Definition and Concepts

Gene therapy is a therapeutic technique involving the delivery of nucleic acids (DNA or RNA) into a patient’s cells to treat, cure, or prevent disease.
It functions by modifying endogenous gene expression, replacing missing or non-functional genes, or correcting abnormal genetic sequences.
The approach fundamentally relies on the unique interaction between the specific disease pathophysiology and the selected gene delivery vehicle.

Mechanisms of Action

Mechanism	Description	Clinical Utility and Examples
Gene Addition (Replacement)	Introduction of a functional copy of a gene to compensate for a defective or missing gene.	Most common for recessive disorders. Example: delivering a functional SMN1 gene in Spinal Muscular Atrophy.
Gene Editing	Precise modification of the endogenous genomic sequence using engineered nucleases.	Utilizes CRISPR/Cas9, Zinc Finger Nucleases (ZFNs), or TALENs for targeted double-stranded breaks or base editing.
Gene Silencing (Knockdown)	Reduction of the expression of a toxic gain-of-function mutant gene or disease-contributing gene.	Uses Antisense Oligonucleotides (ASOs) or RNA interference (RNAi) to degrade targeted mRNA or alter splicing.

Vector Type	Subtype and Characteristics	Advantages and Disadvantages
Viral Vectors	Adeno-Associated Virus (AAV): Non-integrating vector remaining episomal. High tropism for neurons, muscle, and liver.	Low immunogenicity. Overcomes dilution effect in non-proliferating cells, but lacks integration in dividing cells.
Viral Vectors	Lentivirus / Retrovirus: Integrating vectors used mainly for ex vivo therapy in hematopoietic stem cells.	Provides stable long-term expression. Carries a risk of insertional mutagenesis and oncogenesis.
Non-Viral Vectors	Lipid Nanoparticles (LNPs): Spherical structures mimicking cell membranes, used for transient, localized mRNA or siRNA delivery.	Highly effective for liver-targeted therapies. Avoids viral-vector related immune responses.
Non-Viral Vectors	Electroporation: Ex vivo technique opening cell pores for DNA/RNA entry.	Does not use viral components but is restricted to ex vivo laboratory applications.

The therapeutic vector is injected directly into the patient systemically (via intravenous route) or locally (into the eye, central nervous system, or muscle).
It is less invasive but faces challenges regarding targeted delivery and the potential for triggering systemic immune responses.

Autologous cells (usually hematopoietic stem cells or T-cells) are harvested from the patient, genetically modified in the laboratory, expanded, and then re-infused.
This approach offers high precision, overcomes in vivo immunogenicity, and allows cell screening prior to re-infusion.
It requires specialized laboratory infrastructure and often necessitates patient conditioning or chemotherapy regimens prior to cell infusion.

Disease Category	Specific Condition	Approved Product / Strategy	Delivery Route
Neuromuscular	Spinal Muscular Atrophy	Onasemnogene abeparvovec (AAV9 targeting SMN1); Nusinersen (ASO targeting SMN2).	Intravenous; Intrathecal.
Neuromuscular	Duchenne Muscular Dystrophy	Eteplirsen, Golodirsen (Exon-skipping ASOs).	Intrathecal; Subcutaneous.
Hematological	Transfusion-Dependent Thalassemia and Sickle Cell Disease	Betibeglogene autotemcel (Lentiviral beta-globin addition); Exagamglogene autotemcel (CRISPR silencing of BCL11A).	Ex vivo autologous stem cell transfer.
Hematological	Hemophilia A and B	Valoctocogene roxaparvovec (AAV5 for Factor VIII); Etranacogene dezaparvovec (AAV5 for Factor IX).	Intravenous.
Ophthalmology	Leber Congenital Amaurosis	Voretigene neparvovec-rzyl (AAV2 delivering normal RPE65).	Subretinal injection.
Oncology	Refractory B-cell Acute Lymphoblastic Leukemia	CAR-T Cell Therapy (Genetically modifying T-cells to express Chimeric Antigen Receptors targeting CD19).	Ex vivo autologous T-cell infusion.

Insertional Mutagenesis: Integration of viral vectors near proto-oncogenes can trigger oncogenesis, such as leukemia observed in early Severe Combined Immunodeficiency (SCID) trials.
Immunogenicity: Pre-existing antibodies against viral vectors (like AAV) can neutralize the therapy. High-dose AAV can also trigger severe hepatotoxicity or thrombotic microangiopathy.
Genotoxicity and Off-Target Effects: Gene editing tools may cause unintended double-strand breaks at genomic sites sharing sequence homology, posing long-term oncogenic risks.
Economic Barriers: High manufacturing costs lead to exceptionally expensive treatments, severely limiting accessibility in low- and middle-income countries.
Ethical Constraints: Current international consensus strictly prohibits germline gene editing due to the risks of heritable, irreversible genomic alterations.