The Capicua C1 Domain Is Required for Full Activity of the CIC::DUX4 Fusion Oncoprotein.

abstractRearrangements between genes can yield neomorphic fusions that drive oncogenesis. Fusion oncogenes are made up of fractional segments of the partner genes that comprise them, with each partner potentially contributing some of its own function to the nascent fusion oncoprotein. Clinically, fusion oncoproteins driving one diagnostic entity are typically clustered into a single molecular subset and are often treated a similar fashion. However, knowledge of where specific fusion breakpoints occur in partner genes, and the resulting retention of functional domains in the fusion, is an important determinant of fusion oncoprotein activity and may differ between patients. This study investigates these phenomena through the example of capicua (CIC)::double homeobox 4 (DUX4), a fusion between transcriptional repressor CIC and DUX4 genes, which drives an aggressive subset of undifferentiated round cell sarcoma. Using a harmonized dataset of more than 100 patient fusion breakpoints from the literature, we show that most bona fide CIC::DUX4 fusions retain the C1 domain, which is known to contribute to DNA binding by wild-type CIC. Mechanistically, deletion or mutation of the C1 domain reduces, but does not eliminate, the activation of CIC target genes by CIC::DUX4. We also find that expression of C1-deleted CIC::DUX4 is capable of exerting intermediate transformation-related phenotypes compared with those imparted by full-length CIC::DUX4 but was not sufficient for tumorigenesis in a subcutaneous mouse model. In summary, our results suggest a supercharging role for the C1 domain in the activity of CIC::DUX4. SIGNIFICANCE: We show in mammalian settings that the capicua C1 functional domain is a supercharger for CIC::DUX4, a poorly studied fusion oncoprotein which drives a rare sarcoma with dismal outcomes.