Dr. Charlotte Sumner explains the cause of SMA and the two genes, SMN1 and SMN2.

The genetic cause of this disease is a mutation in a gene called SMN and SMN stands for survival motor neuron and in particular, it's caused by a mutation in the SMN1 gene. But all patients with SMA retain a second version of the gene, what we call SMN2. Unfortunately, SMN2 can't compensate fully for the loss of SMN1. It makes less of the SMN protein that's required in motor neurons and as a result because of this reduction in the expression of the SMN protein, motor neurons degenerate in this disease. So that's the underlying cause of SMA, that is a loss or mutation of the SMN1 gene, a retention of this SMN2 gene that makes inadequate amounts of the SMN protein. And because of that reduction in SMN protein, SMA results in the loss of motor neurons and severe, severe muscle weakness.

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