H3 K27M-mutant Research Articles

Spinal Diffuse Midline Glioma H3 K27M-Altered: Report of a Rare Tumor with Extracranial Skeletal Metastases and Review of Literature.

Diffuse midline glioma, H3 K27-altered is a rare and aggressive pediatric brain tumor with a grim prognosis. Diffuse midline glioma is characterized by specific molecular alterations, including H3 K27 mutations, and involves deep midline structures such as the brainstem, cerebellum, spinal cord, and thalamus. These tumors present with a classic triad of symptoms and have limited surgical options due to their challenging locations. Extra-neural metastases are an unusual occurrence in diffuse midline glioma and have been rarely described. Here we report a 17-year-old girl with spinal diffuse midline glioma, H3 K27M-mutant, who presented with multiple metastatic osseous lesions confirmed on biopsy of the thoracic vertebral lesion. Due to the rapid disease progression, the patient was recommended palliative therapy. Extra-neural metastases in diffuse midline glioma are rare, with only 16 reported patients, and no standard therapy exists. An accurate and early diagnosis is necessary to develop a personalized plan of treatment. Further research is needed to gain insights into the molecular pathology of diffuse midline glioma, H3 K27-altered, and improve the quality of life and the outcome of patients with this deadly disease.

Ascites production and prognosis after ventriculoperitoneal shunt for diffuse midline gliomas in children: A case series.

DMG is a highly invasive and lethal type of brain tumor. As these tumors progress, they often compromise the CSF circulation, leading to hydrocephalus. Ventriculoperitoneal shunt (VPS) is commonly employed to manage hydrocephalus; however, the complication of VPS-induced ascites, particularly in the presence of tumor cells, is a significant concern that merits attention. This case series details 3 pediatric patients diagnosed with brainstem DMG harboring the H3 K27M mutation. Each developed hydrocephalus underwent VPS insertion. Post-operatively, all patients developed carcinomatous ascites with tumor cells detected within the ascitic fluid. All 3 patients were diagnosed with intra-abdominal metastasis of DMG H3K27M mutant cancer cells, each presenting with distinct complications. Initially, the patients' primary head tumors responded to treatment, and their hydrocephalus resolved. However, some time after discharge, each patient developed malignant ascites and received palliative chemotherapy to control symptoms and improve quality of life. Despite the interventions, all 3 patients died within 1 month of developing malignant ascites, with central respiratory failure being the direct cause of death. These cases underscore the importance of continuous monitoring of both the CSF and ascitic fluid in patients with gliomas. Regular assessments of biochemical composition, cytology, and other diagnostic tests are crucial for early detection of disease progression. This proactive approach facilitates timely clinical judgment and intervention, potentially improving patient outcomes.

Apatinib combined with temozolomide in diffuse midline glioma: a novel and effective therapy

PurposeDiffuse midline glioma (DMG), H3 K27M-mutant is a type of diffuse high-grade glioma that occurs in the brain midline carrying an extremely poor prognosis under the best efforts of surgery, radiation, and other therapies. For better therapy, we explored the efficacy and toxicity of a novel therapy that combines apatinib and temozolomide in DMG.MethodsA retrospective analysis of 32 patients with DMG who underwent apatinib plus temozolomide treatment was performed. Apatinib was given 500 mg in adults, 250 mg in pediatric patients once daily. Temozolomide was administered at 200 mg/m2/d according to the standard 5/28 days regimen. The main clinical data included basic information of patients, radiological and pathological characteristics of tumors, treatment, adverse reactions, prognosis.ResultsThe objective response rate was 24.1%, and the disease control rate was 79.3%. The median PFS of all patients was 5.8 months, and median OS was 10.3 months. A total of 236 cycles of treatment were available for safety assessment and the toxicity of the combination therapy was relatively well tolerated. The most common grade 3 toxicities were myelosuppression including leukopenia (5.08%), neutropenia (4.24%), lymphopenia (2.12%), thrombocytopenia (1.69%) and anemia (1.27%). Grade 4 toxicities included neutropenia (2.12%), thrombocytopenia (2.12%) and proteinuria (1.69%). All the adverse events were relieved after symptomatic treatment or dose reduction.ConclusionsApatinib plus temozolomide could be an effective regimen with manageable toxicities and favorable efficacy and may outperform temozolomide monotherapy, particularly in newly diagnosed adults with tumors located outside the pons. The novel therapy deserves further investigation in adult DMG patients.

DIPG-23. SAFETY AND TECHNICAL YIELD OF BRAINSTEM BIOPSIES IN CHILDREN – AN UPDATED META-ANALYSIS OF 1000 CASES

Abstract BACKGROUND Brainstem tumors represent ~10% of pediatric brain tumors, ~80% of these are diffuse intrapontine gliomas (DIPG, primarily WHO Grade 4 midline glioma, H3 K27M-mutant). Given invariably poor prognosis in these cases, there continues to be immense variation (across the US and internationally) in performing biopsy of these lesions as opposed to making a presumptive diagnosis based on MRI. Several multi-center studies in recent years have aimed to show the safety profile of biopsy but an updated meta-analysis is lacking. METHODS We found 29 studies of pediatric brainstem biopsy in the last 20 years (2003-2023), representing data from 1002 children. We applied meta-analysis of proportions using a random-effects model to generate point estimates, confidence intervals, and measures of heterogeneity. RESULTS 87% of procedures were stereotactic needle biopsies (of these, 62% with a frame, 14% without frame, and 24% robotic.) Biopsy yield (i.e. the rate at which biopsy resulted in a tissue diagnosis) was 94% (95% CI 92-97). Temporary complications were seen in 6% (95 CI 4-8), most commonly including 1) radiographic evidence of bleeding or air without an associated fixed neurological deficit, 2) cranial nerve dysfunction, and 3) delayed awakening. Permanent complications (excluding death) were seen in 1% (95% CI 0.5-2), most commonly including cranial nerve dysfunction and limb weakness. 5 deaths were reported in the entire pooled cohort of 1002 children (0.5%). Gliomas represented 92% of tumors (95% CI 89-95). Of the gliomas for which WHO grading was provided, 68% were high grade (III/IV). CONCLUSION When counseling families on the merits of brainstem biopsy in children, it is reasonable to state that permanent morbidity is rare (< 2%). If biopsy is performed specifically to facilitate enrollment in clinical trials requiring a molecular diagnosis, the risks of biopsy outlined here should be weighed against potential benefits of trial enrollment.

DIPG-87. TARGETING GENE-REPRESSIVE EPIGENETIC REGULATION TO INDUCE INTERFERON SIGNALING IN DIFFUSE MIDLINE GLIOMA

Abstract BACKGROUND Diffuse intrinsic pontine glioma (DIPG) is a fatal pediatric brainstem tumor with no known effective treatment. DIPG is driven by the H3K27M mutant histone, which promotes epigenetic dysregulation, leading to loss of gene-repressive H3K27me3 marks, global DNA hypomethylation, and a distinct gene expression profile. DIPG cells retain histone methylation at histone H3 lysine 9 (H3K9me2/3), suggesting that the tumor may rely on these marks to repress unwanted expression. Our prior work has demonstrated that treatment with the DNA hypomethylating agent decitabine (DAC) elicits expression of cancer-specific antigens and immune activation, including activation of interferon signaling, but modulating gene-repressive H3K9 methylation in DIPG has not been extensively investigated. METHODS We treated four primary patient-derived DIPG neurosphere cell lines with BIX01294 and UNC0642, inhibitors of H3K9 methyltransferases G9a/GLP, and analyzed gene expression changes by RNA-sequencing and Western blot. We assessed the effect of combination treatments on these four cell lines with both DAC and either BIX01294 or UNC0642 and evaluated impact on gene expression, innate immune signaling, and interferon pathway activation. RESULTS We found that treatment with either of the G9a/GLP inhibitors resulted in the upregulation of genes involved in immune signaling. Given the similarities in the immune signaling response elicited from treatment with DAC and the G9a/GLP inhibitors, we evaluated the effect of combination treatments with both DAC and either BIX01294 or UNC0642. These combinations resulted in an increased induction of genes involved in immune signaling, including STING. CONCLUSIONS Pharmacologic inhibition of DNA methylation and histone H3K9 methylation elicits immune activation in DIPG cells. Upregulation of interferon and STING signaling with this combination treatment may be significant in enhancing the immunogenicity of DIPG cells to allow effective immunotherapy.

DIPG-79. METTL16 AS A THERAPEUTIC VULNERABILITY IN H3K27M MUTANT DMG CELLS

Abstract BACKGROUND Diffuse Midline Glioma (DMG) is a highly aggressive (CNS WHO Grade IV) pediatric brain tumor characterized by the histone mutation H3K27M, resulting in global hypomethylation of histones. H3K27M mutant cells are highly dependent on methionine and methionine Adenosyltransferase 2A (MAT2A), a central regulator of the methionine cycle, representing a key vulnerability in H3K27M gliomas. MAT2A is upregulated by methyltransferase-like protein 16 (METTL16), which deposits N6-methyladenosine (m6A) on a subset of RNA residues to regulate gene expression. MAT2A is a sensor of methyl group donor S-adenosylmethionine (SAM) that, when placed, increases MAT2A intron splicing to compensate for a decrease in MAT2A function. METHODS To investigate our hypothesis, H3K27M histone mutation knockout cell lines were established using CRISPR Cas9 (Jabado lab) in DIPG4 cells. shRNA-mediated knockdown was used to attenuate METTL16 expression in DIPG4 H3K27M cells, and growth was assessed via sulforhodamine B (SRB) assay. RESULTS In silico analysis of normal brain tissue compared to low-grade glioma and high-grade glioma demonstrated increased METTL16 at both the RNA and protein levels (p=<0.0001). Therefore, we postulated that targeting METTL16 would affect H3K27M growth. Results showed increased cell death compared to control cells by day 4 (p=0.0034) and day 8 (p=<0.0001). Furthermore, knockdown of METTL16 in DMG cells with H3K27M decreased m6A levels on poly (A) RNA (p=0.049). CONCLUSIONS Our study highlights the vulnerability of H3K27M mutant cells to METTL16 modulation, elucidating underlying molecular mechanisms and paving the way for innovative therapeutic strategies targeting this axis.

LGG-17. NATURAL HISTORY AND MANAGEMENT OF INCIDENTALLY DISCOVERED BRAIN LESIONS SUSPECT FOR NEOPLASM IN CHILDREN

Abstract BACKGROUND The rise in neuroimaging has resulted in an increase of incidentally discovered brain lesions suspect for a neoplasm (‘incidentaloma’) in children. With the experience in our national hospital, we aim to diversify and broaden the knowledge on the management and natural history of pediatric incidentaloma. METHODS A retrospective analysis of all incidentaloma patients in the Princess Maxima Center for Pediatric Oncology from June 2018 until March 2023. RESULTS Our research population consists of 23 patients (10M/13F) with an incidentaloma. The lesions were mostly located in the cerebellum (21.7%), thalamus and ventricle (both 17%) and cerebral hemisphere (13%) The average age at diagnosis was 7.9 years (range 0-15 years) and the mean duration of follow-up was 4.6 years (range 0-11 years). The initial management was clinical and radiological follow-up in seventeen (74%), resection in five (22%) and biopsy in one patient (4%). Pathological examination performed in the six neurosurgical patients revealed low grade gliomas (LGGs) in three patients, medulloblastoma in one and plexus papilloma in one patient respectively. The pathological analysis of the patient with biopsy only was indecisive. One patient became symptomatic with clear tumor growth on imaging after 6.8 years of follow-up. Biopsy showed a H3K27M mutant diffuse midline glioma. She died 8.1 years after initial diagnosis. In 11 other patients radiographic changes (mostly tumor progression) were observed with a median of 23 months after diagnosis (range 3-70 months). Two of these underwent complete resection of an LGG. CONCLUSIONS For most incidentalomas, clinical and radiological follow-up is sufficient. As late disease progression can occur as seen in our patient who became symptomatic after 7 years, this follow-up should be long-term. Future studies should focus on identifying (radiological) prognostic factors for tumor progression, so follow-up of incidentaloma can be more personalized.

NFKBIA deletions reshape the epigenome antithetical to the IDH mutation and indication of prognosis in diffuse gliomas.

2083 Background: Genetic alterations help predict the clinical behavior of diffuse gliomas, but some variability remains uncorrelated. The NFKBIA gene at 14q13.2 encodes IkBa, a protein that regulates the activity of NF-kB in the cytoplasm. Evidence that, in chromatin, nuclear NFKBIA dynamically interacts with histones H2A and H4 to regulate polycomb-dependent transcriptional repression and, thus, stem cell maturation and lineage specification and our characterization of NFKBIA as a tumor suppressor in glioblastoma prompted our investigation of the relationship of NFKBIA deletions to other genetic markers, alterations in the methylome, and the clinical course of gliomas. Methods: We analyzed the genetic profiles of 2,343 WHO grade 2 to 4 diffuse gliomas in multiple clinically well-characterized patient populations, including national randomized phase III consortium trial RTOG 9802, to determine whether incorporation of NFKBIA deletions could enhance the prognostic value of current molecular descriptions. Results: We demonstrate that haploinsufficient deletions of NFKBIA display distinct patterns of occurrence in relation to other genetic markers, including driver mutations IDH, TERT, ATRX, and PTEN, 1p19q codeletion, and CDKN2A/B deletion, and are disproportionally present at recurrence. NFKBIA haploinsufficiency is associated with unfavorable patient outcomes, independent of genetic and clinicopathologic predictors. NFKBIA deletions reshape the DNA and histone methylome antipodal to the IDH mutation. The methylome changes facilitated through NFKBIA deletions engender a transcriptome landscape partly reminiscent of H3K27M mutant pediatric gliomas and are highly enriched for neural stem cell and neurogenesis genes bearing repressive H3K27 marks. Our findings imply that the NFKBIA deletion in adult gliomas and the H3K27M mutation in pediatric diffuse midline gliomas define aggressive disease subtypes that may share an epigenetic state reflecting a common PRC2 loss-of-function phenotype antipodal to the quasi PRC2 gain-of-function phenotype of IDH mutant gliomas and their favorable clinical course. Lower-grade gliomas harboring NFKBIA deletions behave much like high-grade gliomas. In IDH mutant gliomas, NFKBIA deletions are common in tumors with a clinical course similar to that of IDH wildtype tumors. A sparse, easily interpretable, and externally validated nomogram model for estimating individual patient survival in IDH mutant gliomas, incorporating NFKBIA deletion and current best-established genetic and clinicopathologic factors, confirms that NFKBIA deletions predict comparatively brief survival. Conclusions: NFKBIA haploinsufficiency aligns with distinct epigenome changes, portends a poor prognosis, and should be incorporated into clinical models predicting the disease fate of diffuse gliomas.

782: Treatment Dilemma of H3K27M Mutant Midline Glioma in Adults

782: Treatment Dilemma of H3K27M Mutant Midline Glioma in Adults

Single-cell RNA sequencing of anaplastic ependymoma and H3K27M-mutant diffuse midline glioma.

Anaplastic ependymoma and H3K27M-mutant diffuse midline glioma are two common subtypes of brain tumors with poor long-term prognosis. The present study analyzed and compared the differences in cell types between two tumors by single-cell RNA sequencing (scRNA-seq) technology. ScRNA-seq was performed to profile cells from cancer tissue from anaplastic ependymoma patient and H3K27M-mutant diffuse midline glioma patient. Cell clustering, marker gene identification, cell type annotation, copy number variation analysis and function analysis of differentially expressed genes were then performed. A total of 11,219 cells were obtained from anaplastic ependymoma and H3K27M mutant diffuse midline glioma, and these cells categorized into 12 distinct clusters. Each cell cluster could be characterized with specific cell markers to indicate cellular heterogeneity. Five cell types were annotated in each sample, including astrocyte, oligodendrocytes, microglial cell, neural progenitor cell and immune cell. The cluster types and proportion of cell types were not consistent between the two brain tumors. Functional analyses suggest that these cell clusters are involved in tumor-associated pathways, with slight differences in the cells of origin between the two tumors. In addition, cell communication analysis showed that the NRG3-ERBB4 pair is a key Ligand-receptor pair for anaplastic ependymoma, while in H3K27M-mutant diffuse midline glioma it is the PTN-PTPRZ1 pair that establishes contact with other cells. There was intratumor heterogeneity in anaplastic ependymoma and H3K27M mutant diffuse midline glioma, and that the subtype differences may be due to differences in the origin of the cells.

Usefulness of magnetic resonance imaging characteristics in discriminating H3 K27M-mutant gliomas from wildtype gliomas in spinal cord.

The aim of this study was to determine the usefulness of magnetic resonance imaging (MRI) characteristics in discriminating H3 K27M-mutant gliomas from wildtype gliomas in the spinal cord. Fifty-eight patients with spinal cord gliomas were enrolled in this study. The H3 K27 gene status was identified by Sanger sequencing or immunohistochemistry test of resection tumor specimens. The MR imaging characteristics were evaluated and compared between H3 K27M-mutant and wildtype gliomas using the χ2 test and the Mann-Whitney U test. Of 58 recruited patients, 23 (39.7%) were diagnosed with H3 K27M-mutant glioma. The H3 K27M-mutant gliomas were found to more likely occur in men compared with wildtype gliomas (87.0% vs. 42.9%, p = 0.001). On T2-weighted MR images, the signal-to-noise ratio (SNR) of H3 K27M-mutant gliomas was significantly lower than that of wildtype gliomas (103.9 ± 72.0 vs. 168.9 ± 86.8, p < 0.001). Of 35 wildtype tumors, 60% showed well-defined margin but this feature was not found in all mutant tumors (p < 0.001). The SNR of tumors on contrast-enhanced T1-weighted images of the H3 K27M-mutant gliomas was significantly lower than that of wildtype gliomas (187.7 ± 160.4 vs. 295.1 ± 207.8, p = 0.006). Receiver operating-characteristic analysis revealed that area under curve (AUC) of combination of 1/SNR on T2-weighted images, 1/SNR on contrast-enhanced T1-weighted images, ill-defined margin, and sex reached 0.937 (95% CI, 0.873-1.000) in discriminating H3 K27M-mutant gliomas. The MR imaging characteristics are valuable in discriminating H3 K27M-mutant from wildtype gliomas in the spinal cord and the combination of these imaging features with sex had a high strength in this discrimination.

H3 K27M-altered glioma and diffuse intrinsic pontine glioma: Semi-systematic review of treatment landscape and future directions.

H3 K27M-mutant diffuse glioma is a recently identified brain tumor associated with poor prognosis. As of 2016, it is classified by the World Health Organization as a distinct form of grade IV glioma. Despite recognition as an important prognostic and diagnostic feature in diffuse glioma, radiation remains the sole standard of care and no effective systemic therapies are available for H3K27M mutant tumors. This review will detail treatment interventions applied to diffuse midline glioma and diffuse intrinsic pontine glioma (DIPG) prior to the identification of the H3 K27M mutation, the current standard-of-care for H3 K27M-mutant diffuse glioma treatment, and ongoing clinical trials listed on www.clinicaltrials.gov evaluating novel therapeutics in this population. Current clinical trials were identified using clinicaltrials.gov, and studies qualifying for this analysis were active or ongoing interventional trials that evaluated a therapy in at least 1 treatment arm or cohort comprised exclusively of patients with DIPG and H3 K27M-mutant glioma. Forty-one studies met these criteria, including trials evaluating H3 K27M vaccination, chimeric antigen receptor T-cell therapy, and small molecule inhibitors. Ongoing evaluation of novel therapeutics is necessary to identify safe and effective interventions in this underserved patient population.

EPCO-17. 3D GENOME STRUCTURE REGULATES TRANSCRIPTION IN PEDIATRIC HIGH-GRADE GLIOMA

Abstract INTRODUCTION Pediatric high-grade gliomas (pHGGs), including glioblastoma multiforme (GBM) and diffuse intrinsic pontine glioma (DIPG), are highly morbid brain tumors. Up to 80% of DIPGs harbor a somatic missense mutation in genes encoding Histone H3. To investigate whether the H3K27M mutant protein is associated with distinct chromatin structure affecting transcription regulation, we generated the first high-resolution Hi-C and ATAC-Seq maps of pHGG cell lines, and integrated these with tissue and cell genomic data. METHODS We generated sequencing data from patient-derived cell lines (DIPG n=6, GBM n=3, normal n=2) and frozen tissue specimens (DIPG n=1, normal brainstem n=1). Analyses included cell line RNA-Seq, ChIP-Seq (H3K27ac, H3K27me3, H3K27M) and genome-wide chromatin conformation capture (Hi-C), as well as tissue ATAC-Seq. Publicly available pediatric glioma tissue ChIP-Seq data was integrated with cell data. CRISPR knock-down of target enhancer regions was performed. RESULTS We identified tumor-specific enhancers and regulatory networks for known oncogenes in DIPG and GBM. In DIPG, FOX, SOX, STAT and SMAD families were among top H3K27Ac enriched motifs. Significant differences in Topologically Associating Domains (TADs) and DNA looping were observed at OLIG2 and MYCN in H3K27M mutant DIPG, relative to wild-type GBM and normal cells. Pharmacologic treatment targeting H3K27Ac (BET and Bromodomain inhibition) altered these 3D structures. Functional analysis of differentially enriched enhancers in DIPG implicated SOX2, SUZ12, and TRIM24 as top activated upstream regulators. Distinct genomic structural variations leading to enhancer hijacking and gene co-amplification were identified at A2M, JAG2, and FLRT1. CONCLUSION We show genome structural variations enhancer-promoter interactions that impact gene expression in pHGG in the presence and absence of the H3K27M mutation. Our results imply that tridimensional genome alterations may play a critical role in the pHGG epigenetic landscape and thereby contribute to pediatric gliomagenesis. Further studies examining the impact of the alterations is therefore underway.

EPCO-50. THE TRANSCRIPTIONAL LANDSCAPE OF KEY INNATE DNA SENSORS WITHIN PRIMARY BRAIN TUMORS

Abstract Primary brain tumors are notoriously resistant to immunotherapy. The efficacy of these treatments potentially requires normal expression of DNA innate sensors within both the immune and tumor compartments. Recently, we found that STING1, involved in the CGAS/STING pathway, is epigenetically silenced in the neoplastic cells of primary brain tumors. This silencing can be traced back to the normal brain. We and others have shown that rescue of STING1 expression can be modulated with epigenetic treatments. The mechanism by which DNA damage in cancer leads to an inflammatory response sheds light on the significance of other DNA sensors. We hypothesize that innate immune silencing within the tumor contributes to immunotherapy resistance and the addition of epigenetic treatments could increase efficacy. We evaluated the transcriptional landscape of DNA innate sensors within normal adult brain and in patient derived tumors including glioblastoma, pilocytic astrocytoma, and diffuse midline gliomas, H3K27M mutant. We found low expression of additional cytosolic DNA sensors including cGAS, AIM2, ZBP1 in normal neuronal as well as tumor cells. Since epigenetic silencing is modulated during normal development, we compared the transcriptional innate profiles from normal cortex brain development to primary brain cancers. DNA innate immune sensor expression correlates with pre-oligodendrocyte progenitor cells and oligodendrocyte progenitor cells. These results suggest the innate immune suppression of these key cytosolic DNA sensors is established early in fetal brain development and retained in the context of primary brain tumors. We are currently exploring the epigenetic profiles of fetal brain and brain tumors to determine how these innate immune expression patterns affect tumorigenesis and response to immunotherapy treatment.

H3K27M mutant glioma: Disease definition and biological underpinnings.

High-grade glioma (HGG) is the most common cause of cancer death in children and the most common primary central nervous system tumor in adults. While pediatric HGG was once thought to be biologically similar to the adult form of disease, research has shown these malignancies to be significantly molecularly distinct, necessitating distinct approaches to their clinical management. However, emerging data have shown shared molecular events in pediatric and adult HGG including the histone H3K27M mutation. This somatic missense mutation occurs in genes encoding one of two isoforms of the Histone H3 protein, H3F3A (H3.3), or HIST1H3B (H3.1), and is detected in up to 80% of pediatric diffuse midline gliomas and in up to 60% of adult diffuse gliomas. Importantly, the H3K27M mutation is associated with poorer overall survival and response to therapy compared to patients with H3 wild-type tumors. Here, we review the clinical features and biological underpinnings of pediatric and adult H3K27M mutant glioma, offering a groundwork for understanding current research and clinical approaches for the care of patients suffering with this challenging disease.

H3 K27M-Altered Diffuse Midline Gliomas: A Review

AbstractDiffuse midline glioma H3 K27M-altered is a recently renamed high-grade glioma in the 2021 World Health Organization (WHO) Classification of Central Nervous System Tumors, previously being labelled diffuse midline glioma H3 K27M-mutant in the 2016 update and diffuse intrinsic pontine glioma prior to 2016. After identification of multiple alterations causing H3 K27 hypomethylation, the definition of this tumor subtype was changed. To further characterize this new entity in both the pediatric and adult population, we conducted a review of the current literature, investigating genetic, epidemiological, clinical, radiological, histopathological, treatment and prognostic characteristics, particularly highlighting the differences between adults and children. This tumor is more common in children, and has a poorer prognosis. Additionally, childhood H3 K27-altered gliomas are more common in the brainstem, but more common in the thalamus in adults. Sadly, limited treatment options exist for these tumors, with radiotherapy the only treatment shown to improve overall survival.

Experience with multitargeted antitumor vaccine in a child with diffuse midline glioma, H3 K27M-mutant

Диффузная глиома высокой степени злокачественности (high-grade глиома) — наиболее часто встречающаяся высокозлокачественная первичная опухоль головного мозга с крайне агрессивным течением и неблагоприятным прогнозом.&#x0D; В статье описывается клинический случай 10-летнего ребенка с диффузной срединной глиомой головного мозга с альтерацией в гене H3 K27, которому после стандартного химиолучевого лечения в качестве поддерживающей терапии создан дендритноклеточный вакцинный препарат на основе иммуногенных раковотестикулярных и GD2 антигенов [(CTA+ (cancer-testis antigen) GD2+ (ganglioside)] вакцина). Опухолевый лизат был представлен глиобластомной клеточной культурой с экспрессией GD2 96%. В качестве ростовых и дифференцировочных факторов дендритных клеток, полученных из моноцитов периферической крови, были ГМ-КСФ (гранулоцитарно-макрофагальный колониестимулирующий фактор), ИЛ4 (интерлейкин 4) и ФНОα (фактор некроза опухоли α).&#x0D; Мультитаргетная иммунотерапия CTA+GD2+ аутологичной ДК-вакциной априори решает эволюционную проблему фенотипической гетерогенности и обладает большей клинической и иммунологической эффективностью в отношении high-grade глиом с их постепенным иммуноредактированием.

DIPG-45. ELONGIN B REGULATES H3K27M CHROMATIN INCORPORATION TO PROMOTE MALIGNANCY IN DIFFUSE MIDLINE GLIOMA

Abstract Chromatin dysregulation is both a cause and consequence of aberrant RNA Polymerase II (Pol2) transcription in cancer. Some of the most striking examples of chromatin disruption in cancer include somatic mutations in genes encoding histone H3 (H3.3/H3.1K27M) observed in ~80% of diffuse midline glioma (DMG). H3K27M mutant histones act in a dominant manner to disrupt chromatin regulation and Pol2-dependent transcription in DMG. How H3K27M impinges on the Pol2 transcriptional machinery and whether aberrant Pol2 transcription can reciprocally induce chromatin dysregulation in DMG remain open questions. We conducted epigenome-focused CRISPR dropout screens and identified multiple regulators of Pol2 elongation, including Elongin B and C (ELOB/ELOC) as DMG genetic dependencies. Follow-up studies confirm that knockout (KO) of ELOB inhibits DMG proliferation in neurosphere culture and abrogates DMG tumorigenesis in orthotopic xenografts. Additional chromatin profiling studies reveal that ELOB binding sites are enriched in H3K27M mutant histones, suggesting that ELOB works together with H3K27M to promote malignant gene expression in DMG. Indeed, reciprocal knockout studies suggest that while ELOB chromatin binding patterns were not altered by H3K27M KO, ELOB KO significantly reduced H3K27M chromatin incorporation at thousands of genomic loci. Further RNAseq and PROseq analyses reveal that ELOB KO alters Pol2 transcription resulting in the up-regulation of genes associated with neuronal and muscle differentiation and the repression of glial stem cell genes. Together, these findings suggest that H3K27M chromatin binding patterns and aberrant gene regulation in DMG are, in part, dictated by ELOB and the Pol2 elongation machinery. Finally, analyses of patient datasets indicate that ELOB complex genes are overexpressed in high grade glioma and DMG and correlate with decreased patient survival. Collectively, these studies suggest that a functional interaction between ELOB and H3K27M mutant histones induces DMG-specific aberrations in chromatin and gene regulation to drive malignancy.

DIPG-44. DIFFUSE MIDLINE GLIOMA WITH EXTRACRANIAL METASTASIS

Abstract Diffuse midline gliomas (DMGs) are malignant infiltrative gliomas enriched in the pediatric population and characterized by loss of the H3 K27me3 epigenetic marker, most frequently via mutation of the H3F3A gene. Few cases of extracranial DMG metastasis with redemonstrated H3F3A p.K28M (K27M) mutation in metastatic tissue are reported in the literature. Here, we report two such patients, both females (ages 12 and 15), with DMG and extracranial metastasis who died 5 years after initial diagnosis. The first patient with thalamic DMG underwent biopsy, radiation, chemotherapy, and subsequent decompression of spinal metastasis. Initial tumor biopsy demonstrated mutation in BRAF p.V600E, H3F3A K27M, and TERT promoter. The epidural spinal metastasis had mutations in BRAF p.V600E, H3F3A, TERT, and TP53. The second patient with brainstem DMG, diagnosed by radiographic appearance, underwent conventional external beam radiation therapy, chemotherapy including panobinostat and direct delivery of radioimmunotherapy, intra-arterial delivery of bevacizumab, and subsequent decompression of spinal metastasis. The histology and molecular profile for the spinal metastasis was consistent with DMG H3 K27M-mutant, with mutations in BRAF p.G464V, PTPN11 p.R498L, PDGFRA, and KIT. Bone marrow biopsy demonstrated atypical extrinsic cells with strong and diffuse immunolabeling for OLIG2. Autopsy revealed solid organ and osseous metastases. Both patients significantly exceeded the median life expectancy for DMG, raising the possibility that prolonged overall survival permitted progression to a rarely observed disseminated state of disease. There was absence of TP53/p53 modulating pathway mutation seen in classic DMG in the thalamic biopsy of the first patient, as well as the metastatic disease for the second patient, which may contribute to the prolonged survival observed. Molecular analysis of metastatic disease is important, as clinically and prognostically relevant alterations that vary from the primary site of disease may be detected, which shed light on clonal evolution patterns and further our understanding of disease biology.

DIPG-19. BAF COMPLEX PERTURBATION AS A NOVEL THERAPEUTIC OPPORTUNITY IN H3K27M PEDIATRIC GLIOMA

Abstract Epigenetic dysregulation resulting in stalled development plays a crucial role in pediatric cancer tumorigenesis. Diffuse midline gliomas (DMG) are universally fatal pediatric brain cancers refractory to standard of care treatment modalities. These malignancies are driven by heterozygous mutations in genes encoding histone 3 (H3K27M) which create an aberrant epigenetic landscape that keeps glioma cells in an undifferentiated stem-like state. Consequently, targeting epigenetic regulators to restore the epigenome and force glioma cells to exit this stem-like cell state represents a promising new therapeutic strategy for H3K27M-DMG. To interrogate for epigenetic dependencies, we performed a CRISPR/Cas9 inactivation screen in patient-derived H3K27M-DMG neurospheres using an epigenetically focused sgRNA library and identified several core components of the mammalian BAF (SWI/SNF) chromatin remodeling complex as genetic vulnerabilities. Validation assays revealed that knockout of the BAF catalytic subunit BRG1 results in decreased glioma cell proliferation and tumor growth in orthotopic mouse models. Mechanistically, genome wide localization and DNA accessibility studies combined with regulatory network analysis demonstrated that BRG1 controls the transcription factor and enhancer landscapes that maintain H3K27M-DMG cells in a cycling, oligodendrocyte precursor cell-like state. Single cell transcriptome analysis in vitro and immunofluorescence studies in vivo confirmed that genetic perturbation of this chromatin remodeler promotes progression of differentiation along the astrocytic lineage. Similarly, pharmacological suppression of BRG1 activity, using both catalytic inhibitors as well as recently developed degraders, opposes tumor cell proliferation, stimulates cell state transition, and improves overall survival of patient-derived xenograft models. Interestingly, these effects seem to be restricted to H3K27M mutant glioma, as H3 wildtype glioma cells were less sensitive to BRG1 inhibition both in vitro and in vivo. In summary, we demonstrate that the BAF complex contributes to the maintenance of glioma cells in a proliferative stem-like state and that its therapeutic inhibition has translational potential for children bearing H3K27M-DMG.