Cutaneous T cell lymphoma (CTCL) is a non-Hodgkin lymphoma of skin-homing T lymphocytes. We performed exome and whole genome DNA sequence and RNA sequencing on purified CTCL and matched normal cells. The results implicate mutations in 17 genes in CTCL pathogenesis, including genes involved in T cell activation and apoptosis, NFκB signaling, chromatin remodeling, and DNA damage response. CTCL is distinctive in that somatic copy number variants (SCNVs) comprise 92% of all driver mutations (mean of 11.8 pathogenic SCNVs vs. 1.0 somatic single nucleotide variants per CTCL). These findings have implications for novel therapeutics.

Cohesin extrusion is thought to play a central role in establishing the architecture of mammalian genomes. However, extrusion has not been visualized in vivo, and thus, its functional impact and energetics are unknown. Using ultra-deep Hi-C, we show that loop domains form by a process that requires cohesin ATPases. Once formed, however, loops and compartments are maintained for hours without energy input. Strikingly, without ATP, we observe the emergence of hundreds of CTCF-independent loops that link regulatory DNA. We also identify architectural "stripes," where a loop anchor interacts with entire domains at high frequency. Stripes often tether super-enhancers to cognate promoters, and in B cells, they facilitate Igh transcription and recombination. Stripe anchors represent major hotspots for topoisomerase-mediated lesions, which promote chromosomal translocations and cancer. In plasmacytomas, stripes can deregulate Igh-translocated oncogenes. We propose that higher organisms have coopted cohesin extrusion to enhance transcription and recombination, with implications for tumor development.

SUMMARY In this study, we show that evolutionarily conserved chromosome loop anchors bound by CTCF and cohesin are vulnerable to DNA double strand breaks (DSBs) mediated by topoisomerase 2B (TOP2B). Polymorphisms in the genome that redistribute CTCF/cohesin occupancy rewire DNA cleavage sites to novel loop anchors. While transcription- and replication-coupled genomic rearrangements have been well documented, we demonstrate that DSBs formed at loop anchors are largely transcription-, replication-, and cell type- independent. DSBs are continuously formed throughout interphase, are enriched on both sides of strong topological domain borders, and frequently occur at breakpoint clusters commonly translocated in cancer. Thus, loop anchors serve as fragile sites that generate DSBs and chromosomal rearrangements.

Replication origins, fragile sites, and rDNA have been implicated as sources of chromosomal instability. However, the defining genomic features of replication origins and fragile sites are among the least understood elements of eukaryote genomes. Here, we map sites of replication initiation and breakage in primary cells at high resolution. We find that replication initiates between transcribed genes within nucleosome-depleted structures established by long asymmetrical poly(dA:dT) tracts flanking the initiation site. Paradoxically, long (>20 bp) (dA:dT) tracts are also preferential sites of polar replication fork stalling and collapse within early-replicating fragile sites (ERFSs) and late-replicating common fragile sites (CFSs) and at the rDNA replication fork barrier. Poly(dA:dT) sequences are fragile because long single-strand poly(dA) stretches at the replication fork are unprotected by the replication protein A (RPA). We propose that the evolutionary expansion of poly(dA:dT) tracts in eukaryotic genomes promotes replication initiation, but at the cost of chromosome fragility.

Highlights d TOP2 binding and cleavage complex (TOP2cc) formation is dependent on cohesin d Transcription facilitates the processing of TOP2cc to DSBs that drives translocation d TOP2cc processing is rapid, while commitment to processing is rate-limiting d Cohesin and transcription levels predict TOP2-mediated breakage and translocation

SUMMARY The RAG1 endonuclease, together with its cofactor RAG2, is essential for V(D)J recombination but is a potent threat to genome stability. The sources of RAG1 mis-targeting and the mechanisms that have evolved to suppress it are poorly understood. Here, we report that RAG1 associates with chromatin at thousands of active promoters and enhancers in the genome of developing lymphocytes. The mouse and human genomes appear to have responded by reducing the abundance of “cryptic” recombination signals near RAG1 binding sites. This depletion operates specifically on the RSS heptamer, whereas nonamers are enriched at RAG1 binding sites. Reversing this RAG-driven depletion of cleavage sites by insertion of strong recombination signals creates an ectopic hub of RAG-mediated V(D)J recombination and chromosomal translocations. Our findings delineate rules governing RAG binding in the genome, identify areas at risk of RAG-mediated damage, and highlight the evolutionary struggle to accommodate programmed DNA damage in developing lymphocytes.

Highlights d 53BP1 S25A point mutation rescues lethality of BRCA1 D11 mice without restoring HR d 53BP1 S25A mutation uncouples 53BP1 end-blocking activities pre-and post-resection d RIF1/shieldin blocks BRCA1-independent loading of RAD51 onto single-strand DNA d Shieldin and PTIP associate with 53BP1 to regulate distinct end-resection pathways

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