Comparison of the 3D organization of sperm and fibroblast genomes using the Hi-C approach

The 3D organization of the genome is tightly connected to its biological function. The Hi-C approach was recently introduced as a method that can be used to identify higher-order chromatin interactions genome-wide. The aim of this study was to determine genome-wide chromatin interaction frequencies using the Hi-C approach in mouse sperm cells and embryonic fibroblasts. The obtained results demonstrated that the 3D genome organizations of sperm and fibroblast cells show a high degree of similarity both with each other and with the previously described mouse embryonic stem (ES) cells. Both A- and B-compartments and topologically associated domains (TADs) are present in spermatozoa and fibroblasts. Nevertheless, sperm cells and fibroblasts exhibited statistically significant differences between each other in the contact probabilities of defined loci. Tight packaging of the sperm genome resulted in an enrichment of long-range contacts compared with the fibroblasts. However, only 30% of the differences in the number of contacts are based on differences in the densities of their genome packages; the main source of the differences is the gain or loss of contacts that are specific for defined genome regions. An analysis of interchromosomal contacts in both cell types demonstrated that the large chromosomes showed a tendency to interact with each other more than with the small chromosomes and vice versa. We found that the dependence of the contact probability P(s) on genomic distance for sperm is in a good agreement with the fractal globular folding of chromatin. The similarity of the spatial DNA organization in sperm and somatic cell genomes suggests the stability of the 3D structure of genomes through generations.