FAQs about Duchenne muscular dystrophy

What causes Duchenne?

Duchenne is a genetic disease. Children can either inherit Duchenne from their mother if they carry the genetic mutation in the dystrophin gene, or it can be the result of a random genetic mutation.

Duchenne occurs when there is a fault in the dystrophin gene. This is the gene that the body needs to produce a protein called dystrophin, which is essential for maintaining muscle strength and function.

There are three errors, called mutations, that can prevent any or enough dystrophin being made by the body:

• Exon deletion – this means that one or more exons in the dystrophin gene is missing. This is the cause of around 70% of cases.
• Exon duplication – around 10% of cases are caused by one or more exons having extra copies in the dystrophin gene.
• Other small changes, such as tiny deletions of genetic code. These cause around 20% of cases.

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Why is Duchenne rare in girls?

Genes are found on 'chromosomes'. The Duchenne gene is located on a chromosome called the X chromosome. Boys have one X chromosome and one Y chromosome; girls have two X chromosomes.

Duchenne is inherited in a pattern called 'X-linked inheritance'. The Duchenne gene is 'carried' by women, but does not usually cause problems in girls or women (with rare exceptions, below). This is because of there being two X chromosomes in women: one X chromosome has the 'faulty' Duchenne gene, and the other X chromosome has a normal gene, which compensates for the faulty one.

In contrast, boys with the Duchenne gene do not have a second X chromosome and so they cannot compensate for the faulty gene. Therefore, boys with the Duchenne gene always have symptoms of the disease.

Overall with each pregnancy, there is a 1 in 4 chance that a boy with Duchenne will be born. There is also a 1 in 4 chance that a daughter who carries the Duchenne gene will be born.

On the other hand, if a man with Duchenne was to have children with a non-carrier woman, none of their sons would have Duchenne although all of their daughters would be carriers.

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Does Duchenne affect females?

Duchenne affects approximately 1 in every 3,500 boys that are born but only around 1 in 50 million girls.

This difference is because Duchenne is caused by a fault in the Duchenne gene which is only found on the X chromosome. Boys have one X chromosome and one Y chromosome while girls have two X chromosomes.

A girl can have Duchenne if both of her X chromosomes have faults in the Duchenne gene.

This is extremely unlikely although it could happen in a number of different ways. A girl could inherit a faulty Duchenne gene on one of her X chromosomes from her mother and also develop a spontaneous mutation in her other X chromosome. She could also develop spontaneous mutations in the Duchenne gene on both of her X chromosomes.

It is also thought that the healthy Duchenne gene on one X chromosome can become ‘switched off’ by a process called X-linked inactivation in some girls that have inherited a faulty Duchenne gene on their other X chromosome.

Alternatively, if a man with Duchenne and a woman who carried a defective Duchenne gene ‘a carrier’ had a child, it is possible that a daughter would have Duchenne.

Females can be carriers of Duchenne but generally do not exhibit systems however female carriers of a faulty Duchenne gene can also have very mild symptoms of Duchenne that progress slowly. These women are called ‘manifesting carriers’ because they produce reduced amounts of dystrophin and may have symptoms of fatigue and mild muscle weakness and cramping.

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How long does each stage of Duchenne last?

The progression of Duchenne will be different for each individual and is not determined by genetic variant. People with same genetic variant, even in the same family, can experience different progression. 

Data from our HERUCLES research can be used to identify trends and the average amount of time people with Duchenne spend in each stage but this will not be the same for everyone and there are many factors that can determine progression particularly around the loss of ambulation. According to HERCULES data a person with Duchenne may spend an average of:

• 9.5 years in the ambulatory stage able to walk and stand.
• 1.5 years in the transfer stage able to stand and weight bear to aid transfers.
• The remainder of their lives in non-ambulatory stages.

What is the life expectancy of someone with Duchenne?

This will vary for every person living with Duchenne and will depend on the speed of progression and other factors. Over time and with improvements in care, the life expectancy of a person living with Duchenne has increased significantly with people with Duchenne born after 1990 having an average life expectancy of 28 years. Recent data from our HERCULES research has found an average predicted life expectancy of 34 years.