DIPG-44. DIFFUSE MIDLINE GLIOMA, H3K27-ALTERED: A COMPREHENSIVE CLINICAL AND BIOMOLECULAR LANDSCAPE OF THE NON-PONTINE TUMORS

Abstract

Abstract BACKGROUND Since 2016, the WHO classification of CNS tumors groups all midline gliomas with H3K27me3 loss under the term diffuse midline glioma (DMG), H3K27-altered. Molecular characteristics enable their sub-classification based on driver mutations, co-driver mutations and epigenetic alterations. Predominantly studied in the pons (Diffuse Intrinsic Pontine Glioma, DIPG), where these are mainly pediatric tumors with a median overall survival (OS) of one year, fewer studies have been conducted on non-pontine DMG (npDMG), prompting this study to clinically and biologically characterize these gliomas and compare them with DIPG. METHODS Retrospective monocentric study from 1994 to 2022, including pediatric and adult patients diagnosed with npDMG, H327-altered with neuropathological confirmation at Sainte Anne Hospital in Paris, France. Comparative literature review of cases of npDMG and series of DIPG. RESULTS 108 patients were included (54.7% male, median age 13.05-years [2.4-59.3]). The most frequent location was monothalamic (62.9%), followed by bithalamic, spinal, cerebellar, bulbo-medullar and other. H3K27M mutation was found in 85.2% of cases, EZHIP overexpression in 14.8% and EGFR alteration (mutation and/or amplification) in 16.7%. EZHIP-overexpressing patients were younger than H3K27M-mutated patients, as were EGFR-altered patients compared to those without EGFR alteration. Along with data from the literature, npDMG are older at diagnosis and have a slightly longer median OS (15 months [0.1-247.2]) than DIPG. Positive prognosis factors upon univariate analysis included longer time from symptom onset until diagnosis, treatment with ONC201, re-irradiation upon relapse, microcalcifications, proliferative index >10%, MAP kinase alteration and absence of TP53 mutation and EGFR amplification. However, surgery quality did not impact OS. CONCLUSIONS npDMG is a heterogeneous group of gliomas, affecting all ages, which can develop in diverse locations, with an overall poor prognosis. Despite shared clinical, immuno-histochemical and molecular properties with DIPG, recent molecular biology advancements reveal distinct subtypes within DMG, with varying prognoses, challenging the unified classification.