ABSTRACT: Tumorigenic breast cancer cells characterized by CD44 expression and low or undetectable CD24 levels (CD44+/CD24-/low) may be resistant to chemotherapy and therefore responsible for cancer relapse. Paired breast cancer core biopsies before and after neoadjuvant chemotherapy or lapatinib were obtained and as single cell suspensions stained using antibodies against CD24, CD44, and lineage markers, and then analyzed by flow cytometry. Mammosphere (MS) formation in culture was compared before and after treatment. Global gene expression differences between cancer cells bearing CD44+/CD24-/low cells and all other sorted cells, and between cancer MS and the primary bulk invasive cancers were analyzed. We report that CD44+/CD24-/low tumorigenic breast cancer cells were intrinsically chemoresistant ─ chemotherapy led to increased CD44+/CD24-/low cells, increased self-renewal capacity on MS assays, and enhanced tumorigeneicity in immunocompromised SCID/Beige mice. Conversely, in patients with HER2 overexpressing tumors, the EGFR/HER2 tyrosine kinase inhibitor, lapatinib decreased CD44+/CD24-/low cells, with the majority of these patients after conventional therapy achieving pathologic complete response, a validated surrogate marker for long-term survival. Gene transcription pathways that underlie chemoresistant, MS-forming CD44+/CD24-/low cells involve genes belonging to stem cell self-renewal, Wnt signaling, and early development pathways. Experiment Overall Design: Human breast tumor samples were sorted using flow cytometry to select for cells that were CD44+ and CD24-. Gene expression profiles of these cells were compared with profiles of the other sorted cells (CD24+ and CD44-/CD24-). Experiment Overall Design: Core biopsies of primary breast tumors were taken and placed immediately in cold RPMI-1640 supplemented with 10% heat-inactivated newborn calf serum (HINCS, Invitrogen, Carlsbad, CA). Within an hour, the samples were minced and then digested in 10-15 mL of MEGM with 250-300 units/mL collagenase at 370C. The samples were filtered, washed, and then subjected to hypotonic shock to lyse red blood cells. About 106 single cells were re-suspended, incubated for 15 min at 40C with anti-CD44 (APC), anti-CD24 (FITC), and anti-lineage cocktail antibodies (PE-conjugated anti-CD2, CD3, CD10, CD16, CD18, CD31 and CD 140B) (Pharmingen, San Diego, CA) using the manufacturer’s suggested concentrations. The cells were then washed twice, re-suspended with the viability dye propidium iodide, and analyzed using Dako MoFlo flow cytometry. Side- and forward- scatter were used to eliminate debris and cell doublets, and the Lin- cells were further analyzed by CD44 and CD24 markers.
