Essay Development of gliomas: potential role of asymmetrical cell division of neural stem cells
François Berger, Emmanuel Gay, Laurent Pelletier, Philippe Tropel, and Didier Wion
Asymmetrical cell division is a mechanism that gives rise to two daughter cells with different proliferative and differentiative fates. It occurs mainly during development and in adult stem cells. Accumulating evidence suggests that tumour cells arise from the transformation of normal stem cells. Here, we propose that the asymmetrical mitosis potential of stem cells is associated with the generation of migrating tumour progenitors. Application of this speculative model to glioma proposes that the sites where tumour-initiating stem cells reside are indolent and distinct from the tumour mass, and implies that the tumour mass is continuously replenished with new migrating tumour cells from these clinically silent regions. This hypothesis offers explanations for our inability to cure glioblastoma and points to asymmetrical division as a new potential therapeutic target.
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Despite constant efforts, therapies such as surgery, radiotherapy, chemotherapy, immunotherapy, and gene therapy have not substantially improved the outlook for patients with glioblastoma. The existence of neural stem cells in restricted zones of the adult brain (subgranular layer of the hippocampus and subependymal zone) lends support to a possible stem-cell origin of glioma.1–5 This view is reminiscent of Smyth and Stern’s observations,6 published in 1938, that “subependymal glia may actually be the point of origin of tumours of the thalamus”, and to Globus and Kuhlenbeck’s theory7,8 of the origin of some gliomas, which stressed, in 1942, the “significance of the subependymal cell plate as the seat of embryonal rests which serve as an important source for the development of primary neuroectodermal brain tumors”. The notion that tumours originate from the transformation of normal stem cells, has been supported by the finding that some haematological malignant disorders arise from stem cells in bone marrow.9–12 Furthermore, the presence of adult stem cells—ie, clonogenic cells with self-renewal and multilineage differentiation properties—in several other organs such as epidermis,13 breast,14 and brain15—also suggests a possible stem-cell origin for solid tumours. Although the transformation of stem cells has been documented for leukaemia9 and teratocarcinoma,16,17 evidence supporting the existence of transformed stem cells in the genesis of other cancers remains elusive. However, breast cancers and brain cancers contain a small subset of cells driving tumorigenesis and generating tumour-cell heterogeneity,18,19 which suggests that these tumour-initiating cells might arise from the transformation of breast cells or neural stem cells. Ascertainment of which type of solid tumour arises from transformed stem cells, and identification of the corresponding tumour-initiating stem cells are now two major challenges of cancer research. These points are of
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special concern because cancers derived from stem cells would have a more diverse and invasive phenotype than would those derived from more restricted progenitors.11 Hence, the cell heterogeneity found in most cancers is thought to be generated not only by genetic instability and epigenetic changes, but also by the aberrant differentiation of cancer stem cells.11,12 In this essay we propose that the potential of neural stem cells to divide asymmetrically,20 together with the migratory potential of neural progenitors,21,22 play a part in the intractable nature of glioblastoma.
Hypothesis Symmetrical versus asymmetrical division of stem cells
Cells can divide either symmetrically or asymmetrically.23,24 Unlike conventional symmetrical mitosis that generates two identical daughter cells, asymmetrical cell division leads to the asymmetrical segregation of cell-fate determinants.23–27 Therefore, this mode of cell division generates two daughter cells with different cell fates (figure 1A).23–27 For example, the asymmetrical division of a stem cell gives rise to a selfrenewing stem cell and to a daughter cell, known as the progenitor, engaged in the differentiation process (figure 1A). This mode of cell division is one of the mechanisms used to ensure cell diversity during development. However, two daughter cells generated by symmetrical division can also acquire different fates as a result of their exposure to FB is a neurooncologist, group leader, and professor; EG is a neurosurgeon and professor; and LP is an assistant professor; all at the University of Grenoble. PT is a postdoctoral researcher, French association against Myopathies. DW is a research scientist at INSERM, Grenoble, France. All authors are members of the Preclinical Neuroscience Laboratory, INSERM Unit 318, Grenoble, France. Correspondence: Dr Didier Wion, INSERM U318, CHU Michallon, 38043 Grenoble, France. Tel: +33 4 7676 5653. Fax: +33 4 7676 5619. Email:
[email protected]
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Essay A
identifying the intracellular and environmental cues that specify whether a cancer stem cell will undergo symmetrical or asymmetrical divisions. Asymmetrical division and tumour promotion Asymmetrical mitosis
Stem cell
Symmetrical mitosis Progenitors
B G2M
G1
P
Stem cell G0
P
P P
Asymmetrical mitosis
P
P P
P
Progenitors Progenitor Symmetrical mitosis
Protein kinase Proto-oncogene
Figure 1. Asymmetrical cell division, cancer stem cell and tumour promotion. Asymmetrical division of a stem cell, gives rise to two different daughter cells, one that remains a stem cell and one that is a progenitor (A). Hypothetical asymmetrical segregation of a proto-oncogene in progenitor cells (B).
Figure 1B shows how asymmetrical division of a stem cell can act as a tumour promoter to generate a continuous flux of tumour progenitor cells away from the site where the stem cell resides. In this model, we postulate that stem cells express a protein kinase that phosphorylates a protooncogene expressed in the late G1 phase of cell cycle. Phosphorylation of the proto-oncogene restricts its localisation to the basal side of the cell and ensures its segregation to progenitor cells. By contrast, in the absence of the protein kinase, the proto-oncogene remains nonphosphorylated and therefore becomes uniformly localised into the cytoplasm of progenitor cells. Therefore, in the case of a stem cell that carries an activating mutation in the protooncogene, each round of asymmetrical mitosis leads to the clearance of the oncogenic protein from the stem cell and to its segregation into the progenitor cell. Consequently, the initiated stem cell does not progress towards tumorigenicity whereas the progenitor cell does. Alternatively, the mutated gene might be silent in the stem cell, and be expressed in the progenitor cell only as a consequence of cell differentiation. Hence, in all cases, the population of stem cells remains stable and the migration of resultant proliferating progenitor cells leads to distinct locations for the origin of cancer cells (ie, the niches where the constant pool size of cancer or initiated stem cells resides) and the tumour site (ie, the sites where the amplifying pool of cancer cells develop). Asymmetrical division and glioma
different environmental factors. Investigations of the molecular mechanisms underlying asymmetrical cell division are in progress and have identified the role of evolutionarily-conserved proteins such as PARD3, PARD6A, CDC42, PRKCZ and PRKCL, INSC, GPSM2, NUMB, NOTCH, and LGL.25–31 Asymmetrical divisions initially described in Caenorhabditis elegans and drosophila, have now been extended to mammalian stem cells and progenitors.32–35 This point is of great importance for cancer research because accumulating evidence suggests that some tumours have a stem-cell origin.11 An interesting feature of asymmetrical division is that if a cancer stem cell divides by asymmetrical divisions, the pool of cancer stem cells does not increase, since the asymmetrical division of a cancer stem cell will generate one cancer stem cell, and one cancer progenitor (figure 1A). According to this model, it is the symmetrical division of the progenitor that constitutes the expanding pool of tumour cells. An immediate consequence of this situation is that if tumour progenitors migrate away from the site where cancer stem cells reside, then tumour lesions and cancer stem cells do not necessarily match each other. According to our hypothesis, the site where cancer stem cells reside and proliferate by asymmetrical divisions could be a clinically silent area. However, when cancer stem cells proliferate by symmetrical division, the pool of cancer stem cells increases. This process shows the importance of
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Gliomas are the most common primary brain tumours in humans. They are divided into four grades, with grade IV or glioblastoma multiforme being the most aggressive, heterogeneous, and invasive. In adult mammals, neural stem cells have been isolated from the subependymal zone of the lateral ventricle and from other brain regions such as the subgranular layer of the hippocampal dentate gyrus.4,5,36,37 Here, we postulate that some gliomas arise from tumorigenic neural stem cells, which have the capacity to divide asymmetrically. This asymmetrical division of a cancer neural stem cell is expected to produce two different daughter cells, one that remains in the germinal zone (eg, the subependymal zone) as a cancer neural stem cell, and one that migrates and proliferates away from the subependymal zone as a cancer neural progenitor (figures 1 and 2). The first consequence of this asymmetrical cell division is that the intrinsic chemosensitivity of the two different daughter cells might differ.38 The second important consequence is that the tumour does not reside in the same site as the cancer neural stem cell, but at the location where the proliferating tumour progenitors or tumour glioblasts have migrated. Consequently, the source of tumour cells (eg, one cancer stem cell in the subependymal zone) does not necessarily match the tumour tissue. Moreover, in this speculative model, glioma cells that infiltrate the surrounding tumour border do not come exclusively from the primary tumour
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Essay Search strategy and selection criteria Radiotherapy Gene therapy Cell therapy Surgery
Data for this essay were identified by searches of PubMed. The search terms were “asymmetric(al) division”, “stem cell”, “neural stem cell”, “cancer”, “glioma”. Other reports were identified from the personal collections of the authors. References from selected articles were also considered. Only articles published in English were considered. Glioblastoma G
PR
NSC Asymmetrical division
Figure 2. Asymmetrical division of cancer neural stem cells as a postulated mechanism underlying glioblastoma. In this speculative model, asymmetrical division of a tumour neural stem cell (NSC) gives rise to one tumour NSC, which remains in its niche, and to one migrating and proliferating tumour progenitor (PR). G, tumour glioblast.
mass, but could be cancer progenitors or glioblasts that have migrated towards the glioma-induced lesions from the distant neurogenesis zone where the cancer neural stem cell resides. Hence, the primary tumour mass is continuously replenished by the arrival of new tumour progenitor cells from distant asymptomatic sites, such as the subependymal zone. The suggested ability of tumour progenitor cells to migrate from the niches where tumour stem cells reside towards the tumour mass relies on the known capacity of transplanted neural stem cells and progenitor cells to migrate specifically to brain lesions.21,22,39–43 In fact, the tumour-tracking property of non-tumour neural stem cells has already been used to deliver therapeutic proteins to experimental intracranial brain tumours.21,41–43 It is noteworthy that the deletion of PTEN, frequently associated with gliomas, increases the cellular motility of neural precursors.44 This tropism of tumour progenitor cells for brain lesions could also be involved in the local recurrence of gliomas after surgical resection or radiotherapy that targets the tumour mass, but does not necessarily include the neurogenesis zones where cancer neural stem cell might reside. This process might result in a vicious cycle whereby therapy-associated damage might trigger the migration of new cancer progenitor cells towards the treated sites. In this regard, it is noteworthy that in preclinical studies, brain tumours are established by intracranial injection of glioma cell lines.45–47 Hence, in these experimental models used extensively to investigate various treatments, the source of glioma cells (ie, the injection site where the cells are implanted), and the glioma-induced lesions (ie, the mass of the experimental tumour) match each
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© Du Cane Medical Imaging Ltd/SPL; modified by S Wood
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other. Moreover, in these preclinical models, brain tumours usually consist of a cell population regarded as homogeneous (a glioma cell line), which proliferates by symmetrical divisions. These features could be possible explanations for the fact that some treatments have been successful in animals, but not in humans.
Conclusion Asymmetrical mitosis is an attribute of stem cells and progenitor cells which, if also present in cancer stem cells, could have important implications in cancer research. In gliomas, one consequence of our hypothetical model is that the asymmetrical division of cancer neural stem cells and the migration of cancer progenitors lead to separate locations for the tumour-initiating cell and the actual tumour mass. Moreover, because cancer neural stem cells that divide asymmetrically have a constant pool size, the niche where these cells reside might remain indolent. Preclinical studies will need to use animal models other than the extensively used intracranial injection of glioma cells to account for the consequences of asymmetrical division. Such alternative models exist, and are used to study pathways underlying glioma formation in mice. However, they are not commonly used in investigation of new treatments. Examples of such models include Nf1/Trp53 mutant mice,48 transplacental ethyl-nitrosurea exposure,3 or intracerebral inoculation of avian sarcoma virus.2 Glial tumorigenesis seen in ethylnitrosurea exposure and viral inoculation models suggests that migrating transformed subependymal cells exist.2,3 Hence, even if the role of asymmetrical cell division in tumorigenesis remains speculative, integration of this mode of cell division into the fundamentals of cancer research could lead to the reappraisal of some current experimental models. It could also lead to the development of new therapeutic drugs that target the components of the asymmetrical cell-division machinery, leading to the induction of apoptosis in cancer stem cells. Conflict of interest
None declared. Acknowledgments
We thank AL Benabid for constant support and B Wallace for critical reading of this manuscript. FB and DW are supported by the Ligue Nationale contre le Cancer (Comité de l'Isère) and PT by AFM. This essay is dedicated to the memory of D Horrobin. References
1 Lewis PD. Mitotic activity in the primate subependymal layer and the genesis of gliomas. Nature 1968; 217: 974–75. 2 Vick NA, Lin MJ, Bigner DD. The role of the subependymal plate in glial tumorigenesis. Acta Neuropathol 1977; 40: 63–71. 3 Lantos PL, Cox DJ. The origin of experimental brain tumors: a sequential study. Experientia 1976; 32: 1467–68.
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Essay 4 Taupin P, Gage FH. Adult neurogenesis and neural stem cells of the central nervous system in mammals. J Neurosci 2002; 69: 745–49. 5 Palmer TD, Takahashi J, Gage FH. The adult rat hippocampus contains primordial neural stem cells. Mol Cell Neurosci 1997; 8: 389–404. 6 Smyth GE, Stern K. Tumors of the thalamus: a clinico-pathological study. Brain 1938; 61: 339–74. 7 Globus JH, Kuhlenbeck H. Tumors of the striatothalamic and related regions: their probable source of origin and more common forms. Arch Pathol 1942; 34: 674–734. 8 Globus JH, Kuhlenbeck H. The subependymal plate (matrix) and its relationship to brain tumors of the ependymal type. J Neuropath Exp Neurol 1944; 3: 1–35. 9 Bonnet D, Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat Med 1997; 3: 730–37. 10 Reya T, Morrison SJ, Clarke MF, Weissman IL. Stem cells, cancer, and cancer stem cells. Nature 2001; 414: 105–11. 11 Tu SM, Lin SH, Logothetis CJ. Stem-cell origin of metastasis and heterogeneity in solid tumours. Lancet Oncol 2002; 3: 508–13. 12 Passegué E, Jamieson CHM, Ailles LE, Weissman IL. Normal and leukemic hematopoiesis: are leukemias a stem cell disorder or a reacquisition of stem cell characteristics? Proc Natl Acad Sci USA 2003; 100: 11842–49. 13 Perez-Losada J, Balmain A. Stem-cell hierarchy in skin cancer. Nat Rev Cancer 2003; 3: 434–43. 14 Kordon EC, Smith GH. An entire functional mammary gland may comprise the progeny from a single cell. Development 1998; 125: 1921–30. 15 Steindler DA, Pincus DW. Stem cells and neuropoiesis in the adult human brain. Lancet 2002; 359: 1047–54. 16 Sell S, Pierce GB. Maturation and arrest of stem cell differentiation is a common pathway for the cellular origin of teratocarcinomas and epithelial cancers. Lab Invest 1994; 70: 6–22. 17 Andrews, PW. From teratocarcinoma to embryonic stem cells. Philos Trans R Soc Lond B Biol Sci 2002; 357: 405–17. 18 Al-Hajj M, Wicha MS, Benito-Hernandez A, et al. Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci USA 2003; 100: 3983–88. 19 Singh SK, Clarke ID, Terasaki M, et al. Identification of a cancer stem cell in human brain tumors. Cancer Res 2003; 3: 5821–28. 20 Sommer L, Rao M. Neural stem cells and regulation of cell number. Prog Neurobiol 2002; 66: 1–18. 21 Aboody KS, Brown A, Rainov NG, et al. Neural stem cells display extensive tropism for pathology in adult brain: evidence from intracranial gliomas. Proc Natl Acad Sci USA 2000; 97: 12846–51. 22 Kokaia Z, Lindvall O. Neurogenesis after ischaemic brain insults. Curr Opin Neurobiol 2003; 13: 127–32. 23 Hall PA, Watt FM. Stem cells: the generation and maintenance of cellular diversity. Development 1989; 106: 619–33. 24 Knoblich, JA. Asymmetric cell division during animal development. Nat Rev Mol Cell Biol 2001; 2: 11–20. 25 Rhyu MS, Jan LY, Jan YN. Asymmetric distribution of Numb protein during division of the sensory organ precursor cell confers distinct fates to daughter cells. Cell 1994; 76: 477–91. 26 Fuerstenberg S, Peng CY, Alvarez-Ortiz P, et al. Identification of Miranda protein domains regulating asymmetrical cortical localization, cargo binding, and cortical release. Mol Cell Neurosci 1998; 12: 325–39. 27 Betschinger J, Mechtler K, Knoblich JA. The Par complex directs asymmetric cell division by phosphorylating the cytoskeletal protein Lgl. Nature 2003; 422: 326–30.
514
28 Du Q, Stukenberg PD, Macara IG. A mammalian partner of inscuteable binds NuMA and regulates mitotic spindle organization. Nat Cell Biol 2001; 3: 1069–75. 29 Plant PJ, Fawcett JP, Lin DCC, et al. A polarity complex of mPar-6 and atypical PKC binds, phosphorylates and regulates mammalian Lgl. Nat Cell Biol 2003; 5: 301–08. 30 Chenn A, McConnell SK. Cleavage orientation and the asymmetric inheritance of Notch-1 immunoreactivity in mammalian neurogenesis. Cell 1995; 82: 631–41. 31 Zhong W, Jiang MM, Schoenemann MD, et al. Mouse numb is an essential gene involved in cortical neurogenesis. Proc Natl Acad Sci USA 2000; 97: 6844–49. 32 Morshead CM, Craig CG, van der Kooy D. In vivo clonal analyses reveal the properties of endogenous neural stem cell proliferation in the adult mammalian forebrain. Development 1998; 125: 2251–61. 33 Alvarez-Buyalla A, Garcia-Verdugo JM, Tramontin AD. A unified hypothesis on the lineage of neural stem cells. Nat Rev Neurosci 2001; 2: 287–93. 34 Chenn A, Zhang YA, Chang BT, McConnell SK. Intrinsic polarity of mammalian neuroepithelial cells. Mol Cell Neurosci 1998; 11: 183–93. 35 Shen Q, Zhong W, Jan YN, Temple S. Asymmetric Numb distribution is critical for asymmetric cell division of mouse cerebral cortical stem cells and neuroblasts. Development 2002; 129: 4843–53. 36 Temple S, Alvarez-Buyalla A. Stem cells in the adult mammalian central nervous system. Curr Opin Neurobiol 1999; 9: 135–41. 37 Gage F, Ray J, Fisher JJ. Isolation, characterization, and the use of stem cells from the CNS. Annu Rev Neurosci 1995; 18: 159–92. 38 Bredel M, Zentner J. Brain-tumour drug resistance: the bare essentials. Lancet Oncol 2002; 3: 397–406. 39 Holmin S, Almqvist P, Lendahl U, Mathiesen T. Adult nestinexpressing subependymal cells differentiate to astrocytes in response to brain injury. Eur J Neurosci 1997; 9: 65–75. 40 Fallon J, Reid S, Kinyamu R, et al. In vivo induction of massive proliferation, directed migration, and differentiation of neural cells in the adult mammalian brain. Proc Natl Acad Sci USA 2000; 97: 14686–91. 41 Burns MJ, Weiss W. Targeted therapy of brain tumors utilizing neural stem and progenitor cells. Front Biosci 2003; 1: E228–34. 42 Ehtesham M, Kabos P, Gutierrez MAR, et al. Induction of glioblastoma apoptosis using neural stem cell-mediated delivery of tumor necrosis factor-related apoptosis-inducing ligand. Cancer Res 2002; 62: 7170–74. 43 Ehtesham M, Kabos P, Kabosava A, et al. The use of interleukin 12secreting neural stem cells for the treatment of intracranial glioma. Cancer Res 2002; 62: 5657–63. 44 Li L, Liu F, Salmonsen RA, et al. PTEN in neuronal precursor cells: regulation of migration, apoptosis, and proliferation. Mol Cell Neurosci 2002; 20: 21–29. 45 Stojiljkovic M, Piperski V, Dacevic M, et al. Characterization of 9L glioma model of the Wistar rat. J Neurooncol 2003; 63: 1–7. 46 Tonn, JC. Model systems in neurooncology. Acta Neurochir 2002; 83: 79–83. 47 Barth, RF. Rat brain tumor models in experimental neurooncology: the 9L, C6, T9, F98, RG2(D74), RT-2 and CNS-1 gliomas. J Neurooncol 1998; 36: 91–102. 48 Reilly KM, Loisel DA, Bronson RT, et al. Nf1;Trp53 mutant mice develop glioblastoma with evidence of strain-specific effects. Nat Genet 2000; 26: 109–13.
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