Expanding clinicopathologic knowledge in high-grade glioma with pleomorphic and pseudopapillary features (HPAP): a report of two cases.

High-grade glioma with pleomorphic and pseudopapillary features (HPAP) is a recently identified methylation cluster comprised of relatively circumscribed gliomas enriched for variants in TP53, RB1, NF1, NF2, BRAF and with a more favorable clinical outcome than IDH-wildtype glioblastoma. Here, we present two cases occurring in young adults, one of which occurred in the background of NF2-related schwannomatosis. Both cases demonstrated characteristic histologic features including ependymoma-like areas (Case #1) and an astroblastoma-like phenotype (Case #2), as well as archetypal pseudopapillary structures and pleomorphic tumor cells. High-grade features were present and pathogenic variants in RB1 and TP53 were detected. Cytogenetic analysis revealed aneuploidy involving multiple whole chromosomes, including copy neutral LOH in chromosome 13 (Case #1). Both cases were classified as "no match" using the Heidelberg Brain Tumor Classifier (v12.5 and 12.8). Results from a preliminary classification model ("Bethesda Classifier") were consistent with HPAP. Confirmatory dimensionality reduction (t-SNE) showed clustering within (Case #2) or near (Case #1) the HPAP group. Patient #1 is currently receiving maintenance temozolomide following concomitant chemo-radiotherapy, 10 months post-surgery. Patient #2, treated with temozolomide, remains disease-free at 42 months. Our study highlights additional clinical and pathologic insights into this proposed tumor type and may suggest an association with NF2-related schwannomatosis and evolution from low-grade precursors. These observations support the consideration of HPAP as a distinct clinicopathological entity.