Primary breast cancer patients with high risk clinicopathologic features have high percentages of bone marrow epithelial cells with ALDH activity and CD44 ⁺CD24 ^lo cancer stem cell phenotype

Background: Cancer stem cells (CSCs) are purported to be epithelial tumour cells expressing CD44 ⁺CD24 ^lo that exhibit aldehyde dehydrogenase activity (Aldefluor ⁺). We hypothesised that if CSCs are responsible for tumour dissemination, disseminated cells in the bone marrow (BM) would be positive for putative breast CSC markers. Therefore, we assessed the presence of Aldefluor ⁺ epithelial (CD326 ⁺CD45 ^dim) cells for the presence of the CD44 ⁺CD24 ^lo phenotype in BM of patients with primary breast cancer (PBC). Methods: BM aspirates were collected at the time of surgery from 66 patients with PBC. Thirty patients received neoadjuvant chemotherapy (NACT) prior to aspiration. BM was analysed for Aldefluor ⁺ epithelial cells with or without CD44 ⁺CD24 ^lo expression by flow cytometry. BM aspirates from three healthy donors (HD) were subjected to identical processing and analyses and served as controls. Results: Patients with triple-receptor-negative (TN) tumours had a significantly higher median percentage of CD44 ⁺CD24 ^lo CSC within Aldefluor ⁺ epithelial cell population than patients with other immunohistochemical subtypes (P = 0.018). Patients with TN tumours or with pN2 or higher pathologic nodal status were more likely to have a proportion of CD44 ⁺CD24 ^lo CSC within Aldefluor ⁺ epithelial cell population above the highest level of HD. Furthermore, patients who received NACT were more likely to have percentages of Aldefluor ⁺ epithelial cells than the highest level of HD (P = 0.004). Conclusion: The percentage of CD44 ⁺CD24 ^lo CSC in the BM is higher in PBC patients with high risk tumour features. The selection or enrichment of Aldefluor ⁺ epithelial cells by NACT may represent an opportunity to target these cells with novel therapies.