Backgrounder: Human Gene Transfer Therapy for One Form of Childhood Blindness
A phase I clinical trial, supported by the National Eye Institute (NEI) at the National Institutes of Health (NIH), is assessing the safety of a gene transfer technique in people who have a genetic eye condition known as Leber congenital amaurosis (LCA). People who have LCA are born with severe visual impairment or develop vision loss early in childhood. Previous studies in animal models of LCA found this gene transfer technique, in which a healthy copy of a gene is injected into the eye, to be safe and effective in restoring visual function. The human gene therapy trial, now underway, is being conducted by investigators at the University of Pennsylvania and at the University of Florida.
In a phase I clinical trial, investigators evaluate the safety of a treatment. This is done by first administering a single, low dose in adults. A safety monitoring board, composed of medical experts, is providing independent oversight of the trial. Based on the results of the initial treatment group, the investigators may proceed with their plan to administer a higher dose for a second group of patients. If it is well tolerated, this higher dose will be used in subsequent clinical trials to evaluate the effectiveness of the treatment. Future trials may also assess the treatment in younger patients aged 13 to 18, and a second treatment may be administered.
LCA and the Visual Cycle
LCA is the name given to a family of severe retinal degenerative diseases that cause the deterioration and loss of light-sensing cells in the retina, known as rod and cone photoreceptors. Beneath the retina lies a single layer of cells called the retinal pigment epithelium (RPE). These cells help maintain the health and function of the rods and cones.
NEI-supported investigators have found that the form of LCA studied in this trial is caused by mutations in the RPE65 gene. Normally, the RPE65 gene produces the RPE65 protein. This protein converts dietary vitamin A into a retina-specific form of vitamin A that is shuttled to the rods, where it helps form rhodopsin. Rhodopsin is a visual pigment that absorbs light after it enters the eye, and it requires the RPE65 protein to regenerate after light exposure. Therefore, mutations in the RPE65 gene during LCA disrupt the visual cycle and prevent normal vision.
Gene Therapy for LCA
Although vision loss is severe in this form of LCA, the structure of the retina can remain relatively intact for decades before photoreceptor cells degenerate. This feature of the disease presents a window of opportunity for the development of therapies that could overcome or compensate for RPE65 gene defects.
In 1997, NEI investigators found that mutations in the RPE65 gene caused congenital vision loss in Briard dogs. In 2000, a team of researchers injected the eyes of three Briard dogs with a single dose of a gene transfer therapy containing functional copies of the RPE65 gene. Following this procedure, the dogs had significantly increased vision that allowed them to track movements and avoid objects when walking. Since then, the dogs have retained their vision with no sign of complications. The gene transfer procedure also corrected the dogs' rapid eye movements, called nystagmus, that were associated with LCA.
Gene Delivery: Vectors
One of the biggest challenges of gene therapy is developing a safe and effective gene delivery system, called a vector. Vectors are designed to target affected cells without evoking a damaging immune response. They are created by genetically modifying viruses, which have a natural ability to infect the cells in our body. By removing a virus' own genetic material and inserting the desired therapeutic gene, scientists can create vectors that act like a fleet of microscopic delivery trucks.
Several viruses have been used in vector development, but only a few have the particular features necessary to target retinal cells. For this clinical trial, a modified adeno-associated viral vector is being used, known as rAAV2-hRPE65.
This vector was selected because its properties are well understood. The particular virus strain is very common; approximately 85 percent of the U.S. population has been exposed to it, and it is not associated with any human disease. In addition, AAV vectors can target several retinal cell types and have long-lasting expression in target cells, reducing the need for frequent treatments--a key consideration for treating chronic retinal diseases such as LCA.
Pre-Clinical Safety Studies
Gene therapy is one of the most heavily monitored experimental methods in all of medicine. In addition to the oversight of the U.S. Food and Drug Administration (FDA), the NIH has an advisory body called the Recombinant DNA Advisory Committee (RAC), which reviews all NIH-supported gene-based therapies. These groups rigorously scrutinize proposed clinical trials to assure that risks to participants are minimized as much as possible.
These groups require pre-clinical safety studies before new treatments can be tested in patients. The 2000 study with Briard dogs proved that gene transfer therapy could be successful, but NIH investigators have also conducted numerous additional safety studies to gain regulatory approval. These involved testing the AAV-2 vector to determine how it moves beyond the RPE cell layer to other tissues and organs in the body; studying the responses of tissues and organisms to different doses of the vector; examining the short- and long-term effects of the therapy in animal models; and evaluating the procedure for injecting the therapeutic genetic material into the eye.
These and other pre-clinical studies gave FDA and RAC experts the confidence to approve this clinical trial.
Patient Selection Considerations
A challenge of this therapy is determining how late in the disease process successful therapeutic intervention can occur. Though mutations in the RPE65 gene cause severe vision loss, the structure of the retina remains intact for some indeterminate amount of time. However, a time may come when photoreceptor loss is so great that the disease is no longer treatable with gene transfer.
Using a high-resolution imaging technology known as optical coherence tomography, NEI-supported researchers have studied the structure of the retinas of people who have LCA and those without the disease to determine if it is possible to ascertain retinal health. Adults with LCA had evidence of thinning in some portions of the retina, indicating that cell loss had occurred. The study authors found that for gene transfer therapy to be successful in adults with LCA, researchers must target surviving tissue in the retina. Therefore, any potential patient for this clinical trial must undergo a retinal assessment to determine if their retinal tissue is still viable for gene transfer therapy.
Patients enrolled in this clinical trial receive an injection of fluid between the retina and RPE cell layers. The fluid contains the viral vector with the RPE65 gene inserted. Although this particular gene transfer treatment is new, the use of injections to target retinal diseases is common. Infrequent and minor complications associated with eye injections can typically be resolved.
Clinical Trial Safety
During the clinical trial, researchers monitor the patients' overall health and eye health. They perform systemic clinical examinations and blood testing to look for evidence of an immune reaction and to monitor the presence of the gene and the vector. They also conduct eye examinations to assess visual function and to ensure that the treatment does not affect existing vision. Additionally, an independent data safety and monitoring board reviews the progression of the trial, with a particular emphasis on safety.
Potential Risks and Benefits
Patients in this trial are carefully informed of all the potential risks and benefits of this investigative treatment.
The administration of the viral vector is associated with several potential risks: the vector could spread beyond the injection site to the bloodstream and to tears, or interact with other viruses within the patient, forming a new virus that could produce new side effects; the RPE65 gene could damage the DNA in retinal cells, putting the patient at risk for developing retinal tumors in the future (though this has not occurred in previous animal studies); or a cataract or retinal detachment could develop after the procedure, impacting vision and possibly requiring additional surgery. Other study procedures can also pose risks. For example, the electroretinogram test could cause a minor corneal abrasion, as the electrodes on the surface of the cornea measure the retina's electrical responses to light.
There are several potential benefits of this clinical trial as well. The phase I trial will provide knowledge that will increase the understanding about RPE65-associated LCA, a severe blinding disease. This increase in scientific knowledge may lead to a safe and effective prevention or treatment of vision loss in people who have LCA, and set a precedent for the development of gene-based therapies for other retinal degenerative diseases. Ultimately, if successful, the trial results will reveal the safety of the gene delivery method and the genetic material being used to treat LCA. A successful phase I clinical trial will also allow the investigators to proceed to phase II trials, which will evaluate the effectiveness of the therapy.