Myotonic dystrophy: An inherited disease in which the muscles contract but have decreasing power to relax -- this phenomenon is termed myotonia (irritability and prolonged contraction of muscles). The disease also leads to a mask-like expressionless face, premature balding, cataracts, and heart arrhythmias (abnormalities in heart rhythm). The onset of such problems is usually in young adulthood. However, onset can be at any age and the disease is extremely variable in the degree of severity.
Myotonic dystrophy is due to a trinucleotide repeat (a "stuttering" sequence of three bases) in the DNA. The myotonic dystrophy gene (called DM1), found on chromosome 19q13.3, codes for a protein kinase (an enzyme) that is found in skeletal muscle.
An unusual feature is that the signs and symptoms of the disease usually become more severe with each successive generation. This is because mistakes in the faithful copying of the gene from one generation to the next result in the amplification of a genomic "AGC/CTG triplet repeat." Unaffected individuals have between 5 and 27 copies of AGC/CTG, myotonic dystrophy patients who are minimally affected have at least 50 repeats, while more severely affected patients have an expansion of up to several kilobase pairs.
Myotonic dystrophy is not genetically homogeneous; it is more than one genetic entity. Some families with myotonic dystrophy have a mutation in a gene on chromosome 3q21 (at the DM2 locus). The DM2 mutation is a huge expansion of a tetranucleotide repeat (CCTG) in a noncoding region of the ZNF9 gene. Both DM1 and DM2 appear to cause disease through the effects of the mutant RNA (that contains abnormal repeating sequences).
Myotonic dystrophy is not to be confused with muscular dystrophy. They are distinct and different diseases.