Recent Advances in Glial Tumours

Epidemiological data suggests that glioma incidence in adults is on the increase [1]. Why gliomas occur at all in the brain is still an interesting question – after all the brain is relatively protected from the environment by the blood brain barrier and is thought to have a very low proliferative potential compared with other organs. Recent advances in our understanding of gliomas revolve around the cancer stem cell theory and the cytogenetic and epigenetic control of these tumours. This article briefly outlines the general pathology of gliomas and decribes these recent advances in the glioma field.

General pathology of gliomas

Primary CNS tumours are most often composed of cells that resemble glial cells, and are hence called gliomas. For example, tumours of various. grades with cells resembling astrocytes, termed astrocytomas, are the most common glioma (See Figure 1 for examples of different grades of astrocytoma). The least aggressive of these tumours (grade I) are pilocytic astrocytomas with elongated processes and few mitotic figures, classically occuring in children. Grade II astrocytomas often present in young adults with an average survival of five years after diagnosis and histologically comprise cells with bland nuclei and fibrillary processes. By contrast anaplastic astrocytomas (grade III) show obvious cellular atypia and mitotic activity and have a worse clinical prognosis. The most aggressive astrocytoma grade IV glioblastoma, is seen mainly in elderly adults and has a survival often measured in months. The other major categories of glioma include oligodendrogliomas and ependymomas, which display histological similarities to their putative glial origin [2].

Cancer stem cell theory

The cancer stem cell was originally described in the field of haematological malignancies in acute myeloid leukaemia [3] and has now been described in many solid tumours, including gliomas [4,5,6] By definition, cancer stem cells are a population of cells within a tumour that can divide infinitely and have the capacity to differentiate and therefore show similarities to ‘normal’ stem cells out with the cancer field. These cancer stem cells are of great interest because they may represent the population of cells within a tumour that are resist ant to therapy and responsible for tumour relapse. Whether cancer stem cells actually develop from pre-existing human neural stem cells or represent cells which have reacquired a stem-like state as a by-product of tumourigenesis or in vitro culture conditions remains controversial [4,5,6].

Normal human neural stem cells are thought to reside within the brain mostly in the subventricular zone lining the lateral ventricles and dentate gyrus of the hippocampus (see Figure 2) [7,8]. These stem cells persist throughout adulthood into old age and may divide symmetrically for self-renewal and asymmetrically to produce neurons, astrocytes and oligodendrocytes. Direct isolation of human neural stem cells from the human foetal brain using the cell surface marker CD133 by flow cytometry has been reported to produce human foetal brain cells that differentiate, persist and incorporate into the brain [7]. The precise function of CD133, also known as prominin, remains unclear, however it was originally shown to be a haemopoetic stem cell marker [9]. More recently the Dirks group and others have used CD133 to isolate a population of brain tumour stem cells within glioblastoma.10-14 Original papers suggested that CD133-positive stem-like cells were the only population of cells within the glioblastoma that were capable of producing tumours when transplanted into the brains of immunodeficient mice [13]. More recent studies suggest that this may not always be the case [14].

The question whether there is a universal glioma cancer stem cell, or whether different subtypes of glioma contain different stem cells remains unanswered. Work by Gilbertson’s group suggests that the stem cell may vary according to the nature of the original tumour. He has shown that subtypes of a different type of glial tumour (ependymoma) may derive from radial glia at different locations at different stages of development in the nervous system. It is possible that radial glia may represent the stem cell of ependymomas. These cells express CD133, RC2 and nestin, which are present on radial glia and human neural stem cells [15].

It seems that glioma stem cells are dependent on their microenvironment in order to maintain stem cell properties. There is evidence to suggest that endothelial cells interact and secrete factors in vitro that maintain cells in a stem-like state [16]. Increasing the number of endothelial cells expands the population of self-renewing cells and their tumourigenic properties. It is therefore possible that stem-like-cells survive within a vascular niche.

Recent exciting work has shown that in vitro it is possible to make certain types of glioma initiating stem cells differentiate into neuronal type cells using manipulation of the Bone Morphogenetic Protein (BMP) pathway, which is involved in differentiation of human embryonic stem cells to human neural stem cells [17]. This study demonstrates a major differentiation block in a subset of glioblastoma caused by the Polycomb repressor complex (PRC)-mediated epigenetic silencing of the BMPR1B promoter, analogous to early embryonic neural stem cells. Although this is a radical departure from traditional therapy using surgery, chemo- and radiotherapy, the ability to control glioma cells by forcing them to undergo a differentiation programme represents a major conceptual advance in the field.

Cytogenetics and epigenetics of glioma

The most recent major advance in our understanding of glioblastoma has been achieved by the Cancer Genome Atlas (TCGA) pilot project. This aims to assess the value of large-scale multidimensional analysis of molecular characteristics in human cancer and to provide data rapidly to the research community. They recently reported the interim integrative analysis of DNA copy number, gene expression and DNA methylation aberrations in 206 glioblastomas [18]. They studied alterations in the RTK/RAS/PIK pathway, and p53 and RB signalling pathways, involved in the control of cell cycle and apoptosis in the majority of glioblastomas. RTK/Ras/PIK signalling was altered in 88%, p53 signalling altered in 87% and RB signalling altered in 78% of cases. This study appears to blow apart the conventional dogma that primary and secondary gliomas derive by different restricted genetic pathways. It implicates different players in glioblastoma pathogenesis including ERBB2, NF1 and PIK3R1, which are different components of growth factor and cell cycle control pathways (See Table 1). This study also examined the methylation status of MGMT, a repair protein that specifically removes promutagenic alkyl groups from DNA. This has been implicated in drug resistance since it reduces the cytotoxicity of alkylating agents such as temozolomide – an alkylating agent used to treat glioblastoma. Patients with newly diagnosed glioblastomas with MGMT methylation respond well to temozolomide. The alkylated DNA results in cycles of futile mismatch repair, leading to cell death. The study showed that treated glioblastomas were under a strong selective pressure to lose mismatch repair function predicting that such patients would eventually develop treatment resistance. The authors suggest that combining alkylating agents with an agent designed to target mismatch-repair-deficient cells as a first-line therapy may minimise the emergence of drug resistance [18].

There has been recent progress in gliomas traditionally thought to lack karyotypic abnormalities. Such genetic abnormalities may be key events in the development of these tumours. In particular a tandem repeat has been identified in most pilocytic astrocytomas which produces a novel oncogenic BRAF fusion gene, involved in growth factor stimulated tyrosine kinase path- ways which transforms fibroblasts in vitro [19]. A different type of glioma (subependymoma) also carries cytogenetic abnormalities in a proportion of cases [20]. As the sensitivity of these techniques increases and we head inexorably to whole genome sequencing of tumours, the. number of abnormalities identified will further increase.

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Conclusions

The cancer stem cell theory predicts that not all tumour cells are equal. We may have the ability to control glioma cells by forcing them to differentiate. This represents a major conceptual advance in the field, but it is still controversial. Major new insights into the genetics and epigenetics of gliomas will hopefully direct a new era of patient-specific combined molecular therapies. 

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