miRNAs: effectors of environmental influences on gene expression and disease.

Discovered less than a decade ago, micro-RNAs (miRNAs) have emerged as important regulators of gene expression in mammals. They consist of short nucleic acids, on average approximately 22 nucleotides in length. The miRNAs exert their effect by binding directly to target messenger RNAs (mRNAs) and inhibiting mRNA stability and translation. Each miRNA can bind to multiple targets and many miRNAs can bind to the same target mRNA, allowing for a complex pattern of regulation of gene expression. Once bound to their targets, miRNAs can suppress translation of the mRNA by either sequestration or degradation of the message. Thus, miRNAs function as powerful and sensitive posttranscriptional regulators of gene expression. This review will summarize what is known about miRNA biogenesis, expression, regulation, function, mode of action, and role in disease processes with an emphasis on miRNAs in mammals. We discuss some of the methodology employed in miRNA research and the potential of miRNAs as therapeutic targets. The role of miRNAs in signal transduction and cellular stress is reviewed. Lastly, we identify new exciting avenues of research on the role of miRNAs in toxicogenomics and the possibility of epigenetic effects on gene expression.