miR-99a/100~125b tricistrons regulate hematopoietic stem and progenitor cell homeostasis by shifting the balance between TGFβ and Wnt signaling

Although regulation of stem cell homeostasis by miRNAs is well studied, it is unclear how individual miRNAs, genomically encoded within an organized polycistron, can interact to induce an integrated phenotype. miR-99a/100, let-7 and miR-125b paralogues are encoded in two tricistrons on human chromosome 11 and 21. They are highly expressed in hematopoietic stem cells (HSCs) and acute megakaryoblastic leukemia (AMKL), an aggressive form of leukemia with poor prognosis. Integrative analysis of global gene expression profiling, miRNA target prediction and pathway architecture revealed that miR-99a/100, let-7 and miR-125b functionally converge at the combinatorial block of the TGFβ pathway by targeting four receptor subunits and two SMAD signaling transducers. In addition, downregulation of tumor suppressor genes APC/APC2 stabilizes active β-Catenin and enhances Wnt signaling. By switching the balance between Wnt and TGFβ signaling the concerted action of these tricistronic miRNAs promoted sustained expansion of murine and human HSCs in vitro or in vivo, while favoring megakaryocytic differentiation. We lentivirally transduced cord blood CD34+-hematopoietic stem and progenitor cells (CB-HSPCs) to ectopically express miR-125b-2, miR-99a, let-7c or miR-99a~125b-2 and cultured them in megakaryocytic differentiation medium for 7 days.