Urs Giger, DACVIM, DECVIM-CA (Internal Medicine), DECVCP (Clinical Pathology)
While clinical and routine laboratory and imaging techniques are helpful, specific biochemical and DNA tests have become available for > 120 disorders through various laboratories. Simple test sample requirements and result interpretations are presented with illustrative cases. As it is difficult to keep track of all the diseases, tests and treatments, a WSAVA/VIN web site on hereditary diseases in companion animals for clinicians is being introduced. Some of the major advances is presented in a prior lecture while the treatment and control of diseases will follow in a subsequent lecture.
It is difficult for a clinician to keep up with the rapidly accumulating information on clinical genetics and the large spectrum of disorders and genetic predispositions. Thus, comprehensive update resources are needed. There are several web site that provide some information on many different diseases in companion animals such as "Inherited Diseases in Dogs" (www.vet.cam.ac.uk/idid/); Mendelian Inheritance in Animals http://omia.angis.org.au/home/ [VIN editor: the original link www.angis.org.au/Databases/BIRX/omia could not be accessed on 10/04/2011 and was edited]; Canine Inherited Disease Database www.upei.ca/~cidd/intro.htm; and the FAB list of feline hereditary disorders www.fabcats.org/breeders/inherited_disorders. The WSAVA Committee on Hereditary Diseases is setting up a data base with the Veterinary Information Network (www.wsava.org/ and www.vin.com) with pertinent practical information on clinical features, genetic diagnostics, and management specifically for the clinician.
Beyond physical examination and imaging tools, genetic, metabolic, and other laboratory techniques are used to diagnose hereditary disorders in companion animals. Most genetic defects cause clinical signs early in life. The term congenital does only imply that the disease is present at birth, and does not necessarily mean it is inherited. A common presentation is failure-to-thrive compared to littermates. They are poor doers, often fade (hence the term fading puppy or kitten syndrome), and finally die. Failure-to-thrive should not be confused with growth retardation, dwarfism. In addition to these relatively unspecific clinical signs, some defects may cause specific clinical manifestations. Easy to recognize are malformations that involve any part of the skeleton and lead to disproportionate dwarfism, gait abnormalities, and/or facial dysmorphia. A large number of hereditary eye diseases have been described in dogs, some of which are not recognized until adulthood. Neuromuscular signs may vary from exercise intolerance to ataxia and seizures. Defects of many other internal organs are associated with unspecific clinical signs.
Diagnostic tests are generally required to further support a genetic disorder in a diseased animal. Radiology and other imaging techniques may reveal skeletal malformations or cardiac anomalies, and an ophthalmologic examination may further define an inherited eye disease, although some are not recognized until several years of age. Routine tests such as complete blood cell count, chemistry screen, and urinalysis may suggest some specific hematological or metabolic disorders or rule out many acquired disorders. Furthermore, clinical function studies may more clearly define a gastrointestinal, liver, kidney, or endocrine problem. Histopathology and/or electron microscopy of a tissue biopsy from an affected animal or from the necropsy of a littermate or relative may give the first clue to a genetic defect.
A few laboratories provide special diagnostic tests that allow a specific diagnosis of an inborn error of metabolism. Inborn errors of metabolism include all biochemical disorders due to a genetically determined, specific defect in the structure and/or function of a protein molecule. Disorders of intermediary metabolism typically produce a metabolic block in a biochemical pathway leading to product deficiency, accumulation of substrates, and production of substances via alternative pathways. The most useful specimen to detect biochemical derangements is urine because abnormal metabolites in the blood will be filtered through the glomeruli, but fail to be reabsorbed, as no specific renal transport system exist for most abnormal metabolites. The Metabolic Genetic Disease Laboratory at the university of Pennsylvania offers such tests http://research.vet.upenn.edu/penngen. Similarly Cornell's Comparative Coagulation Laboratory offers functional testing for many bleeding disorders (http://ahdc.vet.cornell.edu/) and the Comparative Neuromuscular Laboratory makes some functional and mostly histological analysis available for muscle and nerve disorders (http://vetneuromuscular.ucsd.edu/). Once the failing system has been identified, the defect can be determined at the protein level. Homozygously affected animals have very low protein activity and/or quantities (0–10%). These tests may also be used to detect carriers (heterozygotes), who typically have intermediate quantities at the protein level (30–70%), but no clinical signs. Unfortunately, protein assays require submission of appropriate tissue or fluid under special conditions to specialized laboratories along with a control sample, and are labor intensive.
The molecular defect has been identified for > 60 canine and > 20 feline hereditary diseases, and, thus, DNA screening tests have been developed. These tests are mutation or DNA marker specific and can, therefore, only be used in animals suspected to have the exact same gene defect. Small animals within the same or a closely related breed will likely have the same disease-causing mutation for a particular disease. However, dogs and cats as well as unrelated breeds of a species with the same disorder will likely have different mutations. On the other hand a few mutations have been found in a couple of breeds or may be widespread within the canine population. For instances different mutations have been found to cause anemia due to pyruvate kinase deficiency in different breeds, while a single mutation in the phosphofructokinase gene has been found to cause hemolytic anemia in English Springer Spaniels, Cocker Spaniels, Whippets, and mixed breed dogs. For many inherited disorders, the defective gene remains unknown; however, for a few, a polymorphic DNA marker that is linked to the mutant allele has been discovered. Some mutation and linkage tests have to be further defined such as hypertrophic cardiomyopathy in Maine Coon and Ragdoll cats or renal dysplasia in a several terrier breeds. At present, mutation-specific and linkage tests are available only for single gene defects in small animals; however, complex genetic traits may also soon be approached by these methods as they are for humans. Many predispositions such as inflammatory, immune-mediated, malignant disorders have a genetic basis. While many more single gene defects are being studied from clinical signs to the molecular defect, current investigations are shifting toward complex genetic traits. The many breed predispositions for various complex genetic traits are particularly attractive to further define their molecular bases.
DNA tests have several advantages over other biochemical tests. The test results are independent of the age of the animals, thus, the tests can be performed at birth or at least long before an animal is placed in a new home as well as before clinical signs become apparent. DNA is very stable and only the smallest quantities are needed; hence, there are no special shipping requirements as long as one follows the specific mailing instructions for biological products. DNA can be extracted from any nucleated cells, e.g., blood, buccal mucosa (using cheek swabs), hair follicle, semen, and even formalinized tissue. For instance, blood can be sent in an EDTA tube or a drop of blood can be applied to a special filter paper; buccal swabs can be obtained with special cytobrushes - the cheek cells and not the saliva is needed and swabs need to be completely dried. The DNA segment of interest, which is surrounding the mutation, is amplified with appropriate DNA primers utilizing the polymerase chain reaction (PCR). The mutant and/or normal alleles are identified by DNA fragment size or base pair differences. These tests are generally simple, robust, and accurate as long as appropriate techniques and controls are used. Furthermore, they can be used not only for the detection of affected animals, but also for carriers from birth on.
Although some hereditary diseases can be successfully treated, of much greater importance is the screening of animals prior to breeding to assure that they are free of known hereditary diseases. Through various genetic screening programs, genetic counseling by veterinarians and with responsible breeders many hereditary disorders can be prevented from future generations. This should not be done without careful consideration of the severity of the disease, the treatment options and efficacy, prevalence of the mutant allele in the breed, and the overall gene pool in the breed
In order to reduce the frequency or eliminate altogether a genetic defect, the further spread of the mutant gene has to be prevented in a family and eventually the entire breed. It is obvious that affected animals of any genetic disease should not be used for breeding. This approach is simple and effectively eliminates disorders with a dominant trait. For recessively inherited disorders, however, the elimination of affected animals is not sufficient and does not markedly reduce the prevalence of a defect within a breed or kennel/cattery. Although it may be safest not to breed any relatives of affected animals, as requested by some kennel clubs, this practice of not using carriers may, because of inbreeding and narrow gene pools in some breeds, eliminate all breeders in an entire kennel or cattery, and may severely reduce the genetic diversity of a breed. This may result in the propagation of other defects in a breed.
Thus, it will be pivotal to detect carriers (heterozygotes which are clinically asymptomatic) and truly "clear" animals (homozygous normal) for simple recessively inherited disorders. Obligate carriers can be readily identified for autosomal (both parents of affected) and X-chromosomal recessive (mother of affected) disorders based upon the production of affected animals. As mentioned above, for some diseases, reliable carrier detection tests are available and many breeders know about them and inform the veterinarian. For instance, carriers have approximately half-normal (~50%) enzyme activity by functional assays, or have a normal and mutant DNA sequence for the diseased gene on a DNA test. Carriers can safely be bred to an animal that tested clear without producing an affected animal for a recessive trait. However, all offspring intended for breeding need to be screened: clears can be bred to clear or carriers as long as their offspring are again tested. This practice is safe and will keep desirable traits and the genetic diversity.
Breeders should, therefore, be encouraged to screen their animals for known genetic diseases before breeding whenever carrier tests are available. The availability of genetic tests can be found on several web sites including the WSAVA website. Unfortunately, many breeders mistrust these newer tests; either they were disappointed by the inaccuracy of early tests, such as the radiographic examination for hip dysplasia, or they fear that the results may become publicly known which could hurt their business. If a carrier is used because of a narrow gene pool and many other desirable traits, it should only be bred with a homozygously normal (clear) animal; all its offspring need to be tested, and only clear animals should be used in future breedings. If carriers are used they need to be bred to clear and again any offspring intended for breeding should be screened. If no carrier tests are available, a test mating between the dog in question and a known carrier or affected could be performed, and no affected and at least 5 and 11 healthy puppies/kittens, respectively, need to be born to "clear" an animal of a carrier state. For many breeders and veterinarians, this approach is ethically unacceptable because it may produce affecteds. Thus, breeders need to be educated by well-informed veterinarians; clinical genetic counseling is labor intense and not necessarily lucrative, but has the potential to affect the health of numerous animals far beyond the kennel or cattery involved and thereby also improve the future health of the breed.
Genetic screening tests permit the identification of animals at risk for many of the single-gene hereditary diseases prior to the development of clinical signs, mitigating the suffering of dogs and cats carrying 2 mutant alleles for autosomal recessive traits. These DNA tests are also extremely valuable to detect carriers and thus select breeding animals that will not cause disease or further spread the disease-causing allele. If an animal with all the desirable qualities is found to be a carrier, it could be safely bred to a clear animal (homozygous normal), as this would not result in any affected offspring. However, all offspring should be tested and only clear animals should be used in future generations. These advancements have far-reaching benefits for promoting canine and feline health permitting the elimination of deleterious gene defects, while preserving desirable traits in a breed.
References are available upon request.