Deleted in Colorectal Cancer (DCC) Gene Plays a Role in Wiring the Mouse Brain and Spinal Cord

CAMBRIDGE, Mass. (April 24, 1997) — A new study has found that Deleted in Colorectal Cancer (DCC), a gene thought to play a role in human colorectal cancer, does not play a role in the development of mouse colon cancer. Instead, the mouse version of the DCC gene, called Dcc, functions as a receptor involved in the wiring of the brain and the spinal cord. DCC was first identified in 1990 as a candidate "tumor suppressor" gene that acts as a brake during normal growth of colonic cells but is missing in most colon cancer cells. The new mouse study, led by Dr. Amin Fazeli in the laboratory of Dr. Robert Weinberg at the Whitehead Institute for Biomedical Research, weakens the candidacy of DCC as a cancer gene and shows that the gene helps establish connections in the developing nervous system.

Dr. Fazeli, Dr. Weinberg and their collaborators, including Dr. Marc Tessier-Lavigne from the Howard Hughes Medical Institute and University of California, San Francisco, and Dr. Jeffrey I. Gordon from Washington University School of Medicine in St. Louis, Missouri, report their results in the April 24 issue of Nature.

In the study, Dr. Fazeli and his colleagues in the Weinberg lab used transgenic technology to "knock out," or inactivate, the Dcc gene in mice and compared the Dcc-deficient mice with their normal littermates. The researchers found that Dcc loss did not predispose mice to intestinal tumors nor did it seem to cause existing intestinal polyps to progress into tumors. Dcc loss also appeared to have no effect on the normal growth and differentiation of gastrointestinal tissues. "These results weaken the candicacy of the Dcc gene as a key player in the formation and growth of intestinal tumors in mice, although they do not rule out definitively its role in human colon cancer pathogenesis," says Dr. Weinberg.

Dcc loss did, however, affect the wiring of the central nervous system of the mice: axons, or nerve fibers, in Dcc-deficient mice had trouble finding the right connections in the brain. Some axons never found their destinations, and others seemed to have lost their way, ending up where they didn't belong. "These results confirm definitively that the mouse Dcc gene plays a role in helping axons find their connections during nervous system development," says Dr. Fazeli. "Given that evolutionary relatives of Dcc have been shown to play similar roles in the worm, C. elegans, and the fruit fly, Drosophila melanogaster, it seems highly likely that the human DCC gene may play a role in establishing connections in the developing nervous system of humans," adds Dr. Fazeli.

DCC was first identified in 1990 as a candidate "tumor suppressor" gene because of its location on a key region of chromosome 18. More than 65 percent of colon-cancer cells are missing this region, leading scientists to suspect that the region contained a tumor suppressor gene. The DCC gene, isolated from this region, seemed a likely candidate; however, its role in colon cancer has not yet been established conclusively.

In the meantime, unrelated research by other scientists, including Dr. Tessier-Lavigne and his colleagues at UCSF and HHMI, had unearthed other possible roles for DCC. Their research in the rat, along with other studies in C. elegans and Drosophila melanogaster, suggested that DCC may be involved in the developing nervous system.

In this study, Dr. Fazeli and his colleagues in the Weinberg and Tessier-Lavigne laboratories sought to elucidate the role of DCC in both colon cancer and nervous system development. First, researchers examined the effects of Dcc loss on the gastrointestinal tract of mice. In one experiment, they inactivated one copy of the Dcc gene and followed the mutant mice for two years to determine if Dcc loss predisposed mice to an increased risk for developing tumors. "We found that this inactivation did not predispose mice to intestinal tumors," says Dr. Fazeli. Next, scientists inactivated both copies of Dcc in mice that already had a large number of colonic polyps—a condition that is similar to patients with Adenomatous polyposis coli (APC) who are at increased risk for colon cancer—to determine whether loss of DCC causes the polyps to progress into tumors. "Dcc loss did not cause tumor progression in mice, nor did it affect the normal proliferation and differentiation of gastrointestinal epithelial cells in newborn mice or the adult mouse gastrointestinal tissue," says Dr. Fazeli.

Even more interesting results emerged when scientists examined the effects of Dcc loss on the central nervous system of mice. Dcc loss seemed in particular to affect a group of neurons called commisural neurons of the spinal cord. "In vertebrates, these neurons project axons that grow from the back of the spinal cord toward the front—toward a group of cells called floor plate cells. This process establishes a 'commissure,' that links neurons in the back of the spinal cord to neurons in other parts of the spinal cord," says Dr. Fazeli.

"These axons are guided to their destinations in part by a nerve chemical called netrin-1 secreted by the floor plate cells; attraction to netrin-1 helps axons find their way to their final destination in their journey from the back to the front," says Dr. Tessier-Lavigne who along with his colleagues discovered this information in a series of seminal studies starting in 1988.

Other studies have associated counterparts of netrin-1 with axonal guidance genes in the worm and the fruit fly. And, other studies by Dr. Tessier-Lavigne's laboratory have suggested that Dcc may be involved in netrin-1 mediated axonal guidance. However until now, the significance of this interaction for a whole animal had not been addressed. "Our results confirm that Dcc is involved in establishing neuronal connections and confirms that Dcc must be a receptor for netrin-1," says Dr. Fazeli. "Dcc-deficient mice showed the same types of deficiencies in spinal cord and brain development as netrin-1-deficient mice."

These deficiencies were revealed when researchers compared the trajectories of fluorescent labeled commissural neurons in normal and mutant mice. They found that while axons in the normal mice found their way from the back to the front, in Dcc deficient mice, the number of commissural axons reaching the floor plate was reduced in some areas, and in other areas, axons never reached the floor plate. "Furthermore, in other sections, there appeared to be misrouting of many of the axons that did manage to reach the front. Instead of reaching the floor plate, these axons ended up elsewhere," says Dr. Fazeli. These results were confirmed when researchers injected dye into the neurons and studied how the dye diffused down the axons. The defects in axonal projections appeared to be limited to the commissural axons and are similar and more severe than those seen in mice that are deficient in netrin-1.

The researchers also found that in Dcc-deficient mice, commissures that link the left side of the brain to the right were completely missing, had failed to cross over to the other hemisphere, or had gotten tangled. In addition, commissures seemed to have formed where they were not supposed to. "Finally, DCC also appears to be involved in migration of whole cells in brain and not just their axons," says Dr. Fazeli.

Citation

Fazeli, A., Dickinson, S. L., Hermiston, M. L., Tighe, R. V., Steen, R. G., Small, C. G., ... & Weinberg, R. A. (1997). Phenotype of mice lacking functional deleted in colorectal cancer (Dec) gene. Nature, 386(6627), 796-804.