WHAT IS DUCHENNE/BECKER MUSCULAR DYSTROPHY?
Duchenne (“doo-shen”) muscular dystrophy (Duchenne) is the most common muscular dystrophy in children. It is a progressive disorder that causes muscles to become weaker over time until it affects the whole body.
Duchenne muscular dystrophy is one of a spectrum of muscle diseases known as dystrophinopathies that can range from mild to severe in presentation. Dystrophinopathies occur when the dystrophin protein (found in the body’s muscle cells) has an altered structure or function, or is entirely absent from the body’s muscle cells. Becker muscular dystrophy is also a dystrophinopathy with many similarities to Duchenne. And while Becker can be variable, depending on when symptoms first present, typically the symptoms are milder in progression compared to Duchenne. PPMD’s work extends to both Duchenne and Becker, but for simplicity we primarily refer to Duchenne. You can think of Duchenne and Becker as different ends of the spectrum of dystrophinopathies.
Duchenne is a genetic disorder caused by a change in the DNA (or gene) that codes for dystrophin. Since the dystrophin gene is found on the X-chromosome, it primarily affects males. Females are typically known as carriers because they have one copy of the changed gene and one copy of the functioning gene. Carrier females often do not have outward symptoms of dytrophinopathy. However, some carrier females can present with varying ranges of physical symptoms and are considered to have dystrophinopathy as well, but are usually termed manifesting carriers.
While there is yet no cure for Duchenne, and many parents are told that there is no hope and little help, Parent Project Muscular Dystrophy exists to offer the support and resources parents may not find anywhere else. There is hope — with more than 40 companies currently working on treatments for Duchenne and more than 20 drugs in clinical trial, there are tremendous reasons to hope. Knowledge is power, and informed and timely care can help people living with Duchenne live longer and with a much greater quality of life than ever before.
HOW COMMON IS DUCHENNE?
Duchenne is not contagious. About one out of every 5,000 boys has Duchenne, and about 20,000 babies worldwide are born with it each year. Duchenne mostly affects boys and reaches across all races and cultures. Parent Project Muscular Dystrophy (PPMD) estimates that there are about 15,000 young men, as well as a few young women, living with Duchenne today in the United States. Although genetic disorders are usually passed down from a parent to a child, Duchenne muscular dystrophy can occur even if no one in your family has had it before.
In the US, a rare disease is defined as a condition that affects fewer than 200,000 people. Although Duchenne is considered rare, there is a huge active community at your fingertips working to end Duchenne.
WHAT CAUSES THE SYMPTOMS OF DUCHENNE?
Duchenne is caused by mutations in the gene that encodes for a muscle protein called dystrophin. Dystrophin is in every single muscle cell in our bodies. Dystrophin acts as the glue that holds muscles together and the “shock absorber” that allows muscles to contract and relax without being damaged.
Without dystrophin, muscles are not able to function or repair themselves properly. As muscles are used for normal day-to-day activity, tiny tears are created in the muscle cells, which cause creatine kinase (CK) to leak out of the cells and results in high levels in the blood. Because there is no dystrophin, the muscles can’t repair themselves by making new muscle, so the damaged muscle is replaced by fat and scar tissue. This causes the person with Duchenne loses muscle function and strength.
Duchenne is a condition that progresses very slowly over time. The first symptoms are often subtle, such as delays in crawling walking and/or speech. Parents may or may not notice these delays, and primary care providers often attribute these delays to something else.
Clear symptoms typically develop between the ages of 2 and 5 years, which is typically when the diagnosis is made. Muscle weakness follows a proximal to distal pattern, meaning muscles near the trunk, such as the hips and shoulders, will be affected before those of the arms and legs. This is why you may notice your child having difficulty walking, running, or climbing stairs. When asked to get off the floor, your child will often use the Gower’s Maneuver, or use their arms to “walk” up their legs to a standing position. They may also appear clumsy and fall a lot, and their calves may appear enlarged or overdeveloped (called pseudohypertrophy, which is replacement of the calf muscle with fat and scar tissues). This is when a primary care provider will likely order a creatine kinase (CK) blood test to check for muscular dystrophy.
WHY DOES MY CHILD HAVE DUCHENNE?
Duchenne is a genetic disorder, which means it is caused by a change in the DNA sequence of a gene. Genes house the instructions for how to make proteins, and proteins are the building blocks of the body. In Duchenne, there is a change (often called a mutation or variant), in the gene that codes for the dystrophin protein, so the dystrophin protein is not made properly.
Duchenne is a “X-linked” disorder, which means the dystrophin gene is on the X-chromosome. It may be helpful to review some basic genetics. Every person has 23 pairs of chromosomes, which contain all of their genes and determine their sex. One of each pair of chromosomes are inherited from their mother, and the other from their father. Females always have two X-chromosomes, so mothers always pass down an X-chromosome to their child. Males have one X-chromosome, and one Y-chromosome, so fathers can pass down either an X or Y-chromosome, which will ultimately determine the sex of the baby. Boys always inherit their X-chromosome from their mother, and their Y-chromosome from their father, whereas girls inherit two X-chromosomes, one from each parent.
Since a boy only has one X-chromosome, if that chromosome contains the Duchenne gene mutation, he will have Duchenne. However, since girls have two X-chromosomes, if one of them has the Duchenne gene mutation, they are usually not affected because they have a second back-up copy of the gene on their other X-chromosome. Females with one X-chromosome with the Duchenne gene mutation are known as carriers of Duchenne. Carriers have the ability to pass on the mutated Duchenne gene to their children.
Since DUCHENNE is genetic, was it inherited from me?
You may have someone in your family who has or had Duchenne. As a genetic disorder, Duchenne can be passed down from a parent to a child, so it could already be in your family’s genes. Many mothers who carry the Duchenne gene often do not know they are carriers. Approximately two-thirds (2/3) of newly diagnosed boys with Duchenne causes have inherited the gene from their carrier mother.
Duchenne can also occur even if no one in your family has had it before. The Duchenne gene is very large, which increases the incidence of mutation during pregnancy. This is why approximately one-third (1/3) of Duchenne cases are caused by a spontaneous mutation, or a new change in the DNA that is not inherited from the mother. The only way to determine if a child with Duchenne has inherited the mutation or has a new mutation is for the mother to have carrier testing.
Genetic testing (also known as DNA testing) is the process of examining a person’s DNA. In Duchenne, the dystrophin gene is analyzed for changes, or mutation, that would prevent the gene from working properly. Genetic testing is typically performed on a blood or saliva sample.
Genetic testing is always necessary and should be offered to every family when a diagnosis of Duchenne is suspected. Different types of genetic tests are able to provide specific information about the change or mutation in the DNA. It is important to know the specific gene change or mutation for many reasons. To learn more about genetic testing, and how genetic counseling can help your family, please click here.