OS9.1 Clinical significance of hypermutation in gliomas

Abstract BACKGROUND Among patients with glioma, little is known about the clinical significance of hypermutation. We sought to define the determinants of hypermutation in gliomas, and to assess the value of this biomarker for predicting response to standard of care and immune checkpoint blockade. MATERIAL AND METHODS We performed comprehensive molecular characterization of 2420 pediatric and adult gliomas with clinical annotation. We determined the clinical and molecular characteristics associated with hypermutation, and assessed the relationship between hypermutation and clinical response to cancer therapies. RESULTS Overall, 114 hypermutated gliomas were identified. Hypermutation occurred predominantly in therapy-responsive subtypes characterized by methylated MGMT promoter, IDH1/2mutation or 1p/19q co-deletion. Hypermutation was almost always associated with prior treatment with temozolomide and molecular defects in DNA mismatch repair (MMR), which were identified in one-third of IDH1/2-mutated recurrent gliomas and was associated with shorter survival in multivariate analyses (hazard ratio 2.11 [95% CI 1.24–3.6], P=0.006). The molecular mechanisms whereby gliomas undergo hypermutation during therapy with alkylating agents were dissected using patient-derived glioma models in vitro and in vivo. Outcomes of hypermutated gliomas treated with immune checkpoint blockade or with standard of care agents will be presented at the conference. CONCLUSION This study establishes that mutation burden and mutation signatures are clinically and biologically significant biomarkers that can be used to predict therapy response and guide treatment decisions in gliomas.