The one-two research punch that allowed the creation of designer mice has earned the 2007 Nobel Prize in Physiology or Medicine. Mario Capecchi, a Howard Hughes Medical Institute investigator at the University of Utah in Salt Lake City, Oliver Smithies of the University of North Carolina, Chapel Hill, and Martin Evans of Cardiff University, 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.
The techniques "truly provided a revolution in mammalian biology," says Raju Kucherlapati, a geneticist at Harvard University. "It is not an exaggeration to say that there is no mammalian biologist today who does not use these tools in one way or another."
Biologists have long studied mutant mice for insights into the mammalian body. But for decades, they were limited to rodents whose DNA had been disrupted in random places by natural mutations or the application of mutagenic chemicals. In fact, most of the time, biologists studying a mutant mouse strain didn't even know which gene was broken. The ability to mutate a specific gene at will seemed a distant dream.
Working independently in the 1980s, however, Capecchi and Smithies each crafted ways to slip foreign DNA into a specific place in the chromosomes of mammalian cells. A similar strategy, exploiting a natural DNA-swapping process called homologous recombination, 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 should 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 genes in mammalian cells via homologous recombination was indeed possible. But the work was in cells in culture dishes, and the technique seemed far too inefficient to be used to make whole animals with genetic alterations.
Enter Martin Evans, then at the University of Cambridge, U.K. He led a group that in 1981 reported growing embryonic stem (ES) cells from mouse embryos. Evans, too, had faced skepticism. Experts had doubted whether such cells, which can become any cell in the body, could be grown in the lab. Even Evans was confused when he first saw the cells in culture, says Elizabeth Robertson of the University of Oxford, U.K., who was a postdoc in the lab. "He came to us and said, 'Someone contaminated my media!' " because there were strange-looking cells growing in it. Lab members had to convince him that the cells were ES cells, she says.
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 many of those 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.
Capecchi and Smithies both quickly saw that ES cells offered an opportunity to generate live animals with a desired mutation in every cell. Researchers could target genes in ES cells and then sort out the cells that carried the desired modification, using them to create chimeras. Some of the chimeras' offspring would have the altered gene in all their tissues, and by breeding these animals together, biologists could create mice that completely lack the two working copies of a given gene. Although they never formally collaborated, Evans "brought the ES cells to my lab in his own pocket," Smithies says, while Capecchi spent time in Evans's lab learning the technology.
Every biologist soon wanted a favorite gene punched out, and a handful of companies quickly began competing with places such as the Jackson Laboratory in Bar Harbor, Maine, to provide knockout strains to drug companies and academic labs. 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. By deactivating specific genes this way, for example, Capecchi and his colleagues went on to identify ones that shape limbs, organs, and the overall mammalian body plan. Both Smithies and Evans developed mice lacking the cystic fibrosis gene, one of many knockout mouse strains created to mimic a human illness. Indeed, there is now a worldwide effort to knock out every mouse gene (Science, 30 June 2006, p. 1862).
Skeptical grant reviewers were not the only hurdle Capecchi overcame on his way to scientific stardom. As a child in war-torn Italy, he survived alone--often begging and stealing on the streets--between the ages of 4 and 9 while his mother, a poet, was imprisoned in the Dachau concentration camp for her anti-Fascist writings. After the war, she tracked down a very malnourished Mario in a hospital, and a few days later they were on a boat to the United States to live with her brother in Pennsylvania. The young Mario expected the streets to be literally paved with gold, he told a press conference in Salt Lake City on the day he won the prize. What he found instead, Capecchi says, "was opportunity."
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