DNA Testing Recommended in Type 1 Myotonic Dystrophy

Researchers at several institutions in Quebec Province in Canada studied 102 people with type 1 myotonic dystrophy (MMD) symptoms in themselves or family members and have confirmed that people with slightly expanded DNA repeat sections on chromosome 19 are unlikely to have MMD1 symptoms by middle age. That, they say, makes genetic testing for this population imperative in determining risks to future children.

A normal maximum number of repeats is 37, and the MMD disease range is generally considered to be between 50 and 4,000.

Marie-Eve Arsenault at Carrefour de Sante in Jonquiere, Quebec, and colleagues report in the April 25 issue of Neurology that most people with 50 to 99 CTG repeats had no symptoms other than cataracts in their eyes.

They found, however, that those with 100 to 200 repeats -- a size to which the smaller segments are likely to expand as the DNA is passed to the next generation -- were much more likely to have MMD symptoms. These included myotonia (delayed muscle relaxation), weakness, excessive daytime sleepiness, and abnormal electromyogram tests, in addition to cataracts.

They recommend genetic testing of at-risk family members. “This is particularly important for young adults of reproductive age,” to allow early detection and genetic counseling, the investigators write.

At-risk family members are often identified when the full range of MMD symptoms is found in a child, leading to the realization that one of the child’s parents transmitted the disease and that the parent’s siblings are at risk for doing so as well.