Oncohistones in brain tumors: the soil and seed

Recurrent somatic mutations in histone 3 (H3) variants (termed 'oncohistones') have been identified in high-grade gliomas (HGGs) in children and young adults and induce tumorigenesis through disruption of chromatin states. Oncohistones occur with exquisite neuroanatomical specificity and are associated with specific age distribution and epigenome landscapes. Here, we review the known intrinsic ('seed') and the extrinsic ('soil') factors needed for their optimal oncogenic effect and highlight the many unresolved questions regarding their effects on development and crosstalk with the tumor microenvironment. The 'seed and soil' analogy, used to explain tumor metastatic niches, also applies to oncohistones, which mainly thrive and flourish in specific chromatin states during very narrow windows of development, creating exquisite vulnerabilities, which could provide effective therapies for these deadly cancers.

Prevalence and clinicopathological features of H3.3 G34-mutant high-grade gliomas: a retrospective study of 411 consecutive glioma cases in a single institution

A recurrent glycine-to-arginine/valine alteration at codon 34 (G34R/V) within H3F3A, a gene that encodes the replication-independent histone variant H3.3, reportedly occurs exclusively in pediatric glioblastomas. However, the clinicopathological and biological significances of this mutation have not been completely elucidated; especially, no such data exist for tumor samples from Japanese patients. We analyzed 411 consecutive glioma cases representing patients of all ages. Our results demonstrated that 14 patients (3.4%) harbored H3F3A mutations, of which four had G34R mutations and 10 had K27M mutations. G34R-mutant tumors were located in the parietal region in two patients and the basal ganglia in one patient. One patient showed multi-lobular extension similar to the pattern observed in gliomatosis cerebri. Regarding neuroradiological features, intratumoral calcification was evident in two cases and all cases showed no or scarce contrast enhancement on MRI. Histopathologically, the four G34R-mutant cases included three glioblastomas and one astroblastoma. We have also investigated alterations in histone methylation including H3K27me3, H3K9me3, and H3K4me3 in G34R-mutant samples by immunohistochemistry. These results indicate that G34R-mutant tumors are likely to show extensive infiltration and alterations in global histone trimethylation might also play an important role in G34R mutant tumors.