Synthesis of (R)-Configured 2′-Fluorinated mC, hmC, fC, and caC Phosphoramidites and Oligonucleotides

A Photoredox Reaction for the Selective Modification of 5-Carboxycytosine in DNA

Covalent epigenetic modifications contribute to the regulation of important cellular processes during development and differentiation, and changes in their genomic distribution and frequency are linked to the emergence of genetic disease states. Chemical and enzymatic methods that selectively target the orthogonal chemical functionality of epigenetic markers are central to the study of their distribution and function, and considerable research effort has been focused on the development of nondestructive sequencing approaches which preserve valuable DNA samples. Photoredox catalysis enables transformations with tunable chemoselectivity under mild, biocompatible reaction conditions. We report the reductive decarboxylation of 5-carboxycytosine via a novel iridium-based treatment, which represents the first application of visible-light photochemistry to epigenetic sequencing via direct base conversion. We propose that the reaction involves an oxidative quenching cycle beginning with single-electron reduction of the nucleobase by the photocatalyst, followed by hydrogen atom transfer from a thiol. The saturation of the C5-C6 backbone permits decarboxylation of the nonaromatic intermediate, and hydrolysis of the N4-amine constitutes a conversion from a cytosine derivative to a T-like base. This conversion demonstrates selectivity for 5-carboxycytosine over other canonical or modified nucleoside monomers, and is thereby applied to the sequencing of 5-carboxycytosine within modified oligonucleotides. The photochemistry explored in this study can also be used in conjunction with enzymatic oxidation by TET to profile 5-methylcytosine at single-base resolution. Compared to other base-conversion treatments, the rapid photochemical reaction takes place within minutes, which could provide advantages for high-throughput detection and diagnostic applications.

Strategies for the Synthesis of Fluorinated Nucleosides, Nucleotides and Oligonucleotides

Fluorinated nucleosides and oligonucleotides are of specific interest as probes for studying nucleic acids interaction, structures, biological transformations, and its biomedical applications. Among various modifications of oligonucleotides, fluorination of preformed nucleoside and/or nucleotides have recently gained attention owing to the unique properties of fluorine atoms imparting medicinal properties with respect to the small size, electronegativity, lipophilicity, and ability for stereochemical control. This review deals with synthetic protocols for selective fluorination either at sugar or base moiety in a preformed nucleosides, nucleotides and nucleic acids using specific fluorinating reagents.

Fluorinated Nucleosides: Synthesis, Modulation in Conformation and Therapeutic Application

Over the last twenty years, fluorination on nucleoside has established itself as the most promising tool to use to get biologically active compounds that could sustain the clinical trial by affecting the pharmacodynamics and pharmacokinetic properties. Due to fluorine's inherent unique properties and its judicious introduction into the molecule, makes the corresponding nucleoside metabolically very stable, lipophilic, and opens a new site of intermolecular binding. Fluorination on various nucleosides has been extensively studied as a result, a series of fluorinated nucleosides come up for different therapeutic uses which are either approved by the FDA or under the advanced stage of the clinical trial. Here in this review, we are summarizing the latest development in the chemistry of fluorination on nucleoside that led to varieties of new analogs like carbocyclic, acyclic, and conformationally biased nucleoside and their biological properties, the influence of fluorine on conformation, oligonucleotide stability, and their use in therapeutics.

An Improved Approach for Practical Synthesis of 5-Hydroxymethyl-2′-deoxycytidine (5hmdC) Phosphoramidite and Triphosphate

5-Hydroxymethyl-2′-deoxycytidine (5hmdC) phosphoramidite and triphosphate are important building blocks in 5hmdC-containing DNA synthesis for epigenetic studies. However, efficient and practical methods for the synthesis of these compounds are still limited. The current research provides an intensively improved synthetic method that enables the preparation of commercially available cyanoethyl-protected 5hmdC phosphoramidite with an overall yield of 39% on 5 g scale. On the basis of facile and efficient accesses to cyanoethyl protected-5hmdU and 5hmdC intermediates, two efficient synthetic routes for 5hmdC triphosphate were also developed.

Intragenomic Decarboxylation of 5-Carboxy-2'-deoxycytidine

Cellular DNA is composed of four canonical nucleosides (dA, dC, dG and T), which form two Watson-Crick base pairs. In addition, 5-methylcytosine (mdC) may be present. The methylation of dC to mdC is known to regulate transcriptional activity. Next to these five nucleosides, the genome, particularly of stem cells, contains three additional dC derivatives, which are formed by stepwise oxidation of the methyl group of mdC with the help of Tet enzymes. These are 5-hydroxymethyl-dC (hmdC), 5-formyl-dC (fdC), and 5-carboxy-dC (cadC). It is believed that fdC and cadC are converted back into dC, which establishes an epigenetic control cycle that starts with methylation of dC to mdC, followed by oxidation and removal of fdC and cadC. While fdC was shown to undergo intragenomic deformylation to give dC directly, a similar decarboxylation of cadC was postulated but not yet observed on the genomic level. By using metabolic labelling, we show here that cadC decarboxylates in several cell types, which confirms that both fdC and cadC are nucleosides that are directly converted back to dC within the genome by C-C bond cleavage.

Direct and Base Excision Repair-Mediated Regulation of a GC-Rich cis-Element in Response to 5-Formylcytosine and 5-Carboxycytosine

Stepwise oxidation of the epigenetic mark 5-methylcytosine and base excision repair (BER) of the resulting 5-formylcytosine (5-fC) and 5-carboxycytosine (5-caC) may provide a mechanism for reactivation of epigenetically silenced genes; however, the functions of 5-fC and 5-caC at defined gene elements are scarcely explored. We analyzed the expression of reporter constructs containing either 2′-deoxy-(5-fC/5-caC) or their BER-resistant 2′-fluorinated analogs, asymmetrically incorporated into CG-dinucleotide of the GC box cis-element (5′-TGGGCGGAGC) upstream from the RNA polymerase II core promoter. In the absence of BER, 5-caC caused a strong inhibition of the promoter activity, whereas 5-fC had almost no effect, similar to 5-methylcytosine or 5-hydroxymethylcytosine. BER of 5-caC caused a transient but significant promoter reactivation, succeeded by silencing during the following hours. Both responses strictly required thymine DNA glycosylase (TDG); however, the silencing phase additionally demanded a 5′-endonuclease (likely APE1) activity and was also induced by 5-fC or an apurinic/apyrimidinic site. We propose that 5-caC may act as a repressory mark to prevent premature activation of promoters undergoing the final stages of DNA demethylation, when the symmetric CpG methylation has already been lost. Remarkably, the downstream promoter activation or repression responses are regulated by two separate BER steps, where TDG and APE1 act as potential switches.

Deformylation of 5-Formylcytidine in Different Cell Types

Epigenetic programming of cells requires methylation of deoxycytidines (dC) to 5-methyl-dC (mdC) followed by oxidation to 5-hydroxymethyl-dC (hmdC), 5-formyl-dC (fdC), and 5-carboxy-dC (cadC). Subsequent transformation of fdC and cadC back to dC by various pathways establishes a chemical intra-genetic control circle. One of the discussed pathways involves the Tdg-independent deformylation of fdC directly to dC. Here we report the synthesis of a fluorinated fdC feeding probe (F-fdC) to study direct deformylation to F-dC. The synthesis was performed along a novel pathway that circumvents any F-dC as a reaction intermediate to avoid contamination interference. Feeding of F-fdC and observation of F-dC formation in vivo allowed us to gain insights into the Tdg-independent removal process. While deformylation was shown to occur in stem cells, we here provide data that prove deformylation also in different somatic cell types. We also investigated active demethylation in a non-dividing neurogenin-inducible system of iPS cells that differentiate into bipolar neurons.

Specificity Enhancement of Deoxyribonucleic Acid Polymerization for Sensitive Nucleic Acid Detection

Specificity of DNA polymerization plays a critical role in DNA replication and storage of genetic information. Likewise, biotechnological applications, such as nucleic acid detection, DNA amplification, and gene cloning, require high specificity in DNA synthesis catalyzed by DNA polymerases. However, errors in DNA polymerization (such as mis-incorporation and mis-priming) can significantly jeopardize the specificity. Herein, we report our discovery that the specificity of DNA enzymatic synthesis can be substantially enhanced (up to 100-fold higher) by attenuating DNA polymerase kinetics via the phosphorothioate dNTPs. This specificity enhancement allows convenient and sensitive nucleic acid detection, polymerization, PCR, and gene cloning with complex systems (such as human cDNA and genomic DNA). Further, we found that the specificity enhancement offered higher sensitivity (up to 50-fold better) for detecting nucleic acids, such as COVID-19 viral RNAs. Our findings have revealed a simple and convenient strategy for facilitating specificity and sensitivity of nucleic acid detection, amplification, and gene cloning.

Electrophilic oligodeoxynucleotide synthesis using dM-Dmoc for amino protection

Solid-phase synthesis of electrophilic oligodeoxynucleotides (ODNs) was achieved using dimethyl-Dmoc (dM-Dmoc) as amino protecting group. Due to the high steric hindrance of the 2-(propan-2-ylidene)-1,3-dithiane side product from deprotection, the use of excess nucleophilic scavengers such as aniline to prevent Michael addition of the side product to the deprotected ODN during ODN cleavage and deprotection was no longer needed. The improved technology was demonstrated by the synthesis and characterization of five ODNs including three modified ones. The modified ODNs contained the electrophilic groups ethyl ester, α-chloroamide, and thioester. Using the technology, the sensitive groups can be installed at any location within the ODN sequences without using any sequence- or functionality-specific conditions and procedures.

2'-O-(2-Methoxyethyl) Nucleosides Are Not Phosphorylated or Incorporated Into the Genome of Human Lymphoblastoid TK6 Cells

Nucleoside analogs with 2'-modified sugar moieties are often used to improve the RNA target affinity and nuclease resistance of therapeutic oligonucleotides in preclinical and clinical development. Despite their enhanced nuclease resistance, oligonucleotides could slowly degrade releasing nucleoside analogs that have the potential to become phosphorylated and incorporated into cellular DNA and RNA. For the first time, the phosphorylation and DNA/RNA incorporation of 2'-O-(2-methoxyethyl) (2'-O-MOE) nucleoside analogs have been investigated. Using liquid chromatography/tandem mass spectrometry, we showed that enzymes in the nucleotide salvage pathway including deoxycytidine kinase (dCK) and thymidine kinase (TK1) displayed poor reactivity toward 2'-O-MOE nucleoside analogs. On the other hand, 2'-fluoro (F) nucleosides, regardless of the nucleobase, were efficiently phosphorylated to their monophosphate forms by dCK and TK1. Consistent with their efficient phosphorylation by dCK and TK1, 2'-F nucleoside analogs were incorporated into cellular DNA and RNA while no incorporation was detected with 2'-O-MOE nucleoside analogs. In conclusion, these data suggest that the inability of dCK and TK1 to create the monophosphates of 2'-O-MOE nucleoside analogs reduces the risk of their incorporation into cellular DNA and RNA.

Synthesis and biological applications of fluoro-modified nucleic acids

Owing to the unique physical properties of a fluorine atom, incorporating fluoro-modifications into nucleic acids offers striking biophysical and biochemical features, and thus significantly extends the breadth and depth of biological applications of nucleic acids. In this review, fluoro-modified nucleic acids that have been synthesized through either solid phase synthesis or the enzymatic approach are briefly summarised, followed by a section describing their biomedical applications in nucleic acid-based therapeutics, ¹⁸F PET imaging and mechanistic studies of DNA modifying enzymes. In the last part, the utility of ¹⁹F NMR and MRI for probing the structure, dynamics and molecular interactions of fluorinated nucleic acids is reviewed.

2'-(R)-Fluorinated mC, hmC, fC and caC triphosphates are substrates for DNA polymerases and TET-enzymes

A deeper investigation of the chemistry that occurs on the newly discovered epigenetic DNA bases 5-hydroxymethyl-(hmdC), 5-formyl-(fdC), and 5-carboxy-deoxycytidine (cadC) requires chemical tool compounds, which are able to dissect the different potential reaction pathways in cells. Here we report that the 2'-(R)-fluorinated derivatives F-hmdC, F-fdC, and F-cadC, which are resistant to removal by base excision repair, are good substrates for DNA polymerases and TET enzymes. This result shows that the fluorinated compounds are ideal tool substances to investigate potential C-C-bond cleaving reactions in the context of active demethylation.

Synthesis of RNA Containing 5-Hydroxymethyl-, 5-Formyl-, and 5-Carboxycytidine

5-Hydroxymethyl-, 5-formyl-, and 5-carboxy-2'-deoxycytidine are new epigenetic bases (hmdC, fdC, cadC) that were recently discovered in the DNA of higher eukaryotes. The same bases (5-hydroxymethyl-, 5-formyl-, and 5-carboxycytidine; hmC, fC, and caC) have now also been detected in mammalian RNA with a high abundance in mRNA. While DNA phosphoramidites (PAs) that allow the synthesis of xdC-containing oligonucleotides for deeper biological studies are available, the corresponding silyl-protected RNA PAs for fC and caC have not yet been disclosed. Here, we report novel RNA PAs for hmC, fC, and caC that can be used in routine RNA synthesis. The new building blocks are compatible with the canonical PAs and also with themselves, which enables even the synthesis of RNA strands containing all three of these bases. The study will pave the way for detailed physical, biochemical, and biological studies to unravel the function of these non-canonical modifications in RNA.

Structural Basis for Excision of 5-Formylcytosine by Thymine DNA Glycosylase

Thymine DNA glycosylase (TDG) is a base excision repair enzyme with key functions in epigenetic regulation. Performing a critical step in a pathway for active DNA demethylation, TDG removes 5-formylcytosine and 5-carboxylcytosine, oxidized derivatives of 5-methylcytosine that are generated by TET (ten-eleven translocation) enzymes. We determined a crystal structure of TDG bound to DNA with a noncleavable (2'-fluoroarabino) analogue of 5-formyldeoxycytidine flipped into its active site, revealing how it recognizes and hydrolytically excises fC. Together with previous structural and biochemical findings, the results illustrate how TDG employs an adaptable active site to excise a broad variety of nucleobases from DNA.