Original ArticlesMorules But Not Squamous Differentiation are a Reliable Indicator of CTNNB1 (β-catenin) Mutations in Endometrial Carcinoma and PrecancersNiu, Shuang MD, PhD*,†; Lucas, Elena MD*,†; Molberg, Kyle MD*,†; Strickland, Amanda MD‡; Wang, Yan MD*; Carrick, Kelley MD*,†; Rivera-Colon, Glorimar MD*,†; Gwin, Katja MD*,†; SoRelle, Jeffrey A. MD*,§; Castrillon, Diego H. MD, PhD*,†,∥,¶; Zheng, Wenxin MD*,†,∥,¶; Chen, Hao MD, PhD*,† Author Information *Departments of Pathology ∥Obstetrics and Gynecology §Next Generation Sequencing Clinical Lab ¶Harold C. Simmons Comprehensive Cancer Center, UT Southwestern Medical Center †Department of Pathology, Parkland Hospital, Dallas, TX ‡Department of Pathology, Feinberg School of Medicine, Northwestern University, Chicago, IL Philip O’Bryan Montgomery endowment fund, UTSW to K.M. Mark and Jane Gibson Distinguished Professorship endowment fund to W.Z. Start-up fund and intramural research fund of the Department of Pathology, UTSW to H.C. and S.N. Conflicts of Interest and Source of Funding: The authors have disclosed that they have no significant relationships with, or financial interest in, any commercial companies pertaining to this article. Correspondence: Hao Chen, MD, PhD, Department of Pathology, UTSW UT Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390 (e-mail: hao.che[email protected]). The American Journal of Surgical Pathology: October 2022 - Volume 46 - Issue 10 - p 1447-1455 doi: 10.1097/PAS.0000000000001934 Buy SDC Metrics Abstract Although collectively regarded as “squamous differentiation (SD)” in endometrial endometrioid carcinoma (EEC) and atypical hyperplasia/endometrioid intraepithelial neoplasia (AH/EIN), morules (often referred to as “squamous morules”) and true SD may represent two distinct phenomena. Here, we explored the distinction between morules versus SD and investigated the association of morules and SD with CTNNB1 mutations. A total of 270 cases of EEC and AH/EIN were studied, including EEC with (n=36) or without (n=36) morules and AH/EIN with (n=80) or without (n=118) morules. Cases were analyzed by immunohistochemistry and selected cases (n=20) by targeted next-generation sequencing panel. Near-perfect agreement was found between morules and glandular β-catenin nuclear staining in AH/EIN and EEC. A strong positive association was found between morules and glandular β-catenin nuclear staining (P<0.0001, Φ=0.59 in AH/EIN; P<0.0001, Φ=0.85 in EEC). There was no association between (1) morules and glandular PAX2 or PTEN aberrant expression or (2) SD and aberrant expression of β-catenin, PAX2 or PTEN (Φ=0.09, β-catenin; Φ=0.16, PAX2; Φ=0.13, PTEN). CTNNB1 mutations were identified in all 20 selected morule-containing cases (100%). Next-generation sequencing was performed on 2 (preprogestin and postprogestin treatment) biopsies from 1 patient, revealing identical mutational profile in morules and glands. In conclusion, (1) SD and morules are distinct biological phenomena; (2) the presence of morules, but not SD, is a reliable indicator of CTNNB1 mutations in EEC and AH/EIN. Our findings demonstrate that SD and morules are distinct biological phenomena. Since morules but not SD are associated with β-catenin mutations, the distinction is clinically relevant and should be included in diagnostic reports. Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.