By David N. Leff
It took more than 1,500 pigs from farm-animal researchers in three European countries to track down the mutant gene that bedevils the swine production industry worldwide.
The industry's porcine nemesis is the Hampshire pig, a black porker with white stripes down its forelegs, which is one of the dozen major breeds raised in the U.S. and elsewhere in the world. Hampshires are the Typhoid Mary of the porcine population, because they harbor a gene mutation that downgrades the quality of their meat.
Molecular geneticist Max Rothschild, distinguished professor of agriculture at Iowa State University in Ames, and coordinator of the U.S. genome program in pigs, told BioWorld Today, "The U.S. swine industry produces 90 [million] to 100 million pigs a year, depending on what year it is. Not very many are purebred Hampshires. More important is how many are part-Hampshire. I would guess 20 percent to 30 percent of the market has some Hampshire in it.
"The breed's mutation is serious to the industry if the meat goes to a processing farm, which cooks - and shrinks - hams and pork loins. This genetic disorder," Rothschild continued, "was originally discovered in France in the 1980s, where hams were quite popular. That's not the case any longer. Because of this problem, the Hampshire breed has taken a real hit over time around the world. It affects the meat-processing industry in direct dollar losses, which I presume they pass on to the consumers - translated into higher prices."
Rothschild's opposite number in Sweden is animal geneticist Leif Anderson, at the Swedish University of Agricultural Sciences in Uppsala. He heads that country's farm-animal genome program.
"It's been known for some time," Anderson told BioWorld Today, "that a mutation occurs in this particular breed of pig, which increases its body's glycogen content considerably - almost double the normal amount. Glycogen, of course," he pointed out, "is the storage form of glucose, which is stored in liver and muscle. So the mutation affects the amount of muscle.
"The consequence of this increased glycogen," Anderson went on, "is that while it increases the proportion of meat content to fat and bone, it degrades meat quality. The meat becomes more acidic, as pH goes down, as does the water-holding capacity. So when you prepare this ham, it loses water while cooking. The industry doesn't like this," he noted. "Most breeders would like to eliminate this mutant gene."
Mutation Yanked Ham From French Cuisine
Anderson is senior author of an article in today's Science, dated May 19, 2000, titled: "A mutation in PRKAG3 associated with excess glycogen content in pig skeletal muscle." Its 17 co-authors, at seven laboratories in France, Germany and Sweden, collected and analyzed the 1,500-plus slaughterhouse pig carcasses for glycogen content.
"French scientists described the mutant Hampshire gene phenotypically in 1985," Anderson recounted, "based on observing this variation in the breed's glycogen content. We mapped the gene's genomic location in '96. And now we have identified the gene itself on the molecular level, as well as the mutation causing its effect in pig muscle. The gene resides on porcine chromosome 15, which corresponds to the long arm of human chromosome 2. The two sequences are about 85 percent identical.
"These findings will have important applications in the pig-breeding industry," Anderson observed, "as we have developed a convenient DNA test - using a simple blood sample - to identify breeding animals that don't have this mutation." The university has patents pending on this DNA test in Europe, and will file for worldwide patent protection "within a year," Anderson said. He added: "We are already negotiating licenses with a number of companies around the world, including America."
He outlined a scenario by which pig breeders could begin to eliminate the deleterious mutant from their swine. "This is a dominant gene," he noted. "They would use the DNA test to simply select animals, preferably those homozygous for the normal variant. It's important to know," he added, "that over 90 percent of Hampshire pigs around the world have this mutation. So you can't eliminate such a large proportion in the population all at once.
"In the first step," he suggested, "breeders would reduce the frequency, probably only by breeding animals that are heterozygous or homozygous for the normal gene. And then they will - during some generations - reduce the frequency to where they have only the normal allele left.
"The Hampshire mutation," Anderson explained, "arises from a single point substitution in the PRKAG3 gene sequence, which replaces the amino acid arginine with glutamine. This is quite a large difference in chemical terms," he pointed out, "but a small change for the protein the gene expresses.
"That protein," Anderson continued, "belongs to a family of proteins called AMP-activated protein kinase, which is known to have a central role in the regulation of energy metabolism. So if the cell is stressed and has a shortage of energy, it tends to stimulate a biochemical process that generates chemical energy."
Pig Connection Urges Diabetics To Exercise
"A very important aspect of our work," he pointed out, "is that it gives new knowledge about the gene, which has a role in regulating the energy metabolism in the muscle. Its mutation produces a disorder of metabolism in muscle. And this can lead to new information concerning ways to diagnose and treat Type II diabetes in humans.
"For a long time it's been known," he observed, "that Type II diabetes patients have cells that tend to be insulin-resistant. They have a defect in the insulin-stimulating uptake of glucose, and that's why their synthesis of glycogen is reduced. It's also known that in muscle there are two ways to take up glucose. One is insulin-mediated. The other is when the muscle contracts. And this muscle-contraction pathway is independent of the insulin-stimulated pathway. That is why Type II patients get improved blood glucose levels by exercising.
"Our data in the Science paper show that this new member of the AMPK family is involved in that glucose stimulation due to muscle contraction. This means," Anderson concluded, "that the gene we now describe may be a clue to developing a treatment that could stimulate this pathway, to help diabetes patients keep their blood glucose on a reasonable level."