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Short Communications |


From the International Agency for Research on Cancer,*
Lyon, France; the Department of Neurosurgery,
University Hospital, Zurich, Switzerland; and the Institute
of Pathology and Molecular Immunology,
Medical Faculty of Porto, Porto, Portugal
| Abstract |
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| Introduction |
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We recently reported that the giant cell glioblastoma, a rare glioblastoma variant characterized by the presence of large, bizarre, multinucleated cells, occupies a hybrid position, sharing with primary (de novo) glioblastomas a short clinical history, the absence of a less malignant precursor lesion, and a 30% frequency of PTEN mutations. They have in common with secondary glioblastomas a younger patient age at manifestation and a high frequency of p53 mutations.6
The gliosarcoma is another morphologically defined glioblastoma variant, originally described in 1895 by Stroebe et al.7 Gliosarcomas comprise approximately 2% of all glioblastomas8,9 and are characterized by a biphasic tissue pattern, with areas displaying glial and mesenchymal differentiation.10,11 Whereas morphological studies suggested an evolution of the sarcomatous component from microvascular proliferations within a highly malignant glioblastoma, two recent genetic studies revealed the presence of identical p53 mutations12 and similar chromosomal imbalances and cytogenetic alterations13 in both tumor areas, suggesting a monoclonal origin. In this study, we screened 19 well-documented cases of gliosarcoma for a variety of genetic alterations in an attempt to identify the genetic profile of gliosarcoma as compared to other glioblastoma subtypes and to elucidate the histogenesis of the sarcomatous component present in this neoplasm.
| Materials and Methods |
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The surgical specimens were obtained from a total of 19 patients
diagnosed in the University Hospitals of Zürich (Switzerland),
Porto (Portugal), and Ribeirão Preto, São Paulo (Brazil).
Gliosarcomas were diagnosed according to the WHO classification of
brain tumors.10
Care was taken to include only classical
cases, showing the typical biphasic pattern with alternating areas of
glial and mesenchymal differentiation. High-grade gliomas with a
mesenchymal component that might have resulted from infiltration of the
dura were excluded. The areas with glial differentiation usually
expressed glial fibrillary acidic protein (GFAP) and showed necrosis
and/or vascular endothelial proliferation. The sarcomatous portions
showed strong reticulin staining as well as signs of malignant
transformation (eg, nuclear atypia, mitotic activity, and necrosis;
Figure 1
). Glioblastomas with focal
sarcomatous appearance but without reticulin staining were not
included. The age and sex of patients are shown in Table 1
. Eleven patients were male and eight
were female (M/F ratio, 1.4). The mean age of patients at first
diagnosis of gliosarcoma was 56 ± 12 years (range, 3276 years).
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Of 12 patients for whom we could obtain detailed clinical data, 10 were diagnosed with gliosarcoma at the first biopsy; the mean preoperative clinical history was 12 ± 16 weeks. In one case (case 221), the first biopsy was histologically classified as glioblastoma (preoperative history, 4 months), and the second biopsy as gliosarcoma. In another case (case 216), the first biopsy showed an anaplastic astrocytoma, the second biopsy a glioblastoma, and only the third biopsy showed the typical features of gliosarcoma.
Polymerase Chain Reaction-Single-Strand Conformational Polymorphism Analysis and Direct Sequencing for p53 Mutations
DNA was extracted as previously described.14 Mutations in exon 58 of the p53 gene were screened using polymerase chain reaction-single-strand conformational polymorphism (PCR-SSCP) as previously described.3 Samples that showed a mobility shift in the PCR-SSCP analysis were further analyzed by direct DNA sequencing. Primer sequences for PCR and DNA sequencing were described previously.3
PCR-SSCP Analysis and Direct DNA Sequencing for PTEN Mutations
Prescreening for mutations in exons 19 of the PTEN gene was carried out by PCR-SSCP as previously described.6 Samples that showed a mobility shift in the SSCP analysis were further analyzed by direct DNA sequencing as previously described.6 In some cases, individual abnormally shifted SSCP bands were cut directly from the dried gels, placed in 100 µl of distilled water, incubated at 80°C for 15 minutes and centrifuged briefly; 1 µl of the supernatant was used for PCR. Sequencing primers used were as follows: 5'-CTC TCC TCC TTT TTC TTC A-3' (sense) and 5'-AGA AAG GTA AAG AGG AGC AG-3' (antisense) for exon 1; 5'-TTT CAG ATA TTT CTT TCC TTA-3' (sense) and 5'-TGA AAT AGA AAA TCA AAG CAT-3' (antisense) for exon 2; 5'-TAA AGC TGG AAA GGG ACG AA-3' (sense) and 5'-TAT CAT TAC ACC AGT TCG TC-3' (antisense) for exon 5; 5'-TTT TTT TTT AGG ACA AAA TGT TT-3' (sense) and 5'-TCA CAT ACA TAC AAG TCA CCA AC-3' (antisense) for exon 8.
Differential PCR for p16 Homozygous Deletion and EGFR, CDK4, and MDM2 Amplification
To assay p16 homozygous deletions in gliosarcomas, differential PCR was carried out, using the STS reference sequence, as reported by Ueki et al,15 with some modifications.4 The average p16/STS ratio, using normal blood DNA, was 1.04, with a standard variation of 0.15. Values of less than 0.2 for the p16/STS ratio indicated deletions of the p16 gene.4 Two primary glioblastomas, which showed a ratio of less than 0.2 in the previous study,4 served as positive controls for p16 deletion.
To detect CDK4 amplification, differential PCR was carried
out as described previously.4
Interferon
(IFN
) was
used as a reference gene. The value for normal blood DNA was 1.07, with
a standard variation of 0.19. A value of more than 2.7 for the
CDK4/IFNG ratio was regarded as positive for CDK4
amplification. This value was calculated according to the method of
Rollbrocker et al.16
One primary glioblastoma, which
showed a ratio higher than 2.7 in a previous study,4
served as the positive control for CDK4 amplification.
MDM2 amplification was detected by differential PCR analysis as previously described.17 Dopamine receptor (DR) was used as the reference gene. The MDM2/DR ratio from normal blood DNA was 0.91, with a standard variation of 0.4. A value of more than 3.02 for the MDM2/DR ratio was regarded as positive for MDM2 amplification. Two primary glioblastomas, which showed a ratio higher than 3.02 in a previous study,17 served as positive controls for MDM2 amplification.
To detect EGFR amplification, differential PCR with the cystic fibrosis (CF) reference gene was carried out as described previously,18 with some modifications.5 The mean EGFR/CF ratio, from DNA from the peripheral blood of healthy adults, was 1.2, with a standard variation of 0.20. The threshold value 2.94 was regarded as evidence of EGFR amplification, according to the method of Rollbrocker et al.16 One primary glioblastoma, which showed EGFR amplification in a previous study,5 was used as a positive control.
Immunohistochemistry
The sections were deparaffinized in xylene and rehydrated in graded ethanol. The endogenous peroxidase was blocked by incubation in 0.3% H2O2 solution in methanol for 30 minutes.
For p53 immunohistochemistry, the sections were boiled three times for 5 minutes in 10 mmol/L sodium citrate buffer (pH 6.0) in a microwave oven. The incubations of anti-human p53 monoclonal antibody (PAb 1801; Genosys Biotechnologies, Cambridge, UK; diluted 1:1000). were carried out overnight at 4°C after blocking of nonspecific binding with 5% skimmed milk for 60 minutes.
For MDM2 immunohistochemistry, the sections were boiled in 10 mmol/L sodium citrate buffer (pH 6.0) for 10 minutes in a steam cooker, subsequently incubated in 5% skimmed milk for 1 hour at room temperature, then incubated overnight at 4°C with the monoclonal antibody to MDM2 (clone IF2; Oncogene Research Products, Cambridge, MA; diluted at 1:2000).
For EGFR immunohistochemistry, sections were pretreated with 0.1% trypsin in 0.1% CaCl2 (pH 7.8) for 15 minutes at 37°C and then incubated in 5% skimmed milk for 60 minutes. Sections were then reacted overnight at 4°C with EGFR monoclonal antibody (NCL-EGFR; Novocastra Laboratories, Newcastle, UK), which recognizes the EGFR ligand binding domain (dilution 1:100).
For RB immunohistochemistry, the sections were boiled three times for 5 minutes in 10 mmol/L sodium citrate buffer (pH 6.0) in a microwave oven. The sections were allowed to cool at room temperature. Sections were incubated overnight at 4°C with the RB monoclonal antibody (clone G3245; PharMingen, San Diego, CA; diluted 1:100), which recognizes an epitope between amino acids 300 and 380 and both phosphorylated and underphosphorylated RB protein.
For c-MET immunohistochemistry, the sections were boiled for 10 minutes in 10 mmol/L sodium citrate buffer (pH 6.0) in a steam cooker and subsequently incubated in 5% skimmed milk overnight at room temperature. Sections were then incubated for 1 hour at room temperature with the c-MET monoclonal antibody (NCL-cMET, diluted 1:100; Novocastra Laboratories).
The reaction was visualized using the Vectastain ABC Kit and diaminobenzidine (Vector Laboratories, Burlingame, CA). Sections were counterstained with hematoxylin. Fractions of positive cells were recorded as follows: -, negative; +, <5%; ++, 550%; +++, >50%.
| Results |
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Miscoding p53 mutations were found in five of 19 (26%)
gliosarcomas analyzed (Table 1)
. One tumor (case 211) contained a
silent mutation (GTG
GTA, Val
Val) in codon 173. In case 210 (Table 1)
, the first biopsy showed the missense mutation CCC
TCT in codon
190; the second biopsy contained a different mutation, CCC
TCC, in
codon 151, which was present in both the gliomatous and
sarcomatous components (Table 2)
. In all
cases, the wild-type base was present along with the mutated base. In
case 216, the mutation was present only in the third biopsy but not in
the first and second biopsies. Of five gliosarcomas with a
p53 mutation, four showed nuclear accumulation of p53
protein in a variable fraction of neoplastic glial and mesenchymal
cells (Table 1)
. One gliosarcoma (case 207) contained a 2-bp insertion
mutation, resulting in a stop codon, and did not show p53
immunoreactivity (Table 1)
.
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SSCP followed by direct DNA sequencing revealed that seven of 19
(37%) gliosarcomas contained a PTEN mutation (Table 1)
. Of
these, three mutations were in exon 5 (phosphatase domain), two were in
exon 8, and one each were in exons 1 and 2. Three were nonsense
mutations leading to a truncated protein, and four were missense
mutations. In all cases, the wild-type base was also detectable. In
case 212, the same mutation was present in both primary and second
biopsies. In cases 215 and 218, identical mutations were detected in
gliomatous and sarcomatous areas (Tables 1 and 2
and Figure 2
).
|
In seven cases (37%), differential PCR revealed a homozygous
p16 deletion (Table 1)
. In one tumor (case 214), the
p16 deletion was detected in both gliomatous and sarcomatous
areas (Tables 1 and 2
and Figure 3
). In
another case (case 221), a p16 deletion was detected in the
first (glioblastoma) and second (gliosarcoma) biopsies. Differential
PCR further revealed amplification of CDK4 in one
gliosarcoma (case 220, Table 1
), again in both gliomatous and
sarcomatous areas (Figure 3)
.
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A cell cycle-related gene alteration (p16 deletion, CDK4 amplification, or loss of RB expression) was found in 10 of 19 (53%) gliosarcomas, but these were mutually exclusive, ie, no biopsy contained more than one of these alterations.
Amplification and Overexpression of the MDM2, EGFR, and c-MET Genes
MDM2 amplification was detected by differential PCR in
one biopsy (case 220), which also showed CDK4 amplification
(Tables 1 and 2
and Figure 3
). In this tumor, MDM2
overexpression was detected immunohistochemically in more than 50% of
neoplastic cell nuclei of both gliomatous and sarcomatous areas.
Differential PCR did not reveal EGFR amplification in any of
the 19 cases, and EGFR overexpression was also absent
immunohistochemically (Table 1)
.
c-MET immunoreactivity presented as strong cytoplasmic and plasma membrane staining in glioma cells in gliomatous areas, but not in sarcomatous areas in all gliosarcomas analyzed.
| Discussion |
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-smooth
muscle actin in sarcomatous portions pointed to vascular smooth muscle
as the potential origin of the mesenchymal tissue
component.22
More recent investigations suggested a common
origin of for the two tissue components; the sarcomatous areas result
from advanced glioma progression with acquisition of a mesenchymal
phenotype.23,24
This view is strongly supported by genetic
analyses, including the present study. Using interphase cytogenetic
analysis, Paulus et al25
detected similar cytogenetical
abnormalities in the gliomatous and sarcomatous components of two
gliosarcomas, but Biernat et al12
were the first to prove
a monoclonal origin by demonstrating the presence of identical
p53 mutations in the two tumor areas. Similar genetic
alterations in both tumor components were subsequently reported by
Boerman et al,13
using comparative genomic hybridization
(CGH), cytogenetic analysis, fluorescence in situ
hybridization, and microsatellite analysis.
The present study extends these findings to a variety of other gene
alterations (Table 2)
. We detected in gliomatous and sarcomatous tumor
areas identical PTEN mutations (two cases), a p53
mutation (one case), homozygous p16 deletion (one case), and
coamplification of MDM2 and CDK4 (one case). In
one biopsy (case 213), only the sarcomatous area showed an unequivocal
p16 deletion, whereas in the gliomatous portion the
p16/sts ratio was 0.31 and thus did not reach the criterion
of p16 deletion. This may be due to an admixture of DNA from
nonneoplastic neural tissue. Taken together, these data firmly
establish the gliosarcoma as a monoclonal tumor with focal aberrant
mesenchymal differentiation. Identical genetic alterations have also
been detected in both the carcinomatous and sarcomatous components of
uterine carcinosarcomas,26
and in epithelial and stromal
components of pulmonary carcinosarcomas.27
In this as well as in previous studies,11
gliosarcoma
typically developed in older patients (Table 3)
and was diagnosed at first biopsy
after a short clinical history, suggesting that these tumors developed
de novo, ie, without an identifiable, less malignant
precursor lesion. Occasionally (case 221) the histological features of
gliosarcoma appeared in the second biopsy of a primary glioblastoma.
Perry et al28
reported that 25 of 32 cases (78%) were
diagnosed as gliosarcoma in the first biopsy, whereas seven (22%)
developed after irradiation for glioblastoma. Rarely, gliosarcomas
develop through progression from low-grade25
or anaplastic
astrocytoma (Ref. 29
and case 216 of this study).
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The most common changes in gliosarcomas detected in cytogenetic studies included gains of chromosome 7 (EGFR gene locus) and loss of chromosome 10, followed by deletions of the long arm of chromosomes 13 and 9.13,30-32 The absence of EGFR amplification in this study suggests that other protooncogenes on chromosome 7 may be involved in the evolution of this glioblastoma variant. CDK6, PDGF-A and c-MET genes on chromosome 7 have been reported to be amplified or overexpressed in malignant gliomas.33-36 c-MET immunohistochemistry in these study shows that c-MET is overexpressed in gliomatous but not in sarcomatous components in gliosarcomas. It remains to be clarified whether amplification of CDK6, PDGF-A genes is involved in the development of gliosarcomas.
The frequency of p53 mutations in gliosarcomas was 26% and thus was significantly lower than in secondary glioblastomas (67%, P = 0.0086) but somewhat higher than in our cohort of patients with primary glioblastomas, but the difference was not significant (26 versus 11%, P = 0.405).3
The 12q1314 chromosomal region contains several genes (MDM2, CDK4, sarcoma amplified sequence SAS and GLI) that have been reported to be coamplified in sarcomas37 and glioblastomas.38 In this study, one gliosarcoma (case 220) showed coamplification of MDM2 and CDK4 in gliomatous and sarcomatous tumor areas.
The progression of cells from G1 to S phase is regulated by
cyclin-dependent kinases (CDKs), their inhibitors, and the
retinoblastoma protein (pRB). In a simplified model, p16 protein binds
to CDK4 and inhibits the formation of CDK4/cyclin D complex. When
activated, this complex phosphorylates the RB protein, thereby inducing
the release of the E2F transcription factor, which in turn activates
genes involved in the late G1 and S phases.39,40
Homozygous p16 deletion, CDK4 amplification, and
loss of RB expression are frequent in
glioblastomas.4,41,42
In this study, approximately
one-half of gliosarcomas showed aberrant expression in one of these
genes (Table 1)
. The frequency of p16 homozygous deletion in
seven (37%) gliosarcomas is similar to that in primary
glioblastomas.4
Our finding that in gliosarcomas
homozygous p16 deletion, CDK4 amplification, and
loss of RB expression were mutually exclusive corresponds to similar
observations in other glioblastomas4,15,42
and indicates
that altered expression of any of these genes may lead to loss of cell
cycle control.
In conclusion, gliosarcomas exhibit clinical features and a genetic profile similar to those of primary (de novo) glioblastomas, ie, advanced patient age, short clinical history, and frequent p16 deletions and PTEN mutations. The unexpected and most striking difference is the absence of amplification/overexpression of the EGFR gene, a genetic hallmark of primary glioblastomas. The presence of identical genetic alterations in both gliomatous and sarcomatous components strongly supports the concept of a monoclonal origin of gliosarcomas.
| Acknowledgements |
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| Footnotes |
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Supported by a grant from the Foundation for Promotion of Cancer Research, Japan.
Accepted for publication November 26, 1999.
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