Gene Therapy: A Revolutionary Approach in Modern Medicine – A Review

Gene therapy is one of the most innovative and promising approaches in modern biomedical science, aiming to treat, prevent, or potentially cure diseases by addressing their underlying genetic causes. Unlike conventional drug-based therapies that primarily alleviate symptoms, gene therapy focuses on correcting, replacing, silencing, or modifying defective genes responsible for disease development. This strategy offers a fundamentally different therapeutic paradigm, particularly for inherited genetic disorders, cancers, and certain acquired diseases. The concept of gene therapy emerged in the late 20th century with advances in molecular biology and recombinant DNA technology. Early developments laid the foundation for understanding gene regulation, gene transfer mechanisms, and the role of genetic mutations in disease pathogenesis. Over the past few decades, significant progress has been made in vector development, gene delivery systems, and genome manipulation techniques, enabling safer and more efficient transfer of therapeutic genes into target cells.

Gene therapy strategies can be broadly classified into in vivo, and ex vivo approaches. In in vivo gene therapy, therapeutic genes are directly delivered into the patient’s body, whereas ex vivo therapy involves genetic modification of cells outside the body followed by reinfusion into the patient. Viral vectors such as adenoviruses, adeno-associated viruses (AAV), and lentiviruses have been widely used due to their high transduction efficiency, while non-viral vectors offer advantages in terms of safety and reduced immunogenicity^.

In recent years, the field has witnessed remarkable clinical success, leading to regulatory approval of several gene therapy products for conditions such as inherited retinal disorders, spinal muscular atrophy, and certain haematological diseases. These approvals mark a major milestone, demonstrating the clinical feasibility and therapeutic potential of gene-based interventions.

Despite these advancements, gene therapy remains a complex and evolving field, facing challenges related to delivery efficiency, immune responses, long-term safety, and ethical considerations. Continued research and technological innovation are essential to overcome these limitations and expand the applicability of gene therapy across a broader range of diseases ^[1,2,3,4,5]

2. History of Gene Therapy.

The concept of gene therapy emerged in the mid-1960s, when scientists proposed that genetic diseases could be treated by introducing functional DNA into a patient’s cells. At the time, this idea remained largely theoretical due to the lack of reliable molecular tools. Subsequent advances in molecular biology and recombinant DNA technology during the following decades gradually enabled the practical exploration of this approach ^[1,3]

One of the earliest attempts at human gene transfer was carried out by Martin Cline in 1980, who introduced foreign DNA into bone marrow cells of patients with thalassemia. Although controversial and clinically unsuccessful, this effort demonstrated the technical feasibility of gene transfer in humans. A breakthrough occurred in 1989 with the first demonstration of stable gene transfer in human cells, leading to the first approved clinical gene therapy trial in 1990 led by Dr. W. French Anderson.

This landmark trial involved a child with adenosine deaminase (ADA) deficiency–related severe combined immunodeficiency (SCID). Autologous T lymphocytes were genetically modified ex vivo using a replication-defective retroviral vector carrying a functional ADA gene and reinfused into the patient. The treatment resulted in partial immune restoration and is regarded as the first clear clinical success of gene therapy.

Following this achievement, gene therapy research expanded rapidly during the 1990s, particularly for inherited immunodeficiencies. By December 2018, more than 2,900 gene therapy clinical trials had been initiated worldwide, most of them early-phase studies focused on safety. However, progress was temporarily hindered by safety concerns such as immune reactions and insertional mutagenesis, prompting improvements in vector design, delivery strategies, and regulatory oversight ^[2,4].

The field entered a new phase with the approval of commercial gene therapies. In 2017, the U.S. FDA approved Luxturna® for inherited retinal dystrophy and Kymriah®, a CAR-T cell–based therapy, for B-cell acute lymphoblastic leukaemia. Additional milestones include the approval of Patisiran, the first RNA interference therapy, and Zolgensma® for spinal muscular atrophy.

Currently, most gene therapies employ viral vectors, particularly adeno-associated viruses for in vivo delivery and lentiviruses for ex vivo applications. Emerging genome-editing technologies, such as CRISPR-Cas systems, are further advancing the field toward precise and durable genetic correction ^[3,4].

3. Types of Gene Therapy

3.1 Gene therapy can be broadly classified based on:

1. Classification based on type on target cell.
2. Classification based on method of gene delivery.
3. Classification based on nature of genetic modification ^[1].

1. Classification Based on Type of Target Cells.

a. Somatic Cell Gene Therapy.

Somatic cell gene therapy involves the introduction of therapeutic genes into non-reproductive (somatic) cells, such as blood cells, muscle cells, liver cells, or epithelial cells. These changes affect only the treated individual and are not inherited by future generations, making this approach ethically acceptable and clinically safer ^[1,3].

Most gene therapy trials conducted so far fall under this category. Diseases such as SCID, haemophilia, cystic fibrosis, cancer, and retinal disorders are commonly targeted using somatic gene therapy ^[2,4].

b. Germ Cell Gene Therapy.

Germ cell gene therapy targets reproductive cells (sperm or ova) or early embryos. Any genetic change introduced at this level becomes heritable and can be passed on to future generations.

Due to ethical, social, and safety concerns—including unpredictable long-term effects—germ cell gene therapy is currently prohibited in humans in most countries. However, it is actively studied in animal models to understand genetic inheritance and disease prevention ^[1,3].

2. Classification Based on Gene Delivery Method.

a. Ex Vivo Gene Therapy

In ex vivo gene therapy, cells are removed from the patient, genetically modified outside the body, and then reintroduced after successful gene integration. This approach allows better control over gene insertion and safety testing before reinfusion.

This method is widely used in CAR-T cell therapy, haematological disorders, and immunodeficiency diseases ^[1,2].

b. In Vivo Gene Therapy.

In in vivo gene therapy, the therapeutic gene is delivered directly into the patient’s body using a vector, most commonly a viral vector such as AAV or adenovirus. The vector carries the functional gene to the target tissue.

This method is commonly used for eye disorders, muscular diseases, and metabolic disorders ^[2,4].

3. Classification Based on Type of Genetic Modification.

a. Gene Replacement Therapy.

Gene replacement therapy involves removing or inactivating a defective gene and replacing it with a functional one. This approach aims to restore normal gene expression permanently.

This method is particularly useful for monogenic disorders caused by a single faulty gene ^[1,3].

b. Gene Addition Therapy.

Gene addition therapy restores cellular function by introducing an additional functional copy of a gene without removing the defective one. The newly added gene compensates for the faulty gene’s function.

This strategy is widely used in cancer gene therapy, where tumour suppressor genes or immune-stimulating genes are added to target cells ^[2,4].

c. Gene Editing–Based Therapy Recent advancements such as CRISPR-Cas9, TALENs, and ZFNs allow precise editing of specific DNA sequences. Unlike traditional gene addition, gene editing directly corrects mutations at their original genomic location ^[3,4].Bottom of Form

4. Overview of Gene Therapy Approaches.

4.1 In Vivo and Ex Vivo Gene Therapy.

Gene therapy strategies are broadly classified into in vivo, and ex vivo approaches based on the site of genetic modification. In vivo gene therapy involves the direct administration of genetic material into the patient, where target cells are transduced within their native tissue environment. This approach is particularly suitable for organs such as the eye, liver, muscle, and central nervous system, where localized or systemic delivery can be achieved efficiently. Approved therapies for retinal dystrophies and haemophilia exemplify the success of in vivo approaches ^[3,4].

Ex vivo gene therapy, in contrast, involves the isolation of patient-derived cells, genetic modification under controlled laboratory conditions, and subsequent reinfusion into the patient. This method allows precise control over gene transfer efficiency and safety testing prior to administration. Ex vivo approaches are widely used in hematopoietic stem cell therapies and chimeric antigen receptor T-cell (CAR-T) therapies for cancer treatment ^[1,2].

4.2 Viral and Non-Viral Gene Delivery Systems.

Efficient and safe delivery of genetic material remains a central challenge in gene therapy. Viral vectors, including adeno-associated viruses (AAVs), lentiviruses, retroviruses, and adenoviruses, are commonly used due to their high transduction efficiency. AAV vectors are particularly favoured for in vivo applications because of their low pathogenicity and long-term gene expression, whereas lentiviral vectors are extensively used in ex vivo therapies due to their stable genomic integration capabilities ^[2,4].

Non-viral delivery systems, such as lipid nanoparticles, polymer-based carriers, and physical methods, offer advantages in terms of safety and cargo capacity but often suffer from lower transfection efficiency. AI-driven interpretation of vector performance data is increasingly used to guide the selection and optimization of both viral and non-viral delivery systems.

between computational scientists, biologists, and clinicians will be critical for realizing the full potential of AI-assisted gene therapy ^[2,4].

5. Vectors for Gene Therapy

In gene therapy, a vector is a carrier system that delivers therapeutic genetic material into target cells. Based on their origin and mechanism of delivery, gene therapy vectors are broadly classified into viral and non-viral vectors. Each category has distinct advantages and limitations that influence their clinical application ^[1,2].

5.1 Viral Vectors

Viruses naturally possess the ability to enter host cells and deliver their genetic material. In gene therapy, this property is exploited by removing disease-causing viral genes and replacing them with therapeutic DNA or RNA. Once inside the host cell, the introduced genetic material can either remain episomal or integrate into the host genome, leading to gene expression.

Retroviruses and lentiviruses integrate their genetic content into the host genome, enabling long-term gene expression. In contrast, adenoviruses and adeno-associated viruses (AAVs) generally do not integrate and instead support transient or semi-stable expression. Herpes simplex virus and vaccinia virus are mainly used for targeting neuronal and cancer cells ^[2,4].

5.1.1 Common viral vectors.

1. Retrovirus
2. Adenovirus (types 2 and 5)
3. Adeno-associated virus (AAV)
4. Herpes simplex virus (HSV)
5. Pox virus
6. Human foamy virus (HFV)
7. Lentivirus

Viral vectors are carefully modified to improve safety and effectiveness in gene therapy, but several challenges remain:

• these vectors are engineered to prevent replication, reducing the risk of uncontrolled viral spread.
• strong immune responses can occur, leading to inflammation in the host.
• viral component expression may cause damage to surrounding tissues.
• unintended integration of genetic material can disrupt normal genes, potentially causing instability or cancer.
• many vectors have limited capacity to carry large therapeutic genes, restricting their use for certain disorder. ^[2,4].

1. Retrovirus vector.

Retroviral vectors are derived from retroviruses, which have RNA genomes that are reverse transcribed into DNA and integrated into the host cell genome. This integration allows for stable, long-term expression of therapeutic genes in dividing cells. By removing genes required for viral replication, retroviral vectors have been made suitable for clinical use. These vectors have been historically applied in the treatment of monogenic disorders, such as X-linked severe combined immunodeficiency (X-SCID), and metabolic diseases like hyperlipidaemia. They are also used in developing experimental cancer vaccines. In contemporary research, retroviral vectors are frequently applied in ex vivo approaches, where patient cells are modified outside the body and reintroduced, allowing precise control over gene delivery. Emerging studies are also exploring combinations of retroviral vectors with genome-editing tools like CRISPR-Cas systems to correct genetic defects at the DNA level ^[1,2,4].

2. Adenovirus vector.

Adenoviral vectors are based on double-stranded DNA viruses, commonly serotypes 2 and 5, and are engineered to remove early genes to prevent replication. These vectors efficiently deliver genes to both dividing and non-dividing cells, with the delivered DNA remaining episomal, leading to transient expression. Adenoviral vectors have been widely used not only in classical gene therapy but also in vaccine development, including viral vector-based vaccines for infectious diseases such as COVID-19. They are also under investigation for cancer immunotherapy, cardiovascular gene therapy, and treatment of respiratory disorders. Due to their capacity to carry relatively large DNA sequences, adenoviral vectors are suitable for delivering complex genetic constructs, including multi-gene cassettes for therapeutic applications. Advanced generations of adenoviral vectors now incorporate modifications to reduce immune responses and improve tissue targeting, making them more versatile for clinical and experimental use ^[2,3,4].

3. Adeno-Associated virus (AAV) vector.

AAV vectors are derived from small, non-pathogenic viruses and are among the safest viral vectors currently in use. They provide long-term gene expression, particularly in non-dividing cells, and certain serotypes integrate at a specific site on chromosome 19, which reduces the risk of random insertional mutagenesis. AAV vectors have been employed in treating genetic disorders such as haemophilia B, cystic fibrosis, alpha-1 antitrypsin deficiency, and inherited retinal diseases. They are also being investigated for applications in neurological diseases, metabolic disorders, and cardiomyopathies. Innovations in AAV research include the development of tissue-specific serotypes and engineered capsids that improve delivery efficiency and reduce immune recognition. Many clinical trials now combine AAV vectors with precise gene-editing techniques, such as CRISPR, to correct disease-causing mutations in situ ^[2,3,4].

4. Herp simplex virus (HSV) vector.

HSV vectors are double-stranded DNA viruses naturally adept at infecting neurons, making them particularly valuable for nervous system applications. They can carry very large DNA fragments, up to 30 kilobases, allowing delivery of complex therapeutic genes or multiple regulatory elements simultaneously. HSV vectors, including modified versions such as Disabled Infectious Single-Cycle (DISC) vectors, enable gene delivery without generating infectious virus particles. They have been extensively studied for treating neurological disorders, brain tumours, chronic pain, and experimental neurodegenerative diseases. HSV vectors are also being explored as vehicles for oncolytic viral therapies, in which the virus selectively targets and kills cancer cells while delivering therapeutic genes to enhance immune responses ^[2,3,4].

5. Lentivirus vectors.

Lentiviral vectors, derived from lentiviruses such as HIV, can infect both dividing and non-dividing cells and stably integrate their genetic material into the host genome, ensuring long-term gene expression. They are commonly used in ex vivo therapies, including CAR-T cell immunotherapy for cancer, and in experimental therapies for central nervous system disorders. Lentiviral vectors have been applied in preclinical and clinical studies targeting Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, spinal cord injuries, and motor neuron disorders. Modern lentiviral vectors are also combined with precise genome-editing strategies, enabling targeted gene insertion or correction with minimal off-target effects. Their ability to accommodate relatively large transgenes makes them versatile tools for complex therapeutic applications ^[1,2,3,4].

5.2 Non-Viral Vectors

Non-viral vectors deliver genetic material without using viruses and are considered safer and more flexible. These systems are easier to manufacture on a large scale and show lower immunogenicity, allowing repeated administration. However, they generally produce lower transfection efficiency and reduced gene expression compared to viral vectors ^[2,4].

Common non-viral delivery methods include naked DNA injection, liposomes, dendrimers, inorganic nanoparticles, electroporation, gene guns, magnetoception, and oligonucleotide-based systems. Advances in nanotechnology and cell-specific targeting strategies are gradually improving their efficiency ^[2] .

Modern non-viral platforms are widely used for the delivery of mRNA, siRNA, antisense oligonucleotides, and CRISPR components. Several biotechnology companies are actively developing lipid nanoparticles and polymer-based systems to overcome delivery challenges, particularly for liver-targeted therapies.

Non-viral gene delivery methods use physical or chemical method to introduce genetic material into cells without relying on viruses ^[2,3,4] .

1. Physical method.

include electroporation, where brief electrical pulses create temporary pores in cell membranes, allowing DNA to enter; this method has shown potential in wound healing and tissue engineering, although its effectiveness can be limited by tissue accessibility. Another technique, the gene gun or ballistic DNA transfer, delivers DNA-coated particles made of gold, tungsten, or silver directly into target cells by physical propulsion, enabling gene transfer into tissues that are difficult to reach by conventional methods. Sonoporation employs ultrasound waves to generate transient defects in cell membranes through acoustic cavitation, with gene delivery efficiency influenced by the intensity and duration of the ultrasound, as well as the concentration of genetic material ^[2,4].

A. Physical Methods for Enhancing DNA Delivery.

a. Electroporation

Electroporation is a widely used technique for introducing DNA into cells through the application of brief, high-voltage electrical pulses. These pulses temporarily disrupt the cell membrane, creating nanoscale pores that allow nucleic acids to pass into the cytoplasm. This method can be applied to a wide range of cell types, including mammalian cells, bacterial cells, and primary cells. Despite its versatility, electroporation often causes significant cell stress or death, which has limited its use in clinical applications. Optimizing pulse duration, voltage intensity, and cell density is crucial to improve efficiency while minimizing cytotoxicity. Electroporation is frequently employed in research settings for gene editing, transfection of stem cells, and delivery of therapeutic genes ^[1,2,4].

b. Gene Gun (Particle Bombardment)

The gene gun, also known as particle bombardment, is a physical DNA delivery method in which DNA molecules are attached to microscopic metal particles, typically gold or tungsten. These DNA-coated particles are accelerated using a high-pressure device, allowing them to penetrate cell membranes and deliver genetic material directly into the nucleus. While effective, careful control is required to avoid off-target insertion of DNA, which could potentially disrupt essential genes such as tumour suppressors. Gene gun technology has been applied in experimental gene therapy, including treatments for X-linked severe combined immunodeficiency (X-SCID). In clinical studies, hematopoietic stem cells (HSCs) treated with retrovirus-mediated gene delivery showed successful correction of T-cell defects in several patients. This approach is particularly useful for targeting cells that are difficult to transfect with conventional methods ^[1,3,4].

c. Sonoporation

Sonoporation utilizes ultrasound waves to transiently increase cell membrane permeability, allowing DNA and other macromolecules to enter the cell. The mechanism involves acoustic cavitation, where the formation and collapse of microbubbles create localized mechanical forces that disrupt the cell membrane. This approach has shown potential in both in vitro and in vivo applications, including gene delivery to tumour cells and vascular tissues. Key parameters, such as ultrasound frequency, intensity, and exposure duration, play a critical role in balancing transfection efficiency and cell viability. Sonoporation is often combined with microbubble contrast agents to improve gene uptake and precision targeting in tissues ^[4].

d. Magnetofection.

Magnetofection is a technique that couples nucleic acids with magnetic nanoparticles to achieve targeted gene delivery. In this method, DNA or RNA is complexed with superparamagnetic particles, and a magnetic field is applied to guide the complexes toward specific cells in culture. This enhances the local concentration of genetic material at the target site, increasing uptake and transfection efficiency. In laboratory studies, magnetofection has been successfully used for primary cells, stem cells, and other hard-to-transfect cell types. For in vivo applications, the DNA-magnetic particle complexes are administered systemically, and external magnets are used to direct them toward the target tissue. Once the complex reaches the desired location, the genetic material can be released from the nanoparticles through enzymatic cleavage, charge interactions, or degradation of the carrier matrix. This method offers precision delivery while minimizing exposure of non-target tissues to the gene construct ^[2,4].

2. Chemical methods

for non-viral delivery rely on molecules that can form complexes with DNA to facilitate cellular uptake. Cationic lipids, used in lipofection, create positively charged complexes with DNA that can fuse with cell membranes, promoting entry into the cell. Similarly, cationic polymers assemble into nanosized complexes called polyplexes, which offer enhanced stability and protection of DNA compared to lipid-based systems. Inorganic nanoparticles, composed of metals, metal oxides, or ceramics, can be coated to bind DNA efficiently, improving both cellular uptake and the expression of delivered genes. These physical and chemical non-viral strategies are increasingly applied in experimental gene therapies, tissue regeneration, and precision medicine, offering alternatives to viral vectors with lower immunogenicity and greater flexibility for repeated administration ^[2,4].

Chemical Methods for Enhancing DNA Delivery

a. Oligonucleotides.

Synthetic oligonucleotides are commonly employed in gene therapy to specifically target and silence disease-related genes. Antisense oligonucleotides bind to complementary sequences of defective mRNA, preventing its transcription and subsequent protein production. Small interfering RNAs (siRNAs) represent another approach, where they guide the cellular machinery to degrade specific mRNA sequences, effectively halting the expression of harmful genes. These strategies allow precise regulation of gene activity and have been explored in therapies for genetic disorders, cancer, and viral infections ^[2,4].

b. Lipoplexes and Polyplexes.

Polyplexes are formed when DNA is complexed with cationic polymers, which help condense the DNA into nanoparticles suitable for cellular uptake. The positively charged surface enhances interaction with negatively charged cell membranes, improving entry into cells. Similarly, lipoplexes are structures created by combining DNA with liposomes—vesicles made of lipid bilayers. Neutral or anionic liposomes can protect the DNA from enzymatic degradation while facilitating its delivery into cells. Both polyplexes and lipoplexes serve as synthetic, non-viral vectors for gene transfer and are often optimized to enhance transfection efficiency, stability, and biocompatibility ^[2].

c. Hybrid method

No single gene delivery method is perfect, so hybrid techniques have been developed to combine the strengths of multiple approaches. Virosomes, for example, merge liposomes with inactive viral particles such as HIV or influenza viruses. This combination allows efficient gene delivery into target cells, such as respiratory epithelial cells, surpassing the effectiveness of either viral or liposomal vectors alone. Hybrid strategies often involve combining viral vectors with cationic liposomes or modifying viral particles to reduce immunogenicity while maintaining high transfection efficiency. These approaches expand the toolbox for gene therapy, enabling the delivery of complex therapeutic constructs to a variety of tissues with improved safety and precision ^[1,3,4].

6. Gene Therapy Products.