As stated by the U.S. Library of Medicine, Spinal Muscular Atrophy (SMA) is an autosomal recessive genetic disease that affects the motor neurons, neurons that are required for movement, in the brain stem and the spinal cord. The loss of the motor neurons means that individuals affected by this disease will loss most motor functions including the ability to stand, sit, and control the movements of their head. There are four different types of SMA, and they are categorized by the time of their onset, but the same different genetic mutation is thought to be responsible for all four types of SMA.
Recently, SMA has been brought to the attention of a large number of people because of five month old Avery Canahuati’s blog written by her parents, Linda and Michael Canahuati. Though Michael and Linda write the blog posts, it is written in Avery’s voice. In all of her posts, Avery encourages the readers to follow her story and ask their doctors about SMA genetic testing.
According to an article in Gene Reviews , the genetic test for SMA looks at levels of survival motor neuron 1 (SMN1) which is one the genes that is most commonly mutated or deleted in SMA. The average person has two copies of the SMN1 gene, while carriers of SMA typically have one copy of the gene. Patients with SMA usually have no copies of SMN1. The genetic test determines how many copies of the gene the perspective parents have to understand if the parent is a carrier for the disease or not.
In most cases, the genetic test can determine if a parent is carrier for SMA, but this test is not always the correct. Sometimes a parent may have two copies of the SMN1 gene, but the copies could be on the same chromosome which means a child could inherit the chromosome that does not have the SMN1 gene. Other carriers may not be detected by the test because they have a mutation in one of their two copies of the SMN1 gene which prevents the SMN1 protein from being produced. Even if only one parent is a carrier, the child can still have SMA if a random mutation occurred in the production of gametes from one of the parents.
On another website, Andy’s Angels , run by parents of child who passed away from SMA, the parents were both tested for SMA before having another child. In this case, the mother’s test revealed that she was a carrier while the father’s test revealed that he was not a carry. After further analysis, it was determined that the test results were incorrect, and the father was actually a carrier.
Though the genetic tests for carriers of SMA do not always come back correctly, it is important that any prospective parents get tested to see if they are carriers for SMA. The results of this test can help couples make decisions about how they will conceive their children, either naturally or through in-vitro fertilization.
