The effect of Bcr-Abl protein tyrosine kinase on maturation and proliferation of primitive haematopoietic cells

Background: Chronic Myeloid Leukaemia (CML) is characterised by the chromos omal translocation resulting in expression of the Bcr-Abl protein tyrosine kinase (PTK) in early stem cells and their progeny. However the precise nat ure of Bcr-Abl effects in primitive CML stem cells remains a matter of acti ve debate. Materials and Methods: Extremely primitive Bcr-Abl fusion positive cells we re purified from patients with CML using multiparameter flow cytometric ana lysis 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 direct ly correlated with cell phenotype. The activation status of a key transcrip tion factor. signal transducers and activators of transcription (STAT-5), w as also analyzed by immunocytochemistry. Results: The most primitive stem cells currently defined (CD34+Lin-Thy+Rh-1 231o) 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 signi ficantly 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, l ate progenitor cells (CD34+Lin+) were cycling significantly less in CML sam ples than normal samples. STAT5, however, was observed to be activated in C ML cells. Conclusions: We conclude that no subpopulation of CML stem cells displays s ignificantly increased cell cycling. Thus, increased cycling cannot be a di rect consequence of Bcr-Abl PTK acquisition in highly enriched stem cells f rom patients with CML. Ln vivo CML need not be considered a disease of unbr idled stem cell proliferation, but a subtle defect in the balance between s elf renewal and maturation.