SINGLE-CHAIN POLYPEPTIDES COMPRISING CREATINE KINASE M AND CREATINE KINASE B

Abstract: This invention relates to single-chain polypeptides and their genetic sequences comprising creatine kinase M and creatine kinase B. The single-chain polypeptide may be expressed recombinantly. A linker peptide may be interposed between the creatine kinase sequences. A linker peptide of about 6 to about 50 amino acids is preferred. The single-chain polypeptide has utility as a control or calibrator for creatine kinase MB assays, for the purification of creatine kinase antibodies, and as an antigen for the preparation of antibodies.

Description: SINGLE-CHAIN POLYPEPTIDES COMPRISING CREATINE KINASE M AND CREATINE KINASE B FIELD OF THE INVENTION This invention relates to recombinantly-expressed, single-chain polypeptides comprising creatine kinase subunits M and B, and their corresponding genetic sequences, and transformed microorganisms carrying the sequences and expressing the polypeptides. BACKGROUND OF THE INVENTION Early and accurate assessment of suspected acute myocardial infarction is critically dependent on the sensitive and specific detection and quantitation in blood, serum or plasma of released cardiac muscle intracellular components in order to distinguish a potentially lethal event in need of emergency measures from non-life threatening conditions such as angina and non-cardiac chest pain such as dyspepsia. Early electrocardiographic changes are neither adequately specific nor sensitive, and the medical profession has come to rely on serum biochemical markers of cardiac tissue injury for early diagnosis. Among the markers released into the circulation from heart tissue following injury or necrosis, creatine kinase MB (CK- MB) is indicative of a cardiac event and is used diagnostically to identify patients undergoing a heart attack. Lee and Goldman (1986, Annals of Internal Medicine 105:221-233) provide a discussion of the utility of CK-MB in the diagnosis of acute myocardial infarction. Creatine kinase is a dimeric enzyme that catalyzes the transfer of high-energy phosphate groups and is found predominantly in tissues that consume large amounts of energy. The enzyme has two subunits, each of which can be either type M (for muscle) or B (for brain), thus, three combinations exist: MM, MB, and BB. The CK-MM isoenzyme is dominant in adult skeletal muscle, whereas CK-BB is found mostly in the central nervous system. In humans, heart 2 muscle tissue creatine kinase is about 85% MM and 15% MB. In order to specifically identify muscle injury of cardiac origin, quantitation of only the CK-MB isoenzyme in circulation is desirable. The amino acid sequence differences between the M and B isoenzymes are exploited in diagnostic tests which specifically measure the cardiac isoenzyme CK-MB. Rapid diagnostic tests employing cardiac markers including CK-MB are described, for example, in U.S. Patent Nos. 5,604,105 and 5,290,678. These and other procedures offer the rapidity of diagnosing myocardial infarction in the emergency room setting and offer significant medical benefit for patients. Numerous other procedures for measuring CK-MB are available, such as automatic clinical analyzers, column chromatography, radioimmunoassay, and electrophoretic methods. Diagnostic tests developed to measure the level of CK-MB in bodily fluids frequently utilize CK-MB as an antigen for the preparation of antibodies used in the assay procedure, as well as purified CK-MB used as controls and calibrators in performing the assay. Assay calibrators are used to prepare a series of dilutions by which a standard curve across the operating range of an assay is prepared; assay controls are used to confirm that an assay is operating properly by ensuring that the assayed values of pre-determined samples fall within an acceptable range around their labeled values. In order for the assay to be calibrated properly, the CK-MB controls and calibrators must remain stable and in a form which is immunodetectable by the antibody or antibodies used in the assay procedure. Rapid diagnostic tests for CK-MB likewise may utilize a positive control to ensure accurate operation of the test. Numerous CK-MB preparations from both natural and recombinant sources have been described. U.S. Patent 5,496,716 describes a stabilized CK-MB composition comprising serum protein plus a CK-MB enzymatic substrate (for example, ATP or creatine) with or without a CK-MB enzymatic product (for example, ADP or phosphocreatine). Stability with a less than a 10% loss by immunoassay after 27 days at room temperature was claimed. Recombinant human CK-MB has been expressed in COS cells as a dimer (Friedman et al., 1993, Clinical Chemistry 39:1598-1601): the subunits were expressed as separate proteins then formed non-covalent dimers within the cells. Medix Biotech Inc. presently offers for sale a recombinant CK-MB product. However, preparation of CK-MB from natural sources is a tedious procedure involving many steps including protein isolation from tissue, with its attendant hazards of infection, and protein purification, with its attendant risks of potential degradation and variable recovery, to provide an isolated CK-MB material of suitable quality and quantity for use as a calibrator or control. CK-MB prepared by recombinant methods involves the expression of separate M and B isoforms and their subsequent association to form the MB dimer which, whether performed in vitro or occurring within a host cell expressing both isoforms, will result in the formation of the MM, BB, and MB dimer; the MB dimer must be subsequently purified from the other dimers. This subsequent protein purification caries the same potential problems as described above. Thus, there exists a need for stable CK-MB calibrators and controls that are easy to prepare and purify, and will meet the requirements of the industry. As will be evident below, a principal object of the present invention is to provide a stable CK-MB preparation for assay and other uses which comprises the CK-M subunit and the CK-B subunit on a single polypeptide chain, prepared as a recombinant construct and expressed in a bacterial expression system as a single polypeptide. A single-chain polypeptide of this invention comprising human creatine kinase M and creatine kinase B is stable, readily purified, uncontaminated with the MM or BB dimers, and has utility for the aforementioned purposes. Moreover, the product is easily produced by the skilled artisan. This ease of production maximizes the reproducibility of the products of the invention. SUMMARY OF THE INVENTION It is a principal objective of the present invention to provide a single-chain polypeptide comprising creatine kinase M and creatine kinase B. The presence of creatine kinase M and creatine kinase B on the same polypeptide chain confers conformational stability and immunostability to the product. The single-chain polypeptide may preferably include a linker sequence interposed between the sequence of creatine kinase M and that of creatine kinase B. The sequence of the linker peptide is chosen based on known structural and conformational 4 contributions of peptide segments so that it does not interfere with the tertiary structure of the product and therefore its aforementioned utilities. A single-chain polypeptide in which creatine kinase M and creatine kinase B are joined, optionally through a linker peptide, provides a stable, reproducible, and easily purified material for the development of CK-MB assays, an antigen for preparing CK-MB antibodies, as well as material for use as controls and calibrators for CK-MB assays. Furthermore, there is no contamination of the preparation with CK-MM or CK-BB isoenzymes. The single-chain polypeptide of the present invention is prepared most readily by recombinant techniques, by constructing a replicable cloning or expression vehicle such as a plasmid carrying the genetic sequence for the single-chain polypeptide, and transforming a host cell, such as E. coli, with the vehicle or plasmid, and expressing the polypeptide in the host cell. The single-chain CK-MB construct preferably contains a genetic sequence for a linker peptide sequence interposed between the creatine kinase M and creatine kinase B nucleotide sequences, the sequence introduced by recombinant means. Certain modifications may be made in the genetic sequence of the gene which codes for creatine kinase molecules, with or without changes in the consequent amino acid sequence of the polypeptide, in order to improve the expression of the polypeptide in the host cell. These changes do not alter the utility of the single-chain polypeptide for use in the aforementioned purposes. It is another object of the present invention to provide a genetic sequence for a single-chain polypeptide comprising the genetic sequences of creatine kinase M and creatine kinase B. The genetic sequence may also include a linker genetic sequence interposed between the genetic sequences of creatine kinase M and creatine kinase B. A host cell may be transformed with the replicable cloning or expression vehicle containing the aforementioned genetic sequence. It is a further object of the present invention to provide a host cell containing a replicable cloning or expression vehicle such as a plasmid carrying the genetic sequence for a single- chain polypeptide chain comprising the genetic sequences of creatine kinase M and creatine 5 kinase B, and capable of expressing a single-chain polypeptide comprising creatine kinase M and creatine kinase B, with or without a linker. These and other aspects of the present invention will be better appreciated by reference to the following drawings and Detailed Description. DETAILED DESCRIPTION OF THE INVENTION Measurement in circulation of the cardiac muscle-associated enzyme creatine kinase MB has proven to be an early and specific indicator of suspected acute myocardial infarction. As such, methods for rapidly and accurately detecting creatine kinase MB in blood have been and are being developed for diagnosing heart attack in an emergency situation, and countless lives have been and will be saved as a result. However, in order to develop accurate and dependable diagnostic assays and to ensure the validity of these assays using assay controls and calibrators, the availability of stable, high-quality human creatine kinase MB controls and calibrators is critical for quality control and testing purposes, as well as CK-MB antigens for raising antibodies for assays. The present invention provides a single-chain polypeptide comprising human creatine kinase M and creatine kinase B. The creatine kinase subunits are thus covalently linked and reside on the same linear polypeptide chain. This polypeptide provides a stable CK-MB to meet the needs of the industry. The single-chain polypeptide may be prepared by recombinant techniques, and preferably includes a linker polypeptide sequence interposed between the creatine kinase M and creatine kinase B sequences. The length and sequence of this linker sequence is limited only in that it does not interfere with the immunodetectability of the product and its other aforementioned utilities. A preferred linker comprises 6 to 50 amino acids. For example, one embodiment of the single-chain creatine kinase MB polypeptide may comprise the creatine kinase M sequence at the N-terminal portion of the polypeptide, with the C-terminus of the creatine kinase M sequence engaged in a peptide bond with the N- 6 terminus of the creatine kinase B sequence. In a second and presently preferred embodiment, a linker peptide sequence is interposed between the creatine kinase M and the creatine kinase B amino acid sequences. Another possible arrangement comprises the creatine kinase M sequence at the N-terminal portion of the polypeptide, its C-terminus engaged in a peptide bond with the N-terminus of the linker peptide, and the C-terminus of the linker peptide then engaged in a peptide bond with the N-terminus of the creatine kinase B sequence. An example of this construct is the amino acid sequence depicted in SEQ ID NO:4. Amino acid residues 1 through 381 comprise creatine kinase M, residues 382-400 comprise the linker peptide sequence, and residues 401-780 comprise creatine kinase B. In this example, the amino acid sequence of the linker is represented in SEQ ID NO:2. It contains 19 amino acids. The amino acid sequences in the above examples correspond to the nucleotide sequences of the cDNA coding for these polypeptides. The genetic sequence in the first example comprises the creatine kinase M genetic sequence at the 5' half of the cDNA, its 3' end followed immediately by the 5' half of the creatine kinase B genetic sequence. In the preferred embodiment wherein a linker is interposed between the creatine kinase M and creatine kinase B sequences, the 5' of the cDNA sequence begins with the creatine kinase M genetic sequence, its 3' end followed by the 5' end of the optional interposed linker genetic sequence, and its 3' end followed by the 5' end of the creatine kinase B genetic sequence, ending at the 3' end of the creatine kinase B cDNA. In the specific example above, the genetic sequence is represented in SEQ LD NO:3. Nucleotides 1 through 1142 (amino acid residues 1-381) code for creatine kinase M, nucleotides 1143 through 1200 (residues 382- 400) code for the linker peptide sequence, and nucleotides 1201 through 2340 (residues 401- 780) code for creatine kinase B. The cDNA sequence of the linker alone is presented in SEQ LD NO:l. As described above, selection of the length and specific sequence of the optional linker polypeptide is limited only in that it must not interfere with the immunodetectability of the single-chain creatine kinase MB polypeptide. It is believed that with a suitable linker sequence, the creatine kinase M and creatine kinase B segments of the single polypeptide 7 chain associate with each other in a similar fashion as they do in the naturally-occurring, non- covalently-bound dimer of creatine kinase M and creatine kinase B, and the attachment of the subunits in the single polypeptide chain maintains the conformation of the association and thus the consistent immunodetectability of the CK-MB. Within this preferred embodiment, a linker of about 6 to about 50 amino acids (and a corresponding number of nucleotides in the genetic sequence) is preferred, for ease and economics of preparation. It is preferred to produce the single-chain creatine kinase MB polypeptide of this invention with a relatively short linker segment because with such products, there is little or no interference with the tertiary structure of the product. Hence there is little or no interference with the availability of epitopes for reaction with readily-available antibodies. For example, a useful linker polypeptide sequence comprises (Gly 4 Ser) 3 which provides a flexible peptide sequence that allows the two subunits to associate. In order to construct the genetic sequence with a linker, an additional 2 codons at each end of the linker are present, which were needed in order to provide unique restriction sites to create the genetic construct of the desired single-chain polypeptide. In one example, codons corresponding to Thr-Ser at the N-terminus of the linker and Ala-Cys at the C-terminus, may be included. Thus, a suitable 19-residue linker may be prepared (genetic sequence SEQ ID NO:l and peptide SEQ ID NO:2). Known and well-understood recombinant methods may be used to prepare the DNA sequence comprising the creatine kinase subunits and the optional linker sequence and to introduce the sequence into a host cell, and standard expression methods are used to express and purify the recombinant polypeptide. These methods are similar to those used for the preparation of fusion proteins such as that described for the two metabolically-coupled yeast enzymes, citrate synthase and malate dehydrogenase (Lindbladh et al., Biochemistry 33:11692-11698 [1994]); in the preparation of fusion proteins for phage display (U.S. Patent 5,516,637); and in the preparation of single-chain polypeptides comprising the antigen-binding site of antibodies (U.S. Patent 4,946,778). Coincidentally, the latter invention describes the 8 preparation of a single-chain antigen-binding polypeptide that specifically recognizes creatine kinase MB, a product unrelated to the present invention. In the instance in which no linker sequence is desired, the creatine kinase M and creatine kinase B nucleotide sequences may be joined through suitable techniques known in the art such as the SOEing method using pairs of partially overlapping primers, for example, as described by Hu et al. (1996, Protein Expression and Purification 7:289-293) in which rare codons in human cardiac troponin T were replaced with synonymous major codons. These methods are well known to the skilled artisan. The recombinant construct is prepared as an expression or cloning vehicle, or plasmid, and introduced into a host cell for expression. Methods for expression of recombinant proteins are known in the art. Once expressed, the single-chain polypeptide may be purified by standard protein purification methods. Several CK-MB assays are commercially available, all of which operate using different formats, instruments, and assay controls and calibrators. Example include the Stratus(R) immunoassay analyzer from Dade, Access(R) from Sanofi, and IMX(R) from Abbott. A single-chain polypeptide of this invention comprising creatine kinase M and creatine kinase B may also be used for the purification of proteins and other substances including antibodies with an affinity for binding creatine kinase M, creatine kinase B, and creatine kinase MB. For example, the single-chain polypeptide of the present invention with or without a linker may be covalently bound to an insoluble matrix or polymer and situated in a chromatography column. A cell or tissue extract suspected of containing a material that binds CK-MB, or an antibody preparation raised against CK-MB, may be passed through the column, whereby it would adhere to the covalently-bound polypeptide. After washing the matrix, the adherent material may be eluted by changing the pH of the eluting buffer or other standard methods used in protein purification. 9 A single-chain polypeptide of the present invention may also be useful for the preparation of monoclonal or polyclonal anti-CK-MB antibodies, using standard methods of animal immunization or hybridoma preparation. The single-chain polypeptide of the present invention comprising creatine kinase M and creatine kinase B has utility for the preparation of sensitive CK-MB assays and for the calibration of such assays. As will be seen from the following non-limiting examples, the single-chain polypeptide exhibits superior performance when compared to other CK-MB calibrators. Example 1 Expression of a single-chain creatine kinase MB polypeptide in E. coli Full-length human creatine kinase M and creatine kinase B subunit cDNAs were amplified by polymerase chain reaction (PCR) from human heart and human brain Quick-clone cDNA libraries (Clontech) using specific primers designed from the published sequences. Creatine kinase M cDNA was linked with creatine kinase B through a linker sequence having SEQ ID:1 by use of the following restriction sites: Creatine kinase M: 5', Ndel, 3', S el; creatine kinase B: 5', SphI, 3', Notl. The construct, with genetic and polypeptide sequences of SEQ ID NO:3 and SEQ ID NO:4, respectively, was inserted into a pET21 (Novagen) expression vector and E. coli BL21(DE3)(Novagen) was transformed. E. coli cells were harvested and lysed following standard procedures. Inclusion bodies containing insoluble single-chain CK-MB were solubilized in 8 M urea and the resulting suspension incubated with chaperonin extract. The CK-MB -chaperonin mixture was allowed to refold by step-dialysis and the addition of coupling buffer. The renatured crude extract was purified by a single-step immunoaffinity chromatography procedure using an immobilized monoclonal antibody to CK-MB, CK-MM, or CK-BB. Single-chain CK-MB polypeptide was eluted with diethylamine. 10 The above purification procedure was accomplished in 3.5 days, in comparison to 5 days by conventional expression and isolation in which both the M and B subunits were expressed within a single cell. Recombinant CK-MB dimers were isolated from MM and BB dimers that also formed in the host cell using affinity purification employing a specific anti-CK-MB monoclonal antibody. Furthermore, the yield of the single-chain polypeptide of the present invention was 160 mg per liter of E. coli culture, as compared with 50 mg per liter from the conventionally expression CK-MB. In addition, electrophoretic analysis showed no contamination of the single-chain polypeptide preparation with CK-MM or CK-BB isoenzymes, as would be expected, though the purified CK-MB prepared by conventional recombinant methods followed by purification of the MB isoenzyme was contaminated with both CK-MM and CK-BB isoenzymes. The E. coli strain expressing the single-chain creatine kinase MB polypeptide as described above has been deposited on February 11, 1998 with the American Type Culture Collection, 10801 University Boulevard, Manassas, NA 20110-2209, and has been designated ATCC number 98648. Example 2 Immunodetectability of the Single-Chain Creatine Kinase MB Polypeptide The single-chain CK-MB described in Example 1 was assayed using the Stratus(R) Immunoassay Analyzer. Immunoreactivity per unit mass was increased by 60% as compared with the conventionally-prepared recombinant CK-MB. While the invention has been described and illustrated herein by references to the specific embodiments, various specific material, procedures and examples, it is understood that the invention is not restricted to the particular material combinations of material, and procedures selected for that purpose. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing 11 description and the accompanying sequences. Such modifications are intended to fall within the scope of the appended claims. Various citations to prior publications are mentioned throughout this specification, each of which is incorporated herein by reference in its entirety.

Claims: 12 WHAT IS CLAIMED IS: 1. A polynucleotide which codes for a single-chain polypeptide comprising creatine kinase M and creatine kinase B. 2. The polynucleotide of claim 1 wherein a linker polynucleotide coding for a linker polypeptide is interposed between the polynucleotides coding for creatine kinase M and creatine kinase B. 3. A replicable cloning or expression vehicle comprising the polynucleotide of claim 1 or 2. 4. The vehicle of claim 3 which is a plasmid. 5. A host cell transformed with a vehicle of claim 3. 6. An E. coli transformed with a replicable cloning or expression vehicle comprising the polynucleotide of claim 1 or 2. 7. The polynucleotide claim 2 identified as (SEQ LD NO:3). 8. A replicable cloning or expression vehicle carrying the sequence set forth in (SEQ ID NO:3). 9. A host cell transformed with the vehicle of claim 8. 10. The host cell of claim 9 which is E. coli. 11. The E. coli of claim 10 which is deposited with the American Type Culture Collection and designated ATCC number 98648. 13 12. The polynucleotide of claim 2 wherein the linker polynucleotide comprises from about 18 to about 150 nucleotides. 13. A single-chain polypeptide comprising creatine kinase M and creatine kinase B. 14. The polypeptide of Claim 13 wherein a peptide linker sequence is interposed between the creatine kinase M and creatine kinase B. 15. The polypeptide of Claim 14 having an amino acid sequence as set forth in (SEQ ID NO:4). 16. The polypeptide of claim 13 wherein the peptide linker sequence comprises from about 6 to about 50 amino acids.

Inventors: SHI QINWEI, TOBIAS ROWEL, SHI, QINWEI, TOBIAS, ROWEL
Applicants: SPECTRAL DIAGNOSTICS INC, SPECTRAL DIAGNOSTICS, INC.
Application year: 1999
Main classification: C12N15
IPC codes: C12N1, C12N9, C12N15