Role of the goat K222-PrP(C) polymorphic variant in prion infection resistance

Classical BSE dismissed as the cause of CWD in Norwegian red deer despite strain similarities between both prion agents

The first case of CWD in a Norwegian red deer was detected by a routine ELISA test and confirmed by western blotting and immunohistochemistry in the brain stem of the animal. Two different western blotting tests were conducted independently in two different laboratories, showing that the red deer glycoprofile was different from the Norwegian CWD reindeer and CWD moose and from North American CWD. The isolate showed nevertheless features similar to the classical BSE (BSE-C) strain. Furthermore, BSE-C could not be excluded based on the PrPSc immunohistochemistry staining in the brainstem and the absence of detectable PrPSc in the lymphoid tissues. Because of the known ability of BSE-C to cross species barriers as well as its zoonotic potential, the CWD red deer isolate was submitted to the EURL Strain Typing Expert Group (STEG) as a BSE-C suspect for further investigation. In addition, different strain typing in vivo and in vitro strategies aiming at identifying the BSE-C strain in the red deer isolate were performed independently in three research groups and BSE-C was not found in it. These results suggest that the Norwegian CWD red deer case was infected with a previously unknown CWD type and further investigation is needed to determine the characteristics of this potential new CWD strain.

Novel polymorphisms in the prion protein gene (PRNP) and stability of the resultant prion protein in different horse breeds

Prion diseases are fatal neurodegenerative disorders in which the main pathogenic event is the conversion of the cellular prion protein (PrPC) into an abnormal and misfolded isoform known as PrPSc. Most prion diseases and their susceptibility and pathogenesis are mainly modulated by the PRNP gene that codes for PrP. Mutations and polymorphisms in the PRNP gene can alter PrPC amino acid sequence, leading to a change in transmission efficiency depending on the place where it occurs. Horses are animals that are considered to be highly resistant to prions. Several studies have attempted to identify polymorphisms in the PRNP gene that explain the reason for this high resistance. In this study, we have analysed 207 horses from 20 different breeds, discovering 3 novel PRNP polymorphisms. By using computer programmes such as PolyPhen-2, PROVEAN, PANTHER, Meta-SNP and PredictSNP, we have predicted the possible impact that these new polymorphisms would have on the horse prion protein. In addition, we measured the propensity for amyloid aggregation using AMYCO and analysed the lack of hydrogen bridges that these changes would entail together with their electrostatic potentials using Swiss-PdbViewer software, showing that an increased amyloid propensity could be due to changes at the level of electrostatic potentials.

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.

Polymorphisms of Codons 110, 146, 211 and 222 at the Goat PRNP Locus and Their Association with Scrapie in Greece

Scrapie is considered an endemic disease in both sheep and goats in Greece. However, contrary to sheep, in goats more than one prion protein (PrP) polymorphism has been recognized as a candidate for resistance breeding against the disease. For an impression, candidates which are circulating, (i) brain samples (n = 525) from scrapie-affected (n = 282) and non-affected (n = 243) animals within the national surveillance program, and (ii) individual blood samples (n = 1708) from affected (n = 241) and non-affected (n = 1467) herds, in a large part of mainland Greece and its islands, were collected and assayed. A dedicated Taqman method was used to test for amino acid polymorphisms 110T/P, 146N/S/D, 211R/Q, and 222Q/K. Highly prevalent genotypes were 110TT, 146NN, 211RR, and 222QQ. The frequencies of polymorphisms in blood and negative brain samples for codons 110P, 211Q, and 222K were 4.0%, 3.0%, and 1.9%, respectively, while 146D (0.7%) was present only on Karpathos island. Codon 110P was exclusively found in scrapie-negative brains, and homozygous 110P/P in two scrapie-negative goats. It is concluded that breeding programs in Karpathos could focus on codon 146D, while in other regions carriers of the 110P and 222K allele should be sought. Case-control and challenge studies are now necessary to elucidate the most efficient breeding strategies.

Canine D163-PrP polymorphic variant does not provide complete protection against prion infection in small ruminant PrP context

E/D₁₆₃ polymorphism of dog prion protein (PrP) has been recently proposed as the variant responsible for canid prion resistance. To further investigate the protective role of this variant against prion replication, the transgenic mouse model OvPrP-Tg532 expressing sheep/goat PrP carrying the substitution D₁₆₂ (equivalent to D₁₆₃ position of dog PrP) was generated and intracranially inoculated with a broad collection of small ruminant prion strains. OvPrP-Tg532 mice showed resistance to classical bovine spongiform encephalopathy (BSE) from sheep and some classical scrapie isolates from sheep and goat but were susceptible to ovine atypical L-BSE and numerous classical scrapie isolates. Strikingly, some of these classical scrapie isolates showed a shift in their prion strain properties. These results suggest that other PrP residues apart from E/D₁₆₃ variant of dog PrP or factors distinct than PrP may participate in prion resistance of canids and that different factors may be required for D₁₆₂ sheep PrP to provide effective protection to sheep against ruminant prions.

Genetic Variation in the Prion Protein Gene (PRNP) of Two Tunisian Goat Populations

Simple Summary

Goat production is contributing to the economic and social development of rural areas in arid lands, within harsh conditions of Southern Tunisia. In this geographic zone, there are two caprine populations: the native goat population and the crossed goat population. Genotyping goats for the prion protein gene (PRNP) allows us to estimate their level of genetic susceptibility to scrapie disease. In the present work, the Sanger sequencing method of the entire PRNP coding sequence was used to determine the different PRNP genotypes and haplotypes in two populations (116 animals). This study represents the first investigation on goats’ PRNP genetic variability in Tunisia, and the results are useful in the design of national breeding programs.

Abstract

Scrapie is a fatal prion disease. It belongs to transmissible spongiform encephalopathies (TSEs), and occurs in sheep and goats. Similarly, to ovine species, the prion protein gene (PRNP) plays a major role in conferring resistance or susceptibility to TSE in goats. This study assesses the variability of PRNP in native and crossed-breed goat populations raised in the Southeast of Tunisia and provides information on the distribution of PRNP haplotypes and genotypes in these goat populations. A total of 116 unrelated goats including 82 native and 34 crossed-breed goats were screened for PRNP polymorphisms using Sanger sequencing. Sequence analysis revealed 10 non-synonymous polymorphisms (G37V, M137I, R139S, I142M, H143R, N146D, R154H, R211Q, Q222K, and S240P), giving rise to 12 haplotypes and 23 genotypes. Moreover, four silent mutations were detected at codons 30, 42, 138, and 179; the former was reported for the first time in goat (nucleotide 60 c→t). Interestingly, the PrP variants associated with resistance (D146 and K222) or with a prolonged incubation time of goat to scrapie (M142, R143, H154, Q211) were absent or detected with low frequencies except for H154 variant, which is present with high frequency (1%, 1%, 4%, 0%, 88%, and 6%, respectively, for native goats, and 0%, 1%, 0%, 1%, 78%, and 1%, respectively, for crossed goats). The analysis of PRNP polymorphisms of goats raised in other regions of the country will be useful in getting a global view of PRNP genetic variability and the feasibility of goat breeding programs in Tunisia.

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.

Classical and Atypical Scrapie in Sheep and Goats. Review on the Etiology, Genetic Factors, Pathogenesis, Diagnosis, and Control Measures of Both Diseases

Prion diseases, such as scrapie, are neurodegenerative diseases with a fatal outcome, caused by a conformational change of the cellular prion protein (PrP^C), originating with the pathogenic form (PrP^Sc). Classical scrapie in small ruminants is the paradigm of prion diseases, as it was the first transmissible spongiform encephalopathy (TSE) described and is the most studied. It is necessary to understand the etiological properties, the relevance of the transmission pathways, the infectivity of the tissues, and how we can improve the detection of the prion protein to encourage detection of the disease. The aim of this review is to perform an overview of classical and atypical scrapie disease in sheep and goats, detailing those special issues of the disease, such as genetic factors, diagnostic procedures, and surveillance approaches carried out in the European Union with the objective of controlling the dissemination of scrapie disease.

PRNP Polymorphisms in Eight Local Goat Populations/Breeds from Central and Southern Italy

In goats, as in sheep, genotypes of the prion protein gene (PRNP) can influence animals' susceptibility to scrapie. Since the polymorphic codons in sheep are well known, a genetic selection plan has been implemented in Europe, in order to reduce the prevalence of susceptible genotypes to scrapie. In Italy, no breeding plan for scrapie resistance in goats has been adopted, yet. Likewise, according to the most recent modification of Regulation EU 999/2001 (Regulation EU 772/2020) of the European Commission (EU), based on all the available experimental and in field data, K222, D146 and S146 polymorphisms could be used as scrapie resistance alleles in genetic management both in scrapie outbreaks and in disease prevention. In order to collect data on the variability of PRNP, the present study aimed to analyze the sequence of the PRNP gene in eight Italian local goat populations/breeds reared in central and southern Italy (Bianca Monticellana, Capestrina, Facciuta della Valnerina, Fulva del Lazio, Garganica, Grigia Ciociara, Grigia Molisana, and Teramana), some of which were investigated for the first time; moreover, two cosmopolitan breeds (Alpine and Saanen) were included. Blood samples were collected from 219 goats. Genomic DNA was extracted from whole blood. DNA was used as template in PCR amplification of the entire PRNP open reading frame (ORF). Purified amplicons have been sequenced and aligned to Capra hircus PRNP. Particularly, the alleles carrying the resistance-related 222 K polymorphism occurred in all populations with a frequency between 2.5% and 12.5%. An additional resistance allele carrying the S146 variant was observed with a frequency of 3.7% only in the Alpine breed. For three of the estimated alleles, we could not establish if the found double polymorphisms in heterozygosis were in phase, due to technical limitations. In this context, in addition to selective culling in scrapie outbreaks according to the European regulation in force, in the future, selection plans could be adopted to deal with scrapie and to control its diffusion, meanwhile paying attention to preserve a high variability of PRNP.

Met166 -Glu168 residues in human PrP β2-α2 loop account for evolutionary resistance to prion infection

Aims

The amino acid sequence of prion protein (PrP) is a key determinant in the transmissibility of prion diseases. While PrP sequence is highly conserved among mammalian species, minor changes in the PrP amino acid sequence may confer alterations in the transmissibility of prion diseases. Classical bovine spongiform encephalopathy (C‐BSE) is the only zoonotic prion strain reported to date causing variant Creutzfeldt‐Jacob disease (vCJD) in humans, although experimental transmission points to atypical L‐BSE and some classical scrapie isolates as also zoonotic. The precise molecular elements in the human PrP sequence that limit the transmissibility of prion strains such as sheep/goat scrapie or cervid chronic wasting disease (CWD) are not well known.

Methods

The transmissibility of a panel of diverse prions from different species was compared in transgenic mice expressing either wild‐type human PrP^C (MDE‐HuTg340) or a mutated human PrP^C harbouring Val₁₆₆‐Gln₁₆₈ amino acid changes (VDQ‐HuTg372) in the β2‐α2 loop instead of Met₁₆₆‐Glu₁₆₈ wild‐type variants.

Results

VDQ‐HuTg372 mice were more susceptible to prions than MDE‐HuTg340 mice in a strain‐dependent manner.

Conclusions

Met₁₆₆‐Glu₁₆₈ amino acid residues present in wild‐type human PrP^C are molecular determinants that limit the propagation of most prion strains assayed in the human PrP context.

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.

MicroRNA Alterations in a Tg501 Mouse Model of Prion Disease

MicroRNAs (miRNAs) may contribute to the development and pathology of many neurodegenerative diseases, including prion diseases. They are also promising biomarker candidates due to their stability in body fluids. We investigated miRNA alterations in a Tg501 mouse model of prion diseases that expresses a transgene encoding the goat prion protein (PRNP). Tg501 mice intracranially inoculated with mouse-adapted goat scrapie were compared with age-matched, mock inoculated controls in preclinical and clinical stages. Small RNA sequencing from the cervical spinal cord indicated that miR-223-3p, miR-151-3p, and miR-144-5p were dysregulated in scrapie-inoculated animals before the onset of symptoms. In clinical-stage animals, 23 significant miRNA alterations were found. These miRNAs were predicted to modify the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways including prion disease, extracellular matrix interactions, glutaminergic synapse, axon guidance, and transforming growth factor-beta signaling. MicroRNAs miR-146a-5p (up in cervical spinal cord) and miR-342-3p (down in cervical spinal cord, cerebellum and plasma), both indicated in neurodegenerative diseases earlier, were verified by quantitative real-time polymerase chain reaction (qRT-PCR). Minimal changes observed before the disease onset suggests that most miRNA alterations observed here are driven by advanced prion-associated pathology, possibly limiting their use as diagnostic markers. However, the results encourage further mechanistic studies on miRNA-regulated pathways involved in these neurodegenerative conditions.

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.

Goats naturally devoid of PrPC are resistant to scrapie

Prion diseases are progressive and fatal, neurodegenerative disorders described in humans and animals. According to the "protein-only" hypothesis, the normal host-encoded prion protein (PrPC) is converted into a pathological and infectious form (PrPSc) in these diseases. Transgenic knockout models have shown that PrPC is a prerequisite for the development of prion disease. In Norwegian dairy goats, a mutation (Ter) in the prion protein gene (PRNP) effectively blocks PrPC synthesis. We inoculated 12 goats (4 PRNP+/+, 4 PRNP+/Ter, and 4 PRNPTer/Ter) intracerebrally with goat scrapie prions. The mean incubation time until clinical signs of prion disease was 601 days post-inoculation (dpi) in PRNP+/+ goats and 773 dpi in PRNP+/Ter goats. PrPSc and vacuolation were similarly distributed in the central nervous system (CNS) of both groups and observed in all brain regions and segments of the spinal cord. Generally, accumulation of PrPSc was limited in peripheral organs, but all PRNP+/+ goats and 1 of 4 PRNP+/Ter goats were positive in head lymph nodes. The four PRNPTer/Ter goats remained healthy, without clinical signs of prion disease, and were euthanized 1260 dpi. As expected, no accumulation of PrPSc was observed in the CNS or peripheral tissues of this group, as assessed by immunohistochemistry, enzyme immunoassay, and real-time quaking-induced conversion. Our study shows for the first time that animals devoid of PrPC due to a natural mutation do not propagate prions and are resistant to scrapie. Clinical onset of disease is delayed in heterozygous goats expressing about 50% of PrPC levels.

Porcine Prion Protein as a Paradigm of Limited Susceptibility to Prion Strain Propagation

Although experimental transmission of bovine spongiform encephalopathy (BSE) to pigs and transgenic mice expressing pig cellular prion protein (PrPC) (porcine PrP [PoPrP]-Tg001) has been described, no natural cases of prion diseases in pig were reported. This study analyzed pig-PrPC susceptibility to different prion strains using PoPrP-Tg001 mice either as animal bioassay or as substrate for protein misfolding cyclic amplification (PMCA). A panel of isolates representatives of different prion strains was selected, including classic and atypical/Nor98 scrapie, atypical-BSE, rodent scrapie, human Creutzfeldt-Jakob-disease and classic BSE from different species. Bioassay proved that PoPrP-Tg001-mice were susceptible only to the classic BSE agent, and PMCA results indicate that only classic BSE can convert pig-PrPC into scrapie-type PrP (PrPSc), independently of the species origin. Therefore, conformational flexibility constraints associated with pig-PrP would limit the number of permissible PrPSc conformations compatible with pig-PrPC, thus suggesting that pig-PrPC may constitute a paradigm of low conformational flexibility that could confer high resistance to the diversity of prion strains.

Characterization of goat prions demonstrates geographical variation of scrapie strains in Europe and reveals the composite nature of prion strains

Bovine Spongiform Encephalopathy (BSE) is the only animal prion which has been recognized as a zoonotic agent so far. The identification of BSE in two goats raised the need to reliably identify BSE in small ruminants. However, our understanding of scrapie strain diversity in small ruminants remains ill-defined, thus limiting the accuracy of BSE surveillance and spreading fear that BSE might lurk unrecognized in goats. We investigated prion strain diversity in a large panel of European goats by a novel experimental approach that, instead of assessing the neuropathological profile after serial transmissions in a single animal model, was based on the direct interaction of prion isolates with several recipient rodent models expressing small ruminants or heterologous prion proteins. The findings show that the biological properties of scrapie isolates display different patterns of geographical distribution in Europe and suggest that goat BSE could be reliably discriminated from a wide range of biologically and geographically diverse goat prion isolates. Finally, most field prion isolates showed composite strain features, with discrete strain components or sub-strains being present in different proportions in individual goats or tissues. This has important implications for understanding the nature and evolution of scrapie strains and their transmissibility to other species, including humans.

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.

Two novel amino acid substitutions in highly conserved regions of prion protein (PrP) and a high frequency of a scrapie protective variant in native Ethiopian goats

BACKGROUND

Polymorphisms of the prion protein gene may influence scrapie susceptibility in small ruminants through modified protein conformation. At least 47 amino acid substitutions and 19 silent polymorphisms have been described in goat PRNP reported from several countries. The objective of this study was to investigate PRNP polymorphisms of native Ethiopian goat breeds and compare the results with other goat breeds.

RESULTS

The analysis of the prion protein gene PRNP in 229 goats belonging to three of the main Ethiopian native goat breeds showed a remarkably high frequency (> 34.6%) of p.(Asn146Ser) in these breeds, a variant involved in scrapie resistance in Cyprus. In addition, two novel amino-acid substitutions p.(Gly127Ala) and p.(Thr193Ile), with frequencies ranging from 1.5 to 7.3% were detected. Both amino acids are well conserved in prion proteins (PrP) of most species and these changes have never been reported before in goats worldwide. Residue 127 is within the N-terminal domain of PrP and is probably involved in the recruitment of neural cell adhesion molecules (NCAM). Residue 193 is within the highly conserved string of 4 threonines that plays a role in determining the efficiency of prion protein conversion towards its pathological form.

CONCLUSION

Two novel coding polymorphisms and a high frequency of a scrapie protective variant indicate a high level of genetic diversity in PRNP of Ethiopian goats. This finding increases the interest in exploring PRNP polymorphisms of native goat breeds in areas where cross breeding with foreign goats has rarely occurred.

Genetic profile of scrapie codons 146, 211 and 222 in the PRNP gene locus in three breeds of dairy goats

Polymorphisms at PRNP gene locus have been associated with resistance against classical scrapie in goats. Genetic selection on this gene within appropriate breeding programs may contribute to the control of the disease. The present study characterized the genetic profile of codons 146, 211 and 222 in three dairy goat breeds in Greece. A total of 766 dairy goats from seven farms were used. Animals belonged to two indigenous Greek, Eghoria (n = 264) and Skopelos (n = 287) and a foreign breed, Damascus (n = 215). Genomic DNA was extracted from blood samples from individual animals. Polymorphisms were detected in these codons using Real-Time PCR analysis and four different Custom TaqMan® SNP Genotyping Assays. Genotypic, allelic and haplotypic frequencies were calculated based on individual animal genotypes. Chi-square tests were used to examine Hardy-Weinberg equilibrium state and compare genotypic distribution across breeds. Genetic distances among the three breeds, and between these and 30 breeds reared in other countries were estimated based on haplotypic frequencies using fixation index FST with Arlequin v3.1 software; a Neighbor-Joining tree was created using PHYLIP package v3.695. Level of statistical significance was set at P = 0.01. All scrapie resistance-associated alleles (146S, 146D, 211Q and 222K) were detected in the studied population. Significant frequency differences were observed between the indigenous Greek and Damascus breeds. Alleles 222K and 146S had the highest frequency in the two indigenous and the Damascus breed, respectively (ca. 6.0%). The studied breeds shared similar haplotypic frequencies with most South Italian and Turkish breeds but differed significantly from North-Western European, Far East and some USA goat breeds. Results suggest there is adequate variation in the PRNP gene locus to support breeding programs for enhanced scrapie resistance in goats reared in Greece. Genetic comparisons among goat breeds indicate that separate breeding programs should apply to the two indigenous and the imported Damascus breeds.

Goats singly heterozygous for PRNP S146 or K222 orally inoculated with classical scrapie at birth show no disease at ages well beyond 6 years

Scrapie is a transmissible spongiform encephalopathy of sheep and goats, and scrapie eradication programs in many parts of the world rely on strong genetic resistance to classical scrapie in sheep. However, the utility of putative resistance alleles in goats has been a focus of research because goats can transmit scrapie to sheep and may serve as a scrapie reservoir. Prior work showed that disease-free survival time was significantly extended in orally inoculated goats singly heterozygous for prion amino acid substitutions S146 or K222, but average durations were only around 3 years post-inoculation. The aim of this study was to investigate whether extended survival would exceed 6 years, which represents the productive lifetimes of most commercial goats. While all control homozygotes were clinically affected by an average of <2 years, none of the NS146 or QK222 goats developed clinical scrapie or had PrP^Sc-positive rectal biopsies. Several NS146 and QK222 goats developed other conditions unrelated to scrapie, but tissue accumulation of PrP^Sc was not detected in any of these animals. The NS146 heterozygotes have remained disease-free for an average of 2734days (approximately 7.5 years), the longest duration of any classical scrapie challenge experiment with any genotype to date. The QK222 heterozygotes have remained disease-free for an average of 2450days (approximately 6.7 years), the longest reported average duration for QK222 goats challenged with classical scrapie. This research is ongoing, but the current results demonstrate S146 and K222 confer strong resistance to classical scrapie in goats.

Caprine PrP variants harboring Asp-146, His-154 and Gln-211 alleles display reduced convertibility upon interaction with pathogenic murine prion protein in scrapie infected cells

Scrapie, the prion disease of sheep and goats, is a devastating malady of small ruminants. Due to its infectious nature, epidemic outbreaks may occur in flocks/herds consisting of highly susceptible animals. Field studies identified scrapie-protective caprine PrP variants, harboring specific single amino acid changes (Met-142, Arg-143, Asp-146, Ser-146, His-154, Gln-211 and Lys-222). Their effects are under further evaluation, and aim to determine the most protective allele. We assessed some of these variants (Asp-146, His-154, Gln-211 and Lys-222), after their exogenous expression as murine-caprine chimeras in a scrapie- infected murine cell line. We report that exogenously expressed PrPs undergo conformational conversion upon interaction with the endogenous pathological murine prion protein (PrP^SC), which results in the detection of goat-specific and partially PK-resistant moieties. These moieties display a PK-resistance pattern distinct from the one detected in natural goat scrapie cases. Within this cellular model, distinct conformational conversion potentials were assigned to the tested variants. Molecules carrying the Asp-146, His-154 and Gln-211 alleles showed significantly lower conversion levels compared to wild type, confirming their protective effects against scrapie. Although we utilized a heterologous conversion system, this is to our knowledge, the first study of caprine PrP variants in a cellular context of scrapie, that confirms the protective effects of some of the studied alleles.

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.

Scrapie, CWD, and Transmissible Mink Encephalopathy

Transmissible spongiform encephalopathies (TSEs), or prions, are neurodegenerative diseases that affect a variety of animal species, including humans. Cruetzfeldt-Jakob disease (CJD) in humans, sheep and goat scrapie, chronic wasting disease (CWD) of cervids, and transmissible mink encephalopathy (TME) of mink are classified as TSEs. According to the "protein-only" hypothesis (Prusiner, 1982),¹ prions are devoid of nucleic acids and consist of assemblies of misfolded host-encoded normal protein, the prion protein (PrP^C). Prion propagation is thought to occur by a templating mechanism during which PrP^C is recruited, converted to a disease-associated isoform (PrP^D), and assembled onto the growing amyloid fibril. This fibular assembly is infectious, with ability to initiate disease processes similar to other pathogenic agents. Evidence indicates that scrapie, CWD, and TME disease processes follow this rule.

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.

Low fraction of the 222K PrP variant in the protease-resistant moiety of PrPres in heterozygous scrapie positive goats

The presence of lysine (K) at codon 222 has been associated with resistance to classical scrapie in goats, but few scrapie cases have been identified in 222Q/K animals. To investigate the contribution of the 222K variant to PrPres formation in natural and experimental Q/K scrapie cases, we applied an immunoblotting method based on the use of two different monoclonal antibodies, F99/97.6.1 and SAF84, chosen for their different affinities to 222K and 222Q PrP variants. Our finding that PrPres seems to be formed nearly totally by the 222Q variant provides evidence that the 222K PrP variant confers resistance to conversion to PrPres formation and reinforces the view that this mutation has a protective role against classical scrapie in goats.

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.

The transmissible spongiform encephalopathies of livestock

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal protein-misfolding neurodegenerative diseases. TSEs have been described in several species, including bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep and goats, chronic wasting disease (CWD) in cervids, transmissible mink encephalopathy (TME) in mink, and Kuru and Creutzfeldt-Jakob disease (CJD) in humans. These diseases are associated with the accumulation of a protease-resistant, disease-associated isoform of the prion protein (called PrP(Sc)) in the central nervous system and other tissues, depending on the host species. Typically, TSEs are acquired through exposure to infectious material, but inherited and spontaneous TSEs also occur. All TSEs share pathologic features and infectious mechanisms but have distinct differences in transmission and epidemiology due to host factors and strain differences encoded within the structure of the misfolded prion protein. The possibility that BSE can be transmitted to humans as the cause of variant Creutzfeldt-Jakob disease has brought attention to this family of diseases. This review is focused on the TSEs of livestock: bovine spongiform encephalopathy in cattle and scrapie in sheep and goats.

PRNP variants in goats reduce sensitivity of detection of PrP(Sc) by immunoassay

Diagnostic analyses often employ single antibody systems but are potentially limited by epitope sequence variation. United States regulatory testing for scrapie primarily uses antibody F99/97.6.1 for immunohistochemistry (IHC) of the prion protein associated with scrapie (PrP(Sc)). Whereas the epitope bound by F99/97.6.1 is highly conserved in sheep, a polymorphism in caprine PRNP results in a glutamine to lysine change at codon 222 and affects PrP detection. This study evaluated the performance of immunoassays (Western blot and IHC) in the presence of PRNP polymorphisms observed in U.S. goat populations. Effects of naturally occurring caprine prion protein alterations at codons 142, 143, 146, 154, or 222 were first evaluated using bacterially expressed recombinant normal cellular prion protein (rec-PrP(C)) and commercially available antibodies (F99/97.6.1, F89/160.1.5, L42, and SAF84). Detection of rec-PrP(C) using F89/160.1.5 was reduced by alterations at 142 and 143; this was also observed in brain PrP(C) from goats expressing these PRNP variants. Effect of allelic variation at 222 was confirmed by Western blot with F99/97.6.1. No differences were observed with L42 or SAF84. IHC of brain demonstrated reduced signal with F89/160.1.5 in animals heterozygous at 143. Decreasing F89/160.1.5 titers were used to demonstrate the impact of PrP(Sc) immunolabeling in preclinical goats and as a surrogate for F99/97.6.1 detection in 222 variants. In the absence of epitope-relevant knowledge of individual goat PRNP, a multi-antibody approach or an antibody that binds an invariant site may provide a more robust immunoassay of PrP(Sc) in classical scrapie, thus reducing the likelihood of false-negative results due to allelic variation.

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

BACKGROUND

The prion protein-encoding gene (PRNP) is one of the major determinants for scrapie occurrence in sheep and goats. However, its effect on bovine spongiform encephalopathy (BSE) transmission to goats is not clear.

METHODS

Goats harboring wild-type, R/Q211 or Q/K222 PRNP genotypes were orally inoculated with a goat-BSE isolate to assess their relative susceptibility to BSE infection. Goats were killed at different time points during the incubation period and after the onset of clinical signs, and their brains as well as several peripheral tissues were analyzed for the accumulation of pathological prion protein (PrP(Sc)) and prion infectivity by mouse bioassay.

RESULTS

R/Q211 goats displayed delayed clinical signs compared with wild-type goats. Deposits of PrP(Sc) were detected only in brain, whereas infectivity was present in peripheral tissues too. In contrast, none of the Q/K222 goats showed any evidence of clinical prion disease. No PrP(Sc) accumulation was observed in their brains or peripheral tissues, but very low infectivity was detected in some tissues very long after inoculation (44-45 months).

CONCLUSIONS

These results demonstrate that transmission of goat BSE is genotype dependent, and they highlight the pivotal protective effect of the K222 PRNP variant in the oral susceptibility of goats to BSE.

Genetic and Pathological Follow-Up Study of Goats Experimentally and Naturally Exposed to a Sheep Scrapie Isolate

Thirty-seven goats carrying different prion protein genotypes (PRNP) were orally infected with a classical scrapie brain homogenate from wild-type (ARQ/ARQ) sheep and then mated to obtain 2 additional generations of offspring, which were kept in the same environment and allowed to be naturally exposed to scrapie. Occurrence of clinical or subclinical scrapie was observed in the experimentally infected goats (F0) and in only one (F1b) of the naturally exposed offspring groups. In both groups (F0 and F1b), goats carrying the R154H, H154H, R211Q, and P168Q-P240P dimorphisms died of scrapie after a longer incubation period than wild-type, G37V, Q168Q-P240P, and S240P goats. In contrast, D145D and Q222K goats were resistant to infection. The immunobiochemical signature of the scrapie isolate and its pathological aspects observed in the sheep donors were substantially maintained over 2 goat generations, i.e., after experimental and natural transmission. This demonstrates that the prion protein gene sequence, which is shared by sheep and goats, is more powerful than any possible but unknown species-related factors in determining scrapie phenotypes. With regard to genetics, our study confirms that the K222 mutation protects goats even against ovine scrapie isolates, and for the first time, a possible association of D145 mutation with scrapie resistance is shown. In addition, it is possible that the sole diverse frequencies of these genetic variants might, at least in part, shape the prevalence of scrapie among naturally exposed progenies in affected herds.

IMPORTANCE

This study was aimed at investigating the genetic and pathological features characterizing sheep-to-goat transmission of scrapie. We show that in goats with different prion protein gene mutations, the K222 genetic variant is associated with scrapie resistance after natural and experimental exposure to ovine prion infectivity. In addition, we observed for the first time a protective effect of the D145 goat variant against scrapie. Importantly, our results demonstrate that the phenotypic characteristic of the wild-type sheep scrapie isolate is substantially preserved in goats carrying different susceptible PRNP gene variants, thus indicating that the prion protein gene sequence, which is shared by sheep and goats, plays a fundamental role in determining scrapie phenotypes.

Human prion protein sequence elements impede cross-species chronic wasting disease transmission

Chronic wasting disease (CWD) is a fatal prion disease of North American deer and elk and poses an unclear risk for transmission to humans. Human exposure to CWD occurs through hunting activities and consumption of venison from prion-infected animals. Although the amino acid residues of the prion protein (PrP) that prevent or permit human CWD infection are unknown, NMR-based structural studies suggest that the β2-α2 loop (residues 165-175) may impact species barriers. Here we sought to define PrP sequence determinants that affect CWD transmission to humans. We engineered transgenic mice that express human PrP with four amino acid substitutions that result in expression of PrP with a β2-α2 loop (residues 165-175) that exactly matches that of elk PrP. Compared with transgenic mice expressing unaltered human PrP, mice expressing the human-elk chimeric PrP were highly susceptible to elk and deer CWD prions but were concurrently less susceptible to human Creutzfeldt-Jakob disease prions. A systematic in vitro survey of amino acid differences between humans and cervids identified two additional residues that impacted CWD conversion of human PrP. This work identifies amino acids that constitute a substantial structural barrier for CWD transmission to humans and helps illuminate the molecular requirements for cross-species prion transmission.

Prion and prion-like diseases in animals

Transmissible spongiform encephalopaties (TSEs) are fatal neurodegenerative diseases characterized by the aggregation and accumulation of the misfolded prion protein in the brain. Other proteins such as β-amyloid, tau or Serum Amyloid-A (SAA) seem to share with prions some aspects of their pathogenic mechanism; causing a variety of so called prion-like diseases in humans and/or animals such as Alzheimer's, Parkinson's, Huntington's, Type II diabetes mellitus or amyloidosis. The question remains whether these misfolding proteins have the ability to self-propagate and transmit in a similar manner to prions. In this review, we describe the prion and prion-like diseases affecting animals as well as the recent findings suggesting the prion-like transmissibility of certain non-prion proteins.

Perspectives of a scrapie resistance breeding scheme targeting Q211, S146 and K222 caprine PRNP alleles in Greek goats

The present study investigates the potential use of the scrapie-protective Q211 S146 and K222 caprine PRNP alleles as targets for selective breeding in Greek goats. Genotyping data from a high number of healthy goats with special emphasis on bucks, revealed high frequencies of these alleles, while the estimated probabilities of disease occurrence in animals carrying these alleles were low, suggesting that they can be used for selection. Greek goats represent one of the largest populations in Europe. Thus, the considerations presented here are an example of the expected effect of such a scheme on scrapie occurrence and on stakeholders.

Genetic resistance to scrapie infection in experimentally challenged goats

In goats, several field studies have identified coding mutations of the gene encoding the prion protein (I/M142, N/D146, S/D146, R/Q211, and Q/K222) that are associated with a lower risk of developing classical scrapie. However, the data related to the levels of resistance to transmissible spongiform encephalopathies (TSEs) of these different PRNP gene mutations are still considered insufficient for developing large-scale genetic selection against scrapie in this species. In this study, we inoculated wild-type (WT) PRNP (I142R154R211Q222) goats and homozygous and/or heterozygous I/M142, R/H154, R/Q211, and Q/K222 goats with a goat natural scrapie isolate by either the oral or the intracerebral (i.c.) route. Our results indicate that the I/M142 PRNP polymorphism does not provide substantial resistance to scrapie infection following intracerebral or oral inoculation. They also demonstrate that H154, Q211, and K222 PRNP allele carriers are all resistant to scrapie infection following oral exposure. However, in comparison to WT animals, the H154 and Q211 allele carriers displayed only moderate increases in the incubation period following i.c. challenge. After i.c. challenge, heterozygous K222 and a small proportion of homozygous K222 goats also developed the disease, but with incubation periods that were 4 to 5 times longer than those in WT animals. These results support the contention that the K222 goat prion protein variant provides a strong but not absolutely protective effect against classical scrapie.