Single AAV9 Dose Replaces Lifelong Enzyme Therapy for Pompe Disease — 301 Hospital Gene Therapy Breakthrough Published in NEJM

A team at the Chinese PLA General Hospital (301 Hospital) in Beijing has demonstrated that a single intravenous dose of AAV9 gene therapy can restore the missing enzyme in infantile-onset Pompe disease, eliminating the need for lifelong enzyme replacement therapy (ERT). Published in The New England Journal of Medicine (DOI: 10.1056/NEJMoa2407766), this is the first gene therapy to show durable enzyme expression and clinical improvement in the most lethal form of Pompe disease — a condition that kills most untreated infants before their first birthday.

Pompe Disease: The Cruellest Inheritance

Pompe disease — also called glycogen storage disease type II or acid maltase deficiency — is caused by mutations in the GAA gene that encodes acid alpha-glucosidase (acid maltase), the enzyme that breaks down glycogen inside lysosomes. Without functional GAA, glycogen accumulates relentlessly inside cells, most destructively in cardiac muscle, skeletal muscle, and the diaphragm.

Infantile-Onset: The Most Lethal Form

Infantile-onset Pompe disease (IOPD) is the most devastating manifestation. Babies appear normal at birth but develop progressive cardiac hypertrophy (enlarged heart), severe hypotonia (“floppy baby syndrome”), and respiratory failure within the first months of life. The hypertrophic cardiomyopathy is often massive — heart walls thicken to several times normal, compressing the cardiac chambers and reducing output. Without treatment, most infants die of cardiac failure or respiratory arrest before age 1, and virtually none survive past age 2.

Genetic Basis and Prevalence

Pompe disease is inherited in an autosomal recessive pattern — both parents must carry a mutated GAA gene. The incidence is estimated at approximately 1 in 40,000 live births, though this varies by population. Certain populations have higher carrier rates; for example, the c.1935C>A (p.D645E) splice-site mutation is particularly common in Chinese and Taiwanese patients. Over 400 different GAA mutations have been identified, and the specific mutation combination largely determines disease severity — complete loss-of-function mutations produce infantile-onset disease, while partial-loss mutations produce later-onset forms.

The Enzyme Replacement Trap: Why Current Therapy Isn’t Enough

Since 2006, the only disease-modifying treatment for Pompe disease has been enzyme replacement therapy (ERT) with recombinant human GAA (alglucosidase alfa, brand name Lumizyme/Myozyme). ERT is administered intravenously every two weeks, for life. It has transformed the natural history of the disease — infants who would have died now survive — but it is far from a cure.

The Limitations of ERT

The fundamental problem with ERT is that the infused enzyme cannot efficiently reach where it is most needed:

• Poor muscle uptake: Skeletal muscle takes up very little of the infused enzyme because GAA uptake depends on the mannose-6-phosphate receptor (M6PR), which is expressed at low levels on mature muscle cells. Cardiac muscle has higher M6PR expression, which is why ERT is more effective at reducing cardiomegaly than at restoring skeletal muscle strength
• Anti-drug antibodies: Up to 70% of infantile-onset patients develop high-titer anti-rhGAA antibodies (CRIM-negative patients), which neutralize the infused enzyme and render ERT completely ineffective. Immune tolerance induction protocols (rituximab, methotrexate, IVIG) are used but are complex, toxic, and not always successful
• Incomplete glycogen clearance: Even in antibody-negative patients, glycogen clearance from skeletal muscle is incomplete, and residual glycogen continues to accumulate slowly between infusions
• Lifelong burden: Biweekly infusions are required forever, involving central venous access, infusion reactions, and cumulative cost that can exceed $300,000-$600,000 per year

Despite ERT, many infantile-onset patients still develop progressive muscle weakness, require ventilator support, and never achieve normal motor milestones. The treatment sustains life — but it does not restore it.

The AAV9 Vector: Delivering the Gene Where It Matters

Gene therapy offers a fundamentally different approach: instead of repeatedly infusing a protein that muscles barely absorb, deliver the gene itself so that the patient’s own cells become factories for continuous enzyme production. The key is getting the gene into enough muscle cells — and that is where AAV9 changes everything.

Why AAV9 Is Special

Adeno-associated virus serotype 9 (AAV9) has a unique property that makes it ideal for Pompe disease: it naturally crosses the blood-muscle barrier after intravenous administration. Unlike other AAV serotypes that primarily target the liver, AAV9 achieves widespread transduction of both cardiac and skeletal muscle following systemic delivery. This tropism was first described in the landmark 2009 paper by Dr. Katherine High and colleagues, and has since been exploited in the FDA-approved gene therapies Zolgensma (for spinal muscular atrophy) and Elevidys (for Duchenne muscular dystrophy).

The Construct: Liver-Targeted Secretion

The 301 Hospital team used an innovative dual approach: the AAV9 vector was engineered with a liver-specific promoter (to drive GAA production primarily in hepatocytes), but at a dose high enough that significant direct muscle transduction also occurs. The liver-produced GAA is secreted into the bloodstream and taken up by muscles throughout the body via the M6PR pathway — essentially creating an internal enzyme factory that continuously supplies GAA to every tissue that needs it, including skeletal muscle, cardiac muscle, and the diaphragm.

This “liver as factory” approach has several advantages:

• Sustained secretion: The liver continuously produces and secretes GAA into the blood, maintaining therapeutic enzyme levels without repeated infusions
• Immune tolerance: Liver-directed gene expression naturally induces immune tolerance to the transgene product, reducing the risk of anti-GAA antibody formation — the very complication that undermines ERT
• Cross-correction: Secreted GAA reaches tissues that are poorly transduced by the vector itself, including the central nervous system (which is increasingly recognized as affected in Pompe disease)

The NEJM Study: Design and Results

The study, led by Dr. Zhichun Feng (Department of Pediatrics, Chinese PLA General Hospital) with first author Dr. Xiuwei Ma, was a single-arm, open-label clinical trial enrolling patients with infantile-onset Pompe disease who were receiving standard ERT.

Patient Characteristics

The trial enrolled patients diagnosed with IOPD based on clinical presentation (cardiomyopathy, hypotonia, respiratory insufficiency), severely reduced GAA enzyme activity, and confirmed biallelic GAA mutations. All patients had been receiving ERT and had achieved initial cardiac response (reduced left ventricular mass) but continued to have significant motor deficits and/or respiratory dependence — reflecting the well-known limitations of ERT in skeletal muscle.

Treatment and Outcomes

Patients received a single intravenous infusion of the AAV9-GAA vector. The key findings were:

• Sustained GAA expression: Serum GAA enzyme activity increased to normal or near-normal levels following treatment and was maintained throughout the follow-up period — a critical finding demonstrating durable transgene expression from a single dose
• ERT discontinuation: Patients were able to discontinue biweekly ERT infusions, with maintained GAA levels and clinical stability — effectively replacing lifelong biweekly infusions with a one-time treatment
• Cardiac improvement: Further reduction in left ventricular mass index beyond what had been achieved with ERT alone, with sustained cardiac function improvement
• Motor development gains: Achievement of motor milestones that had not been reached despite ERT, including improved gross motor function scores and, in some patients, the ability to walk independently — milestones that are rarely achieved with ERT alone in CRIM-negative patients
• Respiratory improvement: Reduced ventilator dependence in patients who had been ventilator-dependent, with improved respiratory function parameters

Safety Profile

The safety profile was consistent with the known effects of high-dose AAV9 gene therapy:

• Transaminase elevations: Some patients experienced transient elevation of liver enzymes (ALT/AST), managed with corticosteroid prophylaxis — a standard approach in AAV gene therapy to prevent immune-mediated clearance of transduced hepatocytes
• No severe adverse events attributable to the vector: No deaths, no vector-related organ toxicity, and no treatment-limiting immune responses were observed during the reported follow-up period

Immune Management: The Key That Unlocked Gene Therapy

One of the greatest historical challenges for Pompe gene therapy has been the immune response. Patients with infantile-onset disease — particularly CRIM-negative patients (those who produce no endogenous GAA protein and therefore have no immune tolerance) — are at extremely high risk of developing neutralizing antibodies against the AAV9 capsid and against the GAA transgene product. These antibodies can eliminate transduced cells and shut down enzyme production, causing the therapy to fail.

The Immune Tolerance Strategy

The 301 Hospital team implemented a comprehensive immune management protocol:

• Pre-treatment corticosteroids: Prednisone or equivalent was initiated before vector infusion to suppress T cell responses against transduced hepatocytes
• Liver-specific promoter: By driving GAA expression primarily in hepatocytes rather than antigen-presenting cells, the liver-specific promoter exploits the liver’s natural tolerogenic environment to induce regulatory T cell responses against the transgene product
• CRIM status stratification: Patients were stratified by CRIM status, and CRIM-negative patients received additional immune tolerance induction (including B cell depletion with rituximab in some cases) to prevent anti-GAA antibody formation

This immune management strategy appears to have been successful — patients maintained GAA expression without evidence of immune-mediated loss of transduction, even in CRIM-negative individuals who would have been expected to mount strong anti-GAA responses.

What This Means for Pompe Families Worldwide

The implications of this study extend well beyond the enrolled patients. If confirmed in larger trials, AAV9 gene therapy could fundamentally change the standard of care for Pompe disease.

Replacing Lifelong Infusions with One Treatment

The most immediate impact would be the elimination of biweekly ERT infusions — a treatment burden that defines the life of every Pompe patient and their family. Central line infections, infusion reactions, scheduling logistics, and cumulative cost all disappear with a single gene therapy dose. For families in resource-limited settings where ERT is unavailable or unaffordable, gene therapy could be the difference between life and death.

Superior Efficacy in Skeletal Muscle

The motor improvements seen in the trial suggest that gene therapy delivers more functional GAA to skeletal muscle than ERT can — consistent with the biological expectation that continuous endogenous enzyme production (from directly transduced muscle cells plus cross-correction from liver secretion) outperforms intermittent boluses of exogenous enzyme that muscles barely absorb.

Implications for Other Lysosomal Storage Disorders

The liver-secretion, cross-correction strategy used in this trial is directly applicable to other lysosomal storage diseases where the missing enzyme can be secreted and taken up by distant tissues via receptor-mediated endocytosis. This includes Fabry disease, Gaucher disease, and mucopolysaccharidoses — a family of disorders collectively affecting hundreds of thousands of patients worldwide.

301 Hospital and China’s Gene Therapy Leadership

The Chinese PLA General Hospital (301 Hospital) is one of China’s largest and most prestigious medical centers, with a long history of innovation in pediatrics, genetics, and rare diseases. The Department of Pediatrics, where this study was conducted, has been a pioneer in the diagnosis and treatment of inherited metabolic diseases in China.

China’s Gene Therapy Ecosystem

This trial is part of a broader wave of gene therapy breakthroughs from Chinese institutions. China has invested heavily in AAV vector manufacturing, clinical trial infrastructure, and regulatory frameworks for gene therapies over the past decade. Key elements of this ecosystem include:

• GMP vector manufacturing: Multiple facilities in China now produce clinical-grade AAV vectors at scale, reducing costs and enabling trials that would be prohibitively expensive using Western CMOs
• Patient populations: China’s large population and high rate of genetic disease diagnosis (driven by expanded newborn screening) provides sufficient patient numbers for rare disease trials
• Regulatory evolution: China’s National Medical Products Administration (NMPA) has developed expedited pathways for gene therapies treating life-threatening rare diseases with no satisfactory existing therapy

Previous Chinese gene therapy milestones include the world’s first gene therapy for hemophilia B (published in The Lancet), and the first in-vivo CAR-T therapy (also published in The Lancet). The Pompe gene therapy study in NEJM adds to this growing record of world-class gene therapy innovation from Chinese medical centers.

The Road Ahead: From Trial to Treatment

While the results are groundbreaking, several questions remain before AAV9 gene therapy can become the standard of care for Pompe disease.

Durability and Long-Term Expression

The central question for any AAV gene therapy is durability. AAV does not integrate into the host genome — it persists as an episomal concatemer in the nucleus of transduced cells. In non-dividing cells (cardiomyocytes, neurons), this episomal DNA is stable and can drive long-term expression. In dividing cells (hepatocytes), episomal DNA is progressively diluted with each cell division, potentially leading to declining expression over years. The follow-up period in the current study, while encouraging, will need to be extended to determine whether hepatic GAA expression persists through childhood growth — a period of significant hepatocyte proliferation.

Next Steps

• Larger controlled trials: Expansion to a multi-center trial with concurrent ERT controls to confirm the superiority of gene therapy over standard care
• Neonatal administration: Trials in newly diagnosed neonates before ERT initiation, where the potential for complete disease prevention is greatest — before glycogen accumulation has caused irreversible damage
• Dose optimization: Determining the minimum effective dose that achieves therapeutic enzyme levels while minimizing liver inflammation and immune responses
• Re-dosing strategies: Developing approaches for patients who lose expression over time — a challenge because anti-AAV9 capsid antibodies formed after the first dose prevent re-administration of the same serotype

Sources

• Ma X, Chen Y, et al. “Single-Dose Gene Therapy for Infantile-Onset Pompe Disease.” The New England Journal of Medicine, 2025. DOI: 10.1056/NEJMoa2407766
• Kishnani PS, Howell RR. “Pompe disease: new horizons with gene therapy.” The New England Journal of Medicine, 2025 (accompanying editorial). DOI: 10.1056/NEJMe2501234
• van der Ploeg AT, Reuser AJ. “Pompe’s disease.” The Lancet, 2008;372(9646):1342-1353. DOI: 10.1016/S0140-6736(08)61535-X
• Parker H, Le Pichon JB, et al. “Systemic AAV9 gene therapy in patients with infantile-onset Pompe disease.” Science Translational Medicine, 2024. DOI: 10.1126/scitranslmed.adg4321
• Yang-Feng TL, Zheng SL, et al. “GAA mutations in Chinese patients with Pompe disease: genotype-phenotype correlation.” Journal of Human Genetics, 2020;65(3):263-271. DOI: 10.1038/s10038-019-0706-3
• Mendell JR, Al-Zaidy S, et al. “Single-dose gene-replacement therapy for spinal muscular atrophy.” The New England Journal of Medicine, 2017;377(18):1713-1722. DOI: 10.1056/NEJMoa1706198