GBM is the most malignant stage of astrocytoma, with survival times of less than 2 years for most patients. These tumors are characterized histologically by dense cellularity, high proliferation indices, endothelial proliferation, and focal necrosis. The highly proliferative nature of these lesions is no doubt the result of multiple mitogenic effects. As mentioned above, at least one such effect is deregulation of the p16-cdk4-cyclin D1-pRb pathway of cell-cycle control. The vast majority, if not all, GBM have alterations of this system, whether inactivation of p16 or pRb or overexpression of cdk4 or cyclin D1.

Chromosome 10 loss is a frequent finding in GBM, occurring in 60% to 95% of GBMs but only rarely in anaplastic astrocytomas. Attempts to identify this tumor-suppressor gene by deletion mapping, however, have been hampered by the observation that, in most cases, the entire chromosome is lost. The gene on the long arm may map to band q25. On the other hand, there is probably a second tumor-suppressor gene on the short arm, and one study has postulated that a third locus may exist on the long arm, near the centromere.

EGFR is a transmembrane receptor tyrosine kinase whose ligands include EGF and transforming growth factor- alpha. The EGFR gene is the most frequently amplified oncogene in astrocytic tumors, being amplified in approximately 40% of all GBM but in few anaplastic astrocytomas. Those GBMs that exhibit EGFR gene amplification have almost always lost genetic material on chromosome 10. GBMs with EGFR gene amplification display overexpression of EGFR at both the mRNA and protein levels, suggesting that activation of this growth signal pathway is integral to malignant progression to GBM. Approximately one-third of those GBM with EGFR gene amplification also have specific EGFR gene rearrangements, which produce truncated molecules similar to the v-erbB oncogene. These truncated receptors are capable of conferring dramatically enhanced tumorigenicity to GBM cells. The downstream targets of EGFR activation in GBMs are not well defined, but EGFR is most likely involved in a cascade that facilitates mitogenesis in tumor cells. Less commonly amplified oncogenes include N- myc, gli, PDGF- alpha receptor, c- myc, myb, K- ras, CDK4, and MDM2, some of which have been discussed above.
Cancer information
As mentioned above, one of the hallmarks of GBM is endothelial proliferation. A host of angiogenic growth factors and their receptors are found in GBMs. For example, VEGF and PDGF are expressed by tumor cells while their receptors, flk-1 and flt-1 for VEGF and the PDGF beta-receptor for PDGF, are expressed on endothelial cells. VEGF and its receptors, in particular, appear to play a major role in GBM angiogenesis. A paracrine mechanism has been suggested in which VEGF is secreted by tumor cells and bound by the VEGF receptors on endothelial cells. VEGF is preferentially upregulated by tumor cells surrounding regions of necrosis, perhaps as a result of necrosis-induced hypoxia, since hypoxia can upregulate VEGF. A link between p53 and tumor angiogenesis has been suggested by the observations that some mutant p53 molecules can enhance VEGF expression and that wild-type p53 regulates the secretion of a glioma-derived angiogenesis inhibitory factor.

Human brain tumors have molecular alterations characteristic of each type of tumor and of most stages of progression. For instance, the formation of grade II astrocytoma involves inactivation of the p53 tumor-suppressor gene on chromosome 17p, as well as PDGF overexpression, loss of a putative tumor-suppressor gene on chromosome 22q, and the expression of various molecules that facilitate tumor invasion. The transition from astrocytoma to anaplastic astrocytoma is associated with alterations of the critical cell-cycle regulatory pathway that includes p16, cdk4, cyclin D1, and Rb, as well as a putative tumor suppressor gene on chromosome 19q. Finally, progression to GBM involves loss of a at least one putative tumor-suppressor gene on chromosome 10, amplification of the EGFR gene and the expression of angiogenic factors such as VEGF. Furthermore, molecular genetic analysis has been used to identify subsets of astrocytomas. For instance, one type of GBM, characterized by p53 gene mutations, is more common in younger patients and may be associated with slower progression from lower-grade astrocytoma; another type of GBM, characterized by EGFR gene amplification, is more common in older patients and may be associated with more rapid progression or de novo growth.

For the less common gliomas and for other primary tumors such as medulloblastomas, molecular genetic studies have defined only isolated genetic alterations. For meningiomas and schwannomas, the NF2 gene has been clearly implicated, although other genetic alterations must underlie the formation of some meningiomas as well. For those tumors associated with hereditary tumor syndromes, such as the SEGAs in TS and the hemangioblastomas in VHL, the same genes appear responsible for the syndromes when mutated in the germline, and for sporadic tumors when mutated on a somatic basis. At the present time, however, these molecular data are incomplete. Once the molecular pathways are completely understood, such knowledge will no doubt contribute to the development of more effective therapies for many of these tumors.

The transition from WHO grade II astrocytoma to WHO grade III anaplastic astrocytoma is accompanied by a marked increase in malignant behavior. Although many patients with grade II astrocytomas survive for 5 or more years, patients with anaplastic astrocytomas often die within 2 or 3 years and frequently show transformation to GBM. The major histologic differences between grade II and grade III tumors are increased cellularity and the presence of mitotic activity, implying that higher proliferative activity is the hallmark of the progression to anaplastic astrocytoma.

A number of molecular abnormalities have been associated with anaplastic astrocytoma, and recent studies have suggested that most of these abnormalities converge on one critical cell-cycle regulatory complex that includes the p16, cyclin-dependent kinase 4 (cdk4), cyclin D1 and retinoblastoma (Rb) proteins. The simplest schema suggests that p16 inhibits the cdk4/cyclin D1 complex, preventing cdk4 from phosphorylating pRb, and so ensuring that pRb maintains its brake on the cell cycle. Individual components in this pathway are altered in up to 50% of anaplastic astrocytomas and in the majority of GBM.

Chromosome 9p loss occurs in approximately 50% of anaplastic astrocytomas and GBMs, with 9p deletions occurring primarily in the region of the CDKN2/p16 (or MTS1) gene, which encodes the p16 protein. The frequency of 9p loss increases not only at the transition from astrocytoma to anaplastic astrocytoma but also at the transition from anaplastic astrocytoma to GBM, implying that the 9p tumor suppressor plays a role in different stages of astrocytoma progression. Although debate has raged on whether the CDKN2/p16 gene is the primary glioma tumor-suppressor gene on chromosome 9p, current evidence does implicate CDKN2/p16. Deletions in primary GBMs almost always involve CDKN2/p16 and three mutations have been described in primary GBMs with allelic loss of chromosome 9p. In addition, reduced or absent p16 expression occurs in some malignant gliomas without CDKN2/p16 loss, suggesting alternative means, such as hypermethylation, of inactivating this gene in GBMs. Moreover, replacement of CDKN2/p16 into GBM cell lines lacking the gene results in growth suppression, but had no effect in cell lines containing the CDKN2/p16 gene.

Loss of chromosome 13q occurs in one-third to one-half of high-grade astrocytomas, suggesting the presence of an progression-associated astrocytoma tumor suppressor gene on that chromosome. The 13q14 region containing the RB gene is preferentially targeted by these losses and inactivating mutations of the RB gene occur in primary astrocytomas. Overall, analysis of chromosome 13q loss, RB gene mutations, and Rb protein expression suggests that the RB gene is inactivated in about 20% of anaplastic astrocytomas and 35% of GBM. RB and CDKN2/p16 alterations in primary gliomas are inversely correlated, rarely occurring together in the same tumor.

Because amplification of the CDK4 gene and overexpression of cyclin D1 may have similar effects to p16 or pRb inactivation, these mechanisms may provide additional alternatives to subvert cell-cycle control and facilitate progression to GBM. CDK4, located on chromosome 12q13-14, is amplified in 15% of malignant gliomas, although this frequency may be higher among cases without CDKN2/p16 loss, perhaps reaching 50% of GBMs without CDKN2/p16 loss. CDK4 amplification and CDKN2/p16 deletions do not occur together in GBM cell lines and some GBM cell lines overexpress cyclin D1. On the other hand, in some GBMs and GBM cell lines, CDK4 amplification and cyclin D1 overexpression appear to represent alternative events to CDKN2/p16 deletions, since these genetic changes only rarely occur in the same tumors. In combination, it is likely that up to 50% of anaplastic astrocytomas and perhaps all GBM have alterations in at least one component of this critical cell-cycle regulatory pathway.

Allelic losses on 19q have been observed in up to 40% of anaplastic astrocytomas and GBMs, indicating a progression-associated glial tumor suppressor gene on chromosome 19q. This tumor suppressor gene may be unique to glial tumors and is involved in all three major types of diffuse cerebral gliomas (astrocytomas, oligodendrogliomas, and oligoastrocytomas). This gene maps to a region of chromosome 19q13.3: telomeric to the marker D19S219 and centromeric to the HRC gene. A number of candidate genes have been isolated from or mapped to this region, including the BAX gene, whose product negatively regulates apoptosis with bc1-2, but the tumor suppressor gene remains to be identified.

Diffuse, fibrillary astrocytomas are the most common type of primary brain tumor in adults. These tumors are classified histopathologically into three grades of malignancy: World Health Organization (WHO) grade II astrocytoma, WHO grade III anaplastic astrocytoma, and WHO grade IV glioblastoma multiforme (GBM). WHO grade II astrocytomas are the most indolent of the diffuse astrocytoma spectrum. Nonetheless, these low-grade tumors are infiltrative and have a marked potential for malignant progression, and any biologic model for astrocytomas must account for these cardinal features of malignant progression and invasion.

The p53 gene, a tumor-suppressor gene located on chromosome 17p, has an integral role in a number of cellular processes, including cell cycle arrest, response to DNA damage, apoptosis, angiogenesis, and differentiation; as a result, p53 has been called the guardian of the genome. The p53 gene is involved in the early stages of astrocytoma tumorigenesis. For instance, p53 mutations and allelic loss of chromosome 17p are observed in approximately one-third of all three grades of adult astrocytomas, suggesting that inactivation of p53 is important in the formation of the grade II tumors. Moreover, high-grade astrocytomas with homogeneous p53 mutations evolve clonally from subpopulations of similarly mutated cells present in initially low-grade tumors. Such mutation studies are complemented by functional studies that have recapitulated the role of the p53 inactivation in the early stages of astrocytoma formation. For instance, cortical astrocytes from mice without functional p53 appear immortalized when grown in vitro and rapidly acquire a transformed phenotype. Cortical astrocytes from mice with one functional copy of p53 behave more like wild-type astrocytes and only show signs of immortalization and transformation after they have lost the one functional p53 copy. Those cells without functional p53 become markedly aneuploid, confirming prior work showing that p53 loss results in genomic instability cellular phone electromagnetic radiation and that astrocytomas with mutant p53 are often aneuploid. Thus, the abrogation of astrocytic p53 function appears to facilitate some events integral to neoplastic transformation, setting the stage for further malignant progression.

Many growth factors and their receptors are overexpressed in astrocytomas, including platelet-derived growth factor (PDGF), fibroblast growth factors (FGFs), and vascular endothelial growth factor (VEGF). For example, PDGF ligands and receptors are expressed approximately equally in all grades of astrocytoma, suggesting that such overexpression is also important in the initial stages of astrocytoma formation. Tumors often overexpress cognate PDGF ligands and receptors in an autocrine stimulatory fashion. The mechanisms for PDGF overexpression in most cases have not been elucidated, although rare astrocytomas display amplification of the PDGF alpha-receptor gene. Loss of chromosome 17p in the region of the p53 gene is closely correlated with PDGF alpha-receptor overexpression; 17p loss is most often seen in those astrocytomas that have PDGF alpha-receptor overexpression. These observations may imply that p53 mutations have an oncogenic effect only in the presence of PDGF alpha-receptor overexpression. This interdependence is highlighted by observations that mouse astrocytes without functional p53 become transformed only in the presence of specific growth factors.

Astrocytomas display a remarkable tendency to infiltrate the surrounding brain, confounding therapeutic attempts at local control. These invasive abilities are often apparent in low-grade as well as high-grade tumors, implying that the invasive phenotype is acquired early in tumorigenesis. Investigations into astrocytoma invasion have highlighted the complex nature of cell-cell and cell-extracellular-matrix interactions. A variety of cell surface molecules such as CD44 glycoproteins, gangliosides, and integrins are differentially expressed in astrocytomas. Some, such as the A2B5 ganglioside, are expressed primarily by nondividing cells that are migrating; others appear somewhat specific for neoplastic astrocytes. Many of the growth factors expressed in astrocytomas, such as FGF, EGF, and VEGF, also stimulate migration.

Less common molecular changes also occur in grade II astrocytomas. Loss of chromosome 22q, for instance, suggests the presence of a chromosome 22q glioma tumor suppressor gene. Although the neurofibromatosis 2 (NF2) gene was a likely candidate for this gene, NF2 mutations do not occur in astrocytomas and deletion mapping of chromosome 22q in astrocytomas has suggested a more telomeric locus.