MTR-19A MACROPHAGE-/MICROGLIAL-RICH TUMOR MICROENVIRONMENT MIMICS PRONEURAL TO MESENCHYMAL TRANSITION IN GLIOBLASTOMA

Abstract

GBM have been classified into four transcriptional subtypes, termed proneural, neural, classical, and mesenchymal, which are characterizedby distinct genomic properties but reports on how transcriptional classification relates to clinical outcome have been conflicting. Methods for classification of new GBMs according to these subtypes have shown poor concordance. We used robust classification methods to classify TCGA samples and found that 37% of GBM could be assigned to multiple expression subtypes, suggesting that mutually exclusive subtype classification may be too strict for deriving robust biologic correlates. We observed that the neural GBM class showed a significant reduction in tumor cellularity, which could be traced to the presence of normal brain tissue. Additionally, mesenchymal GBM were associated with decreased tumor cell fractions which we related to an increase in the microglia. Consequently, microglial marker genes such as ITGAM and AIF1 were found to be highly expressed in thirteen mesenchymal GBM but could not be detected in neurosphere GBM models derived from them. Immunostaining unequivocally identified microglial cells as the source for ITGAM and AIF1 protein, confirming the tumor microenvironment as a key component of the mesenchymal class. GBM was previously described to undergo proneural-to-mesenchymal transition which has been hypothesized as a contributor to therapy resistance. Analysis of 67 matched primary-recurrent GBM pairs showed that most (48 of 67) GBM tumors retained their expression subtype after treatment but cases that showed a transition towards the mesenchymal class at recurrence were specifically associated with a decrease in tumor purity relative to their primary counterparts. We did not find evidence for a tumor cell intrinsic increase in mesenchymal gene expression. Our study provides critical insights into the origin of the neural and mesenchymal subtypes, their transition from other subtypes upon recurrence, and place emphasis on the contribution of the microenvironment in glioma biology.