Abstract 3721: The C1 domain of capicua has functional importance in human CIC-DUX4

Genomic rearrangements between the HMG-box family transcriptional repressor capicua (CIC) and multiple partner genes result in fusion oncoproteins that activate transcription to drive sarcoma growth and progression. In the case of CIC-DUX4, the most common CIC-rearranged fusion oncoprotein, the fusion often retains over 90% of the wild type CIC protein structure including the HMG box and C-terminal C1 domain. Structure-function studies in Drosophila have suggested that the C1 domain of CIC, a non-HMG box DNA binding domain, is important for the transactivating capacity of a synthetic fly-human CIC-DUX4. Consistent with these findings, we performed a retrospective analysis of over 70 breakpoints described from CIC-DUX4 human derived tumors, which revealed that the C1 domain is structurally conserved within the context of the CIC-DUX4 fusion. To further understand the functional role of the C1 domain in human CIC-DUX4 we generated human CIC-DUX4 C1 domain and HMG-box deletion (alone or in combination) mutant constructs and performed a series of studies to test the functional impact on CIC-DUX4 protein levels, subcellular localization, and transcriptional activity. Through these analyses we have found that deletion of the C1 domain does not impact CIC-DUX4 oncoprotein expression or localization but does dramatically reduce target gene expression. In ongoing in vivo studies, we aim to evaluate the significance of the C1 domain in inducing CIC-DUX4 driven tumorigenesis. Collectively, our findings validate the functional importance of the C1 domain in human CIC-DUX4 and to our knowledge provide the first harmonized analysis of breakpoint locations in CIC-rearranged sarcoma. Citation Format: Cuyler Luck, Kyle A. Jacobs, Ross A. Okimoto. The C1 domain of capicua has functional importance in human CIC-DUX4. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3721.