SUMMARY CTCF/cohesin play a central role in insulator function and higher-order chromatin organization of mammalian genomes. Recent studies identified a correlation between the orientation of CTCF-binding sites (CBSs) and chromatin loops. To test the functional significance of this observation, we combined CRISPR/Cas9-based genomic-DNA-fragment editing with chromosome-conformation-capture experiments to show that the location and relative orientations of CBSs determine the specificity of long-range chromatin looping in mammalian genomes, using protocadherin (Pcdh) and β-globin as model genes. Inversion of CBS elements within the Pcdh enhancer reconfigures the topology of chromatin loops between the distal enhancer and target promoters, and alters gene-expression patterns. Thus, although enhancers can function in an orientation-independent manner in reporter assays, in the native chromosome context the orientation of at least some enhancers carrying CBSs can determine both the architecture of topological chromatin domains and enhancer/promoter specificity. The findings reveal how 3D chromosome architecture can be encoded by genome sequence.

The human genome contains millions of DNA regulatory elements and a large number of gene clusters, most of which have not been tested experimentally. The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated nuclease 9 (Cas9) programed with a synthetic single-guide RNA (sgRNA) emerges as a method for genome editing in virtually any organisms. Here we report that targeted DNA fragment inversions and duplications could easily be achieved in human and mouse genomes by CRISPR with two sgRNAs. Specifically, we found that, in cultured human cells and mice, efficient precise inversions of DNA fragments ranging in size from a few tens of bp to hundreds of kb could be generated. In addition, DNA fragment duplications and deletions could also be generated by CRISPR through trans-allelic recombination between the Cas9-induced double-strand breaks (DSBs) on two homologous chromosomes (chromatids). Moreover, junctions of combinatorial inversions and duplications of the protocadherin (Pcdh) gene clusters induced by Cas9 with four sgRNAs could be detected. In mice, we obtained founders with alleles of precise inversions, duplications, and deletions of DNA fragments of variable sizes by CRISPR. Interestingly, we found that very efficient inversions were mediated by microhomology-mediated end joining (MMEJ) through short inverted repeats. We showed for the first time that DNA fragment inversions could be transmitted through germlines in mice. Finally, we applied this CRISPR method to a regulatory element of the Pcdhα cluster and found a new role in the regulation of members of the Pcdhγ cluster. This simple and efficient method should be useful in manipulating mammalian genomes to study millions of regulatory DNA elements as well as vast numbers of gene clusters.