PRRX1-NCOA1-rearranged fibroblastic tumour: a clinicopathological, immunohistochemical and molecular genetic study of six cases of a potentially under-recognised, distinctive mesenchymal tumour

What is new in fibroblastic/myofibroblastic tumors in children

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

PRRX1-fused mesenchymal neoplasm: A novel PRRX1::NCOA1 fusion transcript

PRRX1-fused mesenchymal neoplasm is a recently identified, rare subcutaneous soft tissue neoplasm that is characterized by fusion of PRRX1 (exon 1) with NCOA1 (exon 13) in the majority of reported cases. Although initially considered to be fibroblastic, a possibility of neural or neuroectodermal differentiation has been suggested in a subset of cases. We report a 26-year-old female with a 4.0 cm painless mass located in the subcutis of the left thigh. Microscopically, the tumor was well-circumscribed and multinodular and was composed of relatively monomorphic ovoid to spindle cells arranged in loose fascicles, trabeculae, and cords within alternating myxoid and fibrous matrix, and vascularized stroma. Mitotic figures were scarce and necrosis was not observed. By immunohistochemistry, the neoplastic cells demonstrated focal co-expression of S100 protein and SOX10 and were negative for epithelial membrane antigen, smooth muscle actin, desmin, CD34, STAT6, HMB45, Melan-A, and MUC4. The expression of Rb1 was retained. Targeted RNA-sequencing identified a novel transcript fusion of PRRX1 (exon 1)::NCOA1 (exon 15), which was further confirmed by reverse transcription polymerase chain reaction and Sanger sequencing. The tumor was narrowly excised and no tumor recurrence or metastasis was identified after 13 months of follow-up. In summary, we report a new case of PRRX1-fused mesenchymal neoplasm, expanding the molecular genetic spectrum and providing further support for possible neural or neuroectodermal differentiation of this emerging soft tissue tumor entity.

Novel NCOA2/3-rearranged low-grade fibroblastic spindle cell tumors: A report of five cases

Spindle cell mesenchymal neoplasms are a diverse and often challenging diagnostic group. While morphological impression is sufficient for some diagnoses, increasingly immunohistochemical and even molecular data is required to render an accurate diagnosis, which can lead to the characterization of new entities. We describe five cases of novel mesenchymal neoplasms with rearrangements in the NCOA2 and NCOA3 genes partnered with either CTCF or CRTC1. Three tumors occurred in the head and neck (palate, auditory canal), while the other two were in visceral organs (lung, urinary bladder). All cases occurred in adults (range 33-86) with a median age of 42 and fairly even sex distribution = (male-to-female = 3:2). Morphologically, they had similar features consisting of monotonous, bland spindle to ovoid cells with fascicular and reticular arrangements in a myxohyaline to collagenous stroma. However, immunophenotypically they had essentially a null phenotype, with only two tumors staining partially for CD34 and smooth muscle actin. Targeted RNA sequencing detected in-frame CTCF::NCOA2 (one case), CRTC1::NCOA2 (two cases), and CTCF::NCOA3 (two cases) fusions. Treatment was surgical resection in all cases. Local recurrence and/or distant metastases were not observed in any case (median follow-up, 7.5 months; range, 2-19 months). Given their morphologic, immunohistochemical, and molecular similarities, we believe that these cases may represent an emerging family of low-grade NCOA2/3-rearranged fibroblastic spindle cell neoplasms.

Revealing RB1 loss in an emerging entity: report of two cases of PRRX1-rearranged mesenchymal tumours

AIMS

PRRX1-rearranged mesenchymal tumours are a recently identified and rare subgroup of soft tissue neoplasms with distinct morphological features and genetic alterations. This study aims to further investigate the immunohistochemical profile and underlying genetic alterations in these tumours in order to get more insight on their underlying biology and the unique profile of these tumours.

METHODS

Two new molecular confirmed cases of PRRX1-rearranged mesenchymal tumours were thoroughly studied with immunohistochemical stainings (RB1, CD34, ALK and pan-TRK), fluorescence in situ hybridisation (FISH) RB1/13q12 and RNA-based next-generation sequencing.

RESULTS

Both cases exhibited typical morphological and molecular features, confirming the diagnosis of PRRX1-rearranged mesenchymal tumours. Immunohistochemistry revealed RB1 loss in both cases, which was subsequently confirmed through FISH analysis. Additionally, one case showed focal positivity for CD34, ALK and pan-TRK on immunohistochemistry.

CONCLUSIONS

We identified loss of RB1 in two cases of PRRX1-rearranged mesenchymal tumours. This could suggest a potential association with RB1-deficient soft tissue tumours, although further research is necessary. Furthermore, the finding of focal positivity for CD34, ALK and pan-TRK on immunohistochemistry enriches the immunohistochemical profile of these tumours.

Novel EWSR1::GFI1B gene fusion in angiofibroma of soft tissue

AIMS

Angiofibroma of soft tissue is a benign soft tissue tumour characterised by bland spindle cells and a distinct branching vascular network. The majority of soft tissue angiofibromas harbour AHRR::NCOA2 gene fusions. Here we present three cases of EWSR1::GFI1B-fused soft tissue tumours that are morphologically most reminiscent of soft tissue angiofibroma.

METHODS AND RESULTS

All three cases presented in male patients with an age range of 35-78 years (median = 54 years). Two cases presented as subcutaneous nodules on the trunk (posterior neck and chest wall); one was an intramuscular foot mass. The tumours were unencapsulated nodules with infiltrative margins ranging from 2.2 to 3.4 cm in greatest dimension. Histologically, the tumours contained uniformly bland fibroblastic spindle cells with ovoid to fusiform nuclei and delicate cytoplasmic processes embedded in a myxoid to myxocollagenous stroma. All three cases were characterised by a thin-walled, branching vascular network evenly distributed throughout the tumour. Overt cytological atypia or conspicuous mitotic activity was absent. The spindle cells had an essentially null immunophenotype. By targeted RNA sequencing, an in-frame gene fusion between EWSR1 exons 1-7 and GFI1B exons 6-11 or 7-11 was detected in all three cases. The tumours were marginally excised. For all three cases, there were no documented local recurrence or distant metastases during a limited follow-up period of 6-10 months.

CONCLUSIONS

We propose that EWSR1::GFI1B may represent a novel fusion variant of soft tissue angiofibroma.

"PRRX1-rearranged mesenchymal tumors": expanding the immunohistochemical profile and molecular spectrum of a recently described entity with the proposed revision of nomenclature

Since the publication of the 2020 World Health Organization classification of soft tissue and bone tumors, the classification of "fibroblastic" tumors has expanded to include a novel subset of tumors characterized by PRRX1::NCOA1/2 gene fusions. These tumors defy conventional classification and are morphologically distinct, characterized by a multi-nodular growth of bland spindle cells suspended in a myxo-collagenous stroma with mild cytologic atypia, "staghorn-like" vessels, and variable perivascular hyalinization. Mitotic activity is rare, and necrosis is not identified. Herein, we present six additional cases of PRRX1-rearranged mesenchymal tumors, including five cases with PRRX1::NCOA1 fusion and one case with PRRX1::KMT2D fusion. Three cases (3/6, 50%) demonstrated focal co-expression of S100 protein and SOX10, thereby expanding the immunohistochemical profile of this emerging entity. Like prior reported cases, there was no evidence of malignant behavior on short-term follow-up. The novel fusion, PRRX1::KMT2D, further expands the molecular spectrum of this entity and leads to a proposed revision of the provisional nomenclature to "PRRX1-rearranged mesenchymal tumor" to both accommodate non-NCOA1/2 fusion partners and allow for the possibility of partial neural or neuroectodermal differentiation.

GAB1::ABL1 fusions define a distinctive soft tissue neoplasm, with variable perineurial differentiation, and a predilection for children and young adults

Although well known as a fusion partner in hematological malignancies, fusion genes involving the ABL proto-oncogene 1 (ABL1), mapping to chromosomal region 9q34.12, have only been anecdotally reported in five soft tissue tumors. These neoplasms have been variously reported as perineurioma, angiofibroma, and solitary fibrous tumor, and all have harbored a GAB1::ABL1 gene fusion; however, the nosology and clinicopathological characteristics of soft tissue tumors carrying this rare fusion have not been delineated. We herein describe eight tumors containing the GAB1::ABL1 fusion and review previously reported cases in a series to define their morphological spectrum, address immunohistochemical evidence for a line of differentiation, with special reference to the presence or absence of a perineurial immunophenotype, and gather insight into their behavior. The patients included four females and four males, aged 13-37 years (median, 24 years). Two cases each originated in the shoulder area, trunk, hands, and lower extremities, with a size range of 1.5-8 cm (median, 3.4 cm). Four tumors were deep and four superficial. All tumors were morphologically similar, being composed of bland fibroblast-like spindle to ovoid cells diffusely arranged in a paucivascular fibrous to fibromyxoid stroma with variable resemblance to soft tissue perineurioma. Mitotic activity was generally low (0-8 mitoses in 10 high-power fields [HPFs]; median, 1). All lesions had at least focally infiltrative margins, but they otherwise lacked pleomorphism and necrosis. Immunohistochemistry showed focal reactivity for CD34 (5/7), epithelial membrane antigen (EMA) (3/8), claudin1 (2/3), GLUT1 (4/6), and S100 (2/7); other markers, including MUC4 (0/7), desmin (0/9), and smooth muscle actin (SMA) (0/4), were negative. RNA sequencing revealed a GAB1::ABL1 fusion in all cases with exon 6 of GAB1 fused to exon 2 of ABL1. Treatments included various forms of surgical intervention in seven cases; one tumor was biopsied only. Limited follow-up was available for five patients. One tumor regrew rapidly within 1 month to 1.5 cm after an initial marginal excision and was re-excised with close margins. Four patients were disease-free at 1, 3, 14, and 25 months of follow-up. Metastases have not, to date, been observed. This series characterizes "GAB1::ABL1 fusion-positive spindle cell neoplasm" as a distinct entity, with overlapping features with soft tissue perineurioma and predilection for children and young adults.

Head and Neck Low-Grade Fibromyxoid Sarcoma: A Clinicopathologic Study of 15 Cases

Low-grade fibromyxoid sarcoma (LGFMS) is an uncommon mesenchymal tumor usually arising in the lower extremities and trunk. Only rare examples in the head and neck region have been reported. Fifteen cases of head and neck LGFMS were retrieved. MUC4 was performed on all cases. Results for smooth muscle actins, β-catenin, desmin, S100 protein, Epithelial membrane antigen (EMA) and STAT6 immunohistochemistry, as well as FUS rearrangement status, were recorded when available. Sites included neck (8), supraclavicular region (4) and orbit (1), parapharyngeal space (1) and lower lip (1). The age of the patients ranged from 3 to 97 years (median, 26 years). Tumors displayed classical morphologic features of LGFMS, as described. All cases (15/15) were positive for MUC4, and all cases tested (4/4) harbored FUS rearrangement. Variable positivity for EMA was identified in one case. Follow-up was available in 11 patients, ranging from 2 to 240 months (mean 71.4 months; median, 44 months). Three tumors recurred locally; none metastasized. In conclusion, although distinctly uncommon, LGFMS may arise in the head and neck region and should be distinguished from other more common spindle cell tumors in these locations. The morphologic, immunohistochemical and molecular genetic features of head/neck LGFMS are identical to those occurring elsewhere. The long-term metastatic risk of LGFMS in these locations remains to be fully elucidated.

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 myxo-collagenous 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. This article is protected by copyright. All rights reserved.

The emerging PRRX1-NCOA fibroblastic neoplasm: a combined reappraisal of published tumors and two new cases

Nuclear receptor coactivator (NCOA) family gene fusions have been increasingly discovered in diverse mesenchymal neoplasms, while PRRX1-NCOA-fused fibroblastic tumors still remain insufficiently characterized. We herein present two additional PRRX1-NOCA1-positive cases sharing lobulated hypocellular growth of innocuous spindle-to-stellate cells in a fibromyxoid stroma enriched with polymorphous vessels. A constellation of low cellularity, alternating myxocollagenous matrix, bland cytomorphology, and, especially, unusual collagenous rosettes in one case were morphologically reminiscent of low-grade fibromyxoid sarcoma. In both cases, immunoprofiles were similarly nondescript and negative for all diagnostic markers, including MUC4, emphasizing the diagnostic value of molecular testing. Review of published and current cases highlights a striking predominance of PRRX1-NCOA1, unusual collagenous rosettes, and favorable behavior in this emerging fibroblastic tumor type.

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

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