Flip of genetic switch causes cancer cells in mice to halt, self-destruct
BY LOUIS BERGERON
Killing cancerous tumors isn't easy, as anyone who has suffered through chemotherapy can attest. But a new study in mice shows that switching off a single malfunctioning gene can halt the limitless division of tumor cells and turn them back to the path of their own planned obsolescence.
The surprising possibility that a cell's own natural mechanism for ensuring its mortality could be used to vanquish tumors opens the door to a new approach to developing drugs for cancer, according to Dean Felsher, MD, PhD, associate professor of oncology and of pathology at the School of Medicine. Felsher is the senior author of the study published July 30 in the online version of the Proceedings of the National Academy of Sciences.
"Our research implies that by shutting off a critical cancer gene, tumor cells can realize that they are broken and restore this physiologic fail-safe program," said Felsher.
Cancer can be notoriously resistant to medical treatment. Not only do cancer cells proliferate uncontrollably, they somehow circumvent the mechanism that causes normal cells to die when they get old or malfunction. That makes cancer cells effectively immortal unless doctors manage to squelch them.
The gene Felsher's team studied produces a protein called Myc (pronounced "mick"), which promotes cell division. A mutation of the gene causes cells to overproduce the protein, prompting perpetual cell division and tumor growth. By turning off the mutated gene, researchers found that not only did uncontrolled cell division cease, but the cells also reactivated a normal physiological mechanism, called senescence, which makes it possible for a cell to eventually die.
"What was unexpected was just the fact that cancer cells had retained the ability to undergo senescence at all," said Felsher. Cancer researchers had thought the senescence process had to be irreversibly disrupted for a tumor to develop.
The researchers worked with a series of mice engineered to have Myc-triggered cancers of either the liver, blood or bones, along with a specially constructed version of the Myc gene that they could switch off by feeding the mice antibiotics. When the mice dined on doses of the drugs, invariably, the tumors ceased growing and then diminished, with some disappearing in just a few days.
Although Felsher's lab had previously shown that mouse tumors diminished and disappeared when Myc was switched off, they hadn't been sure how the process worked. Historically, most research involving genetic methods of battling cancer cells has focused on reactivating genes called tumor-suppressor genes, which are generally overcome by a proliferating cancer. No one had explored the idea that senescence might play a key role in diminishing tumors.
Felsher described senescence as acting like a fail-safe mechanism to stop cancer. When a cell detects a deleterious mutation, it launches the senescence process, resulting in the permanent loss of the cell's ability to proliferate, thus halting any cancer.
"In order to become tumor cells, those cells have to overcome senescence," said Chi-Hwa Wu, PhD, postdoctoral researcher in Felsher's lab and first author of the study. Wu had the inspiration to explore whether the sudden diminishment they had observed in the tumors might be due to the reactivation of some latent remnant of the trigger for senescence.
Through a series of experiments looking at enzymes associated with the senescence process, as well as some molecular markers, Wu confirmed her suspicion. And not only was senescence occurring in cells that had been thought to be incapable of it, the process was reactivated in all the different tumors they studied.
Consider it a cell version of the Jekyll-and-Hyde transformation. "It's sort of like Mr. Hyde realizing that there's something wrong with him and then being able to put himself back into his normal state as Dr. Jekyll," Felsher said.
Felsher and Wu see a lot of potential for new approaches to treating cancer, beyond the traditional tactic of trying to kill cancer cells directly. "Maybe part of the strategy should involve figuring out how to get the cancer cells to just be allowed to do what they originally wanted to do anyway, which is not to be proliferating endlessly and growing uncontrolled," Felsher said.
The next step is to see how well the approach works in human cancer cells.
Other authors on the research paper are Jan van Riggelen, PhD, postdoctoral researcher; Alper Yetil, graduate student in cancer biology; Alice Fan, MD, instructor in oncology, and medical student Pavan Bachireddy.
The study was funded by the National Cancer Institute, the National Institutes of Health, the Leukemia and Lymphoma Society, the Burroughs Wellcome Fund, the Damon Runyon Lilly Clinical Investigator Award, the Lymphoma Research Foundation and the Howard Hughes Medical Institute.