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Molecular Oncology, Markers, Clinical Correlates |
Département de Biologie des Tumeurs [P. d. C., E. Bour., E. C. M., A. M., H. M.], Département dOncologie Médicale [J. M. E., J. Y. P., P. P.], Département de Chirurgie [E. Boud., C. N., K. B. C.], Institut Curie, 75005 Paris, and Service de Biochimie Spécialisée, CHU Nantes [M. G. D.], France
| ABSTRACT |
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The sensitivity of the reverse transcription-PCR tested with blood spiked with MCF7 cells was one cell in 5 ml of blood. The immunomagnetic separation step was mandatory to obtain the maximum specificity. Control samples from healthy donors never displayed cycle threshold (Ct) values for MUC1 lower than 38. Circulating cells (Ct, <38) were detected in 3 of 28 (11%) cases in group 1, in 8 of 34 (24%) cases in group 2, and in 27 of 60 cases (45%) in group 3. A semiquantitative estimate of blood-borne cells could be derived from the Ct value when below 32 (the lowest was 28) or by the number of positive aliquots of the same blood sample. Thus, immunomagnetic separation, followed by MUC1-specific RT-PCR, allows the semiquantitative detection of circulating mammary cells. A significant correlation between the presence of MUC1-positive cells and the group of breast tumors was observed. The clinical significance of blood-borne cells in breast cancer, especially at the operable stage, may be investigated by following these patients.
| INTRODUCTION |
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| MATERIALS AND METHODS |
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We have analyzed 10-ml blood samples obtained from 122 patients with
benign or malignant breast tumor. Patients were categorized into three
groups (Table 1)
. Twenty-eight patients had histologically confirmed benign breast
disease (fibroadenoma, cyst, epithelial hyperplasia, papilloma, or
mastopathy; group 1) and serve as a reference group, 34 patients had an
operable breast cancer (group 2, good prognosis), and 60 patients had
an advanced breast cancer [inflammatory, large node invasion (>8 N+),
and metastatic breast cancer; group 3, poor prognosis]. None of the
patients of group 3 had previously received chemotherapy and/or
radiotherapy during the 6 months preceding the analysis of their blood
samples. Samples were obtained in all cases, before any treatment and
in addition for advanced breast cancer, 6 months or more after the
latest treatment (surgery, chemotherapy, or radiotherapy). A second
sample was obtained for some patients, either 24 h after surgery
or after the first cycle of neoadjuvant or adjuvant chemotherapy. The
distribution of all samples is shown in Table 2
. A total of 206 blood samples were thus assayed. The study design was
approved by the ethical committee (Faculty of Medicine of Cochin
University, Paris, France). All patients have given a written informed
consent. All patients were assayed in blind.
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RNA Extraction and cDNA Synthesis.
RNA was extracted from cells immobilized on the beads with Trizol, as
described by the manufacturer (Life Technologies, Inc., Gaithersburg,
MD). Glycogen [20 µg; Boehringer Mannheim, Mannheim, Germany] was
added as RNA carrier to optimize the extraction efficiency. Samples
were stored at -80°C for up to 8 weeks before the assay.
Reverse transcription of RNA was performed in a final volume of 20 µl containing 200 µM of each nucleotide triphosphate, 6.7 mM MgCl2, 5 units of RNase inhibitor (Promega, Lyon, France), 5 µM random Hexamer (Pharmacia, Uppsala, Sweden), and 200 units of Mo-MuLV reverse transcriptase (Life Technologies, Inc.); and 14 µl of the extracted RNA or H2O were added. The samples were incubated at 65°C for 5 min, then 42°C for 30 min.
PCR Conditions.
PCR amplification was performed in the presence of specific target,
doubly labeled fluorigenic probes (Taqman probe) that allow an
automated quantification of the amplified products in real-time with
the ABI Prism 7700 Sequence Detection System (PE Applied Biosystems,
Foster City, CA). Primers and probes were chosen with the assistance of
the Primer Express computer program (PE Applied Biosystems). We
conducted BLASTN searches against dbEST and nr to
confirm the gene specificity of the chosen nucleotidic sequences for
MUC1. MUC1 forward and reverse primers in 5' and
3' orientations were GTGCCCCCTAGCAGTACCG (exon 6) and
GACGTGCCCCTACAAGTTGG (exon 7). The Taqman probe
(AGCCCCTATGAGAAGGTTTCTGCAGGTAATG, exons 67) carrying a 5' FAM
reporter label and a 3' TAMRA quencher group was synthesized by
PE Applied Biosystems (Warrington, United Kingdom).
Of the total reverse transcription volume of 20 µl, 5 µl were used for each PCR. The polymerase amplification was performed in a total volume of 50 µl containing 1x PCR buffer [5 mM MgCl2; 10 mM Tris-HCl (pH, 8.3); 50 mM KCl; 5% glycerol; 200 µM dATP, dCTP, and dGTP; 400 µM dUTP; 200 nM of each primer and 400 nM probe; 1.25 units of AmpliTaqGold DNA polymerase (PE Applied Biosystems); and 0.5 unit UNG. The thermal cycling conditions comprised 2 min at 50°C, 10 min at 95°C, and then 45 cycles at 95°C for 15 s and 60°C for 1 min.
Each experiment was performed in two independent runs (ARN extraction and reverse transcription), each in duplicate (PCR; finally, each blood sample was assayed in quadruplicate). The assessment of quality control was performed in standardized PCR conditions, including in each experiment a positive control (100 ng of reversed ARN of MCF7 cells) and two negative controls (one with no template and one negative reverse transcription control).
| RESULTS |
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To determine the sensitivity of the assay, serial dilutions of total
RNA from the mammary adenocarcinoma MCF7 cell line were performed. A
consistent detection of 5 pg of RNA was observed (Table 3
and Fig. 1
), potentially detecting a single breast cancer cell (estimated 510
pg). These results were confirmed by the dilution of a known number of
MCF7 cells in 5 ml of normal human blood samples. The spiked samples
were processed as described in "Material and Methods," including
the immunomagnetic separation. The observed Cts were 33.73, 35.37, and
37.75 for 20, 2, and 1 cells, respectively, diluted in 5 ml of blood
(results of four duplicate independent experiments). A single cell was
consistently detected in these conditions.
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Detection of MUC1 Transcripts in Peripheral Blood of
Patients with Benign Breast Disease (Group 1).
Group 1 comprised 28 patients with benign breast disease analyzed in
blind. The mean age of these patients was 42 years (range, 2455). Of
these 28 patients, 3 (11%) were considered positive, with a Ct equal
to 37, all three in only one of the four determinations, indicating of
a very low level of MUC1 transcripts in the sample. Seven
patients of group 1 were also analyzed 24 h after surgery; none of
them had detectable MUC1 transcripts. One patient was positive (Ct, 37)
before surgery and negative after surgery. Two patients had a
fibroadenoma and one fibrocystic mastopathy. No clinical history of
malignancy was found.
Detection of MUC1 Transcripts in Peripheral Blood of
Patients with Breast Cancer (Groups 2 and 3).
Patients with breast cancer were divided into two groups according to
the stage of the disease. Group 2 (34 patients) included T1 and T2
breast cancer patients who underwent primary surgery with or without
adjuvant chemotherapy or radiotherapy; group 3 (60 patients) included
patients with advanced breast cancer (inflammatory disease, more than
eight involved nodes, and metastatic disease) treated by chemotherapy.
The mean age was 57 years (range, 3183) and 49 years (range, 2883)
for patients of group 2 and group 3, respectively.
Ten-milliliter blood samples from 34 patients with operable breast
cancer (group 2) were collected before surgery and, for 23 of them,
also 24 h after surgery or before the second cycle of
chemotherapy. Blood samples of 60 patients with advanced breast cancer
(group 3) were collected at diagnosis and, for 45 of them, before the
second cycle of chemotherapy (Table 2)
. The data obtained for these
patients are presented in Table 4
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37. In these conditions, MUC1 transcripts were detected in
8 of 34 patients of group 2 (24%) and in 27 of 60 patients of group 3
(45%). In the latter group, 15 of 37 (41%) patients with advanced
breast cancer and 12 of 23 (52%) patients with metastatic breast
disease were positive for MUC1 transcripts (Table 4)
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A significant correlation between MUC1 positivity and the three groups
of patients was observed (
2 test, P <
0.01).
| DISCUSSION |
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The main issue of the method resides in the balance between high sensitivity and specificity. Real-time detection and quantitation of specific PCR products using Taqman probes is adapted to the specific detection of rare events, because the sensitivity of the assay allowed us to detect 5 pg of RNA. It is highly reproducible, and it eliminates the major risks of contamination encountered with other types of detection (nested PCR and gel electrophoresis; Ref. 18 ). But, as previously described by others, with all tissue-specific genes tested, we observed a basal level of transcription in normal blood, named illegitimate transcription (19, 20, 21, 22, 23, 24, 25, 26) . The sequences of these transcripts are identical to tissue-specific mRNAs found in breast. When we performed serial dilutions of mononuclear cells and breast cancer cells, we observed that the level of expression of MUC1 illegitimate transcripts was about 10,000-fold lower than that observed in human breast cell lines, as described by others (27) . In addition, five normal samples of healthy donors were similarly treated by immunomagnetic separation and RT-PCR, and all displayed a Ct >45 (data not shown). Thus, the specific detection of mammary cells requires an enrichment of mammary cells, with a contamination of <100 blood mononuclear cells. Positive immunomagnetic separation reaches this goal. We used an immunomagnetic separation of epithelial cells with magnetic beads coated with an epithelial-specific monoclonal antibody (Ber-EP4). The immunohistochemical staining of nine human breast cancer specimens showed a consistent staining in all tumor cells by Ber-EP4 (28) . This antibody has already been used to enrich in epithelial colorectal cells (5 , 29 , 30) and breast cancer cell lines (6) with a good yield. We have previously obtained a consistent recovery of 80100% of [3 H]thymidine-labeled MCF7, MDA-MB231, or T47D cells in serial dilutions in normal blood.4 The combination of immunobead isolation of epithelial cells with RT-PCR detection of MUC1 mRNA increases the specificity of the method, because no MUC1 transcripts were detected in normal blood samples in these experimental conditions. Real-time PCR allowed a consistent sensitivity of detection of one cell in 5 ml of blood.
For the clinical evaluation of this method, we defined three groups of patients. The reference group (group1) included patients with benign breast tumors, whereas two groups of breast cancer patients were analyzed: (a) a group of good prognosis, patients with operable breast tumors (group 2); and (b) a group of poor prognosis (group 3), comprising patients with locally advanced breast tumors (inflammatory breast cancer), patients with metastatic breast cancer, and patients with major node involvement. A significant correlation between the frequency of positive cases and the patient group was found. In group 1 (benign breast disease), three patients were positive. In all three cases, the Ct was at the threshold of detection and was reached only in one of four replicates, indicating yet a very low level of MUC1 transcripts. The clinical significance of this result is unclear. The histological diagnosis confirmed that there was no histological feature of malignancy: two patients had a fibroadenoma, and one patient had a fibrocystic mastopathy. False positive cases may occur with a very highly sensitive detection method. MUC1 is not specific of malignant cells and is also expressed in normal and benign breast cells. The detection of normal mammary cells that could have been mobilized by an invasive diagnosis, such as core biopsy or fine-needle aspiration can be eliminated, because only one patient had had a biopsy 4 months before the assay. In addition, none of these cases was found positive 24 h after surgery. Interestingly, one of these patients had a tumor occurring on the contralateral breast 6 months later, the histological examination revealing a papilloma and atypical dysplasia, and another patient had inflammatory alterations at the histological examination, which could explain a release of epithelial cells.
In group 2, eight patients (24%) where positive with a Ct <37. All had an invasive ductal breast carcinoma, with high grade (SBR II or III), with or without nodal involvement. In group 3, 27 patients (45%) had a Ct <37 and, among them, 12 patients (52%) had a metastatic breast cancer. The increasing frequency of positive detection with aggressivity of the disease supports the specificity of the method. In addition, Cts in group 3 were lower than in group 2, and the number of positive replicates was higher than that in group 2. In addition, some patients had a positive detection in all replicates (15%), suggesting that the level of MUC1 transcripts is higher than in group 2.
In conclusion, immunomagnetic separation of epithelial cells from blood of breast cancer patients, combined with real-time RT-PCR of MUC1 transcripts is a noninvasive, sensitive, and specific assay for the detection of circulating mammary cells. The malignancy of such cells, as well as the prognostic value of that assay, has to be established with other molecular probes and with the follow-up of the patients. In patients with advanced breast cancer, the assay will be evaluated as a surrogate end point for assessing the efficacy of chemotherapy.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by a grant from the Ministère de
la Recherche et de la Technologie/Institut National de la Santé
et de la Recherche Médicale (PARMPSTRC 9608). ![]()
2 To whom requests for reprints should be
addressed, at Laboratoire de Physiopathologie et Pharmacologie,
Institut Curie, 26 rue D"Ulm, 75005 Paris, France. Phone:
33-1-44-32-42-70; Fax: 33-1-44-32-40-73; E-mail: Patricia.De-Cremoux{at}curie.net ![]()
3 The abbreviations used are: RT-PCR, reverse
transcription-PCR; Ct, cycle threshold. ![]()
Received 11/ 9/99; revised 4/ 4/00; accepted 5/16/00.
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