Fine-tuning gene action in engineered mice.

For several years, scientists have been able to "knock out" specific genes and create strains of mice that mimic certain disease conditions (SN: 9/4/93, p.148). To do this, geneticists inactivate a gene in very early embryo cells, then add those cells back to developing mouse embryos. The gene is then missing or not functional in offspring of the mice that develop from these altered embryos.

Now, a multinational research team has fine-tuned this type of genetic engineering. A new technique enables scientists to create mice that lack a particular gene at certain stages of development or in certain tissues, says Hua Gu, a geneticist now at a Rockville, Md., laboratory of the National Institute of Allergy and Infectious Diseases.

"[The procedure] opens up entirely new avenues in scientific research that have never been possible before," says Jamey D. Marth of the University of British Columbia in Vancouver. "Every lab that tries to generate... human disease models will use this."

While working with Klaus Rajewsky at the University of Cologne in Germany, Gu and his colleagues wanted to know how an enzyme called polymerase beta affected the function and development of white cells. They couldn't just knock out the gene for this enzyme, because when they did, the mouse embryos died.

So instead, they borrowed genetic material from a bacterial virus, or phage. Some of that material codes for an enzyme, Cre recombinase, that keeps copies of viral DNA separated from each other, enabling them to infiltrate the bacterium's genetic material more easily. This enzyme homes in on a particular bit of DNA consisting of 34 base pairs. Then it snips out that bit and any genes that follow, stopping just short of the next copy of the 34-base-pair bit.

To create their new mouse strain, the scientists started with genetically altered parent strains. One parent strain carried functional polymerase beta genes sandwiched between two 34-base-pair bits. The other made Cre recombinase.

Marth and Paul C. Orban had created that second strain by joining the Cre recombinase gene to a "promoter" gene, which controls the activation of other genes. That promoter worked only in white cells called T cells, so only they made Cre recombinase.

Consequently, mice born after mating these two strains made Cre recombinase enzyme in their T cells. That enzyme then recognized the modified gene for polymerase beta and cut it out of the T cells' DNA, disabling it. The scientists then observed the effect of this loss, they report in the July 1 SCIENCE.