A Preformed Chromatin Architecture Ensures Robust Shh Transcription during Limb Development [ChIP-Seq]

During embryogenesis, enhancer-promoter interactions control gene transcriptional activation. These interactions can be tissue-specific or tissue-invariant and occur mostly within larger insulated regulatory domains called Topologically Associating Domains (TADs). Boundary elements, which delineate the extent of TADs, frequently interact with each other and have been associated with constitutive transcription and CTCF/Cohesin binding. In this work, we set out to investigate the regulatory role of a tissue-invariant, preformed interaction between two boundaries that involve the Shh gene and its unique limb enhancer, the ZRS, located one megabase away. Using CRISPR/Cas9 we specifically perturb CTCF binding sites or constitutive transcription at the ZRS-containing boundary, without altering the enhancer sequence. Using capture-HiC (cHiC) we show that both types of perturbation result in altered preformed chromatin interactions and lead to a reduction of Shh expression in developing limb buds. Finally, we demonstrate that the disruption of the chromatin structure in combination with a hypomorphic ZRS allele results in a dramatic Shh loss- of- function and digit agenesis. We thus propose that preformed chromatin structures can ensure stable enhancer promoter communication during development and robustness of gene transcriptional activation. We used ChIP-seq in E10.5 Wildtype and mutant limb buds to assess the binding of transcription factors CTCF and Rad21, but also of the histone marks H3K27ac and H3K36me3