Gene therapy is the process by which DNA sequences are delivered to cells with the goal of preventing, treating or curing disease. Advances in genomics and molecular biology have revealed that almost all diseases have a genetic component. In some cases, such as Huntington's disease, mutations in a single gene result in disease. In other scenarios, such as hypertension or high cholesterol, certain genetic variations may interact with environmental stimuli to cause disease. Pathological conditions associated with aging frequently result from the loss of gene activity in specific types of cells. Even viral or bacterial infections have a genetic component - the genes of the invading pathogen.
As the genetic and molecular basis for a multiplicity of diseases are elucidated, the prospect of gene-based therapies continues to grow. Although initial efforts in gene therapy focused on delivering a normal copy of a missing or defective gene, current programs are applying gene delivery technology across a broader spectrum of disease conditions. Gene delivery is now being used to:
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Deliver genes encoding therapeutic proteins
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Silence disease-causing genes
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Deliver viral or bacterial genes as a form of vaccination
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Replace missing or defective genes
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Deliver genes that promote the growth of new tissue or stimulate regeneration of damaged tissue
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Deliver genes that catalyze the destruction of cancer cells or cause cancer cells to revert back to normal tissue
The application of gene delivery technology to a growing roster of clinical indications is predicated on significant advances in both genomics and gene delivery systems. As an increasing number of genes are identified and assigned to specific biochemical pathways, the therapeutic utility of these genes becomes more apparent. In parallel with an enhanced understanding of the roles that genes play in health and disease have come improved vehicles, or vectors, for delivering therapeutic genes to target cells.
Gene Delivery Systems
Therapeutic genes can be introduced into target cells in a variety of ways. Gene delivery can be undertaken in cells that have been removed from the body (ex vivo gene therapy) or in the body itself (in vivo gene therapy). Genes can be introduced into cells using viruses, lipid-based (synthetic) vectors or naked DNA. The selection of a particular delivery vehicle depends on a number of factors, including target cell type, duration of gene expression required for therapeutic effect, the size of the DNA encoding the therapeutic gene and the route of administration. While no single vector developed to date is optimal for all clinical indications, the growing number of viral and synthetic vectors will enable gene therapy to be used in treating a wide variety of significant diseases.
Viral Vectors
Viral vectors take advantage of some virus's ability to enter cells and deliver genetic material. In developing viral vectors, DNA encoding some or all of the viral genes is removed and is replaced with a therapeutic gene. Most viral vectors also are engineered so that they are able to enter cells but lose their ability to replicate once inside.
Based on the virus from which it is derived, each type of viral vector has the ability to enter specific types of cells. While first generation viral vectors were thus restricted in their utility, newer vectors have been modified to meet specific objectives. In some cases this may mean engineering the vector so that it enters only one cell type while in other situations an expanded targeting capability may be desired. Modifications are made by changing some of the proteins that are expressed on the virus’s surface. These proteins interact with other molecules on the surface of target cells, similar to a lock-and-key combination. Another approach to targeting vectors is to take advantage of the natural variation that exists in viruses and select the one that meets a specific set of target criteria.
In addition to varying targeting properties, different vectors have specific characteristics that regulate whether they permanently become part of the host cell’s genetic material or exist transiently within the host cell. Depending on the therapeutic aim of a particular gene therapy, persistent or transient expression may be more or less desirable.
Synthetic Vectors
In addition to using viruses as vectors for gene delivery, there are a variety of non-viral vectors also being researched. These synthetic vectors generally are made up of lipids (fat molecules). Cell membranes contain a very high concentration of lipids. When a synthetic vector encounters the membrane of a cell, the lipids in the vector are incorporated into the lipids of the membrane, allowing the DNA contained within the vector to gain entry to the cell. This process is similar to how droplets of fat floating on water coalesce to form larger drops.
While some synthetic vectors encompass a circle of DNA carrying a therapeutic gene, other systems compact the DNA. This compaction makes the vector particles smaller and of more uniform size, properties that are particularly important in developing vectors for systemic gene delivery.
Just as a cell contains proteins and other molecules embedded in the lipids of its membrane, synthetic vectors can be engineered to carry targeting molecules on their surfaces. Such targeting moieties allow synthetic vectors to be directed to specific types of cells, which is important from the standpoint of both safety and systemic delivery.
Gene Expression Systems
In addition to choosing a relevant therapeutic gene and the appropriate gene delivery system, the ability to express (turn on) the delivered gene is a key factor in the development of successful gene therapies. Current approaches utilize a variety of promoters, DNA sequences that act as on/off switches for gene expression. Some promoters are active only in specific cell types, and are used to target gene activity to specific cells. Other promoters are expressed in a wide variety of cell types and may be used when cell-specific expression is not required. Promoter selection also depends on the level of gene expression that is desired. Inducible promoter systems, which can be turned on or off in response to certain drugs or compounds, also are being investigated, as are self-regulating expression systems that provide very high levels of expression. The variety of gene expression systems available and under development expands the potential applications of gene therapy to new indications.
Routes of Administration
Even after the gene, vector and expression system have been optimized, the route through which a gene therapeutic is administered influences the therapeutic effect of the product. In ex vivo gene therapy, gene transfer is performed in cells that have been removed from the patient. Once the therapeutic gene has been introduced into these cells they are transferred back to the patient.
For In-vivo gene therapy approaches, genes are transferred into cells within the patient’s body. The route of administration depends on the target tissue that needs to be treated as well as the mechanism through which the therapeutic gene elicits its effect. For example, therapies targeted to the joint may be administered by intra-articular injection directly to affected joints, vaccines may be administered intramuscularly and candidate therapeutics for several neurological diseases involve local delivery to the central nervous system. Some gene-based therapies may also be administered systemically via an intravenous injection or infusion.
Potential Benefits of Gene Therapy
A primary benefit of gene therapy is the ability to correct the underlying cause of genetic diseases. The delivery or inhibition of genes that play critical roles in the development or progression of diseases such as inflammatory arthritis, Huntington’s disease and congestive heart failure provides a mechanism through which these diseases may be corrected at the molecular level.
Gene therapy also holds the potential to provide patient-friendly treatment regimens for a variety of diseases. Today, patients with inflammatory arthritis and other diseases that are treated by the administration of therapeutic proteins must take daily or weekly injections in order to manage their disease. This is because proteins exist in the blood stream for a limited period of time before they are degraded or eliminated. Because DNA is more stable and functions inside the cell, the delivery of therapeutic genes may result in longer-term expression of therapeutic proteins. Gene-based therapies may require much less frequent dosing compared with therapeutic proteins.
As the genomics revolution identifies new genes and as gene delivery technologies continue to evolve, the role of gene therapy in the treatment of a wide variety of diseases is likely to grow.
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