Eradication of CML stem cells

Abstract

Bcr-Abl tyrosine kinase inhibitors (TKIs) have become the standard of care for patients with chronic myeloid leukemia (CML). Indeed, patients experience high response rates and long-term survival with continuous TKI treatment. However, TKIs rarely cure CML due to their inability to target CML stem cells. Consequently, CML will soon become the most prevalent leukemia with 100,000 patients in the U.S. alone. Long-term treatment with TKIs is extremely expensive, associated with side effects, and development of resistance in some patients. Resistance can fuel the progression to blast crisis (BC), which is associated with almost complete chemo-resistance and extremely poor treatment outcome. During the last decade, significant insights into CML stem cell biology and mechanisms of TKI resistance were gained leading to the development of combinatorial strategies to target CML stem/progenitor cells and to overcome TKI resistance [1,2]. We and others have established Bcl-2 family proteins as key apoptosis regulators and specifically anti-apoptotic Bcl-2 proteins as crucial survival factors for myeloid leukemia cells and stem/progenitor cells. Inhibition of anti-apoptotic Bcl-2 proteins with dual Bcl-2/Bcl-xL or pan-Bcl-2 inhibitors was shown to target CML stem/progenitor cells and enhance the therapeutic efficacy of TKIs [3,4]. The tumor suppressor p53 regulates apoptosis primarily by transcriptional activation of pro-apoptotic Bcl-2 family proteins. Although frequently mutated in solid tumors, p53 mutations are rare in CML. We demonstrated that the activation of p53 via inhibition of its negative regulator, MDM2, in combination with TKIs synergistically targeted quiescent CD34 + BC CML cells [5], and Holyoake recently reported that dual targeting of p53 and c-MYC selectively eliminated CML stem cells [6]. To improve specificity and efficacy, and minimize toxicity, it is important to recognize which Bcl-2 proteins are indispensable for CML stem cell survival. Until recently, most Bcl-2 inhibitors were relatively non-specific and targeted several Bcl-2 proteins. Furthermore, our knowledge of the expression of Bcl-2 family members in hematopoietic and CML stem/progenitor cells is essentially limited to RNA, not protein levels, primarily because stem/progenitor cells account for only a very small portion of total bone marrow (BM) cells. CyTOF (" cytometry by time-of-flight ") combines mass spectrometry and flow cytometry and constitutes a novel single cell proteomics system that can determine the expression of currently over 40 (potentially 120) cell surface and intracellular proteins simultaneously without the spectral overlap, and therefore able to determine the expression of multiple proteins/phosphoproteins in a phenotypically well-defined cell population. Using CyTOF, and an inducible …