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Gene therapy is a field of biomedical research with the goal of influencing the course of various genetic and acquired (so-called multi factorial) diseases at the DNA/RNA level. Cell therapy aims at targeting various diseases at the cellular level, i.e. by restoring a certain cell population or using cells as carriers of therapeutic cargo. For many diseases, gene and cell therapy are applied in combination. In addition, these two fields have helped provide reagents, concepts, and techniques that are illuminating the finer points of gene regulation, stem cell lineage, cell-cell interactions, feedback loops, amplification loops, regenerative capacity, and remodeling.

Gene Therapy Definition:

Gene therapy is the introduction, removal, or change in the content of a person’s genetic code with the goal of treating or curing a disease. Moreover, it is a set of strategies that modify the expression of an individual’s genes or repair abnormal genes. Each strategy involves the administration of a specific nucleic acid (DNA or RNA). Nucleic acids are normally not taken up by cells, thus special carriers, so-called ‘vectors’ are required. Vectors can be of either viral or non-viral nature.

Cell Therapy Definition:

Cell therapy is defined as the administration of living whole cells for the patient for the treatment of a disease. The origin of the cells can be from the same individual (autologous source) or from another individual (allogeneic source). Cells can be derived from stem cells, such as bone marrow or induced pluripotent stem cells (iPSCs), reprogrammed from skin fibroblasts or adipocytes. Stem cells are applied in the context of bone marrow transplantation directly. Other strategies involve the application of more or less mature cells, differentiated in vitro (in a dish) from stem cells.

Overview of Gene Therapy and Cell Therapy

Historically, the discovery of recombinant DNA technology in the 1970’s provided the tools to efficiently develop gene therapy. Scientists used these techniques to readily manipulate bacterial and viral genomes, isolate genes, identify mutations involved in human diseases, characterize and regulate gene expression and produce human proteins from genes (e.g. production of insulin in bacteria revolutionized medicine). Later, various viral and non-viral vectors were developed along with the development of regulatory elements (e.g. promoters that regulate gene expression), which are necessary to induce and control gene expression. Gene transfer in animal models of disease have been attempted and led to early success. Various routes of administrations have been explored (injection into the bloodstream, into the ventricles of the brain, into muscle etc).

The development of suitable gene therapy treatments for many genetic diseases and some acquired diseases has encountered many challenges, such as immune response against the vector or the inserted gene. Current vectors are considered very safe and recent gene therapy trials documented excellent safety profile of modern gene therapy products. Further development 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 many genetic and inherited diseases are elusive today, some success is being observed in the treatment of several types of immunodeficiency diseases, cancers, 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 medical treatment for many diseases, including cancer, immune deficiency and others. Cell therapy is expanding its repertoire of cell types for administration. Cell therapy treatment strategies include: isolation and transfer of specific stem cell populations, induction of mature cells to become pluripotent cells, administration of effector cells and reprogramming of mature cells into iPSCs. Administration of large numbers of effector cells has benefited cancer patients, transplant patients with unresolved infections, and patients with vision problems.

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.

Genome editing

Genome editing (gene editing) has recently gained significant attention, due to the discovery and application of the clustered regularly interspaced short palindromic repeats (CRISPR) system. Actually, genome editing dates back several years and earlier generation genome editing systems are currently tested in clinical trials (such as zinc-finger nucleases). The aim of genome editing is to disrupt a disease-causing mutation or correct faulty genes at the chromosomal DNA. Genome editing can be performed in the patient’s own cells in vitro and edited cells can be administered to the patient (thus genome editing can be combined with cell therapy). However, it is also possible to perform genome editing in vivo by administering the genome editing agent packaged in viral and non-viral vectors.

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