Dynamic readers for 5-(hydroxy)methylcytosine and its oxidized derivatives.

Tet proteins oxidize 5-methylcytosine (mC) to generate 5-hydroxymethyl (hmC), 5-formyl (fC), and 5-carboxylcytosine (caC). The exact function of these oxidative cytosine bases remains elusive. We applied quantitative mass-spectrometry-based proteomics to identify readers for mC and hmC in mouse embryonic stem cells (mESC), neuronal progenitor cells (NPC), and adult mouse brain tissue. Readers for these modifications are only partially overlapping, and some readers, such as Rfx proteins, display strong specificity. Interactions are dynamic during differentiation, as for example evidenced by the mESC-specific binding of Klf4 to mC and the NPC-specific binding of Uhrf2 to hmC, suggesting specific biological roles for mC and hmC. Oxidized derivatives of mC recruit distinct transcription regulators as well as a large number of DNA repair proteins in mouse ES cells, implicating the DNA damage response as a major player in active DNA demethylation.

Author Correction: Landscape of somatic mutations in 560 breast cancer whole-genome sequences.

In the Methods section of this Article, 'greater than' should have been 'less than' in the sentence 'Putative regions of clustered rearrangements were identified as having an average inter-rearrangement distance that was at least 10 times greater than the whole-genome average for the individual sample. '. The Article has not been corrected.

Landscape of somatic mutations in 560 breast cancer whole-genome sequences.

We analysed whole-genome sequences of 560 breast cancers to advance understanding of the driver mutations conferring clonal advantage and the mutational processes generating somatic mutations. We found that 93 protein-coding cancer genes carried probable driver mutations. Some non-coding regions exhibited high mutation frequencies, but most have distinctive structural features probably causing elevated mutation rates and do not contain driver mutations. Mutational signature analysis was extended to genome rearrangements and revealed twelve base substitution and six rearrangement signatures. Three rearrangement signatures, characterized by tandem duplications or deletions, appear associated with defective homologous-recombination-based DNA repair: one with deficient BRCA1 function, another with deficient BRCA1 or BRCA2 function, the cause of the third is unknown. This analysis of all classes of somatic mutation across exons, introns and intergenic regions highlights the repertoire of cancer genes and mutational processes operating, and progresses towards a comprehensive account of the somatic genetic basis of breast cancer.

Clonotypic Features of Rearranged Immunoglobulin Genes Yield Personalized Biomarkers for Minimal Residual Disease Monitoring in Multiple Myeloma.

BACKGROUND: Due to improved treatment, more patients with multiple myeloma (MM) reach a state of minimal residual disease (MRD). Different strategies for MM MRD monitoring include flow cytometry, allele-specific oligonucleotide-quantitative PCR, next-generation sequencing, and mass spectrometry (MS). The last 3 methods rely on the presence and the stability of a unique immunoglobulin fingerprint derived from the clonal plasma cell population. For MS-MRD monitoring it is imperative that MS-compatible clonotypic M-protein peptides are identified. To support implementation of molecular MRD techniques, we studied the presence and stability of these clonotypic features in the CoMMpass database. METHODS: An analysis pipeline based on MiXCR and HIGH-VQUEST was constructed to identify clonal molecular fingerprints and their clonotypic peptides based on transcriptomic datasets. To determine the stability of the clonal fingerprints, we compared the clonal fingerprints during disease progression for each patient. RESULTS: The analysis pipeline to establish the clonal fingerprint and MS-suitable clonotypic peptides was successfully validated in MM cell lines. In a cohort of 609 patients with MM, we demonstrated that the most abundant clone harbored a unique clonal molecular fingerprint and that multiple unique clonotypic peptides compatible with MS measurements could be identified for all patients. Furthermore, the clonal immunoglobulin gene fingerprints of both the light and heavy chain remained stable during MM disease progression. CONCLUSIONS: Our data support the use of the clonal immunoglobulin gene fingerprints in patients with MM as a suitable MRD target for MS-MRD analyses.

Terminal keratinocyte differentiation in vitro is associated with a stable DNA methylome.

The epidermal compartment of the skin is regenerated constantly by proliferation of epidermal keratinocytes. Differentiation of a subset of these keratinocytes allows the epidermis to retain its barrier properties. Regulation of keratinocyte fate-whether to remain proliferative or terminally differentiate-is complex and not fully understood. The objective of our study was to assess if DNA methylation changes contribute to the regulation of keratinocyte fate. We employed genome-wide MethylationEPIC beadchip array measuring approximately 850 000 probes combined with RNA sequencing of in vitro cultured non-differentiated and terminally differentiated adult human primary keratinocytes. We did not observe a correlation between methylation status and transcriptome changes. Moreover, only two differentially methylated probes were detected, of which one was located in the TRIM29 gene. Although TRIM29 knock-down resulted in lower expression levels of terminal differentiation genes, these changes were minor. From these results, we conclude that-in our in vitro experimental setup-it is unlikely that changes in DNA methylation have an important regulatory role in terminal keratinocyte differentiation.

Dynamic binding of RBPJ is determined by Notch signaling status.

Notch signaling plays crucial roles in mediating cell fate choices in all metazoans largely by specifying the transcriptional output of one cell in response to a neighboring cell. The DNA-binding protein RBPJ is the principle effector of this pathway in mammals and, together with the transcription factor moiety of Notch (NICD), regulates the expression of target genes. The prevalent view presumes that RBPJ statically occupies consensus binding sites while exchanging repressors for activators in response to NICD. We present the first specific RBPJ chromatin immunoprecipitation and high-throughput sequencing study in mammalian cells. To dissect the mode of transcriptional regulation by RBPJ and identify its direct targets, whole-genome binding profiles were generated for RBPJ; its coactivator, p300; NICD; and the histone H3 modifications H3 Lys 4 trimethylation (H3K4me3), H3 Lys 4 monomethylation (H3K4me1), and histone H3 Lys 27 acetylation (H3K27ac) in myogenic cells under active or inhibitory Notch signaling conditions. Our results demonstrate dynamic binding of RBPJ in response to Notch activation at essentially all sites co-occupied by NICD. Additionally, we identify a distinct set of sites where RBPJ recruits neither NICD nor p300 and binds DNA statically, irrespective of Notch activity. These findings significantly modify our views on how RBPJ and Notch signaling mediate their activities and consequently impact on cell fate decisions.

Comprehensive evaluation of methods to assess overall and cell-specific immune infiltrates in breast cancer.

BACKGROUND: Breast cancer (BC) immune infiltrates play a critical role in tumor progression and response to treatment. Besides stromal tumor infiltrating lymphocytes (sTILs) which have recently reached level 1B evidence as a prognostic marker in triple negative BC, a plethora of methods to assess immune infiltration exists, and it is unclear how these compare to each other and if they can be used interchangeably. METHODS: Two experienced pathologists scored sTIL, intra-tumoral TIL (itTIL), and 6 immune cell types (CD3+, CD4+, CD8+, CD20+, CD68+, FOXP3+) in the International Cancer Genomics Consortium breast cancer cohort using hematoxylin and eosin-stained (n = 243) and immunohistochemistry-stained tissue microarrays (n = 254) and whole slides (n = 82). The same traits were evaluated using transcriptomic- and methylomic-based deconvolution methods or signatures. RESULTS: The concordance correlation coefficient (CCC) between pathologists for sTIL was very good (0.84) and for cell-specific immune infiltrates slightly lower (0.63-0.66). Comparison between tissue microarray and whole slide pathology scores revealed systematically higher values in whole slides (ratio 2.60-5.98). The Spearman correlations between microscopic sTIL and transcriptomic- or methylomic-based assessment of immune infiltrates were highly variable (r = 0.01-0.56). Similar observations were made for cell type-specific quantifications (r = 0.001-0.54). We observed a strong inter-method variability between the omics-derived estimations, which is further cell type dependent. Finally, we demonstrated that most methods more accurately identify highly infiltrated (sTIL ≥ 60%; area under the curve, AUC, 0.64-0.99) as compared to lowly infiltrated tumors (sTIL ≤ 10%; AUC 0.52-0.82). CONCLUSIONS: There is a lower inter-pathologist concordance for cell-specific quantification as compared to overall infiltration quantification. Microscopic assessments are underestimated when considering small cores (tissue microarray) instead of whole slides. Results further highlight considerable differences between the microscopic-, transcriptomic-, and methylomic-based methods in the assessment of overall and cell-specific immune infiltration in BC. We therefore call for extreme caution when assessing immune infiltrates using current methods and emphasize the need for standardized immune characterization beyond TIL.

Sequential ChIP-bisulfite sequencing enables direct genome-scale investigation of chromatin and DNA methylation cross-talk.

Cross-talk between DNA methylation and histone modifications drives the establishment of composite epigenetic signatures and is traditionally studied using correlative rather than direct approaches. Here, we present sequential ChIP-bisulfite-sequencing (ChIP-BS-seq) as an approach to quantitatively assess DNA methylation patterns associated with chromatin modifications or chromatin-associated factors directly. A chromatin-immunoprecipitation (ChIP)-capturing step is used to obtain a restricted representation of the genome occupied by the epigenetic feature of interest, for which a single-base resolution DNA methylation map is then generated. When applied to H3 lysine 27 trimethylation (H3K27me3), we found that H3K27me3 and DNA methylation are compatible throughout most of the genome, except for CpG islands, where these two marks are mutually exclusive. Further ChIP-BS-seq-based analysis in Dnmt triple-knockout (TKO) embryonic stem cells revealed that total loss of CpG methylation is associated with alteration of H3K27me3 levels throughout the genome: H3K27me3 in localized peaks is decreased while broad local enrichments (BLOCs) of H3K27me3 are formed. At an even broader scale, these BLOCs correspond to regions of high DNA methylation in wild-type ES cells, suggesting that DNA methylation prevents H3K27me3 deposition locally and at a megabase scale. Our strategy provides a unique way of investigating global interdependencies between DNA methylation and other chromatin features.

Temporal uncoupling of the DNA methylome and transcriptional repression during embryogenesis.

DNA methylation is a tightly regulated epigenetic mark associated with transcriptional repression. Next-generation sequencing of purified methylated DNA obtained from early Xenopus tropicalis embryos demonstrates that this genome is heavily methylated during blastula and gastrula stages. Although DNA methylation is largely absent from transcriptional start sites marked with histone H3 lysine 4 trimethylation (H3K4me3), we find both promoters and gene bodies of active genes robustly methylated. In contrast, DNA methylation is absent in large H3K27me3 domains, indicating that these two repression pathways have different roles. Comparison with chromatin state maps of human ES cells reveals strong conservation of epigenetic makeup and gene regulation between the two systems. Strikingly, genes that are highly expressed in pluripotent cells and in Xenopus embryos but not in differentiated cells exhibit relatively high DNA methylation. Therefore, we tested the repressive potential of DNA methylation using transient and transgenic approaches and show that methylated promoters are robustly transcribed in blastula- and gastrula-stage embryos, but not in oocytes or late embryos. These findings have implications for reprogramming and the epigenetic regulation of pluripotency and differentiation and suggest a relatively open, pliable chromatin state in early embryos followed by reestablished methylation-dependent transcriptional repression during organogenesis and differentiation.

Whole-genome DNA methylation profiling using MethylCap-seq.

MethylCap-seq is a robust procedure for genome-wide profiling of DNA methylation. The approach consists of the capture of methylated DNA using the MBD domain of MeCP2, and subsequent next-generation sequencing of eluted DNA. Elution of the captured methylated DNA is done in steps using a salt gradient, which stratifies the genome into fractions with different CpG density. The enrichment reached within the individual eluates allows for cost-effective deep sequence coverage. The profiles together yield a detailed genome-wide map of methylated regions and readily allows detection of DNA methylation in known and novel regions. Here, we describe principles and details of the MethylCap-seq procedure using different sources of starting material.

MBD2/NuRD and MBD3/NuRD, two distinct complexes with different biochemical and functional properties.

The human genome contains a number of methyl CpG binding proteins that translate DNA methylation into a physiological response. To gain insight into the function of MBD2 and MBD3, we first applied protein tagging and mass spectrometry. We show that MBD2 and MBD3 assemble into mutually exclusive distinct Mi-2/NuRD-like complexes, called MBD2/NuRD and MBD3/NuRD. We identified DOC-1, a putative tumor suppressor, as a novel core subunit of MBD2/NuRD as well as MBD3/NuRD. PRMT5 and its cofactor MEP50 were identified as specific MBD2/NuRD interactors. PRMT5 stably and specifically associates with and methylates the RG-rich N terminus of MBD2. Chromatin immunoprecipitation experiments revealed that PRMT5 and MBD2 are recruited to CpG islands in a methylation-dependent manner in vivo and that H4R3, a substrate of PRMT, is methylated at these loci. Our data show that MBD2/NuRD and MBD3/NuRD are distinct protein complexes with different biochemical and functional properties.

DNA methylation immediately adjacent to active histone marking does not silence transcription.

Active promoters generally contain histone H3/H4 hyperacetylation and tri-methylation at H3 lysine 4, whereas repressed promoters are associated with DNA methylation. Here we show that the repressed erythroid-specific carbonic anhydrase II (CAII) promoter has active histone modifications localized around the transcription start, while high levels of CpG methylation are present directly upstream from these active marks. Despite the presence of active histone modifications, the repressed promoter requires hormone-induced activation for efficient preinitiation complex assembly. Transient and positional changes in histone H3/H4 acetylation and local changes in nucleosome density are evident during activation, but the bipartite epigenetic code is stably maintained. Our results suggest that active histone modifications may prevent spreading of CpG methylation towards the promoter and show that repressive DNA methylation immediately adjacent to a promoter does not necessarily repress transcription.

Integrative Proteomic Profiling Reveals PRC2-Dependent Epigenetic Crosstalk Maintains Ground-State Pluripotency.

The pluripotent ground state is defined as a basal state free of epigenetic restrictions, which influence lineage specification. While naive embryonic stem cells (ESCs) can be maintained in a hypomethylated state with open chromatin when grown using two small-molecule inhibitors (2i)/leukemia inhibitory factor (LIF), in contrast to serum/LIF-grown ESCs that resemble early post-implantation embryos, broader features of the ground-state pluripotent epigenome are not well understood. We identified epigenetic features of mouse ESCs cultured using 2i/LIF or serum/LIF by proteomic profiling of chromatin-associated complexes and histone modifications. Polycomb-repressive complex 2 (PRC2) and its product H3K27me3 are highly abundant in 2i/LIF ESCs, and H3K27me3 is distributed genome-wide in a CpG-dependent fashion. Consistently, PRC2-deficient ESCs showed increased DNA methylation at sites normally occupied by H3K27me3 and increased H4 acetylation. Inhibiting DNA methylation in PRC2-deficient ESCs did not affect their viability or transcriptome. Our findings suggest a unique H3K27me3 configuration protects naive ESCs from lineage priming, and they reveal widespread epigenetic crosstalk in ground-state pluripotency.

Partially methylated domains are hypervariable in breast cancer and fuel widespread CpG island hypermethylation.

Global loss of DNA methylation and CpG island (CGI) hypermethylation are key epigenomic aberrations in cancer. Global loss manifests itself in partially methylated domains (PMDs) which extend up to megabases. However, the distribution of PMDs within and between tumor types, and their effects on key functional genomic elements including CGIs are poorly defined. We comprehensively show that loss of methylation in PMDs occurs in a large fraction of the genome and represents the prime source of DNA methylation variation. PMDs are hypervariable in methylation level, size and distribution, and display elevated mutation rates. They impose intermediate DNA methylation levels incognizant of functional genomic elements including CGIs, underpinning a CGI methylator phenotype (CIMP). Repression effects on tumor suppressor genes are negligible as they are generally excluded from PMDs. The genomic distribution of PMDs reports tissue-of-origin and may represent tissue-specific silent regions which tolerate instability at the epigenetic, transcriptomic and genetic level.

Structural insight into gene transcriptional regulation and effector binding by the Lrp/AsnC family.

The Lrp/AsnC family of transcriptional regulatory proteins is found in both archaea and bacteria. Members of the family influence cellular metabolism in both a global (Lrp) and specific (AsnC) manner, often in response to exogenous amino acid effectors. In the present study we have determined both the first bacterial and the highest resolution structures for members of the family. Escherichia coli AsnC is a specific gene regulator whose activity is triggered by asparagine binding. Bacillus subtilis LrpC is a global regulator involved in chromosome condensation. Our AsnC-asparagine structure is the first for a regulator-effector complex and is revealed as an octameric disc. Key ligand recognition residues are identified together with a route for ligand access. The LrpC structure reveals a stable octamer supportive of a topological role in dynamic DNA packaging. The structures yield significant clues to the functionality of Lrp/AsnC-type regulators with respect to ligand binding and oligomerization states as well as to their role in specific and global DNA regulation.

A somatic-mutational process recurrently duplicates germline susceptibility loci and tissue-specific super-enhancers in breast cancers.

Somatic rearrangements contribute to the mutagenized landscape of cancer genomes. Here, we systematically interrogated rearrangements in 560 breast cancers by using a piecewise constant fitting approach. We identified 33 hotspots of large (>100 kb) tandem duplications, a mutational signature associated with homologous-recombination-repair deficiency. Notably, these tandem-duplication hotspots were enriched in breast cancer germline susceptibility loci (odds ratio (OR) = 4.28) and breast-specific 'super-enhancer' regulatory elements (OR = 3.54). These hotspots may be sites of selective susceptibility to double-strand-break damage due to high transcriptional activity or, through incrementally increasing copy number, may be sites of secondary selective pressure. The transcriptomic consequences ranged from strong individual oncogene effects to weak but quantifiable multigene expression effects. We thus present a somatic-rearrangement mutational process affecting coding sequences and noncoding regulatory elements and contributing a continuum of driver consequences, from modest to strong effects, thereby supporting a polygenic model of cancer development.

Corrigendum: A somatic-mutational process recurrently duplicates germline susceptibility loci and tissue-specific super-enhancers in breast cancers.

This corrects the article DOI: 10.1038/ng.3771.

The topography of mutational processes in breast cancer genomes.

Somatic mutations in human cancers show unevenness in genomic distribution that correlate with aspects of genome structure and function. These mutations are, however, generated by multiple mutational processes operating through the cellular lineage between the fertilized egg and the cancer cell, each composed of specific DNA damage and repair components and leaving its own characteristic mutational signature on the genome. Using somatic mutation catalogues from 560 breast cancer whole-genome sequences, here we show that each of 12 base substitution, 2 insertion/deletion (indel) and 6 rearrangement mutational signatures present in breast tissue, exhibit distinct relationships with genomic features relating to transcription, DNA replication and chromatin organization. This signature-based approach permits visualization of the genomic distribution of mutational processes associated with APOBEC enzymes, mismatch repair deficiency and homologous recombinational repair deficiency, as well as mutational processes of unknown aetiology. Furthermore, it highlights mechanistic insights including a putative replication-dependent mechanism of APOBEC-related mutagenesis.

Breast cancer genome and transcriptome integration implicates specific mutational signatures with immune cell infiltration.

A recent comprehensive whole genome analysis of a large breast cancer cohort was used to link known and novel drivers and substitution signatures to the transcriptome of 266 cases. Here, we validate that subtype-specific aberrations show concordant expression changes for, for example, TP53, PIK3CA, PTEN, CCND1 and CDH1. We find that CCND3 expression levels do not correlate with amplification, while increased GATA3 expression in mutant GATA3 cancers suggests GATA3 is an oncogene. In luminal cases the total number of substitutions, irrespective of type, associates with cell cycle gene expression and adverse outcome, whereas the number of mutations of signatures 3 and 13 associates with immune-response specific gene expression, increased numbers of tumour-infiltrating lymphocytes and better outcome. Thus, while earlier reports imply that the sheer number of somatic aberrations could trigger an immune-response, our data suggests that substitutions of a particular type are more effective in doing so than others.

Genome-wide binding of MBD2 reveals strong preference for highly methylated loci.

MBD2 is a subunit of the NuRD complex that is postulated to mediate gene repression via recruitment of the complex to methylated DNA. In this study we adopted an MBD2 tagging-approach to study its genome wide binding characteristics. We show that in vivo MBD2 is mainly recruited to CpG island promoters that are highly methylated. Interestingly, MBD2 binds around 1 kb downstream of the transcription start site of a subset of ∼ 400 CpG island promoters that are characterized by the presence of active histone marks, RNA polymerase II (Pol2) and low to medium gene expression levels and H3K36me3 deposition. These tagged-MBD2 binding sites in MCF-7 show increased methylation in a cohort of primary breast cancers but not in normal breast samples, suggesting a putative role for MBD2 in breast cancer.