Regulation of self-renewal by leukemogenic mutations associated with acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a subtype of acute myeloid leukemia (AML) that accounts for about 5-10% of cases of AML and is characterized by hyperproliferation of promyelocytic progenitors. The genetics of APL are well understood: greater than 95% of cases express the PML-RARα oncogenic fusion protein as a consequence of the chromosomal translocation t(15;17)(g22;q12). Roughly 40% of cases also harbor activating mutations in the receptor tyrosine kinase FLT3 , usually in the form of an internal tandem duplication within the juxtamembrane domain (FLT3-ITD). We characterized the transformative roles of PML-RARα, FLT3-ITD, and additional oncogenic events in the pathogenesis of APL, with a focus on the regulation of self-renewal of the leukemic population, and in particular, the promyelocyte compartment. A murine model of APL in which the PML-RARα fusion is "knocked-in" to the promyelocyte-specific cathepsin G locus served as our experimental system. The extended disease latency of these mice indicates that additional mutations must occur for full transformation to acute leukemia. First, we assessed the relative contributions of PML-RARα and FLT3-ITD to the APL phenotype using accurate genetic models of expression by generating PML-RARα/FLT3-ITD double knock-in animals. In this context, FLT3-ITD did not cooperate with PML-RARα. Because these two oncogenes cooperate in a bone marrow transplant model of APL, we hypothesized that retroviral integration sites may be important in disease development. We therefore cloned retroviral integration sites from transplant mice and identified the transcription factor Gfi-1 as a novel cooperative partner in the pathogenesis of APL. Finally, we analyzed the role of PML-RARα in the process of self-renewal. We observed that bone marrow progenitors expressing PML-RARα derived from non-leukemic mice had certain properties of self-renewal. We hypothesized that the self-renewing and leukemia-initiating population in APL may be a committed myeloid progenitor, and may in fact be a transformed promyelocyte. We demonstrated that in the leukemic state, PR/+ animals have an expanded promyelocyte compartment that is highly enriched for leukemia-initiating activity. These data indicate that in APL, a highly differentiated promyelocyte compartment does in fact possess properties of leukemia stem cells, and that self-renewal ability conferred by PML-RARα is an initiating event during leukemogenesis.