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W.M. Keck Autoimmune Disease Center, The Scripps Research Institute, La Jolla, California 92037 [J-Y. Z., E. K. L. C., E. M. T.]; Department of Biochemistry and Microbiology, Center for Molecular Biology and Gene Therapy, Loma Linda University School of Medicine, Loma Linda, California [C. A. C.]; Department of Biology, School of Life Sciences, Xiamen University, Xiamen, Peoples Republic of China [X-X. P.]; and Division of Biomathematics, Department of Molecular and Experimental Medicine, The Scripps Research Institute [J. A. K.]
| Abstract |
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| Introduction |
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Factors leading to the production of such autoantibodies are not completely understood, but the available data show that many of the target antigens are cellular proteins whose aberrant regulation could lead to tumorigenesis, such as p53, HER-2/neu, and ras, or are proteins whose dysregulation could have tumorigenic potential, including mRNA binding proteins, such as p62, and cell cycle control proteins, such as cyclin B1 (9 , 10) . A previous study showed that lung tumors contained several types of p53 gene mutations, including missense, stop codon, and frame shift mutations, but it was the missense mutations resulting in overexpression of protein with altered function and increased stability that correlated with antibody production (11) . In the case of p62, which is a fetal protein absent in adult tissues, immunogenicity appears to be related to abnormal expression of p62 in tumor cells (12) , and with the onconeural antigens in paraneoplastic neurological disorders, antibody responses are thought be to related to ectopic expression in tumor cells of normally neuron-restricted cellular proteins (6 , 7) . The immune system in certain cancer patients appears to have the capability of sensing these abnormalities, and we have proposed that autoantibodies might be regarded as reporters identifying aberrant cellular mechanisms in tumorigenesis (13) .
Many investigators have been interested in the use of autoantibodies as serological markers for cancer diagnosis, especially because of the general absence of these autoantibodies in normal individuals and noncancer conditions. Enthusiasm for this approach has been tempered by low sensitivity. We showed recently (14) in a study using 777 sera from different types of cancer that autoantibodies to p62 were detected in 11.6 ± 1.15% and antibodies to Koc (15) , another TAA,5 in 12.2 ± 1.17%. However, when the data were analyzed for frequency of antibodies to either p62 or Koc, the frequency rose to 20.5 ± 1.45%. These initial observations were the basis for the current study using several hundred sera from six different types of cancer to determine reactivities to a mini-array of seven selected TAAs. We show that the frequency of antibodies to any individual antigen rarely exceeded 1520%, but with the successive addition of antigens to the panel, there was stepwise increase in the percentage of positive reactors to between 44 and 68% against a combined panel of seven antigens. In addition, breast, lung, and prostate cancers showed separate and distinctive profiles of antibody responses, suggesting that tailor-made TAA panels could be developed for different cancers and that TAA mini-arrays might provide another approach to tumor detection and diagnosis.
| Materials and Methods |
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Expression and Purification of Recombinant TAAs.
Seven antigens, c-myc, p53, cyclin B1, p62, Koc, IMP1, and survivin were selected for the expression of recombinant proteins. Recombinant p62 has been expressed from a clone derived from a cDNA expression library by immunoscreening with antibody from a patient with HCC (4)
. p62 cDNA was subcloned into pET28a vector, producing a fusion protein with NH2-terminal 6x histidine and T7 epitope tags. The recombinant protein expressed in Escherichia coli BL21 (DE3) was purified using nickel column chromatography. Koc cDNA cloned in the pcDNA3 vector (15)
was similarly subcloned to pET28a vector, and recombinant protein was expressed as above. IMP1 construct pCMV5-IMP1 was kindly provided by F.C. Nielsen (16)
and p53 clone (p53SN3) by Yuxin Yin of Columbia University (New York, NY) and subcloned into pET28a for protein expression. cDNA from c-myc was amplified by PCR from human fetal liver tissue and survivin cDNA from human survivin expressed sequence tag clone (BG258433) before subcloning in pET28a vector. Recombinant cyclin B1 had been prepared and used previously (9)
and was isolated from a pGEX construct expressing cyclin B1 with glutathione S transferase fusion partner. The expression of adequate amounts of recombinant protein was examined in SDS-PAGE, and Coomassie Blue staining was used to determine that expression products of expected molecular sizes were produced. In addition, Western immunoblot analysis was used to confirm that the bands seen in SDS-PAGE were reactive with reference antibodies. The antibodies used were rabbit polyclonal anti-IMP1 from F.C. Nielsen (16)
, anti-Koc/IMP3 from F. Müller-Pillasch (15)
, which were raised against specific COOH-terminal peptides of the respective proteins, and anti-p62, which was raised against the full-length protein (4)
. Reactivities of p53, c-myc, and cyclin B1 were determined with monoclonal antibodies obtained from Oncogene Research Products (Boston, MA). Rabbit polyclonal antisurvivin antibody raised against the COOH-terminal peptide was obtained from ProSci, Inc. (Poway, CA).
ELISA.
Purified recombinant proteins were diluted in PBS to a final concentration of 0.5 µg/ml, and 200 µl were pipetted into each well to coat Immulon 2 microtiter plates (Dynatech Laboratories, Alexandria, VA). All human sera were diluted 1:200, incubated with antigen-coated wells at room temperature for 90 min followed by washing with PBS, and developed with horseradish peroxidase-conjugated goat antihuman IgG (Caltag Laboratories, San Francisco, CA) using the substrate 2,2'-azinobis (3-ethylbenztheizoline-6-sulfonic acid; Boehringer Mannheim GmbH, Mannheim, Germany). Additional details of this procedure have been described previously (4
, 14) . Each serum sample was tested in duplicate, and the average A490 nm was used for data analysis.
The cutoff value designating positive reactions was conventionally chosen as the mean absorbance of the 82 normal Chinese human sera +3 SD. Because several hundred test sera were analyzed at different time periods, each run of ELISA included 10 NHS samples and 2 positive control samples. The 10 NHS samples represented a range above and below the mean of the 82 Chinese normals, and the average value of the 10 normals was used in each run to normalize all absorbance values to the standard mean of 82 normals. In addition, all positive sera were confirmed with repeat testing, as were some negative sera.
2 tests were initially used to determine whether the frequencies of positive sera in each cohort of cancer patients was significantly higher than in the normal cohort; statistical significance at
levels 0.05 and 0.01 are reported. Estimated sensitivities and specificities are given with associated SE.
Cancer sera were also tested against four cellular antigens: (a) dsDNA; (b) Sm; (c) SS-A/Ro; and (d) SS-B/La that are known antigenic targets of autoantibodies in the prototype autoimmune diseases, SLE, and SS (17) . Although they are known to be autoantigens in systemic rheumatic diseases, cancer sera have not generally been evaluated for antibodies to these antigens, and this analysis could help to determine the extent of specificity of TAAs in cancer. Enzyme immunoassay kits from Helix Diagnostics (West Sacramento, CA) were used for these analyses.
| Results |
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Table 1
shows the frequency of autoantibodies to the panel of seven TAAs using enzyme immunoassay. The sera that were tested included the 321 patients from China with different types of cancer, 82 normal human sera from the same region in China as the cancer patients, 206 prostate cancer patients from the United States, 264 normal human sera from the United States and as further controls representing noncancer autoimmune diseases, 62 patients with SLE, and 41 with SS. A positive test for antibody to a TAA was taken as an absorbance reading above the mean +3 SD of the 82 normal Chinese sera. As can be seen from Fig. 1
, for as yet unknown reasons, the Chinese normal human sera generally had higher absorbance readings than the United States normal human sera. The decision to establish cutoff values using Chinese normal human sera was in part because the majority of cancer patients came from China and in part to be more rigorous in separating negative from positive data by using the higher set of "normal" values as cutoff.
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Positive reactions to individual TAAs also showed considerable variation in the different cancer cohorts. Positive reactions to c-myc were detected in breast, lung, gastric, and liver cancer but were not different from normals in colorectal and prostate cancer. On the other hand, positive reactions with cyclin B1 were detected in lung, colorectal, gastric, liver, and prostate cancer but not in breast cancer. It is apparent from the data in Table 1
that different profiles of antibody reactivities could be observed using this panel of seven TAAs.
Many cancer sera showed absorbance readings several-fold above the cutoff, indicating that antibody responses in some patients were quite robust and not just mildly elevated (Fig. 1)
. As noted previously by many investigators, the likelihood that antibodies would be detected against any individual TAA did not reach levels of sensitivity, which would be useful as diagnostic biomarkers. However, against the collective panel of seven TAAs, the number of positive reactions increased to 43.8% for breast cancer and 67.9% for lung cancer. For the total of 321 Chinese cancer patients, the sensitivity of the seven TAA panel was 54.2%, and for the United States prostate cancer cohort, it was 46.1%. These data show the utility of the multiple antigen panel in increasing the sensitivity of antibody detection in cancer sera.
Stepwise Increase in Rate of Antibody Positivity with Successive Addition of Antigens.
Table 2
shows that the sequential addition of antigens to the panel resulted in a stepwise increase in the number of positive reactions. This varied from one cancer to another and from antigen to antigen. The addition of p62 to IMP1 in the antigen panel increased the number of positive reactors in lung cancer (7.126.8%) and prostate cancer (8.731%), but there were lower increases in colorectal (13.317.8%) and gastric cancer (16.519.8%). In colorectal and prostate cancer, the addition of survivin did not further increase the number of positive reactors compared with other cancers. These observations suggest that for some cancers, putative TAAs like survivin may not be an optimum candidate for inclusion in a mini-array antigen panel and that specific panels should be developed for different cancers.
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| Discussion |
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This study points to an interesting feature of tumor immunity, that with an increasing number of antigens in a multiantigen immunoassay, the likelihood of detecting antibody in any cancer serum increased from
15 to 26% when one antigen was used to 4468% when seven antigens were used. Earlier studies that might have anticipated these results involved the use of a two-antigen panel (14)
or was restricted to one type of cancer (21)
in contrast to this study using seven known TAAs and several hundred patients with six different types of cancer. The sensitivity of the seven-antigen panel immunoassay system varied for different types of cancer. The factor or factors leading to these differences have not been examined, but from Fig. 2
, it appeared that there was a stronger association between certain antigens and some cancers. In prostate cancer, p62 and cyclin B1 appear to be distinctly more reactive antigens than c-myc, p53, or survivin. In lung cancer, cyclin B1, p62, and p53 appear to be the more reactive antigens, whereas in breast cancer, they are c-myc and Koc. In HCC, c-myc stands out compared with other antigens which are about equal in antigenicity. A clear association of highly reactive antigens with colorectal and gastric cancer is not apparent. However, this initial study is based on seven TAAs, and the selection of antigens, although based on their known association with tumors, is by no means optimized and could be further refined with panels designed for greater specificity for certain types of cancer. There is support at the genetic level for this notion. Gene abnormalities, such as mutations or overexpression, tend to occur in combinations that vary from tissue to tissue (22)
, and if autoimmune responses represent immune system reactions to abnormal gene expression (13)
, one might expect differences in autoantibody profiles from one type of tissue cancer to another. Mini-arrays of TAAs could be constructed to include among others, several other known TAAs, such as HER-2/neu and ras, certain cell cycle proteins, and antigens involved in cell-mediated immune reactions, many of which might have concomitant humoral immune responses (8
, 23)
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The judicious selection of antigens to be included in panels or arrays of TAAs is extremely important because not all cellular proteins recognized as antigens by cancer sera are cancer specific. This was especially apparent in studies in HCC, where it was possible to analyze sera from the same HCC patients many months or years preceding malignancy when the patients had liver cirrhosis, chronic hepatitis, and autoimmune liver disease. Some of the autoantibodies identified with sera obtained when HCC was diagnosed were already present before the development of malignancy (24 , 25) , and these included antibodies against nucleolar antigens fibrillarin, nucleolus organizer region 90 protein, and B23, all of which are also recognized by antibodies from scleroderma and other autoimmune diseases. This feature has also been reported using SEREX technology (26) , in which sera from colon cancer patients used in immunoscreening cDNA expression libraries identified several autoantigens associated previously with noncancer disorders, including hydroxymethylglutaryl-17, poly ADP ribose polymerase, and U1 small nuclear ribonucleoprotein, as well as cellular protein PINCH associated with aging and recognized by antibodies in normal human sera (27) . The definition of TAA can probably be best determined by studies of significant numbers of cancer, as well as noncancer disorders.
Recently, an autoantigen array has been used for characterization of autoantibodies in patients with autoimmune rheumatic diseases, demonstrating potential applications, including the possibility of early diagnosis and characterization of specificity (28) . Our study was not designed to show the predictive potential of autoantibodies as tumor markers but to report the novel finding of increasing sensitivity for detecting autoantibodies in cancer when larger panels of cancer-associated antigens are used. Nevertheless, cancer-associated antigen panels might conceivably be used for early detection of tumors in high-risk individuals. The appearance of antibodies to TAAs has been demonstrated in HCC in serum samples during antecedent liver disease (23 , 29 , 30) . Anti-p53 antibodies were detected in two heavy smokers before clinical detection of lung cancer, and in 1 patient, early treatment resulted in good response, which correlated with total disappearance of p53 antibodies (31 , 32) . Anti-p53 antibodies were detected in workers exposed to vinyl chloride before clinical detection of angiosarcoma (33) , and anti-HER-2/neu antibodies can be detected in patients with early stage breast cancer (34) . Anti-p53 antibodies have been detected in Barretts esophagitis (35) , a condition often predating esophageal cancer and in betel nut chewers with oral leukoplakia (36) , a condition predating oral malignancy. Important future studies would be to determine whether mini-arrays of antibody markers might be useful in identifying early cancer in high-risk individuals. Furthermore, additional efforts should be aimed at increasing both the sensitivity and specificity of antibodies as cancer markers by expanding antigen arrays to include antigens which might be more selectively associated with some tumors and not with others.
| Acknowledgments |
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| Footnotes |
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1 Supported in part by Grant CA56956 from the National Cancer Institute and by technical resources from the Stein DNA Core Facility of The Scripps Research Institute. ![]()
2 Dept. of Biological Sciences, Univ. of Texas at El Paso, TX 79968. ![]()
3 Dept. of Oral Biology, Univ. of Florida Gainesville, FL 32610. ![]()
4 To whom requests for reprints should be addressed, at W.M. Keck Autoimmune Disease Center, The Scripps Research Institute, La Jolla, CA 92037. Phone: (858) 784-8686; Fax: (858) 784-2131; E-mail: emtan{at}scripps.edu ![]()
5 The abbreviations used are: TAA, tumor-associated antigen; SS, Sjögrens syndrome; dsDNA, double-strand DNA; NHS, normal human serum; SLE, systemic lupus erythematosus; HCC, hepatocellular carcinoma. ![]()
Received 8/14/02; revised 11/20/02; accepted 11/22/02.
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