A primary rectal neoplasm with novel DDX5-TFEB fusion.

We report a case of primary rectal neoplasm with a novel DDX5-TFEB fusion in a 14-year-old boy. Histologically, the neoplasm was composed of epithelioid tumor cells with abundant clear to eosinophilic cytoplasm, arranged in nests with rich stromal capillary vasculature. Immunohistochemically, the tumor cells were positive for transcription factor EB (TFEB) and negative for PAX8, TFE3, HMB45, and PCK. TFEB gene rearrangement and low-level amplification were identified using break-apart fluorescence in situ hybridization. Next-generation sequencing identified a heretofore unreported DDX5-TFEB gene fusion, which was confirmed by using reverse transcription-polymerase chain reaction and Sanger sequencing. The major morphological differential diagnoses include perivascular epithelioid cell tumor, microphthalmia-associated transcriptional factor family translocation renal cell carcinoma, and alveolar soft part sarcoma. The morphological, immumophenotypical, and genetic characteristics of this tumor did not fit well with current classification, but it may represent an unusual PEComa-like tumor with a novel DDX5-TFEB fusion.