By Katie Lai

After much mystery, researchers funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases have succeeded in linking the gene defect in myotonic dystrophy (DM) to its biological malfunction. Their findings emphasize how misreading of a gene can lead to improper conduction of electrical impulses in skeletal muscle.

Two different studies were completed. Thomas A. Cooper, M.D., and his team of scientists at Baylor College of Medicine in Texas examined tissue samples from skeletal muscle in patients with myotonic dystrophy. The results revealed that extra genetic material caused by the defect in the DNA sequence affects the chloride channels that control muscle relaxation.

In New York, at the University of Rochester, Charles A. Thornton, M.D., and his colleagues measured electrochemical muscle impulses in a mouse model of the disease. The results indicated that the genetic defect affects the conductance of electrical signals, resulting in delayed muscle control. People with DM have the normal gene with additional information that interferes with the translation of proteins. While further study still needs to be done, these findings are a key step in understanding the causes of muscular dystrophies.

Myotonic dystrophy belongs to a group of genetic diseases called muscular dystrophies characterized by progressive weakness and degeneration of the skeletal or voluntary muscles that control movement. Tens of thousands of people in the United States are affected. An early sign of DM is delayed skeletal muscle relaxation following voluntary contraction.

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Charlet NB, Savkur RS, Singh G, Philips AV, Grice EA, Cooper TA. Loss of the muscle-specific chloride channel in type 1 myotonic dystrophy due to misregulated alternative splicing. Molecular Cell 2002;10:45-53.

Mankodi A, Takahashi MP, Jiang H, Beck CL, Bowers WJ, Moxely RT, Cannon SC, Thorton CA. Expanded CUG repeats trigger aberrant splicing of CIC-1 chloride channel pre-mRNA and hyperexcitability of skeletal muscle in myotonic dystrophy. Molecular Cell 2002;10:35-43.