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All known vertebrate chromatin insulators interact with the highly conserved, multivalent 11-zinc finger nuclear factor CTCF to demarcate expression domains by blocking enhancer or silencer signals in a position-dependent manner. Recent observations document that the properties of CTCF include reading and propagating the epigenetic state of the differentially methylated H19 imprinting control region. To assess whether these findings may reflect a universal role for CTCF targets, we identified more than 200 new CTCF target sites by generating DNA microarrays of clones derived from chromatin-immunopurified (ChIP) DNA followed by ChIP-on-chip hybridization analysis. Target sites include not only known loci involved in multiple cellular functions, such as metabolism, neurogenesis, growth, apoptosis, and signalling, but potentially also heterochromatic sequences. Using a novel insulator trapping assay, we also show that the majority of these targets manifest insulator functions with a continuous distribution of stringency. As these targets are generally DNA methylation-free as determined by antibodies against 5-methylcytidine and a methyl-binding protein (MBD2), a CTCF-based network correlates with genome-wide epigenetic states.

Accumulating evidence converges on the possibility that chromosomes interact with each other to regulate transcription in trans. To systematically explore the epigenetic dimension of such interactions, we devised a strategy termed circular chromosome conformation capture (4C). This approach involves a circularization step that enables high-throughput screening of physical interactions between chromosomes without a preconceived idea of the interacting partners. Here we identify 114 unique sequences from all autosomes, several of which interact primarily with the maternally inherited H19 imprinting control region. Imprinted domains were strongly overrepresented in the library of 4C sequences, further highlighting the epigenetic nature of these interactions. Moreover, we found that the direct interaction between differentially methylated regions was linked to epigenetic regulation of transcription in trans. Finally, the patterns of interactions specific to the maternal H19 imprinting control region underwent reprogramming during in vitro maturation of embryonic stem cells. These observations shed new light on development, cancer epigenetics and the evolution of imprinting.