Publications by authors named "Walt Mahoney"

17 Publications

  • Page 1 of 1

Sensitive and specific assay for the simultaneous detection of Mycoplasma genitalium and macrolide resistance-associated mutations.

Eur J Clin Microbiol Infect Dis 2018 Nov 1;37(11):2137-2144. Epub 2018 Oct 1.

Department of Medical Microbiology, University Medical Centre Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.

Patients infected by Mycoplasma genitalium are often treated empirically with the macrolide azithromycin. Macrolide resistance is becoming quite common; empirical treatment is compromised. Sequencing was initially used to detected azithromycin resistance-associated mutations. As this was laborious, qPCRs have been developed for their detection. In the present study, we describe a fast, sensitive, and specific qPCR assay that enables routine testing of M. genitalium and macrolide resistance-associated mutations in a single assay. M. genitalium positive clinical samples were used to compare (i) the commonly used MgPa assay for the detection of M. genitalium infections (MgPa qPCR), (ii) a combined 23S rRNA gene PCR/sequencing assay (Mg23S qPCR/Sequencing) to identify macrolide resistance-associated mutations, and (iii) our newly developed probe-based melt curve qPCR for simultaneous detection of M. genitalium and macrolide resistance-associated mutations (Macrolide-R/MG ELITe MGB Kit, Elitech Bothel USA in short Mg Macrolide qPCR). Specificity of the qPCR was tested using urogenital samples that were tested positive for a range of other micro-organisms. M. genitalium was detected in 196/236 (83.1%) samples by the MgPa qPCR, versus 172/236 (72.9%) by the combined Mg23S qPCR/Sequencing, and 202/236 (85.6%) by the Mg Macrolide qPCR. The Mg Macrolide qPCR showed high concordance to the Mg23S qPCR/Sequencing assay (201 vs 202 could be genotyped, respectively) for the detection of the macrolide resistant mutations. None of the other urogenital pathogens were tested positive in the Mg Macrolide qPCR, indicating specificity. The Mg Macrolide qPCR is fast, sensitive, specific, and can easily be implemented in the routine diagnostics.
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http://dx.doi.org/10.1007/s10096-018-3350-3DOI Listing
November 2018

Hepatitis E Virus (HEV) Detection and Quantification by a Real-Time Reverse Transcription-PCR Assay Calibrated to the World Health Organization Standard for HEV RNA.

J Clin Microbiol 2017 05 22;55(5):1478-1487. Epub 2017 Feb 22.

Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, Minnesota, USA

Hepatitis E virus (HEV) has emerged as a cause of chronic hepatitis among immunocompromised patients. Molecular assays have become important tools for the diagnosis and management of these chronically infected patients. A real-time reverse transcription-quantitative PCR (RT-qPCR) assay utilizing Pleiades probe chemistry and an RNA internal control for the simultaneous detection and quantification of HEV RNA in human serum was developed based on an adaptation of a previously described and broadly reactive primer set targeting the overlapping open reading frame 2/3 (ORF2/3) nucleotide sequence of HEV. A chimeric bovine viral diarrhea virus construct containing an HEV RNA insert (SynTura HEV) was developed, value assigned with the first World Health Organization (WHO) international standard for HEV RNA (code 6329/10), and used to prepare working assay calibrators and controls, which supported an assay quantification range of 100 to 5,000,000 IU/ml. The analytical sensitivity (95% detection rate) of this assay was 25.2 IU/ml (95% confidence interval [CI], 19.2 to 44.1 IU/ml). The assay successfully amplified 16 different HEV sequences with significant nucleotide mismatching in primer/probe binding regions, while evaluation of a WHO international reference panel for HEV genotypes (code 8578/13) showed viral load results falling within the result ranges generated by WHO collaborative study participants for all panel members (genotypes 1 to 4). Broadly reactive RT-qPCR primers targeting HEV ORF2/3 were successfully adapted for use in an assay based on Pleiades probe chemistry. The availability of secondary standards calibrated to the WHO HEV international standard can improve the standardization and performance of assays for the detection and quantification of HEV RNA.
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http://dx.doi.org/10.1128/JCM.02334-16DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5405265PMC
May 2017

A rapid, instrument-free, sample-to-result nucleic acid amplification test.

Lab Chip 2016 10 23;16(19):3777-87. Epub 2016 Aug 23.

Department of Bioengineering, University of Washington, Seattle, USA.

The prototype demonstrated here is the first fully integrated sample-to-result diagnostic platform for performing nucleic acid amplification tests that requires no permanent instrument or manual sample processing. The multiplexable autonomous disposable nucleic acid amplification test (MAD NAAT) is based on two-dimensional paper networks, which enable sensitive chemical detection normally reserved for laboratories to be carried out anywhere by untrained users. All reagents are stored dry in the disposable test device and are rehydrated by stored buffer. The paper network is physically multiplexed to allow independent isothermal amplification of multiple targets; each amplification reaction is also chemically multiplexed with an internal amplification control. The total test time is less than one hour. The MAD NAAT prototype was used to characterize a set of human nasal swab specimens pre-screened for methicillin-resistant Staphylococcus aureus (MRSA) bacteria. With qPCR as the quantitative reference method, the lowest input copy number in the range where the MAD NAAT prototype consistently detected MRSA in these specimens was ∼5 × 10(3) genomic copies (∼600 genomic copies per biplexed amplification reaction).
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http://dx.doi.org/10.1039/c6lc00677aDOI Listing
October 2016

Isothermal strand displacement amplification (iSDA): a rapid and sensitive method of nucleic acid amplification for point-of-care diagnosis.

Analyst 2015 Nov;140(22):7540-9

Department of Bioengineering, University of Washington, Seattle, WA 98195-5061, USA.

We present a method of rapid isothermal amplification of DNA without initial heat denaturation of the template, and methods and probes for (a) real-time fluorescence detection and (b) lateral flow detection of amplicons. Isothermal strand displacement amplification (iSDA) can achieve >10(9)-fold amplification of the target sequence in <20 minutes at 49 °C, which makes it one of the fastest existing isothermal DNA amplification methods. iSDA initiates at sites where DNA base pairs spontaneously open or transiently convert into Hoogsteen pairs, i.e. "breathe", and proceeds to exponential amplification by repeated nicking, extension, and displacement of single strands. We demonstrate successful iSDA amplification and lateral flow detection of 10 copies of a Staphylococcus aureus gene, NO.-inducible l-lactate dehydrogenase (ldh1) (Richardson, Libby, and Fang, Science, 2008, 319, 1672-1676), in a clean sample and 50 copies in the presence of high concentrations of genomic DNA and mucins in <30 minutes. We also present a simple kinetic model of iSDA that incorporates competition between target and primer-dimer amplification. This is the first model that quantitates the effects of primer-dimer products in isothermal amplification reactions. Finally, we demonstrate the multiplexing capability of iSDA by the simultaneous amplification of the target gene and an engineered internal control sequence. The speed, sensitivity, and specificity of iSDA make it a powerful method for point-of-care molecular diagnosis.
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http://dx.doi.org/10.1039/c5an01632kDOI Listing
November 2015

Malaria prevalence defined by microscopy, antigen detection, DNA amplification and total nucleic acid amplification in a malaria-endemic region during the peak malaria transmission season.

Trop Med Int Health 2011 Jul 29;16(7):786-93. Epub 2011 Mar 29.

Walter Reed Project, Kenya Medical Research Institute, Kisumu, Kenya.

Objectives: To determine the malaria prevalence by microscopy, antigen detection and nucleic acid detection in a defined subpopulation in a Plasmodium falciparum-endemic region during the peak transmission season.

Methods: Blood specimens were collected in a cross-sectional study involving children aged 5-10 years (n = 195) presenting with acute fever to two clinics in Western Kenya. All specimens underwent microscopy, HRP2 and aldolase antigen detection by enzyme immunoassay (EIA), parasite-specific DNA and total nucleic acid (RNA and DNA) by real-time PCR (qPCR) and reverse-transcriptase PCR (qRT-PCR).

Results: Microscopy detected 65/195 cases of malaria infection [95% confidence interval (CI) 52-78]. HRP2 and aldolase EIA had similar sensitivity levels detecting antigen in 65/195 (95% CI, 52-78) and 57/195 (95% CI, 45-70) cases. Discordants in antigen detection vs. microscopy occurred at <470 parasites/μl and <4900 parasites/μl for HRP2 and aldolase, respectively. Detection of total nucleic acid allowed a 3 log lower limit of detection than just DNA detection by real-time PCR in vitro. In clinical specimens, 114/195 (95% CI, 100-127) were qPCR positive (DNA), and 187/195 (95% CI, 179-191) were qRT-PCR positive (DNA plus RNA).

Conclusions: The prevalence of submicroscopic malaria infection was significantly higher when detecting total nucleic acid than just DNA in this outpatient population during the high transmission season. Defining standards for submicroscopic infection will be important for control programmes, diagnostics development efforts and molecular epidemiology studies.
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http://dx.doi.org/10.1111/j.1365-3156.2011.02773.xDOI Listing
July 2011

Development of a multiplex real-time RT-PCR assay for detection of influenza A, influenza B, RSV and typing of the 2009-H1N1 influenza virus.

J Virol Methods 2010 Aug 31;167(2):113-8. Epub 2010 Mar 31.

Associated Regional and University Pathologists (ARUP) Institute for Clinical and Experimental Pathology, 500 Chipeta Way, Salt Lake City, UT 84108, United States.

A high-throughput real-time RT-PCR assay was developed to amplify and detect a conserved region of the hemagglutinin gene of the 2009-H1N1 influenza A virus using a minor groove binder-conjugated hybridization probe. The assay was paired with a separate triplex real-time assay that detects influenza A via the matrix gene, influenza B and RSV in a multiplex format and compared with the Centers for Disease Control and Prevention (CDC) rRT-PCR assay using 143 samples. The 2009-H1N1 portion of the multiplex assay had 100% correlation with the CDC assay, while the triplex assay had a 99% agreement. An additional 105 samples collected from October to November 2009 were also tested using both the individual 2009-H1N1 and triplex assays. Of these 105 samples, eight were positive for the hemagglutinin target in the H1N1 assay and negative for the matrix target in the triplex assay. Discrepant analysis revealed single nucleotide polymorphisms within the matrix gene of 2009-H1N1 virus-positive samples. The limit of detection for the 2009-H1N1 assay was between 750 and 1,500 copies/reaction and no cross-reactivity with other respiratory pathogens was observed. Overall, this multiplexed format proved to be sensitive, robust and easy to use and serves as a useful tool for pandemic testing.
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http://dx.doi.org/10.1016/j.jviromet.2010.03.020DOI Listing
August 2010

5'-MGB probes allow rapid identification of methanogens and sulfate reducers in cold marine sediments by real-time PCR and melting curve analysis.

Oligonucleotides 2009 Sep;19(3):293-8

Nanogen Inc., 21720 23rd Drive SE. #150, Bothell, WA 98021, USA.

The analysis of microorganism communities in uncultured environmental samples requires laborious and cumbersome techniques such as denaturing gradient gel electrophoresis of amplicons generated with 16S rRNA generic primers with subsequent fragment sequencing. We have developed a simple method for genus identification of methanogen archaea and sulfate-reducing bacteria based on a real-time PCR hybridization probe melting curve analysis. The method takes advantage of a recent explosion of microorganism sequencing data conveniently packaged in the Ribosomal Database Project. Specificity of detection is based on a genus-specific real-time PCR fluorescent 5'-MGB-probe melt. As the probes are designed to have destabilizing mismatches with undesired genera, only samples with a proper melting temperature are called positive.
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http://dx.doi.org/10.1089/oli.2009.0196DOI Listing
September 2009

Primers with 5' flaps improve real-time PCR.

Biotechniques 2007 Dec;43(6):770, 772, 774

Nanogen Inc., Bothell, WA 98021, USA.

Primers that contain portions noncomplementary to the target region are usually used to add to the PCR product a utility sequence such as a restriction site or a universal probe binding site. We have demonstrated that primers with short 5'AT-rich overhangs increase real-time PCR fluorescent signal. The improvement is particularly significant for difficult to amplify templates, such as highly variable viral sequences or bisulfite-treated DNA.
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http://dx.doi.org/10.2144/000112631DOI Listing
December 2007

Evaluation of 5'-MGB hybridization probes for detection of Bordetella pertussis and Bordetella parapertussis using different real-time PCR instruments.

Diagn Microbiol Infect Dis 2008 Apr 21;60(4):429-32. Epub 2008 Feb 21.

Nanogen Inc., Bothell, WA 98021, USA.

Rapid and accurate tests capable of distinguishing Bordetella pertussis from milder Bordetella parapertussis infection can aid in treating patients. We evaluated a novel real-time polymerase chain reaction (PCR) method that allows rapid and accurate diagnosis and distinction between B. pertussis and B. parapertussis infections. The method is based on a fluorescent 5'-minor groove-binding molecule hybridization probe that can clearly distinguish between B. pertussis and B. parapertussis by post-PCR melt curve analysis. The reagents worked equally well in several different real-time PCR instruments. The Bordetella detection reagents are combined with internal control components to account for PCR inhibition without compromising assay sensitivity.
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http://dx.doi.org/10.1016/j.diagmicrobio.2007.10.019DOI Listing
April 2008

Type 2 diabetes whole-genome association study in four populations: the DiaGen consortium.

Am J Hum Genet 2007 Aug 26;81(2):338-45. Epub 2007 Jun 26.

Oy Jurilab, and Research Institute of Public Health, University of Kuopio, Kuopio, Finland, and Hope Hospital, Salford, UK.

Type 2 diabetes (T2D) is a common, polygenic chronic disease with high heritability. The purpose of this whole-genome association study was to discover novel T2D-associated genes. We genotyped 500 familial cases and 497 controls with >300,000 HapMap-derived tagging single-nucleotide-polymorphism (SNP) markers. When a stringent statistical correction for multiple testing was used, the only significant SNP was at TCF7L2, which has already been discovered and confirmed as a T2D-susceptibility gene. For a replication study, we selected 10 SNPs in six chromosomal regions with the strongest association (singly or as part of a haplotype) for retesting in an independent case-control set including 2,573 T2D cases and 2,776 controls. The most significant replicated result was found at the AHI1-LOC441171 gene region.
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http://dx.doi.org/10.1086/520599DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1950819PMC
August 2007

Novel DNA probes with low background and high hybridization-triggered fluorescence.

Nucleic Acids Res 2007 26;35(5):e30. Epub 2007 Jan 26.

Nanogen, Inc., 21720 23rd Drive SE, Suite 150, Bothell, WA 98021, USA.

Novel fluorogenic DNA probes are described. The probes (called Pleiades) have a minor groove binder (MGB) and a fluorophore at the 5'-end and a non-fluorescent quencher at the 3'-end of the DNA sequence. This configuration provides surprisingly low background and high hybridization-triggered fluorescence. Here, we comparatively study the performance of such probes, MGB-Eclipse probes, and molecular beacons. Unlike the other two probe formats, the Pleiades probes have low, temperature-independent background fluorescence and excellent signal-to-background ratios. The probes possess good mismatch discrimination ability and high rates of hybridization. Based on the analysis of fluorescence and absorption spectra we propose a mechanism of action for the Pleiades probes. First, hydrophobic interactions between the quencher and the MGB bring the ends of the probe and, therefore, the fluorophore and the quencher in close proximity. Second, the MGB interacts with the fluorophore and independent of the quencher is able to provide a modest (2-4-fold) quenching effect. Joint action of the MGB and the quencher is the basis for the unique quenching mechanism. The fluorescence is efficiently restored upon binding of the probe to target sequence due to a disruption in the MGB-quencher interaction and concealment of the MGB moiety inside the minor groove.
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http://dx.doi.org/10.1093/nar/gkl1136DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1865069PMC
April 2007

Improved biplex quantitative real-time polymerase chain reaction with modified primers for gene expression analysis.

Oligonucleotides 2006 ;16(4):395-403

Nanogen, Inc., Bothell, WA 98021, USA.

Stabilizing modified bases incorporated in primers allows the reduction of housekeeping gene primer concentration not possible with regular primers without sacrificing amplification efficiency. Low primer concentration allows coamplification of the most abundant housekeeping genes with very rare templates without mutual inhibition. Real-time polymerase chain reaction (PCR) coamplification of 18S ribosomal RNA with several genes of interest was used in this study with MGB Eclipse (Nanogen, San Diego, CA) hybridization probes. The results may be useful for high throughput gene expression studies as they simplify validation experiments.
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http://dx.doi.org/10.1089/oli.2006.16.395DOI Listing
January 2007

A novel endonuclease IV post-PCR genotyping system.

Nucleic Acids Res 2006 29;34(19):e128. Epub 2006 Sep 29.

Nanogen, 21720 23rd Drive SE, Suite 150, Bothell, WA 98021, USA.

Here we describe a novel endonuclease IV (Endo IV) based assay utilizing a substrate that mimics the abasic lesions that normally occur in double-stranded DNA. The three component substrate is characterized by single-stranded DNA target, an oligonucleotide probe, separated from a helper oligonucleotide by a one base gap. The oligonucleotide probe contains a non-fluorescent quencher at the 5' end and fluorophore attached to the 3' end through a special rigid linker. Fluorescence of the oligonucleotide probe is efficiently quenched by the interaction of terminal dye and quencher when not hybridized. Upon hybridization of the oligonucleotide probe and helper probe to their complementary target, the phosphodiester linkage between the rigid linker and the 3' end of the probe is efficiently cleaved, generating a fluorescent signal. In this study, the use of the Endo IV assay as a post-PCR amplification detection system is demonstrated. High sensitivity and specificity are illustrated using single nucleotide polymorphism detection.
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http://dx.doi.org/10.1093/nar/gkl679DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1636472PMC
December 2006

Selection of highly productive mammalian cells based on an inducible growth advantage using an antibody/receptor chimera.

J Biosci Bioeng 2002 ;93(4):399-404

Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

In mammalian cell culture, the selection of high producers is a critical step in efficient recombinant protein production. Drug-resistance selection has been commonly used, but does not always give a pure population of high producers. In this study, we propose a novel selection method in which the growth of high producers is specifically promoted. Two plasmids encoding (i) a hybrid receptor composed of the V(H) portion of anti-hen egg lysozyme antibody HyHEL-10 and an N-terminally truncated erythropoietin receptor (V(H)-EpoR), and (ii) a V(L)-EpoR fusion derived from the same construct as in (i), were employed. The second plasmid contained enhanced green fluorescent protein (EGFP) as a model recombinant protein that was flanked by the internal ribosomal entry sequence. Both plasmids were used simultaneously to transfect an IL-3-dependent murine myeloid cell line, 32D. The transfectants, after antigen selection in the absence of IL-3, showed a clear antigen-induced dose-dependent proliferation. In addition, a high EGFP expression level was observed by flow cytometry in comparison with the cells before antigen selection. The results clearly demonstrate the advantage of our method over conventional drug-resistance selection. We propose the term AMEGA (Antigen MEdiated Genetically-modified cell Amplification) for such an approach.
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http://dx.doi.org/10.1016/s1389-1723(02)80074-3DOI Listing
November 2005

Single nucleotide polymorphism genotyping by two colour melting curve analysis using the MGB Eclipse Probe System in challenging sequence environment.

Hum Genomics 2004 Mar;1(3):209-17

Epoch Biosciences, 21720 23rd Drive SE, Bothell, WA 98021, USA.

Probe and primer design for single nucleotide polymorphism (SNP) detection can be very challenging for A-T DNA-rich targets, requiring long sequences with lower specificity and stability, while G-C-rich DNA targets present limited design options to lower GC-content sequences only. We have developed the MGB Eclipse Probe System, which is composed of the following elements: MGB Eclipse probes and primers, specially developed software for the design of probes and primers, a unique set of modified bases and a Microsoft Excel macro for automated genotyping, which ably solves, in large part, this challenge. Fluorogenic MGB Eclipse probes are modified oligonucleotides containing covalently attached duplex-stabilising dihydrocyclopyrroloindole tripeptide (DPI3), the MGB ligand (MGB is a trademark of Epoch Biosciences, Bothell, WA), which has the combined properties of allowing the use of short sequences and providing great mismatch discrimination. The MGB moiety prevents probe degradation during polymerase chain reaction (PCR), allowing the researcher to use real time data; alternatively, hybridisation can be accurately measured by a post-PCR two-colour melt curve analysis. Using MGB Eclipse probes and primers containing modified bases further enhances the analysis of difficult SNP targets. G- or C-rich sequences can be refractory to analysis due to Hoogsteen base pairing. Substitution of normal G with Epoch's modified G prevents Hoogsteen base pairing, allowing both superior PCR and probe-based analysis of GC-rich targets. The use of modified A and T bases allows better stabilisation by significantly increasing the Tm of the oligonucleotides. Modified A creates A-T base pairs that have a stability slightly lower than a G-C base pair, and modified T creates T-A base pairs that have a stability about 30 per cent higher than the unmodified base pair. Together, the modified bases permit the use of short probes, providing good mismatch discrimination and primers that allow PCR of refractory targets. The combination of MGB Eclipse probes and primers enriched with the MGB ligand and modified bases has allowed the analysis of refractory SNPs, where other methods have failed.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3525082PMC
http://dx.doi.org/10.1186/1479-7364-1-3-209DOI Listing
March 2004

Multiplexed SNP genotyping using the Qbead system: a quantum dot-encoded microsphere-based assay.

Nucleic Acids Res 2003 Apr;31(8):e43

Quantum Dot Corporation, 26118 Research Road, Hayward, CA 94545, USA.

We have developed a new method using the Qbead system for high-throughput genotyping of single nucleotide polymorphisms (SNPs). The Qbead system employs fluorescent Qdot semiconductor nanocrystals, also known as quantum dots, to encode microspheres that subsequently can be used as a platform for multiplexed assays. By combining mixtures of quantum dots with distinct emission wavelengths and intensities, unique spectral 'barcodes' are created that enable the high levels of multiplexing required for complex genetic analyses. Here, we applied the Qbead system to SNP genotyping by encoding microspheres conjugated to allele-specific oligonucleotides. After hybridization of oligonucleotides to amplicons produced by multiplexed PCR of genomic DNA, individual microspheres are analyzed by flow cytometry and each SNP is distinguished by its unique spectral barcode. Using 10 model SNPs, we validated the Qbead system as an accurate and reliable technique for multiplexed SNP genotyping. By modifying the types of probes conjugated to microspheres, the Qbead system can easily be adapted to other assay chemistries for SNP genotyping as well as to other applications such as analysis of gene expression and protein-protein interactions. With its capability for high-throughput automation, the Qbead system has the potential to be a robust and cost-effective platform for a number of applications.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC153755PMC
http://dx.doi.org/10.1093/nar/gng043DOI Listing
April 2003

A homogeneous noncompetitive immunoassay for the detection of small haptens.

Anal Chem 2002 Jun;74(11):2500-4

Department of Chemistry and Biotechnology, School of Engineering, University of Tokyo, Japan.

We describe a noncompetitive homogeneous immunoassay for small haptens based on the antigen-dependent reassociation of antibody variable domains and beta-galactosidase (beta-gal) complementation (open sandwich enzymatic complementation immunoassay). As a model system, the reassociation of two fusion proteins, an anti 4-hydroxy-3-nitrophenylacetyl (NP) antibody heavy-chain variable-region fragment fused to an N-terminal deletion mutant of beta-gal (V(H)delta alpha) and the light-chain variable-region fragment fused to a C-terminal deletion mutant of beta-gal (V(L)delta omega), was monitored by the enzymatic complementation between the two. Upon simple mixing of the reagents with the sample, an antigen (NP)-dependent increase in enzymatic activity was observed. When 5-iodo-NP was measured, a 10 times higher sensitivity was observed, probably due to its higher affinity. Compared with our corresponding heterogeneous open sandwich enzyme-linked immunosorbent assay, approximately 1000-fold improvement in the sensitivity was attained, probably due to lower background V(H)-V(L) association. In addition, the assay required less time, handling, sample volume, and assay reagents.
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http://dx.doi.org/10.1021/ac015743xDOI Listing
June 2002
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