MicroRNA Expression Profiling of Hematopoietic and Leukemic Stem Cells in Acute Myeloid Leukemia,

Abstract 3999

Treatment of acute myeloid leukemia (AML) patients with chemotherapy results in complete remission in up to 80% of patients. However, 50% of patients experience a relapse which is thought to be caused by the survival of a population of dormant leukemic stem cells (LSC) that are resistant to chemotherapy. These LSC are responsible for the initiation, maintenance and relapse of AML and eradication of these cells is therefore necessary to cure AML patients. LSC and normal hematopoietic stem cells (HSC) coexist in AML bone marrow and future LSC-specific therapy relies on identification of new targets differentiating normal from malignant stem cells. Our major aim is the development of novel LSC specific microRNA-based therapeutic strategies by identification of microRNAs differentially expressed and functioning in the AML LSC and HSC critical for self-renewal and/or therapy response. We want to use microRNA expression profiling of LSC and HSC within one AML bone marrow sample to identify LSC-specific targets.

MicroRNAs are non-coding single stranded RNAs of 21–25 nucleotides. They regulate gene expression by promoting degradation of mRNA or repressing its translation. MicroRNAs are critical regulators involved in various cell processes, including differentiation, proliferation and apoptosis. Whereas a few studies have been performed comparing microRNA expression of LSC with healthy donor HSC, which is not identical to AML HSC, no such studies have been performed comparing LSC with HSC both present in the same AML bone marrow. This is now possible with the by us identified markers/parameters discriminating LSC from HSC present within one AML bone marrow sample.

Recently we have shown that the small portion of healthy HSC in the CD34+CD38- compartment of AML patients can be identified and purified by using aldehyde dehydrogenase activity. CD34+CD38- stem cells with high aldehyde dehydrogenase activity have no molecular and immunophenotypical aberrancies suggesting a normal phenotype (HSC). Whereas CD34+CD38- stem cells with low aldehyde dehydrogenase activity, harbor leukemia associated aberrancies. This shows that normal HSC have high aldehyde dehydrogenase activity as compared to LSC residing in the same AML bone marrow. We have studied the microRNA expression profile of AML LSC and normal HSC, discriminated by aldehyde dehydrogenase activity, within the patient's bone marrow in order to find microRNAs differentially expressed in LSC and HSC. Besides this, we have studied the microRNA expression profile in the more differentiated blast population of the AML bone marrow.

Cells from the bone marrow of AML patients were purified by flow cytometry based on surface marker expression and aldehyde dehydrogenase activity. Leukemic blasts by being CD34⁺, LSC by being CD34+CD38-ALDHlow and HSC by being CD34+CD38-ALDHhigh. RNA was isolated from the various cell populations and microRNA expression was studied using Agilent microRNA microarrays. Most microRNAs show similar levels of expression in the different cell compartments. However, expression of miR-126 was significantly enhanced while the tumorsupressors miR-451 and miR-34a were downregulated in LSC as compared to leukemic blasts. Comparison of the microRNA expression profile of LSC and HSC showed that expression of miR-21, miR-181a/b and the cluster miR-221/222 was significantly upregulated while others, such as miR-10a and miR-451 were downregulated.

In conclusion, we have identified several microRNAs that are differentially expressed between leukemic blasts, LSC and HSC. Some of these microRNAs have already been linked to stem cell properties such as senescence or chemotherapy resistance while others have unknown functions. Our findings might give more insights in the mechanisms regulating LSC and are important for understanding processes like leukemogenesis and relapse. Furthermore, these microRNAs might in de future prove to be valuable targets for LSC directed therapy while saving the normal HSC.