| Most current research on human brain tumours is focused on the molecular and cellular analysis of the bulk tumour mass. However, there is overwhelming evidence in some malignancies that the tumour clone is heterogeneous with respect to proliferation and differentiation. In human leukemia, the tumour clone is organized as a hierarchy that originates from rare leukemic stem cells. Therefore, the cancer stem cell (CSC) hypothesis suggests that neoplastic clones are maintained exclusively by a rare fraction of cells that have stem cell properties. Although the existence of CSC in human leukemia is established, except for breast cancer, there is little direct evidence for CSC in solid tumours.; We prospectively identified and purified a subpopulation of tumour cells from a variety of human brain tumours that exhibited the stem cell properties of proliferation, self-renewal, and differentiation in vitro. The brain tumour stem cell (BTSC) was exclusively isolated with the cell fraction expressing the neural precursor cell surface marker CD133. CD133 was expressed in a minority of brain tumour cells, and ranged from 0.1 to 30% in tumours of varying phenotype. The increased self-renewal capacity of the in vitro BTSC was highest from the most aggressive clinical samples of medulloblastoma and glioblastoma compared with low-grade gliomas. Conversely, CD133- cells showed no in vitro self-renewal capacity and very limited proliferative ability. The CD133+ cells could also differentiate in culture into tumour cells that phenotypically resembled the tumour from the patient.; In order to test the capacity of the in vitro BTSC to initiate tumours in vivo, we used a xenograft assay to identify human brain tumour initiating cells (BTIC). Only the CD133+ brain tumour fraction contains cells that are capable of tumour initiation in NOD-SCID mouse brains. Injection of as few as 100 CD133+ cells produced a tumour that was serially transplantable and was a phenocopy of the patient's original tumour, whereas injection of 105 CD133- cells engrafted but did not cause a tumour. Therefore, CD133+ brain tumour cells satisfy the definition of cancer stem cells in that they are able to generate a replica of the patient's tumour and they exhibit self-renewal ability in vivo through serial retransplantation. The identification of a brain tumour initiating cell provides new insights into human brain tumour pathogenesis, giving strong support for the CSC hypothesis as the basis for many solid tumours, and establishes a novel cellular target for more effective cancer therapies. |
