Evolutionary comparison reveals that diverging CTCF sites are signatures of ancestral topological associating domains borders

Significance Mammalian chromatin is compartmentalized in topologically associating domains (TADs), genomic regions within which sequences preferentially contact each other. This organization has been proposed to be essential to organize the regulatory information contained in mammalian genomes. We show that Six homeobox genes, essential developmental regulators organized in gene clusters across different animal phyla, share a deeply conserved chromatin organization formed by two abutting TADs that predates the Cambrian explosion. This organization is required to generate separate regulatory landscapes for neighboring genes within the cluster, resulting in very different gene expression patterns. Finally, we show that this extremely conserved 3D architecture is associated with a characteristic arrangement of CCCTC-binding factor (CTCF) binding sites in diverging orientations, revealing a genome-wide conserved signature for TAD borders. Increasing evidence in the last years indicates that the vast amount of regulatory information contained in mammalian genomes is organized in precise 3D chromatin structures. However, the impact of this spatial chromatin organization on gene expression and its degree of evolutionary conservation is still poorly understood. The Six homeobox genes are essential developmental regulators organized in gene clusters conserved during evolution. Here, we reveal that the Six clusters share a deeply evolutionarily conserved 3D chromatin organization that predates the Cambrian explosion. This chromatin architecture generates two largely independent regulatory landscapes (RLs) contained in two adjacent topological associating domains (TADs). By disrupting the conserved TAD border in one of the zebrafish Six clusters, we demonstrate that this border is critical for preventing competition between promoters and enhancers located in separated RLs, thereby generating different expression patterns in genes located in close genomic proximity. Moreover, evolutionary comparison of Six-associated TAD borders reveals the presence of CCCTC-binding factor (CTCF) sites with diverging orientations in all studied deuterostomes. Genome-wide examination of mammalian HiC data reveals that this conserved CTCF configuration is a general signature of TAD borders, underscoring that common organizational principles underlie TAD compartmentalization in deuterostome evolution.