CAMBRIDGE, Mass. -- Researchers at the Massachusetts Institute of Technology report in the August 3 issue of Nature that they have identified a single gene that lends some cancer cells the particularly deadly ability to break off a tumor and travel to other parts of the body.
This process, called metastasis, is responsible for 90 percent of cancer deaths.
If scientists can pinpoint which genes are essential for invasion and metastasis, they can use that information for better diagnosis and therapies.
"I believe that invasion and metastasis are now ripe for a concerted attack because of two scientific advances: our deeper understanding of the molecular basis of cell adhesion based on the past couple of decades of cell biology and the availability of mammalian genomic sequences and methods for exploiting that information," said Richard O. Hynes, director of the Center for Cancer Research at MIT.
"These should allow a systematic search for alterations in genes controlling cell adhesion and migration, as well as other processes that underlie malignancy, and should provide a much deeper understanding of invasion and metastasis -- the processes that really make cancer a killer disease," he said.
Using mouse and human models of melanoma, data from the Human Genome Project, DNA arrays and gene analysis software, Edwin A. Clark, then a postdoctoral fellow in Hynes' laboratory at the MIT Center for Cancer Research, sought to identify precisely which genes or sets of genes are responsible for regulating metastasis.
Clark, now at Millennium Predictive Medicine in Cambridge, Mass.; and Hynes, Daniel K. Ludwig Professor for Cancer Research, Howard Hughes Medical Institute Investigator and president of the American Society for Cell Biology, collaborated with Todd R. Golub and Eric S. Lander of the Whitehead Institute/MIT Center for Genome Research. Golub also is affiliated with the Dana-Farber Cancer Institute in Boston.
NARROWING THE SEARCH
To provide insight into the pattern of gene expression that allows tumors to metastasize, the researchers looked at variants of melanoma cells that were either likely or unlikely to metastasize.
Using new methods of DNA arrays to analyze the expression of multiple genes at a time, the researchers screened for the expression of a total of around 10,000 genes (around 10 percent of the total number of genes) and found 32 genes that increased in their expression in metastatic cells.
About half of those genes fall into three categories -- genes affecting cell adhesion, cytoskeleton and potential regulators of angiogenesis (new blood vessel formation). The cytoskeleton is the cell's internal framework.
The researchers selected one of these called rhoC, a regulator of cytoskeletal structure and cell movement that is itself regulated by cell adhesion and growth factor receptors, to test whether or not it played a central role in inducing metastasis. It turned out that it did.
"If we overexpress rhoC in poorly metastatic cells, they become highly metastatic. If we express an inhibitor of it in highly metastatic cells, then they show reduced metastasis," Hynes said.
The researchers were somewhat surprised to find that a single gene -- the relatively unknown RhoC -- is essential for tumor metastasis. RhoC is a member of the Rho GTPase family that has been shown to regulate numerous cellular functions, most notably cytoskeletal organization in response to extracellular factors.
WHEN CANCER CELLS STRAY
While researchers have learned a great deal in recent years about how primary tumors develop and the cellular and molecular mechanisms that cause the tumor cells to proliferate out of control, little is known about invasion and metastasis.
MIT cancer expert Robert Weinberg says cancer is caused by what he calls a "Holy Trinity" of transformations: a gene that acts like a stuck car accelerator that causes unchecked growth; a gene that disables the braking system that might have halted this unnatural growth; and an enzyme that allows the renegade cells to reproduce forever.
At least two puzzles remain: how do tumor cells convince nearby blood vessels to hook them up to the blood supply they need to grow and flourish; and how do they invade other parts of the body through metastasis?
"It was shown some time ago that metastases arise from rare malignant variant cells within the primary tumors," Hynes said. "Something changes that causes loss of positional control."
Scientists now understand more about the cell surface molecules called adhesion receptors that cells use to attach to their neighbors and to their normal locations.
"Loss of some of these adhesion receptors has been shown to be a key step in the development of malignancy in some human tumors; gain of new adhesion receptors contributes to the wandering behavior of other malignant cells," he said.
INTERNAL CELL FRAMEWORK IS KEY
RhoC was one of three genes that were expressed in all three metastatic melanoma cells from both the human and mouse lines that were studied. All three genes were somehow involved in the cell's cytoskeleton.
"The altered expression of so many genes whose products regulate the actin cytoskeleton suggests an important role for cytoskeleton organization in tumor metastasis," Hynes said. The researchers suspect that rhoC may regulate metastasis by controlling cytoskeletal events necessary for motility.
The MIT Cancer Center and Biology Department are now developing a microarray facility to support this kind of DNA research more broadly. This type of analysis will become much more widespread with the completion of the human genome sequence.
This work was supported by grants from the National Cancer Institute; Howard Hughes Medical Institute, Merck Inc.; Affymetrix Inc.; Bristol-Myers Squibb and Millennium Pharmaceuticals.