Unique molecular differences between acute myelogenous leukemia and normal hematopoietic stem cells

Human a&barbelow;cute m&barbelow;yelogenous l&barbelow;eukemia (AML) is a clonal disorder of immature hematopoietic cells. Heterogeneity is found in the characteristics of malignant cells among afflicted patients. The disease is therefore classified into multiple subtypes. Studies have shown that AML arises from a rare population of leukemic stem cells (LSCs) that are phenotypically defined as CD34+/CD38−/CD117−/CD123+. LSCs are found in nearly all AML subtypes and are sufficient to initiate and maintain leukemic growth in both long-term cultures and NOD/SCID mice. Because LSCs are likely to have a critical role in leukemic disease, we sought to identify the unique molecular properties of LSCs that distinguish them from normal hematopoietic cells (HSCs).; Using gene expression array analysis, we identified several genes differentially expressed in LSCs when compared to HSCs. Among these genes were death-associated protein kinase (DAPK) and interferon regulatory factor 1 (IRF-1), which participate directly or indirectly and promoting cell death. These genes were constitutively expressed in all primitive AML specimens analyzed, but absent in normal primitive cells. Despite the heightened expression of pro-apoptotic signals, these cells remained viable, indicating the possibility of one or more anti-apoptotic signals in these cells.; NFκB, a pro-survival factor usually inactive in normal cells, was found to be constitutively active in LSCs. Inhibition of NFκB using the proteasome inhibitor, MG-132, resulted in strong induction of apoptosis in LSCs but not in HSCs, indicating a potential role for NFκB in LSC survival. Moreover, MG-132 used in conjunction with the chemotherapeutic drug, idarubicin (IDR), greatly enhanced induction of apoptosis relative to treatment with either drug alone. NOD/SCID mouse transplantation analyses were used to demonstrate functional ablation of LSCs upon treatment with MG-132 and IDR in combination. MG-132/IDR-treated LSCs failed to engraft in recipient animals. In contrast, engraftment of normal HSCs was not affected. Together, these data suggest that unique molecular differences between LSCs and HSCs can be used as therapeutic targets.