Genetic variability of the PrP gene in a goat herd in the UK

Large-scale survey of prion protein genetic variability in scrapie disease-free goats from the United States

Scrapie is a slowly progressive neurodegenerative disease of small ruminants caused by an accumulation of an abnormal isoform of prion protein in the central nervous system. Polymorphisms of the prion protein gene (PRNP) strongly modulate scrapie resistance and incubation period in goats. The aim of this study was to identify PRNP genetic variability in goats across the United States. Blood from a total of 6,029 apparent scrapie disease-free goats from 654 operations and 19 breeds were analyzed. Sequencing of PRNP revealed 26 genotypes with different rates based on eight codons. The GG127, RR154, and QQ222 genotypes were predominant and showed a remarkably high rate across all goats. The QK222 and NS146 genotypes, known to be protective against scrapie, were found in 0.6% [with 95% CI = (0.3, 1.2)] and 22.0% [95% CI = (19.1, 25.2)] of goats, respectively. The QK222 genotype was found in 23.1% of Oberhasli goats tested, with 95%CI = (3.9, 68.7)] and 22.0% of Toggenburg goats tested with 95%CI = (9.7, 42.5)], while NS146 was found in 65.5% of Savannah goats tested, with 95%CI = (30.8, 89.9), 36.7% of Boer goats tested, with 95%CI = (33.1, 40.4), 36.3% of Nubian goats tested, with 95%CI = (27.0, 46.7)], and 35.6% of LaMancha goats tested, with 95%CI = (22.8, 50.8%). The MM142 and IM142 genotypes were found more frequently in goats on dairy operations, while the HR143, NS146, and ND146 genotypes were found more frequently in goats on meat operations. Goats in the east region had a higher percentage of goats with RH154, RQ211, and QK222 genotypes than goats in the west region. The results of this study showed high genetic variability of PRNP among the U.S. goat population, with differences by location and breed, and may serve as a rationale for development of goat breeding programs at the national level to mitigate the risk of scrapie.

Prion protein gene variability in Spanish goats. Inference through susceptibility to classical scrapie strains and pathogenic distribution of peripheral PrP(sc.)

Classical scrapie is a neurological disorder of the central nervous system (CNS) characterized by the accumulation of an abnormal, partially protease resistant prion protein (PrP(sc)) in the CNS and in some peripheral tissues in domestic small ruminants. Whereas the pathological changes and genetic susceptibility of ovine scrapie are well known, caprine scrapie has been less well studied. We report here a pathological study of 13 scrapie-affected goats diagnosed in Spain during the last 9 years. We used immunohistochemical and biochemical techniques to discriminate between classical and atypical scrapie and bovine spongiform encephalopathy (BSE). All the animals displayed PrP(sc) distribution patterns and western blot characteristics compatible with classical scrapie. In addition, we determined the complete open reading frame sequence of the PRNP in these scrapie-affected animals. The polymorphisms observed were compared with those of the herd mates (n = 665) and with the frequencies of healthy herds (n = 581) of native Spanish goats (Retinta, Pirenaica and Moncaina) and other worldwide breeds reared in Spain (Saanen, Alpine and crossbreed). In total, sixteen polymorphic sites were identified, including the known amino acid substitutions at codons G37V, G127S, M137I, I142M, H143R, R151H, R154H, R211Q, Q222K, G232W, and P240S, and new polymorphisms at codons G74D, M112T, R139S, L141F and Q215R. In addition, the known 42, 138 and 179 silent mutations were detected, and one new one is reported at codon 122. The genetic differences observed in the population studied have been attributed to breed and most of the novel polymorphic codons show frequencies lower than 5%. This work provides the first basis of polymorphic distribution of PRNP in native and worldwide goat breeds reared in Spain.

Genetic variability of PRNP in Chinese indigenous goats

Polymorphism of the prion protein gene (PRNP) is usually associated with scrapie susceptibility or resistance. To determine the variability of PRNP in Chinese indigenous goat breeds, we isolated genomic DNA from goat blood and amplified and sequenced the coding region of the gene. We identified 10 polymorphic sites that gave rise to 28 haplotypes. Clear frequency differences were found between northern and southern breeds and confirmed by genetic distance analysis, except for the Tangshan dairy goat. Phylogeographic analysis supported the idea that northern and southern breeds might be considered separate clusters, except for the Tangshan dairy goat. The finding of significant differences in allele distribution in northern and southern goats, especially if involved in modulating resistance/susceptibility, needs to be carefully considered for the feasibility of selection plans for resistance to scrapie.

Cell-surface expression of PrPC and the presence of scrapie prions in the blood of goats

Although host-encoded prion protein (PrP(C)) expression in ovine PBMCs and prion infectivity in scrapie-infected sheep blood have been demonstrated, such studies have not been reported in goats. Therefore, this study characterized cell-surface expression of PrP(C) on PBMC subsets derived from normal goats and sheep, by flow cytometry, and determined prion infectivity in blood from a scrapie-infected goat using a transfusion bioassay in goat kids. Cell-surface PrP(C) expression was detected on all subsets of goat PBMCs. The highest PrP(C) cell-surface expression was found in CD2(+) T lymphocytes in goats. Transmission of infection was detected in all three recipients who received whole blood from a goat with classical scrapie. It was concluded that caprine PBMCs express PrP(C) similarly to sheep but with relative differences among PBMCs subsets, and that blood-borne infectious prions can be detected in scrapie-infected goats. Thus, similar to sheep, goat blood may be a suitable diagnostic target for the detection of scrapie infection.

Design and validation of a high-throughput assay to detect codon 146 polymorphisms in the caprine prion protein gene

In sheep, scrapie susceptibility is so strongly associated with single nucleotide polymorphisms (SNPs) in the gene encoding the prion protein (PrP) that this linkage constitutes the basis for selective breeding strategies directed toward controlling the disease. For goats, in contrast, the association between scrapie susceptibility/resistance and variations in the PrP gene is far weaker, with only a few identified SNPs showing an influence on scrapie susceptibility. A recent survey of PrP genotypes in Cypriot goats, however, revealed the existence of a robust association between polymorphisms at codon 146 of the caprine PrP gene and resistance/susceptibility to natural scrapie. Here we describe here a high-throughput assay, based on homogeneous MassExtend technology coupled with mass spectrometry, for genotyping codon 146 of the caprine PrP gene. Our results demonstrate that this assay exhibits high accuracy, reproducibility, and repeatability, thereby making it suitable for large-scale SNP genotyping, as required for scrapie surveillance programs.

Identification of seven haplotypes of the caprine PrP gene at codons 127, 142, 154, 211, 222 and 240 in French Alpine and Saanen breeds and their association with classical scrapie

In sheep, susceptibility to scrapie is mainly influenced by polymorphisms of the PrP gene. In goats, there are to date few data related to scrapie susceptibility association with PrP gene polymorphisms. In this study, we first investigated PrP gene polymorphisms of the French Alpine and Saanen breeds. Based on PrP gene open reading frame sequencing of artificial insemination bucks (n=404), six encoding mutations were identified at codons 127, 142, 154, 211, 222 and 240. However, only seven haplotypes could be detected: four (GIH154RQS, GIRQ211QS, GIRRK222S and GIRRQP240) derived from the wild-type allele (G127I142R154R211Q222S240) by a single-codon mutation, and two (S127IRRQP240 and GM142RRQP240) by a double-codon mutation. A case–control study was then implemented in a highly affected Alpine and Saanen breed herd (90 cases/164 controls). Mutations at codon 142 (I/M), 154 (R/H), 211 (R/Q) and 222 (Q/K) were found to induce a significant degree of protection towards natural scrapie infection. Compared with the baseline homozygote wild-type genotype I142R154R211Q222/IRRQ goats, the odds of scrapie cases in IRQ211Q/IRRQ and IRRK222/IRRQ heterozygous animals were significantly lower [odds ratio (OR)=0.133, P<0.0001; and OR=0.048, P<0.0001, respectively]. The heterozygote M142RRQ/IRRQ genotype was only protective (OR=0.243, P=0.0186) in goats also PP240 homozygous at codon 240. However, mutated allele frequencies in French Alpine and Saanen breeds were low (0.5–18.5 %), which prevent us from assessing the influence of all the possible genotypes in natural exposure conditions.

Histidine at codon 154 of the prion protein gene is a risk factor for Nor98 scrapie in goats

Prion protein gene (PRNP) polymorphisms are involved in modulating the appearance of atypical/Nor98 scrapie in sheep, with the alleles AHQ and AF141RQ strongly associated with occurrence of the disease. The presence of histidine at codon 154 has also been detected in Nor98-affected goats, but statistical analysis of the association between Nor98 and goat PRNP polymorphisms has not been reported previously. Here, a case-control study was carried out on eight Nor98-positive goats and 246 negative herdmates belonging to eight Italian Nor98 scrapie outbreaks. The results revealed that histidine at codon 154 is also strongly associated with the disease in goats.

Atypical PrPsc distribution in goats naturally affected with scrapie

The brain and spinal cord of 48 goats from two Greek herds in which scrapie had been reported were examined. All animals were symptomless at the time of euthanasia. Notably, no lesions were observed either at the level of the obex or at other regions of the brain and spinal cord. Immunohistochemical examination revealed PrPsc labelling of the linear and fine punctuate types, mainly in the cerebral cortices, of 36 goats. Twenty-seven of them were negative by ELISA (designed to detect proteinase-resistant PrP) at the level of the obex but positive in a pooled brain sample, and the majority carried PrP genotypes associated with scrapie susceptibility. Surprisingly, in 16 of the 27 animals, PrPsc deposits were detected only in the rostral parts of the brain. In addition, nine animals which were ELISA-positive at the level of the obex exhibited positive immunoreactivity, but not in the dorsal vagal nucleus. The findings indicate that this unusual scrapie type may have been underdiagnosed previously and may be of importance in scrapie surveillance programmes.

PrP genetics in ruminant transmissible spongiform encephalopathies

Scrapie, bovine spongiform encephalopathy (BSE), and chronic wasting disease (CWD) are prion diseases in ruminants with considerable impact on animal health and welfare. They can also pose a risk to human health and control is therefore an important issue. Prion protein (PrP) genetics may be used to control and eventually eradicate animal prion diseases. The PrP gene in sheep and other representatives of the order Artiodactyles has many polymorphisms of which several are crucial determinants of susceptibility to prion diseases, also known as transmissible spongiform encephalopathies (TSE). This review will present the current understanding of PrP genetics in ruminants highlighting similarity and difference between the species in the context of TSE.

Genetic variability of the PRNP gene in goat breeds from Northern and Southern Italy

AIMS

To determine the variability of the prion protein gene (PRNP) in goats from Northern and Southern Italy.

METHODS AND RESULTS

Genomic DNA isolated from goat blood was polymerase chain reaction (PCR)-amplified for the coding region of the PRNP gene and then sequenced. In total, 13 polymorphic sites were identified: G37V, T110P, G127S, M137I, I142M, I142T, H143R, R154H, P168Q, T194P, R211Q, Q222K and S240P (substitutions I142T and T194P are novel) giving rise to 14 haplotypes. Clear frequency differences between Northern and Southern breeds were found and confirmed by genetic distance analysis.

CONCLUSIONS

Differences in allele distribution were found between Northern and Southern goats, in particular regarding the M142 and K222 alleles, possibly associated to scrapie resistance; philogeographical analysis supported the idea that Northern and Southern breeds may be considered as separate clusters.

SIGNIFICANCE AND IMPACT OF THE STUDY

In Italy only limited studies have been carried out on caprine PRNP genotype distribution; this study is important to fill this lack of information. Moreover the finding of significant differences among allele distributions in Northern and Southern goats, especially if involved in modulating resistance/susceptibility, need to be carefully considered for the feasibility of selection plans for resistance to scrapie.

Histopathological and immunohistochemical features of natural goat scrapie

Histopathological and immunohistochemical examinations were performed on the brain and spinal cord of 37 goats from two Greek herds in which scrapie had been reported. Of the 37 animals, 18 were from a herd consisting only of goats and 19 were from a herd of goats mixed with sheep. The goats studied were grouped on the basis of the presence or absence of clinical signs. Distinctive lesions and PrP(sc) (PrP, prion protein) deposition were found in the central nervous system (CNS) of eight clinically affected animals and six symptomless animals. The lesion profile and PrP(sc) distribution varied both between and within groups, variation being particularly pronounced in the symptomless goats. The results concerning the latter group suggested a poor correlation between the intensity of lesions, the amount of PrP(sc) in the CNS, and the manifestation of clinical signs. Immunohistochemical examination revealed 10 different PrP(sc) types, four of which are reported for the first time in goats. All scrapie-affected animals carried the VV(21)II(142)HH(143)RR(154) genotype, with the exception of two goats that carried the HR(143) dimorphism and had detectable PrP(sc) deposits. The results suggest that the histopathological and immunohistochemical profile of the natural disease in goats is influenced by the PrP genotype and age of the animals but may not be directly associated with the presence or otherwise of clinical signs.

Identification of an allelic variant of the goat PrP gene associated with resistance to scrapie

The association between PrP gene variations and scrapie susceptibility was studied in a single herd of Ionica breed goats. The entire herd comprised 100 animals, 11 of which were clinically affected and showed pathological prion protein (PrPSc) deposition in both their central nervous system (CNS) and lymphoreticular system (LRS). Among asymptomatic goats, nine harboured PrPSc in both CNS and LRS, 19 showed PrPSc only at the LRS level and 61 animals had no PrPSc deposition. Genetic analysis of the PrP gene coding sequence revealed the presence of several polymorphisms, namely G37V, T110P, H143R, R154H, Q222K and P240S. Silent polymorphisms were also found at codons 42, 138, 219 and 232. The effect of PrP polymorphism on scrapie susceptibility was assessed by comparing the genotype distribution at each locus among animals with different pathogenetic and clinical disease stages. Significant differences in the distribution of genotypes were observed for codons 154 and 222, with polymorphism at codon 154 modulating susceptibility to scrapie and lysine at codon 222 being associated with scrapie resistance. The allelic variant encoding lysine at position 222 could be a valuable candidate to select in the framework of appropriate breeding programmes for scrapie resistance in goats.

Identification of prion protein gene polymorphisms in goats from Italian scrapie outbreaks

Susceptibility to scrapie in sheep is influenced by polymorphisms of the prion protein (PrP) gene, whereas no strong association between genetics and scrapie has yet been determined in goats due to the limited number of studies on these animals. In this case–control study on 177 goats from six Italian scrapie outbreaks, the association between PrP alleles and the occurrence of scrapie was studied. Three silent mutations and 11 PrP polymorphisms were identified, of which two polymorphisms (L133Q and M137I) and one silent mutation (T202T) have not been reported previously. Twelve alleles were determined by cloning. Statistical analysis suggested a possible protective role against scrapie for the glutamine to lysine mutation at codon 222.

Immunohistochemical features of PrP(d) accumulation in natural and experimental goat transmissible spongiform encephalopathies

Scrapie is a transmissible spongiform encephalopathy (TSE) or prion disease, which naturally affects sheep and goats. Immunohistochemical epitope mapping of abnormal PrP accumulations (PrP(d)) in brain can help in characterizing sheep TSE sources or strains and in identifying potential bovine spongiform encephalopathy (BSE) infections of sheep. Natural and experimental TSE infections of goats were examined to determine whether the epitope mapping approach could also be applied to aid recognition of BSE infection in goats. Goats experimentally infected with the SSBP/1 or CH1641 sheep scrapie strains or with cattle BSE, together with four field cases of natural TSE in goats, were examined immunohistochemically with six different antibodies. CH1641 and SSBP/1 infections in goats, as in sheep, showed PrP(d) accumulations which were mainly intracellular. Some differences in targeting, particularly of Purkinje cells, was evident in inter-species comparisons of CH1641 and SSBP/1. PrP(d) labelling of goat BSE experimental cases showed extensive intracellular and extracellular accumulations, also similar to those in sheep BSE. Intra-neuronal PrP(d) in both goat and sheep BSE was labelled only by antibodies recognizing epitopes located C-terminally of residue His99, whereas in natural sheep TSE sources, and in sheep and goat SSBP/1, PrP(d) was also detected by antibodies to epitopes located between residues Trp93 and His99. Testing of four natural goat TSE samples showed one case in which epitope mapping characteristics and the overall patterns of PrP(d) accumulation was identical with those of experimental goat BSE. The four natural goat scrapie cases examined showed some degree of immunohistochemical phenotype variability, suggesting that multiple strains exist within the relatively small UK goat population.

Frequencies ofPrPgene haplotypes in British sheep flocks and the implications for breeding programmes

AIMS

To analyse the frequencies of prion (PrP) gene haplotypes in UK sheep flocks and evaluate their relevance to transmissible spongiform encephalopathies (TSEs) and TSE resistance breeding programmes in sheep.

METHODS AND RESULTS

Genomic DNA isolated from sheep blood was PCR amplified for the coding region of the PrP gene and then sequenced. This study has analysed the sequence of PrP between codons 110 and 245 in 6287 ARQ haplotypes revealing a total of eight variant sequences, which represent a higher than expected 41% of all ARQ haplotypes. The additional PrP gene dimorphisms were M112T, L141F, M137T, H143R, H151C, P168L, Q175E and P241S.

CONCLUSION

The results do not suggest a correlation between the occurrence of a specific ARQ haplotype and the scrapie disease status of a flock. The ARQ haplotype variability appears to be different in the UK sheep flocks compared with sheep flocks from outside the UK.

SIGNIFICANCE AND IMPACT OF THE STUDY

Additional PrP dimorphisms may impact on the methodologies used for standard PrP genotyping in sheep breeding programmes. Some of these polymorphisms were found with significant frequencies in the UK sheep flocks and should therefore be considered in breeding programmes.