USP8-Rearranged Mesenchymal Tumors With Myofibroblastic Phenotype: A Comprehensive Clinicopathologic, Genetic, and Epigenetic Characterization

USP8 is one of the members of ubiquitin-specific proteases deconjugating ubiquitin from target proteins. Besides USP6, it can be involved in tumorigenesis of mesenchymal neoplasms by binding to an activating fusion partner. Until now, USP8 fusion genes have been reported in calcified chondroid mesenchymal neoplasms, an inflammatory myofibroblastic tumor, a cardiac neoplasm, and a retroperitoneal sarcoma. In this study, we describe the clinicopathologic and genetic/epigenetic features of 7 USP8-associated tumors. The cohort included 5 male patients aged between 2 and 11 years, and 2 female patients aged 38 and 52 years. Lesions arose in the tongue, finger, hallux, arm, thoracic wall, right ventricle, and leg. Five neoplasms were resected. One was a recent case; the others were without evidence of disease after 0.5 to 3 years. Two lesions were only biopsied, 1 was a recent case and the other had no signs of progression after 4 years. Histology showed nodular or infiltrative lesions comprising bland-looking myofibroblastic spindle cells arranged in mainly short fascicles. The cellularity was variable, and the background was myxoid and/or collagenous. An inflammatory reaction was variably seen. One lesion, however, had features of a chondroid calcified mesenchymal neoplasm. Using RNA-sequencing, the following fusion partners of USP8 were found: SH3KBP1, RASA1, PDGFRA, CRK, PTPN11, and FARP1. Based on RNA-expression analysis, the 2 cases analyzed had a profile of nodular fasciitis; whereas using the Heidelberg Sarcoma Classifier, all cases had a similar methylation profile apart from other soft tissue tumor entities, suggesting that they form a separate group but are closely related to USP6-associated lesions. In conclusion, we broadened the spectrum of USP8-associated mesenchymal lesions in superficial, deep soft tissues and viscera (heart). Almost all lesions in this series displayed a myofibroblastic phenotype and harbored variable USP8 fusion partners. RNA-expression profiling indicated partial clustering with nodular fasciitis, suggesting some biological similarity. However, DNA methylation analysis consistently showed that these tumors formed a distinct epigenetic group, separate from both nodular fasciitis and inflammatory myofibroblastic tumors. Taken together, these findings support the concept of a USP8-rearranged myofibroblastic neoplasm as a potentially distinct entity, but the precise relationship with nodular fasciitis and inflammatory myofibroblastic tumor remains uncertain. Further studies integrating morphology, epigenetics, and transcriptomics are needed to clarify this relationship.