Biology of Blood and Marrow Transplantation 11:12–13 (2005) 䊚 2005 American Society for Blood and Marrow Transplantation 1083-8791/05/1102-0105$30.00/0 doi:10.1016/j.bbmt.2004.11.004
Brain Tumor Stem Cells Peter B. Dirks The Arthur and Sonia Labatt Brain Tumor Research Centre, Division of Neurosurgery and Program in Developmental Biology, The Hospital for Sick Children, Toronto, Ontario, Canada Correspondence and reprint requests: Peter B. Dirks, PhD, Division of Neurosurgery and Program in Developmental Biology, The Hospital for Sick Children, 555 University Ave, Toronto, Ontario, Canada M5G 1X8 (e-mail:
[email protected]).
KEY WORDS Brain tumor
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Stem cell
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Cancer stem cell
Brain tumors are typically composed of morphologically diverse cells that express a variety of neural lineage markers. Study of brain tumors by traditional histopathology has yielded only a limited amount of knowledge of the clinical behavior of the tumor. It is recognized that tumors with vastly different histology have a different prognosis, but often brain tumors that share similar morphology and phenotype can have a very different prognosis and response to treatment. Although major advances have been made in the understanding of the molecular genetic alterations of some types of brain tumors [1,2], particularly medulloblastomas and malignant gliomas, and although some of these identified alterations are now beginning to guide treatment, it is not clear whether all 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 diverse tumor cells are capable of maintaining the growth of the tumor. The cancer stem cell hypothesis suggests that not all 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, possess the ability to extensively proliferate and self-renew. Most of the tumor cells lose the ability to proliferate and self-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. A normal neural stem cell could be the cell of origin for a brain tumor, but this idea remains largely speculative, and there are data that suggest 12
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Medulloblastoma
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Glioblastoma
both neural progenitors and neural stem cells are possible cells of origin [1,2]. Recent evidence, however, suggests that brain tumors contain small numbers of cells with neural stem cell properties [3-7]. These cells have the ability to self-renew, proliferate, and differentiate in vitro. Most of these studies made a stem cell connection on the basis of the ability of the brain tumor cells to form clonogenically derived colonies of cells in the culture, analogous to normal neurospheres [8], that derive from neural stem cells in defined culture conditions (no cell adhesion, serum free, in epidermal growth factor and fibroblast growth factor). The stem cell nature of tumors is defined retrospectively by their ability to form these “neurospheres” and by their self-renewal and multilineage differentiation ability in culture. The first prospective in vitro identification and characterization of a cancer stem cells from human brain tumors of different phenotypes was reported on the basis of cell sorting for CD133 on acutely dissociated brain tumor cell populations [5]. CD133 had been previously used to identify normal human neural stem cells [9]. The brain tumor stem cell (BTSC) represented a fraction of the total cells comprising the tumor and was isolated from low-grade and highgrade tumors from both children and adults. The BTSC was exclusively isolated with the cell fraction expressing the neural stem cell surface marker CD133. Three pieces of evidence suggested that these cells were BTSCs: (1) they generated clusters of clonally derived cells resembling neurospheres, (2) they underwent self-renewal and proliferation, and (3) they differentiated 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, these data supported the appli-
Brain Tumor Stem Cells
cation of principles of leukemogenesis to solid tumors: namely, the principle that only a small subset of cancer stem cells is enriched for clonogenic capacity and that these cells alone are capable of tumor propagation. We believe that further research is required to further purify the BTSCs, because we hypothesize that a more potent BTSC will be found in a CD133 subpopulation. Another report, by Hemmati et al. [4], substantiated this finding of cancer stem cells in pediatric brain tumors. They found that pediatric brain tumors contained neural stemlike cells, termed tumor-derived progenitors, that showed the capacity for sphere formation, self-renewal, and multipotential differentiation. In addition, these cells were shown by reverse transcriptase-polymerase chain reaction to express many genes characteristic of neural stem cells, including musashi-1, Sox2, bmi-1, and CD133. Further reports are emerging that document the presence of BTSCs in human adult brain tumors [6,7]. 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. Despite the intriguing in vitro data, the only true measures of cancer stem cells are their capacity to generate an exact copy of the tumor from which they were derived and to self-renew, and these measures require in vivo data. Galli et al. [7] demonstrated that glioblastoma cell lines, established by culture in neurosphere (neural stem cell) conditions, could proliferate, self-renew, and differentiate into multiple lineages in vitro. Further cerebral injection of 200 000 of these tumor sphere cells into mice intracranially could also generate tumors in vivo and, after repeat culture, could initiate phenotypically similar tumors in a second mouse. Our own further work now also suggests that CD133 can purify a subpopulation of brain tumor cells that are capable of tumor initiation and maintenance in in vivo models. We recently reported the development of a xenograft assay that we used to identify human brain tumor–initiating cells that had the capacity to initiate tumors in vivo [10]. CD133 isolation and in vivo engraftment were performed on
BB&MT
cells that had only an extremely brief time in culture or no culture at all. Only the CD133⫹ brain tumor fraction contained cells that were capable of tumor initiation in nonobese diabetic/severe combined immunodeficiency mouse brains. Injection of as few as 100 CD133⫹ cells produced a tumor that was serially transplantable and was a phenocopy of the patient’s original tumor, whereas injection of 105 CD133⫺ cells produced engraftment but did not cause a tumor. The identification of a brain tumor–initiating cell in vivo provides further insights into human brain tumor pathogenesis and gives strong support for the cancer stem cell hypothesis as the basis for many solid tumors, thus further establishing a novel cellular target for more effective cancer therapies.
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