LG-44: OCCURRENCE OF BRAF V600E MUTATION IN A PEDIATRIC BRAINSTEM GANGLIOGLIOMA WITH NEAR EXCLUSIVE GLIAL COMPONENT IN THE ORIGINAL BIOPSY

A five-year-old male who initially presented with apneic episodes and morbid obesity underwent imaging studies, which revealed a dorsally exophytic medullary tumor in 2014. A biopsy showed a classic pilocytic astrocytoma (PA) with no apparent neoplastic neuronal component. Following completion of chemotherapy, the patient presented again in January of 2016 with worsening symptoms; the tumor was found to have grown in size with brainstem compression. Debulking was performed. Unexpectedly, the histology of this tumor showed a ganglioglioma (GG) with many clusters of enlarged, misshapen, dysmorphic-appearing and occasionally binucleated neurons. Furthermore, BRAF V600E mutations were identified in both prior and recurrent tumors. Recent publications have found that the BRAF V600E mutation is seen in 20-58% of all GGs. In particular, pediatric brainstem GGs appear to harbor this mutation at a rate greater than 50%. Few studies have investigated the distribution of BRAF aberrations among neuronal and glial components in GGs. Pilocytic astrocytomas often possess BRAF abnormalities, though fusion genes (i.e BRAF-KIAA1549) are considerably more common (60-80%) than the BRAF V600E missense mutations (only seen in 10-12% of pediatric brainstem PAs). This case highlights the presence of the BRAF point mutation in both glial and neuronal component of a pediatric brainstem GG, supporting the hypothesis that a glioneuronal lineage precursor is initially affected by the mutation. Additionally, the presence of the BRAF point mutation in a brainstem pilocytic astrocytoma should raise the possibility of a GG in which sampling artifact highlights a near exclusive glial component.