Methods for the Analysis of Topologically Associating Domains (TADs)

Chromatin folding in the 3D space of the nucleus can be explored through high-throughput chromosome conformation capture (Hi-C) approaches. These experiments quantify the number of interactions between any pair of genomic loci in the genome and, thus, allow building genome-scale maps of intra- and inter-chromosomal contacts (contact maps). Statistical and algorithmic analyses of Hi-C data consist in extracting information from these contact maps. One of the most striking patterns observed in intra-chromosomal Hi-C contact maps emerged from genomic regions that exhibit dense intra-region but sparse inter-region contacts. These have been termed topologically associating domains (TADs). The identification of TADs from Hi-C contact maps is of great interest as they have been shown to act as unit of chromosome organization and, potentially, functional activity. Several approaches have been developed to identify TADs (TAD callers). However, results from these methods are often dependent on data resolution and poorly concordant. In this chapter, we present four TAD callers and we provide detailed protocols for their use. In addition, we show how to compare TADs identified by different callers and how to assess the enrichment for TAD-associated biological features. TAD calling has become a key step in the study of chromatin 3D organization in different cellular contexts. Here we provide guidelines to improve the robustness and quality of these analyses.