DNA methylation detection: recent developments in bisulfite free electrochemical and optical approaches.

DNA methylation is one of the significant epigenetic modifications involved in mammalian development as well as in the initiation and progression of various diseases like cancer. Over the past few decades, an enormous amount of research has been carried out for the quantification of DNA methylation in the mammalian genome. Earlier, most of these methodologies used bisulfite treatment. However, the low conversion, false reading, longer assay time and complex chemical reaction are the common limitations of this method that hinder their application in routine clinical screening. Thus, as an alternative to bisulfite conversion-based DNA methylation detection, numerous bisulfite-free methods have been proposed. In this regard, electrochemical biosensors have gained much attention in recent years for being highly sensitive yet cost-effective, portable, and simple to operate. On the other hand, biosensors with optical readouts enable direct real time detection of biological molecules and are easily adaptable to multiplexing. Incorporation of electrochemical and optical readouts into bisulfite free DNA methylation analysis is paving the way for the translation of this important biomarker into standard patient care. In this review, we provide a critical overview of recent advances in the development of electrochemical and optical readout based bisulfite free DNA methylation assays.

Naked-eye and electrochemical detection of isothermally amplified HOTAIR long non-coding RNA.

An inexpensive, simple and rapid sensor platform capable of detecting cancer-related long non-coding RNA (lncRNA) with high accuracy is of great interest in the field of molecular diagnostics. Herein, we report on the development of a new colorimetric and electrochemical assay platform for long non-coding HOX transcript antisense intergenic RNA (HOTAIR) detection. Isothermal reverse transcription-recombinase polymerase amplification (RT-RPA) was performed to amplify HOTAIR sequences from a RNA pool extracted from a designated number of ovarian cancer cells and a small cohort of plasma samples derived from patients with ovarian cancer. During RT-RPA, biotinylated dUTPs were randomly incorporated in the amplified product. Subsequently, HOTAIR amplicons were magnetically purified and isolated followed by a horseradish peroxidase (HRP)-catalyzed colorimetric reaction in the presence of the 3,3',5,5'-tetramethylbenzidine (TMB)/H2O2 system. We finally introduced three potential readout methods for HOTAIR detection - (i) naked-eye visualisation of the color change for a quick screening of the target, (ii) quantitative absorbance measurement by UV-vis, and (iii) amperometric quantification using the electrochemical properties of TMB. The assay has shown excellent reproducibility (% RSD = <5%, for n = 3) and sensitivity (10 cells/ per mL) while detecting HOTAIR in cancer cell lines and patient samples. The expression of HOTAIR in clinical samples was also verified with a standard RT-qPCR method. We believe that our proof of concept assay may find potential relevance for the routine clinical screening of cancer-associated lncRNAs.

A bisulfite treatment and PCR-free global DNA methylation detection method using electrochemical enzymatic signal engagement.

In this paper we report on a bisulfite treatment and PCR amplification-free method for sensitive and selective quantifying of global DNA methylation. Our method utilizes a three-step strategy that involves (i) initial isolation and denaturation of global DNA using the standard isolation protocol and direct adsorption onto a bare gold electrode via gold-DNA affinity interaction, (ii) selective interrogation of methylation sites in adsorbed DNA via methylation-specific 5mC antibody, and (iii) subsequent signal enhancement using an electrochemical-enzymatic redox cycling reaction. In the redox cycling reaction, glucose oxidase (GOx) is used as an enzyme label, glucose as a substrate and ruthenium complex as a redox mediator. We initially investigated the enzymatic properties of GOx by varying glucose and ruthenium concentration to delineate the redox cyclic mechanism of our assay. Because of the fast electron transfer by ruthenium (Ru) complex and intrinsic signal amplification from GOx label, this method could detect as low as 5% methylation level in 50 ng of total DNA input. Moreover, the use of methylation-specific 5mC antibody conjugated GOx makes this assay relatively highly selective for DNA methylation analysis. The data obtained from the electrochemical response for different levels of methylation showed excellent interassay reproducibility of RSD (relative standard deviation) < 5% for n = 3. We believe that this inexpensive, rapid, and sensitive assay will find high relevance as an alternative method for DNA methylation analysis both in research and clinical platforms.

Porous nanozymes: the peroxidase-mimetic activity of mesoporous iron oxide for the colorimetric and electrochemical detection of global DNA methylation.

Nanomaterials (nanozymes) with peroxidase-mimetic activity have been widely used in biosensing platforms as low-cost, relatively stable and prevailing alternatives to natural enzymes. Herein, we report on the synthesis and application of the peroxidase-mimetic activity of mesoporous iron oxide (MIO) for the detection of global DNA methylation in colorectal cancer cell lines. The target DNA was extracted and denatured to get ssDNA followed by direct adsorption onto the surface of a bare screen-printed gold electrode (SPGE). A 5-methylcytosine antibody (5mC) functionalized nanomaterial (MIO-5mC) was then used to recognise the methylcytosine groups present on the SPGE. The MIO-5mC conjugates catalyse the TMB solution in the presence of hydrogen peroxide to give the colorimetric (i.e., naked-eye observation) and electrochemical detection of DNA methylation. The assay could successfully detect as low as 10% difference in the global DNA methylation level in synthetic samples and cell lines with good reproducibility and specificity (%RSD = <5%, for n = 3). This strategy avoids the use of natural enzyme horseradish peroxidase (HRP), traditional PCR based amplification and bisulfite treatment steps that are generally used in many conventional DNA methylation assays. We envisage that our assay could be a low-cost platform with great potential for genome-wide DNA methylation analysis in point-of-care applications.

Epigenetic effect of chronic stress on dopamine signaling and depression.

Because of the complex causal factors leading to depression, epigenetics is of considerable interest for the understanding effect of stress in depression. Dopamine is a key neurotransmitter important in many physiological functions, including motor control, mood, and the reward pathway. These factors lead many drugs to target Dopamine receptors in treating depressive disorders. In this review, we try to portray how chronic stress as an epigenetic factor changes the gene regulation pattern by interrupting Dopamine signaling mechanism.