Researchers at the University of California, San Diego (UCSD) School of Medicine have developed a therapeutic gene and molecular delivery system that provides successful long-term gene therapy that halts chronic heart failure in experimental animals.
A major disease that affects more than 4.5 million Americans, heart failure occurs when the heart loses its ability to pump enough blood through the body. Currently, there is no cure for chronic heart failure other than transplantation or mechanical pump devices including new types of artificial hearts.
In a study published online July 22, 2002 by the journal Nature Medicine, the UCSD researchers describe their modification of a gene called phospholamban (PLN), which regulates the strength of each heartbeat and is known to malfunction in heart failure. With this therapy and development of a long-lasting, highly effective method to deliver the gene directly to heart muscle, the researchers significantly improved heart contraction and relaxation in the animals, ultimately halting deterioration of cardiac function and preventing heart failure.
The therapeutic strategy published in Nature Medicine was successful in hamsters with a naturally occurring genetic mutation that causes them to develop progressive heart failure similar to that in humans. Currently, the research team is testing the therapy in pigs, a larger animal model with heart physiology similar to humans, and they hope to expand the study to human clinical trials in 12-18 months. The gene would be delivered by special catheters and would be performed in a normal hospital setting.
Kenneth Chien, M.D., Ph.D., professor of medicine, director of the UCSD Institute of Molecular Medicine and senior author of the Nature Medicine article, noted that like cancer, heart failure has many causes. Included are such conditions as underlying coronary disease, genetic abnormalities, high blood pressure, irregular heartbeat, and complications of diabetes. As a result, researchers have had difficulty zeroing in on one cause or condition as a target for heart failure prevention.
In recent years, the Chien team has linked heart failure and specifically, defects in the heart's pumping action, to PLN, which is a calcium-cycling regulator gene whose normal function is to regulate the movement of calcium within heart muscle cells. It does so by acting as a brake on the calcium pump, which is responsible for increasing calcium into storage sites so that it can be released with each heartbeat. Hormones that increase heart rate and function, such as adrenaline, release the brake.
This balance between accelerator and brake is disrupted in heart failure. In that setting, because of injury such as a heart attack or genetic defect, the PLN brake becomes defective. It is on too hard, restricting calcium and weakening heart function.
Masahiko Hoshijima, M.D., Ph.D., first author of the Nature Medicine paper and a lead project scientist in Chien's lab, explained that the research team attacked the problem by replacing one amino acid within PLN for another to form a mutant version of the gene that would inhibit its normal counterpart.
"In essence, we inhibited the inhibitor," he said. "The PLN mutation was able to replace the defective PLN gene, generate normal cardiac contraction and prevent heart failure."
In experiments with 30 hamsters that received the mutant PLN gene, compared to 28 that did not, researchers noted a dramatic improvement in the heart's ability to pump blood. The progressive impairment of cardiac contraction was stopped as early as the 5-week point. The hamsters that did not receive the gene therapy developed heart failure comparable to late stage human heart failure.
In addition, the thickness of the heart wall increased in hamsters that received the mutant PLN gene, which reflects a protection of myocardial cells and reduction in the progressive thinning of the ventricular wall that accompanies heart failure. Another indicator of increased heart function was measurement of calcium uptake in the animals that received the PLN mutation. Their calcium activity was enhanced more than 100 percent.
In addition to developing a gene that would prevent the progression of heart failure, the team needed an effective molecular method, called a vector, to deliver the gene through the coronary arteries, directly to the heart. Vectors used to transfer genes within the body are inactivated viral agents.
The UCSD team first used an inactivated adenovirus to serve as the vector, but it was not an ideal choice. More commonly known as the cold virus, adenovirus has a relatively short life within the human body. Within 7 days, the body's immune system killed the virus and eliminated its cargo, the mutant PLN gene.
The UCSD authors noted in their paper that "clearly, for gene therapy to become a feasible therapeutic strategy for chronic cardiac muscle diseases, it will be necessary to express the target gene for many months..."
To achieve this goal, John Ross, Jr., M.D., UCSD professor of medicine and Yasuhiro Ikeda, M.D., Ph.D., the co-first author of the paper, developed a high efficiency gene delivery method, which was then successfully used an adeno-associated viral vector (AAV), a different viral agent than its cousin, adenovirus.
"AAV has few immunostimulatory effects, it became stabilized within the heart muscle cell nucleus and was effective for a much longer period of time than adenovirus," Ross said.
With AAV, there was no cardiac inflammation, indicating that the immune system did not eliminate the gene. For 7 months, the length of the experiment, the progression of heart failure was halted in hamsters that received the mutant PLN gene. The researchers also found that AAV had a high affinity for heart muscle and was attracted to it, rather than diffusing in other tissue within the body. Over 60 percent of the ventricular muscle expressed the transferred gene.
Current experiments by the Chien team will determine how long the mutant PLN gene will continue to be effective within the body. In addition, the investigators are studying the safety of mutant PLN when it's used with beta-blockers, medications frequently taken by heart patients.
An important additional area of development for the new gene therapy is an efficient, catheter-based system that can be used in humans to infuse the AAV vector and its mutant-PLN cargo. Current tests in pigs are investigating potential catheter-based systems.
In addition to Hoshijima, Ikeda, Ross and Chien, additional authors of the Nature Medicine paper are Yoshitaka Iwanaga, M.D., Ph.D., Susumu Minamisawa, M.D., Ph.D., Moto-o Date, M.D., Ph.D., Yusu Gu, M.D., and Mitsuo Iwatate, M.D., Ph.D., UCSD Department of Medicine; Manxiang Li, Ph.D., University of Maryland, Baltimore; and Yibin Wang, Ph.D., and James M. Wilson, Institute for Human Gene Therapy, University of Pennsylvania.
The research was funded by the Jean Leducq Foundation.
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