NAB2-STAT6 Gene Fusion in Meningeal Hemangiopericytoma and Solitary Fibrous Tumor.

Meningeal solitary fibrous tumor (SFT) and hemangiopericytoma (HPC) are considered to be distinct entities in the WHO Classification of CNS Tumours (2007). They harbor NAB2-STAT6 fusions similar to their soft tissue counterparts, supporting the view that they are part of a tumor continuum. We examined 30 meningeal-based tumors originally diagnosed as either SFT or HPC. These showed a spectrum of morphologic features and were diagnosed as SFTs, malignant SFTs, HPCs, or tumors with "intermediate" features. All of the tumors showed nuclear expression of STAT6. SFTs consistently expressed diffuse CD34, while HPCs and intermediate tumors had heterogeneous staining. NAB2-STAT6 fusions were identified in 20 cases, including 7 with exon 4-exon 3, 9 with exon 6-exon 17, and 4 with exon 6-exon 18 fusions. NAB2 exon 4-STAT6 exon 3 fusion correlated with classic SFT morphology and older age and showed a trend toward less mitotic activity; there was also a trend toward more aggressive behavior in tumors lacking NAB2 exon 4-STAT6 exon 3. Thus, despite their clinical and morphologic differences, meningeal-based SFTs, HPCs, and tumors with intermediate features, similar to their soft tissue counterparts, form a histopathologic spectrum unified by STAT6 immunoexpression and NAB2-STAT6 fusion.

Recurrent mutations within the amino-terminal region of ß-catenin are probable key molecular driver events in sinonasal hemangiopericytoma.

Sinonasal hemangiopericytoma (SN-HPC) is an uncommon, site-specific, low-grade mesenchymal neoplasm of probable perivascular myoid cell origin. In contrast to solitary fibrous tumors of soft tissue and sinonasal tract origin, SN-HPCs were recently shown to lack recurrent NAB2-STAT6 fusion variants. Other molecular alterations known to occur in some of soft tissue perivascular myoid cell neoplasms were also absent in SN-HPC; thus, the molecular pathogenesis of SN-HPCs remained unknown. Guided by whole-genome sequencing combined with RNA sequencing of an index case, we analyzed a total of six SN-HPCs for mutations within the amino-terminal region of the gene CTNNB1 (cadherin-associated protein), ß 1, 88 kDa, encoding ß-catenin. All six cases showed missense mutations, with amino acid substitutions clustering at positions 33 to 45, corresponding to the recognition site of the ß-catenin destruction complex. Similar CTNNB1 mutations have been described in a variety of epithelial and mesenchymal neoplasms. These mutations prevent ß-catenin phosphorylation and proteasomal degradation but promote its nuclear accumulation and subsequent increased transcription of Wingless-related integration site target genes. Consistent with these molecular findings, ß-catenin IHC showed consistent diffuse and strong nuclear staining of the tumor cells in all six SN-HPCs. Our results highlight, for the first time, CTNNB1 mutations as the likely initiating molecular events driving SN-HPC tumorigenesis, which places SN-HPC among the growing family of ß-catenin-driven mesenchymal neoplasms.

Aberrant DNA hypermethylation of SDHC: a novel mechanism of tumor development in Carney triad.

Carney triad (CT) is a rare condition with synchronous or metachronous occurrence of gastrointestinal stromal tumors (GISTs), paragangliomas (PGLs), and pulmonary chondromas in a patient. In contrast to Carney-Stratakis syndrome (CSS) and familial PGL syndromes, no germline or somatic mutations in the succinate dehydrogenase (SDH) complex subunits A, B, C, or D have been found in most tumors and/or patients with CT. Nonetheless, the tumors arising among patients with CT, CSS, or familial PGL share a similar morphology with loss of the SDHB subunit on the protein level. For the current study, we employed massive parallel bisulfite sequencing to evaluate DNA methylation patterns in CpG islands in proximity to the gene loci of all four SDH subunits. For the first time, we report on a recurrent aberrant dense DNA methylation at the gene locus of SDHC in tumors of patients with CT, which was not present in tumors of patients with CSS or PGL, or in sporadic GISTs with KIT mutations. This DNA methylation pattern was correlated to a reduced mRNA expression of SDHC, and concurrent loss of the SDHC subunit on the protein level. Collectively, these data suggest epigenetic inactivation of the SDHC gene locus with functional impairment of the SDH complex as a plausible alternate mechanism of tumorigenesis in CT.

Phenotypical and molecular distinctness of sinonasal haemangiopericytoma compared to solitary fibrous tumour of the sinonasal tract.

AIMS: Sinonasal haemangiopericytoma (SN-HPC) is a rare sinonasal mesenchymal neoplasm of perivascular myoid cell origin. Solitary fibrous tumour (SFT) occurs only very rarely in the sinonasal tract. SFT and soft tissue HPC have been considered a single entity. Recently, recurrent gene fusions involving NAB2-STAT6 resulting in differential expression of STAT6 were characterized as central molecular events in SFT. However, no data exist for NAB2-STAT6 status or STAT6 expression in SN-HPC. METHODS AND RESULTS: We examined six SN-HPCs and two sinonasal SFTs by immunohistochemistry and RT-PCR for NAB2-STAT6 fusions. SN-HPC affected three females and three males (mean age: 72 years). They expressed smooth muscle actin, lacked strong CD34 reactivity and were negative for nuclear STAT6 expression. RT-PCR analysis confirmed the absence of NAB2-STAT6 fusions in all cases. Conversely, both sinonasal SFTs (in males aged 39 and 52 years) displayed classical features of pleuropulmonary and soft-tissue SFTs (uniformly CD34-positive with strong nuclear expression of STAT6). RT-PCR revealed NAB2-STAT6 fusions in both cases. CONCLUSIONS: These findings confirm the molecular and phenotypical distinctness of these two entities. While SN-HPC is a site-specific sinonasal neoplasm of as yet unknown molecular pathogenesis, sinonasal SFTs show phenotypical and molecular identity to their pleural/extrapleural counterparts.

Solitary fibrous tumors/hemangiopericytomas with different variants of the NAB2-STAT6 gene fusion are characterized by specific histomorphology and distinct clinicopathological features.

Recurrent somatic fusions of the two genes, NGFI-A-binding protein 2 (NAB2) and STAT6, located at chromosomal region 12q13, have been recently identified to be presumable tumor-initiating events in solitary fibrous tumors (SFT). Herein, we evaluated a cohort of 52 SFTs/hemangiopericytomas (HPCs) by whole-exome sequencing (one case) and multiplex RT-PCR (all 52 cases), and identified 12 different NAB2-STAT6 fusion variants in 48 cases (92%). All 52 cases showed strong and diffuse nuclear positivity for STAT6 by IHC. We categorized the fusion variants according to their potential functional effects within the predicted fusion protein and found strong correlations with relevant clinicopathological features. Tumors with the most common fusion variant, NAB2ex4-STAT6ex2/3, corresponded to classic pleuropulmonary SFTs with diffuse fibrosis and mostly benign behavior and occurred in older patients (median age, 69 years). In contrast, tumors with the second most common fusion variant, NAB2ex6-STAT6ex16/17, were found in much younger patients (median age, 47 years) and represented typical HPCs from deep soft tissue with a more aggressive phenotype and clinical behavior. In summary, these molecular genetic findings support the concept that classic pleuropulmonary SFT and deep-seated HPC are separate entities that share common features but correlate to different clinical outcome.