February 2002

Scientists Discover Key Signaling Protein

For scientists who study hypohidrotic ectodermal dysplasia (HED), the past five years have been the best of times. They have identified the first genes involved in the syndrome, then determined that their protein products, when altered, likely cancel a key developmental signal in the patterned formation of the body's ectodermal derivatives. This work, they say, has provided a rough molecular outline of HED, helping to explain why people with the syndrome often have developmental defects that affect their teeth, hair follicles, and sweat glands.

Now an international team of scientists has reported in Nature another important piece to the puzzle. In a series of experiments that moved back and forth from mouse to human, the group cloned another gene involved in relaying this developmental signal that encodes a so-called adaptor protein. An adaptor protein acts as a platform to recruit other proteins to an activated receptor, influencing where, when, and how signals are transmitted through the cell.

According to Dr. Jonathan Zonana, a human geneticist at Oregon Healthand Science University in Portland, and Dr. Paul Overbeek, a mouse geneticist at the Baylor College of Medicine in Houston, this is important because the adaptor protein inserts itself into the signaling process early on, where it "acts as a critical bridge" linking the upstream receptor to known downstream proteins. "It appears that this adaptor protein has several  important structural domains that interact with other pathways," said Zonana, whose research was supported by the NIDCR and the National Foundation for Ectodermal Dysplasias. "This discovery may help to explain the variability of HED in people and gives us additional clues to understand the normal development of teeth and other ectodermal tissues."

The scientists reported that they identified a disease-causing mutation in this new gene that is present in a large, Middle Eastern family with a history of HED. "Though I hate to make the claim of first, after reviewing the medical literature, this may indeed mark the first human disorder caused by changes in one of these adaptor proteins," said Zonana, noting that additional families with mutations in the gene have not yet been identified.

Ectodermal dysplasia syndromes, or EDS, are characterized by the complete or partial loss of function of at least two tissues derived from the ectoderm, one of the original layers of cells that form before a developing baby is large enough to be seen. These tissues include the buds that erupt into teeth, the follicles that produce hair, and the glands that produce sweat to cool the body. Of the more than 150 clinically recognized forms of EDS, the most common is the hyypohidrotic, which indicates that a person has a diminished capacity to sweat.

Cloning of Genes

Since Charles Darwin first described HED in the 1860s, scientists have had very little to work with to tease out the molecular underpinnings of the syndrome. However, beginning in the 1950s, geneticists identified three different mutations in mice, all of which cause patchy hair and abnormal tooth development that is reminiscent of HED in humans.

By 1998, as the tools for mapping DNA began to dramatically improve, Betsy Ferguson, a research scientist in the Zonana lab, cloned the first of the mouse genes called Tabby, or Eda. This gene, involved in the most common form of HED, encodes a protein that activates, like a key in a lock, a specific receptor on the surface of undifferentiated ectodermal cells. The Overbeek and Zonana labs then turned to a second mouse mutant named downless and isolated a gene in humans and mice known as Edar. As good fortune would have it, the gene encoded the very receptor to which the Eda protein binds to trigger the signal.

But this still left the researchers with an unexplained disconnect between the upstream and downstream signaling proteins. In hopes of possibly bridging the gap, Zonana, Overbeek and colleagues turned to finding the third mouse gene, the subject of the paper published in Nature.

The gene responsible for this abnormal mouse, known as crinkled, had been previously localized to a relatively large stretch of mouse chromosome 13. Starting there, Denis Headon, a doctoral student in Overbeek's mouse genetics laboratory proceeded to narrow down the possible location, giving them several possible candidate genes.

Edaradd

Knowing the area in question of the mouse genome corresponds through evolution to a small region of human chromosome one, Headon next turned to the full draft sequence of the human genome. There, spelled out in the electronic database, the scientists found an interesting structural motif encoding a so-called "death domain."

Despite the dire name, proteins use death domains to transmit signals, not just to kill cells. Because the previously identified Edar receptor also has a death domain in the cytoplasm, the scientists settled on exploring further the new death domain protein, which they later named Edaradd.

"Though these databases are not always perfect, they provide an invaluable tool to speed up the research process," said Overbeek, whose research was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases. "Denis accomplished in a short time what might have taken an entire career just a few years ago."

However, the group still had to prove that Edaradd physically interacted with other proteins in the pathway. After a series of experiments, the collaborators found that Edaradd interacts directly with its receptor, and when mutated, can cancel the signal to other proteins in the pathway.

With these three proteins now plugged into this pathway, Zonana notes that the research will continue to benefit not only people with HED, but biology as a whole. He said a confluence of evidence suggests that this previously unknown developmental pathway is shared among many organisms and might be at least 500 million years old. "This pathway seems to control the early development of hair follicles and teeth in mammals, scales in fish, and, more speculatively, feathers in birds," said Zonana. "To think that we didn't even know about it until the work with HED. It really highlights the importance of studying rare human genetic disorders."

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