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J Thorac Cardiovasc Surg 1994;107:590-0595
© 1994 Mosby, Inc.
General Thoracic Surgery |
Ann Arbor, Mich.
From the Section of Thoracic Surgery, University of Michigan Medical School, Ann Arbor, Mich. 48109.
Address for reprints: David G. Beer, PhD, B560 MSRBII, Box 0686, University of Michigan, Ann Arbor, MI 48109.
Abstract
The development of human adenocarcinoma of the lung involves multiple genetic changes including activation of oncogenes and loss of tumor suppressor genes. Patients whose lung tumors contain K-ras oncogene mutation, accumulation of the protein product of the tumor suppressor gene p53, or erbB-2/neu oncoprotein overexpression have been shown to have a worse prognosis. We examined these three genetic indicators in 29 lung adenocarcinomas to determine whether these markers are present in the same tumors or if they represent molecular changes that define different subsets of patients. P53 nuclear protein accumulation and erbB-2/neu protein overexpression were determined by immunohistochemical analysis of cryostat sections of tumor specimens and corresponding normal lung tissue. K-ras mutations were detected by radiolabeled oligonucleotide probes, specific for the various twelfth codon mutations, hybridized to exon 1 of K-ras, which was amplified by the polymerase chain reaction. Increased nuclear accumulation of p53 protein was found in 11 adenocarcinomas (38%). All of the p53 positive tumors were found to show high level staining and homogeneous expression of erbB-2/neu protein. K-ras mutations were detected in seven tumors (24%), all of which overexpressed erbB-2/neu. The presence of a K-ras mutation did not correlate with p53 accumulation. In total, 93% of the tumors were found to overexpress erbB-2/neu, the highest being in one tumor with erbB-2/neu gene amplification. The presence of K-ras twelfth codon mutation was associated with increased cigarette smoking. In conclusion, erbB-2/neu overexpression is a common event in lung adenocarcinomas. Furthermore, the presence of K-ras mutation and p53 protein accumulation define separate groups of patients. The mechanisms by which these genetic alterations interact or adversely affect prognosis is unknown. (J THORAC CARDIOVASC SURG 1994;107:590-5)
Investigation into the molecular biology of lung cancer has revealed multiple abnormalities in oncogenes and tumor suppressor genes that are consistent with a multistep process of carcinogenesis.
1, 2 Numerous reports exist that implicate specific genetic changes as negative prognostic markers when occurring in lung adenocarcinoma. These markers may identify patients with early-stage lung cancer at risk for recurrence after resection.
K-ras is a commonly activated oncogene in human lung cancer with alterations at the twelfth codon position accounting for 70% to 80% of the activating mutations.
3, 4 The function of K-ras in normal and lung cancer cells is not known, but the oncogene likely functions in growth signal transduction and cellular differentiation.
5, 6 Lung adenocarcinomas with K-ras mutations have been shown to have a significantly worse prognosis than K-rasnegative tumors.
3, 7, 8 In one study, 18 patients with stage I, K-raspositive tumors had a decreased 5-year survival of 53% compared with an 84% 5-year survival in patients with stage I, K-rasnegative tumors.
3
The tumor suppressor gene p53 has also been shown to affect prognosis in lung adenocarcinoma. The normal p53 protein is involved in cell cycle regulation.
9 Mutation of p53 gene leads to a loss of function and to increased protein half-life resulting in nuclear accumulation of the mutant p53 protein.
10 P53 mutation and protein accumulation are associated with a worse prognosis in lung adenocarcinoma.
11, 12 Patients with stage I and II adenocarcinomas demonstrating p53 protein accumulation were found to have a 5-year survival of only 15% whereas those without p53 accumulation had a 5-year survival of 35%.
11
The protooncogene erbB-2/neu encodes a membrane bound, probable autocrine growth factor receptor.
13 Overexpression of erbB-2/neu protein is associated with decreased survival in several human cancers,
14, 15 including lung adenocarcinoma.
16 ErbB-2/neu protein overexpression is associated with gene amplification in breast cancer,
17 whereas gene amplification is rare in lung adenocarcinoma.
14, 18
K-ras oncogene twelfth codon mutations, p53 tumor suppressor gene protein accumulation, and erbB-2/neu oncoprotein overexpression were examined in 29 patients who underwent resection for lung adenocarcinoma. The prevalence and coincidence of these three prognostic markers were examined to determine if they occur in the same tumors and thus identify the same patients or represent independent genetic events in carcinogenesis. Gene amplification as a mechanism for erbB-2/neu protein overexpression was examined. Additionally, correlations between the genetic alterations and age, sex, tumor stage, and smoking history were made.
MATERIALS AND METHODS
Tissue samples
Analyses were performed on specimens from 29 consecutive patients with the final diagnosis of primary lung adenocarcinoma, who were operated on at the University of Michigan Hospital between August 1991 and August 1992. Immediately after resection, normal lung and lung tumor tissues were divided into similar portions. One portion was frozen in liquid nitrogen for isolation of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA); the other was embedded in OCT compound (Miles Inc., Elkhart, Ind.) and frozen in isopentane cooled to the temperature of liquid nitrogen. Samples were then stored at -70° C until analysis.
Detection of K-ras twelfth codon mutation
Genomic DNA was isolated from frozen tissue specimens by standard techniques.
19 Exon 1 of the K-ras gene was amplified by the polymerase chain reaction as described.
7 Polymerase chain reaction products were then vacuum blotted onto nylon membranes (GeneScreen Plus, NEN, Wilmington, Del.) and probed with radiolabeled oligonucleotides (Clon-Tech, Palo Alto, Calif.) homologous to the known twelfth codon mutations. Blots were then washed at high stringency and autoradiograms were prepared.
Immunohistochemistry of p53 and erb B-2/neu
Overexpression of erbB-2/neu protein and accumulation of p53 protein were detected by the avidin-biotin-peroxidase complex method on 5 µm thick cryostat sections.
20 P53 protein was detected with a 1:100 dilution of a mouse monoclonal primary antibody (Ab-2) (clone PAb1801) (Oncogene Science, Inc., Uniondale, N.Y.), which reacts with the N-terminal domain of p53. ErbB-2/neu was detected with a 1:100 dilution of an erbB-2/neu mouse monoclonal antibody (Ab-2) (clone 9G6) (Oncogene Science), which reacts with an extracellular domain epitope.
Detection of erb B-2/neu gene amplification
Southern blot analysis was used to evaluate for erbB-2/neu gene amplification in three tumors that highly overexpressed erbB-2/neu protein as previously described.
20 DNA 10 µg was digested with the restriction enzyme Hind III. Probes used were a 1.6 Kb EcoR1 fragment of human erbB-2/neu cDNA and a 0.8 Kb Pvu II fragment of human MDR-1 cDNA as a single copy reference gene.
Clinical correlations
Relations between p53 and K-ras alterations and age, sex, tumor stage, and smoking history were examined with either the
2 or Fisher's exact test.
RESULTS
Immunohistochemistry was used for the analysis of p53 protein accumulation and erbB-2/neu overexpression in 29 lung adenocarcinomas. P53 protein accumulation was detected in the nuclei of 11 of 29 tumors (38%). Tumors that either stained positively or negatively for p53 were found among all TNM stages
(Table I). In six of 11 p53 positively staining adenocarcinomas all tumor cells demonstrated nuclear protein accumulation, whereas in the other five positive tumors only a subset of the cells demonstrated nuclear protein accumulation (Fig. 1). P53 protein accumulation was not observed in normal lung tissue. ErbB-2/neu protein expression was found in 27 tumors (93%)
(Table I). The staining pattern was identical in all tumors, with every tumor cell showing expression of the erbB-2/neu protein. Three tumors expressed erbB-2/neu at a very high level, whereas erbB-2/neu was either undetectable or expressed only at very low levels in normal lung tissue (Fig. 1).
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No statistically significant differences between K-ras positive and negative or p53 positive and negative tumors were seen in regard to patient sex or age or tumor stage (p > 0.05). Patients with K-ras positive tumors had a longer smoking historya mean of 71 pack-years compared with a mean of 41 pack-years for patients with K-ras negative tumors (p < 0.05). There were four nonsmokers in the cohort, none of whom had K-ras positive tumors. If the analysis is restricted to smokers, K-ras remains associated with increased smoking history with a
2 p value of 0.06. Accumulation of p53 nuclear protein was not found to be associated with smoking history.
DISCUSSION
Alterations in the structure or expression (or both) of K-ras, p53, and erbB-2/neu have all been correlated with worse than expected prognosis in lung adenocarcinoma.
3, 7, 8, 11, 12, 16 The present study has examined these three markers in the same lung adenocarcinoma specimens. The finding of p53 and K-ras twelfth codon abnormalities in 38% and 24% of the tumors, respectively, is consistent with the results of previous studies.
7, 11 Interestingly, p53 and K-ras occurred together in only two tumors (7%), indicating that these changes may be independent events in lung carcinogenesis. Kishimoto and associates
21 reported a similar result when they analyzed for p53 alterations using single-strand conformational polymorphism analysis in a group of non-small-cell lung cancer tumors, which had previously been analyzed for K-ras mutations.
21 P53 and K-ras have also been found to be independent events in colon adenocarcinomas.
22 P53 and K-ras appear to define separate subsets of patients with lung cancer who have potentially decreased survival.
Analysis of the groups of patients with either K-ras or p53 alterations revealed an increased smoking history in the patients with K-ras twelfth codon mutations. This relation remained even if the analysis included only those patients who smoked. Cigarette smoke is known to contain mutagens capable of forming DNA adducts that can result in K-ras mutations. These results are consistent with the results of Reynolds and colleagues,
23 who found a high percentage of K-ras twelfth codon mutations in smokers.
ErbB-2/neu protein was expressed in 93% of the tumors analyzed. This observation is consistent with the findings of Shi and coworkers
24 but appears in contrast with other reports of a much lower frequency of erbB-2/ neu overexpression.
16 These differences likely relate to conditions used during immunohistochemistry. Formalin fixation and paraffin embedding of tissue used by others
16 may alter erbB-2/neu antigens,
14 whereas this study used frozen tissue sections. Additionally, we used a monoclonal antibody directed to the extracellular portion of the protein, whereas others have used either polyclonal or monoclonal antibodies directed toward different epitopes.
16, 24 The high frequency of tumors showing overexpression of erbB-2/neu protein suggests that erbB-2/neu protein overexpression is a very common event in lung adenocarcinomas. We find similar expression of this protein on almost all adenocarcinoma cells within the tumor. This may indicate that an important selection process exists that maintains the expression of this protein in all tumor cells. ErbB-2/neu protein overexpression along with other molecular alterations such as K-ras or p53 gene mutations may play a key role in lung carcinogenesis. ErbB-2/neu protein overexpression is not likely to indicate prognosis in this sample of 29 lung adenocarcinomas inasmuch as it is present in most tumors.
ErbB-2/neu overexpression is commonly associated with gene amplification in breast and ovarian cancer.
14, 17 ErbB-2/neu gene amplification is known to be rare in lung cancer, and erbB-2/neu was reported to occur in only one of 60 non-small-cell lung cancers.
14, 18 Southern blot analysis of the tumors that overexpressed erbB-2/neu to the greatest extent revealed only one tumor with gene amplification and two tumors with single copies of the erbB-2/neu gene. Other possible mechanisms for erbB-2/neu protein overexpression exist and include increased gene transcription, increased messenger RNA stability, and increased protein stability. They are currently being examined in this laboratory.
Future improvements in lung cancer survival will likely require earlier detection of cancer, identifying patients at high risk for recurrence after resection, and the development of novel therapies. Patients at risk may be screened for specific molecular alterations, possibly through sputum samples, with the intent to detect malignancy at its earliest point. Patients with early-stage lung cancer may be identified as being at high risk for recurrence on the basis of these molecular markers and may be given further therapy in addition to resection. Novel therapies beyond standard chemotherapy and radiation therapy, directed at these molecular changes, may be developed. Monoclonal antibodies have been developed against the erbB-2/neu protein,
25, 26 and there are ongoing trials using these antibodies in patients with breast cancer. Because of its abundant expression, erbB-2/neu protein may prove to be an effective target for monoclonal antibody therapy in lung adenocarcinoma.
Appendix: DISCUSSION
Dr. Douglas J. Mathisen (Boston, Mass.).
Did you look at the degree of histologic differentiation of these tumors and whether there is any correlation with your findings? Do you have any information on the survival of these patients?
Dr. Whyte.
We did not specifically evaluate the degree of differentiation, but we did look at the tumor stage and found that there was no significant relation between the genetic alterations and the tumor stage.
Regarding patient outcome, most of these patients were operated on within the last 18 months and most had stage I tumors. The follow-up period is relatively short and I have not evaluated survival yet.
Dr. Martin F. McKneally (Toronto, Ontario, Canada).
Do you envision that these markers will be used to identify patients with stage I disease at poor risk for recurrence and to apply more intensive or more toxic adjuvant therapy?
Dr. Whyte.
That's exactly right. If we can identify patients who are at higher risk, they will be more inclined to get adjuvant therapy. As things are now, patients with stage I disease undergo surgery with no other treatment; if a group of patients at high risk for early recurrence can be identified, these patients can be offered adjuvant therapy.
Footnotes
Read at the Seventy-third Annual Meeting of The American Association for Thoracic Surgery, Chicago, Ill., April 25-28, 1993. ![]()
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