Myotonic dystrophy

Myotonic dystrophy is a common form of adult-onset muscular dystrophy characterized by progressive muscle weakness and the inability to relax muscles after contraction.

Overview

Myotonic dystrophy is an inherited disorder involving progressive muscle wasting and weakness. A hallmark symptom is myotonia—prolonged muscle contractions that make it difficult to release a grip or relax muscles after use. Other common features include cataracts, cardiac conduction defects, and diabetes mellitus.

Types

• Type 1: Often affects distal muscles (lower legs, hands, neck, and face). It includes a congenital form that can be life-threatening at birth.
• Type 2: Tends to be milder than type 1 and primarily affects proximal muscles (neck, shoulders, elbows, and hips).

Causes

Both types are caused by DNA segments that are abnormally repeated many times, creating an unstable region in the gene:

• Type 1: Caused by mutations in the DMPK gene.
• Type 2: Caused by mutations in the CNBP gene.

These mutations result in an unusually long messenger RNA (molecular blueprint) that forms clumps inside the cell. These clumps interfere with the production of other essential proteins, preventing muscle and other tissues from functioning normally.

Inheritance

Myotonic dystrophy is inherited in an autosomal dominant pattern, meaning one copy of the altered gene is sufficient to cause the disorder. In type 1, the disorder exhibits anticipation, a phenomenon where the unstable region of the gene increases in length as it is passed to subsequent generations, causing the condition to appear earlier and with greater severity.