Big-Bottomed Sheep Have A Rare Genetic Mutation That Builds Muscle, Not Fat
DURHAM, N.C. -- Scientists have discovered an elusive, mutated gene named for the Greek goddess, Aphrodite Kallipygos, that causes certain sheep to have unusually big and muscular bottoms. They hope the genetic mutation will illuminate how muscle and fat are deposited in these animals and possibly in humans.
The discovery is especially exciting, said the researchers, because the unusual gene has evaded all the traditional means of detection for nearly a decade. In fact, the gene appears to represent one of numerous stealth genes, called "imprinted," that have yet to be discovered but which could produce a wide range of diseases.
Researchers from the U.S. Department of Agriculture and Duke University Medical Center discovered a gene called "callipyge," (pronounced cal - ah - PEEJ) meaning "beautiful buttocks" in Greek, because the sheep have large, muscular bottoms with very little fat. Such an attribute could prove beneficial in breeding these sheep because it enables them to convert food into muscle 30 percent more efficiently than normal sheep. Moreover, the gene could explain specific processes that give rise to obesity and fat metabolism, said Randy Jirtle, Ph.D., professor of radiation oncology at Duke and co-author of the study.
"These sheep are, in effect, pumping iron without lifting weights," explained Jirtle. "They are converting food into muscle in their hind regions, instead of converting food into fat."
Results of the study, funded by the USDA and the National Institutes of Health, are published in the October 2002 issue of Genome Research.
Excited as they are to have unearthed the gene behind the big-bottomed sheep, the scientists say their discovery has equally dramatic implications for mining the human genome. The callipyge gene appears to be among a rare subset that eludes traditional methods of identification and mapping, said USDA geneticist Brad Freking, Ph.D., lead investigator of the study.
Called imprinted genes because they are literally stamped with markings that inactivate one parent's copy, such genes are quite rare and unusual in the way they operate. They often work only in specific tissues and at defined intervals during an animal's development, said Susan Murphy, Ph.D., a Duke University Medical Center co-author.
"Finding imprinted genes can depend on when and where in the body you search for them," said Murphy. "If you look for an imprinted gene in a mature animal when that gene is only expressed during fetal development, then you may miss it entirely."
The researchers sought an imprinted gene because the big-bottomed sheep inherited a functional copy of the mutated gene from their father alone -- the mother's copy is turned off. Silencing of one parental copy is characteristic of imprinted genes.
For 10 years, the researchers searched likely regions where the callipyge gene and its mutation might reside; namely, in known genes on sheep chromosome 18, according to previous research. But their efforts turned up intact genes with no mutations. Finally, the team of researchers tried a novel approach.
They compared a specific DNA sequence from inbred offspring of the original big-bottomed sheep against the DNA of normal sheep to look for minute genetic variations, called "markers." While they found 600 distinct "markers," only one was unique to the callipyge sheep: a single base change from A to G in the DNA sequence. Further testing showed this mutation alone clearly gave rise to the sheep's big-bottom stature.
Yet the mutation appeared to reside in a "gene desert," where no known gene had previously been mapped, said Freking. Interestingly, when they compared this sequence in the sheep to the same region of humans and mice, they found that the DNA sequences surrounding the callipyge mutation were highly similar in all three species.
"The more similar the region, the more likely the genetic sequence was conserved for an important biological reason," said Jirtle. So, the scientists searched deeper for evidence of a gene's presence. They tested whether the DNA in this conserved region was used as a template to make RNA within the callipyge sheeps' affected tissue. Finding RNA would signal the presence of a gene, since RNA is generally made from a gene in the process of producing a protein.
Surprisingly, they found an RNA "transcript" or copy, suggesting that the mutation is located in a previously unidentified gene.
"This is the first time in animals where a mutation has been found that leads to the identification of a new gene, rather than analyzing a known gene to find its mutation," said Jirtle. "As scientists, we are missing many genes and their mutations by using the traditional approach of linkage analysis to locate and analyze candidate genes."
Now that that they have found the callipyge gene, the next major step is determining how it gives rise to the big-bottomed trait or "phenotype." Researchers have long known that a nearby gene, called DLK-1, is over-expressed in the hind quarters of callipyge sheep. Yet DLK-1 has no mutations. Thus, the researchers speculate that the mutated callipyge gene is inappropriately regulating the expression of DLK-1 and/or other imprinted genes in this domain.
"We believe the regulation of this imprinted domain is flawed, rather than other genes in this domain being mutated," said Jirtle. "Mutated callipyge is having a downstream effect on DLK-1, and potentially on other imprinted genes in this region, that in some manner stunts fat cells from maturing while enhancing hind quarter musculature."
DLK-1 has been studied in other contexts because it is overexpressed in neuroendocrine tumors such as pheochromocytoma and neuroblastoma, and also is involved in the maturation of fat cells and the adrenal gland, said Jirtle. DLK-1 is imprinted, lending further credence to the assumption that callipyge is also imprinted.
"Imprinted genes are like mushrooms, because they are present in groups," said Jirtle. "Moreover, they display a domino effect, in that one mutation of an imprinted gene could knock out five or 10 genes in one hit, especially when they are all regulated as a group."
"These are incredibly powerful and subtle genes that bring you into a whole different realm of thinking about gene regulation," said Jirtle. "In a way, we are at the end of the beginning. We know the specific gene mutation that leads to large bottoms in sheep, but now we have to find how it operates."