Activation of muscle enhancers by MyoD and epigenetic modifiers.

The early 1980s revelation of cis-acting genomic elements, known as transcriptional enhancers, is still regarded as one of the fundamental discoveries in the genomic field. However, only with the emergence of genome-wide techniques has the genuine biological scope of enhancers begun to be fully uncovered. Massive scientific efforts of multiple laboratories rapidly advanced the overall perception that enhancers are typified by common epigenetic characteristics that distinguish their activating potential. Broadly, chromatin modifiers and transcriptional regulators lay down the essential foundations necessary for constituting enhancers in their activated form. Basing on genome-wide ChIP-sequencing of enhancer-related marks we identified myogenic enhancers before and after muscle differentiation and discovered that MyoD was bound to nearly a third of condition-specific enhancers. Experimental studies that tested the deposition patterns of enhancer-related epigenetic marks in MyoD-null myoblasts revealed the high dependency that a specific set of muscle enhancers have towards this transcriptional regulator. Re-expression of MyoD restored the deposition of enhancer-related marks at myotube-specific enhancers and partially at myoblasts-specific enhancers. Our proposed mechanistic model suggests that MyoD is involved in recruitment of methyltransferase Set7, acetyltransferase p300 and deposition of H3K4me1 and H3K27ac at myogenic enhancers. In addition, MyoD binding at enhancers is associated with PolII occupancy and with local noncoding transcription. Modulation of muscle enhancers is suggested to be coordinated via transcription factors docking, including c-Jun and Jdp2 that bind to muscle enhancers in a MyoD-dependent manner. We hypothesize that distinct transcription factors may act as placeholders and mediate the assembly of newly formed myogenic enhancers.