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1 From the Dipartimento Ricerca BioMedica, Section for Molecular Medicine and Gene Therapy, University Campus Bio Medico, Rome, Italy; 2 Department of OtolaryngologyHead and Neck Surgery, Division of Head and Neck Cancer Research, and 3 Wilmer Ophthalmological Institute, Department of Ophthalmology, Johns Hopkins University School of Medicine, Baltimore, Maryland.
| Abstract |
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METHODS. Forty-three uveal melanomas were analyzed by fluorescent in situ hybridization (FISH) with probes for c-myc and the chromosome 8 centromere. Results of the FISH analysis were compared with genetic changes previously detected by microsatellite analysis on chromosomes 3 and 6p.
RESULTS. Thirty uveal melanomas (70%) had extra copies of c-myc, 2 tumors (5%) had loss of c-myc, and 11 tumors (25%) had no abnormalities in c-myc copy number. Of those with extra copies of c-myc, 13 tumors (43%) had amplification of the c-myc gene, 14 tumors (47%) had an intermediate relative increase in the c-myc copy number, and 3 tumors (10%) had a simple gain of chromosome 8. An association between larger tumor size and c-myc amplification was found (P < 0.01). Although extra copies of c-myc were seen in tumors with retention of chromosome 3, remarkably only tumors with monosomy 3 showed amplification of c-myc (P = 0.03).
CONCLUSIONS. The specific amplification of the c-myc oncogene detected in at least 30% of primary uveal melanomas cannot be explained by the simple 8q abnormalities observed in cytogenetic studies. The striking association between c-myc amplification and monosomy 3 suggests a unique pathway of genetic progression in a subset of uveal melanomas.
| Introduction |
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qter,
including the c-myc oncogene locus.5
6
7 The oncogene c-myc participates in the control of cellular proliferation, programmed cell death, and differentiation.8 9 10 c-myc belongs to a family of transcription factors and has been implicated in both activation and repression of transcription.11 Since its initial identification as a proto-oncogene in the early 1980s, alterations in c-myc at the genomic and/or expression level have been demonstrated in many types of cancer.12
Although abnormalities of chromosomal arm 8q have been widely reported in uveal melanoma, only a few studies have investigated the role of the c-myc oncogene in the development of this neoplasm. Multiplication of 8q alleles at the c-myc loci was found by Southern blot analysis in 6 of 11 informative cases.13 Overexpression of the c-myc protein was demonstrated by immunohistochemistry14 15 16 and flow cytometry.17 18 In those studies, c-myc expression was correlated with either poor prognosis16 or better prognosis.14 17 18 However, no published studies have analyzed anomalies of c-myc gene copy number and their numeric relationship with the chromosome 8 centromere in uveal melanoma.
To better understand the role of chromosome 8q abnormalities in uveal melanoma development, we have investigated anomalies in the c-myc gene copy number and their correlation with the genetic abnormalities found on chromosomes 3 and 6 by our published allelotype.19 Forty-three tumors from the original allelotype cohort of 50 uveal melanomas were analyzed by fluorescent in situ hybridization (FISH) with a region specific probe for c-myc and a chromosome enumeration probe for chromosome 8. Extra copies of c-myc were demonstrated in 70% of cases, and of these, 43% showed substantial amplification of the c-myc gene. When these results were compared with the genetic alterations on chromosomes 3 and 6p as detected by the allelotype, all tumors with c-myc amplification were also M3, suggesting a unique pathway of genetic progression in a subset of uveal melanomas.
| Materials and Methods |
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Preparation of Nuclear Suspension
Tumor cells were isolated from 12-µm paraffin sections and
deparaffinized as described previously.20
Cellular
disaggregation was performed in a 0.5% Pepsin (Sigma, St. Louis, MO)
solution in 0.9% NaCl adjusted to pH 1.5 with HCl at 37°C for 30 to
45 minutes, depending on tumor dimension.21
The digested
material was filtered through 40-µm nylon mesh (Sefar America Inc.,
Buffalo, NY) and cytospun at 800 rpm for 5 minutes onto
poly-L-lysinecoated slides (Fisher Scientific,
Pittsburgh, PA).20
The cells were acid dehydrated for 2
minutes by incubation in 70% ethanol, 0.01 N HCl, followed by further
ethanol dehydration and air drying.22
Cells were then
fixed in PBS-buffered 1% formaldehyde, washed in PBS, dehydrated in
ascending ethanol concentrations, and air dried.22
For sample UM58, the two distinct region of the tumor were processed separately as tumor samples A (unpigmented) and B (pigmented).19
FISH Analysis
Slides were incubated in 1 M sodium thiocyanate at 37°C
overnight, followed by ethanol dehydration and air
drying.22
Dual probe hybridization was performed with a
chromosome enumeration probe for chromosome 8 (centromere 8 probe
[CEP8]; Vysis Inc., Downer Grove, IL) and a region specific probe for
8q24.1 (LSI-c-myc; Vysis Inc). Probes and target DNA were codenaturated
at 85°C for 3 minutes and incubated at 37°C overnight. After
hybridization, samples were washed in 2x SSC (pH 7.2) + 0.1% NP40
(Sigma) at 75°C for 2 minutes and 2x SSC (pH 7.2) at RT for 1
minute. Nuclei were then counterstained with 4,6-diamino-2-phenylindole
(DAPI; Sigma) and the antifade compound, p-phenylenediamine
(Vector Laboratory Inc., Burlingame, CA). FISH signals were counted
using a fluorescent microscope equipped with a triple-pass filter. Two
hundred nonoverlapping interphase nuclei were counted for
c-myc and CEP8 signals. Occasionally, split signals were
observed and were counted as one, when the signals appeared to be
derived from sister chromatids in cells in S or G2 phase of the cell
cycle (barely a perceptible distance between them; Fig. 1E
). All 50 tumors from our original allelotype cohort were tested by
FISH analysis, but in seven cases we were unable to obtain an optimal
preparation for FISH. In one case, the preparation lacked a sufficient
number of nuclei to study (at least 200 nuclei). In six cases, >15%
of nuclei had no signal either for c-myc or CEP8. On those
samples, FISH analysis was repeated at least once on a different
nuclear preparation with similar results.
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1 (includes cells with
a relative loss of c-myc with respect to the centromere)
defines a low ratio sector; a ratio value of >1 to
2 (includes cells
with copies of isochrome 8q) defines an intermediate ratio sector; and
a ratio value of >2 (including cells with amplification of
c-myc) define high ratio sector. The percentages of signals
in each sector of the table were totaled (26%, 29%, and 44% for
UM11; Fig. 2A
). The section totals were used to classify the
c-myc anomalies as described previously by Jenkins et
al.23
and Sato et al.24
(see below).
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The mean ± SD percentages of normal nuclei with zero or one, two, three, and more than three c-myc or CEP8 signals are shown in Figure 2B . For both c-myc and CEP8, the mean + 3SD percentage of nuclei with zero or one signal for both c-myc and CEP8 was <55%, and the mean + 3SD percentage with three and more than three signals for both c-myc and CEP8 was <13%. None of the normal nuclei preparation showed >20% nuclei in the intermediate c-myc:CEP8 ratio or > 10% nuclei in the high c-myc:CEP8 ratio.23
Classification of c-myc Anomalies
Results from the normal value study were used to develop the
following conservative criteria for extra copies of the
c-myc gene: (i) Simple gain of a whole chromosome 8 (+8)
without any relative increase in c-myc copy number required
an overall mean c-myc:CEP8 ratio < 1.1 and
13% of
nuclei with three or more signals for c-myc and/or CEP8;
(ii) Intermediate relative increase in c-myc copy number
(IRI) required a mean c-myc CEP8 of
1.1% and
20%
nuclei located in the intermediate c-myc:CEP8 sector; (iii)
Large relative increase in c-myc copy number (amplification)
required a mean c-myc:CEP8 ratio
1.1% and
10% of
nuclei located in the high c-myc:CEP8 sector; (iv) Simple
loss of chromosome 8 required
55% of nuclei with zero or one signal
for both c-myc and/or CEP8; and v) Loss of c-myc
required the overall mean c-myc:CEP8 ratio of
0.90.23
24
Microsatellite Marker Analysis
Tumors from the allelotype, which were noninformative for both
markers on chromosomal arms 3q (n = 2), 6p
(n = 3) or 8q (n = 3), were analyzed
for loss of heterozygosity as described in our published allelotype
with additional microsatellite markers.19
Primer pairs
designed to amplify microsatellite markers were obtained from Research
Genetics (Huntsville, AL). The following two additional markers for
each chromosomal arm were used: 3q, D3S1614 and D3S1593; 6p, D6S273 and
D6S344; and 8q, D8S273 and D8S167. When these results were considered
along with those from the published allelotype,19
8q
allelic imbalance (AI) was found in 28 of the 43 (65%) tumors, M3 in
26 (60%) of tumors, 3p AI in 2 (4.6%) tumors, and 6p AI in 11 (26%)
tumors. M3 and 6p AI were present together in only one case.
Statistical Analysis
Statistical analyses were performed using the SigmaStat 1.02.
The association between the discrete variables was assessed using
Fishers exact test. Mean values were compared using the two-tailed
t-test. Differences were considered statistically
significant for P < 0.05.
| Results |
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Of the 13 uveal melanomas with c-myc amplification, 2 tumors showed >40% of nuclei in the high c-myc:CEP8 sector, 4 had 20% to 31%, and 7 had fewer than 20% of nuclei in such sector. The mean ± SD of the c-myc:CEP8 ratio for the high ratio sector was 3.3 ± 0.32 (range 314).
Tumor UM58 has two histologically distinct regions,19 which were processed separately for FISH. Although the unpigmented region (UM58A) and the pigmented region (UM58B) both showed c-myc amplification, the distributions of the cell populations of the two regions were different. The percentage of cells in the high c-myc:CEP8 ratio sector was 20% for UM58A and 12% for UM58B (Figs. 1E 1F) . In this sector, the high c-myc:CEP8 ratio sector, the percentage of nuclei with 7 or more c-myc signals was 50% for UM58A (17 of 34 nuclei) compared with 4% (1 of 23 nuclei) for UM58B (P = 0.0002).
The FISH results were correlated with tumor size (basal diameter [BD] and apical height [AH]) and tumor localization (choroidal or ciliochoroidal), parameters previously shown to be prognostic.25 A statistically significant association with BD was found for both c-myc amplification and IRI. BD was 13.2 ± 2.9 mm in tumors with c-myc amplification, 13.1 ± 3.2 mm in tumors with IRI, and 9.7 ± 3.4 mm in tumors with no increase in c-myc copy number (P = 0.012 and P = 0.01, respectively). An association was also found between AH and c-myc amplification, but not with IRI. AH was 9.0 ± 4.7 mm in tumors with c-myc amplification and 5.4 ± 2.5 mm in tumors without increase in c-myc copy number (P = 0.03). Extra copies of c-myc were found more often in tumors with ciliary body involvement (86%) than in tumors located solely in the choroid (64%), but the difference was not statistically significant.
Comparison between FISH and Microsatellite Analysis Results
Although extra copies of c-myc were found in tumors
with retention of both chromosome 3 and chromosomal arm 6p (R3-R6p),
increase in c-myc copy number (amplification, IRI and +8)
was more frequent in tumors with M3 (Table 1)
. c-myc amplification was demonstrated in
13 of the 26 tumors with M3 by microsatellite analysis compared with 0
of 10 tumors with 6p AI (P = 0.006, Fishers test) and
in 0 of 7 tumors with R3-R6p (P = 0.03, Fishers
test). Thus, all tumors with specific c-myc amplification
also displayed monosomy of chromosome 3.
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| Discussion |
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Almost half (47%) of the tumors with an increase in c-myc copy number have IRI in the c-myc copy number. Jenkins et al.23 have suggested that the principal mechanism for IRI is isochromosome 8q formation. This hypothesis is consistent with our observation and data from karyotypic studies in which isochromosome 8q is the most commonly detected chromosome 8 abnormality.4 13 26 27 28 29 30 31 32
Amplification of the c-myc oncogene, not explained by isochromosome 8q or trisomy 8 formation, was detected in 43% of the tumors with extra copies of c-myc. In those tumors, the percentage of nuclei containing >3 c-myc signals per chromosome 8 centromere ranged from 11% to 45%. In the presence of isochromosome 8q or trisomy 8, the average c-myc:CEP8 ratio cannot exceed 2. The specific c-myc amplification we have detected is most likely the result of intrachromosomal rearrangement or translocation of a small region of 8q containing the c-myc oncogene rather than an extrachromosomal amplification (double minutes), which has not been observed cytogenetically in this tumor type.
On the basis of our published allelotype, we proposed a bifurcated model for the progression of genetic changes that lead to uveal melanoma.19 The most common pathway involved loss of one copy of chromosome 3 (M3), and the secondary pathway was characterized by alteration of chromosomal arm 6p.19 Further analysis of five tumors (initially noninformative for markers from chromosomal arm 3p or 6p in the original allelotype) with additional microsatellite markers confirmed the mutual exclusivity of M3 and 6p alterations.
In our original allelotype, because all tumors with 8q alterations showed M3 or 6p abnormalities, we proposed that 8q AI imbalance follows chromosome 3 or chromosome 6p alterations. In fact, the presence of 8q abnormalities without M3 or chromosome 6 AI seems to be a very rare event in uveal melanomas. In the present study, only three tumors, which were R3-R6p by microsatellite analysis, had extra copies of c-myc by FISH (IRI in one case, +8 in two cases). CGH studies have reported 8q gain without a concomitant loss of chromosome 3 or a gain of chromosome 6p in just 1 of 21 uveal melanomas (4.7%).5 6 Likewise, in those cytogenetic studies in which both chromosome 3 and chromosome 6 status was reported, only a few tumors were found to harbor solely 8q abnormalities.4 27 28 29 31 32 33
c-myc amplification also appears to follow M3. Remarkably, all 13 tumors with c-myc amplification were M3, but only 13 of the 26 tumors with M3 showed c-myc amplification. c-myc amplification was not present in any tumor without M3. These striking findings suggest that c-myc amplification not explained by isochromosome 8q or trisomy 8 typically follows the loss of one copy of chromosome 3.
Several of our observations are consistent with c-myc amplification occurring later in the genetic progression of uveal melanoma. First, larger BD and greater tumor thickness were associated with amplification of c-myc. Second, for both regions of UM58, microsatellite analysis showed loss of heterozygosity of the same allele for all informative markers on chromosome 3 but amplification of different 8q alleles.19 Third, tumor heterogeneity with respect to the absolute number of copies of c-myc was observed in our tumors with additional copies of c-myc (a representative example is seen for UM11 in Fig. 2A ). As well, histopathologically distinct regions of tumor UM58 showed amplification of c-myc, but the overall percentage of cells with c-myc amplification (cells in the high c-myc/CEP8 ratio sector) was approximately 30% higher in the unpigmented region (UM58A) of the tumor. We believe that UM58A is less differentiated and farther along the genetic progression pathway because it had a total of 10 additional chromosomal arms with LOH when compared with UM58B.
Either extra copies of c-myc (25 cases) or loss of one copy of the c-myc gene locus (2 cases) were demonstrated in all but one of the tumors with AI at 8q loci by microsatellite analysis, demonstrating that allelic amplification can be readily detected by microsatellite analysis in addition to allelic loss. A minimal increase in c-myc copy number was found in 5 tumors (8+ in 2 cases and IRI in 3 cases) with retention of 8q loci by microsatellite analysis. One explanation for the discrepancy is that in those 5 tumors, only a small proportion of cells harbored 8q abnormalities falling below the threshold of detection by microsatellite analysis. It is also possible that the increase in c-myc copy number in these 5 tumors did not lead to a measurable imbalance between the microsatellite alleles tested.
Others have already proposed a potential role for c-myc expression as a prognostic indicator in uveal melanomas. In studies based on protein expression levels, c-myc overexpression correlated with either a poor16 or better prognosis.14 17 18 To date, we have been unable to correlate the molecular data with patient survival, as our overall mean patient follow-up period is less than 3 years.
Although our results cannot exclude the direct involvement of another gene(s) on chromosomal arm 8q in uveal melanoma tumorigenesis, specific amplification of the c-myc oncogene was demonstrated in at least 30% of primary uveal melanomas. Moreover, this level of amplification cannot be explained by the simple 8q abnormalities that have been observed in cytogenetic studies (trisomy 8, isochromosome 8q). The striking association between c-myc amplification and M3 in uveal melanoma further suggests that c-myc amplification (and probably overexpression) generally follows loss of a critical gene(s) on chromosome 3.
| Acknowledgements |
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| Footnotes |
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Submitted for publication October 3, 2000; revised January 29 and March 19, 2001; accepted April 6, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked
"advertisement" in accordance with 18 U.S.C.
1734
solely to indicate this fact.
Corresponding author: Shannath L. Merbs, Wilmer Ophthalmological Institute, Johns Hopkins University School of Medicine, 600 N. Wolfe Street, Maumenee 127, Baltimore, MD 21287. smerbs{at}jhmi.edu
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