Article: Spinal and bulbar muscular atrophy

What is spinal and bulbar muscular atrophy?

Spinal and bulbar muscular atrophy is a disorder of specialized nerve cells that control muscle movement (motor neurons). The condition, which mainly affects males, is characterized by muscle weakness and wasting that usually begins in adulthood and worsens slowly over time. Some men with the disorder also experience unusual breast development (gynecomastia) and may be infertile, or unable to conceive a child.

How common is spinal and bulbar muscular atrophy?

This condition affects fewer than 1 in 50,000 live male births.

What genes are related to spinal and bulbar muscular atrophy?

Mutations in the AR gene cause spinal and bulbar muscular atrophy.

This disorder is caused by a mutation in which a DNA segment, known as a CAG triplet repeat, is expanded within the AR gene. Normally, this DNA segment is repeated up to about 34 times. In people with spinal and bulbar muscular atrophy, the CAG segment is repeated more than 35 times, and may be two or three times its usual length. The abnormally expanded CAG triplet repeat changes the structure of the protein made by the AR gene, disrupting the normal function of motor neurons in the brain and spinal cord. These nerve cells gradually die, leading to the muscle weakness and wasting seen in this condition.

How do people inherit spinal and bulbar muscular atrophy?

This condition is inherited in an X-linked recessive pattern. A condition is considered X-linked if the 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, one altered copy of the gene is sufficient to cause the condition. In females, who have two X chromosomes, a mutation must be present in both copies of the gene to cause the disorder. Males are affected by X-linked recessive disorders much more frequently than females. A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Some females with one altered copy of the AR gene may have mild signs and symptoms related to the condition, including muscle cramps and occasional tremors.

Source: Genetics Home Reference
Cache Date: April 13, 2005