GENO-25HYPERMUTATION AND MALIGNANT PROGRESSION IN AN EXPANDED COHORT OF TEMOZOLOMIDE-TREATED LOW-GRADE GLIOMA PATIENTS

Abstract

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. We previously demonstrated that among ten TMZ-treated patients with initial LGG (IDH1 mutant, 1p19q intact), treatment induced hypermutation in the recurrent tumors of six patients, all six of whom underwent malignant progression to secondary GBM (sGBM) (Johnson & Mazor, Science, 2014). To further explore the relationships among TMZ treatment, hypermutation and malignant progression, we studied tumor evolution in an expanded cohort of 35 TMZ-treated patients by exome-sequencing of paired IDH1/2-mutant LGGs and their post-TMZ recurrences. This expanded cohort allowed us to contrast patterns of hypermutation between 1p19q intact versus co-deleted subgroups. Hypermutated recurrences were common in both subgroups and consistently underwent malignant progression to the highest grade while acquiring somatic mutations in mismatch repair (MMR) genes. In contrast to the 1p19q intact subgroup in which TP53 mutations were present at diagnosis, the hypermutated recurrences of the 1p19q co-deleted subgroup frequently acquired TMZ-associated mutations in TP53. This suggests that TP53 mutations may cooperate with MMR mutations to convert TMZ-induced cytotoxicity to mutagenicity. Once hypermutated clones arose, we found that they dominated subsequent recurrences. In one case, a hypermutated sGBM was resected, but soon recurred with leptomeningeal spread, at which point hypermutated clones were found in the spinal cord. We examined the clonality of hypermutated tumors through exome-sequencing of multiple spatially distinct samples, from which we estimate that hypermutated recurrent tumors can originate from a single initiating cell. Together, these findings further our understanding of the molecular features and clinical behavior of hypermutated clones, raising concerns about the potent mutagenic activity of TMZ and other alkylating agents in LGG patients.