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Childrens Cancer Research Institute, St. Anna Kinderspital, A-1090 Vienna, Austria
| ABSTRACT |
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Experimental Design: The disialo-ganglioside (GD2) molecule was used as the immunological target. The GD2 molecule was described as being specific for neuroblastoma cells, although false positive reactions had been suspected. To verify or disprove the neoplastic nature of the immunologically positive cells, sequential fluorescence in situ hybridization was performed on these cells to search for those genetic aberrations found in the corresponding primary tumors. A total of 115 samples from 40 newly diagnosed patients were evaluated for the presence of GD2+ cells in the BM.
Results: GD2 positivity was detected in 95.2% of stage 4 patients, in 100% of stage 4s patients, and in 38.5% of patients with localized/regional disease. In stage 4 and 4s BM samples, the GD2+ cells were unequivocally identified as tumor cells based on the molecular cytogenetic aberrations found by fluorescence in situ hybridization. However, in BM samples from patients with localized/regional disease, all GD2+ cells were concluded to represent false positivity due to the absence of genetic aberrations.
Conclusions: Automatic search and sequential molecular cytogenetic analysis of the immunologically positive cells provide precise information on both the number and cytogenetic profile of disseminated tumor cells.
| INTRODUCTION |
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Reliable detection of tumor cells in BM samples is fundamental in the initial staging and for the monitoring of therapy response (minimal residual disease) and is presumably also of great prognostic significance. Furthermore, the lack of detectable disseminated tumor cells in stem cell products significantly contributes to the success of autologous BM transplantation in advanced NB (7 , 8) . Therefore, highly specific, sensitive, and quantitative methods are increasingly required for the effective analysis of disseminated tumor cells in hematopoietic samples.
Today, disseminated NB cells are detected by classical cytomorphological examination in clinical practice (9) . Unfortunately, the lack of high sensitivity, difficulties in quantification, and interobserver differences clearly reduce the power of this method of detection. The identification of NB-related gene transcripts, such as tyrosine hydroxylase (10 , 11) or GAGE (12) , has been successfully applied using reverse transcription-PCR. However, the relative quantification of cell type-associated RNA transcripts in diagnostic materials has proven to be complicated and very often impossible.
The immunological detection of antigens expressed by NB cells has been successfully applied for the sensitive demonstration of isolated tumor cells in the BM (13, 14, 15, 16) . Unfortunately, the evaluation of microscopical preparations is time-consuming and complicated by numerous methodological issues (17) . Enzymatic detection systems, for instance, may lead to unspecific reactions mimicking positivity due to endogenous enzyme activity (18) . Because immunological analysis can result in dubious positive findings, unambiguous features disclosing the true nature of the immunologically positive cells should be considered.
We will present an automatic approach to detect and quantify NB cells in the BM, using immunofluorescence labeling by a monoclonal antibody specific for ganglioside GD2 (19) . The fluorescence microscope-based automatic image analysis system allows a consecutive analysis of cytogenetic aberrations by FISH of the selected cells (20) . Previous spiking experiments clearly revealed that not all immunologically positive cells displayed the "tumor typical" cytogenetic aberrations. These cells did not show any genetic aberrations by FISH and therefore had to be considered as falsely positive cells (21) . We analyzed routine BM samples from newly diagnosed NB patients to demonstrate the power and the practical use of the sequential immunological and FISH approach in a clinical setting.
| MATERIALS AND METHODS |
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Immunofluorescence Analyses.
The cells were fixed in 4% formaldehyde/PBS for at least 1 h at 4°C. Before the immunofluorescence staining, slides were incubated with 4% BSA to block unspecific antibody binding for 20 min at 37°C. Slides were incubated for 30 min with an unconjugated mouse monoclonal antibody against the GD2 antigen (monoclonal antibody 14.18; 1:500 dilution; kindly provided by R. A. Reisfeld; The Scripps Research Institute, La Jolla, CA). Detection of the antibody was done using a FITC-labeled goat antimouse antibody (30 min; 1:60 dilution; DAKO, Glostrup, Denmark). Slides were covered with the antifade medium Vectashield (Vector Laboratories, Burlingame, CA) containing DAPI (1 µg/ml).
FISH.
Coverglasses were carefully detached, rinsing the slides in PBS. Residual immunofluorescence was then eliminated by proteinase digestion [50 µg/ml pepsin (pH 1.5)] for 110 min. In situ hybridization was performed with either of the pairs of DNA probes to determine the integrity of 1p36.3 (D1Z1 and D1Z2), MYCN status (N-myc and D2Z), or gain of 17q material [D17Z1 and D17S881-2; obtained from American Type Culture Collection and Oncor Inc. (Gaithersburg, MD)]. Cells and probes were denatured on a heating plate together at 80°C for 10 min. Overnight hybridization at 37°C was followed by two washes in 2x SSC containing 50% formamide for 7 min at 42°C and two washes in 2x SSC. The probes were detected with mouse antibiotin (DAKO) and FITC-conjugated sheep antidigoxigenin antibodies (Roche, Vienna, Austria), followed by a second incubation with tetramethylrhodamine isothiocyanate-conjugated antimouse and FITC-conjugated antisheep antibodies (DAKO). The slides were covered with Vectashield-DAPI. Sequential FISH analyses were performed in up to three different hybridization reactions with each of the above-listed pairs of DNA probes to prove the presence of multiple genetic aberrations within the same tumor cell nuclei. These additional tests were performed to determine whether the immunologically positive cells featured the same cytogenetic markers found in the primary tumor.
Scanning Procedure and Cell Identification.
Slide scanning, cell identification, storage of the exact coordinates on the slide, and the image as well as quantification of cells and repositioning were done with the help of the Metafer 3.0 automatic fluorescence image analysis system equipped with RCDetect software (MetaSystems GmbH, Altlussheim, Germany). This fluorescence object-finding and relocation system is based on a motorized Axioplan2 fluorescence microscope (Zeiss, Göttingen, Germany), a motorized eight-slide scanning stage (Märzhäuser, Germany), a high-resolution charge-coupled device camera, and a personal computer equipped with appropriate cards for accurate stage movement and fast image analysis.
The automatic scanning procedure was performed as follows: DAPI-counterstained nucleated cells were automatically focused, and the number of the nucleated cells was determined (19) . Then, the whole slide was scanned for specific immunofluorescence related to GD2 (FITC). The exact position on the slide (X and Y coordinates) and the color image (DAPI/FITC) of positive cells were stored. Both the number of positive cells and the total number of analyzed cells were given in absolute figures.
Target Cell Repositioning and FISH Evaluation.
The sequential evaluation of the FISH results was done after replacing the slide in the stage and repositioning the cells selected on the basis of their positive immunofluorescence. After capturing the FISH image with the integrated ISIS software, a gallery of images with different immunological and corresponding FISH pictures was created.
Evaluation of Results.
Samples containing at least 1000 strong GD2+ cells/106 MNCs (0.1%), which also fitted the morphological criteria of neoplastic cells, were accepted as being positive without FISH verification. Slides with less than 0.1% GD2+ cells were sequentially analyzed for at least one cytogenetic aberration found in the primary tumor. The neoplastic nature of a given cell could be determined when both the GD2 and the FISH positivity were demonstrated. GD2+ cells without the typical cytogenetic aberration of the primary tumor were declared to be falsely positive and excluded from the evaluation.
| RESULTS |
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In 1 of the 21 stage 4 patients (case 24, Table 1
), a negative initial BM finding was obtained. In the slides from these samples, however, only a mean number of 0.4 x 106 MNCs (± 0.1 x 106, SD) were available for analysis. Thus, less than half of the total number of cells was present as compared with the mean number of analyzed cells in the other cases.
Table 2
demonstrates the quantitative evaluation of patient positivity with respect to clinical stage. In localized/regional disease (stages 13), sequential FISH analysis of GD2+ BM cells had a major influence on BM positivity but was found to have no effect on stage 4 and 4s BM findings. After the elimination of false GD2 positivity by FISH occurring in early-stage disease (stages 13), the rate of tumor cell-positive patients was shown to decrease by 19%, and the rate of positive samples analyzed in this study was shown to decrease by 21%.
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| DISCUSSION |
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In this study, a quantitative method for the demonstration of NB cells is introduced using an immunofluorescence-based automatic microscopic scanning approach with consecutive verification of the neoplastic nature of the respective cells. The cell surface ganglioside GD2 is one of the most reliable immunological markers for NB (25 , 26) , and it serves as an ideal target for automated image analysis. The search method used in our study is based on characteristic fluorescence parameters of the immunolabeled cells. It is performed field by field on the slide, which is followed by storage of the position and the digital image of all positive cells. If necessary, each individual immunopositive cell can subsequently be analyzed morphologically and/or by sequential FISH (20 , 21) . Not only does fluorescence microscopy represent a simple way to determine isolated tumor cells, but it also makes it easier to count the nucleated cells in the sample, which results in a highly accurate tumor cell quantification (19) .
With spiking experiments, we could demonstrate that even 1 cytogenetically proven NB cell in 106 MNCs can be recovered unequivocally (21) . In these spiking experiments, however, the number of GD2+ cells constantly exceeded the number of the spiked tumor cells. FISH to display tumor typical genetic aberrations clearly demonstrated that these cells were of a nontumorous nature. Therefore, one of the most likely explanation of false positive reactions could be a transloading of the tumor cell-derived ganglioside to hematopoietic cells after physical contact with GD2+ tumor cells. Furthermore, the presence of apoptotic cells, which were shown to be positive for tumor-specific markers (27) , has also been implied. These are expected, in both an isolated and an internalized form, to result in sample positivity with an unclear impact on the rate of viable disseminated tumor cells.
In the present study, the vast majority of the BM samples from stage 4 and stage 4s patients and more than one-third of the BM samples from patients with stage 1, 2B, and 3 NB displayed GD2+ cells. The morphological distinction between specific and unspecific GD2 immunolabeling was found to be especially difficult or even impossible when only a few positive cells were found in the BM sample. Even careful morphological analysis after automatic relocation could not support or exclude the true nature of individual GD2+ cells. In all unclear cases, however, at least one tumor typical cytogenetic aberration was demonstrated in the primary tumors that could be specifically applied to clarify the nature of the GD2+ cells in the BM. Genetic differences between primary tumor and disseminated tumor cells could also be addressed applying automated relocation. In samples from stage 4 and stage 4s patients, sequential molecular cytogenetic analysis of MYCN, 1p, and 17q in the same disseminated tumor cells always presented the same aberrations as found in the primary tumor. This finding strongly supported the idea that a targeted analysis of cytogenetic aberrations can be efficiently used to detect rare tumor cells.
Interestingly, none of the GD2 immunofluorescence-positive cells in the BM samples of patients with localized/regional disease displayed numerical chromosomal aberrations characteristic of the individual primary tumors. Therefore, the neoplastic nature of these immunofluorescence-positive cells could be excluded. The false positive rate concerned 38.4% of the patients in this disease group. It is noticeable that previously published results demonstrated GD2 positivity in the BM in a similar number (i.e., in 34%) of patients with localized/regional NBs by immunocytochemistry using the myeloperoxidase detection method (15) . The positive cells in the study of Moss et al. (15) were interpreted as tumor cells without further clarification of their biology. However, endogenous enzymatic activity, active uptake of GD2 by macrophages, and the peculiar transloading capacity of the GD2 molecule described here may all result in false positive findings. The use of immunofluorescence for the demonstration of disseminated tumor cells, on the contrary, offers the following advantages: the likelihood of endogenous enzymocytochemical reactions is eliminated; and immunologically selected doubtful cells can be scrutinized.
Another remarkable finding in this study was the difference in the quantity of tumor cells between stage 4 and stage 4s BM samples. Stage 4s NB, despite the highly proliferative initial phase and fast spreading, generally has a favorable outcome. Despite the different clinical behavior, the tumor cell positivity rate was found to be similar to that in stage 4 patients. Significantly lower numbers of tumor cells in the BM of patients with stage 4s disease can possibly be used to demonstrate a less aggressive disease behavior, although we have used only a small patient collective.
In 18.2% of all analyzed samples, which represent 33.8% of all positive samples, less than 10 tumor cells/106 MNCs were found, indicating that automatic fluorescence microscopy is a powerful approach for the detection of rare tumor cells. Major critical issues of the rare cell detection, i.e., the exact quantification of the tumor cell infiltrate and the presence of false positivity, can also be overcome by this application. In addition to the verification of their neoplastic nature, molecular cytogenetic analysis allows further insights into the biology of disseminated tumor cells.
| FOOTNOTES |
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1 To whom requests for reprints should be addressed, at Childrens Cancer Research Institute, St. Anna Kinderspital, Kinderspitalgasse 6, A-1090 Vienna, Austria. Phone: 43-1-40470-411; Fax: 43-1-408-72-30; E-mail: ambros@ccri.univie.ac.at. ![]()
2 The abbreviations used are: NB, neuroblastoma; GD2, disialo-ganglioside; FISH, fluorescence in situ hybridization; MNC, mononuclear cell; DAPI, 4',6-diamidino-2-phenylindole; BM, bone marrow. ![]()
Received 10/ 2/00; revised 4/ 2/01; accepted 4/16/01.
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