Kennedy's disease, also known as X-linked Spinal Bulbar Muscular Atrophy, is an inherited disorder which causes slowly progressive weakness and wasting of muscles. The symptoms of Kennedy’s disease typically present only in males, however there has been rare cases of females displaying symptoms. It was first described in 1968 by Dr William R Kennedy and his co-workers. The genetic mutation which causes the condition was identified in 1991 by La Spada and others. Although it is a disorder of motor neurones, it is not motor neurone disease, as it is usually more benign.
What are the symptoms?
Symptoms generally begin to be noticed at about 30 or 50 years of age, although cases from the ages of 15 up to 59 have been described.
Common symptoms include:
- muscle twitching (fasciculation)
- muscle weakness and wasting
- fasciculation of the face
- hand tremors
- twitching and wasting of the tongue
- problems with speech and swallowing
The majority of males also show some breast enlargement. In some cases, reduced fertility has also been reported.
Is it hereditary?
Kennedy's disease is inherited, and is passed on by an X-linked genetic mutation (see overleaf for an explanation of how Kennedy's disease is passed on).
What causes it?
Kennedy's disease is caused by a mutation of the Androgen Receptor (AR) gene, the role of which is to moderate the action of the male sex hormones (androgens). How this mutation causes the symptoms seen in Kennedy's disease is not yet understood.
How is it diagnosed?
Kennedy's disease can be diagnosed by genetic testing using a sample of whole blood. The test is rapid and accurate and could also be used to detect pre-symptomatic individuals (i.e., those who have the mutated gene but do not show any signs of the disease), or carriers, and for prenatal diagnosis. Testing is done only after appropriate genetic counselling has been undertaken.
Is there any treatment?
There is as yet no treatment to cure Kennedy's disease or to stop its progression. The severity and course of the disease cannot be predicted, but in most cases, progression is extremely slow, and life expectancy is normal. Support in coping with the effects of the disease is available through the MND Associations in each state as well as other organisations. Research into the cause and cure of the disease is being undertaken, which gives hope for the future.
Kennedy's disease - pattern of inheritance
When a baby is conceived, each parent passes on one copy of each of their genes to the baby. Therefore the baby is a ‘mixture’ of the genetic information of each of his/her parents. The sex of a child is determined by inheritance from their parents of X and Y chromosomes. Males have an X chromosome from their mother and a Y chromosome from their father. Females have two X chromosomes, one from each parent. To be male, a child must inherit a Y chromosome from his father, (and therefore, in the case of Kennedy's disease, does not inherit the mutated X). The sons of men with Kennedy's disease do not get the disease, and do not pass on the mutation.
The daughters of men with Kennedy's disease are called obligatory carriers. Females have two X-chromosomes, one from their father, and one from their mother. Since males have only one X chromosome to pass on, their daughters always receive the X chromosome carrying the genetic mutation.
The sons of female carriers of the genetic mutation have a 50 per cent chance of receiving the mutated gene and developing Kennedy's disease. The daughters of Kennedy's disease female carriers have a 50 per cent chance of inheriting the mutated gene, but because they also have a normal X chromosome, they do not usually present with symptoms of the disease. It is also important to note there is a 50 per cent chance that their female or male children will not inherit the mutant gene from their mother and can therefore not transmit it to their children (See Figure 2). These risks and possibilities apply to every pregnancy.
Figure 1 - Transmission by male with Kennedy's disease
Figure 2 - Transmission by Kennedy's disease carrier daughter