Idiopathic Epilepsy in the Belgian Tervuren: An Update T. R. Famula, PhD  University of California, Davis

Summary:

The American Belgian Tervuren Club (ABTC), recognizing a serious health problem spreading throughout their breed, established a health survey in the early 1980's. The focus of this survey was the growing incidence of seizures, an incidence over 15 percent. In 1996, we began an analysis of 997 complete epilepsy records where the diagnosis of disease was self-reported. Along with seizure status, owners also provided pedigree information, gender, date of birth, feeding habits, other health observations and a variety of descriptive data on seizures.

We began with the simple objective of establishing this as an inherited disorder. Pedigree analysis eliminated a single locus model as an explanation for the patterns of seizures witnessed within families, however, our estimate of heritability (0.77) demonstrates that the key to understanding this illness lies in the genotype of affected dogs. On a practical level, the high heritability estimate is encouraging, suggesting that an organized breeding program can make significant strides in reducing the incidence of this disease. In concert with ABTC, the future will see development of a health database from which periodic predictions of genetic merit can be made for use as breeding decision aids. The application of such statistical procedures alone can significantly reduce the incidence of this disorder, a process identical in approach to that used in many species of livestock to improve milk production or produce leaner beef.

Nevertheless, there is academic interest in a more precise genetic explanation for the inheritance of seizure disorders. With the expansion of the canine genetic map, it should be possible to identify individual genes responsible for idiopathic epilepsy. Such identification of genes, or closely linked genetic markers, has the potential to offer an increase in the accuracy of making selection decisions. For this reason, although we could eliminate a single locus model of epilepsy inheritance, we proceeded with a search for any evidence that a single gene may have a large effect the expression of this disease.

This search begins with statistics. That is, before we invest the time and dollars to find the single gene(s) we assume are responsible for seizures, we must determine if such genes exist with effects large enough to justify the hunt. Complex segregation analysis provides the framework for such a search. With the data provided in the first health survey, there was strong statistical evidence of a single gene with a large effect on the incidence of seizures. Bear in mind that complex segregation analysis is a statistical technique that can only search for the "footprint" of an important locus. However, our analysis suggests that there is an allele with a significant effect on the incidence of the disease with a frequency of 0.14. Interestingly, the genetic model which provided the best fit to the survey data was one where individuals homozygous for the putative "epilepsy gene" would be guaranteed of suffering from seizures. Those presumed to be heterozygotes, however, would have a risk of contracting seizures, but at the same time, could also be free of expressing the disorder. Such evidence argues for a locus that plays an important role in the expression of disease, but is nonetheless a locus that interacts with a collection of other elements in the animal's genome. Of course this evidence is little more than sophisticated, mathematical speculation. To arrive at a more precise understanding of this phenomenon we take the latest step in understanding epilepsy in the Belgian Tervuren.

The specific aim of this phase of our research is the development of a microsatellite genetic marker linked to epilepsy. Our first step was to collect DNA samples with the cooperation of breeders. Beginning in the summer of 1998, breeders from across the country were asked to submit a health questionnaire identifying seizure history, pedigree and other related information. In addition, owners were provided with cytology swabs for collecting buccal epithelial cells for later DNA extraction. At present we have a database of over 425 Belgian Tervuren (many of whom are descendents of dogs in the original seizure survey).

Approximately 16 percent of these dogs have had one or more seizures. In addition, within this data set exist several three-generation families with a high incidence of seizures, as well as the presence of families with a history of freedom from seizures. Of course the contrast of these two groups forms the basis of our search for a microsatellite marker. At present, we have begun with the necessary screening of microsatellites for their utility in the Belgian Tervuren. We began with tetranucleotide repeat microsatellites that, are distributed across established linkage groups in the dog. DNA from 15 unrelated (within the last three generations) animals was used to evaluate the presence of marker polymorphisms within the Belgian Tervuren. At present, we have screened 51 microsatellites, having identified seventeen as highly useful (more than five alleles in the Belgian), thirteen as moderately useful (three to five alleles), and ten with limited utility (less than alleles); eleven microsatellites are monomorphic or fail to amplify in the

Belgian Tervuren. We are in the process of screening up to 100 additional markers, adding to our existing database.

Obviously this is only the beginning of an extensive investigation as we continue to screen markers, while searching for linkage between our markers and the presence of disease. Work in other species, including humans, can only help as we search for that combination of statistical and genetic knowledge that can make the selection of parents of future generations of Tervuren less likely to transmit this debilitating disorder.

This work is supported by the following grant from the AKC Canine Health Foundation:

No. 1613: Development of a Genetic Marker for idiopathic Epilepsy in the Belgian Tervuren (Sponsored in part by the American Belgian Tervuren Club)

Biographical Profile

Thomas R. Famula, PhD, is currently a Professor in the Department of Animal Science at the University of California, Davis. He has been on the faculty since completing his PhD in Animal Breeding in 1981 from Cornell University. Since that time he has worked on a variety of problems in quantitative genetics. This interest has lead to research in the inheritance of health traits in dogs. His expertise is in the statistical aspects of genetics and, because the characters of interest in canine health genetics (e.g., seizures) are often discontinuous, separating "nature" from "nurture" can be statistically challenging.