
Clinical Cancer Research Vol. 6, 2712-2717, July 2000
© 2000 American Association for Cancer Research
Molecular Oncology, Markers, Clinical Correlates |
Differences in 20q13.2 Copy Number between Colorectal Cancers with and without Liver Metastasis
Shigekazu Hidaka1,
Toru Yasutake,
Hiroaki Takeshita,
Masamichi Kondo,
Takashi Tsuji,
Atsushi Nanashima,
Terumitu Sawai,
Hiroyuki Yamaguchi,
Tohru Nakagoe,
Hiroyoshi Ayabe and
Yutaka Tagawa
The First Department of Surgery, Nagasaki University School of Medicine, Nagasaki 852-8501 [S. H., T. Y., H. T., M. K., T. T., A. N., T. S., H. Y., T. N., H. A.], and School of Allied Medical Sciences, Nagasaki University, Nagasaki 852-8520 [Y. T.], Japan
 |
ABSTRACT
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Frequent gains
of 20q have been identified recently in many neoplasias, including
breast, bladder, ovarian, pancreatic, and colon cancers. A high level
of 20q13.2 amplification is associated with poor prognosis in breast
cancer. We investigated the copy number of the 20q13.2 region including
the ZNF217 oncogene in 17 nonmetastatic
colorectal cancers (CRCs), 18 primary CRC tumors with liver metastasis,
and 18 metastatic lesions by two-color fluorescence in
situ hybridization to evaluate the significance of an increased
copy number of 20q13.2 in CRC, especially in those cases with liver
metastasis. The frequency of increased relative copy number of the
20q13.2 region was higher in primary and liver metastatic lesions of
CRC than in CRC lesions without liver metastasis. In particular, a
high-level increase (>3.0-fold) in the relative copy number of 20q13.2
was observed in 2 of 18 (11%) primary CRC lesions with liver
metastasis, 7 of 18 (39%) liver metastatic lesions, and in none of the
cases of primary CRC without liver metastasis. The absolute and
relative copy number of chromosome 20q13.2 was higher in CRCs with
metastasis than in CRCs without metastasis. The percentage of cells
with high-level 20q13.2 amplification was also higher in both
lesions with metastasis per specimen than without metastasis. Our
results suggest that the level of 20q13.2 amplification correlates with
the metastatic potential and tumor progression of CRC. The results also
suggest that 20q13.2 amplification with ZNF217 is
associated with increased metastatic potential.
 |
INTRODUCTION
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Advanced CRC2
is associated with poor prognosis, and surgical resection is the best
available treatment at present. However, recurrence and metastasis are
the major causes of mortality in patients treated by surgical
resection. This behavior of CRC is believed to be dependent on multiple
genetic aberrations. A molecular genetic model of preferential sequence
has been proposed (1)
. However, even with the present
knowledge of the cellular and molecular mechanisms of CRC, no
biological parameter can predict the behavior of cancers.
Recently, gain of chromosome 20q sequences has been reported in many
types of malignant tumors, including breast (2
, 3)
,
bladder (4)
, ovarian (5)
, and pancreatic
cancer (6)
. Karyotypic/cytogenetic data regarding CRC have
been accumulated over the last 10 years (7, 8, 9)
, and
a gain of the long arm of chromosome 20 has been described as a common
genetic aberration (10, 11, 12)
.
We recently reported the numerical chromosome aberration for CRC
(13)
. In that study, we demonstrated that gain of
chromosome 20 is a frequent aberration in primary and metastatic
lesions in patients with CRC associated with liver metastasis
(13)
. Recently, gain of 20q was also identified in 85% of
metastatic lesions of the liver using CGH (14)
. These
findings suggest that aberration of chromosome 20q is associated with
metastasis in CRC.
In human breast carcinoma, DNA amplification at chromosomal region
20q13, which was identified by CGH, was detected in 1218% of lymph
node-negative primary breast cancers and 40% of cell lines
(3)
. Furthermore, a high level of 20q13.2 amplification
was found in 6.8% of primary breast cancers (15)
.
Amplification of the 20q13.2 region was associated with poor prognosis
in lymph node-negative breast cancers (16, 17, 18)
. These data
suggest that the chromosomal segment 20q13 harbors one or more novel
oncogenes. To our knowledge, the relationship between aberration of
this region and the behavior of CRC has not been investigated
previously. The aim of the present study was to investigate the
significance of an increased copy number of 20q13.2 in CRCs, especially
in those CRCs with liver metastasis.
 |
MATERIALS AND METHODS
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Patients and Tumor Samples.
A total of 53 samples were obtained from 35 patients (24 males and 11
females) who underwent surgery between 1989 and 1997 at the First
Department of Surgery, Nagasaki University School of Medicine
(Nagasaki, Japan). None of the cases received adjuvant therapy before
surgery. Seventeen samples of Dukes stage B or Dukes stage C
primary CRC without liver metastasis or recurrence for 5 years after
surgical resection of the primary tumor were selected. Thirty-six
samples were paired samples from 18 patients (18 samples were obtained
from primary tumors with liver metastasis, and the other 18 samples
were obtained from metastatic lesions of the same patients).
Five synchronous cases and 13 metachronous cases were included in these
samples. Synchronous cases were diagnosed with metastasis within 12
months of the diagnosis of the primary tumor. Metachronous cases were
diagnosed with metastasis more than 12 months after diagnosis of the
primary tumor. These cases were designated as primary CRC without liver
metastasis, primary CRC with liver metastasis, and metastatic CRC. The
study protocol was approved by the Human Ethics Review Committee of our
hospital and the Nagasaki University School of Medicine.
Cell Preparation.
All fresh tissue samples were stored at -80°C. Tissue samples were
examined histologically. Specimens were stamped on a glass slide. We
ascertained that malignant cells formed more than 60% of the cells in
these samples. The slides were fixed with a 3:1 methanol:acetic acid
solution at -20°C until use.
FISH.
Two-color FISH was performed using a 20q13.2 locus-specific probe and a
chromosome 20-specific
-satellite DNA biotin-labeled reference probe
(D20Z1; Oncor, Gaithersburg, MD). The 20q13.2 locus-specific probe
including the novel zinc finger gene ZNF217 was
labeled with Spectrum Orange by the vendor (Vysis, Downers Grove, IL).
Tumor samples were denatured with 70% formamide and 2x SSC at 73°C
for 2.5 min and treated with 1.5 µg/ml proteinase K at 37°C for 7.5
min, followed by dehydration using serial concentrations of 70%, 85%,
and 100% ethanol solutions. The probe mixtures were denatured at
70°C for 5 min and incubated on glass slides at 37°C for
hybridization. After hybridization for 16 h, the samples were
washed three times with 50% formamide at 45°C. The reference probe
was stained immunochemically with fluorescein-avidin DCS (Vector
Laboratories, Burlingame, CA). The slides were counterstained with
0.2 µM 4',6-diamidino-2-phenylindole in an
antifade solution. At least 50 nonoverlapping interphase nuclei were
examined under a fluorescence microscope with a dual bandpass filter
and a x400 lens (Nikon Inc., Tokyo, Japan). Each signal was counted
with a single bandpass filter. Control hybridization to normal
peripheral blood lymphocytes and normal epithelial cells was performed
to confirm that the hybridization efficiency of the test and reference
probes was similar.
Data Analysis and Statistical Methods.
Data were expressed as the mean ± SE. The relative copy number
was expressed as the mean level of amplification (the ratio of the
number of signals from the 20q13.2 probe to the number of signals from
the reference probe per individual cell). Low-level
amplification was defined as more than 1.5-fold of the relative copy
number, whereas high-level amplification was defined as more than
3.0-fold of the relative copy number. The ratio of the number of cells
with high-level 20q13.2 amplification (>3.0-fold of the relative copy
number) to the total number of cells counted was the percentage of
high-level amplification. Differences in the frequency of
amplification among the three groups of tumors were examined using the
2
test. Differences in the mean ratio of
amplification and the percentage of amplification among the
three groups were tested for statistical significance using ANOVA and
multiple comparison tests (Scheffe method). P < 0.05
denoted the presence of a statistically significant difference.
 |
RESULTS
|
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Fig. 1
shows representative signals
of the chromosome 20q13.2 locus-specific probe and the chromosome 20
centromere reference probe in one case of CRC with liver metastasis by
using two-color FISH. The mean level of increased 20q13.2 copy number
per case was determined. An increased copy number of the 20q13.2 region
was seen in a total of 7 of 17 (41%) primary CRC lesions without liver
metastasis, 16 of 18 (89%) primary CRC lesions with liver metastasis,
and 17 of 18 (94%) metastatic CRC lesions (Table 1)
. High-level increased copy number of
20q13.2 was observed in 2 of 18 (11%) primary CRC lesions with liver
metastasis and 7 of 18 (39%) metastatic CRC lesions. There was no
high-level increased copy number of the 20q13.2 region in cases of
primary CRC without liver metastasis. The frequency of increased copy
number of the 20q13.2 region was significantly higher in primary CRC
lesions with liver metastasis than in primary CRCs without liver
metastasis (CRCs without liver metastasis versus primary
CRCs with liver metastasis, P < 0.01; CRCs without
liver metastasis versus metastasis, P <
0.001; Table 1
).

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Fig. 1. A representative example of two-color FISH
analysis using a 20q13.2 locus-specific probe (orange)
and a chromosome 20-specific -satellite DNA reference probe
(green). There are 26 copies of the signals of the
chromosome 20 centromere and 723 copies of the chromosome 20q13.2
region in individual tumor cells. The copy number of 20q13.2 is higher
than that of the centromere of chromosome 20. These findings suggest
20q13.2 amplification.
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A comparison of the mean level of increased copy number showed that the
mean ratio of 20q13.2 locus copy number to chromosome 20 centromere
copy number was 1.48 ± 0.20, 2.11 ± 0.54, and
2.66 ± 0.65 in primary CRCs without liver metastasis, primary
CRCs with liver metastasis, and CRC metastatic lesions, respectively
(Fig. 2)
. Furthermore, the mean ratio of
the absolute 20q13.2 locus copy number was 4.28 ± 2.00, 5.88 ± 2.20, and 6.48 ± 2.30 in primary CRCs without liver
metastasis, primary CRCs with liver metastasis, and CRC metastatic
lesions, respectively (Fig. 2)
. These mean absolute and relative copy
numbers of 20q13.2 were significantly higher in primary lesions of CRC
with liver metastasis and CRC metastatic lesions than in primary CRC
lesions without liver metastasis [CRCs without liver metastasis
versus primary CRCs with liver metastasis, P < 0.01 (relative copy number) and P < 0.05 (absolute
copy number); CRCs without liver metastasis versus CRC
metastatic lesions, P < 0.001 (relative copy number)
and P < 0.01 (absolute copy number)]. The mean
relative copy numbers of the 20q13.2 region were also higher in
metastatic CRC lesions than in primary CRC lesions with liver
metastasis (P < 0.01; Fig. 2
). The relative copy
number of the 20q13.2 region in all cases of primary CRC without liver
metastasis was less than 2.0-fold. In contrast, >2.0-fold of the
relative copy number of 20q13.2 was seen in 11 of 18 (61%) primary CRC
lesions with liver metastasis and 16 of 18 (89%) metastatic CRC
lesions. The frequency of cases showing >2.0-fold of the relative copy
number was also higher in metastatic CRC lesions than in primary CRC
lesions with liver metastasis. However, the frequency did not differ
significantly between primary CRC lesions with liver metastasis and
metastatic CRC lesions. The mean copy number was higher in the
metastatic lesions in the liver than in the primary tumors in all but
two cases (data not shown).

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Fig. 2. Comparison of the absolute and relative
copy number of 20q13.2 among the three groups. Bars,
average values. The mean relative copy number of 20q13.2 was higher in
primary CRC lesions with liver metastasis and CRC metastatic lesions
than in CRC lesions without liver metastasis. (CRCs without liver
metastasis versus primary CRCs with liver metastasis,
P < 0.01; CRCs without liver metastasis
versus metastasis, P < 0.001;
primary CRCs with liver metastasis versus metastasis,
P < 0.01).
|
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Intratumor heterogeneity was seen frequently in individual tumor cells.
We examined the percentage of cells with a high-level increase in
relative copy number (>3.0-fold) in each group. The average percentage
of cells with a high-level increase in relative copy number was
6.09 ± 4.20%, 21.15 ± 16.36%, and 38.60 ± 19.45%
in primary CRCs without liver metastasis, primary CRCs with liver
metastasis, and CRC metastatic lesions, respectively (Fig. 3)
. The average percentage
of cells with a high-level increase in relative copy number was
significantly higher in metastatic lesions and in primary tumors with
metastasis than in primary tumors without metastasis (CRCs without
liver metastasis versus primary CRCs with liver metastasis,
P < 0.05; CRCs without liver metastasis
versus metastatic CRC lesions, P < 0.001;
Fig. 3
). Populations with more than 20% of cells showing a high-level
increase in 20q13.2 relative copy number were observed in 9 of
18 (50%) primary CRC lesions with liver metastasis, 16 of 18 (89%)
CRC metastatic lesions, and in no primary CRC lesions without liver
metastasis. We compared the percentage of cells with a 20q13.2
high-level increase in relative copy number in primary and metastatic
lesions from the same patient, and the percentage of cells with a
20q13.2 high-level increase in relative copy number was higher in
metastatic lesions than in primary lesions in all but two cases (data
not shown).

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Fig. 3. Comparison of the percentage of cells with a
high-level increase in the copy number of the 20q13.2 region per total
cells among the three groups. Bars, average values. The
percentage of amplification was significantly higher in primary CRCs
with liver metastasis and in CRC metastases than in CRCs without liver
metastasis (CRCs without liver metastasis versus primary
CRCs with liver metastasis, P < 0.01; CRCs without
liver metastasis versus metastasis,
P < 0.01).
|
|
Comparison of the synchronous and metachronous cases showed that the
percentage of 20q13.2 amplification in synchronous metastatic
cases was higher than that in metachronous cases, although the
difference was not statistically significant (data not shown).
 |
DISCUSSION
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A gain of the long arm of chromosome 20 has been described in many
human tumors. It has been reported that low-level 20q gain is
associated with immortalization, whereas high-level 20q13.2
amplification is associated with chromosomal instability
(19)
, and that gene amplification and overexpression of
STK15/BTAK mapped on 20q13 induce centrosome amplification, chromosomal
instability, and transformation (20)
. These findings
suggest that 20q13.2 amplification may lead to genome instability in
other human epithelial cell types and may contribute to tumor
progression and, ultimately, to metastasis.
In addition, gain of 20q including this region is likely to play an
important role in many other tumors. According to several CGH analyses
in colorectal tumors, gains of chromosome 20q were observed more often
in carcinomas than in adenomas (10
, 11)
. Frequent gains of
chromosome 20q were identified in both primary and metastatic
lesions of CRC with metastasis (21)
. Furthermore, it seems
that aberration of chromosome 20 occurs relatively early and
contributes to tumorigenesis in CRC. However, the relationship between
clinicopathological features and aberration of chromosome 20q remains
unknown. Our analysis of numerical chromosome aberration in CRC showed
that gain of chromosome 20 is a frequent aberration in primary and
metastatic lesions in patients with liver metastasis (13)
.
The total number of cases with gain of the chromosome 20q13.2 region
detected by two-color FISH was markedly higher in both primary lesions
of CRC with liver metastasis and CRC metastatic lesions than in CRC
primary lesions without metastasis. In the metastatic lesion,
the results were similar to those obtained by CGH. The present study
showed that not only numerical aberration of chromosome 20 but also a
change in copy number of the chromosome 20q13.2 region is associated
with liver metastasis in CRC.
The major finding of the present study was the significantly high mean
relative copy number of chromosome 20q13.2 in CRCs with metastasis as
compared with that in CRCs without metastasis. We also showed that the
mean relative copy number was higher in metastatic lesions than in
primary lesions. These results suggest that the mean level of the
relative copy number of the 20q13.2 locus correlates with tumor
progression of CRC. Only high-level gains of the 20q13.2 region may
play an important role in the metastatic potential of CRC.
The percentage of cells showing 20q13.2 amplification per specimen was
higher in metastatic lesions and primary tumors with metastasis than in
CRCs without liver metastasis. Thus, cancer cells with >2.0-fold
20q13.2 amplification as well as a >20% high-level
amplification may have a higher metastatic potential. It seems
that evaluation of the copy number of the chromosome 20q13.2 region by
FISH may be useful for identification of CRCs with a high metastatic
potential.
Comparison of the mean amplification level and percentage of
amplification of the chromosome 20q13.2 region in primary lesions and
metastatic lesions in cases with liver metastasis showed that the mean
level of the relative copy number was higher in almost all cases in
metastatic lesions than in primary lesions. These results
indicate that the number of tumor cells with the same clonality and
20q13.2 amplification was higher in metastatic lesions than in primary
lesions. Interestingly, no amplification case of primary CRC with liver
metastasis showed a high percentage of amplification in the metastatic
lesion. These results suggested that a subpopulation with 20q13.2
amplification was selected more in the metastatic lesion under
intratumor heterogeneity. Therefore, we postulate that cancer cells
with a high level of relative copy number of chromosome 20q13.2 have a
high metastatic potential and that metastatic lesions of the liver
consisted of a clonal cell population with high-level amplification of
this region. However, in a few cases, the level of 20q13.2
amplification in primary lesions was almost similar or lower than that
in metastatic lesions. These results suggest that, in addition to
20q13.2, other unknown factors are involved in liver metastasis of
CRC.
Several known genes, including the Src oncogene,
CAS (cellular apoptosis susceptibility; Ref.
22
), AIB1 (amplified in breast cancer; Ref.
23
), and BTAK (breast tumor amplified kinase;
Ref. 24
), are located on chromosome 20q. Recently, novel
zinc finger gene ZNF217, which is amplified in breast
cancer, was found on 20q13.2 (25)
. According to the recent
analysis of FISH mapping of 20q in CRC metastases, the most frequent
region with high-level gains was located on 20q13.120q13.2
(14)
. However, the candidate genes that are clearly
associated with the metastatic potential of CRC in this region
(20q13.2) remain unknown. Our results suggest that the increased copy
number of the ZNF217 gene may be associated with liver
metastasis in CRC.
In the present study, the level of 20q13.2 in CRC was not very high
(1.33.8-fold) compared with the levels reported previously in
breast cancer and CRC (14
, 18)
. We also found the
increased copy number of chromosome 20
-satellite DNA reference
probe as described previously (13)
. These findings may be
due to differences in the reference probe selected for analysis. The
increased relative copy number at 20q13.2 in CRC may reflect the
coamplification of a different region in the same chromosome arm or
part of large 20q aberrations. The FISH probe used in our study
includes a more than 300-kb sequence in 20q13.2 region
according to information provided by the vendor; thus, it may also
cover other genes, in addition to ZNF217. Therefore, our
results may not be specific for the ZNF217 oncogene. Further
analysis of FISH mapping using a larger number of CRCs would narrow
down the critical region on 20q for localization of a gene or genes
associated with liver metastasis in CRC.
The relative copy number in cases of synchronous metastasis was higher,
albeit insignificantly, than that in metachronous cases. Liver
metastasis occurred within 1 year in almost all metachronous cases.
These findings suggest that occult micrometastasis undetected before
surgery may occur in metachronous cases.
In conclusion, we demonstrated that the mean level of the relative copy
number of 20q13.2 with the ZNF217 oncogene and the frequency
of amplification of this region were higher in both primary and
metastatic colorectal tumors with liver metastasis than in CRCs without
liver metastasis. Our results suggest that the increased level of copy
number of chromosome 20q13.2 is associated with metastasis in CRC.
Further functional analysis of candidate genes on this region could
reveal the mechanism of metastasis in CRC.
 |
ACKNOWLEDGMENTS
|
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We thank Dr. F. G. Issa (Word-Medex, Sydney, Australia) for
careful reading and editing of the manuscript.
 |
FOOTNOTES
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 To whom requests for reprints should be
addressed, at The First Department of Surgery, Nagasaki University
School of Medicine, 1-7-1, Sakamoto Machi, Nagasaki City, Nagasaki
852-8501, Japan. Phone: 81-95-849-7304; Fax: 81-95-849-7306; E-mail: shigekazu.hidaka{at}nifty.ne.jp 
2 The abbreviations used are: CRC, colorectal
cancer; FISH, fluorescence in situ hybridization; CGH,
comparative genomic hybridization. 
Received 12/13/99;
revised 3/21/00;
accepted 3/27/00.
 |
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