USP6 activation in nodular fasciitis by promoter-swapping gene fusions

Physiological roles and therapeutic implications of USP6

Ubiquitin-specific protease 6 (USP6) is a member of deubiquitinating enzyme family, recognized for its essential roles in physiological and pathological processes. USP6 is initially identified as a hominoid-specific enzyme residing on chromosome 17p13. USP6 is involved in regulating cellular functions, signaling pathways, protein degradation, intracellular trafficking, tumorigenesis and immune responses. USP6 is pivotal in signaling pathways, including NF-κB, JAK-STAT, and Wnt, which are fundamental for maintaining cellular homeostasis and mediating stress responses. Dysregulation of USP6 has been implicated in a spectrum of diseases, including bone tumors, breast and colorectal cancers, cranial fasciitis, and neurological disorders such as memory dysfunction. Furthermore, USP6 is involved in emerging therapeutic strategies highlighting its implications for drug development. A number of potential small molecule inhibitors are known to be responsible for suppression of USP6, such as Momelotinib (CYT387), FT385, USP30 Inh-1, -2 and -3, 2,6-Diaminopyridine-3,5-bis(thiocyanate) (PR-619) and so on. This review explores the emerging role of USP6 as a key regulator of cellular signaling pathways, its involvement in disease progression, its physiological functions, and the inhibitors that effectively suppress USP6 activity in detail. The comprehensive study provides insight to enhance our understanding of biological importance and therapeutic interventions of USP6 in drug development.

Mobile subcutaneous tumour on the trapezius

This is a dermatopathology case for learners to test their knowledge of the pathology of subcutaneous nodules and tumours.

Clinicopathological characteristics and favourable prognosis of paediatric cutaneous nodular fasciitis: a case series

Cutaneous nodular fasciitis (cNF) is a rare, benign myofibroblastic proliferation commonly misdiagnosed as a malignancy due to its rapid growth and histological features. We reviewed 15 paediatric patients with cNF, highlighting clinical, histological and molecular characteristics. The patients' median age was 6 years, and there was a predilection for cNF in the head and neck region. Histologically, cNF showed plump spindle cells and myxoid stromal changes. USP6 rearrangement, particularly MYH9-USP6 fusion, was present in the majority of patients. Spontaneous resolution was observed in nonsurgically managed cases. Accurate diagnosis by screening for USP6 rearrangement is essential to avoid overtreatment. This study underscores the benign nature of cNF and the importance of conservative management, especially in cosmetically sensitive areas.

Periorbital nodular fasciitis: a case report

BACKGROUND

Nodular fasciitis (NF), also known as pseudosarcoma fasciitis, is defined as a benign, self-limiting, vascular, or fibrovascular proliferation. It is commonly found on the trunk and upper extremities. NF is rarely found in the orbit and periorbital regions, with only a few case studies reported since it was initially described by Font et al. in 1966. NF usually presents as a rapidly growing solitary tumor and is often mistaken for a sarcoma due to its high cellularity and increased mitotic activity.

CASE PRESENTATION

We present a case of NF in a 41-year-old Saudi man with a 1-year history of a gradually enlarging left medial subbrow mass in the left eye. The mass was not associated with pain, visual disturbances, double vision, or a history of trauma. An external ophthalmic examination revealed a firm, oval mass over the nasal aspect of the superior orbital rim of the left eye. The mass was non-tender, measured 1 × 2 cm in diameter, and fixed to deep tissues. Computed tomography (CT) with contrast revealed a well-demarcated left para-orbital soft tissue round lesion with peripheral enhancement occupying the nasal aspect of the superior orbital rim above the inner canthus. Debulking and incisional biopsies were performed, and the histopathological findings were consistent with those of NF.

CONCLUSIONS

NF is a rare benign tumor that infrequently presents in the periorbital region. It lacks distinctive features for a definitive diagnosis and should be suspected when imaging results are inconclusive. Management typically involves complete local excision or debulking.

Nodular fasciitis: a case series unveiling novel and rare gene fusions, including two cases with aggressive clinical behavior

Nodular fasciitis is a benign myofibroblastic tumor characterized by rapid growth and spontaneous regression. While nodular fasciitis is typically an indolent process, rare cases with benign morphologic features have developed metastases. Conversely, nodular fasciitis with malignant histologic features and benign clinical course have also been reported. In this study, we present seven nodular fasciitis cases with novel USP6 gene fusion partners, in addition to two cases with rare fusions that displayed aggressive clinical behavior. The cohort comprised five females and four males with a median age of 36 years (range 13-59). Tumors were located in the forearm (n = 3), thigh (n = 2), and shoulder, abdominal wall, chest wall, and oral cavity (one each), ranging from 1.4 to 24.0 cm in size (median, 2.2 cm). Except for the clinically aggressive cases, patients presented with painless masses of varying onset from days to months. Of the clinically aggressive cases, one patient presented with a slowly growing subfascial thigh/hip mass over nine years, leading to erosion of the femur and pelvis; the other presented with a painful subfascial thigh mass of several months' duration. Histologically, all cases, including the clinically aggressive ones, showed conventional nodular fasciitis features without nuclear pleomorphism or atypical mitotic figures; one case with aggressive clinical behavior exhibited focal infarction-type necrosis. Break-apart FISH analysis using USP6 flanking probes failed to detect USP6 rearrangement in two cases (false negatives) and was inconclusive in one case. Next-generation RNA sequencing identified USP6 fusions in all cases. The clinically aggressive cases showed fusions with COL1A1 (exon 1) and PPP6R3 (exon 1), while novel fusions were identified in the remaining cases including EIF4A1 (exon 1), FILIP1L (exon 2), NF1 (exon 33), OMD (exon 1), PFN1 (exon 1), RLIM (exon 1), and SETD5 (exon 1). Six patients underwent surgical resection; three were managed conservatively, with two experiencing spontaneous tumor resolution. Of the clinically aggressive cases, one patient had progression of the tumor with erosion of the underlying bone, and the second patient developed local recurrence at 14 months and lung metastasis at 19 months, ultimately dying of disease at 22 months. The remaining patients showed no recurrence or metastasis. Our findings expand the spectrum of USP6 gene fusion partners in nodular fasciitis and, for the first time, report cases with conventional morphology exhibiting aggressive behavior, including death. These observations raise the question of whether a subset of deep lesions with conventional nodular fasciitis histology but unusual clinical features, such as large tumor size, represents malignant nodular fasciitis or alternatively a nodular fasciitis-like myofibroblastic sarcoma.

Intraarticular Nodular Fasciitis of the Elbow Confirmed by UPS6::MYH9 Gene Fusion

Nodular fasciitis is a benign, usually self-limiting myofibroblastic proliferation with a predilection for the upper extremities, trunk, and head and neck, and almost all of which harbor the USP6::MYH9 fusion. Since nodular fasciitis is not widely recognized to arise within the joints, it may therefore cause diagnostic confusion in this uncommon setting. We report an unusual tumor of an 11-year-old patient who presented with a 6-month history of right elbow swelling and pain. Histology revealed that the tumor was composed solely of proliferating bland spindle cells with osteoclast-like giant cells and scattered foam cells. There was only a focal area of immature granulation-like myxoid change, which suggested nodular fasciitis. Molecular analysis using next-generation sequencing revealed gene rearrangement involving USP6 and MYH9, supporting the diagnosis of nodular fasciitis within the elbow joint. This is the first of intraarticular nodular fasciitis within the elbow joint in which USP6::MYH9 gene fusion was identified by molecular analysis.

Palmar Nodular Fasciitis Harboring a Novel SREBF1::USP6 Fusion Gene

The diagnosis of low-grade fibroblastic/myofibroblastic tumors of acral sites can be challenging. These tumors encompass a diverse group of neoplasms with a spectrum of biologic potential ranges from benign to overtly malignant. They often demonstrate significant clinical, radiologic, and immunophenotypic overlap, in which the molecular phenotype may play an important diagnostic role to arrive at the final diagnosis. Herein, we report a case of soft tissue mass lesion presented on the palm of an adult patient for four months. Histologically, the tumor consisted of primarily low-grade spindle cells expressing smooth muscle actin. Molecular testing revealed a novel SREBF1::USP6 fusion gene, confirming the final diagnosis of nodular fasciitis and ultimately expanding its molecular profile. This case highlights the diagnostic value of single, cost-effective, targeted molecular panel to arrive at an accurate diagnosis and provide helpful therapeutic information.

Papillary Thyroid Carcinoma with Fibromatosis/Fasciitis-Like/Desmoid-Type Stroma: Case Report of a Rare Subtype with Cytological and Molecular Study

BACKGROUND

Papillary thyroid carcinoma (PTC) with fibromatosis/fasciitis-like/desmoid-type stroma is a rare subtype of PTC,characterized by two distinct components: a classic papillary carcinoma component and a spindle cell proliferationresembling fibromatosis or nodular fasciitis. This stromal component adds a unique dimension to the tumor'spathology, making diagnosis more challenging and potentially leading to misclassification.

CASE PRESENTATION

We present a case of this rare entity which contributes to the growing body of literature by providing additionalmolecular data, which may shed light on the biological behaviour of the fibromatosis-like stroma and its relationshipwith the papillary carcinoma component. This case underscores the importance of recognizing this subtype, as itsspindle cell proliferation could be mistaken for a separate neoplasm or reactive process, resulting in inappropriatemanagement.

CONCLUSIONS

Increased awareness of this entity will help pathologists avoid diagnostic pitfalls and guide clinicians in developingmore precise treatment plans, addressing both the malignant papillary component and the unique stromal features.This case further enriches the current understanding of the heterogeneity of PTC and highlights the need fortailored management strategies in rare subtypes.

Utility of an anchored multiplex polymerase chain reaction-based fusion assay for diagnosis of soft tissue tumors in cytology

BACKGROUND

Fine-needle aspiration specimens from soft tissue tumors are complicated by lack of tissue architecture and limited material for ancillary testing. There are little data on the feasibility of next-generation sequencing techniques for fusion detection on soft tissue cytology specimens. This study explored the role of an anchored multiplex polymerase chain reaction (PCR)-based gene fusion assay in aiding the diagnosis of mesenchymal neoplasms on cytology samples.

METHODS

The laboratory information system was queried for cytology specimens that had undergone testing by anchored multiplex PCR. After exclusion of epithelial and hematolymphoid neoplasms, clinical and pathologic information was collected on the remaining cases.

RESULTS

There were 1609 cytology specimens tested with anchored multiplex PCR. Of these, 48 (3%) were cytology specimens from mesenchymal tumors. Anchored multiplex PCR was positive for a reportable fusion transcript in 14 of 48 cases (29%); there was no fusion detected in 32 cases (67%), and there was insufficient tissue for analysis in two cases (4%). The detectable fusion partners included ALK (n = 4), STAT6 (n = 4), EWSR1 (n = 3), and one each of SS18, YAP1, and PHF1. Of the cases in which a fusion partner was detected, eight of 14 were disease-defining on cytology preparation, and six of 14 provided molecular confirmation of a metastatic focus of a previously diagnosed tumor.

CONCLUSIONS

The anchored, multiplex PCR-based gene fusion assay is a powerful orthogonal tool in helping diagnose mesenchymal neoplasms on cytology specimens. The material obtained for cytologic analysis yields sufficient quality/quantity of tissue in the majority of cases tested.

Nodular Fasciitis of the Buccal Mucosa with a Novel USP6 Gene Rearrangement: A Case Report and Review of the Literature

Nodular fasciitis is a rare but benign fibroblastic proliferation that typically presents as a solitary lesion with rapid growth and variable mitotic activity. The lesions usually occur on the extremities and occasionally in the head/neck region. Involvement of the buccal mucosa is extremely rare with only few reports in the literature; in this case report, we describe a 41 year old female who presented with a 6-month history of a stable intraoral lump at the junction of the upper and lower lip. Fine needle aspiration revealed an atypical spindle cell population with plump cells. The surgical excision demonstrated a well circumscribed tan-white firm nodule. Histologic examination revealed a spindle cell proliferation that grew in short, intersecting fascicles with focal storiform architecture. The lesion had a pushing border that was not overtly infiltrative and the stroma contained focal myxoid changes giving a "tissue culture" appearance to the cells. Immunohistochemical testing showed the tumor cells were vimentin (+), SMA (+), weakly Calponin (+), and desmin (-), cytokeratin (-), AE1/AE3 (-), S100 (-), ALK (-), STAT6 (-), and beta-catenin (-). Fluorescence in-situ hybridization (FISH) revealed a USP6 gene rearrangement with an atypical probe pattern. Next generation sequencing identified a novel SPTAN1::USP6 fusion gene confirming the diagnosis of buccal nodular fasciitis. Identification of the characteristic histologic features and USP6 gene rearrangements helped support the diagnosis. A review of the literature identified 25 cases of nodular fasciitis involving the buccal mucosa. The occurrence of this tumor in an unusual location may pose difficulties for diagnosis.

Soft tissue aneurysmal bone cyst presenting as an enlarging neck mass: Case report and review of the head and neck literature

Soft tissue aneurysmal bone cysts (STABCs) are rare neoplasms histopathologically identical to aneurysmal bone cysts. These benign lesions are characterized by thin, peripheral ossification and no skeletal continuity. STABC may be difficult to distinguish from myositis ossificans (MO) and malignant entities from imaging and fine needle aspiration, due to rarity and overlapping features. We present a case of a STABC occurring in the paraspinal cervical muscles. The imaging, histopathology, molecular analysis, and treatment are discussed. Four other published cases of STABC of the head and neck are reviewed.

Recurrent USP6 rearrangement in a subset of atypical myofibroblastic tumours of the soft tissues: low-grade myofibroblastic sarcoma or atypical/malignant nodular fasciitis?

AIMS

Low-grade myofibroblastic sarcoma (LGMS) is a rarely metastasizing myofibroblastic tumour mostly affecting extremities and the head and neck of adults. Histologically, it shows long infiltrative fascicles of spindle cells with moderate nuclear atypia. By immunohistochemistry, it stains positive for smooth muscle actin (SMA) and sometimes for desmin. To date, no recurrent genetic abnormalities have been described. Ubiquitin-specific peptidase 6 (USP6) gene rearrangement is typically found in some benign bone and soft-tissue tumours including nodular fasciitis (NF), among others. Nevertheless, rare cases of USP6-rearranged tumours resembling NF with atypical features have been reported.

METHODS AND RESULTS

One index case of LGMS of the deltoid in a 56-year-old man presented the THBS2::USP6 translocation by RNA sequencing (Archer FusionPlex Sarcoma v2 panel). Further screening of 11 cases of LGMS using fluorescent in situ hybridization (FISH) analysis with a USP6 break-apart probe identified two additional cases. These cases were investigated with RNA-sequencing, and a RRBP1::USP6 translocation was detected in one. The other case was not assessable because of low-quality RNA. Noteworthy, rearranged LGMSs presented distinctive features including variable multinodular/plexiform architecture, prominent vasculature with occasional wall thickening, scattered osteoclast-like multinucleated giant cells, and peripheral lymphoid aggregates.

CONCLUSION

Our findings support the notion that among soft-tissue neoplasms with fibroblastic/myofibroblastic phenotype, USP6 rearrangement is not limited to benign tumours, and warrants further investigation of genetic changes in myofibroblastic sarcomas.

Large Nodular Fasciitis of the Shoulder Presenting as Soft Tissue Sarcoma: A Case Report

CASE

This case report describes a patient who presented with clinical and radiographic features of a soft tissue sarcoma of the shoulder. Despite having a painless and relatively large mass, a biopsy and resection revealed nodular fasciitis (NF).

CONCLUSION

This is an unusual case of a painless 10 cm mass that histopathologically was diagnosed as NF in the upper extremity with proximity to the axillary nerve and posterior humeral circumflex vessels. The USP6 rearrangement was helpful in confirming the diagnosis. Careful clinical, radiographic, and pathologic correlation is necessary in diagnosing these relatively rare tumors. In cases where there are discordant findings, molecular markers can be very helpful.

Unusual fusion gene rearrangements in patients with nodular fasciitis: a study of rare and novel USP6 fusion partners with a review of the literature

AIMS

This retrospective non-randomised study aims to identify new and rare fusion partners with USP6 in the setting of nodular fasciitis. It has been proven, that nodular fasciitis can harbour different variants of USP6 fusions, which can be used in routine diagnostics and even determine the biological behaviour of the process.

METHODS

A total of 19 cases of nodular fasciitis examined between 2011 and 2022 at Motol University Hospital in Prague were included into this study. Next to the histopathological evaluation, all cases were assessed using immunohistochemistry, RT-PCR and Anchored multiplex RNA methods. Patient's main demographic characteristics and corresponding clinical data were also analysed.

RESULTS

This study presents one novel (KIF1A) and five rare examples (TMP4, SPARC, EIF5A, MIR22HG, COL1A2) of fusion partners with USP6 among 19 cases of nodular fasciitis.

CONCLUSION

Identification of USP6 fusion partners in nodular fasciitis helps to understand the biology of such lesions. Moreover, it can be useful in routine histopathological practice of soft-tissues diagnostics, especially in preventing possible misdiagnosis of malignancy.

Spindle Cell Lipoma and Pleomorphic Lipoma in the Head and Neck: A Comprehensive Study of Six Cases With Review of Literature

BACKGROUND

Spindle cell lipomas (SL) and pleomorphic lipomas (PL) are rare variants of lipomas, occurring predominantly in the head and neck region. Laryngeal SL/PL is very uncommon and causes obstructive symptoms needing immediate intervention. These tumors are often challenging in radiology due to the admixture of elements and the presence of adipose tissue may help in diagnosis. From a surgeon's perspective, understanding the nuances of SL/PL is paramount. Histology is the gold standard for diagnosis; however, it often causes diagnostic challenges in biopsy.  Method: A retrospective review of the clinical and pathologic features of archival cases of SL/PL was performed.

RESULTS

A total of six cases of head and neck region SL/PL were identified. The age of patients ranged from 21 to 58 years and the male-to-female ratio was 5:1. The tumors were distributed in the nape of the neck (n=3), laryngeal region (n=2), and orbit (n=1). Histology in all the cases showed a low-grade neoplasm composed of a variable amount of spindle cells and adipose tissue. The stroma was myxoid in most cases. CD34 was diffusely positive in all the cases.

CONCLUSION

SLs are a rare and uncommon variant of lipoma with a predilection in the head and neck region. They are low-grade neoplasms with a propensity to recur after years. Having knowledge of this tumor can improve surgical outcomes and better patient care.

Fast-Growing Frontal Subcutaneous Nodule in a Young Woman

A woman in her 20s presented with a firm, mobile, painful, fast-growing subcutaneous nodule in the glabella. What is your diagnosis?

Sinonasal nodular fasciitis: report of a case with USP6 rearrangement

Nodular fasciitis (NF) is a benign myofibroblastic proliferation characterized by rapid growth, a self-limiting course, and USP6 gene rearrangement. Although it can arise in the head and neck region, very few cases of NF involving the sinonasal tract have been reported to date. Herein we report a case of NF involving the nasal cavity presenting as a polypoid well-defined mass causing obstructive symptoms in a male with a history of multiple local surgeries. Although the mass had an unusual clinical presentation, the histopathologic and immunohistochemical findings were consistent with NF. Fluorescent in situ hybridization performed with break-apart probes flanking the USP6 locus on chromosome 17p13 revealed the presence of USP6 gene rearrangement. The patient remained free of disease 124 months after surgical treatment. Considering its remarkably benign behavior despite its alarming clinical and histologic features, the distinction of NF from sinonasal malignant tumors is essential.

Formalin-Fixed, Paraffin-Embedded-Targeted Locus Capture: A Next-Generation Sequencing Technology for Accurate DNA-Based Gene Fusion Detection in Bone and Soft Tissue Tumors

Chromosomal rearrangements are important drivers in cancer, and their robust detection is essential for diagnosis, prognosis, and treatment selection, particularly for bone and soft tissue tumors. Current diagnostic methods are hindered by limitations, including difficulties with multiplexing targets and poor quality of RNA. A novel targeted DNA-based next-generation sequencing method, formalin-fixed, paraffin-embedded-targeted locus capture (FFPE-TLC), has shown advantages over current diagnostic methods when applied on FFPE lymphomas, including the ability to detect novel rearrangements. We evaluated the utility of FFPE-TLC in bone and soft tissue tumor diagnostics. FFPE-TLC sequencing was successfully applied on noncalcified and decalcified FFPE samples (n = 44) and control samples (n = 19). In total, 58 rearrangements were identified in 40 FFPE tumor samples, including three previously negative samples, and none was identified in the FFPE control samples. In all five discordant cases, FFPE-TLC could identify gene fusions where other methods had failed due to either detection limits or poor sample quality. FFPE-TLC achieved a high specificity and sensitivity (no false positives and negatives). These results indicate that FFPE-TLC is applicable in cancer diagnostics to simultaneously analyze many genes for their involvement in gene fusions. Similar to the observation in lymphomas, FFPE-TLC is a good DNA-based alternative to the conventional methods for detection of rearrangements in bone and soft tissue tumors.

Late-Onset Type 1 Diabetes Mellitus and Unexplained Subcutaneous Lesions

Nodular fasciitis is considered a reactive lesion of connective tissue originating from the proliferation of fibroblasts and myofibroblasts. Nodular fasciitis preponderantly localizes within the higher extremities, trunk, head, and neck. We are presenting a report on the case of a 38-year-old Navy pilot who developed nodular lesions in the area of the sternum and upper back and was diagnosed concomitantly with insulin-dependent diabetes mellitus (type 1 diabetes). The patient was treated for diabetic ketoacidosis using intensive insulin therapy protocol, and the nodules were surgically excised. He was discharged from the hospital four weeks later. In our presentation, we intend to highlight the essential characteristics of this rare lesion through a review of the literature and to identify an attainable link between the development of type 1 diabetes and nodular fasciitis.

Unravelling the USP6 gene: an update

Ubiquitin-specific protease 6 (USP6) rearrangements have been identified in aneurysmal bone cyst, nodular fasciitis, myositis ossificans, fibro-osseous pseudotumour of digits and cellular fibroma of tendon sheath. These entities show clinical as well as histological overlap, suggesting they are all clonal neoplastic belonging to the same biological spectrum and referred to as 'USP6-associated neoplasms'. They all show a characteristic gene fusion formed by juxtaposition of the USP6 coding sequences to the promoter regions of several partner genes, leading to USP6 transcriptional upregulation.

Aneurysmal bone cyst: a review

Aneurysmal bone cyst (ABC) is a benign locally destructive bone neoplasm composed of multi-loculated blood-filled cystic spaces. The most common sites of involvement are the meta-diaphysis of the long bones and posterior elements of the vertebrae. Secondary, ABC-like changes can complicate a variety of other benign and malignant primary bone neoplasms, including giant cell tumor, fibrous dysplasia, and osteosarcoma. About two-third of primary ABCs have a rearrangement of the USP6 gene, which is not present in the ABC-like changes that occur secondary to other primary bone tumors (i.e., secondary ABC). Primary ABC of bone carries a variable but generally high rate of local recurrence. This paper provides an overview of the pathophysiology, clinical presentation, radiographic and pathologic findings, treatment, and prognosis of ABC.

Case report: Large-size intramuscular nodular fasciitis, a challenging histopathologic diagnosis confirmed by molecular detection of USP6 gene rearrangement: Case report and literature review

The intramuscular subtype of nodular fasciitis (NF) is rare with lesions normally not more than 2 cm in size and characterized by pseudosarcomatous morphology. We report a case of a 27-year-old man with a large-size intramuscular NF. The patient came for treatment complaining of an increasingly enlarged mass in the left upper arm for 4 months. Magnetic resonance imaging (MRI) confirmed the presence of a well-defined tumor measuring 5 cm within the outer edge of the middle humerus. Microscopically, the neoplasm was rich in fibroblasts and myofibroblasts in an interlaced pattern with high mitotic index and evident multinuclear giant cells. Erythrocyte extravasation was easily seen in the stroma. The tumor border was infiltrative. Immunohistochemically, the tumor cells were positive for smooth muscle actin (SMA) and negative for cytokeratin, desmin, H-Caldesmon, CD34, S100, ALK, and β-catenin. Fibrosarcoma was highly suspected by histopathological and immunohistochemical examination. Molecular detection demonstrated evidence of ubiquitin-specific peptidase 6 (USP6) gene rearrangement in this tumor. Based on the findings, the tumor was diagnosed as intramuscular NF. At 56 months after the initial surgery, the patient had recovered with no evidence of recurrence or metastasis. Large-size intramuscular NF is very rare and easily overdiagnosed as malignant tumor due to its obvious pseudosarcomatoid pathological features. USP6 gene rearrangement detection can effectively avoid this major misdiagnosis.

USP6-associated soft tissue tumors with bone metaplasia: Clinicopathologic and genetic analysis and the identification of novel USP6 fusion partners

Introduction

Among those tumors with consistent USP6 rearrangement, some arise from soft tissue and show bone metaplasia, including myositis ossificans (MO), fibro-osseous pseudotumor of digits (FOPD), soft tissue aneurysmal bone cyst (ST-ABC) and fasciitis ossificans (FO). These lesions are easily confused with malignancies because they show a rapid growth rate and brisk mitoses. Here, we aim to clarify the clinicopathologic and genetic characteristics of this entity and analyze the correlations among the different subtypes in one of the largest cohorts.

Materials and Methods

The clinicopathologic features of 73 cases of MO, FOPD, ST-ABC and FO diagnosed at West China Hospital, Sichuan University from January 2010 to December 2021 were retrospectively analyzed. Forty-three undecalcified samples were analyzed by systematic genetic studies, including fluorescence in situ hybridization (FISH), reverse transcription polymerase chain reaction (RT-PCR), Sanger sequencing and next-generation-based sequencing were performed.

Results

This series included 40 males and 33 females aged 2 to 80 years old (median: 31 years). FOPD occurred in extremal soft tissue, while lower extremities (38/58, 65.5%) were the most commonly involved lesions in the other three subgroups. Histologically, proliferative myofibroblasts/fibroblasts with varying degrees of osteoid tissue were present. Fluorescence in situ hybridization (FISH) results indicated that 22 cases (22/27, 81.5%) were positive for USP6 rearrangement, and 5 cases were negative. Among those cases with positive FISH results, 18 underwent reverse transcription-polymerase chain reaction (RT-PCR) detection that successfully detected common USP6 fusion types. Thirteen cases showed COL1A1::USP6 fusion, one showed MYH9::USP6 fusion, and 4 were negative for common fusion types. Next-generation-based sequencing technology was performed on two lesions with negative RT-PCR results and novel fusion partners SNHG3 and UBE2G1 were discovered.

Conclusions

Our findings revealed that COL1A1 is the most common fusion partner in this entity, unlike primary aneurysmal bone cysts and nodular fasciitis. Notably, we believed that FO may demonstrate more similar clinicopathologic and genetic manifestations with MO/FOPD and ST-ABC instead of nodular fasciitis for involving lower limbs most frequently and showing recurrent COL1A1::USP6 fusion. Additionally, this study also found two novel USP6 fusion partners, which further expanded our knowledge of this neoplastic spectrum.

Infantile Fibrosarcoma and Other Spindle Cell Neoplasms of Infancy. A Review of Morphologically Overlapping yet Molecularly Distinctive Entities

BACKGROUND

Regardless of age at presentation, many soft tissue neoplasms have overlapping histopathologic and immunophenotypic features to serve as a diagnostic challenge.

CASE REPORT

We reported a case of a spindle cell neoplasm in an infant, which was initially considered a vascular anomaly clinically and an eventual biopsy revealed marked inflammation with a spindle cell component that was resolved as an infantile fibrosarcoma with an ETV6 break-apart.

CONCLUSION

The context of this case lead to a further consideration of various other spindle cell neoplasms arising predominantly in the soft tissues during the infancy period as defined by the first two years of age. Though sharing similar morphologic features, these tumors can be categorized into several molecular genetic groups, which have provided both diagnostic and pathogenetic insights as well as treatment options in some cases.

Neoplasia-associated Chromosome Translocations Resulting in Gene Truncation

Chromosomal translocations in cancer as well as benign neoplasias typically lead to the formation of fusion genes. Such genes may encode chimeric proteins when two protein-coding regions fuse in-frame, or they may result in deregulation of genes via promoter swapping or translocation of the gene into the vicinity of a highly active regulatory element. A less studied consequence of chromosomal translocations is the fusion of two breakpoint genes resulting in an out-of-frame chimera. The breaks then occur in one or both protein-coding regions forming a stop codon in the chimeric transcript shortly after the fusion point. Though the latter genetic events and mechanisms at first awoke little research interest, careful investigations have established them as neither rare nor inconsequential. In the present work, we review and discuss the truncation of genes in neoplastic cells resulting from chromosomal rearrangements, especially from seemingly balanced translocations.

Nodular fasciitis of the periorbital soft tissue in an adolescent confirmed by USP6 gene rearrangement

Nodular fasciitis is a benign, idiopathic condition that can simulate both benign and malignant neoplasms. In adults, it generally occurs in the subcutaneous or superficial fascia of the trunk or upper extremities; occurrence in the periorbital region is far less common. We describe a case of a 16-year-old male with a 4-month history of a nodular, non-tender, progressively enlarging mass of the superotemporal periorbita. Histopathologic analysis of the excisional biopsy demonstrated nodular fasciitis, confirmed by molecular cytogenetic analysis that showed rearrangement of USP6.

Retrospective immunohistochemical investigation of suspected non-visceral leiomyosarcoma in dogs

Visceral leiomyosarcoma is well described in dogs, but information about non-visceral locations and prevalence is lacking. The diagnosis of leiomyosarcoma is challenging without a gold standard, and often includes the use of immunohistochemical (IHC) stains. We used defined histopathologic patterns, histochemical staining, and IHC staining for smooth muscle actin (SMA), desmin, and laminin to characterize suspected non-visceral leiomyosarcoma in dogs at a single academic institution. In a retrospective search, we identified 24 dogs with a definitive or suspected histologic diagnosis of leiomyosarcoma in a non-visceral location. Histopathology results and clinical details were obtained. Biopsy sections were reviewed by a single pathologist using standardized histologic criteria, including light microscopic appearance, immunohistochemistry (more than two-thirds of neoplastic cells labeled with SMA and desmin or laminin), and histochemical staining (minimal-to-mild matrix deposition by Masson trichrome). Of the 24 cases of possible non-visceral leiomyosarcomas, 4 were consistent with a definitive diagnosis of non-visceral leiomyosarcoma (3) or leiomyoma (1) based on the established criteria. Only the leiomyoma had more than two-thirds of neoplastic cells label with all 3 markers; all 3 leiomyosarcomas had more than two-thirds of neoplastic cells label with SMA and laminin. Our data highlight the uncommon nature of non-visceral leiomyosarcoma and the importance of IHC for their diagnosis. A definitive diagnosis could not be made based on SMA alone, and desmin was not useful in this cohort. Further studies are needed to clarify the histopathologic, IHC, and clinical features of canine non-visceral SMA-positive mesenchymal tumors.

Cranial Fasciitis in Children: Expanding the Spectrum of USP6-Associated Clonal Transient Neoplasms

Background: Cranial fasciitis (CF) is a benign (myo)fibroblastic proliferation of children. Typical presentation consists of a rapidly growing solitary mass on the temporal or parietal cranium in the first 2 years of age. CF is characterized by a rapid growth followed by a relative slowdown and even growth arrest. The finding of somatic USP6 gene rearrangements demonstrating clonality in CF together with its clinical behavior places it in the category of diseases recently termed "transient neoplasia."Methods: Histological, immunohistochemical, and molecular findings of 18 patients with CF were retrospectively studied.Results: The tumor typically presented as a painless rapidly enlarging mass in the temporal region. Sixty-six percent of the cases harbored USP6 gene rearrangement. Nine patients were treated with gross total resection (GTR) and 9 with subtotal tumor resection (STR). Two patients treated with GTR had recurrence. Five patients treated with STR had progression-free disease for at least 10 months after surgery and in four patients the tumor regressed spontaneously a median 16 months after surgery.Conclusions: In this largest series to date, we reported the clinicopathological, immunohistochemical, and molecular findings of 18 pediatric cases of CF with emphasis on the clinical growth pattern of these tumors.

USP6 rearrangement in pediatric nodular fasciitis

Nodular fasciitis (NF) is a myofibroblastic proliferation that uncommonly presents in pediatric patients. These benign neoplasms can masquerade as more insidious sarcomatous proliferations on both clinical exam and initial histopathologic review, often prompting undue concern in patients, parents and providers. While immunohistochemical analysis of NF can be variable, adding to the diagnostic uncertainty, molecular analysis documenting ubiquitin-specific protease 6 (USP6) gene rearrangement can help confirm the diagnosis as an association between NF and USP6 overexpression was first identified 10 years ago in an analysis that found rearrangements of the involved locus in over 90% of studied samples.¹ In this report, we review one case of nodular fasciitis located on the chin of a 9-year-old in which molecular testing was essential to secure the correct diagnosis, and provide a summary of documented cases of USP6 overexpression in transient pediatric neoplasms. This article is protected by copyright. All rights reserved.

Cellular Fibroma of Tendon Sheath With Novel TNC-USP6 Gene Fusion Clinically Mimicking Arthritis in a 7-Year-Old Boy

Cellular fibroma of tendon sheath (CFTS) is a rare, benign myofibroblastic neoplasm of tenosynovial soft tissues closely resembling nodular fasciitis (NF), but is histomorphologically distinct from classic fibroma of tendon sheath (FTS). We report a case of a pediatric patient with thumb swelling clinically concerning for arthritis with a biopsy demonstrating myofibroblastic proliferation with features consistent with NF/CFTS, and molecular studies confirming the presence of a USP6 gene fusion (TNC-USP6). This case highlights a unique clinical presentation of CFTS in a pediatric patient mimicking an inflammatory or reactive/non-neoplastic musculoskeletal disorder and the increasingly crucial role of molecular testing to differentiate a reactive myofibroblastic process from a neoplasm. Moreover, this report identifies TNC as a new fusion partner to USP6 fusion partner adding to our growing understanding of the USP6-rearranged family of tumors.

Intraneural Nodular Fasciitis of the Femoral Nerve with A Unique CTNNB1::USP6 Gene Fusion: Apropos of a Case and Review of Literature

Nodular fasciitis (NF) is a benign proliferation of fibroblasts and myofibroblasts occurring most commonly in the upper extremities that can mimic a variety of mesenchymal tumors including sarcoma. Although reported in almost all anatomic locations, only 7 cases of intraneural nodular fasciitis have been reported in English literature. The CTNNB1::USP6 gene fusion has not been previously reported in intraneural nodular fasciitis, although it has been reported in three entities including aneurysmal bone cyst, nodular fasciitis, and intravascular fasciitis. We report a case of a 29-year-old female with a 6-month history of left leg weakness, myalgia, and paresthesia of the left foot prompting a clinical diagnosis of a peripheral nerve sheath tumor. Surgical resection was performed, and histologic sections revealed a circumscribed lesion composed of banal spindle cells with variable interstitial collagen and occasional mitotic figures. By immunohistochemistry, the lesional cells were positive for smooth muscle actin, smooth muscle heavy chain myosin, p16, and H-caldesmon and negative for desmin, S-100, SOX10, HMB45, CD34, and beta-catenin. Fluorescence in Situ Hybridization for USP6 gene rearrangement was positive and consistent with the diagnosis of nodular fasciitis. Next-generation sequencing uncovered the presence of a CTNNB1::USP6 gene fusion involving CTNNB1 gene in exon 1 at the genomic position chr3:41241161 and the USP6 gene in exon 1 at the genomic position chr17:5033231. This gene fusion was confirmed by Sanger sequencing. Herein, we report a case that underscores the rare incidence of intraneural nodular fasciitis and highlights the pitfalls associated with the clinical differential diagnoses of intraneural tumors.

Laryngeal Manifestation of Nodular Fasciitis: A Case Report and Literature Review

Nodular fasciitis (NF) are non-neoplastic, fibroblastic lesions, typically located on the trunk and the extremities. The incidence of NF in the head and neck region is 13%-20%. However, a manifestation in the larynx of adult patients is extremely rare. Therefore, the occurrence of NF in this region can lead to diagnostic challenges and a high risk of misdiagnosis as well as potential mishandling when not aware of its possible laryngeal manifestation. Following emergency admission of a 41-year-old woman to the emergency department (ED) due to progressive dyspnea and inspiratory stridor a transnasal flexible laryngeal endoscopic examination revealed a left-lateral, subglottic mass. A subsequently performed CT demonstrated a 2.2 cm x 1.5 cm sized lesion of the subglottic larynx with profound stenosis of the lumen (Myer-Cotton grade III), no extraluminal extension, and no distant metastases. Histopathological processing of a tissue sample obtained by microlaryngoscopy and translaryngeal tracheoscopy revealed a spindle-cell lesion with immunohistochemical and molecular-pathogenic profile of NF. After tumor debulking and steroid infiltration (triamcinolone), a regrowth tendency quickly became apparent, which is why a tracheostomy had to be performed. Laryngectomy was rejected by the patient. After multiple transoral tumor reduction attempts, radiotherapy was performed according to an interdisciplinary tumor board decision to limit regrowth tendency. Subsequently, a substantial reduction of the tumor volume could be seen, although a discreet stenosis of the subglottic tracheal lumen persists in the follow-up. Laryngeal NF poses several challenges due to its rare occurrence in this location. This case report emphasizes the knowledge of this differential diagnosis and also depicts an interdisciplinary therapeutic approach aiming for function-preserving treatment of this benign but potentially relapsing pathology.

Fibroma of tendon sheath is defined by a USP6 gene fusion-morphologic and molecular reappraisal of the entity

Fibroma of tendon sheath (FTS) is an uncommon benign myofibroblastic neoplasm that arises in association with tenosynovial tissue. Fusions of the USP6 gene have been recently documented in a proportion of so-called "cellular FTS" but not in "classic FTS". It remains unknown whether FTS can be defined by a USP6 fusion, regardless of cellularity, and what are USP6 fusion-negative "cellular FTS". Furthermore, FTS with low cellularity seems to be frequently confused with desmoplastic fibroblastoma. We performed a comprehensive analysis, including targeted RNA sequencing, of 58 consecutive cases originally diagnosed as FTS (n = 49), desmoplastic fibroblastoma (n = 6), or nodular fasciitis (n = 3); the latter two at the predilection sites for FTS. After review of the original slides, 28 lesions were morphologically classified as FTS (13 "classic" and 15 "cellular") and 23 as desmoplastic fibroblastoma. Among originally diagnosed FTS at the more cellular end of the spectrum, we identified seven lesions that shared many morphologic features of FTS but, in addition, showed several distinct morphologic features consistent with myofibroma, such as myoid appearance, branching thin-walled vessels, and perivascular growth. Targeted RNA sequencing showed a USP6 fusion in 17 of 18 analyzed FTS, regardless of cellularity, 0 of 5 desmoplastic fibroblastomas and 0 of 4 myofibromas. MYH9, COL1A1, and ASPN were identified as fusion partners in three cases each, and MIR22HG, CTNNB1, SPARC, CAP1, EMP1, LINC00152, NR1D1, and RAB1A in a single case each. FTS, regardless of cellularity, can be defined by a USP6 fusion with a variety of fusion partners. More cellular lesions exhibiting some morphologic features of FTS but lacking a USP6 fusion tend to be myofibromas.

Nodular fasciitis of the breast: clinicopathologic and molecular characterization with identification of novel USP6 fusion partners

Nodular fasciitis is a benign, self-limited, pseudosarcomatous neoplasm that can mimic malignancy due to its rapid growth, cellularity, and mitotic activity. Involvement of the breast is rare and diagnosis on biopsy can be challenging. In this largest series to date, we examined the clinicopathologic and molecular characteristics of 12 cases of nodular fasciitis involving the breast/axilla. All patients were female, with a median age of 32 years (range 15-61). The lesions were 0.4 to 5.8 cm in size (median 0.8). All cases presented as palpable masses, and two patients had overlying skin retraction. Microscopically, lesions were relatively well-circumscribed nodular masses of bland myofibroblastic spindle cells within a variably myxoid stroma. Infiltrative growth into adipose tissue or breast epithelium was frequent. Mitotic figures were present in all cases, ranging from 1 to 12 per 10 high-power fields (median 3). Immunohistochemically, all cases expressed smooth muscle actin and were negative for pan-cytokeratin, p63, desmin, CD34, and nuclear beta-catenin. Targeted RNA sequencing performed on 11 cases identified USP6 gene fusions in eight; one additional case was positive by break-apart fluorescence in situ hybridization. The common MYH9-USP6 rearrangement was detected in four cases; another case had a rare alternative fusion with CTNNB1. Three cases harbored novel USP6 gene fusions involving NACA, SLFN11, or LDHA. All fusions juxtaposed the promoter region of the 5' partner gene with the full-length coding sequence of USP6. Outcome data were available for eight patients; none developed recurrence or metastasis. Five patients elected for observation without immediate excision, and self-resolution of the lesions was reported in three cases. Albeit uncommon, nodular fasciitis should be considered in the differential diagnosis of breast spindle cell lesions. A broad immunohistochemical panel to exclude histologic mimics, including metaplastic carcinoma, is important. Confirmatory detection of USP6 rearrangements can aid in classification, with potential therapeutic implications.

Nodular fasciitis of the breast: Report of two cases illustrating the diagnostic implications for USP6 gene rearrangement and brief review of the literature

Nodular fasciitis is a benign, self-limited, pseudosarcomatous neoplasm that is cytogenetically characterized by recurrent USP6 gene rearrangement. Involvement of the breast by nodular fasciitis is very rare with only a few documented cases. It can clinically, radiologically and histologically mimic a malignancy, posing significant diagnostic challenges to clinicians, radiologists, and pathologists. In this study, we report 2 cases of nodular fasciitis occurring in the female breast, reviewing the literature and emphasizing the application of fluorescence in situ hybridization analysis of USP6 gene rearrangement in its diagnosis and differential diagnosis.

Papillary Thyroid Carcinoma with Desmoid-Type Fibromatosis: Review of Published Cases

Simple Summary

Papillary thyroid cancer (PTC)-desmoid type fibromatosis (DTF) is one of the rarest variants of PTC. The diagnosis is histological, and detecting a mutation of CTNNB1 in the mesenchymal component is highly suggestive of PTC-DTF. The treatment is essentially surgical. We conducted a review of all cases of PTC-DTF found in the English literature and our aim is to describe patient’s characteristics, histology, immunohistochemistry and somatic mutations of every case.

Abstract

Desmoid-type fibromatosis (DTF) is a very rare variant of papillary thyroid carcinoma (PTC). It is essentially a dual tumor with a component of classical PTC with malignant epithelial proliferation (BRAF-mutated) and another component of mesenchymal proliferation (CTNNB1-mutated). We conducted a literature review on PTC-DTF. In total, 31 articles were identified, that together reported on 54 patients. The mean age was 47 years, with a 2.2:1 female predominance. No ultrasound features were found to be helpful in differentiating PTC-DTF from other PTC variants. Of the 43 cases that reported histological details, 60% had locally infiltrative disease (T3b or T4). Around 48% had cervical lymph node metastases, but none had distant metastases. While PTC-DTF may be locally more aggressive than classic PTC, its overall behavior is similar and can include extrathyroidal extension and lymph node metastases, which may contain a stromal component and show extranodal invasion. The mainstay of treatment for PTC-DTF is surgery, and the DTF component is not expected to be sensitive to radioactive iodine. External radiotherapy, non-steroidal anti-inflammatory drugs, tyrosine kinase inhibitors and chemotherapy have also been used in selected cases. Due to the rarity of these tumors and the lack of specific treatment guidelines, management should be discussed in a multidisciplinary team.

Pediatric and Infantile Fibroblastic/Myofibroblastic Tumors in the Molecular Era

Pediatric fibroblastic/myofibroblastic tumors are rare but include a wide variety of benign to malignant tumors. Given their uncommon frequency, they may present as a diagnostic dilemma. This article is focused on using clinical and pathologic clues in conjunction with the increasingly relevant and available molecular techniques to classify, predict prognosis, and/or guide treatment in these tumors.

Molecular Alterations in Pediatric Fibroblastic/Myofibroblastic Tumors: An Appraisal of a Next Generation Sequencing Assay in a Retrospective Single Centre Study

BACKGROUND

Pediatric fibroblastic/myofibroblastic tumors (PFMTs) can be challenging to definitively classify. Large case series or diagnostic updates have not been recently published despite identification of molecular alterations that could improve diagnostic accuracy. Our review of the literature found that over two-thirds of the more than 30 types of PFMTs harbor recurrent molecular alterations. We performed an institutional review of PFMTs to highlight limitations of a predominantly morphological classification, and evaluated the utility of a next-generation sequencing assay to aid diagnosis.

METHODS

PFMTs identified over a period of 12 years were reviewed, categorized per the new WHO classification, and tested using the Oncomine Childhood Cancer Research Assay.

RESULTS

Eighty-seven specimens from 58 patients were reviewed; 50 were chosen for molecular analysis, 16 (32%) lacking definitive classification. We identified alterations, some novel, in 33% of assayed cases. Expected alterations were identified for most known diagnoses and mutations were identified in 6 of 16 tumors (38%) that were initially unclassified.

CONCLUSION

We confirmed a significant subset of PFMTs remain difficult to classify using current criteria, and that a combined DNA/RNA assay can identify alterations in many of these cases, improving diagnostic certainty and suggesting a clinical utility for challenging cases.

Aneurysmal bone cyst with an unusual clinical presentation and a novel VDR-USP6 fusion

Aneurysmal bone cyst is a benign bone neoplasm that most commonly arises from the metaphyses of long bones in the first and second decades of life. Here, we describe a case of an aneurysmal bone cyst that occurred in the distal tibial diaphysis of a 72-year-old female that was concerning for malignancy on imaging, demonstrating cortical breakthrough and soft tissue extension. Histologically, the tumor showed the characteristic morphologic features of aneurysmal bone cyst. Fluorescence in situ hybridization was positive for USP6 rearrangement, and RNA sequencing revealed a USP6 gene fusion with VDR, a novel partner that encodes the vitamin D receptor and that has not been implicated previously in human neoplasia. This case highlights the diagnostic challenges presented by aneurysmal bone cyst in elderly adults, and it expands the genetic spectrum of USP6 rearrangements.

Clinical Utility of Anchored Multiplex Solid Fusion Assay for Diagnosis of Bone and Soft Tissue Tumors

Sarcoma diagnosis has become increasingly complex, requiring a combination of morphology, immunohistochemistry, and molecular studies to derive specific diagnoses. We evaluated the role of anchored multiplex polymerase chain reaction-based gene fusion assay in sarcoma diagnostics. Between 2015 and 2018, bone and soft tissue sarcomas with fusion assay results were compared with the histologic diagnosis. Of 143 sarcomas tested for fusions, 43 (30%) had a detectable fusion. In review, they could be classified into 2 main categories: (1) 31 tumors with concordant morphologic and fusion data; and (2) 12 tumors where the fusion panel identified an unexpected rearrangement that played a significant role in classification. The overall concordance of the fusion assay results with morphology/immunohistochemistry or alternate confirmatory molecular studies was 83%. Collectively, anchored multiplex polymerase chain reaction-based solid fusion assay represents a robust means of detecting targeted fusions with known and novel partners. The predictive value of the panel is highest in tumors that show a monomorphic cell population, round cell tumors, as well as tumors rich in inflammatory cells. However, with an increased ability to discover fusions of uncertain significance, it remains essential to emphasize that the diagnosis of bone and soft tissue neoplasms requires the integration of morphology and immunohistochemical profile with these molecular methods, for accurate diagnosis and optimal clinical management of sarcomas.

Morphologically malignant nodular fasciitis with CALD1-USP6 fusion

Nodular fasciitis is a benign myofibroblastic neoplasm that characteristically enlarges rapidly and then usually regresses spontaneously. While the vast majority of tumors are benign, there are rare reports of morphologically benign nodular fasciitis giving rise to metastases, not predictable on histologic grounds. Here, we report what we believe is an example of morphologically malignant nodular fasciitis, which occurred in the upper extremity of a 7-year-old male. The tumor was composed of short, intersecting fascicles of myofibroblastic cells in a loose myxoid matrix, with keloidal hyalinization and admixed osteoclastic giant cells, all characteristic of nodular fasciitis. However, it additionally exhibited striking nuclear pleomorphism, a feature not compatible with conventional nodular fasciitis. Fluorescence in situ hybridization demonstrated a USP6 translocation, confirmed by next-generation sequencing to be the novel CALD1-USP6 fusion. No other somatic or germline mutations were detected. This case adds to the expanding morphologic and molecular genetic spectrum of nodular fasciitis.

Optimized workflow for digitalized FISH analysis in pathology

BACKGROUND

Effective workflow management in a diagnostic pathology laboratory is critical to achieve rapid turnover while maintaining high quality. Fluorescence in situ hybridization analysis (FISH) is the preferred technique for detecting single chromosomal aberrations in diagnostic surgical pathology.

MATERIAL AND METHODS

FISH analysis applying a rapid hybridization protocol and using an automated whole-slide fluorescence scanning device (3DHISTECH, Sysmex, Switzerland) were implemented in our workflow. By analyzing 42 diagnostic cases, effects of two different scanning profiles on scanning time, and device memory usage were investigated. Manual signal counting (CaseViewer) and software based signal counting (FISHQuant) were compared.

RESULTS

The two scanning profiles, both including a Z-stack function, differed in their exposure time and digital gain. The "low profile" setting (LP) resulted in a significantly shorter scanning time and lower storage volume compared to the "high profile" (HP) setting, making the LP ideal for routine applications. Both signal counting methods (manual versus software based) provided similar cut-offs on a test-cohort of 13 samples.

CONCLUSION

Scanning FISH slides provides good picture quality, reduces the analysis time and allows easy picture archiving and facilitates remote diagnostics, allowing an effective workflow.

Fusion of the Lumican (LUM) Gene With the Ubiquitin Specific Peptidase 6 (USP6) Gene in an Aneurysmal Bone Cyst Carrying a t(12;17)(q21;p13) Chromosome Translocation

BACKGROUND/AIM

Aneurysmal bone cyst is a benign bone lesion with a strong tendency to recur. The rearrangement of chromosome band 17p13/USP6 gene is now considered a characteristic genetic feature of aneurysmal bone cyst, with t(16;17)(q22;p13)/CDH11-USP6 as the most frequent chromosomal aberration/fusion gene. We report a novel variant translocation leading to a new fusion gene in an aneurysmal bone cyst.

MATERIALS AND METHODS

Genetic analyses were performed on an aneurysmal bone cyst found in the tibia of a child.

RESULTS

G-banding chromosome analysis yielded the karyotype 46,XX,t(12;17)(q21;p13)[5]/46,XX[2]. FISH analysis with a USP6 break-apart probe showed rearrangement of USP6. RNA sequencing detected LUM-USP6 and USP6-LUM fusion transcripts which were subsequently verified by RT-PCR/Sanger sequencing. The two genes exchanged 5'- non-coding exons. Thus, promoter swapping between USP6 and LUM had taken place.

CONCLUSION

We report a novel t(12;17)(q21;p13) chromosome translocation which gave rise to a LUM-USP6 fusion in an aneurysmal bone cyst.

Nodular fasciitis on the cheek of a child

Nodular fasciitis is a benign proliferation of myofibroblastic cells affecting subcutaneous tissue, muscles, and fascia. This rare disorder is most commonly observed on the upper extremity of adults. We present a case of nodular fasciitis of the cheek of a 12-year-old girl.

Nodular Fasciitis With Malignant Morphology and a COL6A2-USP6 Fusion: A Case Report (of a 10-Year-old Boy)

Nodular fasciitis is usually a benign lesion genetically characterized by ubiquitin-specific protease 6 (USP6) rearrangements. We present a case of a 10-year-old boy with a 1.5-week history of a painless mass on the right chest wall, which was excised. A histomorphologically malignant tumor with pronounced pleomorphism, atypical mitotic figures, and a myoid immunophenotype was observed. The methylation profile was consistent with nodular fasciitis and fluorescence in situ hybridization confirmed USP6 rearrangement. Using Archer Fusion Plex (Sarcoma Panel) and RNA sequencing, a collagen, type VI, alpha 2 (COL6A2)–USP6 gene fusion was subsequently identified. Furthermore, DNA clustering analysis also showed a match with nodular fasciitis. During the follow-up of 22 months, no recurrence or metastasis occurred. In conclusion, we describe a clinically benign, histomorphologically malignant mesenchymal neoplasm with a myoid immunophenotype, and a genetic and epigenetic profile consistent with nodular fasciitis. In such cases, molecular analysis is a useful adjunct to avoid unnecessary overtreatment.

Ubiquitin-specific Peptidase 6 (USP6)-associated Fibroblastic/Myofibroblastic Tumors: Evolving Concepts

Ubiquitin-specific peptidase 6 (USP6) is a hominoid-specific gene residing on chromosome 17p13 and serves as a deubiquitinating enzyme with a diverse set of functions including intracellular trafficking, inflammatory signaling, cell transformation and protein turnover. USP6 rearrangements were first identified in aneurysmal bone cysts, resulting in promoter swapping and over-expression of wild type USP6. Several morphologically overlapping fibroblastic/myofibroblastic tumors are known to harbor USP6 rearrangements, including nodular fasciitis, cellular fibroma of tendon sheath, myositis ossificans and fibro-osseous pseudotumor of digits. Over the past few years, fusions involving the USP6 gene and various partner genes have been described in these neoplasms. The current World Health Organization Classification of Tumors of Soft Tissue suggests that USP6-rearranged lesions are typically benign and usually self-limited in their growth. This review provides an updated overview of the clinical, histological and molecular genetic features of USP6-associated fibroblastic/myofibroblastic tumors and discusses how these lesions should be best classified.