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Advances in Brief |
Hamon Center for Therapeutic Oncology Research [N. S., A. K. V., I. I. W., J. D. M., A. F. G.] and Departments of Pathology [A. K. V., S. M., A. F. G.], Internal Medicine [J. D. M.], and Pharmacology [J. D. M.], University of Texas Southwestern Medical Center, Dallas, Texas 75235, and Department of Pathology, M. D. Anderson Cancer Center, Houston, Texas 77030 [B. M.]
| ABSTRACT |
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| Introduction |
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The delineation of genetic alterations that occur in MMs may be important for both the development of molecular markers for early detection and the prediction of response to chemotherapy. It is now well recognized that tumorigenesis is a multistep process resulting from the accumulation of sequential genetic alterations (4) . In addition to oncogene activation, the inactivation of tumor suppressor genes has been shown to play an important role in tumorigenesis (4) . Knudsons two-hit model established the paradigm for recessive tumor suppressor genes, whereas both alleles of the affected gene must be inactivated to contribute to the tumor phenotype (5) . Allelic deletion manifested as LOH at polymorphic loci is recognized as a hallmark of tumor suppressor genes inactivated by point mutations or some other mechanism. Using this strategy, previous studies have provided evidence for the presence of putative suppressor genes on chromosomes 1p, 3p, 6q, 9p, and 22q in the development of MMs (6, 7, 8, 9) . The diagnostic distinction between malignant pleural epithelial mesothelioma and metastatic adenocarcinoma is often difficult, and there is a considerable interest in identifying markers that might be of use in the differential diagnosis (10 , 11) .
Evidence for tumor suppressor genes on chromosome 4 has been provided by several studies. Recent allelotyping studies have documented allelic loss on one or both arms of chromosome 4 in several neoplasms including bladder, cervical, colorectal, hepatocellular, and esophageal cancers and squamous cell carcinomas of the head and neck and skin (12, 13, 14, 15, 16, 17, 18) . Phenotypic tumor suppression has been observed with the introduction of chromosome 4 into human glioma cells (19) . Thus, chromosome 4 likely contains one or more tumor suppressor genes that are frequently inactivated in several types of cancer.
A previous cytogenetic study has reported partial or complete loss of chromosome 4 as one of the frequent karyotypic changes in MMs (20) . CGH studies have shown frequent losses on chromosome 4 in SCLC (21 , 22) . Another CGH study has also provided evidence for losses on chromosome 4 in adenocarcinomas and squamous cell carcinomas of the lung (23) . However, the location of the putative oncogenes has not been confirmed or more precisely localized by allelotyping studies. The present study was undertaken to examine the presence and location of regions of common deletion involving chromosome 4 in MM and to compare the pattern with those of NSCLC and SCLC. Using a precise microdissection methodology; DNA from archival paraffin-embedded tissue sections from MMs, NSCLCs, and SCLCs; and cell lines from NSCLC and SCLC, we studied the incidences of LOH on both the p and q arms of chromosome 4.
| Materials and Methods |
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Microdissection and DNA Extraction.
Areas of malignant cells were identified by pathological review of all of the cases of MM, SCLC, and NSCLC. These areas were precisely microdissected under microscopic visualization, avoiding contamination by normal cells (25)
. Stromal cells from the microdissected slides provided a source of constitutional DNA. Approximately 500-1000 tumor and stromal cells were microdissected from each case. The dissected cells were digested using the proteinase K method described previously (25)
, and 5 µl of the DNA samples were used directly for each multiplex PCR reaction.
LOH Analyses.
Microsatellite analysis was used to determine the frequency and pattern of allelic loss on chromosome 4 using 11 polymorphic markers on 4q and 5 markers on 4p. Primer pairs that were used to identify specific dinucleotide and tetranucleotide repeat polymorphisms in genomic DNA were obtained from Life Technologies, Inc. (Gaithersburg, MD). The primer sequences used for LOH studies were obtained from the Genome Database. The relative order of these markers was ascertained from the Genethon map of chromosome 4 (26)
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Two rounds of PCR (multiplex PCR followed by uniplex PCR) were performed to amplify each marker used in this study (27) . A 10°C "touch down" strategy (28) was used spanning the primers annealing temperature followed by 25 cycles at the optimal annealing temperature. The final product was separated on a 6% denaturing polyacrylamide gel and subjected to autoradiography. LOH was scored by visual detection of complete absence of the upper or lower allele of informative cases.
Statistical Analysis.
Fishers exact two-tailed test was used for statistical evaluation of the differences between the percentage of LOH from two different groups. Probability values of P < 0.05 were regarded as statistically significant.
| Results |
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Four cases of MM (cases 1720) and two cases of SCLC (cases 20 and 21) showed no losses at any of a relatively modest number of informative loci on chromosome 4. It is difficult to determine whether there were small deletions in the critical regions, or whether these cases lacked deletions. Thus, it is possible that analyses of the other, more informative cases resulted in an overestimate of the frequency of losses at the critical regions R1R3.
Comparison between LOH data from paraffin-fixed archival tumors from lung cancers and their respective cell lines showed a good overall correlation for both SCLC and NSCLC (Table 1)
. The LOH data identified the same three hot spots in SCLC cell lines as in the tumors (Fig. 2)
, whereas lower frequencies of LOH but presence of the same minimal regions of deletions were detected in NSCLC cell lines. Table 2
shows a comparison of the data on the distribution of LOH at the different regions of chromosome 4 in the three tumor types. In MM and SCLC, the most frequent pattern was loss at all three regions, and 6674% of these tumor types had allelic loss at more than one region. In NSCLC, most samples (72%) lost only one region, especially R3, and none lost all three regions.
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| Discussion |
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In MM and SCLC, we observed frequent losses at three nonoverlapping sites located on the q (two regions) and p (one region) arms of chromosome 4. Losses at these sites occurred at lower frequencies in NSCLC. The most frequently deleted region was at 4q3334 (region R1). The frequency of allelic loss at this region (94%) was higher than loss at any other chromosomal region previously reported for MM (7, 8, 9, 10) . Of interest, SCLC showed a similar high frequency of allelic loss (100%) in this region, but NSCLC showed a lower frequency of allelic loss (41%). Other investigators have also identified this region, which appears to be about 14cM long, as one of the critical regions of deletion in bladder, cervical, and esophageal cancers and squamous cell carcinomas of the head and neck and skin (12, 13, 14, 15, 16, 17, 18) . Our data suggest the location of a tumor suppressor gene at 4q3334 that is commonly altered in MM and SCLC (and in other cancers) but is less frequently altered in NSCLC.
High frequencies of LOH at two other regions of chromosome 4 were noted. One region (R2) included the markers D4S194, D4S1586, and D4S175 (4q2526). Previous studies have shown a common region of deletion at or near this site in squamous cell carcinomas of the head and neck and skin (17 , 18) . Another region that is targeted in MM and SCLC is between markers D4S1546 and D4S404 (4p15.115.3). This region appears to be about 3 cM long and was previously found to be frequently deleted in bladder cancer (14) . In MM and SCLC, the most frequently observed pattern was loss at all three regions. However, in NSCLC, the most frequent pattern was loss at R3 alone.
Our results suggest that inactivation of at least three putative tumor suppressor genes located on both arms of chromosome 4 may play important roles in the pathogenesis of MM and SCLC. The finding of three deletion hot spots on chromosome 4 common to MM and SCLC is of interest, particularly because there are major differences in their pathogenetic mechanisms. Asbestos exposure is the major etiological factor for mesothelioma, whereas heavy smoking is the major risk factor for SCLC. Of particular interest, NSCLC had lower frequencies of loss at these three regions. The diagnostic distinction between epithelial MMs and metastatic adenocarcinomas has always caused clinical and pathological problems, and differences in the patterns of allelic loss may help in the identification of pleural-based malignancies. Although previous cytogenetic and allelotyping studies have clearly shown distinct regions of allelic loss on chromosome 1p, 3p, 6q, 9p, and 22q in MM (6, 7, 8, 9) , no substantial qualitative or quantitative differences between MM and NSCLC have been reported. Our previous findings reported neurofibromatosis type 2 gene mutations in a subset of mesotheliomas and their absence in SCLC and NSCLC cell lines (29) .
Of interest, a recent CGH analysis compared findings between MM and NSCLC (30) . Its results showed loss at 4q as the most frequent change in MM. However, the authors could not detect such alterations in NSCLC. A previous CGH study (23) has provided evidence for the occasional loss on chromosome 4 in adenocarcinomas and squamous cell carcinomas of the lung. CGH is not as sensitive as allelotyping for precisely mapping the location of putative suppressor genes. Nevertheless, the CGH findings reported previously (30) are in agreement with our observations.
In summary, deletions of chromosome 4 are frequent in MM and SCLC but occur at lower frequencies in NSCLC. Our data identified three distinct regions of loss on chromosome 4 that may play important roles in the pathogenesis of MM and SCLC.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by Specialized Program of Research Excellence Grant 1-P50-CA70907 from the National Cancer Institute (Bethesda, MD). ![]()
2 To whom requests for reprints should be addressed, at Hamon Center for Therapeutic Oncology Research, NB8.106, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-8593. Phone: (214) 648-4921; Fax: (214) 648-4924; E-mail: gazdar{at}simmons.swmed.edu ![]()
3 The abbreviations used are: MM, malignant mesothelioma; SCLC, small cell lung carcinoma: NSCLC, non-small cell lung carcinoma; LOH, loss of heterozygosity; CGH, comparative genomic hybridization. ![]()
Received 7/30/98; revised 10/22/98; accepted 11/ 2/98.
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