Science: Transplanting a Gene

Many scientists have boldly stated that they will some day be able to cure hereditary diseases by changing the genetic mechanism of human cells. The day of such genetic engineering may now be a little closer. In a report to Nature on work that the journal hailed as "little short of revolutionary," three American scientists claimed the first successful transplant of bacterial genes into living human tissue.

The cells used by Molecular Biologists Carl Merril, Mark Geier and John Petricciani at the National Institutes of Health in Bethesda, Md. were taken from a victim of the hereditary disease called galactosemia. Because of a defect in the genes in the nuclei of his cells, the victim was unable to produce the essential enzyme that enables the body to metabolize galactose, a simple sugar found in milk and other dairy products. Unless an infant born with the defect is quickly placed on a milk-free diet, he faces malnutrition, mental retardation and even death.

Favorite Tool. To correct this genetic failing in such cells, the scientists used a favorite tool of geneticists: bacteriophages, or viruses that prey on bacteria and may pick up genes from them. The viruses used in the test had a particular virtue: the gene that they had acquired from the common intestinal bacteria Escherichia coli was the one that orders the bacterial cell to manufacture the same galactose-metabolizing enzyme produced in humans.

Hoping to transmit the gene to the human cells, the scientists placed a solution of the gene-bearing viruses in a lab flask containing the cells. Then they incubated the culture at body temperatures in an atmosphere enriched with carbon dioxide. The next step was more subtle: to determine whether the viruses had actually invaded the cells and insinuated their genetic instructions into them. If the genetic transfer had really taken place, the researchers reasoned, the cells would begin issuing their own instructions for making the enzyme.

Clear Implication. To find out if those orders were being given, the scientists used another laboratory trick. Before incubation was completed, they had radioactively tagged the cells' messenger RNA (the single-stranded molecule that carries genetic instructions from one part of the cell to another) so they could later identify it. Then they mixed these radioactive strands with complementary strands of genetic material from viruses carrying the crucial gene, hoping that they would combine; pairing off would take place only if the cellular RNA now had the same genetic structure.

To their great satisfaction, the experimenters found that such hybridization of the viral and RNA strands had occurred. In contrast, when the researchers tested RNA from cells that had not been exposed to the viruses, hybridization virtually ceased. The implication was clear: the cells were indeed ordering up the essential enzyme. Furthermore, the scientists not only found the enzyme and confirmed that it was chemically active but also determined that the cells passed on their enzyme-making ability when they reproduced themselves.