EPN-12: PEDIATRIC SUPRATENTORIAL EPENDYMOMA: COMPARISON BETWEEN IMMUNOHISTOCHEMISTRY, FISH AND DNA METHYLATION ANALYSIS FOR RELA REARRANGEMENT AND HISTOLOGICAL CORRELATION

New molecular markers have been recently identified for supratentorial ependymoma (SE), particularly RELA rearrangements coding for a protein of the NFκB complex. In this study we compared different techniques (FISH, immunohistochemistry (IHC) and DNA methyloma analysis (DNAm)) in the assessment of RELA rearrangements and their correlation with clinical and histological characteristics in a series of 41 pediatric supratentorial ependymoma. Break apart probes for C11orf95 and RELA were developed, NFκB pathway activation having also been analyzed by IHC and DNAm. NFκB IHC and FISH for RELA were positive in 56% (23/41) and 68% (21/31) cases with 96% concordance. Sixty three percent (24/38) cases were RELA- positive by DNAm showing 87% concordance with IHC (33/38) and FISH (27/31). Two cases showed YAP rearrangements and favorable outcome. Twelve tumors clustered apart from SE, and were reclassified by DNAm: 4 subependymoma, 6 PNET, 1 histone H3.3 mutated glioma and 1 infratentorial ependymoma. All patients with tumors reclassified as subependymoma had a favorable outcome despite histological signs of malignancy, and should probably not be graded with classic histological criteria of malignancy. In conclusion FISH and IHC are useful for the diagnosis of SE. DNAm reclassifies a significant proportion of histological ependymoma and highlights a group of tumors (“PNET”) displaying ependymal differentiation and frequent 16q and 22q chromosomal alterations, which should be better characterized in larger collaborative series.