MicroRNAs 17-5p/20a/106a Function as a Master Gene Complex Controlling Monocytopoiesis through AML1 Targeting.

MicroRNAs (miRs) are a novel regulatory class of non-coding, single-stranded RNAs of ~ 22 nucleotides, identified in plants and animals. MiRs repress protein expression at post-transcriptional level, mostly through base pairing to the 3′ untranslated region (UTR) of the target mRNA, thus leading to its degradation and/or reduced translation. MiRs have been shown to control basic biological functions, such as cell proliferation and differentiation. However, little is known on their role in hematopoiesis. Furthermore, in diverse types of cancer miRs may function as “oncomirs”.

We investigated the role of miR-17-5p, -20a and -106a, located in the miR-17-5p-92 and miR-106a-92 clusters respectively, in monocytic-macrophage (Mo) differentiation-maturation. In unilineage Mo culture generated by cord blood (CB) hematopoietic progenitor cells (HPC), these miRs are gradually and markedly downmodulated, whereas AML1 is upmodulated at protein but not at mRNA level. The acute myeloid leukaemia-1 protein (AML1, also known as Runt-related transcription factor 1, Runx1) is the DNA-binding subunit of the hematopoietic transcription factor CBF, which plays a pivotal role in Mo differentiation-maturation. Luciferase assay shows that miR-17/20/106 bind the AML1 mRNA 3′UTR, implying that during monocytopoiesis their decline unblocks AML1 translation. Mo culture transfection with miR-17/20/106 stimulates blast cell proliferation and inhibits differentiation-maturation: this is coupled with AML1 translation blockade, leading in turn to inhibition of M-CSF receptor (M-CSFR) expression. Equivalent results were observed in Mo clonogenic culture of HPCs. Knockdown of endogenous miR-17/20/106 by anti-miR oligonucleotide transfection causes opposite effects. Altoghether, our studies indicate that miR-17/20/106 downmodulation during monocytopoiesis “unblocks” the translation of AML1, which in turn transactivates the M-CSFR, thereby allowing Mo differentiation and maturation. We hence propose that these miRs function as a master gene complex underlying the monocytopoiesis gene program, at least in part through the regulation of two pivotal proteins, AML1 and M-CSFR.

This study also sheds light on other relevant aspects:

• the regulatory action of miR-17/20/106 action on AML1 mRNA is restricted to the Mo lineage, implying a role for lineage-specific factor(s) to modulate the interaction of these miRs with their mRNA target;

• the subtle complexity of mechanisms controlling the transcription/processing of the miR-17-5p-92 cluster, implied by the differential expression patterns of the miRs comprised in the 17-5p-92 polycistron in Mo culture;

• the leukemogenesis mediated by AML1 fusion proteins.