The myotonic dystrophies

Myotonic dystrophy, or dystrophia myotonica (DM), is the most common inheri ted muscle disorder in adults. DM is a multisystem disease in which the mos t disabling feature is muscle wasting that begins in the distal limb and cr anial muscles. The genetic basis for DM is an expanded CTG repeat in the DM PK gene on chromosome 19. The size of the expanded repeat, and the severity of the disease, tend to increase in successive generations. The mechanism by which this unusual mutation leads to muscle wasting, myotonia, cataracts , heart block, and neurobehavioral abnormalities has not been clearly defin ed. Identification of the DM gene has made it easier to delineate other DM- like disorders that are clinically and genetically distinct. The most commo n of these is proximal myotonic myopathy (PROMM), which is characterized by early involvement of proximal limb muscles. The genetic locus for another DM-like disorder, called DM type 2, was recently mapped to chromosome 3.