Conjoint Analysis of DNA Methylation for Tumor Differentiation Using Cationic Conjugated Polymers

Ultrasensitive and visual detection of genetically modified crops using two primers-induced cascade exponential amplification assay

The increased global cultivation area of genetically modified (GM) crops has caused severe controversies over potential health and environmental risks worldwide. There is an urgent need to verify even trace amount of a particular GM material in products. Herein, a two primers-induced cascade exponential amplification reaction combined with cationic conjugated polymers (CCPs)-based visual detection method is developed for rapid and ultrasensitive detection of GM crops. This method only uses two primers to specifically recognize the four regions of the target gene, which is easier for primer design and probably more suitable for the detection of shorter targets. By integrating the two exponential amplification reactions, as low as 5 pg genomic DNA from GM maize can be accurately detected, which is more sensitive than the single amplification-based methods. Taking advantage of the efficient fluorescence resonance energy transfer (FRET) between CCPs and the commercial fluorescent dye SYBR Green I (SG), our method can differentiate as low as 0.01 % GM maize from a large amount of non-GM maize, which is the most accurate method so far. By changing the two primers according to target gene, our method can be modified to the detection of any other GM materials, indicating that our method is promising to be applied in other GM materials-related testing and screening system.

A Label-Free, Mix-and-Detect ssDNA-Binding Assay Based on Cationic Conjugated Polymers

The accurate, simple, and efficient measurement of the concentration of single-stranded DNA (ssDNA) is important for many analytical applications, such as DNA adsorption, biosensor design, and disease diagnosis, but it is still a challenge. Herein, we studied a cationic conjugated polymer (CCP)-based ssDNA assay taking advantage of the obvious fluorescence change of CCPs upon binding ssDNA. Poly(3-(3'-N,N,N-triethylamino-1'-propyloxy)-4-methyl-2,5-thiophene hydrochloride) (PMNT) achieved an apparent dissociation constant (K_d) of 57 ± 4 nM for ssDNA, indicating a very high binding affinity between PMNT and ssDNA. This allowed us to develop a CCP-based ssDNA biosensor with a detection limit of 0.6 nM, similar to the fluorescence-dye-based method using SYBR Green I and SYBR Gold. Our CCP-based biosensor produced smaller differences among ssDNA samples with different base compositions. In addition, the existence of double-stranded DNA (dsDNA) at different concentrations did not interfere with the fluorescence of PMNT, indicating that our CCP-based biosensor was more suitable for the measurement of ssDNA. Compared with fluorescence-intensity-based quantification, our CCP system allowed ratiometric quantification, which made the calibration easier and more robust. We then applied our method to the quantification of ssDNA on AuNPs using both unmodified and thiolated ssDNA, and the accurate quantification of ssDNA was achieved without any fluorophore modification. This method provides an alternative approach for the measurement of ssDNA.

Dual-modality loop-mediated isothermal amplification for pretreatment-free detection of Septin9 methylated DNA in colorectal cancer

Currently, the determination of DNA methylation is still a challenge due to the limited efficiency of enrichment, bisulfite modification, and detection. In this study, a dual-modality loop-mediated isothermal amplification integrated with magnetic bead isolation is  proposed for the determination of methylated Septin9 gene in colorectal cancer. Magnetic beads modified with anti-methyl cytosine antibody were prepared for fast enrichment of methylated DNA through specific immunoaffinity (30 min). One-pot real-time fluorescence and colorimetric loop-mediated isothermal amplification were simultaneously developed for detecting methylated Septin9 gene (60 min). The real-time fluorescence generating by SYTO-9 dye (excitation: 470 nm and emission: 525 nm) and pH indicator (neutral red) was used for quantitative and visualized detection of methylated DNA. This method was demonstrated to detect methylated DNA from HCT 116 cells ranging from 2 to 0.02 ng/μL with a limit of detection of 0.02 ± 0.002 ng/μL (RSD: 9.75%). This method also could discriminate methylated Septin9 in 0.1% HCT 116 cells (RSD: 6.60%), suggesting its high specificity and sensitivity. The feasibility of this assay was further evaluated by clinical plasma samples from 20 colorectal cancer patients and 20 healthy controls, which shows the potential application in simple, low cost, quantitative, and visualized detection of methylated nucleic acids.