Muscular Dystrophy, Becker

Disease Overview

Becker muscular dystrophy is in the category of inherited muscle wasting diseases caused by a gene abnormality (mutation) that results in deficient or abnormal production of the dystrophin protein (dystrophinopathies). The abnormal gene is called DMD and is located on the X chromosome. Becker muscular dystrophy follows x-linked recessive inheritance so it mostly affects males, but some females are affected. Becker muscular dystrophy usually begins in the teens or early twenties and symptoms vary greatly between affected individuals. Muscle deterioration progresses slowly but usually results in the need for a wheel chair. Muscles of the heart deteriorate (cardiomyopathy) in some affected individuals, and this process can become life-threatening. Learning disabilities involving visual abilities may be present.

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Synonyms

• Benign Juvenile Muscular Dystrophy
• BMD
• Progressive Tardive Muscular Dystrophy

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Signs & Symptoms

Symptoms of Becker muscular dystrophy usually begin in the teens or late twenties. Initial symptoms may include cramping during exercise and reduced stamina during exercise. Muscle gradually deteriorates in the hips, pelvis, thighs and shoulders that can lead to walking on toes with the stomach forward. Shortening of muscle fibers can result in the inability to move certain muscles (contractures). The progression of BMD is slower and more variable than Duchenne muscular dystrophy but usually results in the need for a wheel chair. The heart muscle is also affected and can result in feeling breathless, fluid accumulation in the lungs and swelling in the feet and lower legs. Dilated cardiomyopathy is the most common cause of death in individuals with BMD, and most affected individuals die in their mid 40’s.

Approximately 5-10% of female DMD gene carriers have some symptoms of muscle weakness that progress slowly or not at all. Muscle weakness is frequently more severe on one side of the body (asymmetric).

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Causes

Becker muscular dystrophy is caused by abnormalities (mutations) in the DMD gene that is responsible for the production of the dystrophin protein. Dystrophin is necessary for the stability and protection of muscle. The gene mutation causes the dystrophin protein to be shorter than normal and not function normally.

The DMD gene is located on the X chromosome and Becker muscular dystrophy follows x-linked recessive inheritance. Females have two X chromosomes but one of the X chromosomes is “turned off” and all of the genes on that chromosome are inactivated. Females who have a disease gene present on one of their X chromosomes are carriers for that disorder. Carrier females usually do not display symptoms of the disorder because it is usually the X chromosome with the abnormal gene that is “turned off”. A male has one X chromosome and if he inherits an X chromosome that contains a disease gene, he will develop the disease. Males with X-linked disorders pass the disease gene to all of their daughters, who will be carriers. A male cannot pass an X-linked gene to his sons because males always pass their Y chromosome instead of their X chromosome to male offspring. Female carriers of an X-linked disorder have a 25% chance with each pregnancy to have a carrier daughter like themselves, a 25% chance to have a non-carrier daughter, a 25% chance to have a son affected with the disease, and a 25% chance to have an unaffected son.

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Affected populations

Becker muscular dystrophy occurs in approximately 1 in 30,000 male births.

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Disorders with Similar Symptoms

Symptoms of the following disorders can be similar to those of Becker muscular dystrophy (BMD). Comparisons may be useful for a differential diagnosis:

Duchenne muscular dystrophy is an x-linked recessive genetic disease that is also caused by mutations in the DMD gene. The DMD gene mutations that cause Duchenne muscular dystrophy result in little or no dystrophin protein to be made. Symptoms usually begin in early childhood and progress rapidly. The disease is initially characterized by delays in sitting and standing and difficulty climbing. Children often need a wheel chair by the age of twelve. A disease affecting the heart muscle (cardiomyopathy) occurs in the teenage years and can be life threatening. (For more information about this condition, choose “Duchenne” as your search term in the Rare Disease Database.)

Limb-girdle muscular dystrophy is a group of inherited, progressive disorders that are characterized by weakness and of muscles of the hip and shoulder areas. Several different forms of the disorder have been identified that are caused by mutations in certain genes. Some of these disease subtypes follow autosomal recessive inheritance and some follow autosomal dominant inheritance. In most individuals with limb-girdle muscular dystrophy, symptoms begin during childhood, but they may also begin during adolescence or adulthood. Muscle weakness may spread from the lower limbs to the upper limbs or vice versa. Although the disorder typically progresses slowly, some affected individuals experience rapid disease progression. (For more information about this condition, choose “limb girdle” as your search term in the Rare Disease Database.)

Emery-Dreifuss muscular dystrophy is a rare, often slowly progressive form of muscular dystrophy affecting the muscles of the arms, legs, face, neck, spine and heart. The disorder consists of weakness and degeneration of certain muscles, joints that are fixed in a flexed or extended position (contractures), and abnormalities affecting the heart (cardiomyopathy). Major symptoms may include muscle wasting and weakness particularly in the upper legs and arms and contractures of the elbows, Achilles tendons, and upper back muscles. In some cases, additional abnormalities may be present. Emery-Dreifuss muscular dystrophy is inherited as an x-linked, autosomal dominant or autosomal recessive trait. (For more information about this condition, choose “Emery Dreifuss” as your search term in the Rare Disease Database.)

Spinal muscular atrophy is an inherited progressive neuromuscular disorder characterized by degeneration of groups of nerve cells (motor nuclei) within the lowest region of the brain (lower brainstem) and certain motor neurons in the spinal cord (anterior horn cells). Motor neurons are nerve cells that transmit nerve impulses from the spinal cord or brain (central nervous system) to muscle or glandular tissue. Typical symptoms are a slowly progressive muscle weakness and muscle wasting. Affected individuals have poor muscle tone, muscle weakness on both sides of the body without, or with minimal, involvement of the face muscles, twitching tongue and a lack of deep tendon reflexes. SMA is divided into subtypes based on age of onset of symptoms and maximum function achieved. Spinal muscular atrophy is inherited as an autosomal recessive trait and is caused by mutations in the SMN (survival motor neuron) gene. (For more information about this condition, choose “spinal muscular atrophy” as your search term in the Rare Disease Database.)

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Diagnosis

The diagnosis of Becker muscular dystrophy is based on physical symptoms, family history, an elevated concentration of creatine kinase (CK) in the blood indicating destruction of muscle, and molecular genetic testing. DMD is the only gene that has been associated with Becker muscular dystrophy and many different types of DMD gene mutations have been identified in individuals with this condition. Identification of a DMD gene mutation from molecular genetic testing confirms the diagnosis. If molecular genetic testing is performed and a DMD gene mutation is not found, a skeletal muscle biopsy is recommended to examine the appearance of the dystrophin protein.

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Standard Therapies

Treatment

No specific treatment is available for Becker muscular dystrophy but quality of life and lifespan can be improved with appropriate care. Physical and occupational therapy can reduce or delay joint contractures. Surgery is sometimes recommended to treat contractures or scoliosis. Weight control can help to reduce stress on the heart and muscles. Corticosteroids are often prescribed to help slow down the loss of muscle function. Routine monitoring by a cardiologist is recommended.

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Clinical Trials and Studies

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the National Institutes of Health (NIH) in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222

TTY: (866) 411-1010

Email: prpl@cc.nih.gov

For information about clinical trials sponsored by private sources, contact:

www.centerwatch.com

In April of 2008, PTC Therapeutics, Inc. (PTC), announced the initiation of an international pivotal trial of PTC124 in patients with Duchenne/Becker muscular dystrophy (DMD/BMD) due to a nonsense mutation. The primary objective of this registration-directed Phase 2b trial is to demonstrate the efficacy of PTC124 as measured by improvements in the walking ability of patients with this progressive genetic disease.

PTC124 is a novel, orally delivered drug in development for the treatment of patients with genetic disorders due to a nonsense mutation, a type of mutation found in approximately 13% of patients with DMD. In this double-blind study, patients will be randomized to receive placebo, or one of two dose levels of PTC124, three times per day. Eligible patients will be boys with nonsense-mutation-mediated DMD/BMD who are at least 5 years of age and are able to walk at least 75 meters or approximately 80 yards in six minutes. PTC expects to enroll a total of 165 patients at approximately 35 investigational sites; all study subjects will undergo 48 weeks of blinded treatment. Thereafter, all participants, including those who have been receiving placebo, will be eligible to enroll in an open-label PTC124 extension study.

For more information please contact:

or more information:

Jane Baj

PTC Therapeutics, Inc.

(908) 912-9167

jbaj@ptcbio.com

OR

Sheryl Seapy

Pure Communications

(949) 608-0841

sheryl@purecommunicationsinc.com

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References

TEXTBOOKS

Ascadi G. Becker Muscular Dystrophy. In: The NORD Guide to Rare Disorders, Philadelphia: Lippincott, Williams and Wilkins, 2003:622.

JOURNAL ARTICLES

Bushby KM and Gardner-Medwin D. The clinical, gentic and dystrophin characteristics of Becker muscular dystophy. I. Natural history [published erratum appears in J Neurol 1993 Jul;240 (7):453]j Neurol 240:98-104.

Hoffman EP, Fischbeck KH, Brown, et al. Characterization of dystrophin in muscle-biopsy specimens from patients with Duchenne’s or Becker’s muscle dystrophy. N Engl J Med 1998;318:1363-8.

Nolan MA Jones OD, Pederson RL, et al. Cardiac assessment in childhood carriers of Duchenne and Becker muscle dystrophies. Neuromuscul Disord 2003;13:129-32.

Palmucci L, Mongini T, Chido-Piat L, et al. Dystrophinopathy expressing as either cardiomyopathy or Becker dystrophy in the same family. Neurology 2000; 54:529-30.

Quinlivan R, Ball J, Dunckley M, et al. Becker muscle dystrophy presenting with complete heart block in the sixth decade. Neurology 1995;242:398-400.

FROM THE INTERNET

Kork BR, Darras BT, and Urion DK (Updated 10/4/04) Dystrophinopathies In: GeneReviews at GeneTests: Medical Genetics Information Resource (database online). Copyright, University of Washington, Seattle. 1997-2005. Available at https://genetests.org. Accessed 6/05.

McKusick VA ed. Online Inheritance in Man (OMIM). Baltimore, MD: The Johns Hopkins University: Entry No. 300376, Last Update: 4/2/02.

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Programs & Resources

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