Martin Münzel

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.

Tissue Distribution of 5-Hydroxymethylcytosine and Search for Active Demethylation Intermediates

5–Hydroxymethylcytosine (hmC) was recently detected as the sixth base in mammalian tissue at so far controversial levels. The function of the modified base is currently unknown, but it is certain that the base is generated from 5-methylcytosine (mC). This fuels the hypothesis that it represents an intermediate of an active demethylation process, which could involve further oxidation of the hydroxymethyl group to a formyl or carboxyl group followed by either deformylation or decarboxylation. Here, we use an ultra-sensitive and accurate isotope based LC-MS method to precisely determine the levels of hmC in various mouse tissues and we searched for 5–formylcytosine (fC), 5-carboxylcytosine (caC), and 5–hydroxymethyluracil (hmU) as putative active demethylation intermediates. Our data suggest that an active oxidative mC demethylation pathway is unlikely to occur. Additionally, we show using HPLC-MS analysis and immunohistochemistry that hmC is present in all tissues and cell types with highest concentrations in neuronal cells of the CNS.

The Discovery of 5‐Formylcytosine in Embryonic Stem Cell DNA

Cellular development requires the silencing and activation of specific gene sequences in a well-orchestrated fashion. Transcriptional gene silencing is associated with the clustered methylation of cytosine bases (C) in CpG units of promoters. The methylation occurs at position C5 of cytosine to give 5methylcytosine (mC) with the help of special DNA methyltransferases (DNMT). [1] The DNA methylome is significantly reprogrammed at various stages during early development, [2] during the development of primordial germ cells, [2c, 3] or later in a locus-specific way at postdevelopmental stages. [4] Decreasing levels of mC can be established passively by successive rounds of DNA replication in the absence of methyltransferases. Active demethylation, in contrast, is proposed to be a process in which the mC bases are directly converted back into unmodified cytosines in the genome. [5] The recent discovery that mC can be further oxidized to hydroxymethylcytosine (hmC) with the help of TET enzymes [6] has led to the idea that hmC is connected to epigenetic reprogramming, [7] maybe as an intermediate in an, as yet controversial, active demethylation process. [4, 5, 8] Indeed recent data suggest that active demethylation in postdevelopmental phases may proceed through deamination of hmC to give 5-hydroxymethyluridine (hmU), which is then removed from the genome with the help of the base excision repair (BER) system. [9] Chemically, an attractive alternative mechanism for a more global active demethylation could be envisioned through further oxidation of hmC to give either 5formylcytosine (fC) or 5-carboxylcytosine (caC) followed by elimination of a formyl or carboxyl group, respectively

5-Hydroxymethylcytosine, the sixth base of the genome.

5-Hydroxymethylcytosine (hmC) was recently discovered as a new constituent of mammalian DNA. Besides 5-methylcytosine (mC), it is the only other modified base in higher organisms. The discovery is of enormous importance because it shows that the methylation of cytosines to imprint epigenetic information is not a final chemical step that leads to gene silencing but that further chemistry occurs at the methyl group that might have regulatory function. Recent progress in hmC detection--most notably LC-MS and glucosyltransferase assays--helped to decipher the precise distribution of hmC in the body. This led to the surprising finding that, in contrast to constant mC levels, the hmC levels are strongly tissue-specific. The highest values of hmC are found in the central nervous system. It was furthermore discovered that hmC is involved in regulating the pluripotency of stem cells and that it is connected to the processes of cellular development and carcinogenesis. Evidence is currently accumulating that hmC may not exclusively be an intermediate of an active demethylation process, but that it functions instead as an important epigenetic marker.

Low values of 5‐hydroxymethylcytosine (5hmC), the “sixth base,” are associated with anaplasia in human brain tumors

5‐Methylcytosine (5mC) in genomic DNA has important epigenetic functions in embryonic development and tumor biology. 5‐Hydroxymethylcytosine (5hmC) is generated from 5mC by the action of the TET (Ten‐Eleven‐Translocation) enzymes and may be an intermediate to further oxidation and finally demethylation of 5mC. We have used immunohistochemistry (IHC) and isotope‐based liquid chromatography mass spectrometry (LC‐MS) to investigate the presence and distribution of 5hmC in human brain and brain tumors. In the normal adult brain, IHC identified 61.5% 5hmC positive cells in the cortex and 32.4% 5hmC in white matter (WM) areas. In tumors, positive staining of cells ranged from 1.1% in glioblastomas (GBMs) (WHO Grade IV) to 8.9% in Grade I gliomas (pilocytic astrocytomas). In the normal adult human brain, LC‐MS also showed highest values in cortical areas (1.17% 5hmC/dG [deoxyguanosine]), in the cerebral WM we measured around 0.70% 5hmC/dG. 5hmC levels were related to tumor differentiation, ranging from lowest values of 0.078% 5hmC/dG in GBMs (WHO Grade IV) to 0.24% 5hmC/dG in WHO Grade II diffuse astrocytomas. 5hmC measurements were unrelated to 5mC values. We find that the number of 5hmC positive cells and the amount of 5hmC/dG in the genome that has been proposed to be related to pluripotency and lineage commitment in embryonic stem cells is also associated with brain tumor differentiation and anaplasia.

Improved Synthesis and Mutagenicity of Oligonucleotides Containing 5-Hydroxymethylcytosine, 5-Formylcytosine and 5-Carboxylcytosine

5-Formylcytosine (fC or (5-CHO)dC) and 5-carboxylcytosine (caC or (5-COOH)dC) have recently been identified as constituents of mammalian DNA. The nucleosides are formed from 5-methylcytosine (mC or (5-Me)dC) via 5-hydroxymethylcytosine (hmC or (5-HOMe)dC) and are possible intermediates of an active DNA demethylation process. Here we show efficient syntheses of phosphoramidites which enable the synthesis of DNA strands containing these cytosine modifications based on Pd(0)-catalyzed functionalization of 5-iododeoxycytidine. The first crystal structure of fC reveals the existence of an intramolecular H-bond between the exocyclic amine and the formyl group, which controls the conformation of the formyl substituent. Using a newly designed in vitro mutagenicity assay we show that fC and caC are only marginally mutagenic, which is a prerequisite for the bases to function as epigenetic control units.

Biomimetic synthesis of the IDO inhibitors exiguamine A and B

Biomimetic synthesis is an attempt to assemble natural products along biosynthetic lines without recourse to the full enzymatic machinery of nature. We exemplify this with a total synthesis of exiguamine A and the newly isolated natural product exiguamine B. The most noteworthy feature of this work is an oxidative endgame drawing from the complex chemistry of catecholamines, which allows for ready access to a new class of nanomolar indoleamine-2,3-dioxygenase inhibitors.

Efficient Synthesis of 5-Hydroxymethylcytosine Containing DNA

5-Hydroxymethylcytosine ((5-HOMe)dC) was recently discovered as the sixth base in the mammalian genome. The development of a new phosphoramidite building block is reported, which allows efficient synthesis of (5-HOMe)dC containing DNA. Key steps of the synthesis are a palladium-catalyzed formylation and the simultaneous protection of a hydroxyl and amino group as a cyclic carbamate. DNA synthesis is possible under standard conditions, and deprotection can be carried out with dilute NaOH.

Chemical Discrimination between Dc and 5Medc Via Their Hydroxylamine Adducts

The presence of the methylated nucleobase 5MedC in CpG islands is a key factor that determines gene silencing. False methylation patterns are responsible for deteriorated cellular development and are a hallmark of many cancers. Today genes can be sequenced for the content of 5MedC only with the help of the bisulfite reagent, which is based exclusively on chemical reactivity differences established by the additional methyl group. Despite intensive optimization of the bisulfite protocol, the method still has specificity problems. Most importantly ∼95% of the DNA analyte is degraded during the analysis procedure. We discovered that the reagent O-allylhydroxylamine is able to discriminate between dC and 5MedC. The reagent, in contrast to bisulfite, does not exploit reactivity differences but gives directly different reaction products. The reagent forms a stable mutagenic adduct with dC, which can exist in two states (E versus Z). In case of dC the allylhydroxylamine adduct switches into the E-isomeric form, which generates dC to dT transition mutations that can easily be detected by established methods. Significantly, the 5MedC-adduct adopts exclusively the Z-isomeric form, which causes the polymerase to stop. O-allylhydroxylamine does allow differentiation between dC and 5MedC with high accuracy, leading towards a novel and mild chemistry for methylation analysis.

Discovery and Synthesis of New UV-Induced Intrastrand C(4−8)G and G(8−4)C Photolesions

UV irradiation of cellular DNA leads to the formation of a number of defined mutagenic DNA lesions. Here we report the discovery of new intrastrand C(4-8)G and G(8-4)C cross-link lesions in which the C(4) amino group of the cytosine base is covalently linked to the C(8) position of an adjacent dG base. The structure of the novel lesions was clarified by HPLC-MS/MS data for UV-irradiated DNA in combination with chemical synthesis and direct comparison of the synthetic material with irradiated DNA. We also report the ability to generate the lesions directly in DNA with the help of a photoactive precursor that was site-specifically incorporated into DNA. This should enable detailed chemical and biochemical investigations of these lesions.