Abstract 4065: Regulation of luminal breast cancer stem cells by HER2 does not require gene amplification providing a potential molecular explanation for the efficacy of adjuvant trastuzumab in patients with “clinically HER2-negative” breast cancer.

Although current breast treatment guidelines limit the use of HER2 blocking agents such as trastuzumab to women whose tumors display HER2 gene amplification, recent retrospective analysis by several groups have suggested that in the adjuvant setting a wider group of patients whose tumors do not display HER2 amplification may also benefit from this therapy. We hypothesized that these unanticipated results might be explained by the cancer stem cell (CSC) hypothesis. We previously demonstrated that HER2 was an important regulator of CSCs in HER2 amplified breast cancers. We now utilize breast cancer cell lines, mouse xenograft and matched human primary and metastatic tumor samples to demonstrate that HER2 is selectively expressed in and regulates the self-renewal of CSC populations in ER-positive luminal breast cancers which do not display HER2 amplification. In luminal MCF7 and ZR-75-1 breast cancer cells, as well as in primary human tumors, we demonstrate an association between cells which express HER2 and the stem cell marker aldehyde dehydrogenase (ALDH1). In vitro trastuzumab reduced the CSC population as assessed by tumorsphere formation or ALDH expression while no such effects were scene in basal/claudin-low MDA-MB-231 and SUM159 cells. In mouse luminal tumor xenografts, the effects of trastuzumab were highly dependent on the timing of administration. While the effects of trastuzumab on established tumors were limited to those with HER2 amplification, trastuzumab administration begun immediately after tumor inoculation blocked the growth of luminal tumors which did not display HER2 amplification. HER2 expression was increased in luminal tumors grown in mouse bone xenografts, as well as in bone metastasis from patients with luminal breast cancer compared to matched primary tumors. Furthermore, this increase in HER2 protein expression was not due to gene amplification, but rather was mediated by RANK ligand in the bone microenvironment. These studies suggest that the clinical efficacy of adjuvant trastuzumab may relate to the ability of this agent to target the CSC population in a process that does not require HER2 gene amplification. Furthermore, these studies support a CSC model in which maximal clinical benefit is achieved when CSC targeting agents are administered in the adjuvant setting. Citation Format: Korkaya Hasan, Fayaz Malik, Kathleen C. Day, Suthinee Ithimakin, April Davis, Ahmed A. Quraishi, Nader Tawakkol, Kathleen Woods Ignatoski, Dafydd Thomas, Shawn G. Clouthier, Max S. Wicha. Regulation of luminal breast cancer stem cells by HER2 does not require gene amplification providing a potential molecular explanation for the efficacy of adjuvant trastuzumab in patients with “clinically HER2-negative” breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4065. doi:10.1158/1538-7445.AM2013-4065