Gene Regulation and Epigenetic Remodeling in Murine Embryonic Stem Cells by c-Myc

BACKGROUND:The Myc oncoprotein, a transcriptional regulator involved in the etiology of many different tumor types, has been demonstrated to play an important role in the functions of embryonic stem (ES) cells. Nonetheless, it is still unclear as to whether Myc has unique target and functions in ES cells. METHODOLOGY/PRINCIPAL FINDINGS:To elucidate the role of c-Myc in murine ES cells, we mapped its genomic binding sites by chromatin-immunoprecipitation combined with DNA microarrays (ChIP-chip). In addition to previously identified targets we identified genes involved in pluripotency, early development, and chromatin modification/structure that are bound and regulated by c-Myc in murine ES cells. Myc also binds and regulates loci previously identified as Polycomb (PcG) targets, including genes that contain bivalent chromatin domains. To determine whether c-Myc influences the epigenetic state of Myc-bound genes, we assessed the patterns of trimethylation of histone H3-K4 and H3-K27 in mES cells containing normal, increased, and reduced levels of c-Myc. Our analysis reveals widespread and surprisingly diverse changes in repressive and activating histone methylation marks both proximal and distal to Myc binding sites. Furthermore, analysis of bulk chromatin from phenotypically normal c-myc null E7 embryos demonstrates a 70-80% decrease in H3-K4me3, with little change in H3-K27me3, compared to wild-type embryos indicating that Myc is required to maintain normal levels of histone methylation. CONCLUSIONS/SIGNIFICANCE:We show that Myc induces widespread and diverse changes in histone methylation in ES cells. We postulate that these changes are indirect effects of Myc mediated by its regulation of target genes involved in chromatin remodeling. We further show that a subset of PcG-bound genes with bivalent histone methylation patterns are bound and regulated in response to altered c-Myc levels. Our data indicate that in mES cells c-Myc binds, regulates, and influences the histone modification patterns of genes involved in chromatin remodeling, pluripotency, and differentiation

Notch Ankyrin Repeat Domain Variation Influences Leukemogenesis and Myc Transactivation

, cell-based and structural analyses to compare the abilities of activated Notch1-4 to support T cell development, induce T cell acute lymphoblastic leukemia/lymphoma (T-ALL), and maintain T-ALL cell growth and survival., a direct Notch target that has an important role in Notch-associated T-ALL.We conclude that the leukemogenic potentials of Notch receptors vary, and that this functional difference stems in part from divergence among the highly conserved ankyrin repeats, which influence the transactivation of specific target genes involved in leukemogenesis

Max-independent functions of Myc in Drosophila melanogaster

Myc proteins are powerful proto-oncoproteins and important promoters of growth and proliferation during normal development. They are thought to exercise their effects upon binding to their partner protein Max, and their activities are largely antagonized by complexes of Max with Mnt or an Mxd family protein. Although the biological functions of Myc, Mxd and Mnt have been intensively studied, comparatively little is known about the in vivo role of Max. Here we generate Max loss-of-function and reduction-of-function mutations in Drosophila melanogaster to address the contribution of Max to Myc-dependent growth control. We find that many biological activities of Myc do not, or only partly, require the association with Max—for example, the control of endoreplication and cell competition—and that a Myc mutant that does not interact with Max retains substantial biological activity. We further show that Myc can control RNA polymerase III independently of Max, which explains some of Myc's observed biological activities. These studies show the ability of Myc to function independently of Max in vivo and thus change the current model of Max network function

A tumor suppressor activity of Drosophila Polycomb genes mediated by JAK-STAT signaling

A prevailing paradigm posits that Polycomb Group (PcG) proteins maintain stem cell identity by repressing differentiation genes, and abundant evidence points to an oncogenic role for PcG proteins in human cancer. Here we show using Drosophila melanogaster that a conventional PcG complex can also have a potent tumor suppressor activity. Mutations in any core PRC1 component cause pronounced hyperproliferation of eye imaginal tissue, accompanied by deregulation of epithelial architecture. The mitogenic JAK-STAT pathway is strongly and specifically activated in mutant tissue; activation is driven by transcriptional upregulation of Unpaired (Upd, also known as Outstretched, Os) family ligands. We show here that upd genes are direct targets of PcG-mediated repression in imaginal discs. Ectopic JAK-STAT activity is sufficient to induce overproliferation, whereas reduction of JAK-STAT activity suppresses the PRC1 mutant tumor phenotype. These findings show that PcG proteins can restrict growth directly by silencing mitogenic signaling pathways, shedding light on an epigenetic mechanism underlying tumor suppression