Cellular immortality in brain tumours: An integration of
the cancer stem cell paradigm
Ruman Rahman, Rachel Heath, Richard Grundy
To cite this version:
Ruman Rahman, Rachel Heath, Richard Grundy. Cellular immortality in brain tumours: An
integration of the cancer stem cell paradigm. BBA - Molecular Basis of Disease, Elsevier, 2009,
1792 (4), pp.280. .
HAL Id: hal-00486073
https://hal.archives-ouvertes.fr/hal-00486073
Submitted on 25 May 2010
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
Cellular immortality in brain tumours: An integration of the cancer stem cell
paradigm
Ruman Rahman, Rachel Heath, Richard Grundy
PII:
DOI:
Reference:
S0925-4439(09)00026-X
doi:10.1016/j.bbadis.2009.01.011
BBADIS 62921
To appear in:
BBA - Molecular Basis of Disease
Received date:
Revised date:
Accepted date:
26 November 2008
21 January 2009
21 January 2009
Please cite this article as: Ruman Rahman, Rachel Heath, Richard Grundy, Cellular
immortality in brain tumours: An integration of the cancer stem cell paradigm, BBA Molecular Basis of Disease (2009), doi:10.1016/j.bbadis.2009.01.011
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.
ACCEPTED MANUSCRIPT
Invited Review for BBA- Molecular Basis of Disease:
T
Title: Cellular Immortality in Brain Tumours: An Integration of the
RI
P
Cancer Stem Cell Paradigm
NU
SC
Author: Ruman Rahman, Rachel Heath, Richard Grundy
Address: Children’s Brain Tumour Research Centre, Medical School D Floor, School
MA
of Clinical Sciences, Queen’s Medical Centre, Nottingham, NG7 2UH, UK
ED
Corresponding Author: Dr. Ruman Rahman
Email: [email protected]
AC
CE
PT
Tel: (+44)115 8230718
Keywords: Neurogenesis, Neural Stem Cell, Telomerase, Telomeres, Brain Tumour
Stem Cell,
Word Count: (Main Body and Abstract) 5071
1
ACCEPTED MANUSCRIPT
Abstract
Brain tumours are a diverse group of neoplasms that continue to present a formidable
T
challenge in our attempt to achieve curable intervention. Our conceptual framework
RI
P
of human brain cancer has been redrawn in the current decade. There is a gathering
acceptance that brain tumour formation is a phenotypic outcome of dysregulated
SC
neurogenesis, with tumours viewed as abnormally differentiated neural tissue. In
relation, there is accumulating evidence that brain tumours, similar to leukaemia and
NU
many solid tumours, are organized as a developmental hierarchy which is maintained
MA
by a small fraction of cells endowed with many shared properties of tissue stem cells.
Proof that neurogenesis persists throughout adult life, compliments this concept.
ED
Although the cancer cell of origin is unclear, the proliferative zones that harbour stem
cells in the embryonic, post-natal and adult brain are attractive candidates within
PT
which tumour-initiation may ensue. Dysregulated, unlimited proliferation and an
ability to bypass senescence are acquired capabilities of cancerous cells. These
AC
CE
abilities in part require the establishment of a telomere maintenance mechanism for
counteracting the shortening of chromosomal termini. A strategy based upon the
synthesis of telomeric repeat sequences by the ribonucleoprotein telomerase, is
prevalent in ~90% of human tumours studied, including the majority of brain
tumours. This review will provide a developmental perspective with respect to normal
(neurogenesis) and aberrant (tumourigenesis) cellular turnover, differentiation and
function.
Within
this
context
our
current
knowledge
of
brain
tumour
telomere/telomerase biology will be discussed with respect to both its developmental
and therapeutic relevance to the hierarchical model of brain tumourigenesis presented
by the cancer stem cell paradigm.
2
ACCEPTED MANUSCRIPT
Introduction
1. Characteristics of intracranial neoplasms
T
Of all solid cancers, brain tumours have the poorest survival and highest
RI
P
morbidity rates. Although frequently considered collectively, brain tumours represent
a diverse range of tumour types affecting both children and adults. Tumours of the
SC
brain account for less than 2% of all malignancies, yet now approach leukaemia as the
leading cause of cancer-related death in children and the fourth leading cause in adults
NU
[1]. The diversity of brain tumour type is a reflection of the histological complexity of
MA
the central nervous system (CNS) and can present with glial or neuronal phenotypes
or a mixture of cell types.
ED
Medulloblastoma and supratentorial primitive neuroectodermal tumours
(sPNET) are World Health Organisation (WHO) grade IV embryonal tumours,
PT
resulting from the transformation of primitive neuroectodermal cells. The
medulloblastoma is a malignant and invasive tumour with a relatively poor prognosis
AC
CE
and represents the most common malignant brain tumour in children. It is
predominately neuronal in nature and typically located in the cerebellum. In contrast
the rarer sPNETs are more commonly found in the cerebral hemispheres and consist
of poorly differentiated neuroepithelial cells which confer poor prognosis, especially
in children. Despite both being classified as Grade IV tumours, sPNETs have a
significantly worse prognosis [2].
Also classified as a grade IV tumour by the WHO, glioblastoma multiforme
(GBM) represents the most malignant and aggressive form of glioma, arising in the
subcortical white matter as either a primary neoplasm, or from the malignant
progression of a low grade glioma [3]. Typically seen in adults, prognosis is
3
ACCEPTED MANUSCRIPT
extremely poor as the diffuse distribution throughout the brain makes surgical
resection difficult [4].
T
Ependymoma are typically classified as glial tumours, however it is currently
RI
P
debated as to whether ependymoma may actually reflect a separate entity. Composed
of neoplastic ependymal cells, the ependymoma is usually located along the
SC
ventricular system of the paediatric or adolescent brain whereas spinal sites are more
commonly seen in adults. Prognosis is varied and is dependent on surgical resection
NU
[5, 6].
MA
Although an understanding of human brain cancer pathogenesis has increased
in recent decades, insufficient knowledge of the molecular, genetic and cellular
alterations and mechanisms is at the root of treatment failure and relatively limited
ED
diagnostic and prognostic capabilities. As a result, bio-centred and target-specific
PT
brain tumour therapy has not yet been forthcoming.
AC
CE
2. Neuro-developmental biology
The CNS is a highly complex structure which develops from a restricted
number of extensively plastic cells that proliferate and acquire regional identities in
space and time to produce a diverse repertoire of cell types. Collectively, these cells
are defined as neural stem cells (NSCs) on the basis of their potential to differentiate
into mature neurons and glia (astrocytes and oligodendrocytes) and their ability to
self-renew in vitro. This operational definition does not however, necessarily reflect
the function of NSCs in vivo and it remains uncertain whether the cell type(s)
exhibiting ‘stemness’ in vitro faithfully represents the cell exhibiting ‘stemness’ in
vivo.
4
ACCEPTED MANUSCRIPT
Mammalian developmental neuro-biology now entertains an integrated view
of NSCs whereby neuroepithelial cells comprising the neural tube of the developing
T
embryo, give rise first to radial glia, which then transform into astrocyte-like adult
RI
P
multipotent stem cells or directly into mature ependymal cells lining the walls of the
lateral ventricles, the latter of which are post-mitotic in the adult brain [7]. Radial glia
SC
arise from neuroepithelial cells throughout the primitive CNS at the beginning of
neurogenesis and represent the predominant neuronal progenitor [8]. These cells
NU
characteristically display a radial morphology and a mixed primitive cell/glial
MA
immunophenotype. In mammals, radial glial cells disappear from the brain soon after
birth, with a closely-related astrocyte-like cell taking the role as NSC thereafter. The
belief that the genesis of new CNS cells is a rare event in the adult mammalian brain
ED
was established dogma until recently. Two constitutively active germinative layers are
PT
now known to reside in the adult brain: the subventricular zone (SVZ) associated with
the anterior part of the forebrain lateral ventricles and the subgranular zone (SGZ)
AC
CE
within the hippocampus, corresponding to the inner layer of the dentate gyrus [9-11].
New neurons and glia continue to be generated within these restricted regions of the
adult mammalian brain [12]. Evidence that neurogenesis persists in the CNS of the
adult mammalian brain (albeit in restricted domains) has changed our perception of
the cell of origin with respect to neural neoplastic transformation [13, 14].
3. Telomere and telomerase status during normal and dysregulated neurogenesis
In virtually all types of malignant cells, unlimited replicative potential is
conferred in considerable part, by the maintenance of telomere length above a critical
minimum threshold. Telomeres (derived from the Greek telos, meaning end and
meros, a component) are a complex of guanine-rich repeat sequences and associated
5
ACCEPTED MANUSCRIPT
proteins that form a loop structure which caps all eukaryotic chromosome termini [15]
and prevents chromosomes from improper recombination, nuclease degradation and
T
end fusions, thereby contributing to genomic stability. Due to incomplete replication
RI
P
of lagging-strand synthesis at each mitotic division (‘end-replication problem’) [16],
telomeres progressively shorten, a phenomenon that provides a molecular basis of
SC
cellular ageing and hints at a primitive tumour-suppressor mechanism [17]. In germ
cells and cancer cells, unlimited proliferative capacity is achieved through the
NU
addition of telomeric repeats by the telomerase holoenzyme. Telomerase is a
MA
ribonucleoprotein consisting of an RNA subunit (TR) which provides an intrinsic
template for the synthesis of de novo telomeric repeats in a reaction catalyzed by its
enzymatic component (TERT) [18].
ED
NSCs from different regions of the brain have different proliferative capacities
PT
and varying abilities to generate astrocytes, oligodendrocytes and neurons [19, 20].
This may be reflected in the level of telomerase activity and telomere length reserve
AC
CE
within cells from different neuro-anatomical compartments, in particular regions of
high neurogenic activity such as the SVZ and SGZ [21]. Telomerase-mediated
telomere homeostasis is therefore a likely key factor in brain plasticity throughout
development and adulthood.
There is a suggestion here that a fuller understanding of the replicative
histories of cells within tumour hierarchical compartments may provide clues as to the
cancer cell of origin for a given brain neoplasm. Indeed telomerase activity has been
detected in a growing number of brain tumours in the past decade [22-27]. With preclinical and clinical advances in telomerase-therapeutics for cancer, there is
anticipation that this will impact upon paediatric and adult tumours of the CNS.
6
ACCEPTED MANUSCRIPT
4. Brain tumourigenesis and tumour heterogeneity
A developmental relationship between tissue stem cells and cancer cells has
T
been proposed over 50 years ago [28, 29]. However the pioneering demonstration of
RI
P
‘cancer stem cells’ in human cancer was first made in leukaemia [30, 31]. From this
initial discovery in hematopoietic tumours, evidence from most solid cancers studied
SC
defines restricted subsets of tumour cells that share characteristics of the
corresponding normal tissue stem cells [32-36]. This paradigm includes prospective
NU
isolation of a minority population of brain tumour cells, based on the expression of
MA
the cell surface antigen CD133, known to be highly expressed in normal neural stem
cells. These putative brain cancer stem cells were isolated from medulloblastoma,
pilocytic astrocytoma, glioblastoma multiforme and anaplastic ependymoma and
ED
possessed a marked capacity for self-renewal (evidenced by the formation of floating
PT
aggregates termed tumourspheres), proliferation and differentiation [37-40].
Crucially, only the CD133+ fraction was capable of tumour initiation upon orthotopic
AC
CE
transplantation into non-obese diabetic severe combined immunodeficient (NODSCID) mouse brain [41]. The CD133+ fraction produced a tumour that generated a
phenocopy of the original tumour and could be serially transplanted into secondary
recipients, providing evidence of self-renewal capacity in vivo (Singh 2004). Our
growing understanding of the hierarchical organization of the functional
compartments in renewing brain tissues provides a theoretical and technical
framework upon which to base falsifiable questions regarding brain tumour initiation
and cell of origin.
7
ACCEPTED MANUSCRIPT
5. A multidisciplinary approach to a new synthesis
Recent compelling evidence demonstrate that the brain, similar to other organs
T
in which cancers arise, harbours a stem cell population that can give rise to
RI
P
differentiated CNS cells. Prospective identification of neural cells with shared stem
cell-like properties which are capable of initiating tumours in vivo and that are
SC
phenocopies of the original tumour, provides a new synthesis in brain tumour
research. The cellular architecture of brain tumours may thus represent a distorted
NU
mimicry of normal developmental neuro-biology. This concept, coupled with an
MA
increasing understanding of the immortal status of transformed neural cells,
particularly with respect to aberrant telomerase activity and the maintenance of
critically short telomeres, will arguably provide an experimental basis to better
ED
understand and delineate cancer changes that are causal and consequential with
PT
respect to tumour hierarchy and tumour initiation. This review encourages a consilient
approach to investigate telomere/telomerase neurobiology and brain tumour stem cells
AC
CE
within a neurodevelopmental core.
Developmental Neurobiology
1. Neuroepithelial cells, radial glia and early neurogenesis
The origin of all CNS cells can be traced to a single layer of embryological
cells, which constitute the neuroepithelium. The edges of this sheet fold together to
form the neural tube, which later gives rise to the ventricular system and spinal canal.
Initially, neuroepithelial cells divide symmetrically in order to increase the stem cell
pool; thereafter these highly plastic cells proliferate and acquire regional identities in
a spatial-temporal manner [10]. It is likely that neuroepithelial cells differentiate
directly into radial glial cells in the embryonic ventricular zone [42], although this has
8
ACCEPTED MANUSCRIPT
not been confirmed experimentally. Some recent supporting evidence is presented by
the derivation of both neuroepithelial cells and radial glia from human embryonic
T
stem cells; the course of differentiation (with respect to morphology change and
RI
P
lineage-specific marker expression) followed a temporal manner with neuroepithelial
cells generated first, followed later by radial glial cells and finally mature neurons
SC
[43].
The fate and function of a radial glial cell varies considerably from region to
NU
region in the CNS. It had long been accepted that radial glia are both astroglial
MA
progenitors [44, 45] and oligodendrocyte progenitors [46, 47], but recent evidence has
demonstrated that radial glia produce most of the cortical neurons [48-50]. Radial
glial cells thus serve as neural stem/progenitor cells for the majority of neurogenesis
ED
and gliogenesis in the developing brain and gives rise to the astrocytic-like neural
PT
stem cells of the adult brain [48-52]. Whether radial glia transform directly into an
adult multipotent neural stem cell or progenitor, or whether they first differentiate into
AC
CE
an intermediate cell type, is unknown (see question marks, Figure 1). This concept of
early neurogenesis is in dramatic contrast from the long-held view that embryonic,
post-natal and adult NSCs are discrete unrelated populations.
2. Multipotent neural stem cells and neurogenesis in the mature CNS
Although widespread neurogenesis is restricted to the embryonic period, a
limited number of neural stem cells persist throughout adulthood and are restricted
mainly to two neurogenetic domains [13, 14, 53]. The largest of these regions is the
mammalian SVZ situated throughout the lateral walls of the forebrain lateral
ventricles [11]. A similar hierarchical neurogenetic system exists in the SGZ, an area
corresponding to the dentate gyrus of the hippocampal granular layer [54, 55]. Stem
9
ACCEPTED MANUSCRIPT
cell progeny generated within these regions populate the growing CNS parenchyma,
later differentiating into mature neuronal and glial fates, specified in a precise spatial-
T
temporal manner [56]. Cells that express the intermediate filament astroglial marker,
RI
P
glial fibrillary acidic protein (GFAP), have been identified as putative adult neural
stem cells in this region, also known as Type B cells [57, 58] (Figure 1). The
SC
embryonic origin of Type B cells has been confirmed using in vivo lineage tracing
techniques and shows that adult subventricular cells are not related to embryonic
NU
subventricular precursors, but embryonic ventricular radial glia [59, 60].
MA
Further support of the astrocytic nature of NSCs has been confirmed in
experiments in which GFAP+ cells were conditionally ablated in the brain of adult
mouse, resulting in near complete loss of neurogenesis [61, 62]. Type B cells are
ED
proposed to be relatively quiescent with a cell cycle duration of ~24 hours and
PT
constitute a small fraction of the total astrocytic population in the SVZ [57].
Pioneering experiments retrospectively viewed, adds support to this notion; Reynolds
AC
CE
and Weiss in 1992, isolated a small population of cells (<0.1% of total cells) from the
adult striatum that could proliferate and generate multiple clones of cells in freefloating clusters in vitro called neurospheres [63]. It is worth noting that even with
current refined methods to isolate neural stem cells using the neurosphere assay, the
stem cell content is variable; although Type B and Type C cells can produce
neurospheres, not all cells within the neurosphere express stem cell markers [58, 64].
Multipotent Type B astrocytes within the SVZ give rise to fast-cycling
transiently amplifying precursor cells called Type C precursors, which in turn
generate either mitotically active Type A neuroblasts or terminally differentiated
mature glia (astrocytes, oligodendrocytes). Type A neuroblasts migrate towards the
olfactory bulb where they integrate as mature neurons (Figure 1). Similarly, Type B
10
ACCEPTED MANUSCRIPT
astrocytes in the SGZ produce intermediate Type D precursors which give rise to
Type G granule neurons [65]. Over the course of development, NSCs not only feature
T
altered morphology but also exhibit changes to gene expression profiles in a temporal-
RI
P
specific manner [66]. This expression profile may underlie an intrinsic developmental
program as progenitors grown in culture generate progeny at certain times, similar to
SC
progenitors in vivo [67]. In addition to GFAP, Type B cells express other intermediate
filaments such as vimentin and nestin. As NSCs differentiate along developmental
NU
pathways, neuronal-specific (polysialylated neural cell adhesion molecule (PSA-
MA
NCAM), microtubule-associated protein 2 (MAP2), neurofilament protein, tyrosine
hydroxylase) and glial-specific markers (O4 monoclonal antibody that recognizes the
sulfatides and myelin basic protein (MBP)) are expressed in late-amplifying and
PT
ED
mature cell compartments [68] (Figure 1).
3. The role of telomeres and telomerase during neurogenesis
AC
CE
Most studies aimed at understanding the nature of NSCs with respect to self-
renewal, proliferation and differentiation, focus on external cues such as secreted
growth factors [69, 70]. Few studies have addressed the role of intrinsic mechanisms
regulating NSC behaviour [71]. Chromosome integrity is essential for cell viability,
therefore highly proliferative cell types such as tissue stem cells, require active
telomere maintenance strategies to proliferate indefinitely or at least long enough for
the host organism to reach reproductive age. The role of telomere and telomerase
dynamics in NSC compartments has scarcely been addressed. In the mouse, high
levels of TERT and TERC (murine telomerase RNA subunit) mRNA are present in
the developing neural tube as early as E10.5 [72]. Both telomerase subunits continue
to be expressed in different regions of the developing murine CNS and correlates with
11
ACCEPTED MANUSCRIPT
the proliferation of neural progenitors [73-75]. This suggests a role for telomerase
during specification of neural cell types. Consistent with our knowledge of
T
neurogenetic regions of the adult brain, telomerase activity has been shown in neural
RI
P
precursor cells isolated from the SVZ and hippocampus in adult mice [21]. With
reference to stem cell populations in other tissues, the current working hypothesis
SC
states that telomerase activity in NSCs (and actively proliferating early neural
precursors) may combat induced telomere shortening upon mitotic division. However,
NU
human progenitor cells isolated from the developing cortex which exhibit low/absent
MA
levels of telomerase activity, undergo decreased neurogenesis and eventually enter
replicative senescence in vitro [76]. These findings are corroborated by studies in
other tissues where low levels of telomerase activity have been found in tissue stem
ED
cells such as haematopoietic, skin, intestinal crypt and pancreas [77-80]. In all these
PT
cases, telomerase activity is insufficient to completely prevent telomere loss and
senescence. Rather, telomerase in tissue stem cells may slow the rate of telomere
AC
CE
shortening and maintain a replicative reserve pre-determined for the tissue stem cell in
question. It is difficult to argue that this phenomenon is not a primitive tumoursuppressor mechanism and perhaps one that predates protein-mediated tumour
suppression.
Telomere attrition dramatically impairs the in vitro proliferation of adult
NSCs isolated from the SVZ of telomerase-deficient mice, but not that of embryonic
NSCs, even though the latter exhibited critically shortened telomeres. This is
consistent with a recent report describing telomerase levels as high in embryonic
cortical neural progenitor cells, but low in newly generated neurons and mature
neurons [81]. Collectively these results hint at intrinsic differences in the protective
states between adult and embryonic NSCs, with respect to chromosomal stability and
12
ACCEPTED MANUSCRIPT
response to dysfunctional telomeres [82]. A comprehensive understanding of how the
regulation of telomeres/telomerase is tightly linked to cell cycle regulation in tissue
T
stem cells will be crucial to determining how cancer may arise when stem cell
1. Telomeres and telomerase in brain tumours
SC
Cellular Immortality and Tumour Initiation
RI
P
regulation is perturbed.
NU
Tumourigenesis involves multiple oncogenic insults which collectively define
MA
the tumour phenotype. Experimental data from cultured human fibroblasts indicate
that cells must bypass two checkpoints, mortality stage 1 (M1), regarded as cellular
ED
senescence and mortality stage 2 (M2), characterised by widespread apoptosis, prior
to immortalization and unlimited replicative potential [83]. Reactivation hTERT is a
PT
crucial determinant of cellular immortalization and if accompanied by inactivation of
tumour-suppressor genes and/or activation of cellular oncogenes, can subsequently
AC
CE
result in neoplastic formation. Telomerase-mediated telomere maintenance is evident
in virtually all types of malignant cells, and ~90% of tumours show evidence of upregulated telomerase [84, 85]. Brain tumours in which telomerase activity has been
detected include glioblastoma, anaplastic astrocytoma, oligodendroglioma, primitive
neuro-ectodermal tumours, medulloblastoma and ependymoma [23-26]. These data
indicate that hTERT reactivation or overexpression is an essential event for tumour
progression in CNS tumours. The role of hTERT may not be restricted to addition of
telomere repeats as mouse TERT has been shown to promote cell survival (prevents
apoptosis) in neurons, a function that does not require the telomerase RNA subunit
[86]. This extracurricular TERT function may contribute to cancer development and
aging independently of telomere lengthening and telomerase activity.
13
ACCEPTED MANUSCRIPT
In a comprehensive analysis of telomere length, the majority of grade II
astrocytomas, anaplastic astrocytomas and glioblastomas with telomerase activity,
T
exhibited significantly shorter mean telomere lengths relative to corresponding
RI
P
tumours with undetectable telomerase activity and to normal brain tissue. Similarly,
all primitive neuroectodermal tumours exhibited shorter telomere lengths relative to
SC
normal brain tissue [24]. Recent findings from our own laboratory reveal that
paediatric glioblastoma multiforme and ependymoma, tumours of glial origin, have
longer
mean
telomere
length
NU
significantly
than
those
of
supratentorial
MA
neuroectodermal tumours and medulloblastoma, tumours of neuroectodermal origin.
This highlights the consideration required for telomere length with respect to
duration/course of treatment when using anti-telomerase drugs which are predicted to
ED
exert their effects upon telomere shortening (Rahman R et al. unpublished data). The
PT
concurrent shortening of telomeres with telomerase reactivation/overexpression
supports a hypothesis that telomerase is required to maintain both minimum length
AC
CE
and a terminal capping structure at a subset of critically shortened telomeres [87].
The perspective presented by the stem cell origin of brain tumours implies that
the genetic alterations that lead to cancer, accumulate in NSCs rather than mature
neural cells. As NSCs would already exhibit telomerase activity, one would predict
the enzyme to be overexpressed rather than reactivated. Whether telomerase activity
increases during later stages of carcinogenesis is unclear [88]. Similar to selfrenewing tissues where hTERT expression is insufficient to halt progressive telomere
shortening, telomere shortening may continue within a proliferating transformed
neural stem cell. However aberrant and sustained hTERT expression likely results in
telomere stabilization both with respect to telomere length and 3’ telomere capping
structure (Figure 2). Elucidation of telomerase levels and telomere length within
14
ACCEPTED MANUSCRIPT
different neurogenetic subsets will influence strategies to target telomerase in brain
T
cancer.
RI
P
2. The Brain Tumour Stem Cell Paradigm
An acceptance of tumour hierarchical organisation readily leads to a
SC
developmental-centred view of tumourigenesis, where brain tumours represent
aberrantly differentiated tissue, in turn reflecting dysregulated neurogenesis. The stem
NU
cell paradigm for brain tumours presents tumour-initiating events as occurring within
MA
the genome of a cellular entity with neural stem cell-like properties.
The identification of brain tumour stem cells was prospectively demonstrated
by Peter Dirks and colleagues using cells derived from glioblastoma and
ED
medulloblastoma. As few as 100 cells sorted positively for the cell surface antigen
PT
CD133, were capable of initiating infiltrative tumours upon orthotopic transplantation
into SCID mice whereas injection of 100,000 CD133- cells failed to initiate tumours
AC
CE
[41]. In a later study, tumour stem cells isolated from ependymoma manifested a
radial glial phenotype with CD133+/Nestin+/RC2+/BLBP+ positivity confirmed to be
tumourigenic [40].
It is important to note that a definitive demonstration of tumour stem cells in
the brain is lacking as CD133 enriches for, rather than definitively identifies, cancer
stem cells. This caveat has been emphasized with recent studies showing that CD133glioblastoma cells have tumour-initiating ability [89, 90]. In addition, it is the
CD133+/Nestin+/RC2-/BLBP- phenotype that exhibited a tumourigenic phenotype in
medulloblastoma in contrast to the CD133+/Nestin+/RC2+/BLBP+ phenotype in
ependymoma [40], suggesting the characteristics of these two subsets may reflect
different cell of origins. Clearly stem-like properties in a tumour cell cannot be taken
15
ACCEPTED MANUSCRIPT
as direct evidence for neoplastic transformation occurring in a neural stem cell that
originates in a zone of active neurogenesis. The cancer stem cell hypothesis and the
T
cellular origin of cancer are distinct paradigms. It is conceivable that a neural
RI
P
progenitor or mature cell could give the appearance of ‘stemness’ upon neoplastic
transformation (Figure 2).
SC
An alternative focus is that of molecular events that permit tumour initiation
rather than on the cell type that initiated the tumour. The cancer stem cell hypothesis
NU
predicts mechanistic similarities between self-renewal of cancer stem cells and normal
MA
stem cells. In brain tumours there is particular interest in the Notch [91, 92] and
Sonic-hedgehog [93, 94] pathways, as these have been shown to be important for
normal stem cell self-renewal as well as dysregulated growth in cancer.
ED
Overexpression of Notch receptors and their ligands Delta-like 1 (DLK1) and Jagged
PT
1 (JAG1) correlates with the proliferative capacity of human glioma cells, implicating
Notch as a positive effector of self-renewal in adult neural stem cells [95].
AC
CE
Furthermore, overexpression of Notch-2 intracellular domain protein increases
proliferation of medulloblastoma cell lines [96], whereas medulloblastoma growth is
suppressed by the known Notch inhibitor, γ-secretase [97].
Similarly Sonic
Hedgehog (SHH) and its downstream effectors, glioma-associated homologue 1
(GLI1), GLI2 and GLI3 have been shown to specifically regulate self-renewal and
neurogenesis within the external granular layer of the post-natal cerebellum and to
control proliferation of precursors within the adult SVZ [98]. Mutations in this
pathway are thus implicated in the initiation of medulloblastoma [99, 100].
Another emerging concept is that brain tumour stem cells may crucially be
supported by a specific microenvironment which mimics the neural vascular niche
[101]. Differentiated neural cells or blood vessel cells have been implicated as niche
16
ACCEPTED MANUSCRIPT
elements. CD133+ glioblastoma cells secrete increased levels of vascular endothelial
growth factor (VEGF) compared to CD133- counterparts, implicating tumour stem
T
cells as contributing to tumour angiogenesis [102]. However, whether the tumour
RI
P
stem cell niche differs from that of the bulk tumour population or whether it provides
signals which govern aberrant self-renewal is unclear. Future work will validate the
SC
proposition that the tumour stem cell niche represents an attractive therapeutic target.
The cancer stem cell field is at present contentious, in part due to differing
NU
semantic interpretations. The ‘cancer stem cell’ term is often applied to cells
MA
identified by varying methods and criteria. The danger here is the application of the
term to cells with no evidence of cancer-initiating capacity. It is important to clarify
what ‘brain tumour stem cell’ refers to in the present context: these are cells which
ED
demonstrate cancer-reinitiating ability upon orthotopic transplantation, with a
PT
capacity to generate a phenocopy of the original tumour mass (consisting both
tumourigenic and non-tumourigenic cells); show extensive self-renewal ability ex
AC
CE
vivo and in vivo; demonstrate aberrant multipotent differentiation; harbour karyotypic
and/or genetic alterations. At present, this does not exclude the possibility of more
committed progenitors or even mature cells reverting to a stem-like phenotype and
causing tumour-initiation in contexts that differ from those imposed by the prementioned strategies. For example, it may be argued that the mouse may not represent
a physiologically relevant microenvironment for engraftment and growth of human
tumours, thus potentially resulting in an under-estimation of cell types capable of
tumour initiation.
17
ACCEPTED MANUSCRIPT
Towards A New Therapeutic Outlook
The discovery of putative brain tumour stem cells and the growing
T
understanding of neurogenesis and cellular immortality, presents both a novel cellular
RI
P
target and novel molecular targets within. Much larger numbers of brain tumour
samples and prospective multi-parameter cell sorting are needed to determine whether
SC
the cancer stem cell hypothesis is robust as well as to delineate patient subsets within
a particular brain tumour type. Do all tumours under a common nomenclature reflect a
NU
common cell of brain cancer origin? What are the implications for therapy if they do
MA
not? In addition, evidence to date does not address whether the cell type conferring
self-renewal capacity through tumoursphere-formation in vitro is the same cell type
ED
that initiates the neural tumour in vivo; these studies involve tumour-initiating
populations (heterogeneous) rather than (homogenous) tumour-initiating cells.
PT
Similarly, it is important to separate tumour-initiation and tumour-propagation; this
may not involve the same cell type as the tumour-propagating cell may be a much
AC
CE
differentiated progeny of the tumour-initiating cell. It is conceivable therefore, that
improved therapeutic efficacy may be achieved by targeting both the cell-type(s)
which drives malignant progression as well as that which initiates -and maintains the
stem cell pool of- the tumour.
Since telomerase activity has been detected in almost all advanced brain
tumours, the use of telomerase inhibitors may provide an attractive approach to
therapy. These may be most effective in reducing the risk of relapse by targeting
cancer stem cells. As normal neural stem cells would be expected to have longer
telomeres than brain cancer stem cells, it follows that telomere depletion will be
spared in the normal stem cell pool [103, 104]. There is a practical consideration here
with respect to telomere length: if anti-telomerase approaches rely on telomere
18
ACCEPTED MANUSCRIPT
shortening for efficacious results, then the initial telomere length in the target cancer
cell must be a consideration. This invokes the brain tumour cell of origin once again,
T
as telomere dynamics reflect the mitotic history of highly proliferating cells.
RI
P
Elucidation of telomere length and telomerase activity in subsets of cells within the
tumour hierarchy will enable a more targeted anti-telomerase approach, perhaps in
SC
combination therapy. The identification of a greater array of surface markers will
likely aid this characterisation.
NU
Key cell signalling pathways such as Notch and Sonic Hedgehog, that are
MA
involved in the regulation of self-renewal in both normal and cancer stem cells, also
represent putative molecular targets. A more thorough understanding of the
mechanisms of signal transduction in each context, particularly with respect to gene
ED
expression levels will determine whether disrupting self-renewal pathways in brain
PT
tumour stem cells will spare normal neural stem cell pool function and result in
tumour growth inhibition.
AC
CE
In addition, better therapeutics will rely on a better understanding of the
intrinsic mechanisms of resistance in brain tumour cells. The cancer stem cell
hypothesis predicts that only tumourigenic cells are resistant to therapy. Recent work
studying radiation resistance supports this idea as CD133+ glioma stem cells show
preferential activation of the DNA damage response upon radiation treatment (Bao S
2006). However, whether the same cell type that confers radioresistance is the same
cell type that confers chemoresistance is unknown. Tumour resistance may also arise
if brain tumour stem cells, similar to normal stem cells, generally reside in a quiescent
state outside the cycle of division. Although direct evidence is lacking, if shown to be
correct, this would affect the rationale and approach of drugs targeting telomerase and
self-renewal pathways. In addition, brain tumour stem cells may plausibly have high
19
ACCEPTED MANUSCRIPT
levels of ATP-binding (ABC) drug transporters, which can protect cells from
cytotoxic agents [105]. This may result in the efflux of cytotoxic agents delivered to
T
tumour stem cells. If ABC transporters are shown to contribute to tumour resistance,
RI
P
the next generation of chemotherapy regimes may involve chemosensitisers in
conjunction with agents that alter drug-transporter activity.
SC
Understanding the molecular and genetic basis of the cell type(s) surviving
multi-modal therapy will be crucial to our unwavering hope of a curative response in
NU
brain cancer patients (Figure 3). Is each factor contributing to resistance, both
MA
necessary and sufficient for tumour resistance? Or is coercion required involving all
Concluding Remarks
ED
factors of resistance? If the latter, will targeting one mechanism of resistance suffice?
PT
The brain cancer stem cell paradigm without reference to neurodevelopment
AC
CE
and dysregulated cellular proliferation is perhaps reductionist and oversimplified. The
emerging concept of a close relationship between developmental neurobiology,
telomere/telomerase biology and brain cancer stem cells, encourages a consilient
approach to develop the next generation of brain cancer therapeutic interventions. At
one time disparate, these areas of investigative biology should help determine the
cellular culprit of particular brain cancer types, both with respect to the cell type
where neoplastic transformation occurs and to the molecular events that permit it. If
these tumour-initiating cells are intrinsically resistant to conventional therapeutic
strategies, then a comprehensive characterisation of this mechanism will allow more
effective combination therapy to be developed in the future.
20
ACCEPTED MANUSCRIPT
Figure 1: Stem cell theory for the role of telomeres and telomerase in postnatal
neurogenesis and cancer. Radial glial cells (RG, green box) differentiate from
T
embryonic neuroepithelial progenitors at the beginning of neurogenesis and are
RI
P
distinguished by the expression of astroglial lineage markers in the neonatal brain.
Within the constitutively active germinative layers associated with the anterior part of
SC
the forebrain lateral ventricles (SVZ, shown here) and corresponding to the inner
layer of the dentate gyrus within the hippocampus (SGZ, not shown), radial glia give
NU
rise to adult multipotent neural stem cells (Type B astrocytes, dark blue circle). At
MA
later developmental stages, radial glial cell division can also produce ependymal cells
that constitute the epithelia lining the ventricular system of the brain and spinal cord
(light blue circle). Type B neural stem cells retain markers of radial glia, have high
ED
levels of telomerase activity and may have longer telomeres relative to mature
PT
neurons or glia. During postnatal neurogenesis, neural stem cells differentiate and
give rise to fast-cycling early transit amplifying progenitors (Type C putative
AC
CE
precursors, Orange circle), which can give rise to terminally differentiated mature
glia (astrocytes, oligodendrocytes) or late transit amplifying progenitors (Type A
migrating neuroblasts, Yellow star) that are committed to a mature neuronal fate and
that express neuronal-specific markers. As multipotent neural stem cells differentiate
into mature functional neurons and glia, the capability to leave the stem cell
perivascular niche and regenerate neural tissue is decreased. This is due in
considerable part to telomere shortening across time. Decreased mobilization of stem
cells in aged neural tissue is likely to be consequential of senescence (replicative and
stress-induced) and apoptosis. There is increasing awareness that the growth and
recurrence of brain tumours may be due to a transformed CNS cell type with shared
characteristics to a somatic neural stem cell (self-renewal, differentiation) and
21
ACCEPTED MANUSCRIPT
endowed with tumour-initiating ability. These ‘brain tumour stem cells’ may arise in
vivo from a transformed neural stem cell or from an early neural progenitor that
T
acquires the ability to self-renew through mutation. Abbreviations: SC, stem cell; ET,
RI
P
early transit amplifying progenitor; LT, late transit amplifying progenitor; EP,
ependymal cells; SVZ, subventricular zone; SGZ, subgranular zone; GFAP, glial
SC
fabrillary acidic protein; PSA-NCAM, polysialylated neural cell adhesion molecule;
MAP2, microtubule-associated protein 2; O4, monoclonal antibody that recognizes
NU
the sulfatides; MBP, myelin basic protein. Blue rectangles represent stem
MA
cell/progenitor populations; Red rectangles represent terminally differentiated cell
types.
ED
Figure 2: Proposed role for telomerase activity and telomere stabilization in
PT
brain tumour initiation and propagation. (Normal developmental neurogenesis)
The postnatal brain harbours regions that retain mitotic activity throughout adulthood,
AC
CE
in particular the subventricular zone (SVZ) and subgranular zone (SGZ). Mature
neurons and glia are generated in these regions through cell cycle re-entry and
differentiation of tissue-specific stem cells, giving rise to neural lineage-restricted
progenitors. Faithful cell division is safeguarded in neural progenitors and mature cell
types through intact functional cell cycle and senescence checkpoints. (Dysregulated
developmental neurogenesis). hTERT overexpression, loss of tumour suppressors,
oncogenic insults, checkpoint failure and chromosome instability, increase the
tumourigenic potential of the tissue stem cell, ultimately resulting in a transformed
neural stem cell (‘brain tumour stem cell’). It is unclear whether mutational events at
tumour-initiation are necessary and sufficient to propagate differentiation to a brain
tumour progenitor cell and ‘mature’ malignant neural cell, or whether additional
22
ACCEPTED MANUSCRIPT
mutational events drive progression of the tumour. As the transformed neural stem
cell (aberrantly) differentiates, telomere shortening is likely to occur; however due to
T
hTERT acting on critically shortened telomeres, telomere length in the (mature)
RI
P
malignant neural cell would be expected to be stabilized. Although depicted here as
arising in a neural stem cell, it remains plausible that the tumour cell of origin is a
SC
neural progenitor; in the latter scenario, the progenitor cell may acquire the capacity
NU
to self-renew through mutation, increasing the propensity to initiate a tumour.
MA
Figure 3: Brain tumour cells exhibit intrinsic resistance to current treatment
modalities. The cancer stem cell hypothesis presents an attractive framework upon
which to consider a gestalt shift in treatment protocols for CNS tumours. (Top)
ED
Existing therapeutic regimes successfully achieve a substantial degree of tumour
PT
remission prior to tumour relapse fuelled by a brain tumour population(s) with
intrinsic resistance to therapy. There is increasing evidence that the resistant cell type
AC
CE
is a neural stem-like cell with dysregulated self-renewal or neural progenitor which
has acquired aberrant self-renewal capabilities through mutation. Mechanisms of
tumour stem cell resistance are largely based on presumptions of shared properties
with neural stem cells; however evidence is beginning to emerge demonstrating that
brain tumour stem/progenitor cells are intrinsically protected, relative to the bulk
tumour (yellow box). (Bottom) It is likely a new therapeutic paradigm will include
chemosensitizers and cyotoxic agents that target the dysregulated pathways of neural
stem/progenitor cells and mechanisms that mediate resistance, such as ABC-drug
transporters. Characterisation of telomere length/structure and telomerase activation
levels in brain tumour sub-compartments will reveal whether anti-telomerase therapy
will be an attractive proposition for CNS tumours. To accomplish a durable response
23
ACCEPTED MANUSCRIPT
within this new therapeutic paradigm, conventional cytotoxic agents would be used to
de-bulk the tumour mass in a combinatorial strategy with agents for targeted therapy.
T
Nevertheless, a number of challenging, pertinent and often controversial biological
RI
P
questions surround the cancer stem cell paradigm (see section on The Brain Tumour
AC
CE
PT
ED
MA
NU
SC
Stem Cell Paradigm).
24
ACCEPTED MANUSCRIPT
References:
[1]
P. Kleihues, D.N. Louis, B.W. Scheithauer, L.B. Rorke, G. Reifenberger, P.C.
T
Burger, W.K. Cavenee, The WHO classification of tumors of the nervous
[2]
RI
P
system, J Neuropathol Exp Neurol 61 (2002) 215-225; discussion 226-219.
R.J. Packer, Medulloblastoma, J Neurosurg 103 (2005) 299-300; discussion
[3]
SC
300-291.
A. Behin, K. Hoang-Xuan, A.F. Carpentier, J.Y. Delattre, Primary brain
E.C. Holland, Glioblastoma multiforme: the terminator, Proc Natl Acad Sci U
S A 97 (2000) 6242-6244.
F. Nejat, M. El Khashab, J.T. Rutka, Initial management of childhood brain
ED
[5]
MA
[4]
NU
tumours in adults, Lancet 361 (2003) 323-331.
tumors: neurosurgical considerations, J Child Neurol 23 (2008) 1136-1148.
R. Ruda, M. Gilbert, R. Soffietti, Ependymomas of the adult: molecular
PT
[6]
biology and treatment, Curr Opin Neurol 21 (2008) 754-761.
N. Spassky, F.T. Merkle, N. Flames, A.D. Tramontin, J.M. Garcia-Verdugo,
AC
CE
[7]
A. Alvarez-Buylla, Adult ependymal cells are postmitotic and are derived
from radial glial cells during embryogenesis, J Neurosci 25 (2005) 10-18.
[8]
T. Mori, A. Buffo, M. Gotz, The novel roles of glial cells revisited: the
contribution of radial glia and astrocytes to neurogenesis, Curr Top Dev Biol
69 (2005) 67-99.
[9]
A. Alvarez-Buylla, J.M. Garcia-Verdugo, Neurogenesis in adult subventricular
zone, J Neurosci 22 (2002) 629-634.
[10]
F.T. Merkle, A. Alvarez-Buylla, Neural stem cells in mammalian
development, Curr Opin Cell Biol 18 (2006) 704-709.
25
ACCEPTED MANUSCRIPT
[11]
P. Peretto, A. Merighi, A. Fasolo, L. Bonfanti, The subependymal layer in
rodents: a site of structural plasticity and cell migration in the adult
C.S. Zhao, L. Overstreet-Wadiche, Integration of adult generated neurons
RI
P
[12]
T
mammalian brain, Brain Res Bull 49 (1999) 221-243.
during epileptogenesis, Epilepsia 49 Suppl 5 (2008) 3-12.
F.H. Gage, Mammalian neural stem cells, Science 287 (2000) 1433-1438.
[14]
C.G. Gross, Neurogenesis in the adult brain: death of a dogma, Nat Rev
SC
[13]
T. de Lange, Shelterin: the protein complex that shapes and safeguards human
MA
[15]
NU
Neurosci 1 (2000) 67-73.
telomeres, Genes Dev 19 (2005) 2100-2110.
[16]
L. Hayflick, P.S. Moorhead, The serial cultivation of human diploid cell
W.E. Wright, J.W. Shay, Telomere biology in aging and cancer, J Am Geriatr
PT
[17]
ED
strains, Exp Cell Res 25 (1961) 585-621.
Soc 53 (2005) S292-294.
C.W. Greider, E.H. Blackburn, A telomeric sequence in the RNA of
AC
CE
[18]
Tetrahymena telomerase required for telomere repeat synthesis, Nature 337
(1989) 331-337.
[19]
S. Horiguchi, J. Takahashi, Y. Kishi, A. Morizane, Y. Okamoto, M. Koyanagi,
M. Tsuji, K. Tashiro, T. Honjo, S. Fujii, N. Hashimoto, Neural precursor cells
derived from human embryonic brain retain regional specificity, J Neurosci
Res 75 (2004) 817-824.
[20]
D. Moses, Y. Teper, I. Gantois, D.I. Finkelstein, M.K. Horne, J. Drago,
Murine embryonic EGF-responsive ventral mesencephalic neurospheres
display distinct regional specification and promote survival of dopaminergic
neurons, Exp Neurol 199 (2006) 209-221.
26
ACCEPTED MANUSCRIPT
[21]
G.L. Caporaso, D.A. Lim, A. Alvarez-Buylla, M.V. Chao, Telomerase activity
in the subventricular zone of adult mice, Mol Cell Neurosci 23 (2003) 693-
B.K. DeMasters, N. Markham, K.O. Lillehei, K.R. Shroyer, Differential
RI
P
[22]
T
702.
telomerase expression in human primary intracranial tumors, Am J Clin Pathol
[23]
SC
107 (1997) 548-554.
D. Didiano, T. Shalaby, D. Lang, M.A. Grotzer, Telomere maintenance in
S. Hiraga, T. Ohnishi, S. Izumoto, E. Miyahara, Y. Kanemura, H. Matsumura,
MA
[24]
NU
childhood primitive neuroectodermal brain tumors, Neuro Oncol 6 (2004) 1-8.
N. Arita, Telomerase activity and alterations in telomere length in human
brain tumors, Cancer Res 58 (1998) 2117-2125.
L.A. Langford, M.A. Piatyszek, R. Xu, S.C. Schold, Jr., J.W. Shay,
ED
[25]
[26]
PT
Telomerase activity in human brain tumours, Lancet 346 (1995) 1267-1268.
L. Ridley, R. Rahman, M.A. Brundler, D. Ellison, J. Lowe, K. Robson, E.
AC
CE
Prebble, I. Luckett, R.J. Gilbertson, S. Parkes, V. Rand, B. Coyle, R.G.
Grundy, Multifactorial analysis of predictors of outcome in pediatric
intracranial ependymoma, Neuro Oncol 10 (2008) 675-689.
[27]
T. Sano, A. Asai, K. Mishima, T. Fujimaki, T. Kirino, Telomerase activity in
144 brain tumours, Br J Cancer 77 (1998) 1633-1637.
[28]
A.J. Becker, C.E. Mc, J.E. Till, Cytological demonstration of the clonal nature
of spleen colonies derived from transplanted mouse marrow cells, Nature 197
(1963) 452-454.
[29]
J.E. Till, C.E. Mc, A direct measurement of the radiation sensitivity of normal
mouse bone marrow cells, Radiat Res 14 (1961) 213-222.
27
ACCEPTED MANUSCRIPT
[30]
D. Bonnet, J.E. Dick, Human acute myeloid leukemia is organized as a
hierarchy that originates from a primitive hematopoietic cell, Nat Med 3
T. Lapidot, C. Sirard, J. Vormoor, B. Murdoch, T. Hoang, J. Caceres-Cortes,
RI
P
[31]
T
(1997) 730-737.
M. Minden, B. Paterson, M.A. Caligiuri, J.E. Dick, A cell initiating human
SC
acute myeloid leukaemia after transplantation into SCID mice, Nature 367
(1994) 645-648.
M. Al-Hajj, M.S. Wicha, A. Benito-Hernandez, S.J. Morrison, M.F. Clarke,
NU
[32]
MA
Prospective identification of tumorigenic breast cancer cells, Proc Natl Acad
Sci U S A 100 (2003) 3983-3988.
[33]
A.T. Collins, P.A. Berry, C. Hyde, M.J. Stower, N.J. Maitland, Prospective
[34]
PT
10946-10951.
ED
identification of tumorigenic prostate cancer stem cells, Cancer Res 65 (2005)
D. Fang, T.K. Nguyen, K. Leishear, R. Finko, A.N. Kulp, S. Hotz, P.A. Van
AC
CE
Belle, X. Xu, D.E. Elder, M. Herlyn, A tumorigenic subpopulation with stem
cell properties in melanomas, Cancer Res 65 (2005) 9328-9337.
[35]
C.A. O'Brien, A. Pollett, S. Gallinger, J.E. Dick, A human colon cancer cell
capable of initiating tumour growth in immunodeficient mice, Nature 445
(2007) 106-110.
[36]
D. Pinto, H. Clevers, Wnt, stem cells and cancer in the intestine, Biol Cell 97
(2005) 185-196.
[37]
R. Galli, E. Binda, U. Orfanelli, B. Cipelletti, A. Gritti, S. De Vitis, R. Fiocco,
C. Foroni, F. Dimeco, A. Vescovi, Isolation and characterization of
tumorigenic, stem-like neural precursors from human glioblastoma, Cancer
Res 64 (2004) 7011-7021.
28
ACCEPTED MANUSCRIPT
[38]
H.D. Hemmati, I. Nakano, J.A. Lazareff, M. Masterman-Smith, D.H.
Geschwind, M. Bronner-Fraser, H.I. Kornblum, Cancerous stem cells can
T
arise from pediatric brain tumors, Proc Natl Acad Sci U S A 100 (2003)
[39]
RI
P
15178-15183.
S.K. Singh, I.D. Clarke, M. Terasaki, V.E. Bonn, C. Hawkins, J. Squire, P.B.
SC
Dirks, Identification of a cancer stem cell in human brain tumors, Cancer Res
63 (2003) 5821-5828.
M.D. Taylor, H. Poppleton, C. Fuller, X. Su, Y. Liu, P. Jensen, S. Magdaleno,
NU
[40]
MA
J. Dalton, C. Calabrese, J. Board, T. Macdonald, J. Rutka, A. Guha, A. Gajjar,
T. Curran, R.J. Gilbertson, Radial glia cells are candidate stem cells of
ependymoma, Cancer Cell 8 (2005) 323-335.
S.K. Singh, C. Hawkins, I.D. Clarke, J.A. Squire, J. Bayani, T. Hide, R.M.
ED
[41]
PT
Henkelman, M.D. Cusimano, P.B. Dirks, Identification of human brain
tumour initiating cells, Nature 432 (2004) 396-401.
S.C. Noctor, V. Martinez-Cerdeno, A.R. Kriegstein, Neural stem and
AC
CE
[42]
progenitor cells in cortical development, Novartis Found Symp 288 (2007) 5973; discussion 73-58, 96-58.
[43]
R. Nat, M. Nilbratt, S. Narkilahti, B. Winblad, O. Hovatta, A. Nordberg,
Neurogenic neuroepithelial and radial glial cells generated from six human
embryonic stem cell lines in serum-free suspension and adherent cultures, Glia
55 (2007) 385-399.
[44]
P. Levitt, M.L. Cooper, P. Rakic, Coexistence of neuronal and glial precursor
cells in the cerebral ventricular zone of the fetal monkey: an ultrastructural
immunoperoxidase analysis, J Neurosci 1 (1981) 27-39.
29
ACCEPTED MANUSCRIPT
[45]
D.E. Schmechel, P. Rakic, Arrested proliferation of radial glial cells during
midgestation in rhesus monkey, Nature 277 (1979) 303-305.
B.H. Choi, On the origin of oligodendrocytes, Yonsei Med J 26 (1985) 143-
T
[46]
[47]
RI
P
149.
S.M. LeVine, J.E. Goldman, Spatial and temporal patterns of oligodendrocyte
SC
differentiation in rat cerebrum and cerebellum, J Comp Neurol 277 (1988)
441-455.
E. Hartfuss, R. Galli, N. Heins, M. Gotz, Characterization of CNS precursor
NU
[48]
[49]
MA
subtypes and radial glia, Dev Biol 229 (2001) 15-30.
S.C. Noctor, A.C. Flint, T.A. Weissman, R.S. Dammerman, A.R. Kriegstein,
Neurons derived from radial glial cells establish radial units in neocortex,
S.X. Wu, S. Goebbels, K. Nakamura, K. Kometani, N. Minato, T. Kaneko,
PT
[50]
ED
Nature 409 (2001) 714-720.
K.A. Nave, N. Tamamaki, Pyramidal neurons of upper cortical layers
AC
CE
generated by NEX-positive progenitor cells in the subventricular zone, Proc
Natl Acad Sci U S A 102 (2005) 17172-17177.
[51]
T.E. Anthony, C. Klein, G. Fishell, N. Heintz, Radial glia serve as neuronal
progenitors in all regions of the central nervous system, Neuron 41 (2004)
881-890.
[52]
M. Gotz, Y.A. Barde, Radial glial cells defined and major intermediates
between embryonic stem cells and CNS neurons, Neuron 46 (2005) 369-372.
[53]
L. Bonfanti, P. Peretto, Radial glial origin of the adult neural stem cells in the
subventricular zone, Prog Neurobiol 83 (2007) 24-36.
30
ACCEPTED MANUSCRIPT
[54]
P.S. Eriksson, E. Perfilieva, T. Bjork-Eriksson, A.M. Alborn, C. Nordborg,
D.A. Peterson, F.H. Gage, Neurogenesis in the adult human hippocampus, Nat
N.S. Roy, S. Wang, L. Jiang, J. Kang, A. Benraiss, C. Harrison-Restelli, R.A.
RI
P
[55]
T
Med 4 (1998) 1313-1317.
Fraser, W.T. Couldwell, A. Kawaguchi, H. Okano, M. Nedergaard, S.A.
SC
Goldman, In vitro neurogenesis by progenitor cells isolated from the adult
human hippocampus, Nat Med 6 (2000) 271-277.
O. Marin, J.L. Rubenstein, Cell migration in the forebrain, Annu Rev Neurosci
NU
[56]
[57]
MA
26 (2003) 441-483.
F. Doetsch, J.M. Garcia-Verdugo, A. Alvarez-Buylla, Regeneration of a
germinal layer in the adult mammalian brain, Proc Natl Acad Sci U S A 96
A.L. Vescovi, R. Galli, B.A. Reynolds, Brain tumour stem cells, Nat Rev
PT
[58]
ED
(1999) 11619-11624.
Cancer 6 (2006) 425-436.
P. Malatesta, I. Appolloni, F. Calzolari, Radial glia and neural stem cells, Cell
AC
CE
[59]
Tissue Res 331 (2008) 165-178.
[60]
F.T. Merkle, A.D. Tramontin, J.M. Garcia-Verdugo, A. Alvarez-Buylla,
Radial glia give rise to adult neural stem cells in the subventricular zone, Proc
Natl Acad Sci U S A 101 (2004) 17528-17532.
[61]
A.D. Garcia, N.B. Doan, T. Imura, T.G. Bush, M.V. Sofroniew, GFAPexpressing progenitors are the principal source of constitutive neurogenesis in
adult mouse forebrain, Nat Neurosci 7 (2004) 1233-1241.
[62]
T. Imura, H.I. Kornblum, M.V. Sofroniew, The predominant neural stem cell
isolated from postnatal and adult forebrain but not early embryonic forebrain
expresses GFAP, J Neurosci 23 (2003) 2824-2832.
31
ACCEPTED MANUSCRIPT
[63]
B.A. Reynolds, S. Weiss, Generation of neurons and astrocytes from isolated
cells of the adult mammalian central nervous system, Science 255 (1992)
F. Doetsch, L. Petreanu, I. Caille, J.M. Garcia-Verdugo, A. Alvarez-Buylla,
RI
P
[64]
T
1707-1710.
EGF converts transit-amplifying neurogenic precursors in the adult brain into
[65]
SC
multipotent stem cells, Neuron 36 (2002) 1021-1034.
F. Doetsch, The glial identity of neural stem cells, Nat Neurosci 6 (2003)
N. Abramova, C. Charniga, S.K. Goderie, S. Temple, Stage-specific changes
MA
[66]
NU
1127-1134.
in gene expression in acutely isolated mouse CNS progenitor cells, Dev Biol
283 (2005) 269-281.
Q. Shen, Y. Wang, J.T. Dimos, C.A. Fasano, T.N. Phoenix, I.R. Lemischka,
ED
[67]
PT
N.B. Ivanova, S. Stifani, E.E. Morrisey, S. Temple, The timing of cortical
neurogenesis is encoded within lineages of individual progenitor cells, Nat
[68]
AC
CE
Neurosci 9 (2006) 743-751.
P. Dell'albani, Stem cell markers in gliomas, Neurochem Res 33 (2008) 24072415.
[69]
P. Taupin, J. Ray, W.H. Fischer, S.T. Suhr, K. Hakansson, A. Grubb, F.H.
Gage, FGF-2-responsive neural stem cell proliferation requires CCg, a novel
autocrine/paracrine cofactor, Neuron 28 (2000) 385-397.
[70]
V. Tropepe, C.G. Craig, C.M. Morshead, D. van der Kooy, Transforming
growth factor-alpha null and senescent mice show decreased neural progenitor
cell proliferation in the forebrain subependyma, J Neurosci 17 (1997) 78507859.
32
ACCEPTED MANUSCRIPT
[71]
M. Groszer, R. Erickson, D.D. Scripture-Adams, R. Lesche, A. Trumpp, J.A.
Zack, H.I. Kornblum, X. Liu, H. Wu, Negative regulation of neural
T
stem/progenitor cell proliferation by the Pten tumor suppressor gene in vivo,
[72]
RI
P
Science 294 (2001) 2186-2189.
L. Martin-Rivera, E. Herrera, J.P. Albar, M.A. Blasco, Expression of mouse
SC
telomerase catalytic subunit in embryos and adult tissues, Proc Natl Acad Sci
U S A 95 (1998) 10471-10476.
W. Klapper, T. Shin, M.P. Mattson, Differential regulation of telomerase
NU
[73]
Res 64 (2001) 252-260.
[74]
MA
activity and TERT expression during brain development in mice, J Neurosci
T. Ostenfeld, M.A. Caldwell, K.R. Prowse, M.H. Linskens, E. Jauniaux, C.N.
ED
Svendsen, Human neural precursor cells express low levels of telomerase in
PT
vitro and show diminishing cell proliferation with extensive axonal outgrowth
following transplantation, Exp Neurol 164 (2000) 215-226.
K.R. Prowse, C.W. Greider, Developmental and tissue-specific regulation of
AC
CE
[75]
mouse telomerase and telomere length, Proc Natl Acad Sci U S A 92 (1995)
4818-4822.
[76]
L.S. Wright, K.R. Prowse, K. Wallace, M.H. Linskens, C.N. Svendsen,
Human progenitor cells isolated from the developing cortex undergo
decreased neurogenesis and eventual senescence following expansion in vitro,
Exp Cell Res 312 (2006) 2107-2120.
[77]
T.H. Brummendorf, S. Balabanov, Telomere length dynamics in normal
hematopoiesis and in disease states characterized by increased stem cell
turnover, Leukemia 20 (2006) 1706-1716.
33
ACCEPTED MANUSCRIPT
[78]
E. Hiyama, L. Gollahon, T. Kataoka, K. Kuroi, T. Yokoyama, A.F. Gazdar, K.
Hiyama, M.A. Piatyszek, J.W. Shay, Telomerase activity in human breast
C. Moriscot, F. de Fraipont, M.J. Richard, M. Marchand, P. Savatier, D.
RI
P
[79]
T
tumors, J Natl Cancer Inst 88 (1996) 116-122.
Bosco, M. Favrot, P.Y. Benhamou, Human bone marrow mesenchymal stem
SC
cells can express insulin and key transcription factors of the endocrine
pancreas developmental pathway upon genetic and/or microenvironmental
M. Sugihara, K. Ohshima, H. Nakamura, J. Suzumiya, Y. Nakayama, M.
MA
[80]
NU
manipulation in vitro, Stem Cells 23 (2005) 594-603.
Kanda, S. Haraoka, M. Kikuchi, Decreased expression of telomeraseassociated RNAs in the proliferation of stem cells in comparison with
A. Cheng, K. Shin-ya, R. Wan, S.C. Tang, T. Miura, H. Tang, R. Khatri, M.
PT
[81]
ED
continuous expression in malignant tumors, Int J Oncol 15 (1999) 1075-1080.
Gleichman, X. Ouyang, D. Liu, H.R. Park, J.Y. Chiang, M.P. Mattson,
AC
CE
Telomere protection mechanisms change during neurogenesis and neuronal
maturation: newly generated neurons are hypersensitive to telomere and DNA
damage, J Neurosci 27 (2007) 3722-3733.
[82]
S. Ferron, H. Mira, S. Franco, M. Cano-Jaimez, E. Bellmunt, C. Ramirez, I.
Farinas, M.A. Blasco, Telomere shortening and chromosomal instability
abrogates proliferation of adult but not embryonic neural stem cells,
Development 131 (2004) 4059-4070.
[83]
J.W. Shay, W.E. Wright, H. Werbin, Defining the molecular mechanisms of
human cell immortalization, Biochim Biophys Acta 1072 (1991) 1-7.
[84]
T.M. Bryan, T.R. Cech, Telomerase and the maintenance of chromosome
ends, Curr Opin Cell Biol 11 (1999) 318-324.
34
ACCEPTED MANUSCRIPT
[85]
J.W. Shay, S. Bacchetti, A survey of telomerase activity in human cancer, Eur
J Cancer 33 (1997) 787-791.
M.P. Mattson, W. Klapper, Emerging roles for telomerase in neuronal
T
[86]
[87]
RI
P
development and apoptosis, J Neurosci Res 63 (2001) 1-9.
C.M. Counter, A.A. Avilion, C.E. LeFeuvre, N.G. Stewart, C.W. Greider,
SC
C.B. Harley, S. Bacchetti, Telomere shortening associated with chromosome
EMBO J 11 (1992) 1921-1929.
M. Armanios, C.W. Greider, Telomerase and cancer stem cells, Cold Spring
MA
[88]
NU
instability is arrested in immortal cells which express telomerase activity,
Harb Symp Quant Biol 70 (2005) 205-208.
[89]
D. Beier, P. Hau, M. Proescholdt, A. Lohmeier, J. Wischhusen, P.J. Oefner, L.
ED
Aigner, A. Brawanski, U. Bogdahn, C.P. Beier, CD133(+) and CD133(-)
cancer
stem
cells
show
differential
growth
PT
glioblastoma-derived
characteristics and molecular profiles, Cancer Res 67 (2007) 4010-4015.
J. Wang, P.O. Sakariassen, O. Tsinkalovsky, H. Immervoll, S.O. Boe, A.
AC
CE
[90]
Svendsen, L. Prestegarden, G. Rosland, F. Thorsen, L. Stuhr, A. Molven, R.
Bjerkvig, P.O. Enger, CD133 negative glioma cells form tumors in nude rats
and give rise to CD133 positive cells, Int J Cancer 122 (2008) 761-768.
[91]
I. Maillard, W.S. Pear, Notch and cancer: best to avoid the ups and downs,
Cancer Cell 3 (2003) 203-205.
[92]
F. Radtke, K. Raj, The role of Notch in tumorigenesis: oncogene or tumour
suppressor?, Nat Rev Cancer 3 (2003) 756-767.
[93]
K. Lai, B.K. Kaspar, F.H. Gage, D.V. Schaffer, Sonic hedgehog regulates
adult neural progenitor proliferation in vitro and in vivo, Nat Neurosci 6
(2003) 21-27.
35
ACCEPTED MANUSCRIPT
[94]
R. Machold, S. Hayashi, M. Rutlin, M.D. Muzumdar, S. Nery, J.G. Corbin, A.
Gritli-Linde, T. Dellovade, J.A. Porter, L.L. Rubin, H. Dudek, A.P.
T
McMahon, G. Fishell, Sonic hedgehog is required for progenitor cell
[95]
RI
P
maintenance in telencephalic stem cell niches, Neuron 39 (2003) 937-950.
S. Hitoshi, T. Alexson, V. Tropepe, D. Donoviel, A.J. Elia, J.S. Nye, R.A.
SC
Conlon, T.W. Mak, A. Bernstein, D. van der Kooy, Notch pathway molecules
are essential for the maintenance, but not the generation, of mammalian neural
X. Fan, I. Mikolaenko, I. Elhassan, X. Ni, Y. Wang, D. Ball, D.J. Brat, A.
MA
[96]
NU
stem cells, Genes Dev 16 (2002) 846-858.
Perry, C.G. Eberhart, Notch1 and notch2 have opposite effects on embryonal
brain tumor growth, Cancer Res 64 (2004) 7787-7793.
A.R. Hallahan, J.I. Pritchard, S. Hansen, M. Benson, J. Stoeck, B.A. Hatton,
ED
[97]
PT
T.L. Russell, R.G. Ellenbogen, I.D. Bernstein, P.A. Beachy, J.M. Olson, The
SmoA1 mouse model reveals that notch signaling is critical for the growth and
AC
CE
survival of sonic hedgehog-induced medulloblastomas, Cancer Res 64 (2004)
7794-7800.
[98]
V. Palma, D.A. Lim, N. Dahmane, P. Sanchez, T.C. Brionne, C.D. Herzberg,
Y. Gitton, A. Carleton, A. Alvarez-Buylla, A. Ruiz i Altaba, Sonic hedgehog
controls stem cell behavior in the postnatal and adult brain, Development 132
(2005) 335-344.
[99]
L.V. Goodrich, L. Milenkovic, K.M. Higgins, M.P. Scott, Altered neural cell
fates and medulloblastoma in mouse patched mutants, Science 277 (1997)
1109-1113.
[100] S.L. Pomeroy, L.M. Sturla, Molecular biology of medulloblastoma therapy,
Pediatr Neurosurg 39 (2003) 299-304.
36
ACCEPTED MANUSCRIPT
[101] C. Calabrese, H. Poppleton, M. Kocak, T.L. Hogg, C. Fuller, B. Hamner, E.Y.
Oh, M.W. Gaber, D. Finklestein, M. Allen, A. Frank, I.T. Bayazitov, S.S.
RI
P
brain tumor stem cells, Cancer Cell 11 (2007) 69-82.
T
Zakharenko, A. Gajjar, A. Davidoff, R.J. Gilbertson, A perivascular niche for
[102] S. Bao, Q. Wu, S. Sathornsumetee, Y. Hao, Z. Li, A.B. Hjelmeland, Q. Shi,
SC
R.E. McLendon, D.D. Bigner, J.N. Rich, Stem cell-like glioma cells promote
tumor angiogenesis through vascular endothelial growth factor, Cancer Res 66
NU
(2006) 7843-7848.
MA
[103] J.W. Shay, Telomerase in cancer: diagnostic, prognostic, and therapeutic
implications, Cancer J Sci Am 4 Suppl 1 (1998) S26-34.
[104] L.K. White, W.E. Wright, J.W. Shay, Telomerase inhibitors, Trends
ED
Biotechnol 19 (2001) 114-120.
PT
[105] M. Dean, T. Fojo, S. Bates, Tumour stem cells and drug resistance, Nat Rev
AC
CE
Cancer 5 (2005) 275-284.
37
AC
CE
PT
ED
MA
NU
SC
RI
P
T
ACCEPTED MANUSCRIPT
38
AC
CE
PT
ED
MA
NU
SC
RI
P
T
ACCEPTED MANUSCRIPT
39
AC
CE
PT
ED
MA
NU
SC
RI
P
T
ACCEPTED MANUSCRIPT
40