SRM/MRM Assay for the GTPase KRas Protein (KRas)

Abstract

The current disclosure provides for specific peptides, and derived ionization characteristics of the peptides, from the GTPase KRas Protein (KRas) that are particularly advantageous for quantifying the KRas protein directly in biological samples that have been fixed in formalin by the mass spectrometry, or what can also be termed as Multiple Reaction Monitoring (MRM) mass spectrometry. Such biological samples are chemically preserved and fixed wherein the biological sample is selected from tissues and cells treated with formaldehyde containing agents/fixatives including formalin fixed tissue/cells, formalin-fixed/paraffin embedded (FFPE) tissue/cells,

Claims

1. A method for measuring the level of the GTPase KRas Protein (KRas) in a biological sample of formalin-fixed tissue, comprising detecting and quantifying the amount of one or more modified or unmodified KRas fragment peptides in a protein digest prepared from said biological sample using mass spectrometry; and calculating the level of modified or unmodified KRas protein in said sample; and wherein said level is a relative level or an absolute level.

2. The method of claim 1, further comprising the step of fractionating said protein digest prior to detecting and/or quantifying the amount of one or more modified or unmodified KRAS fragment peptides.

3. The method of claim 2, wherein said fractionating step is selected from the group consisting of gel electrophoresis, liquid chromatography, capillary electrophoresis, nano-reversed phase liquid chromatography, high performance liquid chromatography, or reverse phase high performance liquid chromatography.

4. The method of claim 1, wherein said protein digest of said biological sample is prepared by the Liquid Tissue protocol.

5. The method of claim 1, wherein said protein digest comprises a protease digest.

6. The method of claim 5, wherein said protein digest comprises a trypsin digest.

7. The method of claim 1, wherein said mass spectrometry comprises tandem mass spectrometry, ion trap mass spectrometry, triple quadrupole mass spectrometry, MALDI-TOF mass spectrometry, MALDI mass spectrometry, and/or time of flight mass spectrometry.

8. The method of claim 7, wherein the mode of mass spectrometry used is Selected Reaction Monitoring (SRM), Multiple Reaction Monitoring (MRM), and/or multiple Selected Reaction Monitoring (mSRM).

9. The method of claim 1, wherein the KRAS fragment peptide comprises an amino acid sequence as set forth as SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, and SEQ ID NO:7.

10-11. (canceled)

12. The method of claim 1, wherein the tissue is paraffin embedded tissue.

13. The method of claim 1, wherein the tissue is obtained from a tumor.

14-16. (canceled)

17. The method of claim 1, wherein quantifying the KRas fragment peptide comprises comparing an amount of one or more KRas fragment peptides comprising an amino acid sequence of about 8 to about 45 amino acid residues of KRas as shown in SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, and SEQ ID NO:7 in one biological sample to the amount of the same KRAS fragment peptide in a different and separate biological sample.

18. The method of claim 1, wherein quantifying one or more KRAS fragment peptides comprises determining the amount of the each of the KRAS fragment peptides in a biological sample by comparison to an added internal standard peptide of known amount, wherein each of the KRAS fragment peptides in the biological sample is compared to an internal standard peptide having the same amino acid sequence.

19. The method of claim 18, wherein the internal standard peptide is an isotopically labeled peptide.

20. The method of claim 19, wherein the isotopically labeled internal standard peptide comprises one or more heavy stable isotopes selected from .sup.18O, .sup.17O, .sup.34S, .sup.15N, .sup.13C, .sup.2H or combinations thereof.

21. The method of claim 1, wherein detecting and quantifying the amount of one or more modified or unmodified KRAS fragment peptides in the protein digest indicates the presence of modified or unmodified KRAS protein and an association with cancer in the subject.

22. The method of claim 21, further comprising correlating the results of said detecting and quantifying the amount of one or more modified or unmodified KRAS fragment peptides, or the level of said KRAS protein to the diagnostic stage/grade/status of the cancer.

23. The method of claim 22, wherein correlating the results of said detecting and quantifying the amount of one or more modified or unmodified KRAS fragment peptides, or the level of said KRAS protein to the diagnostic stage/grade/status of the cancer is combined with detecting and/or quantifying the amount of other proteins or peptides from other proteins in a multiplex format to provide additional information about the diagnostic stage/grade/status of the cancer.

24. The method of claim 1, further comprising selecting for the subject from which said biological sample was obtained a treatment based on the presence, absence, or amount of one or more KRAS fragment peptides or the level of KRAS protein.

25. The method claim 1, further comprising administering to the patient from which said biological sample was obtained a therapeutically effective amount of a therapeutic agent, wherein the therapeutic agent and/or amount of the therapeutic agent administered is based upon amount of one or more modified or unmodified KRAS fragment peptides or the level of KRAS protein.

26. The method of claim 25, wherein said therapeutic agent binds the KRAS protein and/or inhibits its biological activity.

27. The method of claim 26, wherein the therapeutic agent is selected from Reolysin, and other agents that specifically target KRAS-expressing cancer cells.

28. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1, parts A to C, shows an example of an SRM/MRM assay of a single peptide from the KRas protein performed on a Liquid Tissue lysate from a formalin fixed biological sample with quantitation of the KRas peptide conducted on a triplequadrupole mass spectrometer. The specific characteristics about how to measure this peptide in biological samples that have been fixed in formalin is shown.

DETAILED DESCRIPTION

[0013] In principle, any predicted peptide derived from the KRas protein, prepared for example by digesting with a protease of known specificity (e.g. trypsin), can be used as a surrogate reporter to determine the abundance of KRas protein in a sample using a mass spectrometry-based SRM/MRM assay. Similarly, any predicted peptide sequence containing an amino acid residue at a site that is known to be potentially modified in KRas protein also might potentially be used to assay the extent of modification of KRas protein in a sample.

[0014] KRas fragment peptides may be generated by a variety of methods including by the use of the Liquid Tissue protocol provided in U.S. Pat. No. 7,473,532. The Liquid Tissue protocol and reagents are capable of producing peptide samples suitable for mass spectroscopic analysis from formalin fixed paraffin embedded tissue by proteolytic digestion of the proteins in the tissue/biological sample. In the Liquid Tissue protocol the tissue/biological is heated in a buffer for an extended period of time (e.g., from about 80 C. to about 100 C. for a period of time from about 10 minutes to about 4 hours) to reverse or release protein cross-linking. The buffer employed is a neutral buffer, (e.g., a Tris-based buffer, or a buffer containing a detergent). Following heat treatment the tissue/biological sample is treated with one or more proteases, including but not limited to trypsin, chymotrypsin, pepsin, and endoproteinase Lys-C for a time sufficient to disrupt the tissue and cellular structure of the biological sample and to liquefy the sample (e.g., a period of time from 30 minutes to 24 hours at a temperature from 37 C. to 65 C.). The result of the heating and proteolysis is a liquid, soluble, dilutable biomolecule lysate.

[0015] Surprisingly, it has been found that many potential peptide sequences from the KRas protein are unsuitable or ineffective for use in mass spectrometry-based SRM/MRM assays for reasons that are not immediately evident. This is particularly true for peptides derived from formalin fixed tissue. As it was not possible to predict the most suitable peptides for MRM/SRM assay, it was necessary to experimentally identify modified and unmodified peptides in actual Liquid Tissue lysates to develop a reliable and accurate SRM/MRM assay for the KRas protein. While not wishing to be bound by any theory, it is believed that some peptides might, for example, be difficult to detect by mass spectrometry as they do not ionize well or produce fragments that are not distinct from other proteins. Peptides may also fail to resolve well in separation (e.g., liquid chromatography), or may adhere to glass or plastic ware.

[0016] KRas peptides found in various embodiments of this disclosure (e.g., Tables 1 and 2) were derived from the KRas protein by protease digestion of all the proteins within a complex Liquid Tissue lysate prepared from cells procured from formalin fixed cancer tissue. Unless noted otherwise, in each instance the protease was trypsin. The Liquid Tissue lysate was then analyzed by mass spectrometry to determine those peptides derived from the KRas protein that are detected and analyzed by mass spectrometry. Identification of a specific preferred subset of peptides for mass-spectrometric analysis is based on; 1) experimental determination of which peptide or peptides from a protein ionize in mass spectrometry analyses of Liquid Tissue lysates, and 2) the ability of the peptide to survive the protocol and experimental conditions used in preparing a Liquid Tissue lysate. This latter property extends not only to the amino acid sequence of the peptide but also to the ability of a modified amino acid residue within a peptide to survive in modified form during the sample preparation.

[0017] Protein lysates from cells procured directly from formalin (formaldehyde) fixed tissue were prepared using the Liquid Tissue reagents and protocol that entails collecting cells into a sample tube via tissue microdissection followed by heating the cells in the Liquid Tissue buffer for an extended period of time. Once the formalin-induced cross linking has been negatively affected, the tissue/cells are then digested to completion in a predictable manner using a protease, such as, for example, trypsin, although other proteases can be used. Each protein lysate is turned into a collection of peptides by digestion of intact polypeptides with the protease. Each Liquid Tissue lysate was analyzed (e.g., by ion trap mass spectrometry) to perform multiple global proteomic surveys of the peptides where the data was presented as identification of as many peptides as could be identified by mass spectrometry from all cellular proteins present in each protein lysate.

[0018] An ion trap mass spectrometer or another form of a mass spectrometer that is capable of performing global profiling for identification of as many peptides as possible from a single complex protein/peptide lysate is employed. Ion trap mass spectrometers however may advantageously be used conducting global profiling of peptides. Although an SRM/MRM assay can be developed and performed on any type of mass spectrometer, including a MALDI, ion trap, or triple quadrupole, advantageously a triple quadrupole instrument platform is used for an SRM/MRM assay. That type of a mass spectrometer is suitable instrument for analyzing a single isolated target peptide within a very complex protein lysate that may consist of hundreds of thousands to millions of individual peptides from all the proteins contained within a cell.

[0019] Once as many peptides as possible were identified in a single MS analysis of a single lysate under the conditions employed, then that list of peptides was collated and used to determine the proteins that were detected in that lysate. That process was repeated for multiple Liquid Tissue lysates, and the very large list of peptides was collated into a single dataset. That type of dataset can be considered to represent the peptides that can be detected in the type of biological sample that was analyzed (after protease digestion), and specifically in a Liquid Tissue lysate of the biological sample, and thus includes the peptides for specific proteins, such as for example the KRas protein.

[0020] In one embodiment, the KRas tryptic peptides identified as useful in the determination of absolute or relative amounts of the KRas protein include one or more, two or more, three or more, or four or more of the peptides of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, and SEQ ID NO:7, each of which are listed in Table 1.Each of those peptides was detected by mass spectrometry in Liquid Tissue lysates prepared from formalin fixed, paraffin embedded tissue. Thus, each peptide is a candidate for use in developing a quantitative SRM/MRM assay for the KRas protein in human biological samples, including directly in formalin fixed patient tissue.

TABLE-US-00001 TABLE1 SEQID Peptidesequence SEQIDNO:1 SFEDIHHYR SEQIDNO:2 LVVVGAGGVGK SEQIDNO:3 SALTIQLIQNHFVDEYDPTIEDSYR SEQIDNO:4 QAQDLAR SEQIDNO:5 SYGIPFIETSAK SEQIDNO:6 VEDAFYTLVR SEQIDNO:7 QGVDDAFYTLVR

[0021] The KRas tryptic peptides listed in Table 1 include those detected from multiple Liquid Tissue lysates of multiple different formalin fixed tissues of different human organs including prostate, colon, and breast. Each of those peptides is considered useful for quantitative SRM/MRM assay of the KRas protein in formalin fixed tissue. Further data analysis of these experiments indicated no preference for any specific peptides from any specific organ site. Thus, these peptides may be used for conducting SRM/MRM assays of the KRas protein on a Liquid Tissue lysate from any formalin fixed tissue originating from any biological sample or from any organ site in the body.

[0022] In order to most efficiently implement an SRM/MRM assay for each peptide derived from the KRas protein it is desirable to utilize information in addition to the peptide sequence in the analysis. That additional information may be used in directing and instructing the mass spectrometer (e.g. a triple quadrupole mass spectrometer), to perform the correct and focused analysis of specific targeted peptide(s), such that the assay may be effectively performed.

[0023] The additional information about target peptides in general, and about specific KRas peptides, may include one or more of the mono isotopic mass of the peptide, its precursor charge state, the precursor m/z value, the m/z transition ions, and the ion type of each transition ion. Table 2 shows additional peptide information that may be used to develop an SRM/MRM assay for the KRas protein for one (1) of the KRas peptides from the list in Table 1. Similar additional information described for the one (1) KRas peptide shown by example in Table 2 may be prepared, obtained, and applied to the analysis of the other peptides contained in Table 1.

TABLE-US-00002 TABLE2 Mono Precursor Peptide Isotopic Charge Pregursor Transition Ion SEQID sequence Mass State m/z m/z Type SEQID SFEDI 1202.5468 2 602.281 475.241 y3 NO:1 HHYR 2 602.281 612.3 y4 2 602.281 725.384 y5 2 602.281 840.411 y6 2 602.281 969.453 y7

[0024] The method described below was used to: 1) identify candidate peptides from the KRas protein that can be used for a mass spectrometry-based SRM/MRM assay for the KRas protein, 2) develop an individual SRM/MRM assay, or assays, for target peptides from the KRas protein in order to correlate and 3) apply quantitative assays to cancer diagnosis and/or choice of optimal therapy.

Assay Method

1. Identification of SRM/MRM Candidate Fragment Peptides for the KRas Protein

[0025] a. Prepare a Liquid Tissue protein lysate from a formalin fixed biological sample using a protease or proteases, (that may or may not include trypsin), to digest proteins [0026] b. Analyze all protein fragments in the Liquid Tissue lysate on an ion trap tandem mass spectrometer and identify all fragment peptides from the KRas protein, where individual fragment peptides do not contain any peptide modifications such as phosphorylations or glycosylations [0027] c. Analyze all protein fragments in the Liquid Tissue lysate on an ion trap tandem mass spectrometer and identify all fragment peptides from the KRas protein that carry peptide modifications such as for example phosphorylated or glycosylated residues [0028] d. All peptides generated by a specific digestion method from the entire, full length KRas protein potentially can be measured, but preferred peptides used for development of the SRM/MRM assay are those that are identified by mass spectrometry directly in a complex Liquid Tissue protein lysate prepared from a formalin fixed biological sample [0029] e. Peptides that are specifically modified (phosphorylated, glycosylated, etc.) in patient tissue and which ionize, and thus detected, in a mass spectrometer when analyzing a Liquid Tissue lysate from a formalin fixed biological sample are identified as candidate peptides for assaying peptide modifications of the KRas protein
2. Mass Spectrometry Assay for Fragment Peptides from KRas Protein [0030] a. SRM/MRM assay on a triple quadrupole mass spectrometer for individual fragment peptides identified in a Liquid Tissue lysate is applied to peptides from the KRas protein [0031] i. Determine optimal retention time for a fragment peptide for optimal chromatography conditions including but not limited to gel electrophoresis, liquid chromatography, capillary electrophoresis, nano-reversed phase liquid chromatography, high performance liquid chromatography, or reverse phase high performance liquid chromatography [0032] ii. Determine the mono isotopic mass of the peptide, the precursor charge state for each peptide, the precursor m/z value for each peptide, the m/z transition ions for each peptide, and the ion type of each transition ion for each fragment peptide in order to develop an SRM/MRM assay for each peptide. [0033] iii. SRM/MRM assay can then be conducted using the information from (i) and (ii) on a triple quadrupole mass spectrometer where each peptide has a characteristic and unique SRM/MRM signature peak that precisely defines the unique SRM/MRM assay as performed on a triple quadrupole mass spectrometer [0034] b. Perform SRM/MRM analysis so that the amount of the fragment peptide of the KRas protein that is detected, as a function of the unique SRM/MRM signature peak area from an SRM/MRM mass spectrometry analysis, can indicate both the relative and absolute amount of the protein in a particular protein lysate. [0035] i. Relative quantitation may be achieved by: [0036] 1. Determining increased or decreased presence of the KRas protein by comparing the SRM/MRM signature peak area from a given KRas peptide detected in a Liquid Tissue lysate from one formalin fixed biological sample to the same SRM/MRM signature peak area of the same KRas fragment peptide in at least a second, third, fourth or more Liquid Tissue lysates from least a second, third, fourth or more formalin fixed biological samples [0037] 2. Determining increased or decreased presence of the KRas protein by comparing the SRM/MRM signature peak area from a given KRas peptide detected in a Liquid Tissue lysate from one formalin fixed biological sample to SRM/MRM signature peak areas developed from fragment peptides from other proteins, in other samples derived from different and separate biological sources, where the SRM/MRM signature peak area comparison between the 2 samples for a peptide fragment are normalized to amount of protein analyzed in each sample. [0038] 3. Determining increased or decreased presence of the KRas protein by comparing the SRM/MRM signature peak area for a given KRas peptide to the SRM/MRM signature peak areas from other fragment peptides derived from different proteins within the same Liquid Tissue lysate from the formalin fixed biological sample in order to normalize changing levels of KRas protein to levels of other proteins that do not change their levels of expression under various cellular conditions. [0039] 4. These assays can be applied to both unmodified fragment peptides and for modified fragment peptides of the KRas protein, where the modifications include but are not limited to phosphorylation and/or glycosylation, and where the relative levels of modified peptides are determined in the same manner as determining relative amounts of unmodified peptides. [0040] ii. Absolute quantitation of a given peptide may be achieved by comparing the SRM/MRM signature peak area for a given fragment peptide from the KRas protein in an individual biological sample to the SRM/MRM signature peak area of an internal fragment peptide standard spiked into the protein lysate from the biological sample [0041] 1. The internal standard is a labeled synthetic version of the fragment peptide from the KRas protein that is being interrogated. This standard is spiked into a sample in known amounts, and the SRM/MRM signature peak area can be determined for both the internal fragment peptide standard and the native fragment peptide in the biological sample separately, followed by comparison of both peak areas [0042] 2. This can be applied to unmodified fragment peptides and modified fragment peptides, where the modifications include but are not limited to phosphorylation and/or glycosylation, and where the absolute levels of modified peptides can be determined in the same manner as determining absolute levels of unmodified peptides.

3. Apply Fragment Peptide Quantitation to Cancer Diagnosis and Treatment

[0043] a. Perform relative and/or absolute quantitation of fragment peptide levels of the KRas protein and demonstrate that the previously-determined association, as well understood in the field of cancer, of KRas protein expression to the stage/grade/status of cancer in patient tumor tissue is confirmed [0044] b. Perform relative and/or absolute quantitation of fragment peptide levels of the KRas protein and demonstrate correlation with clinical outcomes from different treatment strategies, wherein this correlation has already been demonstrated in the field or can be demonstrated in the future through correlation studies across cohorts of patients and tissue from those patients. Once either previously established correlations or correlations derived in the future are confirmed by this assay then the assay method can be used to determine optimal treatment strategy

[0045] FIG. 1 shows an example of a single SRM/MRM assay performed on a Liquid Tissue lysate from a formalin fixed biological sample. An SRM/MRM assay was developed for a single peptide for quantitation of the KRas protein on a triplequadrupole mass spectrometer. Specific and unique characteristics about this KRas peptide (sequence SFEDIHHYR) were developed by analysis of all KRas peptides on both an ion trap and triple quadrupole mass spectrometers and are shown in FIG. 1A. That information includes the monoisotopic mass of the peptide, its precursor charge state, the precursor m/z value, the transition m/z values of the precursor, and the ion types of each of the identified transitions. That information must be determined experimentally for each and every candidate SRM/MRM peptide directly in Liquid Tissue lysates from formalin fixed samples/tissue; because, interestingly, not all peptides from the KRas protein can be detected in such lysates using SRM/MRM as described herein, indicating that KRas peptides not detected cannot be considered candidate peptides for developing an SRM/MRM assay for use in quantitating peptides/proteins directly in Liquid Tissue lysates from formalin fixed samples/tissue.

[0046] As shown in FIG. 1B, this particular SRM/MRM assay was performed on a triple quadrupole mass spectrometer. The experimental sample in this experiment was a Liquid Tissue protein lysate prepared from a cell line that had been formalin fixed, paraffin embedded to act as a tissue surrogate. Data from the assay indicates the presence of the unique SRM/MRM signature peak for this KRas peptide in the formalin fixed sample.

[0047] FIG. 1C shows the specific transition ion characteristics for this peptide used to quantitatively measure of the above-mentioned peptide in formalin fixed biological samples. These data indicate absolute amounts of this KRas peptide as a function of the molar amount of the peptide per microgram of protein lysate analyzed. Assessment of KRas protein levels in tissues based on analysis of formalin fixed patient-derived tissue can provide diagnostic, prognostic, and therapeutically-relevant information about each particular patient. In one embodiment, this disclosure describes a method for measuring the level of the GTPase KRas Protein in a biological sample, comprising detecting and/or quantifying the amount of one or more modified or unmodified KRas fragment peptides in a protein digest prepared from the biological sample using mass spectrometry; and calculating the level of modified or unmodified KRas protein in the sample; and wherein the level is a relative level or an absolute level. In a related embodiment, quantifying one or more KRas fragment peptides comprises determining the amount of the each of the KRas fragment peptides in a biological sample by comparison to an added internal standard peptide of known amount, wherein each of the KRas fragment peptides in the biological sample is compared to an internal standard peptide having the same amino acid sequence. In some embodiments the internal standard is an isotopically labeled internal standard peptide comprises one or more heavy stable isotopes selected from .sup.18O, .sup.17O, .sup.34S, .sup.15N, .sup.13C, .sup.2H or combinations thereof.

[0048] The method for measuring the level of the KRas protein in a biological sample described herein (or fragment peptides as surrogates thereof) may be used as a diagnostic and/or prognostic indicator of cancer in a patient or subject. In one embodiment, the results from measurements of the level of the KRas protein may be employed to determine the diagnostic stage/grade/status and/or the prognostic status of a cancer by correlating (e.g., comparing) the level of KRas protein found in a tissue with the level of that protein found in normal and/or cancerous or precancerous tissues.

[0049] Because both nucleic acids and protein can be analyzed from the same Liquid Tissue biomolecular preparation it is possible to generate additional information about disease diagnosis and drug treatment decisions from the nucleic acids in same sample upon which proteins were analyzed. For example, if the KRas protein is expressed by certain cells at increased levels, when assayed by SRM the data can provide information about the state of the cells and their potential for uncontrolled growth, potential drug resistance and the development of cancers can be obtained. At the same time, information about the status of the KRas genes and/or the nucleic acids and proteins they encode (e.g., mRNA molecules and their expression levels or splice variations) can be obtained from nucleic acids present in the same Liquid Tissue biomolecular preparation can be assessed simultaneously to the SRM analysis of the KRas protein. Any gene and/or nucleic acid not from the KRas and which is present in the same biomolecular preparation can be assessed simultaneously to the SRM analysis of the KRas protein. In one embodiment, information about the KRas protein and/or one, two, three, four or more additional proteins may be assessed by examining the nucleic acids encoding those proteins. Those nucleic acids can be examined, for example, by one or more, two or more, or three or more of: sequencing methods, polymerase chain reaction methods, restriction fragment polymorphism analysis, identification of deletions, insertions, and/or determinations of the presence of mutations, including but not limited to, single base pair polymorphisms, transitions, transversions, or combinations thereof.