Detection of proteins released in the bloodstream from tissues damaged by disease can promote early detection of pathological conditions, differential diagnostics, and follow-up of therapy. Despite these prospects and a plethora of candidate biomarkers, efforts in recent years to establish new protein diagnostic assays have met with limited success. One important limiting factor has been the challenge of detecting proteins present at trace levels in complex bodily fluids. To achieve robust, sensitive, and specific detection, we have developed a microparticlebased solid-phase proximity ligation assay, dependent on simultaneous recognition of target proteins by three antibody molecules for added specificity. After capture on a microparticle, solid-phase pairs of proximity probes are added followed by washes, enabling detection and identification of rare protein molecules in blood while consuming small amounts of sample. We demonstrate that single polyclonal antibody preparations raised against target proteins of interest can be readily used to establish assays where detection depends on target recognition by three individual antibody molecules, recognizing separate epitopes. The assay was compared with state-ofthe-art sandwich ELISAs for detection of vascular endothelial growth factor, interleukin-8 and interleukin-6, and it was found to be superior both with regard to dynamic range and minimal numbers of molecules detected. Furthermore, the assays exhibited excellent performance in undiluted plasma and serum as well as in whole blood, producing comparable results for nine different antigens. We thus show that solid-phase proximity ligation assay is suitable for validation of a variety of protein biomarkers over broad dynamic ranges in clinical samples. Molecular & Cellular Proteomics 9:327-335, 2010.Analyses of the plasma proteome, its protein content, their modifications, and interactions, hold great promise to improve detection, classification, and prognostication of pathological conditions such as cancer (1). The attraction of serum or plasma biomarkers lies in their potential to reveal disease processes throughout the body and to guide selection of therapy and follow-up using minimally invasive blood sampling.This optimism is tempered by the molecular complexity of plasma and the fact that the abundance of known plasma proteins varies over at least 12 orders of magnitude (1), posing great challenges for immunoassays used to investigate the plasma proteome. Thus, new assay formats are needed that can offer improved sensitivity and specificity over a broad dynamic range with good precision to assess new protein biomarkers for analysis in plasma, serum, or whole blood.The proximity ligation assay (PLA), 1 first described by Fredriksson et al. (2) in 2002, is an immunoassay for detection of protein molecules via DNA ligation and amplification, offering high specificity and sensitivity. In PLA, pairs of affinity probes directed against the same target molecule are modified by attaching short single-stranded DNA m...
We describe a method, DNA array to protein array (DAPA), which allows the 'printing' of replicate protein arrays directly from a DNA array template using cell-free protein synthesis. At least 20 copies of a protein array can be obtained from a single DNA array. DAPA eliminates the need for separate protein expression, purification and spotting, and also overcomes the problem of long-term functional storage of surface-bound proteins.
Despite intense interest, methods that provide enhanced sensitivity and specificity in parallel measurements of candidate protein biomarkers in numerous samples have been lacking. We present herein a multiplex proximity ligation assay with readout via realtime PCR or DNA sequencing (ProteinSeq). We demonstrate improved sensitivity over conventional sandwich assays for simultaneous analysis of sets of 35 proteins in 5 µl of blood plasma. Importantly, we observe a minimal tendency to increased background with multiplexing, compared to a sandwich assay, suggesting that higher levels of multiplexing are possible. We used ProteinSeq to analyze proteins in plasma samples from cardiovascular disease (CVD) patient cohorts and matched controls. Three proteins, namely P-selectin, Cystatin-B and Kallikrein-6, were identified as putative diagnostic biomarkers for CVD. The latter two have not been previously reported in the literature and their potential roles must be validated in larger patient cohorts. We conclude that ProteinSeq is promising for screening large numbers of proteins and samples while the technology can provide a much-needed platform for validation of diagnostic markers in biobank samples and in clinical use.
The detection of weakly expressed proteins and protein complexes in biological samples represents a fundamental challenge. We have developed a new proximity-ligation strategy named 3PLA that uses three recognition events for the highly specific and sensitive detection of as little as a hundred molecules of the vascular endothelial growth factor (VEGF), the biomarkers troponin I, and prostate-specific antigen (PSA) alone or in complex with an inhibitor--demonstrating the versatility of 3PLA.
Cell-free DNA analysis is becoming adopted for first line aneuploidy screening, however for most healthcare programs, cost and workflow complexity is limiting adoption of the test. We report a novel cost effective method, the Vanadis NIPT assay, designed for high precision digitally-enabled measurement of chromosomal aneuploidies in maternal plasma. Reducing NIPT assay complexity is achieved by using novel molecular probe technology that specifically label target chromosomes combined with a new readout format using a nanofilter to enrich single molecules for imaging and counting without DNA amplification, microarrays or sequencing. The primary objective of this study was to assess the Vanadis NIPT assay with respect to analytical precision and clinical feasibility. Analysis of reference DNA samples indicate that samples which are challenging to analyze with low fetal-fraction can be readily detected with a limit of detection determined at <2% fetal-fraction. In total of 286 clinical samples were analysed and 30 out of 30 pregnancies affected by trisomy 21 were classified correctly. This method has the potential to make cost effective NIPT more widely available with more women benefiting from superior detection and false positive rates.
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