Epigenetic Regulation of MicroRNA Clusters and Families during Tumor Development

SIMPLE SUMMARY: In this review, the history of RNA interference discovery and current knowledge about microRNA biogenesis and post-transcriptional regulation of gene expression is summarized, with a special focus on microRNA clusters and families. Further, strong interplay between microRNAs and basic epigenetic mechanisms, such as DNA methylation and histone modifications, are introduced and associated with deregulated expression of microRNAs during tumor development. Finally, novel strategies for epigenetic-based therapies are discussed. ABSTRACT: MicroRNAs are small non-coding single-stranded RNA molecules regulating gene expression on a post-transcriptional level based on the seed sequence similarity. They are frequently clustered; thus, they are either simultaneously transcribed into a single polycistronic transcript or they may be transcribed independently. Importantly, microRNA families that contain the same seed region and thus target related signaling proteins, may be localized in one or more clusters, which are in a close relationship. MicroRNAs are involved in basic physiological processes, and their deregulation is associated with the origin of various pathologies, including solid tumors or hematologic malignancies. Recently, the interplay between the expression of microRNA clusters and families and epigenetic machinery was described, indicating aberrant DNA methylation or histone modifications as major mechanisms responsible for microRNA deregulation during cancerogenesis. In this review, the most studied microRNA clusters and families affected by hyper- or hypomethylation as well as by histone modifications are presented with the focus on particular mechanisms. Finally, the diagnostic and prognostic potential of microRNA clusters and families is discussed together with technologies currently used for epigenetic-based cancer therapies.