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The Androgen Receptor Gene
The AR gene provides instructions for making a protein
called an androgen receptor. Androgens are hormones (such as testosterone) that
are important for normal male sexual development before birth and during
puberty. Androgen receptors allow the body to respond appropriately to these
hormones. Androgens and androgen receptors also have other important functions
in both males and females, such as regulating hair growth and sex drive.
The receptors are present in many of the body's
tissues, where they attach (bind) to androgens. The resulting androgen-receptor
complex then binds to DNA and regulates the activity of androgen-responsive
genes. By turning the genes on or off as necessary, the androgen receptor helps
direct the development of male sexual characteristics. Androgens and androgen
receptors also have other important functions in both males and females, such
as regulating hair growth and sex drive.
In one region of the AR gene, a DNA segment known as
CAG is repeated multiple times. This CAG segment is called a triplet or
trinucleotide repeat. In most people, the number of CAG repeats in the AR gene
ranges from fewer than 10 to about 37.
The AR Gene and Spinal Bulbar
Muscular Atrophy
Kennedy’s Disease, aka, Spinal Bulbar Muscular Atrophy,
results from a particular type of mutation, an expansion of the CAG
trinucleotide repeat, in the AR gene. This receptor attaches (binds) to a class
of hormones called androgens.
With Kennedy’s Disease, the CAG is abnormally repeated
from 38 to more than 60 times causing a disorder of the specialized nerve cells
that control muscle movement. Researchers believe that a fragment of the
androgen receptor protein containing the CAG repeats accumulates within these
cells and interferes with normal cell functions. This buildup leads to the
gradual loss of motor neurons, which results in muscle weakness and wasting
(atrophy).
People with a higher number of CAG repeats tend to
develop signs and symptoms of Kennedy’s Disease at an earlier age. [ FrequentlyAsked Questions about KD ]
Characteristics of Spinal Bulbar
Muscular Atrophy
Spinal 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). [ CommonSymptoms ]
Inheritance Pattern
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). [ GeneticChart ]
DNA Test
Fortunately, today there is a DNA test to determine if
a person has Kennedy’s Disease. Your doctor can draw some blood and send it to
a DNA lab for testing. Test results are normally returned within three-to-six
weeks. [ TestDetails ]
Note: Most information
provided by the Genetics
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