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.
Delivery Systems (Vectors)
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.
Therapeutic Approaches
In Vivo Therapy
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.
Ex Vivo Therapy
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.
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.
