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. About 95-98% of individuals with SMA have deletion of exon 8 in both copies of the SMN1 gene, and, as a result have no working copies of the SMN1 gene. About 2-5% of individuals with SMA have a missing portion in one copy of the SMN1 gene and a less common genetic change in the other gene, also resulting in no functioning copies of the SMN1 gene.
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 their risk to have 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: