What Is Gene Therapy?

Gene therapy is a medical approach that treats or prevents disease by modifying the genetic material inside a patient's cells. Rather than addressing disease symptoms with pharmacological compounds, gene therapy targets the underlying genetic cause: introducing a functional copy of a defective gene, silencing an overactive gene, or editing the genome sequence directly. The field draws from molecular biology, virology, biomedical engineering, and clinical medicine, and it has moved over several decades from theoretical possibility to approved treatments for a range of genetic diseases.

The history of gene therapy includes early setbacks, including fatal adverse events in trials conducted in the late 1990s and early 2000s, which prompted rigorous regulatory and safety reforms. The subsequent development of safer viral vectors, improved understanding of immunological responses, and the emergence of CRISPR-based editing tools have placed gene therapy on a stable clinical footing. The FDA approved close to 30 viral gene therapy programs by the mid-2020s, covering conditions including spinal muscular atrophy, hemophilia B, beta-thalassemia, and sickle cell disease.

Delivery Mechanisms

Delivering therapeutic genetic material into target cells requires a vector, a vehicle that carries the gene or editing machinery across the cell membrane and, in some cases, into the nucleus. Viral vectors are the most established delivery platforms. Adeno-associated viruses (AAVs) are widely used because they infect a broad range of cell types, provoke a relatively mild immune response, and persist in cells without integrating into the host genome at high frequency. Lentiviral vectors, derived from HIV, integrate into the host cell genome and are used when stable long-term expression is required, such as in correcting hematopoietic stem cells.

Non-viral delivery systems, including lipid nanoparticles (LNPs), have gained clinical prominence following the mRNA vaccine programs. LNPs encapsulate nucleic acid cargo and fuse with cell membranes, releasing their contents into the cytoplasm. They were used to deliver the CRISPR-based therapy for a metabolic disorder caused by CPS1 deficiency, which received FDA authorization and was administered to a patient within six months of development. A comprehensive PMC review of viral and non-viral vectors in gene therapy surveys the current state of each platform's clinical applications and safety profiles.

Types of Gene Therapy

Gene addition introduces a functional copy of a gene into cells that carry a defective version, without altering the existing genomic sequence. This approach was used in the first approved gene therapies for inherited retinal dystrophy (Luxturna) and spinal muscular atrophy (Zolgensma). Gene silencing uses RNA interference or antisense oligonucleotides to reduce the expression of a gene whose overactivity contributes to disease.

Gene editing directly modifies the DNA sequence using programmable nucleases. CRISPR-Cas9 is the most widely used system, enabling targeted double-strand DNA breaks that cells repair by either disrupting or correcting the targeted sequence. In December 2023, the FDA approved Casgevy, the first CRISPR-based gene therapy, for the treatment of sickle cell disease and transfusion-dependent beta-thalassemia. The FDA press announcement on this approval marks a milestone in the clinical deployment of genome editing. PMC reviews of CRISPR-based gene therapies describe the preclinical to clinical development pathway for this class of treatments.

Clinical Development and Safety

Gene therapy trials require careful evaluation of immunogenicity (the risk that viral vectors or editing components trigger immune responses), off-target editing (unintended modifications to non-target genomic sites), and the durability of therapeutic effect over time. Regulatory oversight by the FDA in the United States and the EMA in Europe involves extensive preclinical safety data requirements before human trials can begin. Long-term follow-up studies are required for approved therapies to monitor for delayed adverse events such as insertional oncogenesis with integrating vectors.

Applications

Gene therapy has applications in a wide range of disciplines, including:

• Treatment of inherited monogenic disorders such as hemophilia, muscular dystrophy, and lysosomal storage diseases
• Oncology, including CAR-T cell therapies that engineer patient immune cells to target tumors
• Ophthalmology, with retinal gene therapies targeting inherited forms of blindness
• Infectious disease, with research into HIV functional cures using gene editing of immune cells
• Rare metabolic disorders where enzyme replacement therapy is insufficient