One group, led by Bjorn Olsen, Hersey Professor of Cell Biology at Harvard Medical School and Forsyth Professor and Chair of Oral Biology at Harvard School of Dental Medicine, identifies a gene that when mutated, causes a bizarre bone defect--one that seems to have played a role in Homer's The Iliad. A second team, led by Olsen and scientists in England and Germany, describes a strain of mice in which this gene has been disrupted. And a Texas research team reports molecular evidence bolstering the claim that this gene serves as a "switch" that triggers uncommitted cells in the early embryo to become bone-formers. "This gene is essential for bone-forming cells to arise in the embryo. Without it, there is not a single bone-producing cell in the body," says Olsen.
"This series of papers provides the first molecular insight into how differentiation into bone is regulated," comments Gideon Rodan, head of bone biology and osteoporosis research at Merck Research Laboratories in West Point, Pa., who is writing a minireview on the topic in the same issue of Cell.
The goal of this research is two-fold, Olsen says. First, he is trying to understand the organogenesis of bone in the embryo and child as part of his overall interest in learning how nature puts together a vertebrate body. Second, he hopes that basic knowledge about what controls the activity of bone-producing cells will pay off in better treatments for osteoporosis. "While this work does not have an immediate impact on osteoporosis, it is the kind of information that will allow us eventually to increase the formation of bone in the skeleton," Olsen says.
Olsen's team started the current studies by hunting for the gene causing a rare condition called cleidocranial dysplasia (CCD), hoping that this experiment of nature could teach them something general about bone formation. First described as an inherited condition by a French doctoral student in 1896, CCD is a developmental disorder that stunts the growth of certain bones. Adults with CCD have a hole at the top of their skull, because their infant fontanels never harden. Their skull contains many small bone fragments held together by connective tissue, as opposed to the few large plates that-in normal adults-have fused together into one continuous piece.
Their collar bones develop as tiny stumps or not at all, enabling them to perform a trick known since ancient history: "His shoulders were bent and met over his chest," Homer writes about a man who came to Troy.
While zeroing in on the gene, Olsen's team found a family whose affected members were missing a piece of chromosomes 6. Of two candidate genes the researchers found in that region, they first ignored one--prematurely, it turned out--because another group had implicated it in the immune system. Serendipity got them back on track when immunologists in London genetically engineered a strain of mice to lack this gene. These researchers, expecting problems in immune development, were surprised to find that these mice instead had defective skeletons. Mice lacking one copy of the gene turned out to mirror precisely the human disease, and mice lacking both copies have a skeleton made entirely of cartilage, like little sharks.
Together, the two research teams then showed that the gene, called CBFA1, indeed was the one causing CCD. CBFA1 encodes a protein that can turn on the expression of other genes. The Texas group filled in another piece of the puzzle by showing that this protein can induce laboratory-grown, immature cells to start maturing into bone-producing cells.
The CBFA1 gene is active from the second week of embryonic development, says Olsen, suggesting that it is active in two ways. First, it plays a role in setting up the basic pattern of the body.
That second role is what leaves holes in the skulls of people with CCD. Missing one copy of the CBFA1 gene, they rely on the remaining copy to produce enough of the corresponding protein to build a skeleton, though they are short in stature. But certain spots of the skeleton require that there be more protein than one copy of the gene can supply. In affected people, the growth of the skull simply cannot keep up with the overall growth of the head. "So CCD is really caused by insufficient amounts of bone-forming cells," says Olsen.
The sharklike mice missing both copies of CBFA1 illuminate other aspects of bone formation. Born with a skeleton made of cartilage, they show that cartilage arises quite independently of bone, he adds. However, CBFA1 is essential for replacing cartilage--which serves as a casting mold for later bone formation--with the final product.
Medical Implications
The role of CBFA1 in maintaining bone is still unclear, although it is expressed in adult mice, indicating its possible importance throughout life. Researchers are now asking whether the gene plays a role in osteoporosis, says Olsen. This common disease results from an imbalance between bone production and bone degradation. While researchers have some idea about the molecular underpinning of bone degradation, they know less about bone formation. "These studies give us the tools to investigate the relationship between bone formation and osteoporosis in an experimental way," Rodan says.
This work might also prove useful to efforts going on in Boston and elsewhere at creating live human bone in the laboratory. Being able to manipulate the genes that control the maturation of bone-forming cells may help tissue engineers nudge along that process in their laboratory-grown tissues. "For orthopedic surgeons, it would be great to be able to form pieces of bone for implantation into the patient," says Olsen.
The Harvard researchers studied members of five families, most notably one that dates back to a clan founded by a Chinese immigrant to South Africa. After converting to Islam, he took seven wives and spawned a family of 357 members, 71 of whom have or had CCD. This treasure trove for gene hunters came to the attention of physicians when, in the 1940s, a 7-year old boy was kicked in the face by a horse and a subsequent x-ray film revealed a gaping hole in his skull. Since then, the characteristic skull of people with CCD has been dubbed "Arnold head" after the assumed name of this family's patriarch.
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