Glioma morphology and tumor-induced vascular alterations revealed in seven rodent glioma models by in vivo magnetic resonance imaging and angiography.

Between 15,000 and 20,000 people are diagnosed each year with a primary malignant brain or central nervous system (CNS) tumor in the United States alone (1). Eighty percent of these tumors are gliomas, i.e. tumors originating from the glial cells of the brain, and especially Glioblastoma multiforme (GBM). GBM is the most aggressive glioma type, diagnosed in nearly 10,000 patients each year, and its prognosis is very low. In the best current treatment situation, only 5% of these patients survive the 3-year mark (1). This poor prognosis is due to several factors, such as the resistance to treatment of gliomas and the difficulty in obtaining an early and accurate diagnosis. A clearer understanding of the glioma microenvironment would certainly be crucial for a better diagnosis and an improved patient survival. Indeed, for clinicians to decide on an adequate treatment for the patient, tumor grading must be quick and precise (2), while being virtually non-invasive. From this point of view, non-invasive in vivo imaging tools such as magnetic resonance imaging (MRI) and angiography (MRA) are ideal modalities, since they can be used to assess glioma morphology, growth pattern and the angiogenesis process inherent to any tumor type (3). Morphological imaging can particularly provide detailed information on the tissue structure of the glioma and help the assessment of necrosis, which is characteristic of high-grade gliomas and poor prognosis (4). Diffuse infiltration is also interesting to assess, since gliomas are known to exhibit dramatic invasiveness and proliferation as soon as being grade II (5). MRA techniques can follow the vascular alterations brought by the glioma during its progression (6). Neovascularization is an event that distinguishes the moderately aggressive grade III from the GBM-like grade IV tumors (4, 7), and the nature of vascular alterations can also inform on the prognosis (8, 9). MRA would thus be an interesting imaging modality for the study of gliomas. We were also highly interested in comparing glioma models varying in their degree of malignancy and provide a characterization for these models as extensive as our MRI methods allow. The models that we chose, from slow-growing ethyl-nitrosourea (ENU)-induced gliomas (10) to more aggressive rodent C6, GL261, 9L/LacZ, RG2 and F98 glioma models, as well as the human U87 glioma model (11), are all considered as good glioma models. However, investigators have underlined the importance of using diverse tumor models during preclinical studies (12), since morphological or biochemical differences between models could potentially result in different treatment responses. It was thus in our interest to characterize these models as precisely as possible, to identify if they really are similar in their growth pattern, morphology and angiogenic behavior. A model of spontaneous chemically-induced gliomas was obtained by exposing pregnant females to ENU, which allowed us to study the transition between low-grade, slow-growing gliomas and high-grade tumors (13). Among the existing glioma cell lines, the C6 model has been widely used as a reliable model of GBM (14) and presenting high similarity with human glioma cells in the expression of genes involved in tumor progression (15). 9L/LacZ (16), F98 (11), GL261 (17) and RG2 (11) cells give rise to more aggressive, infiltrative and angiogenic gliomas. Finally, human U87 cells are considered highly tumorigenic and of high interest for angiogenesis studies (18). In summary, we used MRI and non-contrast-enhanced MRA techniques to comprehensively characterize diverse glioma models (ENU-induced, C6, F98, 9L/LacZ, GL261, RG2 and U87), as well as to show that these imaging techniques are sufficient to precisely depict glioma behavior in vivo.