Inhibition of EZH2 Promotes Human Embryonic Stem Cell Differentiation into Mesoderm by Reducing H3K27me3.

Mesoderm derived from human embryonic stem cells (hESCs) is a major source of the mesenchymal stem/stromal cells (MSCs) that can differentiate into osteoblasts and chondrocytes for tissue regeneration. While significant progress has been made in understanding of molecular mechanisms of hESC differentiation into mesodermal cells, little is known about epigenetic factors controlling hESC fate toward mesoderm and MSCs. Identifying potential epigenetic factors that control hESC differentiation will undoubtedly lead to advancements in regenerative medicine. Here, we conducted an epigenome-wide analysis of hESCs and MSCs and uncovered that EZH2 was enriched in hESCs and was downregulated significantly in MSCs. The specific EZH2 inhibitor GSK126 directed hESC differentiation toward mesoderm and generated more MSCs by reducing H3K27me3. Our results provide insights into epigenetic landscapes of hESCs and MSCs and suggest that inhibiting EZH2 promotes mesodermal differentiation of hESCs

Heart enhancers with deeply conserved regulatory activity are established early in zebrafish development.

During the phylotypic period, embryos from different genera show similar gene expression patterns, implying common regulatory mechanisms. Here we set out to identify enhancers involved in the initial events of cardiogenesis, which occurs during the phylotypic period. We isolate early cardiac progenitor cells from zebrafish embryos and characterize 3838 open chromatin regions specific to this cell population. Of these regions, 162 overlap with conserved non-coding elements (CNEs) that also map to open chromatin regions in human. Most of the zebrafish conserved open chromatin elements tested drive gene expression in the developing heart. Despite modest sequence identity, human orthologous open chromatin regions recapitulate the spatial temporal expression patterns of the zebrafish sequence, potentially providing a basis for phylotypic gene expression patterns. Genome-wide, we discover 5598 zebrafish-human conserved open chromatin regions, suggesting that a diverse repertoire of ancient enhancers is established prior to organogenesis and the phylotypic period

Quantitative methods for profiling dynamic chromatin features

Living systems, from entire organisms down to the single cells constituting them are dynamic entities that continuously adapt and respond to their local environment. Cells achieve this through gene expression programs derived from static information encoded in the DNA made dynamic through chemical modifications at the chromatin level, collectively termed the epigenome. Numerous epigenetic regulators have been implicated in early embryonic developmental transitions and pluripotency. Ex vivo, the different states of pluripotency can be recapitulated by embryonic stem cells (ESCs) grown in defined media conditions. Many developmental gene promoters in ESCs display co-occurrence of the activating histone H3 lysine 4 trimethylation (H3K4me3) mark and the repressive H3K27me3 mark. This distinctive bivalent signature is considered to poise expression, allowing timely resolution to an active or inactive state depending on the signal. The distribution of histone modifications and chromatin-associated factors across the genome can be mapped using chromatin immunoprecipitation followed by next-generation sequencing (ChIP-seq). However, traditional ChIP-seq methods fail to quantitatively profile the nuanced global and local epigenetic rewiring that takes place in key developmental stages. This thesis addresses this limitation through the development of a quantitative multiplexed ChIP-seq technology: MINUTE (multiplexed indexed unique molecule T7 amplification end to end sequencing) ChIP. Across the three papers included in this thesis, we reveal the underpinnings of chromatin state dynamics in early mouse and human embryonic development by employing MINUTE ChIP. In Paper I, we first show that MINUTE ChIP enables accurate quantitative comparisons over a wide linear range. By employing it to characterize mouse ESCs grown in 2i and serum conditions, we find that the 2i naïve state is characterized by high global levels of H3K27me3 and low H3K4me3. At bivalent promoters, we observe that while H3K27me3 levels are stably maintained between serum and 2i, H3K4me3 levels are higher in the serum condition. Through quantitative epigenome profiling, in Paper II we find that naïve human ESCs also have broad global gain of Polycomb repressive complex 2 (PRC2)-mediated H3K27me3 and define a previously unrecognized, naïve-specific set of bivalent promoters. Bulk and single-cell transcriptomics confirmed that naïve bivalency maintains key trophectoderm and mesoderm transcription factors in a transcriptionally poised state which is resolved to an active state upon depletion of H3K27me3. Therefore, we discovered that PRC2-mediated repression provides a highly adaptive mechanism to restrict lineage potential during early human development. In paper III we show how quantitative RNA polymerase II occupancy profiles generated by MINUTE ChIP can be integrated with transient transcriptomics data to unravel genome wide transcriptional kinetics in three mESCs pluripotent states: naïve, ground and paused. Taken together, this thesis provides compelling evidence for a broad H3K27me3 hypermethylation of the genome in both naïve mouse and human ESCs and the basis for substantially revising the model for bivalency during embryonic developmen

The Stability of the Induced Epigenetic Programs

For many years scientists have been attracted to the possibility of changing cell identity. In the last decades seminal discoveries have shown that it is possible to reprogram somatic cells into pluripotent cells and even to transdifferentiate one cell type into another. In view of the potential applications that generating specific cell types in the laboratory can offer for cell-based therapies, the next important questions relate to the quality of the induced cell types. Importantly, epigenetic aberrations in reprogrammed cells have been correlated with defects in differentiation. Therefore, a look at the epigenome and understanding how different regulators can shape it appear fundamental to anticipate potential therapeutic pitfalls. This paper covers these epigenetic aspects in stem cells, differentiation, and reprogramming and discusses their importance for the safety of in vitro engineered cell types

Epigenetic regulation of key developmental genes during early mouse development

In undifferentiated ES cells, many Polycomb Repressive Complex 2 (PRC2) target genes carry not only repressive H3K27me3 but are also enriched for conventional indicators of active chromatin including methylated H3K4. This so-called bivalent domain structure is thought to silence key developmental regulators while keeping them poised for future activation (or repression). Consistent with this hypothesis, bivalent genes assemble RNAP II preferentially phosphorylated on Serine 5 residues (poised RNAP II) and are transcribed at low levels. Productive expression is, however, prevented by the action of PRC1. Here, I have focused on the pre-implantation stage of mouse development to evaluate whether bivalent or poised chromatin signatures are indeed specific attributes of emerging pluripotent cells and investigate how the fate of key developmental genes is specified while the first lineage decision event (extra-embryonic lineage formation) occurs. Using blastocyst-derived stem cells and chromatin immunoprecipitation (ChIP), I have shown that lineage-inappropriate genes retain bivalent histone marking in extra-embryonic trophoblast stem (TS). However, and in contrast to ES cells, PRC1 (Ring1B) and poised RNAP II are not recruited to these loci in TS cells, indicating that gene priming is a unique hallmark of pluripotent cells in the early embryo. To investigate the intricate relationship between lineage identity and dynamic chromatin changes, I exploited the potential to convert ES cells into trophoblast-like stem (TSL) cells using a previously established artificial system dependent on doxycycline (Dox) induced repression of an Oct4 transgene. I demonstrated that Suv39h1-mediated H3K9me3 alongside DNA methylation is targeted to PRC2-bound bivalent, lineage-inappropriate genes upon trophectoderm lineage commitment. A change in chromatin conformation was observed upon differentiation of ES cells to TSL cells comparable to that seen in TS cells derived in the traditional manner from the trophectoderm (TE) of blastocyst stage embryos. Most importantly, I have begun to explore when epigenetic differences are specified, at the locus level, from 8-cell stage embryos onwards using newly designed Carrier ChIP technology. This data validated the occurrence of bivalent chromatin domains in vivo and further support the view that alternative strategies operate in the TE to silence key developmental regulators upon blastocyst lineage segregation

Wilms Tumor Chromatin Profiles Highlight Stem Cell Properties and a Renal Developmental Network

Wilms tumor is the most common pediatric kidney cancer. To identify transcriptional and epigenetic mechanisms that drive this disease, we compared genome-wide chromatin profiles of Wilms tumors, embryonic stem cells (ESCs), and normal kidney. Wilms tumors prominently exhibit large active chromatin domains previously observed in ESCs. In the cancer, these domains frequently correspond to genes that are critical for kidney development and expressed in the renal stem cell compartment. Wilms cells also express “embryonic” chromatin regulators and maintain stem cell-like p16 silencing. Finally, Wilms and ESCs both exhibit “bivalent” chromatin modifications at silent promoters that may be poised for activation. In Wilms tumor, bivalent promoters correlate to genes expressed in specific kidney compartments and point to a kidney-specific differentiation program arrested at an early-progenitor stage. We suggest that Wilms cells share a transcriptional and epigenetic landscape with a normal renal stem cell, which is inherently susceptible to transformation and may represent a cell of origin for this disease

Epigenetic Regulation Of Gene Expression Domains In Pluripotent Stem Cells

Transcriptional and histone modification profiles were analysed in detail across several regions of the mouse genome in order to examine how tissue-specificity is determined at early stages of development and how it is maintained during cell commitment. The analysis was mainly focused on a 2 Mb gene-dense region containing 68 known closely situated genes, having very diverse expression patterns. Transcriptional profiling of this region showed that clustered tissue-restricted and housekeeping genes are associated with clearly defined monovalent or bivalent epigenetic domains. In ES cells, the actively transcribed housekeeping genes were enriched for histone H3K4me1, H3K4me2, H3K4me3, H3K9ac and binding of Pol II. Most of the silent tissue-restricted genes were marked by bivalent domains and had background levels of Pol II. In few cases, substantial levels of Pol II were found at inactive genes situated close to housekeeping genes, suggesting that transcriptional read-through could be occurring at these genes in ES cells. Epigenetic profiling of the gene-dense region in LPS-activated B cells showed that B cells have a very similar histone modification profile to ES cells. Specifically, the analysis showed that bivalent domains which were thought to be a defining characteristic of pluripotent stem cells are equally prevalent in B cells, suggesting that the epigenetic marking of silent genes is quite similar in these two very different cell types. Analysis of a large gene-poor region containing four genes encoding GABAA neurotransmitter receptor subunits showed that the this locus acquires a large bivalent domain of approximately 1.3 Mb following differentiation of ES cells into NS cells and astrocytes. The results obtained in this study demonstrate the complex and diverse nature of histone modifications at tissue-restricted genes and suggest that trans-acting factors are responsible for generating highly specific combinations of histone modifications at each individual gene at different stages of cell differentiation

Tagging methods as a tool to investigate histone H3 methylation dynamics in mouse embryonic stem cells

Covalent modification of histones is an important factor in the regulation of the chromatin structure implicated in DNA replication, repair, recombination, and transcription, as well as in RNA processing. In recent years, histone methylation has emerged as one of the key modifications regulating chromatin function. However, the mechanisms involved are complex and not well understood. Histone 3 lysine 4 (H3K4) methylation is deposited by a family of histone H3K4 methyltransferases (HMTs) that share a conserved SET domain. In mammalian cells, six family members have been characterized: Setd1a and Setd1b (the mammalian orthologs of yeast Set1) and four Mixed lineage leukemia (Mll) family HMTs, which share limited similarity with yeast Set1 beyond the SET domain. Several studies demonstrated that the H3K4 methyltransferases exist as multiprotein complexes. To functionally dissect H3K4 methyltransferase complexes, GFP tagging of the core subunit Ash2l and the complex-specific subunits Cxxc1 and Wdr82 (Setd1a/b complexes) Men1 (Mll1/2 complexes), and Ptip (Mll3/Mll4 complexes), was used. The fusion proteins were successfully expressed in mouse embryonic stem cells (ES cells), analyzed by confocal microscopy, Mass Spectrometry (MS) and ChIP-seq. Ptip was the only subunit able to bind mitotic chromatin. Additionally, both Ptip and Wdr82 were found to associate with cell cycle regulators, suggesting a possible role of the two proteins or respective complexes in cell cycle regulation. Mass Spectrometry revealed that Wdr82 and Ptip interact with members of he PAF complex, and ChIP-seq showed that Wdr82, Cxxc1 and Ptip positively modulate pluripotency genes. Thus, Setd1a/b and Mll3/4 complexes might act together in the regulation of embryonic stem cells identity. Protein pull downs identified at least one new Setd1a/b interactor, Bod1l that is orthologous to the yeast protein Sgh1, a component of the Set1C complex. Furthermore, our MS and ChIP-seq data suggested that only Mll2 complex binds to bivalent promoters, wheras Mll2 and Setd1a complexes might function together in a set of promoters

Global Reorganization of Replication Domains During Embryonic Stem Cell Differentiation

DNA replication in mammals is regulated via the coordinate firing of clusters of replicons that duplicate megabase-sized chromosome segments at specific times during S-phase. Cytogenetic studies show that these “replicon clusters” coalesce as subchromosomal units that persist through multiple cell generations, but the molecular boundaries of such units have remained elusive. Moreover, the extent to which changes in replication timing occur during differentiation and their relationship to transcription changes has not been rigorously investigated. We have constructed high-resolution replication-timing profiles in mouse embryonic stem cells (mESCs) before and after differentiation to neural precursor cells. We demonstrate that chromosomes can be segmented into multimegabase domains of coordinate replication, which we call “replication domains,” separated by transition regions whose replication kinetics are consistent with large originless segments. The molecular boundaries of replication domains are remarkably well conserved between distantly related ESC lines and induced pluripotent stem cells. Unexpectedly, ESC differentiation was accompanied by the consolidation of smaller differentially replicating domains into larger coordinately replicated units whose replication time was more aligned to isochore GC content and the density of LINE-1 transposable elements, but not gene density. Replication-timing changes were coordinated with transcription changes for weak promoters more than strong promoters, and were accompanied by rearrangements in subnuclear position. We conclude that replication profiles are cell-type specific, and changes in these profiles reveal chromosome segments that undergo large changes in organization during differentiation. Moreover, smaller replication domains and a higher density of timing transition regions that interrupt isochore replication timing define a novel characteristic of the pluripotent state