Pediatric high-grade gliomas, H3-wildtype, IDH-wildtype: Refining diagnostic criteria and exploring clinical associations.
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BACKGROUND: Diffuse pediatric-type high-grade gliomas (pHGGs), H3-wildtype (H3 WT), and IDH-wildtype (IDH WT), represent a newly recognized, highly malignant brain tumor entity with unique molecular and epigenetic profiles. Despite their recent inclusion in the WHO 2021 classification, their histological and molecular diversity presents significant diagnostic and therapeutic challenges. METHODS: This study analyzed 12 cases of pHGGs, H3 WT and IDH WT, identified through whole-genome methylation profiling. Clinical, radiological, and histopathological evaluations were complemented by immunohistochemical profiling, employing an extended spectrum of antibodies, and molecular studies. Methylation profiling enabled precise tumor classification and correlation with known subtypes such as RTK-1, RTK-2, and MYCN. RESULTS: Among the 12 cases, 7 were classified under the RTK-1 subtype, 3 under RTK2 subtype, and a case each under MYCN, pHGGs, H3 WT, and IDH WT subtype B. Notably, 5/12 cases demonstrated a loss of H3K27me3 expression, contradicting the WHO 2021 recommendation for its retention as a diagnostic criterion. Furthermore, 6/7 RTK-1 subtype cases were linked to either mismatch-repair (MMR) deficiency or radiation-induced gliomas, highlighting an enrichment of these clinical scenarios within this subgroup. MGMT promoter methylation was observed in only 4/12 of these cases, consistent with its low prevalence in this tumor category. CONCLUSIONS: This study provides a comprehensive characterization of pHGGs, H3 WT, IDH WT, and emphasizes the clinical and molecular complexity of this rare tumor entity. The findings challenge current WHO diagnostic criteria regarding H3K27me3 retention and demonstrate the critical role of molecular diagnostics, particularly methylation profiling, in refining classification and guiding clinical management. These results advocate for re-evaluation of existing diagnostic frameworks to better accommodate the observed variability and associations in this challenging tumor subtype.