HiCoP, a simple and robust method for detecting interactions of regulatory regions.

Background: Chromatin physical interactions provide essential information for understanding the regulation of cis-elements like enhancers, promoters, and insulators in cell development and differentiation. The Hi-C assay is a technique detecting chromatin structures of the whole genome, but not sensitive to interactions of regulatory elements. Several methods, like HiChIP, DNase-C, and OCEAN-C, have been developed for enriching interactions of regulatory regions, but all of them have some shortcomings. New simple, efficient, and robust methods are still in need for detecting interactions of regulatory regions.

Results: We developed a new, simple, and robust assay called CoP (Column Purified chromatin) for profiling of open chromatin regions by directly purifying fragmentized crosslinked chromatin with a DNA purification column. The accessible chromatin regions, including active enhancers, promoters, and insulators, were significantly enriched in CoP chromatin. The CoP-seq assay can efficiently detect open chromatin regions, especially active promoters, with a high signal-to-noise ratio. We integrated the CoP-seq and Hi-C technique (HiCoP) to detect interactions of accessible chromatin regions, which represent active cis-regulatory elements in cells. We observed that the HiCoP captured the peaks in the promoters-associated enhancer regions. HiCoP detected more promoter-enhancer (P-E), promoter-promoter (P-P), and enhancer-enhancer (E-E) interactions within 20 kb-5 Mb than Hi-C. Most of the loops identified by HiCoP are associated with the expressed genes.

Conclusion: CoP assay can efficiently enrich open chromatin regions. When CoP assay was integrated with Hi-C assay, it provides a simple, robust, alternative technique for profiling accessible chromatin regions and chromatin conformation simultaneously.