Endometrial endometrioid carcinomas (EECs) with exon 3 CTNNB1 mutations characterize a more aggressive subset of tumors in patients with low-grade low-stage disease. Thus, prospectively identifying these cases may be clinically relevant. The aim of this study was to examine the feasibility of β-catenin and Cyclin D1 immunohistochemistry to identify EECs harboring CTNNB1 mutations and to evaluate the clinicopathologic features of EECs with exon 3 CTNNB1 mutations. Thirty-nine CTNNB1 mutated EECs and 40 CTNNB1 wild-type EECs were identified from a cohort of previously sequenced endometrial carcinomas using a targeted next-generation sequencing panel. Immunohistochemistry for β-catenin and Cyclin D1 was performed on all cases. Immunohistochemistry results were correlated with CTNNB1 mutation status and clinicopathologic parameters. Patients with CTNNB1 mutated EECs were younger than those with CTNNB1 wild-type (56.2 vs. 61.5 y; P=0.033). Nuclear β-catenin expression correlated with exon 3 CTNNB1 mutation (P<0.0001) with a sensitivity of 91% and a specificity of 89%. Cyclin D1 expression correlated with CTNNB1 exon 3 mutation with relatively high specificity (90%) but low sensitivity (29%). Recurrence rate in patients with stage IA disease at diagnosis was significantly higher in patients whose tumors were CTNNB1 mutated compared with CTNNB1 wild-type (30% vs. 0%; P=0.025) and included distant metastases; all recurrent tumors in this group harbored exon 3 mutations and were histologically low grade (5 grade 1, 2 grade 2). Nuclear β-catenin expression appears to be an acceptable proxy for CTNNB1 mutation.

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