Why it’s Important for Pet Owners to Understand Genetic Testing Cynthia O’Connor, DVM, DACT Animal Reproduction Specialist Slade Veterinary Hospital www.sladevet.com |
This is an exciting time with amazing advances in veterinary medicine. The area of molecular genetics has become one of the hottest areas of research bringing the development of genetic testing to dog breeders. So why as a pet owner is this information important? Many pet owners are looking for a happy, healthy pet to add to their family. Because they are not interested in breeding, many mistakenly feel that they do not need to learn about genetic testing. Many of these people also fall into the trap of impulse buying, by purchasing a dog in a pet store or via an internet puppy broker. Many impulse purchases lack research into the health and temperament of the pedigree and these new owners can only hope that the dice will roll in their favor with a happy, healthy pet. Genetic testing offers the opportunity for many people to become more informed consumers. After all, would you consider buying a car, sight unseen, with no history? Then, why would you not put in the same, if not more effort, into investigating a new addition to your family that you will trust to sleep in your child’s bed? To understand the use of genetic testing and their use for breeders and pet owners, we first need to remember some basics about genetics. The term hereditary or genetic is used to describe a disease caused by a DNA mutation that can be passed from parent to offspring. The term congenital is used to describe a disease that is present at birth. It is important to remember that a congenital disease can be genetic; however, not all congenital diseases are genetic. In fact, toxins, infections, and other environmental factors can cause congenital diseases. It is also important to remember that not all genetic diseases are identifiable at birth. A good example of this is progressive retinal atrophy (PRA) in Portuguese Water Dogs (PWDs). Dogs affected with PRA generally do not show signs of blindness until much later in life. Once a disease is recognized as genetic, knowing how it is passed from generation to generation is the next step in preventing and managing genetic diseases. There are several inheritance patterns recognized in dogs including autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and complex (polygenetic) disease. Recessive diseases account for a majority of the diseases for which there is a known inheritance pattern and for which a genetic defect has been identified. However, with continued advances in molecular biology and technology, this will soon be true for complex (polygenetic) disorders as well. We are probably most familiar with autosomal recessive traits. An example of an autosomal recessive trait in Portuguese Water Dogs is storage disease (GM-1 gangliosidosis). With an autosomal recessive trait, PWDs that have two copies of the normal gene are referred to as non-carriers or normal for that particular trait. PWDs that carry one affected gene and one normal gene are referred to as carriers and they do not show any signs of the disease. However, PWDs that are carriers may pass the affected gene onto their puppies. Puppies that receive two affected genes, one from each carrier parent, will develop the disease. These are important points to keep in mind for breeders as well as pet owners. The goal of a breeder is to create healthy puppies while maintaining breed genetic diversity. Breeders can utilize tests such as this to prevent producing an affected puppy, while still utilizing the genes of exceptional carrier and in some instances, affected animals. It is important for pet owners to understand that for autosomal recessive diseases such as this example in PWDs, a carrier dog is a very healthy pet. This does not always hold true for other modes of inheritance, so when evaluating a disease and the use of the genetic test, it is important to understand how the disease is inherited. Health screening tests come in several varieties and vary on what they are able to identify, therefore how they can be used in managing genetic disease. Some tests measure the phenotype, or what can be seen in the animal. This may not directly relate to the genotype, or the genes regulating the disease. Eye examinations by veterinary ophthalmologists for C.E.R.F (Canine Eye Registration Foundation) certification and x-rays of hips for certification free of hip dysplasia by the Orthopedic Foundation for Animals (O.F.A.) are examples of health screening tests of the phenotype of the dog. Most tests of the phenotype only identify affected individuals, and not carriers. Phenotypic tests such as these are utilized for health screening when the disease causing gene(s) is not yet known. Direct gene tests are a direct measurement of the genotype. These detect the exact DNA defect or mutation that is the cause of the disease. Results are often given as Normal, Carrier, or Affected. These can be run at any age, regardless of the age of onset of the disorder. With tests for the genotype, breeders can identify affected, carrier, and genetically normal animals. Most tests of the genotype are for genes that are the sole and direct cause of a disease or condition. Examples of direct gene mutation based tests such as these available for PWDs include GM-1 testing by NYU, prcd-PRA testing by OptiGen, and juvenile dilated cardiomyopathy testing by UPENN. The second type of genetic test is the linked marker test. Markers are segments of DNA that have an identifiable physical location on a chromosome whose inheritance can be followed. A marker can be a gene or it can be some section of DNA with no known function. Because DNA segments that lie near each other on a chromosome tend to be inherited together, markers are often used as indirect ways of tracking the inheritance pattern of a gene that has not yet been identified, but whose approximate location is known. The results of these tests are often given as a pattern such as A, B, or C. The closer the marker is to the disease causing gene, the less chance of a false positive or false negative with this test. Linked marker tests are utilized when the direct gene mutation has not yet been identified. A newer and more complicated application of direct gene tests allows for breeders to identify dogs that carry "susceptibility genes" for certain diseases that are inherited in a more complex manner. This type of direct gene test gives a risk for susceptibility to a disease based on the gene(s) of that individual. Some of these susceptibility genes are necessary for the animal to be affected, though other yet undiscovered genetic or environmental factors are also necessary, making this more complex. Examples of genetic tests for susceptibility of disease are the tests for degenerative myelopathy and juvenile renal dysplasia in several dog breeds. Other susceptibility genes are found to occur at a greater frequency in affected animals, but are not present in all affected animals. An example of this is the susceptibility gene for perianal fistula/anal furunculosis in German Shepherd Dogs. The obvious advantage of genetic testing is the ability to immediately reduce puppy loss from devastating diseases such as juvenile dilated cardiomyopathy in PWDs. Genetic testing allows breeders, based on careful breeding practices, to prevent the production of an affected puppy and a gradual selection away from this disease over several generations. There is nothing more devastating to a pet owner than taking home a beautiful new PWD puppy only to have this puppy die suddenly from this disease at such a young age. The emotional toll that this tragedy takes on the entire family as well as the breeder is immense. With genetic based tests like this, breeders can identify healthy carrier animals so that they can make careful breeding decisions in order to avoid tragedies such as this for pet owners. Pet owners, in turn, can use these tests to help find breeders who are working to identify and prevent diseases like this in their dogs. Unfortunately, breeders can not look into a crystal ball and predict that a puppy they produce will never get sick. However, an educated breeder can discuss with a new puppy buyer the most common inherited diseases in their breed and their line. A buyer should be concerned when during a discussion like this, a breeder states that they have no health concerns in their dogs and so have no need to perform genetic testing. This should hint to the buyer that the breeder is either not very forthcoming with health information, or does not know about the health history of their lines. A puppy buyer, in turn, needs to understand the basics of these diseases and how they are inherited. For a puppy buyer, it is important to remember that for a simple autosomal recessive disease, such as juvenile dilated cardiomyopathy in PWDs, a carrier of the disease is a perfectly healthy pet. Buyers also need to understand that for a disease that is inherited in this way, a breeder who breeds a carrier to a normal dog will not produce the disease and may be making this decision to help preserve the long term genetic health of their line and the breed as a whole. In fact, it has been shown that by quickly eliminating animals that are carriers or affected by a genetic disease, the genetic diversity of the breed is reduced and the incidence of other diseases is increased. Breeders utilize genetic tests as one factor in making well informed breeding decisions and puppy buyers can also utilize them to become well educated consumers in their effort to find a healthy new addition to their family. |
Click on the picture above for health information provided by the Portuguese Water Dog Club of America (PWDCA). |
Click on the picture above for health testing recommendations for PWDs |
Querida Portuguese Water Dogs, LLC |
* Allergy & Skin * Cancer * Endocrine (Addison's Disease) * Eyes Diseases * Gastrointestinal (IBD) * GM1 * Heart * Neonatal * Orthopedic * Seizures * Thyroid |
PWDCA Recommended Pre-Breeding Health Testing: 1. X-ray evaluation of hips to certify free from hip dysplasia (OFA). Please note that while the PWDCA does not yet endorse PennHip, this is an excellent alternative hip dysplasia evaluation. 2. Eye examination by a veterinary ophthalmologist yearly until 10 years of age (CERF or OFA ECR). 3. OptiGen Progressive Retinal Atrophy (PRA) genetic test of both parents with one parent tested non-carrier. 4. One parent genetically tested non-carrier for gangliosidosis (GM-1). 5. One parent genetically tested non-carrier for juvenile dilated cardiomyopathy (JDCM). |
PWD Health Conditions Click below for link: |
Click on the picture above for PennHip information for hip dysplasia screening |