Spinal and Bulbar Muscular Atrophy (SBMA)
Description
Spinal and Bulbar Muscular Atrophy (SBMA), also known as Kennedy‘s disease, is a rare, inherited neurological disorder that primarily affects motor neurons. These neurons control muscle movement, and in SBMA, their degeneration leads to progressive muscle weakness and atrophy, particularly in the arms, legs, and face. This blog delves into the intricacies of SBMA, providing information on its causes, symptoms, diagnosis, management, and strategies for thriving with this condition.
Genes Involved
SBMA is caused by a mutation in the androgen receptor (AR) gene located on the X chromosome. This gene is responsible for producing the androgen receptor protein, which plays a crucial role in the development and function of male sex characteristics. The specific mutation responsible for SBMA is a trinucleotide repeat expansion in the AR gene, causing an excess of the glutamine amino acid within the protein. This leads to the accumulation of the mutated protein within motor neurons, ultimately leading to their dysfunction and death.
Recognizing the Signs and Symptoms
The onset of SBMA symptoms typically occurs in adulthood, often between the ages of 30 and 60. The most common signs and symptoms include:
- Muscle weakness: This often begins in the arms and legs, progressing gradually over time. It can affect the ability to perform daily activities like walking, climbing stairs, and gripping objects.
- Muscle atrophy: Muscles affected by SBMA begin to waste away, leading to a noticeable decrease in muscle mass.
- Bulbar symptoms: These involve the muscles of the mouth, throat, and tongue, resulting in difficulties with speaking, swallowing, and chewing.
- Facial weakness: The muscles in the face may become weak, leading to a droopy appearance around the mouth and eyes.
- Other possible symptoms: These may include fatigue, cramps, tremors, and difficulty breathing. The severity and progression of SBMA vary widely between individuals.
Causes
SBMA is an X-linked recessive disorder, meaning that the mutated gene responsible for the condition is located on the X chromosome. Females have two X chromosomes, while males have one X and one Y chromosome. In females, a mutation in one X chromosome can be compensated for by the normal gene on the other X chromosome. However, males only have one X chromosome, so a mutation in the AR gene will lead to SBMA. The inheritance pattern suggests that:
- Affected fathers: Will not pass the mutated gene to their sons, but all of their daughters will be carriers.
- Carrier mothers: Have a 50% chance of passing the mutated gene to each of their children, regardless of gender.
- Affected mothers: Will pass the mutated gene to all of their sons, who will develop SBMA.
Inheritance/recurrence risk
SBMA is an X-linked recessive disorder, meaning that the mutated gene responsible for the condition is located on the X chromosome. Females have two X chromosomes, while males have one X and one Y chromosome. In females, a mutation in one X chromosome can be compensated for by the normal gene on the other X chromosome. However, males only have one X chromosome, so a mutation in the AR gene will lead to SBMA. The inheritance pattern suggests that:
- Affected fathers: Will not pass the mutated gene to their sons, but all of their daughters will be carriers.
- Carrier mothers: Have a 50% chance of passing the mutated gene to each of their children, regardless of gender.
- Affected mothers: Will pass the mutated gene to all of their sons, who will develop SBMA.
