Non-core Subunits of the PRC2 Complex Are Collectively Required for Its Target-Site Specificity.

The Polycomb repressive complex 2 (PRC2) catalyzes H3K27 methylation across the genome, which impacts transcriptional regulation and is critical for establishment of cell identity. Because of its essential function during development and in cancer, understanding the delineation of genome-wide H3K27 methylation patterns has been the focus of intense investigation. PRC2 methylation activity is abundant and dispersed throughout the genome, but the highest activity is specifically directed to a subset of target sites that are stably occupied by the complex and highly enriched for H3K27me3. Here, we show, by systematically knocking out single and multiple non-core subunits of the PRC2 complex in mouse embryonic stem cells, that they each contribute to directing PRC2 activity to target sites. Furthermore, combined knockout of six non-core subunits reveals that, while dispensable for global H3K27 methylation levels, the non-core PRC2 subunits are collectively required for focusing H3K27me3 activity to specific sites in the genome.

T-cell prolymphocytic leukemia is associated with deregulation of oncogenic microRNAs on transcriptional and epigenetic level.

Deregulation of micro(mi)-RNAs is a common mechanism in tumorigenesis. We investigated the expression of 2083 miRNAs in T-cell prolymphocytic leukemia (T-PLL). Compared to physiologic CD4+ and CD8+ T-cell subsets, 111 miRNAs were differentially expressed in T-PLL. Of these, 33 belonged to miRNA gene clusters linked to cancer. Genomic variants affecting miRNAs were infrequent with the notable exception of copy number aberrations. Remarkably, we found strong upregulation of the miR-200c/-141 cluster in T-PLL to be associated with DNA hypomethylation and active promoter marks. Our findings suggest that copy number aberrations and epigenetic changes could contribute to miRNA deregulation in T-PLL.

DNA (de)methylation in embryonic stem cells controls CTCF-dependent chromatin boundaries.

Coordinated changes of DNA (de)methylation, nucleosome positioning, and chromatin binding of the architectural protein CTCF play an important role for establishing cell-type-specific chromatin states during differentiation. To elucidate molecular mechanisms that link these processes, we studied the perturbed DNA modification landscape in mouse embryonic stem cells (ESCs) carrying a double knockout (DKO) of the Tet1 and Tet2 dioxygenases. These enzymes are responsible for the conversion of 5-methylcytosine (5mC) into its hydroxymethylated (5hmC), formylated (5fC), or carboxylated (5caC) forms. We determined changes in nucleosome positioning, CTCF binding, DNA methylation, and gene expression in DKO ESCs and developed biophysical models to predict differential CTCF binding. Methylation-sensitive nucleosome repositioning accounted for a significant portion of CTCF binding loss in DKO ESCs, whereas unmethylated and nucleosome-depleted CpG islands were enriched for CTCF sites that remained occupied. A number of CTCF sites also displayed direct correlations with the CpG modification state: CTCF was preferentially lost from sites that were marked with 5hmC in wild-type (WT) cells but not from 5fC-enriched sites. In addition, we found that some CTCF sites can act as bifurcation points defining the differential methylation landscape. CTCF loss from such sites, for example, at promoters, boundaries of chromatin loops, and topologically associated domains (TADs), was correlated with DNA methylation/demethylation spreading and can be linked to down-regulation of neighboring genes. Our results reveal a hierarchical interplay between cytosine modifications, nucleosome positions, and DNA sequence that determines differential CTCF binding and regulates gene expression.

Reconstruction of rearranged T-cell receptor loci by whole genome and transcriptome sequencing gives insights into the initial steps of T-cell prolymphocytic leukemia.

T-cell prolymphocytic leukemia (T-PLL) is an aggressive tumor with leukemic presentation of mature T-lymphocytes. Here, we aimed at characterizing the initial events in the molecular pathogenesis of T-PLL and particularly, at determining the point in T-cell differentiation when the hallmark oncogenic events, that is, inv(14)(q11q32)/t(14;14)(q11;q32) and t(X;14)(q28;q11) occur. To this end, we mined whole genome and transcriptome sequencing data of 17 and 11 T-PLL cases, respectively. Mapping of the 14q32.1 locus breakpoints identified only TCL1A, which was moreover significantly overexpressed in T-PLL as compared to benign CD4+ and CD8+ T-cells, as the only common oncogenic target of aberrations. In cases with t(14;14), the breakpoints mapped telomeric and in cases with inv(14) centromeric or in the 3'-untranslated region of TCL1A. Regarding the T-cell receptor alpha (TRA) locus-TCL1A breakpoint junctions, all 17 breakpoints involved recombination signal sequences and 15 junctions contained nontemplated (N-) nucleotides. All T-PLL cases studied carried in-frame TRA rearrangements on the intact allele, which skewed significantly toward usage of distal/central TRAV/TRAJ gene segments as compared to the illegitimate TRA rearrangements. Our findings suggest that the oncogenic TRA-TCL1A/MTCP1 rearrangements in T-PLL occur during opening of the TRA locus, that is, during the progression from CD4+ immature single positive to early double positive thymocyte stage, just before physiologic TCL1A expression is silenced. The cell carrying such an oncogenic event continues maturation and rearranges the second TRA allele to achieve a functional T-cell receptor. Thereafter, it switches off RAG and DNTT expression in line with the mature T-cell phenotype at presentation of T-PLL.

Loss of function mutations of BCOR in classical Hodgkin lymphoma.

BCOR is a component of a variant Polycomb repressive complex 1 (PRC1.1). PRC1 and PRC2 complexes together constitute a major gene regulatory system critical for appropriate cellular differentiation. The gene is upregulated in germinal center (GC) B cells and mutated in a number of hematologic malignancies. We report BCOR inactivating alterations in 4/7 classic Hodgkin lymphoma (cHL) cell lines, subclonal somatic mutations in Hodgkin and Reed-Sternberg (HRS) cells of 4/10 cHL cases, and deletions in HRS cells of 7/17 primary cHL cases. In mice, conditional loss of Bcor driven by AID-Cre in GC B cells resulted in gene expression changes of 46 genes (>2-fold) including upregulated Lef1 that encodes a transcription factor responsible for establishing T-cell identity and Il9r (interleukin-9 receptor), an important member of the cytokine network in cHL. Our findings suggest a role for BCOR loss in cHL pathogenesis and GC-B cell homeostasis.

mIDH-associated DNA hypermethylation in acute myeloid leukemia reflects differentiation blockage rather than inhibition of TET-mediated demethylation.

Isocitrate dehydrogenases 1 and 2 (IDH1/2) are recurrently mutated in acute myeloid leukemia (AML), but their mechanistic role in leukemogenesis is poorly understood. The inhibition of TET enzymes by D-2-hydroxyglutarate (D-2-HG), which is produced by mutant IDH1/2 (mIDH1/2), has been suggested to promote epigenetic deregulation during tumorigenesis. In addition, mIDH also induces a differentiation block in various cell culture and mouse models. Here we analyze the genomic methylation patterns of AML patients with mIDH using Infinium 450K data from a large AML cohort and found that mIDH is associated with pronounced DNA hypermethylation at tens of thousands of CpGs. Interestingly, however, myeloid leukemia cells overexpressing mIDH, cells that were cultured in the presence of D-2-HG or TET2 mutant AML patients did not show similar methylation changes. In further analyses, we also characterized the methylation landscapes of myeloid progenitor cells and analyzed their relationship to mIDH-associated hypermethylation. Our findings identify the differentiation state of myeloid cells, rather than inhibition of TET-mediated DNA demethylation, as a major factor of mIDH-associated hypermethylation in AML. Furthermore, our results are also important for understanding the mode of action of currently developed mIDH inhibitors.

Chromatin Immunoprecipitation.

Chromatin immunoprecipitation (ChIP) is a valuable method to investigate protein-DNA interactions in vivo. Since its discovery it has been indispensable to identify binding sites and patterns of a variety of DNA-interacting proteins, such as transcription factors and regulators, modified histones, and epigenetic modifiers. The Polycomb repressors were the first proteins that have been mapped using this technique, which provided the mechanistic basis for the understanding of their biological function. Cross-linked (XChIP) or native (NChIP) chromatin from tissues or cultured cells is fragmented and the protein of interest is immunoprecipitated using a specific antibody. The co-precipitated DNA is then purified and subjected to analysis by region-specific PCR, DNA microarray (ChIP-on-chip), or next-generation sequencing (ChIP-seq). The assay can therefore produce information about the localization of the analyzed protein at specific candidate loci or throughout the entire genome. In this chapter, we provide a detailed protocol of the basic standard ChIP assay and some remarks about variations.

Tet1 and Tet2 Protect DNA Methylation Canyons against Hypermethylation.

DNA methylation is a dynamic epigenetic modification with an important role in cell fate specification and reprogramming. The Ten eleven translocation (Tet) family of enzymes converts 5-methylcytosine to 5-hydroxymethylcytosine, which promotes passive DNA demethylation and functions as an intermediate in an active DNA demethylation process. Tet1/Tet2 double-knockout mice are characterized by developmental defects and epigenetic instability, suggesting a requirement for Tet-mediated DNA demethylation for the proper regulation of gene expression during differentiation. Here, we used whole-genome bisulfite and transcriptome sequencing to characterize the underlying mechanisms. Our results uncover the hypermethylation of DNA methylation canyons as the genomic key feature of Tet1/Tet2 double-knockout mouse embryonic fibroblasts. Canyon hypermethylation coincided with disturbed regulation of associated genes, suggesting a mechanistic explanation for the observed Tet-dependent differentiation defects. Based on these results, we propose an important regulatory role of Tet-dependent DNA demethylation for the maintenance of DNA methylation canyons, which prevents invasive DNA methylation and allows functional regulation of canyon-associated genes.