Myotonic Dystrophy and the Brain

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.

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 the most likely explanation is called an RNA-gain-of-function mechanism. DNA in the nucleus is transcribed in to another molecule called RNA, which is a messenger carrying information from the nucleus to the cytoplasm where it is made in to protein. In Myotonic Dystrophy the repeat expansion mutation is made into RNA but it does not get out into the cytoplasm. It remains trapped in the nucleus where it sticks to various proteins and appears as spots or foci that can be observed down the microscope. Because these proteins are stuck to the repeat RNA they cannot perform their normal functions correctly within the cell.

Decrease in the activity of DMPK


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

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.


Information supplied by Professor David Brook, department of  Human Genetics, Faculty of Medicine & Health Sciences Queen’s Medical Centre,Nottingham..


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