[MED15-TFE3 renal cell carcinoma: a clinicopathological and molecular analysis]

Objective: To investigate the clinicopathological features, immunophenotype, molecular characteristics, and differential diagnosis of MED15-TFE3 gene fusion renal cell carcinoma (MED15-TFE3 RCC). Methods: A total of 12 MED15-TFE3 RCCs, diagnosed from 2016 to 2023, were collected from the Department of Pathology of Nanjing Jinling Hospital, Nanjing University School of Medicine, Nanjing, China for clinicopathologic, immunohistochemical, fluorescence in situ hybridization (FISH) and RNA sequencing (RNA-seq) analyses and follow-up. In addition, its diagnosis and differential diagnosis were also explored. Results: There were five males and seven females. The patients' ages ranged from 16 to 60 years, with an average age of 40.4 years. The follow-up time ranged from 15 to 92 months, and no recurrence or metastasis was observed. Histologically, 6 cases exhibited extensive cystic structures with almost no solid sheet components, while the remaining 6 cases displayed a cysto-solid growth pattern. The cytoplasm of the tumor cells appeared flocculent, with a clear or faintly eosinophilic appearance, and nucleoli were inconspicuous. Psammoma bodies were observed in 12 cases. There was deposition of basement membrane-like material in 5 cases. All cases showed strong expression of TFE3, GPNMB, Cathepsin K, Melan A, and PAX8, while no expression of CAⅨ or CK7. FISH analyses showed that all 12 cases were positive for the MED15-TFE3 fusion, while the MED15-TFE3 fusion gene and specific fusion sites were detected in 2 cases using RNA-seq. Conclusions: MED15-TFE3 RCC is a type of TFE3-rearranged renal cell carcinoma that exhibits both identifiable diagnostic characteristics and highly deceptive morphology. Its distinct extensive cystic structure can be easily confused with multilocular cystic renal neoplasm of low malignant potential, necessitating careful differentiation in routine practice.