Loss of H3K27 trimethylation is not suitable for distinguishing malignant peripheral nerve sheath tumor from melanoma: a study of 387 cases including mimicking lesions

The utility of DNA methylation profiling in the diagnosis of un-, de- and trans-differentiated melanoma: a series of 11 cases

AIMS

Melanomas are recognised for their remarkable morphological plasticity. Some tumours may lose conventional features and/or acquire non-melanocytic characteristics, referred to as undifferentiated, dedifferentiated and transdifferentiated melanoma. Despite this phenotypical variability, melanomas typically maintain their cancer driver aberrations, affecting genes such as BRAF, NRAS and NF1. Currently, little is known about whether the DNA methylation profile follows the loss or change of differentiation or is retained despite extensive morphological transformation.

METHODS AND RESULTS

In this study we analysed 11 melanoma cases, comprising six males and five females, with a median age of 67 years, including five undifferentiated, four trans-differentiated and two de-differentiated melanomas. Undifferentiated and trans-differentiated tumours either arose in a patient with known melanoma and/or presented in the groin/axilla with molecular alterations consistent with melanoma. Cases with heterologous differentiation resembled chondrosarcoma, osteosarcoma, angiosarcoma and rhabdomyosarcoma both morphologically and immunohistochemically, while undifferentiated tumours resembled undifferentiated pleomorphic sarcoma. Methylome profiling was performed, and unsupervised clustering analysis revealed nine cases (five undifferentiated, three trans-differentiated and one de-differentiated) to cluster closely together with conventional melanomas from a reference set. Two cases clustered separately with a distinct group of conventional melanomas exhibiting H3K27me3 loss.

CONCLUSIONS

Despite loss of differentiation and phenotypical plasticity, methylation patterns seem to be retained in undifferentiated, de-differentiated and trans-differentiated melanomas and represent useful diagnostic tools to enhance diagnostic precision in these diagnostically challenging cases.

Genomic Landscape of Superficial Malignant Peripheral Nerve Sheath Tumor

Superficial malignant peripheral nerve sheath tumors (SF-MPNSTs) are rare cancers and can be difficult to distinguish from spindle cell (SCM) or desmoplastic (DM) melanomas. Their biology is poorly understood. We performed whole-exome sequencing and RNA sequencing (RNA-seq) on SF-MPNST (n = 8) and compared them with cases of SCM (n = 7), DM (n = 8), and deep MPNST (D-MPNST, n = 8). Immunohistochemical staining for H3K27me3 and PRAME was also performed. SF-MPNST demonstrated intermediate features between D-MPNST and melanoma. Patients were younger than those with melanoma and older than those with D-MPNST; the outcome was worse and better, respectively. SF-MPNST tumor mutational burden (TMB) was higher than D-MPNST and lower than melanoma; differences were significant only between SF-MPNST and SCM (P = .0454) and between D-MPNST and SCM (P = .001, Dunn's Kruskal-Wallis post hoc test). Despite having an overlapping mutational profile in some common cancer-associated genes, the COSMIC mutational signatures clustered DM and SCM together with UV light exposure signatures (SBS7a, 7b), and SF- and D-MPNST together with defective DNA base excision repair (SBS30, 36). RNA-seq revealed differentially expressed genes between SF-MPNST and SCM (1670 genes), DM (831 genes), and D-MPNST (614 genes), some of which hold promise for development as immunohistochemical markers (SOX8 and PLCH1) or aids (MLPH, CALB2, SOX11, and TBX4). H3K27me3 immunoreactivity was diffusely lost in most D-MPNSTs (7/8, 88%) but showed variable and patchy loss in SF-MPNSTs (2/8, 25%). PRAME was entirely negative in the majority (0+ in 20/31, 65%), including 11/15 melanomas, and showed no significant difference between groups (P = .105, Kruskal-Wallis test). Expression of immune cell transcripts was upregulated in melanomas relative to MPNSTs. Next-generation sequencing revealed multiple differential features between SF- MPNST, D-MPNST, SCM, and DM, including tumor mutation burden, mutational signatures, and differentially expressed genes. These findings help advance our understanding of disease pathogenesis and improve diagnostic modalities.

Diagnostic Approach to Mesenchymal and Spindle Cell Tumors of the Breast

Mesenchymal and spindle cell tumors of the breast represent a broad and heterogeneous group of lesions that may be sampled on core needle biopsy or surgical excision. Mesenchymal lesions unique to the breast are those that derive from the specialized breast myofibroblast, such as mammary myofibroblastoma and pseudoangiomatous stromal hyperplasia. However, any mesenchymal lesion arising in extramammary soft tissue may also arise in the breast, including fibroblastic, peripheral nerve sheath, adipocytic, and vascular lesions. The spindle cell lesions pose the greatest diagnostic challenge, due to the significant radiographic, morphologic, and immunophenotypic overlap within the category of mesenchymal lesions and more broadly with other nonmesenchymal breast lesions. The distinction is particularly challenging on the limited material of breast core needle biopsies, and caution should be taken before definitively classifying a breast spindle cell lesion on core needle biopsy to avoid unnecessary treatment if misdiagnosed. Consideration of a wide differential diagnosis, adequate sampling of a resection specimen, use of a targeted immunopanel, and selective use of molecular assays are essential steps for accurate classification of mesenchymal lesions in the breast. This review covers the clinical, histologic, and immunophenotypic features of mesenchymal tumors of the breast, with a special emphasis on the differential diagnoses unique to the breast and challenges encountered on breast core needle biopsy.

Radiation-Induced Intraosseous Malignant Peripheral Nerve Sheath Tumor: A Case Report

INTRODUCTION

The significance of radiation therapy in cancer treatment comes with associated complications, including fibrosis, osteonecrosis, and the development of secondary malignancies, such as malignant peripheral nerve sheath tumors (MPNSTs). We emphasize the importance of understanding these complications for an effective patient management.

METHODS

We report a 47-year-old man with a history of squamous cell carcinoma of the tongue, treated with surgery, chemotherapy, and radiation therapy. The patient later presented with symptoms that led to the discovery of an intraosseous MPNST.

RESULTS

Histopathological examination revealed characteristic features of MPNST, including spindle cells arranged is sweeping fascicles with contrasting hypercellular and hypocellular areas, producing a marble-like pattern, with atypical wavy, buckled, hyperchromatic nuclei, and brisk mitotic activity. Immunohistochemical analysis showed patchy positive staining for S100 and SOX10, and a complete loss of H3K27me3 expression. This report underscores the challenge of diagnosing secondary malignancies post-radiation therapy and the importance of careful histological examination to differentiate them from other conditions.

CONCLUSIONS

In conclusion, radiation-induced secondary malignancies are a significant late side effect of radiation therapy that can profoundly impact treatment decision-making and requires a high index of suspicion during post radiation surveillance. Malignant peripheral nerve sheath tumor serves as a pertinent example, highlighting the importance of considering long-term risks when developing optimal management plans for cancer patients.

A Unique Case of High-Grade Dedifferentiated Melanoma Without a Known Primary Site

Melanoma is a malignant neoplasm that arises in melanocytes, pigment-producing cells present primarily in the skin. However, not all malignant melanomas originate from the skin, and the other sites of origin include the uveal (eye) or mucosa (rectal or oral); it can have different patterns of mutations. While targeted therapies and immunotherapies have transformed the treatment of this disease in the metastatic setting, resistance mechanisms can still pose challenges for patients and their healthcare providers. We present a case of a male patient with metastatic melanoma and discuss its clinical presentation, diagnostic workup, treatment options, and outcomes. By exploring the complexities of this multifaceted disease process, clinicians and researchers can gain valuable insights into potential areas for improved management strategies. Ultimately, we should aim to deepen our understanding of melanoma and work towards better patient outcomes.

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.

Integrated Epigenetic and Transcriptomic Analysis Identifies Interleukin 17 DNA Methylation Signature of Malignant Peripheral Nerve Sheath Tumor Progression and Metastasis

PURPOSE

Sarcomas are a complex group of highly aggressive and metastatic tumors with over 100 distinct subtypes. Because of their diversity and rarity, it is challenging to generate multisarcoma signatures that are predictive of patient outcomes.

MATERIALS AND METHODS

Here, we identify a DNA methylation signature for progression and metastasis of numerous sarcoma subtypes using multiple epigenetic and genomic patient data sets. Malignant Peripheral Nerve Sheath Tumors (MPNSTs) are highly metastatic sarcomas with frequent loss of the histone methyltransferase, PRC2. Loss of PRC2 is associated with MPNST metastasis and plays a critical noncanonical role in DNA methylation.

RESULTS

We found that over 900 5'-C-phosphate-G-3' (CpGs) were hypermethylated in MPNSTs with PRC2 loss. Furthermore, we identified eight differentially methylated CpGs in the IL17D/RD family that correlate with the progression and metastasis of MPNSTs in two independent patient data sets. Similar trends were identified in other sarcoma subtypes, including osteosarcoma, rhabdomyosarcoma, and synovial sarcoma. Analysis of scRNAseq data sets determined that IL17D/RD expression occurs in both the tumor cells and the surrounding stromal populations.

CONCLUSION

These results might have broad implications for the clinical management and surveillance of sarcoma.

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.

Synovial Sarcoma of the Gastrointestinal Tract

We report the clinicopathologic and immunohistochemical features of 18 cases of confirmed primary synovial sarcoma of the gastrointestinal tract. The neoplasms arose in 10 women and 8 men ranging in age from 23 to 81 years (mean: 50; median: 57.5 years). The tumors for which size was known ranged from 1.8 to 15.0 cm (mean: 5.2; median: 5.1 cm). Microscopically, 14 synovial sarcomas were of the monophasic type, 2 were biphasic, and 2 were poorly differentiated. Immunohistochemical analysis of 4 cases showed strong, diffuse staining for SS18::SSX (4/4 cases). Pancytokeratin and EMA immunohistochemistry were performed on 13 and 9 tumors, respectively, and each showed patchy-to-diffuse staining. By reverse-transcription PCR, 3 cases were positive for the SS18::SSX1, and 2 cases were positive for the SS18::SSX2 gene fusion. Six cases contained an SS18 gene rearrangement by fluorescence in situ hybridization, and next-generation sequencing identified an SS18::SSX2 gene fusion in one case. Clinical follow-up information was available for 9 patients (4 months to 4.6 years; mean, 2.8 y; median: 29 months), and one patient had a recent diagnosis. Three patients died of disease within 41 to 72 months (mean, 56 months) of their diagnosis. Five patients were alive without evidence of disease 4 to 52 months (mean, 17.6 months) after surgery; of whom 1 of the patients received additional chemotherapy treatment after surgery because of recurrence of the disease. A single patient was alive with intraabdominal recurrence 13 months after surgery. We conclude that synovial sarcoma of the gastrointestinal tract is an aggressive tumor, similar to its soft tissue counterpart, with adverse patient outcomes. It is important to distinguish it from morphologically similar gastrointestinal tract lesions that may have different treatment regimens and prognoses.

Sarcomatoid carcinoma of the common bile duct presenting as painless jaundice

A woman in her 70s presented with painless jaundice and index biopsy of a common bile duct (CBD) mass obtained by endoscopic retrograde cholangiopancreatography was suspicious for malignant peripheral nerve sheath tumour. Treatment consisted of pancreaticoduodenectomy, and final pathology results were consistent with sarcomatoid carcinoma. Postoperative complications included pancreaticojejunal leak, surgical wound infection, bacteraemia, myocardial injury, and significant ulceration and stricturing of the oesophagus. 14 weeks post-pancreaticoduodenectomy, the patient was found to have a perforated viscus, gastroduodenal leak and diffuse small bowel ischaemia-the patient passed away following emergent laparotomy. We aim to add to the limited literature surrounding this rare CBD neoplasm.

Clinicopathological study of malignant peripheral nerve sheath tumors in the head and neck: Case reports and review of literature

BACKGROUND

Malignant peripheral nerve sheath tumor (MPNST) is a rare and aggressive soft tissue sarcoma that poses a major diagnostic and therapeutic challenge.

CASE SUMMARY

We retrospectively reviewed patients with head and neck MPNSTs treated in our hospital from 2000 to 2021. The clinical features, pathological manifestations, treatments, and prognoses were summarized. We also reviewed the literature, focusing on MPNST in the mandible and maxilla. The study population consisted of five women and five men aged 22-75 years (mean age, 49 years). Of the 10 patients, 7 were initial cases and 3 were recurrent cases. All lesions were sporadic. The most common site was the mandible. The most frequently encountered symptoms were a progressive mass and local swelling. Complete or partial loss of trimethylation at lysine 27 of histone H3 (H3K27me3) was evident on staining in four of nine cases (one case was excluded due to lack of tissue for evaluation of loss of H3K27me3). The 2- and 5-year disease-specific survival rates were 86% and 43%, respectively. The average survival time was 64 mo.

CONCLUSION

MPNST is a highly malignant tumor with a poor prognosis, prone to a high risk of recurrence and distant metastasis. Complete surgical resection is the main treatment.

Mesenchymal Neoplasms of the Liver

Mesenchymal neoplasms of the liver can be diagnostically challenging, particularly on core needle biopsies. Here, I discuss recent updates in neoplasms that are specific to the liver (mesenchymal hamartoma, undifferentiated embryonal sarcoma, calcifying nested stromal-epithelial tumor), vascular tumors of the liver (anastomosing hemangioma, hepatic small vessel neoplasm, epithelioid hemangioendothelioma, angiosarcoma), and other tumor types that can occur primarily in the liver (PEComa/angiomyolipoma, inflammatory pseudotumor-like follicular dendritic cell sarcoma, EBV-associated smooth muscle tumor, inflammatory myofibroblastic tumor, malignant rhabdoid tumor). Lastly, I discuss metastatic sarcomas to the liver, as well as pitfalls presented by metastatic melanoma and sarcomatoid carcinoma.

Undifferentiated and Dedifferentiated Metastatic Melanomas Masquerading as Soft Tissue Sarcomas: Mutational Signature Analysis and Immunotherapy Response

The distinction between undifferentiated melanoma (UM) or dedifferentiated melanoma (DM) from undifferentiated or unclassifiable sarcoma can be difficult and requires the careful correlation of clinical, pathologic, and genomic findings. In this study, we examined the utility of mutational signatures to identify patients with UM/DM with particular attention as to whether this distinction matters for treatment because the survival of patients with metastatic melanoma has dramatically improved with immunologic therapy, whereas durable responses are less frequent in sarcomas. We identified 19 cases of UM/DM that were initially reported as unclassified or undifferentiated malignant neoplasm or sarcoma and submitted for targeted next-generation sequencing analysis. These cases were confirmed as UM/DM by harboring melanoma driver mutations, UV signature, and high tumor mutation burden. One case of DM showed melanoma in situ. Meanwhile, 18 cases represented metastatic UM/DM. Eleven patients had a prior history of melanoma. Thirteen of 19 (68%) of the tumors were immunohistochemically completely negative for 4 melanocytic markers (S100, SOX10, HMB45, and MELAN-A). All cases harbored a dominant UV signature. Frequent driver mutations involved BRAF (26%), NRAS (32%), and NF1 (42%). In contrast, the control cohort of undifferentiated pleomorphic sarcomas (UPS) of deep soft tissue exhibited a dominant aging signature in 46.6% (7/15) without evidence of UV signature. The median tumor mutation burden for DM/UM vs UPS was 31.5 vs 7.0 mutations/Mb (P < .001). A favorable response to immune checkpoint inhibitor therapy was observed in 66.6% (12/18) of patients with UM/DM. Eight patients exhibited a complete response and were alive with no evidence of disease at the last follow-up (median 45.5 months). Our findings support the usefulness of the UV signature in discriminating DM/UM vs UPS. Furthermore, we present evidence suggesting that patients with DM/UM and UV signatures can benefit from immune checkpoint inhibitor therapy.

[Histological and molecular characteristics of tumours of the peripheral nervous system]

Tumours of the peripheral nervous system occur sporadically in adults and except for a minority of entities, these tumours are usually benign. The most common are nerve sheath tumours. Because these tumours grow in direct proximity or even invade peripheral nerve bundles, they can lead to severe pain and motion deficits. From the neurosurgical perspective these tumours are technically challenging, and especially for tumours with an invasive growth pattern complete resection may not be possible. Peripheral nervous system tumours that are associated with tumour syndromes such as neurofibromatosis type 1 and 2 or schwannomatosis are a particular clinical challenge. The goal of the current article is to present histological and molecular characteristics of peripheral nervous system tumours. Furthermore, future targeted therapy strategies are presented.

Primary Malignant Peripheral Nerve Sheath Tumor of the Stomach: A Rare Case Report and Review of Literature

Malignant peripheral nerve sheath tumor (MPNST) is a spindle cell sarcoma originating from peripheral nerves or showing differentiation of nerve sheath components. Primary MPNST of the stomach is an extremely rare neoplasm with only a few published reports in the literature. We present the case of a 58-year-old male patient with MPNST in the stomach. The patient was admitted due to upper abdomen discomfort. Gastroscopy revealed a huge ulcer lesion in the stomach, and biopsy revealed a spindle cell malignant neoplasm. No other specific findings were found in the whole-body imaging examination. Subtotal gastrectomy was performed. Histologically, an ulcer-type, push-infiltrating mass composed of dense, woven-like spindle cells with frequent mitosis could be seen. In immunohistochemistry, the tumor cells were negative for expression of H3K27 trimethylation (H3K27me3), keratin (AE1/AE3), epithelial membrane antigen (EMA), CD34, KIT, DOG1 (ANO1), S-100, SOX10, smooth muscle actin, desmin, myogenin, MDM2, CDK4, P16 (CDKN2A) and SS18-SSX (SS18::SSX). Primary MPNST of the stomach was diagnosed based on histological and immunohistochemical results. During the 2.5 years follow-up period after surgery, no recurrence was observed.

Malignant undifferentiated epithelioid neoplasms with MAML2 rearrangements: A clinicopathologic study of seven cases demonstrating a heterogenous entity

Among mesenchymal tumors, MAML2 gene rearrangements have been described in a subset of composite hemangioendothelioma and myxoinflammatory fibroblastic sarcoma (MIFS). However, we have recently encountered MAML2-related fusions in a group of seven undifferentiated malignant epithelioid neoplasms that do not fit well to any established pathologic entities. The patients included five males and two female, aged 41-71 years old (median 65 years). The tumors involved the deep soft tissue of extremities (hip, knee, arm, hand), abdominal wall, and the retroperitoneum. Microscopically, the tumors consisted of solid sheets of atypical epithelioid to histiocytoid cells with abundant cytoplasm. Prominent mitotic activity and necrosis were present in 4 cases. In 3 cases, the cells displayed hyperchromatic nuclei or conspicuous macronucleoli, and were admixed with background histiocytoid cells and a lymphoplasmacytic infiltrate. By immunohistochemistry (IHC), the neoplastic cells had a nonspecific phenotype. On targeted RNA sequencing, MAML2 was the 3' partner and fused to YAP1 (4 cases), ARHGAP42 (2 cases), and ENDOD1 (1 case). Two cases with YAP1::MAML2 harbored concurrent RAF kinase fusions (RBMS3::RAF1 and AGK::BRAF, respectively). In 2 cases with targeted DNA sequencing, mutations in TP53, RB1 and PTEN were detected in 1 case, and PDGFRB mutations, CCNE1 amplifications and CDKN2A/2B deletion were detected in another case, which showed strong and diffuse PDGFRB expression by IHC. Of the 4 cases with detailed clinical history (median follow-up period 8 months), three developed distant metastatic disease (one of which died of disease); one case remained free of disease 3 years following surgical excision. In conclusion, we describe a heterogeneous series of MAML2-rearranged undifferentiated malignant epithelioid neoplasms, a subset of which may overlap with a recently described MIFS variant with YAP1::MAML2 fusions, further expanding the clinicopathologic spectrum of mesenchymal neoplasms with recurrent MAML2 gene rearrangements.

PRAME expression in spindle cell melanoma, malignant peripheral nerve sheath tumour, and other cutaneous sarcomatoid neoplasms: a comparative analysis

Diagnosis of spindle cell/sarcomatoid melanoma may be challenging due to frequent loss of expression of melanocytic marker(s) and histomorphologic resemblance to various mesenchymal tumours, particularly malignant peripheral nerve sheath tumour (MPNST). Overexpression of PReferentially expressed Antigen in MElanoma (PRAME) supports a diagnosis of melanoma when evaluating challenging melanocytic tumours. PRAME expression in MPNST and other cutaneous sarcomatoid neoplasms, however, has not been well characterised. We aimed to determine the utility of PRAME immunostain in distinguishing spindle cell melanoma from MPNST and other sarcomatoid mimics. PRAME expression was scored by extent (0 to 4+) and intensity (0 to 3) of staining. A strong positive correlation was observed between the extent and intensity scores (r = 0.84). An extent score of 4+, defined by staining in 76-100% of tumour cells, was seen in 56% (23/41) of spindle cell melanomas, 18% (7/38) of MPNSTs, 15% (4/27) of cutaneous sarcomatoid squamous cell carcinomas (SCCs), 33% (5/15) of poorly differentiated cutaneous angiosarcomas, 12% (4/33) of atypical fibroxanthomas (AFXs), 4% (1/25) of pleomorphic dermal sarcomas (PDSs), and none (0/16) of the high-grade cutaneous leiomyosarcomas. A significant difference was found between spindle cell melanoma and all other examined sarcomatoid neoplasms except angiosarcoma. While diffuse (and often strong) PRAME expression is more frequently observed in spindle cell melanoma than MPNST, sarcomatoid SCC, AFX, PDS, and high-grade leiomyosarcoma, its limited sensitivity and specificity caution against its use as a standalone diagnostic marker. PRAME may complement other epigenetic or lineage-specific markers and should only be used as part of an immunohistochemical panel when evaluating these sarcomatoid neoplasms.

Loss of H3K27me3 expression in canine nerve sheath tumors

Nerve sheath tumors (NSTs) are characterized by neoplastic proliferation of Schwann cells, perineurial cells, endoneurial and/or epineurial fibroblasts. Diagnosis of NST is often challenging, particularly in distinguishing malignant NST (MNST) from other soft tissue sarcomas, or sometimes between low-grade MNST and benign NST. Recent studies in human pathology have demonstrated loss of trimethylation at lysine 27 of histone 3 (H3K27me3) in a subset of MNSTs using immunohistochemistry. Loss of H3K27me3 expression is rare in other high-grade sarcomas and also appears to be useful in distinguishing benign and low-grade MNSTs from high-grade subsets. In our retrospective study, we performed H3K27me3 immunohistochemistry in 68 canine tumors previously diagnosed as NST. We detected loss of H3K27me3 expression in 25% (n = 17) of all canine NST, including one neurofibroma, whereas 49% (n = 33) of tumors had mosaic loss of expression and 26% (n = 18) retained expression. No statistically significant differences were found between H3K27me3 expression, histopathological features of tumors, and their immunoreactivity for Sox10, claudin-1, GFAP, and Ki67. Because the classification of canine NST is not yet fully established and its correlation with the prognosis and clinical course of the disease is lacking, prospective studies with possible genetic analyses are needed to assess the true diagnostic value of H3K27me3 loss in canine NST.

Cytomorphological Spectrum of Solitary Fibrous Tumor: Revisited

BACKGROUND

Solitary fibrous tumor (SFT) is a tumor of mesenchymal origin. Its diagnosis on cytology is challenging, owing to variation in cellularity, sparsely distributed cellular and stromal components and inapparent vasculature. The cytomorphologic findings have been rarely described in literature with a few case reports and occasional case series. We present the cytomorphologic features of SFT with special emphasis on immunochemical findings.

MATERIALS AND METHODS

We present cytological data from eight cases of histopathologically proven SFTs. The cytomorphologic features, immunochemical markers and differential diagnostic entities on fine needle aspiration cytology (FNAC) arediscussed.

RESULTS

FNA was performed at different anatomical sites. Cytology smears showed variable cellularity with tumor cells arranged in loose clusters and as singly scattered cells. Interlacing fascicles with palisading of cells was noted. The cells were predominantly spindle to elongated, having moderate cytoplasm with elongated wavy nuclei. These nuclei had fine to coarse chromatin, with inconspicuous to prominent nucleoli. There was prominent metachromatically staining, amorphous to fibrillary, collagenous to myxoid matrix materialassociated with tumor cells. Other findings included intranuclear pseudo-inclusions, multinucleated giant cells and atypical mitoses. Cytological diagnosis offered varied from 'spindle cell neoplasm' to 'spindle cell sarcoma' or 'suggestive of sarcoma'.Immunocytochemistry (ICC) done on cell block sections showed positivity for STAT6, CD34 and Bcl-2.

CONCLUSION

Cytological diagnosis of SFT can be challenging. A prudent search for characteristic cytomorphological features is of diagnostic help. The cytomorphology should be interpreted with caution with appropriate ICC panel including STAT6 and CD34.

CDX2 expression in malignant peripheral nerve sheath tumour: a potential diagnostic pitfall associated with PRC2 inactivation

AIMS

Malignant peripheral nerve sheath tumour (MPNST) is a soft tissue sarcoma that exhibits features of Schwann cell differentiation. Heterologous, often mesenchymal-type differentiation occurs in a subset of MPNST, while glandular morphology also is encountered in rare cases. We observed in MPNST unanticipated expression of CDX2, a transcription factor that regulates intestinal epithelial differentiation, and aimed to further characterize this phenomenon.

METHODS/RESULTS

Expression of CDX2 was assessed by immunohistochemistry in a total of 32 high-grade MPNSTs lacking morphological evidence of epithelial differentiation, including twelve tumours (38%) that developed in the setting of neurofibromatosis and four (13%) in the setting of prior radiation therapy. CDX2 was expressed by 14 of 32 MPNSTs (44%), wherein immunoreactivity, varying from weak to strong, was present in 2-95% of neoplastic spindle cells (median 10%, mean 23%). Notably, CDX2 expression was limited to tumours with PRC2 inactivation (22/32; 69%), as evidenced immunohistochemically by diffuse loss of trimethylated histone H3K27. Analysing publicly available RNA-sequencing data from twelve MPNST cell lines, two of which are clonally related, we observed CDX2 expression in all six PRC2-inactivated cell lines, while CDX2 expression was negligible in six cell lines with intact PRC2, amounting to a 58-fold increase in CDX2 expression on average with PRC2 inactivation.

CONCLUSIONS

CDX2 is expressed in a subset of MPNSTs, even in the absence of morphological evidence of epithelial differentiation. CDX2 expression in MPNST is strongly associated with underlying PRC2 inactivation.

Update from the 5th Edition of the World Health Organization Classification of Head and Neck Tumors: Soft Tissue Tumors

The fifth (5th) edition of the World Health Organization (WHO) Classification of Head and Neck Tumors introduces a new chapter dedicated to soft tissue neoplasms commonly affecting the head and neck. While the diversity, rarity, and wide anatomic range of soft tissue tumors precludes a discussion of all entities that may be found in the head and neck, the addition of this new chapter to the head and neck "blue book" aims to provide a more comprehensive and uniform reference text, including updated diagnostic criteria, of mesenchymal tumor types frequently (or exclusively) arising at head and neck sites. Since publication of the previous edition in 2017, there have been numerous advances in our understanding of the pathogenesis of many soft tissue tumors which have facilitated refinements in tumor classification, identification of novel entities, development of diagnostic markers, and improved prognostication. This review will provide a focused discussion of the soft tissue tumors included in the 5th edition WHO Head and Neck classification, with an emphasis on updates.

Undifferentiated and dedifferentiated soft tissue neoplasms: Immunohistochemical surrogates for differential diagnosis

Undifferentiated soft tissue sarcomas (USTS) are described in the current World Health Organization Classification of Soft Tissue and Bone Tumours as those showing no identifiable line of differentiation when analyzed by presently available technologies. This is a markedly heterogeneous group, and the diagnosis of USTS remains one of exclusion. USTS can be divided into four morphologic subgroups: pleomorphic, spindle cell, round cell and epithelioid undifferentiated sarcomas, with this combined group accounting for up to 20% of all soft tissue sarcomas. As molecular advances enable the stratification of emerging genetic subsets within USTS, particularly within undifferentiated round cell sarcomas, other groups, particularly the category of undifferentiated pleomorphic sarcomas (UPS), still remain difficult to substratify and represent heterogeneous collections of neoplasms often representing the common morphologic endpoints of a variety of malignant tumors of various (mesenchymal and non-mesenchymal) lineages. However, recent molecular developments have also enabled the identification and correct classification of many tumors from various lines of differentiation that would previously have been bracketed under 'UPS'. This includes pleomorphic neoplasms and dedifferentiated neoplasms (the latter typically manifesting with an undifferentiated pleomorphic morphology) of mesenchymal (e.g. solitary fibrous tumor and gastrointestinal stromal tumor) and non-mesenchymal (e.g. melanoma and carcinoma) origin. The precise categorization of 'pleomorphic' or 'undifferentiated' neoplasms is critical for prognostication, as, for example, dedifferentiated liposarcoma typically behaves less aggressively than other pleomorphic sarcomas, and for management, including the potential for targeted therapies based on underlying recurrent molecular features. In this review we focus on undifferentiated and dedifferentiated pleomorphic and spindle cell neoplasms, summarizing their key genetic, morphologic and immunophenotypic features in the routine diagnostic setting, and the use of immunohistochemistry in their principal differential diagnosis, and highlight new developments and entities in the group of undifferentiated and dedifferentiated soft tissue sarcomas.

Assessment of H3K27me3 immunohistochemistry and combination of NF1 and p16 deletions by fluorescence in situ hybridization in the differential diagnosis of malignant peripheral nerve sheath tumor and its histological mimics

Background

A definitive diagnosis of malignant peripheral nerve sheath tumor (MPNST) is challenging, especially in cases without neurofibromatosis 1 (NF1), because MPNST lacks specific markers on immunohistochemistry (IHC).

Methods

We performed IHC for histone 3 trimethylated on lysine 27 (H3K27me3) and evaluated the percentage of cells with H3K27me3 loss using measured values at 10% intervals, categorized as complete loss (100% of tumor cells lost staining), partial loss (10% to 90% of tumor cells lost staining), and intact (no tumor cells lost staining). We conducted fluorescence in situ hybridization (FISH) for NF1 and p16 deletions comparing 55 MPNSTs and 35 non-MPNSTs, consisting of 9 synovial sarcomas (SSs), 8 leiomyosarcomas (LMSs), 10 myxofibrosarcomas (MFSs), and 8 undifferentiated pleomorphic sarcomas (UPSs). We assessed the percentage of cells with homozygous and heterozygous deletions and defined “deletion” if the percentage of either the NF1 or p16 deletion signals was greater than 50% of tumor cells.

Results

Among the 55 MPNSTs, 23 (42%) showed complete H3K27me3 loss and 32 (58%) exhibited partial loss or intact. One each of the 9 SSs (11%), 8 LMSs (12%), and 8 UPSs (12%) showed complete H3K27me3 loss and many non-MPNSTs exhibited intact or partial H3K27me3 loss. Among the 55 MPNSTs, 33 (60%) and 44 (80%) showed NF1 or p16 deletion, respectively. Co-deletion of NF1 and p16 was observed in 29 (53%) MPNSTs. Among the 23 MPNTSs showing H3K27me3 complete loss, 18 (78%) and 20 (87%) exhibited NF1 or p16 deletion, respectively. Among the 32 MPNSTs with H3K27me3 partial loss or intact, 15 (47%) and 24 (75%) exhibited NF1 or p16 deletion, respectively. The frequency of NF1 and/or p16 deletion tended to be lower in non-MPNSTs than in MPNSTs. Approximately 90% of MPNSTs included cases with H3K27me3 complete loss and cases showing H3K27me3 partial loss or intact with NF1 and/or p16 deletion. Approximately 50% of MPNSTs showed co-deletion of NF1 and p16 regardless of H3K27me3 loss.

Conclusions

FISH for NF1 and p16 deletions, frequently observed in high-grade MPNSTs, might be a useful ancillary diagnostic tool for differentiating MPNST from other mimicking spindle cell and pleomorphic sarcomas.

Clinicopathological and prognostic significance of H3K27 methylation status in malignant peripheral nerve sheath tumor: correlation with skeletal muscle differentiation

Malignant peripheral nerve sheath tumor (MPNST) is a very aggressive peripheral nerve sheath-derived sarcoma, which is one of the most difficult tumors to diagnose due to its wide spectrum of histological findings and lack of specific immunohistochemical markers. Recently, it has been reported that losses of expression of H3K27me3 and H3K27me2 caused by PRC2 dysfunction may be useful diagnostic markers for MPNST, but there is no consensus on their clinicopathological significance. Here, we investigated the relationship between loss of H3K27 methylation and various parameters and clarified the clinicopathological significance of such loss. We analyzed the clinicopathological and immunohistochemical features in 84 MPNST cases. Complete losses of H3K27me3 and H3K27me2 were observed in 37 (44%) and 29 (35%) cases, respectively. Losses of H3K27me3 and H3K27me2 were significantly correlated with myogenic immunopositivity (H3K27me3 vs. desmin, P = 0.0051; H3K27me3 vs. myogenin, P = 0.0009; H3K27me2 vs. myogenin, P = 0.042). Meanwhile, there were significant correlations between preservation of immunohistochemical neurogenic markers and intact H3K27me3 and H3K27me2 (H3K27me3 vs. S-100 protein, P = 0.0019; H3K27me3 vs. SOX10, P = 0.014; H3K27me2 vs. S-100 protein, P = 0.0011; H3K27me2 vs. SOX10, P = 0.0087). In multivariate analysis, local recurrence, distant metastasis, high FNCLCC grade, and loss of SOX10 expression were independent prognostic factors for overall survival. H3K27me3 and H3K27me2 expression was retained in all 26 cases of rhabdomyosarcoma non-alveolar subtype. In conclusion, we suggest that H3K27me3 and H3K27me2 immunonegativity is useful but not definitive for diagnosing MPNST. Complete loss of H3K27 methylation may be involved in aggressive transdifferentiation from neural differentiation to skeletal muscle differentiation in MPNST.

Mesenchymal Neoplasms of Salivary Glands: A Clinicopathologic Study of 68 Cases

Salivary gland neoplasms are uncommon, and most exhibit epithelial differentiation. Mesenchymal neoplasms of the salivary gland are rare, and the incidence ranges from 1.9% to 5%. The aim of this study is to identify the types and clinical-pathological features of mesenchymal salivary neoplasm and review their differential diagnosis. A retrospective search for mesenchymal neoplasms of salivary glands from our institution's pathology archives from the 2004-2021 period and consultation files of one of the authors (AER) was performed. The clinical data were obtained from available medical records, and the histological slides and ancillary studies were retrieved and reviewed. We identified a total of 68 cases that form the study cohort. Thirty-five patients were male, and thirty-three patients were female, with a mean age of 48 years (range, 7 months-79 years), and the male to female ratio was 1:.94. Sixty-three (92.6%) of sixty-eight tumors were benign and included: 38 (56%) lipomas, 9 (13%) hemangiomas, 7 (10.3%) schwannomas, 3 (4.4%) neurofibromas, 3 (4.4%) lymphangioma, 2 (3%) solitary fibrous tumors, 1 (1.5%) myofibroma. Five of sixty-eight (7.4%) were malignant and included: 3 (4.4%) Adamantinoma-like Ewing sarcomas, 1 (1.5%) malignant peripheral nerve sheath tumor (MPNST), and 1 (1.5%) malignant solitary fibrous tumor. The involved sites included: parotid (55), submandibular gland (5), parapharyngeal space (5), buccal mucosa minor salivary gland (2), and sublingual gland (1). Sixty-seven patients underwent surgical resection. One patient with lymphangioma manifested a recurrence/persistence a week post-surgery. One patient with a parotid hemangioma developed post-operative numbness, and another patient developed chronic postauricular pain after surgery. Two patients with MPNST and one patient with adamantinoma-like Ewing sarcoma underwent neoadjuvant chemoradiation and were disease-free after treatment. The remaining 37 patients with available follow-up ranging from 7 days to 96 months (mean, 18 months) had a favorable outcome and were disease-free after treatment. Mesenchymal neoplasms of salivary gland are rare; most are benign and demonstrate adipocytic, endothelial, and schwannian differentiation; awareness of their development is important for adequate diagnosis. The mainstay of treatment is surgical excision, with the extent determined by tumor type. Adjuvant therapy is reserved for high-grade sarcomas and may be given in a neoadjuvant or adjuvant setting.

Review and update in the diagnosis of peripheral nerve sheath tumors

PURPOSE OF REVIEW

Although tumors with nerve sheath differentiation are vast, the main clinically significant problems faced by the pathologist are the separation of malignant peripheral nerve sheath tumors (MPNSTs) from histologic mimics, the diagnosis of neurofibromatous neoplasms with atypical features, and the separation of cutaneous neurofibromatous neoplasms from melanoma. This review briefly discusses a variety of common nerve sheath tumors and summarizes recent advances on these diagnostic fronts.

RECENT FINDINGS

Much of recent work has focused on abnormalities in polycomb repressive complex 2, and the ways in which these abnormalities may be exploited in the diagnosis of MPNSTs. Progress has been made in the diagnostic and clinical understanding of atypical neurofibromatous neoplasms and low-grade MPNSTs. A number of reports have explored the diagnostic distinction between cutaneous neurofibroma and melanoma.

SUMMARY

New discoveries show promise in the diagnosis of peripheral nerve sheath tumors, but challenges - old and new - remain.

Diagnostic Pathology of Tumors of Peripheral Nerve

Neoplasms of the peripheral nervous system represent a heterogenous group with a wide spectrum of morphological features and biological potential. They range from benign and curable by complete excision (schwannoma and soft tissue perineurioma) to benign but potentially aggressive at the local level (plexiform neurofibroma) to the highly malignant (malignant peripheral nerve sheath tumors [MPNST]). In this review, we discuss the diagnostic and pathologic features of common peripheral nerve sheath tumors, particularly those that may be encountered in the intracranial compartment or in the spine and paraspinal region. The discussion will cover schwannoma, neurofibroma, atypical neurofibromatous neoplasms of uncertain biological potential, intraneural and soft tissue perineurioma, hybrid nerve sheath tumors, MPNST, and the recently renamed enigmatic tumor, malignant melanotic nerve sheath tumor, formerly referred to as melanotic schwannoma. We also discuss the diagnostic relevance of these neoplasms to specific genetic and familial syndromes of nerve, including neurofibromatosis 1, neurofibromatosis 2, and schwannomatosis. In addition, we discuss updates in our understanding of the molecular alterations that represent key drivers of these neoplasms, including neurofibromatosis type 1 and type 2, SMARCB1, LZTR1, and PRKAR1A loss, as well as the acquisition of CDKN2A/B mutations and alterations in the polycomb repressor complex members (SUZ12 and EED) in the malignant progression to MPNST. In summary, this review covers practical aspects of pathologic diagnosis with updates relevant to neurosurgical practice.

Diagnostic Immunohistochemistry of Soft Tissue and Bone Tumors: An Update on Biomarkers That Correlate with Molecular Alterations

The diagnosis of benign and malignant soft tissue and bone neoplasms is a challenging area of surgical pathology, due to the large number, rarity, and histologic diversity of tumor types. In recent years, diagnosis and classification has been aided substantially by our growing understanding of recurrent molecular alterations in these neoplasms. Concurrently, the role of diagnostic immunohistochemistry has also expanded, with the development of numerous biomarkers based on underlying molecular events. Such biomarkers allow us to infer the presence of these events and can therefore substitute for other ancillary molecular genetic techniques (e.g., fluorescence in situ hybridization, polymerase chain reaction, and next-generation sequencing). In this review, we discuss a range of biomarkers currently available for these neoplasms, highlighting the accuracy, staining characteristics, and interpretation pitfalls of each antibody. These include immunohistochemical antibodies that represent reliable surrogates for the detection of gene fusions (e.g., STAT6, CAMTA1, FOSB, DDIT3) and more recently described breakpoint-specific antibodies (e.g., SS18-SSX, PAX3/7-FOXO1). Additionally, discussed are markers that correlate with the presence of gene amplifications (e.g., MDM2, CDK4), deletions (e.g., SMARCB1, SMARCA4), single nucleotide variants (e.g., G34W, K36M), aberrant methylation (H3K27me3), and increased expression as discovered through gene expression profiling (e.g., MUC4, DOG1, ETV4, NKX2.2, NKX3.1).

Genomic Landscape of Malignant Peripheral Nerve Sheath Tumor‒Like Melanoma

Malignant peripheral nerve sheath tumor (MPNST)-like melanoma is a rare malignancy with overlapping characteristics of both neural sarcoma and melanoma. Although the genomics of cutaneous melanoma has been extensively studied, those of MPNST-like melanoma have not. To characterize the genomic landscape of MPNST-like melanoma, we performed a single-center, retrospective cohort study at a tertiary academic cancer center. Consecutive patients with a confirmed histologic diagnosis of MPNST-like melanoma were screened, and those whose tissues were locally available were included in this analysis. Archival tissue from six patients (eight samples) was submitted for whole-exome and transcriptome sequencing analysis. We compared these data with available genomic studies of cutaneous melanoma and MPNST. NF1 was altered (mutated, deleted, or amplified) in 67% of patients. Genes related to cell cycle regulation were frequently altered, with frequent deletion of ZNF331, which, to the best of our knowledge, has not been previously described in cutaneous melanoma. The serine protease inhibitor SERPINB4 was deleted in 100% of the patients. We show that MPNST-like melanoma presents overlapping genomic features with cutaneous melanoma and MPNST, but it is unique by the frequency of loss of function of ZNF331 and SERPINB4.

Loss of H3K27me3 occurs in a large subset of embryonal rhabdomyosarcomas: Immunohistochemical and molecular analysis of 25 cases

Loss of histone 3 lysine 27 trimethylation (H3K27me3) has been described as a diagnostic marker for malignant peripheral nerve sheath tumor (MPNST), also discriminating MPNST with rhabdomyoblastic differentiation (malignant Triton tumor) from rhabdomyosarcoma (RMS). We studied the immunohistochemical expression of H3K27me3 in embryonal RMSs (ERMSs), performed methylation profiling in order to support the diagnosis and RNA-sequencing for comparison of the transcriptome of H3K27me3-positive and -negative cases. Of the 25 ERMS patients, 17 were males and 8 were females with an age range from 1 to 67 years (median, 6 years). None were known with neurofibromatosis type 1. One patient had Li-Fraumeni syndrome. Tumor localization included paratesticular (n = 9), genitourinary (n = 6), head/neck (n = 5), retroperitoneal (n = 4) and lower arm (n = 1). Five MPNSTs served as reference group. All ERMS had classical features including a variable spindle cell component. Immunohistochemical loss (partial or complete) of H3K27me3 was detected in 18/25 cases (72%). Based on methylation profiling, 22/22 cases were classified as ERMS. Using RNA sequencing, the ERMS group (n = 14) had a distinct gene expression profile in contrast to MPNSTs, confirming that the H3K27me3 negative ERMS cases do not represent malignant Triton tumors. When comparing H3K27me3-negative and -positive ERMSs, gene set enrichment analysis revealed differential expression of genes related to histone acetylation and normal muscle function with H3K27me3 negative ERMSs being associated with acetylation. Conclusion: Loss of H3K27me3 frequently occurs in ERMSs and correlates with H3K27 acetylation. H3K27me3 is not a suitable marker to differentiate ERMS (with spindle cell features) from malignant Triton tumor.

The 2020 WHO Classification: What's New in Soft Tissue Tumor Pathology?

The fifth edition of the World Health Organization Classification of Tumors of Soft Tissue and Bone was published in early 2020. The revisions reflect a consensus among an international expert editorial board composed of soft tissue and bone pathologists, geneticists, a medical oncologist, surgeon, and radiologist. The changes in the soft tissue tumor chapter notably include diverse, recently described tumor types (eg, atypical spindle cell/pleomorphic lipomatous tumor, angiofibroma of soft tissue, and CIC-rearranged sarcoma), new clinically significant prognostic information for a variety of existing entities (eg, dedifferentiated liposarcoma and solitary fibrous tumor), and a plethora of novel genetic alterations, some of practical diagnostic relevance (eg, NAB2-STAT6 in solitary fibrous tumor, FOSB rearrangements in epithelioid hemangioma and pseudomyogenic hemangioendothelioma, and SUZ12 or EED mutations in malignant peripheral nerve sheath tumor, leading to loss of H3K27 trimethylation). In this review, we highlight the major changes to the soft tissue chapter in the 2020 World Health Organization Classification, as well as the new chapter on undifferentiated small round cell sarcomas, with a focus on updates in diagnostic categories, prognostication, and novel markers. Recent discoveries in molecular genetics are also discussed, particularly those of immediate utility in differential diagnosis, including protein correlates detectable using immunohistochemistry.

Synovial Sarcoma of the Female Genital Tract: A Protean Mimic of Müllerian Neoplasia

Synovial sarcoma most commonly occurs in the extremities but has rarely been described in the female genital tract. In this series, we describe the clinical, morphologic, immunohistochemical, and molecular features of 7 cases of vulvovaginal synovial sarcoma (vulva, n=6; vagina, n=1). We emphasize their wide morphologic spectrum, which can overlap significantly with other more common tumors at these sites, as highlighted by 2 cases initially diagnosed as other entities (endometrioid carcinoma and malignant peripheral nerve sheath tumor). The average patient age was 41 (range: 23 to 62) years and tumor size ranged from 0.8 to 7 cm. Histologically, the tumors were biphasic (n=6) and monophasic (n=1). All cases were confirmed with fluorescence in situ hybridization or sequencing, and 5/5 cases were positive for the novel immunohistochemical markers SSX and SS18-SSX. In 3 cases with follow-up, 2 patients died of disease and 1 was alive with no evidence of disease. Previously described cases arising in the female genital tract are also reviewed. Vulvovaginal monophasic synovial sarcoma raises a broad differential diagnosis, including smooth muscle tumors, spindled carcinomas, and melanoma. Biphasic synovial sarcoma may mimic Müllerian carcinosarcoma, endometrioid carcinoma with spindled, corded, and hyalinized elements, and mesonephric-like adenocarcinoma. Awareness that synovial sarcoma can occur in the female genital tract with a wide variety of histologic appearances is critical for correctly diagnosing this rare entity. In particular, synovial sarcoma should be considered for any deeply situated "adenocarcinoma" in the vulva, with attention to subtle spindle cell differentiation.

Utility of histone H3K27me3 and H4K20me as diagnostic indicators of melanoma

Histone posttranslational modifications (PTMs) have been shown to be dysregulated in multiple cancers including melanoma, and as they are abundant and easily detectable, they make ideal biomarkers. The aim of this study was to identify histone PTMs that could be potential biomarkers for melanoma diagnosis. Previously, we utilized mass spectrometry to identify histone PTMs that were dysregulated in matched melanoma cell lines and found two modifications, H3 lysine 27 trimethylation (histone H3K27me3) and H4 lysine 20 monomethylation (histone H4K20me), that were differentially expressed in the more aggressive compared to the less aggressive cell line. In this study, we performed immunohistochemistry on tissue microarrays containing 100 patient tissue spots; 18 benign nevi, 62 primary, and 20 metastatic melanoma tissues. We stained for histone H3K27me3 and histone H4K20me to ascertain whether these histone PTMs could be used to distinguish different stages of melanoma. Loss of histone H4K20me was observed in 66% of malignant patient tissues compared to 14% of benign nevi. A majority (79%) of benign nevi had low histone H3K27me3 staining, while 72% of malignant patient tissues showed either a complete loss or had strong histone H3K27me3 staining. When we analyzed the staining for both marks together, we found that we could identify 71% of the benign nevi and 89% of malignant melanomas. Histone H3K27me3 or histone H4K20me display differential expression patterns that can be used to distinguish benign nevi from melanoma; however, when considered together the diagnostic utility of these PTMs increased significantly. The work presented supports the use of combination immunohistochemistry of histone PTMs to increase accuracy and confidence in the diagnosis of melanoma.

Diagnostic Utility of a Custom 34-Gene Anchored Multiplex PCR-Based Next-Generation Sequencing Fusion Panel for the Diagnosis of Bone and Soft Tissue Neoplasms With Identification of Novel USP6 Fusion Partners in Aneurysmal Bone Cysts

CONTEXT.—

Bone and soft tissue tumors are heterogeneous, diagnostically challenging, and often defined by gene fusions.

OBJECTIVE.—

To present our experience using a custom 34-gene targeted sequencing fusion panel.

DESIGN.—

Total nucleic acid extracted from formalin-fixed, paraffin-embedded (FFPE) tumor specimens was subjected to open-ended, nested anchored multiplex polymerase chain reaction and enrichment of 34 gene targets, thus enabling detection of known and novel fusion partners.

RESULTS.—

During a 12-month period, 147 patients were tested as part of routine clinical care. Tumor percentage ranged from 10% to 100% and turnaround time ranged from 3 to 15 (median, 7.9) days. The most common diagnostic groups were small round blue cell tumors, tumors of uncertain differentiation, fibroblastic/myofibroblastic tumors, and adipocytic tumors. In-frame fusion transcripts were identified in 64 of 142 cases sequenced (45%): in 62 cases, the detection of a disease-defining fusion confirmed the morphologic impression; in 2 cases, a germline TFG-GPR128 polymorphic fusion variant was detected. Several genes in the panel partnered with multiple fusion partners specific for different diagnoses, for example, EWSR1, NR4A3, FUS, NCOA2, and TFE3. Interesting examples are presented to highlight how fusion detection or lack thereof was instrumental in establishing accurate diagnoses. Novel fusion partners were detected for 2 cases of solid aneurysmal bone cysts (PTBP1-USP6, SLC38A2-USP6).

CONCLUSIONS.—

Multiplex detection of fusions in total nucleic acid purified from FFPE specimens facilitates diagnosis of bone and soft tissue tumors. This technology is particularly useful for morphologically challenging entities and in the absence of prior knowledge of fusion partners, and has the potential to discover novel fusion partners.

Radiation-associated sarcomas other than malignant peripheral nerve sheath tumours demonstrate loss of histone H3K27 trimethylation†

BACKGROUND AND AIMS

Complete loss of histone H3 lysine 27 trimethylation (H3K27me3) has recently emerged as a biomarker for malignant peripheral nerve sheath tumours (MPNST). Loss of H3K27me3 staining has also been reported in post-radiation MPNST; however, it has not been evaluated in a large series of radiation-associated sarcomas of different histological subtypes. The aim of this study was to assess H3K27me3 labelling by immunohistochemistry in radiation-associated sarcomas and to determine the prevalence of H3K27me3 loss in these tumours.

METHODS AND RESULTS

Radiation-associated sarcomas (n = 119) from two tertiary care referral centres were evaluated for loss of H3K27me3, defined as complete loss of staining within tumour cells in the presence of a positive internal control. Twenty-three cases (19%) showed H3K27me3 loss, including nine of 10 (90%) MPNST, seven of 77 (9%) undifferentiated spindle cell/pleomorphic sarcomas, five of 25 (20%) angiosarcomas, one of five (20%) leiomyosarcomas and one of two (50%) osteosarcomas.

CONCLUSIONS

Complete H3K27me3 loss was present in 19% of radiation-associated sarcomas in our series. Our findings demonstrate that loss of H3K27me3 is not specific for radiation-associated MPNST and may also occur in other histological subtypes of RAS, including radiation-associated undifferentiated spindle cell/pleomorphic sarcoma, angiosarcoma, leiomyosarcoma and osteosarcoma.

H3K27me3 expression and methylation status in histological variants of malignant peripheral nerve sheath tumours

Diagnosing MPNST can be challenging, but genetic alterations recently identified in polycomb repressive complex 2 (PRC2) core component genes, EED and SUZ12, resulting in global loss of the histone 3 lysine 27 trimethylation (H3K27me3) epigenetic mark, represent drivers of malignancy and a valuable diagnostic tool. However, the reported loss of H3K27me3 expression ranges from 35% to 84%. We show that advances in molecular pathology now allow many MPNST mimics to be classified confidently. We confirm that MPNSTs harbouring mutations in PRC2 core components are associated with loss of H3K27me3 expression; whole‐genome doubling was detected in 68%, and SSTR2 was amplified in 32% of MPNSTs. We demonstrate that loss of H3K27me3 expression occurs overall in 38% of MPNSTs, but is lost in 76% of histologically classical cases, whereas loss was detected in only 23% cases with heterologous elements and 14% where the diagnosis could not be provided on morphology alone. H3K27me3 loss is rarely seen in other high‐grade sarcomas and was not found to be associated with an inferior outcome in MPNST. We show that DNA methylation profiling distinguishes MPNST from its histological mimics, was unrelated to anatomical site, and formed two main clusters, MeGroups 4 and 5. MeGroup 4 represents classical MPNSTs lacking H3K27me3 expression in the majority of cases, whereas MeGroup 5 comprises MPNSTs exhibiting non‐classical histology and expressing H3K27me3 and cluster with undifferentiated sarcomas. The two MeGroups are distinguished by differentially methylated PRC2‐associated genes, the majority of which are hypermethylated in the promoter regions in MeGroup 4, indicating that the PRC2 target genes are not expressed in these tumours. The methylation profiles of MPNSTs with retention of H3K27me3 in MeGroups 4 and 5 are independent of mutations in PRC2 core components and the driver(s) in these groups remain to be identified. Our results open new avenues of investigation. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

BRAF mutation, TERT promoter mutation, and HER2 amplification in sporadic or neurofibromatosis-related neurofibromas and malignant peripheral nerve sheath tumors: do these molecules have a signature in malignant transformation?

Peripheral nerve sheath tumors may occur sporadically or related to neurofibromatosis (NF). Unless the mechanisms of tumorigenesis in NF related malignant peripheral nerve sheath tumors (MPNST) are better understood, it remained unclear in sporadic cases. We aimed to investigate the genetic route for malignancy in both individuals with NF-1 and sporadic ones to open a way for targeted therapies in the future. We investigated the role of HER2 with Dual ISH DNA Probe Cocktail test, BRAF mutation (exon 15) and TERT promoter mutation frequency with Sanger sequencing method in respectively 25 sporadic neurofibromas, 25 NF-1 related neurofibromas and 25 MPNST cases from two institutes. Categorical data were analyzed and summarized as frequency and percentage. Statistical analysis was done with SPSS v.22 statistical package, and the statistical significance level was considered as 0.05. We identified TERT promoter mutation only in one sporadic MPNST (4%) and no BRAF mutation in any case. HER2 amplification is found in 10/25 (40%) MPNST cases. No mutations or gene amplification detected in neurofibromas (p < 0.001). MPNSTs are sarcomas with poor prognosis and limited treatment options. TERT promoter mutations and HER2 amplification may play a putative role in therapeutic purposes.

An Algorithmic Immunohistochemical Approach to Define Tumor Type and Assign Site of Origin

Immunohistochemistry represents an indispensable complement to an epidemiology and morphology-driven approach to tumor diagnosis and site of origin assignment. This review reflects the state of my current practice, based on 15-years' experience in Pathology and a deep-dive into the literature, always striving to be better equipped to answer the age old questions, "What is it, and where is it from?" The tables and figures in this manuscript are the ones I "pull up on the computer" when I am teaching at the microscope and turn to myself when I am (frequently) stuck. This field is so exciting because I firmly believe that, through the application of next-generation immunohistochemistry, we can provide better answers than ever before. Specific topics covered in this review include (1) broad tumor classification and associated screening markers; (2) the role of cancer epidemiology in determining pretest probability; (3) broad-spectrum epithelial markers; (4) noncanonical expression of broad tumor class screening markers; (5) a morphologic pattern-based approach to poorly to undifferentiated malignant neoplasms; (6) a morphologic and immunohistochemical approach to define 4 main carcinoma types; (7) CK7/CK20 coordinate expression; (8) added value of semiquantitative immunohistochemical stain assessment; algorithmic immunohistochemical approaches to (9) "garden variety" adenocarcinomas presenting in the liver, (10) large polygonal cell adenocarcinomas, (11) the distinction of primary surface ovarian epithelial tumors with mucinous features from metastasis, (12) tumors presenting at alternative anatomic sites, (13) squamous cell carcinoma versus urothelial carcinoma, and neuroendocrine neoplasms, including (14) the distinction of pheochromocytoma/paraganglioma from well-differentiated neuroendocrine tumor, site of origin assignment in (15) well-differentiated neuroendocrine tumor and (16) poorly differentiated neuroendocrine carcinoma, and (17) the distinction of well-differentiated neuroendocrine tumor G3 from poorly differentiated neuroendocrine carcinoma; it concludes with (18) a discussion of diagnostic considerations in the broad-spectrum keratin/CD45/S-100-"triple-negative" neoplasm.

The Role of Polycomb Repressive Complex in Malignant Peripheral Nerve Sheath Tumor

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive soft tissue sarcomas that can arise most frequently in patients with neurofibromatosis type 1 (NF1). Despite an increasing understanding of the molecular mechanisms that underlie these tumors, there remains limited therapeutic options for this aggressive disease. One potentially critical finding is that a significant proportion of MPNSTs exhibit recurrent mutations in the genes EED or SUZ12, which are key components of the polycomb repressive complex 2 (PRC2). Tumors harboring these genetic lesions lose the marker of transcriptional repression, trimethylation of lysine residue 27 on histone H3 (H3K27me3) and have dysregulated oncogenic signaling. Given the recurrence of PRC2 alterations, intensive research efforts are now underway with a focus on detailing the epigenetic and transcriptomic consequences of PRC2 loss as well as development of novel therapeutic strategies for targeting these lesions. In this review article, we will summarize the recent findings of PRC2 in MPNST tumorigenesis, including highlighting the functions of PRC2 in normal Schwann cell development and nerve injury repair, as well as provide commentary on the potential therapeutic vulnerabilities of a PRC2 deficient tumor cell.

Soft Tissue Special Issue: Biphenotypic Sinonasal Sarcoma: A Review with Emphasis on Differential Diagnosis

Biphenotypic sinonasal sarcoma is an anatomically restricted low-grade malignant neoplasm with dual neural and myogenic differentiation composed of a monotonous population of spindled cells with herringbone/fascicular architecture. These tumors demonstrate a unique immunoprofile with relatively consistent S100-protein and actin expression in conjunction with more variable desmin, myogenin and myoD1 staining. SOX10 is uniformly negative. Genetically, the majority of tumors harbor PAX3-MAML3 fusions, with alternate PAX3 partners including FOXO1, NCOA1, NCOA2 and WWTR1. Although the differential diagnosis of BSNS is broad, careful morphologic inspection together with targeted ancillary studies is often sufficient to arrive at the correct diagnosis. As these tumors have significant local recurrence rates but lack metastatic potential, awareness and accurate diagnosis of this rare and newly described neoplasm is critical for appropriate management.

HYPERsol: High-Quality Data from Archival FFPE Tissue for Clinical Proteomics

Massive formalin-fixed, paraffin-embedded (FFPE) tissue archives exist worldwide, representing an invaluable resource for clinical proteomics research. However, current protocols for FFPE proteomics lack standardization, efficiency, reproducibility, and scalability. Here we present high-yield protein extraction and recovery by direct solubilization (HYPERsol), an optimized workflow using ultrasonication and S-Trap sample processing that enables proteome coverage and quantification from FFPE samples comparable to that achieved from flash-frozen tissue (average R = 0.936). When applied to archival samples, HYPERsol resulted in high-quality data from FFPE specimens in storage for up to 17 years, and may enable the discovery of new immunohistochemical markers.

Histone H3K27 dimethyl loss is highly specific for malignant peripheral nerve sheath tumor and distinguishes true PRC2 loss from isolated H3K27 trimethyl loss

Malignant peripheral nerve sheath tumors contain loss of histone H3K27 trimethylation (H3K27me3) due to driver mutations affecting the polycomb repressive complex 2 (PRC2). Consequently, loss of H3K27me3 staining has served as a diagnostic marker for this tumor type. However, recent reports demonstrate H3K27me3 loss in numerous other tumors, including some in the differential diagnosis of malignant peripheral nerve sheath tumor. Since these tumors lose H3K27me3 through mechanisms distinct from PRC2 loss, we set out to determine whether loss of dimethylation of H3K27, which is also catalyzed by PRC2, might be a more specific marker of PRC2 loss and malignant peripheral nerve sheath tumor. Using mass spectrometry, we identify a near complete loss of H3K27me2 in malignant peripheral nerve sheath tumors and cell lines. Immunohistochemical analysis of 72 malignant peripheral nerve sheath tumors, seven K27M-mutant gliomas, 43 ependymomas, and 10 Merkel cell carcinomas demonstrates that while H3K27me3 loss is common across these tumor types, H3K27me2 loss is limited to malignant peripheral nerve sheath tumors and is highly concordant with H3K27me3 loss (33/34 cases). Thus, increased specificity does not come at the cost of greatly reduced sensitivity. To further compare H3K27me2 and H3K27me3 immunohistochemistry, we investigated 42 melanomas and 54 synovial sarcomas, histologic mimics of malignant peripheral nerve sheath tumor with varying degrees of H3K27me3 loss in prior reports. While global H3K27me3 loss was not seen in these tumors, weak and limited H3K27me3 staining was common. By contrast, H3K27me2 staining was more clearly retained in all cases, making it a superior binary classifier. This was confirmed by digital image analysis of stained slides. Our findings indicate that H3K27me2 loss is highly specific for PRC2 loss and that PRC2 loss is a rarer phenomenon than H3K27me3 loss. Consequently, H3K27me2 loss is a superior diagnostic marker for malignant peripheral nerve sheath tumor.

Expression of PAX3 Distinguishes Biphenotypic Sinonasal Sarcoma From Histologic Mimics

Biphenotypic sinonasal sarcoma (BSNS) is a distinctive, anatomically restricted, low-grade spindle cell sarcoma that shows considerable histologic overlap with other cellular spindle cell neoplasms. This tumor type shows both myogenic and neural differentiation, which can be demonstrated by immunohistochemistry; however, the available diagnostic markers are relatively nonspecific. BSNS is characterized by PAX3 rearrangements, with MAML3 as the most common fusion partner. Our aim was to determine whether immunohistochemistry using a monoclonal PAX3 antibody could distinguish BSNS from potential histologic mimics, as well as to evaluate a widely available polyclonal PAX8 antibody, which is known to cross-react with other paired box transcription factor family members. Immunohistochemistry for PAX3 and PAX8 was performed on whole sections of 15 BSNS (10 with confirmed PAX3 rearrangement) and 10 cases each of the following histologic mimics: malignant peripheral nerve sheath tumor, monophasic synovial sarcoma, spindle cell rhabdomyosarcoma (RMS), solitary fibrous tumor, sinonasal hemangiopericytoma, and cellular schwannoma, as well as alveolar RMS (which harbors PAX3 or PAX7 gene rearrangements). BSNS showed consistent expression of PAX3 (15/15), all multifocal-to-diffuse and most with moderate-to-strong intensity of staining. One single case of spindle cell RMS showed PAX3 expression (1/10), and all other histologic mimics were completely PAX3-negative. In contrast, nuclear staining for PAX8 was present in all 15 BSNS, 7/10 malignant peripheral nerve sheath tumor, 3/10 cellular schwannomas, 2/10 sinonasal hemangiopericytomas, 1/10 synovial sarcoma, 1 spindle cell RMS, and 1 solitary fibrous tumor. All cases of alveolar RMS were positive for PAX8, and most were also positive for PAX3 (8/10). Immunohistochemical expression of PAX3 is highly sensitive (100%) and specific (98%) for BSNS. A polyclonal PAX8 antibody also stains BSNS (likely due to cross-reactivity with PAX3) but has much lower specificity (75%), with frequent expression in numerous mimics.

Very Unusual Sellar/Suprasellar Region Masses: A Review

The cause of sellar region masses in large retrospective series is overwhelmingly pituitary adenomas (84.6%), followed by craniopharyngiomas (3.2%), cystic nonneoplastic lesions (2.8%), inflammatory lesions (1.1%), meningiomas (0.94%), metastases (0.6%), and chordomas (0.5%) (1). While other rare lesions were also identified (collectively 6.0%), single unusual entities in the above-cited series numbered <1-2 examples each out of the 4122 cases, underscoring their rarity. We searched our joint files for rare, often singular, sellar/suprasellar masses that we had encountered over the past several decades in our own specialty, tertiary care specialty pituitary center practices. Cases for this review were subjectively selected for their challenging clinical and/or histological features as well as teaching value based on the senior authors' (MBSL, BKD) collective experience with over 7000 examples. We excluded entities deemed to be already well-appreciated by neuropathologists such as mixed adenoma-gangliocytoma, posterior pituitary tumors, metastases, and hypophysitis. We identified examples that, in our judgment, were sufficiently unusual enough to warrant further reporting. Herein, we present 3 diffuse large cell B cell pituitary lymphomas confined to the sellar region with first presentation at that site, 2 sarcomas primary to sella in nonirradiated patients, and 1 case each of granulomatosis with polyangiitis and neurosarcoidosis with first presentations as a sellar/suprasellar mass. Other cases included 1 of chronic lymphocytic leukemia within a gonadotroph adenoma and 1 of ectopic nerve fascicles embedded within a somatotroph adenoma, neither of which impacted patient care. Our objective was to share these examples and review the relevant literature.