What Does It Mean to Be a Silent Carrier for SMA?

Spinal muscular atrophy (SMA) is a rare genetic disorder passed from parents to their children. SMA affects the spinal nerves by causing them to waste away. This results in the muscles wearing out slowly over time. Some of the muscles often affected are found in the arms, legs, and the muscles we use to breathe.^1,2

SMA occurs due to a change to the SMN1 gene. People can develop SMA at different ages, but children under age 2 account for about 60 percent of cases.^1,3-5

The age you are when you develop SMA is linked to how serious your symptoms will be. Babies that are born with SMA have the most severe symptoms. Adults who develop SMA after age 21 have the mildest, if any, symptoms.^1,3-5

What does it mean to be an SMA carrier?

You can have a working copy and an altered copy of the SMN1 gene. In this case, you likely won’t have symptoms but will be a carrier for SMA. SMA is an autosomal recessive disease. Autosomal recessive means that you can only be affected if both of your parents are carriers.^2,5,6

Getting tested to see if you are a carrier

Modern tests usually make it easy to know if you are a carrier. Many of these tests check for a range of autosomal recessive diseases. They are done by collecting DNA samples from blood or saliva.^3,5,7

SMA is such a serious condition that the American College of Obstetricians and Gynecologists now recommends that everyone who is – or is thinking about becoming – pregnant be tested for SMA. If you are a carrier for SMA, there are also tests that can see if your child will be a carrier or will be born with SMA. These tests can be done on:^3,5,7

• Embryos (just after conception)
• Unborn babies
• Infants

Limits of testing

Although there are a variety of tests to detect SMA carriers, the tests still have some limits. Current tests can detect altered versions of the SMN1 gene. But you can have more than 1 copy of SMN1 on the same chromosome (package of DNA). And the tests cannot detect how many copies there are of the altered SMN1 on the same chromosome.⁸

Another limit of current tests is being able to know the shape of the gene. SMN1 genes take 2 different forms, known as cis or trans. Tests cannot tell whether the gene is the cis or trans shape.⁸

What makes someone a silent carrier

Sometimes people have more than 2 altered copies of cis-SMN1 on the same chromosome. People with this structure are known as silent carriers because the tests cannot detect that they are a carrier.^6,9

The tests are able to detect the presence of altered copies of SMN1. But they cannot detect which chromosome in a pair the altered copy is on. They also cannot tell the altered gene’s shape (cis or trans). Because of this, test results come back negative.^6,9

Unfortunately, silent carriers can still pass on the altered gene. The risk for passing on the gene is also higher because there are multiple copies on the same chromosome.^6,9

There are new testing methods being made to try to work around this problem. Studies have found that there are other rare features of the chromosome. Searching for this feature in the presence of the SMN1 gene may help find silent carriers.^6,9

If you have a family history of SMA, you can discuss testing options with your doctor. Testing can help you make decisions about family planning. There are treatment options available for people with SMA. There are also many tests that can be done before and after a baby is born to know if they might develop SMA. Knowing that you are a carrier can help you prepare.^3,5,7,10