MEDICINE / GENETIC ENGINEERING : Safer Gene Therapy May Be Possible
Scientists have used a new genetic engineering technique to successfully insert genes into heart and other muscle tissue of rats, promising a new, potentially safer way of carrying out gene therapy, a Michigan researcher said Monday.
One potential application of the technique might enable the heart to secrete proteins to stimulate the growth of new blood vessels that would bypass clogged arteries leading to the heart, thereby avoiding the need for bypass surgery. Internist Jeffrey M. Leiden of the University of Michigan Medical Center reported the results at a meeting in Dallas of the American Heart Assn.
The approach could also be used to replace the defective protein that cripples muscular dystrophy victims. The researchers are exploring these possibilities in animal models of the disorders.
Leiden was able to insert a bare piece of DNA into cells, where it could produce a missing or desirable protein.
“It’s a very promising development,” said Gary Nabel, a University of Michigan molecular biologist who was not involved in the research. “Now all (Leiden) has to do is figure out which genes are going to be the most helpful” in treating heart disease.
Two general approaches are now used in studies of gene therapy. In the most common, now being studied on patients with a severe immune deficiency by researchers at the National Institutes of Health, white blood cells are removed from the patient and treated with a specially prepared virus that inserts a healthy copy of a defective gene into the patient’s own genetic material. The altered cells are then reinjected into the patient.
In the second approach, which is unlikely to be attempted in humans anytime soon, the virus is injected directly into the patient, where the virus can insert the gene into many different cells.
“The problem with using viruses is that they . . . can cause persistent infections in animals and humans” with potentially deleterious effects, Leiden said. Furthermore, he added, there is a risk that the virus will insert the added gene at an inappropriate site in the patient’s DNA--deoxyribonucleic acid, the genetic blueprint of life--causing mutations or cancer.
In contrast to those two approaches, Leiden uses a circular piece of bacterial DNA, called a plasmid, into which the desired gene has been inserted. Researchers have long used plasmids to insert new genes into bacteria, but it has been believed that the technique would not work in animals because the plasmids would not be able to get inside mature cells.
But Leiden found that the plasmids were readily absorbed by mature heart muscle cells called cardiac myocytes, where they produced the desired protein. But even after the plasmids are absorbed, he said, they remain separate from the cells’ own DNA so there is no risk of cancer.
Leiden used the technique to insert a bacterial gene for an enzyme called beta-galactosidase into the heart cells. The enzyme has no useful function; it simply serves as a marker to indicate that the gene insertion works. Its presence can be detected by adding a chemical that turns blue when it is altered by the enzyme.
He injected the plasmids into the beating heart muscle of nine rodents. When he removed heart tissue four weeks later and exposed it to the chemical, he found that seven of the nine had taken up the plasmid and were producing the enzyme.
“You can hold the hearts up and see splotches of blue with the naked eye,” he said in a telephone interview.
Earlier this year, molecular biologist Jon A. Wolff of the University of Wisconsin had shown that the same technique could be used in skeletal muscles. “It’s significant that this works in heart muscle as well,” Nabel said. “That’s not something you could take for granted.” Leiden’s results suggest that the absorption of plasmids is a general phenomenon of all types of muscle cells and thus could be widely applicable, he added.
Leiden is now treating rat hearts with the gene for a protein, called angiogenesis factor, that stimulates the formation of new blood vessels in the hopes that it will generate new blood supply to damaged areas.
He and Wolf are also testing the technique for replacement of a defective gene in mice with a type of muscular dystrophy. They hope that it can supplant a technique called myoblast transfer, now being tested in humans, in which cells from a healthy relative are injected into muscle cells to replace the defective gene.
Although myoblast transfer so far seems to be effective in improving muscle function, the patients must have their immune systems suppressed to prevent rejection of the injected cells. Injection of plasmids would be safer because they would not stimulate rejection, and no immune suppression would be necessary.
