BCR/ABL- CD34⁺HLA-DR^- progenitor cells in early chronic phase, but not in more advanced phases, of chronic myelogenous leukemia are polyclonal

Chronic myelogenous leukemia (CML) is characterized by the Philadelphia (Ph) translocation and BCR/ABL gene rearrangement which occur in a pluripotent hematopoietic progenitor cell. Ph-negative (Ph^-) hematopoiesis can be restored in vivo after treatment with α-interferon or intensive chemotherapy, suggesting that normal stem and progenitor cells coexist with the Ph⁺ clone. We have previously shown that Ph^- progenitors are highly enriched in the CD34⁺HLA-DR^- fraction from early chronic phase (ECP) CML patients. Previous studies have suggested that the Ph-translocation represents a secondary clonal hit occurring in an already clonally mutated Ph^- progenitor or stem cells, leaving the unanswered question whether Ph^- CD34⁺HLA-DR^- progenitors are normal. To show the clonal nature of Ph^- CD34⁺HLA-DR^- CML progenitors, we have compared the expression of BCR/ABL mRNA with X-chromosome inactivation patterns (HUMARA) in mononuclear cells and in CD34⁺HLA^-DR⁺ and CD34⁺HLA-DR^- progenitors in marrow and blood obtained from 11 female CML patients (8 in chronic phase and 3 in accelerated phase [AP] disease). Steady-state marrow-derived BCR/ABL mRNA^-, CD34⁺HLA- DR^- progenitors had polyclonal X-chromosome inactivation patterns in 2 of 2 patients. The same polyclonal pattern was found in the progeny of CD34⁺HLA- DR^- derived long-term culture-initiating cells. Mobilization with intensive chemotherapy induced a Ph^-, BCR/ABL mRNA^- and polyclonal state in the CD34⁺HLA-DR^- and CD34⁺HLA-DR⁺ progenitors from 2 ECP patients. In a third ECP patient, polyclonal CD34⁺ cells could only be found in the first peripheral blood collection. In contrast to ECP CML, steady-state marrow progenitors in late chronic phase and AP disease were mostly Ph⁺, BCR/ABL mRNA⁺, and clonal. Further, in the majority of these patients, a Ph^-, polyclonal state could not be restored despite mobilization with intensive chemotherapy. We conclude from these studies that CD34⁺HLA-DR^- cells that are Ph^- and BCR/ABL mRNA^- are polyclonal and therefore benign. This population is suitable for autografting in CML.