Genetics of Myotonic dystrophy

Myotonic dystrophy is the most common form of muscular dystrophy that begins in adulthood, affecting at least $1$‍ in $8000$‍ people worldwide. Myotonic dystrophy results in progressive weakness and muscle wasting. Many people with this disorder suffer from myotonia, or prolonged muscle contractions, and are not able to relax muscles after use. This dystrophy can affect voluntary skeletal muscles as well as involuntary cardiac and smooth muscle. There are two major types of myotonic dystrophy, which are referred to as type 1 (DM1) and type 2 (DM2). Type 1 is often more severe than type 2 and affects the face, neck, hands, and lower legs. Type 2 results in muscle weakness in the neck, shoulders, and hips.

DM1 usually results from an increase in the number of CTG triplet repeats in the DMPK gene, which encodes for a protein kinase, with a larger number of inserted repeats leading to more significant symptoms. DM2, on the other hand, is a result of an increased number of CCTG repeats in the zinc finger ZNF9 gene. Myotonic dystrophy can be difficult to diagnose as it presents in a similar way to other defects in the muscular system, and is also difficult to treat due to its multi-systemic effects.