Abstract 1191: Role of histone deacetylases in silencing functionally relevant microRNAs in CLL

Chronic lymphocytic leukemia (CLL) is characterized by the selective down-regulation of a class of gene regulators, the microRNA (miRs) in comparison to healthy B- lymphocytes. Low levels of miRs are associated with multiple aspects of CLL pathology such as the genesis of disease, apoptotic resistance, progression, and overall survival, suggesting that miRs may function as tumor suppressors in this disease. However, despite the central role of miRs in regulating CLL pathobiology, the mechanisms by which they become under-expressed in CLL are not well known. We hypothesized that the gene repressive action of the histone deacetylases (HDACs) may lead to the silencing of functionally relevant miRs in CLL. HDACs lead to the deacetylation of histones at gene promoters, an action that triggers a loss in the levels of the transcriptionally required histone modification, H3K4me3, which leads to gene silencing. Therefore, we used chromatin immunoprecipitation directed at the H3K4me3 mark to conduct a genome wide analysis of the levels of this transcriptionally activating modification at all miR promoters in normal CD19+ B-cells and in CLL cells before and following HDAC inhibition. Our data demonstrates that normal B-cells displayed the H3K4me3 mark at miR promoters. In contrast, primary CLL samples lacked the H3K4me3 mark at miR promoters but accumulated this activating modification at multiple promoters upon HDAC inhibition. Correspondingly, accumulation of H3K4me3 was associated with an increase in the expression levels of the cognate miRNAs in CLL samples. To determine whether the epigenetically re-expressed miR genes had functional relevance in CLL we first performed a pathway analysis using the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway database. This analysis indicated that the targets of the epigenetically repressed miRs had roles in regulating apoptosis, proliferation, or as transcription factors indicating that the miRs that underwent HDAC-mediated silencing in CLL had important functional roles in regulating multiple, critical cellular processes in CLL. In addition, a detailed functional evaluation of a panel of representative miRs that underwent HDAC-mediated silencing identified both Mcl-1 and E3 ligases that negatively regulate p73-dependent apoptotic signaling among their cellular targets. Finally, our data also identified the presence of HDAC1 at epigenetically silenced miR promoters in CLL suggesting that HDAC1 is an integral component of the repressor complex that silences miR promoters in CLL. Thus our data delineates the complement of miR that is silenced by HDACs in CLL, identifies that such silencing may have functional consequences in facilitating apoptotic resistance or progression in CLL and may identify a new target for therapeutic manipulation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1191. doi:10.1158/1538-7445.AM2011-1191