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Department of Pathology and Laboratory Medicine, University of North Carolina, Chapel Hill, North Carolina 27599-7525 [J. S., W. K. F., B. E. W.]; Laboratory of Genetics, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224 [W. W.]; and Lineberger Comprehensive Cancer Center, University of North Carolina, Chapel Hill, North Carolina 27599-7295 [D. N. R., B. E. W.]
| ABSTRACT |
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30% of human non-small lung cancer cell lines. To determine whether loss of expression of these proteins occurs in primary tumors, we examined their expression in 41 primary lung adenocarcinomas and 19 primary lung squamous carcinomas by immunohistochemistry. These analyses showed that 10% of tumors show a concomitant loss of BRG1 and BRM expression. Moreover, patients with BRG1/BRM-negative carcinomas, independent of stage, have a statistically significant decrease in survival compared with patients with BRG1/BRM. This report provides supportive evidence that BRG1 and BRM act as tumor suppressor proteins and implicates a role for their loss in the development of non-small cell lung cancers. | Introduction |
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Although these and other data appear compelling, establishing BRG1 and BRM as bona fide tumor suppressors also requires demonstrating their loss of expression in primary tumors. Therefore, based on data showing loss of BRG1 and BRM expression in
30% of human NSCLC cell lines and published data showing loss of heterozygosity surrounding the BRG1 and BRM loci in human lung cancers (13)
we conducted immunohistochemical staining for BRG1 and BRM on 60 NSCLC samples taken from patients that underwent curative resections for stage IIIIA tumors. We observed loss of BRG1/BRM expression in 6 of 60 tumor samples (
10%), suggesting that BRG1/BRM loss in tumor cell lines does not arise as a tissue-culture artifact. Consistent with cell line data, loss of BRG1 expression occurred concomitantly with loss of BRM expression. Clinically, loss of BRG1/BRM expression in NSCLC was associated with a significantly worse patient survival compared with the survival of patients with BRG1/BRM-positive tumors. These data additionally support that BRG1/BRM act as tumor suppressor proteins and that loss of function of these proteins may potentially impact the clinical outcome of NSCLC.
| Materials and Methods |
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Tumor Samples.
Tumor samples banked at the University of North Carolina from 19971999 were randomly selected. The specimens were derived from patients with stage IIIIA NSCLC. Internal Review Board approval was obtained before our analysis.
Statistical Analyses.
The Kaplan-Meier (or product limit) method was used to estimate the survivorship function. We used the log-rank test to identify possible differences between estimated survival curves. All analyses were performed with SAS statistical software, version 8.2, from SAS Institute (Cary, NC).
| Results |
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30% (6 of 20) of human lung cancer cell lines (Table 1)
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Using the BRG1 specific antibody, we detected loss of BRG1 expression in 6 of 60 tumors (
10%; Figs. 1,AF
, and 2
). These 6 tumors also showed no positive signal with the second antibody showing that neither BRG1 nor BRM expression was present (Figs. 1, GL
, and 2
). Moreover, these tumors did not show a continuum of staining with either of these antibodies such that the positively stained tumors are distinctly grouped from the negatively stained tumors (Fig. 2)
.
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Although we have previously observed that the loss of expression of both BRG1 and BRM appear limited to adenocarcinoma cell lines (15)
, we also found reduced or absent BRG1 and BRM expression in adenocarcinomas (3 of 41) as well as squamous cell carcinomas (3 of 19; Fig. 2A
). Interestingly, two of these latter carcinomas are adenosquamous type, and loss of BRG1/BRM expression was noted in the area of adenocarcinoma morphology as well as in the area of squamous carcinoma morphology. These results suggest that loss of BRG1 and BRM may not be restricted to adenocarcinomas, as suggested by our original cell line data. Consistent with our previous studies, tumors demonstrated concomitant loss of BRG1 and BRM expression (Fig. 2A)
. However, because the second monoclonal antibody (Transduction Laboratories) detects both BRG1 and BRM proteins (Ref. 15
; Reisman, unpublished data), we cannot determine whether any of the BRG1-positive tumor samples lack only BRM expression.
Loss of BRM/BRG1 Correlates with a Worse Prognosis in NSCLCs.
We compared the survival of the 6 patients who had BRG1/BRM-negative tumors to the 54 patients with BRG1-positive tumors (Fig. 3, B and D)
. During a median 36-month follow-up period, the BRG1-positive patients with stage I, II, and III disease had a survival of 79, 66, and 44%, respectively, consistent with previously published data (Fig. 3
; Ref. 18
). However, the patients with BRG1-negative tumors had 0% survival during this period (Fig. 3D)
. The Kaplan-Meier curves show this survival difference between patients with BRG1-positive and BRG1-negative tumors (Fig. 3B)
. Lung cancer patients with stage I, II, and IIIA disease have progressively worse survival despite best therapy (curative surgical resection). Although the patients with BRG1-negative tumors had primarily stage I (2 of 6) and stage II (3 of 6) disease, they had a statistically worse survival (P < 0.03) than patients with the clinically more advanced stage IIIA (BRG1 positive) tumors (Fig. 3C)
. This difference in survival is not because of difference in treatment or therapies as all 60 patients were treated with surgery alone without any neoadjuvant or adjuvant therapies. As might be expected, patients with BRG1-negative NSCLC also had worse survival than patients with stage-matched BRG1-positive NSCLC.
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| Discussion |
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The loss of function of the SWI/SNF complex will likely produce major phenotypic changes within tumors because of its interaction and requirement in many signal transduction pathways in addition to the RB pathway. BRG1 and BRM have been shown to interact with cyclin E, Myo-D, p53, p107, and p130 (RB2; Ref. 2 , 11 , 21 , 22 ). Their function is also required for the function of estrogen, glucocorticoid, progesterone, and retinoic acid receptors (19) . Another affect of BRG1/BRM loss may come from the SWI/SNF complexs contribution, via BAF60a, to the induction of AP1 responsive genes (23) . The number of genes actually regulated by the SWI/SNF complex is at least 80 and in yeast the SWI/SNF complex regulates 57% of the yeast genome (24) . Thus, the broad changes in the signaling pathways that must occur with loss of BRG1/BRM must have an impact on the tumor phenotype. Our data suggest that this loss may be an important indicator of a patients overall survival.
As with human cancer cell lines, primary tumors appear to have concomitant loss of both BRG1 and BRM protein expression. Because experiments with human tumor cell lines show that BRG1 and BRM have redundant functions (12 , 15 , 25) , their concomitant loss may be required to completely inactivate the SWI/SNF complex. However, biochemical analyses by several groups have shown that BRM and BRG1 are contained in different complexes that, in turn, have apparently different biochemical functions (26 , 27) . Whether the concomitant loss of BRG1 and BRM expression is required to abrogate separate SWI/SNF functions or occurs because BRG1 and BRM share redundant functions important for tumorigenesis, warrants additional investigation.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This work was supported, in part, by NIH Grant CA91048 (to B. E. W.). D. N. R. received support from NIH Grant T32CA09156 and the Kempner Foundation, University of Texas Medical Branch, Galveston, TX. ![]()
2 Present address: Division of Hematology/Oncology Department of Medicine, University of Michigan, Ann Arbor, MI 48108. ![]()
3 To whom requests for reprints should be addressed. Phone: (919) 966-7533; Fax: (919) 966-9673; E-mail: weissman{at}med.unc.edu ![]()
4 The abbreviations used are: RB, retinoblastoma; NSCLC, non-small cell lung cancer. ![]()
Received 9/20/02. Accepted 12/13/02.
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