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.
How does gene therapy work?
Genes are composed of DNA that carries information needed to make proteins – the building blocks of our bodies. Variations in the DNA sequence or code of a gene are called mutations, which often are harmless but sometimes can lead to serious disease. Gene therapy treats disease by “repairing” dysfunctional genes or by providing copies of missing genes.
To reverse disease caused by genetic damage, researchers isolate normal DNA and package it into a vehicle known as a vector, which acts as a molecular delivery truck. Vectors composed of viral DNA sequences have been used successfully in human gene therapy trials. Doctors infect a target cell — usually from a tissue affected by the illness, such as liver or lung cells—with the vector. The vector unloads its DNA cargo, which then begins producing the proper proteins and restores the cell to normal. Problems can arise if the DNA is inserted into the wrong place in the genome. For example, in rare instances the DNA may be inserted into a regulatory gene, improperly turning it on or off, leading to cancer.
Researchers continue to optimize viral vectors as well as develop non-viral vectors that may have fewer unexpected side effects. Nonviral gene delivery involves complexing DNA with an agent that allows it to enter a cell nonspecifically. DNA delivered in this manner is usually expressed for only a limited time because it rarely integrates into the host cell genome.
Initial efforts in gene therapy focused on delivering a normal copy of a missing or defective gene, but current programs are applying gene delivery technology across a broader spectrum of conditions. Researchers are now utilizing gene therapy to :
- Deliver genes that catalyze the destruction of cancer cells or cause cancer cells to revert back to normal tissue
- Deliver viral or bacterial genes as a form of vaccination
- Deliver genes that promote the growth of new tissue or stimulate regeneration of damaged tissue
What diseases could be treated with gene therapy?
About 4,000 diseases have been traced to gene disorders. Current and possible candidates for gene therapy include cancer, AIDS, cystic fibrosis, Parkinson’s and Alzheimer’s diseases, amyotrophic lateral sclerosis (Lou Gehrig's disease), cardiovascular disease and arthritis.
In cases such as cystic fibrosis or hemophilia, disease results from a mutation in a single gene. In other scenarios like hypertension or high cholesterol, certain genetic variations may interact with environmental stimuli to cause disease.
Has gene therapy been successfully used in humans?
Gene therapy is likely to be most successful with diseases caused by single gene defects. The first successful gene therapy on humans was performed in 1990 by researchers at the National Institutes of Health. The therapy treated a four-year-old child for adenosine deaminase (ADA) deficiency, a rare genetic disease in which children are born with severe immunodeficiency and are prone to repeated serious infections.
Since 1990, gene therapy had been tested in human clinical trials for treating such diseases as severe combined immunodeficiency disease (SCID), cystic fibrosis, Canavan's disease, and Gaucher's disease. In 2003, more than 600 gene therapy clinical trials were under way in the United States but only a handful of these are in advanced stages. SCID, in which children lack natural defenses against infection and can only survive in isolated environments, remains the only disease cured by gene therapy.
Are genetic alterations from gene therapy passed on to children?
Gene therapy can be targeted to somatic (body) or germ (egg and sperm) cells. In somatic gene therapy, the patient’s genome is changed, but the change is not passed along to the next generation. In germline gene therapy, the patient’s egg or sperm cells are changed with the goal of passing on changes to their offspring. Existing gene therapy treatments and experiments are all somatic.
Germline gene therapy is not being actively investigated in larger animals and humans for safety and ethical reasons. In September 2000, the American Association for the Advancement of Science (AAAS) called for a moratorium on attempts to cure genetic diseases through human germline gene therapy. While its report supported expanded basic research in the field of clinical gene therapy, AAAS concluded that neither science nor society is ready for germline gene therapy research.
Learn more about federal policies and regulations protecting those who volunteer to participate in biomedical research.
U.S. Department of Energy Office of Science, Office of Biological and Environmental Research, Human Genome Program
American Society of Gene Therapy
Targeted Genetics Corp.