The Ubiquitination-Dependent Interaction of p210 BCR/ABL with β-Catenin Contributes to Chronic Myeloid Leukemogenesis

Abstract 4186

Chronic myelogenous leukemia (CML) is a malignant hematopoietic stem cell disorder that is invariably associated with the kinase activity of BCR/ABL. Although the ABL-encoded kinase activity is essential for disease progression, several studies have shown that BCR encoded sequences are also necessary for BCR/ABL-mediated leukemogenesis. In this current study we have identified a ubiquitin binding domain (UBD) in p210 BCR/ABL which is present in all fusion variants of BCR/ABL (p190, p210 and p230). This UBD does not conform to any known consensus sequence, and is unique to BCR and BCR/ABL. Deletion of the UBD does not impair the auto- or trans-kinase activity of p210 BCR/ABL, nor does it impair the interaction between p210 BCR/ABL and GRB2, or the ability of p210 BCR/ABL to activate ERK1/2. A mutation at residue tyr-177 of p210 BCR/ABL also does not impair the interaction with ubiquitin suggesting that the GRB2 and ubiquitin binding sites are adjacent, but separable. β-catenin has been identified as a binding partner for BCR and BCR/ABL, and the docking site has been mapped to a region of BCR/ABL that contains the UBD. Over-expression of p210 BCR/ABL, but not the ubiquitin binding mutant, leads to an accumulation of β-catenin that is phosphorylated on the serine residues that normally trigger ubiquitin-mediated turnover. This difference cannot be attributed to a difference in the activation status of GSK-3β. Treatment with an E1 inhibitor impairs the interaction between β-catenin and p210 BCR/ABL suggesting that the interaction is ubiquitination-dependent. Consistent with this possibility, previous studies have identified a glycine residue in β-catenin that is required for the BCR interaction. Based on these observations we propose a model in which p210 BCR/ABL may influence the Wnt signaling pathway by binding ubiquitinated β-catenin, and stabilizing it against degradation. In a murine bone marrow transplantation model for CML, mice transplanted with hematopoietic cells that express the ubiquitin binding mutant have significantly increased life spans compared to mice transplanted with p210 BCR-ABL expressing cells. This delayed disease progression may be due to a decrease in levels of β-catenin and less granulocyte macrophage progenitors (GMPs), which are thought to function as leukemic stem cells in the blast phase of CML. The role of ubiquitin-mediated b-catenin binding in p210 BCR/ABL-mediated leukemogenesis is currently being explored.