Johns Hopkins cardiologists have developed a technique for efficiently delivering genes to virtually all the cells in the heart, moving prospects for gene therapy for heart diseases over an imposing barrier.
Using a disabled version of a common virus, the adenovirus, as a carrier, Hopkins researchers "infected" nearly 100 percent of the muscle cells of a rabbit heart with a new gene.
"Our novel delivery system is the first to demonstrate virtually complete transduction of an intact heart and could be adapted to achieve widespread gene transfer in human hearts," says John Lawrence, M.D., assistant professor of cardiology and principal author of a recent paper in Proceedings of the National Academy of Sciences.
Many previous attempts to use an adenovirus to deliver genes have been made by "recombining" or genetically merging a disabled virus with genetic material. Scientists rely on the virus to carry this material in when it infects a cell. Until the Hopkins experiments, though, scientists had only been able to produce local or spotty infection (or transduction) in the heart.
In initial studies in isolated heart cells, Hopkins scientists identified specific factors that enhanced transduction, such as warm heart temperature, high concentration of the virus, and prolonged exposure to the virus.
The researchers put this information to work in whole rabbit hearts kept alive temporarily in the laboratory. Using the coronary arteries, researchers delivered high concentrations of a virus carrying a "reporter" gene--a gene for an enzyme whose presence in cells they could easily detect. They also collected the virus after it had passed through the heart and redelivered it several times.
"Our infection rates more than tripled the most successful previous experiments. We came very close to infecting 100 percent of all cells," says Lawrence. "We believe that the repeated passes through the heart gave the virus time to overcome structural barriers to its spread."
Changes in the function of heart muscle cells can cause a wide variety of diseases, including congestive heart failure, which decreases the heart's ability to contract and efficiently pump blood; familial hypertrophic cardiomyopathy, an inherited condition that over-thickens the muscle of the heart; and long QT syndrome, an inherited irregularity of the heartbeat. To cure them, Lawrence notes, researchers will need to modify the characteristics of individual diseased cells in a manner that can easily be applied throughout the heart. The new technique his laboratory developed makes gene therapy a potential way to do that, he says.
Lawrence says modern cardiac surgery techniques should enable doctors to create conditions similar to the rabbit experiments in human hearts, but further animal research is needed.
To make genes that can have a beneficial effect on the heart, Lawrence notes, scientists also need to learn more about how heart cells control heart functions like contraction and rhythm.
Other authors were Kevin Donahue, Kohei Kikkawa, David Johns and Eduardo Marban. The study was supported by the Four Schools Physician-Scientist Program, the Lucille P. Markey Foundation, the Tanabe Seiyaku Co. Ltd., and the Johns Hopkins University Merck Clinician Scientist Award.
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