Sundeep Kalantry

The murine polycomb group protein Eed is required for global histone H3 lysine-27 methylation.

PcG proteins mediate heritable transcriptional silencing by generating and recognizing covalent histone modifications. One conserved PcG complex, PRC2, is composed of several proteins including the histone methyltransferase (HMTase) Ezh2, the WD-repeat protein Eed, and the Zn-finger protein Suz12. Ezh2 methylates histone H3 on lysine 27 (H3K27), which serves as an epigenetic mark mediating silencing. H3K27 can be mono-, di-, or trimethylated (1mH3K27, 2mH3K27, and 3mH3K27, respectively). Hence, either PRC2 must be regulated so as to add one methyl group to certain nucleosomes but two or three to others, or distinct complexes must be responsible for 1m-, 2m-, and 3mH3K27. Consistent with the latter possibility, 2mH3K27 and 3mH3K27, but not 1mH3K27, are absent in Suz12-/- embryos, which lack both Suz12 and Ezh2 protein. Mammalian proteins required for 1mH3K27 have not been identified. Here, we demonstrate that unlike Suz12 and Ezh2, Eed is required not only for 2m- and 3mH3K27 but also global 1mH3K27. These results provide a functionally important distinction between PRC2 complex components and implicate Eed in PRC2-independent histone methylation.

A RA-dependent, tumour-growth suppressive transcription complex is the target of the PML-RARα and T18 oncoproteins

PML and Tif1a are fused to RARA and Braf, respectively, resulting in the production of PML-RARα and Tif1α-B-Raf (T18) oncoproteins. Here we show that PML, Tif1α and RXRα/RARα function together in a transcription complex that is dependent on retinoic acid (RA). We found that PML acts as a ligand-dependent coactivator of RXRα/RARα. PML interacts with Tif1α and CBP. In Pml–/– cells, the RA-dependent induction of genes such as RARB2 and the ability of Tif1α and CBP to act as transcriptional coactivators on RA are impaired. We show that both PML and Tif1α are growth suppressors required for the growth-inhibitory activity of RA. T18, similar to PML-RARα, disrupts the RA-dependent activity of this complex in a dominant-negative manner resulting in a growth advantage. Our data define a new pathway for the control of cell growth and tumorigenesis, and provide a new model for the pathogenesis of acute promyelocytic leukaemia (APL).

The amnionless gene, essential for mouse gastrulation, encodes a visceral-endoderm-specific protein with an extracellular cysteine-rich domain

Fate-mapping experiments in the mouse have revealed that the primitive streak can be divided into three functional regions: the proximal region gives rise to germ cells and the extra-embryonic mesoderm of the yolk sac; the distal region generates cardiac mesoderm and node-derived axial mesendoderm; and the middle streak region produces the paraxial, intermediate and lateral plate mesoderm of the trunk. To gain insight into the mechanisms that mediate the assembly of the primitive streak into these functional regions, we have cloned and functionally identified the gene disrupted in the amnionless (amn) mouse, which has a recessive, embryonic lethal mutation that interferes specifically with the formation and/or specification of the middle primitive streak region during gastrulation. Here we report that the gene Amn encodes a novel type I transmembrane protein that is expressed exclusively in the extra-embryonic visceral endoderm layer during gastrulation. The extracellular region of the Amn protein contains a cysteine-rich domain with similarity to bone morphogenetic protein (BMP)-binding cysteine-rich domains in chordin, its Drosophila melanogaster homolog (Short gastrulation) and procollagen IIA (ref. 3). Our findings indicate that Amn may direct the production of trunk mesoderm derived from the middle streak by acting in the underlying visceral endoderm to modulate a BMP signaling pathway.

The Polycomb group protein Eed protects the inactive X-chromosome from differentiation-induced reactivation

The Polycomb group (PcG) encodes an evolutionarily conserved set of chromatin-modifying proteins that are thought to maintain cellular transcriptional memory by stably silencing gene expression. In mouse embryos that are mutated for the PcG protein Eed, X-chromosome inactivation (XCI) is not stably maintained in extra-embryonic tissues. Eed is a component of a histone-methyltransferase complex that is thought to contribute to stable silencing in undifferentiated cells due to its enrichment on the inactive X-chromosome in cells of the early mouse embryo and in stem cells of the extra-embryonic trophectoderm lineage. Here, we demonstrate that the inactive X-chromosome in Eed−/− trophoblast stem cells and in cells of the trophectoderm-derived extra-embryonic ectoderm in Eed−/− embryos remain transcriptionally silent, despite lacking the PcG-mediated histone modifications that normally characterize the facultative heterochromatin of the inactive X-chromosome. Whereas undifferentiated Eed−/− trophoblast stem cells maintained XCI, reactivation of the inactive X-chromosome occurred when these cells were differentiated. These results indicate that PcG complexes are not necessary to maintain transcriptional silencing of the inactive X-chromosome in undifferentiated stem cells. Instead, PcG proteins seem to propagate cellular memory by preventing transcriptional activation of facultative heterochromatin during differentiation.

Evidence of Xist RNA-independent initiation of mouse imprinted X-chromosome inactivation

XX female mammals undergo transcriptional silencing of most genes on one of their two X chromosomes to equalize X-linked gene dosage with XY males in a process referred to as X-chromosome inactivation (XCI). XCI is an example of epigenetic regulation. Once enacted in individual cells of the early female embryo, XCI is stably transmitted such that most descendant cells maintain silencing of that X chromosome. In eutherian mammals, XCI is thought to be triggered by the expression of the non-coding Xist RNA from the future inactive X chromosome (Xi); Xist RNA in turn is proposed to recruit protein complexes that bring about heterochromatinization of the Xi. Here we test whether imprinted XCI, which results in preferential inactivation of the paternal X chromosome (Xp), occurs in mouse embryos inheriting an Xp lacking Xist. We find that silencing of Xp-linked genes can initiate in the absence of paternal Xist; Xist is, however, required to stabilize silencing along the Xp. Xp-linked gene silencing associated with mouse imprinted XCI, therefore, can initiate in the embryo independently of Xist RNA.

The Polycomb Group Protein EED Is Dispensable for the Initiation of Random X-Chromosome Inactivation

The Polycomb group (PcG) proteins are thought to silence gene expression by modifying chromatin. The Polycomb repressive complex 2 (PRC2) plays an essential role in mammalian X-chromosome inactivation (XCI), a model system to investigate heritable gene silencing. In the mouse, two different forms of XCI occur. In the preimplantation embryo, all cells undergo imprinted inactivation of the paternal X-chromosome (Xp). During the peri-implantation period, cells destined to give rise to the embryo proper erase the imprint and randomly inactivate either the maternal X-chromosome or the Xp; extraembryonic cells, on the other hand, maintain imprinted XCI of the Xp. PRC2 proteins are enriched on the inactive-X during early stages of both imprinted and random XCI. It is therefore thought that PRC2 contributes to the initiation of XCI. Mouse embryos lacking the essential PRC2 component EED harbor defects in the maintenance of imprinted XCI in differentiating trophoblast cells. Assessment of PRC2 requirement in the initiation of XCI, however, has been hindered by the presence of maternally derived proteins in the early embryo. Here we show that Eed −/− embryos initiate and maintain random XCI despite lacking any functional EED protein prior to the initiation of random XCI. Thus, despite being enriched on the inactive X-chromosome, PcGs appear to be dispensable for the initiation and maintenance of random XCI. These results highlight the lineage- and differentiation state–specific requirements for PcGs in XCI and argue against PcG function in the formation of the facultative heterochromatin of the inactive X-chromosome.

The central role of EED in the orchestration of polycomb group complexes

Polycomb Repressive Complexes 1 and 2 (PRC1 and 2) play a critical role in the epigenetic regulation of transcription during cellular differentiation, stem cell pluripotency, and neoplastic progression. Here we show that the Polycomb Group protein EED, a core component of PRC2, physically interacts with and functions as part of PRC1. Components of PRC1 and PRC2 compete for EED binding. EED functions to recruit PRC1 to H3K27me3 loci and enhances PRC1 mediated H2A ubiquitin E3 ligase activity. Taken together, we suggest an integral role for EED as an epigenetic exchange factor coordinating the activities of PRC1 and 2.

X chromosomes alternate between two states prior to random X-inactivation

Early in the development of female mammals, one of the two X chromosomes is silenced in half of cells and the other X chromosome is silenced in the remaining half. The basis of this apparent randomness is not understood. We show that before X-inactivation, the two X chromosomes appear to exist in distinct states that correspond to their fates as the active and inactive X chromosomes. Xist and Tsix, noncoding RNAs that control X chromosome fates upon X-inactivation, also determine the states of the X chromosomes prior to X-inactivation. In wild-type ES cells, X chromosomes switch between states; among the progeny of a single cell, a given X chromosome exhibits each state with equal frequency. We propose a model in which the concerted switching of homologous X chromosomes between mutually exclusive future active and future inactive states provides the basis for the apparently random silencing of one X chromosome in female cells.

mRNAs for activin receptors II and IIB are expressed in mouse oocytes and in the epiblast of pregastrula and gastrula stage mouse embryos

Activin is a potent inducer of mesoderm in frog embryos. We showed previously that in the mouse, activin beta A is expressed in the uterine decidua near the embryo before and during the first appearance of mesoderm (E4.5-E6.5). Here, using Northern blotting and in situ hybridization, we show that mouse oocytes, E6.5 and E7.5 embryos, and E6.5 and E7.5 decidua contain mRNAs for both activin receptors type II and IIB. The expression of activin receptor type IIB is particularly strong in embryonic ectoderm apparent at E5.5 and continuing through E8.5. These results support the hypothesis that activin derived from the decidua promotes development of mesoderm in the period E5.5-E6.5.

Transcription precedes loss of Xist coating and depletion of H3K27me3 during X-chromosome reprogramming in the mouse inner cell mass

Repression of Xist RNA expression is considered a prerequisite to reversal of X-chromosome inactivation (XCI) in the mouse inner cell mass (ICM), and reactivation of X-linked genes is thought to follow loss of Xist RNA coating and heterochromatic markers of inactivation, such as methylation of histone H3. We analyzed X-chromosome activity in developing ICMs and show that reactivation of gene expression from the inactive-X initiates in the presence of Xist coating and H3K27me3. Furthermore, depletion of Xist RNA coating through forced upregulation of NANOG does not result in altered reactivation kinetics. Taken together, our observations suggest that in the ICM, X-linked gene transcription and Xist coating are uncoupled. These data fundamentally alter our perception of the reactivation process and support the existence of a mechanism to reactivate Xp-linked genes in the ICM that operates independently of loss of Xist RNA and H3K27me3 from the imprinted inactive-X.