STMO-19 Impact of aggressive resection for glioblastoma of the thalamus with histone H3-K27M mutation

Glioblastoma of the thalamus occurs predominantly in childhood and young adulthood, and cases with histone mutations are thought to have a particularly poor prognosis. We studied tumor resection rate, age, type of adjuvant therapy, and histone gene mutations on progression-free survival (PFS) and overall survival (OS) in patients who underwent aggressive removal. Eight cases of thalamic glioblastoma were included in the study. The mean age at surgery was 36.1 years (10–74 years, 3 cases under 18 years). Tumor removal was performed from the parieto-occipital lobe to the thalamus via the lateral ventricles in all cases. In all cases, more than 90% of the contrast-enhancing lesions were removed. Postoperatively, one patient had sensory disturbance of the left upper limb, and the other had incomplete paralysis of the left upper and lower limbs, but both were able to walk with a cane. In the case of the patient with postoperative complications, the tumor was located in the vicinity of the internal capsule. All patients were treated with radiation therapy and temozolomide, and bevacizumab and Novo-TTF were used in cases after approval. All patients were able to return home and return to school or work after initial treatment. The mean progression-free survival (PFS) was 0.87 years, and overall survival (OS) was 1.95 years. Five patients had histone H3-K27M mutations, and three patients had no mutations. PFS and OS were 1.02 years and 0.62 years, respectively, and 2.53 years and 1.20 years, respectively, both of which were longer in patients with mutations (PFS; p=0.16, OS; p=0.23).Aggressive removal of glioblastoma of the thalamus may improve prognosis, especially in patients with histone H3-K27M mutations. In patients with tumors extending to the vicinity of the internal capsule, total removal may cause paralysis and sensory disturbance.

PATH-04. LEPTOMENINGEAL DISEASE IN PATIENTS WITH HISTONE-MUTANT GLIOMAS

INTRODUCTION The 2016 WHO classification describes a subtype of midline gliomas harboring histone 3 (H3) K27M mutations, and the upcoming 2021 edition may include a new subtype of hemispheric diffuse gliomas with H3 G34R/V mutations. Since we began to perform molecular profiling at Memorial Sloan Kettering Cancer Center (MSKCC) we have observed an increased prevalence of leptomeningeal disease (LMD) in this population. However, the incidence and clinical behavior of LMD has not been well defined. METHODS This is a retrospective study of patients with H3-mutant gliomas at MSKCC diagnosed from 01/2012 to 02/2021, either by tumor biopsy or by cerebrospinal fluid (CSF). Histone mutations were identified through next-generation sequencing (NGS). RESULTS We found 40 patients (pts) harboring H3 mutations (K27M mutations in 31 pts, G34R/V in 8, and both in one), with 2/40 (5%) mutations identified through CSF NGS only and rest in tumor tissue. Median age was 20.5 years (4-70); 25 were male. Tumor location was midline for K27M-mutant tumors (thalamus [N=15], brainstem [N=10], spine [N=5], pineal gland [N=1]) and hemispheric for G34R/V-mutant tumors; tumor with both mutations was thalamic. LMD was diagnosed in 22/40 (55%) pts radiographically, including 19/31 (61%) of K27M-mutant pts and 3/8 (38%) of G34R/V-mutant pts (patient with both mutations did not develop LMD). At analysis, 10 patients remain alive. Median time from diagnosis to LMD was 8.8 months (0-44.4), with median OS of 6.5 months (0.3-34) after LMD diagnosis. CONCLUSION More than half of patients with histone-mutant gliomas develop LMD, including over a third of patients with G34R/V mutations. Neuroaxis imaging should be performed in conjunction with CSF studies in histone-mutant gliomas. Besides, CSF NGS represents an important tool to identify molecular profile of tumors when biopsy is not feasible or there is limited tissue for analysis.

HGG-22. EVALUATING THE REGULATION OF BLOOD-BRAIN BARRIER INTEGRITY IN DIPG MOUSE MODELS

Diffuse intrinsic pontine gliomas (DIPGs) are considered to maintain a fairly intact blood-brain barrier (BBB) based on patient imaging tumor histology. In characterizing recently developed DIPG and HGG mouse models, we identified differences in BBB function and increased Angiopoietin1 (Angpt1) in H3 K27M DIPG models. We hypothesize that H3 K27M mutations promote the maintenance of DIPG BBB integrity through upregulation of Angpt1. To determine DIPG and HGG BBB phenotypes we performed an intergrative analysis of vascular histology and endothelial transcriptomes Ongoing studies using electroporation based DIPG mouse models are being performed examine the regulation and function of Angpt1 in DIPG BBB integrity. We have initiated studies comparing H3 K27M DIPG mouse models to H3 WT and G34R cortical HGG mouse models, demonstrating that DIPG models show minimal changes in vascular phenotype, including vessel density, branching, and diameter compared to cortical HGG models. Comparing DIPG and HGG purified endothelial transcriptomes, HGG ECs displayed enrichments of inflammatory signals and proliferation gene sets, and increased expression of tip cell identity genes. We identified Angpt1 as selectively upregulated in H3 K27M mouse models and derived cell lines. Preliminary data suggests Angpt1 supports the maintenance of BBB integrity in DIPG models. BBB phenotype differences are present in DIPG and HGG mouse models. Uncovering mutation specific mechanisms that regulate BBB function in brain tumors will be critical to advance our understanding of brain tumor pathogenesis and treatment response.

Comparison of next-generation sequencing and FISH/IHC for molecular classification in glioma.

e14021 Background: The updated 2016 edition of the WHO Classification of CNS tumors indicates that IDH1 R132H, H3 K27M mutations, and co-deletion of 1p19q are strong stratification and prognostic markers glioma. FISH/IHC, as the commonly detected methods, present specific false-negative rates in the actual condition. Methods: In our study, IDH1 R132H status of 158 cases was assessed by IHC and NGS, and H3 K27M statuses of 83 patients were evaluated by IHC and NGS. 22 positive cases of 1p/19q co-deletion, all confirmed by FISH, were assessed by NGS. Results: For IDH1 R132H, 2 cases were IHC negative and were positive as confirmed by NGS. Another 10 patients with weakly IHC positive results were negative in NGS. Combined with histologic hallmarks, 6 cases of these samples could be diagnosed as glioblastoma, IDH wildtype; 1 case with POLE could be diagnosed as giant cell glioblastoma; 3 cases with BRAF V600E mutation, BRAF fusion, and ATRX mutation, respectively, could be diagnosed as pilocytic astrocytoma. Towards H3 K27M, 3 cases with IHC weakly positive were negative in NGS. Among these samples, 2 cases were diagnosed as glioblastoma, IDH wildtype by molecular and histologic hallmarks, and 1 case was medulloblastoma, SHH. Using NGS, IDH1 R132H /H3 K27M statues can be distinctly distinguished in which that is unknown by IHC. The results of NGS and FISH showed a 90.9%(20/22) consistent rate for the 1p19q co-deletion. 1 case was 1p deletion and intact 19q by FISH while 1p19q co-deletion by NGS because of the 19p deletion. 1 case was 1p19q co-deletion by FISH but 1p19q wildtype by NGS, diagnosed as glioblastoma, IDH wildtype with chr7+/10-. Conclusions: In our study, the agreement between NGS results and clinical pathology diagnosis was approximately 100%. NGS may act as the primary technology of molecular classification in glioma in the future.

Diffuse midline glioma with H3 K27M-mutation in an 83-year-old woman

Diffuse midline gliomas harboring histone H3 K27M mutations are most commonly found in the brainstem of children. This mutation confers a WHO grade IV designation and is associated with a particularly poor prognosis. Although traditionally considered to be a disease of children and young adults, a number of recent reports have described H3 K27M mutations in older adults with diffuse midline gliomas. Here, we present the unusual case of a diffuse midline glioma in the pons and cerebellum of an 83-year-old woman and review the evolving clinical literature on this entity in adults. This case underscores that it may occur even in older adults, in whom prognostic and treatment paradigms used in pediatrics may not be directly applicable.

DIPG-58. HISTONE H3 WILD-TYPE DIPG/DMG OVEREXPRESSING EZHIP EXTEND THE SPECTRUM OF DIFFUSE MIDLINE GLIOMAS WITH PRC2 INHIBITION BEYOND H3-K27M MUTATION

Diffuse midline gliomas (DMG) H3 K27M-mutant were introduced in the 2016 WHO Classification unifying diffuse intrinsic pontine gliomas (DIPG) and gliomas from the thalamus and spinal cord harboring a histone H3-K27M mutation leading to Polycomb Repressor Complex 2 (PRC2) inhibition. However, few cases of DMG tumors presenting a H3K27 trimethylation loss, but lacking an H3-K27M mutation were reported. To address this question, we combined a retrospective cohort of 10 patients biopsied for a DIPG at the Necker Hospital or included in the BIOMEDE trial (NCT02233049) and extended our analysis to H3-wildtype (WT) diffuse gliomas from other midline locations presenting either H3K27 trimethylation loss or ACVR1 mutation from Necker, ICR, the HERBY trial, the INFORM registry study and the St. Jude PCGP representing 9 additional cases. Genomic profiling identified alterations frequently found in DMG, but none could explain the observed loss of H3K27 trimethylation. Similar observations were previously made in the PF-A subgroup of ependymoma, where the H3K27me3 loss resulted from EZHIP/CXorf67 overexpression rather than H3-K27M mutations. We thus analyzed EZHIP expression and observed its overexpression in all but one H3-WT DMGs compared to H3-K27M mutated tumors (EZHIP negative). Strikingly, based on their DNA methylation profiles, all H3-WT DMG samples analyzed clustered close to H3-K27M DIPG, rather than EZHIP overexpressing PF-A ependymomas. To conclude, we described a new subgroup of DMG lacking H3-K27M mutation, defined by H3K27 trimethylation loss and EZHIP overexpression that can be detected by IHC. We propose that these EZHIP/H3-WT DMGs extend the spectrum of DMG with PRC2 inhibition beyond H3-K27M mutation.

Low-Grade Gemistocytic Morphology in H3 G34R-Mutant Gliomas and Concurrent K27M Mutation: Clinicopathologic Findings

Mutations in histone H3 are key molecular drivers of pediatric and young adult high-grade gliomas. Histone H3 G34R mutations occur in hemispheric high-grade gliomas and H3 K27M mutations occur in aggressive, though histologically diverse, midline gliomas. Here, we report 2 rare cases of histologically low-grade gliomas with gemistocytic morphology and sequencing-confirmed histone H3 G34R mutations. One case is a histologically low-grade gemistocytic astrocytoma with a G34R-mutation in H3F3A. The second case is a histologically low-grade gemistocytic astrocytoma with co-occurring K27M and G34R mutations in HIST1H3B. Review of prior histone H3-mutant gliomas sequenced at our institution shows a divergent clinical and immunohistochemical pattern in the 2 cases. The first case is similar to prior histone H3 G34R-mutant tumors, while the second case most closely resembles prior histone H3 K27M-mutant gliomas. These represent novel cases of sequencing-confirmed histone H3 G34R-mutant gliomas with low-grade histology and add to the known rare cases of G34R-mutant tumors with gemistocytic morphology. Although K27M and G34R mutations are thought to be mutually exclusive, we document combined K27M and G34R mutations in HIST1H3B and present evidence suggesting the K27M-mutation drove tumor phenotype in this dual mutant glioma.

RARE-33. LEPTOMENINGEAL DISEASE IN PATIENTS WITH MIDLINE GLIOMAS HARBORING HISTONE H3 K27M MUTATIONS

INTRODUCTION The 2016 WHO classification describes a specific subtype of midline gliomas harboring histone 3 (H3) K27M mutations which compromise 15–40% of gliomas in children and young adults. Since we began to perform molecular profiling at Memorial Sloan Kettering Cancer Center (MSKCC) we have observed an increased prevalence of leptomeningeal disease (LMD) in this population. However, the true incidence and the clinical behavior of this group has not been well-defined. METHODS This is a retrospective study of patients with H3K27M/I diffuse midline gliomas at MSKCC from 01/2012 to 01/2019. Mutations were identified through next-generation sequencing (NGS). RESULTS We identified 24 patients. Median age was 21 (6–70) and 13 were male. Thirteen tumors were in the thalamus, 6 in the brainstem, 4 in the spinal cord, and 1 in the pineal region. H3K27M mutations were detected by NGS from tumor tissue in 21 patients, from cerebral spinal fluid (CSF) circulating tumor DNA (ctDNA) in 2 and from tissue and CSF ctDNA in one. LMD was diagnosed in 17/24 (71%) patients radiographically. Of these, 8 underwent LP: CSF cytology was positive in 2/8 and CSF ctDNA was detected in 3/3. All patients received RT and one or more lines of chemotherapy. At analysis, thirteen patients remain alive. Median time to last follow up for all patients was 14.2 months. Median time from diagnosis to LMD was 13.2 months (0–45.1), with median OS of 6.5 months (0.3–27) after diagnosis of LMD. CONCLUSION Our study showed that more than two-thirds of the patients with H3 K27M diffuse midline developed LMD. Neuroaxis imaging should be performed in conjunction with CSF studies to diagnose LMD. Besides, CSF ctDNA represents an important tool to identify molecular profile of tumors when biopsy is not feasible or limited for the analysis.