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Introduction
CD133 was first presented as a key marker for brain tumour stem cells (BTSCs) in 2003 when in vitro experiments indicated that CD133+ cells possess significantly enhanced ability to differentiate, proliferate and self-renew. The research also showed CD133+ cells could generate new malignancies that were phenotypically and histologically similar to the tumour of origin (1). However, since these findings were published, other researchers have been able to demonstrate tumourigenesis by CD133− cells and challenged the role of CD133 in tumour initiation (2,3). Despite the controversy, the link between CD133 and tumourigenesis has remained a ‘hot’ topic in CSC research (4–6). Current evidence also indicates that CD133 expression in CSCs differs according to cell cycle phases in vitro, although stem cell potency and differentiating capabilities are preserved (7). Indeed, CD133 expression, and thus perhaps also glycosylation, fluctuates during the cell cycle, being low in quiescent cells in G0 or G1 but higher in subsequent phases (7). This has important implications for using CD133 to enrich CSCs as non-dividing invading cells and other quiescent cells within the tumour mass could be missed by immunological detection of CD133.
Several splice variants of CD133 have been identified in man (8). This has potential impact on the CSC detection as CD133 may be present but lacking the antibody specific epitope so cells could be described falsely as CD133−. Indeed, it has been reported that CD133− glioblastoma cells (determined by CD133/1 antibody) express a truncated variant of the protein (9). Post-translational glycosylation of CD133 may also affect antibody detection since the commonly used antibody, CD133/1, is likely to detect CD133-expressing cells only when the epitope AC133 is glycosylated, causing concerns over the limitations of glycosylated CD133 epitopes in isolating CSCs (10). Furthermore, the role of CD133 in brain tumour biology must be elucidated with consideration of the tumour microenvironment. Hypoxic microenvironment is a feature of glioblastoma multiforme (GBM), the most common and malignant of primary glial tumours in the brain (11). Hypoxia is believed to trigger BTSC proliferation and invasion as well as to promote resistance to therapy (12,13). It has been demonstrated by us and others, using the CD133/1 antibody, that CD133 expression increases under hypoxic culture conditions (14–17), suggesting a...