
[Cancer Research 60, 2113-2115, April 15, 2000]
© 2000 American Association for Cancer Research
Alterations of the INK4a/ARF Locus in Human Intracranial Germ Cell Tumors1
Masayuki Iwato,
Osamu Tachibana,
Yasuo Tohma,
Yasuaki Arakawa,
Hisashi Nitta,
Mitsuhiro Hasegawa,
Junkoh Yamashita and
Yutaka Hayashi2
Department of Neurosurgery, Kanazawa University School of Medicine, 920-8641 Kanazawa [M. I., O. T., Y. T., Y. A., M. H., J. Y., Y. H.], and Department of Neurosurgery, Fukui Prefectural Hospital, 910-8526 Fukui [H. N.], Japan
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ABSTRACT
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Little is known about the molecular mechanisms responsible for the
development of intracranial germ cell tumors (ICGTs). Recently, we
demonstrated that the balance of the p53-mdm2 interactions is disrupted
in ICGTs. The p14ARF product, a tumor
suppresser gene located on the INK4a/ARF locus, acts as one
of the major factors affecting p53-mdm2 interactions via its binding to
mdm2 and the stimulation of mdm2 degradation. To evaluate whether
genetic alterations of the INK4a/ARF locus occur in the
genesis of ICGTs, we analyzed the INK4a/ARF genes in 21
ICGTs10 pure germinomas and 11 nongerminomatous germ cell tumors.
Fifteen (71%) of the 21 ICGTs displayed genetic alterations, including
14 homozygous deletions and 1 frameshift mutation. Furthermore, the
frequency of the alterations was higher in pure germinomas [9 (90%)
of the 10] than in nongerminomatous germ cell tumors [6 (55%) of the
11; P = 0.09]. These data suggested that
INK4a/ARF gene abnormalities could play an important
role in the genesis of ICGTs, especially in pure germinoma.
 |
Introduction
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ICGTs3
are rare neoplasms. The reported frequency of the tumor is 0.33.4%
of primary intracranial tumors in Western countries and 2.112.7% in
Japan (1, 2, 3)
, and their etiologies remain largely unknown.
Studies on the histological nature of ICGTs revealed that ICGTs
comprise five interrelated neoplasms: germinoma, teratoma, embryonal
carcinoma, yolk sac tumor, and choriocarcinoma (2
, 4)
.
These five types can be divided into two groups, i.e., pure
germinoma and nongerminomatous germ cell tumors (teratoma, embryonal
carcinoma, yolk sac tumor, and choriocarcinoma). Prognosis varies
between these groups and is highly dependent on the response to
chemotherapy or radiotherapy (2
, 5
, 6)
. In general, pure
germinoma has a better prognosis than nongerminomatous germ cell
tumors.
We recently reported that a disruption in the balance of the p53-mdm2
interactions could play an important role in the tumorigenesis of these
neoplasms. MDM2 gene amplification is found at a moderate
frequency in ICGTs, whereas TP53 gene mutations are very
rare. Furthermore, mdm2 protein is overexpressed at a very high
frequency in ICGTs (7)
.
Increasing knowledge on the molecular genetic mechanisms underlying
many types of tumors has demonstrated a high frequency of genetic
alterations in a single locus, INK4a/ARF
(8, 9, 10)
. The INK4a/ARF locus has two promoters
and encodes two completely different proteins,
p16INK4a and p14ARF (the
mouse homologue is called p19ARF; Refs.
10, 11, 12
). By acting as an inhibitor of cyclin-dependent
kinases, the p16INK4a protein decreases the
phosphorylation of retinoblastoma protein and results in cell
cycle arrest at G1 (13, 14, 15)
. The
other encoded protein, p14ARF, interacts with
mdm2 and stimulates the degradation of mdm2 protein
(16, 17, 18, 19)
. The tumor-suppression function of
p14ARF is dependent upon the presence of
wild-type p53 (20)
, and ARF-deficient mice tend to develop
carcinoma and tumors of the nervous systems (21)
.
Molecular genetic findings of the ICGTs, namely, the disruption of the
balance of p53-mdm2 interactions, led us to speculate that alteration
in the INK4a/ARF locus could be a major factor affecting
p53-mdm2 interaction, and the tumorigenesis of ICGTs.
To evaluate whether INK4a/ARF gene alterations play a role
during ICGTs development, we examined a series of 21 ICGTs to identify
mutations of both the p16INK4a and
p14ARF genes.
 |
Materials and Methods
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Tissue Samples.
Twenty-one intracranial germ cell tumor specimens (10 pure germinomas
and 11 nongerminomatous germ cell tumors, including 4 mature teratomas,
3 immature teratomas, 2 choriocarcinomas, and 2 yolk sac tumors)
were obtained at surgery. All of the cases were diagnosed and followed
up at the Department of Neurosurgery, University Hospital, Kanazawa,
Japan, between 1988 and 1998. The specimens were sent to the pathology
laboratory for routine formalin fixation and paraffin embedding. All of
the tumor specimens were examined microscopically, and only tumor
tissue was dissected under microscope before phenolic DNA extraction.
Homozygous Deletion Analysis of the INK4a/ARF
Genes.
Homozygous deletions of the INK4a/ARF genes were analyzed by
a differential PCR method (22)
. Because the exon2 is a
common open reading frame of both of these genes and the
deletion breakpoint is located between exon3 and exon 1ß
(23)
, we coamplified a 204-bp fragment of INK4a/ARF from
exon2 (primer: ex21) with a 134-bp fragment of the APRT
gene. Primer sequences are listed in Table 1
. One of each pair was
labeled with indodicarbocyanine (Cy5) fluorescent dye (Pharmacia
Biotech, Uppsala) at the 5' end. Differential PCR was performed
in a final volume of 10 µl containing 2 ng of DNA, 50
mM KCL, 1.5 mM of
MgCL2, 10 mM TRIS-HCL (pH
8.3), 200 µM each dNTP, 0.1% gelatin, 5 pmol
of each primer set, 10% DMSO, and 0.25 units of Taq polymerase.
Initial denaturation at 94°C for 3 min was followed by 28 cycles of
denaturation at 94°C for 40 s, annealing at 56°C for 55 s, and extension at 72°C for 55 s on a thermal cycler (thermal
cycler 480, Perkin-Elmer, CA). A final extension step of 10 min
at 72°C was used.
Mutation Analysis of the INK4a/ARF Genes.
For all of the cases without INK4a/ARF gene homozygous
deletion, all of the exons of each gene were analyzed by
fluorescence-based SSCP and direct sequencing (24)
.
PCR was performed in a final volume of 10 µl containing 2ng DNA, 50
mM KCL, 1.5 mM
MgCL2, 10 mM TRIS-HCL (pH
8.3), 200 µM each dNTP, 0.1% gelatin, 5 pmol
of each primer set, 10% DMSO, and 0.25 units Taq polymerase. Initial
denaturation at 94°C for 3 min was followed by 40 cycles of
denaturation at 94°C for 40 s, annealing at 52°C to 60°C for
55 s, and extension at 72°C for 55 s on a thermal cycler
(thermal cycler 480, Perkin-Elmer). A final extension step of 10 min at
72°C was used. PCR products were electrophoretically separated by 8
and 10% acrylamide gels (the methylene-bis-acrylamide:acylamide ratio
was 1:99) at 40 W with water cooling to a temperature of 16°C. Once
separated, the products were analyzed by an automated DNA sequencer
(Pharmacia Biotech Model ALFred) running with a fragment analysis
program (Pharmacia biotech AlleleLinks version 1.00).
The materials with variant SSCPs were reamplified and sequenced
bidirectionally. Sequence analysis was carried out with a semiautomated
sequencer (Model 373, Applied Biosystems, CA). The primers for
SSCP analysis and direct sequencing are listed in Table 1
.
Statistical Analysis.
Two-tailed Fishers exact test was carried out to compare the
frequency of INK4a/ARF alterations in pure germinomas and
nongerminomatous germ cell tumors.
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Results
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To determine the variation in the ratio of the PCR products of the
INK4a/ARF genes to those of the APRT gene in
normal DNA, we studied 60 genomic DNA samples from peripheral
lymphocytes. The mean ratio and SD of the coamplified
INK4a-ARF:APRT were 0.99 and 0.18,
respectively. Cases exhibiting INK4a-ARF:APRT
values lower than 4 SD below this mean ratio, i.e., <0.27,
were considered to have INK4a/ARF gene deletions (Fig. 1)
. The deletions were considered to be homozygous because it was known
that cases carrying hemizygous deletions should have INK4a-ARF values
higher than half of the mean ratio, i.e., >0.45, or 3 SD
below the mean value.

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Fig. 1. Differential PCR-analysis for homozygous deletions of the
INK4a/ARF genes. Representative cases with or without
the INK4a/ARF gene homozygous deletions.
Top, a pure germinoma with the INK4a/ARF
gene homozygous deletion. Bottom, a pure germinoma with
no evidence for deletion of the genes.
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Homozygous deletions of the INK4a/ARF genes were detected in
9 (90%) of 10 patients with pure germinomas, and 5 (45%) of 11
patients with nongerminomatous germ cell tumors. Among the patients who
carried no homozygous deletions, only one showed an altered migration
pattern of SSCP. DNA sequencing confirmed an altered nucleotide
sequence resulting in a frameshift mutation in this patient (Table 2)
.
Although there was no statistical correlation between the frequency of
INK4a/ARF gene alterations and tumor types (pure germinomas
and nongerminomatous germ cell tumors), pure germinomas tended to carry
the INK4a/ARF gene alterations more frequently than the
nongerminomatous germ cell tumors (P = 0.09).
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Discussion
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Little is known about the molecular mechanisms occurring in ICGTs.
Although TP53 gene mutations are rare, some subsets of ICGTs
carry the MDM2 gene amplification (7)
. This
suggests that the disruption of the balance of p53-mdm2 interaction
might play an important role in the tumorigenesis of ICGTs.
In this series, 9 (90%) of 10 pure germinomas and 6 (55%) of 11
nongerminomatous germ cell tumors exhibited INK4a/ARF gene
alterations. The alterations in the ARF gene after the p53
degradation via the stabilized mdm2 could abrogate the normal balance
of each molecule in the ARF-mdm2-p53 pathway. This could break down the
G1-S phase checkpoint and subsequently lead to
cell proliferation. Thus, the high frequency of ARF gene
alterations in ICGTs could confirm the mechanism of ICGTs tumorigenesis
suggested above. Furthermore, another important pathogenesis of ICGTs
in addition to the ARF alterations might be the
INK4a alterations.
The higher frequency of INK4a/ARF alterations in pure
germinomas (90%) compared with that in nongerminomatous germ cell
tumors (55%) may help explain why pure germinomas are more sensitive
to chemotherapy and/or radiotherapy than nongerminomatous germ cell
tumors. In an earlier report (25)
, INK4a gene
transfection and forced expression of p16INK4a in
the p16INK4a-deficient cell line were found to
increase the radiosensitivity of the cells. This is opposed to our
findings, which suggested that pure germinoma, the more radiosensitive
neoplasm, carried INK4a gene alterations. On the other hand,
the abrogation of p53 function was found to increase the response to
radiation in some types of neoplasms (26)
. This was
consistent with our findings, which showed that the ARF
alterations and subsequent abrogations of p53 function were more
frequent in pure germinomas. In ICGTs, disorder in ARF-p53 pathway may
overcome the INK4a-Rb pathway abrogation at the point of
radiosensitivity.
ICGTs and TGTs may share similar cellular origins. Both types of tumors
have the same molecular genetic abnormalities in TP53 gene
and MDM2 genes (7
, 27
, 28)
. However, deletions
or mutations in INK4a/ARF genes are rare in TGTs (29
, 30)
. In contrast, in our study, a high frequency of
INK4a/ARF gene deletions was demonstrated in ICGTs. In
ARF-deficient mice, tumors of the nervous system are likely
to emerge (21)
. The difference in the status of the
ARF gene between ICGTs and TGTs may affect the site of germ
cell tumor development.
The silencing of the INK4a gene by methylation at the
promoter site has been reported in a variety of tumors
(31)
. Recently, it has also been shown that the
ARF gene is silenced by methylation at the
ARF-specific promoter site in some types of cell lines,
although not in natural tumors (32
, 33)
. In our series of
21 ICGTs, cases without deletions or mutations in INK4a/ARF
may carry a promoter site methylation. In the future, in addition to
the analysis for genetic alterations, additional examinations for gene
modifications of this type will be required to evaluate the biological
features of p16INK4a and
p14ARF in, and their influences on, ICGTs.
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ACKNOWLEDGMENTS
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We thank A. Imamura for her technical assistance.
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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 Supported in part by a Grant-in-Aid from the
Ministry of Health and Welfare on Scientific Research for pediatric
brain tumors, and Grant-in-Aid from the Ministry of Education, Science,
Sports and Culture of Japan for Scientific Research B (No. 09470292). 
2 To whom requests for reprints should be
addressed, at Department of Neurosurgery, Kanazawa University School of
Medicine, 13-1 Takaramachi, 920-8641 Kanazawa, Japan. Phone:
81-76-265-2384; Fax: 81-76-234-4262; E-mail: yuh{at}ns.m.kanazawa-u.ac.jp 
3 The abbreviations used are: IGCT, intracranial
germ cell tumor; APRT, adenine phosphoribosyltransferase; SSCP,
single-strand conformational polymorphism; TGT, testicular germ cell
tumor. 
Received 9/10/99.
Accepted 2/29/00.
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