April 24, 2006
UV Light Activates Gene Therapy Near Damaged Cartilage

University of Rochester Medical Center researchers have show that UV light can turn on gene therapy just in the cells that need it.

An early study has demonstrated for the first time that laser light can target gene therapy right up to the edge of damaged cartilage, while leaving nearby healthy tissue untouched, according to an article published in the April edition of the Journal of Bone and Joint Surgery. True repair of injuries to articular cartilage would enable millions of patients, currently consigned to worsening arthritis and joint replacement, to return to athletic exercise.

Study authors say that dramatic progress is being made toward a new form of light-activated gene therapy for cartilage repair that will be safe, fast, easy on patients and compatible with techniques used by most surgeons (e.g. arthroscopy). Beyond knee injuries, researchers believe the technology could one day guide precision gene therapy for cancer or heart disease, restore vision by repairing eye tissue and rebuild skin destroyed by burns.

UV light turns on stress kinase enzymes that turn on DNA polymerase that causes the single stranded DNA in the gene therapy delivery package to get converted into an active form.

The solution to the problem of how to target some cells for gene therapy, while missing their neighbors, came from a strange source: our cellular defenses against sunlight. The sun gives off ultraviolet (UV) light, which can cause destructive changes (genetic mutations) when exposed to sensitive molecules like DNA. If not defended against, the changes in DNA caused by UV light would cause humans to constantly develop cancer, for instance, in exposed tissue. Thus, an SOS system evolved that calls for genetic repairs when UV light causes too many mutations. Specifically, UV light turns on signaling proteins called stress kinases, which activate DNA polymerase, the enzyme that re-builds DNA chains when damaged.

Current technologies can direct UV light with great precision. That, combined with the ability of UV light to turn on DNA polymerase, has granted researchers the ability to turn on gene therapy in one cell, but not its neighbors. In recent years, researchers have been working to develop a system where UV light pre-treats target tissue, so that only the cells exposed to light gain the ability to copy themselves and grow. What remained was to find the right combination of vector and light to make the therapy safe as well as effective.

Recombinant adeno-associated virus (rAAV) turned out to be the right vector because it has evolved to deliver into the cell only a single strand of deoxyribonucleic acids (DNA), not the usual two strands of molecules. A second strand of DNA must be built by DNA polymerase to form active, double-stranded DNA before genes, or a gene therapy, can take effect. Single-stranded delivery is the key rAAV's usefulness as part of light-activated gene therapy because, of the all the cells infected with a gene therapy, only those struck by UV light will turn on DNA polymerase. Only those cells will activate the therapeutic gene, divide and re-grow tissue.

Cell damage from UV light is a concern.

The current study evaluated the ability of long-wavelength ultraviolet light to stimulate gene expression following infection by rAAV. Researchers evaluated the safety and efficacy of long-wavelength ultraviolet laser light to induce light-activated gene therapy in articular cartilage cells (chondrocytes). The study authors believe this is the first demonstration that site-directed gene delivery can safely and effectively treat articular defects in higher animal cartilage cells.

Given the safety concerns found with short wavelength, researchers were excited to find that the new long wavelength system is an order of magnitude more likely to turn on gene therapy as designed than to cause death by mutation (cytotoxity). Along with previous studies, the current research found rAAV to be highly efficient at turning on gene therapy in articular chondrocytes. Pretreatment with 6000 Joules per meter squared, a standard dose of UV light, led to a tenfold increase in the effect of gene therapy in target cells after one week. In addition, nearly half of cells exposed to the light expressed the inserted, therapeutic gene.

If the UV light was an order of magnitude more likely to turn on the gene therapy than to kill the target cells then was the gene therapy activated in every 10 cells per 1 cell that died from the UV? That doesn't sound so exciting. If the 1 cell died then the other 10 cells might have suffered some permanent DNA damage from the UV. But maybe the press release isn't explaining the actual results well.

This approach could be refined to allow the use of smaller doses of UV. Imagine a molecule activated by UV light and delivered along with the gene therapy. Such a molecule, properly chosen, could activate DNA polymerase. Finding such a molecule would be hard. Though development of a better understandnig of how UV light activates DNA polymerase might lead to identification of such a molecule. Mechanisms for amplifying the selective effects of UV light would allow smaller amounts of UV light to be used.

Share |      Randall Parker, 2006 April 24 10:45 PM  Biotech Gene Therapy


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