Spinal muscular atrophy (SMA) is a rare genetic disease in which nerves are lost causing weakness of the muscles. This occurs because special nerve cells in the body that control muscle movement, called motor neurons, deteriorate and stop working
Brain
In people with SMA, signals for muscle movement are still generated
Motor neuron
Individuals with SMA have a genetic fault that causes their motor neurons in the spinal cord to stop working, deteriorate, and die. This means that signals from the brain are not carried to the muscle
Muscle
If the muscles no longer receive signals telling them to move, they become increasingly weaker (atrophied)
A protein called survival motor neuron (SMN) protein is vital for motor neurons to be able to work properly
Without SMN protein, motor neurons in the spinal cord stop working and muscles become weaker
SMN protein is made in the body from the SMN gene
In SMA the main SMN gene known as SMN1 is faulty or missing
There is a second SMN gene known as SMN2 that acts more like a ‘back-up’ and only produces small amounts of SMN protein
In SMA, not enough SMN protein is being produced and the motor neurons stop working causing the muscles to become weaker
SMA is typically inherited, although occasionally it is caused by a random error in the SMN1 gene
Children inherit two copies of the SMN1 gene, one copy from each parent
People with one healthy and one faulty SMN1 gene are known as carriers, they normally show no signs of SMA
If both parents are carriers there is effectively a 25% chance the baby will inherit and develop SMA
SMA affects approximately 1 in 10,000 – 12,000 live births and can impact any race or sex
Around 1 in 50 people are carriers of the disease mutation
References
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NIH (2018a). Genetics home reference. Your guide to understanding genetic conditions. SMN1 gene. Available at: https://ghr.nlm.nih.gov/gene/SMN1. Date accessed: July 2023.
NIH (2018b). Genetics home reference. Your guide to understanding genetic conditions. SMN2 gene. Available at: https://ghr.nlm.nih.gov/gene/SMN2. Date accessed: July 2023.
NIH (2020b). Genetics home reference. Your guide to understanding genetic conditions. What is a gene? Available at: https://medlineplus.gov/genetics/understanding/basics/gene/. Date accessed: July 2023.
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