Finding Right Target Still Tough With Gene Therapy

FRIDAY, June 3, 2005 (HealthDay News) -- For many at the cutting edge of gene therapy, the problem is, literally, how to get it there.

Some of the most challenging kinks still to be worked out in this field have less to do with the gene part of the equation and more to do with the delivery system, better known as vectors.

A vector, usually an inactivated virus, is used to transport the DNA into the cells where it will, theoretically, correct the original problem.

For a long time, unintended side effects were not seen in gene therapy simply because the success rates were so low. With success, however, have come problems.

"We're now seeing the double-edged sword," said David W. Emery, a research associate professor of medicine at the University of Washington in Seattle and author of a paper being presented this week at the American Society of Gene Therapy annual meeting in St. Louis.

Among the problems: inserting the vector in the right place; triggering the immune system to kill the cells where the vector is inserted; and targeting the vector so it only inserts itself into the cells where it is needed.

Emery's paper provided the first evidence that using "chromosomal insulators" could improve the safety of vectors.

Many vectors (in this case, deactivated viruses) contain "enhancers" that will turn on a gene. On rare occasions, the vector lands in or near a critical gene, such as a gene that can cause cancer, and turns that on, too.

This appears to be what happened to three of 10 individuals with SCID (Severe Combined Immunodeficiency) who developed leukemia after their immune systems had been restored by gene therapy.

Fortunately, nature has also provided chromosomes with "insulators" which break the chromosomes up into autonomous regions so that an enhancer in one region doesn't inadvertently turn on a gene in another region.

Emery and his colleagues inserted vectors with and without insulators into mice. The vectors without insulators turned on cancer-causing oncogenes while the ones without insulators did not. This reduced the formation of tumors by about fivefold, Emery said.

The insulators have the added benefit of making sure the vector is not turned off.

Emery is hoping that insulators will become routine practice in the next few years.

While about 75 percent of gene therapy uses viruses, the remaining 25 percent uses non-viral technology. This is a promising approach in that it can avoid the immune problems elicited by viruses.

In working with Duchenne Muscular Dystrophy, for instance, researchers have discovered that one of their major challenges is how to correct defects in every one of billions or trillions of muscle cells in the body without tripping the immune system's "red alert" response.

Various options are being investigated to solve these types of problems, said David Bodine, section chief with the National Human Genome Research Institute of the National Institutes of Health. These include identifying what part of the virus is prompting the immune response and toying with that or with the patient's response to it.

Non-viral approaches may also hold the key. The technique is simple, Bodine said: Put DNA and lipids or fats in a test tube, add water and shake. The end result is something like a salad dressing with DNA dissolved in water inside bubbles of fat (which doesn't dissolve in water). If those fats are inserted into a person, they and the DNA they contain will be taken up by cells.

The science of viral vectors is, at this point, further along than that of non-viral vectors. The latter is, however, a "growth area," Bodine said.

But both still have their wrinkles.

"The technology is easy to understand but it certainly has not been mastered by anyone yet," Bodine added.

More information

Visit the Human Genome Project for more on gene therapy.