Improved cohesin HiChIP protocol and bioinformatic analysis for robust detection of chromatin loops and stripes

Abstract

Chromosome Conformation Capture (3C) methods, including Hi-C (a high-throughput variation of 3C), detect pairwise interactions between DNA regions, enabling the reconstruction of chromatin architecture in the nucleus. HiChIP is a modification of the Hi-C experiment, which includes a chromatin immunoprecipitation step (ChIP), allowing genome-wide identification of chromatin contacts mediated by a protein of interest. In mammalian cells, cohesin protein complex is one of the major players in the establishment of chromatin loops. We present an improved cohesin HiChIP experimental protocol. Using comprehensive bioinformatic analysis, we show that performing cohesin HiChIP with two cross-linking agents (formaldehyde [FA] and EGS) instead of the typically used FA alone, results in a substantially better signal-to-noise ratio, higher ChIP efficiency and improved detection of chromatin loops and architectural stripes. Additionally, we propose an automated pipeline called nf-HiChIP (https://github.com/SFGLab/hichip-nf-pipeline) for processing HiChIP samples starting from raw sequencing reads data and ending with a set of significant chromatin interactions (loops), which allows efficient and timely analysis of multiple samples in parallel, without the need of additional ChIP-seq experiments. Finally, using novel approaches for biophysical modelling and stripe calling we generate accurate loop extrusion polymer models for a region of interest and a detailed picture of architectural stripes, respectively.