Abstract PR008: Location, location, location: Why cellular localization of mutant β-catenin matters in endometrial cancer

Abstract The present study aims to understand the mechanisms of mutant β-catenin in endometrial cancer (EC) patients with exon 3 CTNNB1 mutations. Endometrial cancer patients with exon 3 CTNNB1 mutations tend to be younger in age and have low cancer grades, but they exhibit worse recurrence-free survival than patients with similar clinical characteristics. It is unknown why EC patients with CTNNB1 mutations have worse survival. CTNNB1 encodes the protein β-catenin, which functions primarily at the cell membrane in the adherens junction complex and in the Wnt signaling pathway, where it localizes in the cytoplasm or nucleus. Exon 3 CTNNB1 mutations in EC are classically thought to promote nuclear localization of β-catenin. In this study, we conducted immunohistochemistry for β-catenin localization on 53 EC cases with an exon 3 CTNNB1 mutation from the UNCseq project. Surprisingly, 32 cases had primarily non-nuclear, either cytoplasmic or membranous, localization and 21 cases had primarily nuclear localization. Additionally, we identified 16 cases with intratumoral heterogeneity containing both nuclear and non-nuclear β-catenin localization. To investigate the functional consequences of this differential localization, we performed spatial transcriptomics using the Nanostring GeoMx Whole Transcriptome Analysis. Nuclear and non-nuclear regions of interest (ROI) were selected from 16 CTNNB1 mutant EC cases exhibiting heterogenous β-catenin localization within the same tissue (n=95 total ROIs). We hypothesize that β-catenin nuclear and non-nuclear ROIs will have differing transcriptional outputs in CTNNB1 mutant EC. Differential expression analysis of all ROIs yielded distinct transcriptional profiles based on localization of mutant β-catenin. Gene Set Enrichment Analysis revealed enrichment for Wnt-signaling and the epithelial-to-mesenchymal transition pathways in nuclear ROIs and enrichment for estrogen and androgen signaling pathways in non-nuclear ROIs. Hierarchical clustering of all ROIs yielded two genetically distinct clusters comprised of entirely nuclear or non-nuclear ROIs, with the exception of four ROIs. Higher expression of TACSTD2, encoding the protein Trop2, was enriched in the nuclear ROI cluster. Trop2 is the target of Sacituzumab Govitecan (SG), a drug conjugate with recent success in metastatic triple negative breast cancer patients. Additionally, studies have shown Trop2 to induce nuclear localization of β-catenin in other cancer types. Therefore, we hypothesize that CTNNB1 mutant EC patients with nuclear localization of β-catenin will respond better to SG treatment. To conclude, we established gene expression profiles for areas of nuclear and non-nuclear β-catenin in CTNNB1 mutant EC tissues and identified a novel gene with implications in mechanisms of β-catenin localization which may allow therapeutic targeting of advanced disease. Citation Format: Molly L. Parrish, Andrew B. Gladden, Russell R. Broaddus. Location, location, location: Why cellular localization of mutant β-catenin matters in endometrial cancer abstract. In: Proceedings of the AACR Special Conference on Endometrial Cancer: Transforming Care through Science; 2023 Nov 16-18; Boston, Massachusetts. Philadelphia (PA): AACR; Clin Cancer Res 2024;30 (5Suppl): Abstract nr PR008.