Therapeutic resistance and tumor-initiation: Molecular pathways involved in breast cancer stem cell self-renewal

Abstract

528 Background: Recent evidence supports the existance of a rare subpopulation of ‘cancer stem cells‘ (CSCs) which is chemoresistant and capable of self-renewal and tumor-initiation, resulting in relapse and metastases. We hypothesized that residual breast tumors after conventional chemotherapy (CTx) are enriched for CSCs bearing CD44+/CD24- markers, and show increased self-renewal as demonstrated by mammosphere (MS) forming assays. Molecular pathways like Notch, Wnt, and the polycomb family that regulate normal mammary self-renewal may be in aberrant in CSCs. Methods: Paired breast cancer biopsies from 35 patients were obtained before and after 12 weeks of neoadjuvant CTx (docetaxel 100 mg/m2 or Adriamycin/Cytoxan 60/600 mg/m2, 4 cycles, q3weeks), digested by collagenase, stained with CD24/CD44/lineage antibodies, and analyzed by flow cytometry. MS assays were performed to measure self-renewal ability. Gene expression, using the Affymetrix U133 GeneChip platform, of cancer cells bearing CD44+/CD24- markers vs. all other sorted cells, and between secondary cancer MS vs. the primary bulk invasive cancers were analyzed. Results: CD44+/CD24- cells increased from a median of 4.8% to 14.8% after CTx (p<0.005). Increased self-renewal was demonstrated by an increase in MS capacity after CTx (p=0.03), with a positive correlation between the number of CD44+/24- cells and MS assays (R=0.8, p<0.05). Common molecular pathways shared by CD44+/CD24- cells and MS show increased expression in normal self-renewal pathways - polycomb family (PCGF5), Notch (MAML2), FOXP1, and BBX. In addition, genes governing alternative splicing were increased, including a non-coding RNA (MALAT1) of unknown function, and RNA splicing factors (SFRS3, SFRS21P, SFRS4). Conclusions: Our results with an increase in cells bearing stem cell markers, and increased MS formation of residual tumors provide the first strong clinical evidence for the existance of therapy-resistant cancer stem cells. Post-transcriptional regulation may play a crucial role in modifying gene function involved in cancer stem cell self-renewal. Clinical trials targeting these newly identified pathways may eradicate residual disease and improved cure rates for many breast cancer patients. [Table: see text]