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From the Institute of Pathology*
and Urologic
Clinics,
University of Basel,
Basel, Switzerland
| Abstract |
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| Introduction |
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It appears likely that progression from minimally invasive to deeply invasive cancer goes along with the acquisition of genomic alterations increasing the malignant potential of cancer cells. Previous studies investigating specific alterations have suggested that at least a few molecular changes are more frequent in pT24 than in pT1 carcinomas, including epidermal growth factor receptor (EGFR) overexpression and loss of heterozygosity (LOH) at 3p.1,2 A systematic genome-wide screening for differences between the stages pT1 and pT24 has not been performed. Previous studies attempting a comprehensive analysis of the entire genome by either allelotype studies or comparative genomic hybridization (CGH) have focused on early-stage tumors3,4 or analyzed only a small number of tumors.5,6 In this study, we analyzed a series of 90 invasive urinary bladder carcinomas by CGH to search for genomic alterations that are associated with tumor progression. CGH allows detection of all relative DNA sequence copy number gains and losses of a tumor in one examination.7 The results implicate several genomic alterations that may play a role in bladder cancer progression.
| Materials and Methods |
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Formalin-fixed, paraffin-embedded primary bladder tumors were from the archives of the Institute of Pathology, University Hospital Basel, Basel, Switzerland. Thirty-seven papillary pT1, twenty-four papillary pT24, and twenty-nine solid pT24 carcinomas were randomly selected from a series that had been reviewed by a single pathologist (G. Sauter). The CGH findings of 27 of our pT1 carcinomas have previously been published.4 Tumor stage and grade were defined according to Union Internationale Contre le Cancer (UICC) and World Health Organization classifications.8,9 Only tumors for which histological staging was unequivocal were included in this study. A papillary tumor growth was assumed if at least one unequivocal papilla with similar atypia as in the invasive tumor area was present. The histological tumor grade was grade 2 in 15 and grade 3 in 22 pT1 tumors and grade 2 in 14 and grade 3 in 39 pT24 carcinomas. Of the 53 muscle-invasive tumors, 24 had a papillary and 29 had a solid growth pattern. Sixty-six patients were male, and twenty-four were female. Tumor-specific survival data were available from 67 patients. Tumor-specific 5-year survival was markedly shorter in 30 patients with pT24 carcinomas (50%) than in 37 patients with pT1 carcinomas (89%, P < 0.0001).
DNA Preparation
All tumor blocks were trimmed to enrich for tumor. Twenty 10-µm-thick sections were taken for DNA extraction. The first and the last sections were stained with hematoxylin and eosin. Tumors having an average tumor cell content of less than 75% in these sections were excluded. DNA extraction and labeling was as described.4 One microgram of tumor DNA was nick translated by using a commercial kit (BioNick kit, Life Technologies, Gaithersburg, MD) and Spectrum Green-dUTPs (Vysis, Downers Grove, IL) for direct labeling of tumor DNA. Spectrum Red-labeled normal reference DNA (Vysis) was used for cohybridization.
CGH and Digital Image Analysis
The hybridization mixture consisted of 200 ng of Spectrum Green-labeled tumor DNA, 200 ng of Spectrum Red-labeled normal reference DNA, and 20 µg of Cot-1 DNA (GIBCO, Gaithersburg, MD) dissolved in 10 µl of hybridization buffer (50% formamide, 10% dextran sulfate, 2X SSC, pH 7.0). Hybridization was over 3 days at 37°C to normal metaphase spreads (Vysis). Posthybridization washes were as described.4 Digital images were collected from six to seven metaphases using a Photometrics cooled CCD camera (Microimager 1400, Xillix Technologies, Vancouver, British Columbia, Canada) and a Sun workstation. The VYSIS software program was used to calculate average green-to-red ratio profiles for each chromosome. At least four observations per autosome and two observations per sex chromosome were included in each analysis.
Controls and Threshold Definition
Each CGH experiment included a tumor cell line (Spectrum Green
MPE-600, Vysis) with known aberrations (positive control) and a
hybridization of two differentially labeled sex-mismatched normal DNAs
to each other (negative control). Sex-mismatched normal controls were
also used to test the ability of each metaphase batch to allow for a
linear relationship between fluorescence intensities and DNA sequence
copy numbers. Metaphases were used only if the color ratio of
sex-mismatched normal DNAs was
0.66 at the X chromosome. Thresholds
used for definition of DNA sequence copy number gains and losses were
based on the results of CGH analyses of formalin-fixed normal tissues.
A gain of DNA sequences was assumed at chromosomal regions where the
hybridization resulted in a tumor-to-normal ratio of
1.20.
Overrepresentations were considered amplifications when the
fluorescence ratio values exceeded 1.5 in a subregion of a chromosome
arm. A loss of DNA sequences was presumed at chromosomal regions where
the tumor-to-normal ratio was
0.80. To define an aberration it was
additionally required that the first SD was above (gain) or below
(deletion) 1.00. As some false aberrations were detected in normal
tissues at 1p, 16p, 19, and 22, these G-C-rich regions, known to
produce false positive results by CGH, were excluded from all analyses.
Statistics
Contingency table analysis was used to analyze the relationship between genomic alterations, grade, stage, and growth pattern. A Student t-test was applied to compare the number of genomic alterations between tumors of different grades and stages. A log rank test was applied to examine the relationship between tumor stage and tumor-specific survival.
| Results |
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The total number of aberrations per tumor ranged from 0 to 24
(mean, 7.5). Deletions (3.8 ± 2.7) were slightly more frequent
than gains of chromosomal material (3.4 ± 2.8). Eight tumors had
no detectable aberrations. One pT1G2 carcinoma had a 9p deletion as a
sole aberration. The CGH findings of all 90 tumors examined in this
study are shown in Figure 1, A
(37
papillary pT1 carcinomas), B (24 papillary pT24 carcinomas), C (29
solid pT24 carcinomas). Overall, deletions were most prevalent at
2q33-ter (17%), 4q2531 (10%), 5q (18%, consensus regions 5q1213
and 5q2223), 6q2124 (19%), 8p (29%), 9p (31%), 9q2133 (23%),
10q2526 (17%), 11p1415 (24%), 11q2325 (22%), 13q (11%, 13q21
and 13q3134), 17p (19%), 18q21-ter (12%), Xp (11%), Xq24-ter
(8%), and Y (22%). A gain of chromosomal material was most commonly
seen at 1q (37%, consensus regions 1q2224 and 1q32-ter), 3p2324
(13%), 5p1415 (24%), 6p2122 (16%), 7p1522 (20%), 8q (37%,
8q21.322 and 8q24), 10p1315 (19%), 12q (14%, 12q1521.2 and
12q24), 17q (29%, 17q21 and 17q25), 20p (9%), and 20q13 (28%).
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Interestingly, there were strong associations between some individual
alterations. For example, gains of 11q13 were associated with 9p
deletions. 9p deletions were seen in 6 of 8 tumors (75%) with an 11q13
gain but in only 22 of 82 tumors without 11q gain (27%,
P = 0.005). Gains of 20q were linked to gains of both
17q and 11q13 (Figure 2)
. An 11q13 gain
was observed in 6 of 25 tumors (24%) with a 20q gain but in only 2 of
65 tumors without a 20q gain (3%, P = 0.0018). 17q
gains were present in 13 of 25 tumors (52%) with a 20q gain but in
only 12 of 65 tumors without 20q gain (18%, P =
0.0015). An additional gain of 11q13 or 17q was present in 68% of all
tumors with a 20q gain, as 11q13 gains and 17q gains coincided rarely
(only two tumors had both alterations). Similarly, 1q gains
were associated with 3p gains and 11q deletions (Figure 2)
. An 11q
deletion was present in 15 of 33 tumors (45%) with 1q gain
as compared with only 5 of 57 tumors without 1q gain (9%,
P = 0.0001). And 9 of 33 tumors (27%) with 1q gain had
a 3p gain as compared with only 3 of 57 tumors without 1q gain (5%,
P = 0.0031). There was no association between 3p gains
and 11q deletions (P = 0.32). Sixty-one percent
of all tumors with a 1q gain had an additional 11q deletion or a 3p
gain.
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There was no significant increase in the number of detectable
aberrations with stage and grade of the tumors. pT1 tumors had 3.0
± 2.3 deletions, 2.7 ± 2.8 gains, and 0.4 ± 0.8
amplifications per tumor resulting in a total of 6.1 ± 4.9
alterations. This was only insignificantly less than found in pT24
tumors (3.8 ± 2.9 deletions, 3.8 ± 2.7 gains, 0.4 ±
0.7 amplifications, and 7.9 ± 4.9 total aberrations). Grade 2
carcinomas had 3.0 ± 2.6 deletions, 2.6 ± 2.7 gains,
0.3 ± 0.8 amplifications, and 5.8 ± 5.0 alterations in
total. These numbers were somewhat higher in grade 3 carcinomas
(3.7 ± 2.7 deletions, 3.7 ± 2.7 gains, 0.4 ± 0.7
amplifications, and 7.8 ± 4.8 total aberrations), but the
difference between grade 2 and grade 3 did not achieve significance. To
screen for specific alterations that might be linked to tumor
progression, we compared the CGH findings of 37 papillary pT1 and 24
papillary pT24 carcinomas (Table 1)
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This analysis showed that deletions of 5q, 6q, and 15q as well as gains
of 5p, 7p, and Xq were significantly linked to high stage. These
aberrations were also common in solid pT24 carcinomas. The
relationship between tumor grade and specific alterations was clearly
less prominent. Only gains of 6p were significantly more frequent in
grade 3 than in grade 2 tumors (P = 0.02). 9q
deletions and 11q13 gains were the only alterations showing significant
differences in frequency between papillary and solid muscle-invasive
carcinomas (Table 1)
. Deletions of 5q (P <
0.0038) and 10p gains (P < 0.0353) were linked
to poor outcome within these 67 patients with tumor-specific survival
data. 1q+, 5p+, 6q-, 7p+, 8p-, 8q+, 9p-, 9q-, 11p-, 11q-, 15q-,
17q+, 20q+, and Xq were not significantly associated with the
tumor-specific survival of our patients.
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| Discussion |
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The statistical analyses for comparison of the CGH findings in carcinomas having limited (pT1) and extensive (pT24) invasion was restricted to papillary carcinomas because previous studies had suggested genetic differences between solid and papillary tumors.14 Several alterations were significantly more frequent in pT24 than in pT1 carcinomas, including deletions of 5q, 6q, and 15q as well as gains of 5p, 7p, and Xq. Most of these changes have only rarely been reported in bladder carcinomas, probably because previous studies have either examined specific loci not including these regions or focused on early-stage tumors.3-5 It is tempting to speculate that these loci might carry genes of which a malfunction can contribute to bladder cancer progression. It cannot be excluded that one or several of these loci carry genes with relevance for invasive tumor growth, as alterations that are necessary for invasion may be present only in subpopulations of pT1 carcinomas, not detectable by CGH, but in a much higher fraction of the tumor cells if a cancer is predominantly growing invasively (stage pT24). Deletions of 5q and gains of 10p were significantly related to poor outcome in our patients. However, these alterations were strongly associated with advanced stage, and prognosis was markedly different between pT1 and pT24 tumors. Additional studies on larger sets of patients are required to evaluate whether cytogenetic changes are linked to the risk of progression in pT1 and to tumor-specific survival and metastasis in pT24 carcinomas.
To screen for genetic differences between tumors with different growth patterns, we compared the CGH results of 24 papillary and 29 solid muscle-invasive carcinomas. Deletions of 9q and gains of 11q13 were the only two alterations showing significant differences between papillary and solid tumors. The higher frequency of 9q deletions in papillary than in solid carcinomas is in line with previous studies suggesting that chromosome 9 deletions may be characteristic for papillary tumors.14,15 As 9p deletions were not significantly linked to papillary growth pattern, our data would be consistent with the location of a gene linked to papillary tumor growth on 9q rather than on 9p. This is in agreement with the findings of Simoneau et al16 showing that 9q deletions were more frequent than 9p deletions in papillary pTa tumors. Interestingly, gains of 11q13 were also associated with a papillary growth pattern. A role for cyclin D1 (located at 11q13) overexpression has recently been described in low-grade and low-stage bladder tumors.17,18 In a previous study we analyzed 393 bladder neoplasms for cyclin D1 expression and found that papillary tumor growth but not low grade or low stage was independently linked to cyclin D1 expression.19
The high number of tumors examined in this study enabled us to search for alterations that frequently occur together. Loci that are frequently affected together might carry genes that interact in specific molecular pathways. The significant association between 11q13 gains and 9p deletions is an example of a combination of alterations that might involve genes with known interaction. Although expression analyses have not been performed in this study, it is tempting to speculate that 11q13 amplification, most likely leading to cyclin D1 overexpression,17 may give tumor cells a particularly significant growth advantage if the cyclin D1 inhibitors p16/p15 on 9p are also affected. Even more striking associations could be found for two other groups of alterations: 20q gains were strongly linked to both gains of 17q and 11q13, and 1q gains were associated with both 3p gains and 11q deletions. Although there are currently no clues on the potentially involved genes, it appears possible that genes at these loci could interact with each other. It could be speculated that, for example, overexpression of a gene on 1q will exert a particularly strong oncogenic role if either a tumor suppressor gene on 11q is inactivated or another gene on 3p is co-overexpressed.
It is assumed that circumscribed high-level amplifications are linked to the overexpression of oncogenes. Previous studies using Southern blotting, fluorescence in situ hybridization, conventional cytogenetics, or CGH have described 21 regions of the genome that can be amplified in urinary bladder cancer including the sites of the known oncogenes erbB-2, EGFR, cyclin D1, and c-myc as well as 1q2131, 2q13, 3p2224, 6p22, 8p11, 8q21, 9p21, 10p1314, 12q1315, 13q13, 13q3133, 18p11, 20q, 21p11, 22q1113, Xp1113, and Xq2122.2.4-6,20-24 In this study, 30 high-level amplifications were found at 14 different loci, including 13 regions where amplifications had previously been described in bladder cancer. The only amplification site that was newly detected in this study was located at 5p1415. Although gains involving the entire 5p occur frequently in bladder cancer, this finding is consistent with a target gene in the 5p1415 region. The shortage of new amplification sites found in this set of advanced tumors suggests that the list of genomic regions that can be amplified in urothelial neoplasms is more or less complete. Additional investigations will now have to identify overexpressed target genes at these sites.
Taken together, the results of this and other studies suggest that
several groups of chromosomal alterations may be associated with
specific steps along bladder cancer progression (Figure 3)
. Deletions of 9p, 1q gains, and Y
losses are frequent in bladder tumors of all grades and stages and may
therefore occur early in bladder cancer development.4,25,26
The significantly higher frequency of 9q deletions and 11q13 (cyclin
D1) gains in papillary than in solid tumors is consistent with a role
of genes at these loci for papillary tumor growth. Cytogenetic
alterations such as 2q-, 8p-, 8q+, 9p-, 11p-, 11q-, 17q+, and 20q+
can be found at a comparable frequency in papillary and solid invasive
carcinomas irrespective of the invasion level. Several of these
alterations are also frequent in carcinoma in
situ,27,28
suggesting a considerable similarity
between noninvasive and invasive high-grade neoplasias on the level of
gross genomic alterations. These cytogenetic changes as well as 17p
deletions, which are difficult to detect by CGH because of the small
size of 17p, and p53 mutations may be characteristic for a genetically
unstable bladder neoplasm that is distinct from low-grade pTa tumors.
Cytogenetic changes that are more frequent in pT24 than in pT1
carcinomas, such as 5p+, 5q-, 6q-, 7p+, 15q-, and Xq+, may carry
genes that play a role for tumor progression. Additional studies are
now required to evaluate which are the target genes of cytogenetic
lesions and to translate our knowledge on the genetic background of
bladder cancer into clinical applications.
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| Acknowledgements |
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| Footnotes |
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Supported by Schweizerische Krebsliga (SKL 137-7-1995), Krebsliga Beider Basel, Schweizerischer Nationalfonds (NF 3200043969.95.1), and Stiftung für Klinische Krebsforschung der Onkologischen Abteilung des Kantonsspitals Basel.
Accepted for publication August 3, 1998.
| References |
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