August is SMA awareness month. Given how relatively common SMA is compared to other genetic diseases, such as Cystic Fibrosis, there is not nearly enough awareness among both healthcare providers and the general public. We had never heard of SMA until Sophie was diagnosed, yet since then, we’ve seen just how widespread this disease is. In honor of SMA awareness month, we want to briefly explain the genetics behind SMA. Sophie has been diagnosed with SMA Type 1 (the most severe form), but keep in mind that there are 4 types of SMA (1-4).
- Roughly 1 out of every 40 people are unknowingly carriers of this genetic disease.
- SMA is an autosomal recessive genetic disease which is caused by a deletion or mutation in the Survival Motor Neuron 1 (SMN1) gene.
- Most people have two copies of the SMN1 gene, one inherited from their mother and the other from their father. However, carriers of SMA have one normal copy of SMN1 and one mutated, or defective, copy. Having at least one normal SMN1 copy will allow a person to produce more than enough SMN protein to prevent any symptoms of SMA, so carriers do not show any symptoms of the disease, and generally are unaware that they are carriers. Both of us (Liz and Mike) were tested and confirmed to be carriers of SMA.
- SMA usually occurs in children of couples who are both carriers of SMA (95% of the time), each parent having one normal and one defective SMN1 copy as described above. Because carriers of SMA have an equal chance of passing on a normal or defective copy of SMN1 as parents, there are three possible combinations of parental SMN1 copies that determine if the offspring will be affected by SMA. Children of two SMA carriers have a:
- 25% chance of inheriting two normal copies of SMN1. These children will not have SMA and will not be carriers of SMA.
- 50% chance of inheriting one normal and one defective copy of SMN1. These children will not have SMA but will be carriers of SMA (like their parents).
- 25% chance of inheriting 2 defective copies of SMN1. These children will have SMA.
- Sophie obviously fell into the 25% chance of inheriting 2 defective copies of SMN1, (and was therefore affected with SMA), and any future children we might have will also have a 25% change of being affected (%’s are the same for all children).
- We recently met with a genetics counselor to go over our options for future children. The genetics counselor said something interesting: most people are unknowingly carriers of ~6-7 genetic defects (such as SMA)...it just so happens that both of us have the same genetic defect, which is why Sophie is affected. Other autosomal recessive conditions that you may have heard of include Cystic Fibrosis and Tay Sachs.
- Given that ~1 in 40 people are carriers, ~1/6000 babies is affected with SMA
- Anyone can be tested (via a simple blood test) to see if they are a carrier for SMA. This is something we will be big advocates for as we did not know that we are carriers. If people have the knowledge of their ‘carrier status‘ prior to having children, it gives them the opportunity to talk about family planning with their partner. Cystic Fibrosis carrier status is routinely checked with expecting mothers, but SMA is not (at least not in many OB offices yet). Our hope is that one day, SMA carrier status will fall under the routine category.
SMA research is largely underfunded. The medical community anticipates that a cure/treatment is very conceivable in the short-medium term future. We plan to get involved with both awareness and fundraising efforts in the future, however our first step is to spread the word about just how common and devastating this disease is.
If you are interested in getting tested to see if you are a carrier for SMA, please speak with your doctor and/or a genetic counselor to see where you can get tested in your area!
For more information on the gentetics of SMA, the following link is available: http://www.fsma.org/UploadedFiles/ForMedia/Materials/fsmageneticsbrochure111909.pdf
