There is a hypothesis that cancer may emerge from mutated cells that have stem cell–like properties including the ability to develop into a variety of differentiated cells, to self-renew, and to remain inactive for long periods of time.
This hypothesis is nowhere better demonstrated than in leukemia, for which scientists have identified and characterized disease-initiating cells. These cells not only cause cancer, but they also escape every therapeutic assault. For this reason, most patients with one form of the disease, chronic myelogenous leukemia (CML), must remain on imatinib (Gleevec) for life to keep their leukemia at bay.
Now, research led by Pier Paolo Pandolfi, HMS professor of medicine and pathology at Beth Israel Deaconess Medical Center, has uncovered a striking weakness in leukemia-initiating cells. He has also found that a drug currently used to treat a different form of the disease, acute promyelocytic leukemia (APL), exploits this weakness and eradicates these stem cells in mice. The results are reported in the May 11 Nature.
At the center of this new treatment strategy is a gene called promyelocytic leukemia (PML). Pandolfi, also director of the Cancer Genetics Program and the Division of Cancer Genetics at BID, discovered PML as a medical student at the Instituto Clinica Medica in Italy and, in 1992, implicated it as one half of a fusion protein that causes APL. He likens the PML protein to a set of sturdy brakes that prevent a stem cell from rolling out of its quiescent state into a proliferative state, from which it multiplies and differentiates. Because PML prevents cell proliferation, it also works as a tumor-suppressing protein in prostate, lung, and other cancers.
More recently, Pandolfi and first author Keisuke Ito, HMS research fellow in medicine at BID, wanted to understand the role of PML in hematopoietic stem cells. Ito began the investigation by knocking out PML in mice. The researchers expected that the knockouts would show unchecked cell division with devastating results. “If you remove PML,” said Pandolfi, “what we were thinking was leukemia. Cancer.”
Instead, Ito found the opposite effect. At first, the stem cells did proliferate, as expected. But without PML, they differentiated with abandon. So few stem cells remained behind in their quiescent and undifferentiated state that eventually the stores of stem cells began to be exhausted.
This surprising result inspired a connection. “We already had results from CML patient samples showing that low PML expression levels correlated with good outcomes, so we combined these two things,” said Ito. If low PML levels improve CML patient outcomes and a lack of PML depletes stem cells, they speculated that leukemia-initiating cells might also be depleted by a loss of PML.
According to Ito’s results, they were. In fact, their demise is accelerated by a cancer-causing mutation that makes them super-proliferative. “In the leukemia-initiating cell, not only does it lose PML,” which releases the brake, said Pandolfi, “it also already has the accelerator pushed to the floor because of the oncogene.”
In addition to identifying PML as a target for treating CML in this study, Pandolfi also suggests a treatment regimen based on a drug already approved for use in APL: arsenic trioxide.
Arsenic has a long history of medicinal use, dating all the way back to Hippocrates, who used arsenic to treat ulcers. In 1878, Boston City Hospital doctors used arsenic to treat CML. The intervention, though effective, fell in and out of favor throughout the 20th century as new options such as radiation therapy and chemotherapy came into use. In the 1930s, treatment of CML with arsenic had a brief comeback that was cut short because of toxicity concerns.
But, said Pandolfi, the new formula, approved in 2000 for the treatment of APL, “is a fantastic drug at low doses, the dose given to APL patients”—and also with treatment limited to about two weeks—“it is absolutely well tolerated.” In APL, arsenic trioxide works by selectively degrading PML, thereby destroying the fusion protein that causes the cancer.
Pandolfi and Ito tested arsenic trioxide in a mouse model of CML as part of combination therapy with another drug called cytosine arabinoside (Ara-C), a chemotherapy drug used to treat a variety of blood cancers. This “proof of principle experiment,” said Pandolfi, tested the effect of arsenic trioxide in combination with a “classic chemotherapeutic agent.”
The two-part regimen applied arsenic trioxide to release the brakes on the leukemia-initiating cells by degrading the PML inside of them. As the cells proliferated, they began to exhaust themselves, but they also became vulnerable to Ara-C, which targets dividing cells. When treated with this combination therapy, the mice experienced a complete cure. No detectable leukemia cells, including leukemia-initiating cells, remained.
“This mechanism is exciting,” said co-author David Avigan, HMS assistant professor of medicine and director of Hematologic Malignancies/Bone Marrow Transplant at BID, who is working with Pandolfi’s team to design human clinical trials to test a targeted CML therapy that combines arsenic trioxide with imatinib. But, he cautioned, “We need to see how it plays out in patients. You learn some humility as you go into the clinic.”
Pandolfi’s lab is conducting experiments on their CML mouse model to learn more about how best to combine the two drugs. Clinical trials of arsenic trioxide combined with imatinib are already under way elsewhere, though results are not yet published.
For Avigan, the exciting discovery here is the biological principle Pandolfi’s team uncovered: the findings illustrate one way, through PML, that abnormal leukemia stem cells may be susceptible to attack. “Normal hematopoietic stem cells are fitted with mechanisms to protect themselves. We only have a certain number, and they have to last our whole lives,” he said.
Knowledge of this susceptibility, along with the recent discovery of the mechanism behind arsenic trioxide’s ability to degrade PML, “opens the door for the development of novel PML-targeting drugs,” said Pandolfi.
More broadly, other cancer stem cells may also borrow self-defense techniques from normal stem cells, so Pandolfi’s lab is exploring the role of PML in stem cells in other tissues and in cancer-initiating cells in other tumors. This work is just beginning. “In leukemia,” where the first cancer stem cells were discovered just over a decade ago, “there is a tremendous history and good literature” describing leukemia-initiating cells, said Pandolfi. “Less is known in other forms of cancer.”
