Myotonic dystrophy is a genetic disorder characterized by muscle contractions that struggle to relax, a condition known as myotonia, which involves muscle irritability and prolonged contractions. This disease can also lead to a blank, mask-like facial appearance, early hair loss, cataracts, and irregular heart rhythms. Typically, these symptoms begin in early adulthood, though they can appear at any age and vary widely in severity.The condition is caused by a repeating sequence of three bases in the DNA, known as a trinucleotide repeat. The gene responsible for myotonic dystrophy, referred to as DM1, is located on chromosome 19q13.3 and encodes an enzyme called protein kinase.A unique aspect of this disease is that its symptoms tend to worsen with each new generation. This occurs because errors in replicating the gene lead to an increase in the "AGC/CTG triplet repeat" in the genome. People without the disease typically have 5 to 27 copies of this repeat, while those with mild forms of the condition have at least 50 repeats. More severely affected individuals can have expansions reaching several kilobase pairs.Myotonic dystrophy is not a single genetic disorder but consists of different genetic variations. Some families with the condition have a mutation on chromosome 3q21, known as DM2. This mutation involves a large expansion of a tetranucleotide repeat (CCTG) in a noncoding region of the ZNF9 gene. Both DM1 and DM2 are thought to cause the disease through the impact of mutant RNA with abnormal repeating sequences.It is important to note that myotonic dystrophy is distinct from other forms of dystrophy, as they are separate and different diseases.
