Susan L. Ewart
Genetic Diseases, Breeds, Tests, and Test Sources
Genetic tests are used to identify variation in the genetic code—DNA—that is heritable. Such altered forms of DNA may manifest as disease-causing mutations that occur rarely in a population because of their detrimental effects on health or reproduction. Alternatively, changes in DNA sequence may result in a physical trait, termed phenotype, that is innocuous or even desired, such as a coat color. Despite having no ill effects on health or reproduction, it is the case that some coat colors, such as white spotting, are indeed the result of DNA variants that cause protein malfunction. To encompass both detrimental mutations and DNA changes that by means of selection are no longer rare in the population, the words variant, alternate, and polymorphism are used. These words simply indicate that the DNA sequence is different from the wild type, the type most common in nature. These various forms that DNA can assume at specific sites along a chromosome are termed alleles (Box 212-1).
The complete horse genome has been sequenced by an international consortium led by the Broad Institute, and a high-quality draft assembly is publically available (current assembly EquCab2.0, released September, 2007). The Genome Project Report for the horse can be found at the National Center for Biotechnology Information website (www.ncbi.nlm.nih.gov/genome), along with a single-nucleotide polymorphism (SNP) database for horses. Also maintained by the National Center for Biotechnology Information is the Online Mendelian Inheritance in Animals database, which catalogs genes, inherited disorders, and traits in more than 135 animal species, including horses (www.ncbi.nlm.gov/omia). These are tremendous resources for researchers to identify genes underlying diseases and other phenotypes of horses.
Genetic tests for horses are available for determining the carrier status of a number of genes that control physical features, such as coat color, as well as for genes that cause inherited diseases. In addition, genetic tests can be used for parentage verification. The number of genetic tests for horses has increased rapidly in recent years. Concurrently, the options in commercial laboratories offering tests have grown, but the costs remain reasonable. Finally, breed registries may require DNA-based parentage verification or genetic testing for breed-specific inherited diseases. Together, these factors have resulted in wide interest in and pursuit of genetic testing of horses. Although there are multiple commercial and university laboratories that offer most available genetic tests for horses, breed registries may contract with a specific genetic testing laboratory and only accept results from that laboratory for registry purposes. Therefore it is best to check with the breed registry before submitting samples for genetic testing to confirm any such testing limitations or requirements (Table 212-1).
TABLE 212-1
Commercial Facilities for Horse Genetic Tests
| Ref.* | Laboratory | Location | Website |
| 1 | Animal Genetics, Inc. | Tallahassee, FL | http://www.horsetesting.com |
| 2 | Animal Genetic Testing & Research Laboratory | University of Kentucky, Lexington, KY | http://www.ca.uky.edu/gluck/AGTRL.asp |
| 3 | Animal Health Diagnostic Center | Cornell University, Ithaca, NY | http://ahdc.vet.cornell.edu |
| 4 | Capilet Genetics AB | Västerås, Sweden | http://www.capiletgenetics.com/en |
| 5 | Equinome, Ltd. | Dublin, Ireland | http://www.equinome.com |
| 6 | F.O.A.L. Arabian Foal Association† | N/A | http://www.foal.org |
| 7 | Progressive Molecular Diagnostics | Tioga, TX | http://www.progressivemoleculardiagnostics.com |
| 8 | Veterinary Diagnostic Laboratory | University of Minnesota, St. Paul, MN | http://www.vdl.umn.edu |
| 9 | Veterinary Genetics Laboratory | University of California, Davis, CA | http://www.vgl.ucdavis.edu |
| 10 | VetGen | Ann Arbor, MI | http://www.vetgen.com |
* These numbers are used in Table 212-2 to indicate the specific tests offered by each testing facility.
† Provides opportunity to purchase DNA test kits at a reduced cost, but does not provide genetic testing.
Samples
Genetic tests are conducted on deoxyribonucleic acid (DNA), which is contained in the cell nucleus. All cells have nuclei, with the exception of normal circulating red blood cells. The most common and readily accessible sources of DNA from live animals are anticoagulated peripheral blood, the roots of hairs, cheek swabs or saliva, and nasal swabs. In addition, DNA can be extracted from nearly any tissue collected intraoperatively or at necropsy.
The source of DNA in peripheral blood samples is leukocytes, and the yield of DNA from blood is routinely 10 to 15 µg/mL. The volume of blood collected during routine venipuncture (3 to 7 mL) is therefore more than sufficient for genetic testing. Blood should be collected into an anticoagulant, with EDTA (ethylenediamine tetraacetic acid; lavender-topped Vacutainer tube) being the anticoagulant of choice by most laboratories, and ACD (acid-citrate-dextrose; yellow-topped tube) being preferred by a few. Instructions for storing and shipping blood samples vary by laboratory, with most recommending keeping samples refrigerated at 4° C (39° to 40° F) and shipping them overnight on ice or frozen gel packs. Blood samples sent through the mail must be secured in padded, leak-proof packaging. The submission instructions for the specific laboratory to be used for a given test should be consulted.
The root end of freshly plucked hairs (i.e., the hair bulb) contains as much as 0.5 µg DNA; in contrast, little DNA can be extracted from the shaft portion of hair. Because mane and tail hairs have larger hair bulbs, they are preferred for purposes of DNA extraction. Laboratories that use hair samples as the DNA source generally request 20 to 50 freshly plucked mane or tail hairs per sample (with tail hairs preferred for young horses). Other instructions include abstaining from applying products, such as coat conditioners, to the animal for 2 to 3 days preceding the hair sample collection, ensuring that the root or bulb is present in the plucked hairs, and keeping the hairs dry. Hair samples can be taped to a piece of paper—a laboratory may request that hairs be taped to the submission form—or placed in a sealed paper envelop or sealed plastic bag. Hair samples can be kept at room temperature and shipped by normal mail. See submission instructions for the specific laboratory to be used.
Cheek swabs have long been used as a noninvasive source of sample for DNA isolation from dogs and cats and for research purposes and are used by a few laboratories conducting genetic tests in horses. These laboratories provide cheek swab collection kits and instructions.
Saliva has recently become the sample of choice for many human genetic studies because the collection method is noninvasive, and saliva is rich in DNA from both buccal epithelial cells and white blood cells. Recently, a saliva collection kit for animals has been marketed.1 The reagents in the kit preserve the DNA in saliva, making it stable at room temperature for an extended period of time. Although the cost of saliva collection by means of a kit may exceed that of blood or hair collection, it may be a preferred collection method in some situations. Along with the introduction of saliva collection kits for animals has been the introduction of nasal swab DNA collection kits marketed for livestock.2 The source of DNA in nasal swab samples is both epithelial cells and white blood cells. As with other samples collected for DNA extraction purposes, the submission instructions for the specific laboratory to be used should be consulted.
