LAUSR

Anaplastic pleomorphic xanthoastrocytoma (A-PXA, WHO grade III) is a newly defined entity with highgrade histopathologic features and a propensity for recurrence [6]. While PXA with low-grade histology (WHO grade II) can harbor a recurrent valine-toglutamic acid (p.V600E) point mutation in BRAF in up to 78% of cases [6], the genomic drivers of APXA are poorly understood as the V600E mutation is absent in over half of A-PXAs [4]. Alterations reported to date in V600E-negative cases have included novel BRAF fusions and copy number alterations (Table 1). The efficacy of therapeutic targeting oncogenically activated kinases in BRAF-mutant cancers depends on structural variations in the kinase domain. For example, the BRAF V600E mutation is often sensitive to kinase inhibitors such as vemurafenib, while β3-αC deletions and non-canonical BRAF mutations are often resistant to this small molecular inhibitor [2]. Therefore, from a therapeutic aspect, it is imperative to define the spectrum of BRAF alterations in these aggressive tumors. Here, we report two newly identified A-PXAs with activating mutations in the β3-αC loop of the BRAF kinase domain discovered through whole-exome, whole-genome, and transcriptome sequencing (Michigan Oncology Sequencing Project [MI-ONCOSEQ]) [8]. The first case is a 5-year-old male presenting with a large (11.7 × 7.3 cm) temporoparietal mass with subfalcine and uncal herniation (Fig. 1a). Molecular profiling revealed an oncogenic BRAF in-frame deletion (p.L485_P490delinsF) located adjacent to the β3-αC loop that results in a helix-constraining conformational change in the kinase domain. The second case is a 23-year-old male with a parietal ringenhancing cystic mass. Sequencing revealed a novel 9 bp tandem duplication (p.V504_R506dup) in the β3-αC loop that results in a three codon in-frame insertion in the open reading frame (ORF) of BRAF [see Fig. 1a-d and Online Resource for details and representative images from both cases (Additional file 1)]. Consistently, both cases demonstrated MAPK activation with strong expression of phospho-ERK1/2 in tumor cells (Fig. 1g). Both of the mutations reported here affect the β3αC loop in the kinase domain. To function properly, protein kinases must maintain a level of structural flexibility in order to switch between inactive and active states. This conformational change involves two regulatory regions in the catalytic domain: the