Does CADM1 Immunohistochemistry Distinguish Mycosis Fungoides from its Closest Mimickers?

Abstract

BackgroundImmunohistochemistry (IHC) sometimes fails to distinguish clinically ambiguous cases of evolving patch‐stage mycosis fungoides (MF) from relatively common, non‐neoplastic inflammatory skin diseases (ISD), particularly spongiotic dermatitis (SD), psoriasis (PSO), and pityriasis lichenoides (PL). CADM1 (cell adhesion molecule 1) is a tumor suppressor gene implicated in the pathogeneses of solid skin tumors, visceral malignancies, and peripheral T‐cell lymphomas. Recent data suggest that CADM1 IHC expression could help distinguish early‐stage MF from ISD.DesignThis pilot study sought to determine whether CADM1 IHC distinguishes MF from ISD in clinically‐annotated cases from our archives. MF (n=14), PL (n=10), PSO (n=10), and SD (n=10) were stained with CADM1. Using appropriate controls and antibody optimization, two pathologists established consensus scores in each case for percentage of lymphocytes staining with CADM1 and CADM1 staining intensity. The localization of CADM1‐positive cells (epidermis including junction and dermis) was also noted.ResultsThere were no differences in CADM1 expression pattern or intensity between groups. All cases showed moderate to strong nuclear CADM1 expression in ≥75% of lymphocytes in both the dermis and epidermis. Some intraepidermal lymphocyte nuclei were challenging to detect on CADM1‐stained slides given uniform moderate‐intensity staining of background keratinocyte nuclei.ConclusionThe challenge of distinguishing evolving patch‐stage MF from ISD facilitates demand for novel and readily available diagnostic tests. Although recent data have suggested that differential expression of CADM1 distinguishes MF from ISD, there was no difference in lymphocyte CADM1 expression and distribution between any of the groups included in our pilot study. Furthermore, evaluation of CADM1 expression in intraepidermal lymphocytes was challenging given background keratinocyte expression.