Muscular Dystrophy

Patty Khuly

Summary

Muscular dystrophy is a general term for any one of a number of genetic diseases that can affect the muscles of animals adversely. The most common and well-studied of these muscular diseases (“myopathies”) is the version that affects dogs and humans: Duchenne Muscular Dystrophy. In veterinary medicine, we refer to this disease as “X-linked muscular dystrophy.”

In this form of the disease, dogs who carry a defective gene for the protein “dystrophin” will fail to form normal muscles in its absence or reduced capacity. Dogs become progressively weaker until the critical muscles that govern cardiac and respiratory functions are compromised.

Because the gene is located on the X-chromosome, males are most commonly affected when they inherit this chromosome from their mothers. Females may be carriers (if they carry one affected chromosome) or they can be affected (if they receive a dose of the gene on each X-chromosome), but this relatively rare disease is seldom confirmed in female dogs.

Symptoms and Identification

Affected dogs will display the same progressive muscle weakening we see in human muscular dystrophy patients, usually beginning at around eight weeks of age. Symptoms range from changes in their gait to widespread muscle atrophy, thickening of the tongue and difficulty swallowing. The severity of the disease may vary from one affected dog to another for reasons we don’t yet understand.

Muscle biopsies are necessary in all affected dogs to determine the exact kind of dystrophy and its mode of inheritance, if possible. Since some infectious diseases can mimic muscular dystrophy (toxoplasmosis, for example), it’s important to rule these out––generally via blood tests.

A veterinary neurologist should ideally be sought out for diagnosis of any of these disease processes.

Affected Breeds

For the X-linked, dystrophin-deficiency version of the disease, Golden Retrievers have been determined to be more at risk. However, the trait has been identified in other large breed dogs as well.

Treatment

As of a few years ago, no treatment was available for this dystrophin-related condition. Dogs received anabolic steroids to boost their muscles but these ministrations were largely unhelpful. Since then, however, several exciting new scientific breakthroughs have led to the possibility of a cure for the symptoms of this genetic disease.

In 2006, researchers described an approach using stem cells sourced from the lining of blood vessels of healthy dogs. These healthy cells were then injected into the sick dogs, where they helped form the missing or defective dystrophin protein. Most dogs clinically improved but a couple died of severe complications possibly related to the therapy (for which strong anti-rejection drugs are required).

In March of 2009, a team of scientists discovered that gene therapy might be a more viable route. In this approach, called “exon skipping,” the body is tricked into skipping over the bad segments in the affected gene. Again, most dogs improved significantly after treatment. This is currently considered the most exciting possibility for treating human and animal forms of muscular dystrophy.

Unfortunately, these therapies are considered entirely experimental and are not yet available outside of a research institution.

Veterinary Cost

Sadly, the cost of this therapy is relegated to diagnosis alone, since no treatment is yet available. Muscle biopsies, blood tests and neurologist consultations may nonetheless run into the high hundreds of dollars or more, depending on the form of the muscular dystrophy and the possibility of concurrent or alternative disease processes.

Prevention

Because this disease is genetic in origin, its prevention is accomplished primarily by removing affected animals from the gene pool. This is not so difficult, considering how early these dogs become symptomatic (and the likelihood that they won’t reach reproductive maturity).

The problem remains, however, that carriers will persist until an inexpensive genetic test for the carrier state becomes available. At present, it is recommended that all dogs found to have passed on this trait (and all their progeny) should be spayed and neutered to prevent its hereditary transmission.



References

Cooper, B.J. Animal models of muscular dystrophy: Characteristics and applications. Lab.Animal 22:33-37, 1993.

Kornegay, J.N. et al. Muscular dystrophy in a litter of Golden retriever dogs. Muscle and Nerve, New York, v.11, n.10, p.1056-1064, 1998.

Nguyen, F. et al. Muscle lesions associated with dytrophin deficiency in neonatal Golden retriever puppies. Journal of Comparative Pathology, v.126, p.100-108, 2002.

Valentine, B.A. et al. Canine X-linked muscular dystrophy. An animal model of Duchenne muscular dystrophy: morphologic lesions. Journal of the Neurological Sciences, Amsterdam, v.97, p.1-23, 1990.