SUMMARY TET-family enzymes convert 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) in DNA. Here we show that Tet1 and Tet2 are Oct4-regulated enzymes that together sustain 5hmC in mouse embryonic stem (ES) cells, and are induced concomitantly with 5hmC during reprogramming of fibroblasts to induced pluripotent stem cells. ES cells depleted of Tet1 by RNAi show diminished expression of the Nodal antagonist Lefty1, and display hyperactive Nodal signalling and skewed differentiation into the endoderm-mesoderm lineage in embryoid bodies in vitro. In Fgf4- and heparin-supplemented culture conditions, Tet1-depleted ES cells activate the trophoblast stem cell lineage determinant Elf5 and can colonize the placenta in mid-gestation embryo chimeras. Consistent with these findings, Tet1-depleted ES cells form aggressive hemorrhagic teratomas with increased endoderm, reduced neuroectoderm and ectopic appearance of trophoblastic giant cells. Thus 5hmC is a novel epigenetic modification associated with the pluripotent state, and Tet1 functions to regulate the lineage differentiation potential of ES cells.

The X chromosome-linked transcription factor GATA-1 is expressed specifically in erythroid, mast, megakaryocyte, and eosinophil lineages, as well as in hematopoietic progenitors. Prior studies revealed that genedisrupted GATA-1-embryonic stem cells give rise to adult (or definitive) erythroid precursors arrested at the proerythroblast stage in vitro and fail to contribute to adult red blood cells in chimeric mice but did not clarify a role in embryonic (or yolk sac derived) erythroid cells. To examine the consequences of GATA-1 loss on embryonic erythropoiesis in vivo, we inactivated the GATA-1 locus in embryonic stem cells by gene targeting and transmitted the mutated allele through the mouse germ line. Male GATA-1-embryos die between embryonic day 10.5 and 11.5 (E10.5-E11.5) of gestation. At E9.5, GATA-1-embryos exhibit extreme pallor yet contain embryonic erythroid cells arrested at an early proerythroblast-like stage of their development. Embryos stain weakly with benzidine reagent, and yolk sac cells express globin RNAs, indicating globin gene activation in the absence of GATA-1.Female heterozygotes (GATA-1'1-) are born pale due to random inactivation of the X chromosome bearing the normal allele. However, these mice recover during the neonatal period, presumably as a result of in vivo selection for progenitors able to express GATA-1. Our findings conclusively establish the essential role for GATA-1 in erythropoiesis within the context of the intact developing mouse and further demonstrate that the block to cellular maturation is similar in GATA-1-embryonic and definitive erythroid precursors. Moreover, the recovery of GATA-1+'-mice from anemia seen at birth provides evidence indicating a role for GATA-1 at the hematopoietic progenitor cell level.

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Epigenetic aberrancies likely preclude correct and complete nuclear reprogramming following somatic cell nuclear transfer (SCNT), and may underlie the observed reduced viability of cloned embryos. In the present study, we tested the effects of the histone deacetylase inhibitor (HDACi), trichostatin A (TSA), on development and histone acetylation of cloned bovine preimplantation embryos. Our results indicated that treating activated reconstructed SCNT embryos with 50 nM TSA for 13 h produced eight-cell embryos with levels of acetylation of histone H4 at lysine 5 (AcH4K5) similar to fertilized counterparts and significantly greater than in control NT embryos (p < 0.005). Further, TSA treatment resulted in SCNT embryos with preimplantation developmental potential similar to fertilized counterparts, as no difference was observed in cleavage and blastocyst rates or in blastocyst total cell number (p > 0.05). Measurement of eight selected developmentally important genes in single blastocysts showed a similar expression profile among the three treatment groups, with the exception of Nanog, Cdx2, and DNMT3b, whose expression levels were higher in TSA-treated NT than in in vitro fertilized (IVF) embryos. Data presented herein demonstrate that TSA can improve at least one epigenetic mark in early cloned bovine embryos. However, evaluation of development to full-term is necessary to ascertain whether this effect reflects a true increase in developmental potential.