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From the Departments of Pathology*
and
Surgery,
Università di Verona,
Verona, Italy
| Abstract |
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| Introduction |
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There is almost no information available regarding the chromosomal and molecular alterations giving rise to this disease. Mutations in codon 12 of the K-ras gene have not been found in a total of 11 cases of SCAs analyzed.3-5 Likewise, no tumor was found to stain for p53 protein by immunohistochemistry in seven cases4 and no mutations in p53 were detected in three cases analyzed.5 On the assumption that the sporadic and VHL-associated cases of SCA may involve the same molecular targets, another study analyzed 10 sporadic tumors for allelic loss at the VHL locus and for alterations of the VHL gene; six cases (60%) were found to have loss of heterozygosity (LOH) on chromosome 3p and one case had a somatic silent mutation in the VHL gene.6
The multistep process of carcinogenesis involves the progressive alteration of oncogenes and tumor suppressor genes. As the inactivation of these latter commonly occurs by mutation and/or deletion, the chromosomal regions harboring potential tumor suppressor genes can be identified by LOH analysis using polymerase chain reaction (PCR) amplification of polymorphic microsatellite repeats in tumor and matched-normal DNA. When analysis of LOH is extended to multiple chromosomal arms, a distinct allelotype is generated.
Here we present the allelotype of SCA obtained by genome-wide allelic loss analysis of 21 cases each analyzed with 79 microsatellite markers. This analysis is complemented with by the assessment of microsatellite instability and the mutational analysis of the VHL, K-ras, and p53 genes.
| Materials and Methods |
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Twenty-one serous cystic adenomas of the pancreas were studied from a total of 72 formalin-fixed paraffin-embedded cases available from the files of the Department of Pathology at the University of Verona. The cases subjected to molecular analysis were selected according to strict criteria consisting in an exclusive microcystic histological subtype, which would permit their microdissection to a high cellularity. Thus, all macrocystic lesions, the microcystic variant with low cellularity in which the epithelial component is only a small proportion of unit volume having a relatively high stromal cell content into the intracystic septae, and those tumors with extensive epithelial denudation of the cysts and/or marked stromal cell component were excluded form the study. All of the 21 SCAs under study were well circumscribed, had a central scar, and had cysts <1 cm in diameter. The median patient age was 61 years (range, 49 to 72 years); 19 tumors were from female patients. The average tumor diameter was 5.6 cm (range, 2.5 to 10 cm). Five tumors originated from the head, nine from the body, five from the body-tail, and two from the tail. As no case was associated with VHL syndrome, all tumors were considered sporadic. Normal and tumor DNA were obtained by microdissection in all 21 cases from paraffin-embedded sections as described.7 Based on microscopic assessment, neoplastic cellularity of at least 90% was obtained in all cases. The presence of p53 protein nuclear accumulation was examined using anti-p53 monoclonal antibodies as previously described.8 Nine tumors also had frozen tissue available and in four cases cryostat enrichment to a neoplastic cellularity of >70% was possible.
Allelic Loss Analysis
The primers used were from the ABI Prism Linkage Mapping Set version 1 or 2 (Perkin-Elmer, Emeryville, CA). PCR products were pooled and electrophoresed on an ABI 377 DNA sequencer. Only microsatellites showing two distinct alleles in normal DNA were considered as informative. For these, the intensity of the fluorescent bands of tumor was compared to that of matched-normal DNA. LOH was scored when there was at least a fivefold difference in the ratio of the intensities of the two alleles in tumor and matched-normal DNA. Microsatellites showing differently sized alleles compared with their respective normal sample were scored as instable. All cases were also analyzed for microsatellite instability using the markers BAT-25 and BAT-26.9 In four cases for which high molecular weight DNA was available, chromosomal arms 3p and 10q were analyzed in detail using markers from the ABI Prism Linkage Mapping Set version 1 (Perkin-Elmer) (arm 10q, 11 markers) or 2 (arm 3p, 11 markers).
Mutational Analysis by PCR-Single-Strand Conformational Polymorphism (SSCP)
General conditions used for PCR amplification and SSCP have been described.10 The nine cases for which frozen tissue was available were analyzed for mutations in the VHL, K-ras, and p53 genes. For the VHL gene, exon 1 was amplified in two overlapping fragments, whereas exons 2 and 3 were amplified in their entirety. The primer pairs and the size of the amplified fragments are as follows: exon 1A: VHL 1S, CCCGGGTGGTCTGGATCGCG, and VHLN2AS, AGTTCACCGAGCGCAGCACG (228 bp); exon 1B: VHLN3S, CCGAGGAGGAGATGGAGGCC, and VHLN4AS, GCTATCGTCCCTGCTGGGTC (235 bp); exon 2, VHL5S, CTTTAACAACCTTTGCTTGT and VHL6AS, TTACCACAACAACCTTATCT (195 bp); exon 3: VHL7S, GAGGATTTGGTTTTTGCCCT, and VHL8AS, ACAGTGTAAGTTTCAACAGA (229 bp). Primers for amplification of exon 1 of the K-ras gene11 and exons 5 to 9 of p5312 were as described. Bands exhibiting aberrant migration were cut from the gel, reamplified, and sequenced on an ABI Prism 377 instrument. Results with SSCP analysis were confirmed in two independent experiments.
| Results |
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Each of the 21 cases was analyzed for allelic loss using 79
microsatellite markers on all 22 autosomes (Table 1)
. At least two markers were used per
chromosomal arm with the exception of 5p, 12p, and 12q for which only
one marker was used. Sixty-four percent of the 1,659 markers analyzed
were informative; 85 losses were observed among the 1,066 informative
markers. Representative results are shown in Figure 1
where examples of allelic loss and
retention are shown, which also demonstrates the success of the
enrichment procedure used (>90% in all cases as assessed by
microscopy). All cases showed at least one allelic loss, although the
individual fractional allelic loss varied greatly (range, 0.02 to
0.31); the mean fractional allelic loss was 0.08 calculated as the
number of allelic losses divided by the number of informative markers.
Detailed results for the individual chromosomal arm in each case is
shown in Figure 2
along with the
resulting allelotype of SCAs. Allelic losses on chromosome 10q were the
most frequent event in SCAs, observed in
50% of cases. Allelic loss
involving chromosome 3p was also a relatively frequent event, seen in
40% of cases. Moderately frequent losses (>25% of cases) were
found on chromosomes 1q, 2q, and 7q. No losses were observed on
chromosomes 4p, 8q, 12q, 16p, 18, 20p, or 22, whereas the remaining
chromosomes showed varying, low degrees of allelic loss. No case showed
microsatellite instability of the type seen in mismatch
repair-deficient tumors, as assessed by analysis of the markers BAT-25
and BAT-26.
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Mutational Analysis of VHL, K-ras, and p53
The nine cases with high molecular weight DNA available were
analyzed for alterations in exons 1, 2, and 3 of the VHL
gene; exon 1 and 2 of K-ras; and exons 5 to 9 of
p53 by PCR-SSCP analysis. Although no case had mutations in
either K-ras or p53, two cases (22%) had
potentially inactivating mutations in the VHL gene (Figure 3)
. In particular, tumor SCA1 had a T-A
substitution at codon 80 (AGT-AGA) resulting in Ser to Arg substitution
and case SCA18 had an insertion of T at codon 175 (TAC-TTAC) resulting
in a frameshift. Both mutations were somatic in nature. The PCR-SSCP
analysis was repeated in two independent experiments with identical
results. Our ability to detect mutations in the VHL gene
under the conditions used for PCR-SSCP analysis was also demonstrated
by the analysis of 15 cases of renal clear-cell carcinoma in which
mutations were found in eight cases (53%; data not shown), a frequency
expected from literature data.13
Although case SCA1 was
not informative for the marker analyzed at 3p25, tumor SCA18 showed LOH
at marker D3S1263.
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| Discussion |
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The most common molecular anomaly found in SCA was the loss of genetic
material on chromosome 10q, seen in
50% of cases. LOH on 10q is
also found in approximately one-half of pancreatic ductal cancers,
which are characterized by allelic loss on chromosomal arms 9p, 17p,
and 18q in nearly all cases.14
Interestingly, LOH on
chromosome 10q has been found in follicular thyroid
tumors.15
Allelic loss on 10q has also been correlated
with invasiveness of transitional cell carcinomas of the
bladder.16
One possible candidate gene might be
PTEN, located on the long arm of chromosome 10 and which is
mutated in several cancers.17
It is unfortunate that only
a small number of cases had high molecular weight DNA available for
further localization of the chromosomal regions involved in these
losses.
Allelic loss on chromosome 3p was the second most frequent event found in SCA. As microcystic tumors are part of the von Hippel-Lindau syndrome and the VHL gene is located on chromosome 3p25, it is reasonable to believe that this gene may be altered in sporadic SCA. A previous study found LOH on chromosome 3p25 in six of 10 cases analyzed in at least one of four markers tested.6 Among these 10 cases, one was found to have a somatic silent mutation in the VHL gene.6 Using three markers for chromosome 3p, we found allelic loss in at least one marker in 39% of cases. Analysis of nine cases by PCR-SSCP found inactivating mutations in the VHL gene in two tumors (22%). Mutations in the VHL gene have been observed in 4 of 23 sporadic hemangioblastomas analyzed18,19 and in 4 of 48 cases of sporadic pheochromocytomas.20 Thus the frequency of somatic mutations found in SCA in our analysis is in line with what has been found for other sporadic benign counterparts of familial VHL tumors; ie, mutation of VHL is not an extremely common event. However, alterations in VHL are found in approximately one half of sporadic renal clear-cell carcinomas,13 a cancer type which is also part of VHL syndrome.
Among the three chromosomes showing relatively frequent allelic losses,
arm 7q is of particular interest. Allelic losses on chromosome 7q were
observed in
30% of SCAs. Genetic losses on the long arm of
chromosome 7 have been found at relatively high frequency in other
benign tumors, including uterine leiomyoma21
and, notably,
in ovarian serous tumors.22
A number of different studies,
including in vitro experiments and cytogenetic analysis,
have suggested that one or more tumor suppressor genes are located on
chromosome 7q.23-25
Interestingly, the gene coding for
Cul1 also resides on 7q.26,27
This gene is part
of a complex (Skp1-Cul1-F-Box protein) that has similarities to the VHL
elongin complex28
and its loss results in the
dysregulation of cyclin E.29
One interesting question is whether SCA shares chromosomal or molecular events with pancreatic endocrine tumors or ductal carcinoma. The mean fractional allelic loss of 0.08 in SCA is much lower than that found in ductal cancer (0.36)30 or pancreatic endocrine tumors (0.15),31 and the allelotype of SCA does not resemble that of pancreatic ductal or endocrine tumors.14,32 The chromosomes showing most frequent LOH in our SCAs were 10q (50%) and 3p (40%) followed by 1q, 2q, and 7q (>25%). This pattern is clearly distinct from that observed in pancreatic endocrine tumors, in which the most frequent allelic losses are found on chromosomes 3q, 11p, and 16p with a frequency never exceeding 36%.31 Pancreatic ductal carcinoma is characterized by highly frequent losses at chromosomes 1p, 9p, 17p, and 18q.14,32
In conclusion, the anomalies found by genome-wide allelotyping indicate that chromosomal arm 10q may harbor a tumor suppressor gene involved in the pathogenesis of SCA. Our chromosomal deletion map also suggests that SCA tumorigenesis involves molecular pathways different from those occurring in more common pancreatic tumor types, including mutation of the VHL gene in a subset of sporadic cases.
| Footnotes |
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Supported by grants from the Consorzio Studi Universitari di Verona, Italy; Associazione Italiana Ricerca Cancro, Milan, Italy (to A. S.); co-financed grant from Verona University and Ministero Università e Ricerca Scientifica e Tecnologica (Cofin 9806151968-9906195987-9906218982), Rome, Italy; and European Community grant BIOMED 2 CE Contract No. BMH4-CT98-3805.
Accepted for publication September 13, 2000.
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