AUTHOR CONTRIBUTIONS G.R. designed the experimental approach, performed the experimental work, analyzed the data, coordinated the project and wrote the manuscript. A.H. performed primary tumour growth and exosome education in vivo studies, cancer cell proliferation in vitro studies, cancer cell culture and exosome isolation, coordinated the project and wrote the manuscript. C.M.K. generated CEMIP overexpression, performed molecular cloning work and genetic manipulation of cancer cells, cancer cell culture and exosome isolation, coordinated the project and wrote the manuscript. I.R.M. performed brain slice ex vivo FACS analysis and exosome education in vivo studies, cancer cell culture and exosome isolation, coordinated the project, wrote and reviewed the manuscript. L.S. performed brain slice ex vivo experimental work, tissue processing and immunostaining, ex vivo and in vivo ImageJ data analysis and quantification, cancer cell invasion in vitro studies, western blot analysis, cancer cell culture and exosome isolation, and contributed to figure panel assembly. D.F. performed density gradient exosome isolation, characterization and analysis, western blot analysis, and cancer cell culture. H.S.K. and P.R.O. performed RNA sequencing data analysis. I.S. performed tissue processing and immunostaining, ex vivo and in vivo ImageJ data analysis and quantification, cancer cell culture and exosome isolation. I.C.S. performed western blot analysis and assisted in analysis of human data.

Women of sub-Saharan African descent have disproportionately higher incidence of Triple Negative Breast Cancer (TNBC), and TNBC-specific mortality. Population comparative studies show racial differences in TNBC biology, including higher prevalence of basal-like and Quadruple-Negative subtypes in African Americans (AA). However, previous investigations relied on self-reported race (SRR) of primarily United States (US) populations. Due to heterogenous genetic admixture, and biological consequences of social determinants, the true association of African ancestry with TNBC biology is unclear. To address this, we conducted RNAseq on an international cohort of AAs, west and east Africans with TNBC. Using comprehensive genetic ancestry estimation in this African-enriched cohort, we found expression of 613 genes associated with African ancestry and 2000+ associated with regional African ancestry. A subset of African-associated genes also showed differences in normal breast tissue. Pathway enrichment and deconvolution of tumor cellular composition revealed tumor-associated immunological profiles are distinct in patients of African descent.

Cancer cells metastasize from primary tumors to regional lymph nodes and distant sites via the lymphatic and blood vascular systems, respectively. Our prior work has demonstrated that in primary breast tumors, cancer cells utilize a three-cell complex (known as tumor microenvironment of metastasis, or TMEM) composed of a perivascular macrophage, a tumor cell expressing high levels of the actin-regulatory protein mammalian enabled (Mena), and an endothelial cell as functional “doorways” for hematogenous dissemination. Here, we studied a well-annotated case–control cohort of human invasive ductal carcinoma of the breast and metastatic lymph nodes from a separate breast cancer cohort. We demonstrate that in primary breast tumors, blood vessels are always present within tumor cell nests (TCNs) and tumor-associated stroma (TAS), while lymphatic vessels are only occasionally present in TCN and TAS. Furthermore, TMEM doorways not only exist in primary tumors as previously reported but also in lymph node metastases. In addition, we show that TMEM intravasation doorways are restricted to the blood vascular endothelium in both primary tumors and lymph node metastases, suggesting that breast cancer dissemination to distant sites from both primary tumors and metastatic foci in lymph nodes occurs hematogenously at TMEM doorways. TMEMs are very rarely detected at lymphatic vessels and do not confer clinical prognostic significance, indicating they are not participants in TMEM-associated hematogenous dissemination. These findings are consistent with recent observations that hematogenous dissemination from lymph nodes occurs via blood vessels.

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