Abstract 1654: The low-penetrance breast cancer risk allele MCS5A functions in T-cells through downregulation of the E3-ubiquitin ligase Fbxo10, to alter mammary epithelial cell differentiation

Genome-wide association studies (GWAS) have identified ∼50 low-penetrance, high-population frequency breast cancer susceptibility variants, of which the vast majority are non-protein coding. Mechanisms through which such variants modulate breast cancer risk are largely unknown. The non-coding mammary carcinoma susceptibility locus 5a (MCS5A/Mcs5a) is associated with breast cancer risk in both women and rats (Samuelson et al., 2007 PNAS). We use a comparative genetics approach to functionally characterize this locus. Using mammary gland and bone marrow transplantation assays in congenic rat models, we showed that the Mcs5a resistance allele acts through a non-mammary cell-autonomous immune mechanism. We found that Mcs5a is associated with altered T-cell homeostasis and functions, most notably a 38% and 89% higher abundance of gammadelta T-cells in the spleen and mammary gland, respectively (Smits et al., 2011 Breast Cancer Res). On the molecular genetic level, the non-protein coding Mcs5a resistance allele was found to strongly associate with ∼2-fold and ∼4-fold lower Fbxo10 transcript levels in thymic and splenic T-cells, respectively, but not in other immune cell types or mammary gland (Smits et al., 2011 Nucleic Acids Res). Fbxo10 is an E3-ubiquitin ligase gene, transcriptionally initiating from within Mcs5a. Using flow cytometric analysis of the rat mammary epithelium with antibodies against CD24, CD29, CD31, CD45, cytokeratin (CK) 14, CK19, as well as staining with phalloidin, we identified the luminal and basal/myoepithelial cell populations (Sharma et al., 2011 PLoS ONE). Quantitative analysis of these populations revealed that in a window of cancer susceptibility to carcinogen exposure (∼50-90 days of age), mammary glands from Mcs5a resistant congenic rats have a 47% lower frequency of basal/myoepithelial (CD24medCD29high) cells. Loss of CD29 (Integrin-beta1) expression in the mouse mammary epithelium has been implicated in mammary cancer resistance. To investigate the function of Fbxo10, we generated an Fbxo10 knockout mouse model, which recapitulated both the gammadelta T-cell and the mammary epithelial cell differentiation phenotypes. We hypothesize that Mcs5a controls Fbxo10 expression levels in the thymus, resulting in higher abundance of gammadelta T-cells, especially in the mammary epithelium, which contributes to resistance to carcinoma development through increased immune surveillance and/or altered mammary epithelial cell differentiation. Fbxo10 and its downstream ubiquitination proteins represent novel breast cancer prevention targets within the immune system. This study exemplifies the necessity of mammalian genetic models for deciphering mechanisms through which low-penetrance variants affect cancer susceptibility. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1654. doi:1538-7445.AM2012-1654