TMOD-23. A NOVEL BRAF V600E LOW-GRADE GLIOMA MOUSE MODEL HIGHLIGHTS EXOMIC AND TUMOR IMMUNE ALTERATIONS AND DIFFERING THERAPEUTIC RESPONSES IN LOW- AND HIGH-GRADE GLIOMAS

Pediatric low-grade glioma (pLGG), the most common brain cancer in children, is difficult to treat especially at recurrence. The BRAF V600E mutation is the second most common mutation in pLGG, and in a high-risk group for progression is associated with deletion of the tumor suppressor CDKN2A. A better understanding of the factors contributing to progression, in particular the role of the immune infiltrate is needed, but studies have been hindered by the lack of low-grade glioma mouse models. We utilized transgenic mice with a cre-activatable (CA) allele of BRAF V600E to generate endogenous models for low-grade gliomas. We found that BRAF V600E expression cooperates with hemizygous CDKN2A deletion to induce low-grade gliomas, with tumors forming at a greater latency than by homozygous deletion. Cell line derivatives from low-grade lesions continue to grow slower upon orthotopic injection than those we previously derived from high-grade tumors. Murine LGG can progress to higher grade tumors within the mouse lifespan and we observe exomic changes and alterations in the tumor immune infiltrate associated with progression, the details of which will be discussed at the meeting. High-grade cells’ phenotypic changes within in vivo passage are accompanied by exomic changes. The high-grade glioma immune infiltrate is altered by dual MAPK pathway inhibition with dabrafenib and trametinib. Adding dual immune checkpoint inhibition by anti-PD-L1 and anti-CTLA-4 antibodies significantly extends survival of dabrafenib-trametinib dual treatment. Human BRAF V600E mutant tumors reportedly have a higher tumor immune infiltrate than that of BRAF wildtype gliomas, consistent with our murine RESULTS: Here we present a novel model for BRAF V600E mutant gliomas in mice that has a frequent rate of progression, similar to human BRAF V600E mutant gliomas, and an active immune infiltrate in high grade tumors which makes them susceptible to the immunostimulatory effects of dual checkpoint inhibition.