Tie-1 and Tie-2 define a new class of receptor tyrosine kinases that are specifically expressed in developing vascular endothelial cells. To study the functions of Tie-1 and Tie-2 during vascular endothelial cell growth and differentiation in vivo, targeted mutations of the genes in mice were introduced by homologous recombination. Embryos deficient in Tie-1 failed to establish structural integrity of vascular endothelial cells, resulting in oedema and subsequently localized haemorrhage. However, analyses of embryos deficient in Tie-2 showed that it is important in angiogenesis, particularly for vascular network formation in endothelial cells. This result contrasts with previous reports on Tie-2 function in vasculogenesis and/or endothelial cell survival. Our in vivo analyses indicate that the structurally related receptor tyrosine kinases Tie-1 and Tie-2 have important but distinct roles in the formation of blood vessels.

Trimethylation on H3K27 (H3K27me3) mediated by Polycomb repressive complex 2 (PRC2) has been linked to embryonic stem cell (ESC) identity and pluripotency. EZH2, the catalytic subunit of PRC2, has been reported as the sole histone methyltransferase that methylates H3K27 and mediates transcriptional silencing. Analysis of Ezh2(-/-) ESCs suggests existence of an additional enzyme(s) catalyzing H3K27 methylation. We have identified EZH1, a homolog of EZH2 that is physically present in a noncanonical PRC2 complex, as an H3K27 methyltransferase in vivo and in vitro. EZH1 colocalizes with the H3K27me3 mark on chromatin and preferentially preserves this mark on development-related genes in Ezh2(-/-) ESCs. Depletion of Ezh1 in cells lacking Ezh2 abolishes residual methylation on H3K27 and derepresses H3K27me3 target genes, demonstrating a role of EZH1 in safeguarding ESC identity. Ezh1 partially complements Ezh2 in executing pluripotency during ESC differentiation, suggesting that cell-fate transitions require epigenetic specificity.

SUMMARY c-Myc (Myc) is an important transcriptional regulator in embryonic stem (ES) cells, somatic cell reprogramming, and cancer. Here, we identify a Myc-centered regulatory network in ES cells by combining protein-protein and protein-DNA interaction studies, and show that Myc interacts with the NuA4 complex, a regulator of ES cell identity. In combination with regulatory network information, we define three ES cell modules (Core, Polycomb, and Myc), and show that the modules are functionally separable, illustrating that the overall ES cell transcription program is comprised of distinct units. With these modules as an analytical tool, we have reassessed the hypothesis linking an ES cell signature with cancer or cancer stem cells. We find that the Myc module, independent of the Core module, is active in various cancers and predicts cancer outcome. The apparent similarity of cancer and ES cell signatures reflects in large part the pervasive nature of Myc regulatory network.