Dr. Bollenbacher | Biology 52

Gene Mutation Provides
More Meat on the Hoof

From Science, Volume 227: September 26, 1997

Belgium is not exactly known for wide-open spaces and sprawling ranchlands. So farmers there had to learn to do more with less. Over the last 30 years, they have bred a strain of cattle--the mighty Belgian Blue--that gives 20% more meat per animal on roughly the same food intake as ordinary animals. indeed, the cattle develop such bulging muscles that in extreme cases they have trouble walking and the calves are so big they have to be delivered by cesarean section. Now, three research groups have independently uncovered the genetic cause of this "double-muscling" trait, a discovery that may lead to meatier strains, not just of cattle, but of other agriculturally important animals as well.

In the September issue of Nature Genetics, a pan-European team led by Michel Georges of the University of Liege in Belgium reports that double muscling is caused by a mutation in the bovine version of a recently discovered gene that makes a protein called myostatin. The other two groups, one co-led by Tim Smith of the U.S. Department of Agriculture (USDA) lab in Clay Center, Nebraska, and the other by Sejin Lee of Johns Hopkins University, also found that the myostatin gene is mutated in Belgian Blues and have linked mutations in the gene to double muscling in a second breed of cattle, the Piedmontese, as well. [The Smith team's results are in the September issue of Genome Research, and Lee's are in press in the Proceedings of the National Academy of Sciences (PNAS).]

Discovered just 4 months ago in mice by Lee and his graduate student Alexandra McPherron, myostatin normally serves to limit skeletal muscle growth. Apparently, the mutations block its activity and the animal's muscles grow larger-but without harming meat quality. While some other cattle breeds are also abnormally well muscled, presumably because of as-yet-undiscovered mutations, the muscle fibers in those animals are thicker than normal, toughening the meat. In contrast, the muscles of animals with myostatin mutations have larger numbers of normal-size fibers. Indeed, says Smith, meat from the Belgian Blue is "so tender even round steaks fall apart on the grill." Nevertheless, the meat is lower in fat than that from ordinary breeds.

Given those effects of myostatin mutations, it's not surprising the gene is attracting attention from agricultural scientists. "This is the first gene identified in cattle that controls a combination of muscle size and tenderness," says molecular geneticist Mike Bishop of ABS Global, a biotech firm in Madison, Wisconsin. He notes that beef palatability, as well as yield, might be improved by introducing myostatin gene mutations into cattle or by finding drugs that turn down the gene's activity. Such strategies might also lead to meatier pigs, chickens, and turkeys, as the Lee team found that the myostatin gene has relatives in these and other farm animals.

The meandering cow path to this discovery started in Belgium in the 1950s, Georges says. Cow breeders there, who were under economic pressure from cheaper imports and high production costs, wanted to increase their yields and began to select for the double-muscling trait, which had been reported as early as 1807. Before long, nearly every beef cow in Belgium was a purebred double-muscled animal.

Beginning in the late 1980s, Georges's team spearheaded an effort to isolate the cause of double muscling. "We were so convinced that any gene . . . that had such a spectacular effect on muscular development had to be a very important gene for animal agriculture," he recalls. By 1995, Georges and his colleagues mapped the gene to a region of cow chromosome 2, but then the effort stalled because they still had a lot of DNA to search through.

A break came in May of this year, however, when Lee and McPherron described the myostatin gene and showed that when it is missing in mice, the animals grow into muscle-bound hulks two to three times the size of normal animals. That publication launched a race to find the equivalent bovine gene, as the implications for cattle--if such a gene existed--were obvious.

The group led by Georges--which included researchers from Germany, Spain, and France--used a neat trick involving a third species, humans. The full human gene has not yet been published-it will be in Lee's PNAS paper--but the researchers found that a database of human ESTs (expressed sequence tags) contained sequences similar to those of the mouse gene. Although ESTs are short--100 or 200 bases long--Georges and his colleagues found enough overlapping ones to piece together most of the human gene. After cloning it and mapping its chromosomal location, they played a hunch and compared the site of the human gene with a map of bovine chromosome 2.

The effort paid off. "We then realized," says Georges, "that the position of the myostatin gene on the human map coincided exactly with the position of the double-muscling gene on our bovine genome map." From there, they cloned and sequenced the bovine myostatin genes from both double-muscled and normal cattle.

The sequences revealed that the gene from the double-muscled animals carries an inactivating mutation--an 11-base pair deletion--that results in "virtually complete truncation" of the active region of the protein, Georges says. That lifts the normal repression of muscle growth by myostatin and opens the way for extra brawn.

The Lee team used a similar approach to come up with the Belgian Blue gene. The researchers then guessed that double-muscled Piedmontese cattle would also have a mutated myostatin gene, and when they cloned it, that's what they found. Smith, working with John Baff's team at AgResearch in Ruakura, New Zealand, took a somewhat different tack, using the mouse gene to first find the gene in normal bovine DNA and then in Belgian Blues and Piedmontese, where they, too, found mutations. Similar mutations could also add bulk to other farm animals, for the Lee team has found the gene in all nine species they examined, including mammals, such as the pig, and birds, including chickens and turkeys.

T'he myostatin work may help to identify other genes that influence muscle growth. Piedmontese cattle don't develop the extreme double muscling of Belgian Blues, even though the mutation that the Smith and Lee teams found in their gene is probably sufficient to inactivate the protein. That suggests that the lesser amount of double muscling in Piedmontese cattle is due to other genes that make up for the loss of myostatin.

Despite the interest in using the myostatin gene to improve beef production, researchers warn that it may be a difficult task. One possibility is to use either conventional breeding or genetic engineering to introduce the Belgian Blue mutation into other breeds. So far, however, U.S. breeders have only rarely attempted to do this, even by conventional breeding. This is partly for practical reasons. The need to deliver calves by cesarean section is a serious handicap in the United States, where cattle herds are larger and roam over much wider areas than they do in Belgium.

That problem might be overcome, if researchers can find a less extreme myostatin mutation or identify another gene with a less drastic influence on muscle mass, allowing the calves to be delivered naturally. But there are also worries about whether the public would accept genetically engineered beef. The cattle industry has until now shied away from funding research into transgenic animals for human consumption. "They perceive it as too sensitive and risky an area," Smith says.

Another possibility would be to find some drug that can turn down myostatin activity in animals with the normal gene. And then there may be other genes that can be manipulated. Researchers in at least four countries are mapping the cattle genome, and reproductive physiologist Vernon Pursel of the USDA research labs in Beltsville, Maryland, says "we are getting to the point where there will be a number of genes" like myostatin identified in the near future. Extra helpings of tasty meat at essentially no cost could prove hard to resist.

 --Steven Dickman