Dr. Charlotte Sumner explains how the number of copies of the SMN2 gene may influence disease severity.

Every patient with the SMA retains at least one copy of SMN2. In fact, the copy number of SMN2 varies in SMA patients. Some patients have one copy, others 2, others 3 or 4 most often, and rarely there are individuals who have been described who have 5 copies of SM2. So it's quite astounding how clinically variable this disorder can be and we know in fact that that SMN2 copy number does correlate, generally speaking, with disease severity. So those individuals who have the most severe forms of the disease overall tend to have one or two copies of SMN2 whereas those individuals with mild forms of disease have three or four copies of the gene. And even some individuals have been described who do not have an SMN1 gene, it's mutated but have five copies of SMN2 and don't seem to develop SMA. So you can see here this theme that there is a very important relationship between SMN2 copy number and disease severity. I caution that this is not absolute. In a given patient, we cannot predict based on the SMN2 copy number how severe their disease will be. There are some individuals who have a high copy of SMN2 and yet still have a very severe disease course and others who have a very mild disease course and yet have a low SMN2 copy number. So on an individual basis, it's not very predictive of disease severity and that's important for individuals to understand when they receive this information about SMN2 copy number. However, in a general sense when we look at large numbers of patients there is a relationship.

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