Genomic Screen Captures Genes Preventing Cancer Spread
Cancer is deadliest when it metastasizes: spreads beyond its original site and invades distant organs. Just as cells are equipped with tumor suppressor genes to keep incipient cancers from growing, they also have genes that prevent this dangerous spread. These metastasis suppressor genes have proved difficult to find—but that may be changing.
Over the past 20 years, only a dozen or so metastasis suppressor genes have come to light. But now Howard Hughes Medical Institute investigator Michael Green has developed a systematic method for screening the genomes of cancer cells to detect likely metastasis suppressors.
The group has tested the approach in cell lines developed from mice with metastatic melanoma and identified 22 genes which, when "knocked down" with RNA interference, allowed tumors to metastasize but had no effect on the growth of the original tumors.
One of those 22 genes prevented the spread of melanoma cells, which are highly prone to metastasis. The scientists put that gene through rigorous tests in the lab and validated it as a true suppressor of melanoma metastasis. Further work is needed to validate the remaining 21 candidates, but Green thinks they are highly likely to be suppressors of metastasis for a variety of cancers.
"Metastasis is an important part of cancer biology, but it is a complex process and a tough problem to study," says Green, whose lab is at the University of Massachusetts Medical School in Worcester. "The most exciting part of the paper is not necessarily that we have found this one metastasis suppressor but that we've developed a general approach that can be used to find others - and we can do it with any type of cancer cell."
Like tumor-suppressor genes, those that squelch metastasis are most evident when they are damaged or altered in ways that disrupt their protective function. The first metastasis-suppressors were unearthed in the late 1980s. Reduced activity of some of those genes has been found in cancers through microarray studies that measure gene activity in cells. It is still far from clear, however, how the loss of such a gene, or genes, enables a cancer cell to do the things it must when it metastasizes: that is, operate apart from the original tumor; invade normal tissues; survive a long-distance journey through the blood stream; and establish a thriving colony in some target organ.
In devising a functional screen for metastasis-suppressing genes, Green and his colleagues, including first author Stephane Gobeil, exploited two recent technological advances—RNA interference and three-dimensional cell-culture systems.
Traditional cells cultured in sheets on flat plastic dishes fall short of replicating the natural surroundings of cells in the body, with their neighboring cells, fibrous layers, membranes, and adhesion proteins. Newly developed three-dimensional systems are a boon for studying metastasis, says Green. "They are a system that allows you to mimic early events in the metastasis cascade," he says.
In Green's laboratory, researchers inserted balls of about 1 million cells into collagen, forming a plug that is then coated with an artificial matrix that mimics a cellular basement membrane. Next, the coated plug is embedded in a fibrin matrix within a standard Petri dish. In metastasis assays, cancer cells can be observed as they break off and migrate away from the original cell mass in all directions.