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A New Test for Myotonic Dystrophy: Exposing an Enemy That’s Too Big to See


For release: Wednesday, March 26, 2003

Researchers have developed a genetic test that detects a common form of muscular dystrophy with 99 percent accuracy. The accurate diagnosis of myotonic muscular dystrophy type 2 (DM2) allows researchers to fully describe its clinical features for the first time.

"So many DM2 patients have been undiagnosed or misdiagnosed," says John W. Day, M.D., Ph.D., of the University of Minnesota Department of Neurology and Institute of Human Genetics in Minneapolis. "By establishing a correct diagnosis, we can start to treat people much more effectively and estimate risks in families with a history of the disease."

The study, co-authored by Dr. Day and Laura Ranum, Ph.D., and their colleagues in the University of Minnesota Department of Genetics, Cell Biology and Development and the Institute of Human Genetics, was funded in part by the National Institute of Neurological Disorders and Stroke and appeared in the February 2003 issue of Neurology. 1

The muscular dystrophies (MD) are a group of genetic diseases characterized by progressive weakness and degeneration of the muscles that control movement. Myotonic dystrophy, the most common type of MD in adults, affects the eyes, heart, hormonal systems, blood, and muscles.

DM2 has been difficult to diagnose for two main reasons. First, DM2 affects many different parts of the body, so people with the disease often visit several specialists to treat different symptoms and rarely get a diagnosis of DM2. Second, there has been no simple and reliable test for the disease. Although the DM2 mutation was identified by this group in 2001, the mutation is so large and unstable that standard methods of genetic testing do not reliably detect the abnormality.

Drs. Day and Ranum designed an improved method for detecting the DM2 mutation and then characterized the clinical features of the disease in 379 individuals with genetically confirmed DM2.

The new test, dubbed the DM2 "repeat assay," uses a two-step procedure that allows the detection of large mutations missed with standard testing. The researchers basically combined a genomic test called Southern analysis with a modified version of the polymerase chain reaction (PCR) test.

This repeat assay successfully detected the DM2 mutation in 99 percent of cases, compared to 80 percent by genomic Southern analysis alone.

"Now that we have the ability to genetically diagnose the disorder, we’re realizing that DM2 is much more common than was previously recognized," says Dr. Day. Before this study, only 2 percent of myotonic dystrophy cases were thought to be DM2.

Dr. Day says DM2 may turn out to be as common as the more severe form, DM1, which was thought to account for about 98 percent of myotonic dystrophy cases before this study.

DM1 occurs in about one of every 7,000 people worldwide. DM1 is found in all ethnic groups, but DM2 had previously been found only in people of European origin, particularly those of German descent.

Among DM2 patients with a genetically confirmed diagnosis, the researchers found that the clinical features of DM2 closely resemble those of adult-onset DM1, with common features including progressive weakness, difficulty relaxing muscles after voluntary contraction has stopped, muscle and cardiac problems, cataracts, and insulin insensitivity.

Despite the striking similarities between the two diseases, Dr. Ranum notes that there are important differences, namely that DM1 has a broader range of ages of onset and sometimes presents with significant symptoms at birth, while DM2 usually starts in early adulthood.

"We wanted to define the clinical features of DM2 not only to compare it to DM1, but also to gain a better understanding of the molecular mechanisms of the disease," says Dr. Ranum. "The clinical similarities between DM1 and DM2 tell us that the mutations causing them have similar effects at the cellular level." Both forms of the disease are caused by large, unstable repetitive DNA sequences in genetic regions that lack instructions for proteins.

The researchers plan to continue studying the biological mechanisms of DM2. "We’re excited about this new test and this new information about myotonic dystrophies," says Dr. Ranum. "Our research efforts are now focused on understanding the biology of the mutations that lead to these diseases."

The new DM2 test is available to the general public from Athena Diagnostics of Worcester, Massachusetts.

The NINDS is a component of the National Institutes of Health in Bethesda, Maryland, part of the U.S. Department of Health and Human Services, and is the nation's primary supporter of biomedical research on the brain and nervous system.

Reference:

1 Day JW, Ricker K, Jacobsen JF, Rasmussen LJ, Dick KA, Kress W, Schneider C, Koch MC, Beilman GJ, Harrison AR, Dalton JC, Ranum LPW. "Myotonic dystrophy type 2: molecular, diagnostic and clinical spectrum." Neurology, February 2003, pp. 657-664.

- By Tania Zeigler



 

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Last Modified April 16, 2014