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  • DNA methylation is associated with transcriptional silencing


    DNA methylation is associated with transcriptional silencing of tumor suppressors or other genes important for normal cellular function and plays an important role in the development of cancer and other diseases (such as breast and colorectal cancers) [19]. DNA methyltransferases (MTases) are a family of enzymes that could recognize particular short palindromic sequences and transfer a methyl group from S-adenosyl- L-meth-ionine (SAM) to target Decernotinib synthesis or cytosine residues in the palindromic sequences during the biological DNA methylation process [20]. An abnormal level of DNA MTase leads to the aberrant level of DNA methylation which has been regarded as biomarkers of early cancers [21]. Accordingly, DNA MTase is regarded as a potential target for anticancer therapy and drug screening [22,23]. Therefore, monitoring the DNA MTase activity and screening inhibitors (anti-methylation drugs) are of significance in fundamental biochemical research, clinical diagnostics, drug discovery, and disease therapy. Up to now, many analytical methods including high performance liquid chromatography, radioactive labeling, methylation-specific polymerase chain reaction and gel electrophoresis have been employed for detection of DNA MTase activity [[24], [25], [26], [27], [28]]. However, these methods are time-intensive, laborious, or require isotope labeling. In order to address these issues some new approaches have been introduced such as electrochemical, fluorescence, colorimetry, and bioluminescence based techniques [[29], [30], [31], [32], [33], [34]]. For instance, Zhou et al. developed an electrochemical immunosensing platform for DNA mehtyltransferase activity analysis and inhibitor screening [35]. Zhang et al. develop a sensitive fluorescence method for DNA MTase activity based on RNA polymerase-mediated transcription amplification and duplex-specific nuclease assisted cyclic signal amplification [36]. Jiang and coworkers developed a novel biosensor platform for colorimetric detection of DNA methyltransferase based on terminal protection of the DNA-gold nanoparticle probes by mechanistically covalent trapping of target enzymes [37]. These methods have provided considerable advancements, but the design and modification of these strategies are still not facile enough. Moreover, most of the proposed fluorescence sensors require double-labeled oligonucleotide substrates, which will not only increase the cost of sensing system but also probably reduce the cleavage efficiency. Therefore, substantial efforts were still needed for the development of simple, label-free and efficient methods to meet the increasing demand for detecting DNA MTase activity.
    Results and discussions
    Acknowledgment This work was supported by the Medical Application Technology Tracking Project of Hebei Province (GL2012089). We would like to thank the Langfang People\'s Hospital for supporting this research.
    Introduction Epigenetics is the study of potentially heritable changes in gene expression that does not involve changes to the underlying DNA sequence (Jablonka and Lamb, 2002; Nagase and Ghosh, 2008). DNA methylation is one of the most important epigenetic modifications, which is found in the genomes of diverse organisms including both prokaryotes and eukaryotes, and participates in a variety of important biological processes including regulation of gene expression, gene imprinting, preservation of chromosomal integrity, and X-chromosome inactivation (Li, 2002; Reik and Dean, 2001). DNA methylation occurs mainly at the fifth position of cytosine (5mC) in the dinucleotide CpG and is stably inherited through cell division (Deaton and Bird, 2011). It is carried out by a group of enzymes called DNA methyltransferases (dnmts), which transfer the methyl group from the methyl donor S-adenosylmethionine (SAM) to cytosine residues and generate S-adenosylhomocysteine (SAH) (Lu and Mato, 2012). In general, the dnmts function in two different methylation processes: maintenance (dnmt1) and de novo (dnmt3 subfamily, including dnmt3a, dnmt3b, and dnmt3l) (Bheemanaik et al., 2006). Dnmt1 preferentially transfers methyl groups to the hemi-methylated DNA strands following DNA replication (Cedar and Bergman, 2012; Pradhan et al., 1999). In in vitro conditions, dnmt1 exhibits 5 to 40 fold higher affinity to hemimethylated DNA strands than unmethylated strands (Hermann et al., 2004). Dnmt3a and dnmt3b are responsible for establishing DNA methylation patterns produced through their de novo type DNA methylation activity in embryos development and during germ cell differentiation (Okano et al., 1999). Dnmt3l (Dnmt3-like) is a member of the dnmt3 subfamily. However, it does not possess DNA methylation activity, and it works as a catalytically inactive DNA methyltransferase cofactor (Aapola et al., 2001). Recently, the fourth member of dnmt3 subfamily, dnmt3c, a de novo DNA methyltransferase gene that evolved via a duplication of dnmt3b, was identified in mouse (Barau et al., 2016). In addition, dnmt2 displays RNA methyltransferase catalytic activity, which functions to methylate cytosine 38 in the anti-codon loop of tRNAAsp (Goll et al., 2006).