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Spinal muscular atrophy
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Spinal muscular atrophy is not a disease of the muscle but of the neuron that commands it. It covers several inherited syndromes in which cells degenerate in the anterior horn of the spinal cord, the area where the alpha motor neurons live, the neurons that tell the muscle to contract. As they are destroyed, weakness and atrophy appear in the proximal muscles, the ones closest to the trunk.
The gene that fails is called SMN1, and in 95% of cases it is simply missing. There is a second gene, SMN2, that could stand in for it, but it produces functional messenger RNA only 10% to 15% of the time, so it yields very little SMN protein. The backup copy is there, but it almost always prints badly. Whether the alpha motor neuron survives depends on that scant protein.
This explains something that seems odd but is not: the number of SMN2 copies a person carries correlates with severity. The range runs from death in the first weeks of life to weakness that shows up only in adulthood.
In brief
Spinal muscular atrophy is a group of inherited syndromes in which alpha motor neurons of the anterior horn degenerate, causing proximal weakness and atrophy.
In 95% of cases SMN1 is missing; SMN2 remains, but it yields functional messenger RNA only 10% to 15% of the time, and alpha motor neuron survival depends on that scant SMN protein.
The backup copy is there, but it almost always prints badly.
That is why SMN2 copy number correlates with severity: from death in the first weeks of life to weakness in adulthood.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingSpinal Muscle Atrophy, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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