Myotonic dystrophy is a genetic disorder affecting several body systems. Signs, symptoms, and severity can vary from one individual to another, and may include muscle weakness, abnormal muscle tension, and progressive muscle wasting. It can manifest at any age, but usually has an adult onset. Treatments are symptomatic, as there is currently no cure.
There are two forms of myotonic dystrophy — types 1 and 2. Type 2 is less common, and is usually less severe than type 1. In their article “Myotonic dystrophy”, the Genetics Home Reference note that the worldwide prevalence of the disorder is 1 in 8000. They also note that 98% people affected have type 1, and the other 2% have type 2.
The Gene Involved in Myotonic Dystrophy Type 2
Myotonic dystrophy type 2 is the result of a mutation in a gene called CCHC-type zinc finger, nucleic acid binding protein (CNBP). It codes for a protein of the same name, but its functions are not clear. It is found in muscle, particularly the heart, and appears to play an essential role in muscle function.
The CNBP gene contains a repeating sequence of DNA building blocks called nucleotides. This sequence repeats the following base chemicals: cytosine, cytosine, thymine and guanine. These are called CCTG tetranucleotide repeats. Fewer than 26 repeats is normal, and over 75 repeats results in mytonic dystrophy type 2.
These extraneous repeats result in abnormal messenger RNA. When working properly, messenger RNA copies a gene’s instructions so the right protein can be made. If there is something wrong with the RNA, then abnormal proteins are produced, and the signs and symptoms of myotonic dystrophy type 2 can result.
Inheritance of Myotonic Dystrophy Type 2
Myotonic dystrophy type 2 is inherited in an autosomal dominant manner. Each parent contributes a set of genes to each of their children. In autosomal dominant disorders, only one parent needs to have a mutated gene copy for the child to inherit the disorder.
If one parent has a mutated gene copy, and the other does not, the chances are the following for each pregnancy:
- A 1 in 2 (50%) chance of having a child who is affected by mytonic dystrophy type 2.
- A 1 in 2 (50%) chance of having a child who is completely unaffected.
Anticipation is seen in myotonic dystrophy type 2. This means that with each generation, there is an earlier onset, and an increase in symptom severity. In myotonic dystrophy type 1, this is the result of the repeats getting longer. In type 2, anticipation still happens, but the length of the repeats doesn’t seem to be the cause.
Genetic Testing and Myotonic Dystrophy Type 2
There are genetic tests that can detect the gene mutation involved in myotonic dystrophy type 2. Prenatal testing is also possible. Genetic counseling may be an option for individuals and families considering testing. These professionals are trained to offer information and support without leading people’s decisions.
Read Understanding Genetic Consultation and Understanding Genetic Testing in Healthcare for more details on genetic disorders and testing. For patient information, including details of symptoms and treatment, visit the Myotonic Dystrophy Support Group and Myotonic Dystrophy Foundation websites.
Sources
“Myotonic dystrophy”, last updated: 15th November 2007, Genes and Disease (NCBI) website, accessed: 4th September 2010, no author specified.
“Myotonic dystrophy”, published: 30th August 2010, Genetics Home Reference website, accessed: 4th September 2010, no author specified.
“Myotonic dystrophy”, published: October 2005, The Journal of the Royal College of Physicians of Edinburgh website, accessed: 4th September 2010, author: Longman, C.
“Myotonic Dystrophy Type 2”, last updated: 23rd April 2007, GeneReviews (NCBI) website, accessed: 4th September 2010, author: Dalton, Joline (et al).
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