It only takes seconds: one cancerous cell breaks off from a tumor, slips into the bloodstream and quickly lodges elsewhere in the body. These colonizers may bloom into deadly metastatic cancer right away or lie dormant for years, only to trigger a recurrence decades after the primary tumor is removed.
Metastases cause the vast majority of cancer deaths, but their tiny seeds are so difficult to track that few researchers have managed to study them. Now, scientists from UC San Francisco describe capturing and studying individual metastatic cells from human breast cancer tumors implanted into mice as the cells escaped into the blood stream and began to form tumors elsewhere in the body.
The researchers discovered that genetic programs expressed in these cells were quite distinct from the primary tumors in which they originated and included genes typically expressed in mammary stem cells. The findings, published online Sept. 23 in Nature, could change the way researchers think about how cancer spreads and suggest new drugs to track down and disable its deadly seeds.
For the most part, modern cancer drugs ignore differences between primary and metastatic tumors, said Zena Werb, PhD, professor and vice-chair of anatomy at UCSF, and a senior author on the new study.
“We test drugs for their ability to make primary tumors shrink, but most just don't work on metastases, and this leaves patients open to recurrence,” Werb said. “Patients have their original tumor treated or removed, but then the cancer comes back 20, 30, 40 years later because there were just a few metastatic cells sitting around.”
Catching metastatic cancer cells in the act
No one really knows how dormant metastatic cells can survive incognito for decades, said Devon Lawson, PhD, who led the research team as a UCSF post-doctoral researcher and is now an assistant professor of physiology and biophysics at UC Irvine.
“It's a big black box in the cancer field – mostly because it's very difficult to study,” she said.
As a result, Lawson said, only about 7 percent of all breast cancer funding goes to studying metastatic cancer, despite the fact that it causes virtually all breast cancer deaths.
Previous work by Werb’s group had found a subset of cells at the edges of breast cancer tumors that seemed primed to metastasize. Their close contact with the bloodstream and with proteins in the surrounding tumor microenvironment seemed to turn on genetic programs akin to those of mammary stem cells – the cells that allow breasts to form during puberty and grow during lactation. These genes for self-replication could make these cells particularly apt to generate new tumors elsewhere in the body. But the researchers had yet to catch the cells in the act.
In the new paper, the researchers used a technique called patient derived xenograft (PDX), which involves transplanting human tumor cells into mice. Against the backdrop of healthy mouse tissue, rogue metastatic cells from the human tumor stick out like flares. The researchers developed a new method using flow cytometry that let them capture individual human metastatic cancer cells traveling through the mouse’s blood or lodged elsewhere in its body, then used newly-developed microfluidic technology to characterize the active genes in these rare cells.
“We were able to look at gene expression at a whole new level of resolution,” Lawson said. “We could pull 12 metastatic cells out of the brain and tell you what is special about those 12 cells. Or the two cells we found in the blood. And we discovered there's something really unique about metastatic cells as they arrive in distant tissues.”
Metastases show stem cell qualities
The team compared patterns of gene expression in human cancer cells lodged in different organs of the PDX mice and found stark differences between early-stage and more advanced metastatic colonies. In metastases that had already grown and spread throughout an organ, the cancer cells’ gene activity looked much like that of the primary tumor that had been transplanted into the mice, though with subtly different features specific to the new organ, whether lymph, liver, lung or brain.
In contrast, early-stage metastases and cancer cells traveling through the blood expressed genes typically active in mammary stem cells and quite distinct from primary tumor cells. In addition, these seed cells expressed specific genes that would be expected to keep them in a dormant, undifferentiated state and relatively immune to cell death, which the researchers surmised might help metastatic colonies survive in new and hostile environments.
Remarkably, the same signature pattern of gene activity was found in metastatic cells in mice whose tumors came from genetically and clinically diverse human patients. In other words, the genetic program that makes a cell metastatic did not depend on the genetics of its tumor of origin – suggesting that new techniques might allow researchers to find and specifically target these cells throughout the body in a variety of patient populations.
Read more at UCSF.edu