A system for multiplexed selection of aptamers with exquisite specificity without counterselection.

Aptamers have proven to be valuable tools for the detection of small molecules due to their remarkable ability to specifically discriminate between structurally similar molecules. Most aptamer selection efforts have relied on counterselection to eliminate aptamers that exhibit unwanted cross-reactivity to interferents or structurally similar relatives to the target of interest. However, because the affinity and specificity characteristics of an aptamer library are fundamentally unknowable a priori, it is not possible to determine the optimal counterselection parameters. As a result, counterselection experiments require trial-and-error approaches that are inherently inefficient and may not result in aptamers with the best combination of affinity and specificity. In this work, we describe a high-throughput screening process for generating high-specificity aptamers to multiple targets in parallel while also eliminating the need for counterselection. We employ a platform based on a modified benchtop sequencer to conduct a massively parallel aptamer screening process that enables the selection of highly specific aptamers against multiple structurally similar molecules in a single experiment, without any counterselection. As a demonstration, we have selected aptamers with high affinity and exquisite specificity for three structurally similar kynurenine metabolites that differ by a single hydroxyl group in a single selection experiment. This process can easily be adapted to other small-molecule analytes and should greatly accelerate the development of aptamer reagents that achieve exquisite specificity for their target analytes.

Re-Evaluating the Conventional Wisdom about Binding Assays.

Analytical technologies based on binding assays have evolved substantially since their inception nearly 60 years ago, but our conceptual understanding of molecular recognition has not kept pace. Contemporary technologies, such as single-molecule and digital measurements, have challenged, or even rendered obsolete, core concepts behind conventional binding assay design. Here, we explore the fundamental principles underlying molecular recognition systems, which we consider in terms of signals generated through concentration-dependent shifts in equilibrium. We challenge certain orthodoxies related to binding-based detection assays, including the primary importance of a low dissociation constant (KD) and the extent to which this parameter constrains dynamic range and limit of detection. Lastly, we identify key principles for designing binding assays that are optimally suited for a given detection application.

Tailoring electrode surface charge to achieve discrimination and quantification of chemically similar small molecules with electrochemical aptamers.

Electrochemical biosensors based on structure-switching aptamers offer many advantages because they can operate directly in complex samples and offer the potential to integrate with miniaturized electronics. Unfortunately, these biosensors often suffer from cross-reactivity problems when measuring a target in samples containing other chemically similar molecules, such as precursors or metabolites. While some progress has been made in selecting highly specific aptamers, the discovery of these reagents remains slow and costly. In this work, we demonstrate a novel strategy to distinguish molecules with miniscule difference in chemical composition (such as a single hydroxyl group) - with cross reactive aptamer probes - by tuning the charge state of the surface on which the aptamer probes are immobilized. As an exemplar, we show that our strategy can distinguish between DOX and many structurally similar analytes, including its primary metabolite doxorubicinol (DOXol). We then demonstrate the ability to accurately quantify mixtures of these two molecules based on their differential response to sensors with different surface-charge properties. We believe this methodology is general and can be extended to a broad range of applications.

Directed Evolution of Aptamer Discovery Technologies.

Although antibodies are a powerful tool for molecular biology and clinical diagnostics, there are many emerging applications for which nucleic acid-based aptamers can be advantageous. However, generating high-quality aptamers with sufficient affinity and specificity for biomedical applications is a challenging feat for most research laboratories. In this Account, we describe four techniques developed in our laboratory to accelerate the discovery of high-quality aptamer reagents that can achieve robust binding even for challenging molecular targets. The first method is particle display, in which we convert solution-phase aptamers into aptamer particles that can be screened via fluorescence-activated cell sorting (FACS) to quantitatively isolate individual aptamer particles based on their affinity. This enables the efficient isolation of high-affinity aptamers in fewer selection rounds than conventional methods, thereby minimizing selection biases and reducing the emergence of artifacts in the final aptamer pool. We subsequently developed the multiparametric particle display (MPPD) method, which employs two-color FACS to isolate aptamer particles based on both affinity and specificity, yielding aptamers that exhibit excellent target binding even in complex matrixes such as serum. The third method is an alkyne-azide chemistry ("click chemistry")-based particle display (click-PD) that enables the generation and screening of "non-natural" aptamers with a wide range of base modifications. We have shown that these base-modified aptamers can achieve robust affinity and specificity for targets that have proven challenging or inaccessible with natural nucleotide-based aptamer libraries. Finally, we describe the non-natural aptamer array (N2A2) platform in which a modified benchtop sequencing instrument is used to characterize base-modified aptamers in high throughput, enabling the efficient identification of molecules with excellent affinity and specificity for their targets. This system first generates aptamer clusters on the flow-cell surface that incorporate alkyne-modified nucleobases and then performs a click reaction to couple those nucleobases to an azide-modified chemical moiety. This yields a sequence-defined array of tens of millions of base-modified sequences, which can then be characterized for affinity and specificity in a high-throughput fashion. Collectively, we believe that these advancements are helping to make aptamer technology more accessible, efficient, and robust, thereby enabling the use of these affinity reagents for a wider range of molecular recognition and detection-based applications.

A comparison of EBV serology and serum cell-free DNA as screening tools for nasopharyngeal cancer: Results of the Singapore NPC screening cohort.

We aimed to evaluate the effectiveness of nasopharyngeal cancer (NPC) screening by comprehensive clinical follow-up and adjunctive Epstein-Barr virus (EBV) testing. In a prospective cohort study, 524 individuals with a first-degree family history of NPC were recruited at a university clinical center in Singapore. The cohort was evaluated at baseline and at 6 monthly intervals, with a complete head and neck examination including nasopharyngeal endoscopy. Blood was taken at baseline and at yearly intervals for EBV Viral Capsid Antigen (VCA) IgA, EBV Early Antigen (EA) IgA serology and serum cell-free EBV DNA. Nasopharyngeal biopsy was performed when any irregularity in the nasopharynx was observed, or when EBV markers were elevated. The mean duration of follow-up was 57.7 months, with an average of 8.6 clinical visits per participant. Five participants (0.96%) were identified to have NPC, giving a prevalence of 199 per 100,000 person-years of screening. Four of the five NPC cases identified had asymptomatic T1 disease, at an earlier stage compared to NPC patients diagnosed in the clinic during the same time period (p = 0.0297). All NPC cases identified had elevated EBV-EA IgA titers ≥1:10, with a specificity of 94.6% and a positive predictive value of 15.2%, outperforming EBV-VCA IgA and serum EBV DNA. Two NPC cases were biopsied only because of elevated EBV serology titers, with increasing EBV-EA IgA titers preceding the diagnosis of NPC. In conclusion, screening for NPC is effective in identifying early-stage disease. Adjunctive EBV-EA IgA testing improved the effectiveness of screening.

Shape-based separation of synthetic microparticles.

The functional properties of colloidal materials can be tailored by tuning the shape of their constituent particles. Unfortunately, a reliable, general methodology for purifying colloidal materials solely based on shape is still lacking. Here we exploit the single-particle analysis and sorting capabilities of the fluorescence-activated cell sorting (FACS) instrument, a commonly used tool in biomedical research, and demonstrate the ability to separate mixtures of synthetic microparticles based solely on their shape with high purity. We achieve this by simultaneously obtaining four independent optical scattering signals from the FACS instrument to create shape-specific 'scattering signatures' that can be used for particle classification and sorting. We demonstrate that these four-dimensional signatures can overcome the confounding effects of particle orientation on shape-based characterization. Using this strategy, robust discrimination of particles differing only slightly in shape and an efficient selection of desired shapes from mixtures comprising particles of diverse sizes and materials is demonstrated.

Design and Analysis of a Sample-and-Hold CMOS Electrochemical Sensor for Aptamer-based Therapeutic Drug Monitoring.

In this paper, we present the design and the analysis of an electrochemical circuit for measuring the concentrations of therapeutic drugs using structure-switching aptamers. Aptamers are single-stranded nucleic acids, whose sequence is selected to exhibit high affinity and specificity toward a molecular target, and change its conformation upon binding. This property, when coupled with a redox reporter and electrochemical detection, enables reagent-free biosensing with a sub-minute temporal resolution for in vivo therapeutic drug monitoring. Specifically, we design a chronoamperometry-based electrochemical circuit that measures the direct changes in the electron transfer (ET) kinetics of a methylene blue reporter conjugated at the distal-end of the aptamer. To overcome the high-frequency noise amplification issue when interfacing with a large-size (> 0.25 mm2) implantable electrode, we present a sample-and-hold (S/H) circuit technique in which the desired electrode potentials are held onto noiseless capacitors during the recording of the redox currents. This allows disconnecting the feedback amplifiers to avoid its noise injection while reducing the total power consumption. A prototype circuit implemented in 65-nm CMOS demonstrates a cell-capacitance-insensitive input-referred noise (IRN) current of 15.2 pArms at a 2.5-kHz filtering bandwidth. We tested our system in human whole blood samples and measured the changes in the ET kinetics from the redox-labeled aptamers at different kanamycin concentrations. By employing principal component analysis (PCA) to compensate for the sampling errors, we report a molecular noise floor (at SNR = 1) of 3.1 µM with sub 1-sec acquisition time at 0.22-mW power consumption.

Dual-reporter SERS-based biomolecular assay with reduced false-positive signals.

We present a sensitive and quantitative protein detection assay that can efficiently distinguish between specific and nonspecific target binding. Our technique combines dual affinity reagents with surface-enhanced Raman spectroscopy (SERS) and chemometric analysis. We link one Raman reporter-tagged affinity reagent to gold nanoparticles and another to a gold film, such that protein-binding events create a "hot spot" with strong SERS spectra from both Raman reporter molecules. Any signal generated in this context is indicative of recognition by both affinity labels, whereas signals generated by nonspecific binding lack one or the other label, enabling us to efficiently distinguish true from false positives. We show that the number of hot spots per unit area of our substrate offers a quantitative measure of analyte concentration and demonstrate that this dual-label, SERS-linked aptasensor assay can sensitively and selectively detect human α-thrombin in 1% human serum with a limit of detection of 86 pM.

High-Fidelity Nanopore Sequencing of Ultra-Short DNA Targets.

Nanopore sequencing offers a portable and affordable alternative to sequencing-by-synthesis methods but suffers from lower accuracy and cannot sequence ultrashort DNA. This puts applications such as molecular diagnostics based on the analysis of cell-free DNA or single-nucleotide variants (SNVs) out of reach. To overcome these limitations, we report a nanopore-based sequencing strategy in which short target sequences are first circularized and then amplified via rolling-circle amplification to produce long stretches of concatemeric repeats. After sequencing on the Oxford Nanopore Technologies MinION platform, the resulting repeat sequences can be aligned to produce a highly accurate consensus that reduces the high error-rate present in the individual repeats. Using this approach, we demonstrate for the first time the ability to obtain unbiased and accurate nanopore data for target DNA sequences <100 bp. Critically, this approach is sensitive enough to achieve SNV discrimination in mixtures of sequences and even enables quantitative detection of specific variants present at ratios of <10%. Our method is simple, cost-effective, and only requires well-established processes. It therefore expands the utility of nanopore sequencing for molecular diagnostics and other applications, especially in resource-limited settings.

Treating Tumors at Low Drug Doses Using an Aptamer-Peptide Synergistic Drug Conjugate.

Combination chemotherapy must strike a difficult balance between safety and efficacy. Current regimens suffer from poor therapeutic impact because drugs are given at their maximum tolerated dose (MTD), which compounds the toxicity risk and exposes tumors to non-optimal drug ratios. A modular framework has been developed that selectively delivers drug combinations at synergistic ratios via tumor-targeting aptamers for effective low-dose treatment. A nucleolin-recognizing aptamer was coupled to peptide scaffolds laden with precise ratios of doxorubicin (DOX) and camptothecin (CPT). This construct had an extremely low IC50 (31.9 nm) against MDA-MB-231 breast cancer cells in vitro, and exhibited in vivo efficacy at micro-dose injections (500 and 350 µg kg-1 dose-1 of DOX and CPT, respectively) that are 20-30-fold lower than their previously-reported MTDs. This approach represents a generalizable strategy for the safe and consistent delivery of combination drugs in oncology.

Direct Selection of Fluorescence-Enhancing RNA Aptamers.

RNA aptamers that generate a strong fluorescence signal upon binding a nonfluorescent small-molecule dye offer a powerful means for the selective imaging of individual RNA species. Unfortunately, conventional in vitro discovery methods are not efficient at generating such fluorescence-enhancing aptamers, because they primarily exert selective pressure based on target affinity-a characteristic that correlates poorly with fluorescence enhancement. Thus, only a handful of fluorescence-enhancing aptamers have been reported to date. In this work, we describe a method for converting DNA libraries into "gene-linked RNA aptamer particles" (GRAPs) that each display ∼105 copies of a single RNA sequence alongside the DNA that encodes it. We then screen large libraries of GRAPs in a high-throughput manner using the FACS instrument based directly on their fluorescence-enhancing properties. Using this strategy, we demonstrate the capability to generate fluorescence-enhancing aptamers that produce a variety of different emission wavelengths upon binding the dye of interest.

Strategy for Generating Sequence-Defined Aptamer Reagent Sets for Detecting Protein Contaminants in Biotherapeutics.

Biologic drugs are typically manufactured in mammalian host cells, and it is critical from a drug safety and efficacy perspective to detect and remove host cell proteins (HCPs) during production. This is currently achieved with sets of polyclonal antibodies (pAbs), but these suffer from critical shortcomings because their composition is inherently undefined, and they cannot detect nonimmunogenic HCPs. In this work, we report a high-throughput screening and array-based binding characterization strategy that we employed to generate a set of aptamers that overcomes these limitations to achieve sensitive, broad-spectrum detection of HCPs from the widely used Chinese hamster ovary (CHO) cell line. We identified a set of 32 DNA aptamers that achieve better sensitivity than a commercial pAb reagent set and can detect a comparable number of HCPs over a broad range of isoelectric points and sizes. Importantly, these aptamers detect multiple contaminants that are known to be responsible for therapeutic antibody degradation and toxicity in patients. Because HCP aptamer reagents are sequence-defined and chemically synthesized, we believe they may enable safer production of biologic drugs, and this strategy should be broadly applicable for the generation of HCP detection reagents for other cell lines.

Advancements in Aptamer Discovery Technologies.

Affinity reagents that specifically bind to their target molecules are invaluable tools in nearly every field of modern biomedicine. Nucleic acid-based aptamers offer many advantages in this domain, because they are chemically synthesized, stable, and economical. Despite these compelling features, aptamers are currently not widely used in comparison to antibodies. This is primarily because conventional aptamer-discovery techniques such as SELEX are time-consuming and labor-intensive and often fail to produce aptamers with comparable binding performance to antibodies. This Account describes a body of work from our laboratory in developing advanced methods for consistently producing high-performance aptamers with higher efficiency, fewer resources, and, most importantly, a greater probability of success. We describe our efforts in systematically transforming each major step of the aptamer discovery process: selection, analysis, and characterization. To improve selection, we have developed microfluidic devices (M-SELEX) that enable discovery of high-affinity aptamers after a minimal number of selection rounds by precisely controlling the target concentration and washing stringency. In terms of improving aptamer pool analysis, our group was the first to use high-throughput sequencing (HTS) for the discovery of new aptamers. We showed that tracking the enrichment trajectory of individual aptamer sequences enables the identification of high-performing aptamers without requiring full convergence of the selected aptamer pool. HTS is now widely used for aptamer discovery, and open-source software has become available to facilitate analysis. To improve binding characterization, we used HTS data to design custom aptamer arrays to measure the affinity and specificity of up to ∼10(4) DNA aptamers in parallel as a means to rapidly discover high-quality aptamers. Most recently, our efforts have culminated in the invention of the "particle display" (PD) screening system, which transforms solution-phase aptamers into "aptamer particles" that can be individually screened at high-throughput via fluorescence-activated cell sorting. Using PD, we have shown the feasibility of rapidly generating aptamers with exceptional affinities, even for proteins that have previously proven intractable to aptamer discovery. We are confident that these advanced aptamer-discovery methods will accelerate the discovery of aptamer reagents with excellent affinities and specificities, perhaps even exceeding those of the best monoclonal antibodies. Since aptamers are reproducible, renewable, stable, and can be distributed as sequence information, we anticipate that these affinity reagents will become even more valuable tools for both research and clinical applications.

Multiparameter Particle Display (MPPD): A Quantitative Screening Method for the Discovery of Highly Specific Aptamers.

Aptamers are a promising class of affinity reagents because they are chemically synthesized, thus making them highly reproducible and distributable as sequence information rather than a physical entity. Although many high-quality aptamers have been previously reported, it is difficult to routinely generate aptamers that possess both high affinity and specificity. One of the reasons is that conventional aptamer selection can only be performed either for affinity (positive selection) or for specificity (negative selection), but not both simultaneously. In this work, we harness the capacity of fluorescence activated cell sorting (FACS) for multicolor sorting to simultaneously screen for affinity and specificity at a throughput of 107 aptamers per hour. As a proof of principle, we generated DNA aptamers that exhibit picomolar to low nanomolar affinity in human serum for three diverse proteins, and show that these aptamers are capable of outperforming high-quality monoclonal antibodies in a standard ELISA detection assay.

High-Throughput Discovery of Aptamers for Sandwich Assays.

Sandwich assays are among the most powerful tools in molecular detection. These assays use "pairs" of affinity reagents so that the detection signal is generated only when both reagents bind simultaneously to different sites on the target molecule, enabling highly sensitive and specific measurements in complex samples. Thus, the capability to efficiently screen affinity reagent pairs at a high throughput is critical. In this work, we describe an experimental strategy for screening "aptamer pairs" at a throughput of 106 aptamer pairs per hour-which is many orders of magnitude higher than the current state of the art. The key step in our process is the conversion of solution-phase aptamers into "aptamer particles" such that we can directly measure the simultaneous binding of multiple aptamers to a target protein based on fluorescence signals and sort individual particles harboring aptamer pairs via the fluorescence-activated cell-sorter instrument. As proof of principle, we successfully isolated a high-quality DNA aptamer pair for plasminogen activator inhibitor 1 (PAI-1). Within only two rounds of screening, we discovered DNA aptamer pairs with low-nanomolar sensitivity in dilute serum and excellent specificity with minimal off-target binding even to closely related proteins such as PAI-2.

Integrated electrochemical microsystems for genetic detection of pathogens at the point of care.

The capacity to achieve rapid, sensitive, specific, quantitative, and multiplexed genetic detection of pathogens via a robust, portable, point-of-care platform could transform many diagnostic applications. And while contemporary technologies have yet to effectively achieve this goal, the advent of microfluidics provides a potentially viable approach to this end by enabling the integration of sophisticated multistep biochemical assays (e.g., sample preparation, genetic amplification, and quantitative detection) in a monolithic, portable device from relatively small biological samples. Integrated electrochemical sensors offer a particularly promising solution to genetic detection because they do not require optical instrumentation and are readily compatible with both integrated circuit and microfluidic technologies. Nevertheless, the development of generalizable microfluidic electrochemical platforms that integrate sample preparation and amplification as well as quantitative and multiplexed detection remains a challenging and unsolved technical problem. Recognizing this unmet need, we have developed a series of microfluidic electrochemical DNA sensors that have progressively evolved to encompass each of these critical functionalities. For DNA detection, our platforms employ label-free, single-step, and sequence-specific electrochemical DNA (E-DNA) sensors, in which an electrode-bound, redox-reporter-modified DNA "probe" generates a current change after undergoing a hybridization-induced conformational change. After successfully integrating E-DNA sensors into a microfluidic chip format, we subsequently incorporated on-chip genetic amplification techniques including polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP) to enable genetic detection at clinically relevant target concentrations. To maximize the potential point-of-care utility of our platforms, we have further integrated sample preparation via immunomagnetic separation, which allowed the detection of influenza virus directly from throat swabs and developed strategies for the multiplexed detection of related bacterial strains from the blood of septic mice. Finally, we developed an alternative electrochemical detection platform based on real-time LAMP, which not is only capable of detecting across a broad dynamic range of target concentrations, but also greatly simplifies quantitative measurement of nucleic acids. These efforts represent considerable progress toward the development of a true sample-in-answer-out platform for genetic detection of pathogens at the point of care. Given the many advantages of these systems, and the growing interest and innovative contributions from researchers in this field, we are optimistic that iterations of these systems will arrive in clinical settings in the foreseeable future.

Quantitative selection and parallel characterization of aptamers.

Aptamers are promising affinity reagents that are potentially well suited for high-throughput discovery, as they are chemically synthesized and discovered via completely in vitro selection processes. Recent advancements in selection, sequencing, and the use of modified bases have improved aptamer quality, but the overall process of aptamer generation remains laborious and low-throughput. This is because binding characterization remains a critical bottleneck, wherein the affinity and specificity of each candidate aptamer are measured individually in a serial manner. To accelerate aptamer discovery, we devised the Quantitative Parallel Aptamer Selection System (QPASS), which integrates microfluidic selection and next-generation sequencing with in situ-synthesized aptamer arrays, enabling simultaneous measurement of affinity and specificity for thousands of candidate aptamers in parallel. After using QPASS to select aptamers for the human cancer biomarker angiopoietin-2 (Ang2), we in situ synthesized arrays of the selected sequences and obtained equilibrium dissociation constants (Kd) for every aptamer in parallel. We thereby identified over a dozen high-affinity Ang2 aptamers, with Kd as low as 20.5 ± 7.3 nM. The same arrays enabled us to quantify binding specificity for these aptamers in parallel by comparing relative binding of differentially labeled target and nontarget proteins, and by measuring their binding affinity directly in complex samples such as undiluted serum. Finally, we show that QPASS offers a compelling avenue for exploring structure-function relationships for large numbers of aptamers in parallel by coupling array-based affinity measurements with next-generation sequencing data to identify nucleotides and motifs within the aptamer that critically affect Ang2 binding.

In Vitro Selection of pH-Activated DNA Nanostructures.

We report the first in vitro selection of DNA nanostructures that switch their conformation when triggered by change in pH. Previously, most pH-active nanostructures were designed using known pH-active motifs, such as the i-motif or the triplex structure. In contrast, we performed de novo selections starting from a random library and generated nanostructures that can sequester and release Mipomersen, a clinically approved antisense DNA drug, in response to pH change. We demonstrate extraordinary pH-selectivity, releasing up to 714-fold more Mipomersen at pH 5.2 compared to pH 7.5. Interestingly, none of our nanostructures showed significant sequence similarity to known pH-sensitive motifs, suggesting that they may operate via novel structure-switching mechanisms. We believe our selection scheme is general and could be adopted for generating DNA nanostructures for many applications including drug delivery, sensors and pH-active surfaces.

Thousand-fold volumetric concentration of live cells with a recirculating acoustofluidic device.

The ability to concentrate cells from dilute samples into smaller volumes is an essential process step for most biological assays. Volumetric concentration is typically achieved via centrifugation, but this technique is not well suited for handling small number of cells, especially outside of the laboratory setting. In this work, we describe a novel device that combines acoustofluidics with a recirculating architecture to achieve >1000-fold enrichment of cells in a label-free manner, at high volumetric throughput (>500 µL min(-1)) and with high recovery (>98.7%). We demonstrate that our device can be used with a wide variety of different cell types and show that this concentration strategy does not affect cell viability. Importantly, our device could be readily adopted to serve as a "sample preparation" module that can be integrated with other microfluidic devices to allow analysis of dilute cellular samples in large volumes.