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Does It Run in the Family?
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 will
almost certainly develop the disease. In MMD1, it will very often
be more severe in the child than in the parent. In MMD2, increases
in severity from generation to generation, if they occur, are
smaller.
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A 'Growing' Gene
In 1992, with MDA support, a landmark genetic
discovery was made by three teams of scientists. They found in
people with MMD 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 was found to grow even more as it was passed from
parent to child. This explained the long-observed phenomenon in
MMD whereby children are generally more seriously affected by
the disease 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. It doesn’t, however, fully explain why this phenomenon occurs so often when
mothers have MMD 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.
Then, 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 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.
Today, scientists are investigating how the expanded
areas of DNA cause the various symptoms of MMD. There are many
possibilities. It may be that the expanded DNA section affects
the functioning of more than one gene, or it may cause clumps
of genetic material to build up in the nuclei (control centers)
of cells, affecting many cellular functions.
Ongoing research to answer these questions should
lead to treatments for MMD.
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Genetic Testing
Genetic testing for the expanded DNA that leads
to either type of MMD can be performed. More laboratories offer
type 1 MMD testing than offer tests for type 2. Prenatal testing
is readily available for MMD1 and is offered in a few labs for
MMD2.
You can ask your MDA clinic physician or genetic
counselor to refer you for a genetic test.
A good way to find out more about the inheritance
pattern in your family is to talk to your MDA clinic physician
or a genetic counselor. For more information on how MMD is inherited,
or how it can occur with no family history, see MDA’s pamphlet
"Genetics and Neuromuscular Disease."
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Facts About Myotonic Muscular Dystrophy
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