Jörn Walter

Genomic imprinting : parental influence on the genome

Genomic imprinting affects several dozen mammalian genes and results in the expression of those genes from only one of the two parental chromosomes. This is brought about by epigenetic instructions — imprints — that are laid down in the parental germ cells. Imprinting is a particularly important genetic mechanism in mammals, and is thought to influence the transfer of nutrients to the fetus and the newborn from the mother. Consistent with this view is the fact that imprinted genes tend to affect growth in the womb and behaviour after birth. Aberrant imprinting disturbs development and is the cause of various disease syndromes. The study of imprinting also provides new insights into epigenetic gene modification during development.

Epigenetic reprogramming in mouse primordial germ cells

Genome-wide epigenetic reprogramming in mammalian germ cells, zygote and early embryos, plays a crucial role in regulating genome functions at critical stages of development. We show here that mouse primordial germ cells (PGCs) exhibit dynamic changes in epigenetic modifications between days 10.5 and 12.5 post coitum (dpc). First, contrary to previous suggestions, we show that PGCs do indeed acquire genome-wide de novo methylation during early development and migration into the genital ridge. However, following their entry into the genital ridge, there is rapid erasure of DNA methylation of regions within imprinted and non-imprinted loci. For most genes, the erasure commences simultaneously in PGCs in both male and female embryos, which is completed within 1 day of development. Based on the kinetics of this process, we suggest that this is an active demethylation process initiated upon the entry of PGCs into the gonadal anlagen. The timing of reprogramming in PGCs is crucial since it ensures that germ cells of both sexes acquire an equivalent epigenetic state prior to the differentiation of the definitive male and female germ cells in which new parental imprints are established subsequently. Some repetitive elements, however, show incomplete erasure, which may be essential for chromosome stability and for preventing activation of transposons to reduce the risk of germline mutations. Aberrant epigenetic reprogramming in the germ line would cause the inheritance of epimutations that may have consequences for human diseases as suggested by studies on mouse models.

Active demethylation of the paternal genome in the mouse zygote.

DNA methylation is essential for the control of a number of biological mechanisms in mammals [1]. Mammalian development is accompanied by two major waves of genome-wide demethylation and remethylation: one during germ-cell development and the other after fertilisation [2] [3] [4] [5] [6] [7]. Most previous studies have suggested that the genome-wide demethylation observed after fertilisation occurs passively, that is, by the lack of maintenance methylation following DNA replication and cell division [6] [7], although one other study has reported that replication-independent demethylation may also occur during early embryogenesis [8]. Here, we report that genes that are highly methylated in sperm are rapidly demethylated in the zygote only hours after fertilisation, before the first round of DNA replication commences. By contrast, the oocyte-derived maternal alleles are unaffected by this reprogramming. They either remain methylated after fertilisation or become further methylated de novo. These results provide the first direct evidence for active demethylation of single-copy genes in the mammalian zygote and, moreover, reveal a striking asymmetry in epigenetic methylation reprogramming. Whereas paternally (sperm)-derived sequences are exposed to putative active demethylases in the oocyte cytoplasm, maternally (oocyte)-derived sequences are protected from this reaction. These results, whose generality is supported by findings of Mayer et al. [9], have important implications for the establishment of biparental genetic totipotency after fertilisation, the establishment and maintenance of genomic imprinting, and the reprogramming of somatic cells during cloning.

Conservation of methylation reprogramming in mammalian development: Aberrant reprogramming in cloned embryos

Mouse embryos undergo genome-wide methylation reprogramming by demethylation in early preimplantation development, followed by remethylation thereafter. Here we show that genome-wide reprogramming is conserved in several mammalian species and ask whether it also occurs in embryos cloned with the use of highly methylated somatic donor nuclei. Normal bovine, rat, and pig zygotes showed a demethylated paternal genome, suggesting active demethylation. In bovine embryos methylation was further reduced during cleavage up to the eight-cell stage, and this reduction in methylation was followed by de novo methylation by the 16-cell stage. In cloned one-cell embryos there was a reduction in methylation consistent with active demethylation, but no further demethylation occurred subsequently. Instead, de novo methylation and nuclear reorganization of methylation patterns resembling those of differentiated cells occurred precociously in many cloned embryos. Cloned, but not normal, morulae had highly methylated nuclei in all blastomeres that resembled those of the fibroblast donor cells. Our study shows that epigenetic reprogramming occurs aberrantly in most cloned embryos; incomplete reprogramming may contribute to the low efficiency of cloning.

Embryogenesis: Demethylation of the zygotic paternal genome

In mammals, both parental genomes undergo dramatic epigenetic changes after fertilization to form the diploid somatic genome. Here we show that the paternal genome in the mouse is significantly and actively demethylated within 6–8 hours of fertilization, before the onset of DNA replication, whereas the maternal genome is demethylated after several cleavage divisions. This active demethylation of the paternal genome may be associated with epigenetic remodelling of sperm chroma-tin, in order to establish parent-specific developmental programmes during early embryogenesis.

5-Hydroxymethylcytosine in the mammalian zygote is linked with epigenetic reprogramming

The epigenomes of early mammalian embryos are extensively reprogrammed to acquire a totipotent developmental potential. A major initial event in this reprogramming is the active loss/demethylation of 5-methylcytosine (5mC) in the zygote. Here, we report on findings that link this active demethylation to molecular mechanisms. We detect 5-hydroxymethylcytosine (5hmC) as a novel modification in mouse, bovine and rabbit zygotes. On zygotic development 5hmC accumulates in the paternal pronucleus along with a reduction of 5mC. A knockdown of the 5hmC generating dioxygenase Tet3 simultaneously affects the patterns of 5hmC and 5mC in the paternal pronucleus. This finding links the loss of 5mC to its conversion into 5hmC. The maternal pronucleus seems to be largely protected against this mechanism by PGC7/Dppa3/Stella, as in PGC7 knockout zygotes 5mC also becomes accessible to oxidation into 5hmC. In summary, our data suggest an important role of 5hmC and Tet3 for DNA methylation reprogramming processes in the mammalian zygote.

Dynamic Readers for 5-(Hydroxy)Methylcytosine and Its Oxidized Derivatives

Tet proteins oxidize 5-methylcytosine (mC) to generate 5-hydroxymethyl (hmC), 5-formyl (fC), and 5-carboxylcytosine (caC). The exact function of these oxidative cytosine bases remains elusive. We applied quantitative mass-spectrometry-based proteomics to identify readers for mC and hmC in mouse embryonic stem cells (mESC), neuronal progenitor cells (NPC), and adult mouse brain tissue. Readers for these modifications are only partially overlapping, and some readers, such as Rfx proteins, display strong specificity. Interactions are dynamic during differentiation, as for example evidenced by the mESC-specific binding of Klf4 to mC and the NPC-specific binding of Uhrf2 to hmC, suggesting specific biological roles for mC and hmC. Oxidized derivatives of mC recruit distinct transcription regulators as well as a large number of DNA repair proteins in mouse ES cells, implicating the DNA damage response as a major player in active DNA demethylation.

Promiscuous gene expression in thymic epithelial cells is regulated at multiple levels

The role of central tolerance induction has recently been revised after the discovery of promiscuous expression of tissue-restricted self-antigens in the thymus. The extent of tissue representation afforded by this mechanism and its cellular and molecular regulation are barely defined. Here we show that medullary thymic epithelial cells (mTECs) are specialized to express a highly diverse set of genes representing essentially all tissues of the body. Most, but not all, of these genes are induced in functionally mature CD80hi mTECs. Although the autoimmune regulator (Aire) is responsible for inducing a large portion of this gene pool, numerous tissue-restricted genes are also up-regulated in mature mTECs in the absence of Aire. Promiscuously expressed genes tend to colocalize in clusters in the genome. Analysis of a particular gene locus revealed expression of clustered genes to be contiguous within such a cluster and to encompass both Aire-dependent and –independent genes. A role for epigenetic regulation is furthermore implied by the selective loss of imprinting of the insulin-like growth factor 2 gene in mTECs. Our data document a remarkable cellular and molecular specialization of the thymic stroma in order to mimic the transcriptome of multiple peripheral tissues and, thus, maximize the scope of central self-tolerance.

FGF Signaling Inhibition in ESCs Drives Rapid Genome-wide Demethylation to the Epigenetic Ground State of Pluripotency

Summary Genome-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.

Whole-Genome Bisulfite Sequencing of Two Distinct Interconvertible DNA Methylomes of Mouse Embryonic Stem Cells

The use of two kinase inhibitors (2i) enables derivation of mouse embryonic stem cells (ESCs) in the pluripotent ground state. Using whole-genome bisulfite sequencing (WGBS), we show that male 2i ESCs are globally hypomethylated compared to conventional ESCs maintained in serum. In serum, female ESCs are hypomethyated similarly to male ESCs in 2i, and DNA methylation is further reduced in 2i. Regions with elevated DNA methylation in 2i strongly correlate with the presence of H3K9me3 on endogenous retroviruses (ERVs) and imprinted loci. The methylome of male ESCs in serum parallels postimplantation blastocyst cells, while 2i stalls ESCs in a hypomethylated, ICM-like state. WGBS analysis during adaptation of 2i ESCs to serum suggests that deposition of DNA methylation is largely random, while loss of DNA methylation during reversion to 2i occurs passively, initiating at TET1 binding sites. Together, our analysis provides insight into DNA methylation dynamics in cultured ESCs paralleling early developmental processes.