Low-grade glioma (LGG) represents a histologically and genetically heterogeneous group of slow-growing and invasive tumors, including oligodendroglioma (molecularly defined as 1p/19q/IDH1 mutant) and astrocytoma (TP53/ATRX/IDH1 mutant). The most prevalent anti-glioma drug, temozolomide (TMZ), extends overall survival for high-grade glioma (HGG) patients, but can drive genetic hypermutation (HM) and malignant progression in LGG. We recently reported genetic HM in 6 of 10 recurrent astrocytomas upon TMZ treatment, with all hypermutant recurrences progressing to HGG and harboring TMZ-related mutations in the RB and AKT-mTOR pathways, all of which are drivers of HGG. We have recently expanded our TMZ-treated LGG cohort to include paired primary and recurrent tumors from 24 astrocytomas and 13 oligodendrogliomas, allowing a deeper survey of HM, tumor evolution, and malignant progression in two distinct LGG subtypes. Our preliminary analyses show that 7 of 13 TMZ-treated, initially low-grade oligodendrogliomas hypermutated upon recurrence, with the majority of hypermutant recurrences progressing to HGG and harboring TMZ-related mutations in the p53 (common in pretreatment astrocytoma) and AKT-mTOR pathways. These results suggest a genetic convergence of astrocytoma and oligodendroglioma upon TMZ-induced HM and malignant progression. Initial pathway analysis yields chromatin modifiers as a top-ranked gene class common to HM in both LGG subtypes, with MLL methyltransferases and the SWI/SNF chromatin remodeling complex being the most frequent. Deleterious mutations in each of these families are driving events in an array of cancers, including HGG. The hypothesis of this proposal is 1) TMZ-induced hypermutation pushes initially distinct low grade astrocytomas and oligodendrogliomas down a convergent path to malignant progression and 2) the convergence is mediated, in part, by recurrent mutations in chromatin modifier genes. First, genetic pathways or gene families most associated with TMZ-induced HM in both LGG subtypes will be determined from exome mutation calls from initial and recurrent LGG tumors. Importantly, HM- associated candidate tumor driver pathways will be explored by comprehensive pathway enrichment analysis. Next, I will determine the functional role of 2 candidate driver mutations (CDMs) in a chromatin modifier gene (eg. MLL3) by engineering CDMs in 1p/19q/IDH1 mutant oligoastrocytoma cells. The consequence of the CDMs will be tested by ChIP-seq for CDM protein and CDM-related histone marks (eg. H3K4me1), as well as subsequent genomic expression changes by RNA-seq. Finally, the role of the CDMs on malignant progression will be investigated in vivo. These functional studies serve to expand our understanding of convergent LGG evolution, as well as genetic and epigenetic drivers of malignant progression in the context of TMZ-induced HM.
Temozolomide is the most common and successful anti-glioma drug, but it can drive genomic hypermutation of these two histologically distinct subtypes, as well as malignant progression to high-grade glioma with poorer prognosis. This proposal aims to leverage uniquely robust patient data to study the genomic impacts of temozolomide-driven hypermutation in two low-grade glioma subtypes, astrocytoma and in oligodendroglioma. Expanding our limited understanding of the genetic and epigenetic impacts of temozolomide treatment in glioma hypermutation will help to inform better clinical practices in the future.
|Grimmer, Matthew R; Costello, Joseph F (2016) Cancer: Oncogene brought into the loop. Nature 529:34-5|