Functional anatomy of distant-acting mammalian enhancers

Transcriptional enhancers are a major class of functional element embedded in the vast non-coding portion of the human genome. Acting over large genomic distances, enhancers play critical roles in the tissue and cell type-specific regulation of genes, and there is mounting evidence that they contribute to the aetiology of many human diseases. Methods for genome-wide mapping of enhancer regions are now available, but the functional architecture contained within human enhancer elements remains unclear. Here, we review recent approaches aimed at understanding the functional anatomy of individual enhancer elements, using systematic qualitative and quantitative assessments of mammalian enhancer variants in cultured cells and in vivo. These studies provide direct insight into common architectural characteristics of enhancers including the presence of multiple transcription factor-binding sites and the mixture of both transcriptionally activating and repressing domains within the same enhancer. Despite such progress in understanding the functional composition of enhancers, the inherent complexities of enhancer anatomy continue to limit our ability to predict the impact of sequence changes on in vivo enhancer function. While providing an initial glimpse into the mutability of mammalian enhancers, these observations highlight the continued need for experimental enhancer assessment as genome sequencing becomes routine in the clinic.