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Short Communications |









From the Departments of Pathology*
and
Surgery,
Erasmus University and University
Hospital Rotterdam, Rotterdam, The Netherlands; and the Department
of Pathology,
University of Zürich,
Zürich, Switzerland
| Abstract |
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| Introduction |
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To date no reliable clinical or histopathological markers are available to distinguish benign from malignant PCCs. Metastases, occurring in approximately 10% of the tumors, are the only convincing sign of malignancy. They may already be present at the time of diagnosis or occur only after lag phases as long as 25 to 30 years.1 Such a situation calls for more accurate markers that can predict the clinical behavior of these tumors.
The genetic mechanisms underlying the tumorigenesis of PCCs are poorly understood. Because of the known association with the inherited tumor syndromes multiple endocrine neoplasia type 2 (MEN2), Von Hippel-Lindau disease (VHL) and neurofibromatosis type 1 (NF1),2-4 PCCs have been investigated for involvement of the responsible RET, VHL, and NF1 genes, respectively.5-9 However, only a subset of sporadic lesions harbors somatic mutations in these genes, suggesting that they do not play a prominent role in PCC tumorigenesis.10-13 Furthermore, no mutation has been detected in the c-mos proto-oncogene or in the p16 tumor suppressor gene (TSG) in benign and malignant lesions.14,15 Conflicting results exist with regard to the frequency of TP53 gene alterations in PCCs as well as for the association of c-erbB-2 expression with malignancy.16-19 Other oncogenes that have been found to be overexpressed in PCCs, include c-myc, c-fos, and Bcl-2.19,20
Several loss of heterozygosity (LOH) studies provided evidence for involvement of TSGs on chromosomes 1p, 3p, 11p, 17p, and 22q.21-25 However, the impact of these data are limited, since small series of sporadic PCCs were analyzed and often no correlation with clinical data was presented. Moreover, the vast majority of the genome remains unexamined. We performed comparative genomic hybridization (CGH) analysis on 29 apparently sporadic PCCs, including 19 benign and 10 malignant lesions, to identify chromosome alterations which may be important in tumor development and behavior and might have diagnostic utility. Additionally, fluorescence in situ hybridization (FISH) experiments were carried out to independently confirm some of the CGH data.
| Materials and Methods |
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Altogether, tumors of 29 patients with an apparently sporadic
adrenal (n = 25) or extra-adrenal
(n = 4) PCC were analyzed, including 19
clinically benign and 10 proven malignant lesions (with metastases). A
pheochromocytoma was considered sporadic if there was no family history
of MEN2, VHL, or its constituent tumors and there were no other signs
of MEN 2, VHL, or NF1 recorded by the clinician. Furthermore, previous
analyses did not reveal germline mutations in the RET and
VHL gene in these tumors10,11
(Dannenberg et al,
unpublished data). The average age of the patients was 50.3 years
(range: 2379) and the mean size of the tumors was 7.1 ± 4.6 cm
for benign lesions and 11.0 ± 4.7 cm for malignant lesions. Nine
of 19 benign and 7 of 10 malignant tumors were from female patients.
Except for two lymph node metastases, all specimens were primary
tumors. Clinical follow-up was obtained in all benign cases with
an average follow-up period of 7.6 years. Relevant characteristics of
all patients are listed in Table 1
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Genomic DNA from 16 frozen tumors was isolated using the D-5000 Puregene DNA isolation kit (Gentra Systems, Minneapolis, MN). DNA extraction from 13 formalin-fixed, paraffin-embedded samples was performed by standard detergent-proteinase K lysis, followed by phenol/chloroform extraction and ethanol precipitation, as described elsewhere.26
CGH and Digital Image Analysis
CGH was carried out as previously described.26 One µg of high molecular weight tumor DNA was labeled with Spectrum Green-dUTPs (Vysis, Downers Grove, IL) by nick translation (BioNick kit, Life Technologies, Basel, Switzerland). Archival tumor DNA with fragment sizes of <1 kb was labeled with a platinum/D-Green complex (D-Green-ULS) using the Kreatech Universal Linkage System (ULS) D-Green labeling kit (kindly provided by Kreatech Morwell Diagnostics GmbH, Zürich, Switzerland), according to the manufacturers recommendations. Briefly, 800 ng of tumor DNA and 1.6 µl of D-Green-ULS, adjusted to a total volume of 20 µl, was incubated at 65°C for 20 minutes. To reduce background signals, the ULS-labeled samples were purified using Qiagen nucleotide removal columns (Qiagen GmbH, Hilden, Germany).
The hybridization mixture consisted of 200 ng of Spectrum (or D-) Green-labeled tumor DNA, 200 ng of Spectrum Red-labeled sex matched normal reference DNA (Vysis), and 10 to 20 µg of human Cot-1 DNA (Life Technologies) dissolved in 10 µl of hybridization buffer (50% formamide, 10% dextran sulfate, 2X SSC, pH 7.0). Hybridization to normal metaphase spreads (Vysis) took place for 3 days at 37°C.
Digital images were collected from six to seven metaphases using a Photometrics cooled CCD camera (Microimager 1400; Xillix Technologies, Vancouver, Canada). The QUIPS software program (Vysis) was used to calculate average green-to-red ratio profiles of at least four observations per autosome and two observations per sex chromosome in each analysis. Gains and losses of DNA sequences were defined as chromosome regions where the mean green-to-red fluorescence ratio was above 1.20 and below 0.80, respectively. Over-representations were considered amplifications when the fluorescence ratio values in a subregion of a chromosome arm exceeded 1.5. Because of some false positive results at chromosomes 1p32-pter, 16p, 19, and 22 found in normal tissues, gains of these known G-C-rich regions were excluded from all analyses.
FISH Analysis
To validate CGH data independently, touch preparations of four
frozen PCCs were subjected to fluorescence in situ
hybridization (FISH) analysis using a chromosome 1-specific centromere
probe (pUC1.77) in combination with an 1p2231-specific YAC probe
(929-E-1) (kindly provided by S. Franke, Center for Human Genetics,
University of Leuven, Belgium and J.R. Testa, Fox Chase Cancer Center,
Philadelphia, PA), or a repeat probe (p179), mapping to the 1p36
region.27-29
Chromosomes 3 and 6 were examined using a
chromosome 3 (p
3.5) and 6 (p308) centromere probe, in combination
with a 3q26-specific (pancpin) and 6q22-specific
(cCI644) probe, respectively.30-33
Cell processing,
probe labeling, in situ hybridization, and detection of the
hybridized signals were performed as recently described.34
For each tumor, hybridization signals of 100 interphase nuclei were
scored. Numerical chromosomal aberrations were considered an aneusomy
when the percentage of nuclei showing an abnormal number of centromere
signals exceeded 30%. The presence of only half of the locus specific
signals compared to the number of centromere signals in more than 30%
of the nuclei was interpreted as a deletion.
Statistical Analysis
To calculate the statistical significance of differences in genomic changes between benign and malignant tumors, we used contingency table analysis, which was also used to analyze the relationship between genomic alterations and sex. Regression analysis was carried out to compare the number of chromosomal aberrations with tumor size.
| Results |
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Figure 1A
summarizes all DNA copy
number changes identified in the 29 evaluated sporadic PCCs. Genetic
alterations were observed in all PCCs, and the average number of
chromosome arm aberrations per tumor was 6.3 ± 4.4 (range,
121). Chromosomal losses (mean, 4.3; range, 112) were more frequent
than gains (mean, 1.9; range, 013) and no amplifications could be
detected (Table 1)
. The average number of chromosomal alterations was
marginally associated with malignancy (5.3 ± 2.7
versus 8.2 ± 6.1), principally due to a higher
frequency of gains. The most common chromosome arm copy number changes
included losses of chromosomes 1p11-p32 (86%), 3q (52%), 6q (34%),
3p, 17p (31% each), and 11q (28%), and gains of chromosome 9q (38%)
and 17q (31%). Representative examples of CGH results are shown in
Figure 1B
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Ten of 29 PCCs fell into the category of malignant tumors, since regional or distant metastases occurred in these patients. In these lesions, losses of chromosome 6q were more often observed as compared to clinically benign lesions (60% versus 21%, P = 0.0368). To a lesser degree, losses of 17p were also related to metastatic disease (50% versus 21%), whereas deletions of chromosome 3p and 3q were preferably associated with benign tumors (42% versus 10% and 63% versus 30%, respectively). Although the malignant tumors were significantly larger in size (11.0 ± 4.7 cm versus 7.1 ± 4.6 cm, P = 0.0470) and demonstrated a slightly higher number of chromosome arm copy number changes, there was no correlation between the number of genomic aberrations and the tumor size. No differences were seen between adrenal and extra-adrenal PCCs.
Additionally, a statistically significant correlation was found between the number of chromosomal alterations and female gender (P = 0.0196), predominantly due to a higher frequency of gains of different genomic regions.
Comparison of CGH and FISH
FISH analysis confirmed the CGH results of chromosomes 1p, 3q, and
6q in 4 tumors that showed chromosomal imbalances as detected by CGH
(Table 1)
. Two of the 4 tumors presented with a partial loss of
chromosome 1p (1p1132). In one case, FISH analysis proved the loss of
this region and the retention of 1p36. However, the other tumor
appeared to have a complete loss of chromosome 1p, which could be
expected, since the 1p32-pter region is known to generate some false
positive results in CGH.
In addition, the three benign lesions revealed a tetrasomy in the major
population of cells for all chromosomes analyzed. In contrast, the
malignant tumor turned out to be diploid. In two of the three aneuploid
lesions a subpopulation of diploid tumor cells (as demonstrated by two
centromere signals and only one locus-specific signal per nucleus)
could also be detected. Examples of FISH results are shown in Figure 1C
.
| Discussion |
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The most commonly encountered chromosomal aberrations in our series of 29 sporadic PCCs involved 1p, 3q, 9q, 6q, 3p, 17p, 17q, and 11q. These findings support and extend the hypothesis of Koshla et al22 that multiple genes are involved in the pathogenesis of these tumors. Although LOH studies have shown loss of heterozygosity at 1p, 3p, 11q, 17p, and 22q in syndrome-related and sporadic PCCs,21-25 the here found losses of 3q and 6q and gains of 9q and 17q have not been reported previously. In contrast to a previous LOH analysis,25 we observed only one tumor with loss of 22q. Possible explanations for this discrepancy are the limited sensitivity of CGH for small losses (<10 Mb) and the fact that G-C-rich chromosome regions, like those in chromosome 22, are known to yield false positive CGH results.
We and others21-24 found a very high incidence of 1p loss in PCCs, suggesting that it might be an important tumorigenic event. Deletions of chromosome 1p are common in several other human neoplasms, including adrenocortical carcinomas and neurectodermally derived tumors, such as neuroblastomas.35,36 The smallest common region of overlap as detected by CGH in our study involves the 1p2232 region. Because of known false positive CGH results at 1p32-pter, we verified the CGH data by FISH using a probe mapping to 1p36. Our observations indicate that some PCCs indeed exhibit a partial loss of chromosome 1p. This correlates well with the results of others,23,37 who found evidence for a potential PCC tumor suppressor locus in the middle of the short arm of chromosome 1. There are multiple candidate TSGs in this region, including TGFß-R3, p18INK4C, and PTPRF, none of which has been proven to be relevant in PCC tumorigenesis. One should also consider the possibility that more than a single TSG on 1p contributes to the development of PCCs.
Another interesting finding of our study is the frequent loss of chromosome 3q in PCCs, which is more often encountered in benign than in malignant tumors. This might point to diverging pathways in the development of benign and malignant PCCs, although it is known that in malignant tumors duplications of chromosome arms can occur.38 In that case, LOH will not be detected by CGH. However, it is certainly conceivable that loss of chromosome 3q is an early genetic event in the pathogenesis of PCCs. Thus far, frequent allelic loss of chromosome 3q has been demonstrated only in osteosarcoma, with a putative TSG localized to a region between 3q26.2 and 3q26.3.39 The possible relationship of this candidate TSG for PCC development however, is not clear.
Data on the genetic events that determine the malignant potential of
PCCs are scarce. We observed a strong association of losses of
chromosome 6q with advanced disease stage. Frequent allelic imbalances
at 6q have also been reported in other malignancies40,41
and appear to be related to poor prognosis or metastatic disease in
certain tumors.42,43
Although these data are suggestive of
the existence of several TSGs at 6q, none has been identified so far.
In addition, underrepresentations of chromosome 17p were often observed
in metastasizing lesions. 17p13 contains the TP53 gene, the
TSG most commonly involved in human carcinogenesis. Fourteen of 29 PCCs
evaluated in the present study have previously been
immunohistochemically investigated for TP53
accumulation.20
Comparison of these results with our CGH
results (Table 1)
, indicates that TP53 alterations may play
a role in a subset of these tumors. Involvement of another TSG on 17p,
however, cannot be ruled out and one candidate gene could be the
putative TSG at the medulloblastoma locus 17p13.3.44
Interestingly, three malignant PCCs exhibited simultaneous gains of
chromosomes 5p, 7p and 12q, similar to findings for malignant endocrine
pancreatic tumors.45
In conclusion, our results indicate that the development of PCCs is associated with specific genetic alterations, of which deletions of chromosome 1p (1p2232) and 3q are early genetic events. Losses involving chromosome 3q (minimal overlapping region, 3q21-qter) and 6q (6q22-qter) and over-representations of chromosome 9q (9q3334) and 17q point to new regions that may contain PCC TSGs and oncogenes in addition to the previously reported TSG loci 1p, 3p, 11q, 17p, and 22q.
Further studies are required to narrow down the critical regions in each identified chromosome and to characterize the putative genes involved in tumor initiation and progression of sporadic PCCs.
| Acknowledgements |
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| Footnotes |
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Supported by the Dutch Cancer Society (Koningin Wilhelmina Fonds), the Jan Dekker and Dr. Ludgardina Bouwman Foundation, and the De Drie Lichten Foundation.
Accepted for publication May 10, 2000.
| References |
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