UNDERSTANDING SPINAL MUSCULAR ATROPHY (SMA)

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WHAT IS SMA?

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)

What causes SMA?

  • 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

How do you get SMA?

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

More than 1 in 58 people are carriers of the disease mutation

Carrier for spinal muscular atrophy (SMA) Carrier for spinal muscular atrophy (SMA)

References

References

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NIH (2020b). Genetics home reference. Your guide to understanding genetic conditions. What is a gene? Available at: https://ghr.nlm.nih.gov/primer/basics/gene. Date accessed: November 2022.

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