D-208 Veterinary Medical Center, Department of Small Animal Clinical Sciences, Michigan State University, East Lansing, MI, USA
Over the last decade the number of genetic tests available for companion animals has expanded considerably and will undoubtedly continue to expand. This provides us with new tools for disease identification and elimination. It provides animal owners and breeders with the opportunity to eradicate certain hereditary diseases but also to breed for physical characteristics such as coat colour and coat length. It allows for paternity testing, dog breed identification, DNA fingerprinting and, for avian species that do not have sexual dimorphism, sexing by DNA testing is possible. The banking of DNA is becoming popular, and is to be encouraged, because such samples may prove very useful for future investigations as new genetic traits are identified.
The first genetic tests that were developed were for inherited metabolic diseases where an abnormal metabolite characteristic for the condition could be detected. With advances in molecular biology current tests are DNA-based and typically identify the presence/absence of the genetic trait being tested for. Prior to identification of the DNA change responsible for a particular trait a DNA marker that was closely linked to the trait may have been identified. These linked markers have been used in genetic testing in the past. Linked marker tests are not directly identifying the DNA change that causes the trait being tested for but rather a particular version of a variable portion of DNA that is positioned close to it. This means that such tests have limitations that need to be considered when interpreting them. Fortunately most tests directly identify the actual genetic trait being tested for.
DNA-based tests for hereditary disease genotype the test animal at the DNA locus of interest. For autosomal loci the animal may be homozygous for the normal DNA sequence, homozygous for the mutated DNA sequence or heterozygous. Heterozygotes for autosomal recessive traits will be carriers of the trait. In the case of dominant disease the heterozygotes express the trait.
DNA tests can allow breeders to eliminate hereditary disease from their lines. A DNA sample can be collected at any age allowing genotyping before breeding. When making decisions about breeding strategy the breeder should consider the incidence of the condition in the breeding line. If the condition is present at a low incidence, simply not breeding from affected and (for recessive disease) carrier animals will rapidly eliminate it. If the condition has a high incidence, this strategy is not desirable because eliminating a significant proportion of the breeding population may narrow the gene pool and could potentially increase the incidence of other deleterious genetic characteristics. Additionally, if animals with desirable characteristics are eliminated from the gene pool this could potentially lead to loss of those features from the breeding line. For these reasons a more gradual elimination of the hereditary disease over several generations is preferable. This allows selective breeding to preserve the good genetic characteristic and maintain genetic variability in the breed. This can be considered as breeding to separate the 'good' genes from the 'bad' genes. The aim of genetic testing is to prevent animals affected with the disease from being born. This can simply be achieved for recessive disease by only breeding carriers with homozygous normal animals. Fifty percent of the offspring from a normal x carrier mating will be carriers themselves and if they have other very desirable characteristics they in turn can be used for breeding until eventually the good characteristics are present in dogs that are homozygous normal for the disease trait.
The source of DNA for genetic tests can be from cheek swabs or blood. Obviously it is quicker and cheaper for the dog owner to collect a cheek swab. However, before collecting samples it is important to check the submission requirements of the test laboratory.
Accuracy of DNA Tests
A well designed DNA-based genetic test should be very accurate. However, clearly there is always the possibility of human error. The most likely potential errors come from mixing samples up. If several dogs are sampled at the same time it is important that each sample is labelled clearly and accurately at the time of sampling. Quality control in the test laboratory is also extremely important and adequate controls should be run with the test samples to ensure that the assay is functioning correctly. The majority of tests are based on polymerase chain reaction amplification of DNA. The technique is able to amplify from very low levels of DNA so it is important that there is no cross-contamination of samples. Good laboratory practice should be in place to prevent contamination within the laboratory, and controls set up to detect it if it occurs. Currently there are no established quality controls recommended for laboratories offering companion animal genetic testing. Laboratories running tests vary from commercial laboratories, including those that run other veterinary diagnostic tests, to those that are set up only for DNA-based testing. Some tests are also run out of the laboratory of the investigator who identified the gene mutation.
It is important to educate breeders about the tests and how to interpret them. Each test is highly specific for the trait it was designed to detect and this can lead to confusion. A good example of potential confusion arises with tests for progressive retinal atrophy (PRA). This condition has genetic heterogeneity, meaning it can potentially be caused by a mutation in one of several genes (human inherited retinal degenerative conditions have been mapped to over 200 different genetic loci). As the causal gene mutations for more forms of PRA are identified it has become clear that in some breeds there are concurrently several genetically distinct forms of PRA. Each genetic test will identify only the presence/absence of the gene mutation it was designed for. For this reason a dog might test clear for PRA by a genetic test but still have PRA - just PRA caused by one different gene mutation. The second form of PRA could be in a different gene or even in a different part of the same gene.
Currently genetic tests for hereditary disease are for identifying specific simply inherited conditions. In the future we can expect that genetic factors that predispose for more complex conditions will be identified. The interpretation of genetic tests developed for risk factors for disease will inevitably be more complex than that for simply inherited conditions. However, the increased understanding of disease mechanisms that the discovery of such genetic variations will offer and the ability to test for them can be expected to provide us with even greater tools to use to improve the health of our patients.