Charting differences between tumors and normal tissue is a mainstay of cancer research. However, clonal tumor expansion from complex normal tissue architectures potentially obscures cancer-specific events, including divergent epigenetic patterns. Using whole-genome bisulfite sequencing of normal B cell subsets, we observed broad epigenetic programming of selective transcription factor binding sites coincident with the degree of B cell maturation. By comparing normal B cells to malignant B cells from 268 patients with chronic lymphocytic leukemia (CLL), we showed that tumors derive largely from a continuum of maturation states reflected in normal developmental stages. Epigenetic maturation in CLL was associated with an indolent gene expression pattern and increasingly favorable clinical outcomes. We further uncovered that most previously reported tumor-specific methylation events are normally present in non-malignant B cells. Instead, we identified a potential pathogenic role for transcription factor dysregulation in CLL, where excess programming by EGR and NFAT with reduced EBF and AP-1 programming imbalances the normal B cell epigenetic program.

Thymic antigen-presenting cells (APCs) such as dendritic cells and medullary thymic epithelial cells (mTECs) use distinct strategies of self-antigen expression and presentation to mediate central tolerance. The thymus also harbors B cells; whether they also display unique tolerogenic features and how they genealogically relate to peripheral B cells is unclear. Here, we found that Aire is expressed in thymic but not peripheral B cells. Aire expression in thymic B cells coincided with major histocompatibility class II (MHCII) and CD80 upregulation and immunoglobulin class-switching. These features were recapitulated upon immigration of naive peripheral B cells into the thymus, whereby this intrathymic licensing required CD40 signaling in the context of cognate interactions with autoreactive CD4(+) thymocytes. Moreover, a licensing-dependent neo-antigen selectively upregulated in immigrating B cells mediated negative selection through direct presentation. Thus, autoreactivity within the nascent T cell repertoire fuels a feed forward loop that endows thymic B cells with tolerogenic features.

Cancers require telomere maintenance mechanisms for unlimited replicative potential. We dissected whole-genome sequencing data of over 2,500 matched tumor-control samples from 36 different tumor types to characterize the genomic footprints of these mechanisms. While the telomere content of tumors with ATRX or DAXX mutations (ATRX/DAXX trunc ) was increased, tumors with TERT modifications showed a moderate decrease of telomere content. While normally located at the chromosome termini, telomere sequences are also found in intrachromosomal regions. As such, interstitial telomeric sequences with large blocks of telomere repeats exist in humans and other species, which probably arose from ancestral genome rearrangements or other evolutionary events 19 . Recently, ALT-specific, targeted telomere insertions into chromosomes that lead to genomic instability have also been described 20 . Another source for unexpected telomere repeat sites is the stabilizing function of telomeres at broken chromosomes. After a double-strand break, telomeres can be added de novo to the unprotected break sites ("telomere healing") 21,22 or acquired from other chromosomal positions ("telomere capture") 23,24 .Here, we characterized the telomere landscape of 2,519 tumor samples from 36 different tumor types using whole genome sequencing data from the Pan-Cancer Analysis of Whole Genomes (PCAWG) project 25 . Besides determining telomere content and searching for mutations associated with different telomere maintenance mechanisms (TMMs), we systematically detected 2,683 somatic telomere insertions and show that different TMMs are associated with enrichment of previously undescribed singleton TVRs.not peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/157560 doi: bioRxiv preprint first posted online Jun. 30, 2017; Results Telomere content across cohortsDue to the repetitive nature of telomere sequences, short sequencing reads from telomeres cannot be uniquely aligned to individual chromosomes. However, a mean telomere content for the tumor as a whole can be estimated from the number of reads containing telomere sequences 17,[26][27][28][29] . Here, we extracted reads containing at least six telomere repeats per 100 bases, allowing the canonical telomere repeat TTAGGG and the three most common TVRs TCAGGG, TGAGGG and TTGGGG. The telomere content was defined as the number of unaligned telomere reads normalized by sequencing coverage

Cancers require telomere maintenance mechanisms for unlimited replicative potential. They achieve this through TERT activation or alternative telomere lengthening associated with ATRX or DAXX loss. Here, as part of the ICGC/TCGA Pan-Cancer Analysis of Whole Genomes (PCAWG) Consortium, we dissect whole-genome sequencing data of over 2500 matched tumor-control samples from 36 different tumor types aggregated within the ICGC/TCGA Pan-Cancer Analysis of Whole Genomes (PCAWG) Consortium to characterize the genomic footprints of these mechanisms. While the telomere content of tumors with ATRX or DAXX mutations (ATRX/DAXX trunc) is increased, tumors with TERT modifications show a moderate decrease of telomere content. One quarter of all tumor samples contain somatic integrations of telomeric sequences into non-telomeric DNA. This fraction is increased to 80% prevalence in ATRX/DAXX trunc tumors, which carry an aberrant telomere variant repeat (TVR) distribution as another genomic marker. The latter feature includes enrichment or depletion of the previously undescribed singleton TVRs TTCGGG and TTTGGG, respectively. Our systematic analysis provides new insight into the recurrent genomic alterations associated with telomere maintenance mechanisms in cancer.

In chronic lymphocytic leukemia ( CLL ), a diverse set of genetic mutations is embedded in a deregulated epigenetic landscape that drives cancerogenesis. To elucidate the role of aberrant chromatin features, we mapped DNA methylation, seven histone modifications, nucleosome positions, chromatin accessibility, binding of EBF 1 and CTCF , as well as the transcriptome of B cells from CLL patients and healthy donors. A globally increased histone deacetylase activity was detected and half of the genome comprised transcriptionally downregulated partially DNA methylated domains demarcated by CTCF . CLL samples displayed a H3K4me3 redistribution and nucleosome gain at promoters as well as changes of enhancer activity and enhancer linkage to target genes. A DNA binding motif analysis identified transcription factors that gained or lost binding in CLL at sites with aberrant chromatin features. These findings were integrated into a gene regulatory enhancer containing network enriched for B‐cell receptor signaling pathway components. Our study predicts novel molecular links to targets of CLL therapies and provides a valuable resource for further studies on the epigenetic contribution to the disease.

In the published version of this paper, the members of the Pan-Cancer Analysis of Whole Genomes (PCAWG) Consortium were listed in the Supplementary Information; however, these members should have been included in the main paper. The original Article has been corrected to include the members and affiliations of the PCAWG Consortium in the main paper; the corrections have been made to the HTML version of the Article but not the PDF version. Additional corrections to affiliations have been made to the PDF and HTML versions of the original Article for consistency of information between the PCAWG list and the main paper.

The malignant phenotype combines characteristics that are acquired and inherited from the normal cell of origin. Hematological malignancies and related disease subtypes are thought to arise from diverse cell types that may reflect various developmental stages within the hematopoetic lineage. The contribution of different normal cell states and processes to the biological and clinical features of malignancy is not well understood. In chronic lymphocytic leukemia (CLL), two or three subtypes have been identified by variation in the degree of somatic IGHV mutations and recently uncovered epigenetic differences, respectively, suggesting that these subtypes derive from distinct normal B cell subsets at different stages of maturity. However, in CLL, as well as in most malignancies, the full possible extent of maturity states and the relative contribution of normal versus malignant developmental programs to the malignant phenotype have not been defined in a high-resolution manner. It is widely accepted that epigenetic patterns are important to establish and stabilize cellular phenotypes. Using whole genome bisulfite sequencing and sequence-specific methods, we assessed the dynamic DNA methylation events that occur during the maturation of B cells using six highly purified B cell subsets representing various stages of maturation. We confirmed previous reports that broad epigenetic programming affects about 25% of the genome from naïve to memory B cells, and further revealed that B cell subpopulations of intermediate maturity retained increasing degrees of the maturation program resulting in a singular developmental trajectory. Maturation was driven in part by the activity of a specific set of transcription factors (e.g. AP-1, EBF1, RUNX3, OCT2, IRF4 and NFkB). Using the developmental epigenetic signature defined by transcription factor binding site (TFBS) programming in normal cells to compare to tumor cells of 268 CLL revealed that tumors have the potential to derive from a continuum of possible maturation states that are reflected in the maturation stages of normal cells. Using RNA sequencing to measure gene expression, we found the degree of maturation achieved in tumors closely associates with the acquisition of a more indolent pattern of gene expression, evidenced by progressive downregulation of CLL oncogenes, such as ZAP70, TCL1 and BTK. Further assessment of the level of DNA methylation maturity in an independent sample cohort of 348 CLL cases revealed a quantitative, continuous relationship with increasingly favorable clinical outcomes. Although the majority of methylation differences found between tumor subtypes are naturally present in normal B cells, by identifying changes that are only present in CLL we further uncovered a previously unappreciated pathogenic role of transcription factor dysregulation. Specifically, a blockade in the epigenetic maturation of EBF and AP-1 TFBSs was found to define low-programmed (less mature, poor outcome) CLL cases and was associated with transcriptional and genetic loss of EBF1 and FOS transcription factors in tumor cells. Aberrantly acquired DNA methylation events in CLL were linked to excess activity of specific transcription factor families, namely EGR and NFAT. Intriguingly, we show that recurrent somatic mutations within the DNA binding domain of EGR2 selectively influence the methylation status of its cognate binding sites in mutant cases, establishing a role for this transcription factor in epigenetic dysregulation in CLL. Collectively, this work reveals that a unique epigenetic maturation signature, directed by normal developmental processes, defines individual CLL cases resulting in a spectrum of maturity across tumors. The majority of DNA methylation differences observed between individual CLLs reflects the state of maturity of the founder cell and profoundly influences the disease phenotype. We further propose that in CLL the disease-specific state results, in part, by dysregulation of key transcription factors that imbalance the normal B cell epigenetic program. Disclosures Kipps: Celgene: Consultancy, Honoraria, Research Funding; Gilead: Honoraria, Speakers Bureau; Roche: Consultancy, Honoraria, Research Funding; Pharmacyclics: Consultancy, Honoraria; AbbVie: Consultancy, Research Funding. Stilgenbauer:AbbVie: Consultancy, Other: travel grants, Research Funding; Amgen: Consultancy, Other: travel grants, Research Funding; Boehringer-Ingelheim: Consultancy, Other: travel grants, Research Funding; Celgene: Consultancy, Other: travel grants, Research Funding; Hoffman-LaRoche: Consultancy, Honoraria, Other: travel grants, Research Funding; Genentech: Consultancy, Other: travel grants, Research Funding; Genzyme: Consultancy, Other: travel grants, Research Funding; Gilead: Consultancy, Other: travel grants, Research Funding; GlaxoSmithKline: Consultancy, Other: travel grants, Research Funding; Janssen: Consultancy, Other: travel grants, Research Funding; Mundipharma: Consultancy, Other: travel grants, Research Funding.

Knowledge of the cell-of-origin is essential for the full understanding of the causes of a malignant disease and for the rational design of targeted therapies. The B cell compartment is composed of a highly complex mixture of subtypes, each with distinct phenotypes and roles within the immune system. In chronic lymphocytic leukemia (CLL), heterogeneity in the biology and clinical course of the disease is thought to be linked to divergent cellular origins. We and others have previously shown that the epigenome of CLL, as measured by the global pattering of DNA methylation, is highly clonal and remarkably stable over time and thus represents a powerful approach to trace founder subtype populations. Here we combine epigenomic and transcriptomic analysis using next-generation sequencing approaches to compare CLL cells to highly purified and specific B cell subpopulations at various stages of maturation. We find that B cell maturation involves substantial unidirectional epigenetic programming that occurs as a continuum throughout the transition between naïve to fully-mature memory B cell subpopulations. Combining 258 CLL cases using Illumina 450K analysis reveals that all CLLs arise from a discrete window within the spectrum of B cell maturation that is more similar to mature B cells, with the majority of cases clustering at two distinct points correlating with unmutated IGHV versus highly mutated (<95% homology) IGHV genes; however, a significant number (~20%) of cases arise at various points between these two clusters. Next we show that using RNA-seq, broad differences in global expression patterns mirror the degree of epigenetic programming achieved by individual CLLs. Progressively further programming is paralleled by a transition from an aggressive to indolent expression pattern, indicated by the decrease in the expression levels of genes with known roles in promoting CLL cell survival, such as ZAP70, BTK, TCL1a, MCL1, miR-155 and others. Using DNA methylation and ChIP-seq data to compare the sequence and chromatin features of genomic regions that are programmed in normal B cell maturation versus CLL, reveals that although a myriad of transcription factors and pathways connected to immune cell function are involved in normal epigenetic programming in B cells, aberrant CLL-specific alterations involve excess activity of NFAT and EGR gene families and, paradoxically, a reduction of AP-1 activity. To further investigate the role of immediate-early genes, RNA-seq analysis of in vitro-activated CLL cells revealed a strong association between the degree of epigenetic programming and the specific inducibility of EGR2 and c-FOS, supporting a functional role of these genes in aberrant DNA methylation programming. Finally, in an independent clinically well-annotated cohort of 349 CLL cases, we demonstrate that the degree of epigenetic programming is significantly associated with time to treatment and overall survival in patients. Collectively, this work demonstrates that instead of a distinct cell(s)-of-origin, CLL is rather derived from a continuum of possible programming states, and that the degree of programming achieved by a particular CLL at the time of transformation dictates its global gene expression pattern and clinical outcome. Furthermore, a parallel assessment of B cell maturation with CLL development permits a refinement of the disease-specific, early molecular events, highlighting the dysregulation of particular transcription factors and pathways in the pathogenesis of aggressive versus indolent disease. Citation Format: Christopher C. Oakes, Marc Seifert, Assenov Yassen, Lei Gu, Martina Przekopowitz, Amy Ruppert, Andrius Serva, Sandra Koser, David Brocks, Daniel Lipka, Olga Bogatyrova, Daniel Mertens, Marc Zapatka, Peter Lichter, Hartmut Doehner, Ralf Kueppers, Thorsten Zenz, Stephan Stilgenbauer, John Byrd, Christoph Plass. Progressive epigenetic programming during B cell maturation yields a continuum of clonal disease phenotypes with distinct etiologies in chronic lymphocytic leukemia. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr B25.