MicroRNAs 155 and 125b Physiologically and Pathologically Regulate Hematopoiesis and Immunity

Citation

Chaudhuri, Aadel Ahmed (2012) MicroRNAs 155 and 125b Physiologically and Pathologically Regulate Hematopoiesis and Immunity. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HFSQ-8278. https://resolver.caltech.edu/CaltechTHESIS:09042011-230002591

Abstract

MicroRNAs are a class of ~22 nucleotide RNA molecules with roles in diverse biological processes. Here I focus on two microRNAs, miR-155 and miR-125b, and reveal pathways by which their dysregulation leads to myeloproliferative disorder (MPD) and leukemia, respectively. I begin by searching for miR-155 target genes relevant to MPD. By writing an algorithm to search microarray data for predicted microRNA target genes, I identified 89 candidate target genes for miR-155 in myeloid cells. Literature search whittled this list down to 11, and one gene among them, SHIP1, turned out to be largely responsible for miR-155’s ability to cause MPD. My focus shifted to miR-125b when I noticed that miR-125b was enriched in macrophages and thus might play important roles in that cell type. Indeed, gain- and loss-of-function experiments indicated that miR-125b is a potent activator of macrophage activation, and I identified IRF4 as the primary target gene in this process. Finally I asked whether miR-125b plays pathophysiological roles in the development of the hematopoietic system. Thus I overexpressed miR-125b in the hematopoietic system and, to my surprise, observed a very aggressive myeloid leukemia capable of infiltrating peripheral organs including the lungs, liver, kidneys and brain. To determine whether miR-125b is physiologically necessary for normal hematopoietic development, I designed a loss-of-function sponge vector that acts as a decoy, attracting the microRNA away from its normal targets. Use of the sponge in the mouse hematopoietic system led to significantly decreased overall hematopoietic ouput, indicating that miR-125b is physiologically required for normal hematopoiesis. Next, I assayed in vitro a panel of miR-125b target genes and saw that one, Lin28, was superior to the rest. Indeed, Lin28 gain- and loss-of-function in vivo recapitulated major aspects of miR-125b loss- and gain-of-function, respectively. Thus I identified Lin28 as a primary target of miR-125b in the hematopoietic system. In summary, my work shows that two microRNAs, miR-155 and miR-125b, physiologically and pathologically control hematopoietic development. I also identify important target genes for each of these microRNAs in their respective disease processes. Indeed, therapeutic targeting of these pathways may prove useful in the treatment of cancer.

Thesis Files