Lead Story

Going after the Real Killer:

Metastatic Cancer

Genetic Susceptibility

No one doubts that acquired mutations in individual genes play a critical role in cancer. But, noted Hunter, "You can’t look at these things in isolation." He cites the fact that women with BRCA1 mutations do not always develop cancer. "You have to understand the genetic context."

Hunter has taken a population genetics approach to ask whether there are inherited risk factors associated with metastatic progression in cancer. Using a transgene to induce metastatic mammary tumors in several genetically distinct strains of mice, Hunter has shown that the metastatic efficiency, as measured by the density of pulmonary metastases in these mice, varies enormously with genetic background. Polymorphisms— DNA sequence differences among individuals—account for variations in many normal physiological traits, such as body size and coloring. Polymorphisms also account for different levels of gene expression, and they account for variation in primary tumors from a variety of tissues. In hindsight, then, it is not surprising that inherited genetic differences would affect the development of metastatic cancer.

Kent Hunter, Ph.D., looks for inherited risk factors associated with metastatic progression. (Photo: R. Baer)

In humans, research has shown that metastatic cells, like primary tumor cells, can be characterized by a gene expression "signature" and that this signature can be used to predict the likelihood of metastasis. Although Hunter does not dispute these findings, and has even found the same genetic signatures in mice with high risk of developing metastasis, he does argue against the interpretation that this signature represents only the accumulation of genetic mutation creating metastatic cells. Instead, he has shown that these signatures can be explained by an interaction of both mutation and genetic background and that non-cancerous tissue from animals with high metastatic risk also has similar gene expression profiles.

But, despite the growing body of evidence that he and his colleagues have developed, skeptics remain. Hunter thinks part of the difficulty is in the scientific culture. "We are trained to think in terms of somatic mutation as cancer biologists. And there’s a big divide between susceptibility and somatic genetics in which defects acquired from genetic mutation and rearrangement— not inheritance—are at play.

The Extracellular Matrix

Hunter and his team have been working to identify the genes that underlie risk of metastatic disease. One focus of their work is the extracellular matrix (ECM), the complex molecular environment that cells both secrete and live in, which provides physical scaffolding through which cells migrate as well as transmit signals to and from cells. Many studies, including Hunter’s own work, have shown that changes in the expression of genes encoding ECM molecules predict metastatic progression in both human breast cancer and mouse models. Hunter and his colleagues have begun to identify factors that specifically modulate both ECM-related gene expression as well as metastatic tumor progression. Although we are still far from a mechanistic understanding of how changes in ECM gene expression impact metastasis, the relationship makes some intuitive sense. "I think it has to do with the way cells sense their microenvironment through ECM signaling," said Hunter. For example, the ECM could sequester or modulate the availability of TGF-β and other cytokines involved in growth and immune regulation.

"We’re taught to reduce complexity... But we actually have to embrace it."

Jeffrey Green and his colleagues have also followed up on the evidence for involvement of the ECM in metastasis. Like Hunter, Green has wondered whether it is not the accumulation of new genetic abnormalities that causes a disseminated but dormant tumor cell to proliferate into clinical disease, "but that something else in the immediate environment or within the host may lead to the trigger that allows these cells to proliferate." Green suspects that there may be critical changes in the composition and structure of the ECM that could allow tumor cells to read different stimulatory signals and initiate a proliferative response.

Photo shows Postdoctoral Fellow Daniel Fitzgerald, Ph.D., slicing brain tissue for analysis. (Photo: R. Baer)

But dormancy really just means that the disease is subclinical and that doctors cannot see it. How do you find a dormant cell to study it? "Dormancy," Hunter explained, "gives people the idea that it’s an inactive seed, a spore sitting somewhere. That’s obviously not true— they are cells. We don’t know if they are static, or patrolling the body like a lymphocyte."

Green and Dalit Barkan, Ph.D., a Visiting Scientist, recently reported the development of a three-dimensional culture system model of metastatic cancer that will allow them to address some of the questions of molecular and cellular mechanisms that are so difficult to tackle in this disease. They have shown that cell lines that proliferate in normal cell culture but that can be distinguished by their metastatic potential in vivo can also be distinguished in their three-dimensional culture system. Thus, they have been able to study the transition from quiescence to proliferation of metastatic cells, and they have demonstrated a role for the extracellular microenvironment in regulating the reorganization of internal cellular structure that occurs during the switch from dormancy to proliferation. The molecules involved in this reorganization could represent additional targets for metastatic inhibitors (see "Let Sleeping Micrometastases Lie" in Vol.2, No.2 of CCR connections).

Finding a Cure

"I am not certain that we will ever be able to completely cure metastatic cancer," said Hunter, realistically and without pessimism. "We should also think about treating it the same way people treat heart disease, by looking for ways to reduce the risk of developing metastatic disease." Hunter’s lab has shown that high doses of caffeine suppress metastasis in their mouse model. Although the work does not support a recommendation for cancer patients to drink liters of coffee every day, it does indicate that small-molecule agents might be developed for chronic administration to patients that would reduce the risk of metastasis, a strategy analogous to the administration of statins to reduce the risk of heart disease.

"Ultimately, we may need to rethink how we do clinical trials."

Wakefield’s work with TGF-β, which has effects on so many different physiological systems, has led her to the conclusion that combinations of drugs with different molecular targets will be an important part of the solution. Her work suggests that a combination of a lot of small effects on different cell types involved in the metastatic process would be most effective in combating the disease.

"The major stumbling block," Steeg pointed out, "is how to test our preclinical data in the clinic. Most of our data says that if we use drug X, we can prevent metastasis, but standard clinical trials start with a Phase I trial in highly metastatic patients—so you are asking a drug to melt a golf ball-sized tumor. Most agents will fail in that trial design [even though they might be effective when administered earlier]." Steeg suggests that including biopsies that demonstrate whether the drug had an effect on its target may be a first step. Better imaging tools will also be critical. But, ultimately, we may need to rethink how we do clinical trials.

Lalage Wakefield, D.Phil.
(Photo: R. Baer)

Steeg has recently formed a Center of Excellence to study brain metastases of breast cancer, a disease that combines all of the difficulties in studying metastatic disease with the need to find drugs that cross the blood-brain barrier that normally protects the brain from most blood-borne molecules. The current standard of care, whole brain radiation therapy, may be successful in eradicating the tumors for a time, but it may have serious neurological side effects. The Center’s work, which has been funded by a five-year grant of over $17,000,000 from the Department of Defense Breast Cancer Research Program, is a comprehensive program ranging from target identification to drug delivery methods. The researchers that form this center include neuropathologists, neurosurgeons, neuro-oncologists, molecular biologists specializing in breast cancer, and experts on the blood-brain barrier. "We each have our assignment—we need more model systems, and we need more tissue studies," Steeg concluded (see also "Small Molecule, Big Impact" in Vol.2, No.2 of CCR connections).