Methylation at the 5' position of cytosine in DNA has important roles in genome function and is dynamically reprogrammed during early embryonic and germ cell development. The mammalian genome also contains 5-hydroxymethylcytosine (5hmC), which seems to be generated by oxidation of 5-methylcytosine (5mC) by the TET family of enzymes that are highly expressed in embryonic stem (ES) cells. Here we use antibodies against 5hmC and 5mC together with high throughput sequencing to determine genome-wide patterns of methylation and hydroxymethylation in mouse wild-type and mutant ES cells and differentiating embryoid bodies. We find that 5hmC is mostly associated with euchromatin and that whereas 5mC is under-represented at gene promoters and CpG islands, 5hmC is enriched and is associated with increased transcriptional levels. Most, if not all, 5hmC in the genome depends on pre-existing 5mC and the balance between these two modifications is different between genomic regions. Knockdown of Tet1 and Tet2 causes downregulation of a group of genes that includes pluripotency-related genes (including Esrrb, Prdm14, Dppa3, Klf2, Tcl1 and Zfp42) and a concomitant increase in methylation of their promoters, together with an increased propensity of ES cells for extraembryonic lineage differentiation. Declining levels of TETs during differentiation are associated with decreased hydroxymethylation levels at the promoters of ES cell-specific genes together with increased methylation and gene silencing. We propose that the balance between hydroxymethylation and methylation in the genome is inextricably linked with the balance between pluripotency and lineage commitment.

We report a high-quality draft of the genome sequence of the grey, short-tailed opossum (Monodelphis domestica). As the first metatherian ('marsupial') species to be sequenced, the opossum provides a unique perspective on the organization and evolution of mammalian genomes. Distinctive features of the opossum chromosomes provide support for recent theories about genome evolution and function, including a strong influence of biased gene conversion on nucleotide sequence composition, and a relationship between chromosomal characteristics and X chromosome inactivation. Comparison of opossum and eutherian genomes also reveals a sharp difference in evolutionary innovation between protein-coding and non-coding functional elements. True innovation in protein-coding genes seems to be relatively rare, with lineage-specific differences being largely due to diversification and rapid turnover in gene families involved in environmental interactions. In contrast, about 20% of eutherian conserved non-coding elements (CNEs) are recent inventions that postdate the divergence of Eutheria and Metatheria. A substantial proportion of these eutherian-specific CNEs arose from sequence inserted by transposable elements, pointing to transposons as a major creative force in the evolution of mammalian gene regulation.

SummaryGenome-wide erasure of DNA methylation takes place in primordial germ cells (PGCs) and early embryos and is linked with pluripotency. Inhibition of Erk1/2 and Gsk3β signaling in mouse embryonic stem cells (ESCs) by small-molecule inhibitors (called 2i) has recently been shown to induce hypomethylation. We show by whole-genome bisulphite sequencing that 2i induces rapid and genome-wide demethylation on a scale and pattern similar to that in migratory PGCs and early embryos. Major satellites, intracisternal A particles (IAPs), and imprinted genes remain relatively resistant to erasure. Demethylation involves oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), impaired maintenance of 5mC and 5hmC, and repression of the de novo methyltransferases (Dnmt3a and Dnmt3b) and Dnmt3L. We identify a Prdm14- and Nanog-binding cis-acting regulatory region in Dnmt3b that is highly responsive to signaling. These insights provide a framework for understanding how signaling pathways regulate reprogramming to an epigenetic ground state of pluripotency.

Adverse prenatal environments can promote metabolic disease in offspring and subsequent generations. Animal models and epidemiological data implicate epigenetic inheritance but mechanisms remain unknown. In an intergenerational developmental programming model affecting F2 metabolism, we demonstrate that the in utero nutritional environment of F1 embryos alters the germline DNA methylome of F1 adult males in a locus-specific manner. Differentially methylated regions are hypomethylated and enriched in nucleosome-retaining regions. A substantial fraction is resistant to early embryo methylation reprogramming, potentially impacting F2 development. Importantly, differential methylation is not maintained in F2 tissues, yet locus-specific expression is perturbed. Thus, in utero nutritional exposures during critical windows of germ cell development can impact the male germline methylome, associated with metabolic disease in offspring.