Integrated high‐resolution copy number and histomolecular analysis of diffuse hemispheric glioma, <scp>H3 G34</scp> ‐mutant reveals universal <i>TP53</i> abnormalities

Diffuse hemispheric glioma, H3 G34-mutant (DHG-H3 G34) has been primarily molecularly characterized by methylation profiling and sequencing studies. We describe an integrated histomolecular evaluation including high-resolution copy number profiling of a series of 60 DHG-H3 G34 to further our understanding of the spectrum of genetic changes associated with this tumor type. Cases were clinically tested using an 187-gene mutation and fusion targeted neuro-oncology next-generation sequencing panel (n = 60) and Oncoscan chromosomal microarray (n = 26) by a single laboratory (2018-2022). A subset of cases had immunohistochemical results for OLIG2 (n = 42), p53 (n = 48), and ATRX (n = 46), and methylation array data (n = 8). Median age at testing was 21 years (range, 12-50). No significant difference was noted in clinical, histopathological, and mutational profile between pediatric and adult patients. H3-3A G34 mutations included G34R (n = 56; 94%), G34V (n = 3), and a non-canonical G34E (n = 1). Concurrent mutations most often involved TP53 (n = 55; 92%), ATRX (n = 50; 83%), and PDGFRA (n = 34; 57%). A reportedly primary tumor was confirmed to be hypermutant and had a PMS2 mutation. A single case also showed an FGFR3::FAM184B fusion. All cases with available chromosomal microarray data had unbalanced genomes, which were often complex (14/26; 54%). The most frequent recurrent copy number abnormalities were losses involving 3q, 4q, 10q, 13q, and 18q, and 17p copy-neutral loss of heterozygosity (cnLOH) encompassing TP53. This copy number profile was reminiscent of that seen in Grade 4 IDH-mutant astrocytomas. Collectively, a TP53 abnormality at copy number (12/26, all cnLOH), sequence (55/60) and protein expression (46/48) level was detected in all 60 cases. In conclusion, integrated high-resolution copy number and histomolecular analysis expanded the spectrum of genetic changes associated with DHG-H3 G34, including the presence of universal TP53 abnormalities with frequent cnLOH-a copy number abnormality that has been largely unrecognized-for this new 2021 World Health Organization central nervous system tumor type.