When a child is born with spinal muscular atrophy (SMA), a rare neuromuscular disorder, it’s the result of genes they inherited from both parents. Because SMA is driven by genetic factors, gene therapy has proved effective in treating SMA.
Gene therapy has the potential to change the lives of individuals with SMA, particularly if the treatment is given early. Read more about how gene therapy works and which gene therapies are available now for people with SMA.
SMA is an autosomal recessive disorder. This means that a child needs to inherit a mutant copy of the gene from both parents to display the condition. In the case of SMA, this mutation is almost always a deletion of the survival motor neuron (SMN) 1 gene on chromosome 5.
This SMN1 gene normally provides the body with instructions to make the messenger RNA (mRNA) for SMN protein. Your body needs mRNA to make proteins. Without the proper genetic instructions to make SMN protein, cells in the spinal cord begin to break down. This breakdown leads to the symptoms of SMA, including worsening motor function, muscle weakness, scoliosis (spine curvature), and other progressive SMA symptoms.
Another gene related to SMN protein, SMN2, also plays a role in SMA. The more copies of the SMN2 gene a person has, the less severe their symptoms will likely be.
Read more about the genetics of SMA.
Gene therapy is any technique that changes someone’s genetic code in order to treat a disease. Gene therapy can include gene replacement therapy (replacing a mutant gene with a healthy one), “turning off” a gene that isn’t working right, or introducing a new or modified gene into the body.
These genetic therapies can be delivered in several ways, including using a modified version of a virus, called viral vector technology, which is sometimes used in vaccines.
SMA gene therapy targets one of the major genes associated with the disease, notably SMN1 and SMN2. The U.S. Food and Drug Administration (FDA) has approved three SMA gene therapy methods:
Both Spinraza and Evrysdi target a process for cutting up genetic material, called gene splicing, at the SMN2 gene. Zolgensma is a replacement gene therapy that replaces an SMN1 gene that doesn’t work with a new, fully functional SMN1 gene.
The FDA approved Spinraza for use in December 2016. It can be used to treat all types of SMA. Spinraza is a piece of RNA genetic material that blocks the process that creates a dysfunctional SMA protein and allows the cells to make the functional SMN protein instead. By increasing the production of this normal SMN protein in the cell, it prevents the loss of cells in the central nervous system.
Spinraza is administered by injection into fluid surrounding the spinal cord. Injections are done by a health care professional at a hospital or clinic. After the initial four-dose treatment regimen, the drug is given every four months. The most common side effects found in the clinical trials on Spinraza were upper respiratory infection, lower respiratory infection, and constipation.
Evrysdi was approved for use in August 2020 for people with SMA types 1, 2, and 3. Similar to Spinraza, Evrysdi contains an SMN2-directed RNA splicing modifier and promotes the formation of functional SMN protein. However, this newer formulation has more widespread effects throughout the body than Spinraza.
Research in animals has shown that the levels of SMN protein in the blood may be a biomarker (a molecule that represents the effect of a disease or treatment) of Evrysdi function. This is because the drug increases blood levels of SMN protein in a manner similar to other tissues and organs in the body, including the brain, heart, and spinal cord. This means that a blood draw might be used to monitor the effectiveness of Evrysdi for each individual with SMA.
This drug is given orally at home, once per day. The most common side effects of Evrysdi include fever, diarrhea, rash, ulcers of the mouth area, arthralgia (joint pain), and urinary tract infections.
The FDA approved Zolgensma in May 2019 for use as gene therapy for all types of SMA in children under the age of 2. Zolgensma uses a viral vector and targets the SMN1 gene. This viral vector delivers a fully functional copy of the human SMN1 gene into the motor neuron cells.
Zolgenesma is a one-time intravenous treatment. This drug’s most common side effects are elevated liver enzymes and vomiting.
The future is promising for SMA gene therapy methods. Just a few years ago, there were no preventive treatments for SMA. Now, children with known genetic disease can be treated, and children with SMA are reaching developmental milestones seldom seen before treatment became available. Although gene therapy isn’t a cure, it’s a promising step in the right direction.
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