GLI1-altered mesenchymal tumor: a clinicopathological and molecular analysis of ten additional cases of an emerging entity

A Comprehensive Clinicopathologic and Molecular Reappraisal of GLI1-altered Mesenchymal Tumors with Pooled Outcome Analysis Showing Poor Survival in GLI1- amplified Versus GLI1-rearranged Tumors

GLI1-altered mesenchymal tumor is a recently described distinct pathologic entity with an established risk of malignancy, being defined molecularly by either GLI1 gene fusions or amplifications. The clinicopathologic overlap of tumors driven by the 2 seemingly distinct mechanisms of GLI1 activation is still emerging. Herein, we report the largest series of molecularly confirmed GLI1-altered mesenchymal neoplasms to date, including 23 GLI1-amplified and 15 GLI1-rearranged new cases, and perform a comparative clinicopathologic, genomic, and survival investigation. GLI1-rearranged tumors occurred in younger patients (42 vs. 52 y) and were larger compared with GLI1-amplified tumors (5.6 cm vs. 1.5 cm, respectively). Histologic features were overall similar between the 2 groups, showing a multinodular pattern and a nested architecture of epithelioid, and less commonly spindle cells, surrounded by a rich capillary network. A distinct whorling pattern was noted among 3 GLI1-amplified tumors. Scattered pleomorphic giant cells were rarely seen in both groups. The immunoprofile showed consistent expression of CD56, with variable S100, CD10 and SMA expression. Genomically, both groups had overall low mutation burdens, with rare TP53 mutations seen only in GLI1-amplified tumors. GLI1-amplified mesenchymal tumors exhibit mostly a single amplicon at the 12q13-15 locus, compared with dedifferentiated liposarcoma, which showed a 2-peak amplification centered around CDK4 (12q14.1) and MDM2 (12q15). GLI1-amplified tumors had a significantly higher GLI1 mRNA expression compared with GLI1-rearranged tumors. Survival pooled analysis of current and published cases (n=83) showed a worse overall survival in GLI1-amplified patients, with 16% succumbing to disease compared with 1.7% in the GLI1-rearranged group. Despite comparable progression rates, GLI1-amplified tumors had a shorter median progression-free survival compared with GLI1-rearranged tumors (25 mo vs. 77 mo). Univariate analysis showed that traditional histologic predictors of malignancy (mitotic count ≥4/10 high-power fields, presence of necrosis, and tumor size ≥5 cm) are associated with worse prognosis among GLI1-altered mesenchymal tumors.

GLI1 Coamplification in Well-Differentiated/Dedifferentiated Liposarcomas: Clinicopathologic and Molecular Analysis of 92 Cases

GLI1(12q13.3) amplification is identified in a subset of mesenchymal neoplasms with a distinct nested round cell/epithelioid phenotype. MDM2 and CDK4 genes are situated along the oncogenic 12q13-15 segment, amplification of which defines well-differentiated liposarcoma (WDLPS)/dedifferentiated liposarcoma (DDLPS). The 12q amplicon can occasionally include GLI1, a gene in close proximity to CDK4. We hereby describe the first cohort of GLI1/MDM2/CDK4 coamplified WD/DDLPS. The departmental database was queried retrospectively for all cases of WD/DDLPS having undergone next-generation (MSK-IMPACT) sequencing with confirmed MDM2, CDK4, and GLI1 coamplification. Clinicopathologic data was obtained from a review of the medical chart and available histologic material. Four hundred eighty-six WD/DDLPS cases underwent DNA sequencing, 92 (19%) of which harbored amplification of the GLI1 locus in addition to that of MDM2 and CDK4. These included primary tumors (n = 60), local recurrences (n = 29), and metastases (n = 3). Primary tumors were most frequently retroperitoneal (47/60, 78%), mediastinal (4/60, 7%), and paratesticular (3/60, 5%). Average age was 63 years, with a male:female ratio of 3:2. The cohort was comprised of DDLPS (86/92 [93%], 6 of which were WDLPS with early dedifferentiation) and WDLPS without any longitudinal evidence of dedifferentiation (6/92, 7%). One-fifth (13/86, 17%) of DDLPS cases showed no evidence of a well-differentiated component in any of the primary, recurrent, or metastatic specimens. Dedifferentiated areas mostly showed high-grade undifferentiated pleomorphic sarcoma-like (26/86,30%) and high-grade myxofibrosarcoma-like (13/86,16%) morphologies. A disproportionately increased incidence of meningothelial whorls with/without osseous metaplasia was observed as the predominant pattern in 16/86 (19%) cases, and GLI1-altered morphology as described was identified in a total of 10/86 (12%) tumors. JUN (1p32.1), also implicated in the pathogenesis of WD/DDLPS, was coamplified with all 3 of MDM2, CDK4, and GLI1 in 7/91 (8%) cases. Additional loci along chromosomal arms 1p and 6q, including TNFAIP3, LATS1, and ESR1, were also amplified in a subset of cases. In this large-scale cohort of GLI1 coamplified WD/DDLPS, we elucidate uniquely recurrent features including meningothelial whorl-like and GLI-altered morphology in dedifferentiated areas. Assessment of tumor location (retroperitoneal or mediastinal), identification of a well-differentiated liposarcoma component, and coamplification of other spatially discrete genomic segments (1p and 6q) might aid in distinction from tumors with true driver GLI1 alterations.

Keratin-Positive Giant Cell-Rich Tumor: A Review and Update

Keratin-positive giant cell-rich tumor (KPGCT) is an extremely rare and recently described mesenchymal neoplasm that occurs in both soft tissue and bone, frequently found in young women. It has locally recurrent potential if incompletely excised but low risk for metastasis. KPGCT is histologically similar to conventional giant cell tumors of soft tissue but shows the presence of keratin-positive mononuclear cells. Interestingly, KPGCT also shares some morphological features with xanthogranulomatous epithelial tumors. These two tumors have recently been shown to harbor an HMGA2-NCOR2 fusion, arguing in favor of a single entity. Surgery is the treatment of choice for localized KPGCT. Therapeutic options for advanced or metastatic disease are unknown. This review provides an overview of the current knowledge on the clinical presentation, pathogenesis, histopathology, and treatment of KPGCT. In addition, we will discuss the differential diagnosis of this emerging entity.

The value of GLI1 and p16 immunohistochemistry in the premolecular screening for GLI1-altered mesenchymal neoplasms

Mesenchymal neoplasms with GLI1 alterations have recently been reported in several anatomic locations. Their morphology and immunohistochemistry (IHC) are nonspecific, making their recognition a true challenge. To assess the diagnostic value of GLI1 and p16 IHC for identifying GLI1-altered neoplasms, we evaluated 12 such neoplasms (6 GLI1-amplified and 6 with GLI1-fusions) using the GLI1 IHC. Additionally, we evaluated some of their morphological and molecular mimickers, including glomangiomas, Ewing sarcomas (ES), myxoid liposarcomas, and MDM2/CDK4-amplified sarcomas (well-differentiated liposarcoma/WDLPS, dedifferentiated liposarcoma/DDLPS, and intimal sarcoma). All successfully tested GLI1-altered tumors (11/11) demonstrated at least moderate/strong nuclear and/or cytoplasmic GLI1 IHC positivity. GLI1-amplified tumors exhibited a moderate/strong predominantly nuclear staining, compared to a moderate, patchy, and predominantly cytoplasmic GLI1 positivity in GLI1-fusion tumors. Among their mimics, GLI1 immunoreactivity, either cytoplasmic or nuclear, was observed in intimal sarcoma (3/3) and WDLPS/DDLPS (22/25). GLI1 IHC demonstrated 92% sensitivity and 90.8% specificity in diagnosing GLI1-altered neoplasms. Strong/moderate nuclear/cytoplasmic p16 immunoexpression was noted in all GLI1-amplified tumors compared to none of fused cases. Overall, the GLI1/p16 combination demonstrated a sensitivity and specificity of 100% and 93% for GLI1-amplified tumors. In conclusion, we confirm that GLI1 IHC represents a good, quick, and cheap helpful screening tool. The inclusion of p16 may aid in pre-screening for potential GLI1-amplified neoplasms and provide insights on which tumors warrant further molecular testing.

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.

NR1D1-rearranged soft tissue tumour: A clinicopathological and molecular analysis of four additional cases

AIMS

Nuclear receptor subfamily 1 group D member 1 (NR1D1)-rearranged soft tissue tumour is a newly described entity with an epithelioid morphology and a potential for aggressive behaviour. Largely due to under-recognition, this tumour type has not yet been widely acknowledged. Herein, we report four additional cases to further expand its clinicopathological and molecular spectrum.

METHODS AND RESULTS

Four mesenchymal tumours with NR1D1 rearrangement were identified from our consultation files. There were one male and three females with ages ranging from 19 to 47 years (median = 28.5 years). Tumour occurred in the tongue, neck, hip and index finger, respectively. Histologically, two tumours were composed predominantly of epithelioid cells; one tumour had admixed epithelioid-spindle cells and one tumour consisted of monomorphic small round to ovoid cells. By immunohistochemistry, none of the tumours expressed lineage-specific markers. Targeted RNA-sequencing identified NR1D1 fusions in all four tumours, the partner genes being MAML2, MAML3, KMT2A and NCOA2, respectively. The novel MAML3 and NCOA2 rearrangements were confirmed by fluorescence in-situ hybridisation analysis. On follow-up (2-23 months), one patient experienced local recurrence due to incomplete resection and one patient developed lung metastasis. The other two patients were alive without disease.

CONCLUSIONS

This study adds more support for NR1D1-rearranged soft tissue tumour as an emerging entity. The occurrence of two additional tumours in the head and neck region, description of a small round cell variant and identification of novel MAML3, KMT2A and NCOA2 partners further expand its clinicopathological and molecular spectrum. More studies on larger series are necessary to validate the fully malignant potential of NR1D1-rearranged soft tissue tumour.

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.

Soft Tissue Sarcomas with Chromosomal Alterations in the 12q13-15 Region: Differential Diagnosis and Therapeutic Implications

The chromosomal region 12q13-15 is rich in oncogenes and contains several genes involved in the pathogenesis of various mesenchymal neoplasms. Notable genes in this region include MDM2, CDK4, STAT6, DDIT3, and GLI1. Amplification of MDM2 and CDK4 genes can be detected in various mesenchymal and nonmesenchymal neoplasms. Therefore, gene amplification alone is not entirely specific for making a definitive diagnosis and requires the integration of clinical, radiological, morphological, and immunohistochemical findings. Neoplasms with GLI1 alterations may exhibit either GLI1 rearrangements or amplifications of this gene. Despite the diagnostic implications that the overlap of genetic alterations in neoplasms with changes in genes within the 12q13-15 region could create, the discovery of coamplifications of MDM2 with CDK4 and GLI1 offers new therapeutic targets in neoplasms with MDM2/CDK4 amplification. Lastly, it is worth noting that MDM2 or CDK4 amplification is not exclusive to mesenchymal neoplasms; this genetic alteration has also been observed in other epithelial neoplasms or melanomas. This suggests the potential use of MDM2 or CDK4 inhibitors in neoplasms where alterations in these genes do not aid the pathological diagnosis but may help identify potential therapeutic targets. In this review, we delve into the diagnosis and therapeutic implications of tumors with genetic alterations involving the chromosomal region 12q13-15, mainly MDM2, CDK4, and GLI1.

GLI1-Altered Soft Tissue Tumor in the Tongue-A Case Report and Literature Review

Background: Soft tissue tumors with fusions or amplifications of the GLI1 gene have distinctive molecular characteristics and have recently been considered a unique pathological entity, thus named "GLI1-altered soft tissue tumors." It is a rare mesenchymal neoplasm that involves soft tissues at any site. Case presentation: We report an example of this condition in a 13-year-old Chinese male patient who presented with a mass in the tongue. The tumor was multilobulated; the tumor cells were arranged in nests and sheets, had a rich, delicate fibrovascular network, and were separated by a hyalinized fibrous stroma. The tumor cells were epithelioid to ovoid, with variable eosinophilic to pale vacuolated cytoplasm and round to oval nuclei. Immunostaining revealed that the tumor cells were positive for CDK4 and CD56. Fluorescence in situ hybridization (FISH) for GLI1 translocation was positive, with a high level of amplification of the translocated segment. Literature review: We present a comprehensive literature review of this condition, focusing on its clinical presentation, histological features, immunohistochemical profile, molecular characteristics, and prognosis.

Emerging mesenchymal tumour types and biases in the era of ubiquitous sequencing

New tumour types are being described at increasing frequency, and most new tumour types are now identified via retrospective review of next-generation sequencing data. This contrasts with the traditional, morphology-based method of identifying new tumour types, and while the sequencing-based approach has accelerated progress in the field, it has also introduced novel and under-recognised biases. Here, we discuss tumour types identified based on morphology, including superficial CD34-positive fibroblastic tumour, pseudoendocrine sarcoma and cutaneous clear cell tumour with melanocytic differentiation and ACTIN::MITF fusion. We also describe tumour types identified primarily by next-generation sequencing, including epithelioid and spindle cell rhabdomyosarcoma, round cell neoplasms with EWSR1::PATZ1 fusion, cutaneous melanocytic tumour with CRTC1::TRIM11 fusion, clear cell tumour with melanocytic differentiation and MITF::CREM fusion and GLI1-altered mesenchymal neoplasms, including nested glomoid neoplasm.

GLI1-altered Mesenchymal Tumor Involving the Duodenum: Case Report and Literature Review

GLI1-altered mesenchymal tumor is an emerging entity with distinctive clinicopathologic features. It shows a distinctive monomorphic round to epithelioid morphology, nested to trabecular pattern of growth, and S100+/SOX10-/SMA-immunophenotype. We report an example of this entity arising in the duodenum. A 31-year-old man presented with anemia for 1 year, a mass in the duodenal bulb was found for 9 days. Histopathologic examination revealed the tumor with distinct multilobulated architecture, a monomorphic appearance of round to epithelioid cells arranged in papillary structures, nests, cords, solid, reticular patterns, and hyalinized stroma surrounding a rich capillary network. The neoplastic cells had amphophilic to light eosinophilic or clear cytoplasm, uniform round nuclei with fine chromatin and inconspicuous nucleoli. Immunohistochemical analysis revealed strong positivity for vimentin, S100, CD56, CyclinD1, and negativity for SOX10, SMA, melan-A, HMB-45, synaptophysin, and a variety of other markers. Based on the morphology and immunophenotype, molecular studies were performed, which revealed the presence of an ACTB::GLI1 fusion transcript, confirming the diagnosis of GLI1-altered mesenchymal tumor.

Non-cutaneous syncytial myoepitheliomas are identical to cutaneous counterparts: a clinicopathologic study of 24 tumors occurring at diverse locations

AIMS

Cutaneous syncytial myoepithelioma (CSM) is a rare myoepithelioma variant of skin, characterized by intradermal syncytial growth of spindle cells with a distinct immunophenotype of EMA and S100 positivity and infrequent keratin expression. While CSM was first described as a cutaneous tumor, singular non-cutaneous cases have since been reported in bone. We aimed to investigate the clinicopathological features of this variant across all anatomic sites through a large multi-institutional study.

METHODS AND RESULTS

We complied a total of 24 myoepitheliomas with syncytial growth from our files. The tumors occurred in 12 male and 12 female patients (M:F = 1:1), with a median age of 31 years (range, 9-69 years). While the majority of tumors (75%, n = 18) occurred in skin, a significant subset (25%, n = 6) arose in non-cutaneous sites, including bone (n = 3), bronchus/trachea (n = 2), and interosseous membrane of tibia/fibula (n = 1). Tumor size ranged from 0.4 to 5.9 cm. Clinical follow-up (7 patients; range 14-202 months; median 56.5 months) showed a single local recurrence 8 years after incomplete skin excision but no metastases; all patients were alive at the time of last follow-up without evidence of disease. Histologically, all tumors were pink at low-power and characterized by a syncytial growth of bland ovoid, spindled, or histiocytoid cells with eosinophilic cytoplasm and prominent perivascular lymphoplasmacytic inflammation. One-third displayed adipocytic metaplasia (8/24). Rare cytologic atypia was seen but was not associated with increased mitotic activity. All tumors expressed S100, SMA, and/or EMA. Keratin expression was absent in most cases. Molecular analysis was performed in 16 cases, all showing EWSR1-rearrangments. In total, 15/15 (100%) harbored an EWSR1::PBX3 fusion, whereas 1 case EWSR1 FISH was the only molecular study performed.

CONCLUSION

Syncytial myoepithelioma is a rare but recognizable morphologic variant of myoepithelioma which may have a predilection for skin but also occurs in diverse non-cutaneous sites. Our series provides evidence supporting a reappraisal of the term "cutaneous syncytial myoepithelioma," as 25% of patients in our series presented with non-cutaneous tumors; thus, we propose the term "syncytial myoepithelioma" to aid pathologist recognition and avoidance of potentially confusing terminology when referring to non-cutaneous examples. The behavior of syncytial myoepithelioma, whether it arises in cutaneous or non-cutaneous sites, is indolent and perhaps benign with a small capacity for local recurrence.

Bioinformatics Screen Reveals Gli-Mediated Hedgehog Signaling as an Associated Pathway to Poor Immune Infiltration of Dedifferentiated Liposarcoma

Liposarcomas are the most diagnosed soft tissue sarcoma, with most cases consisting of well-differentiated (WDLPS) or dedifferentiated (DDLPS) histological subtypes. While both tumor subtypes can have clinical recurrence due to incomplete resections, DDLPS often has worse prognosis due to a higher likelihood of metastasis compared to its well-differentiated counterpart. Unfortunately, targeted therapeutic interventions have lagged in sarcoma oncology, making the need for molecular targeted therapies a promising future area of research for this family of malignancies. In this work, previously published data were analyzed to identify differential pathways that may contribute to the dedifferentiation process in liposarcoma. Interestingly, Gli-mediated Hedgehog signaling appeared to be enriched in dedifferentiated adipose progenitor cells and DDLPS tumors, and coincidentally Gli1 is often co-amplified with MDM2 and CDK4, given its genomic proximity along chromosome 12q13-12q15. However, we find that Gli2, but not Gli1, is differentially expressed between WDLPS and DDLPS, with a noticeable co-expression signature between Gli2 and genes involved in ECM remodeling. Additionally, Gli2 co-expression had a noticeable transcriptional signature that could suggest Gli-mediated Hedgehog signaling as an associated pathway contributing to poor immune infiltration in these tumors.

[Special mesenchymal neoplasms of the head and neck: Update from the 2022 WHO]

Similar to the approach adopted in the classification of tumors in other organs, a new feature of the current Word Health Organization (WHO) classification of the head and neck tumors is the exclusion of soft tissue tumors from single organs and their inclusion in a separate chapter devoted to them. This applies to tumors that are principally ubiquitous but show a predilection for the head and neck sites. The only exception to this rule represents those entities that are almost restricted to a specific head and neck site/organ (such as nasopharyngeal angiofibroma, sinonasal glomangiopericytoma, and biphenotypic sinonasal sarcoma), which remain in their respective organ chapters. Included among soft tissue tumors are some old but still underrecognized entities, such as phosphaturic mesenchymal tumors, and newly defined entities, such as GLI1-altered tumors. The aim of including these entities is to enhance recognition of these uncommon but likely under-recognized entities to better characterize them in the future. This review summarizes the main features of these rare entities and discusses their differential diagnoses.

Gene fusions in superficial mesenchymal neoplasms: Emerging entities and useful diagnostic adjuncts

Cutaneous mesenchymal neoplasms are diagnostically challenging because of their overlapping morphology, and, often, the limited tissue in skin biopsy specimens. Molecular and cytogenetic techniques have identified characteristic gene fusions in many of these tumor types, findings that have expanded our understanding of disease pathogenesis and motivated development of useful ancillary diagnostic tools. Here, we provide an update of new findings in tumor types that can occur in the skin and superficial subcutis, including dermatofibrosarcoma protuberans, benign fibrous histiocytoma, epithelioid fibrous histiocytoma, angiomatoid fibrous histiocytoma, glomus tumor, myopericytoma/myofibroma, non-neural granular cell tumor, CIC-rearranged sarcoma, hybrid schwannoma/perineurioma, and clear cell sarcoma. We also discuss recently described and emerging tumor types that can occur in superficial locations and that harbor gene fusions, including nested glomoid neoplasm with GLI1 alterations, clear cell tumor with melanocytic differentiation and ACTIN::MITF translocation, melanocytic tumor with CRTC1::TRIM11 fusion, EWSR1::SMAD3-rearranged fibroblastic tumor, PLAG1-rearranged fibroblastic tumor, and superficial ALK-rearranged myxoid spindle cell neoplasm. When possible, we discuss how fusion events mediate the pathogenesis of these tumor types, and we also discuss the related diagnostic and therapeutic implications of these events.

GLI1 Immunohistochemistry Distinguishes Mesenchymal Neoplasms With GLI1 Alterations From Morphologic Mimics

Glioma-associated oncogene 1 ( GLI1 ) alterations have been described in pericytoma with t(7;12), gastroblastoma, plexiform fibromyxoma, and an emerging class of GLI1 -rearranged or amplified mesenchymal neoplasms including "nested glomoid neoplasm". The immunophenotype of these tumor types is nonspecific, making some cases difficult to diagnose without sequencing. The utility of GLI1 immunohistochemistry (IHC) in distinguishing nested glomoid neoplasms and pericytomas with t(7;12) from morphologic mimics is unknown. To investigate the diagnostic value of GLI1 IHC, we determined its sensitivity and specificity in a "test cohort" of 23 mesenchymal neoplasms characterized by GLI1 alterations, including 12 nested glomoid neoplasms (7 GLI1 -rearranged, 4 GLI1 amplified, and 1 unknown GLI1 status), 9 pericytomas with t(7;12), 1 gastroblastoma, and 1 malignant epithelioid neoplasm with PTCH1 :: GLI1 fusion. GLI1 IHC was 91.3% sensitive in this cohort; all tumors except 2 pericytomas with t(7;12) expressed GLI1. GLI1 was also expressed in 1 of 8 (12%) plexiform fibromyxomas. Nineteen of 22 GLI1-positive tumors showed nuclear and cytoplasmic staining, while 3 showed nuclear staining only. GLI1 IHC was 98.0% specific; among morphologic mimics [40 well-differentiated neuroendocrine tumors, 10 atypical lung carcinoids, 20 paragangliomas, 20 glomus tumors, 20 solitary fibrous tumors, 10 Ewing sarcomas, 10 alveolar rhabdomyosarcomas (ARMS), 10 BCOR -altered sarcomas, 10 myoepitheliomas, 9 myopericytomas, 9 epithelioid schwannomas, 9 ossifying fibromyxoid tumors, 10 biphasic synovial sarcomas, 10 PEComas, 31 gastrointestinal stromal tumors, 10 inflammatory fibroid polyps, 11 pseudoendocrine sarcomas], 5 of 249 tumors expressed GLI1 (2 well-differentiated neuroendocrine tumors, 1 ARMS, 1 Ewing sarcoma, 1 BCOR -altered sarcoma). GLI1 IHC was also performed on a separate cohort of 13 molecularly characterized mesenchymal neoplasms in which GLI1 copy number gain was identified as a putatively secondary event by DNA sequencing (5 dedifferentiated liposarcoma [DDLPS], 2 adenosarcomas, 2 unclassified uterine sarcomas, 1 leiomyosarcoma, 1 ARMS, 1 intimal sarcoma, 1 osteosarcoma); 2 DDLPS, 1 ARMS, and 1 unclassified uterine sarcoma expressed GLI1. Lastly, because pleomorphic sarcomas sometimes show GLI1 amplification or copy number gain, GLI1 IHC was performed on a separate "pleomorphic sarcoma" cohort: GLI1 was expressed in 1 of 27 DDLPS, 1 of 9 leiomyosarcomas, and 2 of 10 pleomorphic liposarcomas, and it was negative in 23 well-differentiated liposarcomas and 9 unclassified pleomorphic sarcomas. Overall, GLI1 IHC was 91.3% sensitive and 98.0% specific for mesenchymal tumor types with driver GLI1 alterations among morphologic mimics. GLI1 expression was less frequent in other tumor types with GLI1 copy number gain. Given its specificity, in the appropriate morphologic context, GLI1 IHC may be a useful diagnostic adjunct for mesenchymal neoplasms with GLI1 alterations.

Distinctive Nested Glomoid Neoplasm: Clinicopathologic Analysis of 20 Cases of a Mesenchymal Neoplasm With Frequent GLI1 Alterations and Indolent Behavior

Recently, it has been recognized that a subset of primary soft tissue neoplasms with GLI1 gene alterations exhibit nested architecture and can mimic glomus tumors or well-differentiated neuroendocrine tumors. Here, we report a series of 20 such neoplasms, which we have provisionally termed "distinctive nested glomoid neoplasm." Eleven patients (55%) were female and 9 were male. The median age at presentation was 41.5 years (range: congenital to 74 y). The anatomic distribution was wide, with body sites including the trunk (7 tumors), lower extremity (5), tongue (4), upper extremity (3), and neck (1). Excluding tumors of the tongue, 10 tumors (62%) arose in deep soft tissue and 6 (38%) arose primarily in the subcutis. Tumor size ranged from 0.9 to 11.1 cm (median: 3 cm). Distinctive nested glomoid neoplasms are composed of nests of round-to-ovoid cells with scant, palely eosinophilic cytoplasm and monomorphic nuclei with vesicular chromatin and small nucleoli. The nests are invested by prominent capillary networks, and they are situated within large lobules separated by irregular, thick fibrous septa. Among 18 tumors for which adjacent non-neoplastic tissue could be assessed, perivascular proliferation of tumor cells was identified in 16 tumors (89%). Microcystic architecture was present at least focally in 8 tumors (40%), and myxoid stroma was identified at least focally in 5 (25%). Seven tumors (35%) showed clear cell features. By immunohistochemistry, some tumors expressed MDM2 (7/15; 47%), S100 (5 of 19; 26%), STAT6 (2 of 5; 20%), and AE1/AE3 (1/5; 20%). Tumors rarely expressed pan-keratin (1/10; 10%) or CAM5.2 (1/10), and all tumors were negative for β-catenin (12 tumors), chromogranin (12), synaptophysin (11), epithelial membrane antigen (10), desmin (10), smooth muscle actin (9), INSM1 (7), and CD34 (6). GLI1 break-apart fluorescence in situ hybridization was performed on 7 tumors, and next-generation sequencing was performed on 15 tumors (10 DNA sequencing only, 1 RNA sequencing only, 4 both DNA and RNA sequencing). Sixteen tumors, including all 15 tested by next-generation sequencing and an additional case tested by fluorescence in situ hybridization only, were found to harbor GLI1 gene alterations: 10 harbored GLI1 gene rearrangements (3 ACTB :: GLI1 , 2 PTCH1 :: GLI1 , 1 HNRNPA1 :: GLI1 , 1 NEAT1 :: GLI1 , 1 TXNIP :: GLI1 , 2 undetermined fusion partners), and 6 harbored GLI1 amplification. Clinical follow-up was available for 10 patients (50%; range: 3 mo to 10 y; median: 6.4 y), including 8 with >1 year of follow-up. Three patients (30%) experienced local recurrence (at intervals of 3 mo to 10 y). None developed distant metastases or died of disease as yet. Overall, our findings support the notion that a subset of GLI1 -altered soft tissue neoplasms are indolent, morphologically distinctive nested glomoid neoplasms that should not be classified as sarcomas.

Xanthogranulomatous epithelial tumors and keratin-positive giant cell-rich soft tissue tumors: two aspects of a single entity with frequent HMGA2-NCOR2 fusions

Xanthogranulomatous epithelial tumor (XGET) and keratin-positive giant cell-rich soft tissue tumor with HMGA2-NCOR2 fusion (KPGCT) are two recently described neoplasms with both distinct and overlapping clinical and histopathologic features. We hypothesized that XGET and KPGCT may be related and represent a histologic spectrum of a single entity. To test this, we sought to characterize the clinical, radiographic, immunohistochemical, ultrastructural and molecular features of additional tumors with features of XGET and/or KPGCT, which we refer to descriptively as keratin-positive xanthogranulomatous/giant cell-rich tumors (KPXG/GCT). The archives were searched for potential cases of KPXG/GCT. Clinical and imaging features were noted. Slides were assessed for histologic and immunohistochemical findings. Ultrastructural and next generation RNA sequencing-based analysis were also performed. Nine cases were identified arising in seven women and two men [median age of 33 years (range: 12-87)]. Median tumor size was 4 cm (range: 2.4-14.0 cm) and tumors presented in the thigh (2), buttock (1), forearm (2), groin (1), cranial fossa (1), ilium (1), and tibia (1). Morphologically, tumors were most frequently characterized by a fibrous capsule, with associated lymphoid reaction, enclosing a polymorphous proliferation of histiocytes, giant cells (Touton and osteoclast-types), mixed inflammatory infiltrate, hemorrhage and hemosiderin deposition, which imparted a variably xanthogranulomatous to giant cell tumor-like appearance. One case clearly showed mononuclear cells with eosinophilic cytoplasm characteristic of XGET. All cases expressed keratin and 7 of 9 were found to harbor HMGA2-NCOR2 fusions including cases with xanthogranulomatous appearance. One patient developed local recurrence and multifocal pulmonary lesions, which were radiographically suspicious for metastases. Shared clinical, histologic and immunohistochemical features, and the shared presence of HMGA2-NCOR2 fusions supports interpretation of KPXG/GCT as a single entity which includes XGET and KPGCT. Given limited clinical follow-up to date and rare cases with apparently aggressive findings, we provisionally regard these tumors as having uncertain biologic potential.

GLI1-Altered Soft Tissue Tumors of the Head and Neck: Frequent Oropharyngeal Involvement, p16 Immunoreactivity, and Detectable Alterations by DDIT3 Break Apart FISH

BACKGROUND

GLI1 is a transcription factor protein that has recently gained recognition in a morphologically distinct group of epithelioid soft tissue tumors characterized by GLI1 fusions or amplifications. The head and neck region, particularly the tongue, is a common location for GLI1-altered tumors. DDIT3 break apart fluorescence in situ hybridization (FISH), commonly used to identify translocations in myxoid/round cell liposarcoma, has been used as a surrogate test to detect both fusions and amplifications of the 12q13.3 region encompassing DDIT3 and GLI1 gene loci.

METHODS

We herein report 5 cases of GLI1-altered soft tissue tumors. Three arose in the oropharynx (base of tongue/vallecula, tonsil) and two arose in the tongue. Given the frequent oropharyngeal location and epithelioid morphology, p16 immunohistochemistry was performed on cases with available material. Commercially available DDIT3 break apart FISH, custom GLI1 specific FISH, and RNA sequencing were performed on select cases.

RESULTS

Two cases showed amplification using DDIT3 FISH which was confirmed using GLI1 specific FISH. The remaining cases harbored ACTB::GLI1, one of which showed rearrangement of the 12q13.3 region by DDIT3 FISH with absence of amplification by GLI1 specific FISH. STAT6 immunoexpression was positive in the GLI1-amplified cases and negative in the GLI1-rearranged cases while MDM2 expression was positive in the 4 cases tested. CDK4 expression was strong and diffuse in the GLI1-amplified cases. p16 immunohistochemistry showed strong nuclear and cytoplasmic staining in 50-70% of tumor cells in all four tested cases.

CONCLUSION

Here we show that GLI1-altered soft tissue tumors are frequently positive for p16 and can occur in tonsillar regions of the oropharynx. As such, positive p16 immunohistochemistry alone cannot be used as evidence for the diagnosis of HPV-related squamous cell carcinoma as strong and diffuse p16 expression may also occur in GLI1-altered soft tissue tumors. Commercially available DDIT3 break apart FISH, which is readily available in many cytogenetic laboratories, may be useful as a sensitive surrogate test for GLI1 fusions and amplifications.