NAB2-STAT6 Fusion Research Articles

Hemangiopericytomas in the Central Nervous System: A Multicenter Study of Korean Cases with Validation of the Usage of STAT6 Immunohistochemistry for Diagnosis of Disease.

Hemangiopericytoma (HPC) in the central nervous system (CNS) is a rare disease with distinctive biological/clinical characteristics compared with meningioma. Cases of HPCs of the CNS were collected, and clinicopathological records were retrospectively reviewed and analyzed. Immunohistochemistry (IHC) for proliferative markers (Ki-67, PHH3) and STAT6 were performed. A total of 140 cases were collected, with mean follow-up of 77months (median 58.8months; range 0.53-540.5months). 1-, 5-, 10-, and 20-year survival rates were 99.1, 94.0, 74.4, and 61.9%, respectively. Thirty-nine patients (27.9%) had recurrent disease. Mean and median times to recurrence were 62.9 and 47.3months with 1-, 5-, 10-, and 20-year recurrence-free survival rates of 98.3, 78.3, 50.1, and 11.0%, respectively. Thirteen patients (9.3%) developed extracranial metastases. No adjuvant radiation therapy, higher histologic grades, failure of gross-total resection, and cases with gamma-knife surgery (GKS) were factors associated with shorter disease-free survival (log-rank test, p=0.02, 0.00, 0.02, 0.00), among which high histologic grade and cases with GKS were significant in multivariable analysis. Strong nuclear STAT6 expression was noted in HPCs in 62 cases of HPCs (60/62, 96.8%), whereas diffuse weak positivity was demonstrated in all meningioma cases. The survival rate in patients with HPC of the CNS is comparable to that of previously reported series. Recurrence remains a critical clinical issue of the disease. Identification of NAB2-STAT6 fusion transcript with surrogate IHC marker is a valuable diagnostic tool in the differential diagnosis of the disease.

Clinicopathological differences between variants of the NAB2-STAT6 fusion gene in solitary fibrous tumors of the meninges and extra-central nervous system.

Investigations on the NAB2-STAT6 fusion gene in solitary fibrous tumors (SFTs) and hemangiopericytomas (HPCs) have increased since its discovery in 2013. Although several SFTs reported without NAB2-STAT6 fusion gene analysis, we reviewed 546 SFTs/HPCs with NAB2-STAT6 fusion gene analysis in this study and investigated differences between the gene variants. In total, 452 cases tested positive for the NAB2-STAT6 fusion gene, with more than 40 variants being detected. The most frequent of these were NAB2 exon 6-STAT6 exon 16/17/18 and NAB2 exon 4-STAT6 exon 2/3, with the former occurring most frequently in SFTs in meninges, soft tissues, and head and neck; the latter predominated in SFTs in the pleura and lung. There was no difference between the histology of SFTs and fusion gene variants. A follow-up analysis of SFTs showed that 51 of 202 cases had a recurrence, with 18 of 53 meningeal SFTs having a local recurrence and/or metastasis within 0-19years. In meninges and soft tissue, SFTs with the NAB2 exon 6-STAT6 exon 16/17/18 tended to recur more frequently than SFTs with the NAB2 exon 4-STAT6 exon 2/3. Clinicopathological data, including yearly follow-ups, are required for meningeal SFTs/HPCs to define the correlation of variants of NAB2-STAT6 fusion gene.

The NAB2–STAT6 gene fusion in solitary fibrous tumor can be reliably detected by anchored multiplexed PCR for targeted next-generation sequencing

Solitary fibrous tumor (SFT) is a mesenchymal tumor of fibroblastic origin, which can affect any region of the body. 10-15% of SFTs metastasize and metastatic tumors are uniformly lethal with no effective therapies. The behavior of SFT is difficult to predict based on morphology. Recently, an intrachromosomal gene fusion between NAB2 and STAT6 was identified as the defining driving genetic event of SFT and different fusion types correlated with tumor histology and behavior. Due to the proximity of NAB2 and STAT6 on chromosome 12, this fusion may be missed by fluorescence in-situ hybridization. We evaluated 12 SFTs from 10 patients. All tumors showed strong nuclear staining for STAT6 by immunohistochemistry (IHC). The same formalin-fixed, paraffin-embedded blocks for IHC were used for gene fusion detection by a next-generation sequencing (NGS)-based assay. Targeted RNA fusion sequencing for gene fusions was performed using the Universal RNA Fusion Detection Kit, the Archer(™) FusionPlex(™) Sarcoma Panel and the Ion Torrent PGM, and data were analyzed using the Archer Analysis Pipeline 3.3. All tumors were positive for NAB2-STAT6 fusion. Six types of fusions were detected: NAB2ex4-STAT6ex2, NAB2ex2-STAT6ex5, NAB2ex6-STAT6ex16, NAB2ex6-STAT6ex17, NAB2ex3-STAT6ex18 and NAB2intron6-STAT6Ex17. The NGS findings were confirmed by RT-PCR followed by Sanger sequencing. No STAT6 fusion was detected in selected morphologic mimics of SFT. The assay also allows for detection of novel fusions and can detect NAB2-STAT6 fusions at a single-base resolution.

NAB2-STAT6 gene fusion and STAT6 immunoexpression in extrathoracic solitary fibrous tumors: the association between fusion variants and locations.

Solitary fibrous tumor (SFT) is a rare mesenchymal neoplasm harboring NAB2-STAT6 fusion, which drives STAT6 nuclear relocation. For extrathoracic SFTs, the clinical relevance of this molecular hallmark remains obscure. We assessed STAT6 immunoexpression for 61 extrathoracic SFTs exclusive of the meninges and head and neck, and 25 had analyzable RNAs to distinguish fusion variants by RT-PCR. The immunohistochemical and molecular findings were correlated with clincopathological features and disease-free survival (DFS). Twenty-eight males and 33 females had SFTs in the body cavities (n=31), extremities (n=17), and trunk (n=13), categorized into 53 non-malignant and 8 malignant tumors. The vast majority (n=57, 93%) exhibited distinctive STAT6 nuclear expression, including malignant ones. The common fusion variants were NAB2ex6-STAT6ex16/17 in 13 SFTs and NAB2ex4-STAT6ex2 in 8, while miscellaneous variants were detected only in 4 SFTs in the limbs and trunk but not in any body cavity-based cases (P=0.026). The worse DFS was univariately associated with malignant histology (P=0.04) but unrelated to tumor size, location, or fusion variant. Conclusively, extrathoracic SFTs mostly harbor NAB2ex6-STAT6ex16/17, followed by NAB2ex4-STAT6ex2. Miscellaneous variants are significantly rare in SFTs within the body cavities. The clinical aggressiveness of extrathoraic SFTs is associated with malignant histology but unrelated to the NAB2-STAT6 fusion variants.

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.

The clinicopathological significance of NAB2-STAT6 gene fusions in 52 cases of intrathoracic solitary fibrous tumors.

NAB2-STAT6 gene fusion drives STAT6 nuclear expression and is the pathognomonic hallmark of solitary fibrous tumors (SFTs). However, no study has systematically analyzed the clinicopathological features, STAT6 immunoexpression status, or the fusion variants of NAB2-STAT6 in intrathoracic SFTs. Fifty-two intrathoracic SFTs were retrieved to appraise histopathology, assess STAT6 immunoexpression, and determine NAB2-STAT6 fusion variants by RT-PCR. Location-relevant histologic mimics served as controls. Thirty-one pleura-based, 12 mediastinal/pericardial, and nine intrapulmonary lesions were histologically categorized into eight malignant, eight atypical, and 36 conventional or cellular SFTs, including two fat-forming and two giant cell angiofibroma-like SFTs. STAT6 distinctively decorated the tumoral nuclei in 51 (98%) SFTs. However, no nuclear staining was observed in the histological mimics. NAB2-STAT6 fusion was detected in 34 SFTs. Twenty-nine (85.3%) exhibited the major NAB2ex4-STAT6ex2/3 variant and 5 (14.7%) the minor NAB2ex6-STAT6ex16/17. NAB2ex4-STAT6ex2 was significantly associated with older age (P=0.01) and pleuropulmonary tumors (P=0.025). After a median follow-up of 33.9 (range, 0.3-174.6) months, adverse outcomes occurred in one atypical and five malignant SFTs, including two local relapses, one intrapulmonary metastasis, and three extrathoracic metastases. Inferior disease-free survival was univariately associated with atypical/malignant histology (P=0.001) and a mitosis >4/10 HPFs (P=0.0012) but was unrelated to fusion variants. In conclusion, the majority of intrathoracic SFTs exhibited STAT6 nuclear staining, and NAB2ex4-STAT6ex2/3 was the predominant fusion type. However, clinical aggressiveness is associated with atypical/malignant histology primarily contributed by increased mitosis but was unrelated to the NAB2-STAT6 fusion variants.

GENO-28ANALYSIS OF NAB2-STAT6 GENE FUSION IN 17 CASES OF MENINGEAL SOLITARY FIBROUS TUMOR/HEMANGIOEPRICYTOMA

BACKGROUND: Solitary fibrous tumor/hemangiopericytoma (SFT/HPC) is a mesenchymal tumor that can affect virtually any region of the body. SFT/HPC occurred in thoracic cavity and soft tissue has been considered a single biological entity termed SFT with characteristic NAB2-STAT6 gene fusion in both of them. Meanwhile meningeal SFT and HPC are still categorized into discrete disease entities, thus still to be investigated in detail, especially for genetic background. The aim of this study is to verify the frequency of NAB2-STAT6 gene fusion and the relationship between fusion variants and clinicopathological findings of SFT/HPC, especially meningeal SFT/HPC. METHODS: We examined the NAB2-STAT6 gene fusion by reverse transcription-PCR with 4 case of meningeal SFT and 13 cases of HPC which were histologically diagnosed. For the detection of the NAB2-STAT6 fusion gene, seven types of forward primers in NAB2 ({"type":"entrez-nucleotide","attrs":{"text":"NM_005967.3","term_id":"95102542","term_text":"NM_005967.3"}}NM_005967.3) and six types of reverse primers in STAT6 ({"type":"entrez-nucleotide","attrs":{"text":"NM_001178078.1","term_id":"296010863","term_text":"NM_001178078.1"}}NM_001178078.1) were designated. RT-PCR of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used as endogenous control. All of the detected NAB2-STAT6 fusion variants were confirmed by Sanger sequencing. RESULTS: We identified fusion genes in 12 out of 17 cases including NAB2 exon 6 – STAT6 exon 17 (4/17, 24%), NAB2 exon 6 – STAT6 exon 16 and NAB2 exon 4 – STAT6 exon 2 (3/17, 18%, respectively). In addition, we also identified the novel fusion genes NAB2 exon 5 – STAT6 exon 16 including two types of fusion variation. CONCLUSIONS: Our results confirmed that meningeal STF and HPC are in a single biological spectrum with NAB2-STAT6 gene fusion as well as non-meningeal SFT/HPC. Moreover, the fact that the major fusion variant in the meningeal SFT/HPC was NAB2 exon 6 – STAT6 exon 16/17, which were in rare in thoracic SFT/HPC might suggest the oran specific tumorigenic procell of SFT/HOC.

NAB2–STAT6 fusion types account for clinicopathological variations in solitary fibrous tumors

Solitary fibrous tumor (SFT) is characterized by the inv12(q13q13)-derived NAB2–STAT6 fusion, which exhibits variable breakpoints and drives STAT6 nuclear expression. The implications of NAB2–STAT6 fusion variants in pathological features and clinical behavior remain to be characterized in a large cohort of SFTs. We investigated the clinicopathological correlates of this genetic hallmark and analyzed STAT6 immunoexpression in 28 intrathoracic, 37 extrathoracic, and 23 meningeal SFTs. These 88 tumors were designated as histologically nonmalignant in 75 cases and malignant in 13, including 1 dedifferentiated SFT. Eighty cases had formalin-fixed and/or fresh samples to extract assessable RNAs for RT-PCR assay, which revealed NAB2–STAT6 fusion variants comprising 12 types of junction breakpoints in 73 fusion-positive cases, with 65 (89%) falling into 3 major types. The predominant NAB2ex4–STAT6ex2 (n=33) showed constant breakpoints at the ends of involved exons, whereas the NAB2ex6–STAT6ex16 (n=16) and NAB2ex6–STAT6ex17 (n=16) might exhibit variable breakpoints and incorporate NAB2 or STAT6 intronic sequence. Including 73 fusion-positive and 7 CD34-negative SFTs, STAT6 distinctively labeled 87 (99%) SFTs in nuclei, exhibited diffuse reactivity in 73, but did not decorate 98 mimics tested. In seven fusion-negative cases, 6 were STAT6-positive, suggesting rare fusion variants not covered by RT-PCR assay. Regardless of histological subtypes, intrathoracic SFTs affected older patients (P=0.035) and tended to be larger in size (P=0.073). Compared with other variants, NAB2ex4–STAT6ex2/4 fusions were significantly predominant in the SFTs characterised by intrathoracic location (P<0.001), older age (P=0.005), decreased mitoses (P=0.0028), and multifocal or diffuse STAT6 staining (P=0.013), but not found to correlate with disease-free survival. Conclusively, STAT6 nuclear expression was distinctive in the vast majority of SFTs, including all fusion-positive tumors, and exploitable as a robust diagnostics of CD34-negative cases. Despite the associations of NAB2–STAT6 fusion variants with several clincopathological factors, their prognostic relevance should be further validated in large-scale prospective studies of SFTs.

Differential diagnosis of solitary fibrous tumors: A study of 454 soft tissue tumors indicating the diagnostic value of nuclear STAT6 relocation and ALDH1 expression combined with in situ proximity ligation assay.

Solitary fibrous tumors (SFTs) are rare mesenchymal neoplasms, displaying variable morphological and clinicopathological features. Supportive immunohistochemical markers such as CD34, CD99, BCL2 and LSD1 are commonly applied in the differential diagnosis of SFTs, although none is sufficiently sensitive or specific enough. The aim of the present study was to examine the most differential markers for the reliable distinction of SFTs from histological mimics. We investigated the expression of STAT6, NAB2, ALDH1, GRIA2 and IGF2 in 454 comprehensive soft tissue tumors, comprising formalin-fixed paraffin-embedded (FFPE) tissue samples from 80 SFTs and 374 other mesenchymal tumors. The Duolink in situ proximity ligation assay (PLA) was adopted for the detection of NAB2-STAT6 fusion proteins. STAT6 was expressed in all 80 SFT cases with a moderate-strong nuclear staining intensity. In contrast, only 4/374 (1%) non-SFT mesenchymal tumors showed a nuclear STAT6 staining pattern. Strong expression of NAB2 and IGF2 was detected in SFT and non-SFT cases. Positive GRIA2 immunoreactivity was found in 64% (SFT) and 8% (non-SFT), respectively. Expression of ALDH1 was moderate-strong in 76% (SFT), whereas only 2 non-SFT lesions showed positive ALDH1 immunoreactivity. Moreover, the presence of NAB2‑STAT6 fusion proteins was indicated in 71/78 (91%) SFT cases by PLA. Nuclear STAT6 and cytoplasmic ALDH1 expression are the most sensitive and specific markers in the differential diagnosis of SFTs. Furthermore, application of Duolink in situ proximity ligation assay can be helpful to detect the NAB2-STAT6 fusion protein in the majority of SFTs.

Multifocal cytokeratin expression in pleural and abdominal malignant solitary fibrous tumors: an unusual diagnostic pitfall

Dear Editor, Solitary fibrous tumor (SFT) is an uncommon mesenchymal fibroblastic neoplasm usually arising in the pleura, but it can virtually affect any extrapleural site. Conventional SFTs are characterized by short, cytologically bland spindle cells arranged in a patternless architecture, a fibro-collagenous stroma, and a prominent hemangiopericytomatous branching vascular network [1]. The vast majority of SFTs are morphologically and clinically benign. However, 5 to 10 % of SFTs will recur and/or metastasize. Proposed prognostic clinical and morphological criteria for SFT include age at presentation, tumor size, cellularity, mitotic activity (>4 mitoses per 10 high-power fields), the presence of necrosis, and the presence of sharply demarcated anaplastic areas. The vast majority of aggressive SFTs demonstrate these clinical and morphological features of malignancy; however, also morphologically benign-appearing SFTs can metastasize, and therefore, the biological behavior of this tumor type still remains largely unpredictable [1, 2]. Immunohistochemistry is commonly used as a valuable adjunct to diagnosis of SFT, alongside convincing histology. Bcl-2, CD99, and CD34 are often positive in SFT (in nearly 100, 75, and 81–95 % of cases, respectively), but positivity for these markers can also be seen at a high frequency in other soft tissue tumors that can mimic SFT, and they are therefore non-specific [1]. Recent studies have discovered that almost all SFTs harbor a NAB2-STAT6 fusion gene [3]. This fusion gene is considered a driving mutation in the initiation of SFTs through EGR1 activation and transcriptional deregulation of EGR1-dependent target genes. The resulting overexpression of STAT6 protein has recently been shown to be a highly sensitive and specific marker for these tumors. Hence, STAT6 immunochemistry should lead to a more accurate diagnosis [4, 5]. We describe three cases of a morphological malignant SFT with unexpected strong cytokeratin expression, and we discuss the diagnostic implications thereof. All tumors were primary. The patientswerewomen aged between 42 and 84 years. In two cases, the tumor was located in the pleura, and in one case, the tumor was situated in the abdomen. Histologically, all three cases showed, at least focally, typical histological features of SFTwith branching hemangiopericytoma-like vessels and a variable fibro-collagenous background. They all met the proposed criteria of morphologically malignant SFTs, with increased mitotic activity, cytonuclear atypia, and tumoral necrosis (Figs. 1 and 2). Immunohistochemically, the atypical spindle cells in all three cases showed strong and patchy to diffuse expression for CD99 and Bcl-2. However, CD34 expression was heterogenous and loss of CD34 expression was observed in the malignant areas of both pleural tumors (Fig. 3). Staining for cytokeratin AE1/AE3 showed strong multifocal positivity (Fig. 4), but none of the cases was positive for cytokeratin 5, calretinin, and Wilms’ tumor protein 1 (WT1). The unforeseen strong positivity for cytokeratin AE1/AE3 together with the clinical setting of a large pleural or abdominal tumor point to sarcomatoid carcinoma or sarcomatoid mesothelioma as major differential diagnoses. The loss of CD34 reactivity contributes further to this diagnostic dilemma. Nevertheless, a histopathological diagnosis of SFT was made for three reasons. First, characteristic CD34 reactivity can be absent or only focally present in SFT cases [1]. Secondly, a few anecdotal single case reports of cytokeratin * Evelyne Lecoutere evelyne.lecoutere@ugent.be

Malignant solitary fibrous tumor with high-grade nuclear atypia: An alternate entity for the undetermined tumor group

Recently, a novel fusion transcript, NAB2–STAT6, and its variants have also been reported to be specific diagnostic markers for solitary fibrous tumors (SFTs). In this study, we validated the existence of the NAB2–STAT6 fusion gene in SFTs and examined its relation with the pathological features. Frozen samples from 9 tumors were assessed for fusion gene. The detected fusion genes exhibited large intron sequences and the insertion of unknown and previously unreported sequences. The fusion genes were not detected in the 2 malignant cases with high-grade nuclear atypia, nuclear pleomorphism and necrosis, that was confirmed by multiplex PCR method. In addition, 1 of the 2 NAB2–STAT6 fusion gene-negative tumors showed amplification of the MDM2 and CDK4 genes. It was suggested that a certain proportion of tumors previously diagnosed as malignant SFTs with high-grade nuclear atypia lacking NAB2–STAT6 should be categorized into a special subtype of SFT, which is genetically different from conventional SFTs, and which cannot be apparently distinguished from dedifferentiated liposarcoma or undifferentiated pleomorphic sarcoma.

Distinct clinicopathological features of NAB2-STAT6 fusion gene variants in solitary fibrous tumor with emphasis on the acquisition of highly malignant potential.

The impact of NGFI-A binding protein 2 (NAB2)-signal transducer and activator of transcription 6 (STAT6) fusion on the biological behavior and the mechanism of acquisition of malignant phenotype in solitary fibrous tumor (SFT) is not well understood. We examined variations of the NAB2-STAT6 fusion gene in 40 cases of SFT using formalin-fixed, paraffin-embedded tissues and secondary genetic alterations of tumor protein p53 (TP53),, platelet-derived growth factor receptor, β polypeptide (PDGFRB), and telomerase reverse transcriptase (TERT) promoters. These gene variations were compared with the clinicopathological features. The 2-year and 5-year disease-free survival rates (DFSRs) were 91% and 83%, respectively. All 40 samples demonstrated nuclear staining for STAT6, including CD34-negative cases. Moreover, p53-positive staining was associated with a lower DFSR and was significantly associated with higher Ki-67 label index, higher mitotic rate (mitosis, >4/high-power field), and the presence of nuclear atypia/pleomorphism. NAB2-STAT6 fusions were detected in all of the cases; the NAB2 exon 4-STAT6 exon 2, the most common genotype, appeared in 18 cases, which was associated with thoracic tumor location and the less aggressive phenotype. In contrast, tumors with NAB2 exon 6-STAT6 exon 16/18 demonstrated an aggressive phenotype. Mutations in TP53 and PDGFRB were detected in 2 and 3 cases respectively, and these occurred in a mutually exclusive fashion. TERT promoter hot spot mutations were observed in 5 cases, which were associated with shorter DFSR. Two dedifferentiated SFT cases harbored both TP53 and TERT promoter mutations. TP53 mutations, which result in its overexpression, in combination with TERT promoter mutations seem to play an important role in the dedifferentiation process.

Recurrent Mutations within the Amino-Terminal Region of β-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), β 1, 88 kDa, encoding β-catenin. All six cases showed missense mutations, with amino acid substitutions clustering at positions 33 to 45, corresponding to the recognition site of the β-catenin destruction complex. Similar CTNNB1 mutations have been described in a variety of epithelial and mesenchymal neoplasms. These mutations prevent β-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, β-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 β-catenin-driven mesenchymal neoplasms.

Solitary fibrous tumor - clinicopathologic, immunohistochemical and molecular analysis of 28 cases.

BackgroundSolitary fibrous tumor is a mesenchymal tumor of fibroblastic type, which can affect any region of the body. Recently, a recurrent gene fusion NAB2-STAT6 has been identified as molecular hallmark. The NAB2-STAT6 fusion leads to EGR1 activation and transcriptional deregulation of EGR1-dependent target genes and is a driving event in initiation of SFT. In this study, we report the clinicopathologic and RT-PCR findings and evaluated expression of STAT6 and EGR1 protein in a cohort of 28 SFTs.Methods28 patients with a median age of 54 years were included with SFTs originating at different sites, most occurring in the lung and pleura (9, 32%), 5 in soft tissues of the lower extremities (18%) and 5 in the head and neck (18%). For detection of the NAB2-STAT6 fusion gene, RT-PCR was performed using RNA extracted from formalin-fixed and paraffin-embedded tissues. Immunohistochemistry was performed on all cases with antibodies against STAT6 and EGR1.ResultsAll patients were treated by surgery, 3 with adjuvant chemo- or radiotherapy. Follow-up data of 18 patients could be obtained of which 2 patients died of metastatic disease 13 months and 52 years after first diagnosis. Sixteen patients have no evidence of disease with a median follow up of 29.5 months (range 7 – 120 months). NAB2-STAT6 fusion transcripts were found in 19/28 cases (68%). The most common fusion was between NAB2 exon 4 and STAT6 exon 3 (11/19, 58%), mainly occurring in pleuropulmonary lesions. All cases showed strong nuclear expression of STAT6 (28/28, 100%) while EGR1 showed low-level variable nuclear expression in all samples, comparable with the EGR1 expression results of the control group.ConclusionsThe identification of the NAB2-STAT6 fusion in SFTs can provide important diagnostic information, especially in cases with aberrant morphology or when biopsy material is limited. STAT6 immunohistochemistry is another useful tool in diagnosing SFT. EGR1 immunohistochemistry indicates low-level protein expression in accordance with EGR1 activation due to distorted NAB2 activity.Virtual slidesThe virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/13000_2014_224

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

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. 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. 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.

STAT6 is amplified in a subset of dedifferentiated liposarcoma

A recurrent intrachromosomal rearrangement on chromosome 12q in solitary fibrous tumor leads to the formation of a NAB2–STAT6 fusion oncogene. As a result, nuclear expression of the cytoplasmic transcription factor STAT6 is found in solitary fibrous tumor and serves as a useful diagnostic marker. STAT6 is located in 12q13, a region containing well-characterized oncogenes that are commonly amplified in dedifferentiated liposarcoma; we have previously reported that STAT6 is expressed in a subset of dedifferentiated liposarcoma. The aim of this study was to determine the frequency of STAT6 expression in dedifferentiated liposarcoma and the underlying genetic mechanism. STAT6 protein expression was analyzed by immunohistochemistry in a well-characterized series of 35 previously unpublished cases of dedifferentiated liposarcoma, all with nuclear MDM2 and/or CDK4 expression by immunohistochemistry and/or cytogenetic features of dedifferentiated liposarcoma. FISH for STAT6 was performed in all cases with STAT6 expression, and a subset of control cases without STAT6 expression. In total 4/35 cases (11%) showed STAT6 expression (three with multifocal staining of moderate to strong intensity and one with weak focal staining). FISH demonstrated amplification of STAT6 in all cases positive for STAT6 by immunohistochemistry; in contrast, FISH performed on four STAT6-negative dedifferentiated liposarcomas demonstrated no STAT6 amplification (P=0.0286). Of the four STAT6 amplified cases, three patients were male and one was female, ranging in age from 51 to 76 years. Tumors were located in the mediastinum (n=2), paratesticular soft tissue (n=1), and perirenal soft tissue (n=1). Three patients received pre-operative chemotherapy +/− radiation therapy. In conclusion, STAT6 is amplified in a subset of dedifferentiated liposarcoma, resulting in STAT6 protein expression that can be detected by immunohistochemistry and may be a potential pitfall in the differential diagnosis of dedifferentiated liposarcoma and solitary fibrous tumor. These findings suggest a role for STAT6-mediated transcriptional activity in some cases of dedifferentiated liposarcoma and highlight the genomic complexity and heterogeneity of dedifferentiated liposarcoma.

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

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. 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: 72years). 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 52years) 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. 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.

GRIA2 is a novel diagnostic marker for solitary fibrous tumour identified through gene expression profiling

The NAB2-STAT6 fusion was recently identified as a consistent finding in solitary fibrous tumour (SFT), resulting in nuclear expression of the C-terminal part of STAT6. Gene expression studies of SFT revealed high expression of the GRIA2 gene. The aim of this study was to examine GRIA2 expression in SFTs and other soft tissue tumours to evaluate its diagnostic utility. Immunohistochemistry was performed on 375 soft tissue tumours. In total, 84 of 105 (80%) SFTs, including 86% of malignant SFTs and 100% of dedifferentiated SFTs, were positive for GRIA2. One SFT known to harbour the NAB2-STAT6 fusion but that was negative for STAT6 by immunohistochemistry was positive for GRIA2. It is of note that 93% of SFTs received in the last 3 years were positive for GRIA2, as compared with only 70% of older cases. The only other tumours that expressed GRIA2 were 15 of 20 (75%) cases of dermatofibrosarcoma protuberans (DFSP), four of nine (44%) myoepitheliomas, one synovial sarcoma (<1% of cells), and one schwannoma. GRIA2 is a useful marker for distinguishing SFT from most mimics. Among other CD34-positive tumours, GRIA2 is also expressed in DFSP; however, clinical and histological features aid in their distinction. GRIA2 shows a limited distribution in other soft tissue tumours.

STAT6 Immunohistochemistry Is Helpful in the Diagnosis of Solitary Fibrous Tumors

Solitary fibrous tumor (SFT) is an uncommon fibroblastic neoplasm. Although histologic characteristics and frequent CD34 expression allow for an accurate diagnosis in the majority of SFT cases, a wide histologic spectrum and an occasional unexpected immunophenotype may pose diagnostic challenges. Molecular analyses have discovered that almost all SFTs harbor an NAB2-STAT6 fusion gene, which is considered specific to this tumor type. Recent studies have suggested that STAT6 immunohistochemistry is a reliable surrogate for detection of the fusion gene. Our aim was to validate these findings by examining a large number of SFT cases and a broad array of 30 different types of non-SFT tumors. A total of 49 SFTs with a range of histologic characteristics and 159 benign or malignant tumors that can mimic SFTs were retrieved and stained for STAT6. All 49 SFTs (100%) showed STAT6 expression that was restricted in the nucleus, mostly in a diffuse and strong manner, irrespective of the tumor sites and histologic patterns. The staining was uniform in most cases but was heterogenous in about 20% of the cases in which zonal staining attenuation was observed, likely reflecting variability in fixation or tissue ischemia. In contrast, only 4 non-SFT tumors (2.5%) exhibited weak nuclear STAT6 expression, whereas the remaining 155 cases showed no staining or often weak reactivity in both the cytoplasm and the nucleus. Therefore, nuclear STAT6 immunoreactivity is a highly sensitive and specific marker of SFTs and can be helpful when diagnosis is inconclusive by conventional methods.

STAT6-positive intraorbital papillary tumor: A rare variant of solitary fibrous tumor?

We experienced a peculiar case of orbital mesenchymal tumor in a 22-year-old Japanese woman. The tumor showed a papillary proliferating pattern, but no typical hemangiopericytomatous staghorn vessels. The tumor was composed of round to oval shaped cells with oval nuclei and mild nuclear atypia. Abundant vascular cores were present in the central portion of papillary proliferations of tumor cells. Immunohistochemistry revealed that the tumor cells were positive for CD34 and bcl2. Moreover, they showed positive nuclear signals of STAT6, which have recently been shown to be specific for solitary fibrous tumors. In the literature, only one case of solitary fibrous tumor with papillary and retiform growth pattern has been reported, but the case partially showed the typical staghorn vessel pattern. Although the definite diagnosis is difficult in the settings of the unusual histology and the deficiency of NAB2–STAT6 fusion, it is possible that this STAT6-positive intraorbital papillary tumor is a very rare variant of solitary fibrous tumor.