Spinal Muscular Atrophy
Spinal Muscular Atrophy
Spinal muscular atrophy, or SMA, is a disease that is caused by abnormally functioning motor neurons that control voluntary movement 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.

What Causes SMA?
Chromosomes are the structures found in our cells that carry the genetic information inherited from our parents. These chromosomes contain all of the genes that provide the instructions for healthy development. Genes, like chromosomes, are inherited in pairs; one copy of each gene is inherited from the mother and one copy is inherited from the father. SMA is caused by changes in a gene called survival motor neuron 1, or SMN1. About 95-98% of individuals with SMA have a missing portion 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 is no functioning copies of the SMN1 gene.

How is SMA inherited?

Autosomal Recessive

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 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.

How is SMA detected?
Testing for SMA involves detecting the number of copies of the SMN1 gene in an individual. An individual with zero copies of the SMN1 gene has no functioning copies of the gene and is likely affected with SMA. An individual who is found to have one copy of the SMN1 gene is considered to be a carrier of SMA. If an individual is found to have two copies of the SMN1 gene, the chance that he or she is a carrier of SMA is reduced. In some cases, individuals are found to have three copies of the SMN1 gene. In these cases, the risk of being a carrier of SMA is significantly reduced.

Copies Results
0 SMA
1 Carrier
2 Reduced Carrier Risk
3 Significantly Reduced Carrier Risk

Why should you consider carrier testing for SMA?
Spinal muscular atrophy is a common genetic condition and can occur in individuals from different ethnic backgrounds. If there is no one in your family with SMA, your risk for being a carrier of SMA is shown in the following table along with the detection rates of the SMA carrier test1.

If someone in your family has SMA or is a known carrier of the condition then your chance of being a carrier increases depending on how closely you are related to that person, regardless of your ethnic background. Your specific risk can be determined by a doctor or a genetic counselor.

 

Ancestry Carrier Risk Detection Rate Carrier Rate if Negative (2 copies) Carrier Rate if Negative (3 copies)
Caucasian 1/35 95% 1/632 1/3,500
Ashkenazi Jewish 1/41 90% 1/350 1/4,000
Asian 1/53 93% 1/628 1/5,000
African American 1/66 71% 1/121 1/3,000
Hispanic 1/117 91% 1/1,061 1/11,000

What if the test results show you are a carrier of SMA?
If your test results show that you have one copy of the SMN1 gene, you are a carrier of SMA.

If you are found to be a carrier for SMA and your partner undergoes testing and is found to be negative for the disorder (with no family history), then your risk of having a child with SMA is reduced.

If both you and your partner are found to be carriers of SMA, there is a 1 in 4 (25%) chance of having a child with SMA in each pregnancy.

If either member of a couple has been tested and is found to be a carrier, the partner should be tested, as well.

The information from the carrier test may help in your family planning, as well as alert other family members to the possible need for SMA testing. You may wish to consult with your doctor or a genetic counselor for further discussion of what the results mean for you and your family.

 

Is there still a risk if you are not found to be a carrier of SMA?
Yes. Although testing for SMA is very accurate, there are limitations. Most individuals with two SMN1 copies have one copy of this gene on each of their chromosomes. However, about 4% of individuals with two copies of the SMN1 gene will have the two copies on one 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; thistest 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.

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. The accuracy of the testing is greater when both parents have been tested for SMA. The accuracy also depends on knowing that the man tested is in fact the biological father.

Is it required that I have a test for spinal muscular atrophy?
Since SMA is a common and severe genetic condition, the American College of Medical Genetics recommends that SMA carrier testing should be offered to all couples prior to conception or early in pregnancy 2. However, testing is completely voluntary and it is your decision whether or not to undergo SMA carrier testing.

Will insurance cover the cost of the sma carrier test?
Health plans and insurance coverage may vary depending on policy, plan or state regulations. It is best to ask your plan representative regarding test coverage prior to having the carrier test. Patients without insurance coverage for this test are encouraged to ask for details about our discount program.