The SMN2 Gene
Another gene also has a role in producing SMN protein. This is the survival motor neuron 2 (SMN2) gene, sometimes referred to as the SMA “back-up gene”. However, most of the SMN protein produced by SMN2 lacks a key building block that is usually produced by SMN1. This means that while SMN2 can make some functional SMN protein, it cannot fully make up for the faulty SMN1 gene in people with SMA. The number of copies of SMN2 can vary from one person to the next and can range from zero to eight. Having more SMN2 copies is generally associated with less severe SMA symptoms. Table 2 shows the generally expected number of SMN2 copies associated with each type of SMA. However, accurate predictions cannot be made about the severity of SMA based on the SMN2 copy number alone. This is likely to be due other genetic factors that affect how much SMN protein is produced by the SMN2 gene(s).
The SMN1 gene is on the fifth chromosome in the region labelled as ‘q’ which is why the main types of SMA are often referred to as 5q SMA.