Researchers at Dana-Farber Cancer Institute and Children's Hospital Boston and their colleagues have isolated rare cancer stem cells that cause leukemia in a mouse model of the human disease.
The leukemia stem cells isolated proved to be surprisingly different from normal blood stem cells -- a finding that may be good news for developing a drug that selectively targets them.
Cancer stem cells are self-renewing cells that are likely responsible for maintaining or spreading a cancer, and may be the most relevant targets for cancer therapy. The discovery provides answers to the longstanding questions of whether cancer stem cells must be similar to normal stem cells, and what type of cell first becomes abnormal in leukemia, the most common form of cancer in childhood. The journal Nature has posted the study's findings online in advance of print publication.
It had been speculated that leukemia begins in a totally undifferentiated stem cell that can become any type of specialized blood cell and has the ability to renew itself almost without limit.
Instead, the scientists showed that they could create leukemia stem cells, which also are self-renewing, from partially committed, non-self-renewing progenitor cells. The latter are short-lived cells that can turn into several types of blood cells, but are more committed than stem cells, which can become any kind of blood cell and also are virtually immortal.
"Our data supports the idea that leukemia stem cells do not have to originate from normal blood stem cells. Furthermore, we have shown that fully developed leukemia stem cells do not necessarily have the same genetic program as normal stem cells," said Scott Armstrong, MD, PhD, of Dana-Farber and Children's Hospital and senior author of the paper. "This is an important finding, because it indicates that in the future we should be able to specifically target leukemia stem cells without killing normal stem cells."
Leukemias are cancers of the blood-forming tissues of the bone marrow in which white blood cells proliferate abnormally, with life-threatening effects. About 35,000 diagnoses of all types of leukemia will be made in 2006, according to the American Cancer Society, with about 22,280 deaths. Some forms of leukemia have a high rate of cure. In other forms, chemotherapy may initially put the patient's disease into remission, but after months or years the cancer reappears and may be fatal.
Many scientists believe that relapses are caused by the survival of a handful of leukemia stem cells mixed in with the population of cancer cells. These cells have gained self-renewal capabilities, and, if not killed by chemotherapy, can lie dormant in the bone marrow and eventually trigger new growth of the leukemia. Current thinking is that cure rates of leukemia and other cancers could be improved if the cancer stem cells could be identified and selectively targeted with designer drugs.
To test this hypothesis, the researchers sought to transform a normal, partially committed progenitor blood cell from a mouse into a leukemia stem cell, and then determine whether that stem cell was more like a normal blood stem cell or instead resembled the progenitor. As a first step, they inserted an abnormal gene, MLL-AF9, which causes a type of acute myelogenous leukemia (AML) in humans, into partially committed mouse blood cells known as granulocyte macrophage progenitors, or GMPs. These genetically altered cells were injected into mice, which subsequently developed AML.