School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA, USA
While clinical and routine laboratory and imaging techniques are helpful, specific biochemical and DNA tests have become available for > 100 disorders through various laboratories. Simple test sample requirements and result interpretations are presented with clinical case examples. As it is difficult to keep track of all the diseases, tests and treatments, a WSAVA website on hereditary diseases in companion animals for clinicians is being introduced.
There are many unique traits of canine and feline breeds and many hereditary disorders and genetic predispositions to disease have been identified. With the recent completion of the canine and feline genome sequences and molecular techniques these genetic traits and defects have been and are being characterized from the clinical signs to the molecular defect. While clinical and routine laboratory and imaging tests are helpful, specific biochemical and DNA tests have become available for > 140 single gene defects through various laboratories in dogs. Moreover, with DNA tests it is now possible to determine the ancestry of mixed breed and purebred dogs, a first example of a complex trait. As it is difficult to keep track of all the diseases, tests and treatments, a web-based database for available DNA tests on hereditary diseases in companion animals for clinicians is being introduced (http://research.vet.upenn.edu/WSAVA-LabSearch).
Because of the increased awareness of breeders, pet owners, and veterinarians of genetic defects and the improved diagnostic abilities in clinical practice, the number of reported hereditary diseases in small animals is rapidly growing. At present, > 900 hereditary diseases in dogs have been adequately documented. For the small animal practitioner, it can be a daunting, nearly impossible task to remember all these diseases and be aware of the many novel tests and their appropriate management and control.
Many of the characteristic breed traits and common and rare diseases seen in veterinary practice have a heritable basis. Recent exciting advances in our current knowledge of the completed dog genome sequences offer the opportunity to clinicians to use these emerging tools in clinical practice and have a positive impact on the health of dogs and in particular the diagnosis, management, and control of hereditary diseases. Many specific breed traits such as size, chondrodysplasia, brachycephaly and many skin and coat color characteristics have recently been defined.
Databases on Hereditary Diseases
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://sydney.edu.au/vetscience/research/disorders/omia.shtml) (VIN editor: link was edited on 2/22/13); "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 has set up a database on genetic tests for hereditary diseases (http://research.vet.upenn.edu/WSAVA-LabSearch; 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 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.
Many 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 few 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 the 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 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. 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. All currently available DNA tests for hereditary diseases in dogs and cats and associated laboratories worldwide can be found at http://research.vet.upenn.edu/WSAVA-LabSearch.
Additional references are available from author. Author's studies were supported in part by grants from the National Institutes of Health (RR002512) and the AKC Canine Health and other foundations.
Contact: Dr. Urs Giger, School of Veterinary Medicine, University of Pennsylvania, 3900 Delancey Street, Philadelphia, Pennsylvania, USA. firstname.lastname@example.org; http://research.vet.upenn.edu/penngen.