LAUSR

High tumor mutational burden (hypermutation) is observed in some gliomas; however, its mechanisms of development and whether it predicts immunotherapy response are poorly understood. Here, we comprehensively analyze the molecular determinants of mutational burden and signatures in 10,294 gliomas including AACR Project GENIE and institutional datasets. We delineate two main pathways to hypermutation: a de novo pathway associated with constitutional defects in DNA polymerase and mismatch repair (MMR) genes, and a more common post-treatment pathway associated with acquired resistance driven by MMR defects in chemotherapy-sensitive gliomas that recur after temozolomide treatment. Experimentally, the mutational signature of post-treatment hypermutated gliomas was only recapitulated by temozolomide-induced damage in cells harboring MMR deficiency. MMR-deficient gliomas exhibited unique features including the lack of prominent T-cell infiltrates, extensive intratumoral heterogeneity, poor survival and low response rate to PD-1 blockade. Moreover, while microsatellite instability in MMR-deficient gliomas was not detected by bulk analyses, single-cell whole-genome sequencing of post-treatment hypermutated glioma cells demonstrated microsatellite mutations. This study shows that chemotherapy can drive acquisition of hypermutated populations without promoting response to PD-1 blockade and supports diagnostic use of mutational burden and signatures in cancer. Citation Format: Mehdi Touat, Yvonne Y. Li, Adam N. Boynton, Liam F. Spurr, Bryan Iorgulescu, Craig L. Bohrson, Isidro Cortes-Ciriano, Jack E. Geduldig, Kristine Pelton, Mary J. Lim-Fat, Sangita Pal, Shakti H. Ramkissoon, Frank Dubois, Charlotte Bellamy, Naomi Currimjee, Kenin Qian, Seth Malinowski, Aniket Shetty, Kin-Hoe Chow, Maite Verreault, Erell Guillerm, Samy Ammari, Frederic Beuvon, Karima Mokhtari, Agusti Alentorn, Caroline Dehais, Caroline Houillier, Florence Laigle-Donadey, Dimitri Psimaras, Alexandre Carpentier, Philippe Cornu, Laurent Capelle, Bertrand Mathon, Jill S. Barnholtz-Sloan, Arnab Chakravarti, Wenya L. Bi, Garrett M. Frampton, Marc Sanson, Brian M. Alexander, Andrew Cherniack, Patrick Y. Wen, David A. Reardon, Aurelien Marabelle, Peter J. Park, Ahmed Idbaih, Rameen Beroukhim, Pratiti Bandopadhayay, Franck Bielle, Keith L. Ligon. Mechanisms and therapeutic implications of hypermutation in gliomas [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5705.