Shedding light on cancer stem cells

Cells created from scratch to trigger breast cancer will bring new evidence to a fierce scientific debate

Like fashion, science sometimes travels in circles. A model of cancer proposed in the 1800s has recently returned to vogue, with huge implications for how we diagnose and treat this group of deadly diseases.

During the 19th century, pathologists noticed that under the microscope, some tumors resemble embryonic tissues: both contain many rapidly dividing cells that appear to be disorganized. By 1875, Julius Cohnheim and Francesco Durante had proposed that tumors arise from embryonic remnants in adult tissues.

Top, sheets of normal human breast cells (whose membranes are stained red) have been grown in a new culture medium. Bottom, the normal cells have been transformed into cancerous cells (with membranes stained green). As many as one in ten are cancer stem cells. Photos: Tan Ince

They planted the seeds for the modern hypothesis that tumors are driven by a rare population of stem cells that can both regenerate themselves indefinitely and give rise to other kinds of cells. But their ideas went out of fashion in the last half of the 20th century with the rise of a competing model of cancer called clonal evolution.

Under this egalitarian model, all cancer cells possess equally destructive potential. Although cancer cells are heterogeneous, all or most of them have the capacity to create a new tumor. A cell becomes cancerous after acquiring a series of mutations, and descendants of that original miscreant evolve while competing with one another for resources. Thus, given the right combination of genetic alterations, any cancer cell can trump its neighbors, expand its territory locally and colonize distant tissues.

Some recent discoveries, however, support the hierarchical model of cancer, in which a handful of stem cells reign supreme. These despots—which become cancerous through a series of mutations—retain control over their descendants, which form the bulk of a tumor. Unlike their offspring, cancer stem cells can live indefinitely and seed new tumors.

The cancer stem cell hypothesis could explain why tumors often return after patients receive chemotherapy or radiation. Such treatments may spare the slow-growing, unspecialized cells at the root of the tumor.

Unsurprisingly, this hypothesis has kicked off an enormous uproar among cancer researchers. Recent work at Whitehead, allowing researchers to create cells that trigger breast cancer in mice, should help to clarify this puzzle.

Solid tumor evidence

The modern retelling of the story begins with Michael Clarke, standing before a room full of medical students at the University of Michigan and delivering a lecture on testicular cancer. The professor of internal medicine made an observation that changed his career: the tumor tissue displayed on the screen above held only a few immature cells surrounded by countless specialized cells.

“That was the eureka moment, when I suspected that solid tumors have stem cells in them,” he recalls.

Although John Dick of the University of Toronto had isolated cancer stem cells from leukemias in the mid-1990s, most scientists assumed they were unique to blood cancers.

But Clarke began hunting for these elusive entities in human breast tumors, aided by the recent discovery that normal adult stem cells typically express telltale CD44 proteins on their surface. For months, Clarke hovered near a cell-sorting machine with postdoctoral fellow Muhammad Al-Hajj, sifting through cancer cells in search of the right protein patterns.

Eventually, Clarke and his colleagues at the University of Michigan succeeded in isolating a population of potent tumor-initiating cells that were dotted with CD44 proteins, yet were missing the CD24 proteins that were typical of more specialized cells. When Al-Hajj injected the breast cancer cells into mice whose immune systems were compromised, hey developed tumors—and they did so after only 100 cells had been introduced instead of the hundreds of thousands that would typically be required to achieve the same effect.

The discovery of cancer stem cells in human breast tumors, which appeared in Proceedings of the National Academy of Sciences in 2003, reinforced Dick’s findings.

“I ignored the discovery of cancer stem cells in leukemias because I thought they might be an idiosyncrasy of blood cancers,” comments Whitehead Member Robert Weinberg. “But when Michael Clarke’s laboratory published compelling evidence of cancer stem cells in breast tumors, I took note.”

Now a professor at Stanford, Clarke is not the only scientist to find himself unexpectedly at the center of the controversy over cancer stem cells. Though journals have published dozens of reviews on the topic since his discovery, just a handful of labs have produced solid data that advances or challenges his findings, but they have all attracted considerable attention. Peter Dirks of the University of Toronto, for example, entered the spotlight in the cancer research community by isolating stem cells from brain cancer in 2004.

Since then, stem cells have continued to pop up in other types of solid tumors. For instance, Clarke and his colleagues pulled them from colon tumor tissue this spring.

Hitting the trifecta

Taken alone, any one of these three discoveries would fail to generate dozens of reviews. The term “cancer” encompasses a multitude of diseases characterized by the abnormal proliferation of cells, so the presence of stem cells in a particular type of tumor doesn’t mean much. In combination, however, the studies suggest a paradigm with enormous clinical ramifications.

“The reason the cancer stem cell hypothesis has taken so much of my time is because of the potential implications for therapy,” says University of Toronto’s Dirks. “It appears that cancer stem cells resist conventional treatments, so we need to find a way to target them.”

Chemotherapy can resemble the arcade game Whac-A-Mole, in which players use a mallet to hit plastic moles that pop up from different holes. The game is seemingly futile, as the moles continue to surface—sometimes in the same spot—after being hit. Similarly, chemotherapy kills many cancer cells and causes tumors to shrink, but they often return after a few months.

That may be because cancer stem cells resist conventional chemotherapy drugs and “rebuild” after their descendants die, explains Dirks.

Studies in leukemia support this hypothesis. For example, Tessa Holyoake, a professor at the University of Glascow, has discovered a mechanism by which blood cancer stem cells keep the drug Gleevec at bay. The stem cells have proteins on their surfaces that prevent this potential killer from accumulating inside by literally pumping it out.

Duke University’s Jeremy Rich has shown that some cancer stem cells also resist radiation treatment. When he exposed brain cancer stem cells to radiation, they cleverly shifted DNA damage repair activities into high gear, thereby dodging destruction.

Scientists are trying to disable these coping mechanisms and find other ways to kill cancer stem cells to prevent tumors from returning. In May, Dirks published the results of a chemical genetic screen, which uncovered several small molecules that inhibit cultures enriched for brain cancer stem cells.

“Looking ahead, we need to build our knowledge of cancer stem cells into the drug discovery process,” says Dirks.

In addition, the cancer stem cell hypothesis may change the way cancer is diagnosed. In a New England Journal of Medicine paper published in January, Clarke and his colleagues demonstrated that the level of expression of 186 genes in cancer stem cells can predict the risk of recurrence in patients with breast cancer, lung cancer and a type of a childhood brain cancer. By isolating cancer stem cells from tumor tissue, he discovered useful information about the stage of the disease.

Which cancers, when?

But scientists caution against tossing out existing cancer paradigms and tools before they acquire more data. For example, neither Clarke nor Dirks is convinced that the cancer stem cell model applies to all types of cancer.

Associate professor Kornelia Polyak of Harvard Medical School and Dana-Farber Cancer Institute couldn’t agree more. Polyak was intrigued by Clarke’s 2003 discovery of breast cancer stem cells, but instead of publishing a review, she decided to test his results.

Her findings, which appeared in Cancer Cell in March, match the clonal evolution model of cancer, rather than the cancer stem cell hypothesis. She discovered that the descendants of the cancer cells with stem cell properties continue to undergo genetic evolution, which suggests that they too can drive tumor progression.

Striving to create better models of breast cancer cells, Tan Ince (above) ended up producing cancer stem cells. Other labs can easily grow the newly created cells for their own experiments, notes Robert Weinberg (below, left). Photos: Sam Odgen

“I do believe there are cells in a tumor that have the features of stem cells and that those cells are more invasive and metastatic than their more differentiated counterparts,” says Polyak. “But I don’t think they’re always rare, and I don’t think they’re the only cells responsible for tumor recurrence and drug resistance.”

If this were the case, she says, then recurrent tumors, presumably composed of the descendents of cancer stem cells, would be sensitive to the same treatment as the original tumor, and acquired drug resistance would never occur.

It’s possible, says Polyak, that mice may have misled Dick, Clarke and Dirks. Mice may provide a hostile environment for differentiated cancer cells that are fully capable of initiating tumors in humans in the right environment.

Cancer stem cells on call

That could change thanks to Whitehead visiting scientist Tan Ince, who recently created breast cancer stem cells from scratch. His findings appeared in Cancer Cell this August.

He didn’t set out to engineer these potent cells. As a postdoctoral researcher and pathologist in the Weinberg lab, Ince was simply trying to create breast cancer models that look like real human tumors under the microscope and behave like those seen in many patients.

Ince developed a recipe for a chemically defined culture medium and managed to grow a type of normal human breast cell that ordinarily dies in culture. He transformed it into a cancer cell by inserting specific genes through a standard procedure.

The engineered cells proved to be extremely powerful. When Ince injected more than 100,000 of them into a mouse with a compromised immune system, the mouse quickly developed massive, deadly tumors. In initial experiments, a few tissue slices revealed a primary tumor structure that resembled that of cancer patients with metastases.

That prompted Ince to wonder whether the cancer cells he created would metastasize if the mouse lived longer. He repeated the experiment in other mice, reducing the number of cells in the injection to as few as 100 in hopes of slowing tumor growth. The cancer cells continued to seed tumors and the tumors metastasized. After submitting the work for review, Ince even generated tumors with an injection of just 10 cells.

“In the process of making a model that reflects a tumor type common in patients, I created tumor-initiating cells,” says Ince, now an independent investigator at Brigham and Women’s Hospital and an instructor at Harvard Medical School. “That was a complete surprise.”

“This work could provide a boon to researchers who study these elusive cancer stem cells by offering a bountiful source of them,” says Weinberg. “Labs can easily grow the newly created cells for use in experiments.”

The cells provide a common platform for discovery. The field can progress more quickly with many researchers working on identical cell lines and repeating each other’s experiments. The cell lines will also make it possible for labs to jump into the field without learning the tedious and expensive cell-sorting techniques required to isolate cancer stem cells from tumors.