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The 2021 5th edition of the WHO Classification of Tumors of the Central Nervous System reflects the discovery of genetic alterations underlying many central nervous system (CNS) neoplasms. Insights gained from technologic advances and novel applications in molecular diagnostics, including next‐generation sequencing and DNA methylation‐based profiling, coupled with the recognition of clinicopathologic correlates, have prompted substantial changes to CNS tumor classification; this is particularly true for pediatric low‐grade gliomas and glioneuronal tumors (pLGG/GNTs). The 2021 WHO now classifies gliomas, glioneuronal tumors and neuronal tumors into 6 families, three of which encompass pLGG/LGNTs: “Pediatric type diffuse low‐grade gliomas,” “circumscribed astrocytic gliomas,” and “glioneuronal and neuronal tumors.” Among these are six newly recognized tumor types: “diffuse astrocytoma, MYB or MYBL1‐altered”; “polymorphous low grade neuroepithelial tumor of the young (PLNTY)”; “diffuse low‐grade glioma‐MAPK altered”; “Diffuse glioneuronal tumor with oligodendroglioma‐like features and nuclear clusters (DGONC)”; “myxoid glioneuronal tumor (MGT)”; and “multinodular and vacuolating neuronal tumor (MVNT).” We review these newly recognized entities in the context of general changes to the WHO schema, discuss implications of the new classification for treatment of pLGG/LGNT, and consider strategies for molecular testing and interpretation. The 5th edition of the WHO Classification of Tumors of the Central Nervous System now classifies gliomas, glioneuronal tumors and neuronal tumors into 6 families, among which are six newly pediatric low‐grade gliomas and glioneuronal tumors: “diffuse astrocytoma, MYB or MYBL1‐altered”; “polymorphous low grade neuroepithelial tumor of the young (PLNTY)”; “diffuse low‐grade glioma‐ MAPK altered”; “Diffuse glioneuronal tumor with oligodendroglioma‐like features and nuclear clusters (DGONC)”; “myxoid glioneuronal tumor (MGT)”; and “multinodular and vacuolating neuronal tumor (MVNT).” We review these newly recognized entities in the context of general changes to the WHO schema, discuss implications of the new classification for treatment of pLGG/LGNT, and consider strategies for molecular testing and interpretation.

Vorasidenib and ivosidenib inhibit mutant forms of isocitrate dehydrogenase (mIDH) and have shown preliminary clinical activity against m IDH glioma. We evaluated both agents in a perioperative phase 1 trial to explore the mechanism of action in recurrent low-grade glioma (IGG) and select a molecule for phase 3 testing. Primary end-point was concentration of d -2-hydroxyglutarate (2-HG), the metabolic product of mIDH enzymes, measured in tumor tissue from 49 patients with m IDH1 -R132H nonenhancing gliomas following randomized treatment with vorasidenib (50 mg or 10 mg once daily, q.d.), ivosidenib (500 mg q.d. or 250 mg twice daily) or no treatment before surgery. Tumor 2-HG concentrations were reduced by 92.6% (95% credible interval (CrI), 76.1–97.6) and 91.1% (95% CrI, 72.0–97.0) in patients treated with vorasidenib 50 mg q.d. and ivosidenib 500 mg q.d., respectively. Both agents were well tolerated and follow-up is ongoing. In exploratory analyses, 2-HG reduction was associated with increased DNA 5-hydroxymethylcytosine, reversal of ‘proneural’ and ‘stemness’ gene expression signatures, decreased tumor cell proliferation and immune cell activation. Vorasidenib, which showed brain penetrance and more consistent 2-HG suppression than ivosidenib, was advanced to phase 3 testing in patients with m IDH LGGs. Funded by Agios Pharmaceuticals, Inc. and Servier Pharmaceuticals LLC; ClinicalTrials.gov number NCT03343197. The dual IDH1/IDH2 inhibitor vorasidenib exhibited better brain permeability and target engagement than ivosidenib in a pilot perioperative randomized clinical trial in patients with IDH1-mutant glioma.

Braf  V600E expression in resident macrophage progenitors leads to clonal expansion of ERK-activated microglia, which causes synaptic and neuronal loss in the brain and results in lethal neurodegenerative disease in adult mice. BRAF mutation begets brain disease Microglia—immune cells in the brain—derive from yolk-sac erythro-myeloid progenitors (EMPs), which are distinct from haematopoietic stem cells (HSCs). Frederic Geissmann and colleagues show that mosaic expression of a mutant BRAF, which activates the RAS–MEK–ERK pathway and causes tumours when expressed in HSCs, results in expansion of tissue-resident macrophages and late-onset neurodegeneration when expressed in EMPs. They show in a mouse model that neurobehavioural abnormalities, astrogliosis, deposition of amyloid precursor protein, synaptic loss and neuronal death are driven by ERK-activated microglia and can be prevented by BRAF inhibition. These results show that, in mice, activation of the MAP kinase pathway in microglia can cause neurodegeneration. These findings may explain the neurodegeneration observed in patients with histiocytosis—disorders of myeloid cell expansion associated with somatic mutations in the RAS–MEK–ERK pathway, such as the BRAF mutation studied here. The pathophysiology of neurodegenerative diseases is poorly understood and there are few therapeutic options. Neurodegenerative diseases are characterized by progressive neuronal dysfunction and loss, and chronic glial activation 1 . Whether microglial activation, which is generally viewed as a secondary process, is harmful or protective in neurodegeneration remains unclear 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . Late-onset neurodegenerative disease observed in patients with histiocytoses 9 , 10 , 11 , 12 , which are clonal myeloid diseases associated with somatic mutations in the RAS–MEK–ERK pathway such as BRAF(V600E) 13 , 14 , 15 , 16 , 17 , suggests a possible role of somatic mutations in myeloid cells in neurodegeneration. Yet the expression of BRAF(V600E) in the haematopoietic stem cell lineage causes leukaemic and tumoural diseases but not neurodegenerative disease 18 , 19 . Microglia belong to a lineage of adult tissue-resident myeloid cells that develop during organogenesis from yolk-sac erythro-myeloid progenitors (EMPs) distinct from haematopoietic stem cells 20 , 21 , 22 , 23 . We therefore hypothesized that a somatic BRAF(V600E) mutation in the EMP lineage may cause neurodegeneration. Here we show that mosaic expression of BRAF(V600E) in mouse EMPs results in clonal expansion of tissue-resident macrophages and a severe late-onset neurodegenerative disorder. This is associated with accumulation of ERK-activated amoeboid microglia in mice, and is also observed in human patients with histiocytoses. In the mouse model, neurobehavioural signs, astrogliosis, deposition of amyloid precursor protein, synaptic loss and neuronal death were driven by ERK-activated microglia and were preventable by BRAF inhibition. These results identify the fetal precursors of tissue-resident macrophages as a potential cell-of-origin for histiocytoses and demonstrate that a somatic mutation in the EMP lineage in mice can drive late-onset neurodegeneration. Moreover, these data identify activation of the MAP kinase pathway in microglia as a cause of neurodegeneration and this offers opportunities for therapeutic intervention aimed at the prevention of neuronal death in neurodegenerative diseases.

The recent discovery of mutations in metabolic enzymes has rekindled interest in harnessing the altered metabolism of cancer cells for cancer therapy. One potential drug target is isocitrate dehydrogenase 1 (IDH1), which is mutated in multiple human cancers. Here, we examine the role of mutant IDH1 in fully transformed cells with endogenous IDH1 mutations. A selective R132H-IDH1 inhibitor (AGI-5198) identified through a high-throughput screen blocked, in a dose-dependent manner, the ability of the mutant enzyme (m1DH1) to produce R-2-hydroxyglutarate (R-2HG). Under conditions of near-complete R-2HG inhibition, the m1DH1 inhibitor induced demethylation of histone H3K9me3 and expression of genes associated with gliogenic differentiation. Blockade of m1DH1 impaired the growth of IDH1-mutant—but not IDH1-wild-type—glioma cells without appreciable changes in genome-wide DNA methylation. These data suggest that m1DH1 may promote glioma growth through mechanisms beyond its well-characterized epigenetic effects.

Radionecrosis is a well-characterized effect of stereotactic radiosurgery (SRS) and is occasionally associated with serious neurologic sequelae. Here, we investigated the incidence of and clinical variables associated with the development of radionecrosis and related radiographic changes after SRS for brain metastases in a cohort of patients with long-term follow up. 271 brain metastases treated with single-fraction linear accelerator-based SRS were analyzed. Radionecrosis was diagnosed either pathologically or radiographically. Univariate and multivariate Cox regression was performed to determine the association between radionecrosis and clinical factors available prior to treatment planning. After median follow up of 17.2 months, radionecrosis was observed in 70 (25.8 %) lesions, including 47 (17.3 %) symptomatic cases. 22 of 70 cases (31.4 %) were diagnosed pathologically and 48 (68.6 %) were diagnosed radiographically. The actuarial incidence of radionecrosis was 5.2 % at 6 months, 17.2 % at 12 months and 34.0 % at 24 months. On univariate analysis, radionecrosis was associated with maximum tumor diameter (HR 3.55, p < 0.001), prior whole brain radiotherapy (HR 2.21, p = 0.004), prescription dose (HR 0.56, p = 0.02) and histology other than non-small cell lung, breast or melanoma (HR 1.85, p = 0.04). On multivariate analysis, only maximum tumor diameter (HR 3.10, p < 0.001) was associated with radionecrosis risk. This data demonstrates that with close imaging follow-up, radionecrosis after single-fraction SRS for brain metastases is not uncommon. Maximum tumor diameter on pre-treatment MR imaging can provide a reliable estimate of radionecrosis risk prior to treatment planning, with the greatest risk among tumors measuring >1 cm.

Erdheim–Chester disease is a rare, non-Langerhans cell histiocytosis histologically characterized by multi-systemic proliferation of mature histiocytes in a background of inflammatory stroma. The disease can involve virtually any organ system; most commonly the bones, skin, retroperitoneum, heart, orbit, lung, and brain are affected. Although a histiocytic proliferation is the histological hallmark of the disease, a wide range of morphological appearances have been described as part of case studies or small series. A comprehensive review of histopathological features in clinically and molecularly defined Erdheim–Chester disease has yet to be characterized. To address this issue and help guide clinical practice, we comprehensively analyzed the pathological spectrum of Erdheim–Chester disease in a clinically and molecularly defined cohort. We reviewed 73 biopsies from 42 patients showing involvement by histiocytosis from a variety of organ systems, including bone (16), retroperitoneum (11), skin (19), orbit (6), brain (5), lung (6), cardiac structures (2), epidural soft tissue (3), oral cavity (2), subcutaneous soft tissue (2), and testis (2). In eight patients, one or more bone marrow biopsies were performed due to clinical indication and an accompanying myeloid neoplasm was detected in six of them. Thirty-eight cases were investigated for genetic abnormalities. Somatic mutations involving BRAF (25/38), MAP2K1 (6/38), ARAF (2/38), MAP2K2 (1/38), KRAS (1/38), and NRAS (1/38) genes were detected. One of the cases with a MAP2K1 mutation also harbored a PIK3CA mutation. We have observed marked heterogeneity in histology and immunophenotype, identified site-specific features, overlap with other histiocytic and myeloid disorders and potential diagnostic pitfalls. We hope that broadening the spectrum of recognized pathologic manifestations of Erdheim–Chester disease will help practicing clinicians and pathologists to diagnose Erdheim–Chester disease early in the disease course and manage these patients effectively.

The widespread application of high-throughput sequencing methods is resulting in the identification of a rapidly growing number of novel gene fusions caused by tumour-specific chromosomal rearrangements, whose oncogenic potential remains unknown. Here we describe a strategy that builds upon recent advances in genome editing and combines ex vivo and in vivo chromosomal engineering to rapidly and effectively interrogate the oncogenic potential of genomic rearrangements identified in human brain cancers. We show that one such rearrangement, an microdeletion resulting in a fusion between Brevican (BCAN) and Neurotrophic Receptor Tyrosine Kinase 1 (NTRK1), is a potent oncogenic driver of high-grade gliomas and confers sensitivity to the experimental TRK inhibitor entrectinib. This work demonstrates that BCAN-NTRK1 is a bona fide human glioma driver and describes a general strategy to define the oncogenic potential of novel glioma-associated genomic rearrangements and to generate accurate preclinical models of this lethal human cancer. The use of mouse models has been an invaluable resource in cancer research but their generation is lengthy and costly. Here the authors describe an approach to generate engineered mouse models carrying specific gene fusions and, as a proof of principle, show that Bcan-Ntrk1 fusion leads to glioblastomas.

Fusions involving CRAF (RAF1) are infrequent oncogenic drivers in pediatric low‐grade gliomas, rarely identified in tumors bearing features of pilocytic astrocytoma, and involving a limited number of known fusion partners. We describe recurrent TRAK1::RAF1 fusions, previously unreported in brain tumors, in three pediatric patients with low‐grade glial‐glioneuronal tumors. We present the associated clinical, histopathologic and molecular features. Patients were all female, aged 8 years, 15 months, and 10 months at diagnosis. All tumors were located in the cerebral hemispheres and predominantly cortical, with leptomeningeal involvement in 2/3 patients. Similar to previously described activating RAF1 fusions, the breakpoints in RAF1 all occurred 5′ of the kinase domain, while the breakpoints in the 3′ partner preserved the N‐terminal kinesin‐interacting domain and coiled‐coil motifs of TRAK1. Two of the three cases demonstrated methylation profiles (v12.5) compatible with desmoplastic infantile ganglioglioma (DIG)/desmoplastic infantile astrocytoma (DIA) and have remained clinically stable and without disease progression/recurrence after resection. The remaining tumor was non‐classifiable; with focal recurrence 14 months after initial resection; the patient remains symptom free and without further recurrence/progression (5 months post re‐resection and 19 months from initial diagnosis). Our report expands the landscape of oncogenic RAF1 fusions in pediatric gliomas, which will help to further refine tumor classification and guide management of patients with these alterations. We describe recurrent TRAK1::RAF1 fusions, previously unreported in brain tumors, in three pediatric patients with low‐grade glial‐glioneuronal tumors. We present the associated clinical, histopathologic and molecular features.

A subset of glioblastomas (GBMs) harbors potentially druggable oncogenic FGFR3 - TACC3 (F3T3) fusions. However, their associated molecular and clinical features are poorly understood. Here we analyze the frequency of F3T3-fusion positivity, its associated genetic and methylation profiles, and its impact on survival in 906 IDH-wildtype GBM patients. We establish an F3T3 prevalence of 4.1% and delineate its associations with cancer signaling pathway alterations. F3T3-positive GBMs had lower tumor mutational and copy-number alteration burdens than F3T3-wildtype GBMs. Although F3T3 fusions were predominantly mutually exclusive with other oncogenic RTK pathway alterations, they did rarely co-occur with EGFR amplification. They were less likely to harbor TP53 alterations. By methylation profiling, they were more likely to be assigned the mesenchymal or RTK II subclass. Despite being older at diagnosis and having similar frequencies of MGMT promoter hypermethylation, patients with F3T3-positive GBMs lived about 8 months longer than those with F3T3-wildtype tumors. While consistent with IDH-wildtype GBM, F3T3-positive GBMs exhibit distinct biological features, underscoring the importance of pursuing molecular studies prior to clinical trial enrollment and targeted treatment.