Correlating SS18-SSX immunohistochemistry (IHC) with SS18 fluorescent in situ hybridization (FISH) in synovial sarcomas: a study of 36 cases

SS18-SSX and SSX c-terminus antibodies for identification of specific fusion oncoprotein in mesenchymal neoplasms

Chromosomal rearrangement can be identified by direct methods or by using immunohistochemistry for a component of the fusion oncoprotein as a surrogate marker. Our aim was to gain insights into the staining profile using novel SS18-SSX and SSX c-terminus antibodies in SS18 fusion tumours and to investigate their potential use in soft tissue tumours harbouring SSX fusion partner outside the spectrum of synovial sarcoma. A retrospective analysis of 310 soft tissue sarcomas [via tissue microarray (TMA)] diagnosed at our Institution between 1999 and 2019 was performed. As controls, whole tissue sections from 14 genetically confirmed synovial sarcomas and one EWSR1::SSX2 rearranged sarcoma diagnosed between 2020 and 2023 were included. Two different antibodies for SSX locus were used: SSX c-terminus and SS18-SSX. Twenty-one of 310 (6.8%) and 25 of 310 (8.1%) sarcomas on the TMA showed nuclear staining with SS18-SSX and SSX, respectively. From the 24 synovial sarcomas, 17 (70.8%) stained positive for both antibodies, and in five of these cases, nuclear staining for SSX was weak. In four (16.7%) cases, only SS18-SSX was positive, and in three (12.5%) cases, only SSX staining was found. Furthermore, SSX nuclear expression was only found in four of 62 (6.5%) myxofibrosarcomas. In the control cohort, 11 of 14 synovial sarcomas (78.6%) showed positive staining for both antibodies. The remaining three cases were negative for SS18-SSX, but demonstrated at least focally strong positivity for SSX. The EWSR1::SSX2 rearranged sarcoma showed strong nuclear positivity for SSX. SS18-SSX and SSX c-terminus antibodies are reliable diagnostic markers that can be used as a surrogate marker for identification of a specific fusion. The SS18-SSX antibody is more specific and shows strong nuclear staining in synovial sarcomas, whereas SSX can present with weak staining and is less specific. However, the latter can be used in soft tissue tumours harbouring SSX fusion partner outside the spectrum of synovial sarcoma.

A therapeutic regimen using neoantigen-specific TCR-T cells for HLA-A*2402-positive solid tumors

The adoptive transfer of TCR-T cells specific to neoantigens preferentially exhibits potent cytotoxicity to tumor cells and has shown promising efficacy in various preclinical human cancers. In this study, we first identified a functional TCR, Tcr-1, which selectively recognized the SYT-SSX fusion neoantigen shared by most synovial sarcomas. Engineered T-cell expressing Tcr-1 (Tcr-T1) demonstrated HLA-A*2402-restricted, antigen-specific anti-tumoral efficacy against synovial sarcoma cells, both in vitro and in vivo. Furthermore, to extend its application, we developed a cooperative therapeutic modality, in which exogenous SYT-SSX fusion neoantigen was loaded into stimuli-responsive nanoparticles (NPs) formed by mPEG-PVGLIG-PCL copolymers (Neo-AgNPs) for tumor targeting delivery. As expected, Neo-AgNPs were proven to have great tumor penetration and local release. In situ, the modification was able to direct engineered Tcr-T1 against other HLA-A*2402-positive malignant cancer cell lines with significant antigen-specific cytotoxicity despite their inherent mutation profiles. With these favorable data, our established cooperative therapeutic modality has great potential for further clinical investigation and provides new insight for future TCR-T cell therapy development.

SS18-SSX Expression and Clinicopathologic Profiles in a Contemporary Cohort of Primary Paratesticular Synovial Sarcoma: A Series of Fourteen Patients

Synovial sarcoma (SS) is a rare genitourinary malignancy with a specific SS18::SSX 1/2 gene fusion in majority of the instances. The paratesticular location of this neoplasm is extremely rare and only 4 cases are reported in the literature. Herein, we describe the clinicopathologic features and molecular profile of paratesticular SS in the largest case series to date and to the best of our knowledge, and the only series to use novel SS18-SSX antibody for immunohistochemistry. Clinicopathologic, immunohistochemical (IHC), molecular, treatment, and follow-up data of the patients were analyzed. There were 14 patients, ranging from 15 to 47 years (mean: 30 y). The tumor size ranged from 4​​​​​​ to 15 cm. The tumors were unilateral, solid, and homogeneous tan-white with monomorphic spindle cell histology. All 14 tumors expressed SS18-SSX and TLE1 IHC and harbored SS18 rearrangement. In addition, the tumor with multifocal SS18-SSX expression had lower break-apart signals in the FISH assay (38% of the tumor cells; range: 29% to 85%). Radical orchiectomy was performed in all 14 patients and adjuvant chemotherapy was administered in 9 patients. Follow-up was available in 9 patients. The follow-up duration ranged from 5 to 24 months (median=10 mo). Four patients died of metastatic disease (range: 5 to 16 mo) and 2 patients who are alive had metastatic disease at the last follow-up. Based on our experience with the largest series to date and aggregate of the published data, paratesticular SS has a poor prognosis despite aggressive therapy. Owing to its rarity, the differential diagnosis is wide and requires a systematic approach for ruling out key morphologic mimics aided with SS18-SSX IHC and molecular confirmation because this distinction carries important therapeutic and prognostic implications. Due to the excellent concordance of SS18-SSX IHC results with FISH results as observed in our study, we would like to suggest inclusion of SS18-SSX in the diagnostic immunohistochemistry panel of all spindle cell sarcomas where synovial sarcoma is considered as a morphologic differential. SS18-SSX-positive staining may be used as a surrogate for FISH assay in a resource-limited setting where molecular assay is not available. Furthermore, IHC has a fairly shorter turn-around-time, is less complex, and of low cost.

Advances of SS18-SSX fusion gene in synovial sarcoma: Emerging novel functions and therapeutic potentials

Synovial sarcoma is a rare type of soft tissue sarcoma that primarily affects adolescents and young adults, featured by aggressive behavior and a high potential for metastasis. Genetically, synovial sarcoma is defined by the fusion oncogene SS18-SSX arising from the translocation of t(X;18)(p11;q11). SS18-SSX fusion gene is the major driver of the oncogenic event in synovial sarcoma. SS18-SSX fusion protein, while not containing any DNA-binding motifs, binds to the SWI/SNF (BAF) complex, a major epigenetic regulator, leading to the disruption of gene expression which results in tumor initiation and progression. Emerging studies on the molecular mechanisms of SS18-SSX associated signaling pathway hold promise for developments in diagnosis and treatments. Advanced diagnostic methods facilitate early and precise detection of the tumor, enabling disease monitoring and prognostic improvements. Treatment of synovial sarcoma typically comprises local surgery, radiotherapy and chemotherapy, while novel managements such as immunotherapy, targeted therapies and epigenetic modifiers are explored. This review focuses on the recent studies of SS18-SSX fusion gene, epigenetic landscape, signaling pathways, diagnostic techniques, and relevant therapeutic advances, aiming to inhibit the oncogenic processes and improve outcomes for patients with synovial sarcoma.

Rare mediastinal small round cell melanoma with synovial sarcoma-like immunophenotype: A potential diagnostic pitfall

Melanoma can pose a significant diagnostic challenge due to the high variability in histological morphology and expression of non-melanocytic immunomarkers. We present a case of a 47-year-old male with an aggressive mediastinal neoplasm and disseminated disease posing several diagnostic challenges. Multiple biopsies were submitted from different anatomic locations and during multiple time points showing an undifferentiated round cell tumor (URCT) with synovial sarcoma-like immunophenotype. SS18::SSX fusion was sought through NGS study for diagnostic confirmation. NGS results revealed NRAS and CDKN2A mutations and absence of fusions, resulting in a new review of the histologic material with a broader immunohistochemical panel, finding strong positivity to melanic antibodies. This case is an illustrative example of a malignant melanoma with small round cell morphology showing aberrant expression of CD99, BCL2, TLE1 and SS18-SSX antibodies exposing a potentially hazardous pitfall highlighting the importance of a wide differential diagnosis and the role of confirmational studies with molecular tests.

Precision Medicine in Cytopathology

Over the last decade, cancer diagnostics has undergone a notable transformation with increasing complexity. Minimally invasive diagnostic tests, driven by advanced imaging and early detection protocols, are redefining patient care and reducing the need for more invasive procedures. Modern cytopathologists now safeguard patient samples for vital biomarker and molecular testing. In this article, we explore ancillary testing modalities and the role of biomarkers in organ-specific contexts, underscoring the transformative impact of precision medicine. Finally, the advent of more than 80 Food and Drug Administration-approved predictive biomarkers signals a new era, guiding cancer care toward personalized and targeted strategies.

The Combined Immunohistochemical Expression of GLI1 and BCOR in Synovial Sarcomas for the Identification of Three Risk Groups and Their Prognostic Outcomes: A Study of 52 Patients

Synovial sarcoma (SS) is a rare soft-tissue tumor characterized by a monomorphic blue spindle cell histology and variable epithelial differentiation. Morphologically, SSs may be confused with other sarcomas. Systemic treatment is more effective for patients with high-risk SSs, patients with advanced disease, and younger patients. However, further studies are required to find new prognostic biomarkers. Herein, we describe the morphological, molecular, and clinical findings, using a wide immunohistochemical panel, of a series of SS cases. We studied 52 cases confirmed as SSs by morphological diagnosis and/or molecular studies. Clinical data (gender, age, tumor size, tumor location, resection margins, adjuvant treatment, recurrences, metastasis, and survival) were also retrieved for each patient. All the available H&E slides were examined by four pathologists. Three tissue microarrays (TMAs) were constructed for each of the tumors, and a wide immunohistochemical panel was performed. For time-to-event variables, survival analysis was performed using Kaplan-Meier curves and log-rank testing, or Cox regression. Statistical significance was considered at p < 0.05. The mean age of our patients was 40.33, and the median was 40.5 years. We found a predominance of males versus females (1.7:1). The most frequent morphological subtype was monophasic. TRPS1, SS18-SSX, and SSX-C-terminus were positive in 96% of cases. GLI1 expression was strong in six and focal (cytoplasmic) in twenty patients. Moreover, BCOR was expressed in more than half of SSs. Positive expression of both proteins, BCOR and GLI1, was correlated with a worse prognosis. Multivariate analysis was also performed, but only BCOR expression appeared to be significant. The combination of GLI1 and BCOR antibodies can be used to group SSs into three risk groups (low, intermediate, and high risk). We hypothesize that these findings could identify which patients would benefit from receiving adjuvant treatment and which would not. Moreover, these markers could represent therapeutic targets in advanced stages. However, further, larger series of SSs and molecular studies are necessary to corroborate our present findings.

Tumor biomarkers for diagnosis, prognosis and targeted therapy

Tumor biomarkers, the substances which are produced by tumors or the body's responses to tumors during tumorigenesis and progression, have been demonstrated to possess critical and encouraging value in screening and early diagnosis, prognosis prediction, recurrence detection, and therapeutic efficacy monitoring of cancers. Over the past decades, continuous progress has been made in exploring and discovering novel, sensitive, specific, and accurate tumor biomarkers, which has significantly promoted personalized medicine and improved the outcomes of cancer patients, especially advances in molecular biology technologies developed for the detection of tumor biomarkers. Herein, we summarize the discovery and development of tumor biomarkers, including the history of tumor biomarkers, the conventional and innovative technologies used for biomarker discovery and detection, the classification of tumor biomarkers based on tissue origins, and the application of tumor biomarkers in clinical cancer management. In particular, we highlight the recent advancements in biomarker-based anticancer-targeted therapies which are emerging as breakthroughs and promising cancer therapeutic strategies. We also discuss limitations and challenges that need to be addressed and provide insights and perspectives to turn challenges into opportunities in this field. Collectively, the discovery and application of multiple tumor biomarkers emphasized in this review may provide guidance on improved precision medicine, broaden horizons in future research directions, and expedite the clinical classification of cancer patients according to their molecular biomarkers rather than organs of origin.

Update on immunohistochemistry in bone and soft tissue tumors: Cost-effectively replacing molecular testing with immunohistochemistry

Soft tissue tumors form part of a challenging domain in diagnostic pathology owing to their comparative rarity, astonishing histologic diversity, and overlap between entities. Many of these tumors are now known to be defined by highly recurrent, or, in some instances, unique molecular alterations. Insights from gene profiling continue to elucidate the wider molecular landscape of soft tissue tumors; many of these advances have been co-opted by immunohistochemistry (IHC) for diagnostic applications. There now exists a multitude of antibodies serving as surrogate markers of recurrent gene fusions, amplifications, and point mutations, which, in certain settings, can replace the need for more resource and time-intensive cytogenetic and molecular genetic analyses. IHC presents many advantages including rapid turnaround time, cost-effectiveness, and interpretative reproducibility. A sensible application of these immunohistochemical markers complemented by a working knowledge of the molecular pathogenesis of bone and soft tissue tumors permits accurate diagnosis in the majority of cases. In this review, we will outline some of these biomarkers while emphasizing molecular correlates and highlighting interpretative challenges and pitfalls.

Immunohistochemistry Update in Dermatopathology and Bone and Soft Tissue Pathology

CONTEXT.—

Immunohistochemistry plays an important role in dermatopathology, particularly for melanocytic lesions and poorly differentiated malignancies. In the field of bone and soft tissue pathology, molecular methods remain the gold standard for diagnosis; however, immunohistochemistry targeting underlying molecular alterations represents a valuable screening tool, especially in areas with limited access to molecular testing.

OBJECTIVE.—

To describe the utility and limitations of new and emerging immunohistochemical stains in the diagnosis of skin, soft tissue, and bone tumors.

DATA SOURCES.—

A literature review of recently described immunohistochemical stains in the fields of dermatopathology and bone and soft tissue pathology was performed.

CONCLUSIONS.—

Immunohistochemistry is an important adjunctive tool for select entities in dermatopathology and bone and soft tissue pathology, and it provides pathologists with valuable evidence of their behavior, underlying molecular alterations, and line of differentiation. Furthermore, immunostains targeting molecular abnormalities have the potential to replace current molecular methods. Many of these recently described stains demonstrate higher sensitivity and specificity; however, limitations and pitfalls still exist, and correlation with morphologic and clinical findings remains essential for diagnosis.

Utility of Immunohistochemistry With Antibodies to SS18-SSX Chimeric Proteins and C-Terminus of SSX Protein for Synovial Sarcoma Differential Diagnosis

Synovial sarcoma is a relatively common soft tissue tumor characterized by highly specific t(X;18)(p11;q11) translocation resulting in the fusion of SS18 with members of SSX gene family. Typically, detection of SS18 locus rearrangement by fluorescence in situ hybridization or SS18 :: SSX fusion transcripts confirms the diagnosis. More recently, immunohistochemistry (IHC) for SS18-SSX chimeric protein (E9X9V) and C-terminus of SSX (E5A2C) showed high specificity and sensitivity for synovial sarcoma. This study screened a cohort of >1000 soft tissue and melanocytic tumors using IHC and E9X9V and E5A2C antibodies. Three percent (6/212) of synovial sarcomas were either negative for SS18-SSX or had scattered positive tumor cells (n=1). In these cases, targeted RNA next-generation sequencing detected variants of SS18 :: SSX chimeric transcripts. DNA methylation profiles of 2 such tumors matched with synovial sarcoma. A few nonsynovial sarcoma tumors (n=6) revealed either focal SS18-SSX positivity (n=1) or scattered positive tumor cells. However, targeted RNA next-generation sequencing failed to detect SS18 :: SSX transcripts in these cases. The nature of this immunopositivity remains elusive and may require single cell sequencing studies. All synovial sarcomas showed positive SSX IHC. However, a mosaic staining pattern or focal loss of expression was noticed in a few cases. Strong and diffuse SSX immunoreactivity was also seen in epithelioid sclerosing osteosarcoma harboring EWSR1 :: SSX1 fusion, while several sarcomas and melanocytic tumors including cellular blue nevus (5/7, 71%) revealed focal to diffuse, mostly weak to intermediate SSX staining. The SS18-SSX and SSX IHC is a useful tool for synovial sarcoma differential diagnosis, but unusual immunophenotype should trigger molecular genetic testing.

SS18-SSX Expression in a Contemporary Cohort of Primary Renal Synovial Sarcoma: A Multi-Institutional Experience of Fourteen Patients

Primary renal synovial sarcoma is a rare aggressive mesenchymal neoplasm of the kidney that accounts for less than 1% of renal sarcomas. Herein, we describe the clinicopathologic and molecular findings of 14 renal synovial sarcoma patients in one of the largest case series to date and to our knowledge, the only renal synovial sarcoma series to use novel SS18-SSX IHC. Clinicopathologic, IHC, molecular, management, and follow-up data were reviewed and analyzed. Macroscopically, the tumors had either homogeneous, tan-white, and solid (n = 10), variegated and solid (n = 3), or variegated and solid-cystic (n = 1) cut surfaces. Spindle cell (n = 10), round cell (n = 3), and round to epithelioid morphologies (n = 1) were observed. SS18-SSX IHC was positive in all 14 tumors (diffuse, n = 10; multifocal, n = 2; focal, n = 2). All the tumors harbored SS18::SSX1/2 gene rearrangement. Metastases to the liver, brain, and lung (n = 1); liver and bone (n = 1); liver and diaphragm (n = 1) were identified. Adjuvant chemotherapy was administered in 11/12 patients. Follow-up was available for 10 patients (time period range: 5 to 24 months). Four patients died of disease, and six patients are alive with no recurrence or metastasis. As SS18-SSX IHC showed an excellent concordance with the FISH results, this may reliably be used in the IHC panel of spindle/round cell sarcomas of the kidney and as a molecular surrogate for renal synovial sarcoma, particularly in a resource-limited setting. Also, the tumors with focal SS18-SSX expression had lower break apart signals in the FISH assay (19% and 23% in two tumors with focal SS18-SSX IHC positivity).

SS18-SSX Antibody: A Useful Tool to Save Time and Reduce Costs in Synovial Sarcoma Diagnosis. Proposal of a Novel Diagnostic Algorithm

Synovial sarcoma is a rare malignant mesenchymal neoplasm mostly affecting young adults, characterized by a specific translocation which results in the fusion of the SS18 gene on chromosome 18 with one of the three highly homologous SSX genes on chromosome X. Its morphological diagnosis, especially in monophasic or poorly differentiated variants, can be challenging because histological features often overlap with other malignant mesenchymal tumors. Until recently, the differential diagnosis mostly relied on the use of cytogenetic or molecular analyses to detect the specific t(X;18)(p11;q11) translocation, thus virtually restricting its correct identification to referral centers with a high histological and molecular pathology workflow. The recently commercialized highly sensitive and fusion-specific SS18-SSX antibody has significantly improved the approach to these tumors, representing a relatively cheap and easy to access tool for synovial sarcoma diagnosis. Through a retrospective analysis of 79 synovial sarcomas and histological mimickers, this study confirms the usefulness of the SS18-SSX antibody in the diagnosis of synovial sarcoma, particularly focusing on its application in the pathological response evaluation after neoadjuvant treatment as well as its time- and cost-saving advantages. Finally, we here propose a new diagnostic algorithm to apply into the routine practice.

Novel characteristics for immunophenotype, FISH pattern and molecular cytogenetics in synovial sarcoma

As a rare and highly aggressive soft tissue sarcoma, the new immunophenotype, atypical FISH pattern and relevant molecular cytogenetics of synovial sarcoma (SS) remain less known, although it is characteristically represented by a pathognomonic chromosomal translocation t (X; 18) (p11.2; q11.2). Methodologically, the morphology was retrospectively analysed by using H&E staining, and immunohistochemical features were investigated by using markers that have been recently applied in other soft tissue tumors. Moreover, FISH signals for SS18 and EWSR-1 break-apart probes were examined. Finally, cytogenetic characteristics were analysed via RT-PCR and Sanger sequencing. Consequently, nine out of thirteen cases that were histologically highly suspected as SS were finally identified as SS via molecular analysis. Histologically, nine SS cases were divided into monophasic fibrous SS (4/9), biphasic SS (4/9) and poorly differentiated SS (1/9). Immunohistochemically, SOX-2 immunostaining was positive in eight cases (8/9) and PAX-7 immunostaining was diffusely positive in the epithelial component of biphasic SS (4/4). Nine cases showed negative immunostaining for NKX3.1 and reduced or absent immunostaining for INI-1. Eight cases showed typically positive FISH signalling for the SS18 break-apart probe, whereas one case exhibited an atypical FISH pattern (complete loss of green signalling, case 2). Furthermore, the SS18-SSX1 and SS18-SSX2 fusion genes were identified in seven cases and two cases, respectively. The fusion site in 8 out of 9 cases was common in the literature, whereas the fusion site in case 2 was involved in exon 10 codon 404 in SS18 and exon 7 codon 119 in SSX1 (which has not been previously reported), which notably corresponded to the complete loss of green signalling in the FISH pattern. Additionally, FISH analysis of the EWSR-1 gene in nine SS cases demonstrated aberrant signalling in three cases that were recognized as a monoallelic loss of EWSR-1 (1/9), an amplification of EWSR-1 (1/9) and a translocation of EWSR-1 (1/9). In conclusion, SS18-SSX fusion gene sequencing is obligatory for a precise diagnosis of SS when dealing with a confusing immunophenotype and atypical or aberrant FISH signalling for SS18 and EWSR-1 detection.

Synovial sarcoma: cytopathology of 51 cases highlighting the application of ancillary molecular testing

INTRODUCTION

Among sarcomas, synovial sarcoma (SS) is defined by its unique SS18 cytogenetic translocation. Fine needle aspiration (FNA) biopsy is in a key position to exploit this uniqueness for diagnostic purposes.

MATERIALS AND METHODS

Our cytopathology files were searched for examples of SS with histopathologic verification. FNA biopsy, imprint smears, and core needle biopsy (CNB) were performed using standard techniques.

RESULTS

Fifty-one cases from 49 patients (male/female ratio, 1:1; age range, 12-79 years; mean age, 40 years) met the inclusion criteria. Of the 51 cases, 44 (86%) were FNAs, 6 were cytology imprints, and 1 was pleural fluid. Eleven aspirates had concurrent CNB. All cases had tissue confirmation. The biopsy sites included extremities (n = 24; 47%), trunk (n = 12; 24%), lung (n = 8; 16%), head or neck (n = 6; 12%), and pleural fluid (n = 1; 2%). The aspirates were from primary (n = 36; 71%), metastatic (n = 12; 24%), and recurrent (n = 3; 5%) neoplasms. The cytologic diagnoses were SS (69%), suspicious for SS (12%), malignancy (10%), spindle cell neoplasm (4%), and malignancy other than SS (6%). In general, smears and imprints contained dense cell aggregates and single cells composed of a monotonous population having fusiform, rounded, or ovoid banal nuclei and scant cytoplasm. Poorly differentiated SS showed both large epithelioid cell and small cell cytomorphology. When performed, SS18 immunohistochemical and genetic testing was positive in all 19 FNA and 3 CNB cases.

CONCLUSIONS

When coupled with appropriate ancillary testing, FNA biopsy allows for a specific, accurate diagnosis of SS in most cases.

GLI1 -Rearranged Enteric Tumor : Expanding the Spectrum of Gastrointestinal Neoplasms With GLI1 Gene Fusions

GLI1 encodes a transcription factor that targets cell cycle regulators affecting stem cell proliferation. GLI1 gene fusions were initially described in pericytomas with a t[7;12] translocation and more recently in gastric plexiform fibromyxomas and gastroblastomas. This study describes the clinicopathologic, immunohistochemical, and molecular features of three intestinal-based neoplasms harboring GLI1 gene fusions. We studied three unique mesenchymal small bowel tumors. Paraffin embedded tumor tissues from these cases and 62 additional tumor samples that included a plexiform fibromyxoma were sequenced using a targeted RNAseq method to detect fusion events. The study patients included two women and one man who were 52, 80, and 22 years of age at the time of diagnosis. The tumors involved the submucosa and muscularis propria of the duodenum, jejunum, and ileum. All 3 tumors contained a proliferation of monotonous oval or spindle cells with scattered, somewhat dilated vessels. Two cases showed epithelioid structures such as glands, tubules, or nests. Immunohistochemical analysis revealed cytokeratin expression in the epithelioid components of both tumors displaying these features, and variable numbers of mesenchymal cells. Diffuse CD56 positivity was seen in the mesenchymal component of 2 tumors and desmin and smooth muscle actin staining in the other tumor. Immunostains for S-100 protein, DOG-1, and CD117 were negative in all cases. GLI1 fusions with different partner genes were detected in all tumors, and in the plexiform fibromyxoma, used as a control. Validation by fluorescence in situ hybridization was performed. None of the tumors have recurred or metastasize after surgery. We describe novel GLI1 fusions in 3 mesenchymal neoplasms of the small intestine, including 2 with biphenotypic features. Thus far, all cases have pursued indolent clinical courses. We propose the term " GLI1 -rearranged enteric tumor" to encompass this group of unique neoplasms of the small intestine that harbor GLI1 gene fusions and expand the spectrum of gastrointestinal neoplasms with these alterations.

Molecular testing of soft tissue tumors

BACKGROUND

The diagnosis of soft tissue tumors is challenging, especially when the evaluable material procured is limited. As a result, diagnostic ancillary testing is frequently needed. Moreover, there is a trend in soft tissue pathology toward increasing use of molecular results for tumor classification and prognostication. Hence, diagnosing newer tumor entities such as CIC-rearranged sarcoma explicitly requires molecular testing. Molecular testing can be accomplished by in situ hybridization, polymerase chain reaction, as well as next generation sequencing, and more recently such testing can even be accomplished leveraging an immunohistochemical proxy.

CONCLUSION

This review evaluates the role of different molecular tests in characterizing soft tissue tumors belonging to various cytomorphologic categories that have been sampled by small biopsy and cytologic techniques.

Expanding the Use of an SS18-SSX Antibody for Molecular Assays in Synovial Sarcoma

Synovial sarcoma is an aggressive malignancy that generally affects adolescents and young adults and is characterized by high rates of recurrence and metastasis, with a 10-year survival rate of about 50%. The fusion oncoprotein SS18-SSX, the product of a pathognomonic chromosomal translocation t(X;18), is the oncogenic driver of this sarcoma, disrupting differentiation through widespread epigenetic dysregulation. Experimental research into SS18-SSX biology has been limited by the lack of an antibody that specifically detects the endogenous fusion oncoprotein as opposed to its native SS18 or SSX components. Recently, a rabbit monoclonal antibody was developed and made commercially available, which specifically detects the fusion junction site epitope of SS18-SSX as found in at least 95% of synovial sarcomas. Here, we characterize a suite of molecular biology assays using this new antibody, both confirming existing and reporting on novel applications. We demonstrate its high sensitivity and specificity for synovial sarcoma diagnosis on patient samples through positive immunohistochemical staining on synovial sarcoma, tissue microarray, and full face sections. In addition, we demonstrate detection of the human SS18-SSX protein when expressed in a genetically engineered mouse model of synovial sarcoma. We also demonstrate nuclear staining of SS18-SSX in synovial sarcoma cells using immunofluorescence, and visualize the interaction between SS18-SSX and the BAF complex member BRG1 through a proximity ligation assay. Lastly, we confirm the interaction between SS18-SSX and promoter regions of target genes through chromatin immunoprecipitation. This antibody represents a breakthrough in sarcoma research and has value in multiple applications to expand the knowledge of synovial sarcoma biology.