Hox11-expressing interstitial cells contribute to adult skeletal muscle at homeostasis.

Interstitial stromal cells play critical roles in muscle development, regeneration and repair and we have previously reported that Hoxa11 and Hoxd11 are expressed in the interstitial cells of muscles attached to the zeugopod, and are crucial for the proper embryonic patterning of these muscles. Hoxa11eGFP expression continues in a subset of muscle interstitial cells through adult stages. The induction of Hoxa11-CreERT2-mediated lineage reporting (Hoxa11iTom) at adult stages in mouse results in lineage induction only in the interstitial cells. However, Hoxa11iTom+ cells progressively contribute to muscle fibers at subsequent stages. The contribution to myofibers exceeds parallel Pax7-CreERT2-mediated lineage labeling. Nuclear-specific lineage labeling demonstrates that Hoxa11-expressing interstitial cells contribute nuclear contents to myofibers. Crucially, at no point after Hoxa11iTom induction are satellite cells lineage labeled. When examined in vitro, isolated Hoxa11iTom+ interstitial cells are not capable of forming myotubes, but Hoxa11iTom+ cells can contribute to differentiating myotubes, supporting Hox-expressing interstitial cells as a new population of muscle progenitors, but not stem cells. This work adds to a small but growing body of evidence that supports a satellite cell-independent source of muscle tissue in vivo.

Early-life pulmonary viral infection leads to long-term functional and lower airway structural changes in the lungs.

Early-life respiratory virus infections have been correlated with enhanced development of childhood asthma. In particular, significant numbers of respiratory syncytial virus (RSV)-hospitalized infants go on to develop lung disease. It has been suggested that early-life viral infections may lead to altered lung development or repair that negatively impacts lung function later in life. Our data demonstrate that early-life RSV infection modifies lung structure, leading to decreased lung function. At 5 wk postneonatal RSV infection, significant defects are observed in baseline pulmonary function test (PFT) parameters consistent with decreased lung function as well as enlarged alveolar spaces. Lung function changes in the early-life RSV-infected group continue at 3 mo of age. The altered PFT and structural changes induced by early-life RSV were mitigated in TSLPR-/- mice that have previously been shown to have reduced immune cell accumulation associated with a persistent Th2 environment. Importantly, long-term effects were demonstrated using a secondary RSV infection 3 mo following the initial early-life RSV infection and led to significant additional defects in lung function, with severe mucus deposition within the airways, and consolidation of the alveolar spaces. These studies suggest that early-life respiratory viral infection leads to alterations in lung structure/repair that predispose to diminished lung function later in life.NEW & NOTEWORTHY These studies outline a novel finding that early-life respiratory virus infection can alter lung structure and function long-term. Importantly, the data also indicate that there are critical links between inflammatory responses and subsequent events that produce a more severe pathogenic response later in life. The findings provide additional data to support that early-life infections during lung development can alter the trajectory of airway function.

Proliferating inverted follicular keratosis of genital skin: Case series of a challenging variant.

BACKGROUND: Inverted follicular keratosis (IFK) is a benign cutaneous epithelial tumor typically presenting as a small papule on the head and neck. We have encountered deep endophytic tumors on genital skin with some characteristics of IFK but also atypical features, raising concern for squamous cell carcinoma (SCC). METHODS: Four such tumors were identified in our database. Histopathologic analysis and ancillary studies were performed. RESULTS: All patients were young women who presented with a solitary 0.5-1.0 cm lesion on the vulva, perineum, or inner buttock. Each showed a squamous proliferation arising from the epidermis, with endophytic growth that was deep and bulbous but not infiltrative. The tumor lobules contained eosinophilic keratinocytes, forming numerous squamous eddies. Small irregular spaces and dyskeratotic cells were frequently found. Nuclear pleomorphism was minimal to absent. All demonstrated wild-type p53 expression and lack of p16 block positivity. In situ hybridizations for human papillomavirus were negative. None of the three cases with follow-up data showed evidence of recurrence. CONCLUSIONS: The absence of infiltrative growth or significant pleomorphism, the presence of numerous squamous eddies, the reassuring immunoprofile, and the lack of evidence of recurrence support a variant of IFK and speak against SCC. We propose the term "proliferating IFK" to highlight the florid squamous proliferation. Recognition of this unusual variant would avoid overdiagnosis of SCC.

Utility of GLI1 RNA Chromogenic in Situ Hybridization in Distinguishing Basal Cell Carcinoma From Histopathologic Mimics.

Basal cell carcinoma (BCC) is the most common human malignancy and is a leading cause of nonmelanoma skin cancer-related morbidity. BCC has several histologic mimics which may have treatment and prognostic implications. Furthermore, BCC may show alternative differentiation toward a variety of cutaneous structures. The vast majority of BCCs harbor mutations in the hedgehog signaling pathway, resulting in increased expression of the GLI family of transcription factors. GLI1 immunohistochemistry has been shown to discriminate between several tumor types but demonstrates high background signal and lack of specificity. In this study, we evaluated the utility of GLI1 RNA chromogenic in situ hybridization (CISH) as a novel method of distinguishing between BCC and other epithelial neoplasms. Expression of GLI1 by RNA CISH was retrospectively evaluated in a total of 220 cases, including 60 BCCs, 37 squamous cell carcinomas (SCCs) including conventional, basaloid, and human papillomavirus infection (HPV)-associated tumors, 16 sebaceous neoplasms, 10 Merkel cell carcinomas, 58 benign follicular tumors, and 39 ductal tumors. The threshold for positivity was determined to be greater than or equal to 3 GLI1 signals in at least 50% of tumor cells. Positive GLI1 expression was identified in 57/60 BCCs, including metastatic BCC, collision lesions with SCC, and BCCs with squamous, ductal, or clear cell differentiation or with other unusual features compared to 1/37 SCCs, 0/11 sebaceous carcinomas, 0/5 sebaceomas, 1/10 Merkel cell carcinomas, 0/39 ductal tumors, and 28/58 follicular tumors. With careful evaluation, GLI1 RNA CISH is highly sensitive (95%) and specific (98%) in distinguishing between BCC and nonfollicular epithelial neoplasms. However, GLI1 CISH is not specific for distinguishing BCC from most benign follicular tumors. Overall, detection of GLI1 RNA by CISH may be a useful tool for precise classification of histologically challenging basaloid tumors, particularly in the setting of small biopsy specimens, metaplastic differentiation, or metastatic disease.

Genomic evidence suggests that cutaneous neuroendocrine carcinomas can arise from squamous dysplastic precursors.

Merkel cell carcinoma (MCC) is an aggressive cutaneous neuroendocrine carcinoma without a known dysplastic precursor. In some cases, MCC is associated with SCCIS in the overlying epidermis; however, the MCC and SCCIS populations display strikingly different morphologies, and thus far a relationship between these components has not been demonstrated. To better understand the relationship between these distinct tumor cell populations, we evaluated 7 pairs of MCC-SCCIS for overlapping genomic alterations by cancer profiling panel. A subset was further characterized by transcriptional profiling and immunohistochemistry. In 6 of 7 MCC-SCCIS pairs there was highly significant mutational overlap including shared TP53 and/or RB1 mutations. In some cases, oncogenic events previously implicated in MCC (MYCL gain, MDM4 gain, HRAS mutation) were detected in both components. Although FBXW7 mutations were enriched in MCC, no gene mutation was unique to the MCC component across all cases. Transcriptome analysis identified 2736 differentially expressed genes between MCC and SCCIS. Genes upregulated in the MCC component included Polycomb repressive complex targets; downregulated transcripts included epidermal markers, and immune genes such as HLA-A. Immunohistochemical studies revealed increased expression of SOX2 in the MCC component, with diminished H3K27Me3, Rb, and HLA-A expression. In summary, MCC-SCCIS pairs demonstrate clonal relatedness. The shift to neuroendocrine phenotype is associated with loss of Rb protein expression, decrease in global H3K27Me3, and increased expression of Merkel cell genes such as SOX2. Our findings suggest an epidermal origin of MCC in this setting, and to our knowledge provide the first molecular evidence that intraepithelial squamous dysplasia may represent a direct precursor for small cell carcinoma.

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

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

p53/CK17 Dual Stain Improves Accuracy of Distinction Between Differentiated Vulvar Intraepithelial Neoplasia and Its Mimics.

Accurate diagnosis of differentiated vulvar intraepithelial neoplasia (dVIN) is challenging, in part due to the sometimes subtle nature of its atypia. Many dVIN lesions demonstrate aberrant p53 staining; however, staining patterns overlap between dVIN and benign/reactive entities. We evaluate a p53/CK17 dual stain in an initial cohort of dVIN (n=30), benign vulvar skin (n=5), lichen sclerosus (LS, n=10), lichen simplex chronicus (LSC, n=10), and pseudoepitheliomatous hyperplasia (PEH, n=10). In the initial cohort, aberrant p53 staining was seen only in dVIN (50%, 15/30). Equivocal p53 staining patterns were seen in dVIN (37%, 11/30), LS (50%, 5/10), LSC (40%, 4/10), and PEH (40%, 4/10). All 30 dVIN cases were positive for CK17 (strong partial-thickness or full-thickness staining), but positive CK17 staining was also seen in LS (70%, 7/10), LSC (50%, 5/10), and PEH (100%, 10/10). In the initial cohort, the combination of aberrant p53 and positive CK17 was seen only for dVIN (50%, 15/30). Forty cases of LS with known follow-up (20 with progression to dVIN, 20 without) were stained to assess prognostic value. Three LS cases showed aberrant p53 staining with CK17 positivity; all progressed to dVIN. Equivocal p53 staining and CK17 positivity were seen in cases with and without progression. The p53/CK17 dual stain is more diagnostically useful than either stain alone. Negative/focal staining for CK17 argues against a diagnosis of dVIN, while aberrant p53 staining with CK17 positivity strongly supports the diagnosis.

Progression of Peritoneal Mesothelioma In Situ to Invasive Mesothelioma Arising in the Setting of Endometriosis With Germline BAP1 Mutation: A Case Report.

Mesothelioma in situ has been proposed as a precursor to malignant mesothelioma arising in the pleura or peritoneum. We report a case of malignant peritoneal mesothelioma which progressed from mesothelioma in situ over a 10-mo period in a 24-yr-old woman with stage IV endometriosis. Initial surgery showed deeply infiltrative endometriosis with progestin effect. Postoperatively the patient had intractable pelvic pain and vaginal discharge. Imaging studies were negative. Repeat laparoscopy 10 mo later revealed vesicular lesions on the omentum and pinpoint white lesions studding the small bowel, appendix, and pelvic peritoneum. A diagnosis of epithelioid mesothelioma was established on biopsy of the omentum and confirmed by immunohistochemistry showing complete loss of BRCA1-associated protein-1 (BAP1) nuclear staining. Retrospectively, BAP1 loss was identified in the cytologically bland, single-layer surface mesothelium of the prior resection specimen, consistent with mesothelioma in situ . The patient underwent genetic testing and was found to have a pathogenic germline mutation in BAP1 .

PRAME Expression in Junctional Melanocytic Proliferations of the Conjunctiva: A Potential Biomarker for Primary Acquired Melanosis/Conjunctival Melanocytic Intraepithelial Lesions.

ABSTRACT: Conjunctival melanocytic proliferations are diagnostically challenging, often complicated by small specimen size, and are separated into 3 broad categories. The first group includes benign nevi and primary acquired melanosis (PAM) without atypia. The second group includes junctional melanocytic proliferations with a risk of progression to invasive melanoma (PAM with atypia). The last category is conjunctival melanoma, of which 65% of tumors arise in the setting of PAM with atypia. Preferentially expressed antigen in melanoma (PRAME) immunohistochemistry has been widely adopted to differentiate cutaneous nevi and melanoma. However, there are limited studies on its utility in the evaluation of conjunctival melanocytic proliferations with little data regarding its potential utility in stratifying PAM. Twenty-eight clinically annotated cases (14 PAM without atypia and 14 PAM with atypia) were retrospectively evaluated with PRAME/MART-1 duplex immunohistochemistry and were assigned the commonly used PRAME immunoreactivity score: 0 for no staining, 1+ for 1%-25% of cells positive, 2+ for 26%-50%, 3+ for 51%-75%, and 4+ for >75%. PAM without atypia showed low (0-3+) PRAME expression in 14 of 14 cases (100%). PAM with atypia showed strong and diffuse (4+) PRAME expression in 12 of 14 cases (86.7%). Seven of eight (87.5%) PAM with severe atypia, 4 of 4 PAM (100%) with moderate atypia, and 1 of 2 PAM (50%) with mild atypia showed 4+ PRAME expression. In addition, all 5 cases that recurred or progressed (all classified as PAM with atypia) showed 4+ PRAME expression. Although additional larger studies are needed, PRAME seems to be a useful adjunct in evaluating junctional melanocytic proliferations of the conjunctiva.

PRAME Expression in Challenging Dermal Melanocytic Neoplasms and Soft Tissue Tumors With Melanocytic Differentiation.

ABSTRACT: Preferentially expressed antigen in melanoma (PRAME) is an immunohistochemical biomarker that is diffusely expressed in most cutaneous melanomas and is negative in most benign nevi. Histologically challenging dermal melanocytic neoplasms, such as cellular blue nevi (CBN) and deep penetrating nevi (DPN), and soft tissue tumors with melanocytic differentiation, such as clear cell sarcoma and perivascular epithelioid cell tumor, may resemble primary or metastatic melanoma. PRAME immunohistochemistry (IHC) was applied to archived formalin-fixed, paraffin-embedded specimens of various dermal melanocytic neoplasms and soft tissue neoplasms with melanocytic differentiation. Staining was graded based on the percentage of melanocytes labeled (0-4+ as previously reported). The gold standard was final pathologic diagnosis using histologic, immunophenotypic, and in some cases molecular findings. Fifty-four cases were evaluated. 62.5% (5/8) of blue nevus-like melanomas and 50% (1/2) of DPN-like melanomas were PRAME positive (4+). Of the other tumors, 100% (20/20) of CBN (including 1 atypical CBN with borderline features); 100% (12/12) of DPN, combined DPN, or borderline DPN; 88.9% (8/9) of perivascular epithelioid cell tumors; and 100% (3/3) of clear cell sarcoma were PRAME negative (0-2+). Within the borderline categories specifically, all 8 tumors (1 borderline CBN and 7 borderline DPN) showed low (0-2+) PRAME expression. Overall, the sensitivity for melanoma in this context was 60%, with a specificity of 97.7%. Although our sample size is limited, the results suggest that IHC staining for PRAME may be useful in supporting a diagnosis of melanoma in the setting of challenging dermal melanocytic neoplasms and other epithelioid neoplasms with melanocytic differentiation. However, PRAME IHC lacks sensitivity in this context.

Immunohistochemical expression of PAX8, PAX2, and cytokeratin in melanomas.

BACKGROUND: Deviations from the classic melanocytic immunophenotype in melanoma can present a diagnostic challenge. PAX8 and PAX2 are common markers for renal or Müllerian differentiation. While most PAX8+ or PAX2+ carcinomas are seldom confused with melanoma, some cases may show a more ambiguous immunophenotype, especially when MiTF family altered renal cell carcinoma (MiTF-RCC) is in the differential diagnosis. Neither PAX8 nor PAX2 expression has been reported in melanoma to date. We aimed to better characterize PAX8, PAX2, and cytokeratin immunoreactivity in a large series of melanomas. METHODS: Tissue microarrays consisting of 263 melanomas were immunostained for PAX8, PAX2, and cytokeratin and graded by an h-score. RESULTS: PAX8 expression was seen in 7.9% of melanomas and was significantly associated with spindle cytomorphology. PAX2 was positive in one (0.4%) melanoma. Cytokeratin positivity was seen in three (1.2%) cases and was associated with metastases. CONCLUSIONS: PAX8 is expressed in a subset of melanomas and may be strong/extensive. As PAX8 positivity does not exclude a diagnosis of melanoma, it should be used in conjunction with other immunohistochemical markers, such as cytokeratin and PAX2, when melanoma, MiTF-RCC, and other PAX8+ tumors are in the differential diagnosis.

Hox5 genes direct elastin network formation during alveologenesis by regulating myofibroblast adhesion.

Hox5 genes (Hoxa5, Hoxb5, Hoxc5) are exclusively expressed in the lung mesenchyme during embryogenesis, and the most severe phenotypes result from constitutive loss of function of all three genes. Because Hox5 triple null mutants exhibit perinatal lethality, the contribution of this paralogous group to postembryonic lung development is unknown. Intriguingly, expression of all three Hox5 genes peaks during the first 2 weeks after birth, reaching levels far exceeding those measured at embryonic stages, and surviving Hoxa5 single and Hox5 AabbCc compound mutants exhibit defects in the localization of alveolar myofibroblasts. To define the contribution of the entire Hox5 paralogous group to this process, we generated an Hoxa5 conditional allele to use with our existing null alleles for Hoxb5 and Hoxc5 Postnatally, mesenchymal deletion of Hoxa5 in an Hoxb5/Hoxc5 double-mutant background results in severe alveolar simplification. The elastin network required for alveolar formation is dramatically disrupted in Hox5 triple mutants, while the basal lamina, interstitial matrix, and fibronectin are normal. Alveolar myofibroblasts remain Pdgfrα+/SMA+ double positive and present in normal numbers, indicating that the irregular elastin network is not due to fibroblast differentiation defects. Rather, we observe that SMA+ myofibroblasts of Hox5 triple mutants are morphologically abnormal both in vivo and in vitro with highly reduced adherence to fibronectin. This loss of adhesion is a result of loss of the integrin heterodimer Itga5b1 in mutant fibroblasts. Collectively, these data show an important role for Hox5 genes in lung fibroblast adhesion necessary for proper elastin network formation during alveologenesis.

Hox5 Paralogous Genes Modulate Th2 Cell Function during Chronic Allergic Inflammation via Regulation of Gata3.

Allergic asthma is a significant health burden in western countries, and continues to increase in prevalence. Th2 cells contribute to the development of disease through release of the cytokines IL-4, IL-5, and IL-13, resulting in increased airway eosinophils and mucus hypersecretion. The molecular mechanisms behind the disease pathology remain largely unknown. In this study we investigated a potential regulatory role for the Hox5 gene family, Hoxa5, Hoxb5, and Hoxc5, genes known to be important in lung development within mesenchymal cell populations. We found that Hox5-mutant mice show exacerbated pathology compared with wild-type controls in a chronic allergen model, with an increased Th2 response and exacerbated lung tissue pathology. Bone marrow chimera experiments indicated that the observed enhanced pathology was mediated by immune cell function independent of mesenchymal cell Hox5 family function. Examination of T cells grown in Th2 polarizing conditions showed increased proliferation, enhanced Gata3 expression, and elevated production of IL-4, IL-5, and IL-13 in Hox5-deficient T cells compared with wild-type controls. Overexpression of FLAG-tagged HOX5 proteins in Jurkat cells demonstrated HOX5 binding to the Gata3 locus and decreased Gata3 and IL-4 expression, supporting a role for HOX5 proteins in direct transcriptional control of Th2 development. These results reveal a novel role for Hox5 genes as developmental regulators of Th2 immune cell function that demonstrates a redeployment of mesenchyme-associated developmental genes.

Mesenchymal Hox6 function is required for mouse pancreatic endocrine cell differentiation.

Despite significant advances in our understanding of pancreatic endocrine cell development, the function of the pancreatic mesodermal niche in this process is poorly understood. Here we report a novel role for mouse Hox6 genes in pancreatic organogenesis. Hox6 genes are expressed exclusively in the mesoderm of the developing pancreas. Genetic loss of all three Hox6 paralogs (Hoxa6, Hoxb6 and Hoxc6) leads to a dramatic loss of endoderm-derived endocrine cells, including insulin-secreting ß-cells, and to mild delays and disruptions in pancreatic branching and exocrine differentiation. Ngn3-expressing pan-endocrine progenitor cells are specified normally in Hox6 mutant pancreata, but fail to mature into hormone-producing cells. Reduced expression of Wnt5a is observed in mutant pancreatic mesenchyme, leading to subsequent loss of expression of the crucial Wnt inhibitors Sfrp3 and Dkk1 in endocrine progenitor cells. These results reveal a key role for Hox6 genes in establishing Wnt mesenchymal-epithelial crosstalk in pancreatic development.

Hox5 interacts with Plzf to restrict Shh expression in the developing forelimb.

To date, only the five most posterior groups of Hox genes, Hox9-Hox13, have demonstrated loss-of-function roles in limb patterning. Individual paralog groups control proximodistal patterning of the limb skeletal elements. Hox9 genes also initiate the onset of Hand2 expression in the posterior forelimb compartment, and collectively, the posterior HoxA/D genes maintain posterior Sonic Hedgehog (Shh) expression. Here we show that an anterior Hox paralog group, Hox5, is required for forelimb anterior patterning. Deletion of all three Hox5 genes (Hoxa5, Hoxb5, and Hoxc5) leads to anterior forelimb defects resulting from derepression of Shh expression. The phenotype requires the loss of all three Hox5 genes, demonstrating the high level of redundancy in this Hox paralogous group. Further analyses reveal that Hox5 interacts with promyelocytic leukemia zinc finger biochemically and genetically to restrict Shh expression. These findings, along with previous reports showing that point mutations in the Shh limb enhancer lead to similar anterior limb defects, highlight the importance of Shh repression for proper patterning of the vertebrate limb.

Sox9 plays multiple roles in the lung epithelium during branching morphogenesis.

Lung branching morphogenesis is a highly orchestrated process that gives rise to the complex network of gas-exchanging units in the adult lung. Intricate regulation of signaling pathways, transcription factors, and epithelial-mesenchymal cross-talk are critical to ensuring branching morphogenesis occurs properly. Here, we describe a role for the transcription factor Sox9 during lung branching morphogenesis. Sox9 is expressed at the distal tips of the branching epithelium in a highly dynamic manner as branching occurs and is down-regulated starting at embryonic day 16.5, concurrent with the onset of terminal differentiation of type 1 and type 2 alveolar cells. Using epithelial-specific genetic loss- and gain-of-function approaches, our results demonstrate that Sox9 controls multiple aspects of lung branching. Fine regulation of Sox9 levels is required to balance proliferation and differentiation of epithelial tip progenitor cells, and loss of Sox9 leads to direct and indirect cellular defects including extracellular matrix defects, cytoskeletal disorganization, and aberrant epithelial movement. Our evidence shows that unlike other endoderm-derived epithelial tissues, such as the intestine, Wnt/ß-catenin signaling does not regulate Sox9 expression in the lung. We conclude that Sox9 collectively promotes proper branching morphogenesis by controlling the balance between proliferation and differentiation and regulating the extracellular matrix.

Expression of SATB2 in primary cutaneous sarcomatoid neoplasms: a potential diagnostic pitfall.

SATB2 can be used as an immunohistochemical marker for osteoblastic differentiation. The differential diagnosis of a cutaneous sarcomatoid neoplasm sometimes includes osteosarcoma when the tumour concomitantly involves the skin, soft tissue, and bone, or when there is a past medical history of osteosarcoma. As the utility of SATB2 immunohistochemistry in these scenarios was unclear, we aimed to determine the frequency and the pattern of SATB2 expression in a variety of cutaneous sarcomatoid neoplasms. SATB2 expression by immunohistochemistry was evaluated by intensity (0-3) and extent (0-100%) of staining to generate an h-score for each case. Expression levels were classified into high-positive (h-score ≥100), low-positive (20-99), and negative (<20) groups. Positive SATB2 expression was observed in 18/23 (78%) atypical fibroxanthomas (AFX), 10/19 (53%) pleomorphic dermal sarcomas, 9/20 (45%) cutaneous sarcomatoid squamous cell carcinomas, 14/39 (36%) sarcomatoid melanomas, 2/13 (15%) poorly differentiated cutaneous angiosarcomas, 10/17 (59%) high-grade cutaneous leiomyosarcomas, and 7/8 (88%) osteosarcoma controls. With the exception of AFX, all cutaneous neoplasms showed significantly lower average h-scores than osteosarcoma. AFX gave the highest average h-score (71) and percentage of high-positive cases (48%) among all examined cutaneous neoplasms. Only two (1.5%) of all cutaneous cases showed strong intensity of staining. Common SATB2 expression in various cutaneous sarcomatoid neoplasms poses a potential diagnostic pitfall when the differential diagnosis includes osteosarcoma. Requirement of strong staining and a high-positive h-score improves the specificity of SATB2 in differentiating these tumours from osteosarcoma.

Clinically Occult Diffuse Pleural Mesothelioma in Patients Presenting With Spontaneous Pneumothorax.

OBJECTIVES: To report histologic features of unsuspected diffuse pleural mesothelioma (DPM) in surgical specimens for pneumothorax and demonstrate how ancillary markers support a diagnosis of malignancy in this context. We explored whether pneumothorax may be a clinical manifestation of mesothelioma in situ (MIS). METHODS: A single-institution database search identified patients who underwent surgical resection for spontaneous pneumothorax (n = 229) and/or were diagnosed with DPM (n = 88) from 2000 to 2020. RESULTS: Spontaneous pneumothorax without clinical, radiologic, or intraoperative suspicion of mesothelioma was the initial presentation in 2 (2.3%) of 88 patients diagnosed with DPM. This represented 0.9% (2/229) of all patients undergoing surgical management of pneumothorax but accounted for a larger proportion of older patients (12.5% older than 70 years). Immunohistochemistry for BAP-1 and/or MTAP confirmed the diagnosis of DPM in 2 cases. Mesothelioma in situ was identified retrospectively by immunohistochemistry in 1 case of spontaneous pneumothorax from a 77-year-old man who developed invasive DPM 25 months later. No additional cases of MIS were identified in 19 surgical lung resections for spontaneous pneumothorax. CONCLUSIONS: Histologic examination of bleb resections with ancillary testing for cases with ambiguous features is essential for detection of early DPM. It is uncertain whether spontaneous pneumothorax may represent a clinical manifestation of MIS.