The astrocyte, one form of glial cell that comprises much of the background substance of the brain and spinal cord, is believed to give rise to a large category of primary brain tumors, the astrocytomas. These neoplasms can occur in all areas of the brain and spinal cord in children and adults. Although the vast majority of astrocytic neoplasms occur sporadically, they can be seen in patients with the familial adenomatous polyposis syndrome, the Li-Fraumeni syndrome, and central neurofibromatosis (see Chaps. 37, 40 and 48). The incidence of astrocytomas is approximately 7.0 per 100,000, 1 which means that nearly 20,000 Americans will have an astrocytoma diagnosed each year. The World Health Organization (WHO) classification2 recognizes four grades of astrocytoma (Fig. 57-1). Grade I astrocytomas are slow-growing, noninfiltrative neoplasms, occurring mainly in children and young adults, and include juvenile pilocytic astrocytomas and gangliogliomas. Grade II astrocytomas are mainly well-differentiated fibrillary astrocytomas, whereas grade III astrocytomas are a more aggressive neoplasm, the anaplastic astrocytoma.3 The most malignant form of astrocytoma, the grade IV tumor, is the glioblastoma multiforme, which is the most common primary malignant brain tumor of adults. Although these tumors most commonly occur in the cerebral hemispheres of older individuals, they can be seen throughout the brain and spinal cord in patients of all ages.
Histology of astrocytomas. (A) WHO grade I astrocytomas are largely represented by pilocytic astrocytomas, which are moderately cellular neoplasms formed of bipolar astrocytes that occasionally produce Rosenthal fibers. (B) WHO grade II astrocytomas, also known as well-differentiated fibrillary astrocytomas, are composed of unipolar and stellate astrocytes with simplified processes that exhibit mild nuclear pleomorphism and a proliferation index of 1 percent or less. (C) Grade III astrocytomas, or anaplastic astrocytomas, are highly cellular neoplasms composed largely of unipolar astrocytes exhibiting nuclear pleomorphism and a brisk proliferation index but lacking in tumor necrosis and vascular proliferation. (D) Grade IV astrocytomas, or glioblastomas, form the most malignant end of the spectrum and are characterized by astrocytic neoplasms exhibiting nuclear pleomorphism, brisk proliferation index, vascular proliferation, and/or necrosis with pseudopalisading.
Chromosomal Abnormalities in Astrocytomas (Table 57-1)
The most consistent chromosomal changes in glioblastomas are gains of chromosome 7, seen in about 80 percent of tumors with abnormal stem lines, losses of chromosome 10 in 60 percent of tumors, losses of 9p in about a third of tumors, and the presence of double minute chromosomes (Dmins) in up to 50 percent of tumors4-9 (Figs. 57-2 and 57-3).
Table 57-1: Chromosomal and Genetic Alterations Characteristic of Specific Types of Brain Tumors |Favorite Table|Download (.pdf) Table 57-1: Chromosomal and Genetic Alterations Characteristic of Specific Types of Brain Tumors
|Tumor Type ||Chromosomal or LOH Abnormality ||Genetic Alteration |
|Glioblastoma ||+7 ||Unknown |
| ||–9p || CDKN2A, CDKN2B |
| ||–10 || PTEN/MMAC1 gene mutation, DMBT1 deletion? |
| ||–17p || p53 gene mutation |
| ||Dmins || EGFR gene amplification and rearrangement |
|Oligodendroglioma ||–1p, –19q ||Unknown |
|Ependymoma ||–22 || NF2 gene mutation? |
|Medullobiastoma ||–17p ||Unknown |
| ||–9q || PTCH gene mutation |
| ||Dmins || c-myc, N-myc gene amplification |
|Meningioma ||–22 || NF2 gene mutation |
|Schwannoma ||–22 || NF2 gene mutation |
Karyotype of glioblastoma. This Giemsa-trypsin–banded karyotype of glioblasoma xenograft D-643 MG shows gain of chromosome 7, a deletion of 9p, and loss of a chromosome 10 (double arrows). Additional, nonspecific changes are marked with single arrows.
FISH of glioblastoma. This interphase nucleus from a glioblastoma contains three chromosome 7 centromere signals (dark) and one centromere 10 signal (light).
Genetic Alterations in Astrocytomas
Loss of heterozygosity (LOH) analyses have confirmed losses of all or part of chromosome 10 in more than 90 percent of tumors (Fig. 57-4) in some series and have narrowed the smallest region of overlapping deletion to 10q25.10,11 Most series also have identified a second region on 10p, and a third site on proximal 10q also has been targeted by some observers.12-14 Li et al.15 and Steck et al.16 have identified a gene located at 10q23 that was mutated or deleted in a subset of gliomas. This gene, called PTEN for phosphatase and tensin homologue deleted on chromosome 10 or MMAC1 for mutated in multiple advanced cancers, is mutated in 24 to 60 percent of glioblastomas with LOH for 10q17-21 and in approximately 40 percent of prostatic and endometrial cancers.22,23 Germ-line mutations of the PTEN/MMAC1 gene are seen in Cowden disease and the Bannayan-Zonana syndrome.24,25 The product of this gene is a protein tyrosine phosphatase, transforming growth factor beta (TGF-β)-regulated and epithelial cell-enriched phosphatase, or TEP1.26 Although mutations of the PTEN/MMAC1 gene are common in high-grade astrocytomas (glioblastomas and anaplastic astrocytomas), they are rarely seen in low-grade astrocytomas.27,28 In addition, among glioblastomas, mutations of this gene are seen more frequently in de novo rather than secondary tumors.29
LOH of glioblastoma chromosome 10. A Southern blot of Taq I-digested DNA (5 μg) from blood (N) and glioblastoma tumor (T) was hybridized to the 10q marker D10S25. This marker showed LOH in tumors 450, 457, 493, and 600 and was uninformative in tumors 519 and 716. The size of the alleles ranged from 1.9 to 3 kb.
Although the PTEN/MMAC1 gene is clearly implicated in a subset of gliomas, the location of this gene is at 10q23, whereas the most frequent region of overlapping deletions in these tumors is at 10q25-6, and the observation that many astrocytomas with LOH for 10q lack mutations of this gene has raised the possibility that another chromosome 10 gene or genes may be involved in gliomas. Candidate genes in the 10q25 region include MXI1 and PAX-2. 30-32 DMBT1, for deleted in malignant brain tumors, is located at 10q25.3-26.1. This gene, which shows homology to the scavenger receptor cysteine-rich superfamily, was shown to be homozygously deleted in 9 of 39 glioblastomas and 2 of 20 medulloblastomas by Mollenhauer et al.33 Although this gene was not expressed in 4 of 5 brain tumor cell lines, the lack of demonstration of point mutations in these tumors raises the possibility that this gene may not be the target of the deletions.
LOH analyses of astrocytomas revealed that approximately one-third of these tumors have loss of all or part of 17p.34-45 Unlike the chromosomal deviations described earlier, which are seen mainly in glioblastomas, LOH for 17p occurs in astrocytomas of all grades. Point mutations of the p53 gene can be demonstrated in the majority of astrocytomas with 17p loss. The mutations are clustered in the same hot spots as are seen in colon, breast, and lung carcinomas. The incidence of TP53 mutations, confirmed by sequence data, is about 25 percent (73 of 295) in glioblastomas, 34 percent (49 of 144) in anaplastic astrocytomas, and 30 percent (33 of 111) in astrocytomas.38-40,45-54 Most of the TP53 studies have concentrated on the conserved exons 5 through 8, but studies that included the entire coding sequence (exons 2–11) have uncovered only a handful of mutations outside of exons 5 through 8.38,40,46,47,50 Similar to colon cancer, codons 175, 248, and 273 are frequently mutated in brain tumors, but the codon that is most frequently mutated in brain is codon 273, whereas in colon it is codon 175.52 TP53 mutations are associated with age of the patient. These alterations are rare among pediatric patients43,45,50,52,55,56 but occur in nearly 50 percent of tumors in young adults, with a much lower incidence (<20 percent) in patients over 50 years of age. Most of the TP53 mutations identified in astrocytomas are G-to-C>A-to-T transitions located at CpG sites and resemble the pattern of mutations found in colon cancer, sarcomas, and lymphomas.
The cytogenetic observation of 9p loss in gliomas prompted evaluation of the α and β interferon genes, which are located at 9p22. Hemizygous or homozygous deletion of interferon genes was reported in glioma cell lines and in biopsies of high-grade astrocytomas, 57,58 but it was not clear in these early studies whether the interferon genes were the target of 9p deletions in gliomas or were simply located near the region of the target gene. In 1994, the CDKN2A and CDKN2B genes, which are located at 9p21, were found to be homozygously deleted in various types of tumors including gliomas.59 By combining data collected on tumor biopsies in several laboratories, the overall incidence of homozygous deletions is 33 percent (98 of 300) in glioblastomas and 24 percent in anaplastic astrocytomas (19 of 79) and for hemizygous deletion or LOH for 9p loci is 24 percent of glioblastomas and 18 percent of anaplastic astrocytomas. The incidence of homozygous deletions of both CDKN2A and CDKN2B is higher in xenografts, approaching 80 percent in some studies.60 Among the 23 low-grade astrocytoma biopsies analyzed, none exhibited homozygous deletion, although 5 showed LOH, altogether there have been only 3 cases of mutations, all in glioblastomas.61-70 The high frequency of homozygous deletions on chromosome 9 and the inclusion of CDKN2A and CDKN2B gene sequences in the deleted region in most cases has led observers to believe that CDKN2A and CDKN2B are the target suppressor genes for 9p loss in gliomas. Unlike the p53 gene, which usually undergoes point mutation, the most common mechanism for CDKN2A gene inactivation in gliomas is homozygous deletion. However, alternative mechanisms such as transcriptional silencing by hypermethylation of CpG islands may be responsible for reduced expression in some gliomas with intact CDKN2A/CDKN2B genes.71
The majority of glioblastomas that possess Dmins contain amplification of the EGFR gene.72 The EGFR gene has been shown to be amplified in one-third to one-half of glioblastomas but only in isolated cases of anaplastic astrocytomas and rarely in other lower-grade tumors. In many glioblastomas, the amplified EGFR gene is also rearranged73,74 (Fig. 57-5). The most common class of mutants bears deletion of exons 2 through 7 of the gene, resulting in an in-frame deletion of 801 bp of coding sequence and generation of a glycine residue at the fusion point. This variant receptor, designated EGFRvIII, has been reported in 17 to 62 percent of glioblastomas.75-80 The tumor cell membrane fractions containing the mutant 140-kDa receptor show a significant elevation in tyrosine kinase activity without its ligand.81 The mutant is still capable of binding with its ligand but at a significantly reduced affinity.82
Southern blot gene amplification, glioblastoma. A Southern blot of EcoRI-digested DNA (5 μg) from blood (N) and glioblastoma tumor (T) was hybridized with EGFR gene probe pE7. The hybridizing fragments, in samples with normal copy number of the gene (all blood and tumor 716), appear as faint bands, and their sizes (in kb) are indicated on the right. In tumors 457 and 519 the gene was amplified, and in 450, 493, and 600 the gene was amplified and rearranged. Arrows indicate variant bands resulting from gene rearrangement.
Relationship between Cell Cycle Regulators in Astrocytomas
In addition to deletions of the CDKN2A and CDKN2B genes as discussed earlier, alterations of other genes involved in cell cycle regulation have been described in subsets of astrocytomas. LOH for 13q or loss of expression of the retinoblastoma (Rb) gene product has been described in 20 to 40 percent of glioblastomas, 12,37,70,83-86 and amplification of the CDK4 gene has been described in up to 15 percent of glioblastomas.64,87 Furthermore, He et al., 86 Biernat, 88 and Ueki et al.70 have shown that most glioblastomas contain only one of these three alterations: (1) CDKN2A/CDKN2B deletion, (2) LOH for 13q or loss of Rb expression, or (3) CDK4 gene amplification or increased expression.
Genetic Alterations in the Progression of Gliomas
It has long been recognized that there are two patterns for the development of glioblastomas. The majority of these tumors occur in patients over 50 years of age, in individuals with no previous indication of a brain tumor. A second group of patients involves younger people whose glioblastomas evolve out of lower-grade astrocytomas. Recent studies have provided molecular markers that in many cases distinguish between these two clinical patterns. The de novo pathway, occurring in older patients, includes tumors over 50 percent of which contain EGFR gene amplification and the majority of which lack TP53 gene mutations.42-44,88 Glioblastomas evolving through progression, in contrast, seldom have EGFR gene amplification, and more than 50 percent contain TP53 gene mutations.41,88-93
Other molecular markers, including LOH for chromosome 10, CDKN2A and CDKN2B deletions, Rb and CDK4 abnormalities, and amplification of other oncogenes do not appear to differ in tumors arising through these two pathways.