The MiR-23a MicroRNA Cluster Inhibits B Cell Development.

Abstract 1465

Poster Board I-488

The miR-23a microRNA (miRNAs) cluster inhibits both [ITALIC]in vitro[/ITALIC] and [ITALIC]in vivo[/ITALIC] B cell development. When murine hematopoietic progenitor cells expressing the 23a cluster miRNAs were cultured in B cell promoting conditions we observed over a five-fold decrease in the generation of CD19+ B cells compared to control cultures. Conversely, we observed over a five-fold increase in CD11b+ myeloid cells. When irradiated mice were transplanted with bone marrow expressing the miR-23a cluster we observed a two-fold decrease in bone marrow and splenic B cells, 8 weeks post-transplant compared to control mice. The miR-23a cluster codes for a single pri-transcript, which when processed yields three mature miRNAs: miR-23a, miR-27a, and miR-24-2. All three mature miRNAs are more abundant in myeloid cells compared to other hematopoietic cells. In vitro miR-24 alone is necessary and sufficient to inhibit B cell development. The promoter for the cluster contains conserved binding sites for the essential myeloid transcription factors PU.1 and C/EBP alpha. Chromatin immunoprecipitations demonstrated that PU.1 and C/EBP alpha are associated with the promoter in myeloid cells. In addition, C/EBP alpha is bound to several highly conserved regions upstream of the promoter. Both PU.1 and C/EBP alpha promote myeloid development at the expense of lymphopoiesis. Our work suggests that the miR-23a cluster may be a critical downstream target of PU.1 and C/EBP alpha in the specification of myeloid cell fate. Although miRNAs have been identified downstream of PU.1 and C/EBP alpha in mediating the development of monocytes and granulocytes, the 23a cluster is the first downstream miRNA target implicated in the regulating lymphoid cell fate acquisition. We are currently identifying targets of miR-24 that may mediate the inhibitory effect on B lymphopoiesis.