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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:

0Affected with SMA
1Carrier for SMA
2Reduced Carrier Risk
3Significantly Reduced Carrier Risk

Is there still a risk to be a carrier if a patient has 2 copies of SMN1?

Yes. Although testing for SMA is very accurate, there are limitations. Most individuals with two SMN1 copies have one copy of the gene on each of their chromosomes. However, some individuals will have two copies on the same chromosome and zero copies on the other chromosome. Since it is possible for these individuals to pass the chromosome with zero SMN1 copies to their children, these individuals are carriers of SMA. This test will not be able to identify these carriers. In addition, this test does not detect other disease-causing mutations in the SMN1 gene. This is why the risk of being a carrier in individuals with two or three copies of the SMN1 gene is reduced but not eliminated.

AncestryCarrier RiskDetection RateCarrier Rate if Negative (2 Copies)Carrier Rate if Negative (3 Copies)
Ashkenazi Jewish1/4190%1/3501/4,000
African American 1/6671%1/1211/3,000

Who Should Be Offered SMA Testing?

Since SMA is a common and severe genetic condition, ACMG and ACOG recommend that SMA carrier testing be offered to all couples prior to conception or early in pregnancy. However, testing is voluntary.

Is There Prenatal Testing For Spinal Muscular Atrophy?

Yes. Prenatal testing by chorionic villi sampling or amniocentesis can be performed to determine whether or not a fetus is affected with SMA.

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