Spinal Muscular Atrophy (SMA) Carrier Screening
What is SMA?
Spinal Muscular Atrophy (SMA) is a disease that causes abnormally functioning motor neurons, which control voluntary movements such as walking, talking, and swallowing. SMA leads to progressive muscle weakness and atrophy, especially in the muscles of the torso, upper legs, and upper arms. Symptoms can begin prior to six months of age, in childhood, and, more rarely, in adulthood. In the most common form of the disease, lifespan is often less than two years of age.
SMA is caused by changes in the survival motor neuron 1, or SMN1, gene. Approximately 95-98% of affected individuals have deletions in the SMA1 gene and 2-5% have specific mutations in the SMA1 gene that result in a decreased production of the SMN protein. When three or more copies of the SMA2 gene are also present, the disease may be milder.1
How is SMA Inherited?
SMA is inherited in an autosomal recessive manner. In most cases, both parents must be carriers of the condition in order to have a child with SMA. Carriers are not themselves affected by the disease, but have a chance of having a child with the disease. If one parent is found to be a carrier, screening is recommended for their partner. If the partner screens negative, then the couple’s risk of having an affected child is reduced (but not eliminated). If both parents are carriers of SMA, there is a 25% chance in each pregnancy of having a child with SMA, a 50% chance of having a child who is an unaffected carrier of the condition, and a 25% chance of having a child who is not a carrier and is unaffected.
Reporting of SMA Carrier Screen Results: