Spinal Muscular Atrophy (SMA) is a genetic condition affecting the nerves that control voluntary muscle movement. Many people are “carriers” of the gene responsible for SMA but do not exhibit the condition themselves. Understanding carrier status is an important step in family planning and assessing the potential impact on future generations. This guide explains what it means to be an SMA carrier, how status is determined, and the available paths forward.
Defining Spinal Muscular Atrophy
Spinal Muscular Atrophy is a neuromuscular disorder characterized by the deterioration of motor neurons in the spinal cord. These nerve cells communicate movement signals from the brain to the body’s muscles. Without this input, muscles progressively weaken and waste away in a process known as atrophy. This can impact a person’s ability to perform actions such as walking, swallowing, and breathing.
The condition presents in several forms, classified by age of onset and symptom severity. The common element across all types is the underlying loss of motor neurons.
The Genetics of SMA Carrier Status
The most common cause of SMA is a mutation in the Survival Motor Neuron 1 (SMN1) gene. This gene provides instructions for creating the SMN protein, which maintains motor neurons. A person is an SMA carrier if they have one functional copy and one mutated or missing copy of the SMN1 gene. Carriers do not experience symptoms because their single working copy produces enough SMN protein for healthy motor neuron function.
Many carriers are unaware of their status until they undergo genetic testing or have a child diagnosed with SMA. An estimated 1 in 40 to 1 in 60 people is a carrier, though frequencies can vary among ethnic backgrounds. Because carriers are healthy, the mutated gene can be passed silently through generations.
Inheritance Risks for Children
SMA is inherited in an autosomal recessive pattern, meaning a child must inherit two mutated SMN1 genes—one from each parent—to have the disorder. When both parents are carriers, the risks for each pregnancy are as follows. There is a 25% chance the child will inherit two mutated genes and be affected by SMA. There is a 50% chance the child will inherit one mutated gene and one functional gene, making them a carrier like their parents. Finally, there is a 25% chance the child will inherit two functional genes, meaning they will be unaffected and not a carrier.
If only one parent is a carrier and the other has two functional SMN1 genes, their child cannot have SMA. The child will, however, have a 50% chance of inheriting the one mutated gene and becoming a carrier.
Identifying SMA Carriers Through Screening
SMA carrier status is determined through a genetic test. This screening is performed on a blood or saliva sample, which is analyzed to count the number of functional SMN1 genes. The test accurately identifies the majority of carriers, though detection rates can vary by ethnicity.
Carrier screening for SMA is recommended for all individuals who are pregnant or planning a pregnancy. It is also suggested for those with a family history of SMA. The test determines if a person has one or two functional copies of the SMN1 gene. A result showing one copy indicates carrier status, and while the screening is highly effective, a negative result greatly lowers the risk but cannot eliminate the possibility of being a carrier.
Navigating a Carrier Diagnosis
A positive result from an SMA carrier screen has implications for family planning. The first step is often a consultation with a genetic counselor. These professionals help couples understand the results, discuss risks for future children, and explore the available reproductive options.
For couples where both partners are carriers, several paths can be considered.
- In vitro fertilization (IVF) with preimplantation genetic testing (PGT) to screen embryos.
- Prenatal diagnosis during pregnancy, such as chorionic villus sampling (CVS) or amniocentesis.
- Using donor sperm, eggs, or embryos from non-carriers.
- Pursuing adoption.
- Conceiving without genetic intervention and preparing for all outcomes.
Support groups and advocacy organizations offer resources and community for families navigating these decisions.