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Special Focus: Pediatric Neuro-Oncology |
Departments of Pathology (M.N., K.S., E.I., T.H., N.K.) and Neurosurgery (H.N., T.S.), Nara Medical University School of Medicine, Nara 634-8521, Japan
2 Address correspondence to Noboru Konishi, Department of Pathology, Nara Medical University School of Medicine, 840 Shijocho, Kashihara, Nara 634-8521, Japan (nkonishi{at}naramed-u.ac.jp).
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Abstract |
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gene. Loss of heterozygosity (LOH) on 1p/19q and 10p/10q was less common in pediatric astrocytic tumors than in those seen in adults, but the frequency of LOH on 22q was comparable, occurring in 44% of diffuse astrocytomas, 40% of anaplastic astrocytomas, and 61% of glioblastomas. Interestingly, a higher frequency of p53 mutations and LOH on 19q and 22q in tumors from children six or more years of age at diagnosis was found, compared with those from younger children. Our results suggest some differences in children compared to adults in the genetic pathways leading to the formation of de novo astrocytic tumors. In addition, this study suggests potentially distinct developmental pathways in younger versus older children.
Key Words: astrocytoma • pediatric tumors • primary glioblastoma • progression • secondary glioblastoma
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Introduction |
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The majority of adult glioblastomas (WHO grade IV) occur in older patients after a short clinical history with no clinical or histopathologic evidence of less malignant precursor lesions (primary or de novo glioblastoma). In younger patients, another type of glioblastoma, so-called secondary glioblastoma, shows a slow progressive development from low-grade diffuse (WHO grade II) or anaplastic astrocytoma (WHO grade III) (Kleihues et al., 2000). Adult glioblastomas display distinct pathways of genetic alterations during progression characterized by EGFR amplification/overexpression and PTEN mutation (Lang et al., 1994a; von Deimling et al., 1993; Watanabe et al., 1996), while, in contrast, secondary glioblastomas developing in younger patients demonstrate frequent p53 mutations (Tohma et al., 1998; Watanabe et al., 1996). Amplification and overexpression of PDGFR- are typical in the pathways leading to secondary glioblastomas (Kleihues et al., 2000). In recent studies, we have observed that primary glioblastomas seem to be characterized by loss of heterozygosity (LOH)3 throughout chromosome 10 (Fujisawa et al., 2000), while secondary glioblastomas preferentially show LOH on chromosomes 10q, 19q, and 22q (Fujisawa et al., 2000; Nakamura et al.,2000, 2005b) and promoter methylation of RB1, TIMP-3, and HRK (Nakamura et al.,2001c, 2005a, 2005b). Pediatric glioblastomas are mostly primary tumors and resemble secondary glioblastomas only in that the patients are young (Sure et al., 1997).
Compared with the extensive efforts that have been directed at characterizing the molecular features of adult astrocytoma progression, relatively few data have been collected on pediatric tumors, reflecting, in part, the fact that these lesions are less common. Although a few studies have suggested that de novo pediatric malignant gliomas rarely show EGFR gene amplification and more commonly exhibit the p53 mutations seen in secondary gliomas (Bredel et al., 1999; Pollack et al., 1997; Raffel et al., 1999; Sung et al., 2000), the question remains as to whether childhood astrocytomas can be classed into genetic subtypes as has been done for adult astrocytomas. In addition, few studies have been conducted on low-grade diffuse astrocytomas of children specifically to assess them for the more common genetic alterations associated with adult astrocytoma progression.
The aim of this study, then, was to investigate whether the genetic alterations documented for adult astrocytomas are, in fact, also operative in children. To address this issue, we collected 44 primary pediatric astrocytic tumors and screened each lesion for genetic alterations that may be involved in tumorigenesis. Our results show different genetic aberrations both between astrocytic tumors in younger and older pediatric patients and between pediatric and adult tumors, supporting the notion of different genetic mechanisms in patient-age-related tumor development.
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Materials and Methods |
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Mutation Analyses of p53 and PTEN
We prescreened for mutations in exons 5-8 of the p53 gene and exons 1-9 of PTEN gene by PCR-single-strand conformational polymorphism (SSCP) analysis. The sequencing primers we used have been described previously (Nakamura et al., 2001b; Watanabe et al., 1998). Samples that showed mobility shifts in SSCP analysis were further analyzed by DNA sequencing; after PCR amplification with the same set of primers used in SSCP, PCR products were sequenced on the Genetic Analyzer 310 (Applied Biosystems, Foster City, Calif.) using ABI PRISM BigDye Terminator Cycle Sequencing Ready Reaction Kits (Applied Biosystems).
Evaluation of EGFR and PDGFR- Amplification
Real-time quantitative PCR was carried out according to previously published methods (Biernat et al., 2004; Nigro et al., 2001). Two sets of primers and fluorescent internal probes for the EGFR genome and the internal control ß-actin were used. Both PCR products have similar sizes, to ensure equal amplification efficiency. Two sets of primers and probes for the PDGFR- and the internal control IFN-82 were also used. To calculate the relative gene dosages and the level of EGFR and PDGFR- amplification, we used the Ct values (the PCR cycle number in which the first fluorescence signal measured above threshold), dividing the difference of Ct values of the target gene and the reference gene of the tumor sample (
CtT) by the difference in Ct values of the target and reference gene of the normal brain sample (CtN). Multiplication by 2 of the ratio of CtT:CtN then yielded the level of amplification.
LOH Assay
Forty-eight highly polymorphic markers were selected from the Genome Database (www.gdb.org/) and the National Center for Biotechnology Information (www.ncbi.nlm.nih.gov/genemap) based on the frequency of heterozygosity, as well as by coverage and flanking of the region of interest. For each marker, we labeled the sense primer with a fluorescent dye and then paired normal and tumor DNA samples from each patient. The paired samples were amplified for 30 cycles with an annealing temperature of 56-58°C. We mixed aliquots of the PCR reactions with a size standard and formamide, denatured them, and subjected them to capillary electrophoresis on a Genetic Analyzer 310 (Applied Biosystems); collected data were analyzed using GeneScan software (Applied Biosystems). We independently repeated the analysis of each marker at least twice and found a variation of no more than 3% in the allelic ratios. Only those samples heterozygous for a given locus were regarded as informative; locus homozygosity and/or microsatellite instability rendered any particular sample noninformative. Samples were considered to show LOH when a peak allele signal from tumor DNA was reduced by 50% compared with its normal-tissue counterpart (Nakamura et al., 2003).
Immunohistochemical Analysis of EGFR and PDGFR- Expression
We assessed the expression of EGFR and PDGFR- immunohistochemically, using monoclonal antihuman EGFR antibody (EGFR113; Novocastra Laboratories, Newcastle upon Tyne, U.K.) and polyclonal antihuman PDGFR- antibody (RB-1691; NeoMarkers, Lab Vision, Fremont, Calif.). After deparaffinization, unstained tissue slides were heated to boiling in a pressure cooker for 5 min in 10 mM sodium citrate (pH 6.0) buffer. They were then incubated overnight at 4°C with anti-EGFR and anti-PDGFR- antibody at a dilution of 1:100 and 1:150, respectively. We visualized binding reactions using the chromagen diaminobenzidine with the Histofine SAB-PO kit (Nichirei, Tokyo, Japan) and hematoxylin counterstaining.
Statistical Methods
Fisher's exact test was used to examine possible associations between genetic alterations and tumor grade. The association between patient age and genetic status was calculated by the Mann-Whitney U-test. Statistical significance was established as P < 0.05.
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Results |
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PTEN mutations were detected in only 2 (11%) of 18 glioblastomas examined (Table 1). Both tumors were in children younger than 6 years who did not have p53 mutations.
EGFR and PDGFR- Gene Amplification/Protein Overexpression
As shown in Table 1, EGFR gene amplification was detected only in one anaplastic astrocytoma (10%) and in two glioblastomas (11%); we saw no amplification in any of the low-grade diffuse astrocytomas. In addition, those tumors positive for EGFR amplification were negative for p53 mutations. In contrast, no amplification was observed for the PDGFR-.
We found that overexpression of EGFR protein was somewhat more frequently encountered. Of the 28 malignant astrocytomas analyzed (anaplastic astrocytomas and glioblastomas combined), 28.6% showed overexpression, although immunopositivity to EGFR appeared to be confined to small foci of tumor cells. Given that the sample sizes were not equal, glioblastomas appeared to be more likely to contain high levels of EGFR than were anaplastic astrocytomas (33% vs. 20%, respectively). Overexpression of PDGFR- protein was less frequently found in three diffuse astrocytomas, two anaplastic astrocytomas, and five glioblastomas, respectively. Two anaplastic astrocytomas and five glioblastomas showed overexpression of PDGFR-, which was mutually exclusive with EGFR overexpression.
Analysis of LOH
Tables 1 and 2 and Fig. 1 also illustrate the frequency of LOH in our collection of tumors. LOH on chromosome 1p was detected in 2 of 18 glioblastomas only (11%). Each case, which had few oligodendroglial components, revealed allelic loss at either D1S548 or D1S508. LOH on chromosome 19q was found in 1 of 16 diffuse and 1 of 10 anaplastic astrocytomas (6.2% and 10%, respectively) and in 4 of 18 glioblastomas (22%); two of the affected glioblastomas, cases 11 and 16, showed a common region of deletion (CRD) on 19q13.1-13.3 between D19S425 and D19S596. It should also be noted that, even though LOH on 1p and 19q is an overall infrequent event in pediatric compared to adult astrocytomas, we did not detect this particular alteration in tumors from children younger than six years of age.
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LOH on chromosome 22 was detected among all classifications of tumors across all age groups but seemed to be more frequent in tumors in children older than six years (Table 2). Seven of 16 grade II diffuse astrocytomas (
44%), 4 of 10 anaplastic astrocytomas (40%), and 11 of 18 glioblastomas (61%) exhibited LOH in at least one locus on 22q. We identified two CRD at 22q12.3 (proximal CRD [PCRD]) and at 22q12.3-13.32 (distal CRD [DCRD]) in the 11 primary glioblastomas studied (Fig. 2). The PCRD was defined with proximal and distal boundaries marked by D22S1176 and D22S1172, respectively, and spanned a distance of 7.5 Mb, while the DCRD was defined with nine markers within a span of 30 Mb. In the five informative grade II diffuse astrocytomas occurring in older patients, the CRD appeared to be defined proximally by marker D22S280 and distally by D22S1172 (Fig. 2). Representative cases are depicted in Fig. 3.
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Discussion |
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The reported frequencies of p53 alterations in pediatric astrocytic tumors vary considerably. Although initial studies indicated that childhood malignant gliomas rarely exhibited p53 mutations (Lang et al., 1994b; Litofsky et al., 1994), recent reports have suggested that a subgroup of anaplastic astrocytomas and glioblastomas of childhood frequently exhibit such mutations (Raffel et al., 1999; Sung et al., 2000; Sure et al., 1997). The overall incidence of p53 mutation in 27% of our lesions is broadly similar to the results obtained from pediatric astrocytic tumors by other authors (Pollack et al., 1997; Sure et al., 1997). It is worth noting that the p53 mutation rate of pediatric glioblastomas is apparently much less than the 60%-80% detected in adult diffuse astrocytomas and secondary glioblastomas.
EGFR amplification occurs in fully one-third of adult glioblastomas, while the majority of high-grade pediatric astrocytomas apparently lack EGFR amplification. These results both refute previously published negative data (Cheng et al., 1999; Raffel et al., 1999; Sung et al., 2000) and corroborate other, positive studies (Sure et al., 1997; Wasson et al., 1990). EGFR gene amplification was not found in the low-grade astrocytomas of our series, which is consistent with what is generally accepted (Kleihues et al., 2000). Therefore, we think it safe to postulate that overexpression/amplification of EGFR does not seem to be a major genetic factor in the development of these tumors, regardless of their clinical history.
The number of specimens in this study did not allow for a meaningful comparison of the frequency of EGFR amplification correlated with different locations or specific histologic subtypes. In one study, EGFR amplification was a feature of supratentorial malignant gliomas of childhood, although found at a lower frequency than in adult tumors (Bredel et al., 1999). Bredel et al. (1999) further reported that, among three tumors showing EGFR amplification, one glioblastoma was of a small-cell type, indicating a possible relationship between EGFR amplification and small-cell histology, at least in adult patients (Burger et al., 2001). All three of the malignant gliomas showing EGFR amplification in our series had a hemispheric location, but they showed the classical glioblastoma histology. In addition, several studies report that, in adult tumors, EGFR amplification and p53 mutations seem to be mutually exclusive. None of the tumors with amplified EGFR exhibited p53 mutations in our study. Based on this, we can hypothesize that astrocytic tumors of both adults and children share at least some molecular mechanisms of tumor progression and, further, that pediatric glioblastomas are more similar to secondary glioblastomas, even if they differ in some clinicopathologic aspects.
PTEN mutations were detected in only two (11%) glioblastoma cases. This lower frequency is similar to that found in the study of adult secondary glioblastomas. In addition, we found no consistent correlation between PTEN mutations and EGFR amplification as in primary glioblastomas, even though our figures are too low for a definite conclusion. Raffel et al. (1999) pointed out that the PTEN mutation was the only alteration of the genes examined that was significantly associated with survival. Due to the small size of our series and low frequency of PTEN mutations, it will be necessary to perform a validation study with a larger cohort of tumors to address the reliability of this finding.
We did not observe amplification of the PDGFR- gene, but this amplification in pediatric gliomas has been previously reported in one study (Di Sapio et al., 2002). PDGFR- gene amplification was detected in only 1 of 38 gliomas, and Di Sapio et al. (2002) suggested a possible role of PDGFR- in the progression of pediatric gliomas. In our series, as in adult cases, PDGFR- overexpression and EGFR overexpression did not occur in the same tumors. In addition, PDGFR- is expressed approximately equally in all grades of tumors. We cannot exclude the possibility that such overexpression could be related to the initial stages of pediatric astrocytoma formation.
LOH analyses have reported losses on 1p/19q in pediatric astrocytomas, and there is evidence in the adult for the presence of tumor suppressor gene(s) on 19q involved in the malignant progression of astrocytomas (Nakamura et al., 2000; Smith et al., 1999). In the present study, LOH at 1p/19q was significantly less common in pediatric tumors compared to adult lesions of similar size and malignancy (Nakamura et al., 2000; Watanabe et al., 2002). These data have several important implications. One possible explanation for lower LOH frequency in pediatric patients is that the histologic distribution of tumors in the pediatric age group differs from that in adults, since LOH is observed most commonly in gliomas with oligodendroglial features. Clearly, pediatric oligodendroglial tumors are uncommon, and the small number of oligodendroglial lesions represented in our series might preclude definitively identifying the association between 1p/19q loss and pathogenesis in this subset. However, our previous LOH study in secondary glioblastomas, which had few oligodendroglial components, showed that 19q loss was more common in secondary glioblastomas than in primary glioblastomas (54% vs. 6%, respectively; Nakamura et al., 2000). Thus, differences in histologic distributions between pediatric and adult tumors do not provide an obvious explanation for the observations noted in the present study.
Genomic loss on 10q appears to be a common chromosomal change overall and showed a similar frequency in pediatric anaplastic astrocytomas and glioblastomas (30% vs. 28%, respectively). The frequency of 10q LOH, however, is somewhat lower than that in adult primary glioblastomas but is closer to the LOH rate seen in secondary glioblastomas in adults (Fujisawa et al., 2000). Multiple tumor suppressor genes on the long arm of chromosome 10 have been implicated in the development of astrocytic tumors (Ichimura et al., 1998), and LOH on 10q has been found more frequently in primary than in secondary glioblastomas; 10q LOH in primary glioblastomas is also associated predominantly with PTEN mutations (Fujisawa et al., 2000; Tohma et al., 1998). Although losses on 10p seem to play little or no role in our pediatric cases, we do note an intriguing concomitant LOH on 10q in the only three tumors that showed LOH in 10p. In each case, this suggests a complete chromosomal deletion, an event detected more commonly in primary glioblastomas (Fujisawa et al., 2000).
We found that 44% of low-grade astrocytomas, 40% of anaplastic astrocytomas, and 61% of glioblastomas exhibited LOH at 22q in our panel. This is comparable to 22q LOH detected in 33% of adult diffuse astrocytomas, in 40% of adult anaplastic astrocytomas, and in 53% of adult glioblastomas; these frequencies are further consistent with a role of perturbation in 22q as an early genetic event in astrocytoma tumorigenesis and progression (Nakamura et al., 2005b). To our knowledge, our study is the first report of a high frequency of LOH at 22q in each grade of pediatric astrocytic tumors. We specifically identified a CRD at 22q12.3 between D22S1176 and D22S1172 in a majority of cases. This chromosomal region includes the tumor suppressor gene TIMP-3 and is a segment frequently deleted in secondary glioblastomas as well as in some diffuse astrocytomas. However, the deleted region is large enough that we cannot exclude involvement of other tumor suppressor gene(s).
Another interesting observation is that astrocytic tumors from adolescents and children older than six years had significantly higher frequencies of 19q and/or 22q deletions than did those from younger children. In addition, glioblastomas from the older children showed 22q LOH at distal region 22q12.3-13.31, as is found in adult primary glioblastomas (Nakamura et al., 2005b). This indicates that glioblastomas in older children follow a molecular pathway distinct from that in very young children, and that the molecular phenotypes of tumors from older children tend to have a greater overlap with that of adult secondary glioblastomas. Since the pattern of progression to glioblastoma is not characteristically observed in pediatric glioblastoma, however, it is not certain whether such tumors are truly analogous to each other. And whereas an age correlation has been noted for p53 mutations, the small number of cases also precludes determination of whether similar chromosomal losses in secondary glioblastomas begin to occur more commonly in older children. Pollack et al. (1997) showed that p53 mutations were rare in tumors from children younger than four years, but in tumors from older children, the percentages were similar to those found in adults. These data are in line with those obtained from studies on adult glioblastomas, suggesting the existence of at least two different subsets of childhood glioblastomas. Although we cannot clearly define genetic pathways characteristic of pediatric astrocytoma progression, specific molecular pathogenic age-related differences might exist in the development of pediatric tumors, especially in younger children. Further efforts to identify minimal common deleted regions or some genetic alterations relevant to pediatric astrocytomas should be supported.
In summation, our data suggest that pediatric astrocytic tumors follow genetic pathways different from those operating in adults. Further, the association between age and genetic alterations among pediatric glioblastomas indicates the probable existence of distinct pathways of molecular tumorigenesis in younger versus older children. Taken together, although the changes detected in pediatric glioblastomas are comparable to those observed in the development of adult secondary glioblastomas, there are sufficient differences in frequency and occurrence to say that the presumed tumorigenic pathways do not always fit.
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Footnotes |
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3 Abbreviations used are as follows: CRD, common region of deletion; DCRD, distal common region of deletion; LOH, loss of heterozygosity; PCRD, proximal common region of deletion; SSCP, single-strand conformational polymorphism.
Received for publication February 9, 2006. Accepted for publication August 17, 2006.
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