European Journal of Gynaecological Oncology Eur. J. Gynaecol. Oncol. 2021 vol. 42(5), 887-892 ©2021 The Author(s). Published by IMR Press. Original Research Concurrent immunohistochemical testing of L1CAM and MMR proteins adds value in risk stratifcation of endometrial cancer: a proof of concept Jaswinder Chalia 1 , Musse Hussein 1 , Mariya Farooqui 1 , Jordan Mattson 2 , Sally A. Mullany 2 , Molly E. Klein 1 , Boris Winterhoff 2 , Mahmoud A. Khalifa 1, * 1 Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, MN 55455, USA 2 Division of Gynecologic Oncology, Department of Obstetrics, Gynecology and Women's Health, University of Minnesota, Minneapolis, MN 55455, USA *Correspondence: mkhalifa@umn.edu (Mahmoud A. Khalifa) DOI:10.31083/j.ejgo4205135 This is an open access article under the CC BY 4.0 license (https://creativecommons.org/licenses/by/4.0/). Submitted: 15 July 2021 Revised: 24 August 2021 Accepted: 15 September 2021 Published: 15 October 2021 Objectives: Histologic classifcation along with clinical stage predomi- nantly drive management of patients with endometrial cancer. How- ever, current clinico-pathologic risk-based stratifcation has proven suboptimal, inciting efforts to identify additional molecular classi- fers, such as L1CAM. This is of particular relevance for the TCGA- defned Nonspecifc Molecular Profle (NSMP) and MMR-defcient (MMR-d) groups of tumors, both of which are classifed as having an intermediate prognosis. In current practice, L1CAM immunostain- ing is reserved for NSMP tumors that have been classifed as MMR- profcient. The aim of this study is to investigate L1CAM testing in tandem, rather than sequential with that of MMR. Methods: A total of 149 MMR-tested endometrial carcinoma cases from 2019–2020 were identifed, of which, 45 had also undergone L1CAM immunostain- ing. Clinical information including grade, stage, and treatment was reviewed. This was correlated with percentage of L1CAM positivity and MMR-status. Results: L1CAM positivity was noted in 7/45 (15.6%) cases with 6/45 (13.3%) additional cases demonstrating only focal positivity. MMR defciency was noted in 24/45 (53.3%) of the cases in which L1CAM was performed. Of the cases that showed L1CAM pos- itivity, 6/7 (85.7%), were found to be MMR-defcient. Within the re- maining group in which L1CAM was not performed, 24/104 (23.1%) of cases showed MMR defciency. Conclusions: Current fndings suggest that L1CAM positivity is not mutually exclusive when correlating with MMR status. Performing L1CAM immunostaining on all endometrial carcinomas may assist in appropriate treatment for patients with L1CAM positivity, and in particular, in MMR-profcient casesclassi- fed within the NSMP category. Keywords L1CAM; MMR; Endometrial cancer; Testing algorithm; Molecular classifcation 1. Introduction As the most commonly diagnosed gynecologic malignancy in the United States, endometrial cancer primary affects post- menopausal women. However, approximately 14% of en- dometrial cancers are diagnosed in premenopausal women, of which one-third occur in women under the age of 40 years. Unlike most other cancers, the incidence of endometrial can- cer (EC) has continued to slowly increase over the past decade with an associated increase in cancer mortality. The increas- ing incidence has been attributed to the rise in obesity, aging population, changing hormonal risk factors, and more im- portantly, rising rates of more aggressive, non-endometrioid histology [1]. Efforts to decrease mortality from endometrial cancer need to focus on improved risk stratification at initial diagnosis, identifying those patients at highest risk for recur- rence who would benefit from adjuvant treatment and im- proving therapeutic options for those who develop recurrent disease. In early stage EC, risk stratification of patients after surgery using clinico-pathologic features typically drive the need for adjuvant therapy. However, stratifying patients into various risk groups based on these criteria continues to be suboptimal [2]. While most early stage, grade 1 and grade 2 endometrioid endometrial carcinomas are associated with good prognosis, a subset of patients with lower stage tumors will still experience relapse and poor outcome [3, 4]. Con- versely, approximately 50% of patients with high grade tu- mors that are classified as high risk experience no recurrence [2]. Additional classifiers, beyond the currently used clinico- pathologic criteria first described in GOG 99, have evolved over the past two decades with the promise of a more clini- cally reliable stratification scheme. Molecular classification of EC was first introduced by The Cancer Genomic Atlas (TCGA) using whole genome sequencing in 2013, with 4 distinct subtypes identified in- cluding: (1) POLEmut: DNA polymerase epsilon exonucle- ase (POLE) domain mutations or ultra-mutated carcinoma; (2) MMRd: microsatellite-instable (MSI) hyper-mutated with deficient mismatch repair (MMR) proteins; (3) Copy- number high: p53 mutant; (4) Copy-number low: Nonspe- cific Molecular Profile (NSMP). These subtypes have been further studied clinically and found to translate into prognos- tic outcomes. The NSMP or copy number low subtype is de- fined by the lack of molecular subtype expression that defines each of the other three groups. Prognostic outcomes are less