Myotonic dystrophy type 1 (DM1), an inherited disease affecting muscles, was one of the first described "expanding repeat" disorders. In these 50 or so conditions, symptoms may appear earlier and worsen from generation to generation, as the mutant gene grows, adding copies of a 3- or 4-base DNA sequence. For many expanding repeat disorders, forty copies seems to be a threshold, causing symptoms when crossed.
In a family with myotonic dystrophy type 1, a grandfather might experience mild weakness in his forearms, while his daughter may have more noticeable arm and leg weakness, slurred speech, and a flat facial expression. Her children have even weaker muscles that contract for too long, creating limitations like being unable to unclench a fist or release a grip.
In MD1, skeletal muscle fibers that contract for too long impair balance and coordination, called ataxia. The condition also causes cataracts, small gonads, frontal balding, fatigue, sleepiness, digestion problems, and cognitive and behavioral impairment. Life may be shortened. MD1 affects about one in 7,500 people, or more than 40,000 people in the US.
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