Human DNA codes for the expression of everything from hair color to metabolism to genetic disorders. DNA with mutations causes some of these disorders. Gene therapy replaces or repairs these mutations, by providing “correct” DNA that is attached to a vector. A vector is a vehicle for the DNA that is put together by researchers. Most vectors are harmless viruses, although researchers are also developing non-viral vectors.
According to the American Medical Association (AMA), “Gene therapy is a novel approach to treat, cure, or ultimately prevent disease by changing the expression of a person’s genes. Gene therapy is in its infancy, and current therapies are primarily experimental, with most human clinical trials still in the research stages.”
Gene therapy is hopeful for the treatment of 4,000 diseases caused by genetic disorders. These include cancer, AIDS, cystic fibrosis, Parkinson’s and Alzheimer’s disease, Lou Gehrig’s disease, cardiovascular disease, and arthritis.
Positives of Gene Therapy
There are many possible functions of gene therapy (AMA). The first, mentioned above, involves replacing defective or missing genes with normal ones. Another possibility is the delivery of genes that can cause the death of cancer cells, or can cause cancerous cells to become normal again. A third function is the delivery of genes from bacteria or viruses to prevent infections from them, as a vaccination. Another function is the delivery of genes to promote new tissue growth or regeneration of damaged tissue.
In 1990, the National Institute of Health performed the first successful gene therapy on a human. It was for a four-year old child with adenosine deaminase (ADA) deficiency, which is caused by a single gene defect. Since that time, human clinical trials have included such diseases as cystic fibrosis, severe immunodeficiency disease (SCID), Canavan’s disease, and Gaucher’s disease. SCID is the only disease that has been cured by gene therapy.
The Journal of Gene Medicine provided the following data on diseases being addressed in gene therapy clinical trials in 2009: Cancer diseases composed 64.5% of worldwide clinical trials, Cardiovascular diseases were 8.7%, monogenic diseases were 7.9%, infectious diseases were 8%, and neurologic diseases were 1.9%. The complete table can be found here.
Shortcomings and Further Research
An article published in The Journal of the American Medical Association (AMA) discusses gene therapy in clinical settings. Gene therapy clinical trials are classified by the stages. For a gene therapy product to be available on the market, the clinical trial must be at Phase 2 or 3. However these phases are “extremely expensive and would require financial support by pharmaceutical or biotechnology companies” (AMA). Most inherited diseases are rare, so “there is little potential for return on investments in expensive research and clinical trials.”
Another problem with gene therapy is that it has only been successful at diseases caused by a single gene mutation. For common diseases like heart disease and cancer that involve a large number of genes, a new approach for gene therapy must be found (AMA). Obstacles include introducing the new gene to a large enough number of cells, preventing the cells from destroying it as foreign, and preventing infectious contaminates from entering at the commercial manufacturer site.
Stem Cells and Gene Therapy
Another possibility, published in The Journal of the American Medical Association, causes for the combination of neural stem cells and gene therapy (JAMA). Spinal cord injuries can be especially overwhelming, yet research has found that some areas of the central nervous system (CNS) continue to produce neurons. These stem cells, which are undifferentiated, have the capacity to become any cell in the body, including neurons. These cells can be isolated and kept in a culture. Then, “when transplanted back into the CNS, these stem cells have the capacity to migrate, to integrate with the host tissue, and to respond to local cues for differentiation.”
One practical application of this theory is the treatment of Parkinson’s disease. Parkinson’s is a result of a decrease in dopamine caused by the destruction of dopamine-producing cells. Stem cells can be grafted to become dopamine-producing cells, and then used to replace the degenerated cells.
“Grafted neural stem cells could potentially replace cells lost to injury, reconstitute the neuronal circuitry, and provide a relay station between the injured pathways above and below the lesion” (AMA). This is a promising theory for the treatment of neurologic diseases, but more research and clinical trials must be done before a “cure” is found.
Genetic therapy has only targeted the cells that are not passed to further generations. This is called somatic gene therapy. In germline gene therapy, sperm and egg cells are changed to pass genes onto the next generation. This type of therapy is not being investigated. In 2000, the American Association for the Advancement of Science (AAAS) provided a report that called for a moratorium on curing diseases using germline gene therapy.