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1.  Analysis of multiple single nucleotide polymorphisms closely positioned in the ovine PRNP gene using linear fluorescent probes and melting curve analysis 
Resistance and susceptibility to scrapie has been associated with single nucleotide polymorphisms located within codons 136, 154 and 171 of the ovine prion protein gene (PRNP). Dual-labelled HyBeacon probes were developed to analyse single and clustered polymorphisms within these and neighbouring codons.
Extracted DNAs and unpurified blood samples were genotyped with respect to polymorphisms in PRNP codons 136, 141, 154 and 171. PCR amplicons were investigated using a LightTyper instrument, measuring the stability of probe/target hybridisation through peak melting temperatures and determining the sequence of nucleotides at polymorphic sites.
The performance of HyBeacon assays was evaluated in a validation study comparing genotypes with those obtained using a primer extension assay (Sequenom MassEXTEND) analysed on a MALDI-ToF mass spectrometer. Over 12,000 sheep samples were successfully genotyped, reliably detecting A136, V136, T136, T137, L141, F141 R154, H154, L168, R171, Q171, H171 and K171 sequence variants using only 4 HyBeacon probes.
HyBeacon assays provide an extremely robust and accurate method for the analysis of single and clustered PRNP polymorphisms in a high-throughput format. The flexibility of the diagnostic tests ensures that samples are correctly genotyped even in the presence of additional sequence variations that flank the polymorphisms of interest. Such sequence variations may also be neutralised using universal bases such as 5-nitroindole if required.
PMCID: PMC1994165  PMID: 17683552
2.  A highly fluorescent DNA toolkit: synthesis and properties of oligonucleotides containing new Cy3, Cy5 and Cy3B monomers 
Nucleic Acids Research  2012;40(14):e108.
Cy3B is an extremely bright and stable fluorescent dye, which is only available for coupling to nucleic acids post-synthetically. This severely limits its use in the fields of genomics, biology and nanotechnology. We have optimized the synthesis of Cy3B, and for the first time produced a diverse range of Cy3B monomers for use in solid-phase oligonucleotide synthesis. This molecular toolkit includes phosphoramidite monomers with Cy3B linked to deoxyribose, to the 5-position of thymine, and to a hexynyl linker, in addition to an oligonucleotide synthesis resin in which Cy3B is linked to deoxyribose. These monomers have been used to incorporate single and multiple Cy3B units into oligonucleotides internally and at both termini. Cy3B Taqman probes, Scorpions and HyBeacons have been synthesized and used successfully in mutation detection, and a dual Cy3B Molecular Beacon was synthesized and found to be superior to the corresponding Cy3B/DABCYL Beacon. Attachment of Cy3, Cy3B and Cy5 to the 5-position of thymidine by an ethynyl linker enabled the synthesis of an oligonucleotide FRET system. The rigid linker between the dye and nucleobase minimizes dye–dye and dye–DNA interactions and reduces fluorescence quenching. These reagents open up new future applications of Cy3B, including more sensitive single-molecule and cell-imaging studies.
PMCID: PMC3413114  PMID: 22495935
3.  Use of DNA and Peptide Nucleic Acid Molecular Beacons for Detection and Quantification of rRNA in Solution and in Whole Cells 
DNA and peptide nucleic acid (PNA) molecular beacons were successfully used to detect rRNA in solution. In addition, PNA molecular beacon hybridizations were found to be useful for the quantification of rRNA: hybridization signals increased in a linear fashion with the 16S rRNA concentrations used in this experiment (between 0.39 and 25 nM) in the presence of 50 nM PNA MB. DNA and PNA molecular beacons were successfully used to detect whole cells in fluorescence in situ hybridization (FISH) experiments without a wash step. The FISH results with the PNA molecular beacons were superior to those with the DNA molecular beacons: the hybridization kinetics were much faster, the signal-to-noise ratio was much higher, and the specificity was much better for the PNA molecular beacons. Finally, it was demonstrated that the combination of the use of PNA molecular beacons in FISH and flow cytometry makes it possible to rapidly collect quantitative FISH data. Thus, PNA molecular beacons might provide a solution for limitations of traditional FISH methods, such as variable target site accessibility, poor sensitivity for target cells with low rRNA content, background fluorescence, and applications of FISH in microfluidic devices.
PMCID: PMC194960  PMID: 12957960
4.  Simultaneous detection of mRNA and protein stem cell markers in live cells 
BMC Biotechnology  2009;9:30.
Biological studies and medical application of stem cells often require the isolation of stem cells from a mixed cell population, including the detection of cancer stem cells in tumor tissue, and isolation of induced pluripotent stem cells after eliciting the expression of specific genes in adult cells. Here we report the detection of Oct-4 mRNA and SSEA-1 protein in live carcinoma stem cells using respectively molecular beacon and dye-labeled antibody, aiming to establish a new method for stem cells detection and isolation.
Quantification of Oct-4 mRNA and protein in P19 mouse carcinoma stem cells using respectively RT-PCR and immunocytochemistry confirmed that their levels drastically decreased after differentiation. To visualize Oct-4 mRNA in live stem cells, molecular beacons were designed, synthesized and validated, and the detection specificity was confirmed using control studies. We found that the fluorescence signal from Oct-4-targeting molecular beacons provides a clear discrimination between undifferentiated and retinoic acid-induced differentiated cells. Using deconvolution fluorescence microscopy, Oct-4 mRNAs were found to reside on one side of the cytosol. We demonstrated that, using a combination of Oct-4 mRNA-targeting molecular beacon with SSEA-1 antibody in flow cytometric analysis, undifferentiated stem cells can be clearly distinguished from differentiated cells. We revealed that Oct-4 targeting molecular beacons do not seem to affect stem cell biology.
Molecular beacons have the potential to provide a powerful tool for highly specific detection and isolation of stem cells, including cancer stem cells and induced pluripotent stem (iPS) cells without disturbing cell physiology. It is advantageous to perform simultaneous detection of intracellular (mRNA) and cell-surface (protein) stem cell markers in flow cytometric analysis, which may lead to high detection sensitivity and efficiency.
PMCID: PMC2682800  PMID: 19341452
5.  Higher blood volumes improve the sensitivity of direct PCR diagnosis of blood stream tuberculosis among HIV-positive patients: an observation study 
Blood stream tuberculosis (TB), caused by Mycobacterium tuberculosis (MTB) is common among HIV-positive patients, turning rapidly fatal unless detected and treated promptly. Blood culture is currently the standard test for the detection of MTB in whole blood but results take weeks; patients deteriorate markedly and often die before a diagnosis of blood stream TB is made. Rapid molecular tests on whole blood, with potential for same day diagnosis of blood stream TB usually show low sensitivity due to the problem of insufficient MTB DNA template when extraction is performed directly on low blood volumes. This study assessed the influence of blood volume on the sensitivity of a HyBeacon PCR assay-the FluoroType® MTB (Hain Lifescience, Nehren, Germany) on direct detection of MTB in whole blood.
Prospective recruitment of HIV-positive patients with clinical suspicion of blood stream TB but not on anti-TB or HIV drug treatment was done. Venous blood samples were collected and DNA extracted using the MolYsis (Molzym, Bremen, Germany) methods; for study A, from duplicate 1 ml (42 patients) and for study B (31 patients) from 9 ml EDTA blood samples. The FluoroType® MTB PCR assay targeting an IS6110 sequence was performed and results compared with blood culture.
The diagnostic sensitivity and specificity of the FluoroType® MTB PCR in study A was 33% and 97%, respectively. Corresponding values in study B were 71% and 96%, respectively. In both studies, one case each of blood culture-negative blood stream TB was detected with the FluoroType® MTB PCR assay. The median time to positivity of blood culture was 20.1 (range 12–32) for study A and 19.9 days (range 15–30) for study B.
Larger blood volumes (9 ml) improved and gave acceptable sensitivity of direct PCR diagnosis of blood stream TB.
PMCID: PMC4326319  PMID: 25656799
Tuberculosis; Blood stream; Direct; Real time; PCR Diagnosis
6.  1,4-linked 1,2,3-Triazole des-peptidic analogues of PNA (TzNA) 
Artificial DNA, PNA & XNA  2010;1(2):68-75.
1,2,3-triazole analogues of PNA (TzNA) in which the amide link in backbone is replaced by triazole ring is synthesized on solid phase by ‘click’ chemistry and such triazolothymine PNA chimeric oligomers are shown to significantly stabilize the derived PNA2:DNA triplexes. With increasing number of triazole units in the backbone, single stranded PNA oligomers exhibit enhanced self-ordering.
PMCID: PMC3116575  PMID: 21686241
PNA analogues; solid phase click chemistry; triazolyl oligimers; XNA; chimeric PNA-XNA; triazole PNA
7.  Label-free hybridization detection of a single nucleotide mismatch by immobilization of molecular beacons on an agarose film 
Nucleic Acids Research  2002;30(12):e61.
We developed a new technique to immobilize a set of molecular beacons on an agarose film-coated slide and found that it has the ability to identify a single nucleotide difference in label-free DNA targets. The annealing properties, specificity and hybridization dynamics of the present technique were compared with those of the conventional technique that directly immobilizes molecular beacons on a planar glass slide. It is demonstrated that the molecular beacon array on an agarose film has high quench efficiency, an excellent discrimination ratio for single nucleotide mismatches and a short detection time. We hypothesize that such a low fluorescence background and high specificity molecular beacon array will find practical applications in label-free, high-throughput mutation analysis and disease diagnosis.
PMCID: PMC117303  PMID: 12060699
8.  Rapid Identification of Candida dubliniensis Using a Species-Specific Molecular Beacon 
Journal of Clinical Microbiology  2000;38(8):2829-2836.
Candida dubliniensis is an opportunistic fungal pathogen that has been linked to oral candidiasis in AIDS patients, although it has recently been isolated from other body sites. DNA sequence analysis of the internal transcribed spacer 2 (ITS2) region of rRNA genes from reference Candida strains was used to develop molecular beacon probes for rapid, high-fidelity identification of C. dubliniensis as well as C. albicans. Molecular beacons are small nucleic acid hairpin probes that brightly fluoresce when they are bound to their targets and have a significant advantage over conventional nucleic acid probes because they exhibit a higher degree of specificity with better signal-to-noise ratios. When applied to an unknown collection of 23 strains that largely contained C. albicans and a smaller amount of C. dubliniensis, the species-specific probes were 100% accurate in identifying both species following PCR amplification of the ITS2 region. The results obtained with the molecular beacons were independently verified by random amplified polymorphic DNA analysis-based genotyping and by restriction enzyme analysis with enzymes BsmAI and NspBII, which cleave recognition sequences within the ITS2 regions of C. dubliniensis and C. albicans, respectively. Molecular beacons are promising new probes for the rapid detection of Candida species.
PMCID: PMC87122  PMID: 10921935
9.  Structure–function relationships of shared-stem and conventional molecular beacons 
Nucleic Acids Research  2002;30(19):4208-4215.
Molecular beacons are oligonucleotide probes capable of forming a stem–loop hairpin structure with a reporter dye at one end and a quencher at the other end. Conventional molecular beacons are designed with a target-binding domain flanked by two complementary short arm sequences that are independent of the target sequence. Here we report the design of shared-stem molecular beacons with one arm participating in both stem formation when the beacon is closed and target hybridization when it is open. We performed a systematic study to compare the behavior of conventional and shared-stem molecular beacons by conducting thermodynamic and kinetic analyses. Shared-stem molecular beacons form more stable duplexes with target molecules than conventional molecular beacons; however, conventional molecular beacons may discriminate between targets with a higher specificity. For both conventional and shared-stem molecular beacons, increasing stem length enhanced the ability to differentiate between wild-type and mutant targets over a wider range of temperatures. Interestingly, probe–target hybridization kinetics were similar for both classes of molecular beacons and were influenced primarily by the length and sequence of the stem. These findings should enable better design of molecular beacons for various applications.
PMCID: PMC140536  PMID: 12364599
10.  Detection of genetic variation using dual-labeled peptide nucleic acid (PNA) probe-based melting point analysis 
Thermal denaturation of probe-target hybrid is highly reproducible, and which makes probe melting point analysis reliable in the detection of mutations, polymorphisms and epigenetic differences in DNA. To improve resolution of these detections, we used dual-labeled (quencher and fluorescence), full base of peptide nucleic acid (PNA) probe for fluorescence probe based melting point analysis. Because of their uncharged nature and peptide bond-linked backbone, PNA probes have more favorable hybridization properties, which make a large difference in the melting temperature between specific hybridization and partial hybridization.
Here, we have shown that full base dual-labeled PNA is apt material for fluorescence probe-based melting point analysis with large difference in the melting temperature between full specific hybridization and that of partial hybridization, including insertion and deletion. In case of narrowly distributed mutations, PNA probe effectively detects three mutations in a single reaction tube with three probes. Moreover, we successfully diagnose virus analogues with amplification and melting temperature signal. Lastly, Melting temperature of PNA oligomer can be easily adjusted just by adding gamma-modified PNA probe.
The PNA probes offer advantage of improved flexibility in probe design, which could be used in various applications in mutation detection among a wide range of spectrums.
Electronic supplementary material
The online version of this article (doi:10.1186/s12575-015-0027-5) contains supplementary material, which is available to authorized users.
PMCID: PMC4632671  PMID: 26539063
Peptide Nucleic Acid (PNA); Fluorescence probe-based melting point analysis; Real-time PCR; Genotyping; Mutation; Probe hybridization; Multiplex
11.  Inhibition of Epithelial Mesenchymal Transition (EMT) With Immunochemogene Treatment in Metastatic Colorectal Cancer 
Epithelial to mesenchymal transition (EMT) causes resistance to epidermal growth factor receptor (EGFR) inhibitors. We used immunochemogene treatment composed of a stealth nanoparticle formulation, consisting of clamp PNA against mRNA of FOXC2, anti-CD44 chimeric MAb, and vinorelbine, in an attempt to eradicate metastatic colorectal cancer (mCRC) cells and inhibit metastasis by blocking EMT.
Tumor cells from patients with stage IV chemoresistant CRC characterized by upregulation of FOXC2, CD44, and bcl-2 were obtained surgically. We synthesized antisense clamp peptide nucleic acid (PNA) oligomers (DNA analogs), in which the 6 mer homopyrimidine triplex [(PNA)2/RNA)] hybridized to the 5-end (Leader), and the 10 mer purine/pyrimidine duplex (PNA/RNA) hybridized to the 3-end (Trailer) of the AUG start codon region on the mRNA of FOXC2. The uncharged and hydrophilic antisense clamp PNA anti-FOXC2 was incorporated in the polar phase, and the vinorelbine molecules were entrapped in the acyl-chains of the lipid phase. This was surrounded by the stealth/biocompatibility polymer layer and biological recognition layer with linked chimeric MAbs against CD44 of the nanoparticle formulation. This was used to treat xenograft animal models developed from CRC cells obtained from the stage IV patients. Tumor cells were analyzed with microarray, single-nucleotide polymorphism (SNP) assay, polymerase chain reaction (PCR), western blot (WB), Southern blot (SB), immunoblotting (LC-MS/MS), immunofluorescence staining, immunohistochemistry (IHC), fluorescent activated cell sorter (FACS), confocal microscopy, transmission electron microscopy (TEM), bromodeoxyuridine (BrdU), MTT, and flow cytometry.
Post-treatment, we observed downregulation of CD44 and Fra-2, and induction of antibody-dependent cellular cytotoxicity (ADCC). The clamp PNA inhibited translation of FOXC2, resulting in activation of Jak2/Stat5a genes, which led to suppression of EMT of cancer cells. This blocked CRC metastatic invasion by reversing the mesenchymal phenotype; reconstituted homotypic adhesion; and promoted differentiation in CRC cells. Undifferentiated epithelial cells undergoing EMT exhibited overexpression of FOXC2, and this expression was lost when these cells returned to their initial differentiated epithelial state, blocking invasion and metastasis. Inhibition of EMT downregulated EGFR and inactivated NF-kB, inhibiting its downstream signaling pathway. Epithelial cell junction proteins claudin 4, claudin 7, and E-cadherin were overexpressed, upregulating beta-catenin; while mesenchymal markers vimentin and fibronectin were downregulated. Downregulation of Twist, Snail, and transcription 3 and 5 blocked the migratory potential of tumor cells, inhibiting metastasis. Calcium-independent cell-cell adhesion molecules EpCAM and TROP2 were upregulated. Vinorelbine blocked tumor cells at G2/M cell cycle, and phosphorylated bcl-2. This circumvented resistance to anoikis, inducing apoptosis in tumor cells due to lack of adhesion, inhibiting invasion and metastasis. In addition to the induction of caspase-dependent apoptosis or programmed cell death (PCD) type I in tumor cells, bcl-2 downregulation caused release of beclin-1 and upregulation of bcl-2–interacting mediator of cell death (BIM), inducing type II PCD or autophagy. TEM exhibited bystander killing effect of tumor cells by adjacent cells, and activated phagocytic cells such as macrophages. DNA synthesis and metabolic activity of tumor cells were inhibited according to BrdU and MTT tests, respectively.
This immunochemogene treatment induced epithelial differentiation by reversing the mesenchymal phenotype, promoted homotypic adhesion, inhibited the multigene signature indicative of EMT, blocking metastatic cell motility/invasiveness, and eradicated mCRC cells resistant to EGFR inhibitors by induction of PCD type-I and type-II, apoptosis and autophagy, leading to a bystander killing effect.
PMCID: PMC3056306
12.  Tiny Molecular Beacons: LNA/2'-O-methyl RNA Chimeric Probes for Imaging Dynamic mRNA Processes in Living Cells 
ACS chemical biology  2012;7(9):1586-1595.
New approaches for imaging dynamic processes involving RNAs in living cells are continuously being developed and optimized. The use of molecular beacons synthesized from 2'-O-methylribonucleotides (which are resistant to cellular nucleases) is an established approach for visualizing native mRNAs in real time. In order to spatially and temporally resolve dynamic steps involving RNA in cells, molecular beacons need to efficiently hybridize to their RNA targets. To expand the repertoire of target sites accessible to molecular beacons, we decreased the length of their probe sequences and altered their backbone by the inclusion of LNA (locked nucleic acid) nucleotides. We named these new LNA/2'-O-methyl RNA chimera oligonucleotides, “tiny molecular beacons”. We analyzed these tiny molecular beacons and found that the incorporation of just a few LNA nucleotides enables these shorter probes to stably anneal to more structured regions of the RNA than is possible with conventional molecular beacons. The ease of synthesis of tiny molecular beacons, and the flexibility to couple them to a large variety of fluorophores and quenchers, renders them optimal for the detection of less abundant and/or highly structured RNAs. To determine their efficiency to detect endogenous mRNAs in live specimens, we designed tiny molecular beacons that were specific for oskar mRNA and microinjected them into living Drosophila melanogaster oocytes. We then imaged the live oocytes via spinning disc confocal microscopy. The results demonstrate that tiny molecular beacons hybridize to target mRNA at faster rates than classically designed molecular beacons, and are able to access previously inaccessible target regions.
PMCID: PMC3448858  PMID: 22738327
13.  Peptide-linked molecular beacons for efficient delivery and rapid mRNA detection in living cells 
Nucleic Acids Research  2004;32(6):e58.
Real-time visualization of specific endogenous mRNA expression in vivo has the potential to revolutionize medical diagnosis, drug discovery, developmental and molecular biology. However, conventional liposome- or dendrimer-based cellular delivery of molecular probes is inefficient, slow, and often detrimental to the probes. Here we demonstrate the rapid and sensitive detection of RNA in living cells using peptide-linked molecular beacons that possess self-delivery, targeting and reporting functions. We conjugated the TAT peptide to molecular beacons using three different linkages and demonstrated that, at relatively low concentrations, these molecular beacon constructs were internalized into living cells within 30 min with nearly 100% efficiency. Further, peptide-based delivery did not interfere with either specific targeting by or hybridization-induced fluorescence of the probes. We could therefore detect human GAPDH and survivin mRNAs in living cells fluorescently, revealing intriguing intracellular localization patterns of mRNA. We clearly demonstrated that cellular delivery of molecular beacons using the peptide-based approach has far better performance compared with conventional transfection methods. The peptide-linked molecular beacons approach promises to open new and exciting opportunities in sensitive gene detection and quantification in vivo.
PMCID: PMC390380  PMID: 15084673
14.  A solid-phase CuAAC strategy for the synthesis of PNA containing nucleobase surrogates 
Artificial DNA, PNA & XNA  2013;4(1):4-10.
The synthesis of an azide containing PNA monomer is described. The monomer was incorporated into two PNA sequences for the purpose of synthesizing an intercalating fluorophore-labeled PNA and a metal binding hairpin using a solid phase copper catalyzed azide-alkyne Huisgen cycloaddition (CuAAC). Click chemistry was performed using 2-ethynylfluorene or 1-ethynylpyrene to add a fluorophore to the PNA, which were tested for their ability to recognize an abasic site on a DNA target. A PNA hairpin possessing azide monomers at each termini was synthesized and reacted with 2-ethynylpyridine to form a hairpin that is stabilized by Ni2+.
PMCID: PMC3654728  PMID: 23422048
click chemistry; pyrene; fluorene; metal-binding; hairpin; on-resin; Huisgen cycloaddition
15.  Development of Peptide Nucleic Acid Probes for Detection of the HER2 Oncogene 
PLoS ONE  2013;8(4):e58870.
Peptide nucleic acids (PNAs) have gained much interest as molecular recognition tools in biology, medicine and chemistry. This is due to high hybridization efficiency to complimentary oligonucleotides and stability of the duplexes with RNA or DNA. We have synthesized 15/16-mer PNA probes to detect the HER2 mRNA. The performance of these probes to detect the HER2 target was evaluated by fluorescence imaging and fluorescence bead assays. The PNA probes have sufficiently discriminated between the wild type HER2 target and the mutant target with single base mismatches. Furthermore, the probes exhibited excellent linear concentration dependence between 0.4 to 400 fmol for the target gene. The results demonstrate potential application of PNAs as diagnostic probes with high specificity for quantitative measurements of amplifications or over-expressions of oncogenes.
PMCID: PMC3622650  PMID: 23593123
16.  From Mechanism to Mouse: A Tale of Two Bioorthogonal Reactions 
Accounts of Chemical Research  2011;44(9):666-676.
Bioorthogonal reactions are chemical reactions that neither interact with nor interfere with a biological system. The participating functional groups must be inert to biological moieties, must selectively reactive with each other under biocompatible conditions, and, for in vivo applications, must be nontoxic to cells and organisms. Additionally, it is helpful if one reactive group is small and therefore minimally perturbing of a biomolecule into which it has been introduced either chemically or biosynthetically. Examples from the past decade suggest that a promising strategy for bioorthogonal reaction development begins with an analysis of functional group and reactivity space outside those defined by Nature. Issues such as stability of reactants and products (particularly in water), kinetics, and unwanted side reactivity with biofunctionalities must be addressed, ideally guided by detailed mechanistic studies. Finally, the reaction must be tested in a variety of environments, escalating from aqueous media to biomolecule solutions to cultured cells and, for the most optimized transformations, to live organisms.
Work in our laboratory led to the development of two bioorthogonal transformations that exploit the azide as a small, abiotic, and bioinert reaction partner: the Staudinger ligation and strain-promoted azide–alkyne cycloaddition. The Staudinger ligation is based on the classic Staudinger reduction of azides with triarylphosphines first reported in 1919. In the ligation reaction, the intermediate aza-ylide undergoes intramolecular reaction with an ester, forming an amide bond faster than aza-ylide hydrolysis would otherwise occur in water. The Staudinger ligation is highly selective and reliably forms its product in environs as demanding as live mice. However, the Staudinger ligation has some liabilities, such as the propensity of phosphine reagents to undergo air oxidation and the relatively slow kinetics of the reaction.
The Staudinger ligation takes advantage of the electrophilicity of the azide; however, the azide can also participate in cycloaddition reactions. In 1961, Wittig and Krebs noted that the strained, cyclic alkyne cyclooctyne reacts violently when combined neat with phenyl azide, forming a triazole product by 1,3-dipolar cycloaddition. This observation stood in stark contrast to the slow kinetics associated with 1,3-dipolar cycloaddition of azides with unstrained, linear alkynes, the conventional Huisgen process. Notably, the reaction of azides with terminal alkynes can be accelerated dramatically by copper catalysis (this highly popular Cu-catalyzed azide–alkyne cycloaddition (CuAAC) is a quintessential “click” reaction). However, the copper catalysts are too cytotoxic for long-term exposure with live cells or organisms. Thus, for applications of bioorthogonal chemistry in living systems, we built upon Wittig and Krebs’ observation with the design of cyclooctyne reagents that react rapidly and selectively with biomolecule-associated azides. This strain-promoted azide–alkyne cycloaddition is often referred to as “Cu-free click chemistry”. Mechanistic and theoretical studies inspired the design of a series of cyclooctyne compounds bearing fluorine substituents, fused rings, and judiciously situated heteroatoms, with the goals of optimizing azide cycloaddition kinetics, stability, solubility, and pharmacokinetic properties. Cyclooctyne reagents have now been used for labeling azide-modified biomolecules on cultured cells and in live Caenorhabditis elegans, zebrafish, and mice.
As this special issue testifies, the field of bioorthogonal chemistry is firmly established as a challenging frontier of reaction methodology and an important new instrument for biological discovery. The above reactions, as well as several newcomers with bioorthogonal attributes, have enabled the high-precision chemical modification of biomolecules in vitro, as well as real-time visualization of molecules and processes in cells and live organisms. The consequence is an impressive body of new knowledge and technology, amassed using a relatively small bioorthogonal reaction compendium. Expansion of this toolkit, an effort that is already well underway, is an important objective for chemists and biologists alike.
PMCID: PMC3184615  PMID: 21838330
17.  Fluorescence detection of single nucleotide polymorphisms using a universal molecular beacon 
Nucleic Acids Research  2008;36(19):e123.
We present a simple and novel assay—employing a universal molecular beacon (MB) in the presence of Hg2+—for the detection of single nucleotide polymorphisms (SNPs) based on Hg2+–DNA complexes inducing a conformational change in the MB. The MB (T7-MB) contains a 19-mer loop and a stem of a pair of seven thymidine (T) bases, a carboxyfluorescein (FAM) unit at the 5′-end, and a 4-([4-(dimethylamino)phenyl]azo)benzoic acid (DABCYL) unit at the 3′-end. Upon formation of Hg2+–T7-MB complexes through T–Hg2+–T bonding, the conformation of T7-MB changes from a random coil to a folded structure, leading to a decreased distance between the FAM and DABCYL units and, hence, increased efficiency of fluorescence resonance energy transfer (FRET) between the FAM and DABCYL units, resulting in decreased fluorescence intensity of the MB. In the presence of complementary DNA, double-stranded DNA complexes form (instead of the Hg2+–T7-MB complexes), with FRET between the FAM and DABCYL units occurring to a lesser extent than in the folded structure. Under the optimal conditions (20 nM T7-MB, 20 mM NaCl, 1.0 μM Hg2+, 5.0 mM phosphate buffer solution, pH 7.4), the linear plot of the fluorescence intensity against the concentration of perfectly matched DNA was linear over the range 2–30 nM (R2 = 0.991), with a limit of detection of 0.5 nM at a signal-to-noise ratio of 3. This new probe provides higher selectivity toward DNA than that exhibited by conventional MBs.
PMCID: PMC2577337  PMID: 18753149
18.  Subnanomolar antisense activity of phosphonate-peptide nucleic acid (PNA) conjugates delivered by cationic lipids to HeLa cells 
Nucleic Acids Research  2008;36(13):4424-4432.
In the search of facile and efficient methods for cellular delivery of peptide nucleic acids (PNA), we have synthesized PNAs conjugated to oligophosphonates via phosphonate glutamine and bis-phosphonate lysine amino acid derivatives thereby introducing up to twelve phosphonate moieties into a PNA oligomer. This modification of the PNA does not interfere with the nucleic acid target binding affinity based on thermal stability of the PNA/RNA duplexes. When delivered to cultured HeLa pLuc705 cells by Lipofectamine, the PNAs showed dose-dependent nuclear antisense activity in the nanomolar range as inferred from induced luciferase activity as a consequence of pre-mRNA splicing correction by the antisense-PNA. Antisense activity depended on the number of phosphonate moieties and the most potent hexa-bis-phosphonate-PNA showed at least 20-fold higher activity than that of an optimized PNA/DNA hetero-duplex. These results indicate that conjugation of phosphonate moieties to the PNA can dramatically improve cellular delivery mediated by cationic lipids without affecting on the binding affinity and sequence discrimination ability, exhibiting EC50 values down to one nanomolar. Thus the intracellular efficacy of PNA oligomers rival that of siRNA and the results therefore emphasize that provided sufficient in vivo bioavailability of PNA can be achieved these molecules may be developed into potent gene therapeutic drugs.
PMCID: PMC2490735  PMID: 18596083
19.  Dual Peptide Nucleic Acid- and Peptide-functionalized Shell Crosslinked Nanoparticles Designed to Target mRNA toward the Diagnosis and Treatment of Acute Lung Injury 
Bioconjugate Chemistry  2012;23(3):574-585.
In this work, multi-functional bio-synthetic hybrid nanostructures were prepared and studied for their potential utility in the recognition and inhibition of mRNA sequences for inducible nitric oxide synthase (iNOS), which are overexpressed at sites of inflammation, such as in cases of acute lung injury. Shell crosslinked knedel-like polymer nanoparticles (SCKs) that present peptide nucleic acids, for binding to complementary mRNAs, and cell penetrating peptides (CPPs), to gain cell entry, along with fluorescent labels and sites for radiolabeling, were prepared by a series of robust, efficient and versatile synthetic steps that proceeded from monomers to polymers to functional nanoparticles. Amphiphilic block graft copolymers having combinations of methoxy- and thioacetyl-terminated poly(ethylene glycol) (PEG) and DOTA-lysine units grafted from the backbone of poly(acrylic acid) (PAA) and extending with a backbone segment of poly(octadecyl acrylate-co-decyl acrylate) (P(ODA-co-DA)) were prepared by a combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and chemical modification reactions, which were then used as the building blocks for the formation of well-defined SCKs decorated with reactive thiols accessible to the surface. Fluorescent labeling with Alexa Fluor 633 hydrazide was then accomplished by amidation with residual acrylic acid residues within the SCK shells. Finally, the PNAs and CPP units were covalently conjugated to the SCKs via Michael addition of thiols on the SCKs to maleimide units on the termini of PNAs and CPPs. Confirmation of the ability of the PNAs to bind selectively to the target iNOS mRNAs when tethered to the SCK nanoparticles was determined by in vitro competition experiments. When attached to the SCKs having a hydrodynamic diameter of 60 ± 16 nm, the Kd values of the PNAs were ca. an order of magnitude greater than the free PNAs, while the mismatched PNA showed no significant binding.
PMCID: PMC3321742  PMID: 22372643
20.  Enzymatic signal amplification of molecular beacons for sensitive DNA detection 
Nucleic Acids Research  2008;36(6):e36.
Molecular beacons represent a new family of fluorescent probes for nucleic acids, and have found broad applications in recent years due to their unique advantages over traditional probes. Detection of nucleic acids using molecular beacons has been based on hybridization between target molecules and molecular beacons in a 1:1 stoichiometric ratio. The stoichiometric hybridization, however, puts an intrinsic limitation on detection sensitivity, because one target molecule converts only one beacon molecule to its fluorescent form. To increase the detection sensitivity, a conventional strategy has been target amplification through polymerase chain reaction. Instead of target amplification, here we introduce a scheme of signal amplification, nicking enzyme signal amplification, to increase the detection sensitivity of molecular beacons. The mechanism of the signal amplification lies in target-dependent cleavage of molecular beacons by a DNA nicking enzyme, through which one target DNA can open many beacon molecules, giving rise to amplification of fluorescent signal. Our results indicate that one target DNA leads to cleavage of hundreds of beacon molecules, increasing detection sensitivity by nearly three orders of magnitude. We designed two versions of signal amplification. The basic version, though simple, requires that nicking enzyme recognition sequence be present in the target DNA. The extended version allows detection of target of any sequence by incorporating rolling circle amplification. Moreover, the extended version provides one additional level of signal amplification, bringing the detection limit down to tens of femtomolar, nearly five orders of magnitude lower than that of conventional hybridization assay.
PMCID: PMC2346604  PMID: 18304948
21.  Improved Fluorescent In Situ Hybridization Method for Detection of Bacteria from Activated Sludge and River Water by Using DNA Molecular Beacons and Flow Cytometry▿  
Fluorescent in situ hybridization (FISH) remains a key technique in microbial ecology. Molecular beacons (MBs) are self-reporting probes that have potential advantages over linear probes for FISH. MB-FISH strategies have been described using both DNA-based and peptide nucleic acid (PNA)-based approaches. Although recent reports have suggested that PNA MBs are superior, DNA MBs have some advantages, most notably cost. The data presented here demonstrate that DNA MBs are suitable for at least some FISH applications in complex samples, providing superior discriminatory power compared to that of corresponding linear DNA-FISH probes. The use of DNA MBs for flow cytometric detection of Pseudomonas putida resulted in approximately double the signal-to-noise ratio of standard linear DNA probes when using laboratory-grown cultures and yielded improved discrimination of target cells in spiked environmental samples, without a need for separate washing steps. DNA MBs were also effective for the detection and cell sorting of both spiked and indigenous P. putida from activated sludge and river water samples. The use of DNA MB-FISH presents another increase in sensitivity, allowing the detection of bacteria in environmental samples without the expense of PNA MBs or multilaser flow cytometry.
PMCID: PMC1828804  PMID: 17277208
22.  Genotyping Single Nucleotide Polymorphisms Using Different Molecular Beacon Multiplexed within a Suspended Core Optical Fiber 
Sensors (Basel, Switzerland)  2014;14(8):14488-14499.
We report a novel approach to genotyping single nucleotide polymorphisms (SNPs) using molecular beacons in conjunction with a suspended core optical fiber (SCF). Target DNA sequences corresponding to the wild- or mutant-type have been accurately recognized by immobilizing two different molecular beacons on the core of a SCF. The two molecular beacons differ by one base in the loop-probe and utilize different fluorescent indicators. Single-color fluorescence enhancement was obtained when the immobilized SCFs were filled with a solution containing either wild-type or mutant-type sequence (homozygous sample), while filling the immobilized SCF with solution containing both wild- and mutant-type sequences resulted in dual-color fluorescence enhancement, indicating a heterozygous sample. The genotyping was realized amplification-free and with ultra low-volume for the required DNA solution (nano-liter). This is, to our knowledge, the first genotyping device based on the combination of optical fiber and molecular beacons.
PMCID: PMC4179081  PMID: 25111240
genotyping; suspended core optical fiber; molecular beacon; multiplexing; DNA detection
23.  A Fluorescence Light-Up Ag Nanocluster Probe that Discriminates Single-Nucleotide Variants by Emission Color 
Journal of the American Chemical Society  2012;134(28):11550-11558.
Rapid and precise screening of small genetic variations, such as single-nucleotide polymorphisms (SNPs), among an individual’s genome is still an unmet challenge at point-of-care settings. One crucial step towards this goal is the development of discrimination probes that require no enzymatic reaction and are easy to use. Here we report a new type of fluorescent molecular probe, termed a chameleon NanoCluster Beacon (cNCB), that lights up into different colors upon binding SNP targets. NanoCluster Beacons (NCBs) are collections of a small number of Ag atoms templated on single-stranded DNA that fluoresce strongly when placed in proximity to particular DNA sequences, termed enhancers. Here we show the fluorescence emission color of a NCB can change substantially (a shift of 60–70 nm in the emission maximum) depending upon the alignment between the silver nanocluster and the DNA enhancer sequence. Chameleon NCBs exploit this color shift to directly detect SNPs, based on the fact that different SNPs produce a different alignment between the Ag nanocluster and the enhancer. This SNP detection method has been validated on all single-nucleotide substitution scenarios in three synthetic DNA targets, in six disease-related SNP targets, and in two clinical samples taken from patients with ovarian serous borderline tumors. Samples with single-nucleotide variations can be easily identified by the naked eye under UV excitation, making this method a reliable and low-cost assay with a simple readout format.
PMCID: PMC3593641  PMID: 22775452
24.  Fluoresceinated Peanut Agglutinin (PNA) is a Marker for Live O2 Sensing Glomus Cells in Rat Carotid Body 
Experiments using live dissociated carotid body (CB) cells for patch clamping, [Ca++]i or other measurements require positive identification of the cell being recorded. At present, cell morphology is usually employed, but several cell types within the carotid body evidence similar morphologic characteristics. Therefore, we sought to develop a method utilizing a vital dye to identify glomus cells before and during experiments that require live cells, such as patch clamp studies. It was previously reported that the binding sites for peanut agglutinin (PNA) were highly expressed by all neuroendocrine-derivatives of the sympathoadrenal neural crest, including glomus cells, small, intensely fluorescent cells, PC-12 cells, and adrenal chromaffin cells in situ (Katz et al. 1995). By utilizing the binding characteristics of galactose-specific lectin peanut agglutinin (PNA) on the outer cell membrane, we tested the possibility that the fluoresceinated PNA may preferentially bind to CB glomus cells. The results to date show: (1) Rhodamine tagged PNA (Rhod-PNA) binds to the live dissociated glomus cells in less than one hour incubation and can be visualized in superfused cells; (2) Rhod-PNA labeled cells are perfectly matched with tyrosine hydroxylase (TH) positive glomus cells; (3) Rhod-PNA did not interfere with Fura-2 for Ca++ imaging; (4) Rhod-PNA bound to glomus cells in [Ca++]i studies does not affect O2 response of glomus cells. Thus fluoresceinated PNA may be a useful marker for live CB glomus studies, without adversely affecting their physiologic response.
PMCID: PMC2958663  PMID: 19536480
PNA; Peanut agglutinin; Lectin; CB; Glomus cells; Marker; O2 sensing; TH; Ca++ imaging; Double immunostaining
25.  Incorporation of thio-pseudoisocytosine into triplex-forming peptide nucleic acids for enhanced recognition of RNA duplexes 
Nucleic Acids Research  2014;42(6):4008-4018.
Peptide nucleic acids (PNAs) have been developed for applications in biotechnology and therapeutics. There is great potential in the development of chemically modified PNAs or other triplex-forming ligands that selectively bind to RNA duplexes, but not single-stranded regions, at near-physiological conditions. Here, we report on a convenient synthesis route to a modified PNA monomer, thio-pseudoisocytosine (L), and binding studies of PNAs incorporating the monomer L. Thermal melting and gel electrophoresis studies reveal that L-incorporated 8-mer PNAs have superior affinity and specificity in recognizing the duplex region of a model RNA hairpin to form a pyrimidine motif major-groove RNA2–PNA triplex, without appreciable binding to single-stranded regions to form an RNA–PNA duplex or, via strand invasion, forming an RNA–PNA2 triplex at near-physiological buffer condition. In addition, an L-incorporated 8-mer PNA shows essentially no binding to single-stranded or double-stranded DNA. Furthermore, an L-modified 6-mer PNA, but not pseudoisocytosine (J) modified or unmodified PNA, binds to the HIV-1 programmed −1 ribosomal frameshift stimulatory RNA hairpin at near-physiological buffer conditions. The stabilization of an RNA2–PNA triplex by L modification is facilitated by enhanced van der Waals contacts, base stacking, hydrogen bonding and reduced dehydration energy. The destabilization of RNA–PNA and DNA–PNA duplexes by L modification is due to the steric clash and loss of two hydrogen bonds in a Watson–Crick-like G–L pair. An RNA2–PNA triplex is significantly more stable than a DNA2–PNA triplex, probably because the RNA duplex major groove provides geometry compatibility and favorable backbone–backbone interactions with PNA. Thus, L-modified triplex-forming PNAs may be utilized for sequence-specifically targeting duplex regions in RNAs for biological and therapeutic applications.
PMCID: PMC3973316  PMID: 24423869

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