Kornelia Polyak, MD, PhD
A Dana-Farber Cancer Institute study challenges the hypothesis that
"cancer stem cells" — a small number of self-renewing cells within a
tumor — are responsible for breast cancer progression and recurrence,
and that wiping out these cells alone could cure the disease.
Instead, the scientists report in the March issue of Cancer Cell
that they have identified two genetically distinct populations of
cancer cells in samples of human breast tumors — one of the types being a
cell recently proposed by other scientists to be a true breast cancer
"If the breast cancer cells were all coming from a single cancer stem
cell, you might be able to cure the disease with just one drug," said
Kornelia Polyak, MD, PhD, of Dana-Farber, senior author of the paper.
"But our findings suggest that the tumor cells come from a 'stem-like'
progenitor cell, and then diverge genetically, so I think you have to
treat both cell types."
The results suggest that both cell types, and probably others, are
involved in the development of breast cancer. While analyzing the
genetics of each cell type, the researchers discovered that the proposed
"cancer stem cells" were driven by an activated molecular pathway that
makes them resemble normal stem cells. Women whose breast tumors are
largely made up of these "stem-like" cells are at higher risk of
On the positive side, the abnormal activated pathway in these cells,
known as the TGF-Beta1 signaling pathway, can be blocked by experimental
drugs now entering clinical trials, said Polyak, who is also an
associate professor at Harvard Medical School. Such inhibitors, in
combination with other therapies, may improve the prognosis in breast
cancers fueled by these cells.
Clonal evolution or cancer stem cells?
According to a longstanding cancer model, known as "clonal
evolution," tumors arise from normal cells that mutate and generate
abnormal offspring that also mutate, forming a mass of genetically
varied cancer cells. However, there has been a new wave of interest in
an alternative explanation — that tumors are initiated and driven by a
single, abnormal type of adult stem cell found in, for example, breast
tissue, resulting in a population of genetically identical tumor cells.
Moreover, several pathways and genes required for normal stem cell
function are activated in cancer cells and play essential roles in the
development of tumors.
According to the cancer stem cell hypothesis, the few self-renewing
stem cells that fuel the cancer are difficult to kill, and their
persistence may explain why tumors so often recur following successful
In 2003, scientists purified what they proposed were breast cancer
stem cells from patients' tumors. The distinctive molecule, or marker,
on the cells' surface, known as CD44+, was identical to the marker on
normal breast cells. When injected into mice lacking an immune system,
the CD44+ cells demonstrated the ability to initiate breast tumors. The
scientists also found closely related cells with a CD24+ marker and
suggested that they were offspring of CD44+ cells.
The team led by Polyak and Michail Shipitsin, PhD, also of
Dana-Farber and HMS, used gene activity analysis to clarify the
relationship of the two cell types. They generated gene libraries from
CD24+ and CD44+ cells purified from normal mammary epithelium and fluids
within the chest, and from primary invasive tumor samples collected
from breast cancer patients.
The findings, the scientists reported, fit more closely with the
clonal model than the cancer stem cell hypothesis. That is, the CD24+
cells were very similar to the CD44+ cells, but not always genetically
identical which they would have been if the CD44+ cells were true stem
cells and the CD24+ their offspring.
"Although CD44+ cells appear to express many stem cell markers, the
genetic difference between CD24+ and CD44+ cells within a tumor
questions the validity of the cancer stem cell hypothesis in breast
cancer, and suggests clonal evolution involving intra-tumoral
heterogeneity as an alternative explanation," the authors wrote.
Moreover, the Polyak team found that the CD44+ cells, but not the
CD24+ cells were driven by the activated TFG-Beta1 pathway. For that
reason, they said, "tumors composed of mostly CD44+ cells may have worse
clinical behavior than tumors mainly composed of CD24+ cells, and these
patients may benefit from therapy targeting the TFG-Beta1 pathway."
Other authors of the paper are from Harvard Medical School; Johns
Hopkins University School of Medicine; GeneGo Inc.; the Vavilov
Institute for General Genetics, Moscow; Harvard School of Public Health;
Brigham and Women's Hospital, and Beth Israel Deaconess Medical Center.
This work was supported by Novartis Pharmaceuticals, Inc., the
National Institutes of Health, and the US Department of Defense.
Dana-Farber Cancer Institute (www.dana-farber.org)
is a principal teaching affiliate of the Harvard Medical School and is
among the leading cancer research and care centers in the United States.
It is a founding member of the Dana-Farber/Harvard Cancer Center
(DF/HCC), designated a comprehensive cancer center by the National