Daily management of gliomas, glioneuronal, and neuronal tumors in the era of the 2021 WHO classification of nervous tumors

Abstract

updates to better classify CNS tumors taking into account the rapid advance of knowledge. Indeed, limits had quickly appeared in the classification of the WHO 2016, and waiting for the publication of a new edition (whose date was unknown) seemed to be prejudicial to the management of patients with a CNS tumor. It is important to note that this consortium has clearly indicated that it does not replace the WHO. Thus, the term NOW has two meanings: literally «now», but also «Not Official WHO». Since its creation, seven updates have been published by c-IMPACT-NOW dealing with specific themes. The themes were chosen by the members of the steering committee and other pathologists (or clinicians) were invited to join this steering committee to participate in the reflection on a given topic. The main themes were: • Clarification of terminology [9,10]; • The grading of gliomas and, in particular, taking into account certain molecular criteria for classifying a grade II or III non-mutant IDH glioma into a grade IV molecular "glioblastoma" (amplification of EGFR and/or mutation of the TERT promoter and/or presence of a gain of chromosome 7 and a loss of chromosome 10) [11]; • Consideration of homozygous deletion of CDKN2A in the grading of IDH-mutant gliomas [12]; • Recognition and histomolecular classification of diffuse low-grade gliomas with altered MYB, MYBL1 or FGFR1 genes or BRAF mutation, V600E [13];• Histomolecular classification of ependymomas [14]; • A summary of the conclusions of the meeting of Utrecht of September 2019 proposing, on the one hand, the inclusion of new entities and, on the other hand, guidelines for the future WHO classification of CNS tumors [15].-2021 Classification: Major Conceptual Changes An effort to simplify and harmonize with the WHO classification of other organ diseases was made:• The «entity» is replaced by «type»and "variant" by "subtype"; whenever possible, the title of a tumour type has been folded. For example, the tumour labelled "third ventricular chordoid glioma" became "chordoid glioma" and the location of this tumour type was given in the definition. In addition, in an effort to standardize, the definition of each tumour type has two components: a component specifying the histopathological characteristics and another reporting the molecular alteration(s) characteristic(s); • The grade is given in Arabic numerals and must be associated with the WHO grade of CNS tumors ("WHO CNS grade"); • The number of mitoses should be given per mm 2 and not for 10 fields with high magnification (HPF), because of the disparity in size of the latter. This correspondence was difficult to establish retrospectively for all CNS tumors, as some older publications did not detail this type of information. In contradiction with what was observed in the previous WHO classifications, where each entity was associated with a grade, in the WHO 2021 classification, a same tumor type can be associated with different grades, for example: astrocytoma, IDH-mutant could be graded as grade 2, 3 or 4 and oligodendroglioma IDH-mutant and 1p/19q co-deleted could be graded as grade 2 or 3. Finally, grading criteria can be mixed: histopathological and molecular. For example, homozygous deletion of CDKN2A is included in the grading of IDH-mutant astrocytomas, its presence conferring a grade 4, regardless of the histopathological aspect of the tumor. Thus, the symbols of the genes are named in italics, in order to be differentiated from the proteins and families of genes, the latter being named in roman font (straight characters). The sequence variation nomenclature follows the recommendations of the Human Genome Variation Society (HGVS) (https://varnomen.hgvs.org/). The ratio of each variation is established with respect to a reference sequence corresponding to the coding sequence of the gene, preceded by the prefix "c." and with respect to the corresponding predicted sequence, preceded by the prefix "p.". No recommendations are made concerning the technique to be used to search for a genetic alteration of interest. In the last decade, new molecular techniques, whether targeted techniques (immunohistochemistry with antibodies recognizing a mutated protein; fluorescence in situ hybridization -FISH -digital plet PCR targeting mutations, or restricted NGS panel DNA or RNA (fusion genes)) or non-target (RNAseq, whole exome sequencing, whole genome sequencing), have shown their interest in the search for molecular alterations associated with a given tumor type. The methyloma analysis technique was developed as a powerful tool for classifying CNS tumours. Indeed, the methylation profile depends largely on the cell of origin, its location and any associated molecular alterations. It is relatively stable between the initial tumor and recurrences. The DKFZ (Deutsches Krebsforschuns-zentrum: German Cancer Research Centre) team in Heidelberg has built a «classification» using artificial intelligence to classify CNS tumours according to their methylation profile. A number of methylation (MC) classes were initially proposed from a series of more than 2000 tumors classified according to WHO 2016. This tool was made freely available (https://www.molecularneuropathology.org/mnp) to download . When the prediction score is greater than 0.9, the diagnosis proposed by the methylation class is generally reliable. A score less than 0.3 is given when no methylation class can be proposed and, finally, when the score is between 0.3 and 0.9, the methionization class can be informative if it is compatible with the morphology, the immunohistochemical and the molecular profile. Major changes are noted in the WHO 2021 classification with the introduction of 22 new tumor types mainly involving tumors in children and young adults and the creation of a new large tumor category entitled «gliomas, glioneuronal and neuronal tumors». The 2021 WHO classification of gliomas, glioneuronal and neuronal tumors is represented in table 1. 1-Gliomas, glioneuronal tumors and neuronal tumors: The WHO 2021 classification includes under the category «gliomas, glioneuronal and neuronal tumors» tumor groups formerly called: gliomas, diffuse astrocytomas and oligodendrogliomas, other astrocytic tumors, other gliomas, neuronal and glioneuronal tumors and ependymomas. Within this new large category, gliomas are classified according to their circumscribed or diffuse character, according to the classical age of occurrence and according to their aggressiveness. Thus, three large groups of diffuse gliomas have been defined: one concerning adult gliomas (which can be of grade 2, 3 or 4), and two of pediatric subtype (either low grade or high grade). Adult diffuse gliomas include three tumor types: IDH-mutant diffuse astrocytomas (grade 2, 3 or 4), IDH-mutant and 1p/19q co-deleted oligodendrogliomas (grade 2 or 3) and glioblastomas, non-mutant IDH (Grade 4). The term "glioblastoma" is no longer used in IDH-mutant gliomas and is replaced by the term "grade 4 IDH-mutant astrocytoma". Each type of diffuse glioma is characterized by recurrent molecular alterations, some of which are essential for diagnosis and others are not essential for diagnosis. Moreover, for IDH-mutant astrocytomas, the grade is mixed, both histopathological and molecular. Thus, the presence of a homozygous deletion of CDKN2A requires to classify glioma in grade 4, regardless of histopathological data. The diagnosis of glioblastoma is based either on the absence of IDH mutation in diffuse glioma with endothelial-capillary proliferation and/or necrosis, or in the absence of these histopathological criteria by the identification of at least one of the following three alterations: mutation of the TERT promotor, combination of a gain of chromosome 7 and a loss of chromosome 10 (+7/ 10), amplification of EGFR [4]. Unfortunately, the WHO 2021 classification does not suggest a mitosis threshold to distinguish grade 2 versus grade 3 IDH-mutant astrocytomas due to the absence of robust publications by several teams into this subject. Similarly, there is no precise threshold of mitosis to retain the diagnosis of anaplastic oligodendroglioma IDH-mutant 1p/19q co-deleted in the absence of endothelial-capillary proliferation. The minimum threshold of 6 mitoses preconized by some authors is however cited. The presence of a homozygous deletion of CDKN2A in an IDH-mutated and 1p/19q co-deleted oligodendroglioma was not retained for the grading of these tumors, despite the very pejorative character of this alteration in the French cohort POLA [15]. The diffuse gliomas of high-grade pediatric subtype include 4 tumor types, including 3 new, for which age and location are important criteria: the diffuse glioma of the impaired H3 K27 midline, the diffuse hemispherical glioma H3 G34-mutated, diffuse glioma pediatric of high grade H3 and IDH wildtype and, finally, the infantile hemispherical glioma. The diffuse glioma of the midline with alteration of H3 K27 is characterized by a loss of nuclear expression of H3K27 (physiological form trimethylated on lysine (K) in position 27 of the protein H3) [16]. This loss of expression is associated with either a K27 H3 mutation, an EGFR mutation (frequent bi-thalamic localization) or an overexpression of EZHIP [17]. These gliomas are grade 4, regardless of histopathological aspects. The diffuse hemispherical glioma H3 G34-mutant does not express OLIG2 and exhibits a loss of ATRX nuclear expression and p53 overexpression. [18] Infantile hemispheric glioma is associated with fusions involving NTRK1/2/3, ALK, ROS1 or MET [19] genes. Their clinical presentation is often similar to that of childhood desmoplastic gangliogliomas, of which they represent the main differential diagnosis. Diffuse low-grade pediatric gliomas include 4 tumor types including three new entities: diffuse astrocytoma with alteration of MYB or MYBL1, angiocentric glioma (most often associated with MYB-QKI fusion), tumor low-grade polymorphic neuro-epithelial young (PLNTY) [29], and low-grade diffuse gliomas with alteration of the MAP kinase pathway (mainly FGFR or BRAF) [13]. This group presents clinical similarities (epileptogenicity, young age of occurrence), phenotypic (CD34 positivity), radiological (cortical localization) and molecular (alteration of the MAP Kinases pathway) with certain glioneuronal tumors [20]. The circumscribed astrocytic glioma group includes pilocytic astrocytoma, pleomorphic xantho-astrocytoma, subependymal giant cell astrocytoma, chordoid glioma and two new tumor types: anaplastic astrocytoma with piloid aspects (HGAP for high-grade astrocytoma with piloid features) whose diagnosis is based on methyloma [21], and astroblastoma with MN1 alteration [22]. Glioneuronal and neuronal tumors contain 14 different types : ganglioglioma, gangliocytoma, infantile desmoplastic ganglioglioma / infantile desmoplastic astrocytoma, dysembryoplasic neuroepithelial tumor, papillary glioneuroal tumor, the glioneuronal tumor with rosettes, dysplastic gangliocytoma of the cerebellum (Lhermitte Duclos disease), central neurocytoma, extraventricular neurocytoma and liponeurocytoma of the cerebellum. In addition, two new tumor types are described: the myxoid glioneuronal tumor (localized at the septum pellucidum whose morphology simulates a dysembryoplastic neuro-epithelial tumor, but with a mutation of the PDFGRA gene) [23], and the multinodular and vacuolated neuronal tumor (associated with a recurrent alteration of the MAPKinase pathway, most commonly MAP2K1) [24,25]. A third type is classified as provisional and noted in italics, it is the «glioneuronal tumor with oligo-like aspects and clusters of nuclei» (DGONC for diffuse glioneuronal tumor with oligodendroglioma-like features and nuclear clusters), whose diagnosis is based on methyloma and frequent monosomy of chromosome 14 [26]. This broad category also includes ependymal tumors that include ependymomas, sub-ependymomas, and myxopapillary ependymomas. Ependymomas are now classified according to their localization: supratentorials, infratentorials and spinal and within the same localization according to the underlying molecular alteration (supratentorials either with ZFTA fusion, or with YAP1 and infratentorials fusion, or PFA type (posterior fossa A) or PFB type). The terminology proposed in 2016 for supratentorial ependymomas with C11orf95-RELA fusion has been modified. Indeed, alternative pathogenic fusions between C11orf95 (now called ZFTA) and other partners other than RELA have been identified, prompting this change. 2-Choroid plexus tumors: There is little change in choroid plexus tumors, but to emphasize their rather epithelial nature, these tumors stand apart as opposed to ependymomas that are now part of the broad category of gliomas, glioneuronal and neuronal tumors. 3-Embryonal tumors: In the chapter of embryonic tumors, the location of the lesion is important, especially the posterior fossa versus other locations. This category includes the medulloblastomas, on the one hand, and other embryonic tumors, on the other hand. Concerning medulloblastomas, we find the main molecular types already present in the 2016 WHO classification, namely: medulloblastomas with activation of the WNT pathway, medulloblastomas with activation of the SHH pathway, which can be either TP53-mutated, i.e., TP53-not mutated, and non-WNT/non-SHH medulloblastomas. However, while the WHO 2021 classification retains these broad categories, it now recognizes among each different molecular subgroups that are based on distinct transcriptomic and/or methylation profiles. There are now 4 subgroups of SHH medulloblastomas (SHH-1 to SHH-4) and 8 subgroups of non-WNT/non-SHH medulloblastomas (subgroups 1-8) [27][28][29][30]. Some of these subgroups are characterized by specific clinicopathological presentations and are of diagnostic, prognostic or predictive interest of response to treatments. This is the case, for example, of the subtypes SHH-1 and SHH-2, which are the prerogative of young children and whose anatomopathological presentation is often of the nodular type extensive or of certain subtypes of non-WNT/non-SHH that are associated with therapeutic resistance [31,32]. Furthermore, the WHO 2021 classification no longer distinguishes the histological types defined previously, namely: classical, nodular/desmoplasic, extensive nodular and large cell/anaplastic. These histological subtypes are now considered as specific histopathological presentations and listed in the chapter «medulloblastoma defined based on microscopic features». Histomolecular diagnosis «in strata» is particularly appropriate for the group of medulloblastomas. Regarding the group of other embryonic tumors, two new histomolecular types are identified following the work of Sturm et al., 2016 for the diagnosis of embryonic tumors. 4-Tumors of the pineal parenchyma: A new tumor type was recognized in addition to the four known tumor types (pineal parenchymal tumor of intermediate differentiation (PPTID), pineoblastoma and papillary tumour of the pineal region); this is the "desmoplasic myxoid tumour of the pineal region with SMARCB1 mutation" [33]. Moreover, the presence of an alteration of the KBTBD4 gene is a diagnostic marker for PPTID. Finally, in pinealoblastomas, several molecular subtypes, whose diagnosis is based on methyloma, are identified (miRNA processing-altered, RB1-altered and MYC/FOXR2-activated); they are characterized by a different prognosis and age of occurrence. 5-Tumors of the cranial and paraspinal nerves: The main changes observed in this tumoral group are represented by the inclusion of paraganglioma (which is no longer part of glioneuronal and neuronal tumors) and the recognition of paraganglioma of the ponytail as a tumor type distinct from paragangliomas observed in other locations and not expressing GATA3 [34]. By analogy to the classification of soft tissue tumours, the aggressive nature of melanotic schwannoma is recognized and this tumour is now called a malignant melanocytic nerve sheath tumour. Finally, a new tumour subtype has been listed among neurofibromas: the "atypical neurofibromatous tumor of unknown biological potential" (ANNUBP), which is associated with neurofibromatosis type 1 and has a characteristic methylation profile [35]. 6-Meningiomas: Meningiomas now represent a single tumor type with 13 histopathological subtypes, up to grade 1, 2 or 3. Grading criteria are applied, regardless of the subtype. 7-Mesenchymal tumours non-meningothelial: Three new tumour types are now recognized: the "tumour" intracranial mesenchymal with FET-CREB fusion" (provisional type, corresponding to tumors called either angiomatoid fibrous histiocytoma or intracranial myxoid mesenchymal tumor) [36,37], "sarcoma with CIC rearrangement" [22], and "primary intracranial sarcoma with DICER1 mutation" [38]. The term hemangioma pericytoma has been definitively deleted in relation to the terminology «solitary fibrous tumor» (grade 1, 2 or 3). 8-Lymphomas and histiocytic tumors: Few modifications involved this group of tumors. The lymphoma group contains 2 subgroups including the lymphomas of the CNS and miscellaneous rare lymphomas. The former consists of primary diffuse large-B-cell lymphoma, immunodeficiency-associated CNS lymphomas, lymphomatoid granulomatosis and intravascular large B-cell lymphoma. The latter contains MALT lymphoma of the dura, other low-grade B-cell lymphomas of the CNS, anaplastic large cell lymphoma and T-cell and NK/T-cell lymphomas. Histiocytic tumors are composed of Erdheim-Chester disease, Rosai-Dorfman disease, juvenile xanthogranuloma, langerhans cell histiocytosis and histiocytic sarcoma. 9-Tumors of the sellar region: Two histopathological types are now retained for craniopharyngiomas (and not two morphological subtypes, as in 2016) justified by distinct clinical, histopathological and genetic characteristics: the «adamantinomatous craniopharyngioma » (associated with a clonal mutation of CTNNB1) [39], and "papillary craniopharyngioma" (with BRAF mutation) [40]. Conversely, because of their histopathological relationship and common expression of TTF1, only one tumor type was retained under the label «pituicytoma/granular cell tumor/fusiform cell oncocytoma» [41,42]. A new chapter is now dedicated to pituitary adenomas called «neuroendocrine pituitary tumors (PitNET)» whose description follows that of the WHO classification of endocrine tumors. Finally, a new tumour type was introduced: The "blastoma of the hypophysis" associated to DICER1 mutations.When analyzing a glial tumor and after taking into account its localization and the age of the patient, the first step consists of screening the cellularity, the morphology, the architecture, the proliferation index, the vascularization, the necrosis… but even this first step may be challenging especially in small biopsies. Using immunohistochemistry in order to differentiate gliomas is useful because of its low cost and the little amount of material used. The limits of the immunohistochemistry consist of the limited number of antibodies available, its high to and the of and diagnostic based on the clinical and morphological features of gliomas, for children and young adults and adults more than are represented in 1 and 2 3 a for immunohistochemical in order to differentiate between the different diffuse gliomas A panel of antibodies the ATRX and antibodies may be useful in in order to differentiate between grade 2, 4 astrocytomas, grade 2, 3 oligodendrogliomas, and gliomas are gliomas expressing or proliferation and chromosome 7 gain and chromosome 10 EGFR amplification and TERT promoter is a glial tumor non-mutant with and/or proliferation or with chromosome 7 gain and chromosome 10 loss or EGFR amplification or TERT promoter are glial tumors expressing ATRX and with a glial tumors, expressing ATRX with IDH mutation by molecular with a are also considered as tumors with or ATRX expression and with or expression are considered as diagnosis a in the 2021 WHO classification of tumors especially in pediatric tumors and the new tumors are challenging because of the of the the of the tumors, the the molecular criteria that are often and the criteria that are from In the the and the nuclear are less criteria in pediatric tumors. techniques are useful in new entities with or analysis of The table 2 the molecular techniques that should be used in the different molecular new has but it also very especially in the of more techniques including immunohistochemistry could represent a in It is to the diagnosis of tumours because of the prognostic of the diagnosis. more specific be when similar but distinct tumors are 2021 WHO classification of tumours modifications concerning especially glial tumours either in adults or The molecular features challenging to in a especially in immunohistochemical have to be as to molecular