Discovery of Two Epilepsy Genes the First Steps in a Longer Journey
The year 2005 ushered in some good news for three breeds. In back to back reports, researchers at the University of Toronto and the University of Missouri reported the discovery of the first genes responsible for two rare forms of epilepsy in dogs. In work supported by the AKC Canine Health Foundation, Drs. Marty Katz, Gary Johnson, and their colleagues at Missouri showed that a mutation in the gene responsible for Northern epilepsy in people causes ceroid lipofuscinosis in English Setters. Drs. Hannes Lohi, Berge Minassian, and their colleagues at Toronto showed that a mutation in the same gene that causes Lafora disease in people also causes myoclonic epilepsy in Miniature Wirehaired Dachshunds and Basset Hounds.
Does this mean we will now be able to eliminate epilepsy in dogs? No, it is an important first step, but these diseases differ in significant ways from the common forms of epilepsy. Both myoclonic epilepsy and ceroid lipofuscinosis are storage diseases. In storage diseases, the mutation interferes with the dog’s ability to process an essential protein in the brain. As a result, abnormal material builds up within the cells interfering with brain function. This accumulated material can be seen under the microscope and provided an important clue that allowed the researchers to quickly focus on the responsible genes. This is very different from the much more common forms of epilepsy seen in most breeds where the pathologist can find no changes in the nerve cells at post-mortem.
Also the clinical signs in these two diseases are very different from the routine epileptic. Most epileptic dogs have tonic-clonic (grand mal) type seizures. Tonic-clonic seizures can be life threatening, but between seizures, the average epileptic dog functions normally. In contrast, dogs with myoclonic epilepsy have less severe seizures that consist of a series of rapid jerks as if they were being startled. Dogs with ceroid lipofuscinosis have classic seizures, but they also suffer from poor coordination, vision loss, and mental retardation between seizures. Their seizures worsen over time, and they die before two years of age.
The first epilepsy gene discovered in people caused a rare form of epilepsy found in an isolated fishing village in Finland. That breakthrough, however, was followed rapidly by the discovery of other human epilepsy genes. The discovery of the genes for epilepsy in these breeds shows the power of the canine genome map to solve genetic diseases, but much work remains to be done before all hereditary epilepsies are relegated to the chapter on rare diseases in veterinary textbooks.
This work was funded by AKC Canine Health Foundation Grant 2304.
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