American Miniature Schnauzer Club, Inc.

 

Myotonia Congenita in Miniature Schnauzers:

An Update after 6 Months of DNA-Testing

 

Drs. Urs Giger, Charles H. Vite, and Dilip P. Bhalerao

School of Veterinary Medicine

University of Pennsylvania

October 7, 2000

 

 

Myotonia Congenita is a hereditary, skeletal muscle disorder, which has recently been discovered in Miniature Schnauzers. Research performed at the University of Pennsylvania School of Veterinary Medicine characterized the clinical course and molecular defect (in collaboration with Vanderbilt University). This led to the development of an accurate DNA-based screening test for Myotonia Congenita in Miniature Schnauzers.

 

Myotonic puppies exhibit clinical signs at a few weeks of age because their muscles are hyperexcitable and contract easily. Affected dogs have prominent muscles in the shoulders and thighs and appear to have difficulty getting up, have a stiff gait, or bunny hop when running. The tongue of affected dogs is enlarged and stiffens when touched. Furthermore, their lower jaws appear to be peak-shaped. They have difficulty swallowing and increased respiratory sounds. Myotonia Congenita in Miniature Schnauzers is caused by a defect in the chloride channel 1. Daily procainamide therapy has been shown to alleviate some of the clinical problems, although dogs with this disease will never be able to exercise and eat normally.

 

Myotonia Congenita in Miniature Schnauzers is inherited as an autosomal recessive trait. Myotonic dogs have both copies of the chloride channel l mutated (the gene sequence contains a single, detrimental base change). Dogs with one mutant and one normal copy, are known as carriers and remain asymptomatic, but can pass on the mutant gene to half of their offspring. In other words, myotonic dogs are produced by asymptomatic parents, who are carriers.

 

Presently, the frequency of the defective gene causing Myotonia Congenita in Miniature Schnauzers remains unknown. However, a popular sire has been discovered to be a carrier, which suggests that the disease is widespread. The University of Pennsylvania School of Veterinary Medicine is interested in determining the frequency of this disorder in Miniature Schnauzers and curtailing the further spread of the disease in future generations.

 

Since March 2000, the Josephine Deubler Genetic Disease Testing Laboratory at the University of Pennsylvania School of Veterinary Medicine has been offering a DNA screening test for Myotonia Congenita. This test allows the detection of affected, carriers, and normal also known as “clear” dogs. A small blood sample (2ml) or 2 cheek swabs are required and instructions for submission are simple to follow. Submissions can be received from anywhere in the world. Instructions are provided by the American and Canadian Miniature Schnauzer Clubs, the Josephine Deubler Genetic Disease Testing Laboratory (at www.vet.upenn.edu/penngen, by phone 215-898-3375, or by email request at penngen@vet.upenn.edu).

 

The test is helpful to confirm a diagnosis of Myotonia Congenita in Miniature Schnauzers with suspicious clinical signs. More importantly, this DNA test is useful for screening potential breeding dogs, thereby guiding breeders in selection of dogs for breeding that are free of the mutant myotonia gene. At this time, we recommend having all breeding Miniature Schnauzers tested. Only clear animals should be used for breeding. However, if a carrier has many other desirable traits, this animal may be used as long as it is bred to a clear dog (thereby no affected puppies are produced. All offspring should be screened to select only the “clear” ones for future breedings). Once all breeding dogs have been screened, future breeders that stem from two clear parents do not need to be tested anymore. Please note that all information will be kept strictly confidential.

 

In our initial survey at the University of Pennsylvania School of Veterinary Medicine, we have screened approximately 200 miniature schnauzers and found 20% to be carriers and 2% to be affected. This means that 78% of all dogs tested are “clear” or normal. However, this does not reflect the true frequency of this disease or the mutated gene but rather represents a biased initial survey of dogs tested. Studies are in progress to determine the relationship of these carrier animals. We encourage the survey of the entire breeding population. Owners of tested dogs are also encouraged to place their dog’s test results in the open registry that is being established by the American Miniature Schnauzer Club, an endeavor that will be extremely helpful to breeders and prospective buyers to assure that no affected dogs will be produced in the future. Clearly, if all dogs were to be tested prior to breeding, myotonia could be successfully eradicated from the Miniature Schnauzer breed.