Golden Retrievers: Man’s Best Solution for Duchenne Muscular Dystrophy

Where It Began

At the tender age of two, John experienced the first signs of abnormality. Having just learned how to walk the past day, he began to feel weak, experiencing a pervasive sense of fragility that spread slowly through his legs until he eventually forgot how to walk.

When his parents enrolled him in school, he found it difficult to memorize all those foreign symbols and abstruse operations. “Repeat ‘A, B, C’ after me”; “Add the 7 and carry the 1,” his teachers said, but all he heard was “gobbledygook.”

Men in white jackets carrying strange instruments came and went, always tapping his knee and looking at him with sad eyes. As each one shook his head and walked out the door, the hopeful smiles on his parents’ faces withered into despondent grimaces, and John wondered what he had done wrong.

Then came the day that John met Dr. Duchenne. John liked him; he was the first one to light up his parents’ faces, even though when he cut out a piece of John’s muscle it hurt him very much. He came to visit him more after that, and the other jacketed men disappeared. But even Duchenne, with his revolutionary muscle biopsy and original ideas, couldn’t save John in time.

John was the first that French physician Guillaume-Benjamin-Amand Duchenne
encountered in a long string of patients all exhibiting the same mysterious symptoms.
Each would start experiencing muscle weakness at a young age, have trouble learning,
and continue to worsen until confined to a wheelchair. Death usually occurred around
age 25 due to defective lung muscles.

Despite never discovering a cause or treatment for the illness, Duchenne was the first
to provide a detailed description of its symptoms and progression (Jay), earning him the condition’s name. Today we know this disease as Duchenne muscular dystrophy (DMD).

Normal Muscle Tissue

 

Tissue Affected By Muscular Dystrophy

 

Since then, the cause of DMD, a degenerative disorder that affects nearly 1 in 3,500 male babies, has been attributed to a lack of dystrophin protein (Kerkis). That is not to say, however, that females cannot be affected, too. Due to the dystrophin gene’s location on the X chromosome, males, who need only one copy of the gene to contract DMD, are twice as likely to be diagnosed with it as females (Darras).

Despite valiant efforts, no cure for DMD currently exists. Scientists now think that hope may lie within a common friend – the golden retriever.

And Then There Was Hope

Golden retrievers are popular among families for their affectionate demeanor, but are more well-known among scientists for their genetic code.

In 1988, Dr. Beth A. Valentine and her collaborators at the Cornell University College of Veterinary Medicine studied a group of dogs suffering from a degenerative disease, coined golden retriever muscular dystrophy (GRMD), which physically resembled DMD. The dogs, like human DMD patients, lacked dystrophin protein and exhibited muscle weakness and gait abnormalities, suggesting similarity between GRMD and DMD (Valentine).

The scientists were excited by this idea, and for good reason. Homology of a physical feature between different animals indicates that the feature stemmed from a common evolutionary ancestor. As a result, the species possessing the homologous features may have similar genetic codes.

Scientists thought that if GRMD and DMD had similar enough genetic coding, then the golden retriever would have the potential to become an excellent animal model for further study of DMD (Kornegay).

To confirm this hunch, scientists led by Dr. N.J.H. Sharp of the Duke University Medical Center extracted the dystrophin gene from both normal dogs and GRMD- inflicted dogs.

After analyzing these genes, Sharp’s research team found that the defective gene (with GRMD) was shorter than the normal gene, leading them to believe that GRMD could be caused by genetic deletions. The scientists then compared these genes to human DMD patients’ dystrophin genes, and what they found proved groundbreaking:

Some of the same genetic mutations occurred in human DMD patients as in the GRMD dogs (Sharp).

Following up on Sharp’s discoveries, Dr. Richard J. Bartlett of the University of Miami School of Medicine and his collaborators used a gene repair technique to promote cellular repair of the dystrophin gene in a six-week-old male dog. Their efforts cumulated with the dog producing complete dystrophin proteins on its own for up to 11 months (Bartlett et al).

What’s Next?

Scientists are hopeful that this treatment will eventually be extended to human patients, though the journey ahead seems far from smooth.

Since the technique uses short synthesized DNA segments, an extraordinarily large dose of these must be delivered into the muscle in order for the treatment to be effective. In addition, too many different types of dystrophin mutations exist for the treatment to be generalized.

Realistically speaking, a cure for DMD is still many years from being approved by the Food and Drug Administration, but animal models such as the golden retriever provide scientists with an excellent method of safely investigating the disease without risking accuracy.

For now the golden retriever remains man’s best friend, but one day in the near future it may be known to us as the incredible animal behind the DMD cure.

In Brief:

• Duchenne muscular dystrophy (DMD) causes muscle weakness and learning disabilities at a young age in about 1/3,500 male babies.
• DMD is caused by a lack of dystrophin protein.
• There is no cure yet for DMD.
• Golden retrievers can contract DMD, possibly leading to a cure for this illness (complete dystrophin proteins have already been produced in male retrievers).

Works Cited

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