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From the Department of Pathology and Clinical
Laboratories*
and the Department of Thoracic
Surgery,
Aichi Cancer Center Hospital,
Nagoya; and the Laboratory of Ultrastructure
Research,
Aichi Cancer Research Institute,
Nagoya, Japan
| Abstract |
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| Introduction |
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Unfortunately, morphological and genetic progression schemes of lung cancers have not been elucidated as clearly as those of colon cancers,2 which are by far the best-investigated example among human cancers in this regard. This has been especially true for adenocarcinomas of the lung, but atypical adenomatous hyperplasia (AAH) is now considered as a possible premalignant lesion.3-7 In addition to solitary AAHs, morphologically low-grade AAH-like lesions and carcinoma in situ (bronchioloalveolar carcinoma component) are often observed at the periphery of invasive adenocarcinomas. It is therefore possible that such morphological heterogeneity represents a cross sectional view of clonal evolutions within individual tumors, although one might argue that AAH-like lesions at the periphery may be nonneoplastic changes resembling reactive proliferation of type II pneumocytes.
It has been suggested that sequential accumulation of multiple genetic alterations occurs in a multistep, sequential progression from adenoma with low-grade dysplasia eventually to invasive and metastatic overt carcinoma.2 A number of chromosomal regions have been shown to be affected in invasive lung cancers, which include 1p, 2q, 3p, 5q, 8p, 9p, 11p, 13q, 17p, 18q, and 22q.8-11 Among these, loss of heterozygosity (LOH) on 2q, 5q, 9p, 18q, and 22q was previously reported to be associated with advanced disease stage and/or tumor metastasis.12,13 In contrast, many studies showed that LOH at 3p and 17p lack such associations,14-18 suggesting that they occur as early molecular events during tumor progression. Furthermore, allelic loss at 3p and 17p was frequently demonstrated in histologically normal epithelium of smokers.19-21 Three distinct putative tumor suppressor genes on 3p25, 3p21.3, and 3p14 have been suggested to be involved,22 whereas the p53 gene at 17p13.1 has been identified as a target for 17p deletions.23 In addition, we have recently shown the presence of a commonly deleted region at 17p13.3.24 Interestingly, LOH at 17p13.3 was observed even in cases without LOH at 17p13.1, suggesting its independent and early occurrence. Nevertheless, previous arguments on genetic progression in lung cancer development, including ours, were essentially based on the statistical correlation between frequencies of each genetic alteration and histological and/or disease progression.
In the present study, we examined topographical distribution of LOH events occurring in the process of neoplastic progression within individual tumors and asked whether previous inferences are indeed applicable to the progression scheme within individual tumors. Accordingly, we examined multiple pathologically well-defined specimens from individual tumors to investigate whether and which genetic alterations are accumulated in relation to morphological progression in individual tumors.
| Materials and Methods |
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Eight cases of adenocarcinoma and two cases of squamous cell carcinoma were selected from the files of the Department of Pathology and Clinical Laboratories, Aichi Cancer Center Hospital. Using serial sections of paraffin-embedded tissues, three individual lesions in each tumor were microdissected with a micromanipulator (Narishige, Tokyo, Japan). Microdissected specimens were digested in a lysis buffer as previously reported25 and used as templates for polymerase chain reactions. Normal control DNAs were extracted from the corresponding lymph nodes without metastasis. In the present study, the term "adenocarcinoma in situ " was used in accordance with the criteria previously used by Hung et al26 and included the lesions with nuclear features falling within the cytological criteria of malignant cells but without invasive or destructive growth.
Assays for LOH
Polymerase chain reaction amplification was performed to examine
LOH at each of the 10 microsatellite markers. Primer sequences were
obtained from the Genome Database through their on-line server
(http//:www.gdb.org). Chromosomal locations and types of repeats of
each microsatellite marker for polymerase chain reaction are listed in
Table 1
. Using AmpliTaq Gold polymerase
(Perkin Elmer, Foster City, CA), polymerase chain reaction
amplification was performed for 45 cycles in a solution of 10 mmol/L
Tris-HCl (pH 8.3), 50 mmol/L KCl, 1.5 mmol/L
MgCl2, 0.2 mmol/L dNTP, and 0.5 µmol/L each of
primers in the presence of [32P]-dCTP.
Polymerase chain reaction products were electrophoretically separated
on 6% denaturing polyacrylamide gels and visualized by exposure to
X-ray films. Microsatellite analysis detects allelic imbalances
including both allelic gains and losses. Karyotypic8,27
and comparative genomic hybridization analyses,28
however, have indicated the frequent occurrence of allelic losses, but
not gains in the regions examined here. This suggests that the observed
allelic imbalance was likely to have resulted from allelic losses
rather than gains.
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| Results |
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We first reviewed a consecutive panel of adenocarcinomas in our
department, and selected cases representative of the following two
types: five adenocarcinoma cases showing heterogeneous morphology
within each tumor mass and three with a relatively homogenous
morphology. Two cases with squamous cell carcinoma, which were
accompanied with adjacent hyperplastic and/or dysplastic regions, were
also included. Detailed histological features of each lesion are
summarized in Table 2
. In each case, the
selection of three portions for analysis was done as follows. In the
five cases with morphologically heterogeneous adenocarcinomas, three
morphologically distinct portions were selected such that they
represent a range from the lowest-grade morphology (designated
hereafter as A) to the most malignant, invasive regions, designated C.
Portions with intermediate-grade morphology, designated B, were also
examined, which mostly showed in situ carcinoma appearances.
In the three cases with morphologically homogeneous
adenocarcinomas, three regions of each tumor were microdissected from
peripheral (A) and central (C) portions as well as from an area
in-between (B), and examined individually. In the cases of squamous
cell carcinoma, portions designated as A were from hyperplastic
bronchial epithelium, B from either high-grade dysplasia or
carcinoma in situ, and C from the most invasive portions,
all of which were contiguous in each tumor. Representative figures are
shown in Figures 1, 2, and 3
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LOH at the TP53 locus in the morphologically most advanced regions in each tumor were present in six of 10 cases, whereas eight carried LOH at 17p13.3. Notably, LOH at 17p13.3 was detected in the absence of LOH at TP53 (cases 4 and 5), confirming our previous study on a different set of lung cancer patients.24 When LOH was present in adenocarcinomas, topographical differences in LOH among the three-microdissected lesions were not seen, and the allelic pattern of deletions was concordant in each lesion.
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We then examined an additional four loci including 2q, 9p, 18q, and 22q, which have been suggested to be related to the progression of lung cancers. LOH at 9p21 was observed in all of the eight informative cases. It was notable that the presence of LOH in cases 1, 5, and 9 was restricted to the portions with higher-grade lesions, suggesting the acquisition of genetic heterogeneity during progression. Although case 7 exhibited homogeneous morphology throughout the tumor, LOH of 9p was detected only in the central (C) and intermediate (B) portions, but not in the peripheral region. Notably, case 7 had low proliferative activity in all of the portions examined, which might have allowed genetically heterogeneous clonal expansion in this morphologically homogenous tumor. LOH at 2q33 was present in all three portions in case 2, whereas it was identified solely in the invasive carcinoma portion of case 9 and in the carcinoma in situ lesion of case 10. LOH at 18q21 and 22q13 was detected in the invasive carcinoma portion of a single case (case 1), although this case also had LOH at 18q21 in all three portions examined.
| Discussion |
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Using multiple microdissected specimens representative of morphological progression within individual tumors, we have shown that LOH at 3p and 17p are present at high frequency even in the lowest-grade portions and that identical alleles were found to be deleted in all portions examined in each affected tumor, even though they are morphologically quite divergent. These results suggest that LOH at 3p and 17p may be crucial steps in the early phase of tumorigenesis and that they are retained throughout successive clonal evolutions. This is supported by the evidence that in the cases without LOH at 3p and/or 17p, all of the portions carried retained alleles, and the LOH was not acquired with the progression to higher-grade morphology and invasive properties. Another important point of this study is that LOH at 17p13.3 was detected even in the absence of 17p13.1, where the p53 gene resides, showing clonal stability throughout morphological progression within individual tumors. The present findings thus support our previous inference that a putative tumor suppressor gene at 17p13.3 may play a role in the early phase of lung carcinogenesis.24
We observed LOH on at least one of the three loci on 3p and/or the two loci on 17p in every tumor examined. However, it is uncertain at present why these presumably early lesions are not present in all tumors. It is possible that LOH at 3p and/or 17p is detected at higher frequency with the aid of additional markers closer to the putative target genes. A number of cell types are thought to be potential precursors of lung cancers and distinct initiation events may be involved depending on which differentiation pathway they are committed. Therefore, it may also be probable that certain cell types do not require inactivation of all of the putative tumor suppressor genes on 3p and 17p.
A similar approach was previously used by Boland et al30 for the analysis of tumor progression schemes in colon cancers. They used multiple microdissected samples from colonic tumors showing cross sections of "adenoma-carcinoma sequence." At one time point a clear and abrupt occurrence of LOH at 5q at the transition phase from normal epithelium to adenoma was revealed. Therefore, it is surmised that LOH at 3p and 17p in the neoplastic progression of lung cancers might have a role as early events comparable to that of 5q in colon cancers.
It should be noted that marked distinctions between LOH at 3p and 17p, and those on other loci such as 9p deletions and the latter, were often present in restricted portions within individual tumors. Shiseki et al13 reported an association between LOH at 9p and the advanced disease stage, whereas Kishimoto et al25 described this as one of the earliest events during tumor progression through analysis of preneoplastic lesions independent from main tumors. In the present study, the highest-grade, most malignant portions were shown to carry 9p deletions at even higher frequency than 3p or 17p deletions. However, it should be noted that the present findings also suggest that the accumulation of LOH at 9p can occur at various stages of tumor progression within individual tumors, indicating a clear distinction with LOH at 3p and 17p. Although a few of the tumors thus far examined showed genetic homogeneity despite great morphological divergence, genetic alterations might be present on loci other than those examined in the present study.
In summary, we examined the topographical distribution of LOH at 3p and17p as well as other loci including 2q, 9p, 18q, and 22q. The present findings suggest early occurrence of 3p and/or 17p deletions and successive clonal expansion during the progression of individual tumors. In contrast, LOH at other loci, such as 9p, seemed to be acquired at various stages during tumor progression. Similar future studies analyzing more genetic loci with a larger number of cases are warranted. Furthermore, as lung cancers result from various genetic and epigenetic alterations, it should also be interesting to examine the topographical differences of other changes such as DNA methylation.
| Footnotes |
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Supported by a Grant-in-Aid for the Second Term Comprehensive Ten-Year Strategy for Cancer Control and a Grant-in-Aid for Cancer Research from the Ministry of Health and Welfare, Japan.
Accepted for publication June 6, 2000.
| References |
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