DURHAM, N.C. -- Scientists at Duke University Medical Center have delivered therapeutic genes throughout a rabbit's heart and have shown that the genes can both boost heart function on their own and also increase sensitivity to heart-stimulating drugs.
According to the researchers, the experiments, reported in the July 1 Journal of Clinical Investigation, are a crucial step in developing a genetic treatment for congestive heart failure. This debilitating and deadly condition develops when heart muscle loses its ability to stretch and contract, usually due to clogged arteries caused by coronary artery disease. People with congestive heart failure often experience fatigue, weakness and an inability to carry out routine daily tasks. There is currently no effective means to reverse heart failure, only to treat symptoms.
The research is funded by grants from the National Institutes of Health and the American Heart Association (AHA).
According to the AHA, about 400,000 new cases of congestive heat failure are recorded every year in the United States. Death rates from congestive heart failure tripled between 1974 and 1994, making it the leading cause of hospitalization among people 65 and older and costing more than $10 billion a year.
Research team leader Walter J. Koch, an associate professor of experimental surgery, and his colleagues have been working for several years to find ways to efficiently deliver genes to the heart to boost heart function.
In their experiments, the scientists first incorporated the therapeutic genes into a live but disabled common cold virus. Then, in a surgical technique that was a key to their success, the scientists injected the virus into the left ventricle of live rabbits while the aorta was clamped for a few seconds. This technique allowed the virus enough time to spread through all the coronary vessels to reach a majority of the heart muscle. Clamping the aorta is sometimes used in human heart surgery, Koch said, making this gene delivery method feasible.
In the current experiments, the effect of the introduced genes lasted for about three weeks. Koch said that he believes the immune system eventually clears the adenovirus from the body, eliminating the therapeutic effect. Koch emphasized that more effective methods of carrying the genes into cells need to be developed before gene therapy for heart failure in people becomes a practical option. The researchers say that with the development of improved viruses and other vehicles to introduce genes into the heart, the technique might be used on congestive heart failure patients within the next decade.
The latest achievement built on earlier basic studies in the lab of Dr. Robert Lefkowitz, a Howard Hughes Medical Institute investigator at Duke. Using mouse models and sophisticated genetic techniques, Lefkowitz and his colleagues identified the molecules responsible for fostering efficient pumping action in the heart and showed which ones can make the heart function more efficiently.
The Duke scientists showed that two key proteins in heart cells - called beta-adrenergic receptors (ßARs) and beta adrenergic receptor kinase (ßARK) -- work together to regulate heart function. Receptors such as ßAR are molecular switches that turn on cell processes when triggered by external substances such as hormones.
In reacting to a failing heart, the body releases large amounts of the hormone norepinephrine directly into the heart. This norepinephrine binds to ßARs on the surface of heart cells, triggering the heart to work up to five times harder than normal. While this overstimulation initially allows the heart to increase the power of its contractions, in heart failure the stimulation quickly becomes self-defeating, because the receptors become desensitized, meaning they no longer respond to hormone stimulation.
This desensitization is caused by ßARK, an enzyme inside the cell that in healthy hearts helps restore normal heart contractions after norepinephrine stimulation. In failing hearts, however, ßARK levels rise and the number of ßARs is greatly reduced.
In 1994, Lefkowitz, Koch and their colleagues showed that mice genetically altered to produce excess beta adrenergic receptors (ßARs) have supercharged hearts that beat faster and stronger than a normal mouse's heart. In 1997, the researchers inserted a gene that encodes the ßAR into an adenovirus.
"Our studies have shown that a malfunctioning beta adrenergic receptor system greatly contributes to heart failure," Koch said. "By contrast, boosting levels of beta adrenergic receptors or inhibiting ßARK can reverse heart failure in genetic mouse models. Now, we are beginning to see the same result when we deliver these genes to rabbits."
In their earlier test tube experiments Koch and his colleagues allowed the virus to infect isolated failing rabbit heart cells. They found that the inserted ßAR genes produced up to 15 times the normal amount of the ßAR protein found in rabbit cells, which restored normal heart signaling.In the current experiments, the animals' heart cells produced the equivalent of 10 times the normal number of ßARs. As a result, they shows a 20 percent increase in the force of their heart contractions.
In addition, the researchers tested the effects of a range of concentrations of ßARs and found that as they increased ßAR, they increased the force of contraction to a maximum of 20 percent.
The researchers tested whether the ßARs would respond to the drug isoproteronol, which stimulates ßARs. When researchers injected the drug into both normal rabbits and rabbits with the additional ßARs, they found the rabbit hearts with the additional ßARs responded to the drug even more so than the normal rabbits.
"Our results with the isoproterenol challenge show that the ßARs we have inserted into the heart are functioning and responding just like the native ßAR," said Koch.
Heart surgeon Donald Glower, and researchers John Maurice, Jonathan Hata, Ashish Shah, David White, Patricia McDonald, Paul Dolber and Katrina Wilson also contributed to the study.
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