Gene therapy involves introducing new, functional genetic material into a patient’s cells to replace or override a defective gene, correcting the underlying cause of many genetic disorders. To achieve this, the genetic material must be transported across the protective outer membrane of the target cell. Viruses, stripped of their harmful properties, are used as indispensable delivery tools known as viral vectors.
The Core Mechanism: Why Viruses Make Ideal Delivery Tools
Viruses are highly efficient delivery vehicles for genetic material, having evolved to recognize, bind to, and penetrate a host cell to deposit their genetic instructions. This process begins when the virus’s outer shell, or capsid, attaches to specific protein receptors on the surface of a human cell. Once attached, the virus injects its genetic payload (DNA or RNA) into the cell’s interior. This genetic material then co-opts the host cell’s internal machinery, reprogramming it to read the viral instructions. The virus’s preference for infecting particular cell types or tissues is known as tropism, which researchers exploit for targeted delivery.
Engineering the Viral Vector
Before therapeutic use, viruses must undergo bioengineering to ensure safety and effectiveness. This involves removing the native viral genes responsible for replication and causing disease, rendering the vector replication-defective. The vector can deliver a gene but cannot multiply or cause an infection. The space within the viral capsid is then filled with the therapeutic gene, which acts as the functional “payload.” This therapeutic DNA contains the instructions needed to produce the missing or corrected protein, allowing the engineered vector to enter cells and carry beneficial genetic instructions.
Key Types of Viral Vectors Used in Therapy
The choice of viral vector depends heavily on the specific disease, the target cell type, and the desired duration of gene expression.
Adeno-Associated Virus (AAV) vectors are currently the most frequently used in clinical applications due to their high safety profile and minimal immune response. AAV is effective for delivering genes to non-dividing cells (e.g., eye, muscle, or nervous system), and the therapeutic gene remains separate from the host chromosome, allowing for long-term expression. However, AAV’s utility is limited by its small packaging capacity, restricting the size of the genetic payload it can carry.
Lentivirus vectors, typically derived from the Human Immunodeficiency Virus, are engineered to integrate their genetic material directly into the host cell’s chromosome. This integration results in a permanent modification, making lentiviruses ideal for therapies requiring stable, lifelong gene expression. They are valuable for modifying frequently dividing cells, such as immune cells and hematopoietic stem cells. Their ability to transduce both dividing and non-dividing cells and their larger cargo capacity distinguish them from AAV vectors.
Adenovirus vectors offer an alternative, characterized by high efficiency in infecting a wide variety of cell types and a large capacity for carrying complex genetic payloads. Unlike lentiviruses, adenoviral vectors do not integrate their genetic material into the host chromosome. This non-integrating nature means the therapeutic gene exists transiently within the cell nucleus, leading to temporary rather than permanent expression. A drawback is their tendency to elicit a stronger immune response, which can limit the duration of the treatment effect.
Methods of Gene Delivery
Engineered viral vectors are delivered to the patient using one of two primary methods, defined by where the genetic modification takes place.
In Vivo Delivery
The in vivo method involves administering the vector directly into the body. Delivery is often accomplished by injecting the vector into a specific tissue or through intravenous infusion for systemic distribution. AAV vectors are frequently used here because their low immunogenicity allows the genetic cargo to reach internal organs while the cells remain inside the patient.
Ex Vivo Delivery
The ex vivo approach requires an intermediate step outside the body for cellular manipulation. Cells (e.g., blood or bone marrow) are removed from the patient and genetically modified in a laboratory setting, often using a lentivirus for stable integration. After modification, the corrected cells are grown and reinfused back into the patient.