Department of Diagnostic Pathology, National Cancer Center Hospital, Tokyo, Japan.
Rare Cancer Center, National Cancer Center Hospital, Tokyo, Japan.
Mod Pathol. 2022 Feb;35(2):228-239. doi: 10.1038/s41379-021-00910-x. Epub 2021 Sep 9.
Synovial sarcoma is characterized by variable epithelial differentiation and specific SS18-SSX gene fusions. The diagnosis is primarily based on phenotype, but fusion gene detection is increasingly being considered indispensable, with SS18 break-apart fluorescence in situ hybridization (FISH) being favored in many laboratories. However, SS18 FISH assay produces negative or atypical results in a minority of cases, leaving uncertainties in diagnosis and management. Here, we analyzed this challenging subset of SS18 FISH-negative/atypical synovial sarcoma using RNA sequencing and monoclonal antibodies that recognize SS18-SSX and the SSX C-terminus. Among 99 synovial sarcoma cases that were previously subjected to SS18 break-apart FISH, eight cases were reported as negative and three cases were indeterminate, owing to atypical signal patterns. Three of these 11 tumors (two monophasic and one biphasic) harbored novel EWSR1-SSX1 fusions, were negative for SS18-SSX staining, and were positive for SSX C-terminus staining. One monophasic tumor harbored a novel MN1-SSX1 fusion, and showed negative SS18-SSX expression and positive SSX C-terminus staining. Another monophasic tumor carried an SS18L1-SSX1 fusion, and was weakly positive for SS18-SSX, while SMARCB1 expression was reduced. The presence of these novel and/or rare fusions was confirmed using RT-PCR and Sanger sequencing. EWSR1-SSX1 was further validated by EWSR1 FISH assay. The remaining six tumors (five monophasic and one biphasic) showed strong SS18-SSX expression, and RNA sequencing successfully performed in three cases identified canonical SS18-SSX2 fusions. Based on a DNA methylation-based unsupervised clustering, the tumors with EWSR1-SSX1 and SS18L1-SSX1 clustered with synovial sarcoma, while the MN1-SSX1-positive tumor was not co-clustered despite classic histology and immunoprofile. In summary, we discovered novel and rare SSX1 fusions to non-SS18 genes in synovial sarcoma. The expanded genetic landscape carries significant diagnostic implications and advances our understanding of the oncogenic mechanism.
滑膜肉瘤的特征是具有可变的上皮分化和特定的 SS18-SSX 基因融合。诊断主要基于表型，但融合基因检测越来越被认为是不可或缺的，许多实验室都倾向于使用 SS18 断裂分离荧光原位杂交（FISH）。然而，SS18 FISH 检测在少数情况下会产生阴性或非典型结果，导致诊断和管理存在不确定性。在这里，我们使用 RNA 测序和识别 SS18-SSX 和 SSX C 末端的单克隆抗体分析了 SS18 FISH 阴性/非典型滑膜肉瘤这一具有挑战性的亚组。在之前进行 SS18 断裂分离 FISH 的 99 例滑膜肉瘤病例中，有 8 例报告为阴性，3 例为不确定，原因是信号模式不典型。这 11 例肿瘤中的 3 例（2 例单相和 1 例双相）携带新型 EWSR1-SSX1 融合，SS18-SSX 染色阴性，SSX C 末端染色阳性。1 例单相肿瘤携带新型 MN1-SSX1 融合，表现为 SS18-SSX 表达阴性和 SSX C 末端染色阳性。另一个单相肿瘤携带 SS18L1-SSX1 融合，SS18-SSX 弱阳性，SMARCB1 表达减少。使用 RT-PCR 和 Sanger 测序证实了这些新型和/或罕见融合的存在。EWSR1-SSX1 通过 EWSR1 FISH 检测进一步验证。其余 6 例肿瘤（5 例单相和 1 例双相）表现出强烈的 SS18-SSX 表达，在 3 例中成功进行 RNA 测序鉴定了典型的 SS18-SSX2 融合。基于基于 DNA 甲基化的无监督聚类，携带 EWSR1-SSX1 和 SS18L1-SSX1 的肿瘤与滑膜肉瘤聚集在一起，而 MN1-SSX1 阳性的肿瘤尽管具有经典的组织学和免疫表型，但并未聚集在一起。总之，我们在滑膜肉瘤中发现了新型和罕见的非 SS18 基因 SSX1 融合。扩展的遗传图谱具有重要的诊断意义，并加深了我们对致癌机制的理解。
