Gene therapy
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Gene therapy is a technique that introduces, alters, or replaces genetic material inside a cell to treat or prevent disease. It can work by replacing a faulty gene, silencing an overactive gene, or introducing a new gene. Vectors (usually modified viruses like AAV or lentiviruses) are the main delivery tools. Applications include monogenic diseases (e.g., SCID, hemophilia), cancer, and some inherited conditions.
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Gene Therapy
Gene therapy is the introduction of a biologically active gene (or editing of an existing gene) into a cell to achieve a therapeutic benefit. The first licensed gene therapy product appeared in 2012 (for lipoprotein lipase deficiency), and the field has since expanded dramatically - with 1,905 active clinical trials globally as of early 2025.
- Thompson & Thompson Genetics and Genomics in Medicine, 9th ed., p. 328
Core Strategies
There are three fundamental approaches:
- Gene addition / replacement - Insert a functional copy of a defective gene into target cells (most common for loss-of-function mutations)
- Gene silencing - Turn off an overactive or harmful gene (e.g., using antisense RNA, siRNA)
- Gene editing - Directly correct a mutation in its native genomic context (e.g., CRISPR/Cas9, ZFN, TALENs)
Delivery Vectors
Viral Vectors
| Vector | Integration | Key Properties | Example Use |
|---|---|---|---|
| Retrovirus | Yes (random) | Integrates into dividing cells; risk of insertional mutagenesis | X-linked SCID |
| Lentivirus | Yes | Infects both dividing and non-dividing cells | ALD, β-thalassemia |
| Adenovirus | No (episomal) | High transduction efficiency; strong immune response | Transient expression |
| AAV (adeno-associated virus) | No (mainly episomal) | Low immunogenicity; long-lived in post-mitotic cells; preferred vector | SMA, hemophilia, LCA |
AAV has become the dominant clinical vector because it forms stable episomes in long-lived cells (neurons, hepatocytes, myocytes) without integrating, minimizing oncogenic risk. As a 2025 review in Molecular Therapy notes, AAV-mediated gene therapy is now the centerpiece of approved products across multiple diseases (Byrne et al., 2025, PMID: 40329530).
Non-Viral Vectors
- Lipid nanoparticles (LNPs) - used for RNA therapies and CRISPR delivery
- Naked DNA / plasmids
- Electroporation of ex vivo cells
Somatic vs. Germline Gene Therapy
- Somatic gene therapy: Targets non-reproductive cells; changes are not heritable. Currently approved and in widespread clinical use.
- Germline gene therapy: Modifies reproductive cells (eggs, sperm, embryos); changes pass to all future generations. No country currently permits this, though scientific debate continues.
Approved / Advanced Clinical Examples
| Disease | Gene / Protein | Vector | Result |
|---|---|---|---|
| X-linked SCID | IL2RG | Retroviral vector, HSCs | Significant improvement in 27/32 patients; newer SIN vectors reduced leukemia risk |
| ADA-SCID | Adenosine deaminase | Retroviral, HSCs | 29/40 patients off enzyme replacement |
| X-linked ALD | ABCD1 | Lentiviral, autologous HSCs | Demyelination arrested in 17/19 boys |
| Spinal Muscular Atrophy (SMA) | SMN1 | AAV, IV injection | Marked improvement in 1,800+ patients; FDA approved (Zolgensma) |
| Hemophilia B | Factor IX | AAV, single IV injection | Stable FIX expression 1-7% of normal for >3 years; >20 patients stopped prophylaxis |
| Leber Congenital Amaurosis | RPE65 | AAV, subretinal injection | FDA approved (Luxturna) for age ≥12 years |
| β-Thalassemia / Sickle Cell | β-globin / HbF induction | Lentiviral or CRISPR (Casgevy) | Elimination of transfusion dependence; vaso-occlusive crises resolved |
Thompson & Thompson Genetics and Genomics in Medicine, 9th ed., Table 14.4
For sickle cell disease, CRISPR/Cas9 technology to boost fetal hemoglobin (HbF) has shown persistence of high HbF levels and elimination of vaso-occlusive pain crises for at least 1 year in trials. - Goldman-Cecil Medicine
Ex Vivo vs. In Vivo Delivery
- Ex vivo: Cells (e.g., hematopoietic stem cells) are removed from the patient, genetically modified in the lab, then reinfused. Safer, more controllable - used in SCID, ALD, thalassemia.
- In vivo: Vector is directly administered into the patient (IV, intrathecal, subretinal). Used for SMA (Zolgensma IV), hemophilia, LCA (subretinal AAV).
Gene Editing Tools
| Tool | Mechanism |
|---|---|
| CRISPR/Cas9 | RNA-guided nuclease; most precise and versatile; used in sickle cell (Casgevy, 2023 FDA approval) |
| ZFN (Zinc Finger Nucleases) | Protein-DNA binding; older, harder to engineer |
| TALENs | Transcription activator-like effector nucleases; intermediate precision |
Requirements Before Applying Gene Therapy
Before gene therapy is considered for a disease (per Box 14.1 of Thompson & Thompson):
- The disease pathophysiology must be well understood
- The responsible gene must be cloned and characterized
- Appropriate target cells must be identifiable and accessible
- Gene expression must be regulatable
- The risk-benefit ratio must be acceptable
Special Applications
- Cancer: CAR-T cell therapy (ex vivo gene modification of T cells to target tumor antigens) - a 2026 review in Nature Reviews Drug Discovery covers in vivo CAR-T as the next frontier (Bot et al., PMID: 41028170)
- Neurological diseases: Parkinson's, Alzheimer's, Huntington's - in trials using AAV vectors
- Infectious disease: HIV gene therapy approaches in development
- iPSC-based gene therapy: Patient-derived stem cells corrected ex vivo then transplanted - avoids immune rejection
Challenges and Limitations
- Immunogenicity - Host immune responses against viral vectors or transgene products
- Insertional mutagenesis - Retroviral/lentiviral integration near oncogenes (led to leukemia in early SCID trials; addressed with newer SIN vectors)
- Transient expression - Episomal DNA diluted in dividing cells
- Cost - Zolgensma (~$2.1M per dose) raises major access and equity concerns
- Off-target editing - CRISPR cuts at unintended genomic sites
- Delivery to target tissue - CNS penetration, immune-privileged sites, large gene packaging limits
Ethical Considerations
The key ethical divide is somatic vs. germline therapy. Somatic therapy affects only the individual; germline changes are heritable and could alter the human species permanently. Additional concerns include:
- Enhancement vs. treatment (e.g., height, intelligence)
- Access equity for ultra-expensive therapies
- Consent (especially in children)
- Long-term unknown effects
No country currently permits germline modification in clinical practice, though the 2018 "CRISPR babies" case in China highlighted the urgency of enforceable international governance.
Current Landscape (2025-2026)
As of H1 2025, there are 1,905 active clinical trials globally. Key recent milestones include:
- Casgevy (exa-cel, CRISPR-based) - FDA approved in late 2023 for sickle cell disease and β-thalassemia, the first CRISPR therapy approved anywhere
- China's BBM-H901 - First AAV-based hemophilia B gene therapy approved in China (2024)
- In vivo CAR-T therapy emerging as the next generation beyond ex vivo cell manufacturing
Gene therapy is firmly transitioning from experimental to standard-of-care for selected monogenic diseases, with the next wave targeting polygenic and common diseases.
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