Abstract 5007: Absence of mammary tissue-resident macrophages is associated with reduced breast cancer susceptibility mediated by the cancer-associated 8q24 gene desert
Authors: Smits Bart M. , Rissman Anna I. , Shunkwiler Lauren , Homer-Bouthiette Collin , Zhao Yang , Peel Benjamine van , Schulte Jennifer , Wilson Robert , Soloff Adam C. 
Published: August 2018

Abstract
Non-protein coding regions have frequently been associated with cancer susceptibility, with the human 8q24 gene desert linked to decreased incidence of breast cancer. To examine the impact of 8q24 upon mammary tissue development and breast cancer dynamics, we created a novel megadeletion (MD) mouse model lacking 430kb of the region orthologous to the human 8q24 gene desert. Homozygous MD knockout (MD-/-) mice have lower MYC expression in multiple organs, including whole mammary gland and mammary tumors, 10% reduced body weight, and strongly reduced susceptibility to luminal and to a lesser degree basal breast cancer compared to wild type animals. The latter finding mirrors human genetic data that associates the 8q24 gene desert variant more strongly with Estrogen Receptor-positive (ER+) than ER-negative (ER-) disease. In this study, we found that the MD-/- alters mammary gland development, resulting in a decrease in ductal branch points, fewer terminal end buds, and lower luminal/basal ratio. Using a reciprocal mammary gland transplantation assay, we found a strong donor effect and weaker host effect of the mutation on mammary gland development, indicating that the non-protein coding locus affects mammary cell-autonomous as well as non-mammary cell-autonomous processes. Interestingly, MD-/- resulted in a marked decrease in immune cell infiltrate in mammary tissues during development and throughout spontaneous tumor induction driven via the PyMT oncogene. Cellular profiling demonstrated a significant loss of mammary tissue-resident macrophage populations, but not monocyte-derived macrophages, with concomitant polarization away from homeostatic, developmental phenotypes which may be subverted to support breast cancer tumorigenesis. Using an integrative computational approach combining flow cytometry-based cell phenotyping with RNAseq-derived differential gene expression, we identified gene set and motif enrichment networks from mammary epithelium samples from wild type and knockout mice regulating mammary gland development, cancer processes, and immune function including inflammatory processes in regions affected by the MD locus. Taken together, these findings suggest that 8q24-mediated disruption of homeostatic mammary macrophage populations, either via MYC or yet undetermined mechanisms, may deprive neoplastic cells of crucial support from the stroma, inhibiting cancer initiation and progression. Our findings not only elucidate how the murine ortholog of the 8q24 gene desert, and likely MYC expression regulation, alters mammary gland development, but also provides novel insight into potential links between mammary development, immune infiltration, and breast cancer susceptibility.
Citation Format: Bart M. Smits, Anna I. Rissman, Lauren Shunkwiler, Collin Homer-Bouthiette, Yang Zhao, Benjamine van Peel, Jennifer Schulte, Robert Wilson, Adam C. Soloff. Absence of mammary tissue-resident macrophages is associated with reduced breast cancer susceptibility mediated by the cancer-associated 8q24 gene desert [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5007.