MyoD regulates p57(kip2) expression by interacting with a distant cis-element and modifying a higher order chromatin structure

The bHLH transcription factor MyoD, the prototypical master regulator of differentiation, directs a complex program of gene expression during skeletal myogenesis. The up-regulation of the cdk inhibitor p57(kip2) plays a critical role in coordinating differentiation and growth arrest during muscle development, as well as in other tissues. p57(kip2) displays a highly specific expression pattern and is subject to a complex epigenetic control driving the imprinting of the paternal allele. However, the regulatory mechanisms governing its expression during development are still poorly understood. We have identified an unexpected mechanism by which MyoD regulates p57(kip2) transcription in differentiating muscle cells. We show that the induction of p57(kip2) requires MyoD binding to a long-distance element located within the imprinting control region KvDMR1 and the consequent release of a chromatin loop involving p57(kip2) promoter. We also show that differentiation-dependent regulation of p57(kip2), while involving a region implicated in the imprinting process, is distinct and hierarchically subordinated to the imprinting control. These findings highlight a novel mechanism, involving the modification of higher order chromatin structures, by which MyoD regulates gene expression. Our results also suggest that chromatin folding mediated by KvDMR1 could account for the highly restricted expression of p57(kip2) during development and, possibly, for its aberrant silencing in some pathologies.