Targeting CIC::DUX4 sarcoma with Minnelide in a dual recombinase-initiated genetically engineered mouse model.

cic:DUX4 sarcoma (CDS) is a lethal cancer driven by a fusion between the tumor suppressor capicua (CIC) and the pioneer transcription factor double homeobox 4 (DUX4). We previously generated 3 genetically engineered mouse models (GEMMs) of CDS with CIC::DUX4 regulated by loxP-STOP-loxP cassettes, however, mice from all 3 models developed spontaneous tumors without Cre recombinase. Here, we established a next-generation GEMM of CDS (dual-flex [dFLEx] CDS) that used a dual recombinase (Cre plus the thermostable mutant of FLP recombinase FLPE) FLEx-switch design to activate CIC::DUX4 expression and initiate sarcomagenesis in a spatially and temporally controlled manner. Because CIC::DUX4 drives sarcoma development by activating an oncogenic transcriptional program, we performed a drug screen on human-derived CDS cell lines using a library of compounds that modulate transcription. This screen identified Minnelide, an inhibitor of RNA polymerase II-mediated transcription, as a selective inhibitor of CDS. Mechanistically, Minnelide acted through xeroderma pigmentosum type B to alter phosphorylation of RPB1, the largest subunit of RNA polymerase II. Subsequently, RPB1 underwent degradation leading to apoptosis of CDS cells. Minnelide demonstrated in vivo efficacy in dFLEx CDS GEMMs and in human CDS xenografts. As Minnelide has already been demonstrated to be safe in clinical trials, these findings identify Minnelide as a potential therapeutic option to test in patients with CDS.