Genetics Northern California

Becker Muscular Dystrophy

General Information
Becker muscular dystrophy (BMD) is a genetic disorder that affects the muscles. The condition is caused by a mutation in the dystrophin gene. A person with BMD does not make enough of a protein, called dystrophin. Dystrophin is made inside the body and is needed to keep muscles healthy. Without enough of this protein, the muscles slowly break down. BMD mainly affects the skeletal (voluntary) muscles and the cardiac (heart) muscles. Mutations in the dystrophin gene can also cause a related condition called Duchenne muscular dystrophy. In a person with Duchenne muscular dystrophy, the dystophin protein is completely (or almost completely) missing, which makes symptoms start earlier.

Becker muscular dystrophy usually affects males because it is an X-linked condition. About 1 in every 18,000 males will have BMD and all racial and ethnic groups are affected equally. Females can be carriers of BMD and may show no signs or have only minor muscle weakness. Women rarely have serious medical complications related to BMD. Sometimes BMD is inherited from the mother, who may be unaware she is a carrier for the condition. Other times BMD occurs due to a de novo mutation.  When a woman is a carrier of BMD, each male child has a 50% chance of having BMD and a 50% chance of not having condition. Each female child from a woman with a BMD mutation has a 50% chance of being a carrier of BMD and a 50% chance of not being a carrier. Every daughter of a man with BMD will be a carrier, but none of his sons will inherit the mutation.

Features of Becker Muscular Dystrophy
The main features of BMD are muscle weakness and muscle wasting (muscle loss). BMD is a progressive condition, with muscle weakness becoming more severe as a person grows older. Most individuals with BMD start to experience muscle weakness or cramping after exercise starting in late childhood or teenage years. Gradually, the muscles in the legs weaken and the symptoms become more noticeable. Some people with BMD, but not all, eventually require the use of a wheelchair. Most individuals with BMD are able to walk into their 20’s or later. Over time, the heart muscles and the breathing muscles can also become weaker. The most common cause of premature death for a man with BMD is related to the breakdown of the heart muscle. The average life expectancy is the mid 40’s, however some men with BMD have a normal lifespan. BMD does not affect intelligence.

Diagnosing Becker Muscular Dystrophy
Becker muscular dystrophy is suspected based on clinical features. Early signs might include exercise intolerance with gradual weakening of the leg muscles. The clinical features a doctor looks for include changes in the way a person walks or runs, how he goes up and down stairs, how he stands up when sitting on the ground, and the way his leg muscles appear. The diagnosis is confirmed by testing creatine kinase (CK) levels, which is a protein found in the blood that shows how fast muscle is breaking down. In males with BMD, the amount of CK is generally 5 times higher than normal. A sample of the muscles (muscle biopsy) was once used to help confirm the diagnosis. Since genetic testing became available, a muscle biopsy is rarely necessary. A family history of BMD can be helpful in making an early diagnosis.

Genetic Testing
Genetic testing may be done to confirm a diagnosis or to identify the specific mutation in a family. About 85% of males with BMD will have a mutation in the dystrophin gene that can be detected. A negative genetic test (no mutation found) does not change the diagnosis, but it means that our current technology is not able to find the specific mutation in that family. If the specific mutation of someone with BMD is known, carrier testing can be offered to females in the family and prenatal diagnostic testing is available during pregnancy. 

Treatment of Becker Muscular Dystrophy
There is currently no cure for BMD, but medical treatment may help slow the progression of the disease. Steroid therapy, when timed correctly, can optimize muscle strength and potentially prolong the ability to walk. In addition, males with BMD should be monitored by a cardiologist. The cardiologist may use medications to manage cardiac symptoms. Female carriers of BMD should also have a cardiac evaluation.

Researchers are currently looking for more effective treatments. To learn more about research opportunities contact one of the support organizations below or go to Clinicaltrials.gov.


Support Resources
Muscular Dystrophy Association- BMD Page
http://mda.org/disease/bmd.html

Parent Project Muscular Dystrophy
http://www.parentprojectmd.org/

References
Gene Reviews on Dystrophinopathies:
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=gene&part=dbmd

NIH: Genetics Home Reference on Duchenne and Becker Muscular Dystrophy:
http://ghr.nlm.nih.gov/condition/duchenne-and-becker-muscular-dystrophy

American Academy of Pediatrics: Cardiovascular Health Supervision for Individuals Affected by Duchenne or Becker Muscular Dystrophy
http://aappolicy.aappublications.org/cgi/reprint/pediatrics;116/6/1569.pdf

Written by:     Elizabeth Stark, MS
Reviewed by:  Kim Barr, MS CGC 
                      Emily Chen, MD, PhD
                      Joyce Kobori, MD
                      SJO Genetics

Last Edited:       September 24, 2010
Last Reviewed:  September 24, 2010