Submitted to: Journal of Veterinary Diagnostic Investigation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2009
Publication Date: 5/1/2009
Publication URL: http://ddr.nal.usda.gov/dspace/bitstream/10113/31105/1/IND44204141.pdf
Citation: Schneider, D.A., Tibary, A., Raudsepp, T., Das, P.J., Orourke, K.I. 2009. Blood chimerism confounds genetic relative susceptibility testing for classical scrapie in sheep. Journal of Veterinary Diagnostic Investigation. 21(3):295-305. Interpretive Summary: This investigation tested the hypothesis that naturally occurring blood chimerism confounds genetic susceptibility testing for classical scrapie in sheep. It has been previously shown that artificial mixing of blood samples from different sheep can confound deduction of the amino acid sequence for the PRioN Protein gene (PRNP) under certain conditions. As yet, no one has demonstrated that a naturally occurring chimera actually does confound interpretation of genetic susceptibility testing as currently carried out by nationally-approved methods and personnel. In this report, we definitively demonstrate that naturally occurring blood chimerism in a sheep confounded interpretation of genetic susceptibility test results by approved laboratories. Standard techniques and strategic tissue sampling were effectively employed in the detection of natural sheep chimeras and in guiding selection of alternative tissue samples more suitable for PRNP codon determination in chimeras.
Technical Abstract: Scrapie is a transmissible spongiform encephalopathy of sheep targeted for eradication in the U.S. Susceptibility of sheep to classical scrapie is linked with certain polymorphisms in the prion protein gene (PRNP), such as disease resistance associated with homozygosity for arginine at codon 171 (R171), and relative disease susceptibility with homozygosity for glutamine (Q171). In this report we positively identify and characterize the extent of naturally occurring chimeric cells in a 2.5 year old, freemartin ewe through blood cell karyotyping and PCR detection of a Y chromosome-specific gene. We document a confounding effect of blood cell chimerism on reporting of genetic susceptibility test results for classical scrapie disease as conducted on coded samples by approved laboratories. PCR sequencing was used to demonstrate the confounding influence of chimeric cells on codon interpretation of PRNP. Commercially available microsatellite analysis was successfully employed to detect the presence of chimeric cells not only in the freemartin ewe but also in archival samples from another chimeric ewe determined to have been born twin to another ewe. The occurrence of chimerism in the general sheep population is thought to be infrequent. Even so, we were able to overcome the confounding effect of chimerism on PRNP codon interpretation by conducting additional tests to identify tissue samples more suitable for PRNP codon determination.