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Cancer stem cells:
lessons from leukemia
Jean C.Y. Wang and John E. Dick
Division of Cell and Molecular Biology, University Health Network, and Department of Molecular and Medical Genetics,
University of Toronto, 620 University Ave, Toronto, Ontario, M5G 2C1, Canada
A fundamental problem in cancer research is identification of the cell type capable of initiating and
sustaining growth of the tumor – the cancer stem cell
(CSC). While the existence of CSCs was first proposed
over 40 years ago, only in the past decade have these
cells been identified and characterized in hematological
malignancies. Recent studies have now described CSCs
in solid tumors of the breast and brain, raising the
possibility that such cells are at the apex of all neoplastic
systems. An appreciation of the biological distinctness
of CSCs is crucial not only for the design of studies to
understand how tumorigenic pathways operate but also
for the development of specific therapies that effectively
target these cells in patients.
Introduction
A fundamental problem in cancer research is identification of the cell type capable of initiating and sustaining
growth of the neoplastic clone in vivo. The key to solving
this riddle lies in determining whether every cell within
the neoplasm has tumor-initiating capacity, or whether
only a rare subset of cells – so-called ‘cancer stem cells’
(CSCs) – is responsible for maintenance of the neoplasm.
The existence of CSCs was first proposed over 40 years ago
[1,2], providing an explanation for the observed functional
heterogeneity within tumors. However, proof of principle
had to await the development of modern research tools for
investigating the behavior of defined cell populations. The
best evidence supporting the existence of CSCs has come
from the study of hematological malignancies. Here, we
review the historical developments that have led to our
current understanding of normal and leukemic stem cell
biology and demonstrate how these studies provide a
paradigm for identification of CSCs from solid tumors.
Stem cells in normal tissues: the hematopoietic system
as a paradigm
In the 1950s and 1960s, seminal studies in mice
established fundamental concepts regarding the nature
of the hematopoietic system and provided a foundation for
later studies in human hematopoiesis (Box 1). In both
mouse and human, hematopoietic cells are organized in a
hierarchy that is ultimately sustained by a small
population of long-lived, quiescent, pluripotent stem
Corresponding author: Dick, J.E. ([email protected]).
cells capable of self-renewal (Figure 1). Succeeding these
cells are lineage-restricted, differentiated progenitors
with reduced self-renewal capacity, which in turn produce
vast numbers of functionally mature, non-proliferating,
short-lived blood cells. The current model of hematopoietic
cell fate commitment postulates that the first lineagecommitment step involves a bifurcation into separate
lymphoid and myeloid pathways, as supported by the
identification of lineage-restricted lymphoid and myeloid
progenitors [3,4]. However, recent studies suggest that
hematopoietic development is more complex and might
not always transit through lineage-restricted stem cells
[5,6]. Regardless, durable hematopoietic reconstitution of
transplantation recipients is mediated solely by the most
primitive stem cells, and consequently, long-term in vivo
repopulation assays are the only definitive means by
which to identify and characterize hematopoietic stem
cells (HSCs). The ingenious design of the hematopoietic
hierarchy allows a lifetime of blood cell production without
exhausting the replicative reserve of the stem cell pool, as
cell divisions occur primarily in committed progenitors
and less frequently in quiescent stem cells. The hierarchical nature of the hematopoietic system also significantly
reduces the lifetime risk of leukemia: because mutations
are linked to cell proliferation, they are more likely to
arise in proliferating progenitors which, due to their
limited life span, will likely undergo terminal differentiation before the required mutations for full cancer
development can accumulate.
Stem cells in leukemia
Just as the fundamental concepts of normal stem cell biology
were derived from early experiments in murine hematopoiesis, hematologic malignancies, in particular chronic
myelogenous leukemia (CML) and acute myelogenous
leukemia (AML), have served as important model diseases
in the establishment of modern concepts of cancer
development.
Clonality and clonal evolution in cancer
The study of CML provided the first evidence of a specific
genetic change associated with a human cancer and of the
clonal nature of these disorders. The discovery of a
consistent chromosomal abnormality in essentially every
dividing hematopoietic cell of patients with CML [7,8]
suggested that the leukemia had arisen from the clonal
expansion of a single cell in which this genetic change had
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Glossary
Box 1. Fundamental lessons in stem cell biology
SCID & NOD/SCID (severe combined immune-deficient & non-obese diabetic/
SCID): immune-deficient mouse strains used in xenotransplantation assays of
human HSC
CFU (colony-forming unit): committed hematopoietic progenitor
LTC-IC (long-term culture-initiating cell): primitive hematopoietic progenitor
assayed in vitro (more primitive than CFU)
SRC (SCID-repopulating cell): primitive human hematopoietic progenitor
capable of repopulating normal hematopoiesis in SCID or NOD/SCID mice
SL-IC (SCID leukemia-initiating cell): human leukemic progenitor capable of
generating leukemia in (NOD)/SCID xenotransplantation assay
Studies in murine hematopoiesis
occurred and that this change was crucial to the
pathogenesis of the disease and not simply correlative.
Clinically, CML follows a predictable progression, in
which an initial chronic phase, characterized by an
excessive production of differentiated but functionally
immature granulocytes, is followed by acceleration of
disease and eventual transformation into a terminal acute
leukemia-like phase with accumulation of primitive blast
cells (blast crisis). The evolution to blast crisis often
involves the acquisition of additional chromosomal
abnormalities [9]. This observation supported the concept,
suggested by animal studies in previous decades, of clonal
evolution in tumors, in which disease progression results
from acquired genetic variability within the original clone,
allowing sequential selection of more aggressive subpopulations. The notion that tumor development occurs via a
multistep progressive conversion of normal cells into
cancer cells is now well accepted [10].
Functional heterogeneity in tumors
The development of quantitative assays enabling measurement of the clonogenicity of malignant hematopoietic cells
began more than 65 years ago in studies employing cell lines
[11]. Subsequently, investigators using primary tumor
tissue demonstrated that only a small subset of cancer
cells is capable of extensive proliferation in vivo [1] and
in vitro [12,13]. Such studies revealed the existence of
functional heterogeneity within tumors and introduced the
concept of tumor stem cells. Subsequently, studies in AML
have been key in elucidating the biological basis of tumor
heterogeneity. AML is a clonal disorder of aberrant
hematopoiesis characterized by an accumulation of functionally immature blasts that fail to differentiate normally.
Despite their morphological homogeneity, the blast cell
population is biologically heterogeneous (for review, see
[14]). Kinetic studies using tritiated thymidine labeling
showed that the majority of AML cells are not actively
proliferating in vivo [15]. Furthermore, only a minority of
highly proliferative leukemic blasts (AML-CFUs) are able to
give rise to colonies in vitro. These observations challenged
the simple-minded notion that cancers result from cell
growth ‘gone amuck’, and suggested that, as in normal
hematopoiesis, the leukemic clone in AML is organized as a
hierarchy in which a small number of proliferating
progenitors continuously replenish the bulk population of
non-cycling leukemic blasts.
Leukemia stem cells
To explain the functional heterogeneity observed in AML
and other types of cancer, two models of tumor stem cell
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The modern era of hematopoiesis and stem cell research was
founded by early studies in mice. Using marker chromosomes to
distinguish donor from host, investigators demonstrated that bone
marrow (BM) contains cells capable of reconstituting all the
hematopoietic tissues of irradiated recipient animals [48]. However,
the crucial question remained as to whether a single stem cell could
reconstitute all blood lineages, or whether there were stem cells for
each lineage. In 1961, Till and McCulloch demonstrated the
formation of multilineage colonies in the spleen following injection
of BM into irradiated mice [49]. Through the use of radiation-induced
chromosomal markers, it was shown that each spleen colony arose
from a single cell, termed the spleen colony-forming unit (CFU-S)
[50], conclusively establishing the existence of multipotent stem
cells. The low frequency of CFU-S (1 in 104 BM cells) implied that the
hematopoietic system is organized as a hierarchy, with mature,
differentiated cells being produced from a smaller number of more
immature precursors. Spleen colonies contained CFU-S that were
able to give rise to new colonies in secondary recipients,
demonstrating that CFU-S are capable of self-renewal [51]. This
key property, together with their capacity for multilineage differentiation and extensive proliferation, suggested that CFU-S could be
considered to be a class of stem cells. Later evidence, including their
inability to differentiate into lymphocytes, would show that CFU-S
are distinct from true pluripotent repopulating hematopoietic stem
cells (HSCs), and that the latter can only be assayed by their ability to
stably reconstitute the hematopoietic system of recipient animals
[52]. The subsequent development of efficient retroviral gene
transfer techniques to uniquely mark HSCs, enabling clonal tracking
of the progeny of individual stem cells [53], as well as more refined
methods for purification of stem cells [54], enabled elucidation of the
surface phenotype of self-renewing HSCs [55]. In addition, these
approaches led to the recognition of functional heterogeneity within
the stem cell compartment [56], the basis of which was later shown
to be the existence of distinct classes of stem cells with differing
functional capacities [57].
Human hematopoietic stem cells
Early progress in the identification and characterization of human
HSCs was hampered by the lack of transplantation assays that would
allow functional testing of candidate stem cell populations in vivo.
The development of quantitative xenotransplantation assays using
immune-deficient mouse recipients to detect primitive human
hematopoietic cells with in vivo repopulating ability (SCID-repopulating cells, SRCs) [58] thus represented a significant advance in the
field of human hematopoiesis research. Studies to characterize SRCs
have shown them to be very primitive cells possessing properties
attributed to HSCs, including multipotentiality, high proliferative
capacity and the ability to self-renew [59]. Recent improvements to
gene transfer techniques that enable efficient marking of primitive
human hematopoietic cells [53], combined with strategies to purify
SRCs [60], have allowed detailed analysis of the clonal behaviour of
human HSCs and pointed to the existence of distinct classes of SRC
with variable proliferative and self-renewal potentials [61–66]. It is
becoming clear that the human stem cell compartment, like its
murine counterpart, is heterogeneous and comprises cell populations with varying capacities for differentiation, proliferation and
self-renewal.
proliferation have been proposed [2,16] (Figure 2). The
stochastic model postulates that the processes of selfrenewal versus differentiation in single cells within a
population occur randomly and are governed by probabilities. Accordingly, every tumor cell will have a low but
equal probability of proliferating extensively and thus the
potential to behave as a stem cell. Importantly, only those
cells that retain self-renewal capacity would have the
ability to sustain neoplastic growth. By contrast, the stem
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CD38–
Thy-1+
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NORMAL
Lymphoid
progenitor
Myeloid
progenitor
HSC
(SRC)
LTC-IC
Transforming
mutations
CFU
Mature
blood cells
LEUKEMIC
LSC
(SL-IC)
CD34+
Leukemic
LTC-IC
CD38–
Thy-1–
c-kit–
IL-3Rα+
Leukemic
CFU
Leukemic blast cells
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Figure 1. Schematic illustration of the normal and leukemic human hematopoietic hierarchies. Human hematopoietic cells are organized in a hierarchy that is sustained by a
small population of self-renewing hematopoietic stem cells (HSCs). HSCs give rise to progressively more lineage-restricted, differentiated progenitors with reduced selfrenewal capacity (LTC-ICs, long-term culture-initiating cells; CFU, colony-forming units), which in turn produce functionally mature blood cells. Disruption of pathways
regulating self-renewal and differentiation through the acquisition of transforming mutations generates leukemic stem cells (LSCs) capable of sustaining growth of the
leukemic clone in vivo. LSCs possess an altered differentiation program, as demonstrated by aberrant expression of some cell-surface markers (indicated in blue) and give
rise to an aberrant developmental hierarchy that retains aspects of its normal counterpart. In vivo reconstitution assays using immune-deficient mouse recipients enable
detection of HSCs and LSCs as SCID-repopulating cells (SRCs) and SCID leukemia-initiating cells (SL-ICs), respectively.
cell model proposes the existence of distinct classes of cells
within a tumor, each with different capacities for selfrenewal and proliferation. Consequently, only a small
definable subset of cells will consistently have the capacity
to initiate tumor growth and reproduce the hierarchy of
cell types that comprise the tumor.
While both models predict that only a limited number of
cells within a tumor will initiate tumor growth, their
underlying biological principles are very different. According to the stochastic theory, the cells within a tumor are
relatively homogeneous, and the genetic changes leading
to the development and progression of malignancy are
operative in all cells. Thus, research to understand
tumorigenic pathways, as well as therapies to eradicate
the tumor, can be directed at the bulk cell population. The
stem cell model, however, postulates that the rare population of tumor-initiating cells is biologically and functionally distinct. Tumorigenic pathways might operate
differently in these cells compared with the bulk cells;
therefore, research must focus on such CSCs to enable
development of targeted cancer treatments that will
effectively prevent disease relapse. The stem cell model
predicts that tumor stem cells with the capacity to initiate
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tumor growth in vivo can be prospectively identified and
purified from the bulk tumor, whereas the stochastic
model predicts that tumor-initiating activity will always
appear in every cell fraction and cannot be enriched.
Adaptation of the available quantitative assays for
normal human stem cells capable of repopulating hematopoiesis in vivo allowed these hypotheses to be directly
tested for AML. Transplantation of primary AML cells
into SCID [17] or NOD/SCID [18] mice (see Glossary) led
to the finding that only rare cells, termed SCID leukemiainitiating cells (SL-ICs), are capable of initiating and
sustaining growth of the leukemic clone in vivo. In
addition to their ability to differentiate and proliferate,
serial transplantation experiments showed that SL-ICs
possess high self-renewal capacity, and thus can be
considered to be AML stem cells. Importantly, SL-ICs
can be prospectively identified and purified as
CD34CCD38K cells in AML patient samples, regardless
of the phenotype of the bulk blast population, and are the
only cells capable of regrowing the leukemia in recipient
mice [17,18]. These findings rule out stochastic processes
as the biological mechanism underlying tumor heterogeneity in AML and show that, like the normal
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Tumor growth
No tumor
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Figure 2. Models of tumor cell proliferation. (a) Tumor cells are heterogeneous, and every cell has a low but equal probability of proliferating extensively and forming new
tumors. Therefore, subpopulations enriched for tumor-initiating activity cannot be consistently isolated. According to this model, the genetic changes leading to
development and progression of malignancy are operative in all cells within the tumor. Existing therapeutic and research approaches aimed at the bulk cells of the tumor are
largely based on this model. (b) Tumor cell are heterogeneous, but most cells have only limited proliferative potential and only a small subset of cancer cells has the ability to
initiate new tumor growth. According to this model, these cancer stem cells (CSCs) are biologically and functionally distinct from the bulk of tumor cells and must be
specifically targeted by cancer treatments to achieve permanent cure. This model is supported by the recent characterization of CSCs in breast and brain tumors (see main
text).
hematopoietic system, AML is organized as a hierarchy of
distinct, functionally heterogeneous classes of cells that is
ultimately sustained by a small number of leukemia stem
cells (LSCs) (Figure 1). These studies provided the first
direct evidence for the cancer stem cell hypothesis.
LSCs have a primitive immunophenotype that is
similar to that of normal HSCs in many respects. For
example, both LSCs and HSCs express CD34 but not
CD38 [18]. However, expression of some cell-surface
antigens on LSCs differs from the usual pattern. In
particular, Thy-1 (CD90) and c-kit (CD117) are expressed
on normal HSCs but are lacking on LSCs [19,20], whereas
the IL-3 receptor a chain (CD123) is a unique marker for
LSCs [21]. The aberrant phenotype of LSCs demonstrates
that, while a differentiation program is retained in AML
stem cells, it is often abnormal. Recently, the use of
lentiviral gene marking to track the behaviour of
individual SL-ICs following serial transplantation has
revealed heterogeneity in their ability to repopulate
secondary and tertiary recipients, pointing to the existence of distinct classes of LSC with differing self-renewal
capacity, similar to what is seen in the normal HSC
compartment [22] (Box 1). However, while LSCs possess
both the machinery for self-renewal and the ability to
regulate this property, certain LSC classes have a much
higher self-renewal capacity than normal HSCs. Overall,
these findings suggest that the pathways that regulate
normal commitment/differentiation and self-renewal processes in hematopoietic cells are not completely abolished
in LSCs. Rather, the effects of transforming mutations are
layered onto the normal developmental framework of
HSCs, resulting in the leukemic clone having an aberrant
developmental hierarchy that retains aspects of its normal
counterpart. This concept is supported by a correlation
between genes required for normal hematopoietic
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development and those perturbed in leukemia [23] and
by the recent demonstration that Bmi-1 is required for selfrenewal of both normal and leukemic HSCs in mice [24,25].
Implicit in the hierarchical nature of AML is the biological
uniqueness of LSCs compared with more mature leukemic
blasts. For example, in contrast to the increased proliferative
activity of leukemic CFUs and long-term culture-initiating
cells (LTC-ICs) in AML, studies employing a variety of
techniques have demonstrated that LSCs, like their normal
counterparts, are quiescent [26–28]. Thus, while the genetic
changes leading to increased proliferation might occur in
stem cells, the effects of these changes are manifested
primarily in downstream progenitors. The quiescent status
of LSCs renders them difficult to eradicate with standard
therapies that typically target proliferating cells. The
development of successful therapies that target the rare
population of LSCs responsible for maintaining the disease
in vivo will depend on a detailed understanding of their
distinct molecular characteristics. Although LSCs share
some phenotypic features with normal HSCs, the recent
identification of leukemia-specific stem cell markers enables
the prospective purification of AML stem cells and elucidation of their unique properties [29,30]. For example, a recent
study demonstrated that the active form of NF-kB, which is
associated with anti-apoptotic activity in human cancer [31],
is expressed in LSCs but not normal primitive hematopoietic
cells [27]. Inhibition of NF-kB in vitro induces apoptosis in
LSCs while sparing normal stem cells [27,32], underscoring
both the importance and specificity of NF-kB signaling in the
survival of AML stem cells. Future studies to characterize
leukemia-specific stem cell properties will not only be crucial
to the development of therapeutic approaches to target LSCs
in vivo but will also provide insight into the key elements of
the leukemogenic process, many of which might be relevant
to other human cancers as well.
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The cell of origin in cancer: studies in AML
A focus of much cancer research is identification of the
normal cell within which cancer initiates, to gain insight
into the first steps of the neoplastic process (Box 2). In
AML, the phenotypic and functional similarities between
LSCs and normal HSCs suggest that the initial transforming events occur in stem cells rather than committed
progenitors. The contrary view that transformation can
occur in a variety cell types in the hematopoietic
hierarchy, including committed progenitors, was originally proposed to explain the observed phenotypic heterogeneity between AML patients [14]. According to this
hypothesis, the phenotype of leukemic blasts is primarily
dependent on the degree of differentiation of the target
cell. The alternative theory, that the genetic events that
lead to leukemic transformation occur in primitive cells
only [18,33], proposes instead that the phenotype of
leukemic blasts is influenced mainly by the nature of the
specific transforming events and their subsequent effects
on the developmental program of the target cell. There are
several arguments that support this latter model
(Figure 3). As discussed above, self-renewal is a key property
of both normal and leukemic stem cells. Presumably, fewer
mutagenic changes would be required to transform stem
cells in which the machinery to specify and regulate selfrenewal is already active, as compared with more committed
progenitors in which self-renewal must be activated
ectopically. In addition, owing to their longevity, there is a
greater opportunity for genetic changes to accumulate in
individual stem cells than in more mature progenitors with
limited life span. For leukemic transformation of a restricted
progenitor to occur, acquisition of self-renewal potential
must be an early event; otherwise, any mutations accrued by
that short-lived cell will be lost from the genetic pool before
the full leukemogenic program can be attained. By contrast,
others have recently argued that stem cells might have
evolved sophisticated inhibitory machinery to keep selfrenewal in check as a cancer defense mechanism. If this idea
is correct, stem cells might actually be less likely to acquire
mutations that affect self-renewal and proliferation, or to
Box 2. The cell of origin versus the cancer stem cell
The term ‘cancer stem cell’ (CSC) refers to the biologically distinct
cell within the neoplastic clone that is capable of initiating and
sustaining tumor growth in vivo (i.e. the cancer-initiating cell). The
CSC results from the accumulation of mutations sufficient to endow
a transformed cell with a full cancer phenotype.
The term ‘cell of origin’ refers to the normal cell in which the initial
transforming event occurs. Tumorigenesis might begin either in a
primitive multipotent stem cell or in a more mature downstream
progenitor. The cell of origin in different tumor types might differ
owing to the unique biology of the tissues in which they arise.
‘CSC’ is sometimes mistakenly used to signify ‘stem cells in which
tumors originate’. Likewise, ‘cell of origin’ has been used incorrectly
to mean ‘cell sustaining tumor growth’. It is important to distinguish
between these independent concepts. The existence of CSCs has
been conclusively established in acute myelogenous leukemia
(AML) and more recently in solid tumors. By contrast, because the
earliest steps in the neoplastic process occur in a ‘black box’, much
research still needs to be carried out before we can identify and
characterize the normal cell types targeted for transformation.
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manifest biological changes as a result of such mutations, as
compared with more mature progenitors [34].
In murine hematopoiesis, investigators have recently
attempted, with conflicting results, to solve the cell-of-origin
puzzle by transducing fusion oncogenes into purified populations of progenitors and HSCs to assess their transformation susceptibility. The fusion oncogene MLL-GAS7
induces mixed-lineage leukemias when expressed in HSCs
or multipotent progenitors, but not in lineage-restricted
progenitors [35]. By contrast, MLL-ENL leads to initiation
of identical myeloid leukemias regardless of the target
cell population used [36]. However, much lower numbers
of transformed HSCs compared with progenitors are
required for tumor initiation in vivo, and tumors arising
from progenitors are oligoclonal not polyclonal, suggesting
that not every progenitor is equally susceptible to
transformation. A recent study using a murine model of
CML showed that inactivation of JunB, a transcriptional
regulator of myelopoiesis, must occur in HSCs and not
more restricted progenitors to induce a transplantable
myeloproliferative disorder [37]. Importantly, loss of JunB
expression in HSCs leads to an increase in numbers of
HSCs and granulocyte/macrophage progenitors but not
other types of restricted progenitors, indicating that
transforming events in the most primitive target cells
can have very specific effects in downstream progeny.
In early human studies to identify the cell in the normal
hematopoietic hierarchy in which the initial leukemogenic
event occurs, investigators using clinical blood samples
attempted to determine whether cells from different lineages were part of the leukemic clone: involvement of
multiple lineages would suggest that the leukemia originated from a primitive multipotent stem cell, while lineage
restriction would point to transformation of a committed
progenitor [38–41]. For example, involvement of all
hematopoietic lineages in chronic-phase CML patients
provides conclusive evidence that CML arises within the
stem cell compartment [42,43]. On the other hand, demonstration of granulocytic/monocytic lineage restriction in
some patients with AML was interpreted as disease origin
from a committed progenitor [44]. A major limitation of
this approach is that apparent lineage restriction of the
leukemic clone could also result from leukemogenic
mutations that arise in a multipotent stem cell and
suppress differentiation to one or more lineages [40].
A second approach used in human studies involves
assessment of clonal involvement of defined primitive
versus more mature cell populations to directly identify
the earliest cell in which leukemogenic events have
occurred. A recent study of patients with t(8;21) AML
demonstrated that, despite the presence of leukemiaspecific AML1-ETO chimeric transcripts in primitive
CD34C Thy-1K CD38K cells from leukemic bone marrow,
these cells give rise to normally differentiating multilineage clonogenic progenitors, whereas more mature
CD34C Thy-1K CD38C cells form exclusively leukemic
blast colonies [45]. These findings suggest that, whereas
the initial t(8;21) translocation occurs in a primitive stem
cell (the ‘cell of origin’), subsequent events occur in the
committed progenitor pool, giving rise to LSCs able to
sustain the leukemia in vivo. However, an alternative
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Loss of self-renewal potential
Normal development
Long-term
repopulating
stem cell
Short-term
repopulating
stem cells
(b)
(a)
Initial transforming
event in self-renewing
stem cell
Restricted
progenitors
Functionally
mature blood cells
(c)
Initial transforming event
reactivates stem cell
self-renewal machinery
Initial transforming
event in nonrenewing cell
Leukemic development
No expansion of pool
of transformed progenitors
Loss of transformed cells
through terminal differentiation
Leukemia
Long-term
leukemia
stem cells
Short-term leukemia
stem cells
Leukemic
progenitors
Aberrantly differentiated
leukemic blast cells;
phenotype depends on
transforming events
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Figure 3. The importance of self-renewal in leukemic initiation and progression. Self-renewal is a key property of both normal and leukemic stem cells. Fewer mutagenic
changes are required to transform stem cells in which the self-renewal machinery is already active (a), as compared with committed progenitors in which self-renewal must
be activated ectopically (b). In addition, self-renewing stem cells are long-lived; thus, there is an increased chance for genetic changes to accumulate in individual stem cells in
comparison with more mature, short-lived progenitors. If a committed progenitor with limited life span acquires a genetic mutation that does not confer increased selfrenewal (c), that cell will likely die or undergo terminal differentiation before enough mutations occur to propagate a full leukemogenic program.
possibility is that the transforming events occur in a
primitive cell but disrupt normal developmental programs
only in downstream progeny. In this case, the ability to
generate leukemia in vivo is retained within the primitive
stem cell pool (Figure 4). Distinguishing between these two
models requires assessment of the ability of purified cell
fractions to initiate and sustain leukemic growth in vivo.
The recent development of xenotransplantation assays
for AML stem cells has enabled a more direct examination
of the cell-of-origin issue in human hematopoietic cells. As
discussed above, the phenotype of SL-ICs is consistent
among patients regardless of AML subtype and is similar
to the phenotype of normal SCID-repopulating cells
(SRCs), suggesting that the cell of origin is a stem cell
rather than a committed progenitor. However, a comparison based on phenotype is potentially problematic owing
to the disruption of normal differentiation pathways by
the leukemogenic process, and thus a functional comparison based on stem cell properties is more reliable. The
recent data suggesting that the LSC pool, similar to the
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normal HSC compartment, is organized as a hierarchy of
distinct stem cell classes with decreasing self-renewal
capacity [22] are the most compelling evidence to date of
the stem cell origin of AML.
Cancer stem cells in solid tumors
Recent studies in solid tumors indicate that the concept of
stem cells in cancer might have broader implications
beyond the field of hematopoiesis. In breast cancer, a
minor, phenotypically distinct tumor cell population has
been isolated that is able to form mammary tumors in
NOD/SCID mice, whereas cells with alternative phenotypes are nontumorigenic [46]. The tumorigenic cells can
be serially passaged, demonstrating self-renewal capacity,
and are able to generate tumor heterogeneity, producing
differentiated, nontumorigenic progeny. Thus, like AML,
breast cancer growth appears to be driven by a rare
population of tumor-initiating cells. Recently, investigators have demonstrated the existence of CD133C cells
in human brain cancers that possess differentiative and
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Initial hit causes
expansion of transformed
stem cell pool.
Subsequent mutations occur
in this expanded
target pool.
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Acquired genetic changes
disrupt normal developmental
programs in downstream leukemic
progenitors, leading to
further expansion
(a)
Self-renewing
stem cell
(b)
Restricted
progenitors
Leukemia
Commitment and
differentiation
Initial hit is retained but
does not cause expansion
of transformed stem cell pool
Additional transforming events in
restricted progenitors lead
to expansion. In order to generate
LSCs, these mutations must
reactivate the cell’s
self-renewal machinery.
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Figure 4. Models of leukemic initiation and progression. Two possible models of leukemic development in which the first transforming mutation occurs in a stem cell are
illustrated. In (a), the initial transforming event results in clonal expansion of the stem cell pool, thus providing more targets for additional mutations. While all of the genetic
changes are acquired in the stem cell compartment, their effects are manifest in downstream progenitors, leading to production of blast cells whose phenotype depends on
the specific nature of the transforming mutations. Importantly, leukemic stem cells (LSCs) with the ability to generate leukemia in vivo are present within the stem cell
compartment. In (b), the initial transforming event does not result in clonal expansion of the stem cell pool, although it persists because the targeted cell is a self-renewing
stem cell. The transformed cell follows its developmental program and becomes progressively more restricted. Subsequent mutations acquired by downstream progeny
must reactivate the cellular self-renewal machinery to generate LSCs that are able to sustain growth of the leukemic clone in vivo. In this model, LSCs are not detected in the
stem cell compartment but are found in more mature cell populations, even though the initiating leukemogenic event occurs in the former.
self-renewal capacities and can initiate tumor growth
in vivo, whereas CD133– cells cannot [47]. The increasing
evidence that rare stem cells drive formation of several
different tumor types raises the possibility that cancer
stem cells are at the apex of all neoplastic systems.
Concluding remarks
Based on these recent studies, the paradigm of cancer as a
hierarchical disease whose growth is sustained by a rare
population of stem cells is re-emerging on a much more solid
footing since being proposed five decades earlier. These
CSCs are self-renewing and retain remnants of normal
developmental programs, giving rise to phenotypic and
functional tumor heterogeneity. Implicit in this model of
cancer development is the notion that CSCs are biologically
distinct from other cells in the tumor and are able to initiate
and sustain tumor growth in vivo, whereas the bulk cells are
not. Thus, the biological function of cancer pathways might
be fundamentally different in the rare CSCs compared with
the bulk tumor cells. Much current cancer research still
treats tumors as homogeneous collections of cells that can
simply be disrupted for biochemistry studies or for geneexpression profiling. Future studies to understand tumorigenic pathways from initiation to metastasis must focus
instead on identifying and characterizing the rare cancerwww.sciencedirect.com
initiating cells, and cancer treatments must be designed to
specifically target these CSCs if they are to effectively cure
and prevent disease relapse.
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