Spinal Muscular Atrophy

Overview

Spinal muscular atrophy (SMA) is an inherited neuromuscular disease, usually caused by having nonworking copies of the SMN1 gene. Too little survival motor neuron (SMN) protein causes irreversible loss of motor neurons—the nerve cells that control voluntary movement—leading to progressive weakness, muscle wasting, and, in severe untreated infantile SMA, respiratory failure and early death. Severity varies widely: some infants develop profound weakness in the first months of life, while others develop symptoms in childhood or adulthood and may remain ambulant for years. FDA: Itvisma approval

In the United States, disease-modifying care now includes the SMN2-splicing therapies nusinersen and risdiplam, intravenous SMN1 gene replacement with onasemnogene abeparvovec-xioi (Zolgensma) for eligible children younger than two years, and, since November 2025, intrathecal onasemnogene abeparvovec-brve (Itvisma) for people aged two years and older. These treatments are paired with respiratory, nutritional, orthopedic, rehabilitation, and psychosocial care. They substantially change prognosis, especially when started before symptoms, but they do not reliably restore motor neurons or muscle already lost and are not established cures. SMA treatment best practices FDA: Itvisma approval

Scope of Recent Research (2020–present)

Research since 2020 has been exceptionally active because SMA has a well-defined genetic cause, effective SMN-raising treatments provide a strong clinical foundation, and newborn screening permits treatment before major nerve injury. The dominant questions are how to deliver SMN safely and durably to the central nervous system across ages and body sizes, whether permanent correction of the backup SMN2 gene is feasible, and how to repair residual weakness after SMN therapy. The field is close to preventing much of the disease when treatment begins extremely early, but it is not yet close to a proven, universal biological cure for people with established SMA. SPR1NT trial Base-editing rescue in mice SMA treatment best practices

Major Breakthroughs and Emerging Therapies

Gene replacement has broadened considerably. Onasemnogene abeparvovec uses an adeno-associated virus type 9 (AAV9) vector to provide a working SMN gene. In the Phase III SPR1NT study, all 14 presymptomatic infants with two SMN2 copies who were treated by six weeks of age survived without permanent ventilation at 14 months, and all sat independently for at least 30 seconds by 18 months; this result illustrates the unusually large benefit of intervening before symptoms. SPR1NT trial Long-term follow-up of the original START cohort has now reported survival and retained benefit out to roughly a decade, although the cohort was very small and many participants later received nusinersen or risdiplam, limiting conclusions about gene therapy alone. LT-001 long-term follow-up

A major 2025 regulatory advance was FDA approval of Itvisma, a more concentrated intrathecal formulation of onasemnogene abeparvovec for adults and children aged two years and older with confirmed SMN1 mutations. Intrathecal delivery places vector directly into cerebrospinal fluid around the spinal cord, aiming to reach motor neurons with a lower vector dose than systemic intravenous administration and to expand one-time gene replacement beyond the youngest children. FDA: Itvisma approval This is an important expansion of disease-modifying treatment rather than proof of a cure, because existing neuron and muscle damage may persist and long-term durability across the lifespan remains under study. FDA: Itvisma approval LT-001 long-term follow-up

RNA-directed treatment continues to evolve. Risdiplam is an oral small molecule that changes SMN2 RNA splicing so that more full-length SMN protein is made. In the SUNFISH Phase III program in type 2 and nonambulant type 3 SMA, 32% of participants improved by at least three points on the Motor Function Measure after 24 months and 58% stabilized or improved, reinforcing the value of sustained systemic SMN production. SUNFISH two-year results An especially innovative proof of concept came from a single prenatal risdiplam case reported in 2025, in which fetal treatment appeared to ameliorate disease through 30 months of follow-up. That result is promising but remains a single case, not evidence that prenatal drug treatment is ready for routine care. Prenatal risdiplam case

Genome editing is the clearest potential route to a true one-time genetic correction, but it remains preclinical. Rather than adding an external SMN1 copy or repeatedly modifying RNA, researchers are using base editors—CRISPR-derived tools that can change a single DNA letter without making a double-strand DNA break—to convert the key SMN2 exon 7 variant into an SMN1-like sequence. In an SMA mouse model, dual-AAV base-editor delivery achieved high on-target editing, increased SMN production, improved motor function, and prolonged mean survival; combining editing with nusinersen extended mean survival further. Base-editing rescue in mice Separate work in patient-derived cells and mice likewise showed that high-fidelity adenine base editors can correct SMN2 while reducing detectable off-target editing. Base editing as genetic treatment These studies support the concept of permanent correction but have not yet established safe editing, delivery, redosing, or long-term follow-up in people.

Muscle-directed add-on therapies seek to address weakness that SMN restoration alone cannot reverse. Apitegromab is an antibody that blocks activation of myostatin, a protein that restrains muscle growth. In the Phase III SAPPHIRE trial in nonambulant type 2 or type 3 SMA already treated with nusinersen or risdiplam, the combined apitegromab-dose groups aged 2–12 improved by 1.8 Hammersmith Functional Motor Scale–Expanded points relative to placebo at 12 months; the 20-mg/kg dose alone did not reach statistical significance against placebo. SAPPHIRE Phase III trial This is not a genetic cure strategy, but it may become an important combination approach for improving residual motor function after SMN-targeted treatment. SAPPHIRE Phase III trial

Clinical Trials and Experimental Approaches

The Phase IIIb SMART trial tested intravenous onasemnogene abeparvovec in 24 symptomatic children weighing 8.5–21 kg, a population beyond the original low-weight clinical-trial experience. At 52 weeks, most evaluable participants maintained or improved motor-function scores, and four gained new motor milestones; however, asymptomatic liver-enzyme elevations occurred in 83% and thrombocytopenia in 71%, emphasizing that expanded-dose gene therapy requires intensive monitoring and often prolonged corticosteroids. SMART Phase IIIb trial

Novartis’s Phase III STEER trial of intrathecal OAV101/onasemnogene abeparvovec in pediatric type 2 SMA was completed, according to the ClinicalTrials.gov record last updated January 13, 2026. The registry describes a single intrathecal dose of 1.2 × 10^14 vector genomes compared with a sham procedure over a 52-week treatment period; results were not posted on that record at the time of its update. STEER trial record The related intrathecal development program provided the clinical basis for the FDA’s November 24, 2025 approval of Itvisma for patients aged two years and older. FDA: Itvisma approval

For muscle-directed therapy, Scholar Rock completed the 188-participant Phase III SAPPHIRE study in December 2024. Its 2025 publication reported statistically significant benefit for the combined apitegromab-dose analysis in younger participants on background nusinersen or risdiplam, with adverse-event rates broadly similar to placebo. SAPPHIRE Phase III trial The study is notable because it tests combination treatment rather than replacing SMN-directed therapy. SAPPHIRE trial record

Methodologies and Scientific Approaches

SMA cure research combines patient-derived fibroblasts and induced pluripotent stem cell–derived motor neurons with genetic mouse models such as Δ7 SMA mice. These systems allow investigators to measure correction of SMN2 splicing, SMN protein restoration, motor-neuron survival, nerve-muscle function, motor behavior, and survival before progressing to human trials. AAV9 and related vectors are central delivery platforms because they can reach the nervous system, while split-editor systems are being used because base editors are too large to fit into one conventional AAV vector. Base-editing rescue in mice Base editing as genetic treatment

Clinical studies increasingly combine functional scales—such as HFMSE, Revised Upper Limb Module, CHOP INTEND, and Motor Function Measure—with biomarkers. Neurofilament light chain, a blood or cerebrospinal-fluid marker of nerve-cell injury, is a leading candidate for measuring neurodegeneration, prognosis, and treatment response, although age-specific interpretation and prospective validation are still needed. SMA biomarker roadmap Neurofilament reference ranges

Leading Institutions and Funding

Key academic and clinical contributors include Boston Children’s Hospital, Harvard Medical School, the Broad Institute, UMass Chan Medical School, Nationwide Children’s Hospital, The Ohio State University, St. Jude Children’s Research Hospital, University College London, and international neuromuscular centers represented in major multicenter trials. The base-editing program, for example, involved investigators from Boston Children’s, Harvard, the Broad Institute, UMass Chan, Ohio State, MIT, and the University of Missouri. Base-editing rescue in mice Major industry developers include Novartis Gene Therapies, Roche/Genentech, Biogen, and Scholar Rock. SMA treatment best practices SAPPHIRE Phase III trial

Patient-led funding remains unusually influential in SMA. Cure SMA reported more than $85 million in cumulative support for basic-research grants and drug-discovery programs, and its 2024 grant cycle anticipated $750,000 across five to six projects, with investigator awards of up to $150,000 over two years. Cure SMA research funding announcement Cure SMA 2024 research RFP At the federal level, NIH’s Wellstone Muscular Dystrophy Specialized Research Center initiative planned $4.8 million for fiscal year 2025 across collaborative neuromuscular research centers, a broader infrastructure that can support SMA-relevant science. NIH Wellstone center initiative

Strengths, Limitations, and Challenges

The strongest evidence in SMA is that early SMN restoration can dramatically alter natural history, particularly after newborn screening and presymptomatic treatment. SPR1NT trial The field also has multiple mechanistically distinct options—RNA splicing modification, gene replacement, gene editing, and muscle targeting—which creates realistic opportunities for tailored and combination treatment. SMA treatment best practices SAPPHIRE Phase III trial

However, timing remains decisive because treatment cannot reliably replace motor neurons already lost. Gene therapy also carries important safety and implementation limits: liver toxicity, low platelet counts, immune responses, corticosteroid exposure, vector-dose constraints, and uncertainty about lifelong durability. SMART Phase IIIb trial FDA: Itvisma approval For base editing, the unresolved barriers are precise delivery to sufficient motor neurons, unintended DNA or RNA edits, immune responses to AAV and editing proteins, and proving durable benefit in large-animal models and then human trials. Base-editing rescue in mice Base editing as genetic treatment

Outlook and Future Directions

As of August 8, 2026, SMA is best described as a disease in which early treatment can often prevent catastrophic progression, not one that has been cured. The next milestones to watch are long-term outcomes after intrathecal gene replacement, confirmation of benefit and safety across adolescents and adults, validated biomarkers that can guide individualized treatment, and first-in-human studies of permanent SMN2 editing. A genuine cure will require not merely sustained SMN production but safe lifelong correction, broad access across ages and disease stages, and a way to address damage that occurred before treatment. FDA: Itvisma approval Base-editing rescue in mice

References

• FDA approval of Itvisma for spinal muscular atrophy — U.S. Food and Drug Administration, 2025.
• Spinal muscular atrophy update in best practices — Neurology Clinical Practice, 2024.
• SPR1NT trial — Strauss et al., Nature Medicine, 2022.
• LT-001 long-term follow-up — Waldrop et al., eClinicalMedicine, 2026.
• SUNFISH two-year results — Baranello et al., Journal of Neurology, 2023.
• Prenatal risdiplam case — Finkel et al., New England Journal of Medicine, 2025.
• Base-editing rescue in mice — Arbab et al., Science, 2023.
• Base editing as genetic treatment — Choudhury et al., Nature Communications, 2023.
• SAPPHIRE Phase III trial — Crawford et al., The Lancet Neurology, 2025.
• SMART Phase IIIb trial — McMillan et al., Neurology, 2025.
• STEER trial record — ClinicalTrials.gov, 2026.
• SAPPHIRE trial record — ClinicalTrials.gov, 2026.
• SMA biomarker roadmap — Glascock et al., Journal of Neuromuscular Diseases, 2023.
• Neurofilament reference ranges — Bayoumy et al., Clinical Chemistry and Laboratory Medicine, 2024.
• Cure SMA research funding announcement — Cure SMA, 2024.
• Cure SMA 2024 research RFP — Cure SMA, 2024.
• NIH Wellstone center initiative — National Institutes of Health, 2024.