Understanding the Biology of Triple Negative Breast Cancer and Breast Cancer Stem Cells in Patients of Diverse Ethnicities

Abstract

Triple negative breast cancer is one of the most aggressive breast cancer subtypes, for which there are no approved targeted therapies. In US alone, the incidence of TNBC is highest in women with African ancestry (AA); in western sub-Saharan Africa, single-institution studies show that TNBC constitutes 40- 80% of all breast cancer cases. There is an urgent need to find actionable targets in TNBC of all ethnicities, but especially in patients with African ancestry, whose tumors are suspected to be more aggressive. Here, we sought to better understand the biology of TNBC by finding genes and pathways that are differentially expressed in the stem cell population of patient derived xenografts (PDX) created with TNBC tumors from Ghanaian (GH), AA and White American (WA) women and the effect of these differentially expressed genes on the stem cell phenotype in these primary tumors. We first established an international, inter-institutional collaboration with a teaching hospital in Ghana (Komfo Anokye teaching hospital, KATH) for the acquisition of patients’ samples. From these samples, we created patient derived xenograft models that serve as a reneweable source of tumor tissue to study the cancer biology. Breast cancer stem cells, the small population of cells that have been shown to mediate breast tumor initiation, metastasis, and resistance to conventional therapy have also been reported to mediate the heterogeneity of TNBC and are especially abundant in TNBC in AA women. We therefore sought to understand the biology of these stem cells in our primary patient samples and their associated PDX models. We report here that, through our collaboration, we now have a better understanding of the diversity of the breast cancers that occur in Ghana and other parts of Africa, we have contributed to an improvement in the diagnosis and treatment of breast cancer patients and have contributed to enriching the human resource at the KATH in Ghana. We have successfully created a cohort of PDXs from TNBC patients in Ghana as well as African-American and White American patients. By sequencing the stem cells in these tumors, we have identified the aldehyde dehydrogenase (ALDH) expressing stem cell population to represent the stem cells in these aggressive tumors from this diverse population. We identified 14 genes that were simultaneously differentially expressed between the two-breast cancer stem cell sub-populations in (ALDH+ vs the CD44+/CD24-/EPCAM+) as well as ALDH+ vs bulk (p-value <0.001, FDR < 0.05 for both comparisons). The 3 most significant genes were matrix metalloproteinase 2 (MMP2) and Protocadherin 7 (PCDH7), both known to be involved in breast cancer metastasis and Probable carboxypeptidase X1 (CPXM1), a carboxylase. Inhibiting MMP2 expression in the PDX cells grown in suspension resulted in significant reduction in the ALDH+ cell population. Also, the ALDH+ and not the CD44+/CD24- cells formed spheres in serum free media. The WNT, MAPK and TGF-beta pathways known to mediate metastasis were all significantly up-regulated in the ALDH+ population with down regulation of biosynthetic pathways which were up-regulated in the CD44+/CD24- population. Further studies are ongoing on pathway modulation in ALDH1+ cells based on these findings. Together, these findings demonstrate the importance of international collaborations with mutual benefits. It also shows that TNBC, being a very heterogeneous disease, may be driven by a population of tumor cells that can be targeted to improve patient outcome.