From molecular diagnostics to personalized testing

Increased Sensitivity of HIV-1 p24 ELISA Using a Photochemical Signal Amplification System

BACKGROUND

In this study we describe a photochemical signal amplification method (PSAM) for increasing of the sensitivity of enzyme-linked immunosorbent assay (ELISA) for determination of HIV-1 p24 antigen. The photochemical signal amplification method is based on an autocatalytic photochemical reaction of a horseradish peroxidase (HRP) substrate, orthophenylenediamine (OPD).

METHODS

To compare the performance of PSAM-boosted ELISA with a conventional colorimetric ELISA for determination of HIV-1 p24 antigen we employed a PerkinElmer HIV-1 p24 ELISA kit, using conventional ELISA alongside ELISA + PSAM.

RESULTS

In the present study, we show that PSAM technology allows one to increase the analytical sensitivity and dynamic range of a commercial HIV-1 p24 ELISA kit, with and without immune-complex disruption, by a factor of approximately 40-fold.

CONCLUSIONS

ELISA + PSAM is compatible with commercially available microtiter plate readers, requires only an inexpensive illumination device, and the PSAM amplification step takes no longer than 15 min. This method can be used for both commercially available and in-house ELISA tests, and has the advantage of being considerably simpler and less costly than alternative signal amplification methods. This method can be used for both commercially available and in-house ELISA tests, and has the advantage of being considerably simpler and less costly than alternative signal amplification methods.

Competitive reporter monitored amplification (CMA)--quantification of molecular targets by real time monitoring of competitive reporter hybridization

Background

State of the art molecular diagnostic tests are based on the sensitive detection and quantification of nucleic acids. However, currently established diagnostic tests are characterized by elaborate and expensive technical solutions hindering the development of simple, affordable and compact point-of-care molecular tests.

Methodology and Principal Findings

The described competitive reporter monitored amplification allows the simultaneous amplification and quantification of multiple nucleic acid targets by polymerase chain reaction. Target quantification is accomplished by real-time detection of amplified nucleic acids utilizing a capture probe array and specific reporter probes. The reporter probes are fluorescently labeled oligonucleotides that are complementary to the respective capture probes on the array and to the respective sites of the target nucleic acids in solution. Capture probes and amplified target compete for reporter probes. Increasing amplicon concentration leads to decreased fluorescence signal at the respective capture probe position on the array which is measured after each cycle of amplification. In order to observe reporter probe hybridization in real-time without any additional washing steps, we have developed a mechanical fluorescence background displacement technique.

Conclusions and Significance

The system presented in this paper enables simultaneous detection and quantification of multiple targets. Moreover, the presented fluorescence background displacement technique provides a generic solution for real time monitoring of binding events of fluorescently labelled ligands to surface immobilized probes. With the model assay for the detection of human immunodeficiency virus type 1 and 2 (HIV 1/2), we have been able to observe the amplification kinetics of five targets simultaneously and accommodate two additional hybridization controls with a simple instrument set-up. The ability to accommodate multiple controls and targets into a single assay and to perform the assay on simple and robust instrumentation is a prerequisite for the development of novel molecular point of care tests.

Microwave-mediated synthesis of labeled nucleotides with utility in the synthesis of DNA probes

A novel method of linking haptens to deoxycytidine 5'-triphosphate via microwave-mediated bisulfate-catalyzed transamination with hydrazine has been developed. This method enables the tethering of small molecule haptens to dCTP via a discrete polyethylene glycol (PEG) spacer, yielding N(4)-aminodeoxycytidine 5'-triphosphate-dPEG-haptens. This synthetic approach employs microwave-catalyzed hydrazinolysis that enables the attachment of spacers via hydrazine linkages. The microwave-mediated introduction of this hydrazine handle provides a significant improvement in yield over those of published thermal methods. The microwave reaction was shown to be scalable, and the final product was amenable to labeling with a wide variety of haptens. The resulting nucleotide triphosphates, N(4)-aminodeoxycytidine 5'-triphosphate-dPEG-haptens, can serve as unique substrates for the enzyme-mediated labeling of DNA probes. The efficacy of incorporation of one such novel nucleotide, N(4)-aminodeoxycytidine 5'-triphosphate-dPEG(4)-DNP, has been demonstrated in nick translation labeling of HER2 and HPV probes. The labeled probes have been shown to be effective in visualizing their target genes in tissue.