Yuanxin Xi

Dnmt3a is essential for hematopoietic stem cell differentiation

Loss of the de novo DNA methyltransferases Dnmt3a and Dnmt3b in embryonic stem cells obstructs differentiation; however, the role of these enzymes in somatic stem cells is largely unknown. Using conditional ablation, we show that Dnmt3a loss progressively impairs hematopoietic stem cell (HSC) differentiation over serial transplantation, while simultaneously expanding HSC numbers in the bone marrow. Dnmt3a-null HSCs show both increased and decreased methylation at distinct loci, including substantial CpG island hypermethylation. Dnmt3a-null HSCs upregulate HSC multipotency genes and downregulate differentiation factors, and their progeny exhibit global hypomethylation and incomplete repression of HSC-specific genes. These data establish Dnmt3a as a critical participant in the epigenetic silencing of HSC regulatory genes, thereby enabling efficient differentiation.

Comparison of sequencing-based methods to profile DNA methylation and identification of monoallelic epigenetic modifications

Analysis of DNA methylation patterns relies increasingly on sequencing-based profiling methods. The four most frequently used sequencing-based technologies are the bisulfite-based methods MethylC-seq and reduced representation bisulfite sequencing (RRBS), and the enrichment-based techniques methylated DNA immunoprecipitation sequencing (MeDIP-seq) and methylated DNA binding domain sequencing (MBD-seq). We applied all four methods to biological replicates of human embryonic stem cells to assess their genome-wide CpG coverage, resolution, cost, concordance and the influence of CpG density and genomic context. The methylation levels assessed by the two bisulfite methods were concordant (their difference did not exceed a given threshold) for 82% for CpGs and 99% of the non-CpG cytosines. Using binary methylation calls, the two enrichment methods were 99% concordant and regions assessed by all four methods were 97% concordant. We combined MeDIP-seq with methylation-sensitive restriction enzyme (MRE-seq) sequencing for comprehensive methylome coverage at lower cost. This, along with RNA-seq and ChIP-seq of the ES cells enabled us to detect regions with allele-specific epigenetic states, identifying most known imprinted regions and new loci with monoallelic epigenetic marks and monoallelic expression.

SIRT7 links H3K18 deacetylation to maintenance of oncogenic transformation

Sirtuin proteins regulate diverse cellular pathways that influence genomic stability, metabolism and ageing. SIRT7 is a mammalian sirtuin whose biochemical activity, molecular targets and physiological functions have been unclear. Here we show that SIRT7 is an NAD+-dependent H3K18Ac (acetylated lysine 18 of histone H3) deacetylase that stabilizes the transformed state of cancer cells. Genome-wide binding studies reveal that SIRT7 binds to promoters of a specific set of gene targets, where it deacetylates H3K18Ac and promotes transcriptional repression. The spectrum of SIRT7 target genes is defined in part by its interaction with the cancer-associated E26 transformed specific (ETS) transcription factor ELK4, and comprises numerous genes with links to tumour suppression. Notably, selective hypoacetylation of H3K18Ac has been linked to oncogenic transformation, and in patients is associated with aggressive tumour phenotypes and poor prognosis. We find that deacetylation of H3K18Ac by SIRT7 is necessary for maintaining essential features of human cancer cells, including anchorage-independent growth and escape from contact inhibition. Moreover, SIRT7 is necessary for a global hypoacetylation of H3K18Ac associated with cellular transformation by the viral oncoprotein E1A. Finally, SIRT7 depletion markedly reduces the tumorigenicity of human cancer cell xenografts in mice. Together, our work establishes SIRT7 as a highly selective H3K18Ac deacetylase and demonstrates a pivotal role for SIRT7 in chromatin regulation, cellular transformation programs and tumour formation in vivo.

NSD2 Links Dimethylation of Histone H3 at Lysine 36 to Oncogenic Programming

The histone lysine methyltransferase NSD2 (MMSET/WHSC1) is implicated in diverse diseases and commonly overexpressed in multiple myeloma due to a recurrent t(4;14) chromosomal translocation. However, the precise catalytic activity of NSD2 is obscure, preventing progress in understanding how this enzyme influences chromatin biology and myeloma pathogenesis. Here, we show that dimethylation of histone H3 at lysine 36 (H3K36me2) is the principal chromatin-regulatory activity of NSD2. Catalysis of H3K36me2 by NSD2 is sufficient for gene activation. In t(4;14)-positive myeloma cells, the normal genome-wide and gene-specific distribution of H3K36me2 is obliterated, creating a chromatin landscape that selects for a transcription profile favorable for myelomagenesis. Catalytically active NSD2 confers xenograft tumor formation upon t(4;14)-negative cells and promotes oncogenic transformation of primary cells in an H3K36me2-dependent manner. Together, our findings establish H3K36me2 as the primary product generated by NSD2 and demonstrate that genomic disorganization of this canonical chromatin mark by NSD2 initiates oncogenic programming.

ZMYND11 links histone H3.3K36me3 to transcription elongation and tumour suppression

Recognition of modified histones by ‘reader’ proteins plays a critical role in the regulation of chromatin. H3K36 trimethylation (H3K36me3) is deposited onto the nucleosomes in the transcribed regions after RNA polymerase II elongation. In yeast, this mark in turn recruits epigenetic regulators to reset the chromatin to a relatively repressive state, thus suppressing cryptic transcription. However, much less is known about the role of H3K36me3 in transcription regulation in mammals. This is further complicated by the transcription-coupled incorporation of the histone variant H3.3 in gene bodies. Here we show that the candidate tumour suppressor ZMYND11 specifically recognizes H3K36me3 on H3.3 (H3.3K36me3) and regulates RNA polymerase II elongation. Structural studies show that in addition to the trimethyl-lysine binding by an aromatic cage within the PWWP domain, the H3.3-dependent recognition is mediated by the encapsulation of the H3.3-specific ‘Ser 31’ residue in a composite pocket formed by the tandem bromo–PWWP domains of ZMYND11. Chromatin immunoprecipitation followed by sequencing shows a genome-wide co-localization of ZMYND11 with H3K36me3 and H3.3 in gene bodies, and its occupancy requires the pre-deposition of H3.3K36me3. Although ZMYND11 is associated with highly expressed genes, it functions as an unconventional transcription co-repressor by modulating RNA polymerase II at the elongation stage. ZMYND11 is critical for the repression of a transcriptional program that is essential for tumour cell growth; low expression levels of ZMYND11 in breast cancer patients correlate with worse prognosis. Consistently, overexpression of ZMYND11 suppresses cancer cell growth in vitro and tumour formation in mice. Together, this study identifies ZMYND11 as an H3.3-specific reader of H3K36me3 that links the histone-variant-mediated transcription elongation control to tumour suppression.

DANPOS: Dynamic analysis of nucleosome position and occupancy by sequencing

Recent developments in next-generation sequencing have enabled whole-genome profiling of nucleosome organizations. Although several algorithms for inferring nucleosome position from a single experimental condition have been available, it remains a challenge to accurately define dynamic nucleosomes associated with environmental changes. Here, we report a comprehensive bioinformatics pipeline, DANPOS, explicitly designed for dynamic nucleosome analysis at single-nucleotide resolution. Using both simulated and real nucleosome data, we demonstrated that bias correction in preliminary data processing and optimal statistical testing significantly enhances the functional interpretation of dynamic nucleosomes. The single-nucleotide resolution analysis of DANPOS allows us to detect all three categories of nucleosome dynamics, such as position shift, fuzziness change, and occupancy change, using a uniform statistical framework. Pathway analysis indicates that each category is involved in distinct biological functions. We also analyzed the influence of sequencing depth and suggest that even 200-fold coverage is probably not enough to identify all the dynamic nucleosomes. Finally, based on nucleosome data from the human hematopoietic stem cells (HSCs) and mouse embryonic stem cells (ESCs), we demonstrated that DANPOS is also robust in defining functional dynamic nucleosomes, not only in promoters, but also in distal regulatory regions in the mammalian genome.

MOABS: model based analysis of bisulfite sequencing data

Bisulfite sequencing (BS-seq) is the gold standard for studying genome-wide DNA methylation. We developed MOABS to increase the speed, accuracy, statistical power and biological relevance of BS-seq data analysis. MOABS detects differential methylation with 10-fold coverage at single-CpG resolution based on a Beta-Binomial hierarchical model and is capable of processing two billion reads in 24 CPU hours. Here, using simulated and real BS-seq data, we demonstrate that MOABS outperforms other leading algorithms, such as Fisher’s exact test and BSmooth. Furthermore, MOABS analysis can be easily extended to differential 5hmC analysis using RRBS and oxBS-seq. MOABS is available at http://code.google.com/p/moabs/.

Broad H3K4me3 is associated with increased transcription elongation and enhancer activity at tumor-suppressor genes

Tumor suppressors are mostly defined by inactivating mutations in tumors, yet little is known about their epigenetic features in normal cells. Through integrative analysis of 1,134 genome-wide epigenetic profiles, mutations from >8,200 tumor-normal pairs and our experimental data from clinical samples, we discovered broad peaks for trimethylation of histone H3 at lysine 4 (H3K4me3; wider than 4 kb) as the first epigenetic signature for tumor suppressors in normal cells. Broad H3K4me3 is associated with increased transcription elongation and enhancer activity, which together lead to exceptionally high gene expression, and is distinct from other broad epigenetic features, such as super-enhancers. Genes with broad H3K4me3 peaks conserved across normal cells may represent pan-cancer tumor suppressors, such as TP53 and PTEN, whereas genes with cell type–specific broad H3K4me3 peaks may represent cell identity genes and cell type–specific tumor suppressors. Furthermore, widespread shortening of broad H3K4me3 peaks in cancers is associated with repression of tumor suppressors. Thus, the broad H3K4me3 epigenetic signature provides mutation-independent information for the discovery and characterization of new tumor suppressors.

RRBSMAP: a Fast, Accurate and User-friendly Alignment Tool for Reduced Representation Bisulfite Sequencing

SUMMARY Reduced representation bisulfite sequencing (RRBS) is a powerful yet cost-efficient method for studying DNA methylation on a genomic scale. RRBS involves restriction-enzyme digestion, bisulfite conversion and size selection, resulting in DNA sequencing data that require special bioinformatic handling. Here, we describe RRBSMAP, a short-read alignment tool that is designed for handling RRBS data in a user-friendly and scalable way. RRBSMAP uses wildcard alignment, and avoids the need for any preprocessing or post-processing steps. We benchmarked RRBSMAP against a well-validated MAQ-based pipeline for RRBS read alignment and observed similar accuracy but much improved runtime performance, easier handling and better scaling to large sample sets. In summary, RRBSMAP removes bioinformatic hurdles and reduces the computational burden of large-scale epigenome association studies performed with RRBS. AVAILABILITY http://rrbsmap.computational-epigenetics.org/ http://code.google.com/p/bsmap/ CONTACT wl1@bcm.tmc.edu SUPPLEMENTARY INFORMATION Supplementary data are available at Bioinformatics online.

SIRT6 deacetylates H3K18ac at pericentric chromatin to prevent mitotic errors and cellular senescence

Pericentric heterochromatin silencing at mammalian centromeres is essential for mitotic fidelity and genomic stability. Defective pericentric silencing has been observed in senescent cells, aging tissues, and mammalian tumors, but the underlying mechanisms and functional consequences of these defects are unclear. Here, we uncover an essential role of the human SIRT6 enzyme in pericentric transcriptional silencing, and we show that this function protects against mitotic defects, genomic instability, and cellular senescence. At pericentric heterochromatin, SIRT6 promotes deacetylation of a new substrate, residue K18 of histone H3 (H3K18), and inactivation of SIRT6 in cells leads to H3K18 hyperacetylation and aberrant accumulation of pericentric transcripts. Strikingly, depletion of these transcripts through RNA interference rescues the mitotic and senescence phenotypes of SIRT6-deficient cells. Together, our findings reveal a new function for SIRT6 and regulation of acetylated H3K18 at heterochromatin, and demonstrate the pathogenic role of deregulated pericentric transcription in aging- and cancer-related cellular dysfunction.