Clinical manifestations

Presentation and course

Symptoms vary based on the involvement of different midline structures and develop over 1 to 2 months (22; 10; 32). Cranial neuropathies can cause expected symptomatology: diplopia, ptosis, facial pain or numbness, facial palsy, tinnitus, hearing loss, vertigo, dysarthria, and dysphagia. Involvement of motor and sensory pathways causes weakness, hypoesthesia, hyperesthesia, and gait abnormalities. High CSF pressure causes headache, nausea, and vomiting. Other CNS symptoms include seizures and persistent hiccups.

H3 K27M mutant diffuse midline gliomas have been extensively described in children and are rarely seen in adults (41). In children, H3 K27M-mutant gliomas occur frequently within the brainstem, particularly the pons, whereas in adults they occur more frequently within the thalamus and spinal cord. Rare adult cases have been reported in the corpus callosum, hypothalamus, pineal region, basal ganglia, and third ventricle. In children, they occur primarily at a median age of 7 years (5 to 11 years) in the pons and at 11 years in the thalamus. In adults, median age at diagnosis is 35 years (range 18 to 82 years). There is no gender preponderance.

In a retrospective study of adult midline gliomas, the H3 K27M mutation was identified in 15% cases (39). Most common midline locations for H3 K27M-mutated tumors were midbrain, pons, and cerebellum. However, other studies in adults have reported these tumors most frequently in the thalamus and spinal cord. In the pediatric population, this mutation is identified in approximately 27% of gliomas (26).

Prognosis and complications

H3 K27M-mutant midline gliomas are associated with poor prognosis and were designated as a grade 4 entity in the 2016 WHO Classification (22). However, there is heterogeneity in the behavior of gliomas with H3 K27M mutation. Prognosis for circumscribed gliomas, thalamic, or diffuse non-midline gliomas with H3 K27M mutation remains uncertain (11; 28; 30; 31).

Prognosis remains dismal even with timely and optimal treatment; overall survival is typically 7 to 11 months in children and 8 to 28 months in adults (17; 37; 41). Overall survival with standard care is slightly longer compared to patients with IDH wildtype glioblastoma but shorter compared to patients with WHO grade 4 IDH-mutant astrocytomas (24). K27M mutation is a significant independent prognostic marker for poor outcome among pediatric glioblastomas (18). As compared to H3.1K27M mutation, H3.3K27M mutation is associated with shorter survival, higher metastatic relapses, and poorer response to radiotherapy (45; 06).

Biological basis

Etiology and pathogenesis

Frequency of IDH mutation in the midline region has been reported to be low (43).

H3 K27M-mutant gliomas are IDH-wildtype, lack 1p/19q co-deletion, and are defined by the presence of K27M mutation in the H3F3A or HIST1H3B/C genes, which encode the histone H3 variants H3.3 and H3.1 in both children and adults (46; 41). These histone mutations are mutually exclusive from the other common mutations that define distinct infiltrating glioma types.

Histones are basic nuclear proteins responsible for the nucleosome structure within eukaryotic cells. Among the 5 classes of histones, some are expressed only during the S phase, whereas others are replacement histones that are replication-independent and expressed in quiescent or terminally differentiated cells.

Supratentorial high-grade gliomas have mutations in H3F3A (located on chromosome 1q) encoding histone H3.3. H3.3 is a replacement histone subclass that is encoded by 2 distinct genes, H3.3A (H3F3A) and H3.3B. This leads to the creation of oncohistones.

Oncohistones in high-grade gliomas show specific associated mutations and brain location. G34R/V is restricted to hemispheric tumors, and K27M occurs in the midline (17; 43; 45; 09). K27M mutations occur in 2 main histone variants H3.3 and H3.1 and result in lysine to methionine exchange at position 27 of H3F3A or HIST1H3B/C genes. G34 mutations of H3.3 result in glycine to arginine exchange at codon 34 of H3F3A (40; 45).

The H3 K27M-mutated diffuse intrinsic pontine gliomas group is further divided into two subgroups, H3.1 K27M and H3.3 K27M (06).

H3.3 K27M mutation. H3.3 K27M mutation occurs at median age 7.5 years. Patients with this mutation have poor prognosis and frequent leptomeningeal dissemination; they are likelier to have an oligodendroglial histologic phenotype, poor response to radiation, and median survival of 9 months (09). These tumors are seen all over the midline structures (46; 06). Approximately 20% of tumors previously categorized as pediatric glioblastomas harbor this mutation (12).

H3.1 K27M mutation. H3.1 K27M mutation occurs at median age 5.5 years. Patients with this mutation are likelier to have an astroglial differentiation, a good response to radiation, and a median survival of 15 months (09). These tumors are almost exclusive to the pons (46; 06).

Diffuse glioma, H3.3 G34-mutant is included as a distinct grade 4 entity (24).

H3 K27M-mutant diffuse midline gliomas frequently share genetic alterations, including TP53 overexpression, ATRX loss, and monosomy 10, ACVR, CCND1-3, and CDK4/CDK6 mutations (41; 10). H3 K27M mutation is mutually exclusive or rarely co-occurs with IDH1 mutation, TERTp mutation, EGFR amplification, and BRAF-V600E mutation (41; 25; 10). Prototypic alterations of adult primary glioblastoma IDHwt (eg, EGFR amplification, CDKN2A/B homozygous deletions, PTEN mutations) are very infrequent in childhood (33; 43; 34; 18). MGMT promoter methylation is found in 75% of G34-mutant subtype and in 3% of K27 mutant tumors (18; 25).

Diagnostic workup

Diagnosis is made with imaging and histologic confirmation. Radiographic criteria alone are used for tumors with midline locations associated with serious procedural complications (15). Imaging involves use of brain MRI with and without gadolinium. MRI abnormalities include diffuse, T2 expansile signal changes in the midline structures.

In most cases, little or no enhancement is noted after the administration of gadolinium at initial diagnosis. Occasionally, areas of necrosis with surrounding serpiginous contrast enhancement are present. A retrospective study to characterize imaging features of 24 diffuse midline gliomas found distinguishing features in H3K27-mutant midline gliomas as compared to their H3wild-type counterparts (01). In H3K27-mutant gliomas, there was an absence of perilesional edema at low frequency of strong contrast enhancement. Contrast enhancement of intermediate or higher and rim enhancement was associated with microscopically detected microvascular proliferation. Necrosis on MRI is less frequently seen than in the H3wt high-grade midline gliomas (01).

Traditional WHO grading for diffuse intrinsic pontine gliomas was not prognostic. Outcomes for grade 2, 3, and 4 tumors are equally poor (04). Next-generation sequencing helps accurately classify these tumors, improves understanding of unique tumor biology, and provides important insights into prognosis and management. Reports have shown that biopsy of these tumors is safe (15). Whether a biopsy at the time of diagnosis should be obtained in the absence of a clinical trial remains contentious. Modern neuronavigation tools allow accurate 3D localization of deep structures in the brain and identification of major vessels prior to surgery, which greatly minimizes surgical complications (15).

Once tissue is obtained, the macroscopic and microscopic evaluation can provide insight into the underlying disease. Macroscopically, these tumors cause enlargement and distortion of anatomical structures by diffuse infiltration. Microscopically, they typically have an astrocytic morphology and rarely an oligodendroglial pattern. Mitotic activity is present in most cases but is not necessary for diagnosis (22). Features such as microvascular proliferation and necrosis may be seen. Immunohistochemistry reveals expression of NCAM1, S100, LIG2, and MAP2. Staining for GFAP is variable, and synaptophysin can be focal. Chromogranin A and NeuN are not typically expressed.

Management

Anatomic location of the tumor severely limits any opportunity for meaningful surgical resection. Treatment usually consists of standard fractionated radiation to a dose of 54-59Gy (over 30 fractions) and temozolomide (09).

Radiation is the mainstay of treatment. Photon-based radiotherapy to a range of 54 to 59.4 Gy given in 30 to 33 fractions of 1.8 Gy daily (6 weeks) as well as hypo-fractionated radiation (9 Gy given in 13 fractions of 3.0 Gy over 3 to 4 weeks) has been utilized with similar survival rates (27; 16; 47). Monotherapy and combination chemotherapy have been tried, with uniformly poor results (14; 44). Temozolomide has limited to no value in H3 K27M-mutant gliomas likely due to lack of MGMT promoter methylation (38; 08).

Future directions

Novel ongoing treatment trials include conventional enhanced therapies, targeted therapies, peptide-based vaccine therapies, other immunotherapeutic approaches, alternating electrical fields, and BET bromodomain inhibitors.

Convection-enhanced delivery. Convection-enhanced delivery is a therapeutic strategy that allows for targeted treatment of a specific region via a cannula that can be placed in difficult-to-access areas and allows for direct intraparenchymal infusion of drugs at a steady rate over a prolonged period of time. Studies have shown safe placement of convection-enhanced delivery catheters into the brainstem in humans (02; 42).

Peptide-based vaccine therapy. Glioma-associated antigens interleukin-13 receptor alpha 2 (IL-13Rα2), EphA2, and survivin are commonly overexpressed in pediatric gliomas (29). A pilot study of subcutaneous vaccination with glioma-associated antigen epitope peptides in HLA-A2-positive children with newly diagnosed brainstem and high-grade gliomas was well tolerated and has preliminary evidence of immunologic and clinical responses (36).

Targeted therapies. K27M mutation drives gliomagenesis by alteration of an important site of posttranslational modification in the histone H3 variants and leads to altered DNA methylation and gene expression profiles (43; 03). Ongoing efforts aim to study the efficacy of therapeutics targeting histone-modifying enzymes for midline gliomas with histone H3 mutations.

Panobinostat. Panobinostat is a general histone deacetylase inhibitor that has shown good in vitro efficacy against diffuse intrinsic pontine gliomas (13; 35). Panobinostat synergizes with histone demethylase inhibitor GSKJ4 in H3.3K27M mutant diffuse intrinsic pontine gliomas cells (13). Together, these data suggest a promising therapeutic strategy for diffuse intrinsic pontine gliomas.

ONC201. ONC201 is a small-molecule selective antagonist of dopamine receptor D2/3 (DRD2/3) that crosses the blood-brain barrier and exhibits P53 independent anticancer efficacy. DRD2 expression within the central nervous system is highest in midline structures of the brain. DRD2 is a G protein-coupled receptor that promotes tumor growth and has emerged as a therapeutic target for gliomas that overexpress this receptor (Li et al 2014; 05). H3 K27M-mutant diffuse midline gliomas have been reported to exhibit elevated expression and dependency on DRD2 as a downstream epigenetic consequence of the mutation (07). It has an exceptional safety profile and has been reported to have clinical and radiographic responses in both recurrent and frontline settings (07). ONC206 is another drug in the same class actively undergoing investigation.

Other agents. Cdk4/6 inhibitors, tyrosine kinase inhibitors, and other agents that more specifically target these mutations in vitro and in xenograft models are being explored. Other potential therapeutic targets tested in in vivo studies include BET bromodomain inhibitors (35).

Multiple phase I and II trials are currently ongoing to test novel agents to treat midline gliomas with histone mutation in frontline and recurrent settings. Novel agents currently being investigated include ONC201 (NCT04617002), ONC206 (NCT04541082), BXQ-350 (NCT04771897), H3.3K27M peptide vaccine with nivolumab (NCT02960230), oral panobinostat in combination with focused ultrasound with microbubbles and neuro-navigator-controlled sonication to temporarily open the blood-brain barrier and allow for a greater concentration of drug to reach the tumor (NCT04804709). An understanding of molecular pathology and ongoing efforts to improve diagnostic tools and therapy hold promise for future advances in prognosis.