TL;DR: The presence of H3F3A/ATRX-DAXX/TP53 mutations was strongly associated with alternative lengthening of telomeres and specific gene expression profiles, suggesting that defects of the chromatin architecture underlie paediatric and young adult GBM pathogenesis.
Abstract: Glioblastoma multiforme (GBM) is a lethal brain tumour in adults and children. However, DNA copy number and gene expression signatures indicate differences between adult and paediatric cases. To explore the genetic events underlying this distinction, we sequenced the exomes of 48 paediatric GBM samples. Somatic mutations in the H3.3-ATRX-DAXX chromatin remodelling pathway were identified in 44% of tumours (21/48). Recurrent mutations in H3F3A, which encodes the replication-independent histone 3 variant H3.3, were observed in 31% of tumours, and led to amino acid substitutions at two critical positions within the histone tail (K27M, G34R/G34V) involved in key regulatory post-translational modifications. Mutations in ATRX (α-thalassaemia/mental retardation syndrome X-linked) and DAXX (death-domain associated protein), encoding two subunits of a chromatin remodelling complex required for H3.3 incorporation at pericentric heterochromatin and telomeres, were identified in 31% of samples overall, and in 100% of tumours harbouring a G34R or G34V H3.3 mutation. Somatic TP53 mutations were identified in 54% of all cases, and in 86% of samples with H3F3A and/or ATRX mutations. Screening of a large cohort of gliomas of various grades and histologies (n = 784) showed H3F3A mutations to be specific to GBM and highly prevalent in children and young adults. Furthermore, the presence of H3F3A/ATRX-DAXX/TP53 mutations was strongly associated with alternative lengthening of telomeres and specific gene expression profiles. This is, to our knowledge, the first report to highlight recurrent mutations in a regulatory histone in humans, and our data suggest that defects of the chromatin architecture underlie paediatric and young adult GBM pathogenesis.
TL;DR: It is proposed that K-to-M substitutions may represent a mechanism to alter epigenetic states in a variety of pathologies and be sufficient to cause specific reduction in methylation through inhibition of SET-domain enzymes.
Abstract: Sequencing of pediatric gliomas has identified missense mutations Lys27Met (K27M) and Gly34Arg/Val (G34R/V) in genes encoding histone H3.3 (H3F3A) and H3.1 (HIST3H1B). We report that human diffuse intrinsic pontine gliomas (DIPGs) containing the K27M mutation display significantly lower overall amounts of H3 with trimethylated lysine 27 (H3K27me3) and that histone H3K27M transgenes are sufficient to reduce the amounts of H3K27me3 in vitro and in vivo. We find that H3K27M inhibits the enzymatic activity of the Polycomb repressive complex 2 through interaction with the EZH2 subunit. In addition, transgenes containing lysine-to-methionine substitutions at other known methylated lysines (H3K9 and H3K36) are sufficient to cause specific reduction in methylation through inhibition of SET-domain enzymes. We propose that K-to-M substitutions may represent a mechanism to alter epigenetic states in a variety of pathologies.
TL;DR: A remarkable picture of tumor type specificity for histone H 3.3.3 driver alterations emerges, indicating that hist one H3.
Abstract: It is recognized that some mutated cancer genes contribute to the development of many cancer types, whereas others are cancer type specific. For genes that are mutated in multiple cancer classes, mutations are usually similar in the different affected cancer types. Here, however, we report exquisite tumor type specificity for different histone H3.3 driver alterations. In 73 of 77 cases of chondroblastoma (95%), we found p.Lys36Met alterations predominantly encoded in H3F3B, which is one of two genes for histone H3.3. In contrast, in 92% (49/53) of giant cell tumors of bone, we found histone H3.3 alterations exclusively in H3F3A, leading to p.Gly34Trp or, in one case, p.Gly34Leu alterations. The mutations were restricted to the stromal cell population and were not detected in osteoclasts or their precursors. In the context of previously reported H3F3A mutations encoding p.Lys27Met and p.Gly34Arg or p.Gly34Val alterations in childhood brain tumors, a remarkable picture of tumor type specificity for histone H3.3 driver alterations emerges, indicating that histone H3.3 residues, mutations and genes have distinct functions.
TL;DR: Although H3K36 trimethylation and ATRX immunohistochemistry cannot be used as surrogate markers for H3F3A mutation status, mutations in H3f3A are associated with increased H3k36 trim methylation, suggesting that methylation at this residue may play a role in the etiology of the disease.
Abstract: Specific H3F3A driver mutations and IDH2 mutations were recently described in giant cell tumor of bone (GCTB) and H3F3B driver mutations in chondroblastoma; these may be helpful as a diagnostic tool for giant cell-containing tumors of the bone. Using Sanger sequencing, we determined the frequency of H3F3A, H3F3B, IDH1, and IDH2 mutations in GCTBs (n=60), chondroblastomas (n=12), and other giant cell-containing tumors (n=24), including aneurysmal bone cyst, chondromyxoid fibroma, and telangiectatic osteosarcoma. To find an easy applicable marker for H3F3A mutation status, H3K36 trimethylation and ATRX expression were correlated with H3F3A mutations. In total, 69% of all GCTBs harbored an H3F3A (G34W/V) mutation compared with 0% of all other giant cell-containing tumors (P<0.001), whereas 70% of chondroblastomas showed an H3F3B (K36M) mutation compared with 0% of other giant cell-containing tumors (P<0.001). Diffuse H3K36 trimethylation positivity was more often seen in mutated H3F3A GCTBs compared with other giant cell-containing tumors (P=0.005). ATRX protein expression was not correlated with H3F3A mutation status. Hotspot mutations in IDH1 or IDH2 were absent. Our results show that H3F3A and H3F3B mutation analysis appears to be a highly specific, although less sensitive, diagnostic tool for the distinction of GCTB and chondroblastoma from other giant cell-containing tumors. Although H3K36 trimethylation and ATRX immunohistochemistry cannot be used as surrogate markers for H3F3A mutation status, mutations in H3F3A are associated with increased H3K36 trimethylation, suggesting that methylation at this residue may play a role in the etiology of the disease.
TL;DR: Overall, the observations suggest that the presence of H3F3A p.Gly34 mutations appear to be an almost essential feature of GCT that will aid pathological evaluation of bone tumours, especially when confronted with small needle core biopsies.
Abstract: Driver mutations in the two histone 3.3 (H3.3) genes, H3F3A and H3F3B, were recently identified by whole genome sequencing in 95% of chondroblastoma (CB) and by targeted gene sequencing in 92% of giant cell tumour of bone (GCT). Given the high prevalence of these driver mutations, it may be possible to utilise these alterations as diagnostic adjuncts in clinical practice. Here, we explored the spectrum of H3.3 mutations in a wide range and large number of bone tumours (n = 412) to determine if these alterations could be used to distinguish GCT from other osteoclast-rich tumours such as aneurysmal bone cyst, nonossifying fibroma, giant cell granuloma, and osteoclast-rich malignant bone tumours and others. In addition, we explored the driver landscape of GCT through whole genome, exome and targeted sequencing (14 gene panel). We found that H3.3 mutations, namely mutations of glycine 34 in H3F3A, occur in 96% of GCT. We did not find additional driver mutations in GCT, including mutations in IDH1, IDH2, USP6, TP53. The genomes of GCT exhibited few somatic mutations, akin to the picture seen in CB. Overall our observations suggest that the presence of H3F3A p.Gly34 mutations does not entirely exclude malignancy in osteoclast-rich tumours. However, H3F3A p.Gly34 mutations appear to be an almost essential feature of GCT that will aid pathological evaluation of bone tumours, especially when confronted with small needle core biopsies. In the absence of H3F3A p.Gly34 mutations, a diagnosis of GCT should be made with caution.