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Spinal muscular atrophy is an inherited neuromuscular disorder that leads to progressive loss of motor neurons in the spinal cord and lower brainstem. Motor neurons are the nerve cells responsible for transmitting signals from the spinal cord to skeletal muscles. When these neurons degenerate, the muscles they control receive fewer signals, resulting in progressive muscle weakness and wasting (atrophy).
In more than 95 percent of cases, spinal muscular atrophy is caused by a deletion or mutation in both copies of the SMN1 gene (survival motor neuron 1), located on chromosome 5. SMN1 is responsible for producing most of the functional survival motor neuron protein, which is essential for motor neuron health and survival.
Humans also carry a nearly identical backup gene called SMN2. SMN2 produces some functional survival motor neuron protein, but far less than SMN1, because most of its genetic transcript is spliced incorrectly. The number of SMN2 copies a person carries (which varies between individuals) partly determines how much functional protein is still produced when SMN1 is lost, and this is a major factor influencing disease severity.
Spinal muscular atrophy follows an autosomal recessive pattern of inheritance. This means a child is affected only when both copies of the SMN1 gene, one inherited from each parent, carry a deletion or mutation. Parents who each carry one altered copy are called carriers and are typically healthy, with no symptoms themselves.
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