Pathologic evaluation of the solid variant of aneurysmal bone cysts with USP6 rearrangement with an emphasis on the frequent diagnostic pitfalls

Solid aneurysmal bone cyst of cuboid: rare location and diagnostic conundrum

The solid variant of aneurysmal bone cyst is an expansile cystic lesion involving long tubular bones with unknown aetiology and usually occurs in the second decade of life. This is a paradigm of fibrocystic lesions. We report the rare case of a male with a biopsy-proven solid variant aneurysmal bone cyst of the cuboid that was mistakenly diagnosed as osteomyelitis. He presented with clinical features suggestive of infective aetiology in the cuboid and was planned for debridement as the radiological investigations were also in agreement. Intraoperatively, the absence of pus and the gross appearance of cuboid raised suspicion and the tissue sampled for biopsy revealed a solid variant of aneurysmal bone cyst. The patient was managed with thorough curettage and antibiotic-filled calcium granule augmentation. The patient returned to his normal functional level without evidence of any recurrence.

Benign Bone Tumors of the Hand: Beyond Enchondromas

Beyond enchondromas, the most common bone tumors of the hand, there are numerous less common benign bone tumors and mimickers with which orthopaedic and hand surgeons should be familiar. These include other benign cartilage tumors, cystic lesions, osteogenic tumors, giant cell tumor, and fibrous dysplasia. Particularly unique lesions include bizarre parosteal osteochondromatous proliferation (Nora lesion), florid reactive periostitis, turret exostosis (acquired osteochondroma), giant cell reparative granuloma (solid aneurysmal bone cyst), and epidermoid cyst. The purpose of this review was to present the epidemiology of this group of lesions as a whole and then focus on the unique radiographic and histological features, natural history, and appropriate management of each of the lesions. Unlike those of enchondromas, malignant bone tumors, and soft-tissue tumors of the hand, we are unaware of a previous comprehensive orthopaedic review focused on these less common benign hand processes.

Brown Tumour in Chronic Kidney Disease: Revisiting an Old Disease with a New Perspective

Osteitis fibrosa cystica (OFC) and Brown Tumours are two related but distinct types of bone lesions that result from the overactivity of osteoclasts and are most often associated with chronic kidney disease (CKD). Despite their potential consequences, these conditions are poorly understood because of their rare prevalence and variability in their clinical manifestation. Canonically, OFC and Brown Tumours are caused by secondary hyperparathyroidism in CKD. Recent literature showed that multiple factors, such as hyperactivation of the renin-angiotensin-aldosterone system and chronic inflammation, may also contribute to the occurrence of these diseases through osteoclast activation. Moreover, hotspot KRAS mutations were identified in these lesions, placing them in the spectrum of RAS-MAPK-driven neoplasms, which were until recently thought to be reactive lesions. Some risk factors contributed to the occurrence of OFC and Brown Tumours, such as age, gender, comorbidities, and certain medications. The diagnosis of OFC and Brown Tumours includes clinical symptoms involving chronic bone pain and laboratory findings of hyperparathyroidism. In radiological imaging, the X-ray and Computed tomography (CT) scan could show lytic or multi-lobular cystic alterations. Histologically, both lesions are characterized by clustered osteoclasts in a fibrotic hemorrhagic background. Based on the latest understanding of the mechanism of OFC, this review elaborates on the manifestation, diagnosis, and available therapies that can be leveraged to prevent the occurrence of OFC and Brown Tumours.

Frequency of translocation of USP-6 in the aneurysmal bone cyst of the jaw

BACKGROUND

This study aims to detect USP6 translocation in aneurysmal cysts located in the jaw and to give an overview of demographic data.

METHODS

The present retrospective cohort study includes 10 patients who underwent surgery due to an aneurysmal cyst of the jaw in our hospital between 2002 and 2021. Unstained formalin-fixed and paraffin-embedded tissue sections cut at 4 μm thickness were subjected to USP6 FISH testing.

RESULTS

All patients underwent surgical treatment. In four of ten patients (40%) USP-6-translocations have been found.

CONCLUSION

Based on the study, it is hypothesized that the aneurysmal bone cyst of the jaw bone may be subject to a different pathomechanism than that of the long bones. Therefore, it seems conceivable that the primary cause of aneurysmal bone cysts in the jaw might differ.

Solid-variant aneurysmal bone cysts in the craniofacial skeleton: the role of genomic analysis

BACKGROUND

Solid variant aneurysmal bone cysts (SVABCs) are a rare but well-described subtype of ABCs. While classic ABCs are readily identified radiographically, SVABCs lack these characteristic radiographic features and thus have a wide differential diagnosis on presentation (including Ewing sarcoma, Langerhans cell histiocytosis, osteosarcoma, metastasis, and giant cell tumor). Genomic/molecular analyses are often necessary for the diagnosis of SVABCs, with USP6 rearrangements being a characteristic finding. We present two cases in which genomic analysis was critical in the diagnosis of SVABCs and revealed unique gene fusions that may provide insight into SVABC pathogenesis.

CASE DESCRIPTIONS

Two 13-year old male children presented to our institution with new mass lesions involving the craniofacial skeleton. Magnetic resonance imaging (MRI) in both cases revealed predominantly solid, avidly enhancing masses, one of the squamous portion of the temporal bone, and the other arising from the sphenopalatine foramen with extension into the ipsilateral maxillary and ethmoid sinuses. Histopathology displayed predominantly solid morphology, and next generation sequencing (NGS) revealed a FAT1-USP6 gene fusion in the temporal lesion, and a MIR22HG-USP6 gene fusion in the maxillofacial lesion, the latter of which was not identified on fluorescence in situ hybridization (FISH). These findings were most consistent with a diagnosis of SVABC in each case.

CONCLUSIONS

These two cases highlight novel gene fusions in atypically located SVABCs and emphasize the ability of NGS to more accurately and consistently identify USP6 gene fusions, particularly in SVABCs that may otherwise be indistinguishable from alternative pathologies.

Clinicopathological and molecular characterisation of USP6-rearranged soft tissue neoplasms: the evidence of genetic relatedness indicates an expanding family with variable bone-forming capacity

AIMS

USP6 rearrangement underpins self-limiting fibroblastic/myofibroblastic neoplasms, including nodular fasciitis (NF), myositis ossificans (MO), aneurysmal bone cyst (ABC), and related variants. The aim of this study was to characterise UPS6 and fusion partners in order to delineate the clinicopathological, genetic and bone-forming features in such lesions of soft tissue (ST).

METHODS AND RESULTS

Break-apart fluorescence in-situ hybridisation (FISH) validated USP6 rearrangement in 31 of 35 NF [comprising three of three fasciitis ossificans (FO) cases, seven of eight cellular variant of fibroma of tendon sheath (C-FTS), four of six MO, three of three ST-ABC, and two of two fibro-osseous pseudotumours of digits (FOPD)]. As determined with FISH and reverse transcription polymerase chain reaction, MYH9-USP6 was the commonest fusion in four C-FTS and 20 NF, including one intravascular case and two infantile (one retroperitoneal) cases. The presence of MYH9-USP6 confirmed the diagnosis of two NFs> 50 mm with prominent ischaemic necrosis. COL1A1-USP6 was predominant in ossifying lesions, including all FO, MO, ST-ABC and FOPD with identified partner genes, and was also present in non-ossifying head and neck NF (HN-NF) and C-FTS in two cases each. A cervical NF of a 14-month-old girl harboured the novel COL1A2-USP6. Ossifying lesions showed considerable genetic and morphological overlaps. Sharing COL1A1-USP6, FO and FOPD showed similar central or haphazard bone matrix deposition. Besides zonation of outward bone maturation, four COL1A1-USP6-positive MO had incipient to sieve-like pseudocysts reminiscent of ST-ABC.

CONCLUSION

MYH9-USP6 is present in some C-FTS and most NF, including rare variants, but is unrelated to bone formation. All bone-forming USP6-rearranged lesions adopt COL1A1 as the 5' partner, indicating close genetic kinships. However, COL1A1/COL1A2 also contributes to the pathogenesis of minor subsets of non-ossifying USP6-rearranged HN-NF and C-FTS.