RT-qPCR Detection of Low-Copy HIV RNA with Yin-Yang Probes.

Accurate monitoring of low levels of viral load (the number of viral particles per milliliter of plasma) in HIV-infected patients is important in terms of evaluation of the progress of antiretroviral therapy. The general approach for detection of low copy HIV RNA is reverse transcription combined with quantitative real-time PCR based on fluorescence detection. The selection of primers and the structure of fluorogenic oligonucleotide probes are crucial for sensitivity and accuracy of the assay. In this chapter, we report the RT-qPCR protocol for detection of low copy HIV RNA using double stranded Yin-Yang DNA probes containing identical fluorescent dyes on each strand of the probe. Dye residues attached to the 3'-end of an oligonucleotide and 5'-end of the complementary oligonucleotide form a self-quenched aggregate in a Yin-Yang duplex probe, and display fluorescence light up upon probe strand displacement with the target sequence amplified in the course of PCR. Among several fluorescent dyes tested (R6G, ROX, Cy5) the ROX labeled Yin-Yang probes showed better fluorescence increase and lower Ct values. All the homo Yin-Yang probes were superior to corresponding dye-quencher probes and allowed reliable detection of 10-10,000 copies of HIV RNA per mL.

DNA i-Motifs With Guanidino-i-Clamp Residues: The Counterplay Between Kinetics and Thermodynamics and Implications for the Design of pH Sensors.

I-motif structures, adopted by cytosine-rich DNA strands, have attracted considerable interest as possible regulatory elements in genomes. Applied science exploits the advantages of i-motif stabilization under acidic conditions: i-motif-based pH sensors and other biocompatible nanodevices are being developed. Two key characteristics of i-motifs as core elements of nanodevices, i.e., their stability under physiological conditions and folding/unfolding rates, still need to be improved. We have previously reported a phenoxazine derivative (i-clamp) that enhances the thermal stability of the i-motif and shifts the pH transition point closer to physiological values. Here, we performed i-clamp guanidinylation to further explore the prospects of clamp-like modifications in i-motif fine-tuning. Based on molecular modeling data, we concluded that clamp guanidinylation facilitated interstrand interactions in an i-motif core and ultimately stabilized the i-motif structure. We tested the effects of guanidino-i-clamp insertions on the thermal stabilities of genomic and model i-motifs. We also investigated the folding/unfolding kinetics of native and modified i-motifs under moderate, physiologically relevant pH alterations. We demonstrated fast folding/unfolding of native genomic and model i-motifs in response to pH stimuli. This finding supports the concept of i-motifs as possible genomic regulatory elements and encourages the future design of rapid-response pH probes based on such structures. Incorporation of guanidino-i-clamp residues at/near the 5'-terminus of i-motifs dramatically decreased the apparent unfolding rates and increased the thermal stabilities of the structures. This counterplay between the effects of modifications on i-motif stability and their effects on kinetics should be taken into account in the design of pH sensors.

Novel homo Yin-Yang probes improve sensitivity in RT-qPCR detection of low copy HIV RNA.

Nucleic acids labeled with a fluorophore/quencher pair are widely used as probes in biomedical research and molecular diagnostics. Here we synthesized novel DNA molecular beacons double labeled with the identical dyes (R6G, ROX and Cy5) at 5'- and 3'-end and studied their photo physical properties. We demonstrated that fluorescence quenching by formation of the homo dimer exciton in such molecular beacons allows using them in homogeneous assays. Further, we developed and evaluated homo Yin-Yang DNA probes labeled with identical dyes and used them for detection of low copy HIV RNA by RT-qPCR. They demonstrated improved sensitivity (LLQ: 10 vs 30 copies mL-1) in comparison to commercially available Abbott RealTime HIV-1 kit based on VIC-BHQ dyes both for model mixtures (naive human plasma with added deactivated HIV-1 virus) and for preliminarily confirmed 36 clinical samples (4 vs 1 positive ones for low-copy samples).

Automated Solid-Phase Click Synthesis of Oligonucleotide Conjugates: From Small Molecules to Diverse N-Acetylgalactosamine Clusters.

We developed a novel technique for the efficient conjugation of oligonucleotides with various alkyl azides such as fluorescent dyes, biotin, cholesterol, N-acetylgalactosamine (GalNAc), etc. using copper-catalysed alkyne-azide cycloaddition on the solid phase and CuI·P(OEt)3 as a catalyst. Conjugation is carried out in an oligonucleotide synthesizer in fully automated mode and is coupled to oligonucleotide synthesis and on-column deprotection. We also suggest a set of reagents for the construction of diverse conjugates. The sequential double-click procedure using a pentaerythritol-derived tetraazide followed by the addition of a GalNAc or Tris-GalNAc alkyne gives oligonucleotide-GalNAc dendrimer conjugates in good yields with minimal excess of sophisticated alkyne reagents. The approach is suitable for high-throughput synthesis of oligonucleotide conjugates ranging from fluorescent DNA probes to various multi-GalNAc derivatives of 2'-modified siRNA.

Specificity of SNP detection with molecular beacons is improved by stem and loop separation with spacers.

Molecular beacons (MBs) are valuable tools in molecular biology, clinical diagnostics and analytical chemistry. Here we describe a novel approach for the design of MBs with nucleotide or non-nucleotide linkers between the stem and loop regions. Such modified MBs have significantly improved specificity and performance for single nucleotide polymorphism (SNP) detection. These advantages are especially distinct, when compared to the classic MBs, in the case of possible interactions between the stem and loop regions. We demonstrated the applicability of such modified MBs for the discrimination of common Factor V, NOS3 and ADRB2 SNPs in model plasmids and in clinical samples. The developed approach could be applicable not only to fluorescently labeled MBs, but also to other biosensors based on nucleic acids with stem-loop structures.

1-Phenylethynylpyrene (PEPy) as a novel blue-emitting dye for qPCR assay.

An alkyl azide derivative of 1-phenylethynylpyrene (PEPy) dye was prepared and used in the functionalization of oligonucleotides via click chemistry. Spectral and photo-physical properties of the PEPy-modified oligonucleotides as a single strand, and in perfect or mismatched duplexes, have been studied. A series of PEPy-Dabcyl fluorogenic TaqMan probes were synthesized and tested in qPCR. PEPy proved to be a superior substitute for AMCA as a short wavelength fluorescent dye for qPCR probes. PEPy probes were shown to significantly reduce Cq (a fractional PCR cycle used for quantification) vs. AMCA labeled probes, thus improving on the reliability of detection. Moreover, a larger increase of fluorescence during amplification was observed in the case of PEPy probes that makes this dye very suitable for an end-point PCR technique. This study broadens the panel of fluorescent dyes suitable for the use in probes for quantitative real-time PCR.

Solid- and solution-phase synthesis and application of R6G dual-labeled oligonucleotide probes.

A novel N-TFA-protected carboxyrhodamine 6G (R6G) phosphoramidite was synthesized for use in an automated DNA synthesis to prepare 5'-labeled oligonucleotides. Deprotection and purification conditions were optimized for 5'-labeled and dual-labeled oligonucleotide probes. As an alternative we synthesized an azide derivative of R6G for CuAAC post-synthetic oligonucleotide labeling. Dual-labeled probes obtained by both methods showed the same efficacy in a quantitative PCR assay. R6G-labeled probes demonstrated superior properties in a qPCR assay in comparison with alternative HEX, JOE and SIMA dyes due to more efficient fluorescence quenching by BHQ-1. We successfully used R6G dual-labeled probes for rotavirus genotyping.