Way to Replace Specifically Targeted Genes Developed

University of Utah researchers have developed a new genetic engineering technique that may allow scientists to selectively modify or replace defective genes.

The laboratory advance, reported in today’s issue of the journal Nature, may facilitate efforts to treat human diseases by replacing abnormal genes. The technique is expected to be tested on human patients within the next several years. It will also contribute to the study of genetic diseases and mammalian development.

The technique allows a new gene to be targeted so that it will search out and replace another gene exactly. This means that it will insert itself into the same location on the same chromosome as the original gene. Previously, scientists have only been able to insert most new genes at random. Such randomly inserted genes may not function properly or may interfere with the functioning of other genes.

“I think it is a huge step,” said Mario R. Capecchi, a professor of biology at the University of Utah in Salt Lake City and the leader of the research team, in a telephone interview.

Capecchi said the promise of the research is that “we can in essence do anything we want to any gene in the cell.” This is because the procedure is general and requires “little knowledge” of the actual gene being targeted. Previous techniques have usually only been applicable to a small number of extensively studied genes.

Genes make up the innermost part of a cell and determine the characteristics that living things inherit from their parents. There are thousands of genes in every cell, programming everything from sex and height to predispositions to certain diseases.

The technique could be used not only to modify or replace genes, but also to boost the concentration of an insufficient gene or to eliminate an extra gene entirely.

Other leading researchers hailed the research at the University of Utah’s Howard Hughes Medical Institute but said that many important questions remain to be answered.

Being able to insert genes “reliably and reproducibly is very important to the concept of gene therapy,” said Dr. R. Michael Blaese of the National Cancer Institute in Bethesda, Md. He said the next step is to determine if the theory--that genes inserted in this fashion are subject to the body’s normal control mechanisms--is correct.

“It is a major advance for studying mammalian development,” said A. Dusty Miller of the Fred Hutchinson Cancer Research Center at the University of Washington in Seattle. Miller cautioned that practical issues for gene therapy, in particular producing large numbers of cells with the desired replacement gene, “are still very difficult to solve.”

The University of Utah researchers conducted their experiments in mouse embryo cells. Using recombinant DNA techniques, they prepared purified copies of two genes--including a gene thought to be involved in the development of a mouse’s ear. Conducting similar experiments with two different genes made the researchers more confident that their results were accurate.

In parallel experiments, the genetically engineered genes were introduced into embryo cells, along with additional genes capable of preventing the cells from being killed by antibiotics that were added subsequently.

Embryo Cells

The technique was designed so that the only embryo cells protected against the antibiotics were the ones in which the new genes replaced the original genes, according to the report. Most of the embryo cells that received no new genes were killed, as were those in which the new genes were inserted in the wrong place. Therefore the scientists could isolate the cells with the properly inserted new genes.

The researchers were then able to grow mice containing the new mouse ear gene, but they have yet to show that this gene can be transmitted from one generation of mice to another, Capecchi said. The ability to transmit a replacement gene between generations is key to using the new method to study genetic diseases and mammalian development.

The purified gene as it was modified “searches the whole genome until it finds that gene, and then it exchanges genetic information with it,” Capecchi said. “The change we started out with in the test tube is introduced into the living mouse. The technology is applicable to any gene.”

Increase Reported

As compared to inserting the genes at random, the new method resulted in a 2,000-fold increase in the concentration of cells containing the selectively inserted genes.

But researchers hoping to apply the technique to gene therapy may be hampered by technical limitations, which dictate that the selectively inserted genes make their way into only a small minority of the original embryo cells--between one in 100,000 and one in 2 million. According to Miller, the greater the number of cells with a properly inserted new gene, the more likely it is that a disease caused by an abnormal gene can be successfully treated.

GENE REPLACEMENT

Researchers have a new technique allowing them to selectively modify or replace defective genes. In the mouse embryo cell are two dozen or more chromosomes, each containing tens of thousands of genes. Using a kind of “courier” to help in the identification, scientists seek a targeted gene and replace it in the same location. The “courier” portion of the new gene then is eliminated. Until now, scientists had been able to insert genes only at random.