Patent Application
20060257915
1. Field of the Invention
The present invention relates to the field of aptamer- and nucleic acid-based diagnostics. More particularly, it relates to methods for the production and use of fluorescence resonance energy transfer (“FRET”) DNA or RNA aptamers for competitive displacement aptamer assay formats. The present invention provides for aptamer-related FRET assay schemes involving competitive displacement formats in which the aptamer contains fluorophores (“F”) (is F-labeled) and the target contains quenchers (“Q”) (is Q-labeled), or vice versa. The aptamer can be F-labeled or Q-labeled by incorporation of the F or Q derivatives of nucleotide triphosphates. Incorporation may be accomplished by simple chemical conjugations through bifunctional linkers, or key functional groups such as aldehydes, carbodiimides, carboxyls, N-hydroxy-succinimide (NHS) esters, thiols, etc.
2. Background Information
Competitive displacement aptamer-FRET is a new class of assay desirable for its use in rapid (within minutes), one-step, homogeneous assays involving no wash steps (simple bind and detect quantitative assays). Others have described FRET-aptamer methods for various target analytes that consist of placing the F and Q moieties either on the 5′ and 3′ ends respectively to act like a “molecular (aptamer) beacon” or placing only F in the heart of the aptamer structure to be “quenched” by another proximal F or the DNA or RNA itself. These preceding FRET-aptamer methods are all highly engineered and based on some prior knowledge of particular aptamer sequences and secondary structures, thereby enabling clues as to where F might be placed in order to optimize FRET results.
The nucleic acid-based “molecular beacons” snap open upon binding to an analyte or upon hybridizing to a complementary sequence, but beacons have always been end-labeled with F and Q at the 3′ and 5′ ends. The present invention provides that F-labeled or Q-labeled aptamers may be labeled anywhere in their structure that places the F or Q within the Forster distance of approximately 60-85 Angstroms of the corresponding F or Q on the labeled target analyte to achieve quenching prior to or after target analyte binding to the aptamer “binding pocket” (typically a “loop” in the secondary structure). The F and Q molecules used can include any number of appropriate fluorophores and quenchers as long as they are spectrally matched so the emission spectrum of F overlaps significantly (almost completely) with the absorption spectrum of Q.
A process in which F and Q are incorporated into an aptamer population is generally referred to as “doping.” The present invention provides a new method for natural selection of F-labeled or Q-labeled aptamers that contain F-NTPs or Q-NTPs in the heart of an aptamer binding loop or pocket by PCR or other enzymatic means. The present invention describes a type of aptamer in which F and Q are incorporated into an aptamer population via their nucleotide triphosphate derivatives (for example, Alexfluor™-NTPs, Cascade Blue®-NTPs, Chromatide®-NTPs, fluorescein-NTPs, rhodamine-NTPs, Rhodamine Green™-NTPs, tetramethylrhodamine-dNTPs, Oregon Green®-NTPs, and Texas Red®-NTPs may be used to provide the fluorophores, while dabcyl-NTPs, Black Hole Quencher or BHQ™-NTPs, and QSY™ dye-NTPs may be used for the quenchers) by PCR after several rounds of selection and amplification without the F- and Q-modified bases. The advantage of this F or Q “doping” method is two-fold: 1) the method allows nature to take its course and select the most sensitive F-labeled or Q-labeled aptamer target interactions in solution, and 2) the positions of F or Q within the aptamer structure can be determined via exonuclease digestion of the F-labeled or Q-labeled aptamer followed by mass spectral analysis of the resulting fragments, thereby eliminating the need to “engineer” the F or Q moieties into a prospective aptamer binding pocket or loop. Sequence and mass spectral data can be used to further optimize the competitive aptamer-FRET assay performance after natural selection as well.
If the target molecule is a larger water-soluble molecule such as a protein, glycoprotein, or other water soluble macromolecule, then exposure of the nascent F-labeled and Q-labeled DNA or RNA random library to the free target analyte is done in solution. If the target is a soluble protein or other larger water-soluble molecule, then the optimal FRET-aptamer-target complexes are separated by size-exclusion chromatography. The FRET-aptamer-target complex population of molecules is the heaviest subset in solution and will emerge from a size-exclusion column first, followed by unbound FRET-aptamers and unbound proteins or other targets. Among the subset of analyte-bound aptamers there will be heterogeneity in the numbers of F- and Q-NTPs that are incorporated as well as nucleotide sequence differences, which will again effect the mass, electrical charge, and weak interaction capabilities (e.g., hydrophobicity and hydrophilicity) of each analyte-aptamer complex. These differences in physical properties of the aptamer-analyte complexes can then be used to separate out or partition the bound from unbound analyte-aptamer complexes.
If the target is a small molecule (generally defined as a molecule with molecular weight of ≦1,000 Daltons), then exposure of the nascent F-labeled and Q-labeled DNA or RNA random library to the target is done by immobilizing the target. The small molecule can be immobilized on a column, membrane, plastic or glass bead, magnetic bead, or other matrix. If no functional group is available on the small molecule for immobilization, the target can be immobilized by the Mannich reaction (formaldehyde-based condensation reaction) on a device similar to a PharmaLink™ column. Elution of bound DNA from the small molecule affinity column, membrane, beads or other matrix by use of 0.2-3.0M sodium acetate at a pH ranging between 3 and 7, although the optimal pH is approximately 5.2.
The candidate FRET-aptamers are separated based on physical properties such as charge or weak interactions by various types of high performance liquid chromatography (“HPLC”), digested at each end with specific exonucleases (snake venom phosphodiesterase on the 3′ end and calf spleen phosphodiesterase on the 5′ end). The resulting oligonucleotide fragments, each one base shorter than the predecessor, are subjected to mass spectral analysis which can reveal the nucleotide sequences as well as the positions of F and Q within the FRET-aptamers. Once the FRET-aptamer sequence is known with the positions of F and Q, it can be further manipulated during solid-phase DNA or RNA synthesis in an attempt to make the FRET assay more sensitive and specific.
The competitive displacement aptamer-FRET assay format of the present invention is unique. The competitive format generally requires a lower affinity aptamer in order to be able to release the F-labeled or Q-labeled target analyte and allow competition for the binding site. This may lead to less sensitivity in some assays.
Referring to the figures,
F-labeled or Q-labeled aptamers (labeled by the polymerase chain reaction (PCR) or other enzymatic incorporation of F-NTPs or Q-NTPs) may be used in competitive or displacement type assays in which the fluorescence light levels change proportionately in response to the addition of various levels of unlabeled analyte which compete to bind with the F-labeled or Q-labeled analytes.
Competitive aptamer-FRET assays may be used for the detection and quantitation of small molecules (<1,000 Daltons) including pesticides, natural and synthetic amino acids and their derivatives (e.g., histidine, histamine, homocysteine, DOPA, melatonin, nitrotyrosine, etc.), short chain proteolysis products such as cadaverine, putrescine, the polyamines spermine and spermidine, nitrogen bases of DNA or RNA, nucleosides, nucleotides, and their cyclical isoforms (e.g., cAMP and cGMP), cellular metabolites (e.g., urea, uric acid), pharmaceuticals (therapeutic drugs), drugs of abuse (e.g., narcotics, hallucinogens, gamma-hydroxybutyrate, etc.), cellular mediators (e.g., cytokines, chemokines, immune modulators, neural modulators, inflammatory modulators such as prostaglandins, etc.), or their metabolites, explosives (e.g., trinitrotoluene) and their breakdown products or byproducts, peptides and their derivatives, macromolecules including proteins (such as bacterial surface proteins from Leishmania donovani, See
If the target molecule is a larger water-soluble molecule such as a protein, glycoprotein, or other water soluble macromolecule, then exposure of the nascent F-labeled and Q-labeled DNA or RNA random library to the free target analyte is done in solution. If the target is a soluble protein or other larger water-soluble molecule, then the optimal FRET-aptamer-target complexes are separated by size-exclusion chromatography. The FRET-aptamer-target complex population of molecules is the heaviest subset in solution and will emerge from a size-exclusion column first, followed by unbound FRET-aptamers and unbound proteins or other targets. Among the subset of analyte-bound aptamers there will be heterogeneity in the numbers of F- and Q-NTPs that are incorporated as well as nucleotide sequence differences, which will again effect the mass, electrical charge, and weak interaction capabilities (e.g., hydrophobicity and hydrophilicity) of each analyte-aptamer complex. These differences in physical properties of the aptamer-analyte complexes can then be used to separate out or partition the bound from unbound analyte-aptamer complexes.
If the target is a small molecule, then exposure of the nascent F-labeled and Q-labeled DNA or RNA random library to the target may be done by immobilizing the target. The small molecule can be immobilized on a column, membrane, plastic or glass bead, magnetic bead, or other matrix. If no functional group is available on the small molecule for immobilization, the target can be immobilized by the Mannich reaction (formaldehyde-based condensation reaction) on a PharmaLink™ column. Elution of bound DNA from the small molecule affinity column, membrane, beads or other matrix by use of 0.2-3.0M sodium acetate at a pH ranging between 3 and 7, although the optimal pH is approximately 5.2.
These can be separated from the non-binding doped DNA molecules by running the aptamer-protein aggregates (or selected aptamers-protein aggregates) through a size-exclusion column, by means of size-exclusion chromatography using Sephadex™ or other gel materials in the column. Since they vary in weight due to variations in aptamers sequences and degree of labeling, they can be separated into fractions with different fluorescence intensities. Purification methods such as preparative gel electrophoresis are possible as well. Small volume fractions (≦1 mL) can be collected from the column and analyzed for absorbance at 260 nm and 280 nm which are characteristic wavelengths for DNA and proteins. The heaviest materials come through a size-exclusion column first. Therefore, the DNA-protein complexes will come out of the column before either the DNA or protein alone.
Means of separating FRET-aptamer-target complexes from solution by alternate techniques (other than size-exclusion chromatography) include, without limitation, molecular weight cut off spin columns, dialysis, gel electrophoresis, thin layer chromatography (TLC), and differential centrifugation using density gradient materials.
The optimal (most sensitive or highest signal to noise ratio) FRET-aptamers among the bound class of FRET-aptamer-target complexes are identified by assessment of fluorescence intensity for various fractions of the FRET-aptamer-target class. The separated DNA-protein complexes will exhibit the highest absorbance at established wavelengths, such as 260 nm and 280 nm. The fractions showing the highest absorbance at the given wavelengths, such as 260 nm and 280 nm, are then further analyzed for fluorescence and those fractions exhibiting the greatest fluorescence are selected for separation and sequencing.
These similar FRET-aptamers may be further separated using techniques such as ion pair reverse-phase high performance liquid chromatography, ion-exchange chromatography (IEC, either low pressure or HPLC versions of IEC), thin layer chromatography (TLC), capillary electrophoresis, or similar techniques.
The final FRET-aptamers are able to act as one-step “lights on” or “lights off” binding and detection components in assays.
Competitive FRET-aptamers that are to be used in assays with long shelf-lives may be lyophilized (freeze dried) and then later reconstituted.
Competitive Aptamer-FRET Assay for Surface Proteins Extracted from Bacteria (L. donovani).
In this example, surface proteins from heat-killed Leishmania donovani were extracted with 3 M MgCl2 overnight at 4° C. These proteins were then linked to tosyl-magnetic microbeads and used in a standard SELEX aptamer generation protocol. After 5 rounds of SELEX, the aptamer population was “doped” during the standard PCR reaction with 3 uM fluorescein-dUTP and purified on 10 kD molecular weight cut off spin columns. Some of the L. donovani surface proteins were then labeled with dabcyl-NHS ester and purified on a PD-10 (Sephadex G25) column. The dabcyl-labeled surface proteins were combined with the fluorescein-labeled aptamer population so as to produce a 1:1 fluorescein-aptamer:dabcyl-protein ratio. Thereafter, unlabeled L. donovani surface proteins were introduced into the assay system to compete with the labeled proteins for binding to the aptamers, thereby producing the “lights off” FRET assay results depicted in
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limited sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the inventions will become apparent to persons skilled in the art upon the reference to the description of the invention. It is, therefore, contemplated that the appended claims will cover such modifications that fall within the scope of the invention.
This application is based upon and claims priority from U.S. Provisional application Ser. No. 60/681,084, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60681084 | May 2005 | US |
