“How Does Genetic Testing Apply to a Breeding Program?”

Paula Henthorn,PhD, University of Pennsylvania School of Veterinary Medicine

Fueled by an increased awareness of genetic diseases in dogs, and by the Human and Canine Genome Projects, we have entered a time of significant advances in the study of dog genetic diseases.  An increased understanding, by breeders and by veterinarians, of how genetic diseases are recognized and studied is critical for efficient progress to be made.  Awareness of genetic disorders and their distinguishing characteristics is the foundation of their control.  With enough information, including accurate diagnosis, knowledge of the mode of inheritance, and identification of asymptomatic carriers of recessively inherited disease, the risks of producing genetically defective offspring can be determined, and appropriate breeding decisions can be made.  Our burgeoning knowledge of the gene content of the canine genome will have a dramatic impact on our ability to identify these carriers of genetic diseases by dramatically increasing the rate at which DNA-based genetic tests are developed.

DNA-based genetic tests identify differences in DNA sequences and are of two different varieties.  One type of test, referred to as a mutation-based test, recognizes disease-causing mutations while a second type of test, the linked-polymorphism test, recognizes DNA differences that are near the disease-causing gene and are used to track normal and mutant alleles of that gene through pedigrees.  While there are differences in how these two types of tests are developed and how they are used, both involve the same basic techniques, based on the availability of the dog’s DNA and on the use of the polymerase chain reaction (PCR).  Polymerase chain reaction (PCR) is a common method of creating copies of specific fragments of DNA.  PCR rapidly amplifies a single DNA molecule into many billions of molecules.

Mutation based tests recognize the specific DNA mutation that causes the genetic disease.  Mutation-based genetic tests:

·         require knowledge of the specific mutation (which itself requires that the normal gene sequence also be known)

·         can be used for carrier detection as well as to detect affected animals

·         may be breed specific, so different tests may be necessary to test for the same genetic disease in different breeds

·         do not require DNA samples from additional members of a pedigree

Tests that recognize variations in DNA sequence outside, but closely linked to the gene involved in the disease are known as linked polymorphism tests.  In other words, a linked polymorphism-based test does not involve the disease gene itself, but relies on the ability to detect a normal variation in DNA sequence (polymorphism) on the same chromosome and near the disease gene.  Therefore, it is not even necessary to understand what gene is involved in the disease in order to perform the test.  This type of test:

·         requires the identification of a specific polymorphism, which itself requires the cloning and usually sequencing of a particular piece of DNA and the preliminary gene mapping studies that demonstrate that the polymorphism is linked to the disease.  DNA from many individuals within pedigrees in which the disease segregates is necessary.  However it is not necessary to know the exact mutation, or even the disease-causing gene.

·         is most accurate for use in families in which the parents of the animal in question are informative and DNA is available from an affected full sibling of the animal to be tested.  For an animal to be informative, it must be heterozygous (have two different alleles) at the linked marker locus.  These are not necessities, but additional research is needed to make the tests accurate for use on individual animals and often for use in different breeds.

·         will be in error a small proportion of the time, which can be predicted if the distance between the disease gene and the linked polymorphic locus and the disease-causing gene.  Many informative pedigrees which contain animals segregating both the disease (or a polymorphism at the disease gene locus) and the linked polymorphism are necessary to get a good estimate of the frequency of recombination between the two loci and thus the likelihood that the test will give an erroneous result.

Which type of test is best, mutation-based or linked polymorphism-based?  It is not a question of which is best, but rather, whether there is any test available.  For the vast majority of diseases with a linked-polymorphism based test, a mutation-based test will eventually be developed.  The important issue is to understand the differences between the two types of tests and the resulting differences in how the test results should be used to advise breeders.

The practicality and success of a genetic screening program depends on the following requisites (after Jolly et al, 1981, Adv.Vet.Sci.&Comp.Med 25:245):

a.                   The disease occurs with sufficient frequency to be of economic or social importance.

b.                   The test for the heterozygote is accurate and affordable. 

c.                   Culling of heterozygotes does not deplete key genetic resources.

d.                   Test and control program should be acceptable to breeders (preceded by educational and public relations programs).

e.                   Genetic counseling is available to breeders.

f.                    Breed societies have rules to insure control is based on test results.

Cooperation between veterinary medicine professionals (veterinarians and researchers) and breeders and breed organizations are essential and rewarding components of the genetic screening programs that allow us to effectively control the incidence of genetic disease.

The frequency of inherited conditions can be reduced through good breeding practices.  For this to occur, there needs to be knowledge of the mode of inheritance (how the disease is inherited), of how to identify the condition as early as possible, and of ways to recognize carriers of the disease who, except in the case of autosomal dominant traits, are not clinically affected (autosomal dominant – a gene on one of the non-sex chromosomes that is always expressed, even if only one copy is present).

When attempting to eliminate a disease that has tests available, it is important not to eliminate carriers out of the first generation.  In one study of Danish animals, removing all affecteds and carriers depleted the diversity of that gene pool.  To maintain diversity, carriers need to be used in one generation and then animals clear of the disease used in the next generation.

When working on a disease of complex inheritance, researchers have to work on one gene at a time to determine the combination of genes necessary to express a disorder.

It is a parent club’s responsibility to promote and support research that may result in the development of a genetic test for the diseases affecting their breed.  Once a test is developed, they should consider subsidizing testing to ensure participation.  In order to have successful genetic screening programs, open registries need to be supported. 

AWS Partners’ recommended resources:

Genetic Tests offered 9/14/01

http://www.akcchf.org/research/genetic.htm

Genetic Testing: A Guide for Breeders

http://www.netpets.com/dogs/reference/genetics/dnatest.html

Canine Molecular Genetic Diseases

http://www.med.umich.edu/hg/RESEARCH/FACULTY/Brewer/compendium.htm

Developing a Healthy Breeding Program

http://www.spinone.com/akc_chf99/23HealthyBreeding.htm

Eliminating Genetic Diseases in Dogs:  A Buyer’s Perspective

http://www.workingdogs.com/eliminating_gen.htm

Canine Inherited Disorders Database

http://www.upei.ca/~cidd/intro.htm