Breast Cancer Initiating Cells in Tamoxifen Treatment and Resistance

  • Ciara O'Brien

Student thesis: Phd


Resistance to endocrine treatments in oestrogen receptor positive (ER+) breast cancer (BC) significantly contribute to patient morbidity and mortality. ER+ BC constitute 60% of all breast cancers although there is considerable clinico-pathological diversity within this group. Breast cancer initiating cells (BCICs) are implicated in tumour relapse and metastasis and are postulated to drive resistance to standard anti-cancer therapies. However little is known about the sensitivity of BCICs to endocrine therapies. We assessed the effect of tamoxifen treatment and acquired tamoxifen resistance on BCIC frequency in vitro and in vivo using breast cancer cell lines and, importantly, patient derived samples of early and metastatic ER+ breast cancer.In ER+ breast cancer, BCICs may be prospectively enriched in vitro by selecting cells by CD44+/CD24lo/ESA+ phenotype or by mammosphere initiating capacity (MIC). However the gold standard assay to determine BCIC frequency is limiting dilution transplantation in vivo. In the past it has been historically difficult to generate xenograft models of ER+ breast cancer using patient samples. In this thesis, using a novel experimental technique, patient-derived xenografts (PDX) of early and metastatic ER+ BC were generated with almost 85% efficiency in NOD/SCID IL2gammaR-/- (NSG) mice. PDX expressed ER and were able to undergo serial in vivo passage, matching the phenotype of the tumour from which they were derived. In this work, two patterns of response to tamoxifen treatment were observed in ER+ cell lines, patient derived breast cancer samples and xenografts during BCIC assays in vitro and in vivo; Limited Sensitivity (LS) or Resistance (R). In the LS group there was no change or a significant diminution in BCIC frequency in the presence of tamoxifen. In the R group, a significant increase in BCIC frequency was observed in the presence of tamoxifen. Furthermore BCIC activity was shown be enhanced by the acquisition of tamoxifen resistance using cell line models. Cellular populations enriched for BCICs in ER+ cell lines were shown to express low levels of ER compared to non-BCICs. Finally Notch (gamma-secretase inhibitor) and EGFR (gefitinib) pathway inhibitors were tested alone or in combination with tamoxifen against a panel of established and novel cell lines and ER+ patient-derived breast cancer samples for anti-BCIC activity.Tamoxifen treatment can increase BCIC frequency in vitro assays of cell lines and patient-derived samples and in vivo using patient-derived xenografts of ER+ breast cancer. However phenotypic diversity of BCIC may be present within the ER+ BC population. A pharmaceutical strategy to effectively treat BCICs alongside standard endocrine therapy is necessary for the effective future treatment of ER+ breast cancer.
Date of Award1 Aug 2012
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorRobert Clarke (Supervisor)


  • oestrogen receptor
  • xenograft
  • treatment resistance
  • tamoxifen
  • breast cancer
  • stem cell

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