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miR-155 targets histone deacetylase 4 (HDAC4) and impairs transcriptional activity of B-cell lymphoma 6 (BCL6) in the Eµ-miR-155 transgenic mouse model

Sandhu, Sukhinder K., Volinia, Stefano, Costinean, Stefan, Galasso, Marco, Neinast, Reid, Santhanam, Ramasamy, Parthun, Mark R., Perrotti, Danilo, Marcucci, Guido, Garzon, Ramiro, Croce, Carlo M.
Proceedings of the National Academy of Sciences of the United States of America 2012 v.109 no.49 pp. 20047-20052
B-lymphocytes, animal models, apoptosis, cell viability, cyclins, gene expression regulation, histone deacetylase, immune response, interleukin-6, leukemia, lymphoma, meta-analysis, mice, microRNA, microarray technology, patients, proto-oncogenes, repressor proteins, transcription (genetics), transcriptomics, transgenic animals
Multiple studies have established that microRNAs (miRNAs) are involved in the initiation and progression of cancer. Notably, miR-155 is one of the most overexpressed miRNAs in several solid and hematological malignancies. Ectopic miR-155 expression in mice B cells (Eμ-miR-155 transgenic mice) has been shown to induce pre–B-cell proliferation followed by high-grade lymphoma/leukemia. Loss of miR-155 in mice resulted in impaired immunity due to defective T-cell–mediated immune response. Here we provide a mechanistic insight into miR-155–induced leukemogenesis in the Eμ-miR-155 mouse model through genome-wide transcriptome analysis of naïve B cells and target studies. We found that a key transcriptional repressor and proto-oncogene, Bcl6 is significantly down-regulated in Eμ-miR-155 mice. The reduction of Bcl6 subsequently leads to de-repression of some of the known Bcl6 targets like inhibitor of differentiation (Id2), interleukin-6 (IL6), cMyc , Cyclin D1 , and Mip1α/ccl3 , all of which promote cell survival and proliferation. We show that Bcl6 is indirectly regulated by miR-155 through Mxd1/Mad1 up-regulation. Interestingly, we found that miR-155 directly targets HDAC4, a corepressor partner of BCL6. Furthermore, ectopic expression of HDAC4 in human-activated B-cell–type diffuse large B-cell lymphoma (DLBCL) cells results in reduced miR-155–induced proliferation, clonogenic potential, and increased apoptosis. Meta-analysis of the diffuse large B-cell lymphoma patient microarray data showed that miR-155 expression is inversely correlated with Bcl6 and Hdac4 . Hence this study provides a better understanding of how miR-155 causes disruption of the BCL6 transcriptional machinery that leads to up-regulation of the survival and proliferation genes in miR-155–induced leukemias.