Utility of combining OLIG2 and SOX10 IHC expression in CNS tumours: promising biomarkers for subtyping paediatric- and adult-type gliomas.

AIMS: The SOX10 transcription factor is important for the maturation of oligodendrocytes involved in central nervous system (CNS) myelination. Currently, very little information exists about its expression and potential use in CNS tumour diagnoses. The aim of our study was to characterize the expression of SOX10 in a large cohort of CNS tumours and to evaluate its potential use as a biomarker. METHODS: We performed immunohistochemistry (IHC) for SOX10 and OLIG2 in a series of 683 cases of adult- and paediatric-type CNS tumours from different subtypes. The nuclear immunostaining results for SOX10 and OLIG2 were scored as positive (≥10% positive tumour cells) or negative. RESULTS: OLIG2 and SOX10 were positive in diffuse midline gliomas (DMG), H3-mutant, and EZHIP-overexpressed. However, in all DMG, EGFR-mutant, SOX10 was constantly negative. In diffuse paediatric-type high-grade gliomas (HGG), all RTK1 cases were positive for both OLIG2 and SOX10. RTK2 cases were all negative for both OLIG2 and SOX10. MYCN cases variably expressed OLIG2 and were all immunonegative for SOX10. In glioblastoma, IDH-wildtype, OLIG2 was mostly positive, but SOX10 was variably expressed, depending on the epigenetic subtype. All circumscribed astrocytic gliomas were positive for both OLIG2 and SOX10 except pleomorphic xanthoastrocytomas, astroblastomas, MN1-altered, and subependymal giant cell astrocytomas. SOX10 was negative in ependymomas, meningiomas, pinealoblastomas, choroid plexus tumours, intracranial Ewing sarcomas, and embryonal tumours except neuroblastoma, FOXR2-activated. CONCLUSION: To conclude, SOX10 can be incorporated into the IHC panel routinely used by neuropathologists in the diagnostic algorithm of embryonal tumours and for the subtyping of paediatric and adult-type HGG.

[Contributions and limitations of FISH analysis for the diagnosis of central nervous system tumors according to the 2021 WHO classification: Feedback from Sainte-Anne Hospital's Department of Neuropathology]. / Apports et limites de la FISH dans le diagnostic des tumeurs du système nerveux central selon la classification de l'OMS de 2021 : retour d'expérience du service de neuropathologie de l'hôpital Sainte-Anne.

The fifth edition of the World Health Organization (WHO) Classification of Tumors of the Central Nervous System has identified many new tumor types and has established, for the first time, essential and desirable diagnostic criteria for each of them. Among these, genetic alterations play an important role associated with morphology. For the first time, epigenetic data can also constitute essential and/or desirable criteria. These genetic abnormalities can be fusions, deletions or gains/amplifications and can thus be detected by fluorescence in situ hybridization techniques. The purpose of this article is to present the advantages and limitations of this technique in reference to its specific use within neuro-oncopathology in light of the 2021 WHO classification.

[The use of immunohistochemical slides for performing FISH as a useful method of conserving tissue samples]. / Ré-utilisation de lames d'immunohistochimie pour la réalisation de FISH : une solution pertinente d'épargne tissulaire.

Diagnostic updates, an increased precision of tumor sub-type classification and the development of new diagnostic biomarkers (immunohistochemistry (IHC), Fluorescence in situ hybridization (FISH) and other molecular pathology techniques), have a significant impact on pathologists' management of tissue samples. The objective of this work was to test and validate the FISH technique on detached IHC slides. An IHC technique was first performed on 30 tissue samples. After detachment of the lamella, a FISH technique was then performed according to the usual protocol with a centromeric probe. A validation cohort (n=10) with duplicate testing using a traditional FISH technique and an IHC slide with a detached lamella was then carried out. Finally, a cohort of 20 "old" cases (IHC carried out over 2years ago) was also tested. Different types of probes (specific locus, break apart) have been used. All the slides were interpreted by a technician and a pathologist. Evaluation criteria were: the general interpretability of the slide ; the percentage of labeled nuclei; intensity of the signal and the presence or absence of autofluorescence. FISH was interpretable in 100% of recently treated cases and 90% of "old" cases with a satisfactory intensity and a high percentage of labeled nuclei, without autofluorescence. The results of our study show that the reuse of IHC slides for performing FISH is a powerful means of economizing tissue samples, especially for small samples and in the absence of archived representative material.

[Analysis of interference criteria for the validation of a method file: HPS staining after fixation with zinc-formalin in comparison to fixation with the classical 4% buffered formalin solution]. / Étude de l'item d'interférence dans un dossier de validation de méthode : cas de la coloration HPS après fixation en formol-zinc et comparaison à la fixation en formol tamponné à 4.

INTRODUCTION: The department of neuropathology of Sainte-Anne Hospital uses zinc-formalin as the fixative agent for its samples. No publication referenced in Pubmed has proven the validity of this fixative agent. In the context of the accreditation of our standard staining (HPS for Hemalun-Phloxin-Saffron), we started a file for the validation of this method in which the fixative agent constitutes an « interfering ¼ substance which can modify the quality of the technique. The aim of this study was to prove that the use of zinc-formalin as a fixative agent is as suitable as the fixation with 4 % buffered formalin. MATERIALS AND METHODS: A cohort of samples fixed by zinc-formalin and by 4 % buffered formalin was performed on fresh samples, then cut and stained by HPS. The slides were interpreted by three pathologists (one of them was outside our centre)  ``blind '' to the fixative agent and they evaluated four criteria (general quality of the staining, components of the extracellular matrix, cytoplasmic details, and nuclear details) and scored them (from 0 to 3) according to the Association française en assurance qualité (AFAQAP) recommendations. RESULTS: The cohort included 43 samples. The results of the analysis showed that for samples fixed by zinc-formalin, three of the four criteria obtained significantly a better score than the samples fixed by classical formalin. DISCUSSION AND CONCLUSIONS: Our results show that the zinc-formalin fixative does not constitute an  ``interfering '' agent for the quality of the HPS staining for neuropathological samples.