programmes has been verified by applying eradication and genetic selection plans. In general terms, the ovine selection plans aim to eliminate and reduce the susceptible allele and to enrich the resistant allele ARR. During outbreaks all susceptible animals are slaughtered, only ARR/ARR resistant rams and sheep and semi-resistant females are preserved. In the occurrence of scrapie positive goats a complete cull of the flock (stamping out) is performed with great economic loss and severe risk of extinction for the endangered breeds. The ability to select scrapie-resistant animals allows to define new breeding strategies aimed to boost genetic progress while reducing costs during scrapie outbreaks. Allelic variants of PRNP can be protective for caprine scrapie, and the knowledge of their distribution in goats has become very important. Over the past few years, the integration of genetic information on goat populations could be used to make selection decisions, commonly referred to as genetic selection. The objective of this review was to summarise the main findings of polymorphisms of the caprine prion protein (PrP) gene and to discuss the possible application of goat breeding schemes integrating genetic selection, with their relative advantages and limitations.

Variation in the prion protein sequence in Dutch goat breeds

Scrapie is a neurodegenerative disease occurring in goats and sheep. Several haplotypes of the prion protein increase resistance to scrapie infection and may be used in selective breeding to help eradicate scrapie. In this study, frequencies of the allelic variants of the PrP gene are determined for six goat breeds in the Netherlands. Overall frequencies in Dutch goats were determined from 768 brain tissue samples in 2005, 766 in 2008 and 300 in 2012, derived from random sampling for the national scrapie surveillance without knowledge of the breed. Breed specific frequencies were determined in the winter 2013/2014 by sampling 300 breeding animals from the main breeders of the different breeds. Detailed analysis of the scrapie-resistant K222 haplotype was carried out in 2014 for 220 Dutch Toggenburger goats and in 2015 for 942 goats from the Saanen derived White Goat breed. Nine haplotypes were identified in the Dutch breeds. Frequencies for non-wild type haplotypes were generally low. Exception was the K222 haplotype in the Dutch Toggenburger (29%) and the S146 haplotype in the Nubian and Boer breeds (respectively 7 and 31%). The frequency of the K222 haplotype in the Toggenburger was higher than for any other breed reported in literature, while for the White Goat breed it was with 3.1% similar to frequencies of other Saanen or Saanen derived breeds. Further evidence was found for the existence of two M142 haplotypes, M142 /S240 and M142 /P240 . Breeds vary in haplotype frequencies but frequencies of resistant genotypes are generally low and consequently selective breeding for scrapie resistance can only be slow but will benefit from animals identified in this study. The unexpectedly high frequency of the K222 haplotype in the Dutch Toggenburger underlines the need for conservation of rare breeds in order to conserve genetic diversity rare or absent in other breeds.

Prion protein genotype survey confirms low frequency of scrapie-resistant K222 allele in British goat herds

Scrapie in goats is a transmissible, fatal prion disease, which is endemic in the British goat population. The recent success in defining caprine PRNP gene variants that provide resistance to experimental and natural classical scrapie has prompted the authors to conduct a survey of PRNP genotypes in 10 goat breeds and 52 herds to find goats with the resistant K222 allele. They report here the frequencies in 1236 tested animals of the resistance-associated K222 and several other alleles by breed and herd. Eight animals were found to be heterozygous QK222 goats (0.64 per cent genotype frequency, 95 per cent CI 0.28 to 1.27 per cent) but no homozygous KK222 goats were detected. The K222 allele was found in Saanen, Toggenburg and Anglo-Nubian goats. The fact that only a few goats with the K222 allele have been identified does not preclude the possibility to design and implement successful breeding programmes at national level.