Grant: Development of a Genetic Marker for Idiopathic Epilepsy in the Belgian Tervuren

This document is designed to introduce the scientific reasonings behind the grant given to the Department of Animal Sciences at the University of California, Davis.

Abstract

Idiopathic epilepsy affects nearly 30 different dog breeds.

Preliminary studies in the Belgian Tervuren indicates that although this is a heritable disorder governed by many genes (polygenic mode of inheritance), there exists a single gene of very large effect on the incidence of epilepsy. The epileptic condition influenced by this particular gene is inherited as an autosomal recessive.

The specific objective of this project is to develop a genetic marker associated with that locus in the Belgian Tervuren by utilizing the emerging canine genetic map.

Such a genetic marker will permit the identification of carriers of epilepsy in Belgian Tervuren, and possibly other breeds as well, with the end result that breeders can make informed, health-based breeding decisions.

Significance of Research

Seizures have been reported in nearly all dog breeds; where a seizure is defined as "a transitory disturbance of brain function" (2). Often, the seizure reflects an isolated incident and does not recur.

Recurrent seizure activity defines epilepsy (16) with the epilepsy categorized by its causality. Idiopathic or primary epilepsy (Idiopathic Epilepsy) describes recurrent seizures with no discernible cause. Acquired or secondary epilepsy is identifiable by a specific initiator of the seizure, such as trauma or hepatic insufficiency (9).

Breeds Affected by Epilepsy

Epilepsy accounts for 0.55% to 2.3% of all ill dogs referred to veterinary teaching hospitals (20). A growing body of evidence supports a hereditary basis for Idiopathic Epilepsy (7,12,13,16). Many AKC recognized breeds are suspected of having a predisposition to Idiopathic Epilepsy. These breeds (compiled from 4,16) include:

In addition, Idiopathic Epilepsy has long been recognized in the Belgian Tervuren (25), a breed with a 17% seizure incidence in the U.S. (7).

With such prevalence among dog breeds and high incidence within certain breeds, a genetic marker identifying deleterious alleles in breeding stock is extremely desirable.

And while prevalence among breeds generally reflects heterogeneity in the allelic causation, it has been demonstrated with other disorders, such as von Willebrand's disease (1), that within a breed the allelic mutation is homogeneous.

The high incidence of Idiopathic Epilepsy in the Belgian Tervuren, coupled with the cooperation and willingness of Belgian Tervuren breeders to eliminate this disorder, translates into an optimal population to initiate a study for a genetic marker linked with Idiopathic Epilepsy. Linkage of Idiopathic Epilepsy with such a marker and the subsequent identification of the affected gene will allow the information to be applied to other breeds also afflicted with Idiopathic Epilepsy.

Background of Research and Preliminary Work

Idiopathic Epilepsy, reported in nearly thirty different dog breeds (4,16), is considered a hereditary disorder (5), especially if the onset of the initial seizure is between 1-5 years of age (15).

A variety of genetic models have been proposed for the inheritance of canine Idiopathic Epilepsy, drawing primarily on human and mouse models of epilepsy. In humans, many of the characterized epileptic syndromes involve single locus modes of inheritance (11,27) though canine models have been consistent with multilocus modes (7,24) excepting the single locus model proposed for Idiopathic Epilepsy in the Keeshond (12).

Belgian Tervuren breeders worldwide have long recognized the necessity of determining the transmission of Idiopathic Epilepsy in the breed (25) due to its high frequency; understandably with a high incidence, a genetic basis was presumed. Although early studies identified a genetic component (25), the mode of inheritance remained elusive. In late 1986, the American Belgian Tervuren Club initiated a confidential survey of its members as to the incidence of seizures in Belgian Tervuren: all members were surveyed and 91% returned completed questionnaires (6).

From this collected data, 997 unique Tervuren over the age of five years of age were analyzed. This data revealed:

• 17% incidence of Idiopathic Epilepsy
• no significant difference in seizuring across sexes
• heritability estimate of 0.77 for Idiopathic Epilepsy using a Bayesian analysis (7); an even higher estimate was obtained with Gibbs sampling

This estimate suggests, congruent with previous reports of Idiopathic Epilepsy in other breeds (22,24), that Idiopathic Epilepsy is a highly heritable disorder in the Belgian Tervuren.

The high heritability estimates confirm that there is sufficient genetic variation in the incidence of Idiopathic Epilepsy to guarantee an effective response to selective breeding and implementation of sound selection decisions. Heritability estimates in excess of 0.5 indicate the presence of a major gene (18).

Although the evidence in the Tervuren suggests that a single locus model is not plausible, multiple statistical tests applied to the data set presented results consistent with a mode of inheritance involving a single autosomal gene of large effect that influences the expression of seizures in the Belgian Tervuren (8).

The statistical tests employed to reach this conclusion included:

• test of global misture distributions for predicted breedgin values
• tests for heterogeneity, polynomial regression and structured exploratory data analysis 

Polygenic inheritance with a single gene of large effect is consistent with epileptic seizure activity in the dog: seizure potential has been characterized as a continuum of susceptibility(9). Such a continuum could represent many genes contributing to a seizure threshold with a single locus exerting great influence on that threshold.

Specific Objectives of the Study

Statistical evidence of a single locus of large effect in a polygenic trait has been exploited in other species to develop genetic markers to assist in selection of particular breeding stock (23). We plan on implementing a similar strategy for the development of a genetic marker useful in identifying carriers of deleterious alleles predisposing offspring to Idiopathic Epilepsy. The high estimate of heritability, coupled with the existence of appropriate family pedigrees, argues for likely success in this endeavor.

Our specific aim for this two year proposal is to find a microsatellite genetic marker linked to Idiopathic Epilepsy in the Belgian Tervuren.

Expected Outcomes and Application to All Dogs

We will utilize the canine genome map that is being developed which is coupled with the ever expanding polymorphic canine microsatellites. Our preliminary data supports that we have a high likelihood of developing such a marker and this will be invaluable to breeders of Belgian Tervuren.

Our long term research objective is to identify and sequence the major gene involved in the Tervuren Idiopathic Epilepsy. This objective is in conjunction with the goal of determining if the other breeds afflicted with Idiopathic Epilepsy will show similar or related mutations.

As ascertained by the annual survey of AKC Parent Club Health Representatives, epilepsy is a major concern to dog breeders.

Identification of a genetic marker and the high heritability of Idiopathic Epilepsy in the various breeds of dogs would allow a significant reduction in the incidence of this debilitating disorder and ultimately improve the health status of the pure-bred dog.

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