Targeting miRNA-551b, a "Stemness"-like microRNA, to Eradicate AML (Stem) Cells
Authors: Martiáñez Tània , Van Gils Noortje , De Leeuw David Christian , Vermue Eline , Rutten Arjo , Denkers Fedor , Verhagen Han J.M.P. , Janssen Jeroen J. W. M. , Ossenkoppele Gert J. , Jimenez Connie , Smit Linda 
Published: October 2019

Abstract
Despite high complete remission (CR) rates achieved after chemotherapy, only 30-40% of patients with Acute Myeloid Leukemia (AML) survive five years after diagnosis. The main cause of this treatment failure is insufficient eradication of a subpopulation of chemotherapy-resistant leukemia cells with stem cell properties, named "leukemic stem cells" (LSCs). LSCs use a variety of mechanisms to resist chemotherapy and targeting them is one of the major challenges in AML treatment. Since miRNAs can target multiple genes/pathways simultaneously, their modulation (downregulation or upregulation) may have great potential for the successful elimination of therapy-resistant leukemic (stem) cells (Martiañez Canales et al. Cancers 2017). Here, we show that miRNA-551b, previously identified by us as a stem cell-like miRNA, can be a potential novel target to specifically eradicate AML stem-like cells.
Aiming at identification of miRNA-based therapy to specifically eradicate LSCs, while sparing normal Hematopoietic Stem Cells (HSCs), we determined expression of miRNAs in normal HSCs, Leukemic Stem Cells (LSCs) and leukemic progenitors (LP) all derived from the same AML patient's bone marrow. Using this approach, we identified miRNA-551b as being highly expressed in normal HSCs residing both in healthy and AML bone marrows. In AML, high expression of miR551b demonstrated to be associated with an adverse prognosis. Moreover, miRNA-551b was highly expressed in immature AML cases and its expression in a cohort of patients coincided with the expression of stem cell genes (De Leeuw et al. Leukemia 2016).
To further elucidate the link between miRNA-551b and AML "stemness" and to test whether downregulation of miRNA-551b affects the survival of AML (stem/progenitor) cells, proliferation and the balance between differentiation and "stemness", we reduced miRNA-551b expression, either by lentiviral transduction of antagomirs or by adding locked nucleotide acid (LNA)-oligonucleotides to AML cell lines and primary AML cells. Downregulation of miRNA-551b in the stem cell-like AML cell line KG1a led to inhibition of cell growth in vitro, which was due to inhibition of proliferation rather than induction of apoptosis. KG1a tumor growth in an in vivo mouse model was also reduced when miRNA-551b was downregulated. In primary AML, miRNA-551b knockdown resulted in a significant decrease in the survival of leukemic progenitors and LSCs, while hematopoietic stem cells (HSCs) and normal progenitors from healthy bone marrows were not affected. These results suggest that a therapeutic approach inhibiting miRNA-551b expression might specifically eradicate leukemic progenitors and LSCs from primary AML, while sparing HSCs. We are currently studying miRNA-551b targets which can be responsible for this specific LSCs elimination.
In conclusion, our results suggest that inhibition of miRNA-551b could be a promising approach to eliminate stem cell-like AML cells, thereby decreasing relapse rates and improving AML treatment outcome.

Disclosures
Ossenkoppele: Pfizer: Consultancy, Honoraria; BMS: Consultancy, Honoraria; Genentech: Consultancy, Honoraria; Jazz: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Karyopharm: Consultancy, Research Funding; Roche: Consultancy, Honoraria; Celgene: Honoraria, Research Funding; Johnson & Johnson: Consultancy, Honoraria, Research Funding; Genmab: Research Funding.