Small noncoding RNAs: biogenesis, function, and emerging significance in toxicology.

In recent years, the discovery of small ncRNAs (noncoding RNAs) has unveiled a slew of powerful riboregulators of gene expression. So far, many different types of small ncRNAs have been described. Of these, miRNAs (microRNAs), siRNAs (small interfering RNAs), and piRNAs (Piwi-interacting RNAs) have been studied in more detail. A significant fraction of genes in most organisms and tissues is targets of these small ncRNAs. Because these tiny RNAs are turning out to be important regulators of gene and genome expression, their aberrant expression profiles are expected to be associated with cellular dysfunction and disease. In fact, an ever-increasing number of studies have implicated miRNAs and siRNAs in human health and disease ranging from metabolic disorders to diseases of various organ systems as well as various forms of cancer. Nevertheless, despite the flurry of research on these small ncRNAs, many aspects of their biology still remain to be understood. The following discussion focuses on some aspects of the biogenesis and function of small ncRNAs with major emphasis on miRNAs since these are the most widespread endogenous small ncRNAs that have been called "micromanagers" of gene expression. Their emerging significance in toxicology is also discussed.