What causes spinal muscular atrophy?

Spinal muscular atrophy (SMA) is primarily caused by degeneration or mutations in the SMN1 gene (Survival Motor Neuron 1 gene). This gene plays a crucial role in the production of a protein that is essential for the survival of motor neurons, which are nerve cells in the spinal cord responsible for controlling voluntary muscle movements. When there is a mutation or deletion in the SMN1 gene, insufficient amounts of this protein are produced, leading to the progressive degeneration of motor neurons. As these neurons deteriorate, it results in muscle weakness and atrophy, characteristic of SMA.

The other options represent different genetic conditions or mechanisms that do not cause spinal muscular atrophy. For example, mutations in the HEXA gene are associated with Tay-Sachs disease, while abnormal hemoglobin structure relates to conditions like sickle cell disease. Loss of myelin in peripheral nerves is related to demyelinating diseases such as multiple sclerosis, neither of which involve the SMN1 gene or result in spinal muscular atrophy. Focusing on the critical role of the SMN1 gene clarifies why its degeneration is the direct cause of SMA.