Histone 3.3 hotspot mutations in conventional osteosarcomas: a comprehensive clinical and molecular characterization of six H3F3A mutated cases.

BACKGROUND: Histone 3.3 (H3.3) hotspot mutations in bone tumors occur in the vast majority of giant cell tumors of bone (GCTBs; 96%), chondroblastomas (95%) and in a few cases of osteosarcomas. However, clinical presentation, histopathological features, and additional molecular characteristics of H3.3 mutant osteosarcomas are largely unknown. METHODS: In this multicentre, retrospective study, a total of 106 conventional high-grade osteosarcomas, across all age groups were re-examined for hotspot mutations in the H3.3 coding genes H3F3A and H3F3B. H3.3 mutant osteosarcomas were re-evaluated in a multidisciplinary manner and analyzed for genome-wide DNA-methylation patterns and DNA copy number aberrations alongside H3.3 wild-type osteosarcomas and H3F3A G34W/L mutant GCTBs. RESULTS: Six osteosarcomas (6/106) carried H3F3A hotspot mutations. No mutations were found in H3F3B. All patients with H3F3A mutant osteosarcoma were older than 30 years with a median age of 65 years. Copy number aberrations that are commonly encountered in high-grade osteosarcomas also occurred in H3F3A mutant osteosarcomas. Unlike a single osteosarcoma with a H3F3A K27M mutation, the DNA methylation profiles of H3F3A G34W/R mutant osteosarcomas were clearly different from H3.3 wild-type osteosarcomas, but more closely related to GCTBs. The most differentially methylated promoters between H3F3A G34W/R mutant and H3.3 wild-type osteosarcomas were in KLLN/PTEN (p < 0.00005) and HIST1H2BB (p < 0.0005). CONCLUSIONS: H3.3 mutations in osteosarcomas may occur in H3F3A at mutational hotspots. They are overall rare, but become more frequent in osteosarcoma patients older than 30 years. Osteosarcomas carrying H3F3A G34W/R mutations are associated with epigenetic dysregulation of KLLN/PTEN and HIST1H2BB.

Differential nuclear ATRX expression in sarcomas.

AIM: Nuclear α-thalassemia/mental retardation X-linked (ATRX) loss and alternative lengthening of telomeres (ALT) are linked in distinct malignancies. We therefore aimed to determine the nuclear ATRX expression correlated with ALT in a comprehensive series of sarcomas. METHODS AND RESULTS: A total of 573 formalin-fixed paraffin-embedded sarcomas comprising 28 entities were investigated for nuclear ATRX expression by immunohistochemistry. Telomere-specific fluorescence in-situ hybridization (FISH) was used to determine the ALT phenotype in 50 sarcomas with complete or heterogeneous ATRX loss. Complete nuclear ATRX loss was detected in 58 of 573 sarcomas, all high-grade, with the highest prevalence in undifferentiated pleomorphic sarcomas (38%) and pleomorphic liposarcomas (38%), followed by dedifferentiated liposarcomas (24%), osteosarcomas (21%), leiomyosarcomas (17%), myxofibrosarcomas (11%) and malignant peripheral nerve sheath tumours (4%). Interestingly, a further 20 sarcomas, all belonging to the aforementioned entities with complete ATRX loss, presented with a heterogeneous ATRX expression pattern. ALT was observed in 41 of 42 sarcomas with complete ATRX loss, but only in two of eight sarcomas with heterogeneous expression. CONCLUSION: Nuclear ATRX loss, either complete or heterogeneous, is encountered in a considerable number of high-grade sarcomas with non-specific genetic alterations. A causal relationship with ALT might be indicated at least in cases with a complete nuclear ATRX loss.

Nuclear relocation of STAT6 reliably predicts NAB2-STAT6 fusion for the diagnosis of solitary fibrous tumour.

AIMS: Nuclear relocation of STAT6 has been shown in tumours with NAB2-STAT6 fusion, and has been proposed as an ancillary marker for the diagnosis of solitary fibrous tumours (SFTs). The aim of this study was to verify the utility of STAT6 immunohistology in diagnosing SFT. METHODS AND RESULTS: A total of 689 formalin-fixed paraffin-embedded tumours comprising 35 pleural SFTs and 654 other mesenchymal tumours were investigated for STAT6 expression using immunohistochemistry. Nine dedifferentiated liposarcomas (DDLSs) and five SFTs were also examined for the presence of NAB2-STAT6 fusion at the protein level using the proximity ligation assay (PLA), and for copy number variants (CNVs) with the Illumina Infinium Human Methylation450 array. Thirty-four of 35 SFTs showed strong nuclear STAT6 expression. Furthermore, five of 68 DDLSs, two of 130 undifferentiated pleomorphic sarcomas and one of 63 cases of nodular fasciitis showed moderate to strong nuclear STAT6 expression. The PLA indicated the presence of NAB2-STAT6 fusion protein in SFTs, but signal was also detected in some DDLSs. Copy number analysis showed an overall low frequency of chromosomal imbalances in SFTs, but complex karyotypes in DDLSs, including amplification of STAT6 and MDM2 loci. CONCLUSIONS: The detection of nuclear relocation of STAT6 with immunohistochemistry is a characteristic of SFTs, and may serve as a diagnostic marker that indicates NAB2-STAT6 fusion and helps to discriminate SFTs from histological mimics.

Integrative DNA methylation and gene expression analysis in high-grade soft tissue sarcomas.

BACKGROUND: High-grade soft tissue sarcomas are a heterogeneous, complex group of aggressive malignant tumors showing mesenchymal differentiation. Recently, soft tissue sarcomas have increasingly been classified on the basis of underlying genetic alterations; however, the role of aberrant DNA methylation in these tumors is not well understood and, consequently, the usefulness of methylation-based classification is unclear. RESULTS: We used the Infinium HumanMethylation27 platform to profile DNA methylation in 80 primary, untreated high-grade soft tissue sarcomas, representing eight relevant subtypes, two non-neoplastic fat samples and 14 representative sarcoma cell lines. The primary samples were partitioned into seven stable clusters. A classification algorithm identified 216 CpG sites, mapping to 246 genes, showing different degrees of DNA methylation between these seven groups. The differences between the clusters were best represented by a set of eight CpG sites located in the genes SPEG, NNAT, FBLN2, PYROXD2, ZNF217, COL14A1, DMRT2 and CDKN2A. By integrating DNA methylation and mRNA expression data, we identified 27 genes showing negative and three genes showing positive correlation. Compared with non-neoplastic fat, NNAT showed DNA hypomethylation and inverse gene expression in myxoid liposarcomas, and DNA hypermethylation and inverse gene expression in dedifferentiated and pleomorphic liposarcomas. Recovery of NNAT in a hypermethylated myxoid liposarcoma cell line decreased cell migration and viability. CONCLUSIONS: Our analysis represents the first comprehensive integration of DNA methylation and transcriptional data in primary high-grade soft tissue sarcomas. We propose novel biomarkers and genes relevant for pathogenesis, including NNAT as a potential tumor suppressor in myxoid liposarcomas.