Development and Application of the Canine Genetic Map   Elaine A. Ostrander, PhD  Clinical Research Division, Fred Hutchinson Cancer Research Center

 Summary:

Genetic maps are the key resources used for determining the location of genes of interest in any organism. The markers that make up the map act as molecular signposts that allow investigators to navigate around the, genome. When analyzed together with the disease status of family members, the genetic map allows one to identify regions of the genome likely to contain genes of interest. For most canine diseases, the underlying genetic cause has yet to be determined.

The development of a high-resolution canine map is a major step forward in the improvement of canine health for two reasons. First, the general location of a gene must be known before experiments can be undertaken to clone that gene and determine the mutations that actually cause the disease. This is, of course, the ultimate goal for every genetic disease. Second, and perhaps more relevant, once the chromosomal location of a disease gene is known, diagnostic tests can be developed and incorporated into canine breeding programs, thus allowing breeders to produce healthier, more long lived dogs.

Maps are made up of "markers," which are inherited in a Mendelian fashion, and which serve as molecular signposts for navigating around the genome. A high resolution map of the canine genome that contains both type I (gene based) and type 11 (anonymous microsatellite-based) markers is necessary to identify disease loci in dogs with high resolution. Microsatellite markers are short segments of DNA that show sequence variation. Because any given individual has two copies of each chromosome, one inherited from each parent, there will often be variability between the two copies of the genetic material that an individual carries. These different copies of the same piece or segment of DNA are referred to as alleles. If identical alleles are inherited from each parent, individuals are homozygous for that marker and if different alleles are inherited the individuals are heterozygotes for that markers. The most useful markers are the ones, that show the greatest levels of variation in a population or have the largest number of alleles.

Genetic maps are constructed by tracing the inheritance of polymorphic markers through the generations of families. Because chromosomes undergo recombination during meiosis, markers or genes located far apart on the same chromosome or located on different chromosomes will likely become disassociated during formation of egg or sperm. For a given portion of a chromosome, the probability of a genetic recombination event occurring between a pair of markers is proportional to the distance between them, with markers close together being less frequently disrupted than those which are far apart. A second type of map development is based on radiation hybrid (RH) mapping. In RH mapping canine chromosomes are randomly fragmented by irradiation in vitro, and the pieces of the genome are then fused with non-irradiated hamster cells. Each radiation hybrid cell line thus contains a random and unique subset of the canine genome. In practice, to place markers on the map, DNA isolated from many independent radiation hybrids are tested and scored for the presence or absence of each marker. Markers or genes which are close together on a chromosome appear together in the same hybrids, while markers far apart on chromosomes have a much greater probability of being separated. The probability is proportional to the distance between the markers or genes. Radiation mapping is thus a useful way to order markers on a chromosome and for determining the distances between markers on chromosomes.

This talk will review recent progress both in our own lab and in the field in general on the development of canine maps. We will discuss specifically progress on the development of new markers for the canine map, and review ways in which the map is growing. The utility of different types of markers for finding disease genes will be examined, and issues related to resolution of the map reviewed. We will summarize examples of how the map is currently being used to identify disease loci and to further the development of diagnostic tests for dogs. We will also discuss the relationship between the canine and human maps, and the way sin which information form each contributes to health care research in the other. Finally, examples of how the map has been used to identify novel disease genes will be discussed.

This research has been supported by the following AKC Canine Health Foundation grants:

No. 1268: A Genetic Map of the Canine Genome

No. 1608: Integrative Mapping of Canine Gene and Microsatellite Loci: A Collaborative Project (Sponsored in part by Ralston Purina Company)

No. 1808: An Integrated Linkage and Radiation Hybrid Map of the Dog

Biographical Profile

Dr. Elaine A. Ostrander is an Associate Member in the Divisions of Clinical Research and Human Biology at the Fred Hutchinson Cancer Research Center, and Head of the Center's Genetics Program. Her interests are in the area of genetic mapping and genomics, with a specific focus on the genetics of disease susceptibility. Ostrander lab studies are aimed at understanding human breast and prostate cancer susceptibility and in the development and application of the canine genetic map. Additional interests include application of the canine map to disease loci in purebred dogs. Dr. Ostrander is Principal Investigator of several research grants and holds academic appointments at the University of Washington in the departments of Zoology and Molecular Biology. Dr. Ostrander is the proud owner of a Border Collie who accompanies her to her lab every day.