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Blood Chimerism Confounds Genetic Relative Susceptibility Testing for Classical Scrapie in Sheep

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Blood Chimerism Confounds Genetic Relative Susceptibility Testing for Classical Scrapie in Sheep University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Other Publications in Zoonotics and Wildlife Disease Wildlife Disease and Zoonotics 2009 Blood chimerism confounds genetic relative susceptibility testing for classical scrapie in sheep David A. Schneider U.S. Department of Agriculture Ahmed Tibary Washington State University Terje Taudsepp Texas A & M University - College Station Pranab J. Das Texas A & M University - College Station Katherine I. O'Rourke U.S. Department of Agriculture, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/zoonoticspub Part of the Veterinary Infectious Diseases Commons Schneider, David A.; Tibary, Ahmed; Taudsepp, Terje; Das, Pranab J.; and O'Rourke, Katherine I., "Blood chimerism confounds genetic relative susceptibility testing for classical scrapie in sheep" (2009). Other Publications in Zoonotics and Wildlife Disease. 145. https://digitalcommons.unl.edu/zoonoticspub/145 This Article is brought to you for free and open access by the Wildlife Disease and Zoonotics at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Other Publications in Zoonotics and Wildlife Disease by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. J Vet Diagn Invest 21:295–305 (2009) Blood chimerism confounds genetic relative susceptibility testing for classical scrapie in sheep David A. Schneider,1 Ahmed Tibary, Terje Raudsepp, Pranab J. Das, Katherine I. O’Rourke Abstract. Classical scrapie disease is a transmissible spongiform encephalopathy of sheep that is enzootic in the United States. Susceptibility of sheep to classical scrapie is linked to single nucleotide polymorphisms in the prion protein gene (PRNP), forming the basis for genetic testing strategies used by national efforts to eradicate scrapie. Such efforts are occasionally hampered by inconclusive results stemming from the detection of ‘‘complex’’ genotypes. Naturally occurring cases of ovine chimerism are thought to account for some of these instances. In the current report, 4 naturally occurring ovine chimeras are documented through cytogenetic and molecular analyses. All 4 of these sheep had chimeric cells circulating in their blood. Blood and alternate tissue samples of ear punch and hair bulbs from one of these chimeras was submitted in batch with similar samples from control sheep for routine scrapie genetic relative susceptibility testing. A complex PRNP genotype was detected in the blood of the chimeric female but not in the alternate tissue samples or in the control sheep samples. The results demonstrate that naturally occurring blood chimerism can confound current testing efforts. The potential impacts of undetected chimeras on current scrapie eradication efforts are discussed. Key words: Chimera; diagnostics; genotyping; microsatellite; PRNP; scrapie; sheep. Introduction conversion of the host’s cellular prion protein (PrPc) to a misfolded form (PrPSc).31 Amino acid substitu- The ‘‘chimaera’’ of Homer’s Iliad is a mythical tions within PrPc naturally occur as a result of single creature composed of parts from different animals.5 nucleotide polymorphisms in its gene, PRNP.Of The modern clinical usage of ‘‘chimera’’ denotes an particular importance to scrapie disease are polymor- animal whose cellular composition persistently in- phisms encoding valine or alanine at codon 136 (V136 cludes cells derived from a nonidentical, or dizygotic, or A136, respectively), arginine or histidine at codon twin. In ruminants, natural chimerism commonly 154 (R154 or H154, respectively), and glutamine, results from an exchange of stem cells made possible arginine, histidine, or lysine at codon 171 (Q171, by the early formation of placental vascular anasto- R171,H171,orK171, respectively).12 Genetic relative mosis between twins.28 Though not as outwardly susceptibility testing categorizes individuals into obvious as the mythical creature, chimeric cells are disease risk groups associated with the inherited nonetheless genetically disposed to express the protein alleles. Of current importance to relative susceptibility isoforms native to the twin sibling. Thus, abnormally for classical scrapie disease in sheep are the alleles complex blood types, a result of immune tolerance to A136R154R171 (hereafter, ARR; which is associated antigens expressed by erythrocytes, have long been with resistance), and A136R154Q171 (ARQ) and recognized as signifying the presence of twin-derived V136R154Q171 (VRQ), which are both associated with erythrocytes circulating in the blood of chimeras.36 susceptibility.20 Scrapie is a slowly progressive, fatal neurologic National programs, including the U.S. Scrapie disease of sheep that is critically associated with Eradication and Surveillance programs, aim to control scrapie disease by increasing the frequency of stock From the United States Department of Agriculture, Agricul- genetically resistant to disease through PRNP geno- tural Research Service, Animal Disease Research Unit, Pullman, type determination (U.S. Department of Agriculture: WA (Schneider, O’Rourke), the Department of Veterinary Clinical Sciences, Center for Reproductive Biology, Washington 2008, National Scrapie Surveillance Plan. Available at State University, Pullman, WA (Tibary), and the Molecular http://www.aphis.usda.gov/vs/nahss/sheep/national_ Cytogenetics Laboratory, Department of Veterinary Integrative scrapie_surveillance_plan_08192008.pdf. Accessed Bio-Sciences, College of Veterinary Medicine, Texas A&M on January 20, 2009).12 Consideration of the PRNP University, College Station, TX (Raudsepp, Das). 1 Corresponding Author: David A. Schneider, Animal Disease genotype significantly affects the valuation of Research Unit, PO Box 646630, 3003 ADBF, Washington State breeding stock, indicating a recognition of the University, Pullman, WA 99164-6630. [email protected] importance of such national programs to the sheep 295 296 Schneider et al. industry.15 Genetic testing is conveniently performed Collection of samples on blood samples, but inconclusive results arising The care and use of ADRU sheep were approved by the from the detection of ‘‘complex’’ PRNP genotypes Institutional Animal Care and Use Committee of Wash- have been reported (,0.1% of U.K. samples).13 Such ington State University. All blood samples were collected genotype complexity has been attributed to sample via jugular venipuncture directly into prelabeled tubes.c contamination and to naturally occurring ovine Skin samples were collected from the ear margin using a chimeras.14,18,26,34 However, many chimeras may be commercially available tissue collection systemd as de- going undetected by current efforts given the scribed by the manufacturer. Hair bulb samples were collected by plucking primary (guard) hairs from either the reported rates of 3–5% chimeras among co-siblings 17,23,25,37 skin-hoof margin of a forelimb or, as in chimera case 4, in some breeds of sheep. In the current from the margin of the pinna using a hemostat wiped clean report, 4 cases of naturally occurring ovine chimeras with alcohol before each use. are characterized, and a confounding effect on current commercially contracted scrapie genetic Cell cultures, chromosome preparations, and relative susceptibility testing in the United States chromosome analysis is demonstrated. Testing methodologies and the Conducted at the Molecular Cytogenetics Laboratorye implications of natural ovine chimerism on genetic (TR, PJD), metaphase chromosome preparations were testing and scrapie eradication efforts are obtained from short-term peripheral blood leukocyte discussed. cultures using a standard protocol.32 In brief, 1 ml of whole blood collected into sterile glass tubes containing Material and methods sodium–heparinc was added to 9 ml of culture medium containing RPMI (Roswell Park Memorial Institute)-1640 Animals medium with GlutaMAX and 25 mmol HEPES (N-2- Chimera case 1 consisted of a black-faced female sheep hydroxyethylpiperazone-N-2-ethanesulfonic acid) buffer,f born twin to a male in the winter of 2005 on a commercial 30% fetal bovine serum,g 1.4% antibiotic–antimycotic U.S. sheep operation. At approximately 18 months of age, solution,f and 1% pokeweed mitogen (lectin from Phyto- results from genetic relative susceptibility testing for lacca americana).h The cells were cultured at 37uC for 72 hr, classical scrapie by a U.S. Department of Agriculture and metaphases were arrested by treating cultures with (USDA)-contract laboratory were reported as ‘‘inconclu- demecolcine solutionh (0.1 mg/ml) for 1 hr, followed by a sive’’ after 2 independent blood sample submissions. A 30-min treatment with Optimal Hypotonic Solution.i The third blood sample was drawn at 20 months of age and cells were fixed in methanol/glacial acetic acid (3:1), submitted to the National Veterinary Services Laboratories dropped into precleaned glass slides, and then air dried. (NVSL),a which subsequently reported that the sample had Chromosomes were stained with 5% Giemsa solutionh in either been contaminated with DNA from another sheep or 0.07 mol Sørensen buffer, pH 6.8. C-banding was had been obtained from a chimera. The female was performed according to a previous study4 by treating slides acquired by the Animal Disease Research Unit (ADRU)b in 0.2 N (normal) HCl for 30 min at room temperature, in for further investigation and was approximately 2.5 years 2.5% Ba(OH)2 for 10 min at 60uC, and in 2X saline–sodium of age
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