Techniques that allow specific detection of target molecules or analytes are important for many areas of research, as well as for clinical diagnostics. Central to most detection techniques are ligands that dictate specific and high affinity binding to a target molecule of interest. In immunodiagnostic assays antibodies mediate specific and high affinity binding, whereas in assays detecting nucleic acid target sequences, complementary oligonucleotide probes fulfill this role. To date, antibodies have been able to provide molecular recognition needs for a wide variety of target molecules and have been the popular choice of the class of ligands for developing diagnostic assays.
Recently, a novel class of oligonucleotide probes, referred to as molecular beacons, that facilitate homogeneous detection of specific nucleic acid target sequences has been described (Piatek et al. (1998) Nature Biotechnol. 16:359-363; Tyagi and Kramer (1996) Nature Biotecnol. 14:303-308). Molecular beacons are nucleic acid sequences that contain a fluorophore and a quencher (FIG. 1; star and filled circle, respectively). By design, molecular beacons are expected to fold into stem-loop structures in which the fluorophore is placed in close proximity to the quencher. When the molecular beacon is illuminated with light corresponding to the excitation wavelength of the fluorescent group, no fluorescence is observed, because energy transfer occurs between the fluorescent group and the quenching group, such that light emitted from the fluorescent group upon excitation is absorbed by the quenching group.
The loop region of molecular beacons is designed to contain a nucleotide sequence complementary to the target sequence of interest. When the molecular beacon is contacted with sequences complementary to the loop, the loop hybridizes to this sequence. This process is energetically favored as the intermolecular duplex formed is longer, and therefore more stable, than the intramolecular duplex formed in the stem region. As this intermolecular double helix forms, torsional forces are generated that cause the stem region to unwind. As a result, the fluorescent group and the quenching group become spatially separated such that the quenching group is no longer able to efficiently absorb light emitted from the fluorescent group. Thus, binding of the molecular beacon to its target nucleic acid sequence is accompanied by an increase in fluorescence emission from the fluorescent group. Molecular beacons undergo intermolecular hybridization upon interaction with the specific target sequence. Molecular beacons have been used for homogeneous detection of specific nucleic acid sequences, both DNA and RNA (Leone et al. (1998) Nucleic Acids Research 26:2150-2155; Piatek et al. (1998) Nature Biotechnol. 16:359-363; Tyagi and Kramer (1996) Nature Biotecnol. 14:303-308).
It is possible to simultaneously use two or more molecular beacons with different sequence specificities in the same assay. In order to do this, each molecular beacon is labeled with at least a different fluorescent group. The assay is then monitored for the spectral changes characteristic for the binding of each particular molecular beacon to its complementary sequence. In this way, molecular beacons have been used to determine whether an individual is homozygous wild-type, homozygous mutant or heterozygous for a particular mutation. For example, using one quenched-fluorescein molecular beacon that recognizes the wild-type sequence and another rhodamine-quenched molecular beacon that recognizes a mutant allele, it is possible to genotype individuals for the .beta.-chemokine receptor (Kostrikis et al. (1998) Science 279:1228-1229). The presence of only a fluorescein signal indicates that the individual is wild-type, and the presence of rhodamine signal only indicates that the individual is a homozygous mutant. The presence of both rhodamine and fluorescein signal is diagnostic of a heterozygote. Tyagi and coworkers have even described the simultaneous use of four differently labeled molecular beacons for allele discrimination. (Tyagi et al. (1998) Nature Biotechnology 16:49-53).
Although useful for the detection of nucleic acid targets, molecular beacons have not been used for detecting other types of molecules. Indeed, there has been no suggestion made in the art that molecular beacons can be used for anything other than detecting specific nucleic acids in mixtures containing a plurality of nucleic acids. Detection of nucleic acids is undeniably important, but in many applications--especially medical diagnostic scenarios--detection of non-nucleic acid molecules, such as proteins, sugars, and small metabolites, is required.
In general, the detection of non-nucleic acid target molecules is a more complicated matter than the detection of nucleic acids, and no single method is universally applicable. Specific proteins may be detected through the use of antibody-based assays, such as an enzyme linked immunoassay (ELISA). In one form of ELISA, a primary antibody binds to the protein of interest, and signal amplification is achieved using a labeled secondary antibody that can bind to multiple sites on the primary antibody. This technique can only be used to detect molecules for which specific antibodies exist. The generation of new antibodies is a time consuming and very expensive procedure and many proteins are not sufficiently immunogenic to generate antibodies in host animals. Furthermore, it is often necessary to measure and detect small molecules, such as hormones and sugars. that are generally not amenable to antibody recognition. In these cases, enzymatic assays for the specific molecule are often required.
The discovery of the SELEX.TM. (Systematic Evolution of Ligands by EXponential enrichment) process enables the identification of nucleic acid-based ligands, referred to as aptamers, that recognize molecules other than nucleic acids with high affinity and specificity (Ellington and Szostak (1990) Nature 346:818-822; Gold et al. (1995) Ann. Rev. Biochem. 64:763-797; Tuerk and Gold (1990) Science 249:505-510). Aptamers have been selected to recognize a broad range of targets, including small organic molecules as well as large proteins (Gold et al. (1995) Ann. Rev. Biochem. 64:763-797; Osborne and Ellington (1997) Chem. Rev. 97:349-370). In most cases, affinities and specificities of aptamers to these targets are comparable or better than those of antibodies. In contrast to antibodies whose identification and production completely rest on animals and/or cultured cells, both the identification and production of aptamers takes place in vitro without any requirement for animals or cells. Aptamers are produced by solid phase chemical synthesis, an accurate and reproducible process with consistency among production batches. Aptamers are stable to long-term storage at room temperature. Moreover, once denatured, aptamers can easily be renatured, a feature not shared by antibodies. These inherent characteristics of aptamers make them attractive for diagnostic applications.
The SELEX process is a method for the in vitro evolution of nucleic acid molecules with highly specific binding to target molecules and is described in U.S. patent application Ser. No. 07/536,428. filed Jun. 11, 1990, entitled "Systematic Evolution of Ligands by Exponential Enrichment," now abandoned; U.S. patent application Ser. No. 07/714,131, filed Jun. 10, 1991. entitled "Nucleic Acid Ligands," now U.S. Pat. No. 5,475,096; U.S. patent application Ser. No. 07/931,473, filed Aug. 17, 1992, entitled "Methods for Identifying Nucleic Acid Ligands," now U.S. Pat. No. 5,270,163 (see also, WO 91/19813), each of which is specifically incorporated by reference herein in its entirety. Each of these applications, collectively referred to herein as the SELEX patent applications, describes a fundamentally novel method for making a nucleic acid ligand to any desired target molecule. The SELEX process provides a class of products which are referred to as nucleic acid ligands or aptamers, each ligand having a unique sequence, and which has the property of binding specifically to a desired target compound or molecule. Each SELEX process-identified nucleic acid ligand is a specific ligand of a given target compound or molecule. The SELEX process is based on the unique insight that nucleic acids have sufficient capacity for forming a variety of two- and three-dimensional structures and sufficient chemical versatility available within their monomers to act as ligands (form specific binding pairs) with virtually any chemical compound, whether monomeric or polymeric. Molecules of any size or composition can serve as targets.
The SELEX method involves selection from a mixture of candidate oligonucleotides and step-wise iterations of binding, partitioning and amplification, using the same general selection scheme, to achieve virtually any desired criterion of binding affinity and selectivity. Starting from a mixture of nucleic acids, preferably comprising a segment of randomized sequence, the SELEX method includes steps of contacting the mixture with the target under conditions favorable for binding, partitioning unbound nucleic acids from those nucleic acids which have bound specifically to target molecules, dissociating the nucleic acid-target complexes, amplifying the nucleic acids dissociated from the nucleic acid-target complexes to yield a ligand-enriched mixture of nucleic acids, then reiterating the steps of binding, partitioning, dissociating and amplifying through as many cycles as desired to yield highly specific high affinity nucleic acid ligands to the target molecule.
The basic SELEX method has been modified to achieve a number of specific objectives. For example U.S. patent application Ser. No. 07/960,093, filed Oct. 14, 1992, entitled "Method for Selecting Nucleic Acids on the Basis of Structure," abandoned in favor of U.S. patent application Ser. No. 08/198,670, now U.S. Pat. No. 5,707,796, describes the use of the SELEX process in conjunction with gel electrophoresis to select nucleic acid molecules with specific structural characteristics, such as bent DNA. U.S. patent application Ser. No. 08/123,935, filed Sep. 17, 1993, entitled "Photoselection of Nucleic Acid Ligands," now abandoned (see U.S. patent application Ser. No. 08/612,895, filed Mar. 8, 1996. entitled "Systematic Evolution of Ligands by Exponential Enrichment: Photoselection of Nucleic Acid Ligands and Solution SELEX, now U.S. Pat. No. 5,763,177), describes a SELEX process-based method for selecting nucleic acid ligands containing photoreactive groups capable of binding and/or photocrosslinking to and/or photoinactivating a target molecule. U.S. patent application Ser. No. 08/134,028, filed Oct. 7. 1993, entitled "High-Affinity Nucleic Acid Ligands That Discriminate Between Theophylline and Caffeine," abandoned in favor of U.S. patent application Ser. No. 08/443,957, now U.S. Pat. No. 5,580,737, describes a method for identifying highly specific nucleic acid ligands able to discriminate between closely related molecules, which can be non-peptidic, termed Counter-SELEX. U.S. patent application Ser. No. 08/143,564, filed Oct. 25, 1993, entitled "Systematic Evolution of Ligands by Exponential Enrichment: Solution SELEX," abandoned in favor of U.S. patent application Ser. No. 08/461,069, now U.S. Pat. No. 5,567,588, describes a SELEX process-based method which achieves highly efficient partitioning between oligonucleotides having high and low affinity for a target molecule.
The SELEX method encompasses the identification of high-affinity nucleic acid ligands containing modified nucleotides conferring improved characteristics on the ligand, such as improved in vivo stability or improved delivery characteristics. Examples of such modifications include chemical substitutions at the ribose and/or phosphate and/or base positions. SELEX process-identified nucleic acid ligands containing modified nucleotides are described in U.S. patent application Ser. No. 08/117,991. filed Sep. 8, 1993. entitled "High Affinity Nucleic Acid ligands Containing Modified Nucleotides," abandoned in favor of U.S. patent application Ser. No. 08/430,709. now U.S. Pat. No. 5,660,985, that describes oligonucleotides containing nucleotide derivatives chemically modified at the 5- and 2'-positions of pyrimidines. U.S. patent application Ser. No. 08/134,028, supra, describes highly specific nucleic acid ligands containing one or more nucleotides modified with 2'-amino (2'-NH.sub.2). 2'-fluoro (2'-F), and/or 2'-O-methyl (2'-OMe). U.S. patent application Ser. No. 08/264,029, filed Jun. 22, 1994, entitled "Novel Method of Preparation of Known and Novel 2' Modified Nucleosides by Intramolecular Nucleophilic Displacement," now abandoned, describes oligonucleotides containing various 2'-modified pyrimidines.
The SELEX method encompasses combining selected oligonucleotides with other selected oligonucleotides and non-oligonucleotide functional units as described in U.S. patent application Ser. No. 08/284,063, filed Aug. 2, 1994, entitled "Systematic Evolution of Ligands by Exponential Enrichment: Chimeric SELEX," now U.S. Pat. No. 5,637,459 and U.S. patent application Ser. No. 08/234,997, filed Apr. 28, 1994, entitled "Systematic Evolution of Ligands by Exponential Enrichment: Blended SELEX," now U.S. Pat. No. 5,683,867, respectively. These applications allow the combination of the broad array of shapes and other properties, and the efficient amplification and replication properties of oligonucleotides with the desirable properties of other molecules.
U.S. patent application Ser. No. 07/964,624, filed Oct. 21, 1992, entitled "Nucleic Acid Ligands to HIV-RT and HIV-1 Rev," now U.S. Pat. No. 5,496,938, describes methods for obtaining improved nucleic acid ligands after SELEX has been performed. U.S. patent application Ser. No. 08/400,440, filed Mar. 8, 1995, entitled "Systematic Evolution of Ligands by Exponential Enrichment: Chemi-SELEX," now U.S. Pat. No. 5,705,337, describes methods for covalently linking a ligand to its target.
The SELEX method further encompasses combining selected nucleic acid ligands with lipophilic compounds or non-immunogenic, high molecular weight compounds in a diagnostic or therapeutic complex as described in U.S. patent application Ser. No. 08/434,465, filed May 4, 1995, entitled "Nucleic Acid Ligand Complexes," U.S. Pat. No. 6,011,020. VEGF nucleic acid ligands that are associated with a lipophilic compound, such as diacyl glycerol or dialkyl glycerol, in a diagnostic or therapeutic complex are described in U.S. patent application Ser. No. 08/739,109, filed Oct. 25, 1996. entitled "Vascular Endothelial Growth Factor (VEGF) Nucleic Acid Ligand Complexes," U.S. Pat. No. 5,859,228. VEGF nucleic acid ligands that are associated with a lipophilic compound, such as a glycerol lipid, or a non-immunogenic, high molecular weight compound, such as polyethylene glycol, are further described in U.S. patent application Ser. No. 08/897,35 1 filed Jul. 21, 1997, entitled "Vascular Endothelial Growth Factor (VEGF) Nucleic Acid Ligand Complexes," U.S. Pat. No. 6,051,698. VEGF nucleic acid ligands that are associated with a non-immunogenic, high molecular weight compound or lipophilic compound are also further described in PCT/US97/18944, filed Oct. 17, 1997, entitled "Vascular Endothelial Growth Factor (VEGF) Nucleic Acid Ligand Complexes." Each of the above described patent applications which describe modifications of the basic SELEX procedure are specifically incorporated by reference herein in their entirety.
It is an object of the present invention to provide methods that can be used to detect virtually any non-nucleic acid target molecule in a test mixture, using nucleic acid reagents that are easily and cheaply manufactured.
It is a further object of the instant invention to provide a method for adapting molecular beacons in order to detect non-nucleic acid target molecules in a test mixture.
Another object of the instant invention is to provide a single, universal assay for virtually any non-nucleic acid target molecule in which measurements of fluorescence emission are used to determine the concentration of the target.