The oncogenic transcription factor FUS-CHOP can undergo nuclear liquid–liquid phase separation

Abstract

Myxoid liposarcoma is caused by a chromosomal translocation resulting in a fusion protein comprised of the N-terminus of FUS (fused in sarcoma) and the full-length transcription factor CHOP (CCAAT/Enhancer Binding Protein Homologous Protein). FUS functions in RNA metabolism and CHOP is a stress-induced transcription factor. The FUS-CHOP fusion protein causes unique gene expression and oncogenic transformation. Though it is clear the FUS segment is required for oncogenic transformation, the mechanism of FUS-CHOP-induced transcriptional activation is unknown. Recently, some transcription factors and super enhancers were proposed to undergo liquid-liquid phase separation and form membraneless compartments that recruit transcription machinery to gene promoters. Since phase separation of FUS depends on its N-terminus, transcriptional activation by FUS-CHOP could result from the N-terminus driving nuclear phase transitions. Here, we characterized FUS-CHOP in cells and in vitro, and observed novel phase-separating properties relative to unmodified CHOP. Our data indicate FUS-CHOP forms phase-separated condensates that colocalize with BRD4, a marker of super enhancer condensates. We provide evidence that the FUS-CHOP phase transition is a novel oncogenic mechanism and potential therapeutic target for myxoid liposarcoma.

Direct digital sensing of protein biomarkers in solution

“…To demonstrate condensate detection by DigitISA, we set out to probe the phase behavior of the protein fused in sarcoma (FUS) (see Methods: Protein preparation). FUS is a DNA/RNA-binding protein that is involved in transcription regulation, RNA transport, and DNA repair 60 , 61 , and has been in the focus of many studies in recent years, due to its ability to phase separate, either on its own 62 or in complex mixtures with nucleic acids 63 , and its implications in neurodegenerative diseases and cancer 64 , 65 . To utilize the DigitISA technique for FUS detection, we used a fluorescently labelled aptamer (see Methods: Sample preparation for DigitISA experiments), that is known to have a mid-range affinity for the FUS protein in the hundreds of nanomolar regime 66 .…”

Biomolecular condensates: Formation mechanisms, biological functions, and therapeutic targets

“…Condensates of FUS/EWS/TAF15 can also promote the abnormal gene transcription and the oncogenic transformation ability in sarcoma 196 . Furthermore, the FUS–CHOP fusion protein can also form condensates with BRD4 and caused myxoid liposarcoma 194 . Besides, heat‐shock factor 1 is a transcriptional regulator of chaperone and is capable of forming condensates.…”

“…196 Furthermore, the FUS-CHOP fusion protein can also form condensates with BRD4 and caused myxoid liposarcoma. 194 Besides, heat-shock factor 1 is a transcriptional regulator of chaperone and is capable of forming condensates. Inhibiting the formation of these condensates can promote the activity of heat shock factor 1 and cell survival and reduce the incidence of sarcoma.…”

Emerging Implications of Phase Separation in Cancer

“…[ 247 ] Furthermore, the oncogenic TF FUS‐CCAAT/enhancer binding protein homologous protein (CHOP) is localized in small nuclear puncta and colocalizes with the SE marker BRD4 in myxoid liposarcoma. [ 248 ]…”

“…Non-small-cell lung cancer, ovarian cancer [188,189] CCDC6-RET fusion RAS signaling overactivation Non-small-cell lung cancer, papillary thyroid cancer [188,189] SHP2 mutants RAS signaling hyperactivation Esophagus cancer [190] DnaJB1-PKAcat fusion Tumorigenic cAMP signaling Atypical liver cancer fibrolamellar carcinoma [191] DACT1 WNT signaling inhibition Breast and prostate cancer [195] Glycogen compartments G6PC YAP signaling activation Liver cancer [194] Stress granules YB1 Cell proliferation Sarcoma [196] KRAS mutants Chemoresistance Pancreatic cancer [198] DDX3X YTHDC1, m6A-mRNAs Gene transcription and mRNA processing Acute myeloid leukemia [243] NUP98-HOXA9 fusion Leukemogenic genes transcription Leukemia [246] FUS-CHOP Oncogenic transcription Myxoid liposarcoma [248] coiled-coil domain-containing protein 6 (CCDC6)-rearranged during transfection (RET) acquire multimerization domains of EML4/CCDC6 and lose the ALK/RET membrane-targeting sequence, forms de novo membraneless cytoplasmic granules. These granules can serve as a subcellular platform to concentrate the RAS-activating complex GRB2/SOS1.…”