What causes Myotonic Dystrophy?

The exact cause of Myotonic Dystrophy is unknown however the genetic change responsible has been identified.

The genetic change (mutation) that causes Myotonic Dystrophy is in the DMPK gene found on chromosome 19.

At one end of the gene is an area where three of the building blocks of DNA, the genetic material, CTG are repeated. An unaffected individual will have 5-35 CTG repeats but in an individual with Myotonic Dystrophy the repeat is unstable and expands. The extent of the expansion ranges from 50 in a mildly affected individual to several thousands in a severely affected individual. This is the basis of genetic tests as the number of CTG repeats can be counted.
Somehow the CTG repeat changes the DMPK gene and results in Myotonic Dystrophy. The gene controls the production of a protein called Mytotonic Dystrophy Protein Kinase which appears to be very important in many systems of the body.

What is a gene? Basic Genetics

Long stretches of this code make up blocks of DNA with specific functions called genes. A gene gives an instruction for a specific characteristic, for example a gene for hair colour or eye colour and the DMPK gene in Myotonic Dystrophy.

How does the repeat expansion cause Myotonic Dystrophy?

This question has not been fully answered yet but there are three possible explanations, one of which or a combination of the three holds the answer.

1. Decrease in the activity of DMPK

In a normal cell the DMPK would send a message in the form of a molecule called RNA out of the nucleus and into the cell.

However the repeat expansion present in the DMPK gene is also present in the message and this prevent it from leaving the nucleus and performing its function. The message RNA builds up in the nucleus of the cell.

Therefore the activity of the DMPK gene is decreased which may be responsible for some of the features of Myotonic Dystrophy.

2. Binding Proteins

Proteins in the cell can become stuck to the areas in the nucleus where the message has accumulated.
Aspects of Myotonic Dystrophy may be caused because these proteins cannot perform their usual functions or because the act abnormally due to their position.

3. Adjacent genes

The genes surrounding the DMPK gene may also be affected by the CTG repeat expansion. For example Six5 is thought to be associated with cataracts a common feature of Myotonic Dystrophy.