Glioblastoma multiforme: the terminator.

G lioblastoma multiforme is the most aggressive of the gliomas, a collection of tumors arising from glia or their precursors within the central nervous system. Clinically, gliomas are divided into four grades; unfortunately, the most aggressive of these, grade 4 or glioblastoma multiforme (GBM), is also the most common in humans. Because most patients with GBMs die of their disease in less than a year and essentially none has long-term survival, these tumors have drawn significant attention; however, they have evaded increasingly cleaver and intricate attempts at therapy over the last half-century. The paper by Gromeier et al. (1) in this issue of PNAS is the newest chapter in this saga, describing a hybrid virus that infects and kills clonal human glioma cell lines, in culture and implanted in athymic mice, without affecting nonneoplastic cells within the brain. For those viewing this battle from a distance, the continued unsuccessful attempts at novel therapies for this disease may be difficult to understand. However, for those treating these patients, and certainly for the patients themselves, the importance and urgency of each attempt is clear. One of the reasons for the resistance of GBM to therapeutic intervention is the complex character of the tumor itself. As the name implies, glioblastoma is multiforme. It is multiforme grossly, showing regions of necrosis and hemorrhage. It is multiforme microscopically, with regions of pseudopalisading necrosis, pleomorphic nuclei and cells, and microvascular proliferation. And it is multiforme genetically, with various deletions, amplifications, and point mutations leading to activation of signal transduction pathways downstream of tyrosine kinase receptors such as epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR), as well as to disruption of cell-cycle arrest pathways by INK4a-ARF loss or by p53 mutations associated with CDK4 amplification or Rb loss (2). These tumors also show intratumor genetic heterogeneity with subclones existing within the tumor cell population (3). It has been estimated that cultured neoplastic and p53-deficient cells may have mutations in any given gene at a rate as high as 1 in 1,000 cells (4). If this is approximately correct for GBMs in vivo, then one would expect a tumor of 109 cells to harbor as many as 106 cells with mutations in any given gene. One of the main reasons that the gliomas are not cured by surgery is the topographically diffuse nature of the disease. In addition to the above-mentioned variability within the tumor proper, the location of the tumor cells within the brain also is variable, resulting in the inability to completely resect this tumor. In 1940, Scherer (5) described the appearance and behavior of glioma cells migrating away from the main tumor mass through the brain parenchyma. The patterns of glioma cell infiltration have since been referred to as the secondary structures of Scherer. These glioma cells migrate through the normal parenchyma, collect just below the pial margin (subpial spread), surround neurons and vessels (perineuronal and perivascular satellitosis), and migrate through the white matter tracks (intrafacicular spread) (Fig. 1). This invasive behavior of the individual cells may correspond to the neoplastic cell’s reacquisition of primitive migratory behavior during central nervous system development. The ultimate result of this behavior is the spread of individual tumor cells diffusely over long distances and into regions of brain essential for survival of the patient. The extreme example of this behavior is a condition referred to as gliomatosis cerebri, in which the entire brain is diffusely infiltrated by neoplastic cells with minimal or no central focal area of tumor per se (6). Furthermore, '25% of patients with GBM have multiple or multicentric GBMs at autopsy (7). Although GBMs can be visualized on MRI scans as mass lesions that enhance with contrast, the neoplastic cells extend far beyond the area of enhancement. Fig. 2 illustrates a typical result of ‘‘gross total resection’’ of a temporal lobe GBM followed 6 months later by recurrence at the surgical margin and elsewhere. Even with repeat surgeries for tumor recurrences, the patients die from tumor spread into vital regions of the brain. The standard of care for treatment of GBM has been essentially unchanged for many decades—surgical resection of as much of the tumor as is safe, followed by radiation therapy and chemotherapy (usually designed to damage DNA or to otherwise inhibit DNA replication). Even under the best of circumstances, in which essentially all of the enhancing tumor seen on MRI scan can be surgically removed and the patients are fully treated with radiation and chemotherapy, the mean survival of this disease is only extended from 2 to 3 months (8) to 1 year (Fig. 3). Because of the poor outcome of the standard treatments for GBM and of the diffuse nature of the disease, a number of clever attempts at novel therapeutic approaches recently have been made with the aim of killing neoplastic cells far from the tumor proper. These approaches have been designed to entice the immune system to reject the tumor, to transfer lethal genes to the tumor cells with gene therapy, or, more recently, to infect with viruses that kill the tumor cells lytically. The immunologic approach has been investigated extensively with many successes in laboratory animals. However, translation of success in rodents to humans has not occurred. Potential explanations for this apparent paradox center on the animal models used in the preclinical experiments. Until recently, animal models for gliomas have consisted of clonal glioma cell lines, maintained in culture, that are injected in the flanks or brains of rodents. These cells grow into mass lesions that eventually kill the animals (9, 10). To what extent either the genetic alterations selected for during passage of the cells in culture or the interactions between tumor cells and the host tissues in these experimental gliomas represents the biology of human gliomas is questionable, especially in the area of immune rejection. Early experiments scored treatment successes as rejection of the implanted allograph by the animals. Since then, syngenic grafts have been used to avoid the non-self-recognition by the host animal (11). Another approach is the transfer of lethal genes to tumor cells by gene therapy. The classic example of this strategy was the retroviral transfer of the herpes