Christine S. Schmidt

Sensitive enzymatic quantification of 5-hydroxymethylcytosine in genomic DNA

The recent discovery of genomic 5-hydroxymethylcytosine (hmC) and mutations affecting the respective Tet hydroxylases in leukemia raises fundamental questions about this epigenetic modification. We present a sensitive method for fast quantification of genomic hmC based on specific transfer of radiolabeled glucose to hmC by a purified glucosyltransferase. We determined hmC levels in various adult tissues and differentiating embryonic stem cells and show a correlation with differential expression of tet genes.

LBR and Lamin A/C Sequentially Tether Peripheral Heterochromatin and Inversely Regulate Differentiation

Eukaryotic cells have a layer of heterochromatin at the nuclear periphery. To investigate mechanisms regulating chromatin distribution, we analyzed heterochromatin organization in different tissues and species, including mice with mutations in the lamin B receptor (Lbr) and lamin A (Lmna) genes that encode nuclear envelope (NE) proteins. We identified LBR- and lamin-A/C-dependent mechanisms tethering heterochromatin to the NE. The two tethers are sequentially used during cellular differentiation and development: first the LBR- and then the lamin-A/C-dependent tether. The absence of both LBR and lamin A/C leads to loss of peripheral heterochromatin and an inverted architecture with heterochromatin localizing to the nuclear interior. Myoblast transcriptome analyses indicated that selective disruption of the LBR- or lamin-A-dependent heterochromatin tethers have opposite effects on muscle gene expression, either increasing or decreasing, respectively. These results show how changes in NE composition contribute to regulating heterochromatin positioning, gene expression, and cellular differentiation during development.

Targeted transcriptional activation of silent oct4 pluripotency gene by combining designer TALEs and inhibition of epigenetic modifiers

Specific control of gene activity is a valuable tool to study and engineer cellular functions. Recent studies uncovered the potential of transcription activator-like effector (TALE) proteins that can be tailored to activate user-defined target genes. It remains however unclear whether and how epigenetic modifications interfere with TALE-mediated transcriptional activation. We studied the activity of five designer TALEs (dTALEs) targeting the oct4 pluripotency gene. In vitro assays showed that the five dTALEs that target distinct sites in the oct4 promoter had the expected DNA specificity and comparable affinities to their corresponding DNA targets. In contrast to their similar in vitro properties, transcriptional activation of oct4 by these distinct dTALEs varied up to 25-fold. While dTALEs efficiently upregulated transcription of the active oct4 promoter in embryonic stem cells (ESCs) they failed to activate the silenced oct4 promoter in ESC-derived neural stem cells (NSCs), indicating that as for endogenous transcription factors also dTALE activity is limited by repressive epigenetic mechanisms. We therefore targeted the activity of epigenetic modulators and found that chemical inhibition of histone deacetylases by valproic acid or DNA methyltransferases by 5-aza-2′-deoxycytidine facilitated dTALE-mediated activation of the epigenetically silenced oct4 promoter in NSCs. Notably, demethylation of the oct4 promoter occurred only if chemical inhibitors and dTALEs were applied together but not upon treatment with inhibitors or dTALEs only. These results show that dTALEs in combination with chemical manipulation of epigenetic modifiers facilitate targeted transcriptional activation of epigenetically silenced target genes.

Cooperative DNA and histone binding by Uhrf2 links the two major repressive epigenetic pathways.

Gene expression is regulated by DNA as well as histone modifications but the crosstalk and mechanistic link between these epigenetic signals are still poorly understood. Here we investigate the multi‐domain protein Uhrf2 that is similar to Uhrf1, an essential cofactor of maintenance DNA methylation. Binding assays demonstrate a cooperative interplay of Uhrf2 domains that induces preference for hemimethylated DNA, the substrate of maintenance methylation, and enhances binding to H3K9me3 heterochromatin marks. FRAP analyses revealed that localization and binding dynamics of Uhrf2 in vivo require an intact tandem Tudor domain and depend on H3K9 trimethylation but not on DNA methylation. Besides the cooperative DNA and histone binding that is characteristic for Uhrf2, we also found an opposite expression pattern of uhrf1 and uhrf2 during differentiation. While uhrf1 is mainly expressed in pluripotent stem cells, uhrf2 is upregulated during differentiation and highly expressed in differentiated mouse tissues. Ectopic expression of Uhrf2 in uhrf1−/− embryonic stem cells did not restore DNA methylation at major satellites indicating functional differences. We propose that the cooperative interplay of Uhrf2 domains may contribute to a tighter epigenetic control of gene expression in differentiated cells. J. Cell. Biochem. 112: 2585–2593, 2011.

Characterization of PvuRts1I endonuclease as a tool to investigate genomic 5–hydroxymethylcytosine

In mammalian genomes a sixth base, 5-hydroxymethylcytosine (hmC), is generated by enzymatic oxidation of 5-methylcytosine (mC). This discovery has raised fundamental questions about the functional relevance of hmC in mammalian genomes. Due to their very similar chemical structure, discrimination of the rare hmC against the far more abundant mC is technically challenging and to date no methods for direct sequencing of hmC have been reported. Here, we report on a purified recombinant endonuclease, PvuRts1I, which selectively cleaves hmC-containing sequences. We determined the consensus cleavage site of PvuRts1I as hmCN11–12/N9–10G and show first data on its potential to interrogate hmC patterns in mammalian genomes.

Selective transduction of recombinant plasmids with cloned pac sites by Salmonella phage P22

SummaryRecombinant plasmids having PstI fragments of P22 DNA inserted in the vector pBR322 can be transduced efficiently by Salmonella phage P22, irrespective of the cloned phage sequences. When the rec function of the donor cells and the corresponding recombination system erf of the infecting phage are simultaneously inactivated, only plasmids containing the P22 pac site can be transduced. By this selective, generalized transduction an EcoRV DNA fragment of the P22 related phage L has been identified that carries a base sequence recognized by phage P22 as a packaging signal. Experiments in which only one of the two recombination systems was inactivated, showed that the bacterial rec system obviously promotes cointegrate formation between plasmid and phage DNA much more efficiently than the phage-coded erf system, allowing the specialized plasmid transduction observed by Orbach and Jackson (1982).

Intrinsic and Extrinsic Connections of Tet3 Dioxygenase with CXXC Zinc Finger Modules.

Tet proteins are emerging as major epigenetic modulators of cell fate and plasticity. However, little is known about how Tet proteins are targeted to selected genomic loci in distinct biological contexts. Previously, a CXXC-type zinc finger domain in Tet1 was shown to bind CpG-rich DNA sequences. Interestingly, in human and mouse the Tet2 and Tet3 genes are adjacent to Cxxc4 and Cxxc10-1, respectively. The CXXC domains encoded by these loci, together with those in Tet1 and Cxxc5, identify a distinct homology group within the CXXC domain family. Here we provide evidence for alternative mouse Tet3 transcripts including the Cxxc10-1 sequence (Tet3CXXC) and for an interaction between Tet3 and Cxxc4. In vitro Cxxc4 and the isolated CXXC domains of Tet1 and Tet3CXXC bind DNA substrates with similar preference towards the modification state of cytosine at a single CpG site. In vivo Tet1 and Tet3 isoforms with and without CXXC domain hydroxylate genomic 5-methylcytosine with similar activity. Relative transcript levels suggest that distinct ratios of Tet3CXXC isoforms and Tet3-Cxxc4 complex may be present in adult tissues. Our data suggest that variable association with CXXC modules may contribute to context specific functions of Tet proteins.

Global DNA hypomethylation prevents consolidation of differentiation programs and allows reversion to the embryonic stem cell state.

DNA methylation patterns change dynamically during mammalian development and lineage specification, yet scarce information is available about how DNA methylation affects gene expression profiles upon differentiation. Here we determine genome-wide transcription profiles during undirected differentiation of severely hypomethylated (Dnmt1−/−) embryonic stem cells (ESCs) as well as ESCs completely devoid of DNA methylation (Dnmt1−/−;Dnmt3a−/−;Dnmt3b−/− or TKO) and assay their potential to transit in and out of the ESC state. We find that the expression of only few genes mainly associated with germ line function and the X chromosome is affected in undifferentiated TKO ESCs. Upon initial differentiation as embryoid bodies (EBs) wild type, Dnmt1−/− and TKO cells downregulate pluripotency associated genes and upregulate lineage specific genes, but their transcription profiles progressively diverge upon prolonged EB culture. While Oct4 protein levels are completely and homogeneously suppressed, transcription of Oct4 and Nanog is not completely silenced even at late stages in both Dnmt1−/− and TKO EBs. Despite late wild type and Dnmt1−/− EBs showing a much higher degree of concordant expression, after EB dissociation and replating under pluripotency promoting conditions both Dnmt1−/− and TKO cells, but not wild type cells rapidly revert to expression profiles typical of undifferentiated ESCs. Thus, while DNA methylation seems not to be critical for initial activation of differentiation programs, it is crucial for permanent restriction of developmental fate during differentiation.

The mechanical stability of nano-hybrid composites with new methacrylate monomers for matrix compositions

OBJECTIVES Dimer acid based metacrylates and TCD-urethane are promoted as new monomers of nano-hybrid resin based composites as alternatives for the conventional BisGMA. Investigations of this study focused on the mechanical and the storage behavior of nano-hybrid resin based composites (RBCs) composed of these new types of monomers in comparison to RBCs using BisGMA. METHODS Flexural strength and modulus were determined in a three-point-bending test. Additionally, the modulus of elasticity was measured on microscopic scale (E(micro)) using an automatic microhardness indenter. Tests were performed on samples after 24 h storage in distilled water, as well as after thermocycling and storing the materials for four weeks in either distilled water, artificial saliva or ethanol. RESULTS The six measured materials showed a pronounced decrease of flexural strength, flexural modulus and E(micro) after four weeks storage in alcohol. Results after four weeks storage in water and saliva could not be proven to be significantly different. The most sensitive factor of influence on all test parameters was the material. SIGNIFICANCE Nano-hybrid composites with new or conventional monomers performed similar in regard to the mechanical properties and the behavior of the materials after aging.

Sequence Comparison among DNA Fragments from Different Sources with pac Site Function for the Packaging Apparatus of Salmonella Phage P22

Two different DNA fragments, deriving from the P22-related bacteriophage LP7 and from the right IS element of transposon Tn10, have been identified which were recognized by the P22-packaging apparatus as pac-like signals. Their nucleotide sequences were compared with the known sequence of P22 gene 3 which contains the P22 pac site. Both sequences show similarity to a particular segment of P22 DNA close to a region identified earlier as part of pac.