Duchenne and Becker muscular dystrophy: types & causes. Part I.
Impact of neuromuscular diseases on education and working opportunities of patients and carers
What is Duchenne- and Becker Muscular Dystrophy?
Duchenne and Becker MD are both X-linked recessive disorders. Both diseases are caused by mutations in the dystrophin gene, which encodes the protein dystrophin. A certain part of the DNA is missing, doubled, or changed so the code cannot be read properly by the body. Males have only one copy of the X chromosome from their mother and one copy of the Y chromosome from their father, that is why mainly man/boys are affected. A woman who has a genetic change in one of her two copies is said to be ‘a carrier’ of Duchenne muscular dystrophy. However, Duchenne and Becker can occur when the gene mutates spontaneously.
Difference between Duchenne and Becker MD
Where Duchenne patients have a complete lack of the gene that produces dystrophin, Becker patients have lower levels of a shorter version of this muscle-protecting protein. Duchenne symptoms start to show when patients are babies or children. People with Becker have less severe symptoms that will often only show later in life when they become adults. This is why Becker can be seen as a milder form of Duchenne. However, some patients are already showing symptoms at a younger age.
Clinical features
An early sign of Duchenne or Becker is that the person is struggling with standing and walking. DMD or BMD people will often have trouble with climbing stairs and have an unsteady way of walking. In some cases, the calves are enlarged, because the muscle tissue gets replaced by fat and scar tissue. In Duchenne, the majority of the children are later in reaching motor milestones, such as walking, hopping and crawling. In a minority of the children, the first symptom is delayed speech. When walking becomes more difficult, children tend to walk on their toes.
Becker symptoms are often initially seen as ‘clumsiness’ in falling often due to muscle weakness in the hips, pelvis and legs. The loss of muscle tissue is not painful in itself, although some patients mention having muscle cramps.
Diagnostic delay
In most countries, the average age of diagnosis of DMD is above 4 years of age and the diagnostic delay around 2.5 years. Parents see symptoms much earlier and some symptoms are already visible when the children are very young. Lack of awareness of DMD in (first line) health care professionals seeing these children is a major factor that contributes to this delay.
Types & Mutations
Duchenne and Becker are caused by mutations (changes) within the dystrophin gene. A gene is made up of coding regions called exons, and the areas between exons are called introns. Making the dystrophin protein involves several steps. The first step is to remove the introns and fitting the exons together, 1 to 79, like puzzle pieces. If there is a missing (deletion) or extra (duplication) piece, the body can have difficulties making dystrophin.
Since the dystrophin gene is one of the largest genes in the body, it can have thousands of changes - or mutations that result in Duchenne or Becker. As some new therapies may only treat certain Duchenne and Becker mutations, it will be important to know which mutation the patient has.
The following table from PPMD USA shows the different types of mutations that can happen in the dystrophin gene, and how commonly the different mutations occur in people with Duchenne and Becker.
|
Type of mutation |
How often mutation causes Duchenne |
How often mutation causes Becker |
|
Large deletions |
60-70% of all cases |
80-85% of all cases |
|
Large duplications |
10% of all cases |
5-10% of all cases |
|
Point mutations and other small changes (including ‘nonsense’ mutations) |
15-30% of all cases |
10-15% of all cases |
Deletions occur when pieces of the gene, called exons, are missing. In most cases, one or more exons missing are the most common type of mutation. There are 79 exons in the dystrophin gene, so there are many deletions that can happen. Certain areas are more likely to have a deletion and are so-called ‘hot-spots’. Exons 44-55 are hot spot regions. If you know the mutation is a deletion, the Exon Deletion Tool can help understand which exons need to be skipped to restore the reading frame, which means the body can make the protein again (albeit a slightly shorter version).
Duplications happen when one or more exons are doubled. This type of mutation is not as common as deletions. Point mutations are smaller changes that do not occur in the entire exon.
Sometimes, just one letter in the DNA code is missing, doubled, or changed. One of the most common point mutations is called a nonsense mutation. This is a premature stop in the gene that stops or hinders the production of the dystrophin protein.
In-Frame or Out-of-Frame deletions
With in-frame mutations, the protein may be shorter than normal, but still functioning. In-frame deletions typically result in Becker because there is some dystrophin present in the cells. In out-of-frame deletions, the reading frame is completely disrupted. In this case, no dystrophin at all is made. This type of deletion is often the case in Duchenne muscular dystrophy.
