Abstract LB328: Measurement of ER and Ki67 coexpression in cytokeratin positive epithelial cells in benign breast disease (BBD) percutaneous biopsies

Abstract Over 1 million women in the U.S. undergo radiologically guided percutaneous biopsies annually. Of these, 75% reveal benign breast disease (BBD), typically surrounded by normal breast lobules, which are proposed to reflect the biology of the breast parenchyma diffusely. With aging, normal lobules undergo involution, and delays in this process are linked to increased breast cancer (BC) risk. Because BBD patients are at increased risk of developing BC, and pathologic classification only stratifies risks among groups of patients, novel methods for individual BC risk prediction are needed to guide precision management. Prior literature has shown that most proliferating epithelial cells in normal lobules do not express estrogen receptor (i.e., ER-/Ki67+), whereas co-expression is frequent in BC precursors and in BCs (i.e., ER+/Ki67+). Thus we developed and applied a novel quantitative dual ER/Ki67 stain to BBD biopsies from the well-characterized contemporary Mayo Clinic BBD cohort (2002-2013) to assess differences by levels of lobular involution and tissue type (BBD vs. normal), as well as associations with BC risk. We identified 100 BBD biopsies preceding BC (cases) and 100 biopsies from cancer-free women (controls), frequency-matched on age and length of follow-up. A multiplex immunofluorescence assay was developed to detect ER/Ki67 coexpression in cytokeratin positive epithelial cells in normal lobules or BBD lesions in FFPE tissue sections. An inverse normal transformation was applied to cell counts and proportions and associations were assessed for levels of lobular involution, tissue type (normal lobule vs. BBD), and case status using t-tests and linear regression models. The final data set included 1152 normal lobules or BBD lesions in biopsies of 177 subjects (88 cases and 89 controls). In univariate analyses, normal lobules showing delayed age related involution included higher counts of ER-/Ki67+, ER+/Ki67- and ER+/Ki67+ cells, and the lowest proportion of ER+/Ki67- cells (p<0.03; all). In separate analyses comparing presence vs. absence, detection of any ER+/Ki67+ cells was suggestively more common in non-involuted vs. involuted lobules. Compared with normal lobules, BBD showed higher counts and proportions of ER+/Ki67- and ER+/Ki67+ cells and lower values for ER-/Ki67+ (all p<0.001). In these preliminary results, marker expression in normal lobules and in BBD did not differ by case-control status. Preliminarily, we find that ER+/Ki67+ cells are increased in normal lobules with delayed age-related involution and in BBD lesions, consistent with their associations with increased BC risk. These findings are consistent with the risk reducing effects of endocrine chemoprevention and may serve as useful biomarkers in prevention therapy. In this analysis, limited to only a few lobules and BBD lesions per biopsy, ER+/Ki67+ cell counts and proportions did not vary significantly by case-control status. Citation Format: Melody L. Stallings-Mann, Robert A. Vierkant, Stacey J. Winham, Bryan McCauley, Matt R. Jensen, Sophie Noel, Laura Pacheco-Spann, Celine M. Vachon, Amy C. Degnim, Derek C. Radisky, Mark E. Sherman. Measurement of ER and Ki67 coexpression in cytokeratin positive epithelial cells in benign breast disease (BBD) percutaneous biopsies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB328.