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Genomic Evolution of Low- and High-Grade Glioma

Malignant transformation (MT) of IDH-mutant low-grade glioma (LGG) to aggressive high-grade tumors is an event of major clinical significance, eventually leading to death in the majority of LGG patients. While patients with primary glioblastoma (GBM) have benefited from increased overall survival due to treatment with the alkylating chemotherapy temozolomide (TMZ), the benefit of TMZ for patients with diffuse low-grade gliomas (LGG) remains unknown. To further explore the relationships among TMZ treatment, hypermutation and malignant progression, we studied tumor evolution in an expanded cohort of untreated and TMZ-treated patients by exome-sequencing of paired IDH1/2-mutant LGGs and their post-TMZ recurrences. Together, these findings further our understanding of the molecular features and clinical behavior of hypermutated clones.

The heterogeneity present in individual tumors, i.e. intratumoral heterogeneity (ITH), is thought to be a major reason why targeted treatments eventually fail. However, we know little about ITH in most human tumors because studies typically analyze one biopsy per tumor without knowledge of where in the tumor the sample was taken. Understanding how malignant and non-malignant cellular populations are distributed in 3D tumor space is foundational knowledge for resolving how human tumors grow, with translational implications for identifying representative biopsy specimens and predicting the impact of targeted therapy.

Diffuse gliomas are incurable brain tumors with variably aggressive molecular subtypes. To determine the extent of ITH and its 3D organization in diffuse glioma, we prospectively collected a median of 10 spatially mapped samples from newly diagnosed and recurrent patient tumors, including aggressive canonical IDH1/IDH2 wild-type (IDH-wtc) glioblastoma, slower-growing IDH1/IDH2 mutant (IDHmut) glioma and a noncanonical IDH-wt glioblastoma (IDH-wtnc) lacking the TERT promoter mutation typical of canonical Glioblastoma.