Epidermal Growth Factor Receptor (EGFR) Protein SRM Assay

Abstract

The current disclosure provides for specific peptides, and derived ionization characteristics of the peptides, from the Epidermal Growth Factor Receptor (EGFR) protein that are particularly advantageous for quantifying the EGFR protein directly in biological samples that have been fixed in formalin by the method of Selected Reaction Monitoring (SRM) mass spectrometry, or what can also be termed as Multiple Reaction Monitoring (MRM) mass spectrometry. Such biological samples are chemically preserved and fixed wherein said 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, FFPE tissue blocks and cells from those blocks, and tissue culture cells that have been formalin fixed and or paraffin embedded. A protein sample is prepared from said biological sample using the Liquid Tissue™ reagents and protocol and the EGFR protein is quantitated in the Liquid Tissue™ sample by the method of SRM/MRM mass spectrometry by quantitating in the protein sample at least one or more of the peptides described. These peptides can be quantitated if they reside in a modified or an unmodified form. An example of a modified form of an EGFR peptide is phosphorylation of a tyrosine, threonine, serine, and/or other amino acid residues within the peptide sequence.

Claims

1. A method for measuring the level of the Epidermal Growth Factor Receptor (EGFR) protein in a biological sample of formalin fixed tissue sample from a human subject, comprising detecting and quantifying by mass spectrometry the amount of an EGFR fragment peptide in a protein digest prepared from said biological sample using mass spectrometry; and calculating the level of modified or unmodified EGFR protein in said sample; wherein said EGFR fragment peptide consists of the amino acid sequence SEQ ID NO:4 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 EGFR 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. (canceled)

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.

7-11. (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 EGFR fragment peptide comprises comparing an amount of said EGFR fragment peptide to the amount of the same EGFR fragment peptide in a different and separate biological sample.

18. The method of claim 1, wherein quantifying said EGFR fragment peptide comprises determining the amount of said EGFR fragment peptide in a biological sample by comparison to an added internal standard peptide of known amount, wherein said EGFR fragment peptide 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 said EGFR fragment peptide in the protein digest indicates the presence of EGFR 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 said EGFR fragment peptide, or the level of said EGFR 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 said EGFR fragment peptide, or the level of said EGFR 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 said EGFR fragment peptide or the level of EGFR protein.

25. The method of claim 24, 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 the amount of said EGFR fragment peptide or the level of EGFR protein.

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

27. The method of claim 26, wherein the therapeutic agent is selected from Tarceva, Iressa, and Erbitux.

28-32. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1, parts A to C, show an example of SRM/MRM assay of a single peptide from the EGFR protein performed on Liquid Tissue™ lysates with quantitation of the EGFR peptide conducted on a triplequadrupole mass spectrometer.

DETAILED DESCRIPTION

[0012] In principle, any predicted peptide derived from EGFR 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 EGFR 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 EGFR protein also might potentially be used to assay the extent of modification of EGFR protein in a sample.

[0013] EGFR fragment peptides may be generated by a variety of means 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 said biological sample and to liquefy said 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.

[0014] Surprisingly, it was found that many potential peptide sequences from the EGFR protein are unsuitable or ineffective for use in mass spectrometry-based SRM/MRM assays for reasons that are not immediately evident. 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 EGFR 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 distinct from other proteins, peptides may also fail to resolve well in separation (e.g., liquid chromatography), or adhere to glass or plastic ware.

[0015] EGFR peptides found in various embodiments of this disclosure (e.g., Tables 1 and 2) were derived from the EGFR 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 EGFR 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.

TABLE-US-00001 TABLE 1 Table 1 SEQ ID Peptide Sequence SEQ ID NO: 1 CDPSCPNGSCWGAGEENCQKLTKIICAQQCSGR SEQ ID NO: 2 CEGPCRK SEQ ID NO: 3 EDSFLQR SEQ ID NO: 4 IPLENLQIIR SEQ ID NO: 5 EISDGDVIISGNK SEQ ID NO: 6 EITGFLLIQAWPENR SEQ ID NO: 7 ELREATSPKANK SEQ ID NO: 8 EYHAEGGK SEQ ID NO: 9 FRELIIEFSK SEQ ID NO: 10 GLWIPEGEK SEQ ID NO: 11 GLWIPEGEKVKIPVAIK SEQ ID NO: 12 HFKNCTSISGDLHILPVAFRGDSFTHTPPLDP QELDILK SEQ ID NO: 13 IICAQQCSGRCRGK SEQ ID NO: 14 IPSIATGMVGALLLLLVVALGIGLFMRRR SEQ ID NO: 15 CEGPCR SEQ ID NO: 16 LFGTSGQKTK SEQ ID NO: 17 LTKIICAQQCSGR SEQ ID NO: 18 NCTSISGDLHILPVAFRGDSFTHTPPLDPQEL DILK SEQ ID NO: 19 PYDGIPASEISSILEKGER SEQ ID NO: 20 PAGSVQNPVYHNQPLNPAPSR SEQ ID NO: 21 SPSDCCHNQCAAGCTGPRESDCLVCR SEQ ID NO: 22 ITDFGLAK SEQ ED NO: 23 TPQHVKITDFGLAKLLGAEEK SEQ ID NO: 24 VCNGIGIGEFK SEQ ID NO: 25 VCNGIGIGEFKDSLSINATNIKHFK SEQ ID NO: 26 VLGSGAFGTVYKGLWIPEGEKVK SEQ ID NO: 27 YSFGATCVKKCPR SEQ ID NO: 28 CRGKSPSDCCHNQCAAGCTGPR SEQ ID NO: 29 DIVSSDFLSNMSMDFQNHLGSCQK SEQ ID NO: 30 EFVENSECIQCHPECLPQAMNITCTGR SEQ ID NO: 31 ELIIEFSKMARDPQR SEQ ID NO: 32 ELVEPLTPSGEAPNQALLR SEQ ID NO: 33 ESDCLVCRKFR SEQ ID NO: 34 GDSFTHTPPLDPQELDILK SEQ ID NO: 35 GENSCKATGQVCHALCSPEGCWGPEPR SEQ ID NO: 36 GKSPSDCCHNQCAAGCTGPRESDCLVCR SEQ ID NO: 37 GRECVDKCNLLEGEPR SEQ ID NO: 38 ILKETEFKK SEQ ID NO: 39 IPLENLQIIR SEQ ID NO: 40 KVCNGIGIGEFK SEQ ID NO: 41 KVCNGIGIGEFKDSLSINATNIK SEQ ID NO: 42 LLQERELVEPLTPSGEAPNQALLR SEQ ID NO: 43 MHLPSPTDSNFYR SEQ ID NO: 44 NVLVKTPQHVKITDFGLAK SEQ ID NO: 45 NVSRGRECVDK SEQ ID NO: 46 NYDLSFLK SEQ ID NO: 47 PKFRELIIEFSK SEQ ID NO: 48 LLQERELVEPLTPSGEAPNQALLR SEQ ID NO: 49 SLKEISDGDVIISGNK SEQ ID NO: 50 TDLHAFENLEIIR SEQ ID NO: 51 TDLHAFENLEIIRGR SEQ ID NO: 52 TKQHGQFSLAVVSLNITSLGLR SEQ ID NO: 53 TLRRLLQER SEQ ID NO: 54 TPLLSSLSATSNNSTVACIDR SEQ ID NO: 55 VAPQSSEFIGA- SEQ ID NO: 56 YLVIQGDER SEQ ID NO: 57 GSTAENAEYLR SEQ ID NO: 58 GSTAENAEY[Phosphoryl]LR SEQ ID NO: 59 GSHQISLDNPDYQQDDFFPK SEQ ID NO: 60 GSHQISLDNPDY[Phosphoryl]QQDDFFPK SEQ ID NO: 61 PAGSVQNPVYHNQPLNPAPSR SEQ ID NO: 62 PAGSVQNPVY[Phosphoryl]HNQPLNPAPSR SEQ ID NO: 63 ELVEPLTPSGEAPNQALLR SEQ ID NO: 64 ELVEPLTPS[Phosphoryl]GEAPNQALLR SEQ ID NO: 65 ELVEPLT[Phosphoryl]PSGEAPNQALLR SEQ ID NO: 66 ELVEPLT[Phosphoryl]PS[Phosphoryl] GEAPNQALLR SEQ ID NO: 67 GSHQISLDNPDYQQDFFPK SEQ ID NO: 68 GSHQISLDNPDY[Phosphoryl]QQDFFPK SEQ ID NO: 69 YSDPTGALTEDSIDDTFLPVPEYINQSVPK SEQ ID NO: 70 YSDPTGALTEDSIDDTFLPVPEY[Phosphoryl] INQSVPK SEQ ID NO: 71 Y[Phosphoryl]SDPTGALTEDSIDDTFLPVPEY INQSVPK SEQ ID NO: 72 Y[Phosphoryl]SDPTGALTEDSIDDTFLPVPEY [Phosphoryl]INQSVPK SEQ ID NO: 73 EYHAEGGK SEQ ID NO: 74 EY[Phosphoryl]HAEGGK

TABLE-US-00002 Table 2 Mono Precursor SEQ ID Isotopic Charge Precursor Transition NO Peptide sequence Mass State m/z m/z Ion Type SEQ ID GLWIPEGEK 1027.534 2 514.774 559.272 y5 NO: 10 2 514.774 672.356 y6 2 514.774 858.435 y7 SEQ ID GSTAENAEYLR 1209.562 2 605.7880249 651.346 y5 NO: 57 2 605.7880249 765.388 y6 2 605.7880249 894.431 y7 SEQ ID ITDFGLAK 863.475 2 432.744 535.323 y5 NO: 22 2 432.744 650.350 y6 2 432.744 751.398 y7 SEQ ID IPLENLQIIR 1207.737 2 604.872 756.472 y6 NO: 39 2 604.872 885.515 y7 2 604.872 998.599 y8 SEQ ID GSTAENAEY[Phosphoryl]LR 1289.529 2 645.7709961 596.7833 Reporter NO: 58 2 645.7709961 660.2747 y4 2 645.7709961 731.3118 y5 2 645.7709961 845.3547 y6 2 645.7709961 974.3973 y7 2 645.7709961 1045.434 y8 2 645.7709961 1146.482 y9 2 645.7709961 1233.514 y10 2 645.7709961 1290.536 y11 SEQ ID ELVEPLTPS[Phosphoryl]GE 2113.046 3 705.3549805 406.2426 y7 NO: 64 APNQALLR 3 705.3549805 811.4779 y7 3 705.3549805 822.411 y15 SEQ ID ELVEPLT[Phosphoryl]PS 2113.046 2 1057.530029 1008.542 Reporter NO: 65 GEAPNQALLR 2 1057.530029 1252.664 y12 SEQ ID GSHQISLDNPD 2314.99 2 1158.501953 1364.555 y10 NO: 68 Y[Phosphoryl]QQDFFPK 2 1158.501953 1478.598 y11

[0016] 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, as for example including but not limited to the protease trypsin. 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. 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 be the best type of mass spectrometer for conducting global profiling of peptides. Although SRM/MRM assay can be developed and performed on any type of mass spectrometer, including a MALDI, ion trap, or triple quadrupole, the most advantageous instrument platform for SRM/MRM assay is often considered to be a triple quadrupole instrument platform.

[0017] 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 EGFR protein.

[0018] In one embodiment, the EGFR tryptic peptides identified as useful in the determination of absolute or relative amounts of the EGFR receptor include one or more, two or more, three or more, four or more, five or more, six or more, eight or more, or ten or more of the peptides of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ 1D NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:18, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, SEQ ID NO:25, SEQ ID NO:26, SEQ ID NO:27, SEQ ID NO:28, SEQ ID NO:29, SEQ ID NO:30, SEQ ID NO:31, SEQ ID NO:32, SEQ ID NO:33, SEQ ID NO:34, SEQ ID NO:35, SEQ ID NO:36, SEQ ID NO:37, SEQ ID NO:38, SEQ ID NO:38, SEQ ID NO:40, SEQ ID NO:41, SEQ ID NO:42, SEQ ID NO:43, SEQ ID NO:44, SEQ ID NO:45, SEQ ID NO:46, SEQ ID NO:47, SEQ ID NO:48, SEQ ID NO:49, SEQ ID NO:50, SEQ ID NO:51, SEQ ID NO:52, SEQ ID NO:53, SEQ ID NO:54, SEQ ID NO:55, SEQ ID NO:56, SEQ ID NO:57, SEQ ID NO:58, SEQ ID NO:59, SEQ ID NO:60, SEQ ID NO:61, SEQ ID NO:62, SEQ ID NO:63, SEQ ID NO:64, SEQ ID NO:65, SEQ ID NO:66, SEQ ID NO:67, SEQ ID NO:68, SEQ ID NO:69, SEQ ID NO:70, SEQ ID NO:71, SEQ ID NO:72, SEQ ID NO:73, and SEQ ID NO:74, 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 of the peptides in Table 1, or any combination of those peptides (e.g., one or more, two or more, three or more, four or more, five or more, six or more, eight or more, or ten or more of those peptides recited in Table 1, and particularly combinations with one or more of the peptides found in table 2) are candidates for use in quantitative SRM/MRM assay for the EGRF protein in human biological samples, including directly in formalin fixed patient tissue.

[0019] The EGFR 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 EGFR protein in formalin fixed tissue. Further data analysis of these experiments indicated no preference is observed for any specific peptides from any specific organ site. Thus, each of these peptides is believed to be suitable for conducting SRM/MRM assays of the EGFR protein on a Liquid Tissue™ lysate from any formalin fixed tissue originating from any biological sample or from any organ site in the body.

[0020] In one embodiment the peptides in Table 1, or any combination of those peptides (e.g., one or more, two or more, three or more, four or more, five or more, six or more, eight or more, or nine or more of those peptides recited in Table 1, and particularly combinations with the peptides also found in Table 2) are assayed by methods that do not rely upon mass spectroscopy, including, but not limited to, immunological methods (e.g., Western blotting or ELISA). Regardless of how information directed to the amount of the peptide(s) (absolute or relative) is obtained, the information may be employed in any of the methods described herein, including indicating (diagnosing) the presence of cancer in a subject, determining the stage/grade/status of the cancer, providing a prognosis, or determining the therapeutics or treatment regimen for a subject/patient.

[0021] Embodiments of the present disclosure includes compositions comprising one or more, two or more, three or more, four or more, five or more, six or more, eight or more, or ten or more of the peptides in Table 1. In some embodiments, the compositions comprise one or more, two or more, three or more, four or more, five or more, six or more, or seven or more of the peptides in Table 2. Compositions comprising peptides may include one or more, two or more, three or more, four or more, five or more, six or more, eight or more, or ten or more peptides that are isotopically labeled. Each of the peptides may be labeled with one or more isotopes selected independently from the group consisting of: .sup.18O, .sup.17O, .sup.34S, .sup.15N, .sup.13C, .sup.2H or combinations thereof. Compositions comprising peptides from the EGFR protein, whether isotope labeled or not, do not need to contain all of the peptides from that protein (e.g., a complete set of tryptic peptides). In some embodiments the compositions do not contain one or more, two or more, three or more, four or more, five or more, six or more, eight or more, or ten or more peptides from EGFR, and particularly peptides appearing in Table 1 or Table 2. Compositions comprising peptides may be in the form of dried or lyophized materials, liquid (e.g., aqueous) solutions or suspensions, arrays, or blots.

[0022] An important consideration for conducting an SRM/MRM assay is the type of instrument that may be employed in the analysis of the peptides. Although SRM/MRM assays can be developed and performed on any type of mass spectrometer, including a MALDI, ion trap, or triple quadrupole, the most advantageous instrument platform for SRM/MRM assay is often considered to be a triple quadrupole instrument platform. That type of a mass spectrometer may be considered to be the most 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.

[0023] In order to most efficiently implement SRM/MRM assay for each peptide derived from the EGFR 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.

[0024] The additional information about target peptides in general, and about specific EGFR 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. Additional peptide information that may be used to develop an SRM/MRM assay for the EGFR protein is shown by example for eight (8) of the EGFR peptides from the list in Table 1 and is shown in Table 2. Similar additional information described for the eight (8) EGFR peptides shown by example in Table 2 may be prepared, obtained, and applied to the analysis of the other peptides contained in Table 1.

[0025] The method described below was used to: 1) identify candidate peptides from the EGFR protein that can be used for a mass spectrometry-based SRM/MRM assay for the EGFR protein, 2) develop individual SRM/MRM assay, or assays, for target peptides from the EGFR protein in order to correlate and 3) apply quantitative assays to cancer diagnosis and/or choice of optimal therapy.

Assay Method

[0026] 1. Identification of SRM/MRM candidate fragment peptides for the EGFR protein [0027] 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 [0028] b. Analyze all protein fragments in the Liquid Tissue™ lysate on an ion trap tandem mass spectrometer and identify all fragment peptides from the EGFR protein, where individual fragment peptides do not contain any peptide modifications such as phosphorylations or glycosylations [0029] c. Analyze all protein fragments in the Liquid Tissue™ lysate on an ion trap tandem mass spectrometer and identify all fragment peptides from the EGFR protein that carry peptide modifications such as for example phosphorylated or glycosylated residues [0030] d. All peptides generated by a specific digestion method from the entire, full length EGFR 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 [0031] 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 EGFR protein [0032] 2. Mass Spectrometry Assay for Fragment Peptides from EGFR Protein [0033] 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 EGFR protein [0034] 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 [0035] 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. [0036] 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 [0037] b. Perform SRM/MRM analysis so that the amount of the fragment peptide of the EGFR 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. [0038] i. Relative quantitation may be achieved by: [0039] 1. Determining increased or decreased presence of the EGFR protein by comparing the SRM/MRM signature peak area from a given EGFR peptide detected in a Liquid Tissue™ lysate from one formalin fixed biological sample to the same SRM/MRM signature peak area of the same EGFR 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 [0040] 2. Determining increased or decreased presence of the EGFR protein by comparing the SRM/MRM signature peak area from a given EGFR 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. [0041] 3. Determining increased or decreased presence of the EGFR protein by comparing the SRM/MRM signature peak area for a given EGFR 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 EGFR protein to levels of other proteins that do not change their levels of expression under various cellular conditions. [0042] 4. These assays can be applied to both unmodified fragment peptides and for modified fragment peptides of the EGFR 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. [0043] 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 EGFR 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 [0044] 1. The internal standard is a labeled synthetic version of the fragment peptide from the EGFR 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 [0045] 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, [0046] 3. Apply Fragment Peptide Quantitation to Cancer Diagnosis and Treatment [0047] a. Perform relative and/or absolute quantitation of fragment peptide levels of the EGFR protein and demonstrate that the previously-determined association, as well understood in the field of cancer, of EGFR protein expression to the stage/grade/status of cancer in patient tumor tissue is confirmed [0048] b. Perform relative and/or absolute quantitation of fragment peptide levels of the EGFR 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

[0049] FIG. 1 shows an example of a single SRM/MRM assay performed on Liquid Tissue™ lysates from formalin fixed cancer tissue. An SRM/MRM assay was developed for a single peptide for quantitation of the EGFR protein on a triple quadrupole mass spectrometer. Specific and unique characteristics about this EGFR peptide (sequence IPLENLQIIR) were developed by analysis of all EGFR 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 tissue; because, interestingly, not all peptides from the EGFR protein can be detected in such lysates using SRM/MRM as described herein, indicating that EGFR 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 tissue.

[0050] As shown in FIG. 1B, this particular SRM/MRM assay was performed on a triple quadrupole mass spectrometer. A control protein lysate where the peptide was known to be present in large amounts was analyzed because this lysate was prepared from a mouse xenograft tumor that resulted from injection of a human-derived cancer cell line into a nude mouse. Thus this xenograft tumor was the positive control. The experimental sample in this experiment was a Liquid Tissue™ protein lysate prepared from standard formalin fixed, paraffin embedded human breast cancer tissue. Data from the assay indicates the presence of the unique SRM/MRM signature peak for this EGFR peptide in both the control sample and the experimental sample. By comparing the SRM/MRM signature peak area between these 2 samples generates relative quantitative measure for the EGFR protein between 2 different biological samples.

[0051] FIG. 1C shows quantitative measurement of the above-mentioned peptide across a collection of ten (10) formalin fixed cancer tissues using an internal standard to achieve absolute quantitation of the EGFR protein across a cohort of cancer-derived patient samples. These data indicate absolute amounts of this EGFR peptide as a function of molar amount of the peptide per microgram of protein lysate analyzed. Assessment of EGFR 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 Epidermal Growth Factor Receptor (EGFR) protein in a biological sample, comprising detecting and/or quantifying the amount of one or more modified or unmodified EGFR fragment peptides in a protein digest prepared from said biological sample using mass spectrometry; and calculating the level of modified or unmodified EGFR protein in said sample; and wherein said level is a relative level or an absolute level. In a related embodiment, quantifying one or more EGFR fragment peptides comprises determining the amount of the each of the EGFR fragment peptides in a biological sample by comparison to an added internal standard peptide of known amount, wherein each of the EGFR 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.

[0052] The method for measuring the level of the EGFR protein in a biological sample described herein (or fragment peptides as surrogates thereof) may be used as a diagnostic indicator of cancer in a patient or subject. In one embodiment, the results from measurements of the level of the EGFR protein may be employed to determine the diagnostic stage/grade/status of a cancer by correlating (e.g., comparing) the level of EGFR receptor found in a tissue with the level of that protein found in normal and/or cancerous or precancerous tissues.