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Ru Chen
Interdisciplinary Biomedical Sciences Program
B.S. 1999, Shanghai Jiao Tong University
M.S, 2005, Seton Hall University

Thesis Advisor: Ian Whitehead, Ph.D.
Department of Microbiology & Molecular Genetics

Tuesday, November 23, 2010
Cancer Center, G Level Seminar Room, 9:00 A.M.


Chronic myelogenous leukemia (CML) is a malignant hematopoietic stem cell disorder that is invariably associated with a balanced reciprocal translocation between chromosomes 22 and 9. The major product of the rearrangement is a 210 kDa in-frame fusion protein (p210 Bcr-Abl) that contains amino-terminal sequences from Bcr, and carboxyl-terminal sequences from the non-receptor tyrosine kinase Abl. This fusion event leads to the constitutive activation of the Abl-encoded kinase activity, which is the principal driving force behind CML. Thus, inhibitors that target the kinase activity produce durable clinical remissions. Although the Abl-encoded kinase activity is essential for disease progression, several studies have shown that Bcr encoded sequences are also necessary for p210 Bcr-Abl-mediated leukemogenesis. Previous studies from our lab have identified ubiquitin as a binding partner for Bcr. The goal of this research was to demonstrate that binding is retained in p210 Bcr-Abl and the function of ubiquitin binding in leukemogenesis. We localized the docking site for ubiquitin to residues 180-191 which is present in all fusion variants of Bcr-Abl (p190, p210 and p230). This putative ubiquitin binding domain (UBD) does not conform to any known consensus sequence, and is unique to Bcr. 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. -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 -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-3 or Pin-1. Treatment with an E1 inhibitor impairs the interaction between -catenin and p210 Bcr-Abl, suggesting that the interaction is ubiquitination-dependent. Consistent with this possibility, previous studies have identified a lysine residue in -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 -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 -catenin and in less granulocyte macrophage progenitors (GMPs), which are thought to function as cancer leukemic stem cells in the blast phase of CML.

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