EPCO-37. LONGITUDINAL TUMOR-WIDE SAMPLING OF GLIOBLASTOMA REVEALS EARLY EVOLUTIONARY DIVERGENCE OF RECURRENCE AND CLUES TOWARDS IDENTIFYING THE BIOLOGICAL TUMOR CENTER

Abstract Therapy resistance in glioblastoma is in part attributed to intratumoral heterogeneity and treatment-resistant cell populations. Understanding heterogeneity requires more complete tumor sampling at diagnosis and at recurrence. We used a whole-tumor sampling approach to obtain 43 spatially mapped biopsies from 3 glioblastomas at diagnosis and recurrence, Pyclone and ClonEvol imputed clonal evolution from exome data, and weighted gene co-expression network analysis inferred gene expression programs from RNA sequencing. Across the cohort, tumor-wide clonal alterations representing evolutionarily early expansions included canonical changes (i.e. Chr7 gain, EGFR amplification) and a diverse set of copy number variations (Chr19/20 gain), driver mutations (i.e. PTEN, KDR), and fusions (LIMCH1::UCHL1, KANK::DOCK8). In 2/3 patients, the founding clone was the only subclone common to primary and recurrence samples and 37% of cancer drivers across the cohort appeared after evolutionary divergence of the primary and recurrent tumors. We identified thirty-two transcriptional programs, six of which were differentially expressed between timepoints. Programs enriched for mitochondrial activity, endothelium/pericytes and classical glioblastoma subtype signature were more highly expressed in primary samples. Programs enriched for mesenchymal-like state, oligodendrocytes and inhibitory interneurons were more highly expressed in recurrence samples. At diagnosis, 11/16 samples showed higher expression of the classical subtype program compared to mesenchymal-like, but at recurrence the mesenchymal program dominated in 18/18 samples. Expression of the programs correlated more strongly with position relative to the contrast-enhancing rather than T2 tumor centroid (p<0.001). These findings reveal novel diversity and complexity in the genetic roots of individual glioblastoma. Recurrences arose from subclones diverging early in evolution of the primary tumor and contained clonal drivers not detected in the primary. Expression data revealed transition from an intratumorally heterogeneous mixture of classical and mesenchymal signatures to tumor-wide mesenchymal at recurrence and suggests that the contrast enhancing centroid may serve as the biological center of the tumor.