Big-bottomed sheep have a rare genetic mutation that builds muscl...( ikely the genetic sequence was conserve...)

ikely 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 crede
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Contact: Rebecca Levine
levin005@mc.duke.edu
919-684-4148
Duke University Medical Center
16-Sep-2002

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