The role of let-7c/miR-125b/miR-99a in the hematopoietic development in the human bone marrow

MicroRNAs (miRNAs) are a class of non-coding RNAs that regulate gene expression post-transcriptionally and in recent studies have been shown to play a key role in governing lineage in hematopoietic progenitors. Although much is known about the molecular signals that direct early hematopoiesis in mice, very little is understood about the hematopoietic process in adult human bone marrow. The main purpose of this thesis was to identify the miRNAs that are involved in the early hematopoietic development in the human bone marrow. Microarray analysis on the hematopoietic stem cells (HSC), lymphoid primed multipotent progenitors (LMPP), and common lymphoid progenitor (CLP) from human bone marrow was used to obtain a differential miRNA expression profile. The second part of the research was focused on validating the results using qRT-PCR on HSCs, LMPPs, and CLPs to confirm the expression of miRNAs. The results revealed microRNAs let-7c and miR-125b expression to be significantly up regulated in HSCs compared to LMPPs, CLPs and even more mature and differentiated (B-cells, T- ix cells, monocytes, macrophages, Natural killer, and erythroid) cells. Furthermore, let-7c and miR-125b are a supposed to be a part of a cluster miRNA including miR-99a, we decided to assess for expression levels of all three miRNAs in all the stem, progenitor and differentiated cells to obtain a comprehensive picture of miRNA expression across the different cells. Our results revealed that let-7c, miR-125b, and miR-99 all to be highly expressed specifically in bone marrow HSCs. Lastly, we also investigated the let-7c/miR-125b/miR-99a expression in stem and progenitor cells in human embryonic stem cells (hESCs). We found let-7c, miR-125b, and miR-99a to be indeed significantly expressed in stem like HSCs derived from hESCs as seen in bone marrow HSCs. Based on the results of our experiments, this very cell stage specific expression pattern of the miRNAs in different populations strongly suggests to us that miRNAs do play some kind of regulatory role in controlling the cell fate determination and cell lineage differentiation. It would be clinically relevant if miRNAs can be efficiently used in the expansion and maintenance of hematopoietic stem cells in vitro in the future.