C-terminal fusion partner activity contributes to the oncogenic functions of YAP1::TFE3.

YAP1 gene fusions are found in a multitude of human tumors and are the likely tumor-initiating events in these tumors. We have previously shown that YAP1 fusion proteins exert TEAD-dependent oncogenic YAP1 activity. However, the contributions of the C-terminal fusion partners to the oncogenic functions of YAP1 fusion proteins are understudied. Here, we expressed eight different YAP1 gene fusions in vivo. Tumors induced by YAP1::TFE3 displayed a significantly different histomorphology compared to tumors induced by other YAP1 fusions or activated non-fusion YAP1. To assess the extent to which TFE3 activity and the functional TFE3 domains (DNA binding: leucine zipper (LZ) and basic-helix-loop-helix (bHLH); activation domain (AD)) contribute to the oncogenic functions of YAP1::TFE3, we generated several mutant variants and performed functional in vitro and in vivo assays. In vitro, mutation or deletion of the TFE3 DNA binding domains (LZ, bHLH) resulted in reduced TFE3 activity but increased YAP1 activity of YAP1::TFE3. In vivo, deletion of the LZ and bHLH domains did not result in a decrease in tumor incidence but induced the formation of more YAP1-like tumors that lacked prominent features of YAP1::TFE3-driven tumors. By contrast, loss of the TFE3 AD almost completely abrogated tumor formation. Our results suggest that the TFE3 domains significantly contribute to the oncogenic activity of YAP1::TFE3.