Brain malignancy

Though currently available clinical treatments and therapies have clearly extended the survival of patients with brain tumors, many of these advances are short lived, particularly with respect to high grade gliomas such as glioblastoma multiforme. The missing link to an efficacious treatment of high grade gliomas is a more complete understanding of the basic molecular and cellular origin of brain tumors. 

However, new discoveries of stem cell and developmental neurobiology have now borne the cancer stem cell hypothesis, drawing off of intriguing similarities between benign and malignant cells within the central nervous system. Investigation of cancer stem cell hypothesis and brain tumor propagation is the current frontier of stem cell and cancer biology. Neurosurgery is also watching closely this promising new area of focus. 

“Molecular neurosurgery”, glioma treatments involving biologics using neural stem cells to target the cancer at the level of individual migratory cell, is a rapidly evolving field. This coming progression of applied cancer stem cell research, coupled with current modalities, promises more comprehensive brain cancer interventions. 

Glioma model

Glioblastomas (GBM) are the most malignant solid tumours (grade IV) of CNS. They are glial lineage neoplasias with a high proliferative and invasive capacity, reaching to occupy an entire lobe of the brain . According with their genesis, they can be differentiated between primary and secondary glioblastoma.

The primary is the most common glioblastoma.This is a new generated tumour after a brief medical history (three months), with no evidence of a less malignant lesion. On the other hand, secondary glioblastoma develops from diffuse astrocytoma, anaplastic astrocytoma or oligodendroglioma and malignant progression. 

Its development time is about five years. It is thought that both types of glioblastomas may be generated from neoplastic cells with characteristic of stem cells (Ohgaki & Kleihues, 2009). In addition, these cancer stem cells called “glioma stem cells” (GSCs) may be the responsible for glioma recurrences due to chemo-and radio resistance.  Glioma stem cells (GSCs) are a subpopulation of neoplastic cells identified in glioma sharing properties with neural stem cells (self-renewal, high proliferation rate, undifferentiating, and neurospheres conformation) and the capacity for leading the tumourigenesis and tumour malignancy. 

The proliferation and the invasion into adjacent normal parenchyma have been attributed to glioma stem cells as well. Indeed, they were related to the angiogenesis process needed for the growth and survival of the neoplasia. The microvascular network in gliomas has to get adapt to metabolic tissue requirements . When the vascular network cannot satisfy cell requirements (Oxygen pressure of 5-10 mm Hg) tissue hypoxia occurs. This situation triggers the synthesis of proangiogenic factors as matrix metalloprotease (MMP-2), angiopoietin-1, phosphoglycerate kinase (PGK), erythropoietin (EPO), and vascular endothelial growth factor (VEGF)-A .

Vascular endothelial growth factor (VEGF) is a major regulator of tumour angiogenesis . VEGF acts as mitogen, survival, antiapoptotic and vascular permeability factor (VPF) for the endothelial cells . The increase of this pro-angiogenic factor, secreted either by neoplastic cells or by cells of the tumour microenvironment, induces the start of angiogenesis, the called “angiogenic switch”.

This event results in the transition from avascularised hyperplasia to outgrowing vascularised tumour and eventually to malignant progression. It has been shown in human glioma biopsies that VEGF overexpression correlates directly to proliferation, vascularization and degree of malignancy, and therefore inversely to prognosis . 

The synthesis of VEGF is mediated by the Hypoxia-Inducible Factor (HIF-1), a critical step for the formation of new blood vessels and for the adaptation of microenvironment to the growth of gliomas . Recent researches have reported that glioma stem cells play a pivotal role inducing the angiogenesis via HIF-1/VEGF . 

By the other hand, hypoxia has been related to clones selection of tumour cells. These clones adapted to the tumour microenvironment have acquired the phenotype of tumour stem cell with increased proliferative and infiltrative capacity . Invasion of adjacent normal parenchyma has been attributed to glioma stem cells as well.
Due to these evidences, GSCs are currently being considered as a potential therapeutic target of the tumours. Recent studies have been focused on the identification of GSCs. In human glioblastomas they have been identified using CD133 marker .

In addition to this, it has been reported that ENU glioma model is a representative model for human glioma due to its location and also to its similar cellular, molecular and genetic alterations  Our experience with this model has proven to be useful to study many aspects of tumourigenesis and neoangiogenesis. 

In previous researches we reported the progression of tumour malignancy associated with vascular structural alterations and blood brain barrier (BBB) disturbances . ENU induced glioma permitted us to identify tumour developmentstages following microvascular changes. In addition, it was possible to study the angiogenesis process. Recently, we have used this model to study the relationship between glioma stem cells and angiogenesis process during the neoplasia development.

Classification and genes

Most recent classifications of brain tumours build on the 1926 work of Bailey and Cushing.² This classification named tumours after the cell type in the developing embryo/fetus or adult which the tumour cells most resembled histologically. The cell of origin of the majority of brain tumours is unknown as no pre-malignant states are recognised, as is the case in some epithelial tumour forms. 

In some tumours, cells may be so atypical that it is difficult to compare them with any normal cell type—hence the use of terms such as glioblastoma. Many unsound or illogical terms have remained in the classifications, as once established in a complex medical setting they are difficult to change. In this paper the terminology and definitions of the World Health Organization classification of 2000 will be exclusively used.

There are more than 120 entities in this classification and here we will concentrate on those that most frequently occur in adults and children. These are the pilocytic astrocytomas, ependymomas, and medulloblastomas in children, and the diffuse astrocytic tumours (including astrocytoma, anaplastic astrocytomas, and glioblastomas), oligodendrogliomas, and meningiomas in adults. 

Tumours of the central nervous system often have a wide morphological spectrum and classification is dependent on the recognition of areas with the characteristic histology for a particular tumour type. Immunocytochemical methods may be required to demonstrate the expression by the tumour cells of an antigen typically expressed by a particular cell type and thus to assist in classification. Unfortunately there are no antibodies that unequivocally identify the different tumour types. The presence or absence of an antigen only adds a further piece of information helping to indicate the tumour type. 

Four malignancy grades are recognised by the WHO system, with grade I tumours the biologically least aggressive and grade IV the biologically most aggressive tumours. The histological criteria for malignancy grading are not uniform for all tumour types and thus all tumours must be classified before the malignancy grade can be determined. Only one or two malignancy grades can be attributed to some tumour types. Brain tumours are well known to progress, becoming more malignant with time. Such progression will initially be focal. A patient’s diagnosis is based on the most malignant part of the tumour. Thus it is of the utmost importance to sample the tumour adequately in order to determine its type and judge its malignant potential. It follows that malignancy grading on biopsies/stereotactic biopsies is always a minimum grading as more anaplastic regions may be present in non-biopsied areas.

Cytotoxic or radiation therapy before histological diagnosis may make classification and malignancy grading extremely difficult or impossible. The clinical implications of tumour classification and malignancy grading have been empirically determined. The application of objective methods of measuring cell proliferation and death in tumours to malignancy grading is conceptually attractive but have yet to be accepted and utilised in the malignancy grading of brain tumours. 

The MIB 1 antibody recognising the same antigen as Ki67 as well as other antibodies identifying antigens associated with proliferation (for example, Cdc6 and Mcm5) can be used efficiently on formalin fixed, paraffin embedded tissues following microwave antigen retrieval.

However, wide variations in the proliferation indices are observed in different areas of individual brain tumours and this has resulted in difficulties in defining relevant proliferation levels. The same applies to the assessment of the numbers of cells undergoing apoptosis. The advances in neuroradiology and parallel improvements in stereotactic and surgical techniques permit the biopsy of just about any neoplastic or non-neoplastic lesion in the central nervous system (CNS). The list of potential diagnoses is thus vast. 

The neuropathologist may be expected to make a diagnosis on the basis of often very small and fragmented biopsies. He thus needs to know the clinical background of the case. Information must be provided: age, neuroradiological findings including location of the tumour, relevant clinical and family history, and whether the patient has received any treatment, including steroids. 

As can be deduced from the above, morphology combined with immunocytochemistry may only provide a differential diagnosis and the most likely diagnosis will then only be reached by considering all the information available at a multidisciplinary team meeting. The vast majority of brain tumours are sporadic. A number of familial syndromes are well documented with an increased incidence of brain tumours (see table 1 and the references therein). However, even in the most common syndromes (neurofibromatosis type 1 and neurofibromatosis type 2), the precise relative risk is difficult to deffine.