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MMD is certainly a disease that runs in families. Both types are inherited in an autosomal dominant pattern, meaning it takes only one flawed gene to cause symptoms of the disease. So, if one parent has the disorder, every child of that person has a 50 percent chance of inheriting the gene that causes it.
If either the type 1 (chromosome 19) or the type 2 (chromosome 3) genetic abnormality is passed on, the child almost will certainly develop the disease. MMD1 very often is more severe in the child than in the parent. In MMD2, this increase in severity between generations also occurs, but not as consistently.
In 1992, with MDA support, a landmark genetic discovery was made by three teams of scientists. They found in people with what is now called MMD1 an area of DNA (the basic genetic material that makes up our genes) on chromosome 19 that’s larger than it should be.
The expanded DNA is in a gene that carries instructions for myotonin protein kinase. The expanded DNA isn’t in the “working” part of the gene — the part that carries instructions for making protein. Instead, in MMD, the genetic flaw is in a part of a gene called the untranslated DNA, an area of DNA that the cell doesn’t use for protein manufacturing.
The experts were puzzled to find that an expanded section of this untranslated DNA could cause so much trouble, and the mystery still isn’t entirely solved.
There was more puzzlement to come. The expanded section of DNA seen in MMD1 was found to grow even more as it was passed from parent to child. This explained the observation that children generally are more seriously affected by MMD1 than are their parents.
The expanding DNA also explains why children with the congenital form of type 1 MMD can be born to parents who have the less severe, adult-onset form. However, it doesn’t fully explain why this phenomenon occurs so often when mothers have MMD1 and so rarely when fathers do.
It may have to do with a difference in the way egg cells, as opposed to sperm, are made in the body.
In 2001, MDA researchers in Minnesota, working with their counterparts in Germany, identified a gene on chromosome 3 that carries instructions for a protein called zinc finger 9. When this gene contains an expanded section of DNA, it too causes a form of MMD.
That type of myotonic dystrophy, MMD2, is found chiefly in Northern Europeans or their descendants. In Germany, MMD2 may be as common as MMD1.
The expanded DNA on chromosome 3 that underlies MMD2 can change size but does not “grow” in size as consistently as it does in MMD1.
Today, scientists are investigating how expanded areas of DNA cause the various symptoms of MMD1 and MMD2. There are many possibilities. As of 2008, experts generally believe that, in both forms of the disease, the DNA expansion leads to expanded strands of RNA and that these RNA expansions have toxic effects on cells.
In addition, direct effects of the DNA expansions on local genes on chromosome 19 or chromosome 3 may play a role.
Ongoing research to answer these questions should lead to treatments for MMD.
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Genetic testing for the expanded DNA that leads to either type of MMD can be performed in several laboratories. Ask your MDA clinic physician or genetic counselor to refer you for a genetic test.
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Facts About Myotonic Muscular Dystrophy
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