Epigenetics and microRNAs.

Epigenetics is defined as mitotically and meiotically heritable changes in gene expression that do not involve a change in the DNA sequence. Two major areas of epigenetics-DNA methylation and histone modifications-are known to have profound effects on controlling gene expression. DNA methylation is involved in normal cellular control of expression, and aberrant hypermethylation can lead to silencing of tumor-suppressor genes in carcinogenesis. Histone modifications control the accessibility of the chromatin and transcriptional activities inside a cell. MicroRNAs (miRNAs) are small RNA molecules, approximately 22 nucleotides long that can negatively control their target gene expression posttranscriptionally. There are currently more than 460 human miRNAs known, and the total number is predicted to be much larger. Recently, the expression of miRNAs has been definitively linked to cancer development, and miRNA profiles can be used to classify human cancers. miRNAs are encoded in our genome and are generally transcribed by RNA polymerase II. Despite the growing evidence for their importance in normal physiology, little is known about the regulation of miRNA expression. In this review, we will examine the relationship between miRNAs and epigenetics. We examine the effects of miRNAs on epigenetic machinery, and the control of miRNA expression by epigenetic mechanisms. Epigenetics is defined as heritable changes in gene expression that do not involve a change in DNA sequence.