Metastatic endometrial stromal sarcoma with sex cord and neuroendocrine differentiation harboring a complex gene fusion

Background: Low-grade endometrial stromal sarcoma (LGESS) is a malignant stromal tumor characterized by an indolent clinical course, often with late recurrence or distant metastasis after a prolonged period of remission. These tumors can exhibit various lineage differentiations, making diagnosis challenging, especially in cases of remote recurrence. Most of these tumors are driven by fusions involving the JAZF1, SUZ12, and/or PHF1 genes. Materials and Methods: A case of metastatic endometrial stromal sarcoma was collected. Clinicopathologic and molecular features were documented. Results: A senior lady with a remote history of an unknown uterine neoplasm. Imaging of the chest revealed bilateral, enlarging pulmonary nodules, which were histologically and immunohistochemically characterized as mesenchymal-like neoplastic cells with sex cord and neuroendocrine differentiation. RNA-based next-generation sequencing identified a complex JAZF1::DLG5::PHF1 fusion, confirming a diagnosis of metastatic endometrial stromal sarcoma with sex cord and neuroendocrine differentiation. Conclusion: This report underscores the propensity of low-grade endometrial stromal sarcoma to metastasize after a prolonged period of remission, in which the tumor exhibited sex cord and unique neuroendocrine differentiation. We also present a complex fusion in which the DLG5 gene acts as a ‘filler’ to maintain the in-frame configuration.