Introduction: Glioblastoma (GBM) is the most common and deadly primary malignant brain tumor, and homozygous deletion of the methylthioadenosine phosphorylase (MTAP) gene occurs in 50% of all GBM patients. The pathogenic consequence of MTAP deletion in GBM, however, remains unclear. MTAP is a metabolic enzyme involved in purine salvage, and MTAP loss results in the accumulation of its metabolite, methylthioadenosine (MTA), a known methyltransferase inhibitor. We sought to determine what impact, if any, MTAP loss has on tumor cell biology and whether this frequent genetic alteration can provide a basis for targeted therapy.
Methods: We used patient-derived primary GBM cells and other GBM cell lines of known MTAP status and retrovirally restored MTAP function or knocked out MTAP using CRISPR gene editing. We utilized DNA methylation (illumina) and gene expression (Affymetrix) arrays as well as quantitative PCR and pyrosequencing to characterize changes in DNA methylation and gene expression following MTAP status changes. We generated orthotopic xenografts to evaluate tumorigenicity and tested the response of tumor cells to select compounds in vivo and in vitro.
Results: We found that MTAP loss resulted in epigenomic reprogramming with differential gene expression and enhanced tumorigenesis. Furthermore, we found that MTAP loss alters the cellular response to temozolomide and to selected epigenome-modulating compounds, while profoundly sensitizing GBM cells to purine deprivation therapy. Analysis of patient data provides supportive evidence that MTAP deletion affects DNA methylation and gene expression patterns in patient tumors and is associated with poor disease outcomes.
Conclusions: These findings suggest a broad impact of MTAP deletion on the epigenetic landscape and implicate MTAP loss as a contributing factor in the pathogenesis of GBM and as a viable focus for targeted therapies.
Patient Care: We are seeking to develop therapeutic strategies that target metabolic vulnerabilities in tumor cells stemming from genetic alterations such as MTAP deletion. Because of the invasiveness of GBM, targeted therapies are crucial for eliminating invasive tumor cells from the normal brain parenchyma.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) know the frequency and consequences of MTAP deletion in GBM, 2) recognize the clinical value of genomic tumor characterization, 3) Consider novel targeted therapy approaches that exploit tumor cell vulnerabilities.