Subungual melanoma with cartilaginous differentiation: Molecular insights.

Melanoma's rare capacity to undergo heterologous differentiation can create significant diagnostic challenges. The molecular mechanisms underlying this phenomenon are not well understood. We present an unusual case of subungual melanoma exhibiting extensive cartilaginous differentiation and provide insights into its molecular and cytogenomic features. Histopathologically, the tumor was predominantly composed of nodules of malignant cartilage in association with a smaller population of nested epithelioid to rhabdoid cells. Immunohistochemically, the tumor cells in both components were positive for S100, SOX10, and PRAME, and were negative for Melan-A and HMB-45. Molecular analysis by whole exome DNA sequence did not detect any pathogenic variants in genes commonly implicated in melanoma. Additional analysis by SNP chromosomal microarray revealed a complex genome characterized by numerous chromosomal losses and gains, including a homozygous deletion of the CDKN2A locus and a heterozygous deletion of the locus containing EXT2, a tumor suppressor implicated in hereditary multiple osteochondromas and secondary chondrosarcomas. This case underscores the importance of recognizing cartilaginous differentiation as a rare manifestation of melanoma, particularly at subungual sites, and suggests that at least some of these melanomas may be driven by non-canonical molecular pathways.

Amplification of Mutant NRAS in Melanocytic Tumors With Features of Spitz Tumors.

NRAS activating mutations are prevalent in melanocytic neoplasia, occurring in a subset of common acquired melanocytic nevi and ∼30% of cutaneous melanomas. In this study, we described a cohort of 7 distinctive melanocytic tumors characterized by activating point mutations in codon 61 of NRAS with amplification of the mutant NRAS allele and shared clinicopathologic features. These tumors occurred predominantly in younger patients, with a median age of 20 years (range, 6-56 years). They presented as papules on the helix of the ear (4 cases) or extremities (3 cases). Microscopically, the tumors were cellular, relatively well-circumscribed, compound, or intradermal proliferations. The tumor cells often extended into the deep reticular dermis and involved the superficial subcutaneous fat in some cases. The melanocytes were epithelioid to spindled with moderate amounts of cytoplasm and conspicuous nucleoli. They were arranged in short plexiform fascicles, nests, and cords. Some cases had occasional pleomorphic and multinucleated melanocytes. Rare dermal mitotic figures were present in all cases. The dermis contained thick collagen bundles and minimal solar elastosis. Follow-up data were available for 5 patients, with a median period of 4.2 years (range, 1-9 years), during which no recurrences or metastases were reported. Our series highlights a clinicopathologically and molecularly distinctive subset of NRAS-mutated tumors with amplification of the mutant NRAS allele.

Chondrolipoma of the Breast: A Myofibroblastoma Variant or a Distinct Lesion?

The entity commonly referred to as chondrolipoma is a rare and enigmatic breast lesion with unclear histogenesis and a complete lack of molecular characterization. It is uncertain whether it represents a hamartoma, choristoma, or a distinct neoplasm, including possibly a variant of mammary-type myofibroblastoma. We report two additional chondrolipomatous lesions of the breast. The lesions had varying histologic and immunohistochemical features similar to myofibroblastoma, including the loss of retinoblastoma (Rb) protein expression in one lesion. Molecular analysis by chromosomal microarray analysis performed on a second lesion did not demonstrate a loss of 13q14 or 16q typical of myofibroblastoma. Our findings further support the concept that at least a subset of breast lesions that historically have been classified as chondrolipoma are related to myofibroblastoma. However, the lack of myofibroblastoma-specific molecular alterations in one lesion suggests chondrolipomas may also have varying origins.

Atypical Spindle Cell/Pleomorphic Lipomatous Tumor With Sarcomatous Transformation: Clinicopathologic and Molecular Analysis of 4 Cases.

Atypical spindle cell/pleomorphic lipomatous tumor (ASPLT) is a recently described adipocytic tumor predominantly affecting the subcutaneous soft tissues of adults. Previous studies have shown that ASPLT follows a benign clinical course with a 4% to 12% local recurrence rate and no risk of dedifferentiation. Herein, we describe the clinicopathologic and molecular findings of 4 cases of ASPLT showing unequivocal sarcomatous transformation. Three patients were male and one was female, aged 65, 70, 74, and 78 years. Two cases presented as mass-forming lesions, while 1 case was incidentally discovered. The tumors measured 30, 55, 80, and 110 mm and occurred in the chest wall (n = 2) or arm (n = 2); all were subcutaneous. Microscopically, they showed a biphasic appearance comprising a low-grade ASPLT component and a high-grade sarcomatous component. The low-grade components showed features in the spectrum of either atypical pleomorphic lipomatous tumor (n = 2) or atypical spindle cell lipomatous tumor (n = 2). The high-grade components displayed leiomyosarcoma-like (n = 2), pleomorphic liposarcoma-like (n = 1) or undifferentiated sarcoma-like (n = 1) morphology. On immunohistochemistry, tumors were negative for MDM2 and showed loss of RB1 expression. In addition, the leiomyosarcoma-like areas seen in 2 cases were positive for smooth muscle actin and H-caldesmon. Single-nucleotide polymorphism array, performed in 3 cases, showed deletions of TP53, RB1, and flanking genes in both components. In contrast, the sarcomatous components showed more complex genomic profiles with rare segmental gains and recurrent loss of PTEN (n = 3), ATM (n = 2), and CDKN2A/B (n = 2) among other genes. Whole exome sequencing identified a TP53 variant in one case and an ATRX variant in another, each occurring in both tumor components. Limited clinical follow-up showed no recurrence or metastasis after 1 to 13 months (median, 7.5 months) postsurgical excision. Altogether, our data support that ASPLT can rarely develop sarcomatous transformation and offer insights into the molecular mechanisms underlying this event.

GLI1-Altered Mesenchymal Tumors.

GLI1-altered mesenchymal tumors comprise an emerging group of neoplasms characterized by fusions or amplifications involving GLI1, a gene that encodes a key regulator of the Hedgehog signaling pathway. In recent years, tumors with GLI1 alterations have been reported across a variety of anatomic sites and a broad age range. Although these tumors can exhibit a wide morphologic spectrum and a variable immunophenotype, they frequently present with monomorphic ovoid cells arranged in distinctive nests with a rich, arborizing vascular network. Recent evidence indicates that they have the potential to metastasize, which suggests that they may be best considered a sarcoma.

GLI1-Altered Mesenchymal Tumors With ACTB or PTCH1 Fusion: A Molecular and Clinicopathologic Analysis.

Mesenchymal tumors with GLI1 fusions or amplifications have recently emerged as a distinctive group of neoplasms. The terms GLI1-altered mesenchymal tumor or GLI1-altered soft tissue tumor serve as a nosological category, although the exact boundaries/criteria require further elucidation. We examined 16 tumors affecting predominantly adults (median age: 40 years), without sex predilection. Several patients had tumors of longstanding duration (>10 years). The most common primary site was soft tissue (n = 9); other sites included epidural tissue (n = 1), vertebra (n = 1), tongue (n = 1), hard palate (n = 1), and liver (n = 1). Histologically, the tumors demonstrated multinodular growth of cytologically uniform, ovoid-to-epithelioid, occasionally short spindled cells with delicate intratumoral vasculature and frequent myxoid stroma. Mitotic activity ranged from 0 to 8 mitoses/2 mm2 (mean 2). Lymphovascular invasion/protrusion of tumor cells into endothelial-lined vascular spaces was present or suspected in 6 cases. Necrosis, significant nuclear pleomorphism, or well-developed, fascicular spindle-cell growth were absent. Half demonstrated features of the newly proposed subset, "distinctive nested glomoid neoplasm." Tumors were consistently positive for CD56 (n = 5/5). A subset was stained with S100 protein (n = 7/13), SMA (n = 6/13), keratin (n = 2/9), EMA (n = 3/7), and CD99 (n = 2/6). Tumors harbored ACTB::GLI1 (n = 15) or PTCH1::GLI1 (n = 1) fusions. The assays used did not capture cases defined by GLI1 amplification. We also identified recurrent cytogenetic gains (1q, 5, 7, 8, 12, 12q13.2-ter, 21, and X). For patients with available clinical follow-up (n = 8), half were disease free. Half demonstrated distant metastases (lungs, bone, or soft tissue). Of cases without follow-up (n = 8), 2 were known recurrences, and 1 was presumed metastasis. Our results imply a more aggressive biological potential than currently reported. Given the possibility for metastasis and disease progression, even in cytologically bland, nested tumors, close clinical surveillance, akin to that for sarcoma management, may be indicated. The term GLI1-altered mesenchymal tumor with malignant potential is proposed.

A novel method to assess copy number variations in melanocytic neoplasms: Droplet digital PCR for precise quantitation of MYC and MYB genes.

INTRODUCTION: While most melanocytic neoplasms can be classified as either benign or malignant by histopathology alone, ancillary molecular diagnostic tests can be necessary to establish the correct diagnosis in challenging cases. Currently, the detection of copy number variations (CNVs) by fluorescence in situ hybridization and chromosomal microarray (CMA) are the most popular methods, but remain expensive and inaccessible. We aim to develop a relatively inexpensive, fast, and accessible molecular assay to detect CNVs relevant to melanoma using droplet digital polymerase chain reaction (ddPCR) technology. METHODS: In this proof-of-concept study, we evaluated CNVs in MYC and MYB genes from 73 cases of benign nevi, borderline melanocytic lesions, and primary and metastatic melanoma at our institution from 2015 to 2022. A multiplexed ddPCR assay and CMA were performed on each sample, and the results were compared. RESULTS: Concordance analysis of ddPCR with CMA for quantification of MYC and MYB CNVs revealed a sensitivity and specificity of 89% and 86% for MYC and 83% and 74% for MYB, respectively. CONCLUSION: We demonstrate the first use of a multiplexed ddPCR assay to identify CNVs in melanocytic neoplasms. With further improvement and validation, ddPCR may represent a low-cost and rapid tool to aid in the diagnosis of histopathologically ambiguous melanocytic tumors.

Anti-PL-7, anti-Ro/SSA, anti-Mi-2, and anti-TIF1-γ correlate with specific patterns of histopathologic features in dermatomyositis: An analysis of 39 skin biopsy specimens from 25 patients.

BACKGROUND: In dermatomyositis (DM), myositis-specific and myositis-associated antibodies have been correlated with clinical features. It is unknown if histopathologic findings in lesional skin biopsies correlate with serologic subtypes of DM. METHODS: A retrospective chart review of patients with DM was performed. Patients with myositis antibodies and DM lesional skin biopsies were included in the study. Skin biopsies were reviewed by blinded dermatopathologists for 20 histopathologic features. RESULTS: There was a statistically significant (p < 0.05) association between anti-PL-7 serology and decreased degree of vacuolar degeneration, necrotic keratinocytes, and thickening of the epidermal basement membrane. Anti-aminoacyl tRNA synthetase (anti-ARS) antibodies had the same significant negative association with degree of vacuolar degeneration, necrotic keratinocytes, and thickening of the epidermal basement membrane. A similar pattern was seen with an anti-cytoplasmic serology; where there was a significant association with an increased degree of vacuolar degeneration and necrotic keratinocytes, and a nonsignificant trend of minimally thickened epidermal basement membrane. There was a statistically significant association between anti-Ro/SSA serology and increased degree of vacuolar degeneration. Anti-TIF1-γ serology was significantly associated with the increased presence of necrotic keratinocytes and pigment incontinence, and displayed a pattern of increased neutrophils. There was a significant association between anti-Mi-2 antibodies and pigment incontinence, as well as between myositis-specific antibodies and pigment incontinence. A statistically significant positive association was found between nuclear antibodies and degree of vacuolar degeneration, thickened epidermal basement membrane, pigment incontinence, and epidermal atrophy. CONCLUSION: In patients with DM, some specific serotypes, including anti-PL-7, anti-Ro/SSA, anti-Mi-2, and anti-TIF1-γ, may have characteristic histopathologic features.

Low-grade fibromyxoid sarcoma of the breast: genetic characterization and immunohistochemical comparison to morphologic mimics.

Spindle cell lesions of the breast elicit a specific, relatively limited differential diagnosis, and accurate classification often requires careful morphologic evaluation and immunohistochemical workup. Low-grade fibromyxoid sarcoma (LGFMS) is a rare malignant fibroblastic tumor with deceptively bland spindle cell morphology. Involvement of the breast is exceedingly rare. We examined the clinicopathologic and molecular characteristics of three cases of breast/axillary LGFMS. In addition, we interrogated the immunohistochemical expression of MUC4, a commonly used marker of LGFMS, in other breast spindle cell lesions. LGFMS presented in women at 23, 33, and 59 years of age. Tumor size ranged from 0.9 to 4.7 cm. Microscopically, they were circumscribed nodular masses composed of bland spindle cells with fibromyxoid stroma. Immunohistochemically, tumors were diffusely positive for MUC4 and negative for keratin, CD34, S100 protein, and nuclear beta-catenin. Fluorescence in-situ hybridization demonstrated FUS (n = 2) or EWSR1 (n = 1) rearrangements. Next-generation sequencing identified FUS::CREB3L2 and EWSR1::CREB3L1 fusions. MUC4 immunohistochemistry performed on 162 additional breast lesions demonstrated only weak and limited expression in a subset of cases of fibromatosis (10/20, ≤30% staining), scar (5/9, ≤10%), metaplastic carcinoma (4/23, ≤5%), and phyllodes tumor (3/74, ≤10%). MUC4 was entirely negative in cases of pseudoangiomatous stromal hyperplasia (n = 9), myofibroblastoma (n = 6), periductal stromal tumor (n = 3), and cellular/juvenile fibroadenoma (n = 21). LGFMS can rarely occur in the breast and should be considered in the differential diagnosis of breast spindle cell lesions. Strong and diffuse MUC4 expression is highly specific in this histologic context. Detection of an FUS or EWSR1 rearrangement can confirm the diagnosis.

PRAME expression in cutaneous melanoma does not correlate with disease-specific survival.

BACKGROUND: Immunohistochemistry-based protein biomarkers can provide useful prognostic information in cutaneous melanoma. The independent prognostic value of Ki-67 has been studied with variable results. PReferentially expressed Antigen in MElanoma (PRAME) immunohistochemistry is a useful new ancillary tool for distinguishing cutaneous nevi from melanoma; however, its prognostic value has not been well studied. We evaluated PRAME as a prognostic marker in cutaneous melanoma, compared to Ki-67. METHODS: We analyzed the immunohistochemical expression of PRAME and Ki-67 in 165 melanocytic lesions, including 92 primary melanomas, 19 metastatic melanomas, and 54 melanocytic nevi using tissue microarrays. PRAME immunostaining was scored based on the percentage of positive nuclei: 0 <1%, 1+ 1%-25%, 2+ 26%-50%, 3+ 51%-75%, and 4+ >75%. The percentage of Ki-67-positive tumor nuclei was used to calculate the proliferation index. RESULTS: PRAME and Ki-67 both showed significantly increased expression in melanomas compared to nevi (p < 0.0001 and p < 0.001, respectively). There was no significant difference in PRAME expression in primary versus metastatic melanomas. By contrast, the Ki-67 proliferation index was higher in metastatic melanoma than in primary melanoma (p = 0.013). Increased Ki-67 index correlated with ulceration (p < 0.001), increased Breslow depth (p = 0.001), and higher mitotic rate (p < 0.0001), whereas increased PRAME expression correlated with higher mitotic rate (p = 0.047) and Ki-67 index (p = 0.007). Increased Ki-67 index correlated with worse disease-specific survival in patients with primary melanoma (p < 0.001), but PRAME expression did not show prognostic significance in disease-specific survival (p = 0.63). In a multivariable analysis of patients with primary melanoma, tumor Breslow depth, ulceration, mitotic rate, and Ki-67 index were each independent predictors of disease-specific survival (p = 0.006, 0.02, 0.001, and 0.04, respectively); however, PRAME expression was not predictive of disease-specific survival (p = 0.64). CONCLUSION: Ki-67 is an independent prognostic marker; although increased PRAME expression correlates with the Ki-67 proliferation index and mitotic rate, PRAME is not an independent prognostic marker for cutaneous melanoma. PRAME and Ki-67 are useful ancillary tools for distinguishing benign from malignant melanocytic lesions.

A Subset of Thoracic SMARCA4-Deficient Undifferentiated Tumors Express GATA3.

Thoracic SMARCA4-deficient undifferentiated tumor (SMARCA4-UT) is a rare and highly aggressive malignant neoplasm characterized by high-grade undifferentiated morphologic features and recurrent inactivating mutations of SMARCA4. These tumors consistently exhibit loss of SMARCA4 (BRG1) while displaying variable expression of other nonspecific markers. Recently, we encountered a SMARCA4-UT demonstrating immunoreactivity for GATA3, and we sought to characterize this phenomenon in a larger series.A total of nine SMARCA4-UTs were examined from 3 large academic institutions. The clinicopathologic and molecular characteristics were studied and GATA3 immunohistochemistry was performed.The cohort included 5 male and 4 female patients, with a median age of 54 years and a median smoking history of 37 pack-years. At initial diagnosis, mediastinal lymph node involvement was observed in 5 patients (56%) while distant metastases were present in 7 patients (78%). The median survival was 6 months. Histologically, the tumors were characterized by sheets of undifferentiated epithelioid and/or rhabdoid cells, accompanied by frequent mitotic figures and necrosis. Immunohistochemically, all tumors displayed a complete loss of BRG1 expression. Notably, 4 of 9 tumors (44%) were positive for GATA3 expression, including one tumor that exhibited strong and diffuse immunoreactivity.GATA3 expression in SMARCA4-UT may pose diagnostic challenges, requiring differentiation from other GATA3-positive tumors. This distinction is crucial for accurate prognostication and treatment decisions.

Histiocytoid melanoma: Diagnostic pitfall and mimicker of non-Langerhans cell histiocytoses including reticulohistiocytoma.

Melanoma and benign histiocytic proliferations can sometimes show considerable clinical and histopathologic overlap. Recently, cases of melanomas resembling xanthogranuloma and Rosai-Dorfman disease have been reported, and herein we report a case of melanoma closely mimicking reticulohistiocytoma. An 84-year-old man presented with a 1 cm purple-red nodule on his arm concerning for squamous cell carcinoma. While the biopsy findings resembled reticulohistiocytoma, the clinical context and regression changes at the lesion perimeter raised stronger concern for melanoma, which was confirmed with immunohistochemistry. We review prior rare reports of melanomas resembling non-Langerhans cell histiocytic proliferations and summarize helpful clinical and histopathologic clues to avoid a diagnostic pitfall when confronted with this unusual quandary.

Sinonasal Myxoma: A Distinct Entity or a Myxoid Variant of Desmoid Fibromatosis?

Sinonasal myxoma (SNM) is a rare benign mesenchymal tumor that arises in the sinonasal cavity or maxilla and almost exclusively affects young children. Currently, it is considered a specific entity, but its molecular characteristics have not been reported. Lesions diagnosed as SNM and odontogenic myxoma/fibromyxoma were identified from the participating institutions, and the clinicopathologic features were recorded. Immunohistochemistry for ß-catenin was performed in all cases with available tissue. Next-generation sequencing was performed in all cases with SNM. Five patients with SNM were identified, including 3 boys and 2 girls with an age range of 20-36 months (mean: 26 months). The tumors were well defined, centered in the maxillary sinus, surrounded by a rim of woven bone, and composed of a moderately cellular proliferation of spindle cells oriented in intersecting fascicles in a variably myxocollagenous stroma that contained extravasated erythrocytes. Histologically, the tumors resembled myxoid desmoid fibromatosis. Three tested cases showed nuclear expression of ß-catenin. In 3 tumors, next-generation sequencing revealed intragenic deletions of APC exons 5-6, 9 and 15, or 16, respectively, with concurrent loss of the other wild-type copy of APC predicted to result in biallelic inactivation. The deletions were identical to those that occur in desmoid fibromatosis, and copy number analysis raised the possibility that they were germline. In addition, 1 case showed the possible deletion of APC exons 12-14, and another case exhibited a CTNNB1 p. S33C mutation. Ten patients with odontogenic myxoma/fibromyxoma were identified, including 4 women and 6 men (mean age: 42 years). Seven tumors involved the mandible and 3 the maxilla. Histologically, the tumors differed from SNM, and all cases lacked nuclear expression of ß-catenin. These findings suggest that SNM represents a myxoid variant of desmoid fibromatosis that often arises in the maxilla. The APC alterations might be germline, and therefore, genetic testing of the affected patients should be considered.

Impact of Histologic Risk Factors on Recurrence Rates for Oral Cavity Squamous Cell Carcinoma.

OBJECTIVES: This study assesses the impact of adverse histologic risk factors including worst pattern of invasion (WPOI), predominant pattern of invasion (PPOI), tumor budding, and tumor infiltrating lymphocytes (TILS), on risk of recurrence in patients with early-stage Oral Cavity Squamous Cell Carcinoma (OCSCC). METHODS: Retrospective chart review was performed at a single institution to identify patients with OCSCC who underwent surgical excision. Inclusion criteria included T1-T2 stage disease based on AJCC seventh edition guidelines, no cervical lymph node involvement, no perineural invasion (PNI), no lymphovascular invasion (LVI), no post-operative adjuvant radiation therapy, and cancer involving only the oral tongue or floor of mouth. Patients were excluded if they had positive final margins. A subset of 35 patients were selected for additional histologic review to determine WPOI, PPOI, TB, and TILS. Bivariable and multivariable cox analysis were performed to determine variables associated with recurrence. RESULTS: One hundred and sixty-one patients met criteria and were included in the analysis. Variables that were significantly associated with recurrence on bivariable analysis included tumors with high risk PPOI (groups 4 and 5; P = .021), TB with 10 or more buds (P = .021), T-stage (P = .001), neck dissection (P = .03), and depth of invasion (DOI) >4 mm (P = .044). Multivariable cox proportional hazards found T-stage (HR: 6.40; 95% CI 1.67, 24.50; P = .007), neck dissection (HR: 0.23; 95% CI 0.07, 0.82; P = .023), and TBs (HR: 1.17; 95% CI 1.05, 1.30; P = .006) to be most predictive of recurrence. CONCLUSIONS: TB is a strong predictor of recurrence. WPOI, PPOI, and TILS were not statistically significant risk factors for recurrence.

Retrospective evaluation of the role of gemcitabine-docetaxel in well-differentiated and dedifferentiated liposarcoma.

OBJECTIVE: Well-differentiated (WDLPS) and dedifferentiated liposarcoma (DDLPS) account for the majority of liposarcomas. Although gemcitabine-docetaxel is used as second-line treatment in soft tissue sarcomas, its efficacy in WDLPS/DDLPS is not established. This study retrospectively analyzed the efficacy of gemcitabine regimens in WDLPS/DDLPS. METHODS: All patients with WDLPS or DDLPS who received gemcitabine-based chemotherapy at our institution between September 2002 and January 2021 were included. Response was evaluated by an independent radiologist using RECIST 1.1. The Kaplan-Meier method was used to estimate distributions of survival outcomes and log-rank tests were used to compare survival outcomes between subgroups. RESULTS: Sixty-five WDLPS/DDLPS patients were included. Seven patients (10.8%) received a gemcitabine-based regimen more than once, totaling 72 treatments. The median age at the start of treatment was 66 years (range 32-80 years). Sixty-five (90.3%) regimens were gemcitabine-docetaxel, and 7 (9.7%) were gemcitabine alone. Majorities of treatments were for disease that was recurrent/metastatic (86.1%), was abdominal/retroperitoneal (83.3%), and had DDLPS components (88.9%), while 25.0% of treatments were for multifocal disease. The overall response rate was 9.7% (7/72). All responses were in patients with documented DDLPS. The median time to progression was 9.2 months (95% CI 5.3-12.3 months). The median overall survival from the start of therapy was 18.8 months (95% CI 13.1-32.4 months). CONCLUSION: Gemcitabine-docetaxel is an efficacious second-line treatment for DDLPS. Though cross study comparisons are not advisable, response to gemcitabine-docetaxel compares favorably to current standard options trabectedin and eribulin. This combination is a valid comparator arm for future second-line trials in DDLPS.

LY6D marks pre-existing resistant basosquamous tumor subpopulations.

Improved response to canonical therapies requires a mechanistic understanding of dynamic tumor heterogeneity by identifying discrete cellular populations with enhanced cellular plasticity. We have previously demonstrated distinct resistance mechanisms in skin basal cell carcinomas, but a comprehensive understanding of the cellular states and markers associated with these populations remains poorly understood. Here we identify a pre-existing resistant cellular population in naive basal cell carcinoma tumors marked by the surface marker LY6D. LY6D+ tumor cells are spatially localized and possess basal cell carcinoma and squamous cell carcinoma-like features. Using computational tools, organoids, and spatial tools, we show that LY6D+ basosquamous cells represent a persister population lying on a central node along the skin lineage-associated spectrum of epithelial states with local environmental and applied therapies determining the kinetics of accumulation. Surprisingly, LY6D+ basosquamous populations exist in many epithelial tumors, such as pancreatic adenocarcinomas, which have poor outcomes. Overall, our results identify the resistant LY6D+ basosquamous population as an important clinical target and suggest strategies for future therapeutic approaches to target them.

Frequent TRPS1 expression in synovial sarcoma is associated with SS18-SSX fusion oncoprotein activity.

Synovial sarcoma is an aggressive translocation-associated soft tissue tumor driven by an SS18-SSX fusion oncoprotein. TRPS1 is a recently identified marker for breast carcinoma, but less is known about its expression in other tumor types. We encountered a case of synovial sarcoma showing strong and diffuse expression of TRPS1. To better characterize this observation, we examined the immunohistochemical expression of TRPS1 in 165 cases of synovial sarcoma, including 70 primary, 21 recurrent, and 74 metastatic tumors, using tissue microarrays. TRPS1 expression was observed in 86% of cases. Among the positive cases, TRPS1 labeled >50% of tumor cells in 57% of cases, and staining intensity was strong or moderate in 68%. Metastatic tumors more frequently demonstrated strong and diffuse TRPS1 expression compared to primary tumors. To understand the mechanism of TRPS1 expression, we interrogated publicly available gene expression and ChIP-seq datasets and found that TRPS1 transcript levels are increased in synovial sarcoma compared to other soft tissue sarcomas. Data from ChIP-seq experiments showed enrichment of SS18-SSX protein at the TRPS1 locus and co-localization with RNA pol II. The TRPS1 locus is also enriched in several histone modifications associated with active transcription. In functional knockdown data, repression of SS18::SSX in synovial sarcoma cell lines is associated with reduced TRPS1 transcript levels, further supporting a model whereby TRPS1 expression is mediated, at least in part, by the SS18-SSX fusion oncoprotein. Knowledge of TRPS1 expression in synovial sarcoma may help avoid potential diagnostic pitfalls in the immunohistochemical workup of tumors.

Pigmented PRRX1::NCOA1-rearranged fibroblastic tumor: A rare morphologic variant of an emerging mesenchymal tumor.

PRRX::NCOAx-rearranged fibroblastic tumor is a recently described, morphologically distinctive subcutaneous fibroblastic tumor with benign behavior. To date, 12 cases have been reported. Here, we report a new case of PRRX::NCOAx-rearranged fibroblastic tumor showing a prominent pigmented component. The lesion occurred on the shoulder of a 23-year-old male. It was an at least 2.5 cm subcutaneous tumor with a multinodular and plexiform appearance. Morphologically, the tumor was characterized by a variably cellular proliferation of uniform oval to spindle cells arranged in fascicles and cords within a myxocollagenous stroma. Irregular, elongated, dilated vessels were prominent at the periphery of tumor nodules. In addition, nests and clusters of pigment-laden epithelioid and dendritic cells were present. Immunohistochemically, the non-pigmented tumor cells showed patchy positivity for factor XIIIa and focal positivity for S100 protein. The pigmented cells were positive for S100 protein, SOX10, MITF, and a pan-melanocytic cocktail (Melan-A, HMB-45, and tyrosinase). Next-generation RNA sequencing identified an in-frame PRRX1::NCOA1 fusion. In summary, this case highlights a rare pigmented variant of PRRX::NCOAx-rearranged fibroblastic tumor, expanding the morphologic spectrum of this newly described mesenchymal tumor.

Spitz nevus with EHBP1-ALK fusion and distinctive membranous localization of ALK.

ALK rearrangements define a histopathologically distinctive yet diverse subset of Spitz tumors characterized by fusiform to epithelioid melanocytes with frequent fascicular growth and ALK overexpression. Molecularly, these tumors are characterized by fusions between ALK and a variety of gene partners, most commonly TPM3 and DCTN1. We describe an unusual case of a Spitz nevus occurring in a 13-year-old female that manifested ALK immunopositivity with cell membrane localization. The proliferation was polypoid and composed of elongated nests of epithelioid melanocytes with enlarged nuclei, prominent nucleoli, and abundant cytoplasm without significant atypia and lacking mitotic figures. The nevus exhibited strong and diffuse expression of p16. Targeted next-generation RNA sequencing revealed an in-frame EHBP1-ALK fusion, which has been reported only once in the literature. EHBP1 encodes an adaptor protein with plasma membrane targeting potential. Together, these findings suggest that the 5' ALK fusion partner in Spitz tumors may dictate the subcellular localization of the ALK chimeric oncoprotein. In summary, this case highlights a rare ALK fusion associated with a distinct immunohistochemical staining pattern and further expands the spectrum of ALK-rearranged melanocytic tumors.