Complete, Programmable Decoding of Oxidized 5-Methylcytosine Nucleobases in DNA by Chemoselective Blockage of Universal Transcription-Activator-Like Effector Repeats

Ultrasensitive TALEs-mediated electrochemical sensor for DNA methylation detection based on RCA reaction and Exo III cyclic amplification-assisted "silver-link" crossing electrode

Pancreatic cancer has become one of the most challenging malignant tumors. This type of cancer shows a high mortality paralleling morbidity and a rather low 5-year survival rate. Early diagnosis of the disease is the key to prevent and treat the pancreatic cancer. Cellular free DNA (cfDNA) methylation detection is a promising non-invasive method in early cancer screening, diagnosis, and prognostic monitoring. However, there are still shortcomings in developing fast, sensitive, low-cost, and quantitative methods for detecting target methylated DNA. Herein, new transcription activator-like effectors (TALEs)-mediated electrochemical sensor by rolling circle amplification (RCA)-assisted "silver-link" crossing electrode was developed for detecting DNA methylation of Ras association domain family 1 isoform A (RASSF1A) tumor suppressor genes (R-5mC). The TALEs-Ni magnetic beads could specifically recognize R-5mC and bind to target methylated DNA to trigger the RCA and catalytic hairpin assembly (CHA), thereby duplexes (H1-H2) were generated. By introducing H3 probe to cause CHA amplification and Exo III-assisted cyclic amplification strategy, large amounts of H2 probes were generated to trigger a new round of RCA reaction, long crossing DNA strands were formed between the gaps of electrodes. The "silver-link" crossing electrode was formed by metallization of "gene-link" and target was detected by recording this change in conductivity. The limit of detection (LOD) was 0.5 fM. The proposed electrochemical sensor showed good stability, high sensitivity and low detected background signal in the R-5mC detection. In addition, the R-5mC detection of cfDNA in plasma samples of cancer patients was satisfactory. This strategy is of great significance in the early screening, treatment and prognosis monitoring of pancreatic cancer.

Experience with German Research Consortia in the Field of Chemical Biology of Native Nucleic Acid Modifications

The chemical biology of native nucleic acid modifications has seen an intense upswing, first concerning DNA modifications in the field of epigenetics and then concerning RNA modifications in a field that was correspondingly rebaptized epitranscriptomics by analogy. The German Research Foundation (DFG) has funded several consortia with a scientific focus in these fields, strengthening the traditionally well-developed nucleic acid chemistry community and inciting it to team up with colleagues from the life sciences and data science to tackle interdisciplinary challenges. This Perspective focuses on the genesis, scientific outcome, and downstream impact of the DFG priority program SPP1784 and offers insight into how it fecundated further consortia in the field. Pertinent research was funded from mid-2015 to 2022, including an extension related to the coronavirus pandemic. Despite being a detriment to research activity in general, the pandemic has resulted in tremendously boosted interest in the field of RNA and RNA modifications as a consequence of their widespread and successful use in vaccination campaigns against SARS-CoV-2. Funded principal investigators published over 250 pertinent papers with a very substantial impact on the field. The program also helped to redirect numerous laboratories toward this dynamic field. Finally, SPP1784 spawned initiatives for several funded consortia that continue to drive the fields of nucleic acid modification.

Sequence-specific DNA labelling for fluorescence microscopy

The preservation of nucleus structure during microscopy imaging is a top priority for understanding chromatin organization, genome dynamics, and gene expression regulation. In this review, we summarize the sequence-specific DNA labelling methods that can be used for imaging in fixed and/or living cells without harsh treatment and DNA denaturation: (i) hairpin polyamides, (ii) triplex-forming oligonucleotides, (iii) dCas9 proteins, (iv) transcription activator-like effectors (TALEs) and (v) DNA methyltransferases (MTases). All these techniques are capable of identifying repetitive DNA loci and robust probes are available for telomeres and centromeres, but visualizing single-copy sequences is still challenging. In our futuristic vision, we see gradual replacement of the historically important fluorescence in situ hybridization (FISH) by less invasive and non-destructive methods compatible with live cell imaging. Combined with super-resolution fluorescence microscopy, these methods will open the possibility to look into unperturbed structure and dynamics of chromatin in living cells, tissues and whole organisms.

Imaging-Based In Situ Analysis of 5-Methylcytosine at Low Repetitive Single Gene Loci with Transcription-Activator-Like Effector Probes

Transcription-activator-like effectors (TALEs) are programmable DNA binding proteins that can be used for sequence-specific, imaging-based analysis of cellular 5-methylcytosine. However, this has so far been limited to highly repetitive satellite DNA. To expand this approach to the analysis of coding single gene loci, we here explore a number of signal amplification strategies for increasing imaging sensitivity with TALEs. We develop a straightforward amplification protocol and employ it to target the MUC4 gene, which features only a small cluster of repeat sequences. This offers high sensitivity imaging of MUC4, and in costaining experiments with pairs of one TALE selective for unmethylated cytosine and one universal control TALE enables analyzing methylation changes in the target independently of changes in target accessibility. These advancements offer prospects for 5-methylcytosine analysis at coding, nonrepetitive gene loci by the use of designed TALE probe collections.

Fluorescence imaging of epigenetic genome modifications

Epigenome contains a lot of information about cell state. Epigenetic analysis includes primarily sequence-based methods, which provide detailed data on distribution of modifications along the genome, but are poorly applicable for screenings. Specific fluorescence labeling and imaging of epigenetic modifications is an attractive complementary approach. It is currently based mainly on histone modifications study. We expect that inclusion of DNA modifications into imaging-based study would empower the method. In this review we discuss methods for fluorescence imaging of DNA modifications (mainly 5-methylcytosine). It opens an easy way to single cell analysis and high-throughput screening. Moreover, tracking epigenome changes in live cells becomes possible with genetically encoded probes.

Programmable tools for targeted analysis of epigenetic DNA modifications

Modifications of the cytosine 5-position are dynamic epigenetic marks of mammalian DNA with important regulatory roles in development and disease. Unraveling biological functions of such modified nucleobases is tightly connected with the potential of available methods for their analysis. Whereas genome-wide nucleobase quantification and mapping are first-line analyses, targeted analyses move into focus the more genomic sites with high biological significance are identified. We here review recent developments in an emerging field that addresses such targeted analyses via probes that combine a programmable, sequence-specific DNA-binding domain with the ability to directly recognize or cross-link an epigenetically modified nucleobase of interest. We highlight how such probes offer simple, high-resolution nucleobase analyses in vitro and enable in situ correlations between a nucleobase and other chromatin regulatory elements at user-defined loci on the single-cell level by imaging.

Designer Receptors for Nucleotide-Resolution Analysis of Genomic 5-Methylcytosine by Cellular Imaging

We report programmable receptors for the imaging‐based analysis of 5‐methylcytosine (5mC) in user‐defined DNA sequences of single cells. Using fluorescent transcription‐activator‐like effectors (TALEs) that can recognize sequences of canonical and epigenetic nucleobases through selective repeats, we imaged cellular SATIII DNA, the origin of nuclear stress bodies (nSB). We achieve high nucleobase selectivity of natural repeats in imaging and demonstrate universal nucleobase binding by an engineered repeat. We use TALE pairs differing in only one such repeat in co‐stains to detect 5mC in SATIII sequences with nucleotide resolution independently of differences in target accessibility. Further, we directly correlate the presence of heat shock factor 1 with 5mC at its recognition sequence, revealing a potential function of 5mC in its recruitment as initial step of nSB formation. This opens a new avenue for studying 5mC functions in chromatin regulation in situ with nucleotide, locus, and cell resolution.

Facile Clamp-Assisted Ligation Strategy for Direct Discrimination and Background-Free Quantification of Site-Specific 5-Formylcytosine

Quantification of site-specific 5-formylcytosine (5fC) in DNA is highly significant to better understand its biological functions. However, it is still a big challenge to precisely discriminate 5fC from cytosine (C), 5-hydroxymethylcytosine (5hmC), 5-methylcytosine (5mC), and 5-carboxycytosine (5caC) owing to their similar structures that will interfere the quantification of 5fC. To solve this issue, a novel peptide nucleic acid (PNA) clamp-assisted ligation amplification strategy coupled with a 5fC-selective chemical conversion route is employed, through which 5fC can be precisely quantified with other interfering signals completely suppressed. As a result, as low as 200 aM of site-specific 5fC-containing DNA target can be accurately determined at single-base resolution in a background-free manner.

Overcoming conservation in TALE-DNA interactions: a minimal repeat scaffold enables selective recognition of an oxidized 5-methylcytosine

Transcription-activator-like effectors (TALEs) are repeat-based proteins featuring programmable DNA binding. The repulsion of TALE repeats by 5-methylcytosine (5mC) and its oxidized forms makes TALEs potential probes for their programmable analysis. However, this potential has been limited by the inability to engineer repeats capable of actual, fully selective binding of an (oxidized) 5mC: the extremely conserved and simple nucleobase recognition mode of TALE repeats and their extensive involvement in inter-repeat interactions that stabilize the TALE fold represent major engineering hurdles. We evaluated libraries of alternative, strongly truncated repeat scaffolds and discovered a repeat that selectively recognizes 5-carboxylcytosine (5caC), enabling construction of the first programmable receptors for an oxidized 5mC. In computational studies, this unusual scaffold executes a dual function via a critical arginine that provides inter-repeat stabilization and selectively interacts with the 5caC carboxyl group via a salt-bridge. These findings argue for an unexpected adaptability of TALE repeats and provide a new impulse for the design of programmable probes for nucleobases beyond A, G, T and C.