MicroRNAs Act as Decoy Molecules to Inhibit the Function of RNA Binding Proteins

RNA Binding Proteins Anna M. Eiring *1 Abstract Altered microRNA (miRNA) expression contributes to aberrant post-transcriptional gene regulation in several types of cancers; however, their role in the progression of chronic myeloid leukemia (CML) from chronic phase (CML-CP) to blast crisis (CML-BC) is still largely unknown. To gain further insight into the role of miRNAs in CML disease progression, we used microarray-based techniques to analyze miRNA expression in CML-BC compared to CMLCP progenitors and in BCR/ABL-expressing myeloid cell lines compared to untransformed controls. Using this method, we identified a discrete number of miRNAs either upregulated (34 miRNAs) or downregulated (14 miRNAs) in both BCR/ABL cell lines and primary patient samples. Among the downregulated miRNAs, we focused our attention on miR-223 because of its reported role in myelopoiesis, miR-15a/16-1 because of their reported role as tumor suppressors, and miR-328, a miRNA with no currently known function. Northern blot and qRTPCR analyses validated the results of our microarray analysis, revealing a marked reduction in miR-223, miR-328, miR-15a, and miR-16-1 expression in 32D-BCR/ABL and K562 cells (5075% inhibition), and expression of these miRNAs was rescued upon treatment of cells with the tyrosine kinase inhibitor imatinib (2 mM; 24h). Interestingly, sequence analysis of both miR-223 and miR-328 revealed homology with the hnRNP E2-mRNA binding site contained in the uORF spacer region of the CEBPA 5’ UTR (CEBPA 5’ uORF). hnRNP E2 is the RNA binding protein responsible for block of myeloid differentiation in CML-BC progenitors, and does so by binding to the CEBPA 5’ uORF to