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UCLA, which has the nation's largest heart transplant program, has achieved a one-year survival rate of 84 percent and a five-year survival rate of 71 percent—both significantly better than results reported by the International Heart Transplant Registry. But heart transplantation still has a long way to go. One of the major barriers to long-term patient survival is graft atherosclerosis, a form of chronic rejection that affects the small arteries of the heart. Preventing this chronic rejection requires the lifetime use of immunosuppressant drugs, which are not organ-specific and so produce harmful side effects without entirely eliminating the threat of rejection. But a UCLA research team headed by cardiothoracic surgeon Hillel Laks has made important early strides using gene transfer to address the issue of rejection. "We are looking at ways to treat the heart at the time of harvesting for transplant by perfusing it with a solution containing the genes we want to introduce into the cells," explains Laks, director of UCLA's heart transplant program. "The idea is that certain changes that you make inside the cell will reduce the risk of rejection and ultimately may help to produce tolerance." Several years ago, investigators demonstrated that injecting genes into the heart results in expression of the encoded proteins. Efforts then focused on how to insert the genes most efficiently so as to achieve maximum expression. Laks, who heads up a research team that includes Drs. Luyi Sen and Ron Brauner, began using reporter genes in animal models and recorded their best results from an intracoronary delivery method, perfusing the heart with a solution rather than injecting the genes with a needle. One of the early questions was whether the gene transfer could occur at the low temperature necessary for heart preservation—4 degrees Celsius. "The myocardial cells are not metabolically active at those hypothermic conditions, but we're showing that efficient gene transfer is still possible," explains Laks. Having demonstrated the feasibility of introducing genes into the cold isolated heart during a short period of perfusion, the researchers tested the ability of functioning genes to suppress the rejection process. To deliver the genes, they used a flu-like virus constructed in the laboratory. Using rabbits undergoing transplants, they performed the first successful functioning gene transfer. The result was a delay in rejection. |
That hurdle cleared, they focused on both determining the most effective genes and the best way to deliver them to achieve prolonged expression. The team learned that some of the desirable proteins are endogenous to the body, but not produced in high enough levels by the new heart. In such cases, the goal is to introduce genes that will boost the production. Thus far, the researchers have achieved positive results in experiments with two genes known to have immunosuppressant properties, interleukin-10 and the transforming growth factor beta (TGF-ß). "The gene is now located within the cells of the new heart, producing the proteins it needs, and the immunosuppressive effect is local rather than systemic," says Laks. Laks' team is also experimenting with biochemical manipulations that will minimize damage to the cell lining that often occurs after the transplant. "When you reperfuse the heart with blood, white cells become adherent to the endothelial cells, which are affected by the period during preservation of the heart when blood is not flowing," Laks explains. "Our goal is to use genes that will reduce this adherence, improving the early function of the heart after the transplant." But the matter of finding the right vector is proving challenging. Because the virus used by the researchers is first-generation, gene expression is transient—lasting approximately two weeks. Then there is the question of whether using a virus for gene delivery might cause unforeseen harm to the patient. All evidence so far shows that the virus currently being used is safe. While looking for a longer-lasting virus, the UCLA researchers are also testing the usefulness of liposomes. As vectors, liposomes offer the advantage of potentially longer expression, less inflammation and more specificity to desired areas, such as endothelial cells. Not incidentally, they are inexpensive to produce, requiring none of the recombinant techniques of viruses. But even the newest generation of liposomes can express only one-fiftieth the amount of protein that viruses can. "We're at the beginning of the process," says Laks. "We still need to do a lot of experiments with viruses and liposomes and to consider how we might be able to administer additional doses after the transplant. In the short term, we may be able to use a combination of gene therapy and pharmacologic agents, reducing the reliance on immunosuppressing medicine. In the long term, we hope to be able to obtain complete tolerance. Without question, if we can develop a mechanism for treating the organ without affecting the whole body, the potential advantages are enormous." —D.G. |
 Hearts
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