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Inactivating mutations in the CCCTC-binding factor (CTCF) in human cancer drive altered methylated genomic states, altered CTCF occupancy at promotor and enhancer regions and deregulate global gene expression. In T-ALL patients, we found that acquired inactivating CTCF events drive subtle and local genomic effects in nearly half of t(5;14)(q35;q32.2) rearranged patients, and especially when CTCF bindings sites are preserved in between the BCL11B-enhancer and the TLX3 oncogene. These solitary intervening sites insulate TLX3 from the enhancer by inducing competitive looping to multiple binding sites near the TLX3 promoter. Loss of CTCF, or deletion of the intervening CTCF site weakens competitive looping while favors TLX3-promoter to BCL11B-enhancer looping that elevates oncogene expression levels and leukemia burden. Collectively, we reveal that loss of CTCF abrogates enhancer insulation, allowing oncogene expression that drives leukemia development.

Background Structural variants in DNA such as translocations are increasingly found as key oncogenic events in tumors, and detection is clinically relevant for therapy choice in many conditions. Despite developments in targeted and whole genome gene sequencing, the robust detection of structural variants remains a challenge, especially in FFPE specimens. Methods Targeted proximity ligation assays selectively amplify and sequence entire genes based on the crosslinking of physically proximal sequences, and thereby enable complete sequencing of genes of interest, including single nucleotide and structural variants. Because these assays are based on the crosslinking and fragmenting of DNA, they have particular advantages in the analysis of FFPE samples, in which DNA is inherently crosslinked and fragmented. Results We will show that we have successfully developed targeted proximity ligation assays on representative clinical FFPE samples with considerably fragmented DNA. By making use of the DNA crosslinks in these samples, we can generate very broad sequence information over 100’s of kb’s, where other targeted NGS technologies are limited by the fragment size. We will show that we can detect NTRK fusions at DNA level, including determination of the fusion partner and the exact translocation breakpoint sequence. The latter can be used as a very sensitive and cancer-specific marker for minimal residual disease (MRD) testing and ctDNA monitoring with PCR. Conclusions Our newly developed targeted proximity ligation assays enable robust detection of structural variation in FFPE samples and the development of personalized MRD/ctDNA tests. Legal entity responsible for the study Cergentis. Funding Cergentis via Horizon2020 SME funding. Disclosure H. Feitsma: Full / Part-time employment: Cergentis. M. Yilmaz: Full / Part-time employment: Cergentis. J. Swennenhuis: Full / Part-time employment: Cergentis. A. Rakszewska: Full / Part-time employment: Cergentis. K. Hajo: Full / Part-time employment: Cergentis. E. Splinter: Full / Part-time employment: Cergentis. M. Simonis: Full / Part-time employment: Cergentis. M. Van Min: Shareholder / Stockholder / Stock options, Full / Part-time employment: Cergentis. All other authors have declared no conflicts of interest.

CCCTC-binding factor (CTCF) is an architectural protein involved in the three-dimensional (3D) organization of chromatin. In this study, we assayed the 3D genomic contact profiles of a large number of CTCF binding sites with high-resolution 4C-seq. As recently reported, our data also suggest that chromatin loops preferentially form between CTCF binding sites oriented in a convergent manner. To directly test this, we used CRISPR/Cas9 genome editing to delete core CTCF binding sites in three loci, including the CTCF site in the Sox2 super-enhancer. In all instances, CTCF and cohesin recruitment were lost, and chromatin loops with distal, convergent CTCF sites were disrupted or destabilized. Re-insertion of oppositely oriented CTCF recognition sequences restored CTCF and cohesin recruitment, but did not re-establish chromatin loops. We conclude that CTCF binding polarity plays a functional role in the formation of higher-order chromatin structure.

SummaryThe relationship between 3D organization of the genome and gene-regulatory networks is poorly understood. Here, we examined long-range chromatin interactions genome-wide in mouse embryonic stem cells (ESCs), iPSCs, and fibroblasts and uncovered a pluripotency-specific genome organization that is gradually reestablished during reprogramming. Our data confirm that long-range chromatin interactions are primarily associated with the spatial segregation of open and closed chromatin, defining overall chromosome conformation. Additionally, we identified two further levels of genome organization in ESCs characterized by colocalization of regions with high pluripotency factor occupancy and strong enrichment for Polycomb proteins/H3K27me3, respectively. Underlining the independence of these networks and their functional relevance for genome organization, loss of the Polycomb protein Eed diminishes interactions between Polycomb-regulated regions without altering overarching chromosome conformation. Together, our data highlight a pluripotency-specific genome organization in which pluripotency factors such as Nanog and H3K27me3 occupy distinct nuclear spaces and reveal a role for cell-type-specific gene-regulatory networks in genome organization.