Abstract PR005: CD73 sequestering mutant β-catenin at the membrane explains recurrence in CTNNB1 mutant endometrial cancer

Somatic missense mutations in exon 3 of CTNNB1 are associated with significantly worse recurrence-free survival in patients with low grade, early stage endometrioid-type endometrial carcinoma (EEC). CTNNB1 encodes for β-catenin, an integral protein in Wnt signaling and core member of E-cadherin-catenin cell-cell adhesions. Although CTNNB1 mutation identifies patients at higher risk for recurrence, not all patients will recur. Nuclear expression of β-catenin protein in CTNNB1-mutated EEC is not robust as is normally seen in other CTNNB1-mutated cancers such as desmoid tumor and colon cancer. This suggests cancer-specific molecular mechanisms may regulate mutant β-catenin in EEC. Previously, we identified a cancer-specific role for CD73, a cell surface 5’nuclotidase, in EEC. CD73 is downregulated and its loss associates with poor survival. Notably, CD73 inhibits tumor progression by increasing the localization of wild-type β-catenin to the membrane. Through fractionation and co-immunoprecipitation studies, using a highly homologous (97.4% identical; 100% identical in exon 3 (UniPort)) Xenopus exon 3 CTNNB1 mutant expressed in HEC-1-A cells, and human tumor data we provide evidence that CD73 sequesters mutant β-catenin at the membrane with E-cadherin, subsequently limiting its nuclear translocation. EEC patients with CTNNB1 mutation and low CD73 expression have increased incidence of recurrence (n = 29). CD73 levels were not significantly different with clinical stage or lymphatic and vascular space invasion, suggesting that CD73 expression independently predicts disease recurrence in patients with CTNNB1 mutant tumors. CTNNB1 mutations in EEC rarely occur in the region of the β-catenin protein that binds to E-cadherin. Thus, mechanisms regulating the movement of wild-type β-catenin to the membrane would be expected to be able to localize mutant β-catenin to the membrane regardless of exon 3 mutation. Immunohistochemistry studies of CTNNB1 mutant tumors (n = 11) show a strong association between CD73 expression and β-catenin localization in cancer cells. Cancer cells expressing membrane CD73 largely express β-catenin at the membrane. Whereas, in cancer cells with CD73 loss or cytoplasmic CD73 expression, β-catenin nuclear staining is increased. While these data are preliminary, CD73 sequestering mutant β-catenin to the membrane suggests a molecular mechanism for why certain patients with CTNNB1 mutant tumors recur. Accordingly, CD73 may serve as a biomarker for identifying patients for more aggressive clinical management to prevent recurrence. Citation Format: Jessica L. Bowser, Katherine C. Kurnit, Luan H. Phan, Jerry B. Harvey, Jiping Feng, SuSu Xie, Andrew B. Gladden, Pierre D. McCrea, Russell R. Broaddus. CD73 sequestering mutant β-catenin at the membrane explains recurrence in CTNNB1 mutant endometrial cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference: Endometrial Cancer: New Biology Driving Research and Treatment; 2020 Nov 9-10. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(3_Suppl):Abstract nr PR005.