Abstracts from USCAP 2021: Bone and Soft Tissue Pathology (33-65) (original) (raw)
Epithelioid hemangioendothelioma (EHE) with YAP1-TFE3 fusion is a recently characterized distinctive variant of EHE that accounts for a small subset (<5%) of cases. This tumor type is composed of nests of epithelioid cells with voluminous pale cytoplasm and often shows focally vasoformative architecture. TFE3 immunohistochemistry (IHC) can be used to support the diagnosis; however, studies have questioned its specificity. Yes-associated protein 1 (YAP1), part of the Hippo signaling pathway, is expressed in normal endothelial cells, but becomes disrupted in EHE variant with YAP1-TFE3, such that only a small N-terminal region of YAP1 is expressed in the fusion protein. We hypothesized that IHC using an antibody directed against the C-terminus of YAP1 would be negative in this tumor type. The purpose of this study was to evaluate YAP1 C-terminus IHC in EHE with YAP1-TFE3 and other epithelioid vascular neoplasms in the differential diagnosis. Design: In total, 72 tumors were included in this study, encompassing 5 tumor types: EHE with YAP1-TFE3 (n=12), conventional (CAMTA1-positive) EHE (n=20), pseudomyogenic hemangioendothelioma (n=10), epithelioid hemangioma (n=20) and epithelioid angiosarcoma (n=10). IHC was performed using a rabbit monoclonal anti-YAP1 C-terminus antibody. CAMTA1, TFE3, and FOSB IHC was performed in selected cases. Results: Results of YAP1 IHC are summarized in Table 1. EHE with YAP1-TFE3 showed complete loss of YAP1 C-terminus expression in 9 of 12 (75%) cases (Figure 1). Three cases showed retained YAP1 expression, 2 with weak cytoplasmic staining and 1 with strong nuclear and cytoplasmic staining. Loss of YAP1 was seen in one conventional EHE (1/20; 5%). Among other epithelioid vascular tumors, only one case showed loss of YAP1. IHC for TFE3 was positive in all cases of EHE with YAP1-TFE3. Weak staining was observed in one conventional EHE. CAMTA1 was negative in all EHE with YAP1-TFE3 tested (0/7).
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