Molecular Detection of FUS-CREB3L2 Fusion Transcripts in Low-grade Fibromyxoid Sarcoma Using Formalin-fixed, Paraffin-embedded Tissue Specimens

A diagnosis of low-grade fibromyxoid sarcoma (LGFMS) remains problematic because of its bland-looking histologic features that can be potentially confused with other benign or low-grade fibromyxoid lesions. Recent cytogenetic and molecular analyses have shown that most LGFMSs have a characteristic chromosomal abnormality, t(7;16)(q33;p11), resulting in the FUS-CREB3L2 fusion gene. However, such assays have only rarely been used to analyze formalin-fixed, paraffin-embedded tumor samples. In the present study, we conducted a reverse transcription-polymerase chain reaction assay to detect the FUS-CREB3L2 fusion transcripts using formalin-fixed, paraffin-embedded tumor tissue specimens from 16 LGFMSs including 3 cases with giant collagen rosettes. The primers were newly designed to specifically amplify most of the junctional regions of the FUS-CREB3L2 fusion gene transcripts previously reported. The FUS-CREB3L2 fusion gene transcripts were detected in 14/16 (88%) cases of LGFMS. A nucleotide sequence analysis of the PCR products revealed that different portions of the FUS exon 6 or 7 were fused with various sites of the CREB3L2 exon 5, resulting in 12 different nucleotide sequences. We also tested a primer set to detect the FUS-CREB3L1 fusion transcript, which is a rare variant of the gene fusion in LGFMS, although no PCR products were identified in any case. The FUS-CREB3L2 fusion transcripts were not detected in any of the 123 other soft-tissue tumors, including desmoid-type fibromatoses, myxofibrosarcomas, soft-tissue perineuriomas, and congenital or adult fibrosarcomas. These data suggest that our reverse transcription-polymerase chain reaction assay is a reliable method to detect FUS-CREB3L2, which can thus help in accurately diagnosing LGFMS.