Canine Genetic Testing in Veterinary Medicine
by Jenny Alonge
In the 1950s, scientists discovered that an additional copy of chromosome 21 causes Trisomy 21 (i.e., Down syndrome). This finding led to genetic testing, which is used commonly in human medicine to detect inherited diseases. Genetic testing is also becoming an important diagnostic tool in veterinary medicine as we expand our knowledge of animal genomes.
Genetically based diseases are common in dogs, with purebred dogs having the most disease-causing genetic mutations, although mixed-breed dogs can also be affected. Selective breeding and inbreeding to achieve specific aesthetic and behavioral traits restrict a breed’s gene pool, increasing the probability of genetic mutations. Testing for genetic markers allows for early disease detection and, in some cases, measures to prevent disease.
Canine genetic mutations
Dogs have 39 chromosome pairs that are composed of hundreds of thousands of genes. Thirty-eight pairs are autosomes (i.e., nonsex chromosomes) and the 39th pair is an allosome (i.e., sex chromosome). Genetic diseases occur when a single gene in the chromosome mutates, or multiple genes mutate in one or more chromosomes. Affected dogs can transfer these mutations to their puppies.
Single gene mutations (i.e., monogenic) can be classified as follows:
- Autosomal recessive — Autosomal recessive genetic diseases are the most common mutation types in dogs. The mother and father must pass on the recessive gene for the puppy to be affected.
- Autosomal dominant — Since the trait is dominant, only one parent needs to contribute a defective gene for the puppy to be affected.
- Sex-linked — Diseases caused by mutations on allsomes increase a particular gender’s risk.
Multiple gene mutations (i.e., polygenic) are more difficult to recognize, and their prevalence is unclear. Polygenic diseases are caused by a number of independently acting or interacting polymorphic genes, and the individual contribution of each gene can vary.
Canine genetic testing
Genetic testing identifies a genetic mutation, which indicates that the dog has a higher risk for a specific disease. Most tests identify single gene defects and many identify only the disease’s genetic markers, which indicate only that mutation is possible. Genetic testing is useful for the following reasons:
- Early detection — Early knowledge that a dog has a disease or has an increased risk of disease development allows you to instigate preventive measures or management strategies to help prolong their life and improve their quality of life.
- Improved breeding practices — Genetic testing is especially important for breeding animals to remove carriers from breeding programs or to breed selectively with genetically normal partners to decrease genetic disease prevalence.
Available canine genetic tests include:
- Hyperuricosuria (HUU) — Increased uric acid levels in the urine predisposes dogs to bladder and kidney stones. HUU is an autosomal recessive defect. The condition can occur in any breed, but is most common in dalmatians, bulldogs, and Black Russian terriers.
- Degenerative myelopathy (DM) — DM is an inherited neurologic disorder causing gradual muscle wasting and incoordination that typically starts in the hind limbs. The condition is associated with a mutation in the SOD1 allele. Clinically affected breeds include Australian shepherd, boxer, Chesapeake Bay retriever, golden retriever, Labrador retriever, standard poodle, and the wire fox terrier.
- Hereditary nasal parakeratosis (HNPK) — HNPK is an inherited, recessive defect that affects the specialized cells in the dog’s nose that crust and crack over the nasal area. Breeds most commonly affected include Labrador retriever, Labradoodle, Australian Labradoodle, and Labrador crosses.
- Multidrug resistance 1 (MDR1) — MDR1 drug resistance is caused by a genetic variant in the ABCB1 gene and can lead to severe complications after taking particular medications. Affected breeds include collies, Australian shepherds, American shepherds, German shepherds, Shetland sheepdogs, and Old English sheepdogs.
- Exercise-induced collapse (EIC) — EIC is a genetic neuromuscular disorder that causes muscle weakness, incoordination, and life-threatening collapse after intense exercise. Affected breeds include Australian Labradoodle, Chesapeake Bay retriever, cocker spaniel, and Labrador retriever.
- Pyruvate kinase deficiency (PKDef) — PKDef is an inherited hemolytic anemia caused by a defect in the enzyme pyruvate kinase. Affected breeds include the beagle, Cairn terrier, pug, and West Highland white terrier.
- Von Willebrand’s disease (vWD) — vWD, an inherited bleeding disorder, comes in three types.
- Type one — Most common in breeds including Doberman pinscher, Shetland sheepdog, and standard poodle
- Type two — Most common in German short-haired and wire-haired pointers
- Type three — Most common in Scottish terriers, Chesapeake Bay retrievers, and Shetland sheepdogs
- Factor VII deficiency — This mild to moderate inherited blood clotting disorder affects breeds including the beagle, giant schnauzer, miniature schnauzer, and Welsh Springer spaniel.
- Juvenile hereditary cataracts (JHC) — JHC causes early onset cataracts in both eyes and affects breeds including the Boston terrier, French bulldog, and Staffordshire bull terrier.
- Progressive retinal atrophy rod-cone degeneration (rcd4-PRA) — This condition causes late-onset retinal photoreceptor cell degeneration, leading to blindness. Affected breeds include the dachshund, French bulldog, golden retriever, Irish setter, and standard poodle.
Genetic testing can help you detect inherited conditions and dogs who have the potential for these conditions, so you can monitor their progress and start management strategies early. These tests are also important to help decrease genetic disease prevalence in breeding animals.
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