Gene therapy and cell therapy are overlapping fields of biomedical research with the goals of repairing the direct cause of genetic diseases in the DNA or cellular population, respectively. These powerful strategies are also being focused on modulating specific genes and cell sub-populations in acquired diseases in order to reestablish the normal equilibrium. In many diseases, gene and cell therapy are combined in the development of promising therapies.
In addition, these two fields have helped provide reagents, concepts, and techniques that are elucidating the finer points of gene regulation, stem cell lineage, cell-cell interactions, feedback loops, amplification loops, regenerative capacity, and remodeling.
Gene Therapy Defined
Gene therapy is defined as a set of strategies that modify the expression of an individual’s genes or that correct abnormal genes. Each strategy involves the administration of a specific DNA (or RNA).
Cell Therapy Defined
Cell therapy is defined as the administration of live whole cells or maturation of a specific cell population in a patient for the treatment of a disease.
Overview of Gene Therapy and Cell Therapy
Historically, the discovery of recombinant DNA technology in the 1970s provided the tools to efficiently develop gene therapy. Scientists used these techniques to readily manipulate viral genomes, isolate genes, identify mutations involved in human diseases, characterize and regulate gene expression, and engineer various viral vectors and non-viral vectors. Many vectors, regulatory elements, and means of transfer into animals have been tried. Taken together, the data show that each vector and set of regulatory elements provides specific expression levels and duration of expression. They exhibit an inherent tendency to bind and enter specific types of cells as well as spread into adjacent cells. The effect of the vectors and regulatory elements are able to be reproduced on adjacent genes. The effect also has a predictable survival length in the host. Although the route of administration modulates the immune response to the vector, each vector has a relatively inherent ability, whether low, medium or high, to induce an immune response to the transduced cells and the new gene products.
The development of suitable gene therapy treatments for many genetic diseases and some acquired diseases has encountered many challenges and uncovered new insights into gene interactions and regulation. Further development often involves uncovering basic scientific knowledge of the affected tissues, cells, and genes, as well as redesigning vectors, formulations, and regulatory cassettes for the genes.
While effective long-term treatments for anemias, hemophilia, cystic fibrosis, muscular dystrophy, Gauscher’s disease, lysosomal storage diseases, cardiovascular diseases, diabetes, and diseases of the bones and joints are elusive today, some success is being observed in the treatment of several types of immunodeficiency diseases, cancer, and eye disorders.
Historically, blood transfusions were the first type of cell therapy and are now considered routine. Bone marrow transplantation has also become a well-established protocol. Bone marrow transplantation is the treatment of choice for many kinds of blood disorders, including anemias, leukemias, lymphomas, and rare immunodeficiency diseases. The key to successful bone marrow transplantation is the identification of a good "immunologically matched" donor, who is usually a close relative, such as a sibling. After finding a good match between the donor’s and recipient’s cells, the bone marrow cells of the patient (recipient) are destroyed by chemotherapy or radiation to provide room in the bone marrow for the new cells to reside. After the bone marrow cells from the matched donor are infused, the self-renewing stem cells find their way to the bone marrow and begin to replicate. They also begin to produce cells that mature into the various types of blood cells. Normal numbers of donor-derived blood cells usually appear in the circulation of the patient within a few weeks. Unfortunately, not all patients have a good immunological matched donor. Furthermore, bone marrow grafts may fail to fully repopulate the bone marrow in as many as one third of patients, and the destruction of the host bone marrow can be lethal, particularly in very ill patients. These requirements and risks restrict the utility of bone marrow transplantation to some patients.
Cell therapy is expanding its repertoire of cell types for administration. Cell therapy treatment strategies include isolation and transfer of specific stem cell populations, administration of effector cells, induction of mature cells to become pluripotent cells, and reprogramming of mature cells. Administration of large numbers of effector cells has benefited cancer patients, transplant patients with unresolved infections, and patients with chemically destroyed stem cells in the eye. For example, a few transplant patients can’t resolve adenovirus and cytomegalovirus infections. A recent phase I trial administered a large number of T cells that could kill virally-infected cells to these patients. Many of these patients resolved their infections and retained immunity against these viruses. As a second example, chemical exposure can damage or cause atrophy of the limbal epithelial stem cells of the eye. Their death causes pain, light sensitivity, and cloudy vision. Transplantation of limbal epithelial stem cells for treatment of this deficiency is the first cell therapy for ocular diseases in clinical practice.
Several diseases benefit most from treatments that combine the technologies of gene and cell therapy. For example, some patients have a severe combined immunodeficiency disease (SCID) but unfortunately, do not have a suitable donor of bone marrow. Scientists have identified that patients with SCID are deficient in adenosine deaminase gene (ADA-SCID), or the common gamma chain located on the X chromosome (X-linked SCID). Several dozen patients have been treated with a combined gene and cell therapy approach. Each individual’s hematopoietic stem cells were treated with a viral vector that expressed a copy of the relevant normal gene. After selection and expansion, these corrected stem cells were returned to the patients. Many patients improved and required less exogenous enzymes. However, some serious adverse events did occur and their incidence is prompting development of theoretically safer vectors and protocols. The combined approach also is pursued in several cancer therapies.