Introduction: Glioblastomas (GBM) infiltrate the brain, making complete removal by surgical resection impossible. Current analyses are predicated on biopsies taken from within the contrast-enhancing (CE) region of GBM without radiographic localization of the sample. However the mixture of tumor and non-neoplastic/reactive neural cells that remain after surgery - in the non-enhancing (NE) FLAIR positive region of the tumor - form the biological context for tumor recurrence and adjuvant therapies. Thus analysis of tissue sampled from both CE and NE regions of the tumor would provide insight into intra-tumoral heterogeneity at the cellular and molecular level.
Methods: In 69 patients with high-grade glioma, multiple radiographically-localized biopsies were obtained from the CE core and the NE infiltrative edge of the tumor prior to debulking. 9 normal brain samples were also acquired as controls from patients undergoing shunt placement. Histological, immunohistochemical and RNA-Seq analysis were performed on these biopsies. A computational approach was used to deconvolve the RNA-seq dataset, and estimate the expression profiles of six different cell types including Olig2+ progenitor cells (OPC), CD44+ astrocytes, CD44- astrocytes, microglia, oligodendrocytes and neurons.
Results: Biopsies taken from CE regions resembled Proneural, Classical or Mesenchymal subtypes of GBM, while biopsies taken from NE regions predominantly resembled the Neural subtype. Comparing the NE regions of GBM to normal brain revealed that differentially expressed genes are distributed among multiple cell types. The NE regions of Proneural tumors were enriched in OPC genes, while the NE region of Classical and Mesenchymal GBM were enriched in astrocytic and microglial genes.
Conclusions: The CE core and NE region of GBM have different molecular and cellular compositions. The differentially expressed genes in NE tissue are distributed across multiple cell types and display a GBM subtype-specific pattern. Understanding these patterns will facilitate development of new therapeutic strategies to target residual disease responsible for tumor recurrence.
Patient Care: This work will lead to greater understanding of the NE FLAIR positive region of GBM. This area is often not resected during surgery and thus forms the biological basis for recurrence. A greater understanding of this area will have implications for adjuvant interventions.
Learning Objectives: By the conclusion of this session, participants should be able to describe how the molecular and cellular composition of the non-enhancing FLAIR positive region of Glioblastoma varies with tumor subtype.