Most cases of MD are caused by gene mutations (changes in the DNA sequence) that affect muscle proteins.
The mutations are usually inherited, but in some cases they occur spontaneously. These spontaneous mutations can then be inherited by an affected person’s offspring.1
There are three different inheritance patterns for most forms of MD. When a mutation that causes MD is inherited by a parent with the condition, this is known as autosomal dominant inheritance.
In contrast, for most autosomal recessive conditions, the gene mutation must be inherited from both parents. Both matching genes must include a mutation to cause the disease.
However, the inheritance pattern for many types of MD is called “X-linked recessive,” meaning that the genetic change is passed from one generation to the next through the X chromosome.2
Girls and women have two X chromosomes (XX), while boys and men have one X and one Y chromosome (XY). This pattern is the reason certain types of MD, whose mutations are on the X chromosome, are more common in males than in females.
- If a female has one X chromosome with the genetic mutation and one X chromosome without, she may not have symptoms or may have milder MD than males have. She is called a “carrier” because she has the genetic mutation and can pass it on to her children.
- Because males have only one X chromosome, if they have the genetic mutation, they will usually have symptoms of MD or more severe MD symptoms than females do.
Hundreds of genes are involved in making proteins that affect muscles. Each form of MD is caused by a genetic mutation that is unique to that type. For instance:
- Duchenne MD results from a genetic mutation that leads to a lack of dystrophin, a protein that helps strengthen muscle fibers and protect them from injury.1 It can be inherited in an X-linked recessive manner.
- Myotonic MD is caused by an abnormal expansion of certain DNA sequences on one of two different genes. Most people may have around 40 repeats of a certain DNA code in their gene, but people with myotonic MD have 4,000 or more repeats. These repeats cause errors in the “instructions” for making muscle proteins, meaning the body may not make enough of the protein or may not make usable forms of the protein. In some cases, the instructions themselves build up to high levels; this buildup causes problems with the protein. Myotonic MD can be inherited in an autosomal dominant manner.
MD is not contagious and cannot be caused by injury or activity.1