👤 Carla Saoud

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children, presenting with heterogeneous clinical and molecular subtypes. While gene fusions are predominantly associated with alveolar RMS, spindle cell RMS, especially congenital and intraosseous variants, are also linked to specific gene fusions. Furthermore, recently, FGFR1 kinase-driven RMSs were published. Here, we describe a case of RMS harboring an EWSR1::NF2 gene fusion, a deletion-driven genetic alteration that has not been previously documented in RMS or other soft tissue tumors. The patient was a 29-year-old female who presented with a lobulated ankle mass. Histologic examination revealed a malignant round cell tumor extensively infiltrating large nerve bundles. Immunohistochemical analysis demonstrated rhabdomyoblastic differentiation, consistent with rhabdomyosarcoma. While some areas showed features resembling the sclerosing and others the embryonal subtypes, the overall findings were considered unclassifiable. Targeted RNA sequencing revealed EWSR1(exon 9):: NF2(exon 7) gene fusion, which was confirmed on whole genome and targeted DNA sequencing. The latter did not yield specific diagnostic insights but revealed mutations in TSC2 (p.T1330M), ZFHX3 (p.A301T), and a NOTCH3 rearrangement, all of unknown oncogenic significance. MYC gene amplification was detected, but there was no evidence of chromosome 8 amplification or chromosome 11p15 loss of heterozygosity. Whole genome sequencing revealed a low tumor mutation burden (2.69/Mb) and showed no other significant potentially oncogenic events. DNA methylation studies using dimensionality reduction and unsupervised clustering placed the case within the embryonal RMS subtype. Although the absence of other oncogenic driver alterations suggests that the fusion may have played a pivotal role in pathogenesis, we cannot exclude the possibility that it represents a passenger alteration rather than a true driver mutation. If the former is true, further studies will be required to determine whether this fusion represents a novel RMS subtype or a rare driver in existing subtypes of RMS. Show less

Myxoinflammatory fibroblastic sarcoma (MIFS) is a rare, low-grade sarcoma affecting with predilection the acral soft tissues of middle-aged adults. Clinically, MIFS is associated with a high rate of local recurrence but infrequent distant metastases. The diagnosis remains challenging due to their wide histologic spectrum and overlap with reactive, benign, and low-grade malignant lesions. Moreover, a significant limitation is that molecular confirmation is achieved in only a subset of cases, due to its broad range of genetic alterations which requires a multiplatform approach. Thus, a definitive diagnosis, especially at nonacral sites and in molecularly negative cases, remains uncertain. Our goal was to perform a detailed clinicopathologic and molecular reappraisal of MIFS managed at a single tertiary cancer center with dedicated orthopedic oncology expertise. Additionally, we examined potential outcomes correlating with specific genetic alterations. A cohort of 33 patients (12 males, 21 females, median age 52 years) was selected. Tumors were tested by FISH, Archer, and/or targeted NGS. VGLL3 amplification was detected in 84%, BRAF fusions in 33% and combined TGFBR3/MGEA5 rearrangements in 32% of cases. Two novel fusions were detected, RRAGB::CCNB3 and FGFR1::ZBTB47. Other events included a YAP1::MAML2 fusion in two cases, one co-existing with a BRAF fusion. Overall, 8 (24%) patients recurred, 4 more than once, while 4 (12%) patients developed metastasis (3 locoregional, 1 pulmonary), all associated with VGLL3 gene amplification. Positive margin status was associated with increased recurrence and reduced disease-free survival (DFS, p = 0.02). Moreover, it emphasizes the impact of multiplatform molecular testing in confirming the diagnosis. The lack of both local recurrence and metastatic potential outside VGLL3 amplifications requires further investigation. Show less