Evaluation of PRDM10 gene rearrangement in unclassifiable undifferentiated soft tissue tumors by immunohistochemistry and molecular methods

Objective: Undifferentiated pleomorphic sarcomas are aggressive soft tissue tumors that do not exhibit any identifiable histological differentiation. Despite being one of the most common soft tissue sarcomas, no specific genetic alteration has been described. In recent years, PRDM10 fusion transcript has been detected in undifferentiated pleomorphic sarcomas by immunohistochemistry and molecular-genetic methods. In our study, we aimed to detect PRDM10 gene rearrangement in pleomorphic soft tissue tumors with immunohistochemical and molecular methods, and evaluate its correlation with clinicopathological findings. Materials and Methods: A total of 106 soft tissue tumors diagnosed as undifferentiated pleomorphic sarcoma and myxofibrosarcoma, myofibroblastic sarcoma, and pleomorphic hyalinizing angiectatic tumor for the differential diagnosis between 2013 and 2022 at the Department of Pathology, Cukurova University Faculty of Medicine were selected for the study. The cases were re-evaluated and scored according to the FNCLCC grading system. As a result of scoring, the cases were divided into two groups, and 33 tumors with lower grades were identified. In these cases, PRDM10 gene rearrangement was investigated by PRDM10 immunohistochemistry and FISH. Results: Five cases (15.2%) showed nuclear and/or cytoplasmic staining on PRDM10 immunohistochemistry, while four cases (12.1%) showed break-apart signals in FISH. In a single case, strong and diffuse (in 90% of tumor cells) nuclear and cytoplasmic staining was observed immunohistochemically, and break-apart signals were detected in FISH. In the other four positive cases, 5-25% of tumor cells showed positivity, but FISH was negative. All four FISH-positive cases had a superficial location, and two of them showed significantly low mitotic activity (1-2 mitoses/50 high power field) and diffuse positivity for CD34. During follow-up, no local recurrence or metastasis was detected in either case. Conclusion: Clinically superficial, morphologically with low mitotic activity and absence of necrosis, immunohistochemically CD34 positive PRDM10-rearranged soft tissue tumors have different clinical and prognostic findings. These tumors are within the same tumor spectrum as superficial CD34-positive fibroblastic tumors; however, it has been concluded that PRDM10 is not specific to these tumors. Keywords: Undifferentiated pleomorphic sarcoma, PRDM10, Superficial CD34-positive fibroblastic tumor