Jerold S. Bell, DVM
As practitioners, we deal with genetic disease every day in our practice in cross-bred and pure-bred dogs. Predictability is the hallmark of genetic disease. With disorders whose progression can be altered, our role as veterinarians is early diagnosis and intervention. Any recommendations that we make to clients based on our knowledge of how our patients will respond to that recommendation is genetic counseling. Genes get turned on and off, and their effects are mitigated based on environmental changes placed on them. These include diet, drugs, infection, and surgery.
Several genetic disorders are frequently seen across all dog breeds, and in mixed-breed dogs. These include hip dysplasia, elbow dysplasia, patella luxation, autoimmune thyroiditis, allergic skin disease, inherited forms of cancer, epilepsy, cataracts, and congenital heart disorders. Other less frequent genetic disorders may be breed-related and limited to specific pure breeds. However, the identification of the molecular basis of genetic diseases is showing us that what we once thought were breed-specific mutations are actually ancient, widespread mutations that can occur in any breed, and can affect any pure-bred or mixed-breed dog. These include genes for: the prcd form of progressive retinal atrophy, choroidal hypoplasia, primary lens luxation, the susceptibility gene for degenerative myelopathy and others.
Genetic tests vary on what they are able to identify, and 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 defect that you are trying to manage. Screening for cataracts, ausculting for heart murmurs, hip and elbow radiographs, urinalysis for crystals or metabolites, skin biopsy for sebaceous adenitis, and observations on behavioral traits are all tests of the phenotype. Most tests of the phenotype only identify affected individuals, and not carriers.
Direct gene tests utilizing the polymerase chain reaction (PCR), are a direct measurement of the genotype. 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. Others test for susceptibility genes that provide liability for the disease. Some of these susceptibility genes are necessary for the animal to be affected, though other yet undiscovered genetic or environmental factors are also necessary. Examples are cord1 PRA in English Springer Spaniels and Miniature Dachshunds, and degenerative myelopathy 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 is the susceptibility gene for perianal fistula/anal furunculosis in German Shepherd Dogs.
Linked-marker based tests do not identify the defective gene, but a marker that lies close to the gene on the same chromosome. If a crossover between the marker and the gene occurs during the production of egg or sperm in meiosis, the marker will no longer be linked to the defective gene. Few false positive and false negative results will occur in all descendents. Due to this phenomenon, linked marker test results must be compared with results from other family members to determine whether they correlate with the known genotype of relatives. Linked marker tests include those for cerebellar ataxia in Italian Spinone and primary hyperparathyroidism in Keeshond.
Genetic Counseling for Owners
The vast majority of our patients are not breeding animals, but they still require genetic counseling for inherited disorders. We need to be knowledgeable about what genetic tests are available, and in what patients they should be run. Patients from breeds with an incidence of von Willebrand's disease and other bleeding disorders should be tested early in life, so that measures can be taken to prevent excessive hemorrhage during surgery or injury. Patients at risk of carrying the mdr-1 mutation should be tested early in life, before drug treatment. If both parents were tested normal prior to breeding (with DNA verification of parentage), then your patient does not need to be individually tested. We counsel owners of large-breed puppies to feed lower calorie foods to provide for a more uniform growth rate and better joint development. Breed related disease should be tested for (or verified results documented on parents) before purchase.
We need to understand the temporal periods when genetic testing will be most accurate, and allow for intervention. Puppy hips should be palpated with a gentle Ortolani procedure at each vaccine visit, and again at spaying or neutering under anesthesia. Juvenile interventional surgery will only benefit those with significant subluxability prior to major growth (for pubic symphysiodesis) or the development of osteoarthritic changes (for triple pelvic osteotomy).
Genetic testing for hypothyroidism is based on the presence of thyroglobulin autoantibodies (TgAA). A dog with normal TgAA levels on two tests at least two years apart between two and six years of age is phenotypically normal. However, TgAA levels should not be measured within 2-3 months post-vaccination, as a transient iatrogenic rise can occur during this period.
Genetic Counseling for Breeders
Breeder genetic counseling recommendations are geared toward preventing the production of affected animals, and reducing the production of carriers. At the same time, recommendations should allow the continuation of breeding lines, to preserve the genetic diversity of the population.
Historically, genetic counseling has ranged from recommendations to not repeat a mating and outbreed, to recommendations to eliminate all relatives of affected dogs. Neither of these serves the best interest of breeds. Outbreeding can prevent the production of affected animals, but it will propagate and further disperse the detrimental recessive genes. Breeders are working with closed studbooks, and must consider how selection affects genetic diversity in the gene pool.
With widely dispersed or high frequency defective genes, it must be recognized that carriers are spread across the gene pool. Eliminating unique breeding lines because some individuals carry a single defective gene may adversely affect gene pool diversity more than a process that allows a limited number of carriers to reproduce. Conversely, with recently mutated or low frequency defective genes, it is advisable to strictly limit breeding, so as to not spread the defective gene in the population.
There are no breeding recommendations that will fit every situation. Protocols for genetic counseling and breeding management of genetic disorders can be based on the known (or unknown) mode of inheritance, and the availability and type of genetic tests. Genetic tests should be used to increase the breeder's options for breeding, and not limit them.
Autosomal Recessive Disorders
In the case of a simple autosomal recessive disorder for which a test for carriers is available, the recommendation is to test breeding-quality stock, and breed carriers to normal-testing individuals. This prevents affected offspring from being produced. The aim is to replace the carrier breeding-animal with a normal-testing offspring that equals or exceeds it in quality. You don't want to diminish breed diversity by eliminating quality animals from the gene pool because they are carriers. Additional carrier testing offspring should not be placed in breeding homes; as the goal is to reduce the frequency of the defective gene in the population. As each breeder tests and replaces carrier animals with normal-testing animals, the problem for the breed as a whole diminishes.
The typical response of a breeder on finding that their animal is a carrier is to remove it from a breeding program. If a majority of breeders do this, it puts the breed's gene pool through a genetic bottleneck that can significantly limit the diversity of the breed. The goal of genetic testing is to allow the superior genes of a breeding individual to be propagated, even if the animal is a carrier. One defective gene that can be identified through a genetic test out of tens of thousands of genes, is not a reason to stop breeding. If an owner would breed an individual if it tested normal for a genetic disease, then a carrier result should not change that decision.
We know that most individuals carry some unfavorable recessive genes. The more genetic tests that are developed, the greater chance there is of identifying an undesirable gene in your patient. History has shown that breeders can be successful in reducing breed-wide genetic disease through testing and making informed breeding choices. However, there are also examples of breeds that have actually experienced more problems as a result of unwarranted culling and restriction of their gene pools. These problems include: reducing the incidence of one disease and increasing the incidence of another by repeated use of males known to be clear of the gene that causes the first condition; creating bottlenecks and diminishing diversity by eliminating all carriers of a gene from the breeding pool, instead of breeding and replacing them; and concentrating on the presence or absence of a single gene and not the quality of the whole animal. Genetic test results should be used to benefit the overall health of breeds, not to limit it.
The problem with simple autosomal recessive disorder for which no carrier test exists is the propagation and dissemination of unapparent carriers in the gene pool. Breeders must assess whether each individual animal in their breeding program is at high risk of being a carrier. An open health registry that is supported by the parent club makes it easier for breeders to objectively assess these matters. By determining the average carrier-risk for the breeding population, breeders can select matings that have a projected risk that is lower than the breed average.
If a quality breeding animal is at high risk of being a carrier, the best advice is to breed to an individual that has a low risk. Using relative-risk assessment as a tool, breeders should replace higher-risk breeding animals with lower-risk offspring that are equal to or better than their parents in quality. A negative aspect of pedigree analysis is that it selects against families, regardless of an individual's normal or carrier status. On the other hand, it allows for the objective risk assessment and continuation of lines that might otherwise be abandoned due to high carrier-risk. An example of an open health database and relative risk analysis program is cerebellar abiotrophy in the Scottish Terrier (http://www.stca.biz/GrandCentral/CACentral-CA.asp).
Autosomal Dominant Disorders
Autosomal dominant genetic disorders are usually easy to manage. Each affected animal has at least one affected parent, but it can be expected that half of the offspring of an affected animal will be free of the defective gene. The recommendation is to not breed affected animals. To produce the next generation of a line, a normal full sibling of an affected animal can be used, or the parent that is normal can be used. A problem with some autosomal dominant disorders is incomplete penetrance, where some individuals with the defective gene may not show the disorder. If a genetic test is available, this is not a problem. Otherwise, relative-risk assessment can identify which individuals are at risk of carrying incompletely penetrant dominant genes.
For sex-linked (also known as X-linked) recessive defective genes, selecting a normal male for breeding loses the defective gene in one generation, regardless of his relationship to affected and carrier relatives. Carrier, affected, or high risk females should not be used, due to the high risk of producing affected male offspring. If a male is affected, he would have received the defective gene from his carrier mother. All of his daughters will be carriers, but none of his sons. Without a test for carriers, you can use relative-risk assessment to breed him to a female that is at low risk of being a carrier. This prevents affected offspring, and a quality son can be selected for replacement. Rare sex-linked dominant disorders are managed the same way as autosomal dominant disorders. The difference is that affected males will always produce all affected daughters.
Polygenic Disorders/Complex Inheritance
Most polygenic disorders have no tests for carriers, but they do have phenotypic tests that can identify affected individuals. These disorders require knowledge of the affected or normal status of full-sibs to prospective breeding animals. Individuals whose siblings are normal, and whose parents' sibs are normal have the greatest chance of carrying a low genetic load for the condition. This "breadth of pedigree" analysis is more important than normalcy in the depth of pedigree (parents and grandparents only.) Affected individuals can be replaced with a normal sib or parent, and bred to a low-liability mate. Breeders can replace the higher risk parent with a quality, lower risk offspring, and repeat the process. For disorders without a known mode of inheritance or carrier test, breeders should be counseled to use the same control methods as with polygenic disorders.
Breeders are the custodians of their breed's past and future. The individual breeder can use genetic tests to 1) identify carriers, 2) work to breed away from the defective gene(s), and 3) ensure (through testing) that the defective gene(s) is not reintroduced in future matings. Breeders should be counseled on how to utilize test results for the best interests of the breed.