SMA is caused by a mutation in the survival motor neuron gene 1 (SMN1).
In a healthy person, this gene produces a protein—called survival motor neuron protein or SMN protein—that is critical to the function of the nerves that control our muscles. Without it, those nerve cells cannot properly function and eventually die, leading to debilitating and often fatal muscle weakness.
SMA may be referred to as a motor neuron disease or a neuromuscular disease.
The SMN1 Mutation
Most people have two copies of the SMN1 gene. Individuals who have one faulty copy and one functioning copy are called carriers. Carriers do not have SMA, but they may pass the faulty gene on to their children.
SMA is an autosomal recessive genetic disorder. This means that, generally, both parents must pass on the mutation for the child to have SMA.
When two carriers have a child, there is a 25% chance that the child will be unaffected, a 50 % chance that the child will also be a carrier, and a 25% chance that the child will have SMA.
The SMN2 Gene
A second gene also has a role in producing SMN protein. This is the survival motor neuron gene 2 (SMN2), often called the SMA “back-up gene.”
Most of the SMN protein produced by SMN2 lacks a key building block that is normally produced by SMN1. This means that SMN2 cannot fully make up for the mutated SMN1 gene.
However, the number of SMN2 genes can vary from person to person, and individuals with more SMN2 copies typically have a less severe form of SMA than those with fewer copies.
One potential treatment for SMA is to cause SMN2 to produce more protein. Another is to change how the protein is produced, so that SMN2-produced protein contains all the key building blocks needed.