Role of the PRNP S127 allele in experimental infection of goats with classical caprine scrapie

Background

Classical scrapie is a transmissible spongiform encephalopathy affecting domestic goats and sheep and associated with accumulation of a misfolded isoform (PrP^Sc) of the PRNP gene product. Multiple PRNP polymorphisms have been reported in goats.1, 2 Experimental scrapie inoculation studies revealed that PRNP polymorphisms at codons 146, 154, 211, and 222 can provide resistance or a prolonged incubation period.3, 4, 5 A recent study identified the association between a polymorphism at codon 127 (G127S) and reduced probability of developing clinical signs of scrapie in goats with PrP^Sc detectable in the brain.6 In that study, the length of the incubation period (time from infection to clinical signs) was not known. The aim of this study was to identify whether goats with the heterozygous (G/S₁₂₇) genotype have an extended incubation period compared with goats homozygous for G₁₂₇ (G/G₁₂₇) following classical caprine scrapie inoculation.

Methods

Intracerebral inoculation was performed with brain homogenates from two natural field cases of scrapie, one G/G₁₂₇ and one G/S₁₂₇, as described (Table S1). Data were combined from previous reports4, 5 to achieve efficient animal use. Because the previous report6 suggested a prolonged incubation time for G/S₁₂₇ goats, a one‐tailed nonparametric exact test (see Appendix S1) was used to assess the recipient genotype hypothesis that G/S₁₂₇ goats have longer incubation times than do G/G₁₂₇ goats when intracerebrally inoculated with classical caprine scrapie from G/G₁₂₇ genotype source material. In the absence of preliminary data, two‐tailed nonparametric exact tests were used for all other analyses.

Results/Discussion

All inoculated goats developed clinical signs of scrapie (Table S1), with disease confirmed by the detection of PrP^Sc in brainstem at the obex and most lymphoid tissues by immunohistochemistry8 (Fig. S1). A significant (P = 0.019) increase in incubation time was observed in G/S₁₂₇ goats following inoculation with the G/G₁₂₇ caprine brain homogenate (range: 647–1333 days). No significant difference in incubation time was observed for G/G₁₂₇ versus G/S₁₂₇ inoculum in G/G₁₂₇ goats (P = 0.30). Incubation times in control G/G₁₂₇ goats inoculated with the G/G₁₂₇ brain homogenate (range: 261–332 days) were significantly shorter than observed in prior work (both P < 0.054, 5). Our findings include the first reported classical scrapie incubation times for G/S₁₂₇ goats, confirming that S127 is not protective in goats.6, 7 These data suggest that eradication programs need to include extended traceback periods and longer post‐exposure monitoring times for infected herds containing G/S₁₂₇ goats.

Supporting information

Appendix S1 Supplementary Methods.

Figure S1 Detection of PrPSc immunolabeling in lymph nodes and brainstem at the obex of G/S127 goats intracerebrally inoculated with classical caprine scrapie.

Table S1 Intracerebral inoculation of goat kids with homozygous (G/G127) or heterozygous (G/S127) brain‐derived inocula prepared from clinically affected goats with naturally acquired classical caprine scrapie.