Frequent PLAG1 gene rearrangements in skin and soft tissue myoepithelioma with ductal differentiation

Gene Fusion-Driven Cutaneous Adnexal Neoplasms: An Updated Review Emphasizing Molecular Characteristics

ABSTRACT

Gene rearrangements or fusions have emerged as critical oncogenic drivers in various cutaneous adnexal neoplasms. This review offers a comprehensive overview of both established and recently identified molecular alterations, with a specific focus on gene fusions. Key alterations discussed include YAP1 rearrangements, CRTC1::MAML2 fusions, BRD3 rearrangements, MYB::NFIB fusions, ETV6::NTRK3 fusions, and PLAG1 rearrangements, alongside rarer fusion transcripts, such as MEF2C::SS18 , FOXK1::GRHL1/2 , GPS2::GRHL , and RARA::NPEPPS . The article highlights the significance of these genetic changes in tumor biology and their potential therapeutic implications for locally advanced and metastatic skin adnexal tumors. It also addresses diagnostic challenges and molecular distinctions, providing updated insights into adnexal tumors driven by these gene fusions.

Comparative transcriptomics of salivary basal cell adenoma and adenocarcinoma sustain linear neoplastic evolution and intertumor heterogeneity: classification and biological implications

It remains uncertain whether basal cell adenoma (BCA) and basal cell adenocarcinoma (BCAC) of the salivary gland represent two distinct neoplasms or temporal stages of a single entity. The issue is central to reconciling their shared phenotypic resemblance and protracted behavior with current pathologic classification. We conducted a transcriptomic analysis on a cohort of both pathologic forms and correlated the findings with the clinicopathologic features using RNA extracted from fresh frozen samples of 25 salivary basal tumors (five BCAs and 20 BCACs) and eight instances of metastatic basal cell carcinomas (BCCs) to parotid glands. Unsupervised analysis revealed shared and intertumoral transcriptome differences within and between BCA and BCAC and distinct segregation from metastatic dermal BCC. Transcriptomic profiling delineated two intermixed subgroups of salivary basal cell neoplasms (SBNs); SBN-I group enriched with adverse pathologic features and SBN-II that lacked any of these features except for a single case. The category with the most instances of adverse pathologic features (SBN-I) manifested upregulations of transcriptional factors linked to cell proliferation pathways (HOXB13, SOX21, MYB, and EN1 genes), while those lacking adverse pathologic features (SBN-II) demonstrated a high expression of the TFAP2B transcription- and differentiation-related pathways. Our transcriptomic findings support common neoplastic evolution and intertumoral heterogeneity of both pathologic forms of basal cell neoplasms and identify molecular pathways of potential biological and clinical significance. We therefore propose a nondeterministic designation of 'basal cell salivary neoplasms, noninvasive (adenoma)/invasive (adenocarcinoma)' as a platform that integrates conventional phenotypic classification and transcriptomic characteristics pending a classification consensus. © 2025 The Pathological Society of Great Britain and Ireland.

Molecular analysis of apocrine mixed tumors and cutaneous myoepitheliomas: a comparative study confirming a continuous spectrum of one entity with near-ubiquitous PLAG1 and rare mutually exclusive HMGA2 gene rearrangements

Myoepithelial neoplasms of the skin and soft tissue still represent a confusing and somewhat controversial field in pathology as it appears that this category includes several different entities. However, recent studies have suggested that both apocrine mixed tumors (AMT) and cutaneous myoepitheliomas (CM) harbor identical chromosomal rearrangements involving the PLAG1 gene and hence may represent a morphological spectrum. The aim of the present study was to share our institutional experience with these tumors and specifically focus on studying their immunohistochemical and molecular features to further assess their relatedness. Eleven cases of AMT and 7 cases of CM were collected and analyzed using immunohistochemistry (IHC), PLAG1 FISH, and Archer FusionPlex assay. There were 14 male and 4 female patients with ages ranging from 26 to 85 years (median 55.8 years, mean 58.5 years). AMTs were mainly located in the head and neck (n = 10), while CMs were mainly located in the acral sites (n = 5). PLAG1 IHC was diffusely strongly positive in 14/17 (82%) cases, whereas a single case of AMT diffusely expressed HMGA2. Both tumor groups showed PLAG1 gene fusions which were detected in 6/13 analyzable samples (AMT, n = 4 and CM, n = 2), and included TRPS1::PLAG1 (n = 3), NDRG1::PLAG1 (n = 1), CTNNB1::PLAG1 (n = 1) and a novel PXDNL::PLAG1 fusion (n = 1). The remaining 5 cases were negative, 5 were not analyzable and the single case positive for HMGA2 by IHC revealed a potential HMGA2 gene rearrangement. The cases were further studied by FISH, with 12/17 cases showing PLAG1 gene rearrangement (AMT, n = 8 and CM, n = 4). Altogether, 14/18 cases showed PLAG1 gene rearrangement by at least one of the methods. PLAG1 immunohistochemistry had a 92% specificity and sensitivity. Our study provided additional data to suggest that AMT and CM share overlapping morphological and immunohistochemical features as well as molecular background characterized by PLAG1 gene fusions and thus represent a morphological spectrum. In addition, we identified a novel PXDNL::PLAG1 fusion and suggested that rare cases may harbor HMGA2 gene alterations which seem to be mutually exclusive with PLAG1 gene fusions. The relatedness of these tumors to salivary gland myoepithelial neoplasms and distinctness from eccrine mixed tumors and other skin and soft tissue myoepithelial neoplasms with EWSR1/FUS fusions is discussed.

[Cutaneous adnexal tumours: Development and synthesis of diagnostic fusion genes]

Cutaneous adnexal tumours are a heterogeneous group of epithelial lesions that includes tumours with follicular, sudoral and/or sebaceous differentiation, or even several combined lines of differentiation. Over the last few years, molecular analysis of these lesions has allowed to identify specific molecular events responsible for tumour development in an increasing number of tumour types. Like other rare neoplasms, such as soft tissue tumours, adnexal tumours display fusion genes resulting from chromosomal translocations that may be specific for the diagnosis if molecular data are properly integrated in the clinical and morphological setting. Molecular testing of adnexal tumours is valuable as it allows to strengthen the robustness of the diagnosis for a group of tumours displaying a wide morphological spectrum. It has allowed to refine the diagnostic criteria and to develop increasingly specific diagnostic immunostainings. Finally, molecular testing has been responsible for the identification of new entities or morphological subtypes of previously known entities. The aim of this review is to provide an update on cutaneous adnexal tumours associated with fusion genes and to evaluate the impact of molecular data on the diagnosis of these lesions.

What is new in fibroblastic/myofibroblastic tumors in children

Fibroblastic and myofibroblastic neoplasms represent about 12% of pediatric soft tissue tumors. Most of these neoplasms in children are either benign or locally aggressive with rare metastasis, while malignant cases are uncommon. Diagnosing these tumors is challenging due to overlapping morphologies and the limited utility of immunohistochemistry. Advances in molecular techniques, especially RNA sequencing, have improved our understanding of the molecular drivers of these tumors, leading to better classification. Key molecular alterations, such as RTK and MAPK activation, are central in the development of tumors like infantile fibrosarcoma (IFS) and inflammatory myofibroblastic tumors (IMT). The identification of alternative fusions in IFS and IMT underscores the importance of an integrated diagnostic approach. Furthermore, new RTK-driven lesions, now included in the WHO's "NTRK-rearranged mesenchymal neoplasms", have been identified. This review provides an update on recent findings in RTK-driven myofibroblastic tumors and highlights novel entities still in need of classification.

Exploring the molecular landscape of cutaneous mixed tumors characterized by TRPS1::PLAG1 gene fusion

The histological similarities between pleomorphic adenomas (PAs) and cutaneous mixed tumors (CMTs) found in certain facial regions can create a diagnostic challenge. Molecular findings reveal common genetic profiles, particularly PLAG1 rearrangements in both PA and CMT. Although molecular distinctions have received limited attention, our observations indicate multiple cases of CMTs carrying the TRPS1::PLAG1 fusion. This clinical experience has driven our investigation into the potential diagnostic utility of TRPS1::PLAG1 fusions for determining tumor origin. Two cohorts consisting of 46 cases of CMT and 45 cases of PA of the salivary glands were obtained from French institutions and reviewed by specialists in each subspecialty. RNA sequencing analysis was conducted to identify the molecular features of cases harboring PLAG1. Clinical, pathological, and molecular data were collected. In this study, cases of CMT exhibited recurrent gene fusions, primarily TRPS1::PLAG1 (74%). These tumors shared characteristic histological features, including tubuloductal differentiation in 55% of cases and squamous metaplasia in varying proportions. In contrast, cases of PA had gene fusions involving PLAG1 with various gene partners, with only one case in which TRPS1::PLAG1 was identified. This disparity was also observed at the transcriptomic level between TRPS1::PLAG1 CMTs and other tumors. However, TRPS1 immunostaining did not correlate with TRPS1::PLAG1 fusion. In conclusion, we report that recurrent TRPS1::PLAG1 fusion CMTs exhibit similar characteristic histological features, including tubuloductal differentiation that is associated with squamous metaplasia in around half of cases. Detection of this fusion could be valuable in correctly identifying the origin of these tumors. © 2024 The Pathological Society of Great Britain and Ireland.

Insights into the molecular alterations of PLAG1 and HMGA2 associated with malignant phenotype acquisition in pleomorphic adenoma

Pleomorphic adenoma (PA) is the most common neoplasm of the salivary gland, presenting with a variety of histological features. In some cases, PA can undergo malignant transformation to carcinoma ex pleomorphic adenoma (CXPA). The transition from PA to CXPA is associated with complex molecular alterations, particularly involving the pleomorphic adenoma gene 1 (PLAG1) and high mobility group protein gene (HMGA2). This review investigates the molecular alterations of PLAG1 and HMGA2 in all domains in the malignant transformation of PA. Our analysis highlights that these markers are key alterations in the etiopathogenesis of PA and CXPA, with gene fusion and amplification being frequently reported mechanisms. Although the exact role of PLAG1 and HMGA2 in the oncogenic process remains unclear, further studies on the HMGA2 and PLAG1, are needed particularly in HMGA2-PLAG1-IGF2 which is proving to be a potential pathway for the development of clinically applicable therapies, especially for CXPA management.

Novel HMGA2::COL14A1 Fusion Identified in Xanthogranulomatous Epithelial Tumor/Keratin-Positive Giant Cell Tumor

Xanthogranulomatous epithelial tumor (XGET)/Keratin-positive giant cell tumor (KP-GCT) represents a spectrum of recently described neoplasms characterized by a proliferation of distinctive mononuclear cells expressing keratin within a background of osteoclast-like giant cells, mixed inflammatory cells, and a variably prominent xanthogranulomatous component. Recent studies demonstrated a recurrent HMGA2::NCOR2 fusion in many cases. We herein describe a case of XGET/KP-GCT arising in the right femoral head of a 19-year-old male harboring a rare novel HMGA2::COL14A1 fusion.

Fibromyxoid aSoft Tissue Tumor With PLAG1 Fusion-The First Case in an Adult Patient

With the expanding possibilities of human genome research in recent years, the number of cases of soft tissue tumors that we are able to classify into the correct subgroups and to reveal their molecular profile is increasing. Among such tumors, we can also consider neoplasms that have a specific fusion of genes, in our case namely the pleomorphic adenoma gene 1 (PLAG1) and its partner. PLAG1 gene fusions were previously associated mainly with salivary gland pleomorphic adenomas, lipoblastomas, myoepithelial tumors, uterine epitheloid, myxoid leiomyosarcomas, and, recently, with PLAG1-rearranged fibromyxoid soft tissue tumors. To our knowledge, we report the first case of a soft tissue tumor with a PLAG1 fusion gene in an adult. In our case, we detected a new H3-3B::PLAG1 fusion in a soft tissue tumor, which originally appeared as nodular fasciitis.

PLAG1-Rearranged Uterine Sarcomas: A Study of 11 Cases Showing a Wide Phenotypical Spectrum Not Limited to Myxoid Leiomyosarcoma-Like Morphology

PLAG1 gene fusions were recently identified in a subset of uterine myxoid leiomyosarcomas (M-LMS). However, we have encountered cases of PLAG1-rearranged uterine sarcomas lacking M-LMS-like morphology and/or any expression of smooth muscle markers. To better characterize their clinicopathologic features, we performed a multiinstitutional search that yielded 11 cases. The patients ranged in age from 34 to 72 years (mean, 57 years). All tumors arose in the uterine corpus, ranging in size from 6.5 to 32 cm (mean, 15 cm). The most common stage at presentation was pT1b (n = 6), and 3 cases had stage pT1 (unspecified), and 1 case each presented in stages pT2a and pT3b. Most were treated only with hysterectomy and adnexectomy. The follow-up (range, 7-71 months; median, 39 months) was available for 7 patients. Three cases (7-21 months of follow-up) had no evidence of disease. Three of the 4 remaining patients died of disease within 55 to 71 months, while peritoneal spread developed in the last patient, and the patient was transferred for palliative care at 39 months. Morphologically, the tumors showed a high intertumoral and intratumoral heterogeneity. M-LMS-like and epithelioid leiomyosarcoma-like morphology were present in 3 and 5 primary tumors, respectively, the remaining mostly presented as nondescript ovoid or spindle cell sarcomas. Unusual morphologic findings included prominently hyalinized stroma (n = 3), adipocytic differentiation with areas mimicking myxoid liposarcoma (n = 2), osteosarcomatous differentiation (n = 1), and undifferentiated pleomorphic sarcoma-like areas (n = 1). The mitotic activity ranged from 3 to 24 mitoses per 10 high-power fields (mean, 9); 3 of 10 cases showed necrosis. In 3 of 11 cases, no expression of smooth muscle actin, h-caldesmon, or desmin was noted, whereas 5 of 5 cases expressed PLAG1. By RNA sequencing, the following fusion partners were identified: PUM1, CHCHD7 (each n = 2), C15orf29, CD44, MYOCD, FRMD6, PTK2, and TRPS1 (each n = 1). One case only showed PLAG1 gene break by fluorescence in situ hybridization. Our study documents a much broader morphologic spectrum of PLAG1-rearranged uterine sarcomas than previously reported, encompassing but not limited to M-LMS-like morphology with occasional heterologous (particularly adipocytic) differentiation. As it is currently difficult to precisely define their line of differentiation, for the time being, we suggest using a descriptive name "PLAG1-rearranged uterine sarcoma."

The Role of PLAG1 in Mouse Brain Development and Neurogenesis

The pleomorphic adenoma gene 1 (Plag1) is a transcription factor involved in the regulation of growth and cellular proliferation. Here, we report the spatial distribution and functional implications of PLAG1 expression in the adult mouse brain. We identified Plag1 promoter-dependent β-galactosidase expression in various brain structures, including the hippocampus, cortex, choroid plexus, subcommisural organ, ependymal cells lining the third ventricle, medial and lateral habenulae and amygdala. We noted striking spatial-restriction of PLAG1 within the cornu ammonis (CA1) region of the hippocampus and layer-specific cortical expression, with abundant expression noted in all layers except layer 5. Furthermore, our study delved into the role of PLAG1 in neurodevelopment, focusing on its impact on neural stem/progenitor cell proliferation. Loss of Plag1 resulted in reduced proliferation and decreased production of neocortical progenitors in vivo, although ex vivo neurosphere experiments revealed no cell-intrinsic defects in the proliferative or neurogenic capacity of Plag1-deficient neural progenitors. Lastly, we explored potential target genes of PLAG1 in the cortex, identifying that Neurogenin 2 (Ngn2) was significantly downregulated in Plag1-deficient mice. In summary, our study provides novel insights into the spatial distribution of PLAG1 expression in the adult mouse brain and its potential role in neurodevelopment. These findings expand our understanding of the functional significance of PLAG1 within the brain, with potential implications for neurodevelopmental disorders and therapeutic interventions.

Genetic Heterogeneity in Cellular Angiofibromas

BACKGROUND

Cellular angiofibroma, a rare benign mesenchymal neoplasm, is classified within the 13q/RB1 family of tumors due to morphological, immunohistochemical, and genetic similarities with spindle cell lipoma. Here, genetic data reveal pathogenetic heterogeneity in cellular angiofibroma.

METHODS

Three cellular angiofibromas were studied using G-banding/Karyotyping, array comparative genomic hybridization, RNA sequencing, and direct cycling sequencing.

RESULTS

The first tumor carried a del(13)(q12) together with heterozygous loss and minimal expression of the RB1 gene. Tumors two and three displayed chromosome 8 abnormalities associated with chimeras of the pleomorphic adenoma gene 1 (PLAG1). In tumor 2, the cathepsin B (CTSB) fused to PLAG1 (CTSB::PLAG1) while in tumor 3, the mir-99a-let-7c cluster host gene (MIR99AHG) fused to PLAG1 (MIR99AHG::PLAG1), both leading to elevated expression of PLAG1 and insulin growth factor 2.

CONCLUSION

This study uncovers two genetic pathways contributing to the pathogenetic heterogeneity within cellular angiofibromas. The first aligns with the 13q/RB1 family of tumors and the second involves PLAG1-chimeras. These findings highlight the diverse genetic landscape of cellular angiofibromas, providing insights into potential diagnostic strategies.

Fusion-driven cutaneous and superficial mesenchymal and adnexal tumors-A clinicopathologic and molecular study of 15 cases, including a novel case of ACTB::ZMIZ2-rearranged adnexal carcinoma

BACKGROUND

While the list of fusion-driven soft tissue neoplasms is expanding rapidly, their importance among cutaneous and superficial mesenchymal and adnexal neoplasms remains poorly understood. This challenge is especially evident in cases with ambiguous histopathology that are difficult to classify based on morphology.

AIMS

Our goal was to investigate the benefits of next-generation sequencing in diagnosing complex cutaneous neoplasms.

MATERIALS & METHODS

Departmental archives were searched for fusion-driven cutaneous neoplasms. Slides were retrieved and clinical information including follow-up was obtained.

RESULTS

Fifteen cases occurred in eight female and seven male patients, with a median age of 26 years (range: 1-83) at diagnosis. Tumors involved the extremities (9), scalp (5), and head and neck (1). Predominant features included myoepithelial (5), nested spindled with clear cytoplasm (2), atypical adnexal/squamoid (2), small round blue cell (2), cellular spindled (3), and fibrohistiocytic morphology (1). Most frequently encountered fusions involved EWSR1 (6) fused to ERG (1), FLI1 (1), CREB1 (2), CREM (1), PBX3 (1), followed by PLAG1 (4) with LIFR (2), TRPS1 (1) and CHCHD7. Additional fusions encountered were YAP1::NUTM1, EML4::ALK, SS18::SSX1 (2), and a novel fusion: ACTB::ZMIZ2. Integration of histologic features and molecular findings led to final diagnoses of primary cutaneous Ewing sarcoma (2), soft tissue myoepithelioma (4), cutaneous syncytial myoepithelioma (1), cutaneous adnexal carcinoma (1), porocarcinoma (1), inflammatory myofibroblastic tumor (1), synovial sarcoma (2), clear cell sarcoma (2), and angiomatoid fibrous histiocytoma (1).

DISCUSSION AND CONCLUSION

Our results show that fusion testing can be a helpful diagnostic tool, especially in cases with unusual or uncommon morphology in superficial sites. Furthermore, it can allow for the identification of potential therapeutic targets in some instances.

Myoepithelial tumors of soft tissue and bone in children and young adults: A clinicopathologic study of 40 cases occurring in patients ≤ 21 Years of age

Myoepithelial tumors of the soft tissue and bone occurring in patients 21 years of age and younger are rare, and their clinicopathologic features remain incompletely understood. We studied a well-characterized series of 40 such tumors. Cases were retrieved from our archives for the period 2009-2022 and re-reviewed. Available immunohistochemical and molecular genetic data was collected. Clinical information including available follow-up was obtained. The tumors occurred in 18 males and 22 females, ranging from 3 months to 21 years of age (median 11.5 years), and involved a wide variety of soft tissue (n = 36) and bone (n = 4) locations. Histologically benign myoepithelial tumors tended to occur in adolescents (median age 14.5 years; range 5-21 years), whereas myoepithelial carcinomas occurred in younger patients (median age 8.5 years; range 3 months-20 years). Microscopically, the tumors showed a complex admixture of epithelioid, plasmacytoid and spindled cells in a variably hyalinized, myxoid, chondroid or chondromyxoid background. Small subsets of histologically malignant tumors had rhabdoid or "round cell" features. Immunohistochemistry showed 35/40 (88%) cases to be positive with at least one keratin antibody. The 5 keratin-negative tumors were uniformly positive for S100 protein and/or SOX10 and expressed EMA (4 cases) and/or p63 (3 cases). EMA, SMA and GFAP were positive in 21/25 (84%), 13/21 (62%), and 8/21 (38%) tumors, respectively. SMARCB1 and SMARCA4 expression was retained in 29/31 (94%) and 22/22 (100%) of cases, respectively. FISH for EWSR1 gene rearrangement was positive in 6/18 (33%) tested cases. Two EWSR1-negative tumors were also FUS-negative. NGS identified EWSR1::POU5F1, FUS::KLF17, and BRD4::CITED1 gene fusions in 3 tested cases. Clinical follow-up (22 patients; median 23 months; range 1-119 months) showed 3 patients with local recurrences and 5 with distant metastases (lymph nodes, lung, and brain). Three patients died of disease, 3 were alive with recurrent or unresectable disease, and 16 were disease-free. Adverse clinical outcomes were seen only in patients with malignant tumors. We conclude that myoepithelial neoplasms of soft tissue and bone are over-repesented in patients ≤21 years of age, more often histologically malignant, and potentially lethal. Histologic evaluation appears to reliably predict the behavior of these rare tumors.

Calcifying Spindle Cell Soft Tissue Tumor With SOX10::PLAG1 Fusion: A Case Report of a Morphologically Distinctive and Potentially Novel Soft Tissue Tumor

The widespread use of advanced molecular techniques has led to the identification of several tumor types with PLAG1 gene fusions some of which also affect the skin and soft tissues. Herein, we present a 38-year-old female with a subcutaneous tumor affecting her forearm, which does not seem to fit into any currently recognized entity. It was a well-circumscribed tumor measuring 6 × 4,5 × 4 cm. It had a thick capsule composed of bland spindle cells forming palisades and Verocay body-like structures within a myxocollagenous background. Scattered calcifications were dispersed throughout the lesion. No cytological atypia, mitotic activity, or necrosis were present. Targeted NGS revealed a SOX10::PLAG1 fusion and fluorescent in situ hybridization confirmed the presence of PLAG1 gene rearrangement. The neoplastic cells showed a diffuse immunohistochemical expression of S100, SOX10, and PLAG1, as well as patchy desmin and CD34 positivity. The methylation profile of this tumor did not match any other entity covered by the DKFZ sarcoma classifier and apart from the gain of chromosome 12, the copy number profile was normal. The tumor was completely excised, and the patient has been free of disease for 4 years since the excision. While more cases are needed to confirm this tumor as a distinct entity, we propose a provisional name "SOX10::PLAG1-rearranged calcifying spindle cell tumor."

Characterization of a Molecularly Distinct Subset of Oncocytic Pleomorphic Adenomas/Myoepitheliomas Harboring Recurrent ZBTB47-AS1::PLAG1 Gene Fusion

Recurrent gene fusions are common in salivary gland tumors including benign tumors, such as pleomorphic adenoma (PA) and myoepithelioma (ME). In cases where chromosomal rearrangement is identified in the pleomorphic adenoma gene 1 (PLAG1) gene, different gene partners are found. Oncocytic metaplasia, characterized by oncocytes with abundant eosinophilic granular cytoplasm and hyperchromatic nuclei, is a well-known phenomenon in salivary gland neoplasms. However, the pure oncocytic variant of PA/ME showed PLAG1 gene rearrangements involving various gene partners at the molecular level, without any recurrent fusion being found. Our study includes 20 cases of PA/ME, with 11 females and 9 males. The age of patients ranged from 37 to 96 years, with a median age of 62.8 years. Most tumors originate from the parotid gland. The median size of the tumor was 26.5 mm (range: 13 to 60 mm). Among the 20 cases, 14 were a pure oncocytic variant of PA/ME, whereas 6 cases showed focal oncocytic or oncocytic-like aspects. Molecular studies on 20 cases of PA/ME were conducted. A novel recurrent ZBTB47-AS1::PLAG1 fusion was identified in 6 of 12 cases with pure oncocytic metaplasia, whereas the other cases had PLAG1 gene fusion with different gene partners. The transcriptomic analysis of the cases harboring ZBTB47-AS1::PLAG1 fusion demonstrated that these tumors have a distinct molecular profile from conventional PA/ME. This study reveals a unique subset in the oncocytic PA/ME spectrum characterized by pure oncocytic morphology with larger oncocytic cells and recurrent ZBTB47-AS1::PLAG1 fusion. It also highlights the transcriptomic distinctness of salivary gland adenomas with pure oncocytic metaplasia in the spectrum of salivary gland neoplasms. Further studies are needed to better understand the oncocytic variant of PA/ME and to determine the true nature of oncocytic cells in PA/ME.

Novel MIR143HG::PLAG1 gene fusion identified in a rectal myxoid leiomyosarcoma

Myxoid leiomyosarcoma (MLS) is a rare but well-documented tumor that often portends a poor prognosis compared to the conventional leiomyosarcoma. This rare sarcoma has been reported in the uterus, external female genitalia, soft tissue, and other locations. However, a definite rectal MLS has not been reported. Recently five cases of MLS were reported to harbor PLAG1 fusions (TRPS1::PLAG1, RAD51B::PLAG1, and TRIM13::PLAG1). In this report, we present a case of rectal MLS with a novel MIR143HG::PLAG1 fusion detected by RNA next-generation sequencing.

SOX10-Internal Tandem Duplications and PLAG1 or HMGA2 Fusions Segregate Eccrine-Type and Apocrine-Type Cutaneous Mixed Tumors

Cutaneous mixed tumors exhibit a wide morphologic diversity and are currently classified into apocrine and eccrine types based on their morphologic differentiation. Some cases of apocrine-type cutaneous mixed tumors (ACMT), namely, hyaline cell-rich apocrine cutaneous mixed tumors (HCR-ACMT) show a prominent or exclusive plasmacytoid myoepithelial component. Although recurrent fusions of PLAG1 have been observed in ACMT, the oncogenic driver of eccrine-type cutaneous mixed tumors (ECMT) is still unknown. The aim of the study was to provide a comprehensive morphologic, immunohistochemical, and molecular characterization of these tumors. Forty-one cases were included in this study: 28 cases of ACMT/HCR-ACMT and 13 cases of ECMT. After morphologic and immunohistochemical characterization, all specimens were analyzed by RNA sequencing. By immunohistochemistry, all cases showed expression of SOX10, but only ACMT/HCR-ACMT showed expression of PLAG1 and HMGA2. RNA sequencing confirmed the presence of recurrent fusion of PLAG1 or HMGA2 in all cases of ACMT/HCR-ACMT, with a perfect correlation with PLAG1/HMGA2 immunohistochemical status, and revealed internal tandem duplications of SOX10 (SOX10-ITD) in all cases of ECMT. Although TRPS1::PLAG1 was the most frequent fusion, HMGA2::WIF1 and HMGA2::NFIB were detected in ACMT cases. Clustering analysis based on gene expression profiling of 110 tumors, including numerous histotypes, showed that ECMT formed a distinct group compared with all other tumors. ACMT, HCR-ACMT, and salivary gland pleomorphic adenoma clustered together, whereas myoepithelioma with fusions of EWSR1, FUS, PBX1, PBX3, POU5F1, and KLF17 formed another cluster. Follow-up showed no evidence of disease in 23 cases across all 3 tumor types. In conclusion, our study demonstrated for the first time SOX10-ITD in ECMT and HMGA2 fusions in ACMT and further refined the prevalence of PLAG1 fusions in ACMT. Clustering analyses revealed the transcriptomic distance between these different tumors, especially in the heterogenous group of myoepitheliomas.

A novel IRF2BP2::CDX2 Gene fusion in digital intravascular myoepithelioma of soft tissue: An enigma!

Soft tissue myoepitheliomas (STM) are benign myoepithelial neoplasms (of nonsalivary gland origin) arising, most commonly within subcutaneous and deep soft tissues of the extremities and rarely within bones. To the best of our knowledge, the intravascular location of STM as well as the identification of a novel IRF2BP2::CDX2 fusion have not been previously reported. Herein, we report a case of spindle cell myoepithelioma arising within the intravascular space of the right index finger in a 52-year-old male of more than 20 years duration. Histopathology demonstrated an intravascular tumefactive lesion composed of predominantly plump banal spindle cells in a fascicular arrangement within a mixed collagenous and chondromyxoid stroma colliding with papillary endothelial hyperplasia (Masson tumor). By immunohistochemistry, the lesional cells were positive for keratin-AE1/3, epithelial membrane antigen, S100, SOX10, glial fibrillary acid protein, calponin and negative for CD34, smooth muscle actin, desmin, p63, and ERG. Fluorescence in situ hybridization for EWSR1 gene rearrangement was negative. Next-generation sequencing detected a novel IRF2BP2::CDX2 fusion involving Exon 1 of the IRF2BP2 gene and Exon 2 of the CDX2 gene confirmed by reverse transcriptase polymerase chain reaction and Sanger sequencing. Further, clinical evaluation for a salivary gland mass in the head and neck region and magnetic resonance imaging (MRI) of the chest, abdomen, and pelvis was performed with no evidence of tumor elsewhere. Taken together, the overall features were considered diagnostic of STM. Our current case underscores the novelty of the IRF2BP2::CDX2 gene fusion in STM and its exceptionally rare intravascular location.

Recurrent 8q11-13 Aberrations Leading to PLAG1 Rearrangements, Including Novel Chimeras HNRNPA2B1::PLAG1 and SDCBP::PLAG1, in Lipomatous Tumors

BACKGROUND/AIM

Structural abnormalities of chromosome bands 8q11-13, resulting in rearrangement of the pleomorphic adenoma gene 1 (PLAG1), are known to characterize lipoblastoma, a benign fat cell tumor, found mainly in children. Here, we describe 8q11-13 rearrangements and their molecular consequences on PLAG1 in 7 lipomatous tumors in adults.

MATERIALS AND METHODS

The patients were 5 males and 2 females between 23 and 62 years old. The tumors, namely five lipomas, one fibrolipoma and one spindle cell lipoma, were examined using G-banding with karyotyping, fluorescence in situ hybridization (FISH; three tumors), RNA sequencing, reverse transcription (RT) PCR, and Sanger sequencing analyses (two tumors).

RESULTS

All 7 tumors had karyotypic aberrations which included rearrangements of chromosome bands 8q11-13 (the criterion for selection into this study). FISH analyses with a PLAG1 break apart probe showed abnormal hybridization signals in both interphase nuclei and on metaphase spreads indicating PLAG1 rearrangement. RNA sequencing detected fusion between exon 1 of heterogeneous nuclear ribonucleoprotein A2/B1 (HNRNPA2B1) and exon 2 or 3 of PLAG1 in a lipoma and fusion between exon 2 of syndecan binding protein (SDCBP) and exon 2 or 3 of PLAG1 in a spindle cell lipoma. The HNRNPA2B1::PLAG1 and SDCBP::PLAG1 fusion transcripts were confirmed using RT-PCR/Sanger sequencing analyses.

CONCLUSION

As 8q11-13 aberrations/PLAG1-rearrangements/PLAG1-chimeras may evidently be a defining pathogenetic feature of lipogenic neoplasms of several histological types and not just lipoblastomas, we suggest that the term "8q11-13/PLAG1-rearranged lipomatous tumors" be generally adopted for this tumor subset.

Keratin-positive fibrotic extraskeletal myxoid chondrosarcoma: a close mimic of myoepithelial tumour

AIMS

Extraskeletal myxoid chondrosarcoma (EMC) is a rare form of adult sarcoma with distinct histology and NR4A3 gene fusion. Immunohistochemically, EMCs are variably positive for S100 protein and neuroendocrine markers. Unlike histologically similar soft-tissue myoepithelial tumours, keratin expression is rare. Prompted by two recent EMC cases with diffuse keratin expression, we investigated the expression of epithelial markers in a molecularly confirmed cohort of EMC and identified two additional similar cases.

METHODS AND RESULTS

Four keratin-positive EMCs occurred in one man and three women aged 46-59 years. All tumours displayed nonclassic histology with prominent stromal fibrosis, and keratin AE1/AE3 was expressed either diffusely (N = 2) or focally (N = 2). In one tumour, keratin expression was limited to the sclerotic area. All tumours coexpressed epithelial membrane antigen and two additionally expressed S100 protein or glial fibrillary acidic protein. All tumours harboured NR4A3 fusions, including TAF15::NR4A3 (N = 1) and EWSR1::NR4A3 (N = 3). Two cases were initially considered as most consistent with myoepithelial tumours based on widespread stromal fibrosis and keratin expression. DNA methylation analysis classified two tumours tested as EMCs.

CONCLUSIONS

We identified a small subset of EMCs characterised by keratin expression and prominent stromal fibrosis. This histological pattern must be recognised in the differential diagnosis of myoepithelial tumours because misclassification may lead to the erroneous prediction of tumour behaviour and may alter patient management. NR4A3 genetic analysis should be considered even in the face of keratin expression and prominent stromal fibrosis.

Translational Implications for Radiosensitizing Strategies in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood and adolescence that includes FP-RMS, harboring the fusion oncoprotein PAX3/7-FOXO1 and FN-RMS, often mutant in the RAS pathway. Risk stratifications of RMS patients determine different prognostic groups and related therapeutic treatment. Current multimodal therapeutic strategies involve surgery, chemotherapy (CHT) and radiotherapy (RT), but despite the deeper knowledge of response mechanisms underpinning CHT treatment and the technological improvements that characterize RT, local failures and recurrence frequently occur. This review sums up the RMS classification and the management of RMS patients, with special attention to RT treatment and possible radiosensitizing strategies for RMS tumors. Indeed, RMS radioresistance is a clinical problem and further studies aimed at dissecting radioresistant molecular mechanisms are needed to identify specific targets to hit, thus improving RT-induced cytotoxicity.

Pediatric fibromyxoid tumor with PLAG1 fusion: An emerging entity with a novel intracranial location

Translocations involving PLAG1 occur in several tumors, most commonly pleomorphic adenoma and lipoblastoma. Recently, a distinctive soft tissue tumor with a PLAG1 fusion has been reported in the pediatric age group. These are low grade tumors with a fibroblastic or mixed fibroblastic and myxoid morphology but no other lines of differentiation. They are typically immunopositive for desmin and CD34. The partner genes for these tumors have included YWHAZ, EEF1A1, ZFHX4l, CHCHD7, and PCMTD1. We report another case of this fibromyxoid tumor with a PLAG1 fusion, this time with COL3A1 as the partner gene. The fusion placed expression of a full-length PLAG1 protein under the control of the constitutively active COL3A1 promoter. Overexpression of PLAG1 was confirmed by diffusely positive immunostaining for PLAG1. The most novel aspect of this tumor is the intracranial location. Opinion has been divided over whether these tumors are a specific entity, or related to lipoblastoma, since that tumor also typically occurs in soft tissue in the pediatric age group and shows many of the same gene fusions. However, lipoblastoma has never been reported in an intracranial location and, thus, our case provides compelling evidence that this fibromyxoid tumor is indeed a distinct entity.

Recent Advances on Immunohistochemistry and Molecular Biology for the Diagnosis of Adnexal Sweat Gland Tumors

Simple Summary

Cutaneous sweat gland tumors form an extremely diverse and heterogeneous group of neoplasms that show histological differentiation to the sweat apparatus. Due to their rarity, wide diagnostic range, and significant morphological overlap between entities, their accurate diagnosis remains challenging for pathologists. Until recently, little was known about the molecular pathogenesis of adnexal tumors. Recent findings have revealed a wide range of gene fusions and other oncogenic factors that can be used for diagnostic purposes and, for some, can be detected by immunohistochemistry. Among other organs containing exocrine glands, such as salivary glands, breasts, and bronchi, most of these biomarkers have been reported in homologous neoplasms that share morphological features with their cutaneous counterparts. This review aims to describe these recent molecular and immunohistochemical biomarkers in the field of sweat gland tumors.

Abstract

Cutaneous sweat gland tumors are a subset of adnexal neoplasms that derive or differentiate into the sweat apparatus. Their great diversity, rarity, and complex terminology make their pathological diagnosis challenging. Recent findings have revealed a wide spectrum of oncogenic drivers, several of which are of diagnostic interest for pathologists. Most of these molecular alterations are represented by gene fusions, which are shared with other homologous neoplasms occurring in organs containing exocrine glands, such as salivary and breast glands, which show similarities to the sweat apparatus. This review aims to provide a synthesis of the most recent immunohistochemical and molecular markers used for the diagnosis of sweat gland tumors and to highlight their relationship with similar tumors in other organs. It will cover adenoid cystic carcinoma (NFIB, MYB, and MYBL1 fusion), cutaneous mixed tumor (PLAG1 fusion), cylindroma and spiradenoma and their carcinomas thereof (NF-κB activation through CYLD inactivation or ALKP1 hotspot mutation), hidradenoma and hidradenocarcinoma (MAML2 fusion), myoepithelioma (EWSR1 and FUS fusion), poroma and porocarcinoma (YAP1, MAML2, and NUTM1 fusion), secretory carcinoma (ETV6, NTRK3 fusion), tubular adenoma and syringo-cystadenoma papilliferum (HRAS and BRAF activating mutations). Sweat gland tumors for which there are no known molecular abnormalities will also be briefly discussed, as well as potential future developments.

Myoepithelioma-like tumour of the vulval region: A case report

Myoepithelioma-like tumours of the vulvar region (MELTVR) are a newly described group of spindle cell neoplasm. Morphologically, they consist of epithelioid to spindled cells in a myxoid to collagenous stroma and can resemble epithelioid sarcomas, myoepithelial carcinomas or extraskeletal myxoid chondrosarcomas. However, they have a distinct pattern of immunohistochemical staining characterised by positivity for EMA, ER, PR and negativity for cytokeratin, GFAP and S100. Nuclear staining for INI-1 is lost. In addition, they lack the characteristic gene arrangements of these other lesions. MELTVR can recur locally when incompletely excised, however, do not appear to metastasize. We report a case of a MELTVR in a 49-year-old woman arising in the right mons pubis.

Methylation profiling reveals novel molecular classes of rhabdomyosarcoma

Rhabdomyosarcomas (RMS) represent a family of aggressive soft tissue sarcomas that present in both children and adults. Pathologic risk stratification for RMS has been based on histologic subtype, with poor outcomes observed in alveolar rhabdomyosarcoma (ARMS) and the adult-type pleomorphic rhabdomyosarcoma (PRMS) compared to embryonal rhabdomyosarcoma (ERMS). Genomic sequencing studies have expanded the spectrum of RMS, with several new molecularly defined entities, including fusion-driven spindle cell/sclerosing rhabdomyosarcoma (SC/SRMS) and MYOD1-mutant SC/SRMS. Comprehensive genomic analysis has previously defined the mutational and copy number spectrum for the more common ERMS and ARMS and revealed corresponding methylation signatures. Comparatively, less is known about epigenetic correlates for the rare SC/SRMS or PRMS histologic subtypes. Herein, we present exome and RNA sequencing, copy number analysis, and methylation profiling of the largest cohort of molecularly characterized RMS samples to date. In addition to ARMS and ERMS, we identify two novel methylation subtypes, one having SC/SRMS histology and defined by MYOD1 p. L122R mutations and the other matching adult-type PRMS. Selected tumors from adolescent patients grouped with the PRMS methylation class, expanding the age range of these rare tumors. Limited follow-up data suggest that pediatric tumors with MYOD1-mutations are associated with an aggressive clinical course.

Coincidence of Intracranial Myoepithelioma and Adrenocortical Carcinoma in a Young Man

Myoepithelial tumors are rare neoplasms that develop from myoepithelial cells in glandular structures and soft tissues. Primary intracranial myoepithelial neoplasms are even rarer with around ten cases reported. On the other hand, adrenocortical carcinoma (ACC) is also uncommon with an annual incidence of 0.7–2 per million and carries a poor prognosis. It is known to have an association with certain familial cancer syndromes. Even in sporadic cases, a significant portion of them had other malignancies before and after diagnosis of ACC. We reported a 34-year-old gentleman who was diagnosed to have ACC without known familial cancer syndrome. After that, he was also found to have right occipital myoepithelioma that was confirmed by excisional biopsy. There was no known association between these two pathologies. This is the first report of coincidence of ACC and intracranial myoepithelioma.

Myoepithelial Carcinoma of Soft Tissue With an EWSR1-KLF15 Gene Fusion in an Infant

Overall, neonatal cancer is uncommon. Because of its rarity and heterogeneity, diagnosis can be challenging. We report a unique case of a myoepithelial carcinoma in a 7 week old girl. Molecular diagnostic workup revealed a EWSR1-KLF15 gene fusion which was previously described in only six cases of myoepithelial tumors so far. All cases occurred in children and adolescents. To our knowledge, this is the first report of a congenital EWSR1-KLF15 fusion positive myoepithelial tumor in an infant.

EWSR1-The Most Common Rearranged Gene in Soft Tissue Lesions, Which Also Occurs in Different Bone Lesions: An Updated Review

EWSR1 belongs to the FET family of RNA-binding proteins including also Fused in Sarcoma (FUS), and TATA-box binding protein Associated Factor 15 (TAF15). As consequence of the multifunctional role of EWSR1 leading to a high frequency of transcription of the chromosomal region where the gene is located, EWSR1 is exposed to aberrations such as rearrangements. Consecutive binding to other genes leads to chimeric proteins inducing oncogenesis. The other TET family members are homologous. With the advent of widely used modern molecular techniques during the last decades, it has become obvious that EWSR1 is involved in the development of diverse benign and malignant tumors with mesenchymal, neuroectodermal, and epithelial/myoepithelial features. As oncogenic transformation mediated by EWSR1-fusion proteins leads to such diverse tumor types, there must be a selection on the multipotent stem cell level. In this review, we will focus on the wide variety of soft tissue and bone entities, including benign and malignant lesions, harboring EWSR1 rearrangement. Fusion gene analysis is the diagnostic gold standard in most of these tumors. We present clinicopathologic, immunohistochemical, and molecular features and discuss differential diagnoses.

Current Diagnosis and Treatment Options for Cutaneous Adnexal Neoplasms with Apocrine and Eccrine Differentiation

Adnexal tumors of the skin are a rare group of benign and malignant neoplasms that exhibit morphological differentiation toward one or more of the adnexal epithelium types present in normal skin. Tumors deriving from apocrine or eccrine glands are highly heterogeneous and represent various histological entities. Macroscopic and dermatoscopic features of these tumors are unspecific; therefore, a specialized pathological examination is required to correctly diagnose patients. Limited treatment guidelines of adnexal tumor cases are available; thus, therapy is still challenging. Patients should be referred to high-volume skin cancer centers to receive an appropriate multidisciplinary treatment, affecting their outcome. The purpose of this review is to summarize currently available data on pathogenesis, diagnosis, and treatment approach for apocrine and eccrine tumors.

Novel morphologic findings in PLAG1-rearranged soft tissue tumors

Oncogenesis in PLAG1-rearranged tumors often results from PLAG1 transcription factor overexpression driven by promoter-swapping between constitutively expressed fusion partners. PLAG1-rearranged tumors demonstrate diverse morphologies. This study adds to this morphologic heterogeneity by introducing two tumors with PLAG1 rearrangements that display distinct histologic features. The first arose in the inguinal region of a 3-year-old, appeared well-circumscribed with a multinodular pattern, and harbored two fusions: ZFHX4-PLAG1 and CHCHD7-PLAG1. The second arose in the pelvic cavity of a 15-year-old girl, was extensively infiltrative and vascularized with an adipocytic component, and demonstrated a COL3A1-PLAG1 fusion. Both showed low-grade cytomorphology, scarce mitoses, no necrosis, and expression of CD34 and desmin. The ZFHX4-/CHCHD7-PLAG1-rearranged tumor showed no evidence of recurrence after 5 months. By contrast, the COL3A1-PLAG1-rearranged tumor quickly recurred following primary excision with positive margins; subsequent re-excision with adjuvant chemotherapy resulted in no evidence of recurrence after 2 years. While both tumors show overlap with benign and malignant fibroblastic and fibrovascular neoplasms, they also display divergent features. These cases highlight the importance of appropriate characterization in soft tissue tumors with unusual clinical and histologic characteristics.

Diagnostic discrepancy in second opinion reviews of primary epithelial neoplasms involving salivary gland: An 11-year experience from a tertiary referral center focusing on useful pathologic approaches and potential clinical impacts

AIMS

In the era of precision medicine, accurate pathologic diagnoses are crucial for appropriate management.

METHODS

We herein described the histologic features and clinical impacts of 66 salivary gland epithelial neoplasms in which the diagnosis was altered after expert review.

RESULTS

The most common revised diagnosis was that of salivary duct carcinoma (SDC, n = 12), adenoid cystic carcinoma (n = 12), and myoepithelial carcinoma (n = 10). The most common initial diagnosis was mucoepidermoid carcinoma (n = 19) with SDC being the most common revised diagnosis (7/19). Thirteen salivary gland carcinomas were initially diagnosed as benign entities, whereas five benign tumors were initially interpreted as carcinoma. The change in diagnosis was considered to be clinically significant in 65 (97%) cases.

CONCLUSIONS

Given their rarity, salivary gland neoplasms are prone to diagnostic inaccuracy and discrepancy. A constellation of histologic features and ancillary studies are useful in reaching the correct diagnosis, which can have significant clinical impacts.

Pediatric fibromyxoid soft tissue tumor with PLAG1 fusion: A novel entity?

The classification of undifferentiated soft tissue tumors continues to evolve with the expanded application of molecular analysis in clinical practice. We report three cases of a unique soft tissue tumor in young children (5 months to 2 years old) displaying a purely fibromyxoid histology, with positive staining for desmin and CD34. In two cases, RNA sequencing detected a YWHAZ-PLAG1 gene fusion, while in the third case, a previously unreported EEF1A1-PLAG1 fusion was identified. PLAG1 fusions have been reported in several pathologic entities including pleomorphic adenoma, myoepithelial tumors of skin and soft tissue, and lipoblastoma, the latter occurring preferentially in young children. In these tumors, expression of a full length PLAG1 protein comes under the control of the constitutively active promoter of the partner gene in the fusion, and the current cases conform to that model. Overexpression of PLAG1 was confirmed by diffusely positive immunostaining for PLAG1 in all three cases. Our findings raise the possibility of a novel fibromyxoid neoplasm in childhood associated with these rare PLAG1 fusion variants. The only other report of a PLAG1-YWHAZ fusion occurred in a pediatric tumor diagnosed as a "fibroblastic lipoblastoma." This finding raises the possibility of a relationship with our three cases, even though our cases lacked any fat component. Further studies with regard to a shared pathogenesis are required.

Poroid adnexal skin tumors with YAP1 fusions exhibit similar histopathologic features: A series of six YAP1-rearranged adnexal skin tumors

BACKGROUND

Adnexal skin tumors are diagnostically challenging with few known molecular signatures. Recently, however, YAP1-MAML2 and YAP1-NUTM1 fusions were identified in poroid adnexal skin tumors.

METHODS

Herein, we subjected eight poroid adnexal skin tumors (three poromas and five porocarcinomas) to fusion gene analysis by whole transcriptome sequencing and next-generation DNA sequencing analysis.

RESULTS

YAP1 fusions were identified in six cases. YAP1-NUTM1 fusions were identified in two poromas and three porocarcinomas. A single case of porocarcinoma harbored a YAP1-MAML2 fusion. Two cases were negative for gene fusion. All cases that harbored YAP1-NUTM1 fusions showed nuclear protein in testis (NUT) expression by immunohistochemistry, with NUT being negative in the YAP1-MAML2-positive case. In this case series, we provide a detailed histopathologic description of six YAP1-fused poroid skin tumors, which we show harbor reproducible histopathologic features, to include broad, bulbous tumor tongues with admixtures of basaloid, poroid cells punctuated by squamatized cuticles and ductules, with uniform tumor nuclei featuring frequent grooves and pseudonuclear inclusions.

CONCLUSIONS

Awareness of the characteristic histopathologic features of YAP1-fused poroid adnexal skin tumor is a step toward a more reproducible classification of adnexal skin tumors as well as a step toward targeted therapy for metastatic and/or unresectable examples of this poroid group of neoplasms.

An atypical chondroid syringoma with malignant degeneration: Utility of comparative genomic hybridization in confirming the diagnosis

Chondroid syringoma (CS) represents the cutaneous counterpart of mixed tumor (pleomorphic adenoma) of salivary glands. Definitive diagnosis is made on histopathology and is based on the presence of characteristic epithelial and stromal components. We report a case of an atypical CS arising on the extremity of an elderly male patient. Histomorphologic features of necrosis and cellular atypia raised suspicion for malignant degeneration, an exceptionally rare circumstance in this context. To further support the diagnosis of malignancy, array comparative genomic hybridization was performed from both low and higher grade areas of the tumor. Both regions demonstrated multiple copy number gains and losses, with additional loss of q7p (TP53), loss of 19p, and loss of heterozygosity on16q demonstrated in the more atypical foci. To our knowledge, this is the first case description of malignant degeneration of a CS with correlative microarray analysis. The findings in this case may prove useful in confirming the diagnosis in future ambiguous cases.

FOXO1 gene involvement in a non-rhabdomyosarcomatous neoplasm

Myoepithelial neoplasms of soft tissue are rare tumors with clinical, morphological, immunohistochemical, and genetic heterogeneity. The morphological spectrum of these tumors is broad, and the diagnosis often requires immunostaining to confirm myoepithelial differentiation. Rarely, tumors show a morphology that is typical for myoepithelial neoplasms, while the immunophenotype fails to confirm myoepithelial differentiation. For such lesions, the term "myoepithelioma-like" tumor was introduced. Recently, two cases of myoepithelioma-like tumors of the hands and one case of the foot were described with previously never reported OGT-FOXO gene fusions. Here, we report a 50-year-old woman, with a myoepithelial-like tumor localized in the soft tissue of the forearm and carrying a OGT-FOXO1 fusion gene. Our findings extend the spectrum of mesenchymal tumors involving members of the FOXO family of transcription factors and point to the existence of a family of soft tissue tumors that carry the gene fusion of the OGT-FOXO family.

Recent advances in smooth muscle tumors with PGR and PLAG1 gene fusions and myofibroblastic uterine neoplasms

Uterine epithelioid and myxoid leiomyosarcomas and inflammatory myofibroblastic tumors are rare mesenchymal neoplasms. Next-generation sequencing recently detected novel PGR fusions in uterine epithelioid leiomyosarcomas that demonstrate characteristic rhabdoid and spindled morphology. PLAG1 gene fusions have also been identified in a subset of myxoid leiomyosarcomas and are associated with PLAG1 overexpression. ALK rearrangements underpin the vast majority of uterine inflammatory myofibroblastic tumors, which demonstrate morphologic, and immunohistochemical features similar to those of inflammatory myofibroblastic tumors elsewhere. This review summarizes the morphologic, immunophenotypic, and molecular genetic features of PGR fusion-positive epithelioid leiomyosarcoma, PLAG1 fusion-positive myxoid leiomyosarcoma, and inflammatory myofibroblastic tumors of the uterus.

NDRG1-PLAG1 and TRPS1-PLAG1 Fusion Genes in Chondroid Syringoma

BACKGROUND/AIM

Chondroid syringoma is a rare benign tumor emanating from sweat glands. Although rearrangements of the pleomorphic adenoma gene 1 (PLAG1) have been reported in such tumors, information on PLAG1 fusion genes is very limited.

MATERIALS AND METHODS

Cytogenetic, fluorescence in situ hybridization, RNA sequencing, array comparative genomic hybridization, reverse transcription polymerase chain reaction, and Sanger sequencing analyses were performed on two chondroid syringoma cases.

RESULTS

Both tumors had structural rearrangements of chromosome 8. An NDRG1-PLAG1 transcript was found in the first tumor in which exon 3 of PLAG1 was fused with exon 1 of NDRG1. A TRPS1-PLAG1 chimeric transcript was detected in the second chondroid syringoma in which exon 2 or exon 3 of PLAG1 was fused with exon 1 of TRPS1.

CONCLUSION

The NDRG1-PLAG1 and TRPS1-PLAG1 resemble other PLAG1 fusion genes inasmuch as the expression of PLAG1 comes under the control of the NDRG1 or TRPS1 promoter.

Myoepithelioma of soft tissue and bone, and myoepithelioma-like tumors of the vulvar region: Clinicopathological study of 15 cases by PLAG1 immunohistochemistry

We demonstrated the clinicopathological findings of 13 myoepitheliomas of soft tissue and bone (MESTBs) and two myoepithelioma-like tumors of the vulvar region (MELTVRs), focusing on the association between nuclear atypia and clinical course, and the utility of immunohistochemistry (IHC) of pleomorphic adenoma gene 1 (PLAG1) for the pathological diagnosis of these tumors. Of the 13 MESTBs, eight, one and four cases exhibited mild, moderate and severe nuclear atypia, respectively. Two cases with venous invasion showed severe nuclear atypia and both died of advanced disease. Two MELTVR cases showed moderate nuclear atypia and had no evidence of disease after surgery. On IHC, 12 of 13 (92.3%) MESTBs showed PLAG1 immunoreactivity and none of the MELTVRs expressed PLAG1. In addition, MELTVRs showed loss of INI1 expression. In contrast, all MESTBs retained INI1 expression. Fluorescence in situ hybridization detected EWSR1, FUS and PLAG1 rearrangement in 5 (38.5%), 0 (0%) and 2 (15.4%) of the 13 MESTBs, respectively. No EWSR1, FUS and PLAG1 rearrangement were observed in the METLVRs. In conclusion, MESTBs with both severe nuclear atypia and venous invasion would be indicative of malignant potential. PLAG1 might be a useful IHC marker in MESTB diagnosis.

Cutaneous Syncytial Myoepithelioma Is Characterized by Recurrent EWSR1-PBX3 Fusions

Cutaneous syncytial myoepithelioma (CSM) is a rare but distinctive benign variant in the family of myoepithelial neoplasms of skin and soft tissue. CSM has unique morphologic and immunohistochemical features, characterized by intradermal syncytial growth of spindled, ovoid, and histiocytoid cells and consistent staining for S-100 protein and EMA, and differs from other myoepithelial tumors by showing only infrequent keratin staining. Rearrangement of the EWSR1 gene is now known to occur in up to half of all skin and soft tissue myoepithelial tumors, with a wide family of documented fusion partners. In 2013, we reported frequent (80%) EWSR1 rearrangements in CSM, but were unable to identify the fusion partner using available studies at that time. After recent identification of an index case of CSM harboring an EWSR1-PBX3 fusion, we used a combination of targeted RNA sequencing and fluorescence in situ hybridization (FISH) studies to investigate the genetic features of a cohort of CSM. An EWSR1-PBX3 fusion was identified in all 13 cases successfully tested. RNA sequencing was successful in 8/13 cases, all of which were found to have identical breakpoints fusing exon 8 of EWSR1 to exon 5 of PBX3. FISH confirmed both EWSR1 and PBX3 rearrangements in 9/9 cases tested, which included 4 confirmed to have EWSR1-PBX3 fusion by RNA-Seq, 3 cases that failed RNA-Seq, and 2 cases examined by FISH alone. Two cases failed RNA sequencing but had no additional tissue remaining for FISH studies. Our findings demonstrate that EWSR1-PBX3 fusions occur in most (and possibly all) cases of CSM.

A morphologic and molecular reappraisal of myoepithelial tumors of soft tissue, bone, and viscera with EWSR1 and FUS gene rearrangements

Myoepithelial tumors (MET) represent a clinicopathologically heterogeneous group of tumors, ranging from benign to highly aggressive lesions. Although MET arising in soft tissue, bone, or viscera share morphologic and immunophenotypic overlap with their salivary gland and cutaneous counterparts, there is still controversy regarding their genetic relationship. Half of MET of soft tissue and bone harbor EWSR1 or FUS related fusions, while MET arising in the salivary gland and skin often show PLAG1 and HMGA2 gene rearrangements. Regardless of the site of origin, the gold standard in diagnosing a MET relies on demonstrating its "myoepithelial immunophenotype" of positivity for EMA/CK and S100 protein or GFAP. However, the morphologic spectrum of MET in soft tissue and bone is quite broad and the above immunoprofile is nonspecific, being shared by other pathogenetically unrelated neoplasms. Moreover, rare MET lack a diagnostic immunoprofile but shows instead the characteristic gene fusions. In this study, we analyzed a large cohort of 66 MET with EWSR1 and FUS gene rearrangements spanning various clinical presentations, to better define their morphologic spectrum and establish relevant pathologic-molecular correlations. Genetic analysis was carried out by FISH for EWSR1/FUS rearrangements and potential partners, and/or by targeted RNA sequencing. Then, 82% showed EWSR1 rearrangement, while 18% had FUS abnormalities. EWSR1-POU5F1 occurred with predilection in malignant MET in children and young adults and these tumors had nested epithelioid morphology and clear cytoplasm. In contrast, EWSR1/FUS-PBX1/3 fusions were associated with benign and sclerotic spindle cell morphology. Tumors with EWSR1-KLF17 showed chordoma-like morphology. Our results demonstrate striking morphologic-molecular correlations in MET of bone, soft tissue and viscera, which might have implications in their clinical behavior.

Soft Tissue Special Issue: Myoepithelial Neoplasms of Soft Tissue: An Updated Review with Emphasis on Diagnostic Considerations in the Head and Neck

Primary myoepithelial neoplasms of soft tissue have been shown to be related to their salivary gland counterparts, with which they often share morphologic, immunophenotypic, and molecular genetic features, such as the presence of PLAG1 rearrangement in both soft tissue mixed tumor and salivary pleomorphic adenoma. However, important distinctions remain between soft tissue and salivary myoepithelial neoplasms, namely differing criteria for malignancy. This review provides an overview of the current understanding of the clinicopathologic and molecular features of soft tissue myoepithelial neoplasms, including discussion of the similarities and differences between soft tissue and salivary counterparts and relevant diagnostic issues specific to head and neck pathology practice.

Novel PLAG1 Gene Rearrangement Distinguishes a Subset of Uterine Myxoid Leiomyosarcoma From Other Uterine Myxoid Mesenchymal Tumors

Genetic alterations in uterine myxoid leiomyosarcoma are unknown. We investigate the clinicopathologic features of 19 uterine tumors previously diagnosed as myxoid leiomyosarcomas in which tumoral RNA was subjected to targeted RNA sequencing. PLAG1, BCOR, BCORL1, HMGA2, and ALK break-apart fluorescence in situ hybridization (FISH) and BCOR, PLAG1, and ALK immunohistochemistry were performed in cases which failed or lacked fusions by sequencing. The diagnosis of myxoid leiomyosarcoma was confirmed in 15 cases after exclusion of 4 tumors with BCOR and ALK rearrangements. These 15 patients presented at a median age of 50 years with stage I (3), II (2), III (2), and IV (1) tumors, respectively; stage was unknown in 7 cases. Tumor size ranged from 10 to 24 cm. Matrix was myxoid in all tumors and also eosinophilic in 2. Cells were spindled, epithelioid, and both in 10, 2, and 3 tumors and showed mild, moderate, and severe nuclear atypia in 3, 8, and 4 tumors, respectively. Mitotic index ranged from <1 to 14/10 HPF, while tumor necrosis was present in 6 (40%). Novel TRPS1-PLAG1 or RAD51B-PLAG1 fusions were detected by sequencing in 4 tumors, 3 of which were also confirmed by FISH. Diffuse PLAG1 expression was seen in 7 tumors, including 4 with PLAG1 rearrangement. No morphologic differences were seen among PLAG1 fusion-positive and fusion-negative tumors. No PLAG1, HMGA2, ALK, BCOR, or BCORL1 rearrangements were detected by FISH in 11 tumors. On the basis of sequencing and FISH results, PLAG1 rearrangements resulting in PLAG1 expression underpin ~25% of myxoid leiomyosarcomas and may serve as a useful diagnostic biomarker. Immunohistochemistry, targeted RNA sequencing, and/or FISH may distinguish myxoid leiomyosarcoma from its morphologic mimics.

Soft-Tissue Myoepithelioma of the Retroperitoneal Space Mimicking a Pancreatic Tumor: A Case Report and Literature Review

Soft-tissue myoepithelioma (STM) is an extremely rare benign tumor with predominant occurrence in head, neck, pelvic girdle and limbs. These tumors lack specific clinical and morphological features and can easily be confused with more common neoplasms. It may lead to incorrect diagnosis and management. Here, we present a clinical case of a young man with retroperitoneal STM which simulated a pancreatic tumor and required a distal pancreatectomy. Performed literature review highlights current data about clinical, morphologic, immunohistochemical and genetic evaluation, treatment and prognosis of STM.

Myoepithelioma of bone: ultrastructural, immunohistochemical and molecular study of three cases

Primary intraosseous myoepithelial tumors are rare neoplasms with only a handful of cases described in the medical literature. To date, intraosseous variant of benign myoepithelioma, due to its rarity, has not been studied ultrastructurally, and only one case of a malignant intraosseous myoepithelioma has been described. Three cases were retrieved from the files at the Massachusetts General Hospital (MGH). A diagnosis of benign myoepithelioma was made in case 1 and malignant epithelioma in cases 2 and 3. Ultrastructurally, intermediate filaments (without dense bodies) were found in each case with an abundance in case 1 and lesser amounts in cases 2 and 3. Surprisingly, cell junctions were not identified in case 1. However, they were found occasionally as intermediate junctions in case 2 and were easily identified as desmosome like junctions in case 3. The nucleus was irregular in the neoplastic cells of benign myoepithelioma which contrasted with cases 2 and 3 where the nuclei were oval yet had visible nucleoli. Herein, we add three new cases, including two new cases of malignant myoepithelioma. We also provide the first ultrastructural description of benign myoepithelioma of bone.

GLI1-amplifications expand the spectrum of soft tissue neoplasms defined by GLI1 gene fusions

GLI1 fusions involving ACTB, MALAT1, and PTCH1 genes have been recently reported in a subset of malignant soft tissue tumors with characteristic monomorphic nested epithelioid morphology and frequent S100 positivity. However, we encountered a group of morphologically similar soft tissue tumors lacking the canonical GLI1 gene fusions and sought to investigate their genetic abnormalities. A combined approach including RNA sequencing, targeted exome sequencing and FISH methodologies were used to identify potential novel genetic abnormalities. Ten patients (five females, five males) with an age range of 4-65 years (median 32.5) were identified. Tumors were located in the soft tissues of the limbs, trunk and head and neck, with one each in the tongue and lung. Histologically, tumors revealed ovoid to epithelioid cells arranged in a distinctive nested-trabecular pattern, separated by thin septa and a delicate vascular network. Two cases showed areas of increased nuclear pleomorphism and focal fascicular spindle cell growth. Four tumors showed a high mitotic count (≥15/10 HPFs), with necrosis seen in three of them. Lymphovascular invasion was noted in two cases. No consistent immunoprofile was detected, with positivity for CD56 (six cases), S100 (four cases), SMA (two cases), and pan-CK (one case). FISH showed GLI1 (12q13.3) gene amplification in all 10 cases, with co-amplification of CDK4 (12q14.1) in nine (90%) and MDM2 (12q15) in eight (80%) cases. Targeted exome sequencing performed in three cases confirmed the GLI1, CDK4, and MDM2 co-amplification. Only one case showed the presence of both GLI1 break-apart and amplification, although no gene partner was detected. Our findings suggest that GLI1 amplification, often associated with co-amplifications of CDK4 and MDM2 genes, may represent an alternative genetic mechanism of GLI1 oncogenic activation akin to GLI1 fusions, defining the pathogenesis of an emerging group of malignant soft tissue tumors with a distinctive nested growth pattern and variable immunoprofile.

Glandular Schwannoma: An Uncommon Variant of Schwannoma with Controversial Histogenesis

Schwannomas are benign, encapsulated nerve sheath tumors arising from Schwann cells; several histologic variants of schwannoma have been described, including the exceedingly rare glandular schwannoma (GS). The histogenesis of the glands in GS (as well as in other peripheral nerve sheath tumors with a glandular component) remains unclear; no consensus exists as to whether glands should be interpreted as metaplastic elements or entrapped reactive glands. We report a case of GS with detailed immunohistochemical analysis of the epithelial component. We believe that our findings raise further questions regarding the histogenesis of glands in GS, casting doubts on the traditional distinction between true metaplasia and glandular entrapment. Further research is warranted in this regard.

Diagnosis of known sarcoma fusions and novel fusion partners by targeted RNA sequencing with identification of a recurrent ACTB-FOSB fusion in pseudomyogenic hemangioendothelioma

Integration of morphological, immunohistochemical, and molecular methods is often necessary for the precise diagnosis and optimal clinical management of sarcomas. We have validated and implemented a clinical molecular diagnostic assay, MSK- Fusion Solid, for detection of gene fusions in solid tumors, including sarcomas. Starting with RNA extracted from formalin-fixed paraffin-embedded tumor material, this targeted RNA sequencing assay utilizes anchored multiplex PCR to detect oncogenic fusion transcripts involving 62 genes known to be recurrently rearranged in solid tumors including sarcomas without prior knowledge of fusion partners. From 1/2016 to 1/2018, 192 bone and soft tissue tumors were submitted for MSK- Fusion Solid analysis and 96% (184/192) successfully passed all the pre-sequencing quality control parameters and sequencing steps. These sarcomas encompass 24 major tumor types, including 175 soft tissue tumors and 9 osteosarcomas. Ewing and Ewing-like sarcomas, rhabdomyosarcoma, and sarcoma-not otherwise specified were the three most common tumor types. Diagnostic in-frame fusion transcripts were detected in 43% of cases, including 3% (6/184) with novel fusion partners, specifically TRPS1-PLAG1, VCP-TFE3, MYLK-BRAF, FUS-TFCP2, and ACTB-FOSB, the latter in two cases of pseudomyogenic hemangioendothelioma, representing a novel observation in this sarcoma. Our experience shows that this targeted RNA sequencing assay performs in a robust and sensitive fashion on RNA extracted from most routine clinical specimens of sarcomas thereby facilitating precise diagnosis and providing opportunities for novel fusion partner discovery.