- Open Access
Embryonic stem cell differentiation: A chromatin perspective
© Rasmussen; licensee BioMed Central Ltd. 2003
- Received: 14 July 2003
- Accepted: 13 November 2003
- Published: 13 November 2003
Embryonic stem (ES) cells hold immense promise for the treatment of human degenerative disease. Because ES cells are pluripotent, they can be directed to differentiate into a number of alternative cell-types with potential therapeutic value. Such attempts at "rationally-directed ES cell differentiation" constitute attempts to recapitulate aspects of normal development in vitro. All differentiated cells retain identical DNA content, yet gene expression varies widely from cell-type to cell-type. Therefore, a potent epigenetic system has evolved to coordinate and maintain tissue-specific patterns of gene expression. Recent advances show that mechanisms that govern epigenetic regulation of gene expression are rooted in the details of chromatin dynamics. As embryonic cells differentiate, certain genes are activated while others are silenced. These activation and silencing events are exquisitely coordinated with the allocation of cell lineages. Remodeling of the chromatin of developmentally-regulated genes occurs in conjunction with lineage commitment. Oocytes, early embryos, and ES cells contain potent chromatin-remodeling activities, an observation that suggests that chromatin dynamics may be especially important for early lineage decisions. Chromatin dynamics are also involved in the differentiation of adult stem cells, where the assembly of specialized chromatin upon tissue-specific genes has been studied in fine detail. The next few years will likely yield striking advances in the understanding of stem cell differentiation and developmental biology from the perspective of chromatin dynamics.
- Embryonic Stem Cell
- Globin Gene
- Inner Cell Mass
- Chromatin Remodel Complex
- Chromosome Inactivation
A great variety of cell-types have been generated by the differentiation of ES cells in vitro. Such cell-types resemble neuronal cells, pancreatic cells, muscle cells and fibroblasts, hematopoietic cells, and many others, but it is unclear at present how closely these cells mimic their normal counterparts created in the course of development in vivo. Indeed, many promising derivatives of ES cells have proven to function poorly in animal engraftment models. These difficulties point to the need for a more sophisticated understanding of the differentiation process and the development of new methods to more exhaustively assess the identities of cells generated by in vitro differentiation. Future methods that achieve successful directed differentiation will rely heavily on a thorough understanding of the developmental pathways that they attempt to recapitulate. Initial attempts to bring stem cell therapy to a state of effectiveness have been hampered by a paucity of knowledge concerning the underlying mechanisms that govern differentiation.
The basic subunit of all chromatin is the nucleosome, which consists of a histone octomer containing a pair of each of the standard histones H2A, H2B, H3, and H4 and 146 base pairs of DNA. Chromatin can be broadly divided into two fractions: euchromatin, which is permissive for transcription, and heterochromatin, which is repressive. Heterochromatin itself occurs in two varieties, constitutive and facultative. DNA within constitutive heterochromatin is obligately silenced. Examples include centromeric regions and inactivated repetitive elements such as Alu, LINE, and SINE elements, and inactivated retroviruses. In contrast, facultative heterochromatin is silenced only in certain contexts. Examples of facultative heterochromatin include tissue-specific genes that are silenced in all but the appropriate tissue, and X chromosomes in female eutherian mammals, which can be either active or silenced for reasons of dosage compensation. Heterochromatin seems to have much more molecular complexity than euchromatin. Most euchromatin consists of standard nucleosomes, but heterochromatin often contains modified nucleosomes in which histone variants substitute for standard histones. Examples include the histone H2A variants macroH2A1  and macroH2A2 [3, 4], the histone H3 variant CENP-A [5, 6] which associates with centromeric heterochromatin, and histones H2A.X and H2A.Z. Histone H2A.X becomes phosphorylated when DNA is damaged and marks sites of double-stranded breaks in DNA , while H2A.Z protects euchromatin from becoming transcriptionally inactive .
Chromatin is further modified by the addition of post-translational modifications upon histones such as methyl-, acetyl-, phosphoryl-, and ADP ribosyl-groups (see reviews: [9–11]). Histone modifications can serve as cis-acting binding sites for auxiliary factors such as heterochromatin protein 1 (HP1), which binds to histone H3 upon methylation at lysine 9 [12, 13], a key bimolecular interaction in heterochromatin that is controlled by Suv39h1 and Suv39h2 histone methyltransferases [14–16]. Histone methylation also occurs upon histone H3 at the lysine 4 position, but in this case, methylation correlates with transcriptional activity . DNA itself can also be methylated, primarily upon cytosines within CpG dinucleotides. Here too, methylation serves to foster bimolecular interactions between heterochromatin and auxiliary proteins such as MeCP2, and MBD1, 2, and 3. On a molecular level, close connections exist between chromatin and the transcriptional apparatus. The importance of transcription factors for gene activation is indisputable, but a remarkable number of transcription factors are now known to interact with transcription-promoting histone acetyltransferase (HAT) proteins . An emerging view is that histone modifications may serve as "switches" to activate or repress gene expression, in much the same way that phosphates toggle the activation or repression of intracellular signal transduction pathways.
As ES cells undergo differentiation, they spontaneously execute developmentally-regulated programs of gene expression and gene silencing that are intimately coordinated with alterations in chromatin structure. Perhaps the best studied of these is X chromosome inactivation (XCI), where a number of step-wise chromatin remodeling events lead to the formation of stably-silenced X chromosomes in differentiating female ES cells. The initiation step of X inactivation is governed by a mutually-antagonistic expression pattern involving the genes Tsix and Xist, which lie adjacent to one another within the X inactivation center (XCI) of the X chromosome . Tsix and Xist give rise to overlapping untranslated RNA transcripts that are retained upon X chromosomes by an unknown mechanism. In undifferentiated cells, Tsix expression prevents the accumulation of Xist RNA. As differentiation commences, Tsix expression is extinguished from the future inactive X chromosome (Xi), which allows Xist to accumulate, spread, and coat the Xi. Expression of Xist RNA is required for initiation of X inactivation but not its maintenance [20, 21]. Shortly after Xist coating, the silencing of X-linked genes can be detected, but this early silencing is unstable and reversible . A number of subsequent (and ordered) chromatin remodeling events then occur to stabilize the inactive state. These include a transient association of the histone methyltransferase complex Eed-Ezh2 which methylates histone H3 at lysine 27 [23, 24]. Another early event in X chromosome inactivation is late replication timing of Xi relative to the active X during S phase of the cell cycle [25, 26]. Subsequently, histones become hypoacetylated [27, 28] upon lysine residues and methylated at lysine 9, an event probably performed by the Suv39h1 and h2 histone methyltransferases [29, 30]. Later, the histone variant macroH2A is incorporated into inactivated X chromosomes within female nuclei. These local macroH2A concentrations are called macrochromatin bodies (MCBs) . MCB formation occurs relatively late in the X chromosome inactivation process [32, 33], and requires prior expression of Xist [20, 34]. Recent evidence points to a functional role for macroH2A1 in gene silencing since it can cause down-regulation of reporter gene activity . In addition, macroH2A1 can inhibit binding of the transcription factor NF-kappaB and retards the action of SWI/SNF chromatin remodeling complexes . A final step in the X inactivation process is marked by the methylation of the CpG islands of silenced X-linked genes . Interestingly, all of these chromatin-remodelling events (with the exception of the action of the Tsix/Xist RNAs) can also occur on autosomes . Therefore, it seems that all post-RNA events of XCI may have been co-opted from pre-existing autosomal systems during the evolution of XCI in placental mammals.
Interestingly, the mammalian nuclear transfer cloning process can reprogram X chromosome inactivation. In this case, inactive X chromosomes from somatic cells were reactivated in cloned embryos but not trophectoderm . Though cloned embryos transit the blastocyst developmental stage (with concomitant formation of the ICM), it is unclear at which stage the somatic donor XCI status was erased (or reactivated), though failure of erasure to occur in trophectodermal cells suggests the possible involvement of the ICM cells or their progenitors in reprogramming events. Another interpretation of these results is that the somatic pattern of XCI is read out in the trophectoderm much like an imprint, a distinct possibility since XCI is imprinted only in the extra-embryonic tissues in the mouse. In either case, it is clear that reprogramming of inactive X chromosomes during the cloning process shows that oocytes or primitive embryonic cells have extensive chromatin remodeling activity.
Mammalian cells contain a variety of chromatin remodeling complexes, which typically are composed of several proteins. Inactivation of chromatin remodeling complexes usually results in developmental arrest at about the blastocyst stage. For instance, loss of SNF5, a shared component of two related mammalian SWI/SNF chromatin remodeling complexes causes developmental arrest at about the time of embryo implantation and is required for ES cell viability . Homozygous deletion of SNF2 β (another component of mammalian SWI/SNF chromatin remodeling complexes), leads to lethality in F9 embryonal carcinoma cells . The polycomb group gene rae28 is required for efficient renewal of hematopoietic stem cells , while the polycomb group gene Ezh2 (which contains a histone methyltransferase SET domain) is required for post-implantation development and ES cell viability .
Direct demonstrations of reprogramming activities within ES cells come from experiments involving intentional cell fusions between ES and somatic cells. Fusions between male ES cells and adult female thymocytes led to demonstrated alterations in the chromatin of inactivated X chromosomes originating from the thymocyte nucleus. These alterations included the abolition of late replication timing of the thymocyte Xi, and destabilization of the Xist RNA transcript. In addition, a silenced Oct4-GFP transgene was reactivated in these fusion cells, a transcriptional state reminiscent of primitive cells . Significantly, ES-thymocyte fusions gained pleuripotency since they contributed to all three germ layers in chimeric embryos . However fusions between ES cells and thymocytes failed to alter methylation of the imprinted Igf2-H19 region, which occurs readily when embryonic germ cells and thymocytes are fused .
Studies of adult stem cells provide ample evidence that chromatin functions in specialized differentiation events. A classic view of development posits that the commitment of primitive multipotent cells to specific lineages is mediated by key transcription factors that activate downstream tissue-specific genes. For instance, retroviral-mediated transfer of transcription factor MyoD into a variety of non-muscle cell types can convert them into cells that resemble striated myoblasts as judged by cellular morphology and expression of muscle-specific markers . This is but one of many demonstrations that transcription factors can effectively initiate developmental programs that culminate in directed differentiation (even trans-differentiation) of cells. In contrast, recent evidence shows that heterochromatin formation can be a key mechanism for lineage restriction during development.
The differentiation of hematopoietic stem cells (HSCs) seems to occur by means of selective silencing (i.e. restriction) of lineage-specific genes. Hematopoietic stem cells and their immediate progenitors exhibit a promiscuous pattern of gene expression. Uncommitted HSCs simultaneously express genes previously thought to be transcribed exclusively in either the myeloid or erythroid lineages . Developmentally-regulated restriction of gene expression also occurs during formation of glial cells of the brain. The formation of oligodendrocytes from glial cell precursors requires exit from mitosis and histone deacetylase (HDAC) activity shortly thereafter. Oligodendrocyte formation is abolished by use of the HDAC inhibitor trichostatin A, suggesting that hypoacetylation of histones in glial stem cells is a normal occurrence during neurogenesis .
The regulation of globin gene expression is also mediated by chromatin remodeling. The locus control region (LCR) is a non-coding regulatory domain that lies adjacent to the globin gene cluster. Lineage-specific expression of globin genes is modulated by a complex set of chromatin events involving the LCR and individual globin gene promoters. Interestingly, DNase I sensitive sites within the LCR that are observed in erythrocyte lineages can be found much earlier in multipotent hematopoietic stem cells, suggesting that chromatin rearrangements precede transcriptional activation of lineage-specific genes during hematopoiesis . This "primed" configuration of the LCR is also evident in embryonic yolk sac, where acetylated histones are present at the LCR and globin gene promoters (whether active or inactive) while in brain, the LCR is hypoacetylated and globin genes are transcriptionally silent . Interestingly, the LCR is present in an acetylated state in ES cells . Together, these data suggest that aspects of globin gene expression may be regulated by a restriction mechanism involving the LCR.
Finally, the intranuclear position of chromatin seems to have a bearing on its regulation. In many mammalian cell types, it has been observed that transcriptionally silent genes reside in a position near the nuclear periphery , or interphase centromeres (chromocenters) , while active genes are maintained near the center of nuclei. Nuclear location of genes may therefore lead to changes in their transcription state and there is some evidence that this is a dynamic process. For instance, the zinc-finger protein Ikaros is expressed in lymphoid cells and seems to be a regulator of gene silencing by recruiting genes to repressive heterochromatin . Deletion of Ikaros leads to a loss of differentiative potential in hematopoietic stem cells in mice .
The dynamic assembly and disassembly of specialized chromatin is a likely mechanism for the epigenetic regulation of gene expression during ES cell differentiation and development in vivo. Put another way, the transcriptional state of a gene can be viewed as a reflection of its underlying chromatin state. Indeed, it seems possible that all stem cells share common epigenetic mechanisms given the recent demonstration that hematopoietic, neural, and embryonic stem cells express a significant number of non-housekeeping genes in common . Because stem cell differentiation is essentially an attempt to achieve tissue-specific patterns of gene expression in vitro, it seems reasonable that studies of chromatin in differentiating ES cells will greatly aid the subsequent clinical development of stem cell therapies. The analysis of chromatin factors and modifications that exhibit tissue-specific genomic distribution patterns will likely yield substantial insights into the use of stem cells of all types.
I thank Stephanie Jacobs and Marina Julian for critical readings of this manuscript.
- Pesce M, Scholer HR: Oct-4: gatekeeper in the beginnings of mammalian development. Stem Cells. 2001, 19: 271-278.View ArticlePubMedGoogle Scholar
- Pehrson JR, Fried VA: MacroH2A, a core histone containing a large nonhistone region. Science. 1992, 257: 1398-1400.View ArticlePubMedGoogle Scholar
- Chadwick BP, Willard HF: Histone H2A variants and the inactive X chromosome: identification of a second macroH2A variant. Hum Mol Genet. 2001, 10: 1101-1113. 10.1093/hmg/10.10.1101.View ArticlePubMedGoogle Scholar
- Costanzi C, Pehrson JR: MACROH2A2, a new member of the MARCOH2A core histone family. J Biol Chem. 2001, 276: 21776-1784. 10.1074/jbc.M010919200.View ArticlePubMedGoogle Scholar
- Palmer DK, O'Day K, Trong HL, Charbonneau H, Margolis RL: Purification of the centromere-specific protein CENP-A and demonstration that it is a distinctive histone. Proc Natl Acad Sci U S A. 1991, 88: 3734-3738.PubMed CentralView ArticlePubMedGoogle Scholar
- Palmer DK, O'Day K, Wener MH, Andrews BS, Margolis RL: A 17-kD centromere protein (CENP-A) copurifies with nucleosome core particles and with histones. J Cell Biol. 1987, 104: 805-815.View ArticlePubMedGoogle Scholar
- Rogakou EP, Pilch DR, Orr AH, Ivanova VS, Bonner WM: DNA double-stranded breaks induce histone H2AX phosphorylation on serine 139. J Biol Chem. 1998, 273: 5858-5868. 10.1074/jbc.273.10.5858.View ArticlePubMedGoogle Scholar
- Meneghini MD, Wu M, Madhani HD: Conserved histone variant H2A.Z protects euchromatin from the ectopic spread of silent heterochromatin. Cell. 2003, 112: 725-736.View ArticlePubMedGoogle Scholar
- Li E: Chromatin modification and epigenetic reprogramming in mammalian development. Nat Rev Genet. 2002, 3: 662-673. 10.1038/nrg887.View ArticlePubMedGoogle Scholar
- Goll MG, Bestor TH: Histone modification and replacement in chromatin activation. Genes Dev. 2002, 16: 1739-1742. 10.1101/gad.1013902.View ArticlePubMedGoogle Scholar
- Jenuwein T, Allis CD: Translating the histone code. Science. 2001, 293: 1074-1080. 10.1126/science.1063127.View ArticlePubMedGoogle Scholar
- Nielsen PR, Nietlispach D, Mott HR, Callaghan J, Bannister A, Kouzarides T, Murzin AG, Murzina NV, Laue ED: Structure of the HP1 chromodomain bound to histone H3 methylated at lysine 9. Nature. 2002, 416: 103-107. 10.1038/nature722.View ArticlePubMedGoogle Scholar
- Jacobs SA, Khorasanizadeh S: Structure of HP1 chromodomain bound to a lysine 9-methylated histone H3 tail. Science. 2002, 295: 2080-2083. 10.1126/science.1069473.View ArticlePubMedGoogle Scholar
- O'Carroll D, Scherthan H, Peters AH, Opravil S, Haynes AR, Laible G, Rea S, Schmid M, Lebersorger A, Jerratsch M, et al: Isolation and characterization of Suv39h2, a second histone H3 methyltransferase gene that displays testis-specific expression. Mol Cell Biol. 2000, 20: 9423-9433. 10.1128/MCB.20.24.9423-9433.2000.PubMed CentralView ArticlePubMedGoogle Scholar
- Rea S, Eisenhaber F, O'Carroll D, Strahl BD, Sun ZW, Schmid M, Opravil S, Mechtler K, Ponting CP, Allis CD, et al: Regulation of chromatin structure by site-specific histone H3 methyltransferases. Nature. 2000, 406: 593-599. 10.1038/35020506.View ArticlePubMedGoogle Scholar
- Peters AH, O'Carroll D, Scherthan H, Mechtler K, Sauer S, Schofer C, Weipoltshammer K, Pagani M, Lachner M, Kohlmaier A, et al: Loss of the Suv39h histone methyltransferases impairs mammalian heterochromatin and genome stability. Cell. 2001, 107: 323-337.View ArticlePubMedGoogle Scholar
- Bernstein BE, Humphrey EL, Erlich RL, Schneider R, Bouman P, Liu JS, Kouzarides T, Schreiber SL: Methylation of histone H3 Lys 4 in coding regions of active genes. Proc Natl Acad Sci U S A. 2002, 99: 8695-8700. 10.1073/pnas.112318199.PubMed CentralView ArticlePubMedGoogle Scholar
- Cheung WL, Briggs SD, Allis CD: Acetylation and chromosomal functions. Curr Opin Cell Biol. 2000, 12: 326-333. 10.1016/S0955-0674(00)00096-X.View ArticlePubMedGoogle Scholar
- Lee JT: Disruption of imprinted X inactivation by parent-of-origin effects at Tsix. Cell. 2000, 103: 17-27.View ArticlePubMedGoogle Scholar
- Csankovszki G, Panning B, Bates B, Pehrson JR, Jaenisch R: Conditional deletion of Xist disrupts histone macroH2A localization but not maintenance of X inactivation. Nat Genet. 1999, 22: 323-324. 10.1038/11887.View ArticlePubMedGoogle Scholar
- Brown CJ, Willard HF: The human X-inactivation centre is not required for maintenance of X-chromosome in activation. Nature. 1994, 368: 154-156. 10.1038/368154a0.View ArticlePubMedGoogle Scholar
- Wutz A, Jaenisch R: A shift from reversible to irreversible X inactivation is triggered during ES cell differentiation. Mol Cell. 2000, 5: 695-705.View ArticlePubMedGoogle Scholar
- Plath K, Fang J, SK Mlynarczyk-Evans, Cao R, Worringer KA, Wang H, de la Cruz CC, Otte AP, Panning B, Zhang Y: Role of histone H3 lysine 27 methylation in X inactivation. Science. 2003, 300: 131-135. 10.1126/science.1084274.View ArticlePubMedGoogle Scholar
- Silva J, Mak W, Zvetkova I, Appanah R, Nesterova TB, Webster Z, Peters AH, Jenuwein T, Otte AP, Brockdorff N: Establishment of histone h3 methylation on the inactive x chromosome requires transient recruitment of eed-enx1 polycomb group complexes. Dev Cell. 2003, 4: 481-495.View ArticlePubMedGoogle Scholar
- Boggs BA, Chinault AC: Analysis of replication timing properties of human X-chromosomal loci by fluorescence in situ hybridization. Proc Natl Acad Sci U S A. 1994, 91: 6083-6087.PubMed CentralView ArticlePubMedGoogle Scholar
- Hansen RS, Canfield TK, Fjeld AD, Gartler SM: Role of late replication timing in the silencing of X-linked genes. Hum Mol Genet. 1996, 5: 1345-1353. 10.1093/hmg/5.9.1345.View ArticlePubMedGoogle Scholar
- Keohane AM, Lavender JS, O'Neill LP, Turner BM: Histone acetylation and X inactivation. Dev Genet. 1998, 22: 65-73. 10.1002/(SICI)1520-6408(1998)22:1<65::AID-DVG7>3.3.CO;2-W.View ArticlePubMedGoogle Scholar
- Keohane AM, O'Neill LP, Belyaev ND, Lavender JS, Turner BM: X-Inactivation and histone H4 acetylation in embryonic stem cells. Dev Biol. 1996, 180: 618-630. 10.1006/dbio.1996.0333.View ArticlePubMedGoogle Scholar
- Heard E, Rougeulle C, Arnaud D, Avner P, Allis CD, Spector DL: Methylation of histone H3 at Lys-9 is an early mark on the X chromosome during X inactivation. Cell. 2001, 107: 727-738.View ArticlePubMedGoogle Scholar
- Mermoud JE, Popova B, Peters AH, Jenuwein T, Brockdorff N: Histone H3 lysine 9 methylation occurs rapidly at the onset of random X chromosome inactivation. Curr Biol. 2002, 12: 247-251. 10.1016/S0960-9822(02)00660-7.View ArticlePubMedGoogle Scholar
- Costanzi C, Pehrson JR: Histone macroH2A1 is concentrated in the inactive X chromosome of female mammals. Nature. 1998, 393: 599-601. 10.1038/31275.View ArticlePubMedGoogle Scholar
- Mermoud JE, Costanzi C, Pehrson JR, Brockdorff N: Histone macroH2A1.2 relocates to the inactive X chromosome after initiation and propagation of X-inactivation. J Cell Biol. 1999, 147: 1399-1408. 10.1083/jcb.147.7.1399.PubMed CentralView ArticlePubMedGoogle Scholar
- Rasmussen TP, Mastrangelo MA, Eden A, Pehrson JR, Jaenisch R: Dynamic relocalization of histone MacroH2A1 from centrosomes to inactive X chromosomes during X inactivation. J Cell Biol. 2000, 150: 1189-1198. 10.1083/jcb.150.5.1189.PubMed CentralView ArticlePubMedGoogle Scholar
- Rasmussen TP, Wutz AP, Pehrson JR, Jaenisch RR: Expression of Xist RNA is sufficient to initiate macrochromatin body formation. Chromosoma. 2001, 110: 411-420. 10.1007/s004120100158.View ArticlePubMedGoogle Scholar
- Perche PY, Vourc'h C, Konecny L, Souchier C, Robert-Nicoud M, Dimitrov S, Khochbin S: Higher concentrations of histone macroH2A in the Barr body are correlated with higher nucleosome density. Curr Biol. 2000, 10: 1531-1534. 10.1016/S0960-9822(00)00832-0.View ArticlePubMedGoogle Scholar
- Angelov D, Molla A, Perche PY, Hans F, Cote J, Khochbin S, Bouvet P, Dimitrov S: The Histone Variant MacroH2A Interferes with Transcription Factor Binding and SWI/SNF Nucleosome Remodeling. Mol Cell. 2003, 11: 1033-1041.View ArticlePubMedGoogle Scholar
- Plath K, Mlynarczyk-Evans S, Nusinow DA, Panning B: Xist RNA and the mechanism of X chromosome inactivation. Annu Rev Genet. 2002, 36: 233-278. 10.1146/annurev.genet.36.042902.092433.View ArticlePubMedGoogle Scholar
- Cohen DE, Lee JT: X-chromosome inactivation and the search for chromosome-wide silencers. Curr Opin Genet Dev. 2002, 12: 219-224. 10.1016/S0959-437X(02)00289-7.View ArticlePubMedGoogle Scholar
- Eggan K, Akutsu H, Hochedlinger K, Rideout W, Yanagimachi R, Jaenisch R: X-Chromosome in activation in cloned mouse embryos. Science. 2000, 290: 1578-1581. 10.1126/science.290.5496.1578.View ArticlePubMedGoogle Scholar
- Klochendler-Yeivin A, Fiette L, Barra J, Muchardt C, Babinet C, Yaniv M: The murine SNF5/INI1 chromatin remodeling factor is essential for embryonic development and tumor suppression. EMBO Rep. 2000, 1: 500-506.PubMed CentralView ArticlePubMedGoogle Scholar
- Sumi-Ichinose C, Ichinose H, Metzger D, Chambon P: SNF2beta-BRG1 is essential for the viability of F9m urine embryonal carcinoma cells. Mol Cel Biol. 1997, 17: 5976-5986.View ArticleGoogle Scholar
- Ohta H, Sawada A, Kim JY, Tokimasa S, Nishiguchi S, Humphries RK, Hara J, Takihara Y: Polycomb group gene rae28 is required for sustaining activity of hematopoietic stem cells. J Exp Med. 2002, 195: 759-770. 10.1084/jem.20011911.PubMed CentralView ArticlePubMedGoogle Scholar
- O'Carroll D, Erhardt S, Pagani M, Barton SC, Surani MA, Jenuwein T: The polycomb-group gene Ezh2 is required for earlymouse development. Mol Cell Biol. 2001, 21: 4330-4336. 10.1128/MCB.21.13.4330-4336.2001.PubMed CentralView ArticlePubMedGoogle Scholar
- Tada M, Takahama Y, Abe K, Nakatsuji N, Tada T: Nuclear reprogramming of somatic cells by in vitro hybridization with ES cells. Curr Biol. 2001, 11: 1553-1558. 10.1016/S0960-9822(01)00459-6.View ArticlePubMedGoogle Scholar
- Tada M, Tada T, Lefebvre L, Barton SC, Surani MA: Embryonic germ cells induce epigenetic reprogramming of somatic nucleus in hybrid cells. Embo J. 1997, 16: 6510-6520. 10.1093/emboj/16.21.6510.PubMed CentralView ArticlePubMedGoogle Scholar
- Choi J, Costa ML, Mermelstein CS, Chagas C, Holtzer S, Holtzer H: MyoD converts primary dermal fibroblasts, chondroblasts, smooth muscle, and retinal pigmented epithelial cells into striated mononucleated myoblasts and multinucleated myotubes. Proc Natl Acad Sci U S A. 1990, 87: 7988-7992.PubMed CentralView ArticlePubMedGoogle Scholar
- Hu M, Krause D, Greaves M, Sharkis S, Dexter M, Heyworth C, Enver T: Multilineage gene expression precedes commitment in the hemopoietic system. Genes Dev. 1997, 11: 774-785.View ArticlePubMedGoogle Scholar
- Marin-Husstege M, Muggironi M, Liu A, Casaccia-Bonnefil P: Histone deacetylase activity is necessary for oligodendrocyte lineage progression. J Neurosci. 2002, 22: 10333-10345.PubMedGoogle Scholar
- Jimenez G, Griffiths SD, Ford AM, Greaves MF, Enver T: Activation of the beta-globin locus control region precedes commitment to the erythroid lineage. Proc Natl Acad Sci U S A. 1992, 89: 10618-10622.PubMed CentralView ArticlePubMedGoogle Scholar
- Forsberg EC, Downs KM, Christensen HM, Im H, Nuzzi PA, Bresnick EH: Developmentally dynamic histone acetylation pattern of a tissue-specific chromatin domain. Proc Natl Acad Sci U S A. 2000, 97: 14494-14499. 10.1073/pnas.97.26.14494.PubMed CentralView ArticlePubMedGoogle Scholar
- Kramer JA, McCarrey JR, Djakiew D, Krawetz SA: Differentiation: the selective potentiation of chromatin domains. Development. 1998, 125: 4749-4755.PubMedGoogle Scholar
- Andrulis ED, Neiman AM, Zappulla DC, Sternglanz R: Perinuclear localization of chromatin facilitates transcriptional silencing. Nature. 1998, 394: 592-595. 10.1038/29100.View ArticlePubMedGoogle Scholar
- Maison C, Bailly D, Peters AH, Quivy JP, Roche D, Taddei A, Lachner M, Jenuwein T, Almouzni G: Higher-order structure in pericentric heterochromatin involves a distinct pattern of histone modification and an RNA component. Nat Genet. 2002, 30: 329-334. 10.1038/ng843.View ArticlePubMedGoogle Scholar
- Klug CA, Morrison SJ, Masek M, Hahm K, Smale ST, Weissman IL: Hematopoietic stem cells and lymphoid progenitors express different Ikaros isoforms, and Ikaros is localized to heterochromatin in immaturely mphocytes. Proc Natl Acad Sci U S A. 1998, 95: 576-62. 10.1073/pnas.95.2.576.View ArticleGoogle Scholar
- Lopez RA, Schoetz S, DeAngelis K, O'Neill D, Bank A: Multiple hematopoietic defects and delayed globin switching in Ikaros null mice. Proc Natl Acad Sci U S A. 2002, 99: 602-607. 10.1073/pnas.022412699.PubMed CentralView ArticlePubMedGoogle Scholar
- Ivanova NB, Dimos JT, Schaniel C, Hackney JA, Moore KA, Lemischka IR: A stem cell molecular signature. Science. 2002, 298: 601-604. 10.1126/science.1073823.View ArticlePubMedGoogle Scholar
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