Which are the causes of Spinal Muscular Atrophy?

Spinal Muscular Atrophy (SMA) is primarily caused by a genetic mutation in the SMN1 gene, which prevents the body from producing enough Survival Motor Neuron (SMN) protein essential for the health and survival of motor neurons. Without sufficient SMN protein, these nerve cells in the spinal cord wither and die, leading to progressive muscle weakness and atrophy. While the primary genetic cause is well-understood, ongoing research continues to explore how modifying genes, such as SMN2, influence the severity of the disease.

What exactly causes Spinal Muscular Atrophy?

At the core of Spinal Muscular Atrophy is a deficiency of the SMN protein. Think of the SMN protein as a vital fuel for motor neurons—the specialized nerve cells that send signals from the spinal cord to your muscles. In individuals with Spinal Muscular Atrophy, the SMN1 gene, which acts as the primary "factory" for this protein, is either missing or mutated. When the factory is broken, the motor neurons cannot receive the instructions they need to function, causing them to degenerate. As these neurons die, the muscles they control receive fewer signals, leading to the weakness and wasting characteristic of the condition.

Is Spinal Muscular Atrophy hereditary?

Yes, Spinal Muscular Atrophy is an autosomal recessive genetic disorder. This means that for a child to be born with the condition, they must inherit one non-working copy of the SMN1 gene from both parents. Parents who carry only one mutated copy and one healthy copy are known as "carriers." Carriers typically do not show any symptoms of Spinal Muscular Atrophy because their healthy gene copy produces enough SMN protein to maintain normal function. If two carriers have a child, there is a 25% chance with each pregnancy that the child will inherit the mutation from both parents and develop the disease.

What role does the SMN2 gene play?

While SMN1 is the primary driver, the SMN2 gene also plays a critical role in Spinal Muscular Atrophy. Humans have a "backup" gene called SMN2, but it is inefficient; it produces only about 10-20% of the functional protein that SMN1 does. The number of SMN2 copies a person has acts as a biological "buffer." Generally, the more copies of SMN2 an individual has, the more functional protein they can produce, which often correlates with a milder clinical presentation of Spinal Muscular Atrophy. Current research is heavily focused on how we can "trick" these backup SMN2 genes into producing more functional protein.

Are there environmental or other risk factors?

Spinal Muscular Atrophy is strictly a genetic condition. It is not caused by environmental toxins, infections, diet, or lifestyle choices, nor is it an autoimmune or metabolic disorder. The risk of having a child with Spinal Muscular Atrophy is determined entirely by the genetic makeup of the parents. While the fundamental cause is understood, researchers are actively investigating why the severity of the disease can vary even among individuals with the same genetic mutation, looking into factors such as:

• Modifier genes that may influence disease progression.


• The timing of motor neuron cell death during development.


• Individual differences in how the body processes the limited amount of SMN protein available.

Currently, 972 people with Spinal Muscular Atrophy have joined the DiseaseMaps community, sharing their unique experiences and helping researchers gather real-world data to better understand the variability of the disease.

Next steps

• Consult a genetic counselor: If you are planning a family or have a history of the condition, a genetic counselor can provide carrier testing and discuss reproductive options.


• Seek a specialist: Connect with a neurologist or neuromuscular specialist who has specific experience in treating Spinal Muscular Atrophy.


• Join a support network: Engage with the community at DiseaseMaps.org to connect with others who understand the day-to-day realities of living with this condition.


• Stay informed on research: Follow updates from organizations like Cure SMA to learn about the latest clinical trials and therapeutic breakthroughs.

Medical disclaimer: This information is for educational purposes only and should not replace professional medical advice, diagnosis, or treatment; always seek the advice of your physician or other qualified health provider with any questions regarding a medical condition.

References

• Orphanet: Spinal muscular atrophy (ORPHA:70)


• NIH Genetic and Rare Diseases Information Center (GARD): Spinal Muscular Atrophy


• Online Mendelian Inheritance in Man (OMIM): Spinal Muscular Atrophy (Entry #253300)


• Cure SMA: Understanding the Genetics of SMA