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Molecular Oncology, Markers, Clinical Correlates |
Department of Otolaryngology-Head and Neck Surgery, Johns Hopkins University, Baltimore, Maryland 21205
| ABSTRACT |
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| INTRODUCTION |
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Microsatellite alterations have been used as markers of clonality (3) and to detect cancer cell DNA in a background of normal cells (4) . Microsatellite analysis can reveal either LOH or MSI in the amplified microsatellite repeat locus. Tumor-specific DNA alterations can be found in the body cavity fluids and blood of patients with various cancers. For example, tumor-specific microsatellite alterations can be consistently detected in the urine of patients with bladder carcinomas (5) and can be detected in the serum of a significant percentage of patients with HNSCC (6) . The purpose of his study was to determine whether tumor-specific microsatellite alterations may also be detectable in the DNA from exfoliated oral cells in saliva of HNSCC patients. We planned to assess a panel of microsatellite markers in a group of subjects with clinically apparent cancer and healthy control subjects as a first step in the development of a screening tool.
| MATERIALS AND METHODS |
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DNA extraction
Tumor
Frozen Tissue.
Representative thin tumor sections were stained with H&E. Microdissection was performed as needed, removing noncancerous tissue to achieve at least a 70% purity of the neoplastic cell population. Ten 12-µm tissue sections were placed in 1% SDS/proteinase K (0.5 mg/ml) and incubated at 48°C for 72 h.
Paraffin-embedded Tissue.
Representive tumor samples were sectioned producing 25 14-µm samples. They were placed on glass slides and microdissected using a dissecting microscope. The samples were placed in xylene overnight for deparaffinization, pelleted in 70% ethanol, dried, and incubated in SDS/proteinase K at 48°C for 72 h.
Blood
Ten ml of blood were brought to 30 ml with TM buffer (2% Tris-EDTA-MgCl). Lymphocytes were collected by centrifugation at 2500 rpm for 15 min and were placed in 1% SDS/proteinase K (0.5 mg/ml) at 48°C for 72 h.
Exfoliative Oral Cells
The 25-ml rinse and swab (test) samples were subjected to centifugation at 2500 rpm for 15 min. The supernatant was discarded, and the cell pellet was retained and placed in 1% SDS/proteinase K (0.5 mg/ml) at 48°C for 72 h. Digested tissue and fluids from all of the sources were then subjected to phenol-chloroform extraction and ethanol precipitation.
Microsatellite Analysis of DNA
A panel of twenty-three tetranucleotide microsatellite repeat PCR primers (Research Genetics, Huntsville, AL) that had been used in similar studies in our laboratory were selected for this study (Table 2)
. Several of these alterations were known to be highly informative because of a study of the allelotype of HNSCC (6)
. Others were identified in studies of lung (7)
and bladder carcinoma (8)
. This experience indicated a high frequency of MSI in microsatellites consisting of (AAAG)n repeats, and, therefore, the panel preferentially included those markers. Prior to amplification, 50 ng of one primer from each pair was end-labeled with [
-32P]ATP (20 mCi; Amersham Life Sciences, Inc., Arlington Heights, IL) and T4 kinase (New England Biolabs, Inc., Beverly, MA) in a total volume of 50 µl. PCR reactions were carried out in a total volume of 12.5 µl containing 10 ng genomic DNA, 0.2 ng labeled primer, and 15 ng of each unlabeled primer. The PCR buffer included 16.6 mM ammonium sulfate, 67 mM Tris (pH 8.8), 6.7 mM magnesium chloride, 10 mM ß-mercaptoethanol, and 1% DMSO, to which were added 1.5 mM deoxynucleotide triphosphates and 1.0 unit Taq DNA polymerase. (Boehringer-Mannheim Biochemicals, Indianapolis, IN). PCR amplifications of each primer set were performed for 3035 cycles consisting of denaturation at 95°C for 30 s, annealing at 5060°C for 60 s, and extension at 72°C for 60 s. One-third of the PCR products were separated on 8% urea-formamide-polyacrylamide gels and exposed to film from 4 to 48 h. For informative cases, allelic loss was documented if one allele was significantly decreased (>50%) in tumor- or oral-test DNA compared with the same allele in the normal (lymphocyte) DNA. MSI was described if an additional band representing a change in repeat number was noted in tumor- or oral-test DNA. All of the samples were assessed by two observers independently (M. F. S., D. S.) and borderline cases were decided by densitometry. All of the identified alterations were confirmed by repeating the PCR reaction and electrophoresis. Samples were coded, so that the observers were blinded to the individual subject.
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| RESULTS |
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Index microsatellite alterations were identified in the tumor of 4 of 6 patients entered in the study after prior radiation therapy. Matching alterations were detected in exfoliative oral samples in all four of the cases.
| DISCUSSION |
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The panel of 23 microsatellites identified at least 1 altered marker for all but 6 of the tumors in this series. Only tumors that displayed alterations at one or more of the loci could be detected in an exfoliative sample, which excluded 14% of tumors from the analysis. When a tumor marker was available, exfoliated cancer cells were detectable in the oral specimens from the vast majority (92%) of cases. Failure to detect tumor cells in three cases displaying a tumor-specific marker may be attributed to a number of factors, such as necrotic tumor yielding little DNA, a high background of normal epithelial cells, mostly submucosal tumor growth, problems with sample handling and processing, or the type (LOH versus MSI), and quantity of altered markers.
Previous studies have demonstrated the capability of scrapings from oral lesions (15) and cytological brushings of laryngeal neoplasms (16) to produce cells with tumor-specific microsatellite alterations. Our protocol included swabbing the tumor bed, but we did not vigorously scrape or brush the index lesion when it was visible. Instead, the harvesting of exfoliative cells was performed in a manner intended to simulate a potential screening protocol. Our population included six cases in which the tumor was not in a region that could be sampled in an office-based swab protocol. The hypopharynx and larynx are easily sampled under general anesthesia, and exfoliative samples from these areas might be obtained in a manner similar to "induced sputum" cytology (17) . If these cases were eliminated from the analysis, the protocol was able to detect cancer in 29 (90%) of 32 oral and oropharyngeal cases for which a tumor marker was available, or 76% of the remaining population of 38 patients overall.
This work constitutes a feasibility trial aiming to demonstrate proof of principle for a molecular detection approach using our panel of microsatellite markers in patients with obvious HNSCC. If the test is validated and refined, molecular analysis of exfoliated oral mucosal cells may be useful as a detection method for patients at risk for developing HNSCC de novo, and as a surveillance tool for patients after completion of therapy. A swab of the tumor bed could be performed after treatment for tumor surveillance, but swabbing the tumor will not be possible when the saliva test is applied to screen at-risk individuals without clinical lesions. An indication of the applicability of the saliva test for screening the at-risk population is found in the successful test results of the three patients with metastatic HNSCC from unknown mucosal primary sites. Oral rinses from these patients produced exfoliated cancer-related cells in each case. Although the oral sample could not identify the site from which the clonal cells originated, it could identify their presence. Directed biopsies of commonly involved mucosal sites are typically harvested in the work-up of patients with HNSCC of unknown primary. These could then be analyzed both histologically and using microsatellite analysis. We have reported the utility of molecular testing to identify cancer cells in histologically benign directed biopsy samples from such patients (18) .
Radiation therapy might prohibit the application of the test in the surveillance setting by altering the rate of exfoliation of both normal and cancer cells in the upper aerodigestive tract or by interfering with the identification of microsatellite alterations. However, successful detection of tumor cells was possible in the test samples of all four of the patients who had undergone previous radiation therapy and who had a marker identified in their tumor. These results support the potential application of the test in postradiation therapy surveillance.
Our panel of markers has been developed with an emphasis on MSI detection and known areas of LOH, featuring a majority of tetranucleotide repeat markers with the (AAAG)n repeat motif (Table 2)
. It was not our goal to streamline the panel of markers at this stage of test development. Given the limited sample size of this pilot cohort, a marker altered in even one tumor was considered potentially valuable. Five microsatellites were the sole markers available for at least one tumor and only two microsatellite alterations had no positive matches between tumor and test samples. Thus, the panel contained few, if any, unnecessary markers. For example, one marker (D20S85), that was detected in only one tumor was also the only available marker for that tumor and was detected in the test sample as well. Markers that display MSI were found to be better than LOH markers for detecting tumor cells in a background of normal cells. Relatively pure clonal populations of tumor cells are required for detection of LOH, because the loss of the signal may be obscured easily by the presence of the alleles in the normal cell majority. Microsatellite loci that display MSI are valuable because the amplified novel tumor DNA band separates from maternal and paternal alleles on gel electrophoresis and serves as a unique positive-signal clonal marker for the tumor and its exfoliated daughter cells. Thirteen markers displayed MSI in multiple tumors. These microsatellites are arguably the most valuable in the panel. If altered in tumor, they were detected in saliva 66% of the time and account for 47% of all of the positive saliva results in the entire study.
Future refinement of the approach may focus on several areas. More microsatellite markers could be added to the panel, thus increasing the yield of detectable tumors above the current 86%. In particular, tetranucleotide markers displaying MSI in a high proportion of tumors seem to be promising candidates for addition to the panel. Tumor-specific promoter methylation markers may also be useful (19) . Microcapillary array technology now allows more rapid and efficient screening of large numbers of samples (12) .
The molecular assessment of exfoliated cells from oral rinsing/swabbing must now be tested in a prospective, blinded fashion in clinical settings requiring actual cancer detection. The fact that no healthy control subject had any DNA alteration is encouraging, indicating the excellent specificity of the microsatellite analysis. Normal appearing mucosa in patients with early cancer or premalignant lesions has been shown to harbor occult microsatellite alterations (20) . It is precisely in these patients that the value of detecting asymptomatic cancers by microsatellite analysis must be prospectively assessed, a setting requiring a highly specific test. A saliva test could be administered by nonspecialists in remote locations as a screening tool to select patients for referral for careful evaluation of the upper aerodigestive tract. Finding early stage, previously undetected disease and prompt identification of persistent disease after therapy using sensitive microsatellite analysis may ultimately save lives.
| FOOTNOTES |
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1 Supported by NIH/National Institute of Dental and Craniofacial Research Grants RO1DE012588-01 and RO1 DE13152-01. ![]()
2 To whom requests for reprints should be addressed, at Division of Head and Neck Cancer Research, Department of Otolaryngology-Head and Neck Surgery, Room 818, Ross Research Building, 720 Rutland Avenue, Baltimore, MD 21205. Phone: (410) 502-5155; Fax: (410) 614-1411; E-mail: 102777.2553{at}compuserve.com ![]()
3 The abbreviations used are: HNSCC, head and neck squamous cell carcinoma; LOH, loss of heterozygosity; MSI, microsatellite instability. ![]()
Received 9/15/00; revised 11/27/00; accepted 11/28/00.
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