Dissecting molecular mechanisms of leukemogenesis with a mouse chronic myelogenous leukemia model

The bcr-abl oncogene plays a critical role in the pathogenesis of chronic myelogenous leukemia (CML). Expression of Bcr-Abl in bone marrow cells by retroviral transduction efficiently induces in mice a myeloproliferative disease resembling human CML. The role of the domains of Bcr-Abl and its activated signaling pathways in induction of myeloproliferative disease is not known. We found that functions of the Abl SH3 domain are either not required or redundant in Bcr-Abl-induced myeloproliferative disease. In addition, c-Abl activated by SH3 deletion induced only lymphoid malignancies in mice, indicating that Bcr sequences in Bcr-Abl play additional roles in inducing myeloproliferative disease beyond simply activating the Abl kinase domain. It has been unclear whether the apparent specificity of Bcr-Abl and v-Abl for myeloid versus lymphoid neoplasms is due to specific intrinsic properties of these Abl oncoproteins, or due to the properties of the target cells expressing them. Under equal bone marrow transduction/transplantation conditions, we found that early in the course of disease, both Bcr-Abl/p210 and v-Abl/p160 expanded early immature hematopoietic cells. Later Bcr-Abl/p210 selectively expanded myeloid cells while v-Abl/p160 primarily induced the rapid in vivo expansion of B-lymphoblastic cells, along with a minor population of myeloid cells. In vitro, Bcr-Abl/p210 induced more growth of myeloid colonies from 5-fluorouracil treated bone marrow than v-Abl/p160. These results indicate that Bcr-Abl/p210 has a greater intrinsic capacity than v-Abl/p160 to induce the neoplastic growth of myeloid cells. In addition, we found that cultured cells expressing Bcr-Abl/p210 had more activated STAT5 than cells that expressed v-Abl/p160. This suggests that activation of STAT5 might be one part of the mechanism of abl oncogene disease specificity. Constitutively activated STAT proteins are found in many transformed cells and tumor cells. The role of this constitutive activation in leukemogenesis is not known. Expression of the constitutively activated STAT3-C mutant in multipotential hematopoietic cells was not sufficient to cause disease or to disturb long-term reconstitution of the hematopoietic system. The constitutively activated STAT5A 1*6 mutant induced disease and transformed cells, but only at a high protein expression level. This implies that molecular events other than activation of STAT5 must also be involved in the development of human hematological neoplasms. Our results also demonstrate that the quantity of an oncogene's expression level should be considered when evaluating its biological functions. Therefore, we developed a method to lower the protein expression level from expression vectors in vivo, for experimental situations and potentially in gene therapy.