Rare Activating BRAF Alteration Involving the β3-αC Kinase Domain in Ganglioglioma.

To the Editor: Ganglioglioma is a rare CNS neoplasm composed of dysplastic ganglion cells and neoplastic glial cells. They often arise in the temporal lobes (>70% of cases) and are highly epileptogenic with early presentation of spontaneous seizures (1). Other reported locations include other areas of the cerebrum, cerebellum, optic nerves, brainstem, spinal cord, pituitary, and pineal glands (2). The neuroimaging is nonspecific and typically shows an intracortical cystic lesion or cysticsolid mass with a wall or nodular enhancement on FLAIR and T2-weighted images (2, 3). Although most gangliogliomas are WHO grade I and do not recur after surgical resection, the clinical course and histological findings are variable. Thus, comprehensive molecular characterization to better understand this tumor category is important. One of the key oncogenic genetic alterations in ganglioglioma is activation of the mitogen-activated protein kinase (MAPK) pathway, which promotes cell proliferation and tumor growth (4). Activating BRAF genetic alterations in the MAPK pathway is implicated in tumorigenesis in both CNS and non-CNS tumors (5). The most well-characterized BRAF mutation is valine to glutamic acid substitution due to nucleotide T1799A (BRAF V600E) mutation, and this is detected in 10%–60% of ganglioglioma cases (2, 6). Additional genetic alterations could involve either other targets of BRAF (gene fusion, in-frame insertions) or MAPK pathway members (KRAS, RAF1, NF1, and FGFR1/2) (6). This result implies heterogeneous genetic alterations in generating a common tumor type. Here, we report a rare genetic mutation in ganglioglioma affecting the BRAF kinase domain in the b3-aC loop. The patient is a 42-year-old man with no significant past medical history presenting with an episode of altered consciousness while driving, which resulted in collision with 2 other vehicles. He was unaware of the accident and transported to a local ER. Bystander description was consistent with a focal impaired awareness seizure, during which he apparently hit cars at low speed and continued driving. Neuroimaging revealed a noninfiltrative, mostly cystic lesion with small peripheral nodular enhancement within the left temporal lobe (3.5 2.8 cm, Fig. 1A). The patient was treated with levetiracetam (Keppra) for seizure prophylaxis and underwent craniotomy for resection of the lesion approximately 1 month later. Histological examination showed a combination of clustered dysplastic neurons with enlarged nuclei and neoplastic glial cells (Fig. 1B, C). There was a lack of normal cytoarchitectural organization (highlighted by NeuN staining, Fig. 1F). Occasional binucleated dysplastic neurons were seen (Fig. 1D). There was a biphasic growth pattern with Rosenthal fibers and eosinophilic granular bodies identified in the coarse fibrillary background (Fig. 1B, E). There was prominent hemosiderin deposition (Fig. 1D, E) and perivascular lymphocytes (Fig. 1E). No tumor necrosis or microvascular proliferation was identified. The tumor cells were negative for IDH1 R132H and BRAF V600E mutations by immunohistochemistry (not shown). Notably, CD34 was positive in many bushy cells (Fig. 1G). The Ki67 proliferation marker labeled up to 3% of tumor cells (not shown). The histological features corresponded to a ganglioglioma, WHO grade I. Molecular profiling of the tumor by the Illumina TruSight Tumor 170 assay showed a BRAF p. V504_R506dup variation. The insertion of 9 nucleotides within the b3-aC loop of the BRAF kinase domain resulted in the duplication of the valine, leucine, arginine residues (codons 504_506) (Fig. 1H). No other mutations in BRAF were detected. The tumor was negative for IDH 1/2, TP53, PTEN, or CDK4 mutations and EGFR amplification. The b3-aC loop in the kinase domain is highly conserved across oncogene families. Maintaining the structural flexibility at the b3-aC loop is critical for conformational changes between active and inactive states (7). In-frame deletion constrains the C loop (aC) in a constant active conformation (7). Additionally, mutation in the R-spine (L505F) was shown to preassemble the active conformation and confers a constitutively activated state (8). Our case shows an in-frame insertion of 3 amino acids in the R-spine of aC helix. Although the mechanistic role of how V504 to R506 insertion/