Highlights d Genomically simple RTs are infiltrated by T cell and myeloid populations d Clonally expanded T cell phenotypes suggest a tumorspecific response d Checkpoint blockade induces tumor regression and immune memory in vivo d Endogenous retrovirus expression is linked to the immunogenicity of RTs

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The "isomorphic subtype of diffuse astrocytoma" was identified histologically in 2004 as a supratentorial, highly differentiated glioma with low cellularity, low proliferation and focal diffuse brain infiltration. Patients typically had seizures since childhood and all were operated on as adults. To define the position of these lesions among brain tumours, we histologically, molecularly and clinically analysed 26 histologically prototypical isomorphic diffuse gliomas. Immunohistochemically, they were GFAP-positive, MAP2-and CD34-negative and nuclear ATRX expression was retained. All 24 cases sequenced were IDH-wildtype. In cluster analyses of DNA-methylation data, isomorphic diffuse gliomas formed a group clearly distinct from other glial/glio-neuronal brain tumours and normal hemispheric tissue. It was most closely related to paediatric MYB/MYBL1 altered diffuse astrocytomas and angiocentric gliomas. 13/25 (52%) of isomorphic diffuse gliomas had copy number alterations of MYBL1 or MYB. Gene fusions of MYBL1 or MYB with various gene partners were detected in 11/22 (50%). Gene fusions were associated with increased RNA-expression of the respective MYBfamily gene in 83%. Integrating copy number alterations and RNA sequencing data, 20/26 (77%) had either MYBL1 (54%) or MYB (23%) alterations. Clinically, 89% of patients were seizure free after surgery and all had a good outcome. In summary, we here define a distinct tumour class with a concise morphology, a typical DNA-methylation profile and frequent MYBL1 and MYB alterations. It occurs both in children and adults and has a benign disease course. For classification, we propose the term "isomorphic diffuse glioma, MYBL1/MYB altered, WHO grade I". DNA-methylation profiling is well suited to identify these tumours.

Background Considering that pediatric high-grade gliomas (HGGs) are biologically distinct from their adult counterparts, the objective of this study was to define the landscape of HGGs in adolescents and young adults (AYAs). Methods We performed a multicentric retrospective study of 112 AYAs from adult and pediatric Ile-de-France neurosurgical units, treated between 1998 and 2013 to analyze their clinicoradiological and histomolecular profiles. The inclusion criteria were age between 15 and 25 years, histopathological HGG diagnosis, available clinical data, and preoperative and follow-up MRI. MRI and tumoral samples were centrally reviewed. Immunohistochemistry and complementary molecular techniques such as targeted/next-generation sequencing, whole exome sequencing, and DNA-methylation analyses were performed to achieve an integrated diagnosis according to the 2016 World Health Organization (WHO) classification. Results Based on 80 documented AYA patients, HGGs constitute heterogeneous clinicopathological and molecular groups, with a predominant representation of pediatric subtypes (histone H3-mutants, 40%) but also adult subtypes (isocitrate dehydrogenase [IDH] mutants, 28%) characterized by the rarity of oligodendrogliomas, IDH mutants, and 1p/19q codeletion and the relative high frequency of “rare adult IDH mutations” (20%). H3G34-mutants (14%) represent the most specific subgroup in AYAs. In the H3K27-mutant subgroup, non-brainstem diffuse midline gliomas are more frequent (66.7%) than diffuse intrinsic pontine gliomas (23.8%), contrary to what is observed in children. We found that WHO grade has no prognostic value, but molecular subgrouping has major prognostic importance. Conclusions HGGs in AYAs could benefit from a specific classification, driven by molecular subtyping rather than age group. Collaborative efforts are needed from pediatric and adult neuro-oncology teams to improve the management of HGGs in AYAs. Key Points 1. Pediatric subtypes (H3G34 and H3K27 mutants) were the most common HGGs in AYAs. 2. Contrary to what is observed in children, non-brainstem diffuse midline gliomas are more frequent than diffuse intrinsic pontine gliomas. 3. WHO grade has no prognostic value contrary to molecular subgrouping.