DNA analysis by fluorescence quenching detection

Development and Evaluation of an Enterovirus D68 Real-Time Reverse Transcriptase PCR Assay

We have developed and evaluated a real-time reverse transcriptase PCR (RT-PCR) assay for the detection of human enterovirus D68 (EV-D68) in clinical specimens. This assay was developed in response to the unprecedented 2014 nationwide EV-D68 outbreak in the United States associated with severe respiratory illness. As part of our evaluation of the outbreak, we sequenced and published the genome sequence of the EV-D68 virus circulating in St. Louis, MO. This sequence, along with other GenBank sequences from past EV-D68 occurrences, was used to computationally select a region of EV-D68 appropriate for targeting in a strain-specific RT-PCR assay. The RT-PCR assay amplifies a segment of the VP1 gene, with an analytic limit of detection of 4 copies per reaction, and it was more sensitive than commercially available assays that detect enteroviruses and rhinoviruses without distinguishing between the two, including three multiplex respiratory panels approved for clinical use by the FDA. The assay did not detect any other enteroviruses or rhinoviruses tested and did detect divergent strains of EV-D68, including the first EV-D68 strain (Fermon) identified in California in 1962. This assay should be useful for identifying and studying current and future outbreaks of EV-D68 viruses.

Rapid genotyping using pyrene-perylene locked nucleic acid complexes

We have developed an assay for single strand DNA and RNA detection which is based on novel pyrene-perylene FRET pairs attached to short LNA/DNA probes. The assay is based on ratiometric emission upon binding of target DNA/RNA by three combinations of fluorescent LNA/DNA reporter strands. Specific geometry of the pyrene fluorophore attached to the 2'-amino group of 2'-amino-LNA in position 4 allows for the first time to efficiently utilize dipole-dipole orientation parameter for sensing of single-nucleotide polymorphisms (SNPs) in nucleic acid targets by FRET. Using novel probes, SNP detection is achieved with advantages of large Stokes shift (115 nm), high fluorescence quantum yields and low limit of target detection values (< 5 nM). Rapid and accurate genotyping of highly polymorphic HIV Pol cDNA and RNA fragments performed herein proves the possibility for broad application of the novel pyrene-perylene FRET pairs, e.g., in imaging and clinical diagnostics.

Fluorescent oligonucleotides containing a novel perylene 2′-amino-α-L-LNA monomer: Synthesis and analytical potential

Herein, a novel fluorescent nucleotide analogue, perylene-2′-amino-α-L-LNA, has been prepared and studied within synthetic oligonucleotides of different sequences. The phosphoramidite reagent was synthesized in 85% overall yield starting from 2′-amino-α-L-LNA nucleoside. Incorporation efficiency of the resulting perylene-2′-amino-α-L-LNA monomer (T*) into synthetic oligonucleotides was significantly improved by replacement of the typically used 1H-tetrazole activator with pyridine hydrochloride. Generally, oligonucleotides containing monomerT* showed high binding affinity towards complementary DNA and RNA targets, batochromically shifted excitation/emission wavelengths with respect to the often applied polyaromatic hydrocarbon pyrene, high fluorescent quantum yields and very low target detection limits (5–10 nM). Fluorescence of single stranded LNA/DNA mixmer oligonucleotide having two incorporations of monomersT* was quenched (quantum yield ΦF= 0.21) relative to duplexes of this probe with complementary DNA and RNA (ΦF= 0.42 and 0.35, respectively). On the contrary, a strong fluorescence quenching upon target binding was demonstrated by two short oligonucleotides of analogues sequences containing monomersT* at 5′- and 3′-terminal positions. We explain the hybridization-induced light-up effect observed for double-labeled probe by a reduction of fluorescence quenching due to precise positioning of the fluorophores within the double-stranded complexes. Furthermore, we propose that a covalent link between twoT* monomers in the double-labeled probe provides a remarkable degree of rigidity in the double helix which enforces positioning of the bulky perylene moieties in the nonpolar groove resulting in reduced fluorescence quenching.

New 9-methyl-8-(4-hydroxyphenyl)adenine derivatives as A1 adenosine receptor antagonists

A new series of 9-methyladenines, bearing different bulky groups at the 8-position, were prepared and their affinity for the four human adenosine receptor subtypes were evaluated. All the synthesized compounds showed affinities at the A1, A2A, and A3AR subtypes ranging from nanomolar to micromolar levels with different degrees of A1selectivity, while they resulted nearly inactive at A2BAR. In particular, 9-methyl-8-[4-(4-methylbenzyloxy)phenyl]- adenine showed A1AR affinity in the nanomolar range and good levels of selectivity versus the other receptor subtypes. Furthermore, a functional assay at mouse ileum allowed to assess the potency of selected compounds at A1AR subtype. Results showed that all the tested derivatives are neutral antagonists and theirKbvalues are in good agreement with theKivalues from radioligand binding assay at human A1AR, confirming that the effect is due to inhibition of this subtype.

A single molecular beacon probe is sufficient for the analysis of multiple nucleic acid sequences

Molecular beacon (MB) probes are dual-labeled hairpin-shaped oligodeoxyribonucleotides that are extensively used for real-time detection of specific RNA/DNA analytes. In the MB probe, the loop fragment is complementary to the analyte: therefore, a unique probe is required for the analysis of each new analyte sequence. The conjugation of an oligonucleotide with two dyes and subsequent purification procedures add to the cost of MB probes, thus reducing their application in multiplex formats. Here we demonstrate how one MB probe can be used for the analysis of an arbitrary nucleic acid. The approach takes advantage of two oligonucleotide adaptor strands, each of which contains a fragment complementary to the analyte and a fragment complementary to an MB probe. The presence of the analyte leads to association of MB probe and the two DNA strands in quadripartite complex. The MB probe fluorescently reports the formation of this complex. In this design, the MB does not bind the analyte directly; therefore, the MB sequence is independent of the analyte. In this study one universal MB probe was used to genotype three human polymorphic sites. This approach promises to reduce the cost of multiplex real-time assays and improve the accuracy of single-nucleotide polymorphism genotyping.

Rapid fixation of non-native alleles revealed by genome-wide SNP analysis of hybrid tiger salamanders

Background

Hybrid zones represent valuable opportunities to observe evolution in systems that are unusually dynamic and where the potential for the origin of novelty and rapid adaptation co-occur with the potential for dysfunction. Recently initiated hybrid zones are particularly exciting evolutionary experiments because ongoing natural selection on novel genetic combinations can be studied in ecological time. Moreover, when hybrid zones involve native and introduced species, complex genetic patterns present important challenges for conservation policy. To assess variation of admixture dynamics, we scored a large panel of markers in five wild hybrid populations formed when Barred Tiger Salamanders were introduced into the range of California Tiger Salamanders.

Results

At three of 64 markers, introduced alleles have largely displaced native alleles within the hybrid populations. Another marker (GNAT1) showed consistent heterozygote deficits in the wild, and this marker was associated with embryonic mortality in laboratory F2's. Other deviations from equilibrium expectations were idiosyncratic among breeding ponds, consistent with highly stochastic demographic effects.

Conclusion

While most markers retain native and introduced alleles in expected proportions, strong selection appears to be eliminating native alleles at a smaller set of loci. Such rapid fixation of alleles is detectable only in recently formed hybrid zones, though it might be representative of dynamics that frequently occur in nature. These results underscore the variable and mosaic nature of hybrid genomes and illustrate the potency of recombination and selection in promoting variable, and often unpredictable genetic outcomes. Introgression of a few, strongly selected introduced alleles should not necessarily affect the conservation status of California Tiger Salamanders, but suggests that genetically pure populations of this endangered species will be difficult to maintain.

The Template-Directed Dye-Incorporation Assay with Fluorescence Polarization Detection (FP-TDI)

INTRODUCTIONThe template-directed dye-incorporation assay with fluorescence polarization detection (FP-TDI) is an economical and robust SNP genotyping method that is easy to optimize and implement in both single-marker and moderate-throughput studies. It utilizes unlabeled, unpurified PCR-grade primers in a dideoxy chain-terminating DNA-sequencing reaction to ascertain the identity of one base immediately 3' to an unlabeled primer, which is annealed immediately upstream of the polymorphic site on the target DNA. The FP-TDI protocol consists of four key steps, all of which can be carried out in the same microtiter plate without further separation or purification. First, PCR amplification of genomic DNA produces the template for the primer extension reaction. Second, a PCR clean-up enzyme cocktail with pyrophosphatase is added directly into the PCR products to remove excess PCR primers, deoxynucleoside triphosphates, and inorganic pyrophosphates. Third, single-base primer extension is carried out using a SNP primer annealed to the target DNA one base upstream from the polymorphic site. Finally, the end product is scanned with an FP plate reader to determine the changes in fluorescence polarization. With the incorporation of pyrophosphatase in the protocol and the use of quenching to analyze the fluorescence data, up to 95% of the assays work well even without optimization.

Multiplex real-time single nucleotide polymorphism detection and quantification by quencher extension

Multiplex quencher extension (multiplex-QEXT) is a novel closed tube single-step method for detection and quantification of several single nucleotide polymorphisms (SNPs) simultaneously. The principle of multiplex-QEXT is that 5' reporter-labeled probes are 3' single-base-extended with TAMRA dideoxy nucleotides if the respective SNP alleles are present. TAMRA can serve as either an energy acceptor (quencher-based detection) or donor [fluorescence resonance energy transfer (FRET)-based detection] for a wide range of different reporter fluorochromes. The extension can therefore be recorded by the respective reporter fluorescence change. We evaluated multiplex-QEXT, analyzing four different SNP loci in the Listeria monocytogenes inlA gene. Probes labeled with the reporters 6-FAM, TET, VIC, and Alexa Fluor 594 were used. Responses for the fluorochromes 6-FAM, TET, and VIC were detected by quenching (decreased fluorescence), while the response for Alexa Fluor 594 was detected by FRET (increased fluorescence). We evaluated the SNP-allele pattern in 252 different L. monocytogenes strains. Multiplex-QEXT gave a good resolution, detecting seven major and five minor groups of L. monocytogenes. Comparison with serotyping showed that multiplex-QEXT gave better resolution. We also evaluated the quantitative aspects of multiplex-QEXT. Quantitative information was obtained for all the fluorochrome/probe combinations in the sample pools. The detection limits for 6-FAM, TET and Alexa Fluor 594 were the presence of the 10% target SNP alleles (P < 0.05), while the detection limit for VIC was the presence of the 5% target SNP alleles (P < 0.05). Currently, overlap in the fluorescence emission spectra is the limiting factor for the multiplexing potential of QEXT. With the emergence of new fluorochromes with narrow emission spectra, we foresee great potential for increasing the multiplex level in the future.

Aspects of the design and analysis of high-dimensional SNP studies for disease risk estimation

The state of readiness for high-dimensional single nucleotide polymorphism (SNP) epidemiologic association studies is described, as background for a discussion of statistical aspects of case-control study design and analysis. Specifically, the important role that multistage designs can play in the elimination of false-positive associations and in the control of study costs will be noted. Also, the trade-offs associated with using pooled DNA at early design stages for additional important cost reductions will be discussed in some detail. An odds ratio approach to relating SNP alleles to disease risk using pooled DNA will be proposed, in conjunction with a simple empirical variance estimator, based on comparisons among log-odds ratio estimators from distinct pairs of case and control pools. Simulation studies will be presented to evaluate the moderate sample size properties of such multistage designs and estimation procedures. The design of an ongoing three-stage study in the Women's Health Initiative to relate 250,000 SNPs to the risk of coronary heart disease, stroke, and breast cancer will provide illustration, and will be used to motivate the choice of simulation configurations.

Multiplex SNP typing by bioluminometric assay coupled with terminator incorporation (BATI)

A multiplex single-nucleotide polymorphism (SNP) typing platform using ‘bioluminometric assay coupled with terminator [2′,3′-dideoxynucleoside triphosphates (ddNTPs)] incorporation’ (named ‘BATI’ for short) was developed. All of the reactions are carried out in a single reaction chamber containing target DNAs, DNA polymerase, reagents necessary for converting PPi into ATP and reagents for luciferase reaction. Each of the four ddNTPs is dispensed into the reaction chamber in turn. PPi is released by a nucleotide incorporation reaction and is used to produce ATP when the ddNTP dispensed is complementary to the base in a template. The ATP is used in a luciferase reaction to release visible light. Only 1 nt is incorporated into a template at a time because ddNTPs do not have a 3′ hydroxyl group. This feature greatly simplifies a sequencing spectrum. The luminescence is proportional to the amount of template incorporated. Only one peak appears in the spectrum of a homozygote sample, and two peaks at the same intensity appear for a heterozygote sample. In comparison with pyrosequencing using dNTP, the spectrum obtained by BATI is very simple, and it is very easy to determine SNPs accurately from it. As only one base is extended at a time and the extension signals are quantitative, the observed spectrum pattern is uniquely determined even for a sample containing multiplex SNPs. We have successfully used BATI to type various samples containing plural target sequence areas. The measurements can be carried out with an inexpensive and small luminometer using a photodiode array as the detector. It takes only a few minutes to determine multiplex SNPs. These results indicate that this novel multiplexed approach can significantly decrease the cost of SNP typing and increase the typing throughput with an inexpensive and small luminometer.

Application of pooled genotyping to scan candidate regions for association with HDL cholesterol levels

Association studies are used to identify genetic determinants of complex human traits of medical interest. With the large number of validated single nucleotide polymorphisms (SNPs) currently available, two limiting factors in association studies are genotyping capability and costs. Pooled DNA genotyping has been proposed as an efficient means of screening SNPs for allele frequency differences in case-control studies and for prioritising them for subsequent individual genotyping analysis. Here, we apply quantitative pooled genotyping followed by individual genotyping and replication to identify associations with human serum high-density lipoprotein (HDL) cholesterol levels. The DNA from individuals with low and high HDL cholesterol levels was pooled separately, each pool was amplified by polymerase chain reaction in triplicate and each amplified product was separately hybridised to a high-density oligonucleotide array. Allele frequency differences between case and control groups with low and high HDL cholesterol levels were estimated for 7,283 SNPs distributed across 71 candidate gene regions spanning a total of 17.1 megabases. A novel method was developed to take advantage of independently derived haplotype map information to improve the pooled estimates of allele frequency differences. A subset of SNPs with the largest estimated allele frequency differences between low and high HDL cholesterol groups was chosen for individual genotyping in the study population, as well as in a separate replication population. Four SNPs in a single haplotype block within the cholesteryl ester transfer protein (CETP) gene interval were significantly associated with HDL cholesterol levels in both populations. Our study is among the first to demonstrate the application of pooled genotyping followed by confirmation with individual genotyping to identify genetic determinants of a complex trait.

Role of excess inorganic pyrophosphate in primer-extension genotyping assays

We have developed and genotyped >15,000 SNP assays by using a primer extension genotyping assay with fluorescence polarization (FP) detection. Although the 80% success rate of this assay is similar to those of other SNP genotyping assays, we wanted to determine the reasons for the failures and find ways to improve the assay. We observed that the failed assays fell into three general patterns: PCR failure, excess of heterozygous genotypes, and loss of FP signal for one of the dye labels. After analyzing several hundred failed assays, we concluded that 5% of the assays had PCR failure and had to be redesigned. We also discovered that the other two categories of failures were due to misincorporation of one of the dye-terminators during the primer extension reaction as a result of primer shortening with a reverse reaction involving inorganic pyrophosphate, and to the quenching of R110-terminator after its incorporation onto the SNP primer. The relatively slow incorporation of R110 acycloterminators by AcycloPol compounds the problem with the R110 label. In this report, we describe the source of the problems and simple ways to correct these problems by adding pyrophosphatase, using quenching as part of the analysis, and replacing R110 by Texas red as one of the dye labels. With this new protocol, we have achieved approximately 95% success rate in assay development without the need for optimization.

Real-time closed tube single nucleotide polymorphism (SNP) quantification in pooled samples by quencher extension (QEXT)

Quencher extension (QEXT) is a novel single step closed tube real-time method to quantify SNPs using reporters and quenchers in combination with primer extension. A probe with a 5'-reporter dye is single base extended with a dideoxy nucleotide containing a quencher dye if the target SNP allele is present. The extension is recorded from the quenching (reduced fluorescence) of the reporter dye. This avoids the influence of the unincorporated dye-labeled nucleotides, resulting in high accuracy and a high signal-to-noise ratio. The relative amount of a specific SNP allele is determined from the nucleotide incorporation rate in a thermo-cycling reaction. We tested the QEXT assay using five SNPs in the Listeria monocytogenes inlA gene as a model system. The presence of the target SNP alleles was determined with high statistical confidence (P < 0.0005). The quantitative detection limits were between 0 and 5% for the targeted SNP alleles on a background of other SNP alleles (P < 0.05). The QEXT method is directly adaptable to current real-time PCR equipment and is thus suited for high throughput and a wide application range.