p53 and c-Myc Are Critical Signaling Hubs That Maintain Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a clonal disorder of the hematopoietic system, leading to increased production of mature and progenitor myeloid cells. Although protein tyrosine kinase inhibitors (TKI) have been successful in managing the disease, there are exceptions where drug resistance and onset of blast crisis occur. Furthermore TKIs are ineffective against leukemic stem cells (LSC) that are responsible for disease initiation and maintenance.

We have shown mRNA changes in primitive hematopoietic cells do not correlate directly to protein changes. Therefore to elucidate fundamental cellular differences between CML and normal cells we employed a proteomic approach (mass spectrometry with isobaric tagging for relative quantification). This approach permits unbiased analyses using direct comparative quantification of peptides and thus proteins from chronic phase CML and normal CD34⁺ human samples. Systematic data analysis identified that the majority of deregulated proteins are connected and regulated by two oncogenes with well defined roles in human disease, p53 and c-myc. The direction of regulation inferred suppression of p53 and up-regulation of c-myc. Altered expression of key proteins was validated using western blotting and immuno-fluorescence approaches. All (6/6) candidate/hub proteins identified using mass spectrometry were confirmed using these orthogonal approaches. Based on our systematic analysis, we targeted the candidate hubs using the drugs RITA (activates p53) and CPI-203 (inhibits c-myc expression; provided by Constellation Pharmaceuticals). In CML CD34⁺ cells, RITA reduced cell expansion in a concentration-dependent manner and induced significant levels of apoptosis as confirmed by positive staining of Annexin V and 4',6-diamidino-2-phenylindole (DAPI) using flow cytometry. CPI-203 also reduced cell expansion, but importantly induced differentiation in addition to apoptosis, as supported by flow cytometric monitoring of levels of carboxyfluorescein succinimidyl ester (CFSE) and CD34. Overlays of CFSE plots for untreated control vs. CPI-203 demonstrated that as cells divided in the presence of CPI-203, there was clear and rapid loss of CD34 expression which was not seen with RITA treatment. By measuring the dose-effect relationship of each drug alone and in combination, we demonstrated potent synergy with combination index (CI) values ranging from 0.034-0.286 based on loss of cell viability. Using flow cytometry we gated on CD34⁺38^- CML cells to enable quantification of the differential effects of each drug alone and in combination against the most primitive and quiescent 1-5% of total CD34⁺ cells. Critically the apoptotic effect was inclusive of primitive CD34⁺38^- cells and quiescent CFSE^max populations. In addition, experiments combining RITA and CPI-203 demonstrated undetectable colony forming cell units at the highest concentrations of drug used. Importantly, it appears that combining these two drugs has negligible effects on normal CD34⁺ cell counts, apoptosis and CFSE profiles. Currently NOD-SCID IL2R gamma null (NSG) repopulation assays are underway to determine if these drugs affect stem cells capable of engrafting immunocompromised mice. Our systems biology approach suggests that altered c-myc and p53 function underlie the most significant cellular differences within CML CD34⁺ cells, which has not been previously demonstrated. We confirm that in CML, p53 and c-myc hub proteins have the ability to modulate downstream defined target proteins thereby enhancing survival and proliferation and thus allowing maintenance of disease.