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GENETICS Evidence Points to Genetic Expansion Behind Vertebrate Fingers, ToesThe evolutionary transition from life in the sea to life on land might have been nudged by a genetic expansion, according to an article appearing in the February Development. HMS researcher Susan Dymecki and her colleagues suggest that a gene previously expressed in the developing brain may also have come to be expressed in the tips of growing limbs, helping to bring about the development of toes and fingers in the first vertebrates. "So the idea is you get expansion of gene expression—not expression of a new gene—just expansion to a new area," says Dymecki, HMS assistant professor of genetics. She and colleagues Scott Baur and Jia J. Mai have recently identified the structure of a gene and also a genetic switch that could have brought about such a genetic expansion.
Comparisons of the embryos of normal and mutant mice by Susan Dymecki and colleagues are yielding clues to limb development and evolution. Until recently, the gene, which codes for a receptor found in the brains and skeletons of all vertebrates living today, was thought to be controlled by a single switch, or promoter. If that were true, a defect in the promoter should affect expression in the brain as well as the skeleton. But the researchers found that while mutant mice carrying such a defect lacked fingers and toes, their brains appeared, for the most part, normal. Two's Company On closer inspection, the researchers found that there was not one but two promoters, one controlling gene expression in the brain, the other, which carried the mutation, in the limbs. The defective promoter was farthest away from the gene. "This is the first time anyone has seen this distal promoter," Dymecki says.
Scott Baur (right), shown with research assistant Jia J. Mai, came into the lab as a BBS rotation student "and really kick-started this project," says Susan Dymecki (left). She and Baur, who is a graduate student, propose that this distant promoter may have evolved more recently, perhaps as a result of a duplication of the one lying closer to the gene. Once formed, the new promoter may have accumulated mutations that enabled it to interact with transcription factors found in developing limb cells. As a consequence, the receptor previously expressed in the brain would have come to be expressed in the limb buds. "This is speculation at this point—we could be wrong," Dymecki says. "But it could also lead to some exciting science." Baur is currently comparing the two promoters to see if he can find signs of a duplication. If so, it would suggest that the origin of life on land could have entailed not just the invention of new genes but also putting old ones to new uses. It is a process nature has used many times before, Dymecki says. The Hox genes, which were first found to regulate body shape in flies and are now known to regulate body and limb development in vertebrates, are thought to have produced their wide variety of evolutionary effects by being expressed in different amounts at different times and places in different animals. Similarly, the newly discovered promoter may have played a role in bringing about the wide variety of vertebrate digits—from the frog's grasping toes to the stubbier toes of a human—by changing when and where the receptor gene was expressed. "These are things we're still fleshing out," Dymecki says. "It's been a real whirlwind just to get this paper out." In fact, their current paper appears back-to-back in Development with a paper by a group at UCLA. The California researchers knocked out the receptor gene in a strain of mice. Intriguingly, the knockout mice exhibited the same phenotype as the HMS mutants. They lacked digits but their brains appeared normal. One explanation for why effects in the brain were masked is that the receptor may play such an important role there that nature has provided a genetic backup to make sure that its job gets done. Accidental Scientist If it weren't for a twist of nature, Dymecki and her colleagues might never have identified the second promoter. Her whole excursion into skeletal development "was definitely serendipity," she says. In the course of studying brain development—her primary interest—Dymecki had generated a series of transgenic mice, each with a piece of DNA wedged into a different part of the genome (see sidebar). When she and her colleagues tried ear tagging one strain, they discovered that the mice were unable to grab the tabletop.
Many bones, including the digits of the hand and foot, develop from a cartilaginous blueprint laid down in early development (top panels). During development, cells destined to become cartilage aggregate loosely, then condense and proliferate to form the blueprint for the adult skeleton. In mutant mice carrying the defective BmprIB promoter, the cells destined to form the digital cartilage did not condense nor did they proliferate normally (top left). Consequently, adult mutant mice failed to form phalanges (bottom left). Susan Dymecki and Scott Baur suggest that BMPRIB, interacting with the protein GDF5, may send signals to genes involved in the condensation and formation of digital cartilage. In addition, BMPRIB and GDF5 may act independently of each other to promote the segmentation of cartilage into individual phalanges. It turned out the mice had failed to develop digits. Suspecting that the piece of DNA had inserted itself into the middle of a gene for skeletal development, the researchers homed in on the gene, which produces IB bone morphogenetic protein receptor (BMPRIB). The protein was known to play a role in skeletal development, specifically the laying down of the cartilaginous blueprint that eventually develops into the bony skeleton (see illustration). But no one had actually mapped out the structure of the gene (BmprIB)—that is, how exactly it is broken up into functional units, or exons. After identifying the structure of BmprIB, Baur was able to determine that the chunk of DNA had become integrated, essentially knocking out what appeared to be the promoter. The lack of any apparent defect in the brain of the mutant—at the time they did not know that even knock-outs show no brain defects—led them to look for a second regulatory element. Dymecki and her colleagues plan to use the transgenic system to see exactly how the limb-region promoter turns on BmprIB during mouse development—in what cells and at what times. Comparing BmprIB regulation in mice and other animals could provide a preliminary step to understanding how the extraordinary array of land-dwelling adaptations have evolved in different species. "We want to understand what regulatory elements are involved and what the evolutionary implications of those elements are in terms of species-to-species variation in digit formation," Dymecki says. "It does make me chuckle that it all came out of a simple transgenic insertion." —Misia Landau Kiss-and-Tell ModelDriving wedges of DNA into the genome to map the fates of cells was not Susan Dymecki's idea, but she has become a master at it. In 1991 researchers showed that by introducing specially engineered reporter genes into cells and then exposing the cells to a particular enzyme, Flp recombinase, they could get particular cells to light up. While they got the system to work in flies, whose embryos are more accessible to manipulation, it was a different matter when it came to mammals. To adapt the system, Dymecki created two types of mouse—one that expresses the specially designed reporter gene and one that expresses the flp gene. By piggybacking flp to particular promoters, she created a whole series of mice, each expressing flp in a different cell type. By mating the flp-carrying mice with the reporter mice, she got specific cells to light up in the offspring. She and her colleagues have used this system to map the fates of developing neurons and plan to use it to study the development of the skeletal system. "Now that we have the BmprIB promoter, we have a way to express flp in the developing digit region," she says. |
