Biomedical subjects

Stuart S Levine

Publications and source records attributed to Stuart S Levine.

7 recordsLinked to original sources

PubMed2006

Control of developmental regulators by Polycomb in human embryonic stem cells.

Polycomb group proteins are essential for early development in metazoans, but their contributions to human development are not well understood. We have mapped the Polycomb Repressive Complex 2 (PRC2) subunit SUZ12 across the entire nonrepeat portion of the genome in human embryonic stem (ES) cells. We found that SUZ12 is distributed across large portions of over two hundred genes encoding key developmental regulators. These genes are occupied by nucleosomes trimethylated at histone H3K27, are transcriptionally repressed, and contain some of the most highly conserved noncoding elements in the genome. We found that PRC2 target genes are preferentially activated during ES cell differentiation and that the ES cell regulators OCT4, SOX2, and NANOG cooccupy a significant subset of these genes. These results indicate that PRC2 occupies a special set of developmental genes in ES cells that must be repressed to maintain pluripotency and that are poised for activation during ES cell differentiation.

Animals↗

PubMed2006

Polycomb complexes repress developmental regulators in murine embryonic stem cells.

The mechanisms by which embryonic stem (ES) cells self-renew while maintaining the ability to differentiate into virtually all adult cell types are not well understood. Polycomb group (PcG) proteins are transcriptional repressors that help to maintain cellular identity during metazoan development by epigenetic modification of chromatin structure. PcG proteins have essential roles in early embryonic development and have been implicated in ES cell pluripotency, but few of their target genes are known in mammals. Here we show that PcG proteins directly repress a large cohort of developmental regulators in murine ES cells, the expression of which would otherwise promote differentiation. Using genome-wide location analysis in murine ES cells, we found that the Polycomb repressive complexes PRC1 and PRC2 co-occupied 512 genes, many of which encode transcription factors with important roles in development. All of the co-occupied genes contained modified nucleosomes (trimethylated Lys 27 on histone H3). Consistent with a causal role in gene silencing in ES cells, PcG target genes were de-repressed in cells deficient for the PRC2 component Eed, and were preferentially activated on induction of differentiation. Our results indicate that dynamic repression of developmental pathways by Polycomb complexes may be required for maintaining ES cell pluripotency and plasticity during embryonic development.

Animals↗

PubMed2005

The core centromere and Sgo1 establish a 50-kb cohesin-protected domain around centromeres during meiosis I.

The stepwise loss of cohesins, the complexes that hold sister chromatids together, is required for faithful meiotic chromosome segregation. Cohesins are removed from chromosome arms during meiosis I but are maintained around centromeres until meiosis II. Here we show that Sgo1, a protein required for protecting centromeric cohesins from removal during meiosis I, localizes to cohesin-associated regions (CARs) at the centromere and the 50-kb region surrounding it. Establishment of this Sgo1-binding domain requires the 120-base-pair (bp) core centromere, the kinetochore component Bub1, and the meiosis-specific factor Spo13. Interestingly, cohesins and the kinetochore proteins Iml3 and Chl4 are necessary for Sgo1 to associate with pericentric regions but less so for Sgo1 to associate with the core centromeric regions. Finally, we show that the 50-kb Sgo1-binding domain is the chromosomal region where cohesins are protected from removal during meiosis I. Our results identify the portions of chromosomes where cohesins are protected from removal during meiosis I and show that kinetochore components and cohesins themselves are required to establish this cohesin protective domain.

Cell Cycle Proteins↗

PubMed2005

Core transcriptional regulatory circuitry in human embryonic stem cells.

The transcription factors OCT4, SOX2, and NANOG have essential roles in early development and are required for the propagation of undifferentiated embryonic stem (ES) cells in culture. To gain insights into transcriptional regulation of human ES cells, we have identified OCT4, SOX2, and NANOG target genes using genome-scale location analysis. We found, surprisingly, that OCT4, SOX2, and NANOG co-occupy a substantial portion of their target genes. These target genes frequently encode transcription factors, many of which are developmentally important homeodomain proteins. Our data also indicate that OCT4, SOX2, and NANOG collaborate to form regulatory circuitry consisting of autoregulatory and feedforward loops. These results provide new insights into the transcriptional regulation of stem cells and reveal how OCT4, SOX2, and NANOG contribute to pluripotency and self-renewal.

Animals↗

PubMed2004

Menin associates with a trithorax family histone methyltransferase complex and with the hoxc8 locus.

The cellular function of the menin tumor suppressor protein, product of the MEN1 gene mutated in familial multiple endocrine neoplasia type 1, has not been defined. We now show that menin is associated with a histone methyltransferase complex containing two trithorax family proteins, MLL2 and Ash2L, and other homologs of the yeast Set1 assembly. This menin-associated complex methylates histone H3 on lysine 4. A subset of tumor-derived menin mutants lacks the associated histone methyltransferase activity. In addition, menin is associated with RNA polymerase II whose large subunit carboxyl-terminal domain is phosphorylated on Ser 5. Men1 knockout embryos and cells show decreased expression of the homeobox genes Hoxc6 and Hoxc8. Chromatin immunoprecipitation experiments reveal that menin is bound to the Hoxc8 locus. These results suggest that menin activates the transcription of differentiation-regulating genes by covalent histone modification, and that this activity is related to tumor suppression by MEN1.

Animals↗

PubMed2004

Division of labor in polycomb group repression.

The epigenetic maintenance of gene expression patterns is essential for developing and maintaining the diverse types of cell that cooperate to form the larger organism. Recent data suggest that proteins of the Polycomb group (PcG) use a combination of posttranslational modifications and structural changes to the underlying chromatin structure to maintain silenced epigenetic states. We are now beginning to understand the mechanisms by which the PcG proteins are able to silence genes and to maintain this silencing over many cell divisions.

Animals↗

PubMed2002

The core of the polycomb repressive complex is compositionally and functionally conserved in flies and humans.

The Polycomb group (PcG) genes are required to maintain homeotic genes in a silenced state during development in drosophila and mammals and are thought to form several distinct silencing complexes that maintain homeotic gene repression during development. Mutations in the PcG genes result in developmental defects and have been implicated in human cancer. Although some PcG protein domains are conserved between flies and humans, substantial regions of several PcG proteins are divergent and humans contain multiple versions of each PcG gene. To determine the effects of these changes on the composition and function of a PcG complex, we have purified a human Polycomb repressive complex from HeLa cells (hPRC-H) that contains homologues of PcG proteins found in drosophila embryonic PRC1 (dPRC1). hPRC-H was found to have fewer components than dPRC1, retaining the PcG core proteins of dPRC1 but lacking most non-PcG proteins. Preparations of hPRC-H contained either two or three different homologues of most of the core PcG proteins, including a new Ph homologue we have named HPH3. Despite differences in composition, dPRC1 and hPRC-H have similar functions: hPRC-H is able to efficiently block remodeling of nucleosomal arrays through a mechanism that does not block the ability of nucleases to access and cleave the arrays.

Amino Acid Sequence↗