Facebook Pixel Code
Dirks Lab
Dirks Lab

Research interests

Brain tumours are typically comprised of morphologically diverse cells that express a variety of neural lineage markers. Study of brain tumours by traditional histopathology has only yielded a limited amount of knowledge of the clinical behaviour of the tumour. It is recognized that tumours with vastly different histology have a different prognosis, but often brain tumours 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 tumours1,2, particularly medulloblastomas and malignant gliomas, and some of these identified alterations are now beginning to guide treatment, it is not clear whether all the tumour cells are equivalent in their ability to maintain the growth of the tumour. Until recently, we lacked a functional assay of the brain tumour cells that could determine which of the morphologically diverse tumour cells are capable of maintaining the growth of the tumour.

The cancer stem cell hypothesis suggests that not all the cells in the tumour have the same ability to proliferate and maintain the growth of the tumour. Only a relatively small fraction of cells in the tumour, termed cancer stem cells, possess the ability to extensively proliferate and self renew. Most of the tumour cells lose the ability to proliferate and self renew and they differentiate into tumour cells that become the phenotypic signature of the tumour. Finding the key cells in the brain tumour population that are able to maintain the tumour will give insight into the mechanism of brain tumourigenesis 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 tumour, but this idea remains largely speculative and there is data that suggest both neural progenitors and neural stem cells are possible cells of origin.

Recent evidence, however, suggests that brain tumours contain small numbers of cells with neural stem cell properties. These cells have the ability to self renew, proliferate, and differentiate in vitro. Most of these studies made a stem cell connection based on the ability of the brain tumour cells to form clonogenically derived colonies of cells in the culture, analogous to normal neurospheres, that derive from neural stem cells in defined culture conditions (no cell adhesion, serum free, in EGF and FGF). The stem cell nature of the tumour is defined retrospectively, by their ability to form these “neurospheres” and by their self renewal and multilineage differentiation ability in culture.

Our laboratory was the first to show prospective in vitro identification and characterization of a cancer stem cell from human brain tumours of different phenotypes. This was based on cell sorting for CD133 on acutely dissociated brain tumour cell populations. CD133 had been previously used to identify normal human neural stem cells. The brain tumour stem cell (BTSC) represented a fraction of the total cells comprising the tumour and was isolated from low grade and high grade tumours 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 supported that these cells are 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 tumour from which they were derived. In defining a class of BTSCs that can be prospectively isolated from a wide range of brain tumours, this data supported the application of principles of leukemogenesis to solid tumours: 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 tumour propagation. We believe that further research is required to further purify the BTSC, as we hypothesize that a more potent brain tumour stem cell will be found in a CD133 subpopulation.

Despite the intriguing in vitro data, the only true measures of a cancer stem cell are their capacity to generate an exact copy of the tumour from which they were derived and to self-renew, requiring in vivo data. Our own further work now also suggests that CD133 can purify a subpopulation of brain tumour cells that are capable of tumour initiation and maintenance in in vivo models. We recently reported the development of a xenograft assay that we used to identify human brain tumour initiating cells that had the capacity to initiate tumours in vivo. CD133 isolation and in vivo engraftment were performed on cells that only had an extremely brief time in culture or no culture at all. Only the CD133+ brain tumour fraction contained cells that were 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. The identification of a brain tumour initiating cell in vivo provides further insights into human brain tumour pathogenesis, giving strong support for the cancer stem cell hypothesis as the basis for many solid tumours, and further establishing a novel cellular target for more effective cancer therapies.