Summary
Aim
Glioblastoma multiforme (GBM) is one of the most frequent human brain tumor and causes dismal outcome. To identify tumor‐associated antigens in GBM patients may find potential diagnostic markers and immunotherapeutic targets. In this study, we identified a gene termed URGCP using the serological identification of antigens by recombinant A2B5 positive glioma cDNA library. The gene product of URGCP is immunogenic in GBM after tested in allogenic patients serum screening.
Methods and results
GBM patients with an auto‐antibody response against URGCP show longer survival than those without URGCP response. In additional, we show that URGCP was high expression in most GBM tissues and cell lines compared with normal brain tissues and majorly co‐expressed with stem cell marker A2B5.
Conclusion
We identified a potential new biomarker of GBM, URGCP. The findings indicate that URGCP is immunogenic in human GBM and suggest its potential use as diagnostic and immunotherapeutic for GBM patients.
Keywords: A2B5, GBM, SEREX, URGCP
Introduction
GBM is one of the most malignant brain tumors with a median survival only 14 months. Various strategies for therapy of glioma have been made, including surgery, chemotherapy, and radiotherapy, without obtaining any obvious improvement 1, 2, 3. However, so far few immunological markers for survival have been introduced into clinical application. Thus, novel clinical immune markers and novel therapeutic concepts are urgently needed for this CNS malignancy.
Cancer stem cells are small subsets of cancer cells and have the ability for self‐renewal and multipotency. These subsets of cancer cells are responsible for most tumor therapy failure and recurrence 4, 5. In GBM, glioma stem‐like cells have been implicated to play one of the important roles in treatment failure. CD133 was well known as a stem cell marker and was universally expressed by most of malignant brain tumors. However, recent studies have defined A2B5+CD133‐tumor‐initiating cells in human glioma 6, 7. The prevalence of A2B5 reactivity in these cells suggests that human glioma contains multiple subsets of cells with the capacity to form tumors and specifically distinct from CD133+ cells.
Serological analysis of recombinant tumor cDNA expression libraries (SEREX) is a method designed to isolate tumor‐associated antigens that have elicited high‐titer IgG responses in human hosts 8, 9. SEREX analysis has identified a number of tumor‐associated antigens that have associated with the etiology, diagnosis, and therapy of cancer. To date, more than 600 tumor‐associated antigens have been identified using by SEREX 9, 10. Most of the SEREX‐based studies aimed at the identification of novel tumor antigens that elicit an antibody response in tumor tissue or cell lines 11, 12, 13, 14, 15, 16. Until now, there was no antigen identified by SEREX based on tumor stem cell, which is notorious for GBM recurrence and malignance 5, 17, 18.
Previously studies have demonstrated that A2B5 was a new marker of glioma stem cell 6, 7. In this study, we firstly present to screen antigen‐specific immune responses associated with glioblastoma by screening cDNA libraries of A2B5‐positive glioma cells. We identified a candidate new biomarker of GBM, URGCP and suggest its use as an immunotherapeutic target for GBM patients.
Materials and methods
Sera and Glioma Tissue
Sera were obtained from patients with GBM and healthy volunteers after they had provided written informed consent. Glioma patients underwent surgery, and the histology was grade II–IV at Department of Neurosurgery, Huashan Hospital, Fudan University. Each serum was centrifuged at 3 000 × g for 5 min and then frozen at –80°C until use. Glioma tissue was collected from operations. Control brain tissue was isolated from nonneoplastic CNS tissues, which were collected during a surgery from patients with intractable epilepsy. The Local Ethical Review Board of the Huashan hospital, Fudan University, approved this study.
cDNA Library
The total RNA was extracted from the A2B5‐positive glioma cells sorting by MACS MicroBead Technology. Using the TRIZOL reagent (Invitrogen), mRNA was extracted and purified on an oligo (dT) column. The first‐strand cDNA synthesis was performed using Hind III Random Primers, MMLV reverse transcriptase. Then the cDNA was ligated to EcoRI and Hind III adaptors and digested with EcoRI and Hind III. Double‐strand cDNA fragments were cloned into T7Select®10‐3b vector (Merck KGaA, Darmstadt, Germany) and then finished in vitro packaging using an vitro packaging kit (Merck).
Immunoscreening
Sera from the mixed GBM patients were diluted in 1% bovine serum albumin/tris‐buffered saline (TBS) and preabsorbed with transformed E. coli lysates and E. coli infected with T7Select®10‐3b phage. Recombinant phages at a concentration of 5 × 108/10 cm plate were amplified for 6 h at 37°C, and then covered with nitrocellulose membranes (Amersham, Buckinghamshire, England) and incubated for an additional 3 h at 37°C to transfer the encoded proteins onto the filter membranes. Membranes were then blocked with 5% (w/v) skim milk/TBS. After washing with TBS containing 0.05% Tween 20 (TBS‐T), membranes were incubated in prepared sera for 15 h at 4°C. This was followed by incubation in horseradish peroxidase (HRP)‐conjugated mouse anti‐human IgG for 1 h at 37°C, and then membranes were washed in TBS‐T and TBS and incubated with ECL RPN 2106 (Amersham) for 1 min and exposed to LAS 4000 to detect antibody‐reactive phage plaques. Positive recombinant clones were picked up and purified by an additional cycle of plating and screening.
Sequence Analysis of Identified cDNA Clones
Immunoreactive phage clones were amplified by PCR using the Ex Taq kit (Takara Shuzo) and sequenced using the Big Dye Terminator Cycle Sequencing Ready Reaction Kit and an ABI Prism automated sequencer (Perkin‐Elmer, Branchburg, NJ, USA). The sequenced DNAs were analyzed by a BLAST search of genetic databases at the National Center for Biotechnology Information.
qRT‐PCR
For the analysis of URGCP messenger RNA (mRNA) expression, complementary DNA (cDNA) synthesis was performed using random primers under standard conditions. mRNA expression was quantified using the 2‐△△Ct method. GAPDH served as the internal control. All reactions were performed in triplicate.
Immunohistochemistry and Immunofluorescence
Immunohistochemistry and Immunofluorescence assay was performed as previously described 19, 20. Briefly, URGCP expression was analyzed using immunocytochemical staining of GBM, low‐grade glioma, and normal brain tissues. The tissue section was incubated with URGCP (1:500) for 12 h, and then washed and incubated with biotinylated goat anti‐rabbit IgG (1:3000) for 30 min at room temperature. The sections were immersed in a solution with the avidin–biotin complex (Vector Laboratories, Burlingame, CA, USA) for 30 min, developed with diaminobenzidine and counterstained with eosin. The sections were scanned at magnification (200 × ) using light microscopy. Two pathologists evaluated the immunoreactivity and staining for each section. For immunofluorescence assay, primary antibodies for cultured tumor cells and clinical samples were anti‐UGRCP, anti‐A2B5, and antinestin. The secondary antibodies were Alexa Fluro 488, 594, or 647‐conjugated donkey anti‐mouse or rabbit or anti‐goat IgG. Nuclei were counterstained with 4′,6‐diamidino‐2‐phenylindole (DAPI). Fluorescence signals were detected with a two‐photon confocal laser‐scanning microscopy.
Elisa
For enzyme‐linked immunosorbent assays, 96‐well flat plates were coated with purified URGCP protein (150 ng/well) at 4°C overnight. After washing three times with PBST, the plates were blocked with FCS. Then, 100 μL/well of 500‐fold diluted sera from normal controls or glioma patients were added to each plate. The plates were washed in PBST after 1.5 h incubation at 37°C and further incubated with 1:1000‐diluted horseradish peroxide (HRP)‐conjugated goat‐anti‐human IgG for 1 h at 37°C. Finally, 100 μL/well of tetramethyl‐benzidine substrate solution was added, and the reaction was stopped by adding 50 μL of 1 mol/L sulfuric acid. Optical absorbance was measured at 450 nm.
GBM mRNA data and Statistical Analysis
The TCGA and Rembrandt mRNA expression microarray data were downloaded from the following portal: http://tcga-data.nci.nih.gov/tcga/homepage.htm and https://caintegrator.nci.nih.gov/rembrandt/home.do. Data were presented as mean values and standard deviation (SD). Statistical differences between two groups were evaluated by the unpaired Student's t‐test by SPSS 14.0. Overall survival analysis was performed according to the Kaplan–Meier method. Tests with P < 0.05 are considered statistically significant.
Result
Serological Screening of A2B5 Positive cDNA Library
The phage expression library was constructed using mRNA derived from the A2B5‐positive glioma cells (Figure 1). To identify glioma‐associated tumor antigens, cDNA expression libraries were screened using mixed sera from 10 GBM patients and 23 reacting clones were recognized by serum IgG antibodies. DNA sequence was analyzed by comparing the homologous sequences from NCBI BLAST database. Finally, we found that these 23 isolated clones comprised 16 independent genes, and the rest of 7 clones were overlapped (Table 1).
Figure 1.
A2B5+ glioma cells sorting and characteristics. (A) A2B5+ glioma cells were sorted by A2B5 MicroBead and proliferated in culture as nonadherent spheres, whereas GBM tumor cells adhered to culture dishes did not form spheres. (B) GBM tumor cells and A2B5+ glioma cells were immunostained for A2B5 and subjected to flow cytometry for quantification of A2B5 expression. (C) A2B5+ primary tumor spheres from GBMs are immunostained for characteristic neural stem cell marker nestin and A2B5.
Table 1.
List of gene fragments obtained by serological analysis of recombinant cDNA libraries from an A2B5‐positive glioma cDNA library
| NO. | Gene name | EntrezGene | Definition |
|---|---|---|---|
| MY‐A2B5‐01 | NUDC | 10726 | Homo sapiens nuclear distribution C homolog (A. nidulans) (NUDC), mRNA |
| MY‐A2B5‐02 | GATC | 283459 | Homo sapiens glutamyl‐tRNA(Gln) amidotransferase, subunit C (GATC), transcript variant 1, mRNA |
| MY‐A2B5‐03 | LOC283459 | 283459 | Homo sapiens hypothetical protein LOC283459, mRNA |
| MY‐A2B5‐04 | SRSF9 | 8683 | Homo sapiens serine/arginine‐rich splicing factor 9 (SRSF9), mRNA |
| MY‐A2B5‐05 | EEF1A1 | 1915 | Homo sapiens elongation factor 1‐alpha 1 (EEF1A1L14) mRNA |
| MY‐A2B5‐06 | EEF1A1 | 1915 | Homo sapiens elongation factor 1‐alpha 1 (EEF1A1L14) mRNA |
| MY‐A2B5‐07 | EEF1A1 | 1915 | Homo sapiens elongation factor 1‐alpha 1 (EEF1A1L14) mRNA |
| MY‐A2B5‐08 | URGCP | 55665 | Homo sapiens Upregulator Of Cell Proliferation, mRNA |
| MY‐A2B5‐09 | MRPS24 | 64951 | Mitochondrial ribosomal protein S24 |
| MY‐A2B5‐10 | MORF4L1 | 10934 | Homo sapiens mortality factor 4 like 1, mRNA |
| MY‐A2B5‐11 | URGCP | 55665 | Homo sapiens upregulator Of cell proliferation, mRNA |
| MY‐A2B5‐12 | RAB3GAP2 | 10933 | Homo sapiens RAB3 GTPase activating protein subunit 2 |
| MY‐A2B5‐13 | MORF4L1P1 | 25782 | Homo sapiens MORF4L1 mRNA |
| MY‐A2B5‐14 | MORF4L1P1 | 25782 | Homo sapiens MORF4L1P1 mRNA |
| MY‐A2B5‐15 | URGCP | 55665 | Homo sapiens upregulator Of cell proliferation, mRNA |
| MY‐A2B5‐16 | USP15 | 9958 | Homo sapiens ubiquitin‐specific peptidase 15 |
| MY‐A2B5‐17 | MCLD | 9958 | MCLD 16S ribosomal RNA, complete sequence |
| MY‐A2B5‐18 | pG1 protein | Homo sapiens pG1 protein mRNA | |
| MY‐A2B5‐19 | NKAIN2 | 154215 | Homo sapiens Na+/K+ transporting ATPase interacting 2 (NKAIN2), mRNA |
| MY‐A2B5‐20 | RP1‐157N22 | 154215 | Human DNA sequence from clone RP1‐157N22 on chromosome 6q22.32‐23.2 |
| MY‐A2B5‐21 | WB20 | WB20 16S ribosomal RNA gene | |
| MY‐A2B5‐22 | WB18 16S | WB18 16S ribosomal RNA gene | |
| MY‐A2B5‐23 | NUDC | 10726 | Homo sapiens nuclear distribution C homolog (A. nidulans) (NUDC), mRNA |
Allogenic Serum Screening on URGCP Antigen
Among the collected 16 gene fragments by SEREX, we firstly focus on URGCP, which was mostly associated with tumor phenotype and involved in cell cycle regulation of cyclin D1 21. To confirm the specificity and relevance of the auto‐antibody to URGCP, the allogenic GBM patient serum screening was performed. We plated the E. coli cells transfected with the URGCP clone and GBM cDNA library containing cells separately on an agar plate. After 12 hours, plaques were transferred to nitrocellulose membranes, reacted with GBM sera, and then scored positive by visual inspection when compared to cDNA library plaques (Figure 2A). The specific immune reaction was detected in 14 of 40 GBM sera (Table 2). To confirm the specificity of antibody reaction in GBM, we applied sera from healthy volunteers. None of the 14 control sera contained antibodies against cDNA library plaques and URGCP (Figure 2A).
Figure 2.
UGRCP‐specific immunal responses. (A) GBM patient sera indicated at the top of the figure were incubated with clones expressing UGRCP antigen and with the A2B5‐positive GBM cDNA library. Positive plaques indicating an antibody reaction show more intensive staining than cDNA library background. Normal control sera contained no antibodies response against cDNA library and URGCP. (B) Serum samples from healthy donors and GBM patients were diluted at 1:100 and were analyzed by enzyme‐linked immunosorbent assay (ELISA) to detect antibodies reactive to recombinant UGRCP protein. The dashed line indicates a cut‐off value (ODmean ± 2 SD) as determined by ELISA assays using sera from healthy individuals. (C) Kaplan–Meier survival curves indicating cumulative survival that patients with UGRCP‐positive response experienced longer survival.
Table 2.
Summary of SEREX results with allogenic GBM patients' sera
| Sera | Sex | Age | Pathology | SEREX |
|---|---|---|---|---|
| G1 | Female | 56 | GBM WHO IV | − |
| G2 | Female | 62 | GBM WHO IV | − |
| G3 | Male | 55 | GBM WHO IV | + |
| G4 | Male | 56 | GBM WHO IV | + |
| G5 | Female | 72 | GBM WHO IV | − |
| G6 | Female | 60 | GBM WHO IV | + |
| G7 | Male | 58 | GBM WHO IV | − |
| G8 | Female | 62 | GBM WHO IV | − |
| G9 | Male | 43 | GBM WHO IV | − |
| G10 | Male | 58 | GBM WHO IV | + |
| G11 | Female | 54 | GBM WHO IV | − |
| G12 | Male | 60 | GBM WHO IV | − |
| G13 | Female | 27 | GBM WHO IV | − |
| G14 | Female | 38 | GBM WHO IV | − |
| G15 | Female | 66 | GBM WHO IV | + |
| G16 | Male | 66 | GBM WHO IV | − |
| G17 | Female | 43 | GBM WHO IV | − |
| G18 | Male | 55 | GBM WHO IV | + |
| G19 | Male | 32 | GBM WHO IV | − |
| G20 | Female | 58 | GBM WHO IV | − |
| G21 | Female | 52 | GBM WHO IV | − |
| G22 | Male | 57 | GBM WHO IV | − |
| G23 | Male | 43 | GBM WHO IV | − |
| G24 | Male | 52 | GBM WHO IV | − |
| G25 | Female | 55 | GBM WHO IV | − |
| G26 | Female | 45 | GBM WHO IV | + |
| G27 | Male | 67 | GBM WHO IV | + |
| G28 | Female | 55 | GBM WHO IV | + |
| G29 | Male | 44 | GBM WHO IV | − |
| G30 | Male | 52 | GBM WHO IV | − |
| G31 | Female | 56 | GBM WHO IV | + |
| G32 | Female | 53 | GBM WHO IV | + |
| G33 | Male | 66 | GBM WHO IV | − |
| G34 | Male | 48 | GBM WHO IV | − |
| G35 | Female | 53 | GBM WHO IV | − |
| G36 | Male | 58 | GBM WHO IV | + |
| G37 | Female | 37 | GBM WHO IV | + |
| G38 | Female | 65 | GBM WHO IV | + |
| G39 | Female | 54 | GBM WHO IV | − |
| G40 | Male | 42 | GBM WHO IV | + |
URGCP Elicited Specific Humoral Responses in GBM Patients
Next, we assessed the frequency of anti‐URGCP immune responses in vivo. Using an ELISA assay, we observed immune reaction of URGCP in 14 healthy donors and 40 GBM patients. The mean OD450 value of anti‐URGCP IgG was higher in GBM patients than those in healthy donors (Figure 2B; P < 0.001). After setting the cut‐off value by mean OD450 plus 2 SD (ODmean + 2SD) from healthy donors, we found that all serum samples from healthy donors tested were negative. URGCP‐specific immune reaction was detectable in 14 of 40 samples from GBM patients (Figure 2B). It was consistent with results obtained from allogenic serum screening in vitro.
Prognostic Value of URGCP Immune Response
To evaluate the role of the URGCP antibody response in GBM survival, we compared the OS of the patients with different immune response of URGCP antibodies. The URGCP‐positive patients showed a better survival than URGCP‐negative patients (Figure 2C). The median survival of URGCP‐positive patients was 19 months, while the URGCP‐negative patients was only 11 months (P = 0.041).
URGCP Expression in Normal and Tumor Tissues and in Tumor Cell Lines
To demonstrate the expression pattern of URGCP transcripts in glioma cell lines and control brain tissues, real‐time PCR assay was performed. Our results suggested that URGCP was expressed highly in most of glioma cell lines (LN229, U87, LN308, A172, T98, U251, and SNB19), but low expression in normal brain tissue (Figure 3A). Consistent with the cell lines' data, we found that the protein expression of URGCP in GBM tissues was also significantly higher than normal brain tissues in an independent set of 227 human GBM tumors from Rembrandt database (Figure 3B).
Figure 3.
Expression of URGCP in normal brain and gliomas. (A) Real‐time PCR analyzed the mRNA expression of URGCP in normal brain and glioma cell lines. (B) The mRNA expression of URGCP in control brain and GBM in Rembrandt database. (C) Immunohistochemistry with URGCP antibody on formalin‐fixed parafin‐embedded brain tissue and different grade glioma samples.
To further analyze URGCP protein expression, IHC assay for URGCP was performed in normal brain and different grade gliomas. In glioma, strong positive expression of URGCP was observed in both cytoplasms and nucleus (Figure 3C). The levels of URGCP expression were increased according to the pathology grade of tumors. That is to say high‐grade tumor was heavily stained, while low‐grade and normal brain tissue were barely stained. In additional, URGCP was also majorly co‐expressed with A2B5 in neurospheres cells and clinical samples (Figure 4). These results suggest that URGCP could serve as a new tumor‐associated biomarker that is expressed frequently in GBMs.
Figure 4.
URGCP was majored co‐expressed with A2B5. (A) A2B5+ primary tumor spheres from GBM are immunostained for characteristic neural stem cell marker A2B5 and for URGCP. (B) Immunofluorescence with URGCP and A2B5 antibody on formalin‐fixed parafin‐embedded GBM samples.
Discussion
Using a combined immunological and molecular approach based on the auto‐antibody response against tumor‐associated antigens, we isolated URGCP, a new biomarker in GBM. Then we reported that URGCP was immunogenic in 14/40 of GBM patients, but not in every healthy control. The obviously high expression in GBM patients compared with the normal brain tissue and co‐localization with A2B5 supports the hypothesis that URGCP may play a key role in GBM.
SEREX was first developed by Michael Pfreundschuh at Homburg's Saarland University 9. By screened diluted serum of the patient with cDNA library from fresh tumor cells, tumor‐associated antigen can be identified. So far, more than 600 tumor‐associated antigens have been identified by SEREX method. Several SEREX studies have focused on glioma specimens, cell lines, and sera from glioma patients. In the initial analysis, Struss et al. 22 screened a human GBM cDNA library with autologous patient serum and identified a gene termed PHF3. GBM patients with auto‐antibody response against PHF3 show significant longer survival than those without PHF3 response 23. To extend the number of cDNA library, 5 cDNA libraries from astrocytomas and other grade gliomas were screened by Rudolf et al. 24. This resulted in the isolation of 10 gene products, including the GFAP, Bax‐inhibitor 1, 3 markers involved in gene regulation and proliferation, SP40, TCP‐1, calnexin, and 2 new gene products. The other studies also applied SEREX to identify glioma antigens, including SH3GL1, GLEA2, and SOX6 23, 24, 25. Our present study was firstly designed to identify cancer antigens for human glioma by SEREX, using a cDNA library from A2B5‐positive glioma cells, which were crucial for the initiation and maintenance of GBM malignant 6, 7. In this screening, we isolated a mew biomarker UGRCP and confirmed its expression and immunoreactivity in human GBM.
URGCP was a novel gene located on chromosome 7, firstly identified by PCR select cDNA subtraction 26. Patients with high expression of URGCP in epithelial ovary cancer and osteosarcoma had shorter survival time, correlated with tumor recurrence and metastasis 27, 28. In additional, overexpression of URGCP in plenty types of cancers promoted cell growth and survival, suggesting that URGCP may be associated with the development of carcinogenesis 27, 29, 30, 31, 32, 33. However, there were no studies about URGCP in glioma. In this study, real‐time PCR analysis showed that the mRNA lever of URGCP was high expressed in human glioma cells compared with normal brain. IHC analyses demonstrated that URGCP was high expressed in both low‐ and high‐grade gliomas. Moreover, URGCP was majorly co‐expressed with stem cell marker A2B5 and confirmed in A2B5 neurosphere cells and in GBM samples.
Taken together, we identified candidate new biomarker of GBM, URGCP. These findings show that URGCP is immunogenic in humans' GBM and indicate its potential use as diagnostic and immunotherapeutic target for GBM.
Conflict of Interest
The authors declare no conflict of interest.
Acknowledgments
This work was supported by the Project for Science and Technology Commission of Shanghai Municipality Grant (13JC1408000), grant from National Natural Science Foundation of China (81272797, 81372708), Shanghai Talents Development Funds (2011063), Innovation Program of Shanghai Municipal Education Commission (13ZZ010), China Postdoctoral Science Foundation (2013M531121); Shanghai Postdoctoral Science Foundation (13R21411300).
The first two authors contributed equally to this work.
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