Gene therapy is a still somewhat experimental treatment that involves introducing genetic material into a person’s cells to fight or prevent disease. Researchers are studying gene therapy for a number of diseases, such as severe combined immuno-deficiencies, haemophilia, Parkinson’s disease, cancer and HIV, via a number of different approaches.
A gene can be delivered to a cell using a carrier known as a “vector.” The most common types of vectors used in gene therapy are viruses. The viruses used in gene therapy are altered to make them safe, although some risks still exist with gene therapy. The technology is still in its infancy, but it has been used with some success.
In most gene therapy studies, a “normal” gene is inserted into the genome to replace an “abnormal,” disease-causing gene. In cancer, some cells become diseased because certain genes have been permanently turned off. Using gene therapy, mutated genes that cause disease could be turned off so that they no longer promote disease, or healthy genes that help prevent disease could be turned on so that they can inhibit the disease.
Other cells may be missing certain genes. Researchers hope that replacing missing or defective genes can help treat certain diseases. For example, a common tumour suppressor gene called p53 normally prevents tumour growth in your body. Several types of cancer have been linked to a missing or inactive p53 gene. If doctors could replace p53 where it’s missing, that might trigger the cancer cells to die.
Gene therapy stalled in the early 2000s as adverse effects came to light in European trials (leukemias triggered by the gene delivery vector) and following the 1999 death of U.S. patient Jesse Gelsinger. But after 30 years of development, and with the advent of safer vectors, gene therapy is becoming a clinical reality.
Types of Gene Therapy
Virtually all cells in the human body contain genes, making them potential targets for gene therapy. However, these cells can be divided into two major categories: somatic cells (most cells of the body) or cells of the germline (eggs or sperm). In theory it is possible to transform either somatic cells or germ cells.
Gene therapy using germ line cells results in permanent changes that are passed down to subsequent generations. If done early in embryologic development, such as during pre-implantation diagnosis and in vitro fertilisation, the gene transfer could also occur in all cells of the developing embryo.
The appeal of germ line gene therapy is its potential for offering a permanent therapeutic effect for all who inherit the target gene. Successful germ line therapies introduce the possibility of eliminating some diseases from a particular family, and ultimately from the population, forever.
Somatic cells are non-reproductive. Somatic cell therapy is viewed as a more conservative, safer approach because it affects only the targeted cells in the patient, and is not passed on to future generations. In other words, the therapeutic effect ends with the individual who receives the therapy. However, this type of therapy presents unique problems of its own.
Often the effects of somatic cell therapy are short-lived. Because the cells of most tissues ultimately die and are replaced by new cells, repeated treatments over the course of the individual’s life-span are required to maintain the therapeutic effect. Transporting the gene to the target cells or tissue is also problematic.
Regardless of these difficulties, however, somatic cell gene therapy is appropriate and acceptable for many disorders, including cystic fibrosis, muscular dystrophy, cancer, and certain infectious diseases. Clinicians can even perform this therapy in utero, potentially correcting or treating a life-threatening disorder that may significantly impair a baby’s health or development if not treated before birth.
Somatic Gene therapy falls into two main categories:
1. In Vivo: Genes are changed in cells still in the body. The gene is transferred to cells inside the patient’s body via direct injection of the gene therapy vector, carrying the desired gene, into the bloodstream or target organ.
2. Ex Vivo: A patient’s cells are removed, modified outside the body and then e-infusing them back into the patient again, as in haematopoietic stem cell transplant and CAR T-cell therapy.
How are the Genes Delivered into Cells?
A carrier molecule called a vector must be used to deliver the therapeutic gene to the patient’s target cells.
The most common gene therapy vectors are viruses because they can recognize certain cells and carry genetic material into the cells’ genes.
Researchers are trying to take advantage of this unique capability. They:
- Remove the original disease-causing genes from the viruses;
- Replace them with the genes needed to stop disease;
- Insert the altered viruses into a person’s diseased cells to deliver their genetic material.
Different Viruses used as Gene Therapy Vectors:
- Adenoviruses – A class of viruses with double-stranded DNA genomes that cause respiratory, intestinal, and eye infections in humans. The virus that causes the common cold is an adenovirus.
- Adeno-Associated Viruses (AAV) – These are small DNA viruses that integrate successfully in one spot of the host’s genome (on chromosome 19 in humans). They can’t replicate by themselves and therefore require a helper virus, either adenovirus or herpes virus. Also they are non-pathogenic (not known to cause disease).
- Herpes Simplex Viruses – A class of double-stranded DNA viruses that infect a particular cell type, neurons. Herpes simplex virus type 1 is a common human pathogen that causes cold sores.
- Retroviruses – A class of viruses that can create double-stranded DNA copies of their RNA genomes. These copies of its genome can be integrated into the chromosomes of host cells. Human immunodeficiency virus (HIV) is a retrovirus.
Stem cells create all other cells with specialized functions in the body. In gene therapy, stem cells can be altered in a laboratory to accept new genes that can help fight disease.
These fatty particles are nanoscale devices that have the ability to carry the new, therapeutic genes to the target cells and pass the genes into the cells’ DNA. Liposomes were developed with nanotechnology.
The current focus in the field of gene therapy is on development of genetic drugs that are capable of treating diseases such as cancer and inflammation.
Using the Immune System
In some cases, the immune system does not attack diseased cells because it does not recognise them as intruders or “non-self.”
Using gene therapy, physicians could potentially infuse tumour cells with genes that make them more recognisable to your immune system. Enhancements could also be made to immune cells to make it easier for them to recognise mutated tumour cells.
The Future of Gene Therapy
Gene therapy has sparked great interest because it offers the possibility of a permanent cure. It’s of high interest in the rare disease community in particular, for single-gene diseases such as sickle-cell disease, severe combined immune deficiency (SCID, a.k.a. “bubble boy” disease), adrenoleukodystrophy and other metabolic diseases, hemophilia and genetic forms of progressive blindness and deafness.
There are as many as 7,000 rare diseases, and gene therapy for each would have to go through a different regulatory process. In the rare disease space, populations are often very small, making it hard to put a trial together. And because the diseases are rare, their natural history is often unknown, making it harder to establish a benefit for gene therapy. There’s no single process: Each disease has its own biological pathway and its own research path to gene therapy. Navigating this takes time and resources.
Potential Advantages of Gene Therapy: May Optimise Cancer Treatments
- Inserting genes into cancer cells to make them more sensitive to chemotherapy,
radiation therapy, or other treatments
- Removing healthy blood-forming stem cells from the body, inserting a gene that makes these cells more resistant to the side effects of high doses of anticancer drugs, and then injecting the cells back into the patient
- Introducing “suicide genes” into a patient’s cancer cells, giving a pro-drug (an inactive form of a toxic drug), activating cancer cells containing these “suicide genes” with the pro-drug to destroy cancer cells
- Preventing cancer cells from developing new blood vessels, or angiogenesis
News & Research
“These are patients who really are without hope. Patients who at best could expect to have a one in 10 chance of having a complete disappearance of their lymphoma,” Dr. Frederick Locke, an oncologist at Moffitt Cancer Center in Tampa, Florida, said. “So the results are really exciting and remarkable.”
“More than 80 percent of the 101 patients who got the treatment were still alive six months later. “Only about half the patients who (went) on this study could expect to even be alive six months after the therapy,” Dr. Locke, added.
The roots of pediatric cancer are hidden deep within a child’s DNA. The St. Jude—Washington University Pediatric Cancer Genome Project is the world’s most ambitious effort to discover the origins of childhood cancer and seek new cures.
To date, results from clinical studies of gene therapies pioneered at St. Jude have been promising. St. Jude researchers are now focused on applying investigational gene therapy approaches to treat X-linked SCID, hemophilia B and a related disorder, hemophilia A. The long-term vision is to apply these technologies to other genetic diseases, such as sickle cell disease, and to explore applications in developing effective immune therapies for cancer.
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