HEY1-NCOA2 expression modulates chondrogenic differentiation and induces mesenchymal chondrosarcoma in mice.

Mesenchymal chondrosarcoma affects adolescents and young adults, and most cases usually have the HEY1::NCOA2 fusion gene. However, the functional role of HEY1-NCOA2 in the development and progression of mesenchymal chondrosarcoma remains largely unknown. This study aimed to clarify the functional role of HEY1-NCOA2 in transformation of the cell of origin and induction of typical biphasic morphology of mesenchymal chondrosarcoma. We generated a mouse model for mesenchymal chondrosarcoma by introducing HEY1-NCOA2 into mouse embryonic superficial zone (eSZ) followed by subcutaneous transplantation into nude mice. HEY1-NCOA2 expression in eSZ cells successfully induced subcutaneous tumors in 68.9% of recipients, showing biphasic morphologies and expression of Sox9, a master regulator of chondrogenic differentiation. ChIP sequencing analyses indicated frequent interaction between HEY1-NCOA2 binding peaks and active enhancers. Runx2, which is important for differentiation and proliferation of the chondrocytic lineage, is invariably expressed in mouse mesenchymal chondrosarcoma, and interaction between HEY1-NCOA2 and Runx2 is observed using NCOA2 C-terminal domains. Although Runx2 knockout resulted in significant delay in tumor onset, it also induced aggressive growth of immature small round cells. Runx3, which is also expressed in mesenchymal chondrosarcoma and interacts with HEY1-NCOA2, replaced the DNA-binding property of Runx2 only in part. Treatment with the HDAC inhibitor panobinostat suppressed tumor growth both in vitro and in vivo, abrogating expression of genes downstream of HEY1-NCOA2 and Runx2. In conclusion, HEY1::NCOA2 expression modulates the transcriptional program in chondrogenic differentiation, affecting cartilage-specific transcription factor functions.

Imaging findings of NTRK­rearranged spindle cell neoplasms: A case series.

NTRK-rearranged spindle cell neoplasms (NTRK-RSCNs) are a new category of soft tissue tumors with NTRK gene fusions. The present study aimed to investigate the radiological features of NTRK-RSCNs and their association with histopathological findings. The present study included six patients with NTRK-RSCNs, whose fusion genes were confirmed using next-generation sequencing. All patients underwent surgery, and their diagnosis and clinical outcomes were investigated. In addition, the magnetic resonance imaging (MRI) features of all tumors and histopathological findings of the resected specimens were assessed. The present study included three women and three men, with a mean age of 22 years (range, 2-43 years). The NTRK gene fusions included four NTRK1 and two NTRK3 fusions. Three patients were preoperatively diagnosed with solitary fibrous tumors. One patient with NTRK3 fusion experienced local recurrence and distant metastases, whereas the other five patients had no local recurrence or metastasis. MRI revealed that all tumors were highly vascular with intra- and peritumoral flow voids of differing degrees. Furthermore, a partially ill-defined border, suggesting infiltration of tumors into the surrounding tissues, particularly fat tissue, was observed in five patients, which was confirmed by histopathological findings. In conclusion, NTRK-RSCNs are highly vascular tumors that can infiltrate the surrounding tissues. These findings suggested that NTRK-RSCNs should be considered in the differential diagnosis of highly vascular-rich mesenchymal tumors, including solitary fibrous tumors and alveolar soft part sarcomas. Furthermore, wide resection may be preferred to completely resect this type of tumor, considering its invasive nature.

Identification of Novel Fusion Genes in Bone and Soft Tissue Sarcoma and Their Implication in the Generation of a Mouse Model.

Fusion genes induced by chromosomal aberrations are common mutations causally associated with bone and soft tissue sarcomas (BSTS). These fusions are usually disease type-specific, and identification of the fusion genes greatly helps in making precise diagnoses and determining therapeutic directions. However, there are limitations in detecting unknown fusion genes or rare fusion variants when using standard fusion gene detection techniques, such as reverse transcription-polymerase chain reaction (RT-PCR) and fluorescence in situ hybridization (FISH). In the present study, we have identified 19 novel fusion genes using target RNA sequencing (RNA-seq) in 55 cases of round or spindle cell sarcomas in which no fusion genes were detected by RT-PCR. Subsequent analysis using Sanger sequencing confirmed that seven out of 19 novel fusion genes would produce functional fusion proteins. Seven fusion genes detected in this study affect signal transduction and are ideal targets of small molecule inhibitors. YWHAE-NTRK3 expression in mouse embryonic mesenchymal cells (eMCs) induced spindle cell sarcoma, and the tumor was sensitive to the TRK inhibitor LOXO-101 both in vitro and in vivo. The combination of target RNA-seq and generation of an ex vivo mouse model expressing novel fusions provides important information both for sarcoma biology and the appropriate diagnosis of BSTS.

Case of mesenchymal tumor with the PPP6R3-USP6 fusion, possible nodular fasciitis with malignant transformation.

Nodular fasciitis (NF) is a self-limiting benign disease that is characterized by rapid proliferation of fibroblastic and myofibroblastic cells. The characteristic gene fusion containing the USP6 gene is a genetic hallmark of NF and MYH9-USP6 is the most frequent fusion, suggesting that NF is not a reactive condition but a neoplastic disease. Malignant transformation of NF has been reported rarely as a single case associated with the PPP6R3-USP6 fusion. Here we report a case of soft part tumor of which the histological feature was a typical NF but showed aggressive and non-regressing growth with local invasion. Targeted RNA sequencing and fluorescence in situ hybridization analysis identified PPP6R3-USP6 with gene amplification. These findings indicate that the present case is the second case of malignant NF, and we suggest potential malignant transformation in certain NF cases.