Members of the human FET family of RNA‐binding proteins, comprising FUS, EWSR1, and TAF15, are ubiquitously expressed and engage at several levels of gene regulation. Many sarcomas and leukemias are characterized by the expression of fusion oncogenes with FET genes as 5′ partners and alternative transcription factor‐coding genes as 3′ partners. Here, we report that the N terminus of normal FET proteins and their oncogenic fusion counterparts interact with the SWI/SNF chromatin remodeling complex. In contrast to normal FET proteins, increased fractions of FET oncoproteins bind SWI/SNF, indicating a deregulated and enhanced interaction in cancer. Forced expression of FET oncogenes caused changes of global H3K27 trimethylation levels, accompanied by altered gene expression patterns suggesting a shift in the antagonistic balance between SWI/SNF and repressive polycomb group complexes. Thus, deregulation of SWI/SNF activity could provide a unifying pathogenic mechanism for the large group of tumors caused by FET fusion oncoproteins. These results may help to develop common strategies for therapy.

FET fusion oncoproteins containing one of the FET (FUS, EWSR1, TAF15) family proteins juxtaposed to alternative transcription‐factor partners are characteristic of more than 20 types of sarcoma and leukaemia. FET oncoproteins bind to the SWI/SNF chromatin remodelling complex, which exists in three subtypes: cBAF, PBAF and GBAF/ncBAF. We used comprehensive biochemical analysis to characterize the interactions between FET oncoproteins, SWI/SNF complexes and the transcriptional coactivator BRD4. Here, we report that FET oncoproteins bind all three main SWI/SNF subtypes cBAF, PBAF and GBAF, and that FET oncoproteins interact indirectly with BRD4 via their shared interaction partner SWI/SNF. Furthermore, chromatin immunoprecipitation sequencing and proteomic analysis showed that FET oncoproteins, SWI/SNF components and BRD4 co‐localize on chromatin and interact with mediator and RNA Polymerase II. Our results provide a possible molecular mechanism for the FET‐fusion‐induced oncogenic transcriptional profiles and may lead to novel therapies targeting aberrant SWI/SNF complexes and/or BRD4 in FET‐fusion‐caused malignancies.

The therapeutic options for patients with relapsed or metastatic myxoid liposarcoma (MLS) remain scarce and there is currently no targeted therapy available. Inhibition of the HSP90 family of chaperones has been suggested as a possible therapeutic option for patients with MLS. However, the clinical effect of different HSP90 inhibitors vary considerably and no comparative study in MLS has been performed. Here, we evaluated the effects of the HSP90 inhibitors 17-DMAG, AUY922 and STA-9090 on MLS cell lines and in an MLS patient-derived xenograft (PDX) model. Albeit all drugs inhibited in vitro growth of MLS cell lines, the in vivo responses were discrepant. Whereas 17-DMAG inhibited tumor growth, AUY922 surprisingly led to increased tumor growth and a more aggressive morphological phenotype. In vitro, 17-DMAG and STA-9090 reduced the activity of the MAPK and PI3K/AKT signaling pathways, whereas AUY922 led to a compensatory upregulation of downstream ERK. Furthermore, all three tested HSP90 inhibitors displayed a synergistic combination effect with trabectidin, but not with doxorubicin. In conclusion, our results indicate that different HSP90 inhibitors, albeit having the same target, can vary significantly in downstream effects and treatment outcomes. These results should be considered before proceeding into clinical trials against MLS or other malignancies.

Background: FET (FUS, EWSR1, and TAF15) fusion oncoproteins are characteristic for several sarcomas and leukemias, including myxoid liposarcoma and Ewing sarcoma. FET oncoproteins interact with the SWI/SNF chromatin remodeling complex subtypes cBAF, PBAF, and GBAF, but their impact on SWI/SNF compositions, interactions, and downstream epigenetic effects remains elusive. Methods: We employ a comprehensive immunoprecipitation and quantitative mass spectrometry approach to determine the impact of FET oncoproteins on SWI/SNF composition and their interactomes. Validation of complex composition and interaction partners is performed by glycerol gradient sedimentation assays and co-immunofluorescence analysis. Furthermore, we determine the differential chromatin accessibility and gene regulation in FET sarcomas using assay for transposase-accessible chromatin sequencing and RNA sequencing, respectively. Results: Our data show that FET sarcomas have distinct SWI/SNF complex compositions, with different subunit paralogs and subtype-specific components that utilize distinct sets of interaction partners, including specific transcription factors. We show that FET oncoproteins cause no major disruption of the SWI/SNF complex composition. Instead, FUS::DDIT3-bound SWI/SNF complexes in myxoid liposarcoma cells are enriched in PBAF and GBAF components as well as most interaction partners. Conclusions: These data suggest that FET oncoproteins act together with fully assembled and functional SWI/SNF complexes and recruited interaction partners. Finally, our data reveal that the SWI/SNF compositions, interactomes, and epigenetic background contribute to the tumor type in FET sarcoma. Trial registration Clinical trial number: not applicable. Graphical