Parents of children with spinal muscular atrophy (SMA) must learn a lot very quickly. Genetics is one of the more complex topics you may have had to study up on to better understand your child’s condition. SMA is an inherited disease caused by a mutation in the two survival motor neuron genes, SMN1 and SMN2.
SMN genes make SMN proteins, which are necessary for motor neurons to work. Their job is to send signals from the brain to the spinal cord and muscles, telling them to move. When motor neurons do not have enough SMN protein, they die, causing muscle weakness and degeneration (wasting away).
Read more about the SMN1 and SMN2 genes and their roles in the type, severity of symptoms, and treatment of SMA.
In people with SMA, differences in how many copies they have of each SMN gene and what type of mutation has occurred determine:
Both SMN1 and SMN2 genes are responsible for making SMN proteins to help motor neurons work. Gene expression — the way information stored in a gene turns into a function — is also important. In a person without SMA, the SMN1 gene makes 85 percent to 90 percent of the protein needed for muscles to work. The SMN2 gene makes only around 10 percent to 15 percent of the protein.
The SMN2 gene makes many types of SMN proteins. However, just one version (isoform D) is the full-length, functional protein. The other versions are smaller and broken down by the nerve cells quickly. Isoform D is very similar to the protein made by the SMN1 gene.
Generally, people without SMA have two copies of the SMN1 gene and one or two copies of the SMN2 gene. However, some people may have no copies of the SMN2 gene, and others have up to eight copies.
SMA develops when there is a mutation in both copies of the SMN1 gene. Most people with SMA (95 percent to 98 percent) have deletions in the SMN1 gene. This means that a piece of the gene is missing entirely. When a deletion occurs, the SMN1 gene cannot make any SMN protein. The remaining 2 percent to 5 percent of people with SMA have smaller, more specific mutations that result in less SMN protein being made.
The SMA type and disease severity depend mainly on the number of copies of SMN2 a person has. The more copies a person has, the more SMN protein they can make. The age of onset (when symptoms start to show up) is also influenced by the SMN2 copy number.
Genetic testing can be done to find out if a person carries an SMN1 mutation. These mutations are recessive, meaning that a person must inherit two mutated copies (one from each parent) to develop SMA. According to the American College of Obstetricians and Gynecologists, between 1 in 40 and 1 in 60 people are carriers of SMA. If both parents have one mutated copy of the SMN1 gene, there is a 25 percent chance they will have a child with SMA.
The descriptions of developmental milestones and age of onset are based on historical averages of people with those types of SMA. These predictions were developed before SMA treatments became available. Long-term studies to predict differences in abilities in children who receive early treatment are not yet available.
SMA type 0 is noticeable in fetuses that do not move much during pregnancy. After birth, the disease is associated with severe muscle weakness, occasionally congenital heart defects, and the inability to move the muscles in the face. Typically, infants with type 0 SMA have only one copy of the SMN2 gene, which cannot make enough protein for the nerve cells to function properly.
SMA type 1 — also known as Werdnig-Hoffmann disease — is diagnosed when SMA symptoms are noticeable between birth and 6 months of age. Generally, babies have muscle weakness, difficulty breathing, and weak cries. They also cannot swallow or suck well and have difficulty eating. Babies with type 1 SMA have only one or two copies of the SMN2 gene.
SMA type 2 — also known as intermediate SMA, or Dubowitz disease — develops in babies between 3 and 15 months of age. Typically, they aren’t able to stand or walk on their own because the muscles in their legs and arms are weak. Babies with type 2 SMA usually have three copies of the SMN2 gene.
SMA type 3 — also known as Kugelberg-Welander disease — accounts for 30 percent of SMA cases. It typically develops in children (juvenile onset). However, the symptoms can be noticed anywhere from 18 months into adulthood. People with type 3 SMA can become independent and move well on their own, but they may still have some muscle weakness that makes some activities difficult. These individuals usually have three or four copies of the SMN2 gene.
SMA type 4 develops later in life — typically at age 30 and older — and makes up only 5 percent of SMA cases, according to the Muscular Dystrophy Association. This is a mild form of SMA because individuals have anywhere from four to eight copies of the SMN2 gene. The extra copies of the SMN2 gene can compensate for the mutations in SMN1, letting people with type 4 SMA maintain their mobility.
Although there is currently no cure for SMA, gene therapies have been approved for treating the disease. Nusinersen (Spinraza) and onasemnogene abeparvovec-xioi (Zolgensma) are two treatments approved by the U.S. Food and Drug Administration for treating SMA.
Both of these medications are gene therapies given to modify the function of the SMN genes. Nusinersen is given by injection into the spinal canal, and it is thought to increase the functional SMN protein levels made by the SMN2 gene. It’s given to infants with SMA and children between the ages of 2 and 12. Onasemnogene abeparvovec-xioi is a one-time injection for infants under the age of 2. It’s thought that it acts by replacing the SMN1 gene with an SMN gene that is effective, helping make more functional SMN protein.
In addition, ongoing clinical trials are looking at new treatments for SMA in hopes of improving the quality of life for those living with the disease.
Read more about how gene therapy works.
On mySMAteam, the social network for people with spinal muscular atrophy and their loved ones, more than 2,200 members come together to ask questions, give advice, and share their stories with others who understand life with SMA.
Do you have questions about SMN1 and SMN2 genes? Do you have a child with SMA who is receiving gene therapy? Share your thoughts in the comments below, or start a conversation by posting on mySMAteam.