Diffuse glioma overview based on the 2021 WHO classifi cation part 2 – pediatric type

Summary

The fifth edition of the WHO Classification of Tumors of the Central Nervous System categorizes diffuse gliomas into the pediatric type, which mainly affects children, and into the adult type of diffuse gliomas occurring dominantly in adult patients. Recently characterized tumors, defined for the first time according to the 2021 WHO classification, have been included in the group of diffuse gliomas of the pediatric type. At the same time, established gliomas have undergone changes in their diagnostic criteria, reflecting the latest insights from molecular-genetic research. This second part of the review presents a comprehensive summary of tumors classified into the pediatric-type diffuse gliomas group according to the fifth edition of the WHO Classification of Tumors of the Central Nervous System published in 2021.

Keywords:

diffuse glioma – WHO CNS 2021 – integrated diagnostics – PLNTY – angiocentric glioma – diffuse astrocytoma – diffuse midline glioma – diffuse hemispheric glioma – low-grade glioma – high-grade glioma

This is an unauthorised machine translation into English made using the DeepL Translate Pro translator. The editors do not guarantee that the content of the article corresponds fully to the original language version.

 

Introduction

The current classification according to the 5th edition of the WHO Classification of Central Nervous System Tumours from 2021 [1,2] reflects the different biological nature of diffuse gliomas primarily occurring in adult and paediatric patients and introduces their division into two age groups - diffuse gliomas of paediatric type and diffuse gliomas of adult type. The distinction between these two groups is based on the molecular genetic alterations recurrently occurring in these entities, whereas paediatric-type gliomas can also occur in adult patients and vice versa. Childhood diffuse gliomas are further divided into low-grade and high-grade malignancies [1], and both groups are summarized in this second part of the review (Table 1).

Similar to diagnostics, therapeutic options are also developing significantly. Currently, targeted therapy and immunotherapy are recommended as preferred treatments in cases of recurrent or progressive disease. Approximately 10-15% of paediatric gliomas are found to have a point mutation in BRAF V600E, which leads to constitutive activation of the MEK/ERK pathway. Combination therapy targeting BRAF and the downstream MEK pathway (dabrafenib/trametinib) has proven successful in several clinical trials in high-grade adult gliomas. In the paediatric population, experience is still limited, although promising results have been reported in small case studies [3]. Currently, a phase II trial testing the BRAF blocker vemurafenib [4] and a phase III trial testing dordaviprone in patients with diffuse midline glioma, H3 K27-altered glioma, are ongoing in paediatric recurrent or progressive high-grade gliomas [5].

Gene fusions involving NTRK1, NTRK2 and NTRK3 encoding TRK fusion proteins (TRKA, TRKB, TRKC) can be affected by TRK inhibitors (larotrectinib and entrectinib). The observed objective response rate in TRK fusion-positive tumours is relatively high (objective response rate [overall response rate; ORR] up to 93%) and the observed toxicity is acceptable. Currently, testing for both agents has already progressed to phase II clinical trials [6].

Gliomas that have been shown to have a high mutational load (hypermutant tumours) may respond to immunotherapy with checkpoint inhibitors (nivolumab, pembrolizumab). However, evidence for their efficacy is currently limited to small retrospective cohorts and case series.

A deeper molecular understanding of paediatric gliomas has led to the testing of many other agents targeting the MAPK/ERK or mTOR pathways. These include MEK inhibitors (selumetinib, binimetinib) or second-generation RAF inhibitors (tovorafenib). Agents that target the mTOR pathway (everolimus) are under investigation. An ongoing Pediatric Neuro-Oncology Consortium (PNOC) study is evaluating the effect of the combination of trametinib and everolimus in recurrent paediatric gliomas [7]. An FGFR inhibitor (erdafitinib) has been investigated in tumours containing FGFR activating changes. In low-grade gliomas, dual isocitrate dehydrogenase (IDH) inhibitor (vorasidenib) therapy appears promising, although evidence of its efficacy in the paediatric population has not yet been presented.

Although targeted therapy is already significantly influencing the treatment paradigms of childhood gliomas, it should be noted that, apart from tuberous sclerosis-associated subependymal giant cell astrocytoma (everolimus) and glioma with BRAF V600E mutation (dabrafenib +  trametinib), targeted therapy has not yet found its place in primary treatment. However, it is currently being evaluated in a number of ongoing prospective clinical trials [8,9].

 

Paediatric- type diffuse low-grade gliomas

Diffuse astrocytoma, MYB- or MYBL1-altered, WHO G1

Diffuse astrocytoma, MYB- or MYBL1-altered, is a rare diffusely infiltrating astrocytic differentiated glioma with alterations in the MYB or MYBL1 genes and without evidence of IDH mutation or histone H3 alteration (Table 2). It typically occurs in children and young adults with pharmacoresistant epilepsy [2,10]. It is characterized by supratentorial localization, predominantly in the temporal lobe, with involvement of the cortex or subcortical region. Occasionally, cases with localization in the brainstem have been published [11]. It is an indolent tumour with a good prognosis [12,13] and only rare anaplastic transformation [14].

The characteristic microscopic picture includes a unimorphic proliferation of cytologically bland glial cells of astrocytic differentiation, for which it was initially termed an isomorphic astrocytoma [13]. The tumour itself has only slightly increased cellular density compared to the brain tissue it infiltrates. The characteristic immunophenotype is the absence of OLIG2, CD34 and MAP2 expression [12].

 

Angiocentric glioma, WHO G1

Angiocentric glioma is a rare diffusely growing tumour predominantly located in the temporal cortex or frontal lobe and is typically associated with pharmacoresistant epilepsy [15]. Angiocentric glioma is characterized by MYB gene alteration, most commonly the MYB::QKI fusion (Table 3) [16]. The prognosis of patients with angiocentric glioma is favourable, the vast majority of patients are cured by complete resection, with recurrence seen in only a minority of cases [15].

The neoplasia itself consists of variably arranged, mainly spindle cell proliferation with a minimally segmented arrangement into characteristic angiocentric formations with radial arrangement of cells around the vessels (Figure 1). Spindle-shaped tumour cells contain regular, small nuclei without significant cytonuclear atypia or significant mitotic activity. The immunophenotype of the neoplasia is also specific, with expression of astrocytic markers as well as luminal or punctate expression of epithelial membrane antigen (EMA), characteristic of neoplasms of ependymal differentiation [17].

 

Polymorphous low-grade neuroepithelial tumour of the young (PLNTY), WHO G1

Polymorphous low-grade neuroepithelial tumour of the young (PLNTY) is a recently described neoplasia associated with pharmacoresistant epilepsy typically seen in paediatric patients [18]. PLNTY usually occurs in the temporal lobe, either cortically or subcortically. Characteristically, PLNTY is composed of a solid and cystic component with numerous calcifications [19]. Similar to other tumours associated with pharmacoresistant epilepsy, PLNTY may be associated with focal cortical dysplasia (FCD IIIB according to ILAE) [20,21].

Morphologically, PLNTY is characterized by variably heterogeneous differentiation with constant, at least focal presence of oligodendroglial differentiation (Figure 2) and calcification. Molecular genetically, PLNTY is characterized as an IDH-wildtype glioma with strong diffuse expression of CD34 and alterations in the MAP kinase cascade (Table 4) [18,22]. It is a potentially curable disease with a low tendency to recur after complete resection, and only one case of malignant reversal of PLNTY to glioblastoma (GBM) has been described [23].

 

Diffuse low-grade glioma,MAPK pathway-altered

The group of diffuse low-grade gliomas with MAPK pathway alterations includes diffuse glial neoplasms genetically defined by alterations in MAPK pathway genes in the absence of mutations in IDH1/2 and histone H3 genes and the absence of homozygous deletion of the CDKN2A gene [1,2,10]. They predominantly occur in the supratentorial region; however, cases have been described throughout the craniospinal axis [11,24]. The most common alterations detected are FGFR1 gene alterations and BRAF V600E mutations (Table 5) [25]. Diffuse low-grade glioma,MAPK pathway-altered is a group of low-grade gliomas predicted to have a good prognosis, mainly based on localization, morphology and molecular alterations. The grade of the entity has not been assigned at present [2].

Morphologically, these are low cellularity glial proliferations of astrocytic or oligodendroglial differentiation with mild cytonuclear atypia and absence of other morphological signs of anaplasia, while tumours with BRAF V600E mutation are morphologically more astrocytic differentiated, while tumours with FGFR1 alteration present with oligodendroglial differentiation [24,25]. Immunohistochemically, tumour elements are OLIG2 positive, with variable expression of GFAP and sporadic reaction with CD34.

 

Paediatric-type diffuse high-grade gliomas

Diffuse midline glioma, H3 K27-altered

Diffuse midline glioma, H3 K27-altered (DMG) is a genetically heterogeneous group of grade 4 IDH-wildtype gliomas with demonstrable loss of H3 K27me3 expression based on H3 K27M mutation [26], EGFR alteration [27] or EZHIP overexpression (Table 6) [28]. By definition, DMG primarily affects midline structures - brainstem, thalamus, spinal cord, cerebellum, hypothalamus and pineal gland; however, postmortem studies have also demonstrated tumour infiltration into the cerebral hemispheres and leptomeningeal dissemination in a significant proportion of DMG patients [29]. Although this is a neoplasia occurring primarily in paediatric patients, a few cases of DMG in the adult population have been described [30]. DMG can have both low-grade and high-grade morphological features; however, by definition it is a grade 4 neoplasia with a very poor prognosis. Less than 10% of patients achieve 2-year survival [26]. More prognostically favourable are DMG variants with genetically characteristic H3.1 and H3.2 K27M subtypes or overexpression of EZHIP [28,31].

The histological picture includes a wide spectrum of differentiations with minimal focal astrocytic differentiation (Figure 3). Other possible differentiation components include giant cell elements, pilomyxoid component, oligodendroglial, sarcomatous, ependymal, epithelioid, rhabdoid, embryonal/primitive neuroectodermal component, as well as atypical ganglion cell component [30]. It is the marked morphological heterogeneity that underscores the need to test for DMG-defining alterations in all IDH-wildtype diffuse gliomas affecting midline structures [32]. However, the loss of H3 K27me3 expression alone is not specific for DMG, as it has also been described in IDH-mutated gliomas, especially oligodendrogliomas with the canonical IDH1 R132H mutation, and innumerable IDH-wildtype gliomas [33,34], which conditions further specification of genetic alterations (Table 6) to discriminate between DMG and GBM.

 

Diffuse hemispheric glioma, H3 G34-mutant

Diffuse hemispheric glioma, H3 G34-mutant (G34-DHG), is a rare high-grade diffuse glial neoplasia arising in the cerebral hemispheres. It is a neoplasia defined by a specific genetic mutation G34R/V in the H3F3A gene, which distinguishes it from the group of morphologically indistinguishable IDH-wildtype diffuse gliomas (Table 7) [35]. This specific alteration predicts aggressive biological behaviour regardless of histological morphology [32]. Although this neoplasia occurs in paediatric patients or young adults with a median age of 15.8 years [36], its occurrence in adult patients cannot be excluded [37]. The prognosis of patients diagnosed with G34-DHG is slightly better compared to both GBM and DMG, with a median survival of 17.3 months in a recent systematic review. At the same time, the prognosis of adult patients with G34-DHG is similarly better compared to paediatric patients with DMG [36]. Similarly to GBM and DMG patients, some patients with G34-DHG also experience leptomeningeal dissemination during disease progression [36,38,39].

Morphologically, G34-DHG is very heterogeneous. On one side of the spectrum, it is morphologically indistinguishable from GBM, on the other side it may mimic embryonal tumours/primitive neuroectodermal tumours, while morphological appearance does not influence prognosis. On the other hand, the immunophenotype of the neoplasia is characterized by a demonstrable loss of ATRX expression associated with ATRX gene mutation, nuclear hyperaccumulation of p53 associated with TP53 gene mutation and absence of nuclear expression of OLIG2. At present, the indication criteria for rational testing of the G34R/V mutation of the H3F3A gene are not clearly established [32], however, early work suggests testing in patients younger than 50 years of age, especially in tumours with a PNET-like component and abnormal expression of ATRX, p53, and OLIG2 [40].

 

Diffuse paediatric-type high-grade glioma, H3-wildtype and IDH-wildtype

Diffuse paediatric-type high-grade glioma, H3-wildtype and IDH-wildtype (pHGG) is a diffuse glial neoplasia typically found in children and adolescents and is genetically defined by the absence of mutations in the IDH1/2 genes and histone H3 genes (Table 8). pHGG was formerly called paediatric glioblastoma, but the use of the term glioblastoma in the paediatric population has recently been abandoned to distinguish the different biological nature of paediatric and adult gliomas [1,2]. pHGG occurs most commonly in supratentorial localization, and less frequently in the brainstem or cerebellum. The histologic picture oscillates from the morphology characteristic of GBM to primitive neuroectodermal differentiation, and the two morphologic extremes may be intermingled [41,42]. pHGGs are aggressive neoplasia with poor prognosis that differ within molecular subgroups. The prognosis is worst for pHGG MYCN with a median survival of 14 months, slightly more favourable for RTK1 with a median survival of 21 months, and most favourable for RTK2 with a median survival of 44 months [41].

pHGG comprises three distinct molecular subgroups differing in molecular alterations, methylation profile and prognosis - pHGG RTK1, RTK2 and MYCN [41]. Amplification of PDGFRA (evident in approximately 33% of cases) is characteristic of pHGG RTK1, amplification of EGFR (approximately 50% of cases) and mutations of the TERT promoter (approximately 64% of cases) are common in pHGG RTK2, and amplification of MYCN (approximately 50% of cases) is frequent in pHGG MYCN, while these are not subgroup specific and may occur in a low percentage in a subgroup other than the one for which they are characteristic [32,42].

 

Infant-type hemispheric glioma

Infantile hemispheric glioma (iHG) is a new entity of supratentorial diffuse gliomas occurring in early childhood, mainly before 1 year of age, and containing characteristic fusions in the receptor tyrosine kinase genes - NTRK, ROS1, ALK and MET1 leading to aberrant expression of the kinase domain controlling tumorigenesis (Table 9) [43,44]. Diagnostic testing for these characteristic alterations is recommended for sensitivity to targeted therapy [32]. The neoplasia itself can infiltrate adjacent leptomeninges and disseminate leptomeningeally [43,44]. Grading is not currently applied in this type of glioma (WHO).

The histological picture of iHG is very heterogeneous, including morphologically low-grade and high-grade neoplasia, mainly with astrocytic spindle cell, less frequently gemistocytic and ependymal differentiation. Occasionally, a gangliocytic component or primitive neuroectodermal differentiation is described [2].

 

Conclusion

Paediatric diffuse gliomas are a group of tumours whose classification has recently undergone revolutionary progress. Firstly, it has been clearly distinguished as a separate group of tumours from morphologically related adult tumours. At the same time, new diagnostic entities have been defined based on the understanding of their gliomagenesis and the demonstration of characteristic genetic alterations that offer therapeutic potential for the use of targeted therapy with a possible positive impact on prognosis.

Grant support

This work was supported by the MU Grant Agency (MUNI/A/1379/2022) and the Medical Research Agency of the Ministry of Health (NU23-03-00100).

 

Declaration of Conflict of Interest

The authors declare that they have no conflict of interest in relation to the subject of the study.

 

Tables

Table 1. Overview of diffuse gliomas according to WHO 2021 incl. WHO CNS grade assigned to each diagnostic unit and diagnostic genetic alterations.

Table 2. Diagnostic criteria for diffuse astrocytoma, MYB- or MYBL1-altered according to WHO 2021.

Table 3. Diagnostic criteria for angiocentric glioma according to WHO 2021.

Table 4. WHO 2021 diagnostic criteria for polymorphous low-grade neuroepithelial tumour of the young.

Table 5. WHO 2021 diagnostic criteria for diffuse low-grade glioma, MAPK pathwayalterated.

Table 6. Diagnostic criteria for diffuse midline glioma, H3 K27-altered according to WHO 2021.

Table 7. Diagnostic criteria for diffuse hemispheric glioma, H3 G34-mutant according to WHO 2021.

Table 8. Diagnostic criteria for diffuse paediatric type high-grade glioma, H3-wildtype and IDH-wildtype according to WHO 2021.

Table 9.Diagnostic criteria for infant-type hemispheric glioma according to WHO 2021.

Table 1. Overview of diffuse gliomas according to WHO 2021 incl. WHO CNS grad assigned to each diagnostic unit and diagnostic genetic alterations.

Overview of diffuse gliomas according to WHO 2021	Grade	Molecular genetic diagnostic features
Childhood type of diffuse low-grade gliomas
Diffuse astrocytoma, MYB- or MYBL1-altered	1	alteration MYB, MYBL1
Angiocentric glioma	1	MYB alterations
Polymorphic low-grade adolescent neuroepithelial tumor	1	alteration of MAPK cascade
Diffuse low-grade glioma with MAPK pathway alteration	Unassigned	alteration of MAPK cascade
Childhood type of diffuse high-grade gliomas
Diffuse midline glioma, H3 K27-altered	4	loss of H3 K27me3 expression H3 K27M EGFR mutations/amplification overexpress EZHIP
Diffuse hemispheric glioma, H3 G34-mutated	4	H3.3 G34V/R alteration TP53 alteration
Diffuse high-grade glioma of childhood type, H3-wildtype and IDH-wildtype	4	EGFR alteration alterations PDGFRA MYCN amplification
Haemispheric glioma of infantile type	Unassigned	fusion of NTRK, ROS1, MET1 and ALK genes

ALK, anaplastic lymphoma kinase; ATRX, alpha thalassemia/mental retardation syndrome X-linked; EGFR, epidermal growth factor receptor; EZHIP, EZH inhibitory protein; MAPK, mitogen-activated protein kinase; MET1 - methyltransferase 1; MYCN - neuroblastoma MYC oncogene; NTRK - neurotrophic tropomyosin kinase receptor; PDGFRA - platelet-derived growth factor receptor alpha; ROS1 - C-ros oncogene 1; TP53 - tumor protein p53

 

 

Table 2. Diagnostic criteria for diffuse astrocytoma, MYB- or MYBL1-altered according to WHO 2021.

Home	diffusely growing glioma without histological signs of anaplasia
	absence of mutation in genes
	absence of mutation in histone H3 genes
	Methylation profile corresponding to diffuse astrocytoma, MYB- or MYBL1-altered and/or structural variants of MYB or
Supporting	absence of OLIG2 and MAP2 expression

IDH1/2, isocitrate dehydrogenase 1/2; MAP2, microtubule-associated protein 2; OLIG2, oligodendrocyte transcription factor 2

 

 

Table 3. Diagnostic criteria for angiocentric glioma according to WHO 2021.

Home	diffusely growing glioma with at least focal angiocentric growth
	monomorphic spindle cells with combined astrocytic and ependymal immunophenotype
Supporting	MYB gene alteration
	absence of anaplastic histological features - microvascular proliferations or necrosis
	methylation profile corresponding to diffuse astrocytoma, MYB- or MYBL1-altered

 

 

Table 4. WHO 2021 diagnostic criteria for polymorphic low-grade neuroepithelial tumour of adolescents.

Home	diffusely growing glioma
	focal oligodendroglial differentiation
	mitotic spore figures
	strong expression of CD34
	absence of mutation in IDH1/2 genes
	alterations in the MAP kinase cascade
Supporting	absence of the 1p/19q nucleus
	calcification

1p/19q - combined loss of the short arm of chromosome 1 and loss of the long arm of chromosome 1; 19IDH1/2 - isocitrate dehydrogenase 1/2; MAP - mitogen-activated protein

 

 

Table 5: WHO 2021 diagnostic criteria for diffuse low-grade glioma with MAPK pathway alteration.

Home	diffusely growing glioma without histological signs of anaplasia
	genetic alteration of the MAPK pathway
	absence of mutations in IDH1/2 and histone H3 genes
	absence of homozygous deletion of the CDKN2A gene
Supporting	development of symptoms in childhood, adolescence or young adults
	absence of morphological features or methylation profile suggesting another diagnostic entity with BRAF or FGFR gene alteration

CDKN2A - cyclin-dependent kinase inhibitor 2A; FGFR - fibroblast growth factor receptor; IDH1/2 - isocitrate dehydrogenase 1/2; MAPK - mitogen-activated protein kinase

 

 

Table 6: Diagnostic criteria for diffuse midline glioma, H3 K27-altered according to WHO 2021.

Home	diffusely growing glioma
	localisation in midline structures
	absence of mutation in genes
	loss of H3 K27me3 expression conditional H3 K27M or K27I mutations in histone H3 genes (H3F3A, HIST1H3B/C) (H3 K27-mutated subtype) EGFR mutation or amplification (EGFR-mutated subtype) increased EZHIP expression (H3-wildtype subtype with increased EZHIP expression) and/or methylation profile corresponding to one of the subtypes of diffuse midline glioma
Supporting	a molecular test result that allows differentiation of the H3.1 or H3.2 pK27-mutated subtype from the H3.3 pK27-mutated subtype

EGFR - epidermal growth factor receptor; EZHIP - EZH inhibitory protein; H3F3A - H3 histone, family 3A; HIST1H3B/C - histone cluster 1 H3 family member b/c; IDH1/2 - isocitrate dehydrogenase 1/2

 

 

Table 7: Diagnostic criteria for diffuse hemispheric glioma, H3 G34-mutated according to WHO 2021.

Home	diffusely growing glioma
	localization in the cerebral hemispheres
	absence of mutation in genes
	G34R/V mutation of the H3-3A gene (H3F3A) and/or methylation profile corresponding to diffuse hemispheric glioma, H3 G34-mutated
Supporting	loss of ATRX expression, absence of OLIG2 expression and diffuse strong p53 expression

ATRX - alpha thalassemia/mental retardation syndrome X-linked; H3F3A - H3 histone, family 3A; IDH1/2 - isocitrate dehydrogenase 1/2; OLIG2 - oligodendrocyte transcription factor 2

 

 

Table 8.Diagnostic criteria for diffuse high-grade glioma of childhood type, H3-wildtype and IDH-wildtype according to WHO 2021.

Home	diffusely growing glioma
	absence of mutation in genes
	absence of mutation in histone H3 genes
	Methylation profile corresponding to pHGG RTK1/RTK2/MYCN and/or characteristic molecular alteration PDGFRA amplification EGFR amplification MYCN amplification
Supporting	presence of vascular proliferations and necroses, typically palisading
	preserved nuclear expression of H3 K27me3

EGFR - epidermal growth factor receptor; IDH1/2 - isocitrate dehydrogenase 1/2; MYCN - neuroblastoma MYC oncogene; PDGFRA - platelet-derived growth factor receptor alpha; pHGG - diffuse high-grade glioma of childhood type, H3-wildtype and IDH-wildtype; RTK - receptor tyrosine kinase

 

 

Table 9.Diagnostic criteria for hemispheric glioma infantile type according to WHO 2021.

diffusely growing glioma

localization in the cerebral hemispheres

occurrence in early childhood

methylation profile corresponding to infantile-type hemispheric glioma and/or characteristic fusions of tyrosine kinase receptor genes - NTRK, ROS1, ALK and MET1

ALK, anaplastic lymphoma kinase; MET1, methyltransferase 1; NTRK, neurotrophic tropomyosin kinase receptor, ROS1, C-ros oncogene