95% of patients with SMA have an abnormality in survival motor neuron 1 (SMN1) on chromosome 5q11.2-13.3; abnormalities include deletions, mutations, or rearrangements. SMN1 abnormalities reduce production of SMN protein, which results in loss of anterior horn cells in the spinal cord.
Two genes are responsible for encoding and producing SMN: SMN1 and SMN2. SMN1 is the primary producer of SMN, making 10 times the amount of SMN compared with SMN2. Whereas patients with SMA lose SMN1 production, those who have more copies of SMN2 have less severe disease, because higher SMN2 production partially compensates for lost SMN1 production.
Although most patients with SMA have carrier parents, sporadic cases occur 3% of the time.
In addition, there are cases of SMA that are linked to other genes, such as UBA1, which causes X-linked SMA that presents similar to SMA type 1.
For more on the pathophysiology of SMA, read here.
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Cite this: Stephen L. Nelson. Fast Five Quiz: Can You Diagnose the Different Types of Spinal Muscular Atrophy? - Medscape - Dec 06, 2018.