Abstract A078: Characterization of normal human male mammary cells and their <i>de novo</i> transformed derivatives

Abstract Men normally develop and maintain bi-layered mammary glands that appear structurally and histologically similar to those that develop in women, but with reported deficient lobule formation. Men also develop breast cancer, mostly of the ER+ subtype and an age-associated increasing onset, but at a 100-fold lower incidence and overall worse outcome. However, the classification and treatment of male breast cancers are largely based on strategies developed for female patients, despite their known hormonal and other differences. This is likely due, at least in part, to the fact that adult male mice, unlike humans do not develop full mammary glands. Thus, experimental access to mouse models have been lacking and studies of “normal” male mammary tissue have thus been largely restricted to analysis of human male gynecomastia samples. We now report the utility of multiple systems for analyzing normal adult human female mammary cell properties to this source of freshly obtained and viably cryopreserved normal human male mammary tissue. Histological examination of male mammary tissues (3 donors) confirmed their reported bi-layered structure surrounded by fibroblasts and lack of gross evidence of lobules. Use of procedures created for analyzing female breast tissue indicates that the human male mammary glands contain the same subsets but in different relative proportions; i.e., the proportion of EpCAM+CD49f- mammary cells (Luminal Cells, LCs) is relatively increased and of the EpCAM+CD49f+ cells (Luminal Progenitors, LPs) and EpCAM-CD49f+ cells (Basal Cells, BCs) is correspondingly decreased, consistent with a lack of alveolae. However, the freshly isolated male LPs and BCs contain a similar frequency of EGF-responsive colony-forming cells (CFCs) as female LPs and BCs. Subcutaneous transplantation of the male mammary cells in both male and female immunodeficient mice resulted in the generation of hollow mammary structures 4 weeks later as previously shown for female cells, and these were also found to contain LC, LP and BC populations including some with detectable levels of CFCs suggesting their derivation from a male mammary stem cell population (Eirew et al Nat Med 2008). We have also found evidence from qPCR and RNA-seq analyses of reduced progesterone signaling and alveologenesis in male LPs and LCs. These findings suggest hormonal differences may play a major role in regulating the altered ratios of cell types present in the normal adult human male mammary gland in comparison to premenopausal female. Interestingly, preliminary experiments also indicate that normal human male mammary cells transduced with KRASG12D without, or with BMI1+MYC+TP53R273C , generate analogous slowly-growing ER+, and rapidly-growing, highly aggressive tumours, respectively, in male immunodeficient mice. Taken together, our findings demonstrate the potential of established cell culture, xenotransplantation and molecular analytical tools to reveal the altered mechanisms that account for the different structure and biology of human male mammary tissue. Citation Format: Shengsen Ding, Susanna Tan, Ebrahim Eskandari-Nasab, Shrinka Sen, Martin Hirst, Connie Eaves. Characterization of normal human male mammary cells and their de novo transformed derivatives [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr A078.

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