Cancer stem cells in nervous system tumorsnecessary是什么意思
Sheila K Singh1,2,Ian D Clarke1,2,Takuichiro Hide1,2and Peter B Dirks*,1,2
1The Arthur and Sonia Labatt Brain Tumor Rearch Centre,The Hospital for Sick Children,Toronto,Canada;2Program
in Developmental Biology,Department of Laboratory Medicine and Pathobiology,Division of Neurosurgery,University of Toronto, Toronto,Canada
Most current rearch on human brain tumors is focud on the molecular and cellular analysis of the bulk tumor mass.However,evidence in leukemia and more recently in solid tumors such as breast cancer suggests that the tumor cell population is heterogeneous with respect to prolifera-tion and differentiation.Recently,veral groups have described the existence of a cancer stem cell population in human brain tumors of different phenotypes from both children and adults.Thefinding of brain tumor stem cells (BTSCs)has been made by applying the principles for cell culture and analysis of normal neural stem cells(NSCs)to brain tumor cell populations and by identification of cell surface markers that allow for isolation of distinct tumor cell populations that can then be studied in vitro and in vivo.A population of brain tumor cells can be enriched for BTSCs by cell sorting of dissociated suspensions of
tumor cells for the NSC marker CD133.The CD133þcells, which also expresd the NSC marker nestin,but not differentiated neural lineage markers,reprent a minor-ity fraction of the entire brain tumor cell population,and exclusively generate clonal tumor spheres in suspension culture and exhibit incread lf-renewal capacity. BTSCs can be induced to differentiate in vitro into tumor cells that phenotypically rembled the tumor from the patient.Here,we discuss the evidence for and implications of the discovery of a cancer stem cell in human brain tumors.The identification of a BTSC provides a powerful tool to investigate the tumorigenic process in the central nervous system and to develop therapies targeted to the BTSC.Specific genetic and molecular analys of the BTSC will further our understanding of the mechanisms of brain tumor growth,reinforcing parallels between normal neurogenesis and brain tumorigenesis. Oncogene(2004)23,7267–7273.doi:10.1207946 Keywords:brain tumor stem cell;neural stem cell;flow cytometry;CD133爱情大魔咒歌词
Introduction
Brain tumors are typically comprid of morphologi-cally diver cells that express a variety of neural lineage markers.Study of brain tumors by traditional histo-pathology has only yielded a limited amount of knowl-edge of the clinical behavior of the tumor.It is recognized that tumors with vastly diff
erent histology have a different prognosis,but often brain tumors that share similar morphology and phenotype can have a very different prognosis and respon to treatment. Brain tumors of the same histologic type can also have a very different behavior in patients of different ages. Although major advances have been made in the understanding of the molecular genetic alterations of some types of brain tumors(Maher et al.,2001; Wechsler-Reya and Scott,2001;Zhu and Parada, 2002;Gilbertson,2004),particularly medulloblastomas and malignant gliomas,and some of the identified alterations are now beginning to guide treatment,it is not clear whether all the tumor cells are equivalent in their ability to maintain the growth of the tumor.Until recently,we lacked a functional assay of the brain tumor cells that could determine which of the morphologically diver tumor cells are capable of maintaining the growth of the tumor.The cancer stem cell hypothesis suggests that not all the cells in the tumor have the same ability to proliferate and maintain the growth of the tumor.Only a relatively small fraction of cells in the tumor,termed cancer stem cells,posss the ability to proliferate and lf-renew extensively.Most of the tumor cells lo the ability to proliferate and lf-renew and they differentiate into tumor cells that become the phenotypic signature of the tumor.Finding the key cells in the brain tumor population that are able to maintain the tumor will give insight into the mechanism of brain tumorigenesis and will allow us to trace back to the cell of origin in the normal brain.dead and gone
Relationship between neural stem cells(NSCs)
and cancer stem cells
Stem cells are functionally defined as lf-renewing, multipotent cells that exhibit multilineage differentiation (Fuchs and Segre,2000;Weissman,2000;Reya et al., 2001).Somatic stem cells are thought to lf-renew to generate all the mature cell types of a particular tissue through proliferative expansion of progenitor cells followed by differentiation into mature cell types.So far,rigorous identification and isolation of tissue-specific
*Correspondence:PB Dirks,Hospital for Sick Children,Suite1504, 555University Ave.,Toronto,Ontario,Canada M5G1X8;
E-mail:
peter.dirks@sickkids.ca Oncogene(2004)23,7267–7273
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stem cells has been prospectively accomplished in only a few organ systems (Reya et al .,2001).The discovery that multipotential,lf-renewing NSCs exist throughout life in the adult mammalian brain has only re-emerged in the past decade (Gage,2000;Temple,2001;Gage,2002),reflecting a rediscovery of 1960s evidence that suggested that neurogenesis was occurring in the adult brain (Gross,2000).The neurosphere culture system and analysis first ud by Reynolds and Weiss (1992)to identify NSCs has permitted in vitro characterization of NSCs,but in a retrospective manner,as the multi-potential floating clusters of cells (the neurospheres)are inferred to have been derived from clonal expansion of a single NSC.Prospective study of this cell has been previously limited by lack of cell surface markers necessary for its isolation,until recent reports of NSC enrichment using antibodies to the cell surface protein CD133(Uchida et al .,2000).Also,neural crest stem cells can be prospectively bad on cell surface markers (Morrison et al .,1999).
The concept of the cancer stem cell aro from the obrvation of striking similarities between the lf-renewal mechanisms of stem cells and cancer cells (Reya et al .,2001;Pardal et al .,2003).In hematologic malignancies such as leukemia (Blair et al .,1997;Bonnet and Dick,1997)and multiple myeloma (Matsui et al .,2004),and in solid tumors such as breast cancer (Al-Hajj et al .,2003),rare cells were isolated with a remarkable potential for lf-renewal,and the cells alone were found to d
rive the formation and growth of tumors.Since normal somatic stem cells must lf-renew and maintain a relative balance between lf-renewal and differentiation,cancer can be contextualized as a dia of unregulated lf-renewal (Reya et al .,2001).NSCs posss a lf-renewal machinery that is primed and can be harnesd to create a cancer cell,and their longevity targets them for the accumulation of genetic mutations.Therefore,NSCs and also probably their cloly related downstream rapidly proliferating progenitors should be further investigated as possible targets of transforma-tion in the formation of brain tumors.mona lisa
Do brain tumors ari from a transformed NSC:what is the evidence?
大雁塔 英文
The traditional hypothesis has been that brain tumors ari from the dedifferentiation of a mature brain cell in respon to genetic alterations.This hypothesis prevailed becau it was felt that the postnatal brain had no proliferation.It has also been considered for some time that brain tumors may ari from a transformation event in a resident immature brain cell.With the discovery of adult NSCs (early 1990s)(Reynolds and Weiss,1992;Kilpatrick and Bartlett,1993;Lois and Alvarez-Buylla,1993),it became conceivable that a normal NSC or progenitor cell that resides in the brain may be the target for transformation leading to a brain tumor.
The potential that an NSC may be transformed into a brain tumor has been considered bad on the
obrvations of tumors occurring in the brain’s putative stem cell or proliferative zones in a number of older experimental systems.The ‘subependymal plate’or subventricular zone was suspected many years ago to contain ‘embryonal rests’that were thought to give ri to brain tumors,particularly tho occurring in the brain adjacent to the walls of the ventricular system (Globus and Kuhlenbeck,1944).This idea was then considered further after the identification of mitotic cells in the subependymal regions of adult rodents and primates in the 1960s (Smart,1961;Altman,1963;Lewis,1968).In the 1970s,periventricular tumors were then demonstrated to occur in the subventricular region after intraventricular inoculation with avian sarcoma virus,with a much higher rate of tumors occurring in neonatal rats versus adult rats (Copeland et al .,1975;Copeland and Bigner,1977;Vick et al .,1977).The incidence of brain tumors of a variety of types in mice inoculated with a pellet of carcinogen was much greater if the pellet was placed in the subventricular region versus the peripheral cortex (Hopewell and Wright,1969).A single do of ethylnitrosurea administration to pregnant rats also induced periventricular tumors in the offspring (Koestner et al .,1971).
The primitive cytoarchitecture and embryonic fea-tures of many malignant brain tumors have also suggested that the tumors may ari from transformed neural stem or progenitor cells (Bailey and
Cushing,1926;Rubinstein,1972).Childhood medulloblastoma,in particular,and other solid pediatric malignancies such as retinblastoma and neuroblastoma,have been considered as developmental process gone awry,with suspected origin from progenitors or ‘blast cells’in the tissue of origin (Maris and Denny,2002).Since medulloblastoma cells often remble proliferating external granule cells of the postnatal cerebellum,this tumor has long been thought to ari from a primitive pluripotential cerebellar stem cell or precursor cell (Kadin et al .,1970;Rubinstein,1972).looppa
The cell origin of brain tumors remains a subject of ongoing debate in the current scientific literature:do brain tumors ari from the dedifferentiation of a normal brain cell or from the transformation of a normal NSC or progenitor cell (Figure 1)?Several further lines of evidence suggest that brain tumors ari from the transformation of a normal NSC or progenitor cell,all of which rely on the recognition of the many functional and genetic similarities shared by somatic stem cells and cancer cells (Pardal et al .,2003).Histological studies of brain tumors note the abnce of expression of differentiated cell markers in primitive tumors,as well as the prence of immunostaining for nestin (Dahlstrand et al .,1992;Tohyama et al .,1992),a marker of NSCs (Lendahl et al .,1990).Brain tumors can be very heterogeneous,being comprid of cells expressing phenotypes of more than one neural lineage,implicating a multipotential cell of origin.Microarray anal
ysis of human medulloblastomas suggests a simi-larity of gene expression with normal developing brain cells (Pomeroy et al .,2002).In a recent study,transfection of a medulloblastoma cell line
with
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transcription factors specified a factor-dependent adop-tion of either neuronal or glial cell fates (Buzanska et al .,2001).Finally,mutations that dysregulate the pathways that control normal stem cell lf-renewal (such as the sonic hedgehog pathway)cau brain tumors,empha-sizing the mechanistic similarities between normal stem cell and cancer cell lf-renewal (Dahmane et al .,2001;Ruiz i Altaba et al .,2002;Pardal et al .,2003).
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Recent experiments in mice further suggest that neural progenitors may be transformed into brain tumors.By investigating the mechanisms underlying gliomagenesis,Holland and co-workers have found that undifferentiated neural precursor cells may be more nsitive to transformation than differentiated cells (Holland et al .,1998;Holland et al .,2000).Using a retroviral system that allows for gene expression in apparently different brain cell populations driven by different promoters,they directed the expression of oncogenes to brain cells expressing glial fibrillary acidic protein (GFAP,presumed to be expresd by differ-entiated astrocytes)or cells expressing nestin (presumed to mark neural stem or progenitor cells),and they found that malignant glial tumors ari most efficiently after oncogene transfer to nestin-expressing brain cells.The interpretation of cell of origin remains unclear as a GFAP-positive cell in the subventricular zone has recently been suggested to be the adult NSC (Doetsch et al .,1999).The same rearchers also suggest that a differentiated neural cell such as an astrocyte may be equally permissive to transformation as a progenitor,if it has a key genetic alteration such as loss of tumor suppressor Ink4a-Arf (Bachoo et al .,2002;Uhrbom et al .,2002).The data support a view that dysregula-tion of specific genetic pathways,rather than cell of origin,may dictate the emergence and phenotype of high-grade gliomas.
Although brain tumors may ari from a dediffer-entiated cell that has accumulated a ries of oncog
enic mutations,an NSC may be en as a more permissive and likely compartment for transformation,since it already has the lf-renewal machinery primed and it has a long lifespan favoring the accumulation of mutations.A progenitor cell is also a possible target if the genetic alteration allows it to reacquire the ability
to
Figure 1On the left,normal NSCs in the brain undergo highly regulated lf-renewing divisions to regenerate themlves and they also generate lineage-committed progenitor cells and then differentiate into the three main neural lineages,neurons,astrocytes and oligodendrocytes.On the right,multiple mutational events target a stem cell,progenitor cell or mature brain cell results in the formation of a BTSC.The BTSC has enhanced lf-renewal and proliferative capacity and also differentiates aberrantly into other cells in the tumor,which then have a limited proliferative
ability
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lf-renew.A mutational event occurring in a progenitor may not be as dangerous as in a stem cell,as this cell normally has limited lf-renewal ability and it typically becomes clonally exhausted as it generates differentiated cells.Whether the transforming event of a brain tumor occurs in an NSC or in a more differentiated cell type that has reacquired stem cell characteristics remains to be proven.It is conceivable that an astrocytoma aris from an oncogenic event in an astrocyte-restricted progenitor or mature astrocyte,leading to an astrocyte phenotype.However,it is also possible that the oncogenic event occurred in an NSC and this event caus a block in differentiation (Pereira et al .,1998)that allows the transformed cell to differentiate down astrocytic lineages but not neuronal or oligodendrocyte lineages.The cell of origin may come down to a question of probability,any normal brain cell can give ri to a tumor,but the number and potency of the genetic alteration may determine the ea at which the cell becomes transformed.The structural organization of the brain,the integrity of which is demanded to prerve function,suggests that differentiated and morphologi-cally complex cells such as neurons must be exceedingly resistant to proliferation.
Prospective identification of a cancer stem cell from human brain tumors
Regardless of the possible cell of origin,recent evidence suggests that brain tumors contain small numbers of cells with NSC properties.The cells have the ability to lf-renew,proliferate and differe
ntiate,and uniquely maintain the tumor growth.Although qualitative reports of sphere formation in cultured brain tumor cells exist (Mackillop et al .,1985)and stem-like cells have been obrved in brain tumor cultures (Ignatova et al .,2002),a prospectively isolated population of cancer stem cells from brain tumors had not been described until recently (Singh et al .,2003).The existence of cancer stem cells,first proven in the context of acute myeloid leukemia (AML),required sorting by surface markers to distinguish leukemic stem cells from the remaining AML cells,which had limited prolifera-tive potential (Blair et al .,1997;Bonnet and Dick,1997).This established a hierarchy of clonally derived popula-tions within the cancer,originating from malignant cancer stem cells,which have extensive proliferative potential,to differentiated cancer cells,which have limited proliferative potential.The relative paucity of good NSC markers prented an obstacle to the establishment of this type of hierarchy within brain tumors.Nestin,the best CNS stem cell marker to date,was a nonspecific cytoplasmic intermediate filament,and the nature of cell sorting required a cell surface marker.An excellent candidate was found in CD133,a novel 120kDa five-transmembrane cell surface protein originally shown to be a hematopoietic stem cell marker,and recently found to be a marker of normal human NSCs (Corbeil et al .,1998;Uchida et al .,2000;Tamaki et al .,2002).
The first prospective in vitro identification and characterization of a cancer stem cell from human brai
n tumors of different phenotypes was reported bad on cell sorting for CD133on acutely dissociated brain tumor cell populations (Figure 2)(Singh et al .,2003).The brain tumor stem cell (BTSC)reprented a fraction of the total cells comprising the tumor and was isolated from low-and high-grade tumors from both children and adults.The BTSC was exclusively isolated with the cell fraction expressing the NSC surface marker CD133.There are three pieces of evidence that support that the cells are BTSCs:(1)they generate clusters of clonally derived cells rembling neurospheres,(2)they lf-renew and proliferate,and (3)they differentiate to recapitulate the phenotype of the tumor from which they were derived.In defining a class of BTSCs that can be prospectively isolated from a wide range of brain tumors,we support the application of principles of leukemogenesis to solid tumors:namely,the principle that only a small subt of cancer stem cells is enriched for clonogenic capacity,and that the cells alone are capable of tumor propagation.Importantly,we have recent unpublished data that support the fact that BTSCs can also initiate tumors in vivo .
Another recent report by Hemmati et al .(2003)substantiated the finding of cancer stem cells in pediatric brain tumors.They found that pediatric brain tumors contained neural stem-like cells,termed ‘tumor-derived progenitors’,that showed the capacity for sphere formation,lf-renewal and multipote
ntial differentia-tion.In addition,the cells were shown by RT–PCR to express many genes characteristic of NSCs,including musashi-1,Sox2,bmi-1and CD133.Further reports are emerging documenting the prence of BTSCs in human adult brain tumors.The description of BTSCs from a number of groups describes a class of cells that may drive tumorigenesis in an increasing number of brain tumors.The application of principles for the study of normal NSCs to brain tumor cell populations establishes a link between normal neurogenesis and brain tumorigenesis.Brain tumors are not only phenotypically heterogeneous but are also functionally heterogeneous.Brain tumors exhibit phenotypic heterogeneity,being compod of cells expressing both undifferentiated and differentiated markers.In vitro analysis of brain tumor cells sorted for CD133expression indicates that the capacity for tumor cell lf-renewal and proliferation exclusively resides in the minority CD133þcell fraction.The results suggest that CD133reprents a new cell surface marker of the BTSC –a novel cell type with incread potential for lf-renewal that drives brain tumorigenesis.The data also suggest that brain tumors are comprid of populations of proliferating tumor stem cells that are differentiating into the more mature cell types that characterize the tumor.Purification of CD133þcells in brain tumors implies that a hierarchy may exist in the tumor cell population,as not all tumor cells were capable of maintaining the tumor in culture.This apparent hierarchy may be functionally elucidated as more surface markers for NSCs emerge and further tumor subpopulations identified.As normal NSCs
are
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also found in the CD133population of the normal human fetal brain,and that tumors of all phenotypes studied contained nestin þ/CD133þ/lineage Àcells,it suggests that the cell of origin for a brain tumor may be a normal NSC.Future investigations of the BTSC may lead to further insight of this possibility.Much further work is required to determine the ability of BTSCs to initiate and maintain tumor growth in vivo ,and to determine how the cells determine brain tumor phenotype.英文句子
BTSC:a new target for future brain tumor therapy Current diagnostic and prognostic criteria for brain
tumors rely on the histopathological and molecular features of the global tumor population,and may not be sufficient to determine the key molecular alterations in a rarer tumor stem cell fraction.Molecular analysis of the BTSC population may lead to the identification of novel pathways important for the proliferation,lf-renewal and differentiation of the cells that opens up new targets for therapy.It will likely be important to study the effect of activation of oncogenes or inactivation of tumor suppressor genes in the stem cell as oppod to other brain cells,as the alterations are predicted to have different effects in stem cells versus differentiated cells.Rapid prospective purification of the BTSC may allow clinicians to pinpoint the transformed cell within a lineage hierarchy,and target it with appropriately tailored drug and molecular therapies.The cancer stem cell hypothesis predicts that even if a brain tumor
shows
Figure 2Cancer stem cells are identified from solid human brain tumors after acute dissociation of the solid tumor mass and then cell sorting for surface markers.CD133identified a small subpopulation of the brain tumor cells that was uniquely able to propagate the tumor in stem cell and tumorigenic assays.CD133Àcells cannot expand the tumor population becau they lack the ability to lf-
renew
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