DNA manipulation goes large-scale.

Geneticists have scored another victory on the playing field of the mouse genome. In what they call chromosome engineering, researchers have succeeded in deleting, inverting, or rearranging not single genes but large, selected blocks of mouse DNA.

In genetic engineering, investigators routinely pop extra genes into mice and knock out specific genes to create mice that develop without those genes' proteins. Now, with their new skills, researchers from Baylor College of Medicine and Texas A&M University, both in Houston, have robbed a mouse chromosome of 10 percent of its DNA.

"That's a mighty big chunk of DNA," marvels Kenneth Paigen, director of Jackson Laboratory in Bar Harbor, Maine, which collects mutant mice.

Chromosome engineering will speed the search for new genes, especially those that normally prevent uncontrolled proliferation of cells, says Allan Bradley, a Howard Hughes Medical Institute researcher at Baylor. The technique may also help create rodent examples of many human difficulties, since chromosomal rearrangements often cause failed pregnancies, familial diseases, and cancers.

"We're going to have quite a few different types of applications using this technology," says Mario Capecchi of the University of Utah in Salt Lake City.

Bradley and his colleagues, who describe their experiments in the Dec. 14 Nature, use an unusual enzyme produced by a virus that infects bacteria. The enzyme, Cre recombinase, or simply Cre, recognizes short viral DNA sequences called loxP sites.

When Cre encounters two loxP sites that scientists have inserted into a mouse chromosome, the enzyme cuts out the intervening DNA (SN: 7/9/94, p.20). Depending on the orientation of the loxP sites, the enzyme then either inverts the DNA fragment and places it back in the chromosome or discards the snipped DNA, says Bradley.

Since every cell contains two copies of most chromosomes, the loxP insertions sometimes land on different copies of a particular chromosome. In those cases, Cre uses the loxP sites to define a region of one chromosome that it will cut off and attach to the other chromosome. "It puts one piece of chromosome onto another," says Capecchi.

To create mice with manipulated chromosomes, Bradley's group alters the DNA of stem cells and injects these immature cells into early-stage embryos. If the altered stem cells develop into the mouse's reproductive cells, all cells in the embryo's offspring will have the modified chromosomes.

Deleting large chunks of one chromosome does not generally kill the offspring, says Bradley, because the second copy usually contains a spare of each missing gene. The deletion does make it easier to search for tumor-suppressing genes, for example, since both copies of such genes must be deactivated before cancer results.

Bradley's group removed about one-tenth of one copy of chromosome 11 from a mouse strain. The deleted region resembles the part of human chromosome 17 that contains genes implicated in suppressing breast cancer. By mutating genes in the same region of the unaltered copy of chromosome 11 and observing whether cancer develops, investigators hope to identify the tumor-suppressor genes.

This search technique can also help identify genes whose functions are normally masked by dominant counterparts on the other copy of the chromosome. Chromosome engineering promises "to revolutionize mouse genetics," asserts Bradley.