A Sequence Listing accompanies this application.
Low-cost and easy-to-use tests to detect SARS-CoV-2, the causative agent of the COVID-19 pandemic, are essential tools to contain the spread of the virus and ensure that patients receive timely treatment. Such tests can be implemented at the point of care or in the home to enable distributed testing and more rapid results. Conventional PCR-based testing, however, is limited to centralized labs with sophisticated equipment. The increasing demand for PCR-based testing reagents further suggests that diagnostic assays that utilize reagents outside the PCR pipeline could be valuable tools to increase testing capacity.
Survey of conventional diagnostics currently approved for use in the United States reveals that they require multiple days to return results, they require expensive equipment, or they lack sensitivity and specificity. Moreover, tests require trained personnel to run them, making in-home use challenging. These requirements substantially increase both the cost and time required to return assay results. Accordingly, there remains a need in the art for rapid, inexpensive, and highly sensitive diagnostic tests for SARS-CoV-2, the causative agent of the COVID-19 pandemic, that require neither sophisticated laboratory equipment nor biosafety level 3 containment.
In a first aspect, the present invention provides aptasensors for detecting SARS-CoV-2. The aptasensors comprise: (a) a target-binding sequence that is complementary to a SARS-CoV-2 target nucleic acid or to the complement thereof; and (b) an aptamer. In the absence of the SARS-CoV-2 target nucleic acid, the aptasensor forms a stem-loop structure in which a first portion of the target-binding sequence forms a single-stranded toehold and a second portion of the target-binding sequence base-pairs with a portion of the aptamer to form a stem, such that the aptamer cannot fold into its active form. However, binding of the target-binding sequence to the SARS-CoV-2 target nucleic acid disrupts the stem-loop structure, allowing the aptamer to fold into its active form and bind to its cognate ligand.
In a second aspect, the present invention provides methods of detecting SARS-CoV-2 in a sample. The methods comprise: (a) amplifying the SARS-CoV-2 target nucleic acid in the sample; (b) contacting the amplified nucleic acid with an aptasensor disclosed herein and the cognate ligand of its aptamer; and (c) detecting any signal produced by the aptamer binding to its cognate ligand. In these methods, detection of the signal indicates that SARS-CoV-2 is present in the sample.
In a third aspect, the present invention provides kits for detecting SARS-CoV-2 comprising the aptasensors disclosed herein.
The patent or patent application file contains at least one drawing in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.
The present invention provides compositions and methods for rapid, highly sensitive detection of SARS-CoV-2, the causative agent of the COVID-19 pandemic. In the methods, a SARS-CoV-2 target nucleic acid is amplified and is then bound by a sequence-specific aptasensor for detection. These methods offer several advantages. For example, the use of an aptasensor for detection confirms that the amplified nucleic acid comprises the target sequence, reducing the risk of false positive results. The aptasensors described herein produce a strong fluorescence signal that can be detected by eye or using inexpensive and readily available equipment, such as a smartphone camera. Consequently, the methods of the present invention do not need to be performed at a centralized lab. Further, the inventors have demonstrated that these methods can detect SARS-CoV-2 in samples containing as few as 2 copies of viral RNA. Aptasensors
In a first aspect, the present invention provides aptasensors for detecting SARS-CoV-2. The aptasensors comprise: (a) a target-binding sequence that is complementary to a SARS-CoV-2 target nucleic acid or to the complement thereof; and (b) an aptamer. In the absence of the SARS-CoV-2 target nucleic acid, the aptasensor forms a stem-loop structure in which a first portion of the target-binding sequence forms a single-stranded toehold and a second portion of the target-binding sequence base-pairs with a portion of the aptamer to form a stem, such that the aptamer cannot fold into its active form. However, binding of the target-binding sequence to the SARS-CoV-2 target nucleic acid disrupts the stem-loop structure, allowing the aptamer to fold into its active form and bind to its cognate ligand.
As used herein, the term “aptasensor” refers to a single-stranded oligonucleotide that functions as a molecular sensor. The aptasensors of the present invention form an inhibitory stem-loop structure that is disrupted when the aptasensor binds to a target nucleic acid, allowing the aptasensor to produce a detectable signal. The aptasensors used with the present invention may comprise single-stranded RNA or single-stranded DNA.
The aptasensors of the present invention comprise an inhibitory stem-loop. Within this stem-loop, the stem is typically about 10-25 nucleotides in length. In some embodiments, the stem is 12-21 nucleotides in length. In some embodiments, the stem is about 20 nucleotides in length. In some embodiments, the stem comprises bulges, i.e., non-base paired nucleotides within the stem. For example, the aptasensors described in Example 1 comprise a stem with two bulges that are four and eight bases from the top base pair of the stem. In other embodiments (exemplified by the aptasensors described in Example 2), the stem does not comprise bulges. The loop of the inhibitory stem-loop structure may be about 6-10 nucleotides in length and is typically about 8 nucleotides in length. However, the length of the loop may be decreased to make the hairpin stronger or be increased to make the hairpin weaker due to entropic effects.
The aptasensors of the present invention comprise two functional components: a target-binding sequence and an aptamer. The “target-binding sequence” is an oligonucleotide that is complementary to a SARS-CoV-2 target nucleic acid or to the complement thereof. Within the unactivated aptasensor structure, a first portion of the target-binding sequence exists as a toehold (i.e., a single-stranded overhang), while a second portion forms a stem by base-pairing with a complementary portion of the aptamer. Binding of the toehold to a target nucleic acid thermodynamically drives the remaining stem-forming portion of the target-binding sequence to bind to the target nucleic acid, disrupting the stem-loop structure of the aptasensor. The toehold portion of the target-binding sequence should be at least 4 nucleotides in length. In some embodiments, the toehold is 8-30 nucleotides in length. In certain embodiments, the toehold is 15 nucleotides in length. The portion of the target-binding sequence that forms a stem by base-pairing with a complementary portion of the aptamer (i.e., the b domain in
The “aptamer” portion of the aptasensor is an oligonucleotide that is capable of binding to a specific cognate ligand when it is in its active form. An aptamer is in its “active form” when it has folded into the proper three-dimensional structure for binding to its cognate ligand.
As is schematically depicted in
In some embodiments, the aptamer comprises an inner clamp within the aptamer core, as depicted in
The aptamers of the present invention serve as reporters in that they produce a detectable signal upon binding to their cognate ligand. A “detectable signal” is a signal that can be detected over any background noise. Suitable detectable signals include, without limitation, fluorescence signals, luminescence signals, colorimetric signals, wavelength absorbance, and radioactive signals.
In some embodiments, the detectable signal is a colorimetric signal. A “colorimetric signal” is a signal that produces a color change. One example of a system that generates a colorimetric signal is 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) system, wherein ABTS interacts with a DNA catalyst to generate a colored byproduct. Advantageously, a colorimetric signal may be visible by eye, such that no special equipment is required to visualize it. However, in some cases, it may be desirable to quantify the colorimetric signal using a device such as a spectrophotometer.
In some embodiments, the detectable signal is a fluorescence signal. “Fluorescence” is the emission of light by a substance that has absorbed light or another form of electromagnetic radiation. Any aptamer that produces a fluorescence signal upon binding to its cognate ligand may be used with the present invention. In Example 1, the inventors utilize the aptamers Broccoli and Corn in their aptasensors. Thus, in some embodiments, the aptamer is Broccoli or Corn. The binding of these aptamers activates the fluorescence of their cognate ligands (see
The SARS-CoV-2 target nucleic acid that is bound by the aptasensors may comprise any portion of the SARS-CoV-2 genome. The SARS-CoV-2 genome is comprised of single-stranded positive-sense RNA. Suitable target sequences include those found in any of the major genes (i.e., the S, E, M, and N genes), in any of the 13-15 open reading frames, or in any non-coding region of the SARS-CoV-2 genome. Ideally, the target nucleic acid comprises a sequence that is specific to SARS-CoV-2, meaning that it is not present in the genome of other organisms. In the Examples, the inventors designed aptasensors that detect the SARS-CoV-2 genes Orflb, RdRp, spike, E, and N. The sequences of their aptasensors are provided in Tables 1-7, 10, 12, and 15-18 as SEQ ID NOs:1-118, 121-136, and 151-248. Thus, in some embodiments, the SARS-CoV-2 target nucleic acid is a portion of a SARS-CoV-2 gene selected from the group consisting of: Orflb, RdRp, spike, E, and N. In some embodiments, the aptasensor comprises a sequence selected from SEQ ID NOs:1-118, 121-136, and 151-248. However, the aptasensors provided herein (i.e., SEQ ID NOs:1-118, 121-136, and 151-248) can tolerate mutations, particularly in the toehold domain. The aptasensors may also comprise mutations in the b and c stem-forming domains (see
“Percentage of sequence identity” is determined by comparing two optimally aligned sequences over a comparison window, wherein the portion of the polynucleotide sequence in the comparison window may comprise additions or deletions (i.e., gaps) as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. The percentage is calculated by determining the number of positions at which the identical nucleic acid base or amino acid residue occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison and multiplying the result by 100 to yield the percentage of sequence identity. Protein and nucleic acid sequence identities are evaluated using the Basic Local Alignment Search Tool (“BLAST”), which is well known in the art (Karlin and Altschul, 1990, Proc. Natl. Acad. Sci. USA 87: 2267-2268; Altschul et al., 1997, Nucl. Acids Res. 25: 3389-3402). The BLAST programs identify homologous sequences by identifying similar segments, which are referred to herein as “high-scoring segment pairs,” between a query amino or nucleic acid sequence and a test sequence which is preferably obtained from a protein or nucleic acid sequence database. Preferably, the statistical significance of a high-scoring segment pair is evaluated using the statistical significance formula (Karlin and Altschul, 1990), the disclosure of which is incorporated by reference in its entirety. The BLAST programs can be used with the default parameters or with modified parameters provided by the user.
In a second aspect, the present invention provides methods of detecting SARS-CoV-2 in a sample. The methods comprise: (a) amplifying the SARS-CoV-2 target nucleic acid in the sample; (b) contacting the amplified nucleic acid with an aptasensor disclosed herein and the cognate ligand of its aptamer; and (c) detecting any signal produced by the aptamer binding to its cognate ligand. In these methods, detection of the signal indicates that SARS-CoV-2 is present in the sample.
Any sample can be tested for the presence of SARS-CoV-2 using the methods described herein. In some embodiments, the sample is obtained from a subject, e.g., a human or animal subject. In such cases, the sample may comprise saliva, a nasopharyngeal swab, blood, serum, or sputum. Other suitable samples include, without limitation, food samples, drinking water, environmental samples, agricultural products, plastic and packaging materials, paper, clothing fibers, and metal surfaces. In certain embodiments, the methods are used in food safety and biosecurity applications, such as screening food products and materials used in food processing or packaging for the presence of the virus. In some embodiments, the sample is heat inactivated (e.g., at 65° C.) or frozen (e.g., at −80° C.) prior to testing.
In the first step of the present methods, the SARS-CoV-2 target nucleic acid is amplified. Amplification may be performed using any known nucleic acid amplification method. In some embodiments, the amplification step is performed using a PCR-based method. Suitable PCR-based methods include, without limitation, standard PCR, quantitative PCR (qPCR), PCR-restriction fragment length polymorphism (PCR-RFLP), asymmetrical PCR, transcript mediated amplification (TMA), self-sustained sequence replication (3SR), and ligase chain reaction (LCA). In preferred embodiments, the amplification step is performed using an isothermal amplification method. Suitable isothermal amplification methods include, without limitation, nucleic acid sequence-based amplification (NASBA), loop-mediated isothermal amplification (LAMP), reverse transcription loop-mediated isothermal amplification (RT-LAMP), strand displacement amplification (SDA), recombinase polymerase amplification (RPA), reverse transcription recombinase polymerase amplification (RT-RPA), helicase-dependent amplification (HDA), reverse transcription helicase-dependent amplification (RT-HDA), nicking enzyme amplification reaction (NEAR), signal mediated amplification of RNA technology (SMART), rolling circle amplification (RCA), isothermal multiple displacement amplification (IMDA), single primer isothermal amplification (SPIA), and polymerase spiral reaction (PSR). In some embodiments, the amplification method involves performing reverse transcription and transcription in a single reaction. In the Examples, the inventors provide aptasensors that can be used to detect amplicons generated using the isothermal amplification methods NASBA, RT-RPA, and RT-LAMP. Thus, in some embodiments, the amplification method is selected from NASBA, RT-RPA, or RT-LAMP.
To allow for detection using the aptasensors disclosed herein, the amplification method must produce a single-stranded product. Some amplification methods, such as NASBA and LAMP, produce single-stranded regions that are suitable for binding. However, methods that produce a double-stranded DNA (dsDNA) product must be adapted, e.g., by supplying a higher concentration of one of the primers (akin to asymmetric PCR) or by adding a T7 promoter that facilitates transcription of dsDNA products into ssRNA.
At a minimum, the amplicons detected using the aptasensors disclosed herein should be at about 10 nucleotides in length, excluding primer binding sites. For example, an amplicon that is 12 nucleotides in length can hybridize with an aptasensor comprising a 4 nucleotide toehold, a 6 nucleotide b domain, and 2 nucleotide c domain, assuming that the melting temperature is at least room temperature.
In the second step of the present methods, the amplified nucleic acid is contacted with an aptasensor disclosed herein and the cognate ligand of its aptamer. The cognate ligand used with the present invention may be any ligand that generates a detectable signal upon binding to the aptamer portion of the aptasensor. Suitable cognate ligands include, without limitation, -Difluoro-4-Hydroxybenzylidene)-2-Methyl-1-(2,2,2-Trifluoroethyl)-1H-Imidazol-5(4 H)-One (DFHBI-1T), 3,5-difluoro-4-hydroxybenzylidene-imidazolinone-2-oxime (DFHO), (Z)-3-((1H-benzo[d]imadazol-4-yl)methyl)-5-(3,5-difluoro-4-hydroxybenzylidene)-2-methyl-3,5-dihydro-4H-imidazol-4-one] (BI), and 3,5-difluoro-4-hydroxybenzylidene-imidazolinone-2-oxime-1-benzoimidazole (OBI).
In the final step of the present methods, any signal produced by the aptamer binding to its cognate ligand is detected. The detection method used in this step may be quantitative (i.e., measure the amount of the SARS-CoV-2 target nucleic acid present in the sample) or qualitative (i.e., simply determine whether the SARS-CoV-2 target nucleic acid is present in the sample at a detectable level). In embodiments in which the detectable signal is a colorimetric signal, detection may be performed by eye or using a spectrophotometer. In embodiments in which the detectable signal is a fluorescent signal, detection may be performed using a fluorescence instrument, such as a fluorometer, fluorospectrometer, or fluorescence spectrometer.
Alternatively, a fluorescent signal may be detected using a simple electronic reader comprising readily available components, as is described in Example 1 in the section titled “Detection equipment”. For example, the electronic reader may measure a fluorescence signal produced from a reaction that is placed into the reader between a light source (i.e., that supplies light of an appropriate wavelength to excite a fluorophore cognate ligand) and electronic sensors (i.e., that detect any emission produced by an excited fluorophore cognate ligand). In some cases, the light source is a light emitting diode (LED) light source. In some cases, the electronic reader may be configured to measure the output of a freeze-dried, paper-based reaction. In other cases, it may be configured to measure the output of a liquid reaction. In some cases, the output is read using onboard electronics that provide low-noise measurements of signal changes.
In the Examples, the inventors demonstrate that the methods of the present invention are highly sensitive. For instance, the inventors achieved a limit of detection of 2 copies of SARS-CoV-2 per 30-μL reaction or 0.13 aM using RT-LAMP primers amplifying the spike gene of SARS-CoV-2 and a single Broccoli aptasensor (see
Additionally, the inventors demonstrate that their aptasensors rapidly produce a detectable signal upon binding to a SARS-CoV-2 target nucleic acid. Thus, in some embodiments, the signal, if present, is detectable in less than 1 hour. In some embodiments, the signal, if present, is detectable in less than 50 minutes, less than 40 minutes, less than 30 minutes, less than 20 minutes, less than 15 minutes, or less than 10 minutes.
To verify that the sample has been correctly processed for use in these methods and reduce false negative results, it may be advantageous to use a positive control. Thus, in some embodiments, the methods further comprise amplifying a control nucleic acid in the sample and detecting the amplified control nucleic acid. The “control nucleic acid” may be any nucleic acid that is expected to be present in all of the samples tested. For example, when a sample is from a human patient, a human gene product can be used as a control nucleic acid. Following amplification, the control nucleic acid can be detected using any means of nucleic acid detection known in the art. Suitable methods for detecting nucleic acids include, without limitation, ethidium bromide staining, quantitative PCR, fluorometer detection, sequencing, and the like.
In some embodiments, the control nucleic acid is detected using an aptasensor. In the Examples, the inventors tested aptasensors that detect the control nucleic acids human RNase P mRNA, beta actin (ACTB) mRNA, and 18S rRNA. The sequences of these “control aptasensors” are provided in Tables 11, 13, 14, 20, 22, and 23 as SEQ ID NOs: 119-120, 137-150, and 249-256. Thus, in some embodiments, the control nucleic acid is selected from the group consisting of: human RNase P mRNA, beta actin (ACTB) mRNA, and 18S rRNA. In some embodiments, the control nucleic acid is detected using an aptasensor comprising a sequence selected from SEQ ID NOs: 119-120, 137-150, and 249-256.
In some embodiments, the methods involve the detection of two or more different nucleic acids (e.g., one or more SARS-CoV-2 target nucleic acids and, optionally, one or more control nucleic acids). This can be accomplished using a two-channel assay that utilizes two or more different aptamer-ligand pairs with different spectral properties, as described in Example 2.
The RNA genome of SARS-CoV-2 may not be accessible in an unprocessed sample. Thus, in the embodiments, the methods further comprise isolating, purifying, or extracting RNA prior to step (a). Suitable extraction methods for isolating viral RNA from saliva samples include, without limitation, protease K treatment, Triton X-100 processing, and use of ARCIS reagents. In some embodiments, the extraction is performed using a commercially available kit (e.g., PureLink RNA extraction kit). In preferred embodiments, any virus present in the sample is heat inactivated prior to step (a). Heat inactivation serves the dual-purpose of extracting the viral genome from virions and killing the virus, making this method safer to perform outside of a biosafety level 3 laboratory. Heat inactivation is performed by heating the sample to a temperature sufficient to kill the virus and to release its genomic RNA. For example, heat inactivation may be performed by subjecting the sample to a high temperature for at least about 3 minutes, about 5 minutes, about 10 minutes, about 20 minutes, or about 30 minutes. The temperature used for heat inactivation is preferably between about 60° C. and 150° C., and more preferably between about 60° C. and 100° C. In some embodiments, the inactivating step comprises heating the sample to about 65° C. for about 30 minutes. In other embodiments, the inactivating step comprises heating the sample to about 98° C. for about 5 minutes.
In some embodiments, the methods are adapted for high-throughput and/or rapid detection. For example, the method may utilize a high-throughput format, such as a multi-well plate (e.g., a 6-, 12-, 24-, 48-, 96-, 384-, or 1536-well plate). For convenience, the multi-well plate may be pre-aliquoted with a master mix for the amplification reaction (e.g., RT-LAMP enzyme, buffer, and primers) or for the aptasensor readout reaction (e.g., aptasensor RNA, cognate ligand, and buffer), as described in Example 2. The method may also utilize a device configured for rapid detection in a clinical setting or in the field. Such devices may comprise, for example, a preserved paper test article or test tubes comprising the aptasensor. In some embodiments, the aptasensor is freeze-dried (e.g., on a paper test article or in a test tube) to render it stable at room temperature. Kits
In a third aspect, the present invention provides kits for detecting SARS-CoV-2 comprising the aptasensors disclosed herein. Optionally, the kits can further include instructions and/or additional reagents for performing the SARS-CoV-2 detection methods described herein.
In some embodiments, the kits further comprise primers that can be used to specifically amplify the SARS-CoV-2 target nucleic acid. The primers in the kit may be suitable for use with any amplification method. In some embodiments, the primers in the kit are designed for use in an isothermal amplification method, such as NASBA, LAMP, RT-LAMP, RPA, RT-RPA, HDA, or RT-HDA.
In some embodiments, the kits further comprise reagents that allow for the detection of a control nucleic acid, such as primers that specifically amplify the control nucleic acid and/or an aptasensor that binds to the control nucleic acid.
The present disclosure is not limited to the specific details of construction, arrangement of components, or method steps set forth herein. The compositions and methods disclosed herein are capable of being made, practiced, used, carried out and/or formed in various ways that will be apparent to one of skill in the art in light of the disclosure that follows. The phraseology and terminology used herein is for the purpose of description only and should not be regarded as limiting to the scope of the claims. Ordinal indicators, such as first, second, and third, as used in the description and the claims to refer to various structures or method steps, are not meant to be construed to indicate any specific structures or steps, or any particular order or configuration to such structures or steps. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to facilitate the disclosure and does not imply any limitation on the scope of the disclosure unless otherwise claimed. No language in the specification, and no structures shown in the drawings, should be construed as indicating that any non-claimed element is essential to the practice of the disclosed subject matter. The use herein of the terms “including,” “comprising,” or “having,” and variations thereof, is meant to encompass the elements listed thereafter and equivalents thereof, as well as additional elements. Embodiments recited as “including,” “comprising,” or “having” certain elements are also contemplated as “consisting essentially of” and “consisting of” those certain elements.
Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. For example, if a concentration range is stated as 1% to 50%, it is intended that values such as 2% to 40%, 10% to 30%, or 1% to 3%, etc., are expressly enumerated in this specification. These are only examples of what is specifically intended, and all possible combinations of numerical values between and including the lowest value and the highest value enumerated are to be considered to be expressly stated in this disclosure. Use of the word “about” to describe a particular recited amount or range of amounts is meant to indicate that values very near to the recited amount are included in that amount, such as values that could or naturally would be accounted for due to manufacturing tolerances, instrument and human error in forming measurements, and the like. All percentages referring to amounts are by weight unless indicated otherwise.
No admission is made that any reference, including any non-patent or patent document cited in this specification, constitutes prior art. In particular, it will be understood that, unless otherwise stated, reference to any document herein does not constitute an admission that any of these documents forms part of the common general knowledge in the art in the United States or in any other country. Any discussion of the references states what their authors assert, and the applicant reserves the right to challenge the accuracy and pertinence of any of the documents cited herein. All references cited herein are fully incorporated by reference, unless explicitly indicated otherwise. The present disclosure shall control in the event there are any disparities between any definitions and/or description found in the cited references.
The following examples are meant only to be illustrative and are not meant as limitations on the scope of the invention or of the appended claims.
EXAMPLES
Example 1
In the following Example, the inventors describe their rapid, low-cost, highly sensitive method for detection of SARS-CoV-2. As illustrated in
A general schematic depicting this method is shown in
Libraries of aptasensors targeting multiple regions of the SARS-CoV-2 genome were designed for use in this method. The main target regions for these sensors are shown in
Aptasensors based on the Broccoli and Corn aptamers were designed computationally using a custom algorithm. The resulting sensor transcripts were then screened for function by challenging them with synthetic versions of the SARS-CoV-2 genomic targets after in vitro transcription.
Tests were also conducted with multiple other libraries of Broccoli aptasensors targeting different regions of the SARS-CoV-2 genome, including: (A) the sense orientation of Orflb, (B) the sense orientation of the N gene, (C) the sense orientation of the E gene, and (D) the sense orientation of the N gene. ON/OFF ratios for libraries of aptasensors targeting Orflb, the N gene, and E gene are shown in
Results from aptasensors based on the Corn aptamer are shown in
The top-performing aptasensors were next coupled to isothermal amplification reactions to ensure that they could reach the clinically relevant detection limit of SARS-CoV-2 RNA. A custom primer design algorithm was implemented to select NASBA and RPA primers having optimal specificity, secondary structure, and sequence composition to enable amplification of the region about the binding site of the aptasensors. Experiments were first conducted to screen the resulting primers in 6-4, NASBA reactions supplied with 185 copies of SARS-CoV-2 RNA obtained from cultured virions. Reactions were incubated at 41° C. using 10 different NASBA primer pairs designed to amplify the Orflb target region (see Table 8 for primer sequences). The resulting amplicons were then added to solutions containing the Orflb Broccoli aptasensor and DFHBI-1T.
The optimal primer pair was then combined with the Orflb Broccoli aptasensor for a series of experiments supplying the amplification reactions with different concentrations of cultured SARS-CoV-2 RNA. These experiments revealed that the Broccoli aptasensor could provide significant fluorescence output for sample concentrations down to 23 RNA copies/μL, which corresponds to only 28 copies of RNA supplied to the 6-μL NASBA reaction. In addition, significant fluorescence was observed from the aptasensors immediately during the 37° C. measurement in the plate reader (
To demonstrate the robustness of the approach, a second set of NASBA primers was designed for the RdRP gene target of SARS-CoV-2 (see Table 9 for primer sequences). Primer screening experiments in
In preparation for experiments using RT-RPA for amplification, we have also validated a library of aptasensors targeting an RT-RPA amplicon. This particular amplicon is generated through an RT-RPA reaction amplifying the antisense orientation of the N gene of SARS-CoV-2 and it appends a T7 promoter site to the viral sequence to enable subsequent in vitro transcription. In previous experiments using toehold switches for sequence verification, the RT-RPA primers have provided a detection limit of 0.5 aM, corresponding to 15 copies of SARS-CoV-2 RNA in the 50 μL RT-RPA reaction.
Parallel sample control reactions that detect nucleic acids expected to be present in all samples are valuable for ensuring proper sample processing during tests. Aptasensors for sample controls were implemented to detect the human RNase P mRNA (SEQ ID NOs:119-120; see Table 11).
To reduce the likelihood of false-positive RT-LAMP assays, we also developed a library of aptasensors for detection of the DNA products of RT-LAMP amplification assays. It is noted that the LAMP primer sequences provided herein for
The capacity of these diagnostic tests to detect SARS-CoV-2 RNA using isothermal reactions close to human body temperature suggests that these systems could be used for in-home assays. Accordingly, the use of readily available components to detect fluorescence from activated Broccoli aptamers was explored.
As demonstrated herein, a simple assay has been developed that provides specific detection of SARS-CoV-2 RNA down to sample concentrations of 23 RNA copies/μL. In this assay, a target RNA is amplified using an isothermal amplification method, such as NASBA, RT-RPA, or RT-LAMP. Then, to reduce the possibility of false positive results due to non-specific amplification, computer-designed aptasensors are employed to verify the sequence of the amplified products and produce a strong and readily visible fluorescence signal for a positive test. Importantly, the reactions in this assay can be accomplished using simple heating procedures and incubation near human body temperature, facilitating the transition to in-home use. Furthermore, the assays can be visualized using simple, readily available equipment that can be obtained for $20 to $24. Each target RNA can be detected in the reactions for as little as $3.09 per result. This assay can be used to detect more than one analyte at the same time by harnessing the different optical properties of various aptamer/fluorogen combinations. This capability can be used to reduce assays costs and reduce the likelihood of false positive results. In addition to Broccoli and Corn, other aptamers (e.g., Red and Orange Broccoli) can be used in the general toehold-mediated aptasensor design described herein. Inclusion of additional aptamers could be used to increase assay multiplexing capacity or allow the assay results to be interpreted by other fluorescence detection systems.
In the following Example, the inventors describe an improved aptasensor design and an improved SARS-CoV-2 detection assay that employs reverse transcription loop-mediated amplification (RT-LAMP) for isothermal amplification. Further, they describe multiplexed and one-pot variations of the RT-LAMP-based assay, and they validate their method against clinical samples.
The improved aptasensor design makes a few key changes over the aptasensors described in Example 1. This design features an improved sensing hairpin. The previous aptasensors employed a hairpin that spanned 20 nucleotides and featured two bulges, 4 and 8 bases from the top of the stem, to reduce the likelihood of premature transcriptional termination. The improved aptasensors do not have any bulges within the stem, which reduces signal leakage without decreasing transcriptional efficiency. The stem itself is varied from 12 bp to 21 bp, depending on the properties of the target RNA and the particular output aptamer. For output aptamers that have a middle stem-loop that does not have a fully conserved sequence (e.g. Broccoli, Red Broccoli, and Orange Broccoli), we designed appropriate RNA sequences for each aptasensor that will ensure that this middle stem-loop folds into a strong secondary structure and avoids pairing elsewhere within the aptasensor (see cyan region of
In addition to these changes, we also tested the updated fluorogens BI [full name: (Z)-3-((1H-benzo[d]imadazol-4-yl)methyl)-5-(3,5-difluoro-4-hydroxybenzylidene)-2-methyl-3,5-dihydro-4H-imidazol-4-one] and OBI [full name: 3,5-difluoro-4-hydroxybenzylidene-imidazolinone-2-oxime-1-benzoimidazole (OBI)]. These new fluorogens provided stronger fluorescence when bound to the aptamers, with BI pairing with Broccoli aptasensors and OBI pairing with Red Broccoli aptasensors.
We have developed an improved SARS-CoV-2 detection assay employing reverse transcription loop-mediated amplification (RT-LAMP) for isothermal amplification of the SARS-CoV-2 genome and the sample control RNA (either ACTB mRNA or 18S rRNA). RT-LAMP has an operating temperature of —60° C. to 65° C. and uses a set of four to six primers to generate DNA products containing exposed, single-stranded loop domains. These loop domains are targeted by two to four of the LAMP primers and are essential for the amplification process. We thus designed aptasensors that targeted these exposed loop domains, as illustrated in
Since RT-LAMP produces amplicons with two loop regions that have unrelated sequences, we also implemented a scheme to target the two independent loop regions in the same reaction (
Validation data from of the dual loop detection scheme is presented in
We proceeded to evaluate a combined two-pot assay wherein RT-LAMP is performed first on a sample of RNA for 10 minutes to 60 minutes at 60° C. to 65° C. The resulting RT-LAMP product is then diluted into a second pot or reaction vessel containing the aptasensors designed to target the loop domains of the RT-LAMP amplicons. Detection limit tests were performed with the two-pot RT-LAMP aptasensor assay using different combinations of primers and different aptasensors to gauge its sensitivity. In general, the sensitivity of the assay is equivalent to RT-qPCR tests conventionally used for SARS-CoV-2 detection in clinical samples.
A limitation of the two-pot RT-LAMP assays of the previous section is that they only detected one target amplicon at a time. Taking advantage of the different spectral properties of aptamers with their companion fluorogens, we implemented a two-channel two-pot RT-LAMP assay capable of simultaneously detecting two different targets. Such as assays can lead to reduced costs and processing time. The two-channel reaction employs a first-step RT-LAMP reaction where the primers for both targets are present. These targets are then amplified over the course RT-LAMP for 10 to 60 minutes at 60° C. to 65° C. The reaction products are then diluted into a second pot containing aptasensors for each target and their companion fluorogens.
The two-channel, two-pot reaction was tested with input concentrations of 0.478 aM of SARS-CoV-2 RNA and 150 aM of ACTB mRNA. RT-LAMP was performed in a single reaction with 12 total primers for amplification of both SARS-CoV-2 and ACTB RNAs simultaneously. The resulting products were then added to detection reactions containing different combinations of aptasensors and fluorogens. Fluorescence output from each of the reactions was measured in the Broccoli channel with green fluorescence and the Corn channel with orange fluorescence (
One-pot diagnostic assays where all reaction steps occur in the same reaction vessel and do not require the additional reagents to be added after the start of the reaction are highly desirable. Such assays reduce processing time, time to result, and the likelihood of cross-contamination. Accordingly, we have developed one-pot RT-LAMP/aptasensor assays for detection of SARS-CoV-2 RNA. The one-pot assay process is schematically illustrated in
Results from a one-channel reaction are shown in
In other cases, the one-pot assay is performed with lx concentrations of the RT-LAMP primers (i.e., 1.6 μM for FIP and BIP, 0.2 μM for F3 and B3, and 0.4 μM for LF and LB primers). In some cases, the one-pot assay is performed with no ions added with 2 μM of BI. In other cases, 2 μM of BI is used with 40 mM KI and 1 mM MgCl2. In some cases, the assay is performed with only one aptasensor provided at a concentration of 0.5 μM.
We next exploited the multiplexing capabilities of our aptasensor systems by implementing a two-channel, one-pot RT-LAMP/aptasensor assay. This assay again combines RT-LAMP components with aptasensor reagents. Primers for both the SARS-CoV-2 N gene and the ACTB mRNA control were provided at 1.5× the standard RT-LAMP concentration. The fluorogens BI and DFHO were present at 2 μM and 0.5 μM, respectively. No additional ions were added. The SARS-CoV-2 N gene was targeted with dual Broccoli aptasensors at 0.5 μM concentration, while a single Corn aptasensor at 0.5 μM was used for detection of ACTB mRNA.
The general SARS-CoV-2 detection assay developed by our lab requires the following steps. A patient saliva sample is subjected to viral RNA extraction and amplified by reverse transcription loop-mediated isothermal amplification (RT-LAMP). The amplified product is then detected using aptamer-based sensors referred to as aptasensors, which are designed to detect SARS-CoV-2 genetic material and the human actin mRNA sequence. The latter transcript serves a sample control to ensure proper sample processing. The computer-designed aptasensors recognize specific target sequences and form the active structure of the output aptamer only after binding to the viral target RNA. Fluorescence from the aptasensors is generated when a non-fluorescent dye ligand interacts with the aptamer and generates a fluorescently active conformation once bound to the aptamer binding site. The material costs for the assay are approximately $4/test when detecting one SARS-CoV-2 target and the actin control mRNA.
Following FDA EUA requirements, we have validated the assay using a panel of 30 positive and 30 negative patient samples. These samples were obtained from the Biodesign Institute clinical testing laboratory and were provided by patients as saliva samples. RNA from the samples was first extracted using a PureLink RNA extraction kit and SARS-CoV-2 RNA concentrations quantified via RT-qPCR using the TaqMan 2019-nCoV Assay Kit. The extracted RNA was supplied to the RT-LAMP reactions and incubated at 61° C. for 45 minutes. During the amplification, SARS-CoV-2 RNA in the spike gene was amplified at the same time as the actin control mRNA. Following RT-LAMP, the DNA product was then added to a mixture containing a Broccoli aptasensor for the spike gene, a Corn aptasensor for actin mRNA, and the fluorogenic dyes DFHBI-1T for Broccoli and DFHO for Corn. This readout reaction was then measured in 384-well plates in a plate reader at 37° C. while monitoring the green and yellow fluorescence from the Broccoli and Corn aptasensors, respectively.
We investigated multiple fast extraction methods for isolating viral RNA from saliva samples, including protease K treatments, Triton X-100 processing, and use of ARCIS reagents. From these studies, we found that the simplest method, a brief 5-minute heating step at 98° C., provided the best combination of extraction speed and assay results. We then applied the 98° C. extraction method to a panel of 10 positive and 10 negative clinical saliva samples. Aliquots of 12 μL of the saliva sample were heated at 98° C. for 5 minutes and 1.5 μL of the resulting product was added to RT-LAMP reactions at a final volume of 30 μL. After incubation for 45 minutes at 61° C., the RT-LAMP product was added the aptasensor/dye solution for readout.
To increase the throughput and ease of implementation of the tests, we have implemented a streamlined approach for 384-well assay processing. Since the assay employs two separate reaction steps, RT-LAMP and aptasensor readout, we developed master mix formulations for reactions that can be stably stored at −20° C. and rapidly added to 384-well plates at the time of use. For the RT-LAMP reactions, the master mix contains the RT-LAMP enzyme, buffer, and primers. It can be stored for multiple weeks in the freezer and remain active. Moreover, the mix can be provided pre-aliquoted into the wells of a 384-well plate and stored at −20° C. Upon thawing, the reactions can be started in the plate immediately after addition of the RNA sample to each well. For the aptasensor readout reactions, master mixes containing the aptasensor RNAs, buffer, DFHBI-1T, and DFHO also remained stable under -20° C. storage and could be aliquoted into a 384-well plate prior to measurement. In addition to the master mix formulations, we also optimized the assay by reducing the RT-LAMP step from 45 minutes to 30 minutes without affecting assay sensitivity.
Results from the parallelized 384-well assay are shown in
Analysis of the Broccoli fluorescence curves and Corn fluorescence from the plate revealed that the high-throughput assay was highly effective at identifying SARS-CoV-2. Of the positive samples, 176 out of 176 were correctly identified. Similarly, 192 out of 192 negative samples were correctly assigned. In the same plate, we also tested the specificity of the assay against potential confounding viruses, in particular multiple other human coronaviruses and influenza. These confounding samples showed no activation of the Broccoli aptasensor, further demonstrating the excellent specificity of the SARS-CoV-2 test.
This application claims priority to U.S. Provisional Patent Application No. 63/070,543, filed Aug. 26, 2020, the contents of which are incorporated by reference in their entireties.
This invention was made with government support under GM126892 and R21 AI136571 awarded by the National Institutes of Health and 2029532 awarded by the National Science Foundation. The government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind |
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PCT/US2021/047675 | 8/26/2021 | WO |
Number | Date | Country | |
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63070543 | Aug 2020 | US |