COLLABORATION

The Root of the Matter

Do cancer stem cells drive cancer recurrence? And how to you kill them? UCCC scientists are collaborating on crucial studies to find out.

By Lynn Gorham

 

Ask cancer survivors what their greatest fear is, and chances are they'll reply: "The cancer coming back." Recurrence rates can vary widely, from 5 to 95 percent depending on how far the original tumor had spread, its particular molecular characteristics and other clinical factors. But what makes cancer come back?

One answer may get to the root of the cancer problem: cancer stem cells, or CSCs.

CSCs can be thought of as generals in a war. There aren't very many of them, they hang at the back, and send in the troops to inviade. They build the armies.

Many scientists believe that CSCs keep tumors growing, invading and spreading into new places. Traditional cancer treatments—chemotherapy and radiation—target fast-dividing cells. CSCs divide slowly. Chemo and radiation don't kill them, and they live on to make new tumors in close-by and distant places. CSCs also may make up less than 0.1 percent, or one in 1,000 cells, of a solid tumor's bulk.

"It might take just one cell to start a new tumor," said Dr. Xiao-Jing Wang, director of the University of Colorado Cancer Centre Head and Neck Cancer Program and professor of Pathology and John S. Gates Chair of Head and Neck Cancer Research at the University of Colorado Denver School of Medicine (UCD SOM). "That has been shown in a leukemia experimental system. If you see a complete destruction of a tumor but have one or two surviving cancer stem cells, you would have risk of recurrence."

In fact, Dr. Kathryn Horwitz, a UCCC breast cancer researcher and distinguished professor at UCD SOM, recently showed that a single breast cancer stem cell can give rise to mammary tumors in mice.

"Until we identify what makes cancer stem cells survive, we won't be able to cure solid tumors," said Dr. Antonio Jimeno, assistant professor of medical oncology at UCD SOM.

"More complex than we thought."

In 2007, the University of Colorado Denver established the Charles C. Gates Regenerative Medicine and Stem Cell Biology Program with a $6 million gift from the Gates Frontiers Fund. Cancer stem cell research, the donors said, was to be a key focus of the program. Dr. Dennis Roop, a national expert in stem cell biology with a focus on skin cells, was recruited to lead the program. Its focus is broad, covering everything from cancer and heart disease to Parkinson's disease, strokes and paralysis.

"We have to understand normal stem cells, normal development of organisms and tissues, and many of the normal pathways of maintaining a state of equilibrium, because we know if mutations occur in these pathways, they can cause cancer, said Roop, professor of dermatology at UCD SOM. "You can't separate cancer away from normal cell biology."

Adult epithelial stem cells—which keep tissues like skin and the lining of the oral cavity, colon, lungs and other organs regenerating and therefore alive—divide slowly and live long. They may accumulate genetic damage from carcinogens we're exposed to in air, water and food. With enough genetic damage, they give rise to what we call cancer—cells that divide very quickly and out of control.

"Normal cells check how they are doing," Jimeno explained. "If there are too many gene alterations, they commit suicide. Cancer cells don't look at DNA damage. They keep dividing despite mutation after mutation, which in turn is advantageous because it gives them new abilities to evade our current treatments."

Where normal adult epithelial stem cells divide evenly, making one daughter cell and one stem cell, CSCs may divide unevenly, creating multiple damaged daughters and CSCs, a theory some scientists say explains rapid solid tumor growth. Combine this with the idea that CSCs can change hats midstream and lie dormant for years before rearing up again, and you might begin to fathom why people still die from solid tumors like breast, lung and prostate cancer 38 years after President Nixon declared war on cancer.

"Cancer is so much more complex than we thought 30 years ago," Jimeno said. "It has been a tough opponent because it is tremnedously complex and it has a phenomenal ability to adapt. It can shut down and weather the storm."

Cancer stem cells: Myth or fact?

Two papers published in the past year question the rarity of cancer stem cells and the claim that only CSCs give rise to tumors—two tenants pro-CSC scientists hold a key to identifying them. In both studies, cells were transplanted into mice with wiped-out immunize systems. A melanoma experiment in these mice showed as many as one-quarter of cells could start tumors, which conflicts with the idea that CSCs are rare. But the results could have been skewed by the mice's level of immune compromise, UCCC researchers said.

"The question is in a human patient, not a mouse patient," Wang said. "Do human patients ever get to that state of immune compromise outside of stem cell transplant patients?"

Wang is the creator of the world's first genetically modified mouse model for head and neck cancer. The mice have intact immune systems, but have genes altered to give rise to specific tumors. She is then able to study the tumors' molecular makeup, genes and proteins in action.

"Our preliminary data suggest that, at least in head and neck cancer, new tumor development follows the cancer stem cell theory," Wang said. "Certain tumors may not be initiated by cancer stem cells. It's also possible that non-stem cells can acquire 'stem-like' properties. We just need to be open minded."

Roop said solid data exists pointing to cancer stem cell populations in nearly every major tumor type, but adds that many questions remain to be answered.

"It's possible that not all cancer stem cells are the same," he said. "Some could be closer to a normal adult stem cell and may be more susceptible to traditional therapy, which would explain why some cancers do not recur. Perhaps if the cancer stem cell has many gene mutations, it makes the tumor less susceptible."

Roop and Dr. Mayumi Fujita have developed techniques to isolate and characterize cancer stem cells in melanoma. Fujita's experiments show a correlation between the number of cells expressing gene mutations that are linked to chemotherapy resistance and more aggressive melanoma tumors. She has also show that as few as 10 of these mutated cells can give rise to a new tumor in mouse models. Roop is also partnering with Wang on experiments to see if they can sort stem cells in skin and head and neck cancer.

"If we can prove we can sort the cells, we can ask is this cancer development really stem-cell driven?" Wang said. "We can transplatn the cells into the mouse models, then go back and see how many cells do we really need to give rise to a tumor."

The ultimate experiment

 

Cancer science is molecular science, said Dr. Yosef Refaeli, assistant professor of dermatology at UCD SOM, and new technology is making cancer science pick up speed.

"The big question is what is the molecular basis of cancer stem cells?" he said. "Then the question is how do you distinguish them? Then how do you poison them? We all have our own approach. In a year, it will be mindboggling what people can do. There are always multiple minds at play."

Refaeli has developed a tool for screeing old drugs against leukemia stem cells that has proven fruitful. Three of 80 drugs he tested—drugs that had been given up as useless by the National Cancer Institute—have killed leukemia stem cells.

"He has data to suggest he can do the same for skin cancer, for squamous cell cancer, and we are hopeful that this approach will work for lung, head and neck and melanoma cancer," Roop said.

Refaeli's technology can be used to isolate stem cells, make more of them and elicit antibodies that successfully attack them.

"In the future, doctors would likely debulk the tumor first using surgery, traditional chemotherapy and radiation, thenuse these antibodies to go after any residual cancer stem cells, which we think are responsible for tumors recurring," Roop said.

Refaeli also stumbled upon a way to retain cells' "stemness" without any genetic modification, and he's using the technology to develop a universal stem cell line.

"That will mean you have no need for genetic matching with bone marrow transplants, or with organ transplants with concurrent stem cell transplants," he said. "The blood will already by educated on the organ. The new immune system emerges and does not see the difference between itself and the recipient, which will end the problem of organ rejection."

Refaeli's technologies are complementary to Jimeno's novel animal system: a xenograft model of head and neck cancer involving transplanting tumors from human patients into mice to make more tumors, then studying what drives that particular tumor and how to stop it. It's the only model of its kind in the world.

"We have the potential of using Yosef's technology to make a humanized mouse model that contains the same immune system as the patient where the tumor arose," Roop said. "In my opinion, that is the ultimate experiment, and we're in the position to do that."

Jimeno is the only group member who treats human patients. A key member of the UCCC Phase I Clinical Trials program, he takes care of people in the morning, then his animal "patients" in the afternoon.

"The beauty of the xenograft model is when I put a patient's tumor in a mouse, I know what it is," he said. "And I know it contains human CSCs. I see the patient every-other week in clinic and the mice twice a week. I can treat the mice with the same drug that the patient has received as part of his standard care. I can see how they compare, and learn."

Jimeno is bringing to the Phase I Program drugs that have shown promise in controling CSCs, derived from his experience with a group at Johns Hopkins that tested some of the first anti-CSC drugs. He just received positive results from an inhibitor of a cancer pathway he suspects head and neck cancer stem cells use, which he tested both in first-in-man clinical trials and in animal models of head and neck cancer in his laboratory.

"This is a true example of the potential and integration of our preclinical and clinical programs," he said. "Only by narrowing the bench-to-bedside gap will we ultimately impact the care to our patients.

"We want to answer the questions: Are cancer stem cells generating cancer, driving it or sustaining it? It's likely that not all cancer stem cells are the same."

Jimeno said he believes the Colorado program is experiencing a perfect storm of the right people and technology together in the same place at the right time.

"We have to perfect team," he said. "We have complementary animal models. We have clinicians who understand the lab and lab scientists who understand what clinicians need to improve the curability of cancer."

Roop recalled that in August, 2006, when he agreed to accept his current position, "I bravely stood up at a press conference and said we will make this a first-rate center, but I honestly had no idea how we would do it."

The field has moved so fast, he said. With 35 independent investigators and 300 people engaged in collaborative stem cell research at the Anschutz Medical Campus, National Jewish Health and University of Colorado Boulder, the UC Denver Stem Cell Program is keeping pace.

"The young talent we have is amazing," Roop said. "In the near future, people on the East and West coasts will say, 'Where did these people come from?'"