UNDERSTANDING SPINAL MUSCULAR ATROPHY (SMA)

BOOK A DOCTOR'S VISIT TODAY

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

Around 1 in 50 people are carriers of the disease mutation

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

References

References

Anderton RS, Mastaglia FL. Expert Rev Neurother. 2015;15(8):895–908.

Chabanon A, et al. PLoS One. 2018;13(7)e0201004. doi: 10.1371/journal.pone.0201004.

Coovert DD, et al. Hum Mol Genet. 1997;6(8):1205–14.

Cure SMA (2023). About SMA. Available at: https://www.curesma.org/about/. Date accessed: July 2023.

Dimitriadi M, et al. Proc Natl Acad Sci U S A. 2016;113(30):E4377–86.

Kolb SJ and Kissel JT. Neurol Clin. 2015;33(4):831–46.

Kolb SJ, et al. Ann Neurol. 2017;82(6):883–91.

Glascock J, et al. J Neuromuscul Dis. 2018;5(2):145–58.

Govoni A, et al. Mol Neurobiol. 2018;55(8):6307–18.

Lefebvre S, et al. Cell. 1995;80(1):155–65.

Lin CW, et al. Pediatr Neurol. 2015;53(4):293–300.

National Institute for Health (NIH) (2020a). Genetics home reference. Your guide to understanding genetic conditions. Spinal muscular atrophy. Available at: https://ghr.nlm.nih.gov/condition/spinal-muscular-atrophy#genes. Date accessed: July 2023.

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.

NIH (2020c). Genetics home reference. Your guide to understanding genetic conditions. Inheriting genetic conditions. Available at: https://medlineplus.gov/genetics/understanding/inheritance/. Date accessed: July 2023.

Prior TW, et al. Spinal Muscular Atrophy. 2000 Feb 24 [Updated 2019 Nov 14]. In: Adam MP, Ardinger HH, Pagon RA, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2020.

Verhaart IEC, et al. Orphanet J Rare Dis. 2017;12:124.

Verhaart IEC, et al. Additional file 2: Table S1. Overview carrier frequencies of SMA. Supplementary material to Verhaart IEC, et al. Orphanet J Rare Dis. 2017;12:124. Available at: https://ojrd.biomedcentral.com/articles/10.1186/s13023-017-0671-8. Date accessed: July 2023.

Zayia LC and Tadi P. Neuroanatomy, motor neuron. In StatPearls [Updated July 2022]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK554616/. Date accessed: July 2023.