The Biology of Primary Brain Tumors

Astrocytic Tumors

Diffusely infiltrating astrocytomas are so named because they display morphologic and some biochemical evidence of astroglial differentiation. These are the most common tumors of adults and children, occur throughout the CNS, and exhibit a wide range of histopathologic appearances and clinical behaviors. These tumors are organized by the World Health Organization (WHO) according to tumor grade. Histologically, malignancy is manifested as hypercellularity, cellular atypia, endothelial proliferation, necrosis, and invasion of normal adjacent tissue. WHO grade I tumors are variants of astrocytoma that are generally benign, and WHO grade IV, also known as glioblastoma multiforme (GBM), is the most aggressively malignant. Prognosis is closely associated with pathologic grade. WHO grades II and III exhibit intermediate grades of malignancy, but the evidence of increased mitotic activity in grade III tumors is likely to be a key contributor to poor prognosis. Although low-grade tumors can exhibit a circumscribed growth pattern, all pathologic grades of glioma can exhibit invasiveness, and this characteristic compromises the possibility of treating these tumors with surgery alone. The known propensity of some astrocytic tumors that present as low-grade tumors to progress over time to higher grade tumors has provided insight into the pathogenesis of these tumors, suggesting that they are closely related and that progression is associated with the acquisition of sequential genetic alterations (see Figure 40-1 ; Table 40-2 ). High-grade astrocytic tumors, GBMs that arise in this manner, are called secondary GBMs. The finding of selected genetic changes (e.g., TP53 mutation) in lower grade tumors that are also present in higher grade tumors along with additional mutations typically found only in higher grade tumors (e.g., epidermal growth factor receptor [EGFR] amplification) suggests that specific genetic alterations are associated with particular pathologic features characteristic of the corresponding grade. Genetic changes currently thought to be important in this regard are shown in Figure 40-1 and Table 40-2 , an adaptation of the pathogenesis model first proposed for colon cancer.

Mutation of the TP53 gene is likely to be an early event associated with the change of normal cells to low-grade neoplasia. Commonly mutated residues are codons 248 and 273. TP53 has an important role in stabilizing the genome, and the genetic instability resulting from its loss may contribute to the accumulation of multiple mutations in a single cell that are required for the development of highly malignant tumors. Inactivation of TP53 by mutation or epigenetic mechanisms may occur in up to 75% of astrocytomas. MDM2, encoding MDM2, a protein that inhibits the ability of p53 to promote transcription by targeting the protein for degradation, is amplified in approximately 10% of gliomas, and these invariably have a wild-type TP53 gene. A second gene that is also likely to be mutated early in gliomagenesis is IDH1 . Mutations of IDH1 were first identified in studies sequencing glioma cell genomes. Mutations of IDH1 are found both in lower grade glioma and in GBM . IDH1 encodes isocitrate dehydrogenase, a Krebs cycle enzyme, and its role in tumorigenesis is an area of intense investigation.

Loss of heterozygosity (LOH) of Ch9p21 at the site of the CDKN2A and CDKN2B loci leads to homozygous deletion of these adjacent genes in approximately 60% of GBM. This deletion results in the loss of the p15 (INK4B), p16 (INK4A), and p14 (ARF) tumor suppressor proteins. CDKN2A and CDKN2B encode cyclin-dependent kinase inhibitors, p16 (INK4A) and p15 (INK4B), respectively, which bind to the cyclin-dependent protein kinases CDK4 and CDK6 inhibiting the catalytic activity of the cyclin D–CDK complex. p1 (ARF) is expressed from a distinct transcript as the result of alternative splicing of the CDKN2A gene and functions to keep MDM2 in the nucleolus so that it cannot degrade p53. Loss of p14 (ARF) results in the enhanced degradation of p53. Other cytogenetic changes including +7p/q, +19q, and –1p/q are widely recognized in high-grade brain tumors, but the best understood is clearly the deletion of chromosome 10, where PTEN is located. The PTEN protein product, PTEN, is a lipid phosphatase that antagonizes the function of the phosphatidylinositol-3-kinase (PI3K) family of lipid messengers and consequently inhibits downstream signaling through AKT1, a serine/threonine kinase that is a key regulator of critical cell functions including cell proliferation and survival.

Several different receptor tyrosine kinases (RTKs) and their cognate ligands have been implicated in the malignant behavior of astrocytic tumors, and especially GBM. These include PDGF/PDGFR, EGF and TGF-α/EGFR, IGF/IGFR, and others. Although amplification of EGFR is found in approximately 50% of GBMs, this change is rarely found in WHO grade III and never in grade I or II tumors. Also, small deletions and rearrangements of the EGFR gene are commonly found in GBM. As many as 50% of tumors in which amplification of EGFR is detectable also have a rearrangement of EGFR in which exons 2 to 7 are deleted. This results in an in-frame deletion that encodes a constitutively active EGFR ( EGFRvlll ), which, along with EGFR amplification in tumor cells, predicts a poor outcome. Both antibodies and small-molecule therapeutics have been used to target EGFR in glioma, but resistance to such therapies is rapidly manifested when these treatments are used alone. Although members of the PDGF pathway tend to be overexpressed in all grades of astrocytic tumors, amplification and mutation are rare. Activation of RTKs in glioma leads to autophosphorylation of these receptors, which in turn leads to the docking of a series of proteins on phosphorylated sites of the receptors and downstream activation of various effectors. Activation of RAS by the docking protein SOS1 leads to initiation of the RAF/MEK/MAPK (ERK) pathway that leads to the transcriptional activation of genes important for proliferation. Docking of GAB1 to another phosphorylated site on autophosphorylated RTKs leads to activation of PI3K, which in turn initiates a cascade of phosphorylation leading to PDK1 phosphorylation of AKT1. Activated AKT1 in turn phosphorylates both a number of pro-apoptotic proteins, inactivating them, and a series of transcription factors, activating and leading to transcription of other genes important for cellular proliferation. About 80% of GBMs exhibit activation of AKT1, primarily as a result of RTK activation or deletional inactivation of PTEN .

It is now widely recognized that not all GBMs exhibit the same constellation of genetic alterations, and, interestingly, a second clinical presentation of GBM seems to be associated with a distinctive genetic profile (see Figure 40-1 ; Table 40-3 ). Approximately 90% of patients with GBMs present without evidence of a precursor lesion (de novo, primary GBM). Although high levels of EGFR expression are frequently found in primary GBM, these are rarely detectable in secondary GBM, which is associated with lower grade precursor lesions. In primary GBM, amplification of MDM2 is a more common mechanism for inactivation of TP53 than it is in secondary GBM. The CDKN2A and PTEN loci are also frequently inactivated by mutation in primary GBM, whereas IDH1 is more commonly mutated in secondary GBM.

It has not yet been possible to recognize a characteristic pathology or clinical presentation associated with the genetic heterogeneity that is now well documented to accompany the histologic and cytologic variation characteristic of GBM. Current interpretation of the extensive genomic evaluation of glioma gene structure and gene expression suggests that among histologically indistinguishable tumors, there are definable subtypes of glioma characterized by distinctive molecular alterations (see Table 40-3 ). These classifications may be of prognostic significance, although proven targets for therapeutic intervention have not yet been identified.