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Pathophysiological and Experimental Pathology, Departments of Pathology [T. K., S. H., S. M., Y. M., K. Sue.], and General Surgery [K. Sugio, I. Y., K. Sugim.], Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan
| ABSTRACT |
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| INTRODUCTION |
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Neoplastic transformation is considered to be the result of a multistep accumulation of genetic abnormalities, including either activation of oncogenes or inactivation of tumor suppressor genes. p53 tumor suppressor gene mutation is common and frequent among genetic abnormalities in various human cancers, suggesting that the occurrence is a fundamentally important step in carcinogenesis, and it may even play a key role in the clinical prognosis. Regarding non-small cell lung cancers, although many workers have investigated the relationship of p53 abnormalities and prognosis, the results have differed; hence, the clinicopathological significance of p53 alteration has remained unknown. The findings conflicted perhaps because of the methodology used in examinations, such as immunohistochemistry and genomic analysis, regardless of histological subtypes. An important issue to be considered is that there is little information regarding if and when p53 mutation occurs during tumor progression. Although recent studies (3, 4, 5, 6, 7) suggest that BAC likely derives from an atypical adenomatous hyperplasia, a putative premalignant lesion, we find no documentation as to when p53 mutation occurs.
In the present study, we examined (a) the frequency of p53 mutation and (b) the clinicopathological background in each histological subtype, classified according both to the current WHO criteria, and the minor modification, including a proposed subgroup of BAC-invasive. On the basis of our data, we propose that BAC-invasive type should be classed independently from the mixed subtype of lung adenocarcinoma, with regard to both p53 mutations and clinical prognosis. We also found that histopathological subtyping with this modification is a more productive and independent prognostic indicator than is p53 mutation.
| MATERIALS AND METHODS |
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Exons 29 of the p53 gene were analyzed using PCR-SSCP and p53-specific oligonucleotide primers (Table 2)
. Thereafter, semi-nested or nested PCR was done as described (9)
, but with some modifications. The first PCR reaction was performed in a 20-µl reaction mixture containing 20 mM Tris (pH 8.4), 50 mM KCl, 1.5 mM MgCl2, 200 µM of deoxyribonucleotide triphosphate, 0.2 µM of outer primer pairs, 1 unit of Taq DNA polymerase (Takara Shuzo Co., Kyoto, Japan), and 1 µl of template DNA. The first PCR product was diluted to a ratio of 1:50 in distilled water, and 1 µl of the dilution, as a template, was applied to the second PCR. The PCR reaction conditions for the first PCR were 95°C for 20 s, annealing temperature 5960°C for 20 s, and 72°C for 20 s (35 cycles), whereas the second PCR was run at 95°C for 20 s, 59°C for 20 s, and 72°C for 20 s (30 cycles). A Perkin-Elmer 9600 Thermal Cycler (Perkin-Elmer Co., Norwalk, CT) was used.
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DNA Sequencing.
All of the mutations detected by PCR-SSCP were confirmed by direct DNA sequencing, as described (9)
, but with minor modifications. Any band showing any aberrant mobility shifts was excised from the gel and eluted into water at 80°C for 5 min, followed by reamplification with the same inner primers and conditions, as described above. The samples were subjected to subsequent direct DNA sequencing, using a Thermo Sequenase core sequencing kit (Amersham, CA) and SQ-5500 DNA sequencer (Hitachi, Japan).
Immunohistochemical Analysis of P53 Protein.
An immunohistochemical study was done using monoclonal antihuman P53 antibody (DO7; Novocastra, Newcastle, United Kingdom). For the antigenic retrieval of the antibody, the sections were autoclaved for 5 min at 121°C in 0.1 M citrate buffer solution (pH 6.0). After treating the sections with 1.5% milk solution to reduce the nonspecific absorption of antibody, the sections were reacted with the primary monoclonal antibody diluted to 1:100 overnight at 4°C. The tissue sections were treated with biotin-labeled antimouse antibody and then with 0.1% H2O2-methanol solution, followed by the streptavidin-biotin-peroxidase complex method (10)
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The P53-labeling index of cancer cells in each cancer tissue was determined by counting the number of P53-positive cells among at least 300 cancer cells.
Statistical Analysis.
To estimate the correlation between the frequency of p53 mutation and clinicopathological data, including histological subtypes, tumor size, smoking status, and pathological stage,
2 test, Students t test, and the Mann-Whitney U test were used. All of the Ps were based on two-hypothesis testing, and statistical significance was assumed at a level of P < 0.05. Survival curves were obtained using the Kaplan-Meier method, and the statistical significance of differences was calculated using the log-rank test. Multivariate analysis was performed to identify independent prognostic factors and to assess the hazard ratio with the Cox proportional hazards model, using the Statistical Package for Social Science (SAS). In this model, seven factors potentially related to survival (age at surgery, gender, histological subtype, p53 gene status, tumor size, smoking history, and pathological stage) were included, and the model selection for identifying the subset of significant variables was based on the stepwise method for background selection. Discriminant analysis was also examined between an independent prognostic factor obtained from multivariate analysis and other factors. In this analysis, a stepwise method for background selection was used.
| RESULTS |
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The Frequency of p53 Mutation in Each Histological Subtype.
Of 145 cases, 51 (35%) had a mutation in the p53 gene in exons 29 (Tables 1
and 3
), 5 (9.8%) in exon 4, 26 (51%) in exon 5, 4 (7.8%) in exon 6, 8 (15.7%) in exon 7, 7 (13.7%) in exon 8, and 1 (2%) in exon 9. Cases 7 and 29 had two different mutations (Table 3)
. No mutation was found in exons 2 and 3. Regarding the relation of p53 mutation to histological subtypes, p53 mutations were found in 3 of 27 (11%) BACs with invasion, 1 (25%) of acinar adenocarcinomas, 27 (46%) of papillary adenocarcinomas, and 20 (53%) of solid adenocarcinomas (Table 1)
, but no mutation was found in the case of BAC alone (P < 0.01; Table 1
).
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Immunohistochemical Overexpression of P53 Protein and the Relation to p53 Gene Mutation.
In Table 4
, 61 of the 145 cases (42%) showed overexpression of P53 protein. The concordance rate of overall cases between P53 immunohistochemistry and p53 gene status was 67% with a statistical significance (P < 0.001). However, the concordance rates were higher in BAC without and with invasion (94% and 85%, respectively; P < 0.05) than that of the acinar, papillary, and solid adenocarcinomas ranging from 50 to 58%.
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T:A transversions were observed in 9 (18.8%) of 48 non-BAC types, but in none of 3 BACs with invasion.
Survival Rate, Histological Subtype, and p53 Gene Status.
Analyses for the relationship between histological subtypes, p53 gene status, and survival rates revealed that all of the patients with BAC with invasion, acinar, papillary, or solid type had a poorer prognosis than in cases with BAC alone, which was statistically significant (P = 0.001; Fig. 4A
). We also studied the relationship between p53 gene status and overall survival. Of the 145 cases, patients with p53 mutations had a shorter survival period than did those without any p53 mutation (Fig. 4B
; P < 0.05); however, among patients with either acinar, papillary, or solid type, no statistical significance was noted with regard to p53 gene status (Fig. 4C
). Examining the relationship between the type of p53 mutation and survival by stage, no statistical significance was apparent regarding survival time between patients with p53 null mutations and missense mutations in any stage (data not shown).
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| DISCUSSION |
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Although a significantly poor prognosis was observed in patients with p53 mutation, the frequency of p53 mutation did not significantly affect the prognosis in patients with non-BAC. Furthermore, multivariate analysis for overall survival revealed that p53 mutation was not a significant prognostic factor, whereas the histological subtype is an independent and significant indicator. Together with the discordance between p53 mutation and P53 immunoreactivity shown in Table 4
, these findings suggest that the enhanced expression of P53 is likely attributable to mechanisms other than p53 mutation in non-BAC, and there is a lack of significant correlation between p53 mutation and the prognosis in patients with non-BAC. Inversely, BAC and BAC-invasive showed strong concordance between p53 mutation and P53 immunoreactivity, which suggests that the prolonged half-life of P53 seemed largely attributable to the mutated p53 allele.
The discordance between p53 gene status and P53 immunoreactivity within each subtype of lung adenocarcinoma has been unclear. In vitro studies suggested that the type of mutation, including null mutation, other cellular proteins associated with P53, including murine double minute 2 or viral oncoproteins such as simian virus 40 large T antigen, and DNA damage may have an effect.
Many studies (11, 12, 13)
revealed that p53 mutations are more frequent in squamous cell carcinoma, well-known smoking-related malignancies, than in adenocarcinomas. In the present study, the frequency of p53 mutation in papillary and solid adenocarcinoma was relatively higher than in BAC groups and close to findings in the case of the squamous cell carcinoma, where the reported rate was from 60 to 68% (11, 12, 13)
. G:C
T:A transversion in the p53 gene is a smoking-related mutation (13
, 14)
and is assumed to arise as a direct consequence of benzopyrene diol epoxide-DNA adducts (15)
. In the present study, the relationship of smoking history with p53 mutation in all of the patients showed a correlation coefficient, and the ratio of patients who used tobacco was significantly higher in patients with non-BAC. These results suggest that a history of tobacco smoking was strongly associated with p53 mutation in these subtypes, similar to findings with squamous cell carcinoma of the lung. Conversely, BAC and BAC-invasive showed a poorer correlation to smoking than did non-BAC.
Histological subtypes reflecting p53 status are useful prognostic markers for determining survival time after surgical intervention. p53 mutation was not a significant prognostic factor, yet discriminant analysis revealed that p53 mutation was a significant factor for discriminating histological subtype.
Our findings also suggest that BAC-invasive should be classed independently from the mixed subtype in the WHO criteria, because this type of adenocarcinoma has a better prognosis than other mixed types composed mainly of non-BAC (Fig. 4A)
. There are reports that showed the size of the scar or the area of lepidic growth component is prognostically important in lung adenocarcinoma (16
, 17)
; however, in the present study, no statistical significance was found between the area of lepidic growth component and patients survival (data not shown). This conflict may be related to the fact that we did not limit the tumor size of subjects examined in this study, which has been suggested to be an important prognostic factor, within 3 cm in diameter.
In conclusion, BAC is independent from other histological subtypes from the point of clinicopathological and molecular evidence. Evaluation of the histological subtype based on the current WHO classification can predict the clinical prognosis more so than analyzes of the p53 gene mutation. We propose that patients with non-BAC type adenocarcinoma should be prescribed adjuvant therapies regardless of the p53 gene status.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 To whom requests for reprints should be addressed, at Pathophysiological and Experimental Pathology, Department of Pathology, Graduate School of Medical Sciences, Kyushu University 60, 3-1-1, Higashi-ku, Fukuoka 812-8582, Japan. Phone: 81-92-642-6060; Fax: 81-92-642-5965; E-mail: sueishi{at}pathol1.med.kyushu-u.ac.jp ![]()
2 The abbreviations used are: BAC, bronchioloalveolar carcinoma; SSCP, single-strand conformation polymorphism. ![]()
Received 2/15/00; revised 2/23/01; accepted 3/22/01.
| REFERENCES |
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