It has been determined that within the heterogeneous leukemic population, there is a rare subset of leukemia stem cells that are responsible for continued maintenance and propagation of the tumor. Recent evidence suggests that during disease pathogenesis, more mature progenitor cells are capable of acquiring stem cell-like characteristics including properties of self-renewal and the capacity to differentiate into progeny that lack self-renewing potential. Acute promyelocytic leukemia (APL) is associated with the t(15;17)(q22;q12) chromosomal translocation that produces the PML-RARa fusion protein, and is unusual among the various subtypes of AML in that it is characterized by hyperproliferation of differentiated promyelocyte progenitors. In a mouse model in which PML-RARa is expressed under the control of the endogenous murine cathepsin G promoter, we observed that whole bone marrow expressing PML-RARa derived from pre-leukemic mice has certain properties of self-renewal, such as serial replating potential in methylcellulose cultures, and serial transplantation of disease into recipient animals. Using multiparameter flow cytometry, we demonstrated that preleukemic PML-RARa animals have an expansion of mature Gr-1+c-kit-CD34− granulocytes that are not capable of self-renewal. However, in the leukemic state, the expanded population shifted to a more immature population with an immunophenotype of Gr-1+c-kit+CD34+. This population was capable of transplanting disease to secondary recipients that recapitulated the disease of the primary donor, and thus contained leukemia-initiating activity. Morphologically, these cells resembled promyelocytes, and their promyelocytic identity was confirmed by quantitative PCR detection of the expression of azurophilic primary granule components associated with the promyelocyte stage of development, including myeloperoxidase, neutrophil elastase, and cathepsin G. However, these cells lacked expression of the secondary granule protein gelatinase B associated with the more mature myelocyte stage of granulocytic differentiation. Additionally, these leukemia-initiating promyelocytes responded to differentiation and cell death signals mediated by administration of all-trans retinoic acid (ATRA). These data indicate that in APL, a highly differentiated promyelocyte compartment does in fact possess properties of leukemia stem cells. This suggests that an important step in disease pathogenesis mediated by PML-RARa is the acquisition of properties of self-renewal in a terminally differentiating progenitor that normally lacks this potential.
Disclosure: No relevant conflicts of interest to declare.