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Genetics Home Reference: your guide to understanding genetic conditions
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Spinal and bulbar muscular atrophy

Reviewed December 2012

What is spinal and bulbar muscular atrophy?

Spinal and bulbar muscular atrophy, also known as Kennedy disease, is a disorder of specialized nerve cells that control muscle movement (motor neurons). These nerve cells originate in the spinal cord and the part of the brain that is connected to the spinal cord (the brainstem).

Spinal and bulbar muscular atrophy mainly affects males and is characterized by muscle weakness and wasting (atrophy) that usually begins in adulthood and worsens slowly over time. Muscle wasting in the arms and legs results in cramping; leg muscle weakness can also lead to difficulty walking and a tendency to fall. Certain muscles in the face and throat (bulbar muscles) are also affected, which causes progressive problems with swallowing and speech. Additionally, muscle twitches (fasciculations) are common. Some males with the disorder experience unusual breast development (gynecomastia) and may be unable to father a child (infertile).

How common is spinal and bulbar muscular atrophy?

This condition affects fewer than 1 in 150,000 males and is very rare in females.

What genes are related to spinal and bulbar muscular atrophy?

Spinal and bulbar muscular atrophy results from a particular type of mutation in the AR gene. This gene provides instructions for making a protein called an androgen receptor. This receptor attaches (binds) to a class of hormones called androgens, which are involved in male sexual development. Androgens and androgen receptors also have other important functions in both males and females, such as regulating hair growth and sex drive.

The AR gene mutation that causes spinal and bulbar muscular atrophy is the abnormal expansion of a DNA segment called a CAG triplet repeat. Normally, this DNA segment is repeated up to about 36 times. In people with spinal and bulbar muscular atrophy, the CAG segment is repeated at least 38 times, and it may be two or three times its usual length. Although the extended CAG region changes the structure of the androgen receptor, it is unclear how the altered protein disrupts nerve cells in the brain and spinal cord. Researchers believe that a fragment of the androgen receptor protein containing the CAG segment accumulates within these cells and interferes with normal cell functions. The nerve cells gradually die, leading to the muscle weakness and wasting seen in this condition. People with a higher number of CAG repeats tend to develop signs and symptoms of spinal and bulbar muscular atrophy at an earlier age.

Related Gene(s)

Changes in this gene are associated with spinal and bulbar muscular atrophy.

  • AR

How do people inherit spinal and bulbar muscular atrophy?

This condition is inherited in an X-linked pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. In males (who have only one X chromosome), a mutation in the only copy of the gene in each cell causes the disorder. In most cases, males experience more severe symptoms of the disorder than females (who have two X chromosomes). Females with a mutation in one copy of the AR gene in each cell are typically unaffected. A few females with mutations in both copies of the gene have had mild features related to the condition, including muscle cramps and occasional tremors. Researchers believe that the milder signs and symptoms in females may be related to lower androgen levels.

A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Where can I find information about diagnosis or management of spinal and bulbar muscular atrophy?

These resources address the diagnosis or management of spinal and bulbar muscular atrophy and may include treatment providers.

  • Gene Review: Spinal and Bulbar Muscular Atrophy (http://www.ncbi.nlm.nih.gov/books/NBK1333)
  • Genetic Testing Registry: Bulbo-spinal atrophy X-linked (http://www.ncbi.nlm.nih.gov/gtr/conditions/C1839259)
  • MedlinePlus Encyclopedia: Muscle Atrophy (http://www.nlm.nih.gov/medlineplus/ency/article/003188.htm)

You might also find information on the diagnosis or management of spinal and bulbar muscular atrophy in Educational resources (http://www.ghr.nlm.nih.gov/condition/spinal-and-bulbar-muscular-atrophy/show/Educational+resources) and Patient support (http://www.ghr.nlm.nih.gov/condition/spinal-and-bulbar-muscular-atrophy/show/Patient+support).

General information about the diagnosis (http://ghr.nlm.nih.gov/handbook/consult/diagnosis) and management (http://ghr.nlm.nih.gov/handbook/consult/treatment) of genetic conditions is available in the Handbook. Read more about genetic testing (http://ghr.nlm.nih.gov/handbook/testing), particularly the difference between clinical tests and research tests (http://ghr.nlm.nih.gov/handbook/testing/researchtesting).

To locate a healthcare provider, see How can I find a genetics professional in my area? (http://ghr.nlm.nih.gov/handbook/consult/findingprofessional) in the Handbook.

Where can I find additional information about spinal and bulbar muscular atrophy?

You may find the following resources about spinal and bulbar muscular atrophy helpful. These materials are written for the general public.

You may also be interested in these resources, which are designed for healthcare professionals and researchers.

What other names do people use for spinal and bulbar muscular atrophy?

  • bulbospinal muscular atrophy, X-linked
  • KD
  • Kennedy disease
  • Kennedy's disease
  • Kennedy spinal and bulbar muscular atrophy
  • SBMA
  • X-linked spinal and bulbar muscular atrophy

For more information about naming genetic conditions, see the Genetics Home Reference Condition Naming Guidelines (http://ghr.nlm.nih.gov/ConditionNameGuide) and How are genetic conditions and genes named? (http://ghr.nlm.nih.gov/handbook/mutationsanddisorders/naming) in the Handbook.

What if I still have specific questions about spinal and bulbar muscular atrophy?

Ask the Genetic and Rare Diseases Information Center (http://rarediseases.info.nih.gov/GARD/).

What glossary definitions help with understanding spinal and bulbar muscular atrophy?

androgens ; atrophy ; brainstem ; cell ; chromosome ; class ; DNA ; gene ; gynecomastia ; infertile ; inheritance ; inherited ; motor ; mutation ; protein ; receptor ; sex chromosomes ; wasting

You may find definitions for these and many other terms in the Genetics Home Reference Glossary (http://www.ghr.nlm.nih.gov/glossary).

References

  • Atsuta N, Watanabe H, Ito M, Banno H, Suzuki K, Katsuno M, Tanaka F, Tamakoshi A, Sobue G. Natural history of spinal and bulbar muscular atrophy (SBMA): a study of 223 Japanese patients. Brain. 2006 Jun;129(Pt 6):1446-55. Epub 2006 Apr 18. (http://www.ncbi.nlm.nih.gov/pubmed/16621916?dopt=Abstract)
  • Finsterer J. Bulbar and spinal muscular atrophy (Kennedy's disease): a review. Eur J Neurol. 2009 May;16(5):556-61. Review. (http://www.ncbi.nlm.nih.gov/pubmed/19405197?dopt=Abstract)
  • Finsterer J. Perspectives of Kennedy's disease. J Neurol Sci. 2010 Nov 15;298(1-2):1-10. doi: 10.1016/j.jns.2010.08.025. Epub 2010 Sep 16. Review. (http://www.ncbi.nlm.nih.gov/pubmed/20846673?dopt=Abstract)
  • Gene Review: Spinal and Bulbar Muscular Atrophy (http://www.ncbi.nlm.nih.gov/books/NBK1333)
  • Katsuno M, Banno H, Suzuki K, Adachi H, Tanaka F, Sobue G. Clinical features and molecular mechanisms of spinal and bulbar muscular atrophy (SBMA). Adv Exp Med Biol. 2010;685:64-74. Review. (http://www.ncbi.nlm.nih.gov/pubmed/20687495?dopt=Abstract)
  • Monks DA, Rao P, Mo K, Johansen JA, Lewis G, Kemp MQ. Androgen receptor and Kennedy disease/spinal bulbar muscular atrophy. Horm Behav. 2008 May;53(5):729-40. doi: 10.1016/j.yhbeh.2007.12.009. Epub 2008 Jan 5. Review. (http://www.ncbi.nlm.nih.gov/pubmed/18321505?dopt=Abstract)
  • Poletti A, Negri-Cesi P, Martini L. Reflections on the diseases linked to mutations of the androgen receptor. Endocrine. 2005 Dec;28(3):243-62. Review. (http://www.ncbi.nlm.nih.gov/pubmed/16388114?dopt=Abstract)
  • Schmidt BJ, Greenberg CR, Allingham-Hawkins DJ, Spriggs EL. Expression of X-linked bulbospinal muscular atrophy (Kennedy disease) in two homozygous women. Neurology. 2002 Sep 10;59(5):770-2. (http://www.ncbi.nlm.nih.gov/pubmed/12221177?dopt=Abstract)

 

The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? (http://ghr.nlm.nih.gov/handbook/consult/findingprofessional) in the Handbook.

 
Reviewed: December 2012
Published: December 22, 2014