What Is Becker Muscular Dystrophy (BMD)?

Becker muscular dystrophy (BMD) is a genetic disease characterized by progressive muscle weakness and wasting. It is named after Peter Emil Becker, a German doctor who published the first formal reports of the disease in the 1950s.

Causes of BMD

BMD is caused by mutations in a gene called DMD. This gene provides instructions for producing a protein known as dystrophin. This protein, working in concert with other cellular components, acts sort of like a “shock absorber” in muscle cells, helping to reduce wear and tear during the intensive processes of muscle movement. When the protein is missing or defective, muscles become more susceptible to damage from everyday movements, ultimately resulting in muscle damage that accumulates over time.

Mutations that cause BMD result in an abnormally low amount of dystrophin protein being made, or lead to the production of an abnormal version of the protein that doesn’t work quite right.

In contrast, mutations in DMD that result in no dystrophin protein being made usually cause another form of muscular dystrophy called Duchenne muscular dystrophy, or DMD. Both BMD and DMD are dystrophinopathies — diseases caused by abnormalities in dystrophin — but since there is some functional dystrophin in BMD, these patients tend to have a later age at disease onset, and milder symptoms overall, than those with DMD.

Inheritance of BMD

Most of the time, mutations in the DMD gene are passed from parents to their biological children, but about one-third of DMD mutations occur de novo — that is, they only are found in the affected individual, not in either of the biological parents.

The DMD gene is located on the X chromosome, which is one of the two sex-determining chromosomes in humans: with rare exceptions, males have one X chromosome and one Y chromosome, while females have two X chromosomes. As such, males inherit one copy of the DMD gene, while females inherit two. BMD and other dystrinopathies are therefore substantially more common in males, since females with one mutated copy of DMD typically have a second, functional copy that can compensate for the defective gene. BMD is estimated to affect as many as one out of every 18,000 male births.

Individuals with one mutated copy of DMD and one functional copy are called “carriers,” because they themselves will not develop BMD, but they can pass the disease-causing mutation to their biological offspring. Most carriers show no symptoms of BMD-like disease, but as many as 22% of carriers display some disease symptoms.

Symptoms of BMD

The age at onset of symptoms in BMD varies widely from person to person, and can range from 5 to 60 years old. Some people with BMD may have symptoms that become apparent during early childhood, while others may have no noteworthy manifestations of the disease until adulthood.

The first noticeable symptom usually is muscle weakness, which typically first affects the proximal muscles, which are those closer to the trunk of the body. Patients may experience cramping when doing strenuous activities, or might find they have more difficulty than usual with jumping, climbing, or running stairs. The legs are usually affected before the arms in BMD. Most patients will experience some amount of difficulty walking by the time they are teenagers, and eventually will rely on a walking aid or wheelchair to help them get around.

Certain muscles, most commonly the calves, may appear enlarged due to the buildup of scar and fat in damaged muscle tissue. Patients also may experience contractures, when muscles become shortened and result in deformity. Around 20–25% of people with BMD exhibit cognitive impairment. BMD also can cause damage to the heart muscle, resulting in heart disease (cardiomyopathy), which is the most common cause of death among those with the condition.

Management and prognosis of BMD

There currently is no cure for BMD, but proper medical care can help to improve life quality and longevity. The average survival for people with BMD is about 40 or 50 years.

Medical treatment usually involves regularly giving patients anti-inflammatory medications called corticosteroids, which can help improve lung function, delay cardiomyopathy, and prolong survival. Other treatments usually are tailored based on the needs of the individual; for example, ACE inhibitors and beta-blockers — two classes of medicine that lower blood pressure — may help manage cardiomyopathy in patients with heart disease.

Outside of medications, a number of non-pharmacological interventions are available that may provide support to people with BMD. Physical therapy can help maintain motor function, for example, and occupational therapy can help patients devise strategies for navigating day-to-day life with their illness.

 

Last updated: Jan. 19, 2022, by Marisa Wexler MS

 


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