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BackgroundChronic Myeloid Leukaemia (CML) is characterised by the chromosomal translocation resulting in expression of the Bcr-Abl protein tyrosine kinase (PTK) in early stem cells and their progeny. However the precise nature of Bcr-Abl effects in primitive CML stem cells remains a matter of active debate.Materials and MethodsExtremely primitive Bcr-Abl fusion positive cells were purified from patients with CML using multiparameter flow cytometric analysis of CD34, Thy, and lineage marker (Lin) expression, plus rhodamine-123 (Rh-123) brightness. Progenitor cells of increasing maturity were examined for cycling status by flow cytometry and their proliferative status directly correlated with cell phenotype. The activation status of a key transcription factor, signal transducers and activators of transcription (STAT-5), was also analyzed by immunocytochemistry.ResultsThe most primitive stem cells currently defined (CD34+Lin-Thy+ Rh-123lo) were present as a lower proportion of the stem cell compartment (CD34+Lin-) of CML patients at presentation than of normal individuals (2.3% ± 0.4 compared with 5.1% ± 0.6 respectively). Conversely there was a significantly higher proportion of the more mature cells (CD34+Lin-Thy-Rh-123 hi) in CML patients than in normal individuals (79.3 ± 1.8 compared with 70.9 ± 3.3). No primitive subpopulation of CML CD34+Lin-cells was cycling to a significantly greater degree than cells from normal donors, in fact, late progenitor cells (CD34+ Lin+) were cycling significantly less in CML samples than normal samples. STAT5, however, was observed to be activated in CML cells.ConclusionsWe conclude that no subpopulation of CML stem cells displays significantly increased cell cycling. Thus, increased cycling cannot be a direct consequence of Bcr-Abl PTK acquisition in highly enriched stem cells from patients with CML. In vivo CML need not be considered a disease of unbridled stem cell proliferation, but a subtle defect in the balance between self renewal and maturation.

The Effect of Bcr-Abl Protein Tyrosine Kinase on Maturation and Proliferation of Primitive Haematopoietic Cells