Enhancer RNAs: similarities with both lncRNAs and mRNAs reveal novel functions

Cells produce numerous types of RNAs. Among these, transcripts produced by RNA polymerase II include protein-coding mRNAs as well as a variety of long noncoding RNAs. In this latter group, enhancer (e) RNAs constitute a class of RNAs transcribed from enhancer sites. Although eRNAs are typically unstable and degraded rapidly, multiple roles related to enhancer function have been suggested. But eRNAs also share similarities with mRNAs, such as in a limited number the presence of translated open reading frames. Indeed, other "noncoding" RNAs have also been found to contain coding sequences, and together these transcripts blur the line between coding and noncoding. Here, we review current models of eRNA function, the discoveries that led to them, and additional functions, specifically the potential for translation. We also review the characteristics of proteins encoded by such "noncoding" transcripts, and their possible implications regarding the function and evolution of both eRNAs and mRNAs.