Oliver Smithies (top), Mario Capecchi (left) , Martin Evans (right).

Cardiff University (top); Tim Kelly (left); UNC Chapel Hill (right)

A Knockout Nobel

The pioneering work that allows biologists to modify specific genes in mice with relative ease has won the 2007 Nobel Prize in Physiology or Medicine. Mario Capecchi of the University of Utah in Salt Lake City, Oliver Smithies of the University of North Carolina, Chapel Hill, and Martin Evans of the University of Cardiff, U.K., will share the prize for developing the techniques to make knockout mice, animals that lack a specific gene or genes. Such mice have allowed scientists to learn the roles of thousands of mammalian genes and provided laboratory models of human afflictions in which to test potential therapies.

Working independently in the 1980s, Capecchi and Smithies each crafted ways to slip foreign DNA into a specific place in a cell's chromosomes. Both scientists recognized that if they inserted DNA that was very similar to that of a target gene, the cell would take it up and swap it for the target gene. Because the inserted DNA could be modified in advance, say, by disabling it with a mutation, scientists could change specific genes at will. A similar strategy had been used to alter genes in yeast and other organisms, but most people assumed it wouldn't work in mammals. Indeed, in the early 1980s, Capecchi's grant application was rejected by the National Institutes of Health in Bethesda, Maryland, with the advice that he forget the idea.

He persevered, using money cobbled together from other projects. And a few years later, both he and Smithies, then working at the University of Wisconsin, Madison, showed that targeting specific mouse genes was indeed possible. The process was very inefficient, though, and most people still doubted that it would have much practical use.

Enter Martin Evans, then at the University of Cambridge, U.K. He led a group that reported in 1981 that they could grow embryonic stem (ES) cells from mouse embryos. These cells can become any kind of cell in the body, including eggs and sperm. A few years later, Evans and his colleagues showed that they could produce live mice by injecting cultured ES cells into a developing embryo. The result is a chimera, an animal whose tissues are a mix of the ES cells and those from the host embryo. In some chimeras, the added ES cells by chance produce the animal's sperm or eggs, and when these chimeras mate, some of their offspring carry the stem cells' genes in all their tissues.

Embryonic stem cells were the key to getting around the inefficient gene swapping that had stymied Capecchi and Smithies. "With ES cells, the frequency didn't matter as much," Smithies says. "If it was one in a million, you just used a million cells." By targeting genes in ES cells--and then sorting out the cells that carried the desired modification to create chimeras--scientists could create mice that completely lack a working copy of a given gene.

To date, researchers have knocked out at least 11,000 genes in mice, observing what goes wrong in development or adulthood and thereby gaining a sense of what the gene does (Science, 30 June 2006, p. 1862). By deactivating specific genes this way, for example, Capecchi and his colleagues went on to identify ones that shape the overall mammalian body plan. Smithies and Evans also both quickly created rodents lacking the cystic fibrosis gene, one of many knockout mouse strains created to mimic a human illness.

The technique "has been transformational in our understanding of biology," says geneticist Allan Bradley, director of the Wellcome Trust Sanger Institute in Cambridge, U.K., who worked in Evans's lab when ES cells were first derived. Adds Smithies, "You can't open a journal today without finding a paper that uses these techniques."

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