Outsmarting Cancer Stem Cells
The standard protocol for cancer chemotherapy is to give the "maximum tolerated dose"—a regimen that's brutal to experience, yet doesn't always kill the cancer. "We're incredibly good at shrinking tumors, and incredibly bad at curing them," says Richard White, MD, PhD, an oncologist and clinical fellow in the Stem Cell Program at Children's Hospital Boston. "It's like fighting a moving target."
Many scientists are convinced that tumors are formed and fed by a small but critical group of cells known as cancer stem cells. These cells, they believe, can endlessly replace themselves and form other tumor cell types, just as normal stem cells form different tissues in a developing embryo. They could represent the most appropriate target in cancer therapy.
Scott Armstrong, MD, PhD, a pediatric oncologist at Children’s and the Dana-Farber Cancer Institute and affiliate member of the Stem Cell Program at Children’s, likens cancer stem cells to queen bees: A hive collapses only if the queen is destroyed; if she isn’t, the colony re-forms. White makes another analogy. “If you want to get rid of a tree, you could cut off the branches and hope it dies, but it usually won’t,” he says. “Or you could cut out the root, which will kill the tree. Current chemotherapy removes the branches, not the root.”
Research has accelerated dramatically since the first cancer stem cells were identified in a human leukemia in the 1990s. But exactly what a cancer stem cell is or isn’t has yet to be defined. “Some say they’re the cells from which a tumor originates,” says Carla Kim, PhD, who researches lung stem cells in the Program. “Others think they’re specialized cells within a tumor that help maintain it.”
Scientists are also debating the true “stemness” of these cells. “Calling them cancer stem cells is confusing, because they don’t always come from normal stem cells in the tissue,” says Kim. “Sometimes molecules involved only in normal stem cell function are reactivated in cancer cells.”
Ultimately, the most important question is how these cells help tumors thrive and spread, and how to eliminate or neutralize them. Kim, White, and Armstrong are exploring these questions, looking at three different cancers.
In a room filled with tanks full of tiny zebrafish—some striped, others yellowish, still others genetically engineered as albinos—White points out his latest creation, dubbed Casper. This ghost-like breed, with its clearly visible internal organs, is helping White study metastatic disease — the unpredictable, often fatal spread of cancer to another part of the body.
“A breed of zebrafish, called "casper," has transparent skin throughout its life. The fish, says Richard White, MD, PhD, is helping researchers learn about disease processes, like tumor metastasis, more easily than before.”
“Once you’ve developed metastatic disease, I generally can’t cure you,” White says. “I can only delay its progress. That’s the paradox of cancer: Shrinking a tumor, and curing one that has spread, seem to be somewhat unrelated. So why is metastatic disease so incurable?”
White’s studies, conducted in the laboratory of Leonard Zon, MD, Director of the Stem Cell Program at Boston Children’s Hospital, focus on metastatic melanoma. Because Casper’s skin is transparent, White can directly observe melanoma’s spread by viewing the fish under a microscope. Some tumor cells quickly break off to go elsewhere in its body. The mobility of these tumor cells, at least in melanoma, may be integral to metastasis, he believes.
“These cells share certain genetic programs with stem cells; they’re inherently mobile and raring to go,” White says. “We’re trying to find molecules that block this capacity to move.” As he finds such compounds, he can test them in Casper and observe how effectively melanoma’s spread is checked.
Taking another approach, Markus Frank, MD, an affiliate member of the Stem Cell Program at Children’s, in the Transplantation Research Center of Children’s and Brigham and Women’s Hospital, works with mice bearing human melanomas. In 2008, he showed that melanoma stem cells carry a protein on their surface, ABCB5, that makes them chemotherapy-resistant. But they also showed that melanoma stem cells can be targeted for destruction—and tumors inhibited—by using antibodies against that very protein. In early 2010, Frank and colleagues further demonstrated that melanoma stem cells help the cancer evade our immune defenses by actually lulling the immune system into shielding the cancer from immune attack. In revealing this clever dodge, the study also suggests several possible ways of countering it.
New light on lung cancer:
Although lung cancer is the number one cause of cancer death worldwide, it remains one of the most poorly funded cancers in terms of research, largely because of the stigma associated with smoking. Kim, who started her lab at Children’s in 2006, is one of the few people in the world who’s carved out a niche in lung stem cell research, studying both cancer and normal lung cells.
During her postdoctoral fellowship, Kim was the first to isolate bronchioalveolar stem cells (BASCs), a type of lung stem cell, from adult mice. She also found that the most common genetic mutation in lung cancer appears to transform BASCs into the bad guys of adenocarcinoma, an aggressive form of lung cancer.
By transplanting lung tumor cells from one mouse to another, Kim has observed that in certain cases, only the BASC-like cells from tumors can grow when implanted in laboratory animals. These cells, she says, may be the stem cells in lung cancer. She’s now examining the activity of their genes to see what makes them different from normal lung stem cells.
“What molecules control these cells, versus the remaining tumor, versus normal lung?” she asks. “If we can answer this, we’ll have a better plan to attack lung cancer through new therapies, or combinations of existing ones.”
Kim is also searching mouse lungs for other cells with “stem” properties, observing which cell types are involved in repairing lung injuries. She hopes to learn whether normal lung stem cells could be used as a therapy for cystic fibrosis, the defective lungs of premature infants and other pulmonary problems. “There are many broad applications in lung stem cell studies that go beyond cancer,” she says.
Targeting leukemia stem cells:
Scott Armstrong has just spent two weeks on service in Children’s oncology unit. He saw more than 30 young patients during that duty, many suffering from leukemia, the most common childhood cancer. These children have abnormally high numbers of immature, dysfunctional white blood cells that crowd out the bone marrow, interfering with its ability to produce healthy blood cells.
Today the most common leukemias have cure rates of approximately 80 percent. But children with certain rare forms of the disease may initially go into remission, only to suffer a fatal relapse. Mixed lineage leukemia (MLL) is one example, sharing features of two major childhood blood cancers, acute lymphoblastic leukemia (ALL) and acute myelogenous leukemia (AML). Armstrong, who studies MLL, believes that cancer stem cells could be initiating relapse and is keen to find new and better therapies.
Armstrong recently showed that in MLL, certain progenitor cells that give rise to white blood cells inappropriately acquire stem-like, self-renewing qualities through a rearrangement of their chromosomes. This causes the halves of two different blood proteins to fuse. When this hybrid protein was injected into progenitor cells from mice, it activated genes that made the cells turn cancerous and stem-cell-like. “The chromosome translocation gives progenitors an ability to copy themselves that they shouldn’t have, which eventually results in leukemia,” Armstrong explains. “We don’t yet know precisely how the hybrid protein activates this self-renewing program, but we think a specific enzyme, Dot1, is key, making it a potential therapeutic target.”
Molecular mechanisms like this one can take years to map out, even for a single cancer. But Armstrong believes that such mapping is possible. “You can imagine, in the future, having a collection of 30 drugs to attack specific cancer stem cells in specific cancers,” he says. “You’d isolate the stem cells, identify the programs controlling their survival, and get the targeted therapy off the shelf.”
The road ahead:
Scientists don’t necessarily agree that all or even most tumors are driven by stem cells, and while it seems fairly clear that stem-like cells can initiate relapse, the origins, characteristics and role of such cells probably vary from cancer to cancer. What scientists are learning is that cancer stem cells are as diverse as cancer itself.
Ultimately, researchers envision two sets of cancer treatments—classic chemotherapy, which shrinks most of a tumor, and new therapies aimed at cancer stem cells. Understanding what makes stem cells tick, and what might make them prone to turn cancerous, is crucial. “It’ll help us find new ways to detect, image and treat tumors,” says Kim. “Even if we’re not directly studying a tumor, we’re probably learning things to assist cancer research just by studying stem cell biology.”
Two fundamental processes in biology—stem cell generation and carcinogenesis—are in fact closely related. The laboratories of George Q. Daley, MD, PhD and Richard Gregory, PhD, are exploring this relationship at the molecular level, showing how a factor called Lin-28, which is associated with breast and lung cancer, makes a cell more prone to de-differentiate—revert to a less mature, unspecialized, stem-like state. Gregory is seeking drugs that mimic the effect of Lin-28 to help generate pluripotent stem cells, as well as drugs that block Lin-28 to inhibit cancers. Read more.
It may seem paradoxical that stem cells can be so dangerous—driving a cancer—yet are also sought after for cures. “On the one hand, we’re saying we want to use stem cells to cure diseases like Parkinson and Alzheimer disease,” acknowledges White. “And here I am saying, ‘Well, stem cells are part of the problem with your cancer; we’re going to try killing those cells. In both cases, figuring out how these cells get regulated is the key. The two can’t be divorced; they have to be studied together.”